Analysis of the thermal expansion behavior of oriented polyoxymethylene sheets
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The applicability of the molecular composite model is extended to include the analysis of the thermal expansion behavior of oriented (anisotropic) polyoxymethylene. The model accounts for changes in the morphological and structural aspects of polyoxymethylene during solid state-induced orientation. The results are reported and analyzed in terms of three-dimensional surfaces. Apart from the effect of temperature, it has been found that the crystalline phase orientation distribution, λ, is the most crucial parameter affecting the thermal expansion behavior of anisotropic polyoxymethylene followed by the volume fraction crystallinity. The size and the geometry of the crystalline phase have negligible effects on the thermal expansion behavior of oriented polyoxymethylene. Above the glass-transition temperature, a minimum in the transverse thermal expansion coefficient is exhibited in the vicinity of λ = 2 for volume fraction crystallinities ≤0.75. It is sufficient to align 75% of the crystallites with their c-axis at ±10° to the draw direction, to attain the ultimate degree of anisotropy in the thermal expansion behavior of oriented polyoxymethylene sheets.
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