Fourthly, polymer crystallization
As mentioned above, depending on crystallinity, it can be divided into polymers with a tendency to crystallize and without a tendency to crystallize. However, polymers with a tendency to crystallize can be either crystalline or amorphous. An analysis of formation and organization of crystals shows that form of polymer crystallization is a folded chain structure: first, chain bundle consists of molecules, and chain bundle folds into a “ribbon”, then “ribbon” overlaps into a plate, and finally, plate and plate are stacked in large crystals.
To clarify characteristics of crystalline polymers and their crystallization mechanism, relevant issues are described below.
(1) Main characteristics of crystalline polymer
- 1. Differences in physical properties of crystalline and non-crystalline polymers
After crystallization of polymer, its density increases. For example, relative density of polycarbonate after complete crystallization is 1.33, and relative density with no crystallization is 1.20. Due to increase in density and uneven density of crystalline polymers, refraction occurs when light passes through. The more crystals, more serious refraction, and light cannot pass through directly, so transparency and light transmission are poor. Amorphous polymers such as polystyrene, plexiglass, polysulfone, etc., are transparent when light passes through them, just like water, due to their uniform density. The difference between crystalline and non-crystalline polymers in appearance is to observe transparency of thick-walled polymer products. Opaque or translucent are crystalline, and transparent are non-crystalline. Of course, there are exceptions, for example, ionic polymers are crystalline but very transparent, ABS is non-crystalline but opaque.
- Physical and mechanical properties of crystalline polymers
Whether a polymer crystallizes has a great influence on its physical and mechanical properties, and degree of crystallization (the so-called crystallinity refers to mass fraction of crystalline phase in a crystalline polymer) has a greater influence on physical and mechanical properties. The difference in bending strength , tensile strength, hardness, hardness, wear resistance, chemical corrosion resistance and electrical properties, etc., crystalline type is better than amorphous type, and high crystallinity is better than low crystallinity. Conversely, low crystallinity polymers have better softness and folding resistance, higher elongation, and higher toughness. In molding process, crystallization mechanism is often used to control molding conditions to produce products with specific properties.
- Comparison of temperature-strain curves for crystalline and non-crystalline polymersin
Because there is an amorphous part in crystalline polymer, it can also be reflected in temperature strain curve that strain between glass transition temperature Tg and melting point Tm is very small, but it reaches melting limit (from melting to complete melting)) later if molecular mass is not too large, it will immediately go into a liquid state (Tm > Tf); if molecular weight is relatively large, a highly elastic state will appear between Tm and Tf, and this can occur with a further increase in temperature to viscous flow temperature Tf Flow (Tm < Tf). Figure 1-1 applies to most polymers. From a technological point of view, if strength of crystalline polymer with a molecular weight of M1 is sufficient for application, a polymer with a higher molecular weight of M2 is not chosen, which is much more convenient for processing. Because in order to make M2 molecular weight polymer flowable, it is necessary to raise temperature above melting point to increase processing temperature, and highly elastic state will appear at an extremely high temperature. If it is accidentally supercooled and exists in product, product will generate internal stress, deformation, deformation and cracking.
Figure 1-1 Temperature versus strain curves for crystalline and amorphous plastics
Dashed line indicates amorphous plastic, solid line indicates crystalline plastic, M is molecular weight, M2 > M1
(2) Polymer crystallization conditions
Polymers can crystallize, but not all polymers and not in all cases. What polymers can crystallize? Whether a polymer can crystallize or not depends mainly on its own structure. Through practical analysis, following factors are known to promote crystallization.
- The higher regularity of polymer chain, simpler structure, easier it crystallizes.
Like polyethylene, polyvinyl alcohol, polytetrafluoroethylene, etc., their molecular chains are relatively simple and regular, which means that molecular chains are relatively neatly arranged and these polymers are usually crystalline. Another example: polyvinyl chloride is non-crystalline, while polyvinylidene chloride is crystalline. This is due to fact that chlorine atoms of first are in main chain, and spatial arrangement is disordered, which greatly reduces regularity of chain, while two chlorine atoms in main chain of second are fixed in space, and not random. , and chain is very regular and easily arranged. Definite lattice, so easy to crystallize. As a rule, polyvinylidene chloride has a high degree of crystallinity and high mechanical strength.
