We analyze the swelling behavior of N-isopropylacrylamide (NIPAM)-based microgels incorporating the non-thermoresponsive comonomer N-tert-butylacrylamide (NtBAM) using photon correlation spectroscopy (PCS) and atomic force microscopy (AFM). Previous thermodynamic analysis of PNIPAM-co-NtBAM microgel swelling relied on the classical Flory-Rehner theory. However, this approach struggled to accurately describe swelling curves at higher NtBAM content. Our present work combines the original expression for the Flory-Huggins interaction parameter \(\chi _{FH}\) for NtBAM with a recently adapted Hill-like model for the interaction parameter \(\chi _{Hill}\) that accounts for cooperative effects in the volume phase transition in poly(NIPAM) microgels. This approach outperforms other methods in fitting quality. The observed results are revealing an exponential decrease in hydrodynamic radius upon increasing NtBAM content for swollen microgels. In addition, an exponential decay of the number of water molecules leaving the polymer chain during the volume phase transition is found, which can be attributed to the steric influence of one NtBAM monomer on the hydration of neighboring NIPAM monomers. The molar interaction enthalpy \(\Delta H_{SP}= \text {-49 kJ / mol}\) and entropy \(\Delta S_{SP} = \text {-177 J /( mol K)}\) were obtained from the fits of the swelling curves.
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