Chapter 15: Problem 44
Contrast compression, injection, and transfer molding techniques that are used to form plastic materials.
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Chapter 15: Problem 44
Contrast compression, injection, and transfer molding techniques that are used to form plastic materials.
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Briefly explain how each of the following influ\(\Theta\) ences 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
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?
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 time-dependent and initial (i.e., time = 0 ) stresses, respectively, and \(t\) and \(\tau\) denote elapsed time and the relaxation time, respectively; \(\tau\) is a time-independent 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 \(0.5 ;\) 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 \(3.5 \mathrm{MPa}\) (500 psi), which dropped to \(0.5 \mathrm{MPa}\) (70 psi) after \(30 \mathrm{~s}\).
For thermoplastic polymers, cite five factors that favor brittle fracture.
Fifteen kilograms of polychloroprene is vulcanized with \(5.2 \mathrm{~kg}\) of sulfur. What fraction of the possible crosslink sites is bonded to sulfur crosslinks, assuming that, on the average, \(5.5\) sulfur atoms participate in each crosslink?
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