Effects of carbon nanotubes on crystallization and melting behavior of poly(L-lactide) via DSC and TMDSC studies

In this work, multiwalled. carbon nanotubes (MVTNTs) were surface-modified and grafted with poly(L-lactide) to obtain poly(L-lactide)-grafted MWNTs (i.e. MWNTs-g-PLLA). Films of the PLLA/MWNTs-g-PLLA nanocomposites were then prepared by a solution casting method to investigate the effects of the MWNTs-g-PLLA on nonisothermal and isothermal melt-crystallizations of the PLLA matrix using DSC and TMDSC. DSC data found that MWNTs significantly enhanced the nonisothermal melt-crystallization from the melt and the cold-crystallization rates of PLLA on the subsequent heating. Temperature-modulated differential scanning calorimetry (TMDSC) analysis on the quenched PLLA nanocomposites found that, in addition to an exothermic cold-crystallization peak in the range of 80-120 degrees C, an exothermic peak in the range of 150-165 degrees C, attributed to recrystallization, appeared before the main melting peak in the total and nonreversing heat flow curves. The presence of the recrystallization peak signified the ongoing process of crystal perfection and, if any, the formation of secondary crystals during the heating scan. Double melting endotherms appeared for the isothermally melt-crystallized PLLA samples at 110 degrees C. TMDSC analysis found that the double lamellar thickness model, other than the melting-recrystallization model, was responsible for the double melting peaks in PLLA nanocomposites. Polarized optical microscopy images found that the nucleation rate of PLLA was enhanced by MWNTs. TMDSC analysis found that the incorporation of MWNTs caused PLLA to decrease the heat-capacity increase (namely, Delta C-p) and the C-p at glass transition temperature. (c) 2007 Wiley Periodicals, Inc.