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Chapter 15: Problem 21

Would you expect the tensile strength of polychlorotrifluoroethylene to be greater than, the same as, or less than that of a polytetrafluoroethylene specimen having the same molecular weight and degree of crystallinity? Why?

Short Answer

Expert verified
Answer: Polytetrafluoroethylene (PTFE) has a higher tensile strength than polychlorotrifluoroethylene (PCTFE) due to its more symmetrical structure and tighter packing, which result from the presence of only fluorine atoms. The presence of chlorine atoms in PCTFE disrupts this symmetry and reduces molecular packing, leading to a lower tensile strength.

Step by step solution

01

Understand the molecular structures of polychlorotrifluoroethylene and polytetrafluoroethylene

Before comparing the tensile strength of these materials, it's essential to have a basic understanding of their molecular structures. Polychlorotrifluoroethylene (PCTFE) is a polymer formed by replacing one fluorine atom in -CF2- group of polytetrafluoroethylene (PTFE) with a chlorine atom, resulting in a -CClF- group. PTFE, on the other hand, consists only of perfluorinated ethylene units (-CF2-) with no chlorine atoms.
02

Analyze the structural implications of the chlorine substitution in polychlorotrifluoroethylene

In PCTFE, the presence of the chlorine atom disrupts the symmetry of PTFE. This results in a less uniform and less tightly packed structure because of the larger size of the chlorine atom compared to the fluorine atom and the less favorable interactions between chlorine and fluorine atoms. In PTFE, the regular and symmetrical nature allows the chains to pack closely together, resulting in a more crystalline structure.
03

Understand the impact of molecular packing on tensile strength

The degree of crystallinity and molecular packing can impact a polymer's tensile strength. In general, a more crystalline and tightly packed material will be stronger and more resistant to break, while a less crystalline and more amorphous structure will be weaker.
04

Compare the tensile strength of polychlorotrifluoroethylene and polytetrafluoroethylene

Given the difference in structure between PCTFE and PTFE, we can infer that the tensile strength of PCTFE with the same molecular weight and degree of crystallinity might be less than that of PTFE. This is because the presence of the chlorine atom in PCTFE reduces the overall molecular packing, making it more susceptible to break under stress. In conclusion, one would expect the tensile strength of polychlorotrifluoroethylene to be less than that of polytetrafluoroethylene specimen having the same molecular weight and degree of crystallinity due to the structural differences induced by the presence of chlorine atoms in PCTFE.

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Most popular questions from this chapter

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

For each of the following pairs of polymers, plot and label schematic stress- strain curves on the same graph [i.e., make separate plots for parts (a) to \((\mathrm{c})]\) (a) Polyisoprene having a number-average molecular weight of \(100,000 \mathrm{~g} / \mathrm{mol}\) and \(10 \%\) of available sites crosslinked; polyisoprene having a numberaverage molecular weight of \(100,000 \mathrm{~g} / \mathrm{mol}\) and \(20 \%\) of available sites crosslinked (b) Syndiotactic polypropylene having a weightaverage molecular weight of \(100,000 \mathrm{~g} / \mathrm{mol}\); atactic polypropylene having a weight- average molecular weight of \(75,000 \mathrm{~g} / \mathrm{mol}\) (c) Branched polyethylene having a numberaverage molecular weight of \(90,000 \mathrm{~g} / \mathrm{mol}\); heavily crosslinked polyethylene having a number- average molecular weight of \(90,000 \mathrm{~g} / \mathrm{mol}\)

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 cite the reason(s) for your choice; and (3) if it is not possible to decide, state why. (a) Linear and isotactic poly(vinyl chloride) with a weight-average molecular weight of \(100,000 \mathrm{~g} /\) mol; branched and atactic poly(vinyl chloride) having a weight-average molecular weight of \(75,000 \mathrm{~g} / \mathrm{mol}\) (b) Graft acrylonitrile-butadiene copolymer with \(10 \%\) of possible sites crosslinked; alternating acrylonitrile-butadiene copolymer with \(5 \%\) of possible sites crosslinked (c) Network polyester; lightly branched polytetrafluoroethylene

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

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