Temperature-induced phase transition in hydrogels of interpenetrating networks of poly(N-isopropylacrylamide) and polyacrylamide
Abstract
Collapse transition in hydrogels of interpenetrating polymer networks (IPNs) of thermoresponsive poly(N-isopropylaaylamide) (PNIPAm) and hydrophilic polyacrylamide (PAAm) was studied by a combination of NMR spectroscopy, small-angle neutron scattering (SANS) and DSC. The influence of IPNs composition and preparation on the phase transition was examined with respect to possible application of these responsive materials for drug delivery and molecular separation.
Heterogeneous nature and an existence of compact globular structures with gyration radius similar to 20 nm was revealed in all investigated IPN hydrogels by SANS. The increasing content of PAAm component in IPNs shifts the transition toward higher temperatures and the fraction p(max) of polymer units with significantly reduced mobility detected by NMR as well as the enthalpy change Delta H in DSC measurements are reduced.
Reversed order of adding components during IPNs preparation also significantly affects parameters of the phase transition and collapsed structures. IPN PAAm/PNIPArrr 42/58 shows substantially reduced Delta H values of the transition in comparison with IPNs PNIPAm/PAAm.
A certain portion of water (HDO) bound in globular structures was established from measurements of H-1 NMR spectra and spin-spin relaxation times T-2 in all PNIPAm/PAAm collapsed hydrogels and in collapsed hydrogel of the neat PNIPAm, while bound water was not detected by NMR in PAAm/PNIPAm hydrogel. A slow exchange regime between bound and free water was revealed in most cases.
T-2 relaxation times of bound HDO are then up to one order of magnitude smaller in comparison with "free" HDO. Interestingly enough, the bound HDO appears already at temperatures when polymer segments just begin to form collapsed structures.
The fraction of bound water slowly decreases with time.