Effect of small-scale architecture on polymer mobility

TL;DR

This study investigates how small-scale architectural differences in polyolefins influence polymer mobility, using an exact enumeration scheme to predict segmental motion probabilities and friction coefficients under various conditions.

Contribution

It introduces a novel enumeration scheme for simulating segment interactions and motion in polymers, linking small-scale architecture to macroscopic viscosity and mobility predictions.

Findings

Segmental motion probability varies with architecture, temperature, and pressure.

The model qualitatively matches experimental viscosity data.

Friction coefficients are inversely related to segmental motion probability.

Abstract

Processes on different length scales affect the dynamics of chain molecules. In this work, we focus on structures on the scale of a monomer and investigate polyolefins, i.e. hydrocarbon chains with different small scale architectures. We present an exact enumeration scheme for the simulation of interactions and relative motion of two short chain sections on a lattice and employ it to deduce the probability for segmental motion for polymers of four different architectures in the melt. The probability for segmental motion is inversely proportional to the monomeric friction coefficient and hence the viscosity of a polymer. Combining our simulation results with an equation of state for the thermodynamic properties of the polymers, we are able to make predictions about the variation of the friction coefficient with temperature, pressure, and small scale architecture. To compare our results with experimental data, we have determined monomeric friction coefficients from experimental viscosity data for the four polyolefins considered in this work. For temperatures well above the glass transition temperature, we find that our simple approach gives a good qualitative representation of the variation of the friction coefficient with chain architecture, temperature and pressure.