H. P. Wang

Confined Crystallization of Polyethylene Oxide in Nanolayer Assemblies

The design and fabrication of ultrathin polymer layers are of increasing importance because of the rapid development of nanoscience and nanotechnology. Confined, two-dimensional crystallization of polymers presents challenges and opportunities due to the long-chain, covalently bonded nature of the macromolecule. Using an innovative layer-multiplying coextrusion process to obtain assemblies with thousands of polymer nanolayers, we discovered a morphology that emerges as confined polyethylene oxide (PEO) layers are made progressively thinner. When the thickness is confined to 20 nanometers, the PEO crystallizes as single, high-aspect-ratio lamellae that resemble single crystals. Unexpectedly, the crystallization habit imparts two orders of magnitude reduction in the gas permeability.

Impact of Nanoscale Confinement on Crystal Orientation of Poly(ethylene oxide)

Using a layer-multiplying coextrusion process to fabricate films with thousands of alternating polymer nanolayers, we report here a new crystalline morphology in confined polymer nanolayers and an abrupt transition in the crystallization habit. At higher temperatures, poly(ethylene oxide) crystallizes as large, in-plane lamellae. A 5 °C change in the crystallization temperature produces an on-edge lamellar orientation. The results point to a transition from heterogeneous nucleation to substrate-assisted nucleation. This may be a general phenomenon that accounts for previously unexplained differences in the preferred chain alignment of confined polymer crystals.