【Basic Knowledge of Glossary Definitions for “Polymers and Materials”-014】Glossary definitions related to polymers
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1. Forced high-elastic deformation: It refers to the large deformations that occur in glassy polymers under external forces. Its nature is similar to that of high-elastic deformation in rubber, but the manifestation differs; to distinguish it from ordinary high-elastic deformation, it is commonly referred to as forced high-elastic deformation. Same essence: both involve segmental motion. The difference lies in the fact that forced high-elastic deformation does not return to its original state on its own; heating is required, and a much greater external force is needed, with this phenomenon occurring between Tb and Tg. High elastic deformation of rubber 2. Mean square end distance: the average of the end distances or the average of the squares of the end distances. 3. Cold drawing: The stretching process of crystalline polymers and glassy polymers involves stages such as elastic deformation, yield, development of large deformations, and strain hardening. The later stages of stretching exhibit strong anisotropy; the large deformations that occur before fracture do not recover spontaneously at room temperature, but they can return to their original state when heated. Therefore, essentially, the large deformations resulting from these two types of stretching processes are both forms of hyperelastic deformation, and they are commonly referred to together as cold drawing. 4. Pseudoplastic fluids: Fluids whose rheological behavior is independent of time, and whose viscosity decreases as the shear rate increases. 5. Creep: It refers to the phenomenon in which, under a certain temperature and a relatively small constant external force, the deformation of a material gradually increases over time.
Mean square end distance? What exactly does it refer to, and what impact does it have on the properties of polymers?
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How is the creep of polymer materials measured?
Creep refers to the phenomenon in which the degree of deformation of a material increases over time under a constant load (an unchanging external load). Creep occurs not only in plastics (polymer materials) but also in metal materials. Creep reflects the rheological properties of a material under load, that is, its tendency to flow when subjected to stress； For plastics and other polymer materials, it reflects their inherent viscoelasticity. Moreover, creepiness also indicates the stability of plastics under temperature changes. Creep is related to the temperature level and the magnitude of the external force; when the temperature is too low and the external force is too small, creep is minimal and occurs slowly, making it difficult to detect in a short period of time ； Excessively high temperatures, excessive external forces, and rapid deformation progression also prevent the occurrence of creep phenomena from being detected ； Under appropriate external forces, usually not far above the glass transition temperature of the polymer, the chain segments can move under these forces; however, due to the high internal friction encountered during movement, they can only move slowly, and as a result, a noticeable creep phenomenon can be observed. Creep is the result of the interaction between a material’s elasticity and viscosity; materials with high elasticity exhibit higher creep stress and a higher creep temperature. If viscosity is greater than elasticity, the opposite occurs, allowing us to understand what might happen in practice. So in general, especially for plastics or pressure-sensitive adhesives, the material should have good creep properties (good fluidity) during processing, whereas in terms of actual application it should have poor creep properties (stability; creep implies changes in the material). Creep is influenced by factors such as the material structure, molecular weight, and molecular chains. To avoid creep, it is necessary to find ways to stabilize the material, such as increasing its elasticity to raise the critical stress, adding antioxidants to improve its resistance to aging, and maintaining a stable molecular structure.
So, how is creep determined?
There are many factors that affect creep, such as temperature, humidity, pressure level, the structure of the product, thickness, and so on. But it seems that there is no unified testing standard similar to ASTM or ISO at the moment. Generally speaking, however, thermoset plastics have higher creep resistance than thermoplastics. Those reinforced with glass fiber perform better than those made from pure material, and PPS among thermoplastics has good creep resistance.