Polymers can exist in four physical states - three amorphous and one crystalline.
Each temperature interval of a polymer corresponds to its own physical state, which is determined by the peculiarities of the mobility of atoms, groups of atoms, segments of macromolecules and supramolecular structures at a given specific temperature.
In a polymer, the transition from one physical state to another occurs over time. The phenomenon of transition of a substance from one equilibrium state to another in time is called relaxation. The rate of relaxation processes is characterized by relaxation time.
For polymers, the relaxation time can be very long and it significantly affects their behavior.
Amorphous polymers can be in three relaxation (physical) states:
– glassy,
– highly elastic,
– viscous flow.
Crystalline polymers when the temperature rises, they also transform into a different physical state; first into highly elastic, and then into viscous fluid.
A glassy polymer and a highly elastic polymer are in a solid state of aggregation, while a viscous polymer is already in a liquid state of aggregation (polymer melt). Highly elastic state – special condition, which exists only in polymers.
Transitions in amorphous polymers from one physical state to another are non-phase; the transition from a crystalline state to a highly elastic state is a phase transition.
Transitions of a polymer from one physical state to another occur in a certain temperature range. The average temperatures of these intervals are called transition temperature. The temperature of transition from the glassy state to the highly elastic state and vice versa is called the glass transition temperature ( T WITH). T C = T P, where T P – softening temperature.
The temperature of transition from a highly elastic state to a viscous-flow state and back is called the fluidity temperature T T. Interval T WITH - T T corresponds to a highly elastic state. The temperature of the phase transition from a crystalline state to an amorphous state (to a highly elastic state or directly to a viscous fluid state) is called the melting point T PL. The temperature of the phase transition from the amorphous to the crystalline state is called the crystallization temperature T KR. For polymers T PL > T KR.
Each physical state of polymers has its own behavior under load, i.e. view deformation.
Boundaries of coexistence physical conditions polymers can be installed using the thermomechanical method. Using this method, the transition temperature is determined from the thermomechanical curve (TM curve).
The properties of a polymer depend not only on the chemical composition of the polymer and the shape of the macromolecule, but also on their relative position. Macromolecules of different polymers have different chemical composition, length, shape and degree of flexibility. The flexibility of macromolecular chains is significantly influenced by intermolecular interaction forces. These forces limit to a certain extent the freedom of movement of individual chain links.
The nature of the rotation of the chain is determined by the kinetic energy of the macromolecule, and to change both the nature of the rotation and the shape of the chain, it is necessary to impart to it a certain amount of energy (for example, thermal), which is called the energy barrier of the macromolecule. Depending on the spatial arrangement of the macromolecule relative to each other, the degree of their flexibility and the elasticity of the polymer change, which, in turn, determines the nature of the deformation of the material under mechanical influence.
Based on the degree of order in the arrangement of macromolecules, two types of phase states of polymers are distinguished: amorphous and crystalline. Amorphous the phase is characterized by a chaotic arrangement of the macromolecule in the IMC with some ordering of the structure, observed at relatively short distances commensurate with the size of the macromolecule. Crystalline the phase is characterized by an ordered arrangement of macromolecules in the polymer, and orderliness is maintained at distances exceeding the size of the macromolecule by hundreds and thousands of times (Fig. 1).
Crystalline zone
Amorphous zone
Rice. 1. Schematic representation of a polymer globule
Amorphous and crystalline polymers differ significantly in their properties.
Amorphous polymers with a linear or branched macromolecule structure can exist in three physical states:
1. glassy. This state is characterized by the strongest bonding forces between molecules and, as a consequence, the least flexibility of the macromolecule. The lower the temperature of a polymer in a glassy state, the fewer units have mobility, and at a certain temperature, called the brittle temperature, glassy polymers collapse without deformation (or small deformation), like low-molecular-weight glasses.
