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Polymers are an integral part of many industries and are used in conjunction with other materials and components to make a vast array of the world’s products. There are also many different types of polymers, many of which can exhibit different properties depending on how they are formed, tuned and processed. The ability to change and tune the properties of different polymers has enabled them to be used in many applications, and it is this ability that has enabled polymer-based products to become widespread in many markets. Here, we’re going to look at a few ways in which these properties are changed.
Many fundamental properties that can be changed with polymers. Polymers are very long carbon-based molecules that have a wide range of beneficial structural (and sometimes thermal) properties. Most of the ways to change the structure of polymers revolves around linking the chains together, branching the polymer chains and increasing the molecular weight of the polymer chains. While these are simple structural changes, they can be responsible for changing the heat conductivity, heat capacity, crystallinity, thermal expansion, permeability, elastic, tensile strength, refractive index, wear resistance and electrical insulating properties of the polymer. It is the case, that a simple change in the chemical make up of the polymer can drastically change its properties.
Overall, the main ways to change and tune the properties of a polymer include changing the length of the polymer chains, creating branched chains from linear polymer chains, crosslinking the polymer chains and adding plasticizers into the polymer. The first two mostly rely on the processing conditions, whereas the latter two rely more on additives to the polymer (be it during polymerization or afterward).
Changing the Processing Parameter
Many different parameters that can be controlled during the polymerization reaction(s) themselves and controlling the reaction conditions can change the molecular weight, length and degree of branching in the polymer chains. The processing conditions which can affect the properties of the polymer include the temperature and pressure used during the polymerization, the solvent in which the polymer is polymerized, the types of monomer units used, the concentration of monomers in the reaction, the reagent which is used to initiate the polymerization reaction, and how they are collected (i.e. are they collected in a manner that promotes linear alignment or not).
The change in both polymer lengths and the degree of branching can lead to a change in the molecular weight of the polymer. Because these changes are down to the processing conditions and starting materials, the properties can be easily controlled and tuned. Changing the length of the polymer is often utilized to change the melting point of the polymer. When the polymer chains are longer, they become more tangled, which means that they stick together better. So, it becomes harder to break the bonds between the chains, which increases the melting point. The reverse scenario is applicable when lower melting points are required (less entanglement, easier to break). As well as changing the melting point, the chain length can also be used to determine the matter state of the polymer at room temperature. Examples include making the polymer to be a viscous liquid, a soft solid, a stretchy/elastic solid, etc. Longer chains can also increase the density because when they are tangled, there can be a greater mass per area ratio.
Changing the degree of branching can be used to change the molecular weight, the density and the stretchiness/elasticity of the polymer. The plastics which are linear (unbranched) stack together better at the molecular level, which increases the density of the polymer (more molecules in a given area), as well as increase the tensile strength of the polymer (through stronger van der Waals interactions), and are more elastic/stretchy in nature because the polymer molecules can slide over each other easier. On the other hand, branched chains are less stretchy, less dense and have lower tensile strengths for the opposite reasons (less movement over each other, lower molecules per area, and weaker van der Waals interactions).
Addition of Other Chemicals
The other two ways to change the properties of a polymer are through cross-linking the polymer and the addition of plasticizers. While cross-linking is technically on the process side (in terms of changing the chemicals used), polymers only cross-link when a specific cross-linker is added during the polymerization reaction. So, it is dependent on the addition and not the process conditions per se, as the cross-linking only occurs after the cross-linker has been added (albeit under specific reaction conditions). By comparison, plasticizers are added after the polymers have been synthesized and are used to change the properties of the polymer-based product.
Cross-linking the polymer changes the chemical nature of the polymer if a chemical is used (in some cases there are physical cross-link can occur where the polymer chains interlink and are held via intermolecular interactions rather than chemical bonds). Cross-linking the polymers not only increases the tensile strength of the polymer, but it also changes the elastic properties of the polymer. The degree of elasticity of the cross-linked polymer is dependant upon the polymer itself and what the cross-linker molecules are, but in general, cross-linked polymers return to their original shape much easier than non-cross-linked polymers after being stretched.
On the other hand, plasticizers are additives that are often used in polyvinyl chloride (PVC) polymers and change the elastic properties of a polymer by acting as a molecular lubricant (which decreases the friction between molecules as they pass over each other). Polymers without plasticizers are much harder and rigid (as the molecules don’t move as easily), whereas those which have added plasticizers are more elastic, flexible and softer. The addition of plasticizers into a polymer also increases the free volume of the polymer, causing them to have lower glass transition temperatures (which also plays a part in them being softer).
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