The Effect of a Secondary Process on Polymer Crystallization Kinetics – 3. Co-poly (lactic acid)

The crystallization kinetics of a copolymer of L-lactic acid with 4% D-lactic acid has been studied using FTIR spectroscopy by measuring the absorbance of the crystalline carbonyl stretching band at 1759 cm-1. Copolymerization greatly reduced the rate of crystallization and the technique directly measured the relative crystallinity over extended periods with sufficient accuracy to test the validity of the Avrami equation. This was modified to account for the simultaneous presence of a secondary process from the onset of crystallization, such that the overall fractional crystallinity, Xt, is related to the lapsed time, t, by
X_(t )=X_(p,∞) ( 1-exp⁡〖-Z_p t^n 〗)( 1+ k_s t^(1/2))
where Xp,∞ is the final fractional crystallinity achieved by the primary process, Zp is the primary composite rate constant incorporating nucleation and growth, and ks is the secondary rate constant.
The additional crystallinity produced by the secondary processes is sufficient to account for the observed fractional constant n values on analysis of the total development of crystallinity with time. It was concluded that the analysis using the Avrami equation should be restricted to the time dependence of the crystallinity produced by the primary process alone.