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Polymer or large molecule or polymer are very large molecules that are formed by connecting several hundred monomers to each other and forming long chains. The word polymer is of Greek origin and is composed of the word “poly” meaning several and “mer” meaning part or part.
A monomer is the smallest repeating unit of a polymer. For example, if you consider a rosary as a polymer chain, the rosary beads are its monomers.
It is a process in which monomer molecules are linked together to form a large polymer molecule. Polymerization is the process of linking monomers together through a series of chemical reactions.
Polymers can be classified in different ways, the most common of which are as follows. Of course, polymers are divided into two types, engineering and common, depending on their construction and application, and each is described in separate sections.
According to their origin, polymers are divided into three categories: natural, synthetic, and semi-synthetic.
Natural polymer: There are polymers that are made only from materials found in nature, such as wool, silk, natural rubber, cellulose, protein, starch, etc.
Synthetic or synthetic polymer: Polymers that are made in the laboratory from low molecular weight molecules. Such as nylons, polyethylene, polyester, synthetic rubbers, PVC, Teflon, etc.
Semi-synthetic polymer: Polymers that are made from simple chemical modification of natural polymers in order to modify properties. Such as cellulose acetate, cellulose nitrate and…
Plastics are divided into two categories, thermoplastics and thermosets, according to their behavior against heat.
Thermoplastics: Thermoplastics or thermoplastics are polymers that are easily molded into various shapes by heat and then cooled at room temperature and can be used. That is, they will soften when exposed to heat and harden again when cooled. Such as polyethylene, PVC, polypropylene, polystyrene and…
Thermoplastics consist of long chains, each of which may have groups or side chains. Thermoplastics do not undergo any kind of chemical reaction during the process and their shaping due to heat. Thermoplastics, when exposed to heat, have the ability to flow and are easily molded into different shapes.
Thermosets: Thermosets or thermosets are hard and non-meltable polymers against heat. These polymers do not soften due to heat and it is not possible to re-mould them. Thermosets are cross-linked polymers that cannot be reused.
In thermoset resins, molecular active groups form crosslinks between molecules during the manufacturing process. These transverse bonds or so-called curing of the material do not allow the material to soften due to heat. Thermoset materials are generally supplied in the form of semi-polymerized compounds or polymer-monomer mixed liquid so that production units can make them into final products and then bake them.
In other words, thermostats are baked through methods such as heating or radiation, etc. The baking process is irreversible due to the cross links that were mentioned. That is, by reheating, unlike thermoplastics, these materials remain solid until they start to degrade.
Phenolics, urea, melamine, epoxy, alkyds, polyesters, silicone and urethane are among the most important thermostats.
Structurally, polymers are divided into three categories: linear, branched and cross-linked polymers.
Linear polymers: In this type of polymers, monomers are connected to each other and form long chains. In this type of polymers, there are no side branches and their molecules are close to each other and their density, melting point and tensile strength are usually high. such as HDPE and nylon.
Branched polymers: This category of polymers have long and straight chains, but they also have different side branches. For this reason, their molecules are arranged irregularly and therefore have a lower density, tensile strength and low melting point.
Polymers with crosslinks and networks: In this category of polymers, repeating units form 3D networks by connecting to each other. The connection between them is formed through transverse pions. And because of the way the units are connected and their network structure, they are hard, rigid and fragile. Among the most important of them, we can mention melamine, formaldehyde resin, cured rubbers, etc.
Polymers are divided into three categories, crystalline, amorphous and semi-crystalline, in terms of chain arrangement and crystallinity.
Crystalline: When very regular polymer chains are put together with a special shape, they form a crystal. A crystalline polymer, its molecular chains are regularly locked in each other and therefore show high strength. For crystalline polymers, the melting temperature is defined. That is, at a certain temperature or temperature range, crystalline polymers start to melt and show it with Tm.
Amorphous: They are the opposite of crystalline polymers, that is, the molecular chains are placed in a random and irregular shape and do not follow a specific shape. This has caused them to have high elasticity and flexibility properties. Unlike crystalline polymers, amorphous polymers do not have a melting point and the temperature at which they flow is indicated by Tg or glass transition temperature. One of the important features of amorphous polymers is relatively low thermal resistance, low dimensional stability, and relatively high creep.
Semi-crystalline: In reality, it can be said that all polymers have crystalline and amorphous regions at the same time. That is, there is no 100% amorphous polymer and especially 100% crystalline polymer. However, these values can be brought closer to 100 through methods. Usually, amorphous polymers are up to 90% and crystalline polymers are up to 80% amorphous and crystalline in nature.
Polymers are classified into homopolymers and copolymers based on the type of constituent monomers.
Homopolymer: A polymer that has only one specific monomer that is repeated in its chains. These types of polymers are called homopolymers, such as polyethylene (only containing ethylene monomer), polyvinyl chloride (only including vinyl chloride monomer) and…
Copolymer: Copolymers are polymers that are composed of 2 different types of monomers and there are two types of monomers in their repeating units, such as nylon 66. If the number of available monomers increases to 3, they are called terpolymers, such as ABS or acrylonitrile butadiene. Styrene
In terms of application and physical properties, polymers are divided into 4 categories: plastic, rubber or elastomer, fiber and resin.
Plastic: Polymers that have been transformed into hard and rigid materials using heat and pressure are representative plastics and are mostly semi-crystalline. Like PMMA, PE, etc., their intermolecular force is between fibers and rubber.
Elastomers or rubbers: solid polymers with good elastic properties and high elasticity. In this type of polymers, the polymer chains are connected by the weakest intermolecular force, and most of their structure is amorphous. This weak intermolecular force is the reason for the properties of those above them. Like rubbers and…
Fibers: If there is a long polymer string whose length is at least 100 times its diameter, they are called fibers. These polymers have very high tensile properties and show a high degree of crystallinity, like nylon.
Resins: They are low molecular weight polymers that are used as adhesives, sealants, etc. Most of them are available in liquid form and they are usually cooked during or after use to increase their strength. For example, glues that are used for general purposes harden after being exposed to air for a while. They are called air curing adhesives.
Tacticity means placement of side groups in space.
Isotactic: A head-to-tail configuration in which the groups are all on the same side of the chain is called isotactic.
Atactic: If these groups are randomly placed on both sides of the chain, it is called atactic.
Syndio Tactic: If the groups on both sides of the chain are placed in the form of one in the middle, they are called Syndio Tactic.
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