2. Highly elastic the state is characterized by less strong bonding forces between macromolecules, their greater flexibility and, as a consequence, the ability of long chain molecules to continuously change their shape. In a highly elastic state, small stresses cause a rapid change in the shapes of the molecule and their orientation in the direction of the force. After the load is removed, the macromolecules, under the influence of thermal movements, take on the most energetically favorable forms, as a result of which the original dimensions of the polymer are restored (reversible deformation). In this case, the position of only individual links and sections of the chains changes, and the macromolecules themselves do not perform translational motion relative to each other. Polymers whose amorphous phase is in a highly elastic state over a wide temperature range are called elastomers or rubbers(for example, the temperature range of the highly elastic state of natural rubber is from –73 to +180 °C, organosilicon rubber is from –100 to +250 °C).
3. Viscous the state is characterized by the disappearance of bonding forces between macromolecules, as a result of which they are not able to move relative to each other. This can occur when the polymer is heated to a certain temperature, after which the highly elastic (or glassy) state is replaced by a viscous flow state. A highly elastic state is a characteristic feature of the IUD.
Crystalline polymers are distinguished by the fact that they contain, along with the crystalline phase, an amorphous phase. Due to the very large length of the molecules and the likelihood of weakening the forces of intermolecular interaction in individual sections of the chains in the polymer, as a rule, a continuous crystalline phase cannot form. Along with the ordered sections of the chains, sections with randomly located links appear, which leads to the formation of an amorphous phase in the crystalline polymer. The main condition that determines the possibility of crystallization of polymers is the linear and regular structure of macromolecules, as well as a sufficiently high mobility of units at the crystallization temperature. If the substituting atoms are small, then polymers can crystallize even if they are randomly arranged, for example, fluorine atoms in polyvinyl fluoride
(−CH 2 −CH−) n
In the presence of lateral, substituting hydrogen atoms of groups (C 6 H 5 ~, CH 3 ~, etc.), crystallization is possible only if the macromolecules have a folded shape, their orientation relative to each other is difficult and crystallization processes require dense packing of molecules , do not leak – the polymer is in an amorphous state.
For the formation of a crystalline phase, it is necessary that the macromolecules have a relatively straightened shape and have sufficient flexibility; in this case, the orientation of the macromolecules occurs and their dense packing is achieved. Polymers whose macromolecules lack flexibility do not form a crystalline phase.
Crystallization processes develop only in polymers that are in a highly elastic and viscous flow state. The following types of polymer crystal structures exist:
Lamellar,
Fibrillar,
Spherulitic.
Lamellar crystal structures are a multilayer system of flat thin plates, the macromolecules in which are folded many times. Fibrils, consisting of straightened chains of macromolecules, have the shape of a ribbon or thread . Spherulites- more complex crystalline structures built from fibrillar or lamellar structures growing radially at the same speed from one center. As a result of this growth, the crystal takes the shape of a sphere ranging in size from tenths of a micron to several millimeters (sometimes up to several centimeters).
Crystalline polymers include polyethylene ( low pressure), polytetrafluoroethylene, stereoregular polypropylene and polystyrene, a number of polyesters.
Crystalline polymers have greater strength than amorphous ones. Crystallization imparts rigidity to the polymer, but due to the presence of the amorphous phase, which is in a highly elastic state, crystalline polymers are elastic.
When heated to a certain temperature, crystalline polymers transform directly into the viscous flow state of amorphous polymers.
The considered patterns of phase states of polymers relate to polymers with a linear or branched structure of macromolecules.
In IMCs with a spatial structure, the phase states are determined by the frequency of cross-linking (the number of valence bonds between macromolecules).
Polymers with highly interlinked (three-dimensional) polymers are rigid and under all conditions form an amorphous phase, which is in a glassy state. IUDs with rare cross-links (mesh) form an amorphous phase, which is mainly in a highly elastic state.
The physical and phase states in which materials are located during operation have vital importance for their characteristics.
Physical states of polymers
The physical state of a substance is determined by the packing density of atoms and molecules, on which the nature of their thermal motion depends.
