The gelation behavior of a poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PON) triblock terpolymer and a poly(N-isopropylacrylamide)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (NON) triblock copolymer was studied by rheology over the concentration range 1-5 wt %. In comparison to the NON copolymer, gelation of the PON terpolymer was achieved at a much lower concentration, with a much sharper sol-gel transition. This is due to a stepwise gelation of PON terpolymers involving micellization at room temperature and gelation at elevated temperatures. The separation of micellization and gelation leads to the formation of a two-compartment network as observed by cryoTEM. The results highlight the intricate and tunable nanostructures and new properties accessible from ABC terpolymer hydrogels.

Three poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEP-PEO-PNIPAm, "PON") triblock terpolymers were synthesized using a combination of anionic and reversible addition-fragmentation chain transfer polymerization, and their micellization and micellar aggregation properties in dilute aqueous solution were studied by dynamic light scattering (DLS) and cryo-TEM. The PEP and PEO blocks had molecular weights of 3000 and 25 000, respectively, while the PNIPAm block was varied with molecular weights equal to 4000, 10 000, and 21 000. In dilute aqueous solution the terpolymers formed well-defined micelles with hydrophobic PEP cores surrounded by hydrophilic PEO-PNIPAm coronas at temperatures below the lower critical solution temperature (LCST) of PNIPAm. DLS revealed that the PON micelles prepared by a dialysis technique were significantly smaller than those prepared by a thin-film hydration technique. In either case, at temperatures above the LCST of PNIPAm, the micelles associated to form larger aggregated structures. The critical micellar aggregation temperature for these PON triblock terpolymers was higher than the typical LCST for a PNIPAm homopolymer and depended on the PNIPAm block length and polymer concentration. As the molecular weight of PNIPAm decreased from 21 000 to 4000, the critical micellar aggregation temperature increased from 36 °C to above 60 °C. The critical micellar aggregation temperature was also higher at lower polymer concentrations. These results demonstrate how the inclusion of a temperature-sensitive endblock in a linear ABC triblock terpolymer with a strongly hydrophobic component can lead to tunable and reversible micellar aggregation behavior.

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