Spherulitic crystallization behavior of poly(∈-caprolactone) with a wide range of molecular weight

Poly(∈-caprolactone) (PCL) with a wide range of molecular weight (MW) has been prepared via fractionation by either precipitating PCL/chloroform solutions into different amounts of methanol or adding methanol into PCL/tetrahydrofuran (THF) solutions. The samples with M_n ranging from 1900 to 64 700 were used to investigate the MW effects on the spherulite growth rate, nucleation density, and equilibrium melting point (T_m⁰) of PCL. The variation of spherulite growth rate with MW exhibited a maximum rather than a conventional monotonic drop. The existence of such a maximum was rationalized by considering the interplay between the effects of MW on T_m⁰ and segmental mobility. A growth kinetic formula proposed by Hoffman was employed to extract the crystal surface free energy product (σσ_e). In contrast to the conventional Lauritzen - Hoffman analysis which was based on the growth rates measured at different crystallization temperatures (T_c) for a given MW, the present analysis was based on the growth rates measured for different MW at a given T_c. The spherulite nucleation density was found to be higher for the sample with larger MW, and this observation was interpreted based on the individual effects of MW on the primary nucleation rate and spherulite growth rate. An increase in MW promoted the nucleation rate much more significantly compared with its effect on the growth rate, and this in turn led to a higher nucleation density. An unusual morphology due to the segregation of uncrystallizable short chains into the interfibrillar regions of the spherulites was also observed for PCL with M_n = 1900.