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Chapter 15: Problem 14

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

Short Answer

Expert verified
Answer: The tensile modulus of a semicrystalline polymer is influenced by factors such as molecular weight, degree of crystallinity, deformation by drawing, and annealing of undeformed or drawn materials. Higher molecular weight leads to stronger intermolecular forces, increasing the tensile modulus. Increased crystallinity and deformation by drawing both enhance the resistance to deformation, while the effect of annealing depends on the material and specific annealing conditions.

Step by step solution

01

(a) Molecular weight

Higher molecular weight in a polymer leads to stronger intermolecular forces, such as van der Waals forces and entanglements between the polymer chains. This increased interaction between the polymer chains results in greater resistance to deformation, thus increasing the tensile modulus of the semicrystalline polymer.
02

(b) Degree of crystallinity

As the degree of crystallinity increases, the number of ordered regions in the polymer structure also increases. These ordered regions, or crystallites, are usually more resistant to deformation than the amorphous regions. Therefore, an increase in the degree of crystallinity leads to an increase in the tensile modulus of the semicrystalline polymer.
03

(c) Deformation by drawing

When a semicrystalline polymer is subjected to the process of drawing, it leads to the alignment of the polymer chains along the direction of deformation. This alignment of chains introduces greater order in the amorphous regions and can potentially increase the degree of crystallinity. As a result, the tensile modulus of the deformed polymer increases due to the enhanced resistance to the applied force.
04

(d) Annealing of an undeformed material

Annealing is a heat treatment process that allows an undeformed polymer to relax and reorganize its structure. During annealing, the polymer chains can move and rearrange themselves to form a more energetically stable, crystalline structure. This increase in the degree of crystallinity ultimately leads to a higher tensile modulus for the annealed undeformed polymer.
05

(e) Annealing of a drawn material

When a drawn polymer is annealed, the applied heat allows the polymer chains to relax and disentangle themselves. This relaxation process can potentially reduce the degree of crystallinity achieved during the drawing process. However, the effect of annealing on the tensile modulus depends on the specific material and annealing conditions. In some cases, it might increase the tensile modulus by promoting further crystallization, whereas in other cases, it might cause a decrease due to the relaxation of stresses introduced during the drawing process.

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