Molecular origin of extensional strain hardening in phenyl-containing polymer melts

We recently reported unexpectedly extensional strong strain hardening (SH) in poly(4-vinylbiphenyl) (PVBP) when subjected to uniaxial flow. We postulate that this behavior is due to a molecular rearrangement mechanism (supported by WAXS measurements) that involves flow-induced π- π stacking of the phenyl groups, which results in an enhancement of friction drag between polymer chains [PRL 119, 247801 (2017)]. To further elucidate the molecular mechanism of SH, we measure the time evolution of the molecular alignment using an extensional rheometer housed in a custom-built oven designed to facilitate in-situ synchrotron X-ray experiments. Based on the anisotropic 2D WAXS profiles, we defined two alignment factors, one for the backbone (A_bb) and one for the π-π stacking (A_ππ). Perfect correlation between A_bb and A_ππ are observed during extensional flow start-up and flow cessation. Moreover, linear relations between both A_bb and A_ππ and transient stress (σ_E⁺) are observed at stress values below 1 MPa. The resemblance between this behavior and the stress-optical rule observed in many polymeric liquids led us to propose a stress-WAXS rule (SWR): A_bb=C_bbσ_E⁺ and A_ππ=C_ππσ_E⁺. Finally, we will discuss the validity of the SWR in PVBP and polystyrene.