sábado, 26 de junio de 2010

Hybrid materials for batteries: gels as ionic conductors.

Electrolytes for use in lithium batteries have to be free of Si–OH-functionalities, as otherwise hydrogen forms, which must be strictly avoided. Si–OH-functionalities can be prevented either by choosing appropriate solvents and catalysts during synthesis or by the reaction of remaining Si–OH with tri-substituted alkoxysilanes.

By specific choice of functionalized alkoxysilanes and addition of plasticizers conductivities of up to 1023 S cm21 can be achieved186 while a good mechanical stability is also maintained. These materials are electrochemically stable up to 4.2 V. Prototype battery production based on an up-scaled ORMOCER1 electrolyte separator from Fraunhofer ISC has started at Varta Company.

Recent developments at the Fraunhofer ISC aim at systems which can be applied without adding liquid plasticizers. Such electrolytes will have enhanced dimensional stability. So, very thin electrode foils without further encapsulation measures can be used. Such electrolytes have reached conductivities of about 5 6 1025 S cm21 at room temperature until now. These values are below those for systems containing liquid plasticizers but they are sufficient for only 20 mm thick layers which are achieved in the battery concept. This conductivity in addition to an electrochemical stability of 4.2 V shows the very high potential of this new electrolyte for thin film lithium–polymer batteries (see Fig. 21).

In parallel and in cooperation at LEPMI-INPG, organic– inorganic gels have also been synthesized by interchanging some alkoxy groups of Si(OR)4 precursors with polyethyleneglycol (PEG) chains. The PEG was used to solvate small cations such as lithium, leading to a good ionic conductivity. 181 However the slow hydrolysis of the Si–O–PEG bonds leads to a degradation of such materials. This stability problem can be avoided by using AMINOSILS.181 These compounds were recently synthesized via the hydrolysis and condensation of Si(OR)3R9 precursors (R9 = –(CH2)n–NH2). The nonhydrolyzable alkylamino group can solvate, via the amino group, anions such as CF3SO3 2 rendering free for conduction the associated counter ions (protons). The resulting gels exhibit a rather good protonic conductivity at room temperature (s = 1.4 6 1025 S cm21 for Si(OR)3(CH2)3NH2,(CF3SO3H)0.1 based systems).181,187

Among electrochemical devices, electrochromic displays using transition metal oxides (WO3, TiO2, MnO2, IrO2) as active electrodes can be built by using protonic conductor gels as electrolytes. However, in such acidic conditions and upon electrochromic solicitations the oxide layers are corroded, because the stability of many oxides lies in the 4 to 12 pH range. To overcome this problem, new proton vacancy conducting transparent polymers which work in a higher pH range were developed.183 However when the different components (POE, sulfamide, guanidinium cation) are mixed without covalent bonding between the different phases, the resulting polymer electrolyte is in a metastable amorphous state.

Slow crystallization responsible for a drastic decrease in conductivity occurs in a few month of storage. In order to overcome both acidity and crystallinity problems, new proton vacancy conductors based on anion-grafted ormosils have been synthesized via a sol–gel process. These ionic conductors are based on a three component system: a solvating polymer (a,v-di-(3- ureapropyltriethoxysilane)poly(oxyethylene-co-oxypropylene)), a proton source (3-methanesulfonamidopropyltrimethoxysilane) and a deprotonating agent or proton vacancy inducer (imidazolium cations introduced through 3-(2-imidazolin-1- yl)propyltriethoxysilane, where imidazoline is used as a strong base).183 These materials are obtained by the copolymerization of sulfonamide-containing groups, partially deprotonated, and POE as an internal plasticizer. All organics groups are anchored to trialkoxy silanes which, through hydrolysis and condensation reaction, lead to a silica based backbone. In the presence of a deprotonating agent, conductivity is greatly enhanced, being now solely due to the motion of proton vacancies. The conductivity is 1025 S cm21 at 80 uC.

Asignatura: CRF
Fuente: www.rsc.org/materials Journal of Materials Chemistry

Hybrid materials for dental applications

Inorganic–organic hybrid materials can be used as filling composites in dental applications. As schematized in Fig. 13(a), these composites feature tooth-like properties (appropriate hardness, elasticity and thermal expansion behaviour) and are easy to use by the dentist as they easily penetrate into the cavity and harden quickly under the effect of blue light.

Moreover, these materials feature minimum shrinkage, are non-toxic and sufficiently non-transparent to X-rays. However, the composition of the hybrid material and the chemistry behind it depends strongly on its later application: as filler/particles, as matrix materials, as composites, as glass ionomer cements or as bonding.

Traditional plastic filling composites had long-term adhesion problems and a high degree of polymerisation shrinkage resulting in marginal fissures. The dual character of the ORMOCER1s as inorganic–organic copolymers is the key for improving the properties of filling composites. The organic, reactive monomers are bound in the sol–gel process by the formation of an inorganic network. Thus, in the subsequent curing process, polymerisation takes place with less shrinkage.

Furthermore, abrasion resistance, in particular, is significantly enhanced by the existing inorganic Si–O–Si network. For example, in dental fillers organic functionalities including ring-opening reactions, such as functionalized spyrosilanes, are commonly included in the hybrid network. Other systems are based on multiacrylate silanes, offering a high organic density. In addition, mechanical properties of the composite can be tuned through variation of the spacer between the silicon atom and the reactive functionality. All these possibilities are already taken into account, and most of these hybrids include various fillers in their composition. As examples of available commercial filling composites based on dental ORMOCER1s from Fraunhofer ISC one can appoint ''Definite1'' and ''Admira1'' (Fig. 13(b)). In the case of the Admira1 product, a specifically designed dentineenamel bonding, an adhesive ORMOCER1 developed in cooperation with VOCO GmbH, is used to make this product especially advantageous. In glass ionomer cement based dental composites blue light polymerisable carboxyl functionalised ORMOCER1s have been developed. In this case, the cement forming reaction compensates the shrinkage resulting from organic crosslinking reaction of e.g. methacryl functionality.
Asignatura: CRF
Fuente: www.rsc.org/materials Journal of Materials Chemistry

Hybrid materials as barrier systems

The interest in hybrid materials as barrier systems has beenincreased in the last decades as a result of the requirements to develop much more sophisticated materials in fields such as solar cells, optics, electronics, food packaging, etc. New barrier coating materials based on ORMOCER1 have been developed by Fraunhofer ISC which together with a vapour-deposited SiOx layer guarantee sufficient protection to ensure a long durability of encapsulated solar cells (see Fig. 11(a)).

This new inorganic–organic hybrid coating represents a whole encapsulation system since apart from the physical encapsulation it acts as an adhesive/sealing layer barrier against water vapour and gases, as well as an outside layer for weatherability. All these functions are combined in one composite (''one component encapsulant'') and in this way the overall cost reduction for encapsulation reaches about 50 per cent. The flexible nature of this hybrid material results in an optimized encapsulation process and especially in good protection of the edge area, the most difficult part for protection. Furthermore, this new material can be used from the roll, thus providing easy handling and automation in the production of flexible modules (Fig. 11(b)). Thin barrier layers like SiOx combined with ORMOCER1 based barrier coatings save material and energy consumption in the production of the new encapsulating material. Therefore, a material and energy saving encapsulation technology is achieved. This result should encourage the application of thin film solar cells in the building industry as well as the number of users of solar cells.

Especially flexible thin film solar cells cover broader fields of application because they can be adapted to non-planar surfaces. This means that photovoltaic (PV) devices could be a standard integrated part of construction components, in e.g. roofing materials, fac¸ades, and balustrades. Thus, a larger number of potential users could benefit from solar technology. Upcoming and future products require flexible films that have high barrier or even ultra-high barrier properties. The encapsulation film for solar modules is one important example which illustrates that there is still a great need for the development of improved polymer barrier systems. The basis of most of the barrier coatings are polycondensates of some percentage of Al alkoxides with phenyl- and epoxyfunctionalised alkoxysilanes.

Further developments on patternable barriers/passivation with good dielectric properties for application in roll-to-roll processing of polymer-electronic devices and systems are on the way for up-scaled technology and production at Fraunhofer ISC.141 Sol–gel chemistry has been extensively applied to produce thin oxide coatings with appropriate protective behavior onto metal substrates.142 Nevertheless, protection of metallic silver reflectors, for example, should resist not only gaseous oxidation but also mechanical and chemical attacks during handling, cleaning or weathering of the metal parts. Laser labs have developed a silica-based hybrid material to protect silvercoated light reflectors installed in laser pumping cavities as shown in Fig. 12.143 These metallic reflectors require a protective overlayer in order to preserve the high-reflectivity front surfaces for long periods of operation under intense broadband flashlamp light and typical airborne contaminants.

The organically modified silica coating has been optimized in terms of thickness and composition to enhance metal resistance to oxidation and tarnishing under UV-irradiation and ozone-attack. To fulfil these requirements, the hybrid sol– gel material not only must act as an oxidation dense barrier but also needs to be chemical-resistant, time-stable and to remain transparent. On the other hand, industrial protective coating deposition onto large-sized and multi-shaped metallic parts is allowed by using the dip-coating technique. The protection efficiency is mainly related to the density of the hybrid coating, and can be managed varying the sol–gel chemistry conditions (hydrolysis rate) and oxide content.

