Multi-Responsive PAA/PNIPAM Semi-IPN Hydrogel Properties Controlled by PAA Topology

Polymer topology has emerged as a key strategy for advancing hydrogel design, offering the ability to impart tunable and enhanced properties through structural control beyond chemical composition. In this work, we specifically focus on the impact of polymer topologies such as linear and cyclic forms of poly(acrylic acid) (PAA) incorporated in poly(N-isopropylacrylamide) (PNIPAM) based semi-interpenetrating polymer networks. Topologically distinctive PAAs (linear and cyclic) were synthesized and characterized through ¹H NMR, SEC, and DSC. Both linear and cyclic PAA with comparable molecular weights were incorporated during the PNIPAM hydrogel preparation enabling a direct evaluation of topological effects. Swelling experiments revealed that hydrogels containing cyclic PAA demonstrated significantly lower and controlled swelling at varying pH conditions compared to their linear counterparts due to the structural constraints imposed by cyclic architecture. Rheological analyses showed that introduction of PAA topologies decreased the mechanical strength of pure PNIPAM hydrogel. Cyclic PAA/PNIPAM in the hydrated (swelling) state showed better network stability than linear PAA analogue due to additional mechanical crosslinking effect, while under thermal responsive phase transition (de-swelling), linear PAA/PNIPAM showed the highest mechanical strength. This study confirms the complex role of topological PAA in tuning the property of PNIPAM hydrogels under various conditions.