Page 88 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Structure (Morphology) 51

Mer: Repeat unit.
Modulus: Ratio of stress to strain, which is a measure of the stiffness of a polymer.
Monodisperse: Polymer mixture made up of molecules of one specific molecular weight.

Pendant groups: Groups attached to the main polymer backbone, like methyl groups in
polypropylene.
Polydisperse: Polymer mixture containing chains of varying lengths.
Radius of gyration (S): Root-mean-square distance of a chain end to a polymer’s center of
gravity.
1/2
2 ½
Root-mean-square distance ([r ] ): Average end-to-end distance of polymer chains; l(n ).
Side-chain crystallization: Crystallization related to that of regularly spaced long pendant
groups.
Single polymer crystals: Lamellar structure consisting of folded chains of a linear polymer.
Spherulites: Three-dimensional aggregates of polymer crystallites.
Stiffening groups: Groups in a polymer backbone that decrease the segmental motion of the
polymer chain.
Syndiotactic: Polymer in which the pendant groups are arranged alternately on each side of
the carbon backbone.
Tacticity: Arrangement of the pendant groups in space; that is, isotactic, syndiotactic, and
atactic.
Thermoplastic: A linear polymer that softens when heated.
Thermoset: A network polymer containing chemical cross-linking that does not soften when
heated.
van der Waals forces: Secondary forces based on the attraction between groups.
Viscosity: Measure of the resistance of a polymer or polymer solution to fl ow.

EXERCISES

(To answer some of these questions you may need to look at other parts in the book for structures
and specifi c details.)
1. Make sketches or diagrams showing (a) a linear polymer, (b) a polymer with pendant groups,
(c) a polymer with short branches, (d) a polymer with long branches, and cross-linked poly-
mers with (e) low, and (f) high cross-linked density.
2. Which has (a) the greater volume for the same weight of material and (b) the lower softening
point: HDPEC or LDPE?
3. What is the approximate bond length of the carbon atoms in (a) a linear and (b) a cross-linked
polymer.
4. What is the approximate contour length of a HDPE chain with an average degree of polymeri-
zation (chain length) of n = 2,000 and of a PVC chain of the same number of repeating units?
5. Which of the following are monodisperse polymers with respect to chain length? (a) hevea rub-
ber, (b) corn starch, (c) cellulose from cotton, (d) an enzyme, (e) HDPE, (f) PVC, (g) a specifi c
DNA, (h) nylon-66, or (i) a specifi c RNA?
6. What is the average degree of polymerization of LDPE having an average molecular weight of
28,000?
7. What is the structure of the repeating unit in (a) polypropylene, (b) poly(vinyl chloride, and
(c) hevea rubber?
8. Which of the following is a branched chain polymer: (a) HDPE, (b) Isotactic PP, (c) LDPE, or
(d) amylose starch?
9. Which of the following is a thermoplastic: (a) ebonite, (b) Bakelite, (c) vulcanized rubber,
(d) HDPE, (e) celluloid, (f) PVC, or (g) LDPE?
10. Which has the higher cross-linked density: (a) ebonite or (b) soft vulcanized rubber?

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