Dynamical behavior of microgels of Interpenetrated Polymer Networks

TL;DR

This study investigates the dynamic behavior of IPN microgels composed of PNIPAM and PAAc using light scattering, revealing how isotopic substitution, pH, and temperature influence their swelling, relaxation, and transition to glassy states.

Contribution

It extends the Flory-Rehner theory to describe IPN microgel swelling and explores the temperature-dependent fragility transition across the volume phase transition.

Findings

H/D substitution slows dynamics due to hydrogen bonding effects.

Swelling behavior fits extended Flory-Rehner theory for IPN microgels.

Two different approaches to glass transition are identified: Arrhenius and super-Arrhenius.

Abstract

Microgel suspensions of Interpenetrated Polymer Network (IPN) of PNIPAM and PAAc in D$_2$O, have been investigated through dynamic light scattering as a function of temperature, pH and concentration across the Volume Phase Transition (VPT). The dynamics of the system is slowed down under H/D isotopic substitution due to the different balance between polymer/polymer and polymer/solvent interactions suggesting the crucial role played by H-bondings. The swelling behavior, reduced with respect to PNIPAM and water, has been described by the Flory-Rehner theory, tested for PNIPAM microgel and successfully expanded to higher order for IPN microgels. Moreover the concentration dependence of the relaxation time at neutral pH has highlighted two different routes to approach the glass transition: Arrhenius and super-Arrhenius (Vogel Fulcher Tammann) respectively below and above the VPT and a fragility plot has been derived. Fragility can be tuned by changing temperature: across the VPT particles undergo a transition from soft-strong to stiff-fragile.