Extensional Rheology of Unentangled Azobenzene Polymers: Synergetic Effect of π–π Interactions and Side-Chain Self-Dilution

The transient strain hardening during extension is critical to the industrial processing of polymers. However, the molecular mechanism of strain hardening of linear polymers under fast extension is still controversial. Both extension-enhanced and extension-reduced monomeric frictions have been proposed to explain the experimental observations in different polymers. In this study, we systematically studied the extensional rheology of unentangled azobenzene polymers (Pazo) and their copolymers and revealed the possibility of synergetic contribution of the side-chain self-dilution and π–π stacking to the strain hardening. In the slow flow regime (Rouse Weissenberg number WiR < 0.5), the strain hardening during extension is dominated by extension-enhanced friction due to the side-chain π–π stacking. The importance of side-chain self-dilution grows as WiR increases, and there is a critical side-chain length for the solvation effect to play a role under fast extension (WiR > 0.5). The strain hardening under all extension conditions weakens as the molar fraction of the azobenzene monomer in the copolymer decreases. However, azobenzene content as low as 0.2 in the copolymer is sufficient to generate evident π–π stacking-induced friction enhancement. The in situ wide-angle X-ray scattering (WAXS) experiments reveal a distinct enhancement of π–π stacking in Pazo in the transverse direction of extension due to side-chain flexibility, in contrast to the enhancement in the stretching direction in polymers with short rigid side chains containing benzene rings.