| Summary: | Non-isocyanate polyurethane (NIPU) networks physically modified with octa(3-hydroxy-3-methylbutyldimethylsiloxy)POSS (8OHPOSS, 0–10 wt%) were conditioned in environments of different relative humidities (up to 97%) to study water–polymer interactions. The equilibrium sorption isotherms are of Brunauer type III in a water activity range of 0–0.97 and are discussed in terms of the Guggenheim (GAB) sorption model. The study shows that the introduction of 8OHPOSS, even in a large amount (10 wt%), does not hinder the water affinity of the NIPU network despite the hydrophobic nature of POSS; this is attributable to the homogenous dispersion of POSS in the polymer matrix. The shift in the urethane-derived carbonyl bands toward lower wavenumbers with a simultaneous shift in the urethane N-H bending bands toward higher wavenumbers exposes the breakage of polymer–polymer hydrogen bonds upon water uptake due to the formation of stronger water–polymer hydrogen bonds. Upon water absorption, a notable decrease in the glass transition temperature (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>T</mi></mrow><mrow><mi>g</mi></mrow></msub></mrow></semantics></math></inline-formula>) is observed for all studied materials. The progressive reduction in <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>T</mi></mrow><mrow><mi>g</mi></mrow></msub></mrow></semantics></math></inline-formula> with water uptake is driven by plasticization and slaving mechanisms. POSS moieties are thought to impact slaving indirectly by slightly affecting water uptake at very high hydration levels.
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