A linked polymer, often referred to as a crosslinked polymer, is a material where individual polymer chains are chemically bonded together, creating a robust, interconnected network.
Based on the provided reference, crosslinked polymers are polymers in which long polymer chains are crosslinked together to form a 3D matrix of interconnect polymer chains. This structural change significantly alters the material's properties compared to linear or branched polymers. The process ensures that a polymer has been crosslinked when the entire portion of polymer has been exposed to the cross-linking method, resulting in a unified, three-dimensional structure rather than a collection of independent chains.
Understanding Linked Polymers
Unlike typical linear or branched polymers where chains can slide past each other, the cross-links in linked polymers act like molecular bridges. These connections tie the chains together into a rigid, continuous network.
Key characteristics resulting from this structure include:
- Enhanced Stability: The network structure resists deformation and solvent attack.
- Increased Mechanical Strength: They are typically harder, stronger, and less flexible than their non-crosslinked counterparts.
- Improved Thermal Properties: Cross-links often increase the material's melting point or prevent melting altogether, causing them to decompose at high temperatures rather than melt (thermosets).
- Reduced Solubility: The 3D network makes them insoluble in most solvents, although they can swell by absorbing them.
The Crosslinking Process
The process of creating a linked or crosslinked polymer involves forming chemical bonds between adjacent polymer chains. This can be achieved through various methods:
- Chemical Additives: Introducing molecules (crosslinking agents) that react with specific sites on the polymer chains.
- Heat: Applying heat can sometimes initiate crosslinking reactions, especially in thermosetting polymers.
- Radiation: Exposure to high-energy radiation (like UV or electron beams) can create radicals that lead to bond formation between chains.
The extent of crosslinking significantly influences the final properties. A higher density of cross-links results in a more rigid and brittle material, while lower density leads to more flexible, elastic materials (like rubbers).
Applications of Linked Polymers
The unique properties of linked polymers make them indispensable in numerous applications:
- Tires and Rubber Products: Vulcanized rubber (crosslinked natural or synthetic rubber) exhibits elasticity and durability.
- Adhesives and Coatings: Many strong, durable adhesives and protective coatings rely on crosslinked polymer networks.
- Thermoset Plastics: Materials like epoxies, phenolic resins, and polyesters used in composites, electrical insulation, and structural components are crosslinked.
- Hydrogels: Some linked polymers can absorb large amounts of water, forming hydrogels used in contact lenses, diapers, and drug delivery systems.
The formation of this 3D matrix of interconnecting polymer chains is the defining feature that differentiates linked or crosslinked polymers from other polymer types.
