Enhancement of Transient Elongational Hardening of Poly(alkyl methacrylate) via Increase of Side-Chain Length Distribution

In this study, we investigated the effect(s) of side-chain length distribution on the transient elongational hardening behavior of polymer melts. For either entangled or unentangled systems, we successfully prepared a series of samples, poly(butyl methacrylate), poly(methyl methacrylate-co-hexyl methacrylate), and poly(methyl methacrylate-co-lauryl methacrylate), abbreviated as PBMA, P(MMA-co-HMA), and P(MMA-co-LMA), respectively, that have the same average length of side chains and similar weight-average molecular weight. These samples exhibited quite different nonlinear elongational behavior despite a similarity of their linear viscoelastic behavior. The transient strain hardening is enhanced on an increase of the side-chain length distribution, namely, in the order of PBMA < P(MMA-co-HMA) < P(MMA-co-LMA), consistently for either the unentangled or entangled series. Given this consistency, the detailed hardening behavior is different, where the unentangled samples exhibit thickening at the Rouse Weissenberg number WiR > 0.1 and thinning at the higher WiR > 1, which can be explained by a combination of the finite extensional nonlinear elasticity and the frictional reduction when chains are highly stretched and coaligned. The degree of friction reduction relies on the length distribution according to the interpenetration of side chains. In comparison, the entangled samples exhibit consistent thinning even when WiR is lower than one, which is attributable to the concurrence of the flow-induced disentanglement.