Furthermore, hybrid material compared to pure inorganic allows one to enhance the chemical resistance through incorporation of hydrophobic surface functions, such as methyl groups, which also reduce coating stress allowing a thicker film deposition onto eventually deformable substrates. This hybrid layer preserves the high reflectance of silver over a broad spectral range and enables silver reflectors to withstand a very corrosive medium with no appreciable degradation.
Asignatura: CRF
Fuente: www.rsc.org/materials Journal of Materials Chemistry
Ver: http://nanocompositescrf.blogspot.com/

General strategies for the design of functional hybrids

Independently of the types or applications, as well as the nature of the interface between organic and inorganic components, a second important feature in the tailoring of hybrid networks concerns the chemical pathways that are used to design a given hybrid material. General strategies for the synthesis of sol–gel derived hybrid materials have been already discussed in details in several reviews.7,8,11 The main chemical routes for all type of hybrids are schematically represented in Fig. 3.

Path A corresponds to very convenient soft chemistry based routes including conventional sol–gel chemistry, the use of specific bridged and polyfunctional precursors and hydrothermal synthesis.

Route A1: Via conventional sol–gel pathways amorphous hybrid networks are obtained through hydrolysis of organically modified metal alkoxides (vide infra section III) or metal halides condensed with or without simple metallic alkoxides. The solvent may or may not contain a specific organic molecule, a biocomponent or polyfunctional polymers that can be crosslinkable or that can interact or be trapped within the inorganic components through a large set of fuzzy interactions (H-bonds, p–p interactions, van der Waals). These strategies are simple, low cost and yield amorphous nanocomposite hybrid materials. These materials, exhibiting infinite microstructures, can be transparent and easily shaped as films or bulks. They are generally polydisperse in size and locally heterogeneous in chemical composition. However, they are cheap, very versatile, present many interesting properties and consequently they give rise to many commercial products shaped as films, powders or monoliths. These commercial products and their field of application will be discussed in section III-2. Better academic understanding and control of the local and semi-local structure of the hybrid materials and their degree of organization are important issues, especially if in the future tailored properties are sought. The main approaches that are used to achieve such a control of the materials structure are also schematized in Fig. 3.

Route A2: The use of bridged precursors such as silsesquioxanes X3Si–R9–SiX3 (R9 is an organic spacer, X = Cl, Br, OR) allow the formation of homogeneous molecular hybrid organic–inorganic materials which have a better degree of local organisation.8g,e,15 In recent work, the organic spacer has been complemented by using two terminal functional groups (urea type).15–17 The combination within the organic bridging component of aromatic or alkyl groups and urea groups allows better self-assembly through the capability of the organic moieties to establish both strong hydrogen bond networks and efficient packing via p–p or hydrophobic interactions.15–17 Route A3: Hydrothermal synthesis in polar solvents (water, formamide, etc.) in the presence of organic templates had given rise to numerous zeolites with an extensive number of applications in the domain of adsorbents or catalysts. More recently a new generation of crystalline microporous hybrid solids have been discovered by several groups (Yaghi,18 Ferey,19–23 Cheetham and Rao24). These hybrid materials exhibit very high surface areas (from 1000 to 4500 m2 g21) and present hydrogen uptakes of about 3.8 wt% at 77 K.18–24 Moreover, some of these new hybrids can also present magnetic or electronic properties.20,25 These hybrid MOF (Metal Organic Frameworks) are very promising candidates for catalytic and gas adsorption based applications.18

Path B corresponds to the assembling (route B1) or the dispersion (route B2) of well-defined nanobuilding blocks (NBB) which consists of perfectly calibrated preformed objects that keep their integrity in the final material.7 This is a suitable method to reach a better definition of the inorganic component. These NBB can be clusters, organically pre- or post- functionalized nanoparticles (metallic oxides, metals, chalcogenides, etc.), nano-core–shells26 or layered compounds (clays, layered double hydroxides, lamellar phosphates, oxides or chalcogenides) able to intercalate organic components.27–29 These NBB can be capped with polymerizable ligands or connected through organic spacers, like telechelic molecules or polymers, or functional dendrimers (Fig. 3). The use of highly pre-condensed species presents several advantages: N they exhibit a lower reactivity towards hydrolysis or attack of nucleophilic moieties than metal alkoxides; N the nanobuilding components are nanometric, monodispersed, and with better defined structures, which facilitates the characterization of the final materials. The variety found in the nanobuilding blocks (nature, structure, and functionality) and links allows one to build an amazing range of different architectures and organic–inorganic interfaces, associated with different assembling strategies. Moreover, the step-by-step preparation of these materials usually allows for high control over their semi-local structure. One important set of the NNB based hybrid materials that are already on the market are those resulting from the intercalation, swelling, and exfoliation of nanoclays by organic polymers. Their applications will be described in section IV.

Path C Self assembling procedures. In the last ten years, a new field has been explored, which corresponds to the organization or the texturation of growing inorganic or hybrid networks, templated growth by organic surfactants (Fig. 3,Route C1).30–36 The success of this strategy is also clearly related to the ability that materials scientists have to control and tune hybrid interfaces. In this field, hybrid organic– inorganic phases are very interesting due to the versatility they demonstrate in the building of a whole continuous range of nanocomposites, from ordered dispersions of inorganic bricks in a hybrid matrix to highly controlled nanosegregation of organic polymers within inorganic matrices. In the latter case, one of the most striking examples is the synthesis of mesostructured hybrid networks.33 A recent strategy developed by several groups consists of the templated growth (with surfactants) of mesoporous hybrids by using bridged silsesquioxanes as precursors (Fig. 3, Route C2). This approach yields a new class of periodically organised mesoporous hybrid silicas with organic functionality within the walls. These nanoporous materials present a high degree of order and their mesoporosity is available for further organic functionalisation through surface grafting reactions.37
Route C3 corresponds to the combination of self-assembly and NBB approaches.7 Strategies combining the nanobuilding block approach with the use of organic templates that selfassemble and allow one to control the assembling step are also appearing (Fig. 3). This combination between the ''nanobuilding block approach'' and ''templated assembling'' will have paramount importance in exploring the theme of ''synthesis with construction''. Indeed, they exhibit a large variety of interfaces between the organic and the inorganic components (covalent bonding, complexation, electrostatic interactions, etc.). These NBB with tunable functionalities can, through molecular recognition processes, permit the development of a new vectorial chemistry.38,39

Path D Integrative synthesis (lower part of Fig. 3). The strategies reported above mainly offer the controlled design and assembling of hybrid materials in the 1 A° to 500 A° range. Recently, micro-molding methods have been developed, in which the use of controlled phase separation phenomena, emulsion droplets, latex beads, bacterial threads, colloidal templates or organogelators leads to controlling the shapes of complex objects in the micron scale.30,34 The combination between these strategies and those above described along paths A, B, and C allow the construction of hierarchically organized materials in terms of structure and functions.30,34 These synthesis procedures are inspired by those observed in natural systems for some hundreds of millions of years. Learning the ''savoir faire'' of hybrid living systems and organisms from understanding their rules and transcription modes could enable us to design and build ever more challenging and sophisticated novel hybrid materials.
Asignatura: CRF
Fuente: www.rsc.org/materials Journal of Materials Chemistry

Materiales Nanocompuestos, materiales del presente

Ahorro de costos e incremento de desempeño Laura Flórez-Consultora Editorial, Abril 2007

Los nanocompuestos ya no son materiales de laboratorio o de aplicación especializada, sino que están posicionándose como opciones competitivas de mercado, con resultados tangibles en ahorro de costos e incremento de desempeño y con un abanico creciente de proveedores.

Pese a que hace cinco o diez años se preveía que los nanocompuestos serían protagonistas de una nueva era en plásticos, es posible que nadie haya acertado a imaginar la celeridad con la que han entrado a formar parte de aplicaciones comercialmente exitosas. Una eficacia extraordinaria en la transferencia de tecnología, guiada sin duda por la promesa económica subyacente, ha hecho posible que hoy se vean multiplicados los casos de implementación de nanotecnología en plásticos. Todo esto en beneficio de la resistencia, propiedades de barrera, peso y costo de los productos moldeados.

Si se observan las tendencias en desarrollo en plásticos reforzados es claro que los nanocompuestos son, de lejos, los que más atención e inversión han acaparado. Y el mensaje que entregan es que ya no son materiales de laboratorio o de aplicación especializada, sino que están posicionándose como opciones competitivas de mercado, con resultados tangibles en ahorro de costos e incremento de desempeño y con un abanico creciente de proveedores. Empresas del tamaño de Arkema, Bayer MaterialScience y Solvay ofrecen nanocompuestos como parte de su portafolio, y en América Latina la brasileña Braskem ya ha registrado dos patentes en nanotecnología. En los últimos tres años es notable el crecimiento en el número de proveedores que ofertan nanoaditivos y nanocompuestos; la mayor parte de ellos ha surgido como un joint venture entre proveedores tradicionales de resina y grupos de expertos en aplicación de nanotecnología. Los mayores esfuerzos se han hecho en mejorar la consistencia en los compuestos obtenidos, en incrementar la pureza y en mejorar las capacidades de exfoliación (separación de capas de las nanoarcillas).

Nanocompuestos listos para usar

Naturalnano, en Rochester, N.Y, se encuentra en el proceso de comercializar sus masterbatches de nanotubos Pleximer, empleados para incrementar la resistencia y flexibilidad de aplicaciones de nylon y polipropileno, a la vez que reducen el peso y los costos necesarios de capital. Son nanotubos de arcilla del tipo halloisita, funcionalizados, concentrados, y mezclados con varios materiales poliméricos, empleando un proceso propietario de la compañía. Con el uso de este material se eliminarían los problemas tradicionales de exfoliación asociados a los nanocompuestos. Cathy Fleicher, presidente y directora de tecnología de la empresa, afirma: "aunque hay un mercado de rápido crecimiento, hay restricciones por la complejidad, calidad e inversión necesarias para manufacturar nanocompuestos. Pleximer cambia esta ecuación y reduce las complicaciones y los problemas de control de calidad, con un aditivo que puede ponerse directamente en las extrusoras o líneas de producción, disminuyendo los problemas normalmente asociados con otros nanocompuestos".

