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Unveiling the Molecular Secrets of Rubber: Structure, Properties, and Applications

Structure of Rubber Elastomers

The molecular structure of rubber elastomers exhibits the following characteristics:

  1. Long Chain Molecules: Repeating units (monomers) form the molecules, creating long chains. These chains are flexible, with highly mobile segments, and the glass transition temperature (Tg) is below room temperature. The molecular weight of rubber polymers significantly influences their properties.
  2. Weak Intermolecular Forces: Rubber molecules are held together by relatively weak van der Waals forces. This makes rubber amorphous in its unstressed state, allowing the molecules to move easily relative to each other, contributing to its high flexibility and elasticity.
  3. Chemical Structure and Synthesis: Synthetic rubber molecules often have carbon atoms linked by covalent bonds. This structure allows for various chemical modifications to customize the properties of the rubber.
  4. Synthetic Rubber Production: Petrochemical sources typically produce synthetic rubber by polymerizing monomers such as butadiene and styrene.
  5. Synthesis Process and Microstructure: The way rubber is synthesized determines its microstructure and the arrangement of its molecular model. It also affects how synthesis residues influence the performance of the final rubber products. Advanced polymer chemistry techniques allow precise control over the molecular weight and chain configuration, enhancing the specific properties of synthetic rubber.

Physical and Rheological Properties

Polymer physics and rheology explain the characteristics of raw rubber. The formulation design ensures that the rubber meets product requirements and processing techniques. Physical testing of rubber products assesses processing performance and formulation design flaws, guiding factory production and improving acceptance rates to maximize product service life.

Molecular Movement and Elasticity

On a microscopic level, the atoms and segments of the long-chain molecules in rubber are in constant motion due to thermal vibrations, resulting in an irregular, random coil shape. The distance between the two ends of a molecule is much shorter than its fully extended length.

An unstretched piece of rubber appears as a tangled mass of linear molecules. In an undeformed state, the entropy is maximized. When rubber is stretched, its molecules align to varying degrees in the direction of the stretch. Work is required to maintain this alignment, making rubber resistant to stretching.

When the external force is removed, the rubber contracts back to its maximum entropy state. Thus, the elasticity of rubber primarily arises from changes in system entropy, known as “entropic elasticity.”

Stress-Strain Properties

The stress-strain curve of rubber is typical of elongation crystallization. The main component is the entropy change due to the system becoming more ordered. As molecules straighten, the isolating effect of side chains on the molecular chain disappears, and intermolecular attraction becomes significant, aiding resistance to further deformation. Therefore, rubber exhibits high tensile strength when fully stretched.

Temperature Dependence of Stress

Rubber’s stress under constant strain is a function of temperature, increasing proportionally with temperature. This temperature dependence is known as the Joule effect, illustrating the fundamental difference between metallic elasticity and rubber elasticity. In metals, each atom is held in a strict lattice by interatomic forces, and deformation work changes atomic distances, affecting internal energy, referred to as “energetic elasticity.” This range of elastic deformation is much smaller than the “entropic elasticity” in rubber, driven primarily by entropy changes in the system. For more details, see Rheology Fundamentals.

Nonlinear Stress-Strain Behavior

In general usage, rubber’s stress-strain curve is nonlinear, meaning its elastic behavior cannot be simply determined by Young’s modulus. Rubber deformation relates to temperature, deformation speed, and time.

Deformation and Temperature Relationship

Rubber’s molecular deformation cannot occur instantaneously, as intermolecular attraction must be overcome by atomic vibrational energy. If the temperature decreases, these vibrations become less active, slowing deformation. At very low temperatures, the vibrational energy is insufficient to overcome attraction, making rubber a hard solid. Similarly, increasing deformation speed at a constant temperature produces effects akin to lowering the temperature, causing rubber to behave as a hard solid under high deformation speeds.

Creep and Permanent Deformation

Under stress, rubber’s molecular chains gradually break, causing “creep” – an increasing deformation over time. Once the deforming force is removed, this creep results in small irreversible deformations, known as “permanent set.”

Thermal Properties of Rubber

  1. Thermal Conductivity: Rubber is a poor thermal conductor, with a thermal conductivity coefficient of about 2.2-6.28 W/m²·K at 25 mm thickness. It is an excellent insulating material, and its thermal insulation improves further when made into microporous or sponge forms, reducing the thermal conductivity to 0.4-2.0 W/m²·K.
  2. Thermal Expansion: Due to the large free volume between rubber molecular chains, their internal rotations become easier with increasing temperature, causing the volume to expand. Rubber’s linear expansion coefficient is about 20 times that of steel, a critical consideration in the design of vulcanized rubber molds, as the linear dimensions of rubber products will be 1.2-3.5% smaller than the mold. The hardness and raw rubber content also significantly affect the shrinkage rate, which is inversely proportional to hardness and directly proportional to rubber content.

Theoretical Shrinkage Rates of Various Rubbers

The shrinkage rates in descending order are:

  • Fluororubber
  • Silicone rubber
  • Butyl rubber
  • Nitrile rubber
  • Neoprene rubber
  • Styrene-butadiene rubber
  • Natural rubber

When used at low temperatures, the volumetric shrinkage of rubber products, such as oil seals, may cause leakage, and products bonded to metal may develop excessive stress leading to premature failure.

Electrical Properties of Rubber

General-purpose rubbers are excellent electrical insulators. Natural rubber, butyl rubber, EPDM, and styrene-butadiene rubber have good dielectric properties, making them widely used in insulating cables. Nitrile rubber and neoprene have poorer dielectric properties due to polar atoms or groups in their molecules. However, adding conductive carbon black or metal powders to rubber can provide enough conductivity to dissipate static electricity or even make the rubber conductive.

Gas Permeability (Gas Tightness)

Rubber’s gas permeability is the product of gas solubility and diffusivity in rubber. Gas solubility decreases with an increasing solubility parameter of the rubber, while the diffusion rate depends on the number of side chain groups in the rubber molecules.

Different rubbers have significantly varying gas permeability rates. Polyether rubber and butyl rubber have low gas permeability, with butyl rubber being only 1/20th as permeable as natural rubber. In contrast, silicone rubber has the highest gas permeability. Gas permeability increases rapidly with temperature.

The type and amount of carbon black filler in rubber products have little impact on gas permeability. However, the amount of plasticizer significantly affects it. Therefore, the plasticizer content should be minimized for rubber products requiring high gas tightness.

Flammability of Rubber

Most rubbers are flammable to varying degrees. Rubbers containing halogens, such as neoprene and fluororubber, have some flame resistance. For example, neoprene and chlorosulfonated polyethylene are difficult to burn once the external flame is removed, and fluororubber is completely self-extinguishing. Adding flame retardants (e.g., phosphates or halogen-containing substances) can enhance the flame resistance of rubber compounds.