Phase Transition in Hydrogels of Thermoresponsive Interpenetrating Polymer Networks

We compared behavior of hydrogels of two types of thermoresponsive interpenetrating polymer networks (IPN), PNIPMAm/PNIPAm and PVCL/PNIPAm, during their phase transition. One or two transitions, depending on the ratio of both IPN components, were found by NMR and DSC. SANS results showed that compact multi-chain globules with gyration radius similar to 20 nm are formed in collapsed IPN hydrogels. A certain portion of spatially restricted bound water (HDO) was established for all studied IPNs in the phase transition region and at temperatures above the phase transition by NMR methods. Fraction of bound water slowly decreases with time. The separate signal of bound water in NMR spectra appears already in pretransition region at temperatures when polymer segments just begin to form collapsed structures. Higher content of bound water as found for collapsed PVCL/PNIPAm hydrogels in comparison with PNIPMAm/PNIPAm hydrogels is in accord with swelling experiments and lower values of the shear mechanical modulus, and shows the decisive role of bound water in this respect.