Confined Crystallization in Nanolayered Films Eric Baer, Case Western Reserve University, DMR 0423914.
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The design and fabrication of ultra-thin polymer layers is of increasing importance due to the rapid development of nanoscience and nanotechnology. We discovered a unique morphology that emerges as polyethylene oxide (PEO) layers are made progressively thinner. This was achieved by using an innovative layer-multiplying process to obtain assemblies with thousands of PEO nanolayers confined between layers of another polymer, in this case poly(ethylene-co-acrylic acid) (EAA),
Direct observation by atomic force microscopy revealed that when the thickness confinement occurs on the 20 nm size scale, the PEO layers crystallize as single, high aspect ratio lamellae that resemble large, impermeable single crystals (Figure 1A). If the layers are slightly thicker, for example 100 nm, the PEO crystallizes as stacks of three to five long thin lamellae oriented in the plane of the layer (Figure 1B). The oriented crystal structure was confirmed by small angle and wide angle x-ray scattering (Figure 2).
This is the first time that a “single crystal” structure has been obtained in a continuous melt process.
100nm PEO layer
20nm PEO layer
Figure1 AFM images of EAA/PEO Nanolayers
PEO “Single Crystal”
from melt process
Figure2 Wide angle X-ray scattering pattern of PEO “single crystal” in EAA/PEO Nanolayers
Test condition: 23oC, dry
Figure3 Effect of layer thickness confinement on oxygen permeability, P(O2) of PEO Layer.
Confined Crystallization in Nanolayered FilmsEric Baer, Case Western Reserve University, DMR 0423914
Unexpectedly, the unique crystallization habit of PEO nanolayers imparts two orders of magnitude reduction in the PEO gas permeability. Structurally, the PEO lamellae resemble large impermeable platelets that are aligned perpendicular to the gas diffusion direction (Figure 3). Analysis of the increased gas diffusion pathway gives the lamellar aspect ratio as high as 120, corresponding to a lateral dimension of more than 2 microns. This is comparable to the size of single crystals grown from solution.
Confined crystallization is conventionally studied with specifically synthesized block copolymers. The facile coextrusion process offers a new route to a wealth of scientific and technological opportunities for confined crystallization. Polymer nanolayers can now be incorporated into conventional polymeric films to utilize their unique properties in the design and execution of packaging strategies that address growing environmental and energy concerns. Packaging with enhanced gas barrier and selectively could dramatically reduce the amount of food waste, thereby reducing world hunger, greenhouse gas generation and the load on global water and energy supplies