Polymer transparency

Polymer transparency also requires that the crystalline regions be smaller than the wavelength of light, since these regions also represent changes in refractive index. Larger crystallites will scatter light at their interfaces and make the PET opaque. 

Polymer Transparency Oxygen barrier (mL/m d bar) Water absorbtion (%) Food contact material B iodegradability ... 

Polymer, transparent, elastic substance. Practically no odor. Little adhesive power. Resists the usual solvents. 

Styrene-butadiene copolymers are often blended with other polymers. Transparent blends can be made with styrene, styrene-acrylonltrlle copolymers, or styrene-methyl methacrylate copolymers. Blends with styrene have low impact strength even at low styrene levels, while blends with styrene-methyl methacrylate copolymers can have greatly Improved impact strength. Blends with high impact polystyrene, polypropylene, and polycarbonate are opaque. 

Neutralization of ethylene copolymers containing up to 5%-10% acrylic or methacrylic acid copolymer with a metal salt such as the acetate or oxide of zinc, magnesium, and barium yields products referred to as ionomers. (Commercial products may contain univalent as well as divalent metal salts.) lonomers are marked by Du Pont under the trade name Surlyn. These have interesting properties compared with the nonionized copolymer. Introduction of ions causes disordering of the semicrystalline structure, which makes the polymer transparent. lonomers act like reversibly cross-linked thermoplastics as a result of microphase separation between ionic metal carboxylate and nonpolar hydrocarbon segments. The... 

Transparency, in general drops with crystallinity (e.g. polyethylene), and with an increase of crystallite size which causes light scattering. Most fillers, colorants and auxiliary additives lead to opacity. Transmittance depends on the refractive index, so that some fillers may preserve full or partial transparency (translucent). There are also dyes that dissolve in the polymer, so that a colored transparent polymer is produced. It is also possible to find stabilizers (including antioxidants or UV absorbers) that do not affect the polymer transparency. Any chemical change in the polymer like degradation or oxidation, or diffusion of some components, may reduce light transmission. 

Ultraviolet spectrophotometry This technique has been used extensively for polymers of the polystyrene or polyacrylate types. It has also been used for the estimation of optical properties of polymers (transparency, haze, color, and color stability) and quantitative analysis of additives (antioxidants, UV stabilizers, etc.). 

Polymer Transparency (visible light) (ASTM D791) (%)... 

A survey of various polymer classes indicated duit two classes, fluorinated polymers and siloxanes, exhibit fitirly good transparency at 1S7 nm (5). Since this observation was r rted, several groiqis of researchmis have been exploiting both of these chemistries in the design of new polymers for lS7-nm lidiogr hy. Fluorinated materials have become the more popular of the approaches and to this end several types of fluorinated monomers have been explored to achieve the required polymer transparency (6S). 

Heterocyclic monomers can also be polymerized by chemical oxidation with FeCE, producing conductive polymers. Transparent electrically conductive film... 

Amorphous polymers (transparent in the solid state to be precise, it is not a solid but rather a supercooled liquid) are usually easy to dissolve in the good solvent. In contrast, crystalline and semicrystalline polymers (opaque in the solid state) are sometimes not easy to dissolve. Within a crystallite, polymer chains are folded into a regular, thermodynamically stable arrangement. It is not easy to unfold the chain from the self-locked state into a disordered state in solution even if the latter state is thermodynamically more stable. Heating may help the dissolution because it facilitates the unfolding. Once dissolved, polymer chains take a random-coil conformation unless the chain is rigid. 

The procedures to fabricate masters are handled later in this section. Practically, aU higher accuracy methods used to fabricate microlenses can be used to produce masters. In aU of the quoted methods, it is important to choose polymers transparent in the IR range. 

Ultimate loss will be obtained by using a polymer with fluorine and deuterium in its structure, where the intrinsic loss of the polymer (i.e., molecular vibrational loss and Rayleigh scattering loss) will be reduced to an ideal level. Table 7.7 shows the loss factors and the loss limit estimate for deuterated and fluorinated alkyl methacrylate polymer, trideutero-hexafluorobutyl, pentadeutero-methacrylate polymer. This polymer will have a refractive index almost the same as the fluorinated methacrylate polymers. As shown in this table, the lowest loss around 6 dB/km will be attained if a POF using this polymer as a core is developed. As discussed, the value of 6 dB/km is the limit of the polymer transparency in the visible wavelengths that has been obtained to date. 

In addition, highly water-absorbing and oil-absorbing resins are of interest. These have developed rapidly in recent years by unique grafting and cross-linking of hydrophilic polymers. Transparent polymeric materials with optical functions are also noteworthy. Some are biocompatible, such as poly(2-hydroxyethyl methacrylate), which serves as a material for soft contact lenses. Plastic optical fibers are also widely used as substitutes for glass and quartz devices in various fields of technology, especially the biomedical and communication sciences. 

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