Introduction: Why Are Crosslinked Polymers Superior to Linear Polymers?
Crosslinked polymers have distinct advantages over their linear counterparts. Linear polymers consist of long chains held together by relatively weak van der Waals forces or hydrogen bonds. These polymers are thermoplastic, meaning they can be reshaped with heat as the bonds between chains break, allowing the material to flow and be remolded.
In contrast, crosslinking forms covalent bonds between adjacent polymer chains, creating an interconnected three-dimensional (3D) structure (Figure 1). These covalent bonds in crosslinked polymers result in a thermosetting material, meaning it cannot be reshaped once set.
The key differences between linear and crosslinked polymers lie in their:
• Mechanical properties
• Thermal stability
• Chemical resistance
Crosslinked polymers excel in terms of strength and stability. Figure 1 illustrates the structural differences between linear and crosslinked polymers.
Types of Crosslinking in Polymers
There are various types of crosslinking in polymers, which can be broadly categorized into three types:
1. Chemical Crosslinking
2. Physical Crosslinking
3. Ionic Crosslinking
1. Chemical Crosslinking
Chemical crosslinking involves the formation of covalent bonds between polymer chains, creating a permanent 3D network. This process is driven by chemical reactions, such as condensation, addition, or free radical polymerization.
Figure 2 demonstrates how covalent bonds form during chemical crosslinking.
This type of crosslinking can be initiated through heat, catalysts, or other chemical agents, including:
• Peroxides: e.g., benzoyl peroxide, dicumyl peroxide
• Diisocyanates: e.g., hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI)
• Epoxy resins: e.g., bisphenol A diglycidyl ether (DGEBA), triglycidyl isocyanurate (TGIC)
• Sulfur: Used in vulcanizing rubber, forming bridges between polymer chains
• Platinum catalysts: Common in silicone crosslinking via addition-curing
2. Physical Crosslinking
Physical crosslinking relies on non-covalent interactions, such as hydrogen bonding, van der Waals forces, or crystallization. Unlike chemical crosslinking, physical crosslinking is reversible and depends on external factors such as temperature, pressure, or solvent conditions. This type of crosslinking is beneficial for applications that require flexibility and responsiveness to environmental stimuli.
Figure 3 illustrates the concept of physical crosslinking.
Examples of physical crosslinking include:
• Entanglement of long chains in high molecular weight polyethylene, leading to increased viscosity
• Crystalline regions in semi-crystalline polymers like poly(vinyl alcohol) (PVA), which form strong physical links
Conclusion: Why Crosslinking Matters
Crosslinked polymers offer significant enhancements in mechanical strength, thermal stability, and chemical resistance, making them indispensable in high-performance applications across industries. From the durable components of automotive materials to the flexible yet resilient structures used in electronics, crosslinking has transformed the potential of polymeric materials.
In the case of delicate applications like smartphone manufacturing—where thermal stability is crucial for protecting sensitive components—crosslinked polymers, particularly UV-curable resins, have shown unparalleled benefits.
The evolution of crosslinking, whether chemical or physical, proves that even with traditional methods, continual innovation leads to new breakthroughs in materials science.[prisna-wp-translate-show-hide behavior="show"][/prisna-wp-translate-show-hide]
Post time: Sep-13-2024
