Induced orientational effects in relaxation of polymer melts

We study stress relaxation in bidisperse entangled polymer solutions. Shorter chains embedded in a majority of longer ones are known to be oriented by coupling to them. We analyze the mechanism for this both by computer simulation and theoretically. We show that the results can be understood in terms of stress fluctuations in a polymer melt and chain screening. Stress fluctuations are frozen on the relaxation time of the longer chains, and these will induce strong orientational couplings in the shorter chains.

Figure 1

(Color online) The decay of the average birefringence for a simulation using reptational motion in a polymer melt with two different chain lengths. The top plot, $L_l=256$, and the middle plot, $L_s=64$. The lower plot is the average amount of chain length left in the tube at time $t$, $l\left(t\right)$ for chain length $L_s$.

Figure 2

(Color online) The residual birefringence $r\left(t\right)$ (line) and the average amount of chain left in a tube $l\left(t\right)$ (points) are shown. As is clear, the two curves closely follow each other.