Structure Of Polymers

Polymers are chemical compounds that are made synthetically, and they are not a variety of polymers that occur naturally. Therefore, many significant polymer products that are used in different fields or industries are made either entirely synthetically or by mixing synthetic and natural polymer compounds. Polymers are composed of small molecules that are known as monomers. Through the process called polymerization, the monomers break down and join in a myriad of ways to form different kinds of polymers. The polymer structure is of three kinds linear, branched and network structures. Branched and network polymers are also called cross-linked polymer structures. 

The Polymer Structure

Polymers are made up of small and simple molecules that repeat themselves throughout the polymer structure. These repeating molecules are called monomers. One polymer may be made up of several hundred or million monomers that form various structures. The polymer structure could have a linear chain, a branched-chain or a network of polymer chains. These structures are held together by covalent bonds. Copolymers are those that are made up of one or two different kinds of monomers. There are mainly two types of polymer structures which include polymer chains which can be linear, branched or networked structures.

The Polymer Structure: Chains

The polymer structure is maintained through a series of carbon and hydrogen atoms. The carbon atom in its outer shell has four electrons, and each electron is able to form a covalent bond with a foreign atom. Therefore, in a polymer structure, there are two carbons that form three common bonds and also bond with other atoms.

To understand further, the example of polyethylene can be used. Polyethylene is made from ethane gas which has two carbon atoms that share electrons with the other carbon atoms. When ethane molecules are brought together, they divide into two molecules and bind with other fragmented ethane molecules. This results in the formation of a chain of ethane molecules which is called polyethylene.  

Forming long chains is important for the production of polymers. Although the polymer chain is shown in two dimensions often, it actually has a three-dimensional structure. Elongation of polymers happens when stress is applied. As the chain of the polymer increases, the state of the polymer changes from liquid to solid, which is waxy. This gives the polymer its toughness, strength and flexibility. 

While straight chains are common, polymers do not always form linear structures. Most often, the polymers are tangled. This is because the bonds formed between the molecules of a polymer are Van Der Wals forces which can be easily broken down when heated. When heated, there is increased mechanical movement. This can be seen in thermoplastic materials, which can be remelted easily.

Certain polymer forms join together to form a large network during the process of polymerization. In this type of polymerization, raw materials are heated at first which breaks their existing bonds and forms stronger bonds. The polymers produced in this manner are called thermosetting plastics or polymers. Bonds are formed at more than two places for branched or networked structures of polymers. These joint structures form rings and fold on top of each other to form crystalline structures in a partial manner. 

Once the mass has set and opened, free radical carbons bond with hydrogen molecules, polymerization ends and no movement occurs between the molecules anymore. At this stage, there will be a big molecule. The degree of movement between molecules after polymerization decides whether the polymer is strong or not. This is the reason why thermosetting polymers have more strength than thermoplastic polymers, which have free movement of molecules, and they also melt easily when heated. 

What is Monomer?

A molecule that reacts with other similar molecules is called a monomer. Monomers are the building block of polymers. 

Cross-Linked Polymer Structure: Branched and Network Polymers

In cross-linking, covalent bonds are formed that hold multiple chains of a polymer together. Cross-linked polymers are those whose molecules are bonded with one another, and the bond is either a physical one or a chemical one. However, most cross-linked polymers have chemical bonds. Cross-linking is beneficial because it makes the polymer sturdier and more rigid with higher melting points. The cross-linked structure of polymers is usually three dimensional. One example of a cross-linked polymer is Bakelite which is used in epoxy resin glues, electric plugs, plastic crockery and melamine. 

The cross-linked structure of polymers allows them to have thermosetting properties. A compound is said to have thermosetting properties when it can neither be dissolved, nor it can be melted. Therefore, it becomes difficult to work with a cross-linked polymer as it can withstand a lot of common processing methods like extrusion and injection—the most common methods of making cross-linked polymers are step polymerization and vulcanization. 

Conclusion

There are different types of polymers based on molecular and structural composition. Based on structural composition, there are linear branched and network polymers. Linear polymers form a straight chain, while branched and network polymers make intertwined structures that seem like small worms in a Petri dish. Branched and network polymers are also called cross-linked polymer structures. There are different kinds of variations possible based on how the polymer bonds. Therefore, there are a plethora of polymer compounds available as industrial products. The polymer structure determines the usability and strength of the compound.