Chapter 15: Problem 44
Contrast compression, injection, and transfer molding techniques that are used to form plastic materials.
Chapter 15: Problem 44
Contrast compression, injection, and transfer molding techniques that are used to form plastic materials.
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Get started for freeIn your own words, briefly describe the phenomenon of viscoelasticity.
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), (b), and (c)] (a) Isotactic and linear polypropylene havng a weight-average molecular weight of \(120,000 \mathrm{~g} / \mathrm{mol}\); atactic and linear polypropyene having a weight-average molecular weight of \(100,000 \mathrm{~g} / \mathrm{mol}\) (b) Branched poly(vinyl chloride) having a degree of polymerization of 2000 ; heavily crosslinked poly(vinyl chloride) having a degree of polymerization of 2000 (c) Poly(styrene-butadiene) random copolymer having a number-average molecular weight of \(100,000 \mathrm{~g} / \mathrm{mol}\) and \(10 \%\) of the available sites crosslinked and tested at \(20^{\circ} \mathrm{C}\); poly(styrene- butadiene) random copolymer having a number-average molecular weight of \(120,000 \mathrm{~g} / \mathrm{mol}\) and \(15 \%\) of the available sites crosslinked and tested at \(-85^{\circ} \mathrm{C}\). Hint: poly(styrene- butadiene) copolymers may exhibit elastomeric behavior.
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 \(60 \mathrm{~s}\).
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 \(195 \mathrm{MPa}\).
The vulcanization of polyisoprene is accomplished with sulfur atoms according to Equation 15.4. If \(57 \mathrm{wt} \%\) sulfur is combined with polyisoprene, how many crosslinks will be associated with each isoprene repeat unit if it is assumed that, on the average, six sulfur atoms participate in each crosslink?
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