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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Crystallization of polymers | 3/3 | https://en.wikipedia.org/wiki/Crystallization_of_polymers | reference | science, encyclopedia | 2026-05-05T10:46:59.767344+00:00 | kb-cron |
=== Thermal and mechanical properties === Below their glass transition temperature, amorphous polymers are usually hard and brittle because of the low mobility of their molecules. Increasing the temperature induces molecular motion resulting in the typical rubber-elastic properties. A constant force applied to a polymer at temperatures above Tg results in a viscoelastic deformation, i.e., the polymer begins to creep. Heat resistance is usually given for amorphous polymers just below the glass transition temperature. Relatively strong intermolecular forces in semicrystalline polymers prevent softening even above the glass transition temperature. Their elastic modulus changes significantly only at high (melting) temperature. It also depends on the degree of crystallinity: higher crystallinity results in a harder and more thermally stable, but also more brittle material, whereas the amorphous regions provide certain elasticity and impact resistance. Another characteristic feature of semicrystalline polymers is strong anisotropy of their mechanical properties along the direction of molecular alignment and perpendicular to it. Above the glass transition temperature amorphous chains in a semi-crystalline polymer are ductile and are able to deform plastically. Crystalline regions of the polymer are linked by the amorphous regions. Tie molecules prevent the amorphous and crystalline phases from separating under an applied load. When a tensile stress is applied the semi-crystalline polymer first deforms elastically. While the crystalline regions remain unaffected by the applied stress, the molecular chains of the amorphous phase stretch. Then yielding, which signifies the onset of plastic deformation of the crystalline regions, occurs. The molecular mechanism for semi-crystalline yielding involves the deformation of crystalline regions of the material via dislocation motion. Dislocations result in coarse or fine slips in the polymer and lead to crystalline fragmentation and yielding. Fine slip is defined as a small amount of slip occurring on a large number of planes. Conversely, coarse slip is a large amount of slip on few planes. The yield stress is determined by the creation of dislocations and their resistance to motion. After yielding, a neck is formed in the amorphous region and propagates down the sample length. During necking, the disordered chains align along the tensile direction, forming an ordered structure that demonstrates strengthening due to the molecular reorientation. The flow stress now increases significantly following neck propagation. Mechanical anisotropy increases and the elastic modulus varies along different directions, with a high modulus observed in the draw direction. Drawn semi-crystalline polymers are the strongest polymeric materials due to the stress-induced ordering of the molecular chains. Other defects, such as voids, occur in the semi-crystalline polymer under tensile stress and can drive the formation of the neck. The voids can be observed via small angle x-ray scattering. Unlike crazes these voids do not transfer stresses. Notably, cavitation is not observed under compressive stress or shearing. Evidence suggests that cavitation also impacts the onset of yielding. The voids are associated with the breaking of the amorphous phase. The strength of the crystalline phase determines the importance of cavitation in yielding. If the crystalline structures are weak, they deform easily resulting in yielding. Semi-crystalline polymers with strong crystalline regions resist deformation and cavitation, the formation of voids in the amorphous phase, drives yielding. As done in crystalline materials, particles can be added to semi-crystalline polymers to change the mechanical properties. In crystalline materials the addition of particles works to impede dislocation motion and strengthen the material. However, for many semi-crystalline polymers particle fillers weaken the material. It has been suggested that for particles to have a toughening effect in polymers the interparticle matrix ligament thickness must be smaller than a certain threshold. Crystalline polymers polypropylene and polyethylene display particle strengthening. Plastics are viscoelastic materials meaning that under applied stress, their deformation increases with time (creep). The elastic properties of plastics are therefore distinguished according to the time scale of the testing to short-time behavior (such as tensile test which lasts minutes), shock loading, the behavior under long-term and static loading, as well as the vibration-induced stress.
=== Optical properties === Crystalline polymers are usually opaque because of light scattering on the numerous boundaries between the crystalline and amorphous regions. The density of such boundaries is lower in polymers with very low crystallinity (amorphous polymer) or very high degree of crystalline polymers, consequentially, the transparency is higher. For example, atactic polypropylene is usually amorphous and transparent while syndiotactic polypropylene, which has crystallinity ~50%, is opaque. Crystallinity also affects dyeing of polymers: crystalline polymers are more difficult to stain than amorphous ones because the dye molecules penetrate through amorphous regions with greater ease.
== See also == Liquid-crystal polymer Modeling of polymer crystals
== References ==