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Unformatted text preview: 1) a) T g is always lower than T m (if the polymer has a T m ). Below T g , the polymer is glassy i.e. there is not enough thermal energy for it to move. Therefore, there can be no crystallization or melting below T g . b) Commercial polypropylene is isotactic and thus semicrystalline. Commercial polystyrene is atactic and amorphous. Pure amorphous polymers are typically transparent whereas semicrystalline polymers are translucent or even opaque. c) In one of the lectures, we noted that Tg is marked by an abrupt change in either heat capacity or thermal expansion. Hence, a couple ways to measure Tg: i) Calorimetry. See how temperature changes with heat input (related to heat capacity). ii) Volume measurement as a function of temperature. Thermal expansion can be measured this way. Another way to get a measure of Tg: iii ) Mechanical property measurement, for example measurement of the storage modulus E. T g is marked by a relatively abrupt drop in the T g due to transition from glassy to rubbery. d) Upper and lower critical solution temperatures. They are transition temperatures in the phase behavior of a binary mixture. Above the UCST or below the LCST, the mixture is homogeneous one phase at all compositions. e) Because polymers are large chain molecules, there is little entropy gained in mixing two polymers. Therefore, the enthalpic interaction must be very favorable for the two to mix. f) M w is always larger than M n because the larger molecules count for more (they are weighted more) in the averaging by weight. For a proof of this, see the end of solution to exam #1. g) x n is the number-average degree of polymerization, i.e. the average number of repeat units per molecule. It is related to M n by x n = M n / M o where M o is the mol. wt. of the repeat unit. In step polymerization, x n is 1/(1-p) where p is the conversion. Important: This is only for the case of step polymerization where the reacting species start with stoichiometric / equal quantity. x n can be measured by measuring M n and knowing what the repeat unit is. h) Because the principle of GPC is the use of porous beads to create separation by size or volume, not by mass. For the same polymer dissolved in the same solvent, volume and mass are directly related, i.e. larger volume means larger mass. However, if you are comparing different polymers , larger volume doesnt always mean larger mass. i) Ideal: = 0.5 and a = 0.5. Poor: > 0.5 and a < 0.5. Good: < 0.5 and a > 0.5 j) No. The K in the light scattering is called the optical constant. It has a completely different physical meaning from that of the Mark-Houwink constant K. The units are also different. k) A couple possible interpretations: i) [ ] = (lim c 0) [( o ) / o ]/c. [ ] is therefore the initial slope of viscosity change versus concentration. [ ] tells us how the viscosity will increase from the pure solvent viscosity as we add a tiny amount of polymer....
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