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Chapter 15: Problem 11

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 (f) Annealing of a drawn material

Short Answer

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Answer: (a) Molecular weight influences the tensile modulus by increasing chain entanglements, which effectively transmit stress and resist deformation under an applied force. (b) Degree of crystallinity increases tensile modulus as crystalline regions have stronger intermolecular forces and stiffer material behavior. (c) Deformation by drawing increases tensile modulus along the draw direction by increasing molecular orientation and tightening polymer chains, while decreasing it perpendicular to the draw direction. (d) Annealing undeformed material can increase tensile modulus by allowing better molecular packing through relaxation and recrystallization. (f) Annealing drawn material can decrease tensile modulus by allowing the polymer chains to partially revert to their random coiled conformation due to relaxation and disentanglement.

Step by step solution

01

(a) Effect of Molecular Weight

The tensile modulus of a semicrystalline polymer is influenced by its molecular weight. As the molecular weight increases, the entanglement of polymer chains becomes more pronounced. This leads to an increase in tensile modulus, as higher molecular weight materials have more chain entanglements that can effectively transmit stress and resist deformation under an applied force.
02

(b) Effect of Degree of Crystallinity

A higher degree of crystallinity in a semicrystalline polymer leads to a higher tensile modulus. This is because crystalline regions are tightly and regularly packed, resulting in stronger intermolecular forces and stiffer material behavior. As the degree of crystallinity increases, the fraction of the material with these strong intermolecular forces also increases, leading to an overall increase in tensile modulus.
03

(c) Effect of Deformation by Drawing

Deformation by drawing influences the tensile modulus of a semicrystalline polymer by increasing the degree of molecular orientation along the draw direction. This molecular orientation further aligns and tightens the polymer chains, resulting in an increase in tensile modulus along the draw direction and a decrease in tensile modulus perpendicular to the draw direction.
04

(d) Effect of Annealing Undeformed Material

Annealing an undeformed semicrystalline polymer allows the polymer chains to relax and, in some cases, recrystallize. This can lead to an increase in crystallinity and better molecular packing, resulting in an increase in tensile modulus. However, the specific change in tensile modulus depends on the annealing conditions and initial state of the undeformed material.
05

(f) Effect of Annealing Drawn Material

Annealing a drawn semicrystalline polymer can lead to a decrease in tensile modulus due to relaxation and disentanglement of the polymer chains. During drawing, the material experiences an increase in tensile modulus due to molecular orientation. However, annealing allows the polymer chains to partially revert to their random coiled conformation, leading to a decrease in tensile modulus, especially along the draw direction.

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Most popular questions from this chapter

The vulcanization of polyisoprene is accomplished with sulfur atoms according to Equation \(15.4 .\) If \(45.3 \mathrm{wt} \%\) sulfur is combined with polyisoprene, how many crosslinks will be associated with each isoprene repeat unit if it is assumed that, on the average, five sulfur atoms participate in each crosslink?

Nylon 6,6 may be formed by means of a condensation polymerization reaction in which hexamethylene diamine \(\left[\mathrm{NH}_{2}-\left(\mathrm{CH}_{2}\right)_{6}-\right.\) \(\left.\mathrm{NH}_{2}\right]\) and adipic acid react with one another with the formation of water as a byproduct. What masses of hexamethylene diamine and adipic acid are necessary to yield \(20 \mathrm{kg}\) of completely linear nylon \(6,6 ?\) (Note: the chemical equation for this reaction is the answer to Concept Check \(15.12 .\) )

For the vulcanization of polyisoprene, compute the weight percent of sulfur that must be added to ensure that \(10 \%\) of possible sites will be crosslinked; assume that, on the average, 3.5 sulfur atoms are associated with each crosslink.

For some viscoelastic polymers that are subjected to stress relaxation tests, the stress decays with time according to \(\sigma(t)=\sigma(0) \exp \left(-\frac{t}{\tau}\right)\) where \(\sigma(t)\) and \(\sigma(0)\) represent the timedependent and initial (i.e., time \(=0\) ) stresses, respectively, and \(t\) and \(\tau\) denote elapsed time and the relaxation time; \(\tau\) is a time independent constant characteristic of the material. A specimen of some viscoelastic polymer with the stress relaxation that obeys Equation 15.10 was suddenly pulled in tension to a measured strain of \(0.5 ;\) the stress necessary to maintain this constant strain was measured as a function of time. Deter\(\operatorname{mine} E_{r}(10)\) for this material if the initial stress level was 3.5 MPa \((500\) psi), which dropped to \(0.5 \mathrm{MPa}(70 \mathrm{psi})\) after \(30 \mathrm{s}\).

For thermoplastic polymers, cite five factors that favor brittle fracture.
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