Effect of substrate on the isothermal crystallization kinetics of confined poly(ε-caprolactone) nanolayers

Layer multiplying coextrusion was used to create films with hundreds or thousands of alternating layers of two polymers. The constituent polymers were chosen to create a temperature window in which a crystallizable polymer could melt and recrystallize within the glassy confinement of an amorphous polymer. This enabled the study of polymer crystallization behavior under nanoscale confinement. In this article, we examined the crystallization of poly(ε-caprolactone) (PCL) confined by polystyrene and poly(methyl methacrylate). We used AFM, WAXS, and SAXS to demonstrate that confined PCL nanolayers crystallized as large in-plane lamellae of high aspect ratio. This phenomenon, previously observed only for poly(ethylene oxide) (PEO), may be more general to crystalline polymers. We found that the in-plane PCL lamellae were at least as effective as PEO lamellae in reducing the oxygen permeability by more than 2 orders of magnitude. The substrates examined did not affect the crystallization habit of PCL but surprisingly had a large effect on the crystallization kinetics. The results supported the hypothesis that heterogeneous nuclei could diffuse to the interface from the confining polymer during melt processing, thereby substantially increasing the crystallization rate. In the absence of these additional nuclei, the crystallization kinetics was quantitatively described by a model that considered truncation of the growing spherulite. The retardation in crystallization rate over a very large range in layer thicknesses was described by the change in layer thickness only without any change in the linear growth rate. To our knowledge, this is the first time that the model has been quantitatively verified by experiment.