One question per week: Popularizing basic knowledge of polymers – What are the factors that affect the crystallization ability of polymers?

1. Molecular structural regularity: The higher the regularity, the easier it is to crystallize. Linear polymers crystallize more easily than those with more branches. 2. Strength of intermolecular forces: Molecules with stronger intermolecular forces crystallize more easily than those with weaker forces. For example, in nylon 3, the flexibility of its molecular chain is high; the molecules can rotate easily, allowing for a regular arrangement and facilitating crystallization.

1. The influence of the chain structure. The higher the symmetry of the molecular chain structure, the easier it is to crystallize. The better the regularity of the molecular chain, the easier it is to crystallize, such as isotactic or syndiotactic polymers. The greater the flexibility of the molecular chain, the easier it is for crystallization to occur. In the case of branching or cross-linking, these factors disrupt the regularity of the chains and limit their mobility; therefore, the lower the degree of branching and cross-linking, the easier it is for crystallization to take place. The smaller the steric hindrance of the side-chain substituent, the easier it is to crystallize. 2. Intermolecular forces: Intermolecular forces generally reduce the flexibility of molecular chains, thereby hindering crystallization ； And once crystals are formed, the intermolecular forces further contribute to the stability of the crystal structure. 3. Molecular weight: For the same polymer, under identical crystallization conditions, the lower the molecular weight, the easier it is for the molecular chains to move, and thus crystallization occurs more readily. 4. Environmental factors: Crystallization occurs at higher temperatures; the greater the mobility of molecular chains, the more complete the crystallization. Stretching crystalline polymers facilitates crystallization.

The factors affecting polymer crystallization can be divided into two categories: the regularity of the internal structure, and external factors such as concentration, solvent, and temperature. The more regular the structure, the easier it is to crystallize; conversely, it is harder, resulting in an amorphous polymer. Structural factors are the most important. To improve the crystalline orientation of polymers, structurally, it is possible to: increase the symmetry of the molecular chains ； Increase the stereoregularity of the molecular chain ； Increase the order of arrangement of repeating units, that is, random copolymerization ； Increase the hydrogen bonds within the molecular chain ； Reduce the degree of branching or cross-linking of the molecular chain ； From an external factors perspective, methods that can be applied in the factory include annealing; a slow cooling rate can increase crystallinity ； Be aware of the impact of stress. Materials such as rubber and fibers accelerate crystallization under stress. Selection of solvent. It does not crystallize easily in good solvents.

The crystallization ability of polymers is closely related to their molecular structure (chemical structure, intermolecular forces, and chain flexibility); those with symmetrical structures, good regularity, and strong intermolecular interactions tend to crystallize more easily. That is, the simpler the molecular chain structure, the higher the symmetry, the less steric hindrance, and the better the regularity, the faster the crystallization rate.

The person on the 3rd floor is really amazing – setting it at such a high level and still asking people whether they want to learn it or not. Give them 100 stars instead

In the end, the factors that affect polymer crystallization are three in number: near-range structure, long-range structure, and condensed-state structure. Therefore, I classify the factors affecting polymer crystallization into two categories: atomic-level factors and molecular-level factors. For some synthetic polymers, the synthesis process is the main factor determining their approach distance, and thus it is an inherent factor affecting crystallization. This is the foundation of all other factors. Only after this step is completed can other conditions be considered. Of course, for natural polymers, the factor that affects their near-range crystallization is undoubtedly their growth environment.

Internal factors: the symmetry and regularity of the molecular chain; external factors: time and temperature.

Choose the answer for the third floor: 1. The impact of the chain structure. The higher the symmetry of the molecular chain structure, the easier it is to crystallize. The better the regularity of the molecular chain, the easier it is to crystallize, such as isotactic or syndiotactic polymers. The greater the flexibility of the molecular chain, the easier it is for crystallization to occur. In the case of branching or cross-linking, these factors disrupt the regularity of the chains and limit their mobility; therefore, the lower the degree of branching and cross-linking, the easier it is for crystallization to take place. The smaller the steric hindrance of the side-chain substituent, the easier it is to crystallize. 2. Intermolecular forces: Intermolecular forces generally reduce the flexibility of molecular chains, thereby hindering crystallization ； And once crystals are formed, the intermolecular forces further contribute to the stability of the crystal structure. 3. Molecular weight: For the same polymer, under identical crystallization conditions, the lower the molecular weight, the easier it is for the molecular chains to move, and thus crystallization occurs more readily. 4. Environmental factors: Crystallization occurs at higher temperatures; the greater the mobility of molecular chains, the more complete the crystallization. Stretching crystalline polymers facilitates crystallization.