Anomalous Molecular Weight Dependence of Chain Dynamics in Unentangled Polymer Blends with Strong Dynamic Asymmetry

TL;DR

This study investigates how blending unentangled polymers with different dynamics alters their chain relaxation behavior, revealing deviations from classical models and establishing a link between molecular weight dependence and relaxation nonexponentiality.

Contribution

It demonstrates that blending can induce entangled-like dynamics in unentangled polymers and connects molecular weight effects with relaxation behavior using experimental and simulation data.

Findings

Unentangled polyisoprene shows entangled-like behavior at low temperatures in blends.

A relationship between relaxation time dependence and Rouse correlator nonexponentiality is established.

Molecular dynamics simulations confirm experimental observations.

Abstract

We address the general question of how the molecular weight dependence of chain dynamics in unentangled polymers is modified by blending. By dielectric spectroscopy we measure the normal mode relaxation of polyisoprene in blends with a slow matrix of poly(ter-butylstyrene). Unentangled polyisoprene in the blend exhibits strong deviations from Rouse scaling, approaching 'entangled-like' behavior at low temperatures in concomitance with the increase of the dynamic asymmetry in the blend. The obtained results are discussed in the framework of the generalized Langevin equation formalism. On this basis, a non trivial relationship between the molecular weight dependence of the longest chain relaxation time and the nonexponentiality of the corresponding Rouse correlator is found. This result is confirmed by molecular dynamics simulations.