The ageing of poly(ε-caprolactone)

Partially crystallized poly(ϵ-caprolactone) has been stored for up to 6 months at various temperatures from −18 to 50 °C and the change in tensile properties, crystallinity and melting behaviour followed with storage time. The Young modulus, yield and drawing stress were observed to increase with time and at a rate which increased with storage temperature. These changes in tensile properties could be accounted for by the increase in crystallinity and were attributed to a thickening of the lamellae which reinforced the morphology and increased the stiffness of the polymer. The thickening of the lamellae accounted for the shift of the melting endotherms to higher temperatures with time. The stem lengths increased with the square root of the storage time and the rate increased with temperature corresponding to an activation energy of 40 ± 5 kJ mol−1. It is considered that ageing occurred by a process of secondary crystallization by extension of the ‘fold surface’ into the adjacent melt and the thickening of the lamellae. The time dependence of growth can only be explained by small segments of the chain being incorporated onto the crystal on the time scale of the local segmental mobility which is independent of chain entanglements. This does not have the characteristics of a nucleation controlled process but is a thermally activated diffusion process the rate of which increases with temperature. © 2015 Society of Chemical Industry