Abstract
A new method for the estimation of the initial portion of the crystallization exotherm, normally obscured due to the response time of the DSC in the case of fast crystallization, was developed. This method allowed a more accurate analysis of the DSC isothermal crystallization data. Poly(propylene glycol)(PPG)-poly($\varepsilon$-caprolactam) (PCL)-PPG triblock copolymers with various PPG contents were synthesized via anionic copolymerization of $\varepsilon$-cairo-lactam and the macroactivators which contain PPG units. Isothermal crystallization behaviors of the synthesized block copolymers were allalyzed with Avrami equation by using above-mentinned non-linear curve fitting program and conventional Avraml plot. At PPG contents greater than 10%, the overall crystallization rate decreased because of the uncrystallizable PPG unit. The induction time for crystallization increased, and the ratio of primary crystallization to overall crystallization increased at the same degree of undercooling. On the other hand, crystal growth rate increased with PPG content. The presence of PPG unit in the block copolymer has two opposing effects on crystallization. First, as the amount of PPG unit increases it interferes with nucleation. As a result the induction time for crystallization increases and the crystallization rate decreases. Secondly, it increases the mobility of the block copolymers because of the ether linkage and allows faster diffion of the copolymer chain to the crystal surface. Consequently the crystal growth rate after nucleation increases. Between the two effects the first one seems to be dominating and the overall crystallization rate decreases with increasing PPG content in block copolymers. Furthermore there are some different crystallization behaviors between the block copolymers and PCL homo polymer. Nucleation of block copolymers of PPG content up to 10% occurs faster than in the case of homo PCL because small amounts of PPG unit may act as heterogeneous nuclei. Also the chain mobility increased with the introduction of PPG unit, therefore the overall crystallization rate was faster than homo PCL. But if the PPG content exceeds 10% the overall crystallization rate of the block copolymers becomes slower than homo PCL.