Solvin, joint venture en Europa entre BASF y Solvay, lanzó el nanocompuesto NanoVin, que combina PVC y nanopartículas de arcilla para mejorar las propiedades de viscosidad, flujo y plasticidad del material. De acuerdo con la empresa el NanoVin es un "material inteligente", pues su viscosidad se reduce cuando se aplica un esfuerzo cortante e incrementa cuando el esfuerzo se remueve. Estas propiedades son interesantes para aplicaciones en contacto con el cuerpo humano en cojinería o tapicería de la industria automotriz, o en aplicaciones de imitación de cuero. El material viene listo para usar.

En la última edición de premios automotrices de la Sociedad de Ingenieros Plásticos, SPE, el nanocompuesto de PP y PS Elan XP, producido por la empresa alemana Putsch GmbH desde hacia apenas seis meses, estuvo considerado entre los finalistas. De acuerdo con el jurado, el material proveía un acabado mate uniforme, muy buena resistencia a las rayaduras y un encogimiento en molde inferior al 1%. El compuesto remplaza partes automotrices interiores hasta ahora moldeadas con polímeros estirénicos y ahorra costos hasta en 50%. El compuesto fue obtenido con adiciones cercanas a 2% del aditivo Nanofil SE 3000, de Süd-Chemie AG, que gracias a su alta relación area-superficie (700 m2/g) asegura una distribución fina y estable.
La alemana Merck anunció una alianza con Nano Terra LLC, una empresa de co-desarrollo en nanotecnología, para crear nuevas propiedades físicas en aditivos químicos especiales, actualmente manufacturados y comercializados por Merck. De acuerdo con voceros de la empresa, esto permitirá incrementar la precisión y el control con que pueden ser usados los productos de Merck, y permitirá nuevas aplicaciones a niveles económicos que hasta ahora no habían sido posibles.

Nanocor, uno de los pioneros en suministro de nanocompuestos, presentó sus masterbatches de poliolefina conteniendo de 40% a 60% de las nanoarcillas montmorillonitas Nanomer, desarrolladas por la empresa. La empresa asegura que los masterbatches pueden mejorar la propiedades mecánicas entre 8 y 12%, al igual que la resistencia a la llama. Pueden emplearse en PP, TPO, PEBD y EVA.

Nano desde el origen
En julio del año pasado la petroquímica Braskem anunció el depósito de su segunda patente de nanotecnología en Brasil. La nueva patente tiene que ver con el desarrollo de un nuevo proceso de producción de nanocompuestos de polipropileno y polietileno a través de reacciones de polimerización "in situ", es decir, directamente en los reactores. De acuerdo con Luiz de Mendoça, vice presidente y responsable del negocio de poliolefinas, este desarrollo está orientado a incrementar la participación de productos de mayor valor agregado en el portafolio de la empresa. La tecnología estará inicialmente orientada al segmento de empaques, para incrementar propiedades de sello, barrera y vida en estante. En el futuro también servirá a la manufactura de piezas de ingeniería en los sectores automotriz y de electrodomésticos.
Arkema desarrolló los copolímeros acrílicos de bloque Nanostrength, que pueden mezclarse fácilmente en dispersiones a nanoescala con varios materiales poliméricos, para lograr combinaciones únicas de resistencia al impacto, transparencia y rigidez. Además están diseñados para ser usados como agentes compatibilizantes. El copolímeros SBM, de poliestireno, polibutadieno y PMMA sindiotáctico, generan una estructura polar y no-polar combinada, cuyos bloques se repelen. De esta forma si se introducen en una resina se "auto-organizan" en dominios muy pequeños, formando nanoestructuras. El componente de butadieno, que actúa como un caucho, previene la propagación de grietas e incrementa la tenacidad de la aplicación. El copolímero es compatible con epóxicos, estirénicos, PC, PPE, PVC y PVDF.

PolyOne comercializó recientemente Nanoblends, un compuesto basado en nylon 6 que se fabrica por la polimerización "in-situ" de caprolactama y nanoarcillas funcionalizadas, afines al nylon. Esta tecnología de aleación, que ha sido licenciada de Toyota, evita la re-aglomeración posterior del compuesto durante el procesamiento y reduce a la mitad el uso de nanoarcilla.

Las nanopartículas están cobrando también importancia en la catálisis de los procesos de polimerización. Empresas como Dow Química, Mitsubishi Chemical y Univation Technologies han llevado a cabo catálisis metalocénica y de sitio simple con partículas de arcilla y otros materiales de tamaño nanométrico, obteniendo resinas más eficientes y de mejor desempeño a un costo de producción menor. El uso de nanopartículas como catalizadores incrementa la actividad de catálisis notablemente, con lo que se requieren menos cargas de catalizadores, e incluso puede eliminarse la necesidad de activadores. Los polímeros resultantes son en sí nanocompuestos listos para ser usados en películas, aplicaciones de moldeo por inyección o productos extruidos.

Nanotubos de carbono
Gracias a su forma y estructura, los nanotubos de carbón, descubiertos en 1991, tienen propiedades intrínsecas únicas. Son cuatro veces más ligeros y cinco veces más resistentes que el acero, su conductividad eléctrica es equivalente a la del cobre, su conductividad térmica es extremadamente alta y son tan duros como el diamante. En los plásticos, los nanotubos de carbono se emplean para mejorar las propiedades de resistencia, tenacidad, flexibilidad y conductividad, y se prevee que su aplicación está en la fabricación de plásticos muy robustos, hechos de la misma forma que el concreto reforzado, en aplicaciones como aspas de turbinas, que puedan ser más ligeras y largas que las actualmente disponibles. Los primeros usos se han hecho en aplicaciones deportivas, haciendo palos de hockey o bates de beisbol que son más resilientes, resistentes y livianos.

Los principales problemas en la producción de nanotubos tienen que ver con la consistencia, la capacidad de producción y los altos costos por kilogramo. Bayer MaterialScience afirma ser una de las únicas tres empresas en el mundo que pueden proveer el aditivo con calidad reproducible, y ha desarrollado un método para sintetizar los nanotubos de carbono que promete ser útil para producción a gran escala de manera efectiva en costos, además de asegurar una pureza de 95% del material. Actualmente la
empresa tiene una planta piloto con capacidad anual de 30 toneladas, y entre sus planes está ampliar la producción a 3.000 toneladas por año.

Arkema anunció que suministrará sus nanotubos de carbono multi-paredes, bajo el nombre de Graphistrength C100, a la empresa Nanoledge, para aplicaciones comerciales en el sector de esparcimiento y deportes. Arkema está desarrollando sus ventas de nanotubos de carbono en los sectores de termoplásticos, resinas epóxicas, elastómeros y recubrimientos.

Idaho Space Materials desarrolló un revolucionario proceso de manufactura de nanotubos de pared sencilla, que de acuerdo con la empresa se destaca por su pureza, por la ausencia de metales pesados y su alta capacidad de producción. No usa ningun catalizador de origen metálico, por lo que se evita la contaminación de este tipo. La empresa anunció que otorgará precios especiales para clientes en altos volúmenes y para instituciones de investigación sin ánimo de lucro.

Por su parte, la empresa norteamericana NanoDynamics, localizada en Búfalo, New York, anunció el uso de sus nanotubos de carbono en la síntesis de otros materiales de refuerzo, como fibras de grafito o acero. Se espera que este "pre-refuerzo" facilite la incorporación de nanotubos en aplicaciones finales.
Asignatura: CRF
Fuente: www.nanotecnologia.com.pe/.../materialesnanocompuestos.pdf

Organic/inorganic nanocomposite gels employed as electrolyte supports in Dye-sensitized Photoelectrochemical cells


Dye sensitized photoelectrochemical cells (DSPEC) are studied with increasing interest by several groups around the world [1–11], ever since the original work of Graetzel [1], where it was announced that visible light can be efficiently converted into electricity using mesoporous titania films and a tris(2,2'-bipyridine) ruthenium derivative as photosensitizer. The original DSPEC's employed a liquid electrolyte containing the I3−/I− redox couple, which is a standard choice [1, 12– 15] when combined with TiO2 and Ru-bipyridyl photosensitizers. Most recent efforts are directed towards the choice of a solid electrolyte [5] since liquid electrolytes present many practical disadvantages. On the contrary, a solid electrolyte allows for a sandwich thin film configuration, which is ideal for most applications. In the present work we present a sol-gel procedure for the deposition of a nanocomposite inorganic/organic thin film, combining silica and poly(ethylene glycol)- 200 (SiO2/PEG-200), enriched with I3−/I−, which supports a solid sandwich thin film DSPEC. The sol-gel procedure is ideal for this application, since in the original sol (liquid) phase, the material can enter into the pores of the mesoporous semiconductor and greatly increase the interface between the semiconductor and the electrolyte.

