Evolution of the structural polymorphs of poly(l-lactic acid) during the in vitro mineralization of its hydroxyapatite nanocomposites by attenuated total reflection fourier transform infrared mapping coupled with principal component analysis

Abstract

The evolution of the structural polymorphs of poly(l-lactic acid) (PLLA) during the in vitro mineralization of its hydroxyapatite (HA) nanocomposites is crucial for understanding the degradation mechanism of PLLA under physiological conditions. It was studied by attenuated total reflection Fourier transform infrared (ATR FTIR) mapping coupled with principal component analysis (PCA). The intensity ratios of the band pairs 1211/1186 cm−1 and 1384/1361 cm−1 (i.e., I1211/I1186 and I1384/I1361) of the first principal component (PC1) were used to trace the variations of the conformations and the states of order of PLLA, respectively. Two alternate transitions between amorphous and crystalline PLLA, as well as between α- and α′-PLLA were found for both porous and compact PLLA/HA nanocomposites during the mineralization. However, for PLLA states of order, more and more amorphous forms were observed with degradation, while for PLLA conformations, the alternate transition between α- and α′-PLLA was gradually hindered as mineralization proceeds. Our results demonstrate that the change of PLLA states of order is greater than the one of PLLA conformations, indicating that the hydrolyzation of amorphous PLLA is the major way of the mineralization of PLLA/HA nanocomposites. This developed method is sensitive to the changes of the structural polymorphs of PLLA and can be used to trace their evolution during the degradation.