1. The figure below shows a TEM micrograph of a rubber toughened polystyrene, PS, in which the rubber particles are indicated by arrows. The rubber, which has a Tg of -60℃, can decrease the Tg of the PS (which has a Tg of 100℃), however
The fracture toughness of the sample is 15 kJ/m2 vs 0.4 for pure PS. A typical method for measuring this property involves placing a small crack in the sample, applying a stress and measuring its resistance to brittle fracture, as seen in the figure below. Discuss the mechanism(s) by which the rubber increases the fracture toughness of HIPS relative to pure PS.
. How much rubber must be added in order to decrease the Tg of PS to 65℃?
2. Consider unidirectionally reinforced glass/epoxy composites. The fibers are continuous and 60% by volume. The tensile strength of glass fibers is 1 GPa and the Young’s modulus is 70 GPa. The tensile strength of the epoxy matrix is 60 MPa and its Young’s modulus is 3GPa. Compute the Young’s modulus and the tensile strength of the composite in the longitudinal direction.
3. A polyethylene sample was cooled from 160 C, where is a liquid to a temperature of
126 C. The following table gives the density of poly(ethylene) as a function of time at the temperature of 126 C. Note that the clock was started when the polymer reached the temperature of 126 C. Why is the density changing as a function of time? What information can you extract from this data? Analyze this data with the appropriate theory. Is the theory applicable to the whole time scale or only to a part of it?
4. Discuss the mechanism(s) by which the toughness increases from 0.5 to 2.2 to 7 MNm-3/2, respectively, for a pure epoxy resin, versus a rubber-toughened epoxy versus a glass fiber reinforced epoxy.
5. O-rings made of the semicrystalline polymer DelrinTM (polyoxymethylene) are in hydraulic pumps and other systems which results in their exposure to hydraulic fluids and water. A comparison of injection molded o-rings, IM to those that had been compression molded, CM, showed that the IM o-rings exhibited much more significant swelling, loss of dimensional tolerance and wear.
(a) Explain these differences in terms if the morphology of the IM vs. CM o-rings.
(b) How do you think adding a small percentage (eg., 2 wt%) of layered silicate nanoparticles to the polymer would affect the swelling, dimensional stability and wear properties?
(c) How do you characterize the dispersion of nanosilicates and morphologies in polymer–clay nanocomposites.
6. A linear homopolymer was crystallized from the melt at crystallization temperatures (Tc) in the range 270K to 330K. Following complete crystallization, DSC was used to measure melting points, and they are listed in the table below. Determine graphically the equilibrium melting temperature, Tom.
Small-angle x-ray scattering experiments using Cu Kα radiation gave the positions of the first maxima as:
Using the Bragg equation calculate the values of the long period for each crystallization temperature. The degree of crystallinity was measured to be 45 percent for all samples. The lamellar thickness, l, can be calculated from l = d * degree of Crystallinity. Calculate the lamellar thickness in each case and determine graphically the fold surface energy, σe, if the enthalpy of fusion per unit volume is 1.5×108 J/m3.
7. Toothbrushes are available with soft, medium and hard bristles, generally made of polyamides (i.e. nylons). (a) Propose at least two distinctly different ways by which the bristles can be controlled to provide soft, medium or hard performance. (b)If you bristles from different toothbrushes, what tests or experiments would you perform to determine the ways that were actually used to control hardness? (c) Why polyamides?
8. Thermoplastic polyurethane(a multiphase copolymer) samples were fabricated using injection molding and compression molding, resulting in one being clear and one opaque. On the attached XRD and DMA data, please indicate which corresponds to the clear sample and which corresponds to the opaque. Provide a reason for your choice, including reasons for the relative differences in the data.
10. Match each statement on the left with one topic from the box
a) This staining agent is often used to improve contrast between phases for TEM.
b) The square root of the cohesive energy density.
c) Crystal orientation in fibers.
d) Useful technique for studying domain sizes in block copolymers, blends, and nanocomposites from 5-100 nm.
e) Defines a reinforcement efficiency to obtain a better estimate than the inverse rule of mixtures.
f) The phase separation kinetics for microstructures found between the spinodal and bimodal curves.
g) Polymerization of monomer II in the presence of polymer I.
(B) True or False
a) Increasing molecular weight increases entropy of mixing
b) As the difference in solubility parameters between two components increases, the FloryHuggins interaction parameter, χ1, decreases.
c) Most polymer blends have a phase behavior exhibiting a lower critical solution temperature.
e) If the Gibbs free energy of mixing, ΔGm, is less than zero, than phase separation will occur.
f) If the Flory-Huggins interaction parameter, χ1, is negative, the Flory Huggins theory will always yield a negative value of ΔGm.
g) The Stress Intensity Factor, K, is proportional to the stress near a crack tip.
h) Liquid crystalline polyaramid can be characterized by a two or more first order endothermic peak in DSC and a skin-core microscopic morphology.
l) Polymer-polymer phase behavior is controlled by the following four factors: (1) choice of monomer, (2) molecular architecture, (3) composition, and (4) molecular size.
11. Poly(cy) (Tg=140℃) is mixed with Poly(clones) (Tg= –40℃) at a 65/35 weight ratio. You measure the glass transition temperature of the blend and find two distinct glass transitions, one at –5℃ and one at 100 ℃.
(a) What is the composition of each phase?
(b) What is the volume fraction of each phase (assume equal densities)?
12. Briefly describe (2 or 3 lines) how you would produce the following structures from a given sample of polyethylene (HDPE):
a) single crystals
b) oriented crystals
c) spherulites
d) shish kebab struture
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