Titanium (IV) isopropoxide, tetramethoxysilane (TMOS),polyoxyethylene(10) isooctylphenyl ether (Triton X-100), poly(ethylene glycol)-200 (PEG-200), were purchased from Aldrich and used as received. Cis-bis(isothiocyanato) bis(2,2'-bipyridyl-4,4' -dicarboxylato)-ruthenium (II) [16] (RuL2(NCS)2) was provided by Solaronix SA (rue de l'Ouriette 129, 1170 Aubonne VD, Switzerland). The rest of the reagents were from Merck, while Millipore water was used in all experiments. Optically transparent electrodes (OTE) were cut from an indium-tin-oxide (ITO) coated glass (<>
The diameter of the nanoparticles employed in the present work, as estimated by using AFM images, was around 30 nm. X-ray diffusion study of TiO2 powder made with the above procedure showed that films consist of anatase nanocrystals. When the TiO2 film was taken out of the oven and while it was still hot, it was dipped into an 1mM ethanolic solution of (RuL2(NCS)2) and was left there for about 24 hours. Then it was copiously washed with ethanol, dried in a stream of N2 and studied by absorption spectrophotometry. The dye is steadily attached on the TiO2 film, obviously, by means of its carboxylate groups. Figure 1 shows the absorption spectra of the TiO2 film with and without the adsorbed dye. The absorption of visible light by the film is, obviously, possible only through the dye. The oscillating part in the TiO2 absorption spectrum is due to interference fringes. 2.2. Synthesis of the nanocomposite SiO2/PEG-200 film containing electrolyte. On the top of the TiO2/dye layer, a thin composite organic/inorganic film containing I3−/I− has been deposited under the following procedure, which was carried out at ambient conditions. TMOS was partially hydrolyzed by mixing with acidified water (HCl, pH 3.0) at a molar ratio TMOS:water = 1:2. The mixture was stirred for one hour. It was originally turbid but it became clear in the course of proceeding hydrolysis. Then to 1ml of this sol, we added 5 g of an aqueous PEG-200 solution containing the redox-couple. In particular, I2 was diluted in pure PEG-200 while KI was diluted in water. Then the two solvents were mixed and produced a transparent solution. Only if I2 is first diluted in PEG-200, it can finally produce a transparent solution, since it is not directly soluble in water. Different PEG-200/water ratios have been obtained for the purpose of the present work, while the overall concentration was 0.03M for I2 and 0.3M for KI. After mixing with prehydrolyzed TMOS, the solution was stirred for 4 hours, when it was judged ready for application. During that time, a condensation procedure goes on by −Si − O−Si−Polymerization, slowly producing a gel. 4 hours of waiting time under stirring still leaves the solution at an early stage of gelation. A thin film of this composite material was deposited by dip-coating. As before, the back inactive side of the glass support was covered with a tape before dipping and was pealed off afterwards. 2.3. Application of the counter-electrode that ends the fabrication of the cell. On the top of the SiO2/PEG-200/electrolyte layer, while the electrolyte layer was still in the fluid phase, we placed an ITO electrode covered with a thin Pt film and pressed against the underlying support. −Si − O− bridges help binding the counter electrode so that the composite SiO2/PEG- 200 material additionally acts in holding the parts of the cell together in a stable thin sandwich configuration.
Pt was applied prior to cell binding by vacuum evaporation on the ITO slide. Its presence is necessary to improve cell performance. A schematic diagram of the cell cross section is shown in Figure 2. Absorption measurements were made with a Cary 1E spectrophotometer. Incident Photon to Current Efficiency (IPCE %) values [21] have been measured by illumination of the samples with a 250 Watt Phillips tungsten halogen lamp through a filter monochromator (Oriel-7155). The lamp spectrum satisfactorily simulates solar radiation at the surface of the earth. The number of incident photons was calculated by employing a radiant power/energy meter (Oriel-70260).
Asignatura: CRF
Fuente: downloads.hindawi.com/journals/ijp/2002/562391.pdf
Ver: http://nanocompositescrf.blogspot.com/

Organic Hybrid Nanocomposites for Photorefractivity at Communication

This paper reports new photorefractive polymeric nanocomposites photosensitized with HgS or PbS nanocrystals and operating at the communication wavelength of 1.3 ím. To our knowledge, it is the first report of a polymeric photorefractive medium with spectral response at a communication wavelength. The nanocomposites involving HgS are prepared by an in-situ nanochemistry approach, whereas those involving PbS were prepared using competitive nanochemistry. Photoconductivity experiments were employed in the characterization of photocharge generation quantum efficiency provided by the semiconductor nanocrystals. The photorefractive nature of the composites is confirmed using electric field dependent two-beam coupling. In the case of nanocomposite containing PbS nanocrystals, a net gain in excess of the associated absorption loss is observed

Photorefractivity, a multifunctional property derived from the combination of photoconductivity and electrooptic activity, has been the focus of an extensive body of research due to its potential application to real-time optical information processing; beam clean up and amplification, dynamic interferometry, phase conjugation, and pattern recognition.1-3 Due to their large optical nonlinearities, low dielectric constants and low cost, polymeric photorefractive (PR) materials in particular have attracted a significant amount of attention during the past 10 years, and tremendous progress has been made in their development.4-19 Although many dyes are well suited for visible wavelengths, none have been reported to photosensitize PR polymeric composites at the infrared wavelengths of 1.31 or 1.55 ím, commonly used in optical communications. Although considerable effort has been made to alter the absorption spectra of recognized organic photosensitizers through functionalization of the parent chromophore, the observed spectral shifts have been meager and unpredictable.

With the advent of nanocrystal (NC) technology, a new means by which to photosensitize polymer composites has been realized, establishing a novel class of inorganic-organic hybrid photoconductive materials.20-22 Perhaps the primary advantage gained through this approach concerns the ease with which the spectral properties of NCs are modified, made possible by the quantum size effect where the magnitude of the optical bandgap is inversely proportional to the size of the nanocrystal.23-26 This characteristic in conjunction with the wide range of materials available, including narrow band-gap semiconductors, permits this approach to be used for ultra-violet and visible as well as for applications involving infrared wavelengths. In addition, superior photocharge generation efficiency, ¼, associated with inorganic semiconductor materials incorporated into these hybrid composites, may result in a notable PR figureof- merit obtained with a relatively small applied electric field. We recently reported photorefractivity in the visible spectral range using nanocomposites of a polymer, poly N-vinylcarbazole (PVK), and CdS nanocrystals.16,17 Here, we report on the incorporation of narrow band-gap semiconductor NCs into a PR polymeric matrix to produce, to our knowledge for the first time, photorefractivity at a communication wavelength (1.3 ím in the present case). Two composites were studied; in both cases the photoconductor, PVK, was the primary continuous medium for the transport of the photogenerated charge-carriers (holes). The Tg of the composite was decreased to below ambient temperature through the inclusion of a plasticizer tritolyl phosphate (TCP), allowing
for room-temperature electric field alignment of a second-order nonlinear optical (NLO) chromophore, N-(4-nitrophenyl)-(s)- prolinol (NPP). Although NPP exhibits traditional electrooptic activity (Pockels effect), modulation of the refractive index in the PR operation occurs primarily through birefringence since the chromophore can align with respect to the periodic spacecharge field.13,16 The composites were photosensitized at 1.31 ím through the inclusion of nanocrystals composed of lead sulfide (NCPbS) or mercury sulfide (NCHgS). Assuming complete conversion of mercury acetate to NCHgS and neglecting the weight of the passivating layer associated with the NCPbS, the sample composition was PVK:TCP:NPP:nanocrystal)

Experimental Section
For PR samples doped with NCHgS, an in-situ nanochemistry approach was used where fabrication involved solvent casting PVK, TCP, NPP, and mercury acetate on to an indium tin oxide (ITO) coated substrate. Upon complete removal of the solvent, the samples were exposed to H2S converting the mercury acetate to NCHgS and acetic acid. The byproduct was removed under vacuum, and the sample was subsequently heated above its melting temperature and sandwiched with another ITO coated substrate. By varying the temperature and the time duration of H2S exposure, it is possible to exercise a large degree of control over the eventual size of NCHgS. Transmission electron microscopy as well as X-ray diffraction (XRD) was used to determine particle size;27 the average size of NCHgS in our.

samples was _10 nm. In the case of samples containing NCPbS, the nanocrystals were synthesized using wet chemical techniques. Surface passivation of the NCPbS was accomplished through their encapsulation with a suitable organic moiety, in the present case p-thiocresol. Here, the eventual size of the particle was controlled by adjusting the ratio of sulfide (S2-) to p-thiocresol in solution, thus taking advantage of the competitive reaction rate of these species with solvated lead ions.28 Upon purification, the NCPbS was suspended in an organic solvent, and solvent cast together with PVK, TCP and NPP on to an ITO coated substrate. Upon removal of the solvent, the samples were heated and sandwiched between a second piece of ITO. The mean diameter of NCPbS was determined to be _50 nm. The mechanism by which charge transport occurs across the inert passivating layer is the subject of current debate and in recent literature several possibilities have been proposed.13,16,29,30 Samples for photoconductive characterizations were similarly fabricated, but here films approximately 5 ím in thickness were spin-coated onto an ITO coated substrate and counter electrodes were fabricated via high-vacuum deposition of metallic silver.

