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

The role of the molecular weight in the crystallizationand meltmemory of poly(epsilon-caprolactone) (PCL) was investigated. To thisend, 10 PCL samples of synthetic and commercial origin and differentchain ends, covering a number-average molecular weight (M (n)) 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-depthcharacterization. The thermal transitions, morphology, crystallizationkinetics, structural parameters, and memory effects were evaluatedas a function of M (n). The melting temperatureand the equilibrium melting temperature saturate at a particular M (n). Instead, the crystallization temperatureand the degree of crystallinity display an optimum M (n) at which the parameters reach a maximum, describinga bell-shaped behavior as a function of M (n). Similarly, the primary nucleation rate, spherulitic growth rate,and overall crystallization rate exhibit a bell-shaped behavior asa function of M (n), attributed to a competitionbetween nucleation and diffusion. SAXS analysis, which includes thelong period and lamellar thickness determination, revealed that at M (n) < 2.0 kg/mol, PCL crystallizes in an extended-chainconformation, while at M (n) >= 2.0kg/mol, folded chains are already present. In line with these results,the morphological study showed that the PCL crystallizes as axialitesat M (n) < 2.0 kg/mol and at higher M (n) as spherulites. The melt memory effect ofPCL, studied by self-nucleation experiments, increases with M (n) due to the simultaneous increase of entanglementsand the number of chain folding within the constituent crystallinelamellae per chain. Successive self-nucleation and annealing (SSA)experiments revealed that the PCL samples exhibit a similar SSA profile,indicating that the M (n) does not influencethe intermolecular interactions.