Solid-state 13C NMR analyses of the structures of crystallized and quenched poly(lactide)s: Effects of crystallinity, water absorption, hydrolytic degradation, and tacticity

Abstract

The effects of crystallinity, water absorption, hydrolytic degradation, and tacticity on the solid structure and chain mobility of poly(lactide)s were investigated by solid-state 13C NMR spectroscopy. The following results were obtained from the line shapes of the carbonyl and methine carbons in 13C NMR spectra and their spin-lattice relaxation behavior. The crystallized poly( l-lactide) (PLLA) specimens in the dried, hydrated, and hydrolyzed states had two components, rigid and mobile components which can be, respectively, assigned to the crystalline and non-crystalline components. Upon water absorption, the chain mobility in the non-crystalline component of PLLA-C remained unvaried, reflecting a very small effect of the incorporated water molecules at room temperature. In contrast, the elevated chain mobility in the crystalline component and unclear splitting of carbonyl carbon strongly suggest that the water molecules are incorporated in the crystalline lattice. Upon removal of the non-crystalline components by hydrolytic degradation of crystallized PLLA, the chain mobility was slightly elevated in both crystalline and non-crystalline components by the lowered crystalline thickness and shortened non-crystalline chains. The non-crystalline specimens, PLLA (PLLA-Q) and poly( dl-lactide) (PDLLA), could be analyzed to contain two components, rigid and soft components, with the similar conformation but different restricted states of chains which cause high and low chain mobility. The insignificant difference in the spectral shapes and T 1C values between PLLA-Q and PDLLA strongly suggests that the effects of difference in the chain regularity and interaction on the spectral shapes and T 1C values are very low.