Results and Discussion
Characterization of the photocharge-generating quantum efficiency, ¼, of the nanocrystal:polymer composites at 1.31 ím was accomplished using a simple dc photocurrent technique. The electric field dependencies of ¼ for both composites are depicted in Figures 1 and 2 and were determined according to the equation where Ncc is the number of charge carriers generated per unit volume, Nph is the number of photons absorbed per unit volume in the sample, Jph is the photocurrent density, h is Planck's constant, c is the speed of light, R is the absorption coefficient of the sample, and d is the sample thickness. An attempt was made to fit the data to the Onsager formalism;17,20,31 however, a convergence was not observed indicating the mechanism of charge-generation and subsequent separation to be more complex than that assumed by this model. The PR nature of both composite materials is demonstrated through asymmetric two-beam-coupling (TBC) experiments. Holographic gratings were written by two coherent laser beams from a Lepton IV Series Laser (Micro Laser System, Model # L41310D-30) operating at 1.31 ím with p-polarization. The writing beams were of approximately equal intensity and intersected in the samples at incident angles of 60° and 38° (in air) respectively, creating an intensity grating with a periodic spacing of ¤ ) 4.75 ím. The TBC gain coefficient ¡ is given in terms of the experimentally determined quantities ç0 and â, as where L is the length of the optical path of the beam experiencing gain inside the sample; â is the ratio of the writing beam intensities before the sample; and ç0 is the beam-coupling ratio, defined as ç0 ) P1/P0 where P1 is the intensity of the signal with the pump, and P0 is the intensity of the signal without the pump.3,32,33

In Figure 3 the electric field dependence of ¡ for the NCPbS sample (thickness ) 170 ím) is presented. The solid line is a fit derived from the standard theory of photorefractivity.34-36 Here, the magnitude of the space-charge electric field, jEscj takes the form where m ) xI1I2/(I1 + I2) is the modulation depth; Eq ) eNpr/ (_0_rKG) is the trap-density-limited space-charge field, where Npr is the effective trap density, _0 is the permittivity of free space, KG is the grating wave vector, and e is the fundamental unit charge; Ed ) kBTKG/e is the diffusion field, where kB is Boltzmann's constant and T is the absolute temperature; and óp and ód are photoconductivity and dark conductivity, respectively. Assuming the general condition Ed , E0, Eq is met, ¡ can be expressed as where _p ) arctan((E0 2 + EdEq)/E0Eq) is the phase of the grating vector, ì is the wavelength of the incident radiation, n is the refractive index, and R is a coefficient describing the effective NLO activity including the birefringence as well as the traditional electrooptic contributions. As is evident in the figure, a reasonably good theoretical fit is realized for the data and a value of Npr ) 1.06 _ 1017 cm-3 is obtained as the effective trap density. The maximum TBC gain coefficient, measured just prior to the dielectric breakdown of the sample, was ¡ ) 36.1 cm-1 and the magnitude of the space charge field, Esc, was 18.8 V/ím where E0 ) 52.7 V/ím (corrections were made for reflection at the glass/air interface and for the absorption attributed to ITO). From a practical point of view, the optical amplification, ¡, must exceed the absorption loss, R, of the PR sample in question.37-39 In this case, the optical loss of the sandwiched sample (glass/ITO/polymer composite/ITO/glass) at 1.31 ím was measured to be R ) 4.2 cm-1 yielding a maximum net gain coefficient of ¡ - R ) 30.9 cm-1. An example of asymmetric exchange of energy is depicted in the inset in Figure 3, which confirms the PR nature of the NCPbS composite. Also evident in the figure is that when the external electric field is removed from the sample, the intensities of the writing beams after the sample do not return to exactly their original values, indicating the presence of a secondary grating. While uniform illumination of the sample does not affect the secondary grating, it is reversible in that it is observed to decay over a time span of _30 min. The mechanism responsible for this secondary grating is the subject of current studies. The electric field dependence of TBC gain coefficient, ¡, for the NCHgS sample (thickness ) 155 ím) is illustrated in Figure 4. The data presented here was as well fit to eqs 3 and 4 yielding an effective trap density of Npr ) 9.25 _ 1016 cm-3, again producing a good fit of the data. Here it was calculated that an electric field of E0 ) 90 V/ím produced a space charge field, Esc, of 21.0 V/ím and yielded a TBC gain coefficient of ¡ ) 4.35 cm-1. Unlike the NCPbS composite, the optical amplification associated with the composite doped with NCHgS did not exceed that of the absorption losses where R ) 9.48 cm-1. Graphical illustration of the asymmetric exchange of energy in the case of the NCHgS sample is presented in the inset of Figure 4, thereby confirming the PR nature of this composite.

In summary, it has been demonstrated that inorganic nanocrystals can be effectively used to photosensitize PR polymeric composites for operation at the infrared wavelength of 1.31 ím commonly used in telecommunications. Moreover, the quantum efficiency of the photocharge generation process associated with NCPbS or NCHgS doped composites has been characterized at this wavelength.
Asignatura: CRF
Fuente: groups.physics.umn.edu/fastspin/profiles/jooho-jp013805h.pdf

Organic/Inorganic Hybrid Nanoparticles

Hybrid materials composed of inorganic nanoparticles and organic surface groups possess interesting optical,50 magnetic,51 and blending52 properties. These hybrids containing nanoparticles have been prepared by other synthetic routes by trapping colloids within cross-linked matrixes,53 "grafting to" particles with functional molecules/ polymers,25 or "grafting from" particles using a living or controlled polymerization process.54 Controlled radical polymerization techniques have also been introduced to colloidal materials by the attachment of ATRP initiating groups to the particle surface. Subsequent ATRP of vinyl monomers yielded core-shell particles with well-defined homopolymers and block copolymers tethered to a colloidal initiator.55,56 The properties of hybrid nanoparticles prepared from this method can be tuned by varying the particle size of the colloidal initiator, changing the composition of the particle core, or tethering (co)polymers with novel composition/functionality. An interesting feature of hybrid nanoparticle ultrathin films has been the formation of ordered two-dimensional arrays of particles, with a spacing dependent on the radius of gyration of the tethered (co)polymer. The general methodolgy for the synthesis of hybrid nanoparticles from ATRP is presented in Scheme 11.

Siloxane-based nanoparticles have been successfully applied to ATRP systems to prepare well-defined hybrid nanoparticles. In the first step of the process, nanoparticles were synthesized via the base-catalyzed hydrolysis and condensation of tetralkoxysilanes (i.e., the Sto¨ber process)57,58 or by microemulsion polymerization of trialkoxysilanes.59 Condensation reactions of surface silanol groups with functional silanes yielded colloidal initiators bearing benzyl chloride, 2-bromopropionate, or 2-bromisobutyrate groups. The synthesis of hybrid nanoparticles was then conducted by using the colloidal initiators in the ATRP of various vinyl monomers.

5.1. Hybrid Nanoparticles from Silica Colloids.
A modification of the Sto¨ber process was developed to prepare silica (SiO2) colloidal initiators for ATRP.55 Dynamic light scattering (DLS) and transmission electron microscopy (TEM) revealed that functional silica particles with an average effective diameter (Deff) of 70 nm were obtained. Silica colloids with benzyl chloride groups on the surface were used in the ATRP of Sty.

DLS and TEM confirmed that Deff of the pSty hybrid nanoparticles increased with monomer conversion. SEC of pSty chains cleaved from the particle confirmed the synthesis of well-defined polymers of low polydispersity (Mw/Mn <> 1 ím) has been conducted using CRP. Previously, surface-initiated polymerizations from micron-sized particles had been conducted using conventional radical polymerization.61-64

However, by the use of CRP from particles, polymer coatings of controlled thickness and functionality were prepared. In particular, hybrids from larger particles were synthesized as potential chromatographic stationary phases65-68 and templated supports.69 In the preparation of stationary phases for liquid chromatography, thin films (_100 Å) of poly(acrylamide) were grown from benzyl chloride functional porous silica particles (Deff ) 5 ím and pore sizeave ) 860 Å) using ATRP. The successful separation of various proteins using these poly(acrylamide) particles pointed to the successful grafting of polymers, without significant clogging of particle pores.65 Similarly, poly(methacrylates) possessing nucleotide side-chain groups were grown from porous silica particles using ATRP. The immobilization of oligonucleotides has been demonstrated as an attractive approach for the templated synthesis of nucleic acids, with the primary goal being control of both the degree of polymerization and sequence distribution in the final product. Toward this endeavor, methacrylate derivatives of uridine and adenosine were synthesized and polymerized using a 2-bromoisobutyrate functional silica particle.69 In a different templated system, poly- (benzyl methacrylate) was grafted to a silica surface using ATRP and treated with hydrofluoric acid to prepare hollow polymeric colloids.70 Hybrid particles with tethered pSty were also prepared using ATRP from 2-chloro-2-phenylacetate functional silica particles.71
Asignatura: CRF

Synthesis of Nanocomposite Organic/Inorganic Hybrid Materials Using Controlled/“Living” Radical Polymerization

The preparation of hybrid organic/inorganic nanocomposites comprised of well-defined polymers was reviewed. In particular, synthetic methods using controlled/"living" radical polymerization techniques, such as stable free-radical/nitroxide-mediated polymerizations, atom transfer radical polymerization, and reversible addition-fragmentation chain-transfer polymerization were described. The various approaches taken to prepare hybrid copolymers, nanoparticles, polymer brushes, dispersed silicate nanocomposites, and nanoporous materials were discussed.

