Molecular mechanisms driving the microgels behaviour: a Raman spectroscopy and Dynamic Light Scattering study

TL;DR

This study explores the molecular mechanisms behind the volume-phase transition and aggregation behaviors of PNIPAM/PAAc interpenetrated microgels using Raman spectroscopy and dynamic light scattering, revealing complex dual-step processes.

Contribution

It provides new insights into the molecular interactions and transition mechanisms in IPN microgels, especially the double-step transition involving both PNIPAM and PAAc.

Findings

PNIPAM hydrophobicity increases with PAAc interpenetration.

A crossover concentration triggers aggregation phenomena.

Double-step transition mechanism in IPN microgels involving coil-globule and PAAc steric effects.

Abstract

Responsive microgels based on poly(N-isopropylacrylamide) (PNIPAM) exhibit peculiar behaviours due to the competition between the hydrophilic and hydrophobic interactions of the constituent networks. The interpenetration of poly-acrilic acid (PAAc), a pH-sensitive polymer, within the PNIPAM network, to form Interpenetrated Polymer Network (IPN) microgels, affects this delicate balance and the typical Volume-Phase Transition (VPT) leading to complex behaviours whose molecular nature is still completely unexplored. Here we investigate the molecular mechanism driving the VPT and its influence on particle aggregation for PNIPAM/PAAc IPN microgels by the joint use of Dynamic Light Scattering and Raman Spectroscopy. Our results highlight that PNIPAM hydrophobicity is enhanced by the interpenetration of PAAc promoting interparticle interactions, a crossover concentration is found above which aggregation phenomena become relevant. Moreover we find that, at variance with PNIPAM, for IPN microgels a double-step molecular mechanisms occurs upon crossing the VPT, the first involving the coil-to-globule transition typical of PNIPAM and the latter associated to PAAc steric hindrance.