Effect of Chain Architecture on the Structure, Dynamics, and Rheology of Thermoresponsive Poloxamer Hydrogels and Associated Blends

Poloxamers, ABA triblock polymers composed of a poly(propylene oxide) (PPO) midblock (B) and poly(ethylene oxide) end blocks (A), are widely studied for biomedical applications. Aqueous poloxamer 407 (P407) undergoes a solution-to-gel transition with increasing temperature, driven by the formation and ordering of micelles onto periodic lattices; however, the gel temperature and resulting modulus have limited tunability. Here, reverse P407 (RP407), a BAB polymer of the same composition and molar mass but with an inverted architecture, is synthesized via anionic polymerization. The micellization and gelation temperatures of RP407 are higher than those of P407, and the PPO end blocks allow for intermicelle bridging; however, both single-component solutions favor body-centered cubic (BCC) packings. Furthermore, aqueous RP407 displays a "soft-gel" region with interesting rheological behavior, including viscoelastic aging and thermal hysteresis. Combining P407 and RP407 yields solutions with intermediate transition temperatures and alters the micelle size and packing. While the single-component solutions produce BCC packings, the blends form close-packed structures and larger micelles of higher aggregation numbers. Blends of P407 with an analogous AB diblock (E111P32) display similar behavior, whereas RP407/diblock blends form intermediate-sized BCC-packed micelles. These differences in packing and aggregation alter the local environments within the gels, which could have implications for applications such as drug delivery and protein stabilization.