Topic 7_Elastomers_Rubber Elasticity

Topic Overview

  • Rubber Elasticity: Understanding thermodynamic principles governing rubber elasticity, effects of temperature on elastomers, and using equations based on statistical thermodynamics and empirical equations to predict tensile or retractive forces.

Elastomers

  • Definition: Elastomers are materials that can undergo large (>400%) reversible deformations under small stresses.

  • Applications: Used in dynamic and static applications such as:

    • Dynamic Applications: Tires, Gaskets, Belts, Hoses, Sports Equipment, Engine mounts.

    • Static Applications: Tires, Engine mounts, Belts, Squash balls.

Fundamental Properties of Elastomers

  • Key Characteristics:

    1. Must be macromolecular.

    2. Should be amorphous (especially at low strains).

    3. Glass transition temperature (Tg) must be below the operating temperature.

    4. Low secondary forces between molecules for flexibility.

    5. Moderate degree of crosslinking to form a network.

Crosslinking and Elastomeric Networks

  • Natural state of rubber: Not useful; flows under applied force without structure memory.

  • Crosslinking process: Creates a 3D network making rubber more stable. A rubber band can be seen as one large molecule. Average molecular weight between crosslinks is crucial in vulcanizates.

Vulcanization

  • Types of Cures:

    • Sulfur Cures: Wide range of properties, cost-effective, only for unsaturated materials.

    • Peroxide Cures: High temperature stability, applicable to varied polymers.

  • Chemical Elements in Curation: Sulfur (S), Zinc oxide (ZnO) as accelerators.

Crosslinked Polymer Networks

  • Definition: Vulcanization refers to converting individual polymer chains into networks, generally yielding average molecular weight of 4,000-10,000 between crosslinks.

  • Mechanical Properties: Unique mechanical behavior includes large reversible extensions (>400%) and increased modulus with temperature due to crosslinks that prevent molecular bulk slippage.

Thermoplastic Elastomers

  • Structure: Triblock copolymers with a soft elastomeric segment and hard amorphous blocks, allowing flow at high temperatures and forming physical crosslinks when cooling.

  • Examples: Polystyrene-block-polybutadiene-block-polystyrene, segmented polyurethanes (Spandex, Lycra).

Chain Conformation and Elasticity

  • Conformations: Polymer chains have numerous conformations; end-to-end vector varies from 0 to a maximum rod-like form.

  • Gaussian Distribution: Probability of conformational changes is modeled by a Gaussian function.

Molecular Origin of Rubber Elasticity

  • Governing Parameters:

    1. Random processes statistics (Brownian motion).

    2. Bond arrangement preferences due to steric and energetic factors.

  • Characteristic of Vulcanized Elastomers: Deform with minimal volume change (incompressibility) and differ from hard solids in elasticity.

Thermodynamics of Rubber Elasticity

  • Retraction Force: Derived as a function of end-to-end distance. Change in Helmholtz free energy relates force and dimension.

  • Entropy Reduction: Stretching an elastomer reduces its conformational entropy, increasing restorative force as the chain becomes more ordered.

Gough-Joule Effect

  • Phenomenon: When a rubber band is stretched adiabatically, its temperature increases; applying heat with a static load causes it to shorten, described by classical thermodynamics.

Tensile Properties of Vulcanized Elastomers

  • Elasticity Source: Flexibility of chain segments between crosslinks contributes to elasticity.

  • Modeling Stress-Strain: Statistical thermodynamic approaches help model extension/compression based on tensile or compressive forces.

Tensile Properties of Elastomeric Compounds

  1. Statistical Thermodynamics: Chain conformation entropy as a function of end-to-end distance and distribution derived between crosslinks.

  2. Phenomenological Approach: Mooney-Rivlin model relates stress to deformation; empirical constants help refine models.

Examples and Applications

  • Calculation Examples: Given a retractive stress under specific conditions, estimate stress at different elongation and temperature.

  • Mooney-Rivlin Fit: Involves fitting empirical data from tensile tests to the theoretical model.

Knowledge Check

  • Understanding Restorative Force: Explain the mechanics behind restorative forces in rubber bands during compression and stretching, including influencing factors.