How does cross linking alter the properties of a polymer?
Polymers are widely used in various industries due to their unique properties, such as flexibility, strength, and durability. However, the inherent properties of polymers can be significantly altered through a process called cross linking. Cross linking involves the formation of covalent bonds between polymer chains, which can lead to a wide range of modifications in the polymer’s structure and functionality. This article aims to explore how cross linking alters the properties of a polymer, highlighting the benefits and potential drawbacks of this process.
Enhanced Mechanical Properties
One of the most notable effects of cross linking is the enhancement of a polymer’s mechanical properties. By forming covalent bonds between polymer chains, cross linking increases the intermolecular forces, making the polymer more resistant to deformation and stress. This results in improved tensile strength, elasticity, and hardness. For example, cross linking in polyethylene (PE) increases its tensile strength and resistance to impact, making it suitable for applications such as geomembranes and plastic pipes.
Increased Chemical and Thermal Stability
Cross linking also contributes to the improved chemical and thermal stability of polymers. The covalent bonds formed during the cross linking process create a three-dimensional network structure, which hinders the penetration of chemicals and reduces the polymer’s susceptibility to thermal degradation. This makes cross linked polymers more resistant to environmental factors, such as UV radiation, chemicals, and high temperatures. As an example, cross linked polyvinyl chloride (PVC) is widely used in outdoor applications due to its excellent resistance to weathering and chemicals.
Modified Solubility and Swelling Behavior
The solubility and swelling behavior of polymers can be significantly altered through cross linking. Cross linking reduces the polymer’s porosity, which limits the penetration of solvents and decreases swelling. This property is particularly important in applications where the polymer is exposed to aggressive solvents or aqueous environments. For instance, cross linked polyurethane (PU) is used in waterproofing applications due to its low swelling and excellent resistance to solvents.
Increased Permeability to Gases and Vapors
While cross linking reduces the permeability of liquids, it can actually increase the permeability of gases and vapors. This is because the cross linked network creates a more open structure, allowing for easier diffusion of gases through the polymer. This property is advantageous in applications where gas permeability is desired, such as in gas separation membranes. However, it is crucial to balance the permeability to gases and vapors with the desired barrier properties to ensure the polymer’s effectiveness in the specific application.
Conclusion
In conclusion, cross linking is a powerful tool that can significantly alter the properties of polymers. By forming covalent bonds between polymer chains, cross linking enhances mechanical properties, increases chemical and thermal stability, modifies solubility and swelling behavior, and alters permeability to gases and vapors. While the benefits of cross linking are numerous, it is essential to carefully design the cross linking process to achieve the desired properties for a specific application. By understanding the effects of cross linking, researchers and engineers can develop polymers with tailored properties for a wide range of applications.
