Rheology of SiO2/(acrylic polymer/epoxy) suspensions. II. Nonlinear stress relaxation

Large deformation, nonlinear stress relaxation modulus G(t,gamma) was examined for the SiO2 suspensions in a blend of acrylic polymer (AP) and epoxy (EP) with various SiO2 volume fractions (phi) at various temperatures (T). The AP/EP contained 70 vol.% of EP. At phi <= 30 vol.%, the SiO2/(AP/EP) suspensions behaved as a viscoelastic liquid, and the time-strain separability, G(t,gamma) = G(t)h(gamma), was applicable at long time. The h(gamma) of the suspensions was more strongly dependent on gamma than that of the matrix (AP/EP). At phi = 35 vol.% and T = 100 degrees C, and phi >= 40 vol.%, the time-strain separability was not applicable. The suspensions exhibited a critical gel behavior at phi = 35 vol.% and T = 100 degrees C characterized with a power law relationship between G(t) and t; G(t) proportional to t(-n). The relaxation exponent n was estimated to be about 0.45, which was in good agreement with the result of linear dynamic viscoelasticity reported previously. G(t,gamma) also could be approximately expressed by the relation G(t,gamma) proportional to t(-n') at phi = 40 vol.%. The exponent n' increased with increasing gamma. This nonlinear stress relaxation behavior is attributable to strain-induced disruption of the network structure formed by the SiO2 particles therein.