The melting of poly (L-lactic acid)

The effect of crystallization temperature and time on the melting of a stereo-copolymer of L-lactic acid has been measured using differential scanning calorimetry and hot stage microscopy. The temperature of the last trace of crystallinity increased with time and temperature. Increasing the heating rate to 50 degrees C min(-1) and correcting for thermal lag enabled the m.pt of the as-crystallized material to be determined without premature melting and recrystallization of the sample on heating. The measured melting points enabled the equilibrium m.pt. of the copolymer to be determined as 205 +/- 2 degrees C. Changes due to lamellae thickening occurred by secondary crystallization from the initial onset of crystal. lization increasing with increasing temperature consistent with the process being diffusion controlled The addition of small chain segments emerging from the top lamellar surface occurred by reptation and growth increased with the square root of the lapsed time. This is consistent with the process of secondary crystallization.