91Ó°ÊÓ

In your own words, briefly describe the phenomenon of viscoelasticity.

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 \(30 \mathrm{s}\).

(a) Contrast the manner in which stress relaxation and viscoelastic creep tests are conducted. (b) For each of these tests, cite the experimental parameter of interest and how it is determined.

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

Briefly explain how each of the following influences the tensile or yield strength of a semicrystalline polymer and why: (a) Molecular weight (b) Degree of crystallinity (c) Deformation by drawing (d) Annealing of an undeformed material

Normal butane and isobutane have boiling temperatures of -0.5 and \(-12.3^{\circ} \mathrm{C}(31.1\) and \(\left.9.9^{\circ} \mathrm{F}\right),\) respectively. Briefly explain this behavior on the basis of their molecular structures, as presented in Section 14.2.

The tensile strength and number-average molecular weight for two poly(methyl methacrylate) materials are as follows: $$\begin{array}{cc}\hline \begin{array}{c}\text { Tensile Strength } \\\\\text { (MPa) }\end{array} & \begin{array}{c}\text { Number-Average } \\ \text { Molecular Weight (g/mol) }\end{array} \\\\\hline 50 & 30,000 \\\150 & 50,000 \\\\\hline\end{array}$$ Estimate the tensile strength at a numberaverage molecular weight of \(40,000 \mathrm{g} / \mathrm{mol}\).

List the two molecular characteristics that are essential for elastomers.

Which of the following would you expect to be elastomers and which thermosetting polymers at room temperature? Justify each choice. (a) Linear and highly crystalline polyethylene (b) Phenol-formaldehyde (c) Heavily crosslinked polyisoprene having a glass-transition temperature of \(50^{\circ} \mathrm{C}\) \(\left(122^{\circ} \mathrm{F}\right)\) (d) Lightly crosslinked polyisoprene having a glass-transition temperature of \(-60^{\circ} \mathrm{C}\) \(\left(-76^{\circ} \mathrm{F}\right)\) (e) Linear and partially amorphous poly(vinyl chloride)

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?