Stress relaxation of bi-disperse polystyrene melts

Abstract

We present start-up of uniaxial extension followed by stress relaxation experiments of a bi-disperse 50% by weight blend of 95k and 545k molecular weight polystyrene. We also show, for comparison, stress relaxation measurements of the polystyrene melts with molecular weight 95k and 545k, which are the components of the bi-disperse melt. The measurements show three separated relaxation regimes: a fast regime, a transition regime and a slow regime. In the fast regime the orientation of the long chains is frozen and the stress relaxation is due to stretch relaxation of the short chains primarily. Conversely in the slow regime the long chains have retracted and undergo relaxation of orientation in fully relaxed short chains.

Nematic effects and strain coupling in entangled polymer melts under strong flow

“…We investigate two systems: a pure melt of short polystyrene (95 kg/mol) chains and a bidisperse melt composed of a 50/50 wt mixture of short and long (545 kg/mol) polystyrene chains. Note that both chain populations are entangled with the number of entanglements per chain Z L ≈ 41 and Z S ≈ 7 for the long and short chains respectively [10]. In both systems a fraction of the short chains are deuterated and thus allows for direct comparison of the short chain relaxation in these two scenarios.…”

“…We label these two sample series Short-in-Short (SiS) and Short-in-Long (SiL) respectively. Synthesis and chromatography of the monodisperse polystyrenes, PS-545k and PS-95k used in this work have already been described along with characterisations of both shear and extensional rheology [10,11]. The main characteristics of the sample constituents are summarised in Table I.…”

“…The main characteristics of the sample constituents are summarised in Table I. As shown in [10] extensional steady state flow conditions are established beyond a Hencky strain of = 3 and a total of five strain rates were rheologically tested probing different flow regions separated by the time constants of the constituent chains. Here we focus our attention to = 3 and to the highest strain rate investigated,˙ = 0.1 s −1 .…”

“…We note that we also find nematic effects at lower strain rates which will be presented elsewhere. The details of the stretching experiments are given in [10]. However, for the SANS experiments it is vital to quench the samples fast enough to trap the relevant molecular configurations.…”

“…In the figure the Rouse and reptation time of the monodisperse linear short components are indicated. In [10] the rheology data of the blend is separated into 3 domains approximately defined in the following time intervals: a fast regime (0-20s), a transition regime (20-700s) and a slow regime for times longer than 700s. It is hypothesised that the fast regime is dominated by fast stretch relaxation (note that the blend initially relaxes faster than the pure short chain melt) and the slow regime by long chain relaxation in a sea of essentially relaxed short chains.…”