Confined Crystallization of PEO in Nanolayered Films Impacting Structure and Oxygen Permeability

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Abstract

Layer-multiplying coextrusion or “forced assembly” of thousands of alternating polystyrene (PS) and poly(ethylene oxide) (PEO) nanolayers was used to study crystallization in a confined, two-dimensional space. Atomic force microscopy and small-angle X-ray scattering revealed that as the thickness of the confined PEO layer decreased from the microscale to the nanoscale, the morphology of the PEO changed from three-dimensional spherulites to two-dimensional discoids and to “in-plane” lamellar stacks. Finally, when the confinement occurred on the 25 nm size scale of the usual lamellar thickness, the PEO layers crystallized as single lamellae with large aspect ratio that resembled large single crystals. This crystallization habit imparted more than 2 orders of magnitude reduction in oxygen permeability of the PEO layers. The oxygen permeability directly correlated with the orientation of the lamellar crystals. The dramatic decrease in oxygen permeability arose from a reduction in diffusivity due to increased tortuosity of the diffusion pathway through the oriented lamellae. However, the confined crystalline morphology did not affect the diffusion jump length of oxygen molecules, as reflected by the constant activation energy regardless of the layer thickness. At 85% relative humidity, the moisture sorption of PEO layers was consistently about 8%, independent of the layer thickness. Nevertheless, the lamellar orientation was not affected, and the dramatic decrease in PEO permeability was preserved.

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