Interplay of network architecture and ionic environment in dictating pNIPAM microgel thermoresponsiveness

TL;DR

This study systematically explores how network architecture and ionic strength influence the thermoresponsive behavior of pNIPAM microgels, providing insights into their stability and responsiveness in saline environments.

Contribution

It offers a comprehensive experimental analysis of various microgel architectures under different ionic conditions and evaluates the applicability of theoretical models to these systems.

Findings

Network topology significantly affects salt tolerance thresholds.

Crosslinker distribution influences hysteresis and flocculation behavior.

Theoretical models are assessed for their validity in describing microgel responses.

Abstract

The utility of non functionalized poly(N-isopropylacrylamide) (pNIPAM) microgels in physiological and environmental applications is strictly dependent on their reversible thermoresponsiveness and stability in saline media. Despite their importance, a unified understanding of how network topology specifically crosslinker concentration and distribution regulates ionic sensitivity remains fragmented in the literature. This work systematically investigates the interplay between network topology and ionic strength (0 to 100 mM NaCl) across eight distinct microgel architectures, ranging from ultra-low crosslinked (ULC) to core-corona and homogeneously crosslinked (HC) variants. Utilizing dynamic light scattering across 22 batches, we analyzed critical thermoresponsive properties, including volume phase transition temperature (VPTT) shifts, salt tolerance thresholds, hysteresis indices, and flocculation kinetics (only at extreme salinity, 1000 mM NaCl and at 25 deg C). This comprehensive investigation enables a multidimensional analysis of how ionic strength, the presence or absence of crosslinkers (MBA), spatial crosslinking distribution, and thermodynamic states dictate microgel behavior across varying temperatures. Finally, we evaluate the applicability of this experimental library to established theoretical frameworks, specifically the Flory Rehner and Flory Rehner Donnan models, addressing ongoing debates regarding their validity in describing complex microgel systems.