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Chapter 15: Problem 13

In your own words, describe the mechanisms by which semicrystalline polymers (a) elastically deform and (b) plastically deform, and (c) by which elastomers elastically deform.

Short Answer

Expert verified
Answer: Semicrystalline polymers elastically deform through the reversible stretching and bending of polymer chains in the amorphous regions, while the crystalline regions remain unchanged. They plastically deform when the applied force surpasses the elastic limit, causing the crystalline regions to break apart and polymer chains to slide past one another, leading to irreversible deformation. Elastomers primarily elastically deform due to the entropic effect, wherein polymer chains stretch and straighten upon deformation, decreasing the system's entropy, and return to their original, more disordered state when the force is released.

Step by step solution

01

Explain Semicrystalline Polymers and Elastomers

Semicrystalline polymers are a class of polymers that consist of both crystalline (ordered) and amorphous (disordered) regions. The crystalline regions are formed by the arrangement of polymer chains in a regular pattern, whereas the amorphous regions consist of randomly oriented polymer chains. Elastomers, on the other hand, are a type of polymer that exhibit rubber-like elasticity. They have a high degree of flexibility and can return to their original shape after being deformed.
02

Discuss Elastic Deformation of Semicrystalline Polymers

(a) Elastic deformation in semicrystalline polymers occurs when the polymer is subjected to a force that is within its elastic limit. In this case, the polymer chains in the amorphous regions undergo reversible stretching and bending, while the crystalline regions remain relatively unchanged. Once the force is removed, the polymer chains return to their original state, and the material goes back to its initial shape.
03

Discuss Plastic Deformation of Semicrystalline Polymers

(b) Plastic deformation occurs in semicrystalline polymers when the applied force exceeds the elastic limit of the material. In this case, the crystalline regions begin to break apart and disordered chain segments slide past one another, leading to irreversible deformation. This process is known as "yielding" and results in permanent changes to the shape of the material. Additionally, plastic deformation also involves the rearrangement of the polymer chains in both crystalline and amorphous regions, which may include the formation and breaking of intermolecular bonds.
04

Discuss Elastic Deformation of Elastomers

(c) For elastomers, the elastic deformation mechanism is mainly due to the entropic effect. Elastomers have a high degree of cross-linking between the polymer chains, which provides them with their rubber-like elasticity. When an elastomer is deformed, the polymer chains are stretched and straightened, resulting in a decrease in the entropy of the system, as the chains become more ordered. Upon releasing the force, the entropy increases as the chains return to their more disordered state, and the material goes back to its original shape. This process is highly reversible, allowing elastomers to repeatedly undergo elastic deformations without significant damage.

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Most popular questions from this chapter

List two important characteristics for polymers that are to be used in fiber applications.

For each of the following pairs of polymers, do the following: (1) state whether it is possible to decide whether one polymer has a higher tensile strength than the other; (2) if this is possible, note which has the higher tensile strength and then cite the reason(s) for your choice; and (3) if it is not possible to decide, then state why. (a) Syndiotactic polystyrene having a numberaverage molecular weight of \(600,000 \mathrm{~g} / \mathrm{mol}\); atactic polystyrene having a number-average molecular weight of \(500,000 \mathrm{~g} / \mathrm{mol}\) (b) Random acrylonitrile-butadiene copolymer with \(10 \%\) of possible sites crosslinked; block acrylonitrile-butadiene copolymer with \(5 \%\) of possible sites crosslinked (d) Network polyester; lightly branched polypropylene

Make two schematic plots of the logarithm of relaxation modulus versus temperature for an amorphous polymer (curve \(C\) in Figure 15.8). (a) On one of these plots demonstrate how the behavior changes with increasing molecular weight. (b) On the other plot, indicate the change in behavior with increasing crosslinking.

Briefly explain how each of the following influences the tensile modulus of a semicrystalline polymer and why: (a) Molecular weight (b) Degree of crystallinity (c) Deformation by drawing (d) Annealing of an undeformed material (e) Annealing of a drawn material

Briefly explain the difference in molecular chemistry between silicone polymers and other polymeric materials.
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