Extensional Strain Hardening Induced by π-π Interactions in Barely Entangled Polymer Chains: The Curious Case of Poly(4-vinylbiphenyl).

Aromatic π-π interactions between phenyl groups of adjacent chains in poly(4-vinylbiphenyl) (PVBP) have profound effects on the dynamics of this polymer. We report two unexpected nonlinear viscoelastic responses of PVBP when subjected to uniaxial flow. One is the unprecedented observation of extensional strain hardening (SH) in a barely entangled polymer melt. An even more intriguing finding is that SH of lightly (or even barely) entangled melts occurs at strain rates one order of magnitude below the coil-stretch transition predicted by Rouse theory (ϵ[over ˙]{H}=0.5/τ{R}).We postulate that this behavior is due to a molecular rearrangement mechanism (supported by x-ray diffraction measurements) that involves flow-induced π-π stacking of the phenyl groups, which results in an enhancement of the friction coefficient between polymer chains.