- The steric hindrance of substituents in polymer chain is low, and molecular force between chains is high, which promotes crystallization.
For example, polyvinyl alcohol can crystallize well because substituents are relatively small and effect of steric hindrance is negligible, which promotes crystallization. However, polystyrene and plexiglass cannot crystallize, since substituents are large, and neighboring molecular chains are difficult to access due to steric hindrances, while side groups on chain are asymmetric, which prevents molecules from separating. forming a certain crystallization sequence Cannot crystallize. Of course, if it is oriented polystyrene or oriented plexiglass, then it can still crystallize due to regular spatial arrangement. The strong intermolecular force makes chains easily accessible and ordered, so they are easy to crystallize. For example, polyethylene terephthalate.
- Favorable external conditions favor crystallization.
Temperature influence: The crystallization process is a process in which arrangement of polymer chains changes from incorrect to correct. This change in arrangement can only be completed when chain segments can move, and movement of chain segments It also occurs only above glass transition temperature Tg of polymer, so temperature that allows polymer to crystallize should bemust be above its Tg.
Influence of cooling rate: when a polymer melt is rapidly cooled to a solid state, its chain segments do not have time to bring themselves into an ordered state to form position required for crystallization, and disordered state is still maintained at this time. The polymer thus formed is non-crystalline or amorphous. For example, when extruding a polyethylene terephthalate film by a flat film method, it can be slowly cooled in a water tank at a temperature of 50-60°C to obtain a film with a high degree of crystallinity; another example is blown polypropylene film that is quench cooled. It is possible to obtain an amorphous packaging film with higher transparency.
Stretch Effect: In case of stretch, molecular chains will be neatly and tightly arranged along direction of external force, which helps to align polymer chains. Therefore, stretching is also beneficial for crystallization. For example, flat filament extrusion from woven fabric uses a stretching process to make it high tensile strength and crystalline flat filament. The tension mentioned above means that direction of external force corresponds to direction of molecular chain, which promotes crystallization and increases melting point; when direction of stretch is perpendicular to molecular chain, it does not promote crystallization. crystallization and melting point is reduced.
(3) Crystallization characteristics of polymers
- System of coexistence of crystalline region and amorphous region
The basic unit of polymer structure is dual. It can order entire macromolecular chain into a lattice or chain segments into a lattice. Most crystalline polymers rearrange and fold into crystals. However, form of movement of chain segments is extremely complex, and its movement cannot be free from limitations of long chains of macromolecules. Due to complexity of polymer molecules with branched chains or end groups, it is very difficult to obtain crystals with complete crystal formation. Therefore, crystallization of polymer is incomplete and uneven. There are not only crystalline parts, but also non-crystalline parts. It is a system in which crystalline regions and amorphous regions coexist.
- The melting point of a crystalline polymer is a range of temperatures (also known as melting point).
Low molecular weight crystals have a certain melting point. For example, melting point of ice is 0°C. During melting process, although outside air temperature can be much higher than 0°C, temperature of ice does not rise until it has melted. This suggests that melting of ice occurs at a fixed temperature. However, melting of crystalline polymers does not occur at a certain fixed temperature, but upon meltingthe temperature fluctuates, which from beginning of melting to end of melting can vary by 10-20 degrees and even more. That is, crystalline polymers have a relatively wide range of melting points. People stipulate melting point of all crystals, which is called upper limit of melting limit.
The reason why melting point of crystalline polymers is in temperature range is because crystalline polymers are a system in which crystalline and amorphous regions coexist, and structures of crystalline and non-crystalline parts are very different, so there must be an internal stress at junction, which facilitates melting of crystals, which will lower melting point. The degree of reduction is related to magnitude of internal stress: greater internal stress, more melting temperature will fall, and vice versa, lower internal stress, less obvious drop in melting temperature. Thus, degree of decrease in melting point of same piece of polymer is not same, so a range of temperatures is necessary for its melting.
