Hierarchical assembly in PLA-PEO-PLA hydrogels with crystalline domains and effect of block stereochemistry

Understanding the development of microstructure (e.g., structures with length scales roughly 0.5–500 μm)in hydrogels is crucial for their use in several biomedical applications. We utilize ultra-small-angle neutron scattering (USANS)and confocal microscopy to explore microstructure of poly(lactide)-poly(ethylene oxide)-poly(lactide)(PLA-PEO-PLA)triblock copolymer hydrogels with varying L/D-lactide ratio. We have previously found that these polymers self-assemble on the nanoscale into micelles. Here, we observe large-scale structures with diverse morphologies, including highly porous self-similar networks with characteristic sizes spanning approximately 120 nm–200 μm. These structural features give rise to power-law scattering indicative of fractal structures in USANS. Mass fractal and surface fractal structures are found for gels with L/D ratios of 80/20 and 50/50, respectively. Confocal microscopy shows microscale water-filled channels and pores that are more clearly evident in gels with a higher fraction of L-lactide in the PLA block as compared to the 50/50 hydrogels. Tuning block stereochemistry may provide a means of controlling the self-assembly and structural evolution at both the nanoscale and microscale, impacting application of these materials in tissue engineering and drug delivery.