Nonlinear Stress Relaxation of ``Quasi-monodisperse'' Miscible Blends of \textit{cis}-Polyisoprene and Poly(\textit{ptert}-butylstyrene)

Viscoelastic relaxation was examined for entangled miscible blends of \textit{cis}-polyisoprene (PI) and poly(\textit{ptert}-butylstyrene) (PtBS). The terminal relaxation times of PI and PtBS therein, $\tau_{\mathrm{PI\thinspace }}$and $\tau _{\mathrm{PtBS}}$, changed with the composition $w_{\mathrm{PI}}$ and the molecular weights $M_{\mathrm{PI}}$ and $M_{\mathrm{PtBS}}$. This ratio became unity when the $w_{\mathrm{PI}}$, $M_{\mathrm{PI}}$, and $M_{\mathrm{PtBS}}$ values were chosen adequately. For example, in a blend with $w_{\mathrm{PI}} \quad =$ 0.75, $M_{\mathrm{PI}} \quad =$ 321k, and $M_{\mathrm{PtBS\thinspace }}=$ 91k at $T \quad =$ 40ûC, $\tau _{\mathrm{PI}}$/$\tau_{\mathrm{PtBS}} \quad =$1 and $M$/$M_{\mathrm{e}} \quad =$ 55 and 8.3 for PI and PtBS. Under small strains, this blend exhibited sharp, single-step terminal relaxation as similar to monodisperse homopolymers, thereby behaving as a ``quasi-monodisperse'' material. Under large step strains, the blend exhibited moderate nonlinear damping known as the type-A damping for entangled monodisperse homopolymers. Nevertheless, PI had $M$/$M_{\mathrm{e}} \quad =$ 55 in that blend, and homopolymers having such a large $M$/$M_{\mathrm{e}}$ ratio exhibit very strong type-C damping. Thus, as compared to homopolymers, the nonlinearity was suppressed in the PI/PtBS blend having the large $M$/$M_{\mathrm{e}}$ ratio. This suppression is discussed in relation to the slow Rouse retraction of the coexisting PtBS chains (having $M$/$M_{\mathrm{e}} \quad =$ 8.3 in the blend).