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Elasticity of Elastomers

Elasticity of Elastomers. Materials which undergo recoverable deformation of a few 100 % are called elastomers and exhibit rubber like elasticity Elastomers → Long chain molecules with some cross-links Cross-links keeps molecules from slipping past one another permanently

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Elasticity of Elastomers

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  1. Elasticity of Elastomers • Materials which undergo recoverable deformation of a few 100 % are called elastomers and exhibit rubber like elasticity • Elastomers → Long chain molecules with some cross-links • Cross-links keeps molecules from slipping past one another permanently • Relative mobility restricted to points between cross-links • Stress  strain • Strained equilibrium configuration reached swiftly and hence time dependence ignored (as a first approximation) i.e. Strain  f(t) • Stress  strain

  2. Mobile segments Cross-link points

  3. Tensile stress Heating Tensile stress S t r e t c h e d state Unstretched state • C-C bond angle is same • No change in enthapy • Randomly coiled molecules •  large number of configurations of equal potential energy •  high Sconfig & low Free Energy • Reduction in number of configurations •   in Sconfig

  4. Enthalpy change on stretching = 0 • I law of Thermodynamics • II law of Thermodynamics • F → Force applied to stretch • T → Constant temperature • L → Length of molecule • S → Entropy • Entropy change for stretch L0→ L • Change in number of configurations0 → 

  5. N0→ number of chain segments between cross linking points Equation Of State For a Rubbery material Constant at constant temperature

  6. Bond stretching in straightened out molecules Experimental Theory

  7. Equation of state • → Stress •  → Strain or • If  is continuous and single valued: ve term for elastomers

  8. Summary of salient features • In the elastic range E↑ as T ↑ (for a metal E↓ as T ↑) • During adiabatic stretching a elastomer gets warmer (metal gets cooler) • An unstretched elastomer → positive coefficient of thermal expansion elastomer stretched more than a few percent → negative coefficient of thermal expansion

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