Bulk crystallization behavior of poly(ε-caprolactone) with a wide range of molecular weight

Bulk crystallization kinetics and crystallizability of poly(ε-eaprolaclone) (PCL) with number average molecular weight (M_n) ranging from 1900 to 64700 were investigated. Most PCL samples crystallized rapidly at 30 and 40 C, such that the crystallization started before the equilibration of DSC signal. As a consequence, the relative crystallinily required for Avrami analysis could not be calculated accurately from the recorded DSC exotherms. An alternative method is proposed to evaluate the Avrami crystallization rate constant from the peak time of the isothermal crystallization exotherm. The calculated crystallization rate constants of PCL displayed a maximum in variation with molecular weight (MW). Such behavior was attributed to interplay between the MW effects on the thermodynamic driving force and the segmental mobility associated with crystallization. A kinetic formula proposed by Hoffman was used to obtain the crystal surface free energy product (σσ_c. In contrast to the conventional Lauritzen- Hoffman analysis based on crystallization rates measured at ditlerent crystallization temperatures (T_c) for a given MW, the present analysis was based on rates measured for different MW at a given T_c. MW variation of degree of crystallinity also displayed a maximum rather than a monotonic drop. Crystallizability was suggested to be controlled by the presence of uncrystallizable short chains and the entanglements in the polymer. The fraction of the uncrystallizable short chains played the main role in controlling the crystallinity for the low MW samples, while entanglement density was the principal factor for the high MW samples.