Direct extrusion 346 INDEX

A difficulty arises because the photonic crystal structures for the visible region are not easy to fabricate. However, in this chapter we describe a facile extrusion method for fabricating polymers with a ID structure like that shown in Figure 2. The nanolayered polymeric structures can consist of many thousands of layers and have a modulation in the nonlinear refractive index in the direction normal to the surface of the layers. Such materials are the nonlinear analogue of polymeric multilayer interference mirrors. (10,11,12) They are also the ID analogue of the 2D photonic crystals studied by Lin et. al. (13) The latter workers demonstrated that photonic crystals do indeed provide an effective method for converting an intensity dependent refractive index into an intensity dependent transmission. 

A similar morphology to that in sheet extrusion is formed in pipe extrusion, too. The spherulite size and crystallinity increase with increasing bath temperatiue. The molecular orientations in the axial, radial and tangential directions are high at the surfaces and decrease toward the interior. The orientation factors are higher when the melt flow index (MFI) of the PP is lowered or the extrusion speed is increased. 

Values for K and n can be obtained several ways. The most direct and accurate method is to utilize a capillary rheometer. Since most extrusion facilities do not have this equipment, other methods can be used to obtain approximate values. The extruder itself can be used to determine K and n as described in a reference paper (4). Additionally, K can be approximated from the polymer melt index, which is a fairly common value used in the extmsion industry. Equation 7 (5) below shows the relationship between viscosity (rj) and polymer melt index (MI) ... 

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