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Chapter 15: Problem 24

Which of the following would you expect to be elastomers and which thermosetting polymers at room temperature? Justify each choice. (a) Epoxy having a network structure (b) Lightly crosslinked poly(styrenebutadiene) random copolymer that has a glass transition temperature of \(-50^{\circ} \mathrm{C}\) (c) Lightly branched and semicrystalline polytetrafluoroethylene that has a glass transition temperature of \(-100^{\circ} \mathrm{C}\) (d) Heavily crosslinked poly(ethylenepropylene) random copolymer that has a glass transition temperature of \(0^{\circ} \mathrm{C}\) (e) Thermoplastic elastomer that has a glass transition temperature of \(75^{\circ} \mathrm{C}\)

Short Answer

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Question: Categorize each of the following polymers as elastomers or thermosetting polymers at room temperature: (a) Epoxy having a network structure (b) Lightly crosslinked poly(styrenebutadiene) random copolymer with Tg = \(-50^{\circ} \mathrm{C}\) (c) Lightly branched and semicrystalline polytetrafluoroethylene with Tg = \(-100^{\circ} \mathrm{C}\) (d) Heavily crosslinked poly(ethylenepropylene) random copolymer with Tg = \(0^{\circ} \mathrm{C}\) (e) Thermoplastic elastomer with Tg = \(75^{\circ} \mathrm{C}\) Answer: (a) Thermosetting polymer (b) Elastomer (c) Neither elastomer nor thermosetting polymer (d) Thermosetting polymer (e) Thermosetting polymer at room temperature

Step by step solution

01

(a) Epoxy having a network structure

Epoxy resins are known to have a high degree of crosslinking, which makes their structure rigid and strong. This structural property categorizes them as thermosetting polymers at room temperature. Justification: Network structure with high degree of crosslinking leads to rigid and strong materials, characteristic of thermosetting polymers.
02

(b) Lightly crosslinked poly(styrenebutadiene) random copolymer with Tg = \(-50^{\circ} \mathrm{C}\)

The glass transition temperature of this polymer is lower than room temperature. Moreover, it is lightly crosslinked, which implies that it can be easily deformed. Thus, it belongs to the elastomer category. Justification: Tg lower than room temperature and light crosslinking imply elasticity, characteristic of elastomers.
03

(c) Lightly branched and semicrystalline polytetrafluoroethylene with Tg = \(-100^{\circ} \mathrm{C}\)

Although the glass transition temperature is lower than room temperature, the semicrystalline structure and light branching make it a less flexible material compared to typical elastomers. It might not be classified as an elastomer, but it is also not a thermosetting polymer. Justification: Low Tg but semicrystalline structure and light branching prevent this material from being elastic enough to be classified as an elastomer.
04

(d) Heavily crosslinked poly(ethylenepropylene) random copolymer with Tg = \(0^{\circ} \mathrm{C}\)

The glass transition temperature is lower than room temperature, but the high degree of crosslinking in this polymer makes it rigid and robust. Therefore, it can be considered a thermosetting polymer at room temperature. Justification: Despite the low Tg, heavy crosslinking leads to rigid and strong materials, characteristic of thermosetting polymers.
05

(e) Thermoplastic elastomer with Tg = \(75^{\circ} \mathrm{C}\)

The glass transition temperature of this thermoplastic elastomer is higher than room temperature, which implies that it behaves like a rigid and strong material at room temperature, making it a thermosetting polymer in these conditions. Justification: Tg higher than room temperature leads to rigid and strong behavior, characteristic of thermosetting polymers at room temperature.

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Most popular questions from this chapter

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 timeindependent constant characteristic of the material. A specimen of a viscoelastic polymer whose stress relaxation obeys Equation \(15.10\) was suddenly pulled in tension to a measured strain of \(0.6\); the stress necessary to maintain this constant strain was measured as a function of time. Determine \(E_{r}(10)\) for this material if the initial stress level was \(2.76\) MPa (400 psi), which dropped to \(1.72 \mathrm{MPa}\) (250 psi) after \(60 \mathrm{~s}\).

(a) Compare the fatigue limits for polystyrene (Figure 15.11) and the cast iron for which fatigue data are given in Problem 8.20. (b) Compare the fatigue strengths at \(10^{6} \mathrm{cy}\) cles for poly(ethylene terephthalate) (PET, Figure 15.11) and red brass (Figure 8.34).

The tensile strength and number-average molecular weight for two polyethylene materials are as follows: \begin{tabular}{cc} \hline Tensile Strength (MPa) & Number-Average Molecular Weight \((\mathrm{g} / \mathbf{m o l})\) \\ \hline 85 & 12,700 \\ 150 & 28,500 \\ \hline \end{tabular} Estimate the number-average molecular weight that is required to give a tensile strength of \(195 \mathrm{MPa}\).

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.

On the basis of the curves in Figure \(15.5\), sketch schematic strain-time plots for the following polystyrene materials at the specified temperatures: (a) Crystalline at \(70^{\circ} \mathrm{C}\) (b) Amorphous at \(180^{\circ} \mathrm{C}\) (c) Crosslinked at \(180^{\circ} \mathrm{C}\) (d) Amorphous at \(100^{\circ} \mathrm{C}\).
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