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

Large deformation, nonlinear stress relaxation modulus G(t, γ) was examined for the SiO₂ suspensions in a blend of acrylic polymer (AP) and epoxy (EP) with various SiO₂ volume fractions (ϕ) at various temperatures (T). The AP/EP contained 70 vol.% of EP. At ϕ ≤ 30 vol.%, the SiO₂/(AP/EP) suspensions behaved as a viscoelastic liquid, and the time-strain separability, G(t, γ) = G(t)h(γ), was applicable at long time. The h(γ) of the suspensions was more strongly dependent on γ than that of the matrix (AP/EP). At ϕ = 35 vol.% and T = 100°C, and ϕ ≥ 40 vol.%, the time-strain separability was not applicable. The suspensions exhibited a critical gel behavior at ϕ = 35 vol.% and T = 100°C characterized with a power law relationship between G(t) and t; G(t) ∝ 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, γ) also could be approximately expressed by the relation \(G(t,\gamma) \propto t^{-n^{\prime}}\) at ϕ = 40 vol.%. The exponent n ^′ increased with increasing γ. This nonlinear stress relaxation behavior is attributable to strain-induced disruption of the network structure formed by the SiO₂ particles therein.