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The amazing properties of semi-crystalline polymers: why are their properties so special?
Semi-crystalline polymers are known for their unique structures and properties. From optical to mechanical, these materials exhibit unique properties. They are used in a wide range of applications in daily life, including plastic bottles, fibers, etc., but their real charm lies in the scientific principles they contain invisibly.
Crystallization process of polymer
The crystallization process of polymers is an important process for the partial alignment of their molecular chains. These chains fold during cooling to form ordered regions called lamellae, which further form larger globular structures called spherulites. Crystallization can occur when polymers are cooled from the molten state, mechanically stretched, or when the solvent evaporates.
Crystallization affects the optical, mechanical, thermal and chemical properties of polymers, and crystallinity typically ranges from 10% to 80%.
Among them, the degree of crystallinity is closely related to the size and directionality of the molecular chain. Taking polyethylene (PE), polyethylene terephthalate (PET) and polypropylene (PP) as examples, these polymers exhibit a variety of properties in different environments.
The mechanism of crystallization
Solidization from melt
In the molten state, the molecular chains of polymers will form irregular simulated structures, and some polymers can still maintain disordered structures after cooling to form amorphous solids. However, with other polymers, the chains rearrange on cooling and form partially ordered regions, with sizes in the range of about 1 micron. Within these ordered regions, the polymer chains are both aligned and folded, so these regions are called semi-crystalline.
Nucleation of crystallization
Nucleation is the first step in the crystallization process, which usually starts with some small nanoscale regions where thermal motion causes some of the chain segments to align in parallel. These "seeds" can selectively grow or decompose and are affected by impurities and additives, which represents non-homogeneous nucleation.
How crystals grow
Growth from molten crystals
Crystal growth requires the addition of further folded segments and must occur within a range below the melting temperature and above the glass transition temperature. This process has profound effects on the mechanical properties of the polymer.
When the temperature distribution is static and isotropic, the lamellar crystals grow radially, forming larger and irregular spherulites that exhibit unique color patterns.
Stretching crystallization of polymer
During the extrusion process to make fibers and films, polymers are subjected to tensile stress in the nozzle, causing their molecular parts to align. This stretching affects the material's properties and makes the fibers significantly stronger in the machine direction.
Interestingly, some polymers that do not crystallize in the molten state also partially align when stretched, a phenomenon that has a large impact on the properties of the resulting material.
Applications and their performance
Through different crystallization mechanisms, semi-crystalline polymers exhibit different properties. For example, these polymers exhibit great heterogeneity in thermal stability, mechanical properties, and optical properties.
The applications of semi-crystalline polymers cover a wide range of fields from consumer products to engineering materials, and their properties also depend on the degree of crystallinity and the arrangement of the molecular chains.
Conclusion
The structure and properties of semi-crystalline polymers make them an integral part of modern materials science. By gaining a deeper understanding of their crystallization mechanisms, we can better exploit the potential of these materials and further advance science and technology. However, beyond existing knowledge, can we explore more potential applications that have yet to be discovered?
