6.3 Branched, crosslinked, and network polymers

Polymer architectures come in various forms, from simple linear chains to complex networks. These structures, including branched and crosslinked polymers, greatly influence material properties. Understanding these architectures is crucial for designing polymers with specific characteristics.

Synthesis methods for complex polymers involve techniques like chain transfer, copolymerization, and post-polymerization modifications. These approaches allow for the creation of branched, crosslinked, and network polymers, each with unique properties suited for diverse applications in industries from packaging to aerospace.

Polymer Architectures and Synthesis

Types of polymer architectures

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• Linear polymers consist of a single backbone chain with no branches or crosslinks (polyethylene, polypropylene, nylon)
• Branched polymers have a main backbone chain with side branches
  • Types of branching include short chain branching, long chain branching, star polymers, comb polymers, and dendritic polymers
• Crosslinked polymers are linear or branched polymers with intermolecular covalent bonds connecting the chains
  • Crosslinks can be formed during polymerization or post-polymerization
• Network polymers are highly crosslinked polymers with a three-dimensional structure
  • Crosslink density is high enough to form a continuous network
  • Thermosets such as epoxy resins and phenol-formaldehyde resins are examples of network polymers

Synthesis of complex polymers

• Branched polymers can be synthesized by chain transfer to polymer during free radical polymerization, copolymerization with a branching monomer (divinylbenzene), or grafting side chains onto a linear backbone
• Crosslinked polymers are created through copolymerization with a multifunctional monomer (divinylbenzene, ethylene glycol dimethacrylate) or post-polymerization crosslinking using radiation, heat, or chemical agents
  1. Vulcanization of rubber with sulfur
  2. Electron beam or UV radiation crosslinking
• Network polymers are formed by polymerization of multifunctional monomers (epoxy resins, phenol-formaldehyde resins) with simultaneous polymerization and crosslinking
  • High crosslink density leads to network formation

Properties and Applications

Effects of polymer structure on properties

• Branching reduces crystallinity and increases amorphous content, lowers density and increases free volume, improves processability and melt flow properties, but reduces mechanical strength compared to linear polymers
• Crosslinking improves mechanical strength and dimensional stability, increases solvent resistance and reduces solubility, raises glass transition temperature ($T_g$) and melting temperature ($T_m$), and reduces creep and stress relaxation
• Network formation results in high mechanical strength and rigidity, insolubility in most solvents, high thermal stability and resistance to softening at elevated temperatures, but also brittle behavior and limited elongation at break

Applications of advanced polymers

• Branched polymers: Low-density polyethylene (LDPE) for flexible packaging and insulation, branched polycarbonate for improved impact resistance in automotive and aerospace applications
• Crosslinked polymers: Crosslinked polyethylene (PEX) for plumbing and heating systems, vulcanized rubber for tires, seals, and gaskets, crosslinked hydrogels for contact lenses and wound dressings
• Network polymers: Epoxy resins for adhesives, coatings, and composite materials, phenol-formaldehyde resins for electrical insulators and laminates, polyurethane foams for insulation and cushioning applications