Effect of Tacticity on Segmental and Chain Dynamics of Poly(methyl methacrylate)

This study examines how the segmental and chain dynamics of poly(methyl methacrylate) (PMMA) depend on tacticity. Dielectric response spectroscopy reveals a much stronger β-relaxation peak, more clearly separated from the α-relaxation process at T < Tg, for syndiotactic PMMA (sPMMA) than for isotactic PMMA (iPMMA). Linear viscoelastic measurements show similar rubbery plateaus for sPMMA and iPMMA, suggesting comparable entanglement molecular weights. In contrast, nonlinear extensional rheology exhibits substantially weaker strain hardening in sPMMA than in iPMMA. We propose that locally stiffer and more curved chain conformations in sPMMA reduce packing efficiency at T < Tg, facilitating flipping of planar ester groups that manifests as an enhanced β-relaxation in dielectric response spectroscopy. The same local stiffness probably leads to lower stretchability of the sPMMA chains, thereby leading to the weaker strain hardening at Weissenberg number much higher than one.