Main Difference – Linear vs Crosslinked Polymer
A polymer is a substance that has a molecular structure built up mainly from a large number of similar units bonded together. These repeating units represent the monomer that was used for the formation of the polymer. Since there are many different polymers, they can be divided into different groups depending on several parameters. We can divide polymers into two groups depending on the basic structure of polymer: linear polymers and crosslinked polymers. The main difference between linear polymers and crosslinked polymers is that linear polymers are straight chain structures whereas crosslinked polymers are branched structures.
Key Areas Covered
1. What is a Linear Polymer
– Definition, Structure, and Properties
2. What is a Crosslinked Polymer
– Definition, Structure, and Properties
3. What is the Difference Between Linear and Crosslinked Polymer
– Comparison of Key Differences
Key Terms: Atactic Polymers, Crosslinked Polymers, Isotactic Polymers, Linear Polymers, Monomer, Polymer, Syndiotactic Polymers
What is a Linear Polymer
A linear polymer is a macromolecule made out of many monomer units arranged in a straight line. A linear polymer consists of a single continuous chain of repeat units. This chain is called the backbone of the polymer. This liner polymer can have side groups attached to the backbone. These side groups are called pendant groups. But these side groups are not side chains.
In a linear polymer, the pendant groups can be arranged in different patterns. These patterns are described under the concept of tacticity – the relative regularity of a polymer chain. According to the tacticity of a polymer, these linear polymers can be divided into three main groups as isotactic polymers, syndiotactic polymers, and atactic polymers. Isotactic polymers have their pendant groups on the same side of the polymer chain. Syndiotactic polymers have their pendant groups in an alternating pattern. Atactic polymers have the pendant groups in a random manner.
Figure 1: Amylose is a Linear Polymer of Glucose Monomers
The backbone can be made out of either the same monomer or from different monomers. If it is the same polymer, it is called a linear homopolymer. If the backbone is made out of different monomers, it is called a linear copolymer. These copolymers can be found in different forms such as alternating copolymers (where the polymer chain is composed of regular alternating monomers), periodic copolymers (where the monomers are arranged in a repeating sequence) and, block copolymers (where the blocks of different monomers are arranged in a linear chain).
These macromolecules are closely packed due to less steric hindrance. This gives the linear polymers a high density, high tensile strength, and a high melting point.
What is a Crosslinked Polymer
Crosslinked polymers are macromolecules that has covalent bonds between polymer molecules. A crosslink is a bond between two polymer chains. These crosslinks can be either ionic bonds or covalent bonds. These crosslinks can be formed either during the process of polymerization or after polymerization.
Since crosslinks between polymer chains are stronger than normal intermolecular attractions, crosslinking forms a stable and stronger polymer material. Crosslinked polymers can be found in both synthetic polymers and as naturally occurring polymers. Crosslinks can be formed from chemical reactions using crosslinking reagents. The most common example of crosslinked polymers is the vulcanized rubber. Since natural rubber is not enough stiff and rigid, rubber is vulcanized. It is heated with sulfur, so sulfur molecules form covalent bonds in the rubber polymer chains, connecting the chains with each other. This makes rubber a stiff, rigid material and durable.
Figure 2: Vulcanization of Rubber
The degree of crosslinking gives the number of crosslinking per mole of a material. The degree of crosslinking is usually measured by swelling experiment. Here, the material is placed in a container with a suitable solvent. Then the change of mass or the change of volume is measured. If the degree of crosslinking is low, the material swells more.
Difference Between Linear and Crosslinked Polymer
Definition
Linear Polymer: A linear polymer is a macromolecule made out of many monomer units arranged in a straight line.
Crosslinked Polymer: A crosslinked polymer is a macromolecule that has covalent bonds between the polymer molecules.
Structure
Linear Polymer: Linear polymers are straight chain structures.
Crosslinked Polymer: Crosslinked polymers are network structures.
Melting Point
Linear Polymer: Linear polymer materials have high melting points due to the close packing of polymer chains.
Crosslinked Polymer: Crosslinked polymers do not melt at high temperatures. They tend to become a soft material that eventually burns.
Degree of Crosslinking
Linear Polymer: The degree of crosslinking of linear polymers is zero.
Crosslinked Polymer: The degree of crosslinking of crosslinked polymers is a positive value which gives the number of crosslinks present in a mole of polymer material.
Conclusion
Polymers are macromolecules that are made out of many monomers bonded to each other. Polymers is a very diverse group that include many different types of polymers. These polymers can be grouped according to different parameters. The basic structure of a polymer can be used to categorize polymers into linear polymers and crosslinked polymers. The main difference between linear polymers and crosslinked polymers is that linear polymers are straight chain structures whereas crosslinked polymers are branched structures.
References:
1. “Conceptual intro to polymers .” The Polymer Science Learning Center, Available here.
2. “Cross-Link.” Wikipedia, Wikimedia Foundation, 2 Oct. 2017, Available here.
3. “What is a polymer?” Polymer science learning center, Available here.
Image Courtesy:
1. “Vulcanization of POLYIsoprene V.2” By Jü – Own work (CC0) via Commons Wikimedia
2. “Amylose 3Dprojection.corrected” By glycoform – Own work (Public Domain) via Commons Wikimedia
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