Effect of epoxy oligomer on viscoelasticity of acrylic polymer

Linear and nonlinear viscoelasticity of an adhesion material for integrated circuit chips were investigated to control the processability. The material consists of acrylic polymer (AP) and epoxy oligomer (EP). EP content in AP/EP blend is 70 vol %. From the linear viscoelasticity, it was found that the iso-free volume state of AP/EP blend was 20°C lower than that of AP and the entanglement molecular weight Me of AP/EP was three times higher than that of AP. Nonlinear stress relaxation modulus G(t,γ) showed that the time-strain separability, G(t,γ) = G(t)h(γ), was applicable at long time above a characteristic time τk, where G(t) is linear relaxation modulus and h(γ) is the damping function. The τk value was estimated to be 10 s for AP/EP and below 0.1 s for AP at an iso-free volume state. (h(γ) for AP and AP/EP behaved like a usual linear homopolymer.) The time evolution of the elongational viscosity ηE(t) of each sample showed that AP/EP system exhibited strong strain hardening at $\dot {\varepsilon} \geq 0.3 \ {\rm s}^{-1}$, although AP did not show strain hardening at strain rate measured, when the data were compared at an iso-free volume state. These results strongly suggest that the strain hardening behavior of AP/EP is attributable to enhancement of the stretch of AP polymer chains by diluting EP oligomer chains.