Positional isomeric effects of coupling agents on the temperature-induced gelation of triblock copolymer aqueous solutions
While the importance of molecular linkers to design functional polymers and adjust their performance has been acknowledged, their role in thermogelling polymers is still unclear. Herein, we demonstrate that the di-functional positional isomers as the coupling agents in the middle of polymer chains greatly affect the microscopic conformation and macroscopic properties of thermogelling polymers. To enable such an examination, we synthesized BAB-type triblock copolymers methoxy poly(ethylene glycol)-b-poly(D,L-lactic acid-co-glycolic acid)-b-methoxy poly(ethylene glycol) (mPEG-PLGA-mPEG) linking with different coupling agents. All the polymers were soluble in water at ambient temperature and the polymer solutions underwent a sol–gel transition upon heating. After replacing hexamethylene diisocyanate (HMDI), a conventional coupling agent, with positional isomers of phthaloyl dichloride (o-PC, m-PC and p-PC), the polymer/water system exhibited lower sol–gel transition temperatures and larger gel moduli due to the phenyl rigidity of PC. We revealed that the polymer coupled with o-PC had a smaller coil size owing to its 60° linkage. This spatial conformation difference further resulted in a reduced thermal gelation temperature and an increased gel modulus of the polymer/water system compared with the copolymers linking with m-PC and p-PC isomers. In addition, the formation of a percolated micelle network via micellar aggregation was responsible for their sol–gel transitions with increasing temperature. The present work offers more insight into the relationship between the molecular structure of polymer chains and the thermogelation behaviors of polymer aqueous solutions.