The states of a substance differ in its ability to have and retain constant temperature given shape and volume. The solid, liquid and gaseous states of low molecular weight substances are known. Transitions of substances from one state to another are accompanied by changes in many physical properties, which is explained by a change in the nature and level of thermal movement and interaction of their molecules.
IN hard in its state, a substance is capable of having a constant volume and maintaining its given shape; V liquid In this state, the substance also has a constant volume, but is not able to maintain its shape, since it loses it even under the influence of gravity. Finally, in gaseous state, a substance is unable to have either a constant volume or a constant shape.
Polymers can only exist in condensed states: solid and liquid.
The type of physical state of the polymer depends on the ratio of the energies of intermolecular interaction and thermal motion. In cases where the energy of intermolecular interaction is much greater than the energy of thermal motion of macromolecules, the polymer is in a solid state. The liquid state is realized when both energies are comparable in magnitude. In this case, the thermal movement of macromolecules is able to overcome intermolecular interaction, and the polymer exhibits the properties of a liquid.
The impossibility of the existence of polymers in a gaseous state is explained by the fact that the total energy of intermolecular interaction, due to the large length of macromolecules, is always higher than the energy of the strongest chemical bond in them. It follows from this that before the intermolecular interaction weakens so much that the polymer passes into the gaseous state, the chemical bonds inside the macromolecule are broken and it is destroyed.
Another fundamental difference between polymers and other substances is their ability to exist in two solid states: glassy and highly elastic. The highly elastic state exists only in polymers; it is unknown for other materials.
Thus, polymers can exist in three physical states: glassy, highly elastic And viscous. Transitions from one state to another occur in a certain temperature range (Fig. 2.1). For convenience, a fixed temperature is used, which is calculated from experimental data.
Rice. 2.1. Typical thermomechanical curve of a linear amorphous polymer: T s- glass transition temperature; T t- flow temperature; I, Nor III - temperature regions of three physical states (glassy, highly elastic and viscous, respectively)
Shown in Fig. 2.1 the curve is called thermomechanical. There are three regions on it in which the state and behavior of the polymer are different: the region / corresponds to the glassy state, II - highly elastic and III - viscous flow state of the polymer. In each of these states, the polymer has properties characteristic of it. The transition from glassy to highly elastic state occurs at the glass transition temperature T s, and the transition from a highly elastic state to a viscous-flow state - at the flow temperature T t. The glass transition and flow temperatures are the most important characteristics polymers, at these temperatures occur dramatic changes most of their physical properties. Knowing these temperatures, it is easy to establish temperature conditions processing and operation of polymer materials. By purposefully changing them, it is possible to reduce the processing temperature or expand the temperature range in which products made from a given polymer can be used.
Changes in mechanical, electrical, thermophysical and other properties of polymers at transition temperatures from one state to another occur smoothly, which is explained by a gradual change in the interaction of sections of macromolecules: links, segments, blocks.
From Fig. 2.1 it can be seen that above the flow temperature the deformation of the polymer is very large, i.e. it flows like a liquid. As a rule, polymers are processed in a viscous-flow state or close to it.
The flow of polymers, like other processes, has its own characteristic features, distinguishing these materials from other substances. Unlike low-molecular high-viscosity liquids, the viscosity of which does not change during flow, the viscosity of polymers increases during flow, which is associated with some straightening of chain macromolecules that occurs.
This phenomenon is widely used in polymer processing. Thus, the processes of fiber formation and production of films from polymers under isothermal conditions are based on an increase in the viscosity of the polymer during flow through a die.
The viscous flow state is a consequence of the intensification of the thermal movement of macromolecules with increasing temperature. As a result, at a certain temperature it becomes possible for them to move relative to each other.
When the temperature of the polymer decreases below the fluid temperature, it changes from a viscous flow to a highly elastic state. The process of deformation of polymers in a highly elastic state is reversible, and the magnitude of the deformation does not depend on temperature. This property of polymer materials is widely used. The most typical example of using the reversibility of polymer deformation and the independence of its value from temperature is wide application rubbers and rubbers. Their ability to undergo large, reversible deformations is well known.
