Structural characterization of PLA-PEO-PLA solutions and hydrogels: Crystalline vs amorphous PLA domains

We have shown that we can significantly modify the nanoscale structure of solution and gels of ABA triblock copolymers in a solvent selective for the mid B block by making simple changes to the stereochemistry of the A block. We have also shown that the length of the A block can be used as an additional variable to further modify and thereby control the sizes of the nanoscale domains formed by these polymers in the presence of the solvent. Our systems are poly(lactide)-poly(ethylene oxide)-poly(lactide) solutions and gels, which have been previously shown to have tunable release characteristics and mechanical properties suitable for applications in tissue engineering and drug delivery. We have performed SANS to understand the self-assembly of these polymers in aqueous solution as a function of block length and stereospecificity of the PLA block as well as polymer concentration. A significant difference in structure and association behavior was seen between polymers made from amorphous D/L-lactic acid as compared to those with crystalline L-lactic acid blocks. In the former case, spherical micelles with radii of 10-14 nm form, whereas the latter forms assemblies of nonspherical "lamellar micelles" with characteristic radii of 11-15 nm and thicknesses of 8-10 nm. In both cases, increasing PLA block length leads to a larger characteristic size. Both polymers form an associative network structure at higher concentrations, leading to gelation.