Solid State NMR study of stereocomplexes formed by enantiomeric star-shaped PEG-PLA copolymers in water

Solid-state NMR was applied to samples obtained by freeze-drying hydrogels of 1:1 (PEG(65)-NHCO-PLLA(13))(8)/(PEG(65)-NHCO-PDLA(13))(8) or (PEG(65)-NHCO-PDLA(13))(8) only star block copolymers (where PEG, PLLA, and PDLA stand for poly(ethylene glycol), poly(L-lactide), and poly(D-lactide), respectively) in order to get insight into the different structural and dynamic properties of stereocomplexed poly(lactide) (PLA) aggregates with respect to single enantiomer ones responsible for the improved mechanical and degradation properties of the corresponding hydrogels. (13)C MAS NMR experiments together with (13)C relaxation time measurements indicated that the PLA domains in (PEG(65)-NHCO-PLLA(13))(8)/(PEG(65)-NHCO-PDLA(13))(8) were highly crystalline, whereas those in (PEG(65)-NHCO-PDLA(13))(8) were mainly amorphous. On the basis of (1)H relaxation and spin-diffusion experiments, similar average dimensions were determined for the PLA aggregates in the two samples. PLA stereocomplexation was found to strongly affect the conformational behavior of PEG chains. Under the assumption that freeze-drying preserves the structure of at least the PLA aggregates, the results obtained are of value for understanding self-aggregation of PEG PLA star block copolymers in water.