# The Stiffness for Viscous Deformation in the Interlamellar Amorphous Region of Polyethylene

## TL;DR

This paper uses a spring-dashpot model to study how different parts of polyethylene's amorphous region respond to stress relaxation.

## Contribution

The study reveals that stiffness in the short-term Maxwell branch is not consistently affected by SCB content or density.

## Key findings

- Stress drop during relaxation can be simulated without changing short-term branch parameters.
- Short-term branch stiffness does not correlate monotonically with SCB content or mass density.
- MR tests and the spring-dashpot model can provide insights into stiffness in different amorphous regions of polyethylene.

## Abstract

A spring–dashpot model, consisting of a spring branch and two Maxwell (named long- and short-term) branches, was used to simulate stress drop during the relaxation stages of multi-relaxation (MR) tests. This work shows that the stress drop at relaxation in a deformation range around the peak stress could be closely simulated without changing the parameter values for the short-term branch. This possibility was confirmed using three ethylene/1-hexene copolymers and one ethylene homo-polymer, among which the main differences are mass density and short-chain branch (SCB) content. The work examined the influence of SCB content and mass density on the stiffness of the two Maxwell branches, and the results showed that, unlike the long-term branch counterpart, stiffness of the short-term branch is not a monotonic function of the SCB content or the mass density. This led to a discussion on the possible relationship between the stiffness of the two Maxwell branches and the deformation resistance of the amorphous phase at different locations of the microstructure, i.e., in the interlamellar region and as part of the network structure. The paper concludes that a combination of the MR test and the spring–dashpot model could provide information that is related to the stiffness in different parts of PE’s amorphous phase, though further work is needed to verify this conclusion.