The key difference between cross linked polymer and linear polymer is that the monomer units of linear polymers have end-to-end links, resembling the beads in a necklace, whereas cross linked polymers are made up of chains that are joined together by a series of covalent bonds, called cross-links.
Polymers are the compounds consisting of small repeating units connecting together to form long-chain molecules. The repeating units or the building blocks of a polymer are monomers. Polymers can be broadly classified into three parts based on their chemical and thermal nature, namely; (a) thermoplastic polymers, (b) thermosetting polymers, and (c) elastomers. Thermoplastics are plastics that can change the shape under the application of heat. Unlike thermoplastics, thermosets cannot tolerate repeated heating cycles. Elastomers are the rubbers that exhibit excellent elastic properties, unlike the two types mentioned above. According to the structure, there are three types of polymers as linear, branched and cross linked polymers. Thermoplastic polymers are linear molecules, whereas thermosets and elastomers are cross linked polymers.
CONTENTS
1. Overview and Key Difference
2. What is a Cross Linked Polymer
3. What is a Linear Polymer
4. Side by Side Comparison – Cross Linked Polymer vs Linear Polymer in Tabular Form
5. Summary
What is a Cross Linked Polymer?
A cross linked polymer is a polymer that has chains linked together by a network of covalent bonds. Cross links can be short or long, but in most of the polymers, these bonds are short. Thermosets and elastomers have cross links. The properties of cross linked polymers mainly depend on the degree of cross linking. To be specific, if the degree of cross linking is low, the polymer will behave as an uncross-linked polymer and show softening behavior. However, if the degree of cross-linking is high, the softening behavior of the polymer will be much harder. A good example of using cross-linking to improve the properties of rubbers is the vulcanization process.
Figure 1: Cross-linked polyisoprene (vulcanized natural rubber using sulfur as the cross-linking agent)
During vulcanization, adding vulcanization agents such as sulfur, metal oxides, etc., increase the cross-links between rubber chain molecules. And thus, improves the tensile strength and hardness of rubbers. Many rubber product manufacturing processes use vulcanization. Unlike rubbers, thermoset polymers such as urea formaldehyde become hard and brittle materials during the process of cross-linking. That is because cross-linking makes the polymer chemically set, and this reaction is irreversible. Furthermore, the solubility parameter of the cross-linking polymers varies with the cross-linking density. If a polymer has a low degree of cross-linking, it will tend to swell in the liquid.
What is a Linear Polymer?
A linear polymer is a thermoplastic polymer that consists of long-chain molecules. Here, the monomer units have end-to-end links, resembling beads in a necklace. Polyethylene is an example of a linear polymer where ethylene units act as monomers. Sometimes these linear chains have branched structures. Generally, linear and branched-chain structures of the same polymer exhibit similar properties.
Figure 02: Polyethylene
Since they are thermoplastics, heat can soften linear polymers. The softening temperature is a unique feature of linear polymers. The softening temperature of rubbers or viscous fluids is below room temperature, whereas that of hard, brittle solids or ductile solids is above room temperature. Moreover, a linear polymer is a thermoplastic polymer that consists of long-chain molecules. Here, the monomer units have end-to-end links, like beads in a necklace.
Polyethylene is an example of a linear polymer where ethylene units act as monomers. Sometimes these linear chains have branched patterns. Generally, linear and branched-chain structures of the same polymer exhibit similar properties.
What is the Difference Between Cross Linked Polymer and Linear Polymer?
Cross Linked Polymer vs Linear Polymer |
|
| Cross linked polymer is made up of chains that are joined together by a series of covalent bonds. | Linear polymer is made up of monomers joined together end-to-end, resembling the beads in a necklace. |
| Thermoplastics | |
| Thermosets and elastomers | Thermoplastics |
| Heating of Polymers | |
| Cannot tolerate repeated heating cycles | Can tolerate repeated heating cycles |
| Recyclability | |
| Cannot be recycled (cannot be remolded) | Highly recyclable (can be remolded/ reshaped) |
| Type of Bond Between Molecular Chain | |
| Permanent primary bonds | Temporary secondary bonds |
| Examples | |
| phenol-formaldehyde, polyurethanes, silicones, natural rubber, butyl rubber, chloroprene rubber | acetals, acrylics, acrylonitrile-butadiene-styrene (ABS), polyamides, polycarbonate, polyethylene |
Summary – Cross Linked Polymer vs Linear Polymer
In brief, there are two categories of polymers based on their structure: linear polymers and cross linked polymers. The monomers of linear polymers have end-to-end links, resembling beads of a necklace. Hence, all thermoplastics belong to linear polymers and have no permanent cross-links between polymer chains. However, cross linked polymers have permanent bonds between adjacent polymer chains. All elastomers and thermosets belong to cross linked polymers. Thus, this is the difference between cross linked polymer and linear polymer.
Reference:
1. Groover, M. P. (2007). Fundamentals of modern manufacturing: materials processes, and systems. John Wiley & Sons.
2. Alger, M. (1996). Polymer science dictionary. Springer Science & Business Media.
3. Dyson, R. W. (Ed.). (1987). Specialty polymers. Blackie.
Image Courtesy:
1. “Vulcanization of POLYIsoprene V.2” By Jü – Own work (CC)0 via Commons Wikimedia
2. “Polyethylene-repeat-2D” (Public Domain) via Commons Wikimedia