The ability of polymers to be in a highly elastic state distinguishes them from all other materials, which cannot be in this state under any conditions.
It is no secret that other materials, such as plasticine, are also capable of large deformations. However, they are all deformed irreversibly. You can pull a rod out of a piece of plasticine, and it will retain the shape given to it.
A polymer material in a highly elastic state can also be stretched, but after removing the load it will return to its original state, that is, a polymer in a highly elastic state deforms reversibly. In this case, long chain macromolecules make a transition from one conformational state to another due to the movement of their individual sections.
Highly elastic deformation is a consequence of the flexibility of macromolecules and the mobility of their individual parts. The return of the polymer to its original state after removing the load occurs within a noticeable period of time, i.e. it can be observed and thus studied relaxation characteristics polymer.
In their highly elastic state, polymers have another feature that distinguishes them from all other solid materials. In this state, as the temperature increases, the elastic modulus of polymers increases, while for other materials it decreases. The fact is that due to the thermal movement of macromolecules and their links in a highly elastic state, they twist, which prevents the deformation of the polymer. This resistance is greater the higher the temperature, since with increasing temperature the thermal movement of macromolecules becomes more intense.
The nature of the deformation of polymers in a highly elastic state depends on the rate of deformation, i.e., the rate of application of the load. Since the manifestation of highly elasticity requires time to overcome the forces of intermolecular interaction, then at a high rate of deformation, the highly elasticity does not have time to manifest itself, and the material behaves like a glassy body. This must be taken into account when using polymers for the manufacture of products that must maintain elasticity under operating conditions under dynamic loads and low temperatures.
When the temperature of the polymer decreases below the glass transition temperature, there is no mechanical impact on it, as can be seen from Fig. 2.1, strain changes. At this temperature, macromolecules are not capable of conformational changes, and the polymer loses the ability not only for viscous flow, but also for highly elastic deformation. This means that the polymer is in a glassy state.
It should be noted the difference between the glass transition processes of polymers and low molecular weight substances. The glass transition of a low molecular weight liquid occurs when the entire molecule loses its mobility. For the polymer to transition to a glassy state, loss of mobility even by segments of the macromolecule is sufficient. For low-molecular-weight liquids, the glass transition and brittleness temperatures are practically the same, but for polymers they are different, which is explained by the fact that parts of the macromolecules retain their mobility in the glassy state.
There are often cases when a polymer in a glassy state is capable of significant deformations (sometimes up to several hundred percent). This is the so-called forced highly elastic deformation; it is associated with a change in the shape of flexible macromolecules, and not with their movement relative to each other. Such deformation, being forced, disappears when the polymer is heated, when at a temperature above the glass transition temperature the mobility of macromolecules increases and they return to their original conformational state.
A comparison should be made between the forced elasticity of polymer materials and the cold flow of metals. Both processes occur when the materials are in a solid state. However, a polymer sample that exhibits forced high elasticity restores its shape and size when heated. This is the basis for the creation of “intelligent” polymers with shape memory. Unlike polymers, heating metals that have been drawn in a cold state, that is, those that have exhibited cold flow, does not allow their shape and size to be restored.
It should be noted that for some polymers the fluidity temperature and sometimes the glass transition temperature cannot be detected, since when heated, the thermal destruction of such polymers occurs before they have time to transform into a viscous-flow or highly elastic state. Such polymers can only exist in a glassy state. An example is the natural polymer cellulose, as well as a number of ethers based on it (in particular, such a technically important one as nitrocellulose, which is the basis of ballistic powders).
Modern science makes it possible to control the glass transition and flow temperatures of polymers. Thus, plasticization of nitrocellulose with nitroglycerin reduces the glass transition and flow temperatures and creates conditions for processing this polymer into products of a given shape and size.
