Raymond B. Seymour

Electric Properties of Polymers

Some of the more important dielectric properties are dielectric loss, loss factor, dielectric constant (or specific inductive capacity), dc conductivity, ac conductivity, and electric breakdown strength. The term dielectric behavior usually refers to the variation of these properties within materials as a function of frequency, composition, voltage, pressure, and temperature.

Thermal Properties of Polymers

Linear amorphous polymers are glasslike at low temperatures and become leathery at temperatures slightly higher than the glass transition temperature (T g ). These leathery polymers become rubbery at slightly higher temperatures, and crystalline polymers melt at the melting point (T m ).

Chemical Structure of Polymers

Polymers such as proteins and nucleic acids were present when the first unicellular organism appeared on earth. Other natural polymers, such as cellulose and starch, have been utilized for food, shelter, and clothing for thousands of years. Cellulose, polyisoprene, and shellac were converted to useful man-made plastics, fibers, and elastomers in the 19th century, but these conversions were based primarily on empirical knowledge.

Properties of Polyolefins

The principal polyolefins are low-density polyethylene (ldpe), high-density polyethylene (hdpe), linear low-density polyethylene (lldpe), polypropylene (PP), polyisobutylene (PIB), poly-1-butene (PB), copolymers of ethylene and propylene (EP), and proprietary copolymers of ethylene and alpha olefins. Since all these polymers are aliphatic hydrocarbons, the amorphous polymers are soluble in aliphatic hydrocarbon solvents with similar solubility parameters. Like other alkanes, they are resistant to attack by most ionic and most polar chemicals; their usual reactions are limited to combustion, chemical oxidation, chlorination, nitration, and free-radical reactions.

Physical Structure of Polymers

The molecular weight or molar mass of proteins and nucleic acids (DNA, RNA) is identical for each specific species, e.g., the molecular weights of all casein molecules from a specific source are identical. These polymers are members of a homologous series and are said to be monodisperse or molecularly homogeneous.

Tests for Properties of Polymers

The frequency of failure (breakdown) of polymeric materials has decreased and will continue to decrease as polymer scientists and technologists recognize the importance of significant tests. In addition to knowing the glass transition temperature T g and the melting point T m , scientists must know the results of many other laboratory tests before a polymer can be recommended for a specific application.

Mechanical Properties of Polymers

When a polymer is used as a structural material, it is important that it be capable of withstanding applied stresses and resultant strains over its useful service life. Polymers are viscoelastic materials, having the properties of solids and viscous liquids. These properties are time- and temperature-dependent.

Effect of Additives on Polymers

The properties of polymers may be improved by the presence of appropriately selected additives. With the exception of some cast plastics, such as polymethyl methacrylate (PMMA), and some fibers, such as unpigmented cotton, most commercial polymers are mixtures of the polymer with one or more additives.

Diffusion and Permeation of Gas and Vapors in Polymers

The diffusion of gases and vapors in polymers and the permeability of polymers to gases and vapors are not only of practical significance in packaging and coatings but may also be used to demonstrate the kinetic agitation of the diffusate and the permeate molecules. The diffusion process, such as the dissolution of a polymer in a solvent; the plasticization of a polymer, such as polyvinyl chloride (PVC); and the passing of gases through films, is dependent on random jumps and hole filling by small diffusate molecules.

Chemical Resistance of Polymers

If the chemistry of polymer molecules were different from that of simple compounds resembling the repeating units (model compounds), the study of the chemical resistance of organic polymers would be difficult. Fortunately, Nobel laureate Paul Flory found that the rate of esterification of molecules with terminal hydroxyl and carboxyl groups is essentially independent of the size of the molecules. Thus it is customary to assume that the rates of most reactions of organic molecules are similar regardless of the size of the molecule.