Lyogel Equilibria with Helmholtz Equations of State. Part II: Assessment of PC-SAFT Parametrizations and Network Models for pNIPAM Lyogels

Abstract This work investigates the equilibrium swelling of poly(N-isopropylacrylamide) (pNIPAM)-based lyogels in multicomponent solvent systems using a Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT)-based thermodynamic framework coupled with polymer-network elasticity. Literature parameter sets for the butyl esterification system were assessed and refined, and different network models were compared within a sequential fitting strategy that links solvent, polymer-solution, and gel data. The results show that the framework can reproduce key swelling trends and supports the transfer of calibrated network parameters between related solvent descriptions. At the same time, the study highlights important limitations: quantitative fitting of lower critical solution temperature (LCST) behavior requires a temperature-dependent interaction parameter, finite-extensibility effects are negligible for the investigated systems, and the transferability of fitted interaction parameters to mixed solvents is limited. Overall, the work provides a practical framework for gel-equilibrium calculations while emphasizing the need for careful interpretation of fitted parameters.