Crosslinking is a common process in polymer materials. Gelation is also a type of crosslinking. However, it specifically forms a gel rather than a simple crosslinked polymer material.
What is Crosslinking?
Crosslinking is the formation of covalent bonds between two polymer chains. These chemical bonds can be either ionic bonds or covalent bond – most commonly they are covalent bonds. Crosslinked polymers are polymers having crosslinks between polymer chains. These bonds form during the polymerization process (formation of the polymer material). Sometimes crosslinks form after the completion of polymerization as well.
Since crosslinks between polymer chains are stronger than normal intermolecular attractions, polymers formed from crosslinking is stable and stronger. These polymers occur in both synthetic forms and as naturally occurring polymers. Crosslinks are created from chemical reactions in the presence of crosslinking reagents. The most common example of crosslinked polymers is vulcanized rubber. Since natural rubber is not stiff or rigid enough, the rubber is vulcanized. Here, rubber is heated with sulfur, so sulfur molecules form covalent bonds in the rubber polymer chains, connecting the chains with each other. Then the rubber becomes a stiff and rigid material which is durable.
The amount of crosslinking gives the degree of crosslinking per mole of a material. We can measure the degree of crosslinking via swelling experiment. In this experiment, the material is placed in a container with a suitable solvent. Then the change of mass or the change of volume is measured. Here, if the degree of crosslinking is low, the material swells more.
What is Gelation?
Gelation is the formation of a gel from a mixture of polymers. Here, branched polymers cause the formation of linkages between branches. This is a type of crosslinking, and it leads to the formation of a large polymer network. During this process of network formation, a single macroscopic molecule forms at some point, and we call this point as the gel point. Here, the mixture loses its fluidity and viscosity. Meanwhile, it becomes very large. The gel point of a system can be determined easily by observing a sudden change in viscosity. After the completion of the formation of this infinite network material, we can call it a “gel”, and this gel does not dissolve in the solvent. However, the gel can undergo swelling.
A gel can form in two ways: physical linking or chemical crosslinking. Among these methods, the physical gelation process involves physical bonding between polymer molecules. The physical bonds may include attraction forces which are not chemical bonds. However, the chemical crosslinking process involves covalent bond formation between polymer molecules.
What is the Difference Between Crosslinking and Gelation?
The key difference between crosslinking and gelation is that crosslinking is the formation of ionic or covalent bonds between polymer chains, whereas gelation refers to the formation of a gel. Moreover, crosslinking forms due to the addition of crosslinking agent while gelation forms due to the sudden change of viscosity via the addition of crosslinking agents. Gelation is also a type of crosslinking.
Below infographic tabulates more differences between crosslinking and gelation.
Summary – Crosslinking vs Gelation
Crosslinking is a common process in polymer materials. Gelation is also a type of crosslinking. However, it specifically forms a gel rather than a simple crosslinked polymer material. The key difference between crosslinking and gelation is that crosslinking is the formation of ionic or covalent bonds between polymer chains, whereas gelation refers to the formation of a gel.
1. “Gelation.” – ScienceDirect Topics, Available here.