Time-resolved SANS analysis of micelle chain exchange behavior: thermal crosslink driven by stereocomplexation of PLA-PEG-PLA micelles

Time-resolved small-angle neutron scattering (TR-SANS) was used to study the dynamic chain exchange behavior of the micelle mixture from poly(L-lactide)-b-poly(ethylene glycol)-b-poly(L-lactide), PLLA-PEG-PLLA, and its enantiomeric copolymer, PDLA-PEG-PDLA. The mixture of enantiomeric micelle solutions underwent a sol-to-gel transition as a consequence of temperature increase to yield a thermo-responsive hydrogel. The mechanism of gelation is hypothesized as a chain exchange process between L-micelles and D-micelles followed by the formation of stereocomplex crystals from PLLA and PDLA blocks. Stereocomplex crystals that have different physicochemical properties from PLLA or PDLA crystals restrain further chain exchange ability, which results in a network structure. We investigated the changes in micelle re-organization given by the SANS intensity as a function of q-range over time ranging from one to twenty minutes. TR-SANS data supported the hypothesis that single and stereo-mixed micelle systems undergo equilibrium and non-equilibrium chain exchange behaviors, respectively. We observed considerable differences in the scattering profile for the stereo-mixed micelles due to the structural re-organization from the original micelle to a bridged micelle network. The molecular weight of the hydrophilic PEG block was shown to influence the intermicellar interaction and chain exchange rate; short PEG micelles exhibited faster chain exchange behavior which was correlated to the lower sol-to-gel transition temperature observed.