The isocyanate chemistry has been used to prepare, without adding any solvent, organic-rich hybrid O/I systems. α–ω hydroxy-terminated prepolymers (soft segments, denoted SS) can be end-capped with γ-isocyanato propyl triethoxy silane (γ-IPS), or previously reacted with a diisocyanate (DI) and then end-capped with y-amino propyl triethoxy silane (γ-APS), or with γamino propyl methyl diethoxy silane (γ-APMDES). With this second pathway, a double distribution of molecules is present. The aim of this work is to investigate the morphologies and structural properties of different organic-rich hybrid organic/inorganic materials. Two types of α-ω hydroxy prepolymers have been used : hydrogenated polybutadiene, HPBD, and polycaprolactone, PCL.
The inorganic phase is obtained through the hydrolysis and condensation of the silane groups under acidic conditions and with [H2O]/Si = 3. Assuming a complete conversion of SiOH groups, the SiO2 content never exceed 10% wt. The extent of crosslinking is estimated by the soluble fraction able to be extracted by tetrahydrofurane. Times for gelation were obtained by rheological measurements and/or from the appearance of insoluble fractions. In-situ small angle X-ray scattering (SAXS) measurements show that phase separation between an organic-rich phase and an inorganic-rich one can appear before or after gelation depending on the acid catalyst concentration and the nature of SS.
The nanometric silica-rich particles in the final morphologies, after a post-cure at 150°C, were also studied by means of SAXS measurements. Viscoelastic measurements show one or two main relaxation peaks depending on the phase separation process during reaction and the polarity of the initial SS. However for full crosslinked SS/γ-IPS hybrid networks, the relaxed modulus does not depend on SS nature and it is well described by the affine network theory. Silica particles have a small size and probably a high functionality in elastically active network chains (EANC). The fully cured SS/DI/silane hybrid networks equilibrium moduli are higher than SS/γ-IPS one. This can be understood if some silane end-linked DI participate to the EANC.