Weak non-linearities of amorphous polymer under creep in the vicinity of the glass transition

TL;DR

This study investigates the weak non-linear creep behavior of amorphous poly(etherimide) near its glass transition, revealing a stress-dependent acceleration consistent with recent theoretical predictions.

Contribution

It provides experimental evidence for a specific non-linear creep acceleration law with an exponent close to 2, supporting recent theoretical models of amorphous polymer creep.

Findings

Creep acceleration varies as exp[-(Σ/Y)^n] with n≈2.

Macroscopic acceleration correlates with local configurational rearrangements.

Development of stress disorder observed during creep at larger compliances.

Abstract

The creep behavior of an amorphous poly(etherimide) (PEI) polymer is investigated in the vicinity of its glass transition in a weakly non linear regime where the acceleration of the creep response is driven by local configurational rearrangements. From the time shifts of the creep compliance curves under stresses from 1 to 15~\si{\mega\pascal} and in the temperature range between $T_g -10K$ and $T_g$, where $T_g$ is the glass transition, we determine a macroscopic acceleration factor. The macroscopic acceleration is shown to vary as $e^{-(\Sigma/Y)^n} $ with $n=2 \pm 0.2$, where $\Sigma$ is the macroscopic stress and $Y$ is a decreasing function of compliance. Because at the beginning of creep, the stress is homogeneous, the macroscopic acceleration is thus similar to the local one, in agreement with the recent theory of Long \textit{et al.} (\textit{Phys. Rev. Mat.} (2018) \textbf{2}, 105601 ) which predicts $n=2$. For larger compliances, the decrease of the of $Y$ is interpreted as a signature of the development of stress disorder during creep.