Linear polymers have a specific complex of physico-chemical and mechanical properties. The most important of these properties are: the ability to form high-strength anisotropic highly oriented fibers and films, the ability for large, long-lasting reversible deformations; ability in a highly elastic state to swell before dissolving; high viscosity solutions.
This complex of properties is due to high molecular weight, chain structure, and also the flexibility of macromolecules. When passing from linear chains to branched, rare three-dimensional grids and, finally, to dense mesh structures, this complex of properties becomes less pronounced. Strongly crosslinked polymers are insoluble, non-meltable and incapable of highly elastic deformations.
Polymers can exist in crystalline and amorphous states. A necessary condition for crystallization is the regularity of sufficiently long sections of the macromolecule. In crystalline polymers, a variety of supermolecular structures (fibrils, spherulites, single crystals, whose type largely determines the properties of the polymer material, may appear.) Supramolecular structures in non-crystallized (amorphous) polymers are less pronounced than in crystalline polymers.
Properties and most important characteristics
Uncrystallised polymers can be in three physical states: vitreous, highly elastic and viscously flowing. Polymers with a low (below room) transition temperature from the glassy to the highly elastic state are called elastomers, with high – plastics. Depending on the chemical composition, structure and mutual arrangement of macromolecules, the properties of polymers can vary within very wide limits.
Thus, 1,4.-cispolybutadiene, constructed from flexible hydrocarbon chains, at a temperature of about 20 ° C, is an elastic material that transforms into a vitreous state at a temperature of -60 ° C; polymethyl methacrylate, built from more rigid chains, at a temperature of about 20 ° C – a solid glassy product, which turns into a highly elastic state only at 100 ° C. Cellulose – a polymer with very rigid chains connected by intermolecular hydrogen bonds, cannot generally exist in a highly elastic state to the temperature of its decomposition.
Large differences in the properties of polymers can be observed even if the differences in the structure of macromolecules at first glance are not large. Thus, stereoregular polystyrene is a crystalline substance with a melting point of about 235 ° C, and non-stereoregular is not capable of crystallization at all, and softens at a temperature of about 80 ° C.
Polymers can enter into the following main types of reactions:
- the formation of chemical bonds between macromolecules (the so-called crosslinking), for example, in the vulcanization of rubbers, tanning of the skin;
- the decomposition of macromolecules into separate, shorter fragments, the reactions of lateral functional groups of polymers with low molecular substances, which do not affect the main chain (the so-called polymer-analogous transformations);
- intramolecular reactions occurring between the functional groups of one macromolecule, for example intramolecular cyclization.
Crosslinking often occurs simultaneously with destruction. An example of polymer-analogous transformations is the saponification of poly (ethyl acetate), which leads to the formation of polyvinyl alcohol. The rate of reactions of polymers with low molecular weight substances is often limited by the rate of diffusion of the polymers into the polymer phase.
This is most evident in the case of crosslinked polymers. The rate of interaction of macromolecules with low molecular substances often depends significantly on the nature and location of neighboring links relative to the reacting link. The same is true for intramolecular reactions between functional groups belonging to the same chain. However, it sounds a lot more difficult than it is. Figuring how to get big slots wins is a lot more difficult, trust me, I am chemist and gambler.
Some properties of polymers, for example, solubility, viscous flow, stability, are very sensitive to the action of small amounts of impurities or additives reacting with macromolecules. So, to convert a linear polymer from soluble to completely insoluble, it is sufficient to form one or two transverse bonds on one macromolecule.
The most important characteristics of polymers are chemical composition, molecular weight and molecular weight distribution, degree of branching and flexibility of macromolecules, stereoregularity and others. Properties of polymers significantly depend on these characteristics.