Composition-Driven Phase Separation, Micellar Ordering, and Sulfadiazine Release in Poly(vinylpyrrolidone)–Tetronic 908 Hydrogels

We investigate the phase behavior, nanostructure, swelling, and sulfadiazine (SDZ) release of aqueous PVP K90-Tetronic 908 (T908) blends with concentration and T908:PVP ratio. Phase diagrams (20 °C–80 °C, 5–30 wt %) reveal a percolation threshold at ∼25 wt %: below it, mixtures form clear solutions or coexisting liquid phases; above it, a gel forms at physiological temperature. Phase coexistence─demixing into a T908-rich clear liquid and a PVP-rich turbid phase, with T908 inserted into the PVP network─occurs across most compositions at low temperature (20 °C–35 °C), and persists across the full range (20 °C–80 °C) near 1:1 T908:PVP ratios; there, the PVP-rich phase is dense and cohesive, liquid at low temperature and gelling upon heating. Small-angle neutron scattering (SANS) supports this picture at the nanoscale: individual micelle size is largely unaffected by PVP content. In the T15:P15 blend, T908 forms free micelles at 20 °C─where a low-q upturn confirms nanoscale microsegregation─and a body-centered cubic (BCC) gel at 37 °C; however, only part of the nominal 15 wt % T908 enters the ordered BCC lattice: 15 wt % alone is insufficient for gelation, but local crowding in the PVP-rich domain raises T908 above this threshold there. The remaining T908 likely resides in the coexisting T908-rich liquid phase, below its local aggregation concentration, as unimers rather than micelles. Swelling and SDZ release both reflect this phase-separated architecture, though not proportionally: T908 substantially enhances water uptake versus pure PVP, but blend swelling is nonadditive, and the 1:1 (w/w) blend─despite lower net swelling than pure T908─released more SDZ per gram (0.12 ± 0.04 mgSDZ/gformulation) than the 5:1 blend (0.083 ± 0.011 mgSDZ/gformulation) or pure PVP (0.068 ± 0.006 mgSDZ/gformulation), pointing to the dense phase as a preferential drug reservoir.