What Are Elastomers?

Elastomers are polymers that can be stretched to many times their original length and recover their shape almost completely when the load is removed. They are amorphous polymer networks used above their glass transition temperature, so the chains between junction points remain mobile while the network as a whole resists flow. This combination separates them from the other two broad polymer classes: thermoplastics, which flow when heated because their chains are not connected, and thermosets, which are crosslinked so densely that large deformation is impossible. Natural rubber is the archetype, and the term is often used interchangeably with rubber, although it also covers silicones, polyurethanes, and thermoplastic block copolymers.

The engineering importance of elastomers comes from a property set no other material class provides at comparable cost: recoverable strains of hundreds of percent, low elastic modulus, high damping, gas and liquid sealing under compression, and electrical insulation. Those properties underpin tires, seals, cable jacketing, vibration isolators, and soft robotic actuators.

Network Structure and Entropic Elasticity

The restoring force in an elastomer is thermodynamic rather than energetic. A relaxed chain adopts a random coil because that conformation has the largest number of accessible configurations; stretching it reduces configurational entropy, and the free energy penalty pulls the chain back. Classical rubber elasticity theory relates the shear modulus to the number density of network strands and to temperature, which predicts the counterintuitive result that an elastomer under constant load contracts when heated. Real networks depart from the ideal model because entanglements act as additional temporary constraints and because dangling ends and loops carry no load, so molecular simulation is used to separate these contributions. Studies of crosslink and entanglement contributions in model silicone networks quantify how much of the modulus each mechanism supplies, and the textbook treatment of crosslinked polymers and rubber elasticity sets out the statistical mechanics behind it.

Chemically and Physically Crosslinked Classes

Elastomers divide by how their network junctions are formed. Chemically crosslinked rubbers are vulcanized, a process Charles Goodyear introduced in 1839, in which sulfur or peroxide creates covalent bridges between chains during curing; once cured, these materials cannot be remelted. Physically crosslinked thermoplastic elastomers instead use block copolymer architecture, in which hard glassy or semicrystalline domains, such as polystyrene blocks in styrene-butadiene-styrene, aggregate to form junctions that dissociate on heating and reform on cooling, making the material recyclable and injection moldable. Composition determines the service environment: natural rubber and styrene-butadiene rubber for mechanical performance and abrasion resistance, nitrile rubber for oil contact, ethylene propylene diene monomer for weathering and electrical insulation, silicone for wide temperature range and biocompatibility, and fluoroelastomers for aggressive chemicals and high temperature.

Reinforcement, Failure, and Aging

Unfilled elastomers are weak, so nearly all commercial compounds contain reinforcing filler, usually carbon black or precipitated silica at loadings of tens of parts per hundred rubber. Filler particles form a percolating structure that raises stiffness and tear strength while introducing nonlinear effects, including the Payne effect, a drop in dynamic modulus with strain amplitude, and the Mullins effect, a softening on the first loading cycle. Analysis treating elastomer networks as complex networks with heterogeneous connectivity has been used to connect this structural disorder to measured mechanical response. Long-term performance is limited by degradation: ozone attack on unsaturated backbones, thermo-oxidative crosslinking that hardens the material, compression set in seals, and swelling in contact with solvents.

Applications

Elastomers have applications in a wide range of fields, including:

• Tires, belts, hoses, and vibration isolation mounts
• Static and dynamic seals, gaskets, and O-rings
• Cable insulation, jacketing, and high-voltage outdoor insulators
• Medical tubing, catheters, and implantable device encapsulation
• Dielectric elastomer actuators and stretchable electronics substrates
• Soft robotics, including pneumatic grippers and compliant manipulators