Plastic just like natural polymers such as starch and protein are made up of many same small molecules (such as glucose and amino acids), these artificially polymerized polymers are also made up of many same small molecules; these Small molecules, called "monomers" of plastics. Some plastics have only one monomer, and some plastics have two or more monomers.
Generally, the name of plastic is based on its monomer. For example, a polymer that uses ethylene as a monomer is called polyethylene; a polymer that uses polypropylene as a monomer is called polypropylene. Plastic made by the polymerization of two monomers, ethylene, and polypropylene, is called poly(polypropylene-co-ethylene).
The characteristics of plastics are ever-changing because engineers can change the physical and chemical properties of plastics in many ways, such as:
Molecular weight: Plastic does not have a fixed molecular weight like oxygen and calcium carbonate, because its monomer number is not fixed. Some plastic molecules may have only a few hundred monomers, and some may reach more than thousands. The higher the molecular weight of the plastic, the lower the molecular mobility. The phenomenon that appears in the macroscopic view is the improvement of mechanical properties such as hardness. Also, the molecular weight distribution range will affect the characteristics of the plastic.
Functional group: The functional group on the monomer may be polar (such as polyvinyl chloride), maybe non-polar (such as polypropylene methyl), which will affect the inter molecular force; these functional matrix sizes, Occupying space varies, which will affect the gaps between molecules and mobility. Therefore, the functional group will affect the polymer's physical properties such as thermoplastic, hardness, tension, crystallinity, whether it is fragile, etc.
Number and length of branches: If there are many branches in a long-chain plastic molecule, the distance between molecules will be larger than that of molecules without branches, the space for molecular activity is also larger, and the arrangement of molecules is less easy to regulate. The phenomenon that appears in the macroscopic view is that the softness increases and the density and crystallinity decrease. This is also the difference between LDPE and "HDPE."
Processing method: During processing, through directional stretching, all molecules can be arranged in the direction of the force, reducing the gap between the molecules, improving the mechanical properties of plastics, such as BOPP; foaming through the gas allows the molecules The gap becomes larger and the density decreases, making the plastic lighter but fragile, but the thermal insulation effect is also better because the material is filled with air, such as Styrofoam.
Addition or blending: Through various additives or doping other polymers, the properties of plastics will be further changed. For example, a plasticize is usually a compound that is compatible with the plastic to which it is added, but its molecular weight is much smaller than that of a plastic molecule and is a liquid at room temperature. Plasticizes can increase the distance between plastic molecules and improve the mobility of plastic molecules, so they can turn hard plastics into soft plastics. Another example is the blending of two compatible plastics, which can also shorten the length and fill up, or produce unexpected effects, such as PPE with a high glass transition temperature (Tg=215°C) and difficult to process. If the glass is blended Polystyrene with a low transition temperature (Tg=100°C) can reduce its processing temperature and increase its stability against UV rays.
Co-polymerization: Through co-polymerization, different types of plastic monomers can be combined with chemical bonds to achieve the purpose of improving the characteristics of plastics. For example, co-polymerization of polystyrene, acrylonitrile, and polyethylene can obtain ABS co-polymer: this process cross-links the long-chain molecules of polyethylene with many short-chain molecules of polystyrene-acrylonitrile co-polymer stand up. These three monomers give the ABS co-polymer many advantages: acrylonitrile makes this co-polymer chemically resistant and has a certain surface hardness; butadiene makes this co-polymer rubbery; polystyrene makes the polymer rigid and fluid.