Effect of Molecular Weight on the Crystallization and Melt Memory of Poly(ϵ-caprolactone) (PCL)

Abstract

The role of the molecular weight in the crystallization and melt memory of poly(ϵ-caprolactone) (PCL) was investigated. To this end, 10 PCL samples of synthetic and commercial origin and different chain ends, covering a number-average molecular weight (Mn) range between 0.48 and 70.5 kg/mol, were analyzed. Polarized light optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering (SAXS) were employed for in-depth characterization. The thermal transitions, morphology, crystallization kinetics, structural parameters, and memory effects were evaluated as a function of Mn. The melting temperature and the equilibrium melting temperature saturate at a particular Mn. Instead, the crystallization temperature and the degree of crystallinity display an optimum Mn at which the parameters reach a maximum, describing a bell-shaped behavior as a function of Mn. Similarly, the primary nucleation rate, spherulitic growth rate, and overall crystallization rate exhibit a bell-shaped behavior as a function of Mn, attributed to a competition between nucleation and diffusion. SAXS analysis, which includes the long period and lamellar thickness determination, revealed that at Mn < 2.0 kg/mol, PCL crystallizes in an extended-chain conformation, while at Mn ≥ 2.0 kg/mol, folded chains are already present. In line with these results, the morphological study showed that the PCL crystallizes as axialites at Mn < 2.0 kg/mol and at higher Mn as spherulites. The melt memory effect of PCL, studied by self-nucleation experiments, increases with Mn due to the simultaneous increase of entanglements and the number of chain folding within the constituent crystalline lamellae per chain. Successive self-nucleation and annealing (SSA) experiments revealed that the PCL samples exhibit a similar SSA profile, indicating that the Mn does not influence the intermolecular interactions.