Question of the week: Popularizing knowledge about polymers. What are the factors that affect the flexibility of polymer chains?

The flexibility of polymer chains depends primarily on the structure of the main chain and the side-chain groups. Regarding the main chain structure, the ease with which rotation occurs within the chain segments determines the degree of flexibility. In hydrocarbon substances, polarity is low, intermolecular interactions are minimal, and the rotational barriers within the chain are low; as a result, they exhibit good flexibility, such as polyethylene. If the main chain contains conjugated carbon-carbon double bonds, internal rotation is not possible, resulting in a significant increase in the rigidity of the molecular chain, as in polyacetylene. If the main chain contains ring structures, internal rotation is also not possible, and the rigidity of the molecular chain increases significantly; for example, Kevlar has much higher rigidity and heat resistance than ordinary nylon. Furthermore, in heterochain polymers, the internal rotation barriers for single bonds such as C-O, C-N, and C-Si are lower than those for C-C bonds; therefore, polyethers and polyesters possess good flexibility. Fluoroether rubber also significantly improves the low-temperature flexibility of fluororubber due to the introduction of ether bonds. For side-chain groups, the strength of the interactions and steric hindrance effects determine the degree of flexibility. In the case of polar side chains, the stronger the interactions with other non-bonding atoms, the higher the rotational barrier within the chain, and thus the lower the flexibility of the chain. This applies to side chains containing electron-withdrawing groups such as CN and Cl, as seen in polymers like polyacrylonitrile and polyvinyl chloride. For non-polar side groups, the larger the volume, the greater the steric hindrance effect, and the poorer the chain flexibility.

The flexibility of polymers is related to their molecular structure; polymers with C-O, C-N, and C-Si bonds in their structure exhibit good flexibility, such as polyesters, polyamides, polyurethanes, and polydimethylsiloxanes.

It is mainly related to the molecular size and the various groups present in the molecule: for example, the presence of more heterocyclic and aromatic groups in the molecule results in lower flexibility. Additionally, there are carbon-oxygen, carbon-nitrogen, and carbon-sulfur bonds in the molecule, and oxygen, sulfur, and nitrogen all possess strong intermolecular forces

The property whereby polymer chains can change their conformation is known as flexibility, and this is the main reason why many properties of polymers differ from those of small molecules. The main chain structure has a significant impact on the flexibility of polymers. For example, due to the large Si-O-Si bond angle and longer Si-O bond length, internal rotation is relatively easy; as a result, polydimethylsiloxane has excellent flexibility and is an excellent synthetic rubber. Due to the inability to undergo internal rotation, polymer chains containing aromatic heterocyclic structures in their backbone have poor flexibility, but they exhibit high-temperature resistance. The strength of the side group polarity has a significant impact on the flexibility of the polymer chain. The weaker the polarity of the side groups, the greater the forces acting between them; internal rotation of single bonds becomes difficult, resulting in poor chain flexibility. The length of the chain also affects flexibility; if the chain is very short, there are few single chains available for internal rotation, resulting in a limited number of molecular conformations, which inevitably leads to rigidity.

