Silica/pHEMA nanocomposites

Since the mid-1980s, when the first sol-gel derived hybrids were obtained by mixing linear polymer chains with silica precursors [47], many works have been conducted trying to probe structure and to explore potential applications of this type of materials, as demonstrated by several review papers [48-50]. This low temperature process, which is particularly well suitable to combine with organic species, offers an interesting way of preparing hybrid materials. The most commonly used silica precursor is tetraethoxysilane (TEOS). The sol–gel reactions involving this silicon alkoxide can be summarized as follows:

(i) Hydrolysis

≣Si－O－C2H5 + H²O→≣Si－OH＋C2H5OH (1) (ii) Condensation

≣Si－OH＋HO－Si≣→≣Si－O－Si≣＋H2O (2) and/or

If these sol– gel reactions are complete, full condensed silica is obtained through a chemical process that can be summarized by the following equation:

Si(O－C2H5)4＋2 H2O→SiO2＋4C2H5－OH (4)

At high pH values, where condensation reactions are favored, discrete, large, and highly condensed silica particles are produced, whereas low pH conditions favor hydrolysis reactions and lead to a finer ramified polymeric silicate structure. A wide variety of types of organic polymers have been employed in the syntheses of hybrids of silica [51]. Particularly the polymer pHEMA is an interesting choice because, in addition to being easily soluble in the water-alcohol mixtures employed in sol-gel method, its pendant hydroxyl groups lead to the formation of hydrogen bonds and eventual condensation with silanol groups [52, 45], thus favoring the production of structurally homogeneous materials within a wide range of compositions[53]. Many studies have been devoted to the preparation of pHEMA-based composite materials with organic HEMA monomers and inorganic precursors such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) through sol-gel processes [54-55].

One of the most important problems in studying the hybrid nano-composite materials is their structural analysis. The correlation between material evolution and composition–structure–property dependence, as well as the processes of nanostructure formation, aggregation and development of self-organized materials has been investigated.

The nanostructure surface of silica and hybrids has been studied by several research groups [56-57]. The nano-structure of the hybrid, either with the nano-silica uniformly dispersing in the polymer phases or with phase separation of inorganic and organic phases occurring, depends on the processing conditions such as the type of catalyst, the pH value, the water quantity, the solvent system, and the reaction temperature [58]. In order to investigate the type and the extent of the interactions between inorganic and organic domains, which play a determinant role in controlling the properties of hybrid materials, Fontana et al [59] have characterized the hybrids obtained from the polymerization of pHEMA, modified by silicon

tetraethoxide (TEOS) by Raman and Brillouin spectroscopes to explain tentatively the changes of the properties, included of Brillouin scattering and refractive index, were on the basis of the structural change in the polymeric network, due to the presence of the inorganic domains. Moreover, Wu wt al. [60] have used XRD, FT-IR, BET, SEM and AFM techniques to investigate the influence of the main silica precursors and the type and quantity of organic components (HEMA) on the structure of hybrid materials by sol-gel synthesis. It was found that the type of precursors is of paramount importance for the formation of strong chemical bonds between inorganic and organic components of synthesized materials.

In inorganic/organic nano-coomposites, the inorganic molecules and organic molecules interconnect by chemical covalent bonds, hydrogen bonds or physical interaction.

The formation of covalent bonds such as Si-C or specific secondary interactions such as hydrogen bonds between organic and inorganic moieties may be a technique for structural control. Structural control by introducing hydrogen bonds into a system depends on the use of polymers containing specific functional groups susceptible to interacting with Si–OH groups of silica gel. Particularly the polymer pHEMA is an interesting choice because its hydroxyl groups, in addition to forming hydrogen bonds, will eventually form Si–O–C bonds by condensation with silanol groups [61-62], although Si–O–C bonds may be, theoretically, unstable to hydrolysis under sol–gel conditions [52]. On the other hand, the formation of covalent bonds between organic and inorganic moieties may be a technique for structural control. Some studies [63] developed during the past years have emphasized the insertion of covalent bonds linking organic polymers and silica network by introducing coupling agents, which contain both polymerizable and hydrolysable groups. In order to improve the understanding of structural control of hybrid materials synthesized from mixtures of tetramethoxysilane, water and pHEMA, Costa and Vasconcelos [64] introduced

reactions to evaluate the effect of primary interactions between organic and inorganic components. The results reveled that thhe presence of MTMOS prevented crack formation and macroscopic phase separation, though the formation of larger fluctuating composition domains with increasing pHEMA content.

An alternative way to synthesize the inorganic/organic hybrids is by directly mixing inorganic colloidal particles with polymers. The inorganic colloidal particles can be pre-produced through the sol–gel process with a pure inorganic precursor such as alkoxysilanes. Vigier et al. [45] compared two types of pHEMA/silica nano-composites prepared by undergoing free radical polymerization of HEMA either in the presence of HEMA-functionalized SiO2 nano-particles (Type 1) or during the simultaneous in situ growing of the silica phase through the acid-catalyzed sol– gel polymerization of tetraethoxysilane (TEOS) (Type 2). They demonstrated that Type 1 systems exhibit classical particle-matrix morphology, but where particles tend to form aggregates. Type 2 systems possess a finer morphology characterized by a very open massfractal silicate structure, which is believed to be bicontinuous with the organic phase at a molecular level. Similar discovers were also observed by Yang et al. [65] that the structure of the colloidal silica/pHEMA hybrid consisted of nano-silica uniformly dispersed in the pHEMA phase with slight inter-molecular hydrogen bonding. Furthermore, the structure of TEOS/pHEMA hybrid was similar to a semi-interpenetrated network with pHEMA chains tethered into the nano-silica network by inter- and intra-molecular hydrogen bonding. Consequently, the TEOS/pHEMA hybrid gels exhibited a smoother surface, higher transparency, and better thermal stability than the colloidal silica/pHEMA hybrid gels.