Macromolecules, Vol.52, No.10, 3697-3715, 2019
Thermogelling of Amphiphilic Block Copolymers in Water: ABA Type versus AB or BAB Type
Some amphiphilic block copolymers exhibit sol gel transition in water upon heating, which affords a promising injectable thermogel in clinic and an interesting soft matter system. While ABA-, BAB-, and AB-type block copolymers have all been reported, little is known about the comparative study of supermolecular structures of these polymer types in the physical hydrogels, which hinders the understanding of the universal mechanism of structural changes during thermogelling. Herein, a thermogellable aqueous system of ABA triblock copolymer poly(D,L-lactide-co-glycolide)-b-poly(ethylene glycol)-b-poly(D,L-lactide-co-glycolide) was investigated by both experiments and computer simulations, with the corresponding AB diblock copolymer and BAB triblock copolymer as controls. The copolymers were synthesized via ring-opening polymerization, and their thermogelling behaviors in water were analyzed with transmission electron microscopy, three-dimensional dynamic light scattering, and so forth. Fluorescence resonance energy transfer, temperature-dependent C-13 NMR, and rheological measurements were also carried out to investigate the internal structures and their evolutions during the sol gel transition. A dynamic Monte Carlo simulation was operated to analyze the thermogelation further. We found two states with different structures in the thermogel window of the ABA block copolymer. The formation of a hydrophobic channel evolved from the semibald micelle was revealed as the key universal cue triggering the physical gelation for all types of thermogellable copolymers. Based on our structural studies, the molecular design principles for the thermogellable copolymers have been established.