The mixing of polymers under the action of shear deformations

The mixing of polymers under the action of shear deformations is an effective method for producing compositions with improved performance properties.

Constantly increasing demands on products made of polymeric materials, such as, for example, high heat resistance, improved mechanical and dielectric properties, water and chemical resistance, stability of polymer products during long-term operation, frost, benzo and oil resistance make it necessary to either synthesize new polymers, or obtaining compositions of already existing polymers by various methods. While the synthesis of new polymers, due to the limited number of previously unused monomers, is a very problematic task, the combination of polymers by mixing allows them to vary their properties and produce materials with the required characteristics. In the general case, the mixing of polymers is a complex physicochemical process that occurs under the action of mechanical and temperature effects. As a rule, the mechanical action leads to the grinding of the material, and after reaching the formed particles of a certain size, the formation of a new surface stops, and a plastic flow of material occurs, that is, the process of mass transfer in a solid under the influence of an external force field occurs. As a result of grinding, a decrease in the degree of crystallinity of a substance is usually observed, and sometimes its complete amorphization, as confirmed by X-ray diffraction and IR-spectroscopy data. As a result of amorphization, the dissolution rate and solubility of substances increase, the temperature and heat of fusion of crystals decrease, and the temperature range of fusion increases. The need to continuously improve the methods and technology of polymer processing is one of the main problems of obtaining polymeric materials, the successful solution of which allows us to purposefully regulate their properties. In this regard, the use of the method of joint action on the material of high pressure and shear deformations is of considerable interest. The advantages of this method are the possibility of mixing and processing of polymers, as well as their physical and chemical modification, in the solid state in the absence of organic solvents, which makes the process environmentally friendly. When polymers are mixed under conditions of high-temperature shear deformations, materials are formed that combine a variety of properties, such as heat resistance and elasticity, hydrophobicity and hydrophilicity, crystalline and amorphous structures, etc. The essence of the method is based on the destruction of the initial components during the creation of a complex-stressed state under the action of all-round compression and shear deformation. The physical principle underlying the method is that the energy stored in the material, when pressure is applied under the influence of shear deformations, is realized in the formation of a new surface, and for this it suffices to create only a small shear force. A feature of the method of high-temperature shear deformations is that the destruction of polymeric materials occurs with the formation of fine powder. At the same time, the grinding process is not a gradual crushing of the material to smaller particles, but is carried out sharply in a limited spatial and temporal interval, when the release of elastic energy occurs through the formation of a new surface, which indicates spontaneous destruction and suggests a branched mechanism for the development of destruction centers. A classic example of equipment capable of creating shear deformations is Bridgman anvils, which allow varying the pressure in the range from 0.1 to 20 GPa ^[1] . When working on Bridgman anvils, the test substance in the form of a disk is placed between two anvils and compressed to the required pressure, and then one of these anvils rotates relative to the other, creating an almost unlimited plastic flow in the material placed between the anvils, and the angle of rotation determines the amount of shear deformation. In specially designed diamond anvils, the pressure can reach values ​​up to 550 GPa ^[2] . Mechanical action, therefore, leads to the forced transfer of reactive particles and ensure their contact. Thus, carrying out solid-phase reactions on Bridgman anvils includes three main stages: grinding of the material necessary to ensure optimal particle size, mixing of reagents, ensuring maximum contact between the particles, and directly a chemical reaction. If Brdgmen's anvils are used for laboratory research, the extruder is widely used to process polymers of various classes. The extruder also refers to the type of equipment, the principle of which is based on the joint effect of pressure and shear deformation on the material. The speed of the processes occurring in the extruder is determined by the nature of mixing, temperature gradients, diffusion of reagents and products. These characteristics during the process in the extruder can be adjusted. Currently, single- and twin-screw extruders-reactors of continuous action. Their use allows combining usually separated processes - grinding, mixing, chemical interaction and molding of products. Mixing polymers under the action of shear deformations can also be implemented in a Brabender-type mixer. On this type of equipment, pressure and shear deformations are created, unlike in an extruder in a closed chamber, as a result of the rotation of two augers. Thus, the interest in the problem of polymer transformations under conditions of high-temperature shear deformations, first of all, is due to the possibility of wide practical use of this method, which allows to obtain compositions that have a higher homogeneity of the components compared to traditional methods. At the same time, there is an undoubted scientific aspect, associated mainly with the need to obtain a deeper understanding of the structural changes in polymers that arise when they are deformed in a complexly stressed state. So, when mixing or extruding polymeric materials, the processes of destruction or modification can proceed, that is, the processes arising as a result of mechanochemical action.