The ageing of poly(ϵ-caprolactone)

Abstract

Partially crystallized poly( -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.

Secondary crystallization kinetics

“…Local segments are not constrained by the tube or entanglements and the mean square displacement is proportional to square root of time. The development of secondary crystallization followed a square root dependence on the lapsed time and as a result local diffusion of the chain segments play a part in the nucleation of the secondary growth nuclei.…”

“…The surface free energy σ e is large compared to the side surface free energies, σ , and as a result ξ is of the order of 10–20 nm. This is larger than the distance between adjacent chain entanglements, 1–3 nm, and as a result the length of the chain segments which makes up the thickness of the lamellae are constrained by these entanglements until they move.…”

“…Evidence have been derived from recent experimental for primary and secondary crystallization being consecutive and not concurrent, the extent of the primary process with time following an Avrami equation, and the extent of the secondary process is dependent on the square root of the lapsed time. …”

The isothermal crystallization of poly(ether ether ketone) by two‐dimensional differential scanning calorimetry correlation mapping

“…It is also nucleation rate dependent in the temperature range studied and the rate decreases as the temperature increases. Recently it has been shown that secondary crystallization is diffusion controlled and its rate increases with increasing temperature . Accordingly its contribution to the overall crystallinity must increase as its rate increases and the primary crystallization slows down.…”

“…Recently it has been shown that secondary crystallization is diffusion controlled and its rate increases with increasing temperature. 30 Accordingly its contribution to the overall crystallinity must increase as its rate increases and the primary crystallization slows down. The observed fractional n values which increase with temperature are considered to arise from the increasing contribution of secondary crystallization to the overall crystallinity.…”