Structure, rheology, and microrheology of mixed-micelle hydrogels of PEO-PPO-PEO copolymers

PEO-PPO-PEO block copolymers, particularly commercially available Pluronic® F127 and Pluronic® P123, exhibit thermoresponsive self-assembly, forming micelles and gels with tunable viscoelastic properties that have been utilized in drug delivery and personal care formulations. Formulations that combine multiple PEO-PPO-PEO copolymers with differing molecular weights and block lengths may provide a route to access systems with new properties that are optimized for specific delivery applications. This study investigates the rheology and microstructure of F127/P123 blends in water across a range of concentrations and temperatures by integrating bulk rheology, dynamic light scattering (DLS) microrheology, and small-angle X-ray scattering (SAXS). Our results demonstrate that increasing the F127 content enhances elasticity of gel samples at higher concentrations and temperatures, attributed to stronger intermicellar interactions from the longer PEO chains, whereas P123-rich blends exhibit more fluid-like behavior. Intriguingly, discrepancies between bulk rheology and microrheology measurements reveal that microstructural mechanisms governing gelation are not fully captured at the macroscopic scale. Specifically, microrheology uncovered an unexpected trend: decreasing the F127 ratio led to reduced probe particle mobility, contradicting bulk modulus trends and suggesting localized structural constraints. SAXS analysis further elucidates the relationship between micellar packing and phase transitions, providing nanoscale insights into the system’s structural evolution. These findings underscore the importance of multiscale characterization in bridging the gap between micro- and bulk rheology, offering critical insights for the design and optimization of Pluronic®-based formulations in biomedical and industrial applications.