Accelerated crystallization of poly(l-lactic acid) by silk fibroin nanodisc

Abstract

Slow crystallization of poly(l-lactic acid) (PLLA) limits the widespread applications of PLLA because of its insufficient thermal and mechanical properties. In this study, a biobased filler, silk fibroin nanodisc (SFN), has been included in the PLLA matrix with the aim of accelerating the crystallization process of PLLA. Detailed investigation of the effect of SFN inclusion on the crystallizability of PLLA has been performed by polarizing optical microscope (POM) observations and differential scanning calorimetry (DSC) measurements. For isothermal crystallization studies by DSC, extremely quick cooling (~300 °C/min) from the melt to the crystallization temperature was conducted in order to avoid nucleation during the cooling process. The experimental results show that the small amount of SFN (only 1%) can significantly enhance the crystallizability of PLLA. The glass transition temperature and the melting temperature of PLLA were not influenced by the inclusion of SFN. However, the nucleation rate and the final degree of crystallinity were significantly enhanced. The effect of optical purity on the acceleration ability of SFN was examined in this research by utilizing two different types of PLLA having different contents of the D moiety (0.5 and 1.4%). As a result, it was found that the extent of enhancement is much larger in the case of highly optical pure PLLA (0.5% D moiety). The nucleating effect of silk fibroin nanodisc on the crystallization behavior of poly(l-lactic acid) (PLLA) was investigated by polarizing optical microscope observations and the differential scanning calorimetry measurements. The experimental results show that the small amount of SFN (only 1%) reduces the induction period and the crystallization half-time while it increases the final degree of crystallinity. It was found that the SFN works as a nucleation agent by increasing the number of nuclei about four times while it does not change the growth rate of spherulites.