Dielectric properties amorphous polymers

An exhaustive review of the dielectric properties of polymers at microwave frequencies was carried out in 1985 by Burr [121). In this paper, loss spectra of several polymers were given as a function of frequency or temperature. Figure 8.9 shows the classical evolution of loss spectra with temperature for amorphous and semi-crystalline polymers. 

WUiiatna and D. C Watts, Multiple dielectric relaxation processes in amorphous polynKis as a fiinctioo of frequency, temperature and applied pressure, Dielectric Properties of Polymers (F. E Karasz, ed.), Plenum Press, New York-LotKion. 1972, pp. 17-44. 

In the present account we shall not seek to duplicate the detailed accounts of the dielectric properties of polymers contained in the texts of McCrum, Read and Williams, Hedvig and Jonscher or in the reviews by Williams. Instead we shall give a structured account which incorporates the recent developments in experimental techniques and theoretical interpretations and gives illustrations of representative dielectric behaviour for selected amorphous, crystalline and liquid crystalline polymers, and for polymers in solution. 

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. 

Polystyrene is a clear, amorphous polymer with a high stiffness and good dielectric properties. It is easily cross-linked by ionizing radiation. Often small amounts of divinyl benzene may enhance the degree of cross-linking.98... 

The effect of diluents on the viscoelastic behavior of amorphous polymers is more complex at temperatures below T, i.e., in the range of secondary relaxation processes. Mechanical, dielectric and NMR measurements have been performed to study the molecular mobility of polymer-diluent systems in this temperature range (see e.g. From extensive studies on polymers such as polycarbonate, polysulfone and polyvinylchloride, it is well known that diluents may suppress secondary relaxation processes. Because of the resulting increase in stiffness, these diluents are called antiplasticizers . Jackson and Caldwell have discussed characteristic properties... 

In order to demonstrate convincingly that this is a general experimental fact of glass-formers, experimental data for many different materials and (for a particular material) experimental data for several dielectric relaxation times are presented herein. The glass-formers include both molecular liquids and amorphous polymers of diverse chemical structures. All show the property of temperature-pressure superpositioning of the dispersion of the structural a-relaxation at constant xa. 

Diffusion of small impurity molecules is an important issue of polymers particularly with respect to their mechanical, dielectric, and optical properties as well as ageing, and has stimulated extensive theoretical and experimental work within the past years [90,91], Diffusion in amorphous polymers is often random on a macroscopic scale, this must not necessarily... 

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... 

PROPERTIES OF SPECIAL INTEREST The Structure of polyquinolines can be altered from a semirigid chain to a rigid one during the synthesis. Although largely an amorphous polymer, some substituted rigid-rod members exhibit crystallinity in low amounts. Polyquinolines are found to posses excellent thermal and oxidative stability, good mechanical properties, low dielectric constants, low values of moisture absorption, and low thermal expansion coefficients. 

The Hostatec has low water absorption. Dielectric properties of films from polyesteresterketone Hostatec are high. This amorphous polymer has the electric inductivity 3, 6, loss factor 10 and the specific voliune resistance 10 Ohmxcm these values remain still up to 60 °C. 

Dielectric relaxation and photon correlation spectroscopic measurements (to be discussed later) have found that the stretched exponential time correlation fimction of the Q -relaxation of amorphous polymers, exp[ — it/Taf"], has Pa that varies from polymer to polymer. For example, PS and PIB have Pa equal to 0.36 and 0.55 respectively. It was recognized that this difference in the stretched exponent Pa of PIB and PS is the origin of their contrasting viscoelastic properties 12,IQ, 121,122). 

The commercially important properties of Et>-Nb copolymers include low density, high transparency and low color, high moisture barrier and low moisture absorption, low optical distortion, excellent feature replication, resistance to polar solvents, high purity, shatter resistance, good biocompatibiUty, extremely low dielectric loss, high temperature capability, and compatibility with polyethylenes. The resins also have the low shrinkage and warpage typical of amorphous polymers. 

---