1. Introduction
The synthesis of novel materials with improved properties and performance is a continually expanding frontier at the interface of chemistry and materials science. In this pursuit, the ability to control molecular structure on atomic and macroscopic dimensions is a key parameter in designing materials with preprogrammed activity. A significant advance in this area has been the synthesis of nanocomposites where the structural order within the material can be controlled on nanometer/submicron scales. While materials possessing such structural complexity are common in nature, robust and versatile methods to prepare synthetic nanocomposites remains an exciting challenge that is being tackled by research groups around the world.1 One approach to prepare nanocomposites has been the incorporation of well-defined organic and inorganic components into a singular material. In particular, the inclusion of well-defined polymers to inorganic substrates is of significance, because the functionality, composition, and dimensions of these macromolecules enable the design of specific properties into the resulting hybrid.2

Well-defined organic polymers have been attached to inorganic (co)polymers, particles, surfaces, glassy networks and interpenetrating polymer networks to prepare organic/inorganic hybrid materials. Additionally, polymers of controlled size, composition, and architecture have been used as shape templates in the synthesis of mesoporous inorganic networks (Figure 1). Polymers, such as poly(tetramethylene oxide) and poly(oxazolines), have been used to synthesize hybrid organic/inorganic nanocomposites.3 However, recent developments in controlled/"living" radical polymerization (CRP) have provided another valuable methodology to introduce well-defined organic (co)polymers to a variety of inorganic substrates. The scope of this review will cover hybrid organic/inorganic nanocomposites that have been made using CRP. For more fundamental discussions of organic/inorganic hybrid materials, the reader is directed to other reviews.2-5

2. Controlled Radical Polymerization
CRP has proved to be a versatile and robust method to prepare well-defined organic polymers. In the past decade, several techniques have been developed to synthesize well-defined polymers via radical polymerization. A major difference between conventional radical [i.e., azobis(isobutyronitrile)- or peroxide-initiated processes] and controlled radical polymerizations is the lifetime of the propagating radical during the course of the reaction. In conventional radical processes, radicals generated by decomposition of the initiator undergo propagation and bimolecular termination reactions within a second. In contrast, the lifetime of a growing radical can be extended to several hours in a CRP, enabling the preparation of polymers with predefined molar masses, low polydispersity, controlled compositions, and functionality.6,7 The mechanism invoked in CRP processes to extend the lifetime of growing radicals utilizes a dynamic equilibration between dormant and active sites with rapid exchange between the two states. Unlike conventional radical processes, CRP requires the use of persistent radical (deactivator) species, or highly active transfer agents to react with propagating radicals.

These persistent radicals/transfer agents react with radicals (deactivation or transfer reactions with rate constant, kd) to form the dormant species. Conversely, propagating radicals are generated from the dormant species by an activation reaction (with rate constant, ka).

2.1. Classification of CRP Systems. In the past decade, the field of CRP has seen tremendous development as evidenced by the wide range of materials that have been prepared using these techniques. In particular, three methods of considerable importance are the following: stable free-radical polymerization [SFRP; e.g., nitroxide-mediated processes (NMP)], metal-catalyzed atom transfer radical polymerization (ATRP), and degenerative transfer [e.g., reversible addition-fragmentation chain transfer (RAFT)]. While these three systems possess different components, general similarities in the CRP processess can be seen in the use of initiators, radical mediators (i.e., persistent radicals or transfer agents), and in some cases catalysts (Scheme 1). It is important to note that while SFRP and ATRP are subject to the persistent radical effect (PRE),8,9 degenerative processes, such as RAFT, do not conform to the PRE model because of the transfer-dominated nature of the reaction.

2.1.2. SFRP. In this type of CRP, alkoxyamine initiators10 (-PnX; eq 1 in Scheme 1) and nitroxide persistent radicals (X0; eq 1 in Scheme 1) have been effectively used to polymerize styrenes and acrylates. In certain systems, alkoxyamines have also been generated in situ by the initial use of conventional radical initiators (AIBN and peroxides) and nitroxide persistent radicals, which also led to a CRP process.11 A widely used nitroxide in the polymerization of styrene (Sty) is 2,2,6,6-tetramethylpiperidinyloxy (TEMPO), although recently developed nitroxides can also polymerize acrylates in a controlled fashion.12,13 The current limitation in this system lies in the inability to successfully polymerize methacrylate monomers, because of â-hydrogen elimination to the nitroxide radicals. Additionally, thiuram disulfides and dithiocarbamate "iniferter" systems have been used as agents for CRP with limited success.14,15

2.1.3. ATRP. In these polymerizations, radicals are generated by the redox reaction of alkyl halides (-PnX; eq 2 in Scheme 1) with transition-metal complexes (Y; eq 2 in Scheme 1).16-18 Radicals can then propagate but are rapidly deactivated by the oxidized form of the transition-metal catalyst (X-Y0; eq 2 in Scheme 1). Initiators typically used are R-haloesters (e.g., ethyl 2-bromoisobutyrate and methyl 2-bromopropionate) or benzyl halides (e.g., 1-phenylethyl bromide and benzyl bromide). A wide range of transition-metal complexes, such as Ru-, Cu-, and Fe-based systems, have been successfully applied to ATRP. For Cu-based systems, ligands such as 2,2¢-bipyridine and aliphatic amines have been employed to tune both the solubility and activity of various ATRP catalysts. ATRP has been successfully applied for the controlled polymerization of styrenes, (meth)acrylates, (meth)acrylamides, acrylonitrile, and 4-vinylpyridine. ATRP systems are currently limited to monomers that do not strongly coordinate
to the catalyst.18

2.1.4. Degenerative Transfer. Radical polymerizations based upon a degenerative transfer system rely upon the rapid and reversible exchange of highly active transferable groups (-X; eq 3 in Scheme 1) and growing polymeric radicals (-Pm 0 and –Pn 0; eq 3 in Scheme 1). While conventional radical initiators (AIBN and peroxides) are used in this reaction, these compounds only serve as a radical source to drive reversible exchange reactions between active and dormant states. Transfer agents in this process contain moieties for both initiation and transfer which are generated in the presence of radicals. Controlled radical polymerizations from degenerative transfer reations have been done using alkyl iodides,19 unsaturated methacrylate esters,20 or thioesters as the transfer agents. In particular, the use of thioesters in the radical polymerization of vinyl monomers results in a RAFT polymerization.21 The RAFT process has proven to be a versatile method to polymerize functional styrenes, (meth)acrylates, and vinyl esters.

2.2. Synthetic Methodologies To Prepare Nanocomposite Hybrids Using CRP. A variety of CRP techniques have been developed to incorporate welldefined organic polymers to inorganic substrates. A key advantage of CRP processes is the facile functionalization/ deposition of initiator and polymerizable moieties onto inorganic polymers and surfaces.

2.2.1. Synthesis of Hybrid Homopolymers and Block, Graft, and Random Copolymers. In the preparation of organic/inorganic nanocomposite materials, hybrid copolymers are of particular interest because of the inherent incompatibility of the two segments. Thus, phase separation of these segments yields a variety of controlled nanostructures depending on the degree of incompatibility of the components, the composition, and the degree of polymerization in the final copolymer.

A wide range of polymeric structures have been made using different combinations of organic and inorganic components. In the simplest case, a vinyl monomer possessing an inorganic moiety (R ) inorganic moiety; Scheme 2a) can be homopolymerized in the presence of initiator (R-X) to prepare a hybrid homopolymer with organic main-chain and inorganic side-chain groups.

Alternatively, inorganic vinyl monomers can be copolymerized in the presence of organic monomers and R-X to make random copolymers (Scheme 2b). Chain extension of an organic macroinitiator prepared by CRP with an inorganic monomer yields a block copolymer (Scheme 2c). Conversely, an inorganic macroinitiator can be used in the CRP of an organic vinyl monomer to also prepare a hybrid block copolymer (Scheme 2d). Inorganic macroinitiators can be synthesized by end-capping growing inorganic chain ends obtained from ionic or step-growth processes with coupling agents that contain CRP initiator groups. In a similar fashion, hybrid graft copolymers are prepared by chain extending a multifunctional inorganic macroinitiator with an organic vinyl monomer (Scheme 2e). Additionally, inorganic macromonomers possessing vinyl end groups can be copolymerized with organic vinyl monomers by CRP, yielding graft copolymers with organic main-chain and inorganic side-chain
groups (Scheme 2f).

2.2.2. Synthesis of Hybrid Polymers from Inorganic Initiators. Another type of hybrid polymer is prepared by the use of a (multi)functional inorganic compound as an initiator for CRP. Various polymeric structures are possible depending on the functionality of the initiator. Monofunctional inorganic initiators would result in an end-functional polymer, while multifunctional initiators yield branched/star polymers (Scheme 3).

2.2.3. Synthesis of Nanocomposite Particles, Modified Surfaces, Dispersed Silicates, and Nanoporous Materials. The tethering of well-defined organic polymers to inorganic substrates such as particles (Scheme 4a) and surfaces (Scheme 4b) can be conducted using CRP. The attachment of prepared polymers was achieved by grafting to curved and flat surfaces.22-26 However, by the use of CRP methods, grafting organic polymers from surfaces can be performed by attachment of an initiator group (-X; Scheme 4) and polymerization of a vinyl monomer. The chemistry required for the functionalization of CRP initiator groups on curved and flat surfaces depends on the type of inorganic substrate used. In the case of gold and silicon wafers, the incorporation of initiating moieties to surfaces requires the tethering of functional thiols, or chloro/alkoxysilanes, respectively. By the use of CRP from inorganic surfaces, ultrathin films of precise thickness and composition can be grown. Organic/inorganic nanocomposites can also be prepared by CRP from the surfaces of layered silicates (Scheme 4c). CRP of an organic vinyl monomer between the inorganic layers yields a nanocomposite of inorganic platelets dispersed in a matrix of well-defined polymer. Finally, polymers of controlled size and compositions can be prepared by CRP and used as templates in the network polymerization of inorganic resins (Scheme 4d). When the molar mass and composition of the polymer template are tuned, using CRP enables control of both the hydrodynamic volume and the compatibility of the polymer with the inorganic matrix, respectively. Network formation around the polymeric templates, followed by degradation of the organic components, yields an inorganic glass with nanosized pores dispersed throughout the material.

