Chain-Stretch Relaxation from Low Frequency Fourier Transform Rheology

TL;DR

This paper introduces a MAOS protocol to analyze chain-stretch relaxation in linear polymer melts, revealing low-frequency third harmonic features that align with a parameter-free molecular model, enhancing structural insights via standard rheometry.

Contribution

The study demonstrates that third harmonic analysis in MAOS can effectively characterize chain-stretch relaxation, expanding the capabilities of rheometry for polymer structure analysis.

Findings

Third harmonics obey time-temperature superposition.

GLaMM model closely matches experimental third harmonic data.

Low-frequency third harmonic features are accessible with standard rheometers.

Abstract

Medium or large amplitude oscillatory shear (M/LAOS) is sensitive to polymer chain structure, yet poses unsolved challenges for 'a priori' structural characterisation. In this letter, we present a MAOS protocol applied to near-monodisperse linear polymer melts, from which chain-stretch relaxation, a key structural feature, is clearly discernible. The third harmonics of MAOS frequency sweeps are decomposed into real and imaginary components and found to obey time-temperature superposition. The GLaMM molecular tube-based model of linear entangled melt rheology and structure, which has no free parameters, closely follows the form of our experimental results for the third harmonics and contains discriminatory features which depend only on the polymer's chain stretch relaxation time. Significantly, these third harmonic features occur at low frequency and are readily accessible with standard rheometers. For materials where phase transitions restrict the use of time temperature superposition, this method greatly increases the scope of rotational rheometry for structural analysis of polymers. Although the theory shows good qualitative agreement with experimental data, we find fundamental differences in magnitude and the frequency dependence of the third harmonics which must be resolved in order to fully understand the molecular basis of the stress response.