Structure of Two-Compartment Hydrogels from Thermoresponsive ABC Triblock Terpolymers

Aqueous dispersions of a poly­(ethylene-alt-propylene)-b-poly­(ethylene oxide)-b-poly­(N-isopropyl­acrylamide) (PON) triblock terpolymer with block molecular weights of 3 000–25 000–10 000 and polymer concentrations ranging from 1 to 5 wt % were investigated at several temperatures from 25 to 55 °C using cryogenic scanning electron microscopy (cryo-SEM), cryogenic transmission electron microscopy (cryo-TEM), and small-angle neutron scattering (SANS). The cryo-SEM and cryo-TEM micrographs revealed that PON triblock terpolymer self-assembled into spherical micelles with PEP cores and PEO–PNIPAm coronae at room temperature and subsequently formed a two-compartment micellar network consisting of distinct spherical PEP and PNIPAm cores upon heating above the critical gelation temperature (42 °C). The formation of two discrete spherical PEP and PNIPAm hydrophobic domains was supported by detailed SANS analysis of the PON triblock samples in D2O, as the resulting intensity profiles can be successfully fitted using a scattering equation based on the two-compartment network structure. The two-compartment structure was further confirmed using contrast-matching SANS experiments on a PONd7 triblock sample with similar block molecular weights and a partially deuterated PNIPAm block. An important result of the SANS profiles in the gel state was the emergence of two distinct scattering peaks, which could be accounted for by considering spatial correlations between PEP and PNIPAm micellar cores. This study confirms the hypothesis that the formation of two-compartment networks in ABC terpolymer hydrogels results in better gelation properties, in comparison to other physically associated hydrogels, and can further guide the design and development of advanced hydrogel systems with enhanced performance.