Molecular structure and aggregation state of polymers
The properties of polymers are closely related to their molecular structure and aggregation state. Molecular structure determines the basic properties of polymers, while aggregation state is the spatial arrangement of molecular chains, which directly affects the physical and processing properties of polymers. The molecular structure of a polymer includes the chemical composition, molecular weight and distribution, and geometric shape of the molecular chains. Chemical composition is fundamental. For example, polyethylene is composed of repeating -CH2- units, while polyvinyl chloride (PVC) has significantly different properties from polyethylene due to the presence of chlorine atoms. Molecular weight significantly influences the strength and toughness of polymers. A higher molecular weight increases the forces between molecular chains, resulting in higher strength and toughness, but also reduces fluidity and increases processing difficulty.
Molecular chains can be categorized into three geometric shapes: linear, branched, and cross-linked. Different molecular chain shapes impart distinct properties to polymers. Linear molecular chains, such as polyethylene and polypropylene, have a regular structure with tightly packed molecules. These chains exhibit good crystallinity and plasticity, allowing them to melt and flow upon heating, making them suitable for molding processes like injection molding and extrusion. Branched molecular chains, due to the presence of branches, increase the distance between molecules and decrease crystallinity. Low-density polyethylene, for example, has superior flexibility and transparency to linear high-density polyethylene. Cross-linked molecular chains, such as vulcanized rubber, use chemical bonds to connect molecular chains into a three-dimensional network structure. These chains are insoluble and infusible, offering significantly improved heat resistance and elasticity. Once formed, they cannot be reprocessed.
The aggregate states of polymers are primarily categorized as crystalline and amorphous (or amorphous) states, with some polymers also existing in a liquid crystal state. Crystalline polymers, such as polyethylene and nylon, have molecular chains arranged regularly in space, forming a crystalline structure that exhibits a high melting point, strength, and hardness. The higher the crystallinity, the greater the material’s heat resistance and rigidity, but also decreases its toughness and transparency. Amorphous polymers, such as polyvinyl chloride and polystyrene, have disordered molecular chains and no fixed melting point. They gradually soften upon heating, exhibiting good transparency and impact resistance but poor heat resistance. Liquid crystal polymers, under certain conditions, exhibit a molecular chain arrangement intermediate between the crystalline and amorphous states, combining both fluidity and orientation, making them suitable for manufacturing high-strength, high-modulus fibers and films.
Intermolecular forces, including van der Waals forces and hydrogen bonds, significantly influence the aggregation state of polymers. The stronger the intermolecular forces, the easier it is for the molecular chains to arrange tightly together to form crystals, and the higher the material’s strength and heat resistance. For example, polyamide (nylon) contains amide bonds in its molecular chains, which allow for hydrogen bonding between molecules, resulting in high crystallinity and excellent mechanical properties and wear resistance. Polyolefin polymers, on the other hand, rely solely on van der Waals forces, resulting in weaker intermolecular forces, relatively low crystallinity, and poor heat resistance. Furthermore, the flexibility of the molecular chain also influences the aggregation state. Molecular chains with good flexibility easily fold and arrange to form crystals. For example, polyethylene has a highly flexible molecular chain with a crystallinity of over 90%. However, polystyrene, due to the greater rigidity of the benzene rings, has a less flexible molecular chain and is typically amorphous.
External conditions such as temperature, pressure, and molding process will change the aggregation state of polymers, thereby affecting their performance. During the injection molding process, increasing the mold temperature can promote the orderly arrangement of molecular chains and increase crystallinity; while rapid cooling will inhibit crystallization and form an amorphous structure. For example, when polypropylene is molded in a high-temperature mold, it has high crystallinity and good rigidity; when molded in a low-temperature mold, it has low crystallinity and improved toughness. The increase in pressure will also promote the close arrangement of molecular chains and increase crystallinity. For example, the crystallinity of high-pressure polyethylene is higher than that of low-pressure polyethylene. In addition, through the stretching orientation process, the polymer molecular chains can be arranged along the stretching direction to form an oriented structure, which significantly improves the strength and modulus of the material in the orientation direction. For example, the strength of polyester fiber is greatly improved after stretching orientation treatment. Therefore, an in-depth understanding of the relationship between the molecular structure and aggregation state of polymers can provide a theoretical basis for material selection, molding process optimization and product performance improvement.
