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Source of internal haze
December 15, 2007
The attached figure, showing light passing through a polymer containing crystalline regions is designed to show the source of internal haze in substrates composed of a single semi crystalline polymer. In the polymer there are crystalline regions called lamellar crystals which form larger semi crystalline aggregates called spherulites. The spherulites are composed of the lamellar crystals embedded in amorphous polymer chains. The light scattering causing the internal haze is from two primary sources. First, at the transition from the amorphous to crystalline phases there is a density change which causes some of the transmitted light to be defracted or scattered causing an increase in haze. Second there can be form scattering from the spherulites if the size of the spherulite is about that of the wave length of the light passing through the sample. If the spherulites are small relative to the lights wavelength, or if they are large relative to the lights wavelength then they will not increase light scatting. This is how nucleation works to lower haze, by forming many small spherulites. In amorphous polymers such as polystyrene (PS), polycarbonate (PC), polymethyl methacrylate (PMMA)there are no crystals and the polymers are crystal clear and any internal haze present would be due to small particles (such as catalyst) in the polymer.
The way the polymer substrate is manufactured will affect the level of the internal haze depending on how it affects level and size of the crystalline regions as well as the density of the amorphous phase. Orientation, especially solid state orientation will increase the average density of the amorphous phase and decrease light scattering at the crystal/amorphous phase boundary and decrease the haze. Casting heat transfer conditions of semi crystalline polymers will affect crystal nucleation and growth rates and affect internal haze. We will spend more time on these aspects of substrate manufacture in the coming days to better explain the impact of processing on substrate optical properties.
Interfacial instabilities in coextruded substrates as well as the density jump between layers can also impact internal haze and the quality if an image transmitted through the substrate and will be addressed in upcoming posts
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Posted by Eldridge M. Mount on December 15, 2007 | Comments (0)