Observation of Polylactide Stereocomplex by Atomic Force Microscopy

Stereocomplex (SC) interaction between poly(lactide)s (PLAs) was investigated by atomic force microscopy (AFM). Poly(L, L-lactide) (PLLA) and poly(D, D-lactide) (PDLA) were synthesized and grafted on a gold substrate, respectively. The chemical graft modification on the substrates was confirmed by contact angle measurement. In order to observe the force on stereocomplexation between PLAs, PLLA was also grafted on the tip of an AFM cantilever and moved toward the PLLA or PLDA on the substrate. When the cantilever was retracted from the substrate, a rupture (discrete deflection of the cantilever) was observed on the order of hundreds of piconewtons (pN), which reflects an attractive interaction between PLAs on the substrate and on the AFM tip. Force mapping measurement was then performed on the substrates in order to estimate distribution of the rupture force. The distribution of the rupture force between the PDLA and PLLA shifted to higher values than that between the PLLA and PLLA due to the stereocomplex formation. Conclusively, we firstly indicated that interaction between PLAs would be enhanced about two-fold by forming SC on the PLLA/PLLA interface.