Summary
      The manuscript deals with the development and characterization of hybrid materials based on poly(hydroxyethyl acrylate) (hereafter PHEA) reinforced by the inclusion of an amorphous silica phase. Both phases were simultaneously synthesized: the organic phase underwent a free radical polymerization reaction induced by the small addition of a thermal initiator (benzoil peroxide); besides, silica (SiO2) was polymerized by an acid catalyzed sol-gel reaction of the silicon alkoxide tetraethoxysilane (hereafter TEOS). The sol-gel reaction conditions where silicon dioxide is formed influence the final silica structure: degree of condensation, linear versus branched intermediate species, average size, and so on. Some of the key parameters to control SiO2 topology on sol-gel derived composites include the catalyst nature used to increase the alkoxide reactivity (as well as its amount, pH), the available water to hydrolyze the silica precursor (referred to the stoichiometric amount needed to fully hydrolyze one molecule of TEOS) and ratio between the organic and inorganic phases on the final hybrid. The former (catalyst) and the second (water) conditions were fixed so as to synthesize materials with silica average sizes around tens of nanometres (nanocomposites); the latter, the relative ratio between organic and inorganic phases, was systematically changed.
      Previous works pointed that the reaction rate for the silica polymerization must be faster than that corresponding to PHEA; otherwise, a macro-phase separation takes place. The linear tend to increase of the density measures when silica does agrees with that hypothesis. The fact that the silicon dioxide network is formed at the presence of a mixture of small polymer species (monomers and oligomers) gives rise to wonder what is the role of the hydrophilic/hydrophobic character of the solution which surrounds the germinal silica nuclei. In order to evaluate that influence, another set of hybrids was synthesized with a systematic variation of the organic phase by the copolymerization with a hydrophobic monomer, ethyl acrylate (hereafter EA). 
      Several techniques are used to characterize some physical and chemical properties of the nanocomposites: microscopy, infrared spectroscopy, calorimetry, dynamic mechanical analysis, thermogravimetry, swelling on solvents,… Most of the trends shown by these experimental data are explained on the basis of a couple of physical phenomena. The most important one is the inorganic threshold from which both phases are co-continuous (silica percolation at 20wt% for PHEA-SiO2 hybrids). 
      Above all the results, it is worth highlighting the improvement of mechanical properties (referred to the neat polymer) on both dried and swollen states as the silica content increases. Indeed, it was the basic request when research started: to find a way to enhance the elastic modulus of swollen PHEA keeping as long as possible its high water uptake capability (for instance,  the equilibrium water content of a sol-gel derived PHEA-based nanocomposite with a 30wt% SiO2 equals 38%, referred to its polymer mass). The elastic modulus increase also takes place bellow and above the glass transition, albeit it is more significant at the rubbery state (sometimes up to 3 decades higher).
      Besides, thermal stability tends to decay with the inclusion of a silica phase, presumably because of the degradation catalysis by not condensed species (silanols), which is additionally confirmed by infrared spectroscopy. The existence of silanols also gives rise to an irreversible increase of the elastic modulus by means of heat treatments at temperatures below the onset of the thermal degradation due to further condensation reactions.
      Polymeric macromolecules confinement appears at high values of the inorganic phase, where the organic phase is more hindered due to silica percolation. Polymeric chains are trapped upon the surfaces of rich silica domains, and a gradation of mobility is established from those silica surfaces to the bulk-like polymer. It is experimentally probed by the fall of the heat capacity at the glass transition temperature, as well as by the broadening of the relaxation measured by calorimetry and dynamic mechanical analysis.
      The issue of the sort of interactions between phases (physical and/or chemical bonds) is mainly addressed by water immersion assays. The polymer state under water immersion of hybrids depends on the continuity of silica: before silica percolates, the water uptake does not vary dramatically (in fact, it slightly increases due to the hydrophilicity of the silica surfaces); however, above the silica percolation threshold, the polymer state is similar to that corresponding to pure PHEA equilibrated at high relative humidities: the fall of the water uptake capability is the outcome of the PHEA hindrance to swell (the disentanglement of hydroxyl micelles), and thus not to the (significant) loss of -OH groups of the polymer due to hybrid condensation reactions between silanols and hydroxyls corresponding to PHEA lateral groups.
Finally, it is introduced a methodology to prepare a new kind of scaffolds made by nanocomposites whose pore morphology consists of a cylindrical channel mesh, which are perpendicular between themselves. The procedure is based on the well-known method of intermediate templates, this time prepared by a stack of woven fabrics which are first pressed and afterwards sintered. After the filling of the holes left inside the template by the monomeric solution and subsequent thermal polymerization, templates are removed by the selective solvent of the material it is made up. A suitable template preparation is found to be crucial, since the sintering of localized points between neighbour fabrics gives rise to transversal holes which make that porous structure tridimensionally interconnected. Porosity values equal 60%.