STRUCTURE OF VINYLESTER NETWORKS - A RUBBER ELASTICITY STUDY

Five networks based on a diglycidylether of bisphenol A-dimethacrylate prepolymer of molar mass 1100 g.mol-1 crosslinked with 35%, 40%, 45%, 50%, and 55% styrene mass fractions were characterized by Fourier tra nsform infrared spectroscopy, cross-polarization/magic-angle spinning, nuclear magnetic resonance spectroscopy, styrene extraction, and rubb er elasticity measurements. Various structural models were built up fr om the available analytical data, and their crosslink density was comp ared to the experimental data derived from rubber elastic moduli. Glob ally, network models based on a purely statistical approach but taking into account the incomplete cure conversion make it possible to predi ct the trends of modulus variations. The kinetic data on copolymerizat ion show, however, that the material is diphasic, one phase being comp osed of the vinylester network and the other of practically pure polys tyrene. We tried to take this heterogeneity into account in modulus pr edictions using a simple mixture rule, but the method failed for high styrene mass fractions (s much greater than 0.45), for which the exper imental modulus value is about 50% higher than the predicted one. The results suggest the existence of a morphological transition for a crit ical styrene weight fraction between 40% and 45%.