6.3 Branched, crosslinked, and network polymers

Polymer Architectures and Synthesis

Polymer chains don't all look the same. Some are straight lines, some have branches sticking off them, and some are tied together into massive three-dimensional networks. The architecture of a polymer controls nearly everything about how it behaves: how strong it is, whether it dissolves in solvents, how it responds to heat, and how easily you can process it. This section covers the main structural types and how they're made.

Types of Polymer Architectures

Linear polymers are the simplest architecture: a single backbone chain with no branches or crosslinks. Polyethylene (in its high-density form), polypropylene, and nylon are common examples. Because the chains can pack together efficiently, linear polymers tend to have higher crystallinity and density.

Branched polymers have a main backbone with side chains hanging off it. Branching comes in several forms:

• Short-chain branching — small side groups, typically a few carbons long, that disrupt chain packing and reduce crystallinity
• Long-chain branching — longer side chains that significantly affect melt viscosity and flow behavior
• Star polymers — multiple linear arms radiating from a central point
• Comb polymers — many branches spaced regularly along the backbone
• Dendritic polymers — highly branched, tree-like structures with repeated branching points

Crosslinked polymers have covalent bonds connecting separate chains to each other. These crosslinks can form during polymerization or be added afterward. The key distinction from branching is that crosslinks are intermolecular bonds tying different chains together, not just side chains on one backbone.

Network polymers are crosslinked polymers taken to the extreme. The crosslink density is high enough that the entire material forms one continuous three-dimensional structure. Thermosets like epoxy resins and phenol-formaldehyde resins fall into this category. Once formed, they can't be melted or reshaped.

Synthesis of Complex Polymers

There are several routes to building these architectures, depending on the target structure.

Branched polymers can be made by:

• Chain transfer to polymer during free radical polymerization — a growing radical abstracts a hydrogen from an existing chain, creating a new radical site on that chain where a branch starts growing. This is how low-density polyethylene (LDPE) gets its branches.
• Copolymerization with a branching monomer — adding a small amount of a multifunctional monomer like divinylbenzene introduces branch points.
• Grafting — attaching preformed side chains onto a linear backbone through chemical reactions.

Crosslinked polymers are created through two main approaches:

1. Copolymerization with a multifunctional monomer — monomers like divinylbenzene or ethylene glycol dimethacrylate have two or more reactive groups, so they link separate chains together as polymerization proceeds.
2. Post-polymerization crosslinking — crosslinks are introduced after the polymer is already formed. Common methods include:
   • Vulcanization — heating natural or synthetic rubber with sulfur, which forms sulfur bridges between chains
   • Radiation crosslinking — using electron beams or UV light to generate radicals that form crosslinks
   • Chemical crosslinking agents — adding reactive molecules that bond to functional groups on different chains

Network polymers form when multifunctional monomers polymerize and crosslink simultaneously. Epoxy resins, for example, are made by reacting an epoxy prepolymer (with two or more epoxide groups) with a multifunctional curing agent. Because each monomer can react in multiple directions, the growing structure quickly becomes a dense, interconnected network.

Properties and Applications

Effects of Polymer Structure on Properties

Each architecture produces a distinct set of material behaviors.

Branching disrupts the regular packing of chains, which:

• Reduces crystallinity and increases amorphous content
• Lowers density and increases free volume between chains
• Improves processability and melt flow (branched chains slide past each other more easily)
• Reduces mechanical strength compared to linear versions of the same polymer

A clear example: high-density polyethylene (HDPE, mostly linear) has a density around 0.94–0.97 g/cm³, while low-density polyethylene (LDPE, heavily branched) sits around 0.91–0.94 g/cm³. The branching in LDPE makes it more flexible but weaker.

Crosslinking ties chains together, which:

• Improves mechanical strength and dimensional stability
• Increases solvent resistance and reduces solubility (crosslinked polymers swell in solvents rather than dissolving)
• Raises the glass transition temperature ($T_g$) because chain mobility is restricted
• Reduces creep and stress relaxation under sustained loads

Network formation (high crosslink density) produces:

• High mechanical strength and rigidity
• Insolubility in virtually all solvents
• High thermal stability with resistance to softening at elevated temperatures
• Brittle behavior and limited elongation at break (the tradeoff for all that rigidity)

Applications of Advanced Polymers

Branched polymers:

• LDPE for flexible packaging films and wire insulation
• Branched polycarbonate for improved impact resistance in automotive and aerospace components

Crosslinked polymers:

• Crosslinked polyethylene (PEX) for plumbing and radiant heating systems
• Vulcanized rubber for tires, seals, and gaskets
• Crosslinked hydrogels for contact lenses and wound dressings

Network polymers:

• Epoxy resins for adhesives, protective coatings, and fiber-reinforced composite materials
• Phenol-formaldehyde resins for electrical insulators and laminates
• Polyurethane foams for thermal insulation and cushioning