Do you know what makes crosslinked polymers an edge above linear polymers?
Linear polymers have long chains held by weaker van der Waals force or hydrogen bonding. They are thermoplastic because heat can break the bonds between the polymer chains. This in turn allows the chains to flow past each other so that the material is remolded.
Crosslinking involves the formation of covalent bonding between adjacent polymer backbone. It creates an interconnected three-dimensional structure (Figure 1). The chemical bonds in the crosslinked polymer are stronger making them thermosetting in nature.
The key difference in linear and crosslinked polymers leads to variations in:
- Mechanical properties,
- Thermal properties, and
- Chemical properties
This makes crosslinked polymers helpful in scenarios where high strength and stability are paramount. Figure 1 below shows a structure of linear and crosslinked polymers.
Read on to learn more about:
- Types of Crosslinking
- Effects of Crosslinking on Polymer Properties
- Applications of Crosslinked Polymers
- Advantages and Disadvantages of Crosslinking
- Future Trends in Crosslinking Technology
Types of Crosslinking
There are different types of crosslinking in polymers. They can be broadly categorized into three types:
Chemical crosslinking
Chemical crosslinking involves the formation of covalent bonds between polymer chains. This creates a permanent 3D network. The mechanism relies on chemical reactions like:
- condensation,
- addition, or
- free radical polymerization
Figure 2: Schematic Representation of Chemical Crosslinking Forming Covalent Bonds1
→ Peroxides: , dicumyl peroxide.
→ Diisocyanates: Hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI).
→ Epoxy resins: Diglycidyl ether of bisphenol A (DGEBA), triglycidyl isocyanurate (TGIC).
→ Sulfur: Used in rubber vulcanization to form a bridge between polymer chains.
→ Platinum catalysts: Used in silicone crosslinking by the addition-cure method.
Physical crosslinking
Physical crosslinking relies on non-covalent interactions like:
- hydrogen bonding,
- van der Waals forces, or
- crystalline domains
Unlike chemical crosslinking, physical crosslinks are reversible. It is dependent on external factors like temperature, pressure, or solvent conditions. This method is used where flexibility and responsiveness to external stimuli are required.
Figure 3: Schematic Representation of Physical Crosslinking1
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- Physical entanglements: Long chains become entangled in high molecular weight polyethylene. This leads to increased viscosity and improved mechanical properties.
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- Hydrogen bonding agents: Polyvinyl alcohol (PVA) forms hydrogen bonds.
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- Crystalline structures: Polyethylene with crystalline regions.
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- Ionic interactions: Ionic interactions contribute to physical crosslinking in polyelectrolytes. For example, in sodium polyacrylate.
- Self-assembly and supramolecular interactions: In supramolecular polymers, non-covalent interactions contribute to physical crosslinking. For example, π-π stacking or host-guest interactions.
Radiation-induced crosslinking
Radiation-induced crosslinking is done with high-energy radiations to induce crosslinking in polymers. For example, gamma rays, ultraviolet (UV) rays, and electron beams. The energy from radiation breaks existing bonds and generates reactive species. These form new crosslinks.
Figure 4: Radiation-induced Crosslinking of Polymers2
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Understanding crosslinking mechanisms is crucial for tailoring polymer properties. The method chosen depends on the desired properties and applications.
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Self-healing And Mechanochromic Polymers With Dynamic Carbon–Carbon Covalent Bonds
