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Chapter 14: Problem 22

Explain briefly why the tendency of a polymer to crystallize decreases with increasing molecular weight.

Short Answer

Expert verified
Answer: The tendency of a polymer to crystallize decreases with increasing molecular weight because as the molecular weight increases, the chains become longer and more entangled. This results in reduced mobility of the chains and makes it more difficult for them to align and pack together to form a crystalline structure. Therefore, the entangled chains have a harder time overcoming conformational barriers and adopting the ordered arrangement required for crystallization.

Step by step solution

01

1. Define Crystallization

Crystallization is the process by which a polymer transforms from a disordered, amorphous state into a more ordered, crystalline state. This occurs as a result of the close packing of polymer chains, which leads to a reduction in free volume and an increase in density. The polymer's properties, such as mechanical strength, stiffness, and thermal stability, are generally improved in the crystalline state as compared to the amorphous state.
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2. The Role of Molecular Weight in Polymer Chains

Molecular weight is a measure of the total mass of atoms present in a single polymer chain. Polymers with high molecular weights have longer chains and more monomer units. The molecular weight is an important factor that affects various physical properties of the polymer, such as melt viscosity, tensile strength, and stiffness. In the context of crystallization, the length of the polymer chains directly influences the ability of the chains to pack together and form a crystalline structure.
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3. Factors Affecting Crystallizability of Polymers

There are several factors that affect the crystallizability of a polymer. These include the chain regularity, chain stiffness, and attractive forces between chains. In relation to molecular weight, the main factor to consider is the entanglement of polymer chains. As the molecular weight increases, the chains become longer and more entangled, which hinders their mobility and ability to align and pack together. This makes the crystallization process more difficult, as it takes more time and energy for the chains to disentangle and rearrange themselves into a crystalline structure.
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4. The Relationship between Molecular Weight and Crystallization Tendency

As the molecular weight of a polymer increases, the chains become longer and more entangled. This results in a reduced mobility of the chains and a consequent decrease in their ability to align and pack together to form a crystalline structure. Overall, the tendency of a polymer to crystallize decreases with increasing molecular weight because it becomes more difficult for the entangled chains to overcome their conformational barriers and adopt the ordered arrangement required for crystallization.

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Most popular questions from this chapter

For a linear freely rotating polymer molecule, the total extended chain length \(L\) depends on the bond length between chain atoms \(d\), the total number of bonds in the molecule \(N\), and the angle between adjacent backbone chain atoms \(\theta\), as follows: $$ L=N d \sin \left(\frac{\theta}{2}\right) $$ Furthermore, the average end-to-end distance \(r\) for a randomly winding polymer molecule in Figure \(14.6\) is equal to $$ r=d \sqrt{N} $$ A linear polytetrafluoroethylene has a numberaverage molecular weight of \(500,000 \mathrm{~g} / \mathrm{mol}\); compute average values of \(L\) and \(r\) for this material.

Calculate the number-average molecular weight of a random nitrile rubber [poly (acrylonitrile-butadiene) copolymer] in which the fraction of butadiene repeat units is \(0.30\) assume that this concentration corresponds to a degree of polymerization of 2000 .

Sketch cis and trans structures for (a) butadiene, and (b) chloroprene. Use twodimensional schematics per footnote 11 of this chapter.

The density of totally crystalline polypropylene at room temperature is \(0.946 \mathrm{~g} / \mathrm{cm}^{3}\). Also, at room temperature the unit cell for this material is monoclinic with the following lattice parameters: $$ \begin{array}{ll} a=0.666 \mathrm{~nm} & \alpha=90^{\circ} \\ b=2.078 \mathrm{~nm} & \beta=99.62^{\circ} \\ c=0.650 \mathrm{~nm} & \gamma=90^{\circ} \end{array} $$ If the volume of a monoclinic unit cell, \(V_{\text {mono }}\) is a function of these lattice parameters as $$ V_{\text {mono }}=a b c \sin \beta $$ determine the number of repeat units per unit cell.

Compute repeat unit molecular weights for the following: (a) poly(vinyl chloride), (b) poly(ethylene terephthalate), (c) polycarbonate, and (d) polydimethylsiloxane.
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