Asignatura: CRF
Fuente: www.eng.uc.edu/~gbeaucag/.../MatyjaszewskiReviewPolyonCer.pdf
Ver: http://nanocompositescrf.blogspot.com/

An Overview of Mesoporous Materials and Organic-Inorganic Hybrid

1.1. Introduction
In the past twenty years, sol-gel science1,2 has undergone a spectacular development. The various stages of the sol-gel process have been scrutinized in considerable detail and established a sound basis for technological development. A turning point was reached with the emergence of organic-inorganic nanocomposite materials via the sol-gel process that opened gateway to whole classes of new materials. In 1985, Wilkes et al.3 initiated the work to develop novel organic-inorganic hybrid network materials by reacting metal alkoxides with end- functionalized condensational polymeric/oligomeric species through a sol- gel process. Their first successful example has been the incorporation of poly(dimethylsiloxane) (PDMS) oligomers into the silica matrix.3 In 1990, Wei et al.4 pioneered the synthesis of vinyl polymer- metal oxide hybrid materials.

In the early 1990s, another new material was made by Kresge et al.5 by templating silica species with surfactant molecules leading to the formation of ordered mesoporous silica oxides. These new products, known under the group name M41S, with the hexagonal MCM-41 being the most prominent member, dramatically expanded the range of pore sizes accessible in the form of an ordered pore system. Soon enough, research in this area has been extended to many metal oxides systems other than silica and also the novel organic-inorganic hybrid mesoporous materials etc. Our lab at Drexel has developed a nonsurfactant templating (i.e., small organic molecules) pathway in making mesoporous materials via the sol- gel process. In the following sections, we will briefly review the work on mesoporous materials and organic- inorganic hybrid sol- gel materials.

1.2. Mesoporous Molecular Sieves
Meso, the Greek prefix, meaning "in between", has been adopted by IUPAC to define porous materials with pore sizes between 2 and 50 nm.6 Along with two other classes of porous solids (defined by pore size), microporous (<> 50 nm),7 they have been used technically as adsorbents, catalyst and catalyst supports owing to their high surface areas and large pore volumes. Typical macroporous example include porous vicor and hydrogels. Mesopores are present in aerogels, pillared layered clays which show disordered pore systems with broad pore-size distributions. Well known members of the microporous class are the zeolites, which provide excellent catalytic properties by virtue of their crystalline inorganic, e.g., aluminosilicate network. However, their applications are limited by the relatively small pore openings.8,9 Thus, a persistent demand has developed for larger pores with well-defined pore structures.

1.2.1. Periodic Mesoporous Silicates
The design and synthesis of organic, inorganic and polymeric materials with controlled pore structure are important academic and industrial issues. In many applications, the precise control of pore dimensions, sometimes to a fraction of an angstrom, is the dividing line between success and failure. Zeolites and zeolite-like molecular sieves (zeotypes) often fulfill the requirements of an ideal porous materials such as narrow pore size distribution and a readily tunable pore size in a wide range.

However, despite the many important commercial applications of zeolites where the occurrence of a well-defined micropore system is desired, there has been a persistent demand for crystalline mesoporous materials because of their potential applications as catalysts in processes for heavy oil cracking and catalytic conversion for large molecules, separation media or hosts for bulky molecules for advanced materials applications. Until recently, most mesoporous materials were amorphous and often with broad pore size distributions.

The breakthrough came in 1992 with the discovery of MCM (Mobil Composition of Matter)-41 mesoporous materials.5 These new (alumino) silicate materials, with welldefined pore sizes of about 2-10 nm, break past the pore-size constraint (<> 1000 m2 g-1) and narrow pore size distributions. Instead of using small organic molecules as the templating compound as in the case of zeolites, Mobil scientists employed long chain surfactant molecules as the structure-directing agent during the synthesis of these highly ordered materials.5 Rather than individual molecular directing agent participating in the ordering of the reagents of forming the porous materials, assemblies of molecules, dictated by solution energetics, are responsible for the formation of these pore systems. This supramolecular directing concept has led to a family of materials whose structure, composition, and pore size can be tailored during synthesis by variatio n of the reactant stoichiometry, nature of the surfactant molecule, or by post-synthesis functionalization techniques. For example, MCM-41 contains regular arrangements of hexagonal pores in a honeycomb arrangement; MCM-48 which exhibits cubic symmetry that can be envisaged as a gyroid minimal surface10,11 and MCM-50 is a layered silicate (Figure 1-1).

Following the initial announcement of MCM-41, there has been a surge in research activity, echoed by published literature (For recent reviews, see Reference 12). Interestingly, Di Renzo et al.13 recently discovered a patent from 1971 in which a synthesis procedure similar to the one used by the Mobil group was described as yielding "low-bulk density silica". The patent procedure was reproduced, and the product had all the features of a well-developed MCM-41 structure, as shown by transmission electron microscopy, X-ray diffraction and nitrogen adsorption. However, in the original patent only few of the remarkable properties of the materials were actually discovered. It was the Mobil scientists who really recognized the spectactular features of these ordered mesoporous oxides.

Scientists have postulated that the formation of these molecular sieve materials concerns the concepts of structural directing agent or template. Templating has been defined, in a general sense, as a process in which an organic species functions as a central structure about which oxide moieties organize into a crystalline lattice.14-16 Strictly speaking, a template is a structure (us ually organic) around which a material (often inorganic) nucleates and grows in a "skin- tight" fashion, so that upon the removal of the templating structure, its geometric and electronic characteristics are replicated in the (inorganic) materials.12f The above definition has also been elaborated to include the role of the organic molecules as:14
(1) space- filling species,
(2) structural directing agents,
(3) templates.

In the simplest case of space filling, the organic species merely serves to occupy a void about which the oxide crystallizes. Therefore, the same organic molecule can be used to synthesize a variety of structures or vice versa. Structural direction requires that a specific framework is formed from a unique organic compound. And this does not imply that resulting oxide structure mimics identically the form of the organic molecule. In true templating, however, in addition to the structural directing component, there is an intimate relationship between the oxide lattice and the organic form such that the synthesized lattice contains the organic "locked" into position. Thus, the lattice reflects the identical geometry of the organic molecule.

In M41S materials, a liquid crystal templating (LCT) mechanism has been proposed in which supramolecular assemblies of alkyltrimethylammonium surfactants serve as one component of the operative template for the formation of mesophase.5,17 This mechanism behind the composite mesophase formation is best, although not fully, understood for the synthesis under high pH conditions. Under these conditions, anionic silicate species, and cationic surfactant molecules, cooperatively organize to form hexagonal, lamellar or cubic structures. The earliest model relied on the liquid crystal templating of the surfactant.17 Here the composite hexagonal mesophase is suggested to form by condensation of silicate species around a preformed hexagonal surfactant array or by adsorption of silicate on organic arrays thus initiating the hexagonal ordering in both the surfactant template molecules and the final product. Surprisingly, hexagonal mesostructures may also be synthesized using a cationic surfactant such as cetyltrimethylammonium chloride under extremely acidic conditions where the silicate species are positively charged. The pore size in MCM-41 materials can be controlled from 1.5 to 10 nm by the hydrophobic alkyl chain length of ionic surfactants or with the aid of auxiliary organic compounds (i.e., trimethylbenzene) as spacers and fillers. Strong electrostatic interactions between the ionic surfactants and the inorganic species result in MCM-41 matrix with limited pore wall thickness of 0.8-1.3 nm that are influenced little by the pH conditions.

Neutral template molecules, such as primary amines (with carbon tail lengths between C8 and C18) have also been employed to direct mesoporosity in silicates.18 It is suggested that a neutral silicate would interact with micellar aggregates through hydrogen bonding between hydroxyl groups of hydrolyzed silicate species and the polar surfactant headgroups. The resultant framework structures are shown to have thicker silicate walls (i.e., 1.7-3.0 nm) and therefore enhanced thermal and hydrothermal stability compared to MCM-41. However, the final mesoporous materials (namely, HMS) has only short-range hexagonal ordering. The use of these surfactants, although offering advantages, does suffer from several drawbacks also associated with cationic surfactants, being expensive and toxic and therefore not optimal for large-scale production. Polyethylene oxide neutral templates (PEO, C11-15EO9-30), are a convenient alternative to primary amines being nontoxic, low cost and biodegradable.19 Named MSU, these PEO template materials again show only short-range order. However, later on, ordered mesoporous materials with large wall thickness values of 3.1-6.4 nm have been made using alkyl PEO oligomeric surfactants in strongly acidic media.19d

Other newly developed methods include the use of nonsurfactant templates20 and copolymer precursor pathways.21 The nonsurfactant templated synthesis discovered in our lab utilize small organic molecules such as D-glucose, D- fructose, dibenzoyl tartaric acid (DBTA) etc., as the structure-directing agent. By simply varying the concentration of the template molecules, mesoporous materials with different pore sizes (i.e., 2-6 nm) can be obtained. The template can be easily removed by washing with water or by solvent extraction. These products possess high surface areas of ~1000 m2 g-1, pore volumes as large as ~1.0 cm3 g-1, and narrow pore size distributions. In addition to low cost, environmental friendliness, and easy removal of templates, this new approach also provide many other advantages such as mild synthesis conditions and good compatibility with enzymes which has led to a novel technology for immobilization of enzymes and other bioactive substances.22

1.2.2. Non-Silica Periodic Mesostructured Materials
Stucky and co-workers first extended the surfactant templating strategy to the synthesis of non-silica based mesostructure, mainly metal oxides.23 It was found that for the formation of an mesophase to be successful, three conditions should be satisfied:

1) the inorganic precursor should have the ability to form polyanions or polycations allowing multidendate binding to the surfactant;

2) the polyanions or polycations should be able to condense into rigid walls;

3) a charge density matching between the surfactant and the inorganic species is necessary to control the formation of a particular phase.

