Polymerization-Induced Crystallization to Form Stretchable Hydrogels with Banded Spherulites and Circularly Polarized...

Reported here is the synthesis of stretchable hydrogels with large spherulites of different morphologies by polymerization-induced crystallization of dopant molecules. By varying the concentrations of chemical crosslinker and initiator, or the light intensity for photopolymerization, the stiffness of polyacrylamide network is tunable to regulate the crystallization of dibenzo-24-crown-8-ether molecules that form spherulites in the hydrogels. Regular spherulites are formed in relatively stiff gels, whereas banded spherulites with twisted crystal fibers are obtained in soft gels. The structure of spherulites is investigated by microscopy and scattering measurements. The formation of twisted crystal fibers is related to dynamic variations of crystallization pressure and network impedance. The gels with regular spherulites show stronger fluorescence and phosphorescence than those with banded spherulites. A remarkable fact is that the latter gels exhibit circularly polarized luminescence (CPL) with dissymmetry factor up to +1.5 × 10^-2. This luminescence arises from the clusterization-triggered emission of the network constrained by the crystals, while the twisted fibers render the achiral clusterluminogens with CPL. The mutual influences between polymer network and crystal growth account for the collective functions of the composite gels. The design principle and chiral transfer mechanism should open opportunities for developing other soft materials with tailored crystals and optical properties.