Elucidations of Crystallization Pathway in Poly(Lactic Acid) Racemate by Solid-State NMR Spectroscopy: Analogies between Glassy and Crystalline Structures

Polymer crystallization is a process which connects the initial amorphous state with the final semicrystalline state. It is important to elucidate the amorphous structure which determines the crystallization pathway. In this work, we report quantitative analysis of the spatial proximity of poly(Lactic acid) (PLA) racemate before and after stereocomplex (SC) crystallization by using ¹³C selective isotope labeling and two-dimensional solid-state (ss) NMR techniques. It is found that i) the PLA racemate forms SC structure prior to crystallization (chiral recognition), ii) fraction of the chiral recognition segments (f) is extremely high, 94% in a low molecular weight (M) racemate while a high M one possesses only f = 10%, and iii) the f value for the former and latter is surprisingly in accordance with the f value after SC crystallization and SC crystallinity, respectively. From the observed analogies between the initial glassy and final crystalline structures, it is concluded that pre-existing chiral recognition fraction governs the formations of SCC.