Structures and interactions in collapsed hydrogels of thermoresponsive interpenetrating polymer networks

Abstract

Temperature-induced collapse of hydrogels of interpenetrating polymer networks (IPNs) poly(N-vinylcaprolactam)/poly(N-isopropylacrylamide) (PVCL/PNIPAm) and poly(N-isopropylmethacrylamide) (PNIPMAm)/PNIPAm, where both components are thermoresponsive, was studied by combination of 1H nuclear magnetic resonance (NMR) spectroscopy, small-angle neutron scattering (SANS), differential scanning calorimetry (DSC), and dynamic mechanical measurements. Behavior of studied hydrogels (one or two transitions) was found to depend on the ratio of both IPN components. For hydrogels of IPNs containing around 50 mol% of PNIPAm monomer units, separate transitions were revealed for both components. From SANS curves, it follows that compact three-dimensional multi-chain globules are formed in PNIPMAm/PNIPAm and PVCL/PNIPAm IPN hydrogels at temperatures above the phase transition, with a gyration radius of 14–28 nm. A certain portion of spatially restricted bound water (HDO) was established for all the studied IPNs at temperature above the volume phase transition from measurements of 1H NMR spectra, spin-spin relaxation times (T2), and diffusion coefficients (D) of HDO. Slow exchange regime between bound and free water was revealed. Spin–spin relaxation times (T2) and diffusion coefficients (D) as obtained for the bound HDO are up to 2 orders of magnitude smaller in comparison with “free” HDO. Higher content of bound water as found for collapsed hydrogels of IPN PVCL/PNIPAm in comparison with PNIPMAm/PNIPAm hydrogels is in accordance with swelling experiments and lower values of the shear mechanical modulus; this shows the decisive role of bound water in this respect.