Structure-property correlation of hydrogels obtained via radical polymerization using the central cores of multiarm star polymers as crosslinkers

To improve the mechanical properties of a hydrogel, the construction of uniform network structures and/or the incorporation of energy-dissipating structures is important. In this study, we focused on gel synthesis using multiarm star polymers with a microgel core, which is expected to establish the abovementioned structures in vinyl polymer hydrogels. A series of star poly(N-isopropylacrylamide)s (PNIPAAms) with different arm molecular weights and vinyl group contents in the core were synthesized via an arm-first method using reversible addition-fragmentation chain transfer (RAFT) polymerization. The obtained star polymers were employed as crosslinkers to prepare polyacrylamide (PAAm) hydrogels by free radical polymerization. The content of vinyl groups in the core was critical for producing a hydrogel, and significantly affected the mechanical properties of the produced gels, which is indicative of the high effectiveness of the star polymer core as a crosslinker. The molecular weight of the arm chains of the star polymers also played a pivotal role in controlling the mechanical properties of the produced gels: moderately long arm chains, which form hydrogen bonding, were shown to act as energy-dissipating units. An equally important feature is the nearly even dispersion of the star crosslinkers in the network structure, as confirmed by SAXS, which achieved an increase in toughness without impairing the elongation upon increasing the main chain monomer concentration in the gelation reaction.