Secondly, it is also related to history of structure of crystalline polymers. The lower crystallization temperature and higher cooling rate, less perfect crystal structure and wider temperature range at which polymer melts. This is because when polymer crystallizes at a relatively low temperature and high cooling rate, mobility of molecular chain is low, and it is too late to fully adjust position, causing a large internal stress, and distribution is uneven. , and voltage is large., when heated, it breaks down at a lower temperature - melting, and place with a small voltage will break down at a higher temperature, so it shows a higher melting limit. Conversely, if it crystallizes at a higher temperature and cooling rate is slower, chain mobility is good, which is conducive to chain layout, so internal stress uneven distribution phenomenon is not so serious, so melting point is higher, and melting limit is narrower. In practice, "annealing" method is often used to reduce internal stresses, whereby melting temperature range is narrowed, and melting temperature is actually increased.
- Effect of stretching on crystallization
The crystal structure is a very regular structure. After stretching polymer, molecular chains can be arranged in parallel, which greatly improves their regularity and prepares better conditions for further crystallization. Thus, after stretching polymer, crystallization rate will be greatly increased. For example, polyester (polyethylene terephthalate) has fastest crystallization temperature at 90°C, but if it is stretched at 80-100°C, its crystallization rate can be increased compared toyu with that without stretching.About a thousand times higher polyester fiber has better crystallinity. For already crystallized polymers, stretching can also disrupt crystallization. For example, in process of stretching polyethylene, crystallization before stretching has no direction. After applying stress, crystallization of molecular chains aligned in direction of stretching will not be broken, but crystal structure of molecular chains and direction of tension are perpendicular will be broken by pulling force.
(4) Characteristics of molding and processing of crystalline polymer
- For molding, a large amount of heat is required.
Crystalline polymers are heated to a viscous flow state before they can be molded and processed. Macromolecular chains, regularly densely packed, separate, that is, chain segments separate. At this time, a certain amount of energy is released. required to break force in chain segments and separate them. , and there must be a certain amount of energy to rotate inside chain segment. Due to strong force between molecular chains of crystalline polymers, rigidity is high and it is difficult to rotate inside, so a large amount of heat from outside must be applied during molding so that crystalline polymers realize state of viscous state. separation of chain segments.
- Loudness difference
The volume of any polymer consists of two parts: one part is intrinsic volume of molecule, which is a fixed value and can be calculated according to chemical structure; other is an "empty" volume, size of which depends on temperature at a certain moment and chain State of motion. As temperature decreases, ability of molecules to move weakens, distance between molecules decreases, and volume of voids decreases accordingly. When temperature drops to Tg, movement of chain segments stops, distance between molecules is smaller, and they become closer to each other. How emptiness is forced out And froze. It can be seen that shrinkage of polymer volume during its freezing is due to a decrease in voids.
For crystalline polymers, from melting to solidification, chain segments change from disordered to ordered. Closely packed, reduction in space volume is more significant. Therefore, molded article has a large shrinkage, it is difficult to guarantee dimensional accuracy, and article is prone to "air holes". After molding polymer and cooling it at a certain temperature and pressure, geometric size of product is smaller than geometric size of mold, which is shrinkage. The shrinkage rate depends on factors such as polymer type, product shape, thickness, and pressure holding time. In general, crystalline polymers have a higher shrinkage rate than amorphous polymers. Therefore, when designing a mold, it is necessary to take into account degree of shrinkage. Table 1-1 byindicates degree of shrinkage of conventional plastics.
Table 1-1 General Reference Table for Plastic Shrinkage
- Influence of cooling rate on polymer crystallinity
Especially for thick-walled products, cooling rate has greatest effect on crystallinity. Due to low thermal conductivity, cooling rate is not constant, and surface crystallinity is lower than inside, so difference between surface and internal crystallinity is subject to stress, leading to defects such as cracks , warping and cracking of product. Such articles are usually heat treated to minimize difference in crystallinity between surface and interior.