Refractive indices semicrystalline polymers

Amorphous polymers characteristically possess excellent optical properties. Unlike all the other commercially available fluoropolymers, which are semicrystalline, Teflon AF is quite clear and has optical transmission greater than 90% throughout most of the UV, visible, and near-IR spectrum. A spectrum of a 2.77-mm-thick slab of AF-1600 is shown in Figure 2.5. Note the absence of any absorption peak. Thin films of Teflon AF have UV transmission greater Ilian 95% at 200 mm and are unaffected by radiation from UV lasers. The refractive indexes of Teflon AF copolymers are shown in Figure 2.6 and decrease with increasing FDD content. These are the lowest refractive indexes of any polymer family. It should be noted that the abscissa could also be labeled as glass transition temperature, Tg, since Tg is a function of the FDD content of the AF copolymer. Abbe numbers are low 92 and 113 for AF-1600 and AF-2400. 

Experimental refractive indices were used whenever available in these calculations. For semicrystalline polymers such as polyethylene, typical refractive indices of the semicrystalline specimens were used instead of taking the amorphous limit as was done whenever possible in Section 8.C. Whenever experimental refractive indices were not available, the best possible estimate was made for the refractive index, in most cases by using Equation 8.6. 

All three commercial amorphous fluoropolymers. Teflon AF, Hyflon AD, and Cytop posses a unique set of properties. All dissolve in fluorinated solvents and thus may be spin coated to produce thin hlms and coatings. The polymers may also be extruded and molded using traditional polymer processing techniques. Note that the polymers are not soluble in hydrocarbon solvents or water and retain the chemical and thermal stability of perfluorinated polymers such as Teflon . These polymers have lower density than the well-known semicrystalline perfluorinated polymers such as pTFE that results in lower refractive index, lower thermal conductivity, higher gas permeability, and lower dielectric constant. The polymers are transparent and have excellent mechanical properties below their Tg due to their amorphous character. The presence of a heterocyclic ring in the polymer backbone of these materials is key... 

Crystallinity is important in determining optical properties because the refiaetive index of the crystalline region is always higher than that of the amorphous component irrespeetive of whether the amorphous component is in the glassy or rubbery state. This difference in refractive indices of the component phases leads to high scattering and consequently, the translucency or haziness of semicrystalline polymers. For a purely amorphous polymer, this does not occur, and hence amorphous polymers are usually transparent. Therefore the state of polyethylene terephthalate can be explained as follows ... 

PMMA is amorphous there are therefore no regions of differing indices of refraction in the polymer. Hence, it is transparent. At room temperature, polyethylene is semicrystalline, having both crystalline and amorphous components that differ in their refractive indexes. Consequently, PE is translucent. At 135°C, PMMA is still amorphous, while polyethylene also becomes amorphous since its melting point is 135°C. Both polymers are then transparent. 

Typically one aims to predict the refractive index of the amorphous limit for a semicrystalline polymer. After that the effect of crystallinity may be calculated by using either 1 ll or Rqd combined with an estimate of the change in volume due to crystallization. A large collection of refractive index data on polymers may be found in Physical Properties of Polymers Handbook by the American Institute of Physics (22). A representative albeit necessarily short list is given below in Table 1. 

Refractive Index and Orientation Fluctuations in Semicrystalline Polymers. The crystals of semicrystalline pol5nners are optically anisotropic. The simplest spherulite-based model for predicting light scattering is a sphere of radius r with a different polarizability in the radial direction Uj. than in the tangential direction (25). Assuming vertically polarized incident light, the intensities of the two scattered components (vertical and horizontal) are... 

Light scattering is a classical technique used to study solutions (with the necessary difference in refractive index between solvent and solute). This is described in Chapter 2. It is also used for characterization of superstructures (spherulitic, axialitic, etc.) in semicrystalline polymers. The technique is called small-angle light scattering (SALS) and information is obtained about type and size of superstructure (Chapter 7). 

The common contrast modes include polarized light, phase contrast, differential interference contrast, and Hoffman modulation contrast [5]. Depending on the nature of the polymer, such as refraction index, sample thickness, and optical anisotropies in the materials, different modes of transmission optical microscopy can be employed by mounting special accessories in a classic optical microscope to overcome different problems. For example, a polarizer and analyzer can be mounted before and after the sample to construct a polarized light microscope, commonly used for semicrystalline polymers a phase plate and phase ring can be added to construct a phase contrast optical microscopy, which is common for studying a noncrystafline multiphase polymer system. 

With semicrystalline polymers having moderate to high crystallinity, Tg may be poorly resolved. Other static methods used to measure Tg are the following refractive index gas difiusion/solubility thermal conductivity chain mobility [nuclear magnetic resonance (NMR)] specific volume (dilatometry). Experimentally observed Tg is a function of several variables including molecular weight, plasticizer content, test rate/fi equency, sample size, copolymers/blends, cross-linking, crystallinity, and tacticity. 

Using Eq. (B2), the unoriented refractive index of a semicrystalline polymer may be calculated from a knowledge of its intrinsic crystalline and noncrystalline densities and refractive indices. First, the volume fraction crystallinity, Vcr, is calculated from the densities ... 

If the intrinsic properties of a semicrystalline polymer are known, Eq. (E2) can be used to calculate the volume fraction crystallinity by simply measuring the polymer s refractive index, while Eq. (E3) can be used to obtain the value of the noncrystalline orientation function by measuring birefiingence and crystalline orientation by an independent teehnique [eg., wide angle X-ray (5)]. For a singlephase polymer like polystyrene, the orientation funetion ean be directly calculated from the measured birefiingence value and the known intrinsic birefiingence ... 

A separate problem for quantitative measurements is that aligned interfaces in optically isotropic materials can also affect the state of polarization. This form birefringence is usually small but inaeases with the density of interfaces and with the difference in the refractive index across the interface. It is difficult to calculate unless the exact stmcture is known. Copolymers or semicrystalline polymers with some phase dimension close to the wavelength of light may be affected. 

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