4.7. Basic Information on the Structure and Properties of Polymers

Reactions of polymer formation. The overwhelming majority of organic materials used for electrical insulation belong to the group of polymers. Polymers are high-molecular-weight compounds whose molecules consist of a large number of structurally repeating units – monomers.

The reaction of forming a polymer from monomers is called polymerization. During polymerization, a substance may pass from a gaseous or liquid state into a state of a very viscous liquid or a solid. The polymerization reaction is not accompanied by the elimination of any low-molecular-weight byproducts; in this reaction the polymer and the monomer have the same elemental composition. Polymerization of compounds with double bonds usually proceeds by a chain mechanism. For a chain reaction to begin, active particles must first be generated in the initially inert mass. In chain reactions, one active particle draws thousands of inactive molecules into the reaction, forming a long chain. The primary active centers are free radicals and ions.

Radicals are parts of a molecule formed when an electron pair is broken, containing an unpaired electron (for example, the methyl СН₃–, phenyl С₈Н₆–, and ethyl С₂Н₅– groups, etc.). The formation of initial radicals and ions can occur under the action of heat, light, various types of ionizing radiation, or specially introduced catalysts.

Let us consider, as an example, the polymerization of ethylene (СН₂ = СН₂), which under normal conditions is in the gaseous state. Suppose that the initiator of polymerization is some free radical R–, which, having a free valence, possesses high reactivity. Such a radical attaches to an ethylene molecule, opening the double bond, and thereby converts it into a new radical with a free valence bond at the end:

The resulting complex is very active and is, in turn, capable of attaching a new molecule to form a longer radical. The polymerization reaction continues until the polymer chain is terminated. In the reaction system, near the growing polymer chain, another free radical or another growing polymer chain may appear nearby. Then they combine with each other, and the polymer chain stops growing further.

Linear and network polymers. Depending on the spatial structure of the macromolecules, polymers are divided into two main types – linear and network (three-dimensional). In linear polymers the macromolecules are chain-like sequences of repeating units. In this case the ratio of the molecule's length to its transverse dimensions is very large and can be on the order of 1000.

In network polymers the macromolecules are linked into a common network, which leads to an unbounded increase in molecular weight, which then characterizes not an individual macromolecule but a certain region of the polymer. In such spatially cross-linked materials, individual macromolecules lose their individuality. Therefore network polymers are sometimes called polymer bodies.

There are substantial differences between the properties of linear and network polymers. As a rule, linear polymers are comparatively flexible and elastic, and most of them soften and melt easily upon moderate heating. Network polymers have greater rigidity, they melt only at high temperatures, and many of them undergo chemical decomposition before reaching the melting point (burning, charring, and so on).

Because of these properties, linear polymers are referred to in practice as thermoplastic materials, and network polymers as thermosetting.

Owing to weak intermolecular interaction, linear polymers in most cases are able to swell and dissolve in solvents of suitable composition, forming viscous solutions from which strong films and fibers can then be obtained. Network polymers are difficult to dissolve, and a significant fraction of them are insoluble. Typical network polymers include phenol-formaldehyde and epoxy resins, and highly vulcanized rubber (ebonite, escapon).

Flexibility and chemical bonding. The high flexibility of linear polymers is determined by two equally important factors – the size of the macromolecules and the nature of the chemical bonds between the atoms. For example, in polyethylene, as in other saturated hydrocarbons, each carbon atom forms four covalent bonds directed toward the corners of a regular tetrahedron. The normal tetrahedral angle between the bonds is 109°28′, so the straightened chain of a polyethylene macromolecule looks like a snake. Such a snake cannot be stretched without breaking it, since covalent bonds have a definite length and directionality.

Thermal impulses are not able to cause motion of the entire macromolecule within the structure of the polymer. However, thanks to the flexibility of the molecular chains, individual sections of the macromolecule, called segments, can take part in thermal motion. The smaller the size of the segments capable of independent movement, the greater the flexibility of the macromolecule. In linear polymers, segments usually consist of several dozen units. Segmental mobility of the molecular chains is one of the causes of relaxation dielectric losses in polymers. Depending on the temperature, amorphous polymers can exist in three physical states.

1. Glassy state. In this state the material is brittle, and under very large mechanical stresses it deforms only slightly before failure. The temperature at which a polymer, upon heating, ceases to be glassy and acquires high elasticity, or, conversely, at which it passes into the glassy state upon cooling, is called the glass-transition temperature (T_g).
2. Highly elastic state. In this state materials exhibit very large elastic deformation under relatively small mechanical stresses. For example, rubbers can stretch almost tenfold. Upon further heating and reaching the temperature T_f, the polymer transitions into the flow state.
3. Viscous-flow state. In this state the material, under the influence of small forces, exhibits irreversible plastic deformation, which can be used for its processing.

As the temperature decreases, linear polymers again pass through all the stages listed above. Being in one stage or another is determined by changes in the structure of the substance and by the cohesive forces between the macromolecules of linear polymers.

Network polymers at an advanced stage of polymerization are completely inert to changes in ambient temperature.

Composition of polymer chains. By chemical composition, polymers can be divided into organic and organoelement polymers. Organic polymers include those high-molecular-weight compounds whose main chain consists of carbon or a combination of carbon with oxygen, nitrogen, sulfur, and phosphorus.

Organoelement polymers are those polymers whose main chain does not contain carbon atoms, but is flanked by organic groups. The most common representatives of these materials are organosilicon compounds.

Electrical properties. The structure of the macromolecules largely determines the electrical properties of polymers. All chemical bonds of carbon with other elements are polar to some degree, owing to differences in the electronegativities of the atoms involved in the bond. The total dipole moment of a molecule is determined by the vector sum of the dipole moments of the individual bonds. If a molecule has a symmetric structure, the dipole moments of the individual bonds can balance one another, so that the total dipole moment is zero.

Substances with asymmetrically constructed units of polymer molecules are dipolar and usually possess some hygroscopicity and low or moderate electrical characteristics. High-molecular-weight hydrocarbons with symmetrically constructed molecules are practically nonpolar or weakly polar, their hygroscopicity is negligibly small, and therefore they have a low dielectric loss tangent and low specific conductivity.

Heat resistance. Most organic polymers can operate for extended periods only at temperatures below 100°C. Above this temperature, rapid thermal aging of the material generally occurs. Therefore the main problem in the chemistry of high-molecular-weight compounds has always been to create more heat-resistant materials while retaining the flexibility, elasticity, and other characteristic properties of organic substances. At present, industry also produces highly heat-resistant high-molecular-weight materials, for example, fluorine-containing polymers, organosilicon compounds, and polyimides.