Architectural Effects of Poly(ε-caprolactone)s on the Crystallization Kinetics

Abstract

A series of hyperbranched poly(ε-caprolactone)s (HPCLs) with molecular architectural variation, which refers to the different lengths of the linear backbone segments consisting of 5, 10, and 20 ε-caprolactone monomer units (thereby referred to as HPCL-5, HPCL-10, and HPCL-20, respectively) and the different numbers of branching points, were synthesized without significant variation in the molecular weights, and linear poly(ε-caprolactone) (LPCL) of the same chemical structure and similar molecular weight was used as the linear counterpart to HPCLs for comparison. The specific molecular architectures of these samples were characterized by 1H NMR end-group analyses. The nonisothermal crystallization exotherms of HPCLs and LPCL were measured by DSC and further analyzed by the modified Avrami method. Regardless of the samples, the Avrami exponents ranged from 2.4 to 3.3, indicating that the nucleation and growth mechanisms of the samples are apparently identical. On the other hand, the crystallization kinetics was found to be fairly affected by the molecular architectures. All the kinetic parameters estimated from DSC exotherms such as the initial slope, Si, and from the modified Avrami analyses such as the corrected rate constant, Kc, and crystallization half-time, t1/2, indicated that HPCLs with longer linear segments and fewer branches showed the faster crystallization, whereas LPCL exhibited intermediate crystallization rate between HPCL-10 and HPCL-20, i.e., HPCL-5 < HPCL-10 < LPCL < HPCL-20. The slower crystallization was linked with the frequent presence of heterogeneous branching points, hindering the regular chain packing, in the backbone of HPCLs with shorter linear segments. In addition, the faster crystallization of HPCL-20 compared with LPCL was attributed to the higher cooperative chain mobility in their melt state as evaluated by activation energy of flow. A number of spherulites developed during isothermal process from the melt were observed for all samples by POM, and the radial growth rates of these spherulites were evaluated to correspond well with the results of Si, Kc, and t1/2 from the nonisothermal crystallization analyses.