Amorphous polymers optical properties

Most Kaminsky catalysts contain only one type of active center. They produce ethylene—a-olefin copolymers with uniform compositional distributions and quite narrow MWDs which, at their limit, can be characterized by M.Jratios of about 2.0 and MFR of about 15. These features of the catalysts determine their first appHcations in the specialty resin area, to be used in the synthesis of either uniformly branched VLDPE resins or completely amorphous PE plastomers. Kaminsky catalysts have been gradually replacing Ziegler catalysts in the manufacture of certain commodity LLDPE products. They also faciUtate the copolymerization of ethylene with cycHc dienes such as cyclopentene and norhornene (33,34). These copolymers are compositionaHy uniform and can be used as LLDPE resins with special properties. Ethylene—norhornene copolymers are resistant to chemicals and heat, have high glass transitions, and very high transparency which makes them suitable for polymer optical fibers (34). 

Because of the capacity to tailor select polymer properties by varying the ratio of two or more components, copolymers have found significant commercial appHcation in several product areas. In fiber-spinning, ie, with copolymers such as nylon-6 in nylon-6,6 or the reverse, where the second component is present in low (<10%) concentration, as well as in other comonomers with nylon-6,6 or nylon-6, the copolymers are often used to control the effect of sphemUtes by decreasing their number and probably their size and the rate of crystallization (190). At higher ratios, the semicrystalline polyamides become optically clear, amorphous polymers which find appHcations in packaging and barrier resins markets (191). 

Polycarbonates are an unusual and extremely useful class of polymers. The vast majority of polycarbonates are based on bisphenol A [80-05-7] (BPA) and sold under the trade names Lexan (GE), Makrolon (Bayer), CaUbre (Dow), and Panlite (Idemitsu). BPA polycarbonates [25037-45-0] having glass-transition temperatures in the range of 145—155°C, are widely regarded for optical clarity and exceptional impact resistance and ductiUty at room temperature and below. Other properties, such as modulus, dielectric strength, or tensile strength are comparable to other amorphous thermoplastics at similar temperatures below their respective glass-transition temperatures, T. Whereas below their Ts most amorphous polymers are stiff and britde, polycarbonates retain their ductiUty. 

Structure and Crystallinity. The mechanical—optical properties of polycarbonates are those common to amorphous polymers. The polymer may be crystallized to some degree by prolonged heating at elevated temperature (8 d at 180°C) (16), or by immersion ia acetone (qv). Powdered amorphous powder appears to dissolve partially ia acetone, initially becoming sticky, then hardening and becoming much less soluble as it crystallizes. Enhanced crystallization of polycarbonate can also be caused by the presence of sodium phenoxide end groups (17). 

The introduction of a polymer network into an FLC dramatically changes phase and electro-optic behavior. Upon addition of monomer to the FLC, the phase transitions decrease and after polymerization return to values close to that observed in the neat FLC. The phase behavior is similar for the amorphous monomers, HDD A and PPDA. The electro-optic properties, on the other hand, are highly dependent on the monomer used to form the polymer/FLC composite. The ferroelectric polarization decreases for both HDDA and PPDA/FLC systems, but the values for each show extremely different temperature dependence. Further evidence illustrating the different effects of each of the two polymers is found upon examining the polarization as both the temperature and LC phase of polymerization are changed. In PPDA systems the polarization remains fairly independent of the polymerization temperature. On the other hand, the polarization increases steadily as the polymerization temperature of HDDA systems is increased in the ordered LC phases. 

The refractive index is the most important optical property and its effect in determining the appearance of the polymer composite has already been referred to above. Amorphous fillers such as glass fibres and beads have only one refractive index, but most mineral fillers are crystalline and have anisotropic crystal structures resulting in a number of different indices, and this can cause complex and undesirable interference effects [27]. 

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. 

FEP and PFA despite being melt-processible are crystalline (between 50 and 70%). The crystallinity results in poor optical properties (low clarity) and a very poor solubility in organic solvents. The latter makes the preparation of thin optical coatings exceedingly difficult.10 TEFLON AF, an amorphous fluoropolymer, contains in its molecule a bulky dioxole ring, which hinders crystallization. As a result, the polymer has an exceptionally high clarity and excellent optical properties. Its refractive index is the lowest of any plastic.11... 

Ethylene-norbornene copolymers, which have thermoplastic properties when heated above their glass transition temperatures of ca. 200-250°C, have been commercialized by Ticona GmbH under the trade name TOPAS (Tliermoplas-tic Olefin Polymer of Amorphous Structure). Their properties - exceptional transparency, low double refraction, high stiffnes and hardness, low permeability for moisture and excellent biocompatibility - make these ethylene-norbornene copolymers particularly valuable as engineering polymers, for optical applications and as materials for food and medical packaging. 

Before these results were published, polymer physicists and chemists mainly investigated only two phase-states, amorphous and crystalline. At the present time, along with these two states, the third phase-state of condensed systems, i.e. the liquid crystalline state, became very important. Here the situation turned out to be the same as in the case of low molar mass liquid crystals. In spite of the fact that historically the low molar mass substances in liquid crystalline state had been known for about a century, the intensive study of their properties began only after they had found an important practical application owing to a sharp change in optical properties of liquid crystals in electromagnetic fields (for visual displays) and as sensitive temperature indicators (in medicine). 

The optical properties of semicrystalline polymers are often anisotropic. On the other hand, amorphous polymers are normally isotropic unless directional stresses are frozen in a glassy specimen during fabrication by a process such as injection molding. Anisotropy can often be induced in an amorphous polymer by imposing an electric field (Kerr effect), a magnetic field (Cotton-Mouton effect), or a mechanical deformation. Such external perturbations can also increase the anisotropy of a polymer that is anisotropic even in the absence of the perturbation. 

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