Effect of microstructure on polymer chain dynamics in polyethylene glycol solutions

TL;DR

This study investigates how microstructure and additives influence the segmental dynamics and rheological properties of polyethylene glycol solutions, revealing effects of concentration, nanoclay, and cellulose derivatives on polymer mobility and viscoelastic behavior.

Contribution

It provides new insights into the microstructural effects on PEG dynamics, including the impact of nanoclay and CMC on diffusion and rheology, and identifies a characteristic interaction time scale.

Findings

PEG diffusion follows Rouse scaling at low concentrations

Nanoclay slows probe diffusion and causes heterogeneity

CMC restores Rouse scaling and homogenizes diffusion

Abstract

Effect of microstructures and interactions on segmental dynamics in polyethylene glycol (PEG) solution in water is probed with macro-scale oscillatory rheology and micro-scale diffusion of a fluorescent probe. PEG solution fluorescence recovery after photobleaching (FRAP) curves have immobile fractions which increase with PEG concentration, for PEG volume fraction (c) > 0.2, indicating structuring. PEG solution micro-scale diffusion coefficients follow Rouse scaling D {\ alpha} c^(-0.54) for c < 0.8, resembling unentangled neutral polymers in good solvent. Addition of nanoclay bentonite (B) in PEG matrix slows down probe diffusion 3-7 times, with heterogeneous dynamics. With addition of carboxymethyl cellulose (CMC) in PEG matrix, probe diffusion is homogeneous with negligibly small enhancement in diffusion time. The macroscale storage modulii for PEG, PEG+B, and PEG + CMC solutions scale as viscous fluid-like, followed by short elastic plateau. Beyond the elastic plateau, the scaling is a concentration-dependent power law greater than the Rouse scaling of 0.5 for 0.1<c<0.2 PEG solutions. We identify a time scale {\tau}_b due to intermolecular interactions in PEG, such that for {\omega}>{\tau}_b^(-1) , Rouse scaling is recovered. Addition of CMC to PEG also restores Rouse scaling. Addition of B gives contributions from both polymer matrix and network of B particles, leading to departure from pure Rouse behaviour. Static microstructural studies reveal clay aggregation due to depletion interactions on increasing the concentration of clay particles in PEG matrix, which leads to a non-monotonic concentration dependence of storage modulus with c.