The main chain of synthetic polymers is primarily a carbon chain formed by carbon atoms bonded together through covalent bonds. Since single bonds can rotate freely, linear polymer chains are unlikely to remain in a straight configuration due to these rotations. In fact, linear long-chain polymers are in a naturally curled state, with their molecules entangled with one another, which gives them flexibility. When an external force is applied to the molecules, the curled molecules can be straightened out, but once the external force is removed, the molecules return to their original curled state; therefore, synthetic polymers all possess a certain degree of elasticity. Since synthetic polymers are all long-chain macromolecules in a naturally curled state, it is difficult for them to arrange themselves neatly into a periodic crystalline structure. Unlike small molecules, synthetic polymers do not easily form complete crystals. However, on a local scale, molecular chains may arrange themselves in an orderly manner to form a crystalline state, namely short-range order. Therefore, polymer crystals often contain both crystalline and amorphous regions, and thus the degree of crystallinity is commonly used to measure the proportion of the crystalline portion within the entire polymer. Crystalline polymers generally have higher heat resistance and mechanical strength than amorphous polymers, and they also possess a certain melting point. Therefore, to improve these properties of polymers, it is necessary to increase their degree of crystallinity. Polymer structures exhibit heterogeneity, or polydispersity, which is distinctly different from the structure of small molecules. The structure of small molecules is determined, as is their molecular weight. However, for synthetic polymers, once the degree of polymerization n of each individual polymer is determined, its molecular weight is also determined. However, in polymerization reactions, the resulting polymers are not homogeneous; rather, they are mixtures of polymers with different degrees of polymerization. Therefore, it is impossible to determine the molecular weight of these polymers in such cases. Experimental determination of the molecular weight of polymers is merely the statistical average of the molecular weights of polymers with varying degrees of polymerization in the sample. The aforementioned structural characteristics of synthetic polymers endow them with special properties such as thermoplasticity, thermosetting behavior, wear resistance, insulation, low relative density, and high specific strength. When long-chain polymers are heated, the molecules are not heated evenly; some parts are heated, some parts are less heated, and there are even some parts that are not heated at all. Therefore, when polymers are heated, they do not melt and turn into a liquid immediately; instead, they go through a softening process first before becoming liquid. Of course, this is the effect of external factors; internal unevenness within the molecule is also an important reason. After cooling, the liquid hardens into a solid; when heated again, it softens and flows. This property of linear polymers is called thermoplasticity. It not only facilitates the processing of polymeric materials, but also allows the process to be repeated multiple times. The linear polymers listed in Table 8-1 are all thermoplastic. After being heated until they soften, they can be processed into plastic products of various shapes, or made into fibers; thus, they are very easy to process. When monomers undergo polymerization, linear polymers are first formed; under certain conditions, crosslinking occurs between the molecular chains, causing the structure to change from linear to three-dimensional. Thermoset polymers do not melt or flow when heated; however, when heated to a certain temperature, their structure is destroyed. This property is known as thermosetting. Therefore, once polymers of this type are processed into shape, they cannot return to their original state by heating. Synthetic polymers mainly contain elements such as C, H, O, N, S, and halogens, which is why they are much lighter than metal materials. Generally, the relative density of polymers ranges from 1 to 2; the lightest polypropylene plastic has a relative density of only 0.91 ； The relative density of foam plastic is only 0.01, making it 100 times lighter than water, which makes it an excellent material for life saving. Polymer materials have a low relative density but high strength; the strength of some engineering plastics exceeds that of steel and other metal materials. For example, the strength of fiberglass-reinforced plastic is 1.7 times greater than that of alloy steel, 1.5 times greater than that of aluminum, and 1 time greater than that of titanium steel. Due to their light weight, high strength, corrosion resistance, and low cost, polymer materials have gradually replaced metal materials in many applications; the advent of all-plastic cars is a typical example. Why do polymer materials have such high strength? Polymer molecules have a large molecular weight, contain a large number of atoms, and their molecular chains are entangled with one another; as a result, there are many points of contact between the atoms in these molecular chains, leading to strong intermolecular forces. This force is called an intermolecular force, or van der Waals force. If the conditions for forming hydrogen bonds are met, hydrogen bonds can also be formed between molecular chains. The presence of strong intermolecular forces in polymers is the main reason why polymeric materials exhibit high strength. The molecular chains of polymers are entwined with each other, and the genes located on many of these molecular chains are enclosed within them. When reagent molecules are added, only the groups that are exposed can readily react with those reagent molecules, while the genes that are enclosed inside do not react easily. As a result, polymers have poor chemical reactivity and appear to be more stable in the presence of chemical reagents. Polymers possess properties such as acid and corrosion resistance. The well-known \"plastic king\" polytetrafluoroethylene does not deteriorate even when boiled in aqua regia, and its acid resistance far exceeds that of gold. Polytetrafluoroethylene is an excellent acid- and corrosion-resistant material. In polymers, the molecular chains consist of atoms bonded together by covalent bonds. These molecules cannot be ionized, nor can they transfer electrons within the structure. Therefore, polymers are insulating; plastic is used to make wire coatings, electrical sockets, and the like. In addition, polymers are resistant to various types of radiation such as α, β, γ, and X-rays; they can resist radiation. For naming synthetic polymers, one method is to add the prefix “poly-” before the monomer, such as polyethylene, polyvinyl chloride, etc ； Another approach is to add the word “resin” after the simplified name of the monomer; for example, phenol-formaldehyde resin, which is obtained by the polycondensation of formaldehyde and phenol, as well as urea-formaldehyde resin, epoxy resin, and so on. Businesses prefer to use product names as it is more convenient. Table 8-2 lists the trade names of some synthetic polymers.

