Sub-Rouse Dynamics in Poly(isobutylene) as a Function of Molar Mass

Abstract

We report on the molar mass dependence of the segmental, sub-Rouse, and terminal relaxation processes of poly(isobutylene). For this purpose, seven samples were synthesized with molar masses in the range from 2.8 to 172 kg·mol–1 and investigated by temperature-modulated differential scanning calorimetry, dielectric spectroscopy, and rheology. Rheology provided access to the terminal times that were found to scale as τterm ∼ Μ3.2. On the other hand, the sub-Rouse dielectric process shows a very weak molar mass dependence (τsub – Rouse/τSM ∼ M0.1), that is, distinctly different from the Rouse and terminal dynamics. We infer that the sub-Rouse mode has a lengthscale intermediate to the statistical segment length (∼1.25 nm) and the length of an entanglement strand (∼2.3 nm).