Depending on the chemical structure of the polymer chains or chemicals introduced into the polymer, Crosslinks may form between the polymer chains. This phenomenon is referred to as cross-linking the polymer. Crosslinks greatly affect the physical properties of a polymer, because they increase its molecular weight and limit the movements of the polymer chains in relation to each other. The formation of only a small number of crosslinks significantly reduces the solubility of the polymer. The polymer, however, retains the ability to absorb the solvent, which still dissolves the non-cross-linked part of the material. As a consequence, the solvent does not completely dissolve the polymer, and only by absorbing the solvent causes the material to swell.
An example of cross-linking a polymer is transformation, which occurs during the curing of thermosetting polymers. These polymers are generally produced as semi-solids, which, as a result of heating in the mold, transform into highly cross-linked products, hard, infusible, with a three-dimensional spatial network of bonds connecting the polymer chains with each other (drawing).
Drawing. The process of converting a non-cross-linked polymer into a highly cross-linked polymer: 1 - polymer not cross-linked, 2 - highly cross-linked polymer (thick lines indicate cross-links).
Crosslinking of the polymer also takes place during the curing of chemosetting polymers.
Another important process, where cross-linking of the polymer takes place, there is vulcanization. The vulcanization process discovered by Goodyear consists in heating the rubber to approx. 5% addition of sulfur, as a result, the polymer chains are joined together by transverse sulfur bridges. Rubber vulcanization significantly increases its abrasion resistance. This property can be further enhanced by the addition of zinc oxide and carbon black. Vulcanized rubber is no longer thermoplastic, because long chains connected by cross-links form a continuous structure, which, as a result of advanced vulcanization, becomes a three-dimensional giant particle.
The vulcanization of rubber produces a product called rubber. When much more sulfur is used (above 30% the composition of the rubber mixture) a material called ebonite is obtained, i.e. hard rubber. The raw materials for the production of rubber are also chemical compounds with properties similar to natural rubber, obtained by chemical synthesis, called synthetic rubbers.
The preparation of polymers leads to products of heterogeneous molecular weight, as not all molecules polymerize to the same degree. In order to obtain commercial products with the same properties, strictly controlled reaction conditions are therefore needed. The production of the high-molecular polymer requires the use of the purest raw materials, and moreover, the reaction proceeds with very high yield, because cleaning the final product is difficult or even impossible.
The polymerization process can be carried out by various methods. They include, among others. block polymerization (in mass), in solution (solvent), emulsion, bead (pearl):
• Block polymerization requires the use of undiluted monomer itself, which gradually transforms into a polymer throughout its mass;
• solution polymerization requires the dissolution of the monomer in the liquid, which is also a solvent for the polymer;
• emulsion polymerization takes place in an aqueous environment, in which it is dispersed by means of emulsifiers (dispersed) very fine droplets monomer; the polymerization produces a fine-grained water dispersion of the polymer; Emulsion polymers are very fine grained (diameter below 0,04 mm);
• suspension polymerization, that is, pearl, it also takes place in the aquatic environment, the monomer is only mechanically dispersed in the water; the pure polymer is in the form of a relatively coarse powder as a result of polymerization (diameter approx. 0,15 mm) or pearls.