The complex solution chemistry often observed for transition metal ions as a result of the multitude of possible coordination numbers and oxidation states has a pronounced influence on the observed composite mesoporous materials as expected.

Depending also on the surfactant concentration, lamellar, cubic and hexagonal mesostructure could be synthesized in the pH range of 6-7. However, the overwhelming majority of products were layered. Regardless of the composite symmetry, removal of the surfactant caused structural breakdown probably due to the lack of complete condensation of the inorganic framework. The first stable mesoporous transition metal oxide was prepared by controlled hydrolysis of titanium isopropoxide in the presence of tetradecylphosphates using acetylacetone as a chelating agent.24 Through this "modified sol- gel method", titania mesostructure (named TMS for "tech molecular sieves) with a narrow pore size distribution and a high surface area of about 200 m2 g-1 has been obtained. However, IR spectroscopy measurements revealed the presence of phosphate groups after calcinations at 623 K. Mesoporous niobium oxide (Nb-TMS) and tantalum oxide (Ta-TMS) have also been made through a ligand-assisted templating (LAT) path.25-27 In this approach, the surfactants were pretreated with the metal alkoxides in the absence of water to form metal- ligated surfactants. Upon addition to the alkoxide-surfactant solution, water actedas both a solvent and a reactant, to initiate surfactant self-assembly and alkoxide hydrolysis/condensation, respectively.

Wormlike mesoporous alumina have been prepared with surface areas greater than 400 m2 g-1 using PEO surfactants.19c Higher surface areas (i.e., 710 m2 g-1) were obtained by mixing aluminum alkoxides with carboxylic acids in low molecular weight alcohols in the presence of a controlled amount of water.28 A narrow pore size distribution centered around 20 Å was observed. However, the material showed no longrange order.

Zirconium oxide is an interesting material catalytically and much effort has been devoted to the generation of a mesoporous form. Studies showed that both Zrisopropoxide29 and zirconium sulfate30 can be used as the inorganic precursor, while the former seemed to result in more stable but less ordered materials. In the Zr(SO4)2 case, a hexagonal form was successfully stabilized by post-synthesis treatment with potassium phosphate at room temperature followed by air calcination at 623 K.30 Many other non-silica mesostructured systems have been studied extensively in the recent years. A list of successful synthesis includes V,31 W,31-33 Zr,28,29,34-36 Sn,37 Hf38 oxides, etc. However, it still remains a major challenge to stabilize their structure or to discover new or improved pathways for these mesoporous molecular sieves.

1.2.3. Organic-Inorganic Hybrid Mesoporous Materials
Besides the extension from silicate to non-silica mesoporous materials, one other important way of modifying the physical and chemical properties of mesoporous silica materials has been the incorporation of organic components, either on the silicate surface, inside the silicate wall, or trapped within the channels. Organic functionalization of these solids permits the tuning of surface properties (e.g., hydrophilicity, hydrophobicity, binding to guest molecules), alteration of the surface reactivity, protection of the surface from attack, and modification of the bulk properties of the materials, and at the same time stabilizing the materials towards hydrolysis. For example, mesoporous silica having thiol groups on the pore surface showed high adsorption efficiency for heavy metals such as Hg, Ag, and Cr ion.39 Sulfonic acid groups grafted mesoporous materials exhibited high catalytic activity for selective formation of bulky organic molecules.40

These hybrid materials are generally synthesized via two methods.41 The first is the post-synthesis grafting method in which pore wall surface of the pre-fabricated inorganic mesoporous materials is modified with organosilane compounds after the surfactant removal. The original structure of the mesoporous support is usually maintained after grafting. Another method is the direct co-condensation of a tetraalkoxysilane and one or more organoalkoxysilanes with Si-C bonds through sol-gel process.

Grafting of the mesopore surface with both passive17,42 (i.e., alkyl and phenyl) and reactive43 (i.e., amines, nitriles etc.) surface groups have been studied. The former can be used to tailor the accessible pore sizes, increase surface hydrophobic ity, while the latter permits further functionalizations. Multiple grafting has also been demonstrated. In order to minimize involvement of the external surface in reaction processes and to optimize selectivity, researchers have tried to graft external surface first through passive groups, before functionalizing the internal silanol groups.44 The "one-spot" cocondensation approach has been studied extensively. Co-condensation using ionic,45 neutral surfactant46 and non-surfactant templates47 have all been demonstrated. Each of the two functionalization methods has certain advantages. If uniform surface coverage with organic groups is desired in a single step, the direct method may be the first choice.

It also provides better control over the amount of organic groups incorporated in the structure. However, products obtained by post-synthesis grafting are often structurally better defined and hydrolytically more stable. Although pore sizes can be controlled to some extent by both methods, it is more easily achieved by grafting.

A recent development in this field has been the study of organic- inorganic species covalently bonded inside the mesoporous wall structure. The surfactant template synthesis of these materials use a precursor that has two trialkoxysilyl groups connected by an organic bridge.48-50 The new technique allows stoichiometric incorporation of organic groups in silicate networks, resulting in higher loading of organic functional groups than by grafting or direct synthesis method. One can envision further tunability of the mechanical, surface chemical, electronic, optical, or magnetic properties of the hybrid composite by introducing suitable functional groups into the walls.

1.3. Organic-Inorganic Hybrid/Nanocomposite Materials by the Sol-Gel Process
Organic-inorganic hybrid composites are one of the most important classes of synthetic engineering materials. By definition, a composite is a material composed of two or more physically distinct components, the intent of which is to achieve better properties than can be obtained by a single homogeneous materials.51 Aside from the intrinsic physical properties of the components, composite materials can also display special new properties as a result of the nature and degree of interfacial interaction between the two components. One of the frontiers in composites engineering is the development of viable methods for the efficient design and synthesis of organic-inorganic composites with nanometer-scale architecture. The term "nanocomposites" was proposed for the first time by Theng in 1970.52 According to Rustum Roy's definition,53 a nanocomposite is an interacting mixture of two phases, one of which is in the nanometer size range, typically, 1-20 nm in at least one dimension.

Organic and inorganic materials are usually quite different from each other in their properties. Inorganic materials such as glass and ceramics are hard but not impact resistant, that is, they are brittle, whereas organic polymer/oligomers are resilient. However, organic polymers generally suffer from some of the inherent drawbacks such as instability to heat and tendency of natural degradation upon aging. In general, inorganic species usually have good mechanical and thermal stability as well as optical properties. Organic moiety would provide flexibility, toughness, hydrophobicity and new electronic or optical properties. Organic-inorganic hybrid materials could then have desired combinations of the features of both organic and inorganic components. Because of the new and different properties of these nanocomposite materials that the traditional macroscale composites do not have, the preparation, characterization, and applications of organic/inorganic hybrid materials have become a fast expanding area of research in materials science. However, since the traditional processing conditions for inorganic materials usually involve high temperature, the survival of organic compounds is completely impossible. Thus, the mild-conditioned sol- gel process slowly established its popularity in making organic- inorganic hybrid materials. The sol- gel process, which is mainly based on inorganic polymerization reactions, is a chemical synthesis method initially used for the preparation of inorganic materials such as glasses and ceramics. Their unique low temperature synthesis condition allows it to be well adopted for the preparation of organic/inorganic hybrid materials and have proven to be effective. It brings both inorganic and organic components into intimate mixing that lead to new morphologies and unique properties. The extent of phase mixing can vary, but domain sizes are typically on the nanometer scale. Even though many of the network systems are comprised of components having very different refractive indexes, the resulting materials can often be prepared optically transparent due to the small scale lengths over which phase separation may exist. As a result, these composites can find applications in many fields which are far beyond the scope of application of traditional composite materials which tend to be opaque. Several different methods can be used in making these hybrid materials via the sol-gel process based on their macromolecular structures and phase connectivities:3
1. Hybrid networks synthesized by using low molecular weight organoalkoxysilanes as one or more of the precursors to introduce the Si-C bond;
2. Co-condensation of functionalized organic species with metal alkoxides and establishing covalent bonding;
3. Infiltration of preformed oxide gels with polymerizable organic monomers/polymer or vise versa, entrap organic molecules as guest within inorganic gel matrix (as host);
4. In situ formation of the inorganic species within a polymer matrix, especially for inorganic particles;
5. Hybrid networks can also be formed by interpenetrating networks and simultaneous formation of organic and inorganic phases.

The properties of a composite product depend not only on the properties of the individual components, but are also greatly influenced by such factors such as the phase's size, shape and interfacial properties. According to the nature of the interface between the organic and inorganic components, these composites can also be simply classified into two major families:54 16
(1) Class I---organic and inorganic components are linked through covalent bond;
(2) Class II---does not contain covalent bond between organic and inorganic components.

The scope of research and applications of these composite materials have ranged from inorganically modified organic polymers to inorganic glasses modified by organic compounds.55 Potential applications for these materials include: scratch and abrasiveresistant coatings;56 electrical and nonlinear optical (NLO) properties;57 adhesives and contact lens materials;58 reinforcement of elastomers and plastics;59 catalyst and porous supports, adsorbents;60 hybrid solid-state dye laser and chemical/biomedical sensor materials;61 photochromic hybrid material62 etc. To date, even though the number of commercial sol-gel hybrid products is comparatively small, but the promises of new technological uses are still high.
Asignatura: CRF
Fuente: idea.library.drexel.edu/bitstream/1860/39/1/feng_thesis.pdf