Effect of α-Substituent in Poly(methyl)acrylate on Segmental and Chain Dynamics

This study compares the dynamic difference between poly(alkyl methacrylate) and poly(alkyl acrylate) to reveal the effect of the α-position methyl group (α-CH 3 ) on segmental and chain dynamics. We tune the molecular weights of poly(methyl methacrylate) (PMMA) and poly(methyl acrylate) (PMA), either entangled and unentangled, and entangled poly(isopropyl methacrylate) (PiPMA) and poly(isopropyl acrylate) (PiPA) to ensure that their linear viscoelasticity is normalizable, which allow us to compare their nonlinear extensional behaviors in a rigor and valid way: The strain hardening is much stronger for the PMA and PiPA samples without the α-CH 3 groups, which is attributed to different degree of freedom in exploring dihedral conformation. Namely, poly(alkyl acrylate) predominantly adopts cis side-chain conformations, whereas poly(alkyl methacrylate) tends to form a mix of cis and trans conformations, owing to a stabilization of trans conformation by intramolecular CH−π interactions. This conformational difference leads to a stiffer backbone and less stretchability in poly(alkyl methacrylate) than in poly(alkyl acrylate). When the alkyl group is the same, the contrast (between poly(alkyl acrylate) and poly(alkyl methacrylate) reduces with increasing alkyl group size.