The internal rotation of polymer chains is not completely free; there are interactions between the atoms within the molecules, and these are influenced by factors such as the molecular structure (short-range structure), temperature, the interactions among molecules in the aggregated state, the interactions between polymers and the solvent in solution, as well as external force fields. Below is an introduction to the effect of short-range structure on the flexibility of macromolecules. Main chain structure: The main chain structure has a significant effect on the flexibility of polymer chains. (1) Carbon-chain polymers: a Unsaturated carbon-chain polymers are more flexible than saturated carbon-chain polymers. PB>PE IR>PP PVC>CR b Polymers with benzene rings in their main chains and those with conjugated double bonds have poor flexibility. Polyphenylene oxide (PPO), polybenzene, polyacetylene (2) Heterochain polymers and elemental polymers: Si-O > C-N > C-O > C-C. Silicon rubber, nylons, esters, olefins. Polarity, volume, and symmetry of side groups – Side group polarity:  The greater the polarity, the less flexible the chain is. PAN > PVC > PP  Degree of polarity: As the number of polar groups increases, flexibility decreases. When the chlorine content is low, polyvinyl chloride is a rubber with good elasticity; as the chlorine content increases, the flexibility of the chains decreases, the elasticity declines, and it eventually becomes a hard material. Polyethylene > Polyvinyl chloride > 1,2-Polydichloroethylene  Symmetry: When the substituents are symmetrically distributed, the flexibility is better. Polyvinylidene chloride > Polyvinyl chloride (2) Non-polar side groups  When the side groups are flexible, the longer they are, the greater the flexibility of the chain. Polymethyl methacrylate < Polyethyl methacrylate < Propyl methacrylate  When the side groups are rigid, their volume increases, leading to greater steric hindrance and reduced flexibility. PE > PP > PS > Polyvinylcarbazole  When the side groups are symmetric, flexibility increases. PP < PIB Branching and crosslinking (1) Branching:  Long branching – this enhances physical entanglement between molecular chains; as a result, molecular chain mobility is hindered and flexibility decreases.  Short branching – the distance between molecular chains increases, and flexibility improves. (2) Cross-linking:  Mild cross-linking – does not affect the mobility of the chains, and flexibility remains unchanged.  High cross-linking – chain mobility is restricted, resulting in reduced flexibility. Hydrogen bonds and intermolecular forces: If hydrogen bonds are formed within or between molecules in a polymer, the rigidity of the molecular chains increases. Polyethylene > Polyoxymethylene; copolymer structure. The types of copolymers include alternating copolymers, random copolymers, block copolymers, and graft copolymers. The study of copolymers is introduced in polymer chemistry. Typical copolymers include SBS, ABS, and AS. Their properties vary significantly depending on their composition. For example: those primarily composed of butadiene exhibit good toughness; ABS, which is mainly made up of styrene, has a smooth surface finish; those rich in acrylonitrile possess good rigidity and high strength. Other factors such as end groups, sequence structure, and stereochemistry have little to no effect on flexibility. Among these, the three-dimensional configuration affects the structure, thereby influencing the mobility of the chains in the aggregated state. However, what is emphasized here is the mobility of the “chain” as an independent unit—that is, the flexibility of the “chain” rather than its mobility under various states.

The flexibility of polymer molecular chains is an important factor affecting radiation cross-linking. Polymers with highly flexible molecular chains cross-link more easily at lower gelation doses; conversely, polymers with stiffer molecular chains experience greater difficulties in radiation cross-linking. For some polymers with particularly stiff molecular chains, radiation cross-linking can only occur at high temperatures, when the molecular chains have sufficient mobility. The flexibility of the molecular chains also affects the relationship between the degree of cleavage and the irradiation dose. By introducing a β factor that characterizes the flexibility of the molecular chains into the equation relating the sol fraction to the radiation dose, the Charlesby-Pinner equation was modified to yield a new equation for this relationship with a broader range of applicability. The flexibility of molecular chains also affects the mechanism of radiation cross-linking. As can be seen from various polymer examples, polymers with high flexibility have lower Tg values and smaller intramolecular rotation steric factors; for such polymers, the H-type mechanism predominates in radiation cross-linking ； Rigid chain polymers have a higher Tg and a larger intramolecular rotational steric factor; the mechanism of radiation cross-linking for such polymers is primarily of the T-type (or Y-type).

It mainly depends on the structure of the polymer chains. 1. Those containing many groups that affect steric hindrance have poor flexibility. For example, the main chain contains many benzene rings, and there are numerous side chains. 2. Those containing many polar groups have poor flexibility, such as Cl, etc. 3. Polymers containing ether groups and many double bonds have better flexibility.

The 2nd, 4th, 8th, and 10th floors are so awesome! Regarding this issue, it is mentioned in polymer textbooks. It is basically a branched structure, involving the length of the molecular chain and the presence of functional groups.