First understand the intrinsic factors that affect the crystallization of polymers:
1. Symmetry of polymer chains: good symmetry and easy crystallization. (does not require high symmetry)
For example, PTFE and PE have good symmetry and are easy to crystallize. The highest crystallinity of PE is as high as 95%. When PE is chlorinated, the symmetry is destroyed and the crystallization ability is greatly reduced. The high-prepared branched polyethylene has a lower crystallization ability than the low-pressure prepared linear polyethylene.
2. The regularity of the polymer chain: Generally consider a polymer containing an asymmetric center or a polymer having a cis-transformation, which has good regularity and is easy to crystallize. (Not all links are regular, allowing some irregularities (branches, links, irregularities), not too much, regularity.)
For example: random polymers such as PS, PMMA and PVA are amorphous polymers.
Polypropylene PP is divided into three types, isotactic, syndiotactic and random PP. The isotacticity affects the crystallization ability of PP. The isotactic PP crystallinity is high, while the random PP is amorphous polymer. Transparent. Generally, poly-α-olefins need to be considered isotactic.
For polydienes, it is necessary to consider cis-trans isomerization, and the all-cis or all-trans structure has a certain crystallization ability. When the cis-trans is randomly arranged, the regularity of the chain is destroyed and the crystallization ability is lowered. Generally, the cis polymer has a lower crystallinity than the trans polymer.
Special case: Polychlorotrifluoroethylene, which is neither a symmetric polymer nor an isotactic polymer, but has a crystallinity of up to 90%. Random PVC also has a weak crystallization ability.
3. The link is small and the flexibility of the chain: the chain link is small, the crystal nucleus is easy to form, the flexibility is good, and the crystallization ability is improved.
4. Cross-linking: light cross-linking, the crystallization ability of the polymer is decreased, but it can also be crystallized (the cross-linked polymer is not crystallizable), and when the degree of cross-linking is increased to a certain extent, the polymer loses its crystallization ability.
5, intermolecular forces, the formation of hydrogen bonds between molecules, is conducive to stable crystal structure.
For example, PVA is an amorphous polymer, but after hydrolysis, the obtained polyvinyl alcohol can crystallize, why? Because of the hydrogen bond.
6. Crystallization ability of the copolymer: random copolymerization, and the crystallization ability is lowered. In the block copolymer, each block remains relatively independent, and the crystallizable block will form its own crystal region.
Note: The crystallization of the material is affected by internal and external factors, so the polymer with crystallinity can be either crystalline or amorphous.
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