Elastomers and Their Mechanical Behavior

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Questions and Answers

What is the primary characteristic of elastomers?

They can withstand very high temperatures without deforming.

They possess a high melting point.

They are highly crystalline in structure.

They can stretch 5-10 times their original length and quickly retract. (correct)

Which condition is NOT a requirement for a polymer to be classified as an elastomer?

It must be produced from synthetic materials. (correct)

It must have a very low degree of crystallinity.

It must be above its glass transition temperature.

It should be lightly crosslinked.

What is the effect of vulcanization on rubber?

It enhances the rubber's ability to stretch

It increases the crystallinity of rubber.

It lowers the melting point of rubber.

It promotes crosslinking using sulfur. (correct)

What characterizes the ideal structure of an elastomeric network?

An amorphous network with junction points from which chains emanate. Signup and view all the answers

What fundamental assumption is made in the statistical theory of elastomer deformation?

The displacement vector components relate directly to the specimen’s overall dimensions. Signup and view all the answers

How does the configuration of cis-1,4-polyisoprene affect crystallinity in natural rubber?

Cis configuration reduces crystallinity. Signup and view all the answers

What happens to the structure of elastomers during short-term loading?

Entanglements can act as effective crosslinks. Signup and view all the answers

Which of the following statements about the melting point of natural rubber is true?

It is approximately 35 °C. Signup and view all the answers

What does the symbol $M_c$ represent in the expression for polymer density?

Number-average molar mass of the chain lengths between crosslinks Signup and view all the answers

How does the shear modulus $G$ relate to the average molar mass $M_c$?

G increases as Mc is reduced Signup and view all the answers

What effect do loops have on the mechanical behavior of elastomers?

Do not affect the modulus Signup and view all the answers

Which statement is true about the effect of temperature on the modulus of elastomers?

Modulus behaves oppositely to other materials upon heating Signup and view all the answers

What aspect of elastomer deformation is primarily affected when the modulus increases with reduced $M_c$?

Entropy changes Signup and view all the answers

Which of the following statements is incorrect regarding network defects in elastomers?

Chain ends enhance elasticity of the network Signup and view all the answers

In the context of the given content, what does the term 'network density' refer to?

Density of polymer chains between crosslinks Signup and view all the answers

What role do entanglements play in the behavior of elastomers?

They act as physical crosslinks Signup and view all the answers

What is the equation for the change in entropy per unit volume during deformation?

$ \Delta S = -\frac{1}{2}N k (\lambda_1^2 + \lambda_2^2 + \lambda_3^2 - 3) $ Signup and view all the answers

Which expression correctly represents the entropy of an individual chain before deformation?

$ S = c - k \beta^2 (x^2 + y^2 + z^2) $ Signup and view all the answers

What determines the value of N in the context of polymer networks?

The number of crosslink junctions and entanglements. Signup and view all the answers

Which of the following statements is incorrect regarding the change in Helmholtz free energy during deformation?

The change in Helmholtz free energy increases with higher temperatures. Signup and view all the answers

What happens to the entropy of a chain after deformation according to the provided equations?

It decreases when $ \lambda_i < 1 $ for any i. Signup and view all the answers

Which of these correctly describes the probability function W(x,y,z)?

It gives the probability of finding a single chain's end at any distance r. Signup and view all the answers

For isothermal deformation, how is the reversible work of deformation defined?

$ w = -\frac{1}{2}NkT(\lambda_1^2 + \lambda_2^2 + \lambda_3^2 - 3) $ Signup and view all the answers

Which condition is a limitation of the Gaussian distribution in the context of polymer chains?

It fails when chains become extended beyond typical lengths. Signup and view all the answers

Study Notes

Elastomers

Elastomers, also known as crosslinked rubbers, can be stretched 5-10 times their original length and return to their original shape when the stress is removed.
Three key requirements for elastomers:
- The polymer must be above its glass transition temperature (Tg).
- The polymer must have a very low degree of crystallinity (x→0).
- The polymer should be lightly crosslinked.
Examples include ethylene/propylene rubbers (copolymerization reduces crystallinity).
Natural rubber is formed by the polymerization of cis-1, 4-polyisoprene. The cis configuration reduces crystallinity, leading to a low melting point (~35 °C).
Vulcanization is a process that crosslinks rubbers with sulfur, typically at temperatures between 120-180°C. A curing agent or accelerator may be used, with the amount designated by m (1 or 2).

Mechanical Behavior of Elastomers

Elastomers exhibit unique deformation due to their behavior as an "entropy spring".
Elastomer deformation has been studied since the 19th century.
Analysis can be done thermodynamically.
Key assumptions:
- Freely jointed chains.
- Change in displacement vector is proportional to change in specimen dimensions (x' = λ₁x, y' = λ₂y, z' = λ₃z).
- Constant volume (λ₁λ₂λ₃ = 1).
Entropy of an individual chain (S = c − kß²r²).
The change in entropy (ΔS) during deformation is related to the extension ratios.
- ΔS = -1/2Nk (λ₂² + λ₂² + λ₃² - 3)

Limitations and Use of Theory

The theory assumes freely jointed chains.
The theory can be limited when chains become extended.
The number of junctions (N) affects the polymer network. Crosslinks can be chemical or physical in nature
Chain ends and loops do not strongly contribute to network strength.
Density (ρ) of the polymer can be expressed as NMc / NA
- Mc = average molar mass of chain length between crosslinks.
- NA = Avogadro's constant
Parameter G relates work of deformation to extension ratios, also referred to as shear modulus for elastomers
G increases as chain length between crosslinks (Mc) decreases—meaning the material is becoming stiffer as crosslink density increases (tighter network).
In contrast to most other materials, the modulus of an elastomer (G) will typically increase with temperature.
Various network defects like entanglements, chain ends, and loops will have some mechanical impact.
- Entanglements act like crosslinks and increase modulus.
- Loops and chain ends have no noticeable effect on network elasticity.

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Description

This quiz explores the characteristics and mechanical properties of elastomers, including their unique ability to stretch and return to form. Key concepts include their temperature requirements, crystallinity, and the process of vulcanization. Dive into the fascinating world of polymers and elastomers!