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(a) molecular weight;

(b) degree of crystallinity;

(c) deformation by drawing;

(d) annealing of an undeformed material.

8.46* The tensile strength and number-average molecular weight for two

polyethylene materials are as follows:

Estimate the number-average molecular weight that is required to give a tensile

strength of 195 MPa.

8.47* For each of the following pairs of polymers, do the following: (1) state whether

or not it is possible to decide if one polymer has a higher tensile modulus than the other; (2) if this is possible, note which has the higher tensile modulus and then cite the reason(s) for your choice; and (3) if it is not possible to decide, then state why.

(a) Syndiotactic polystyrene having a number- average molecular weight of 400,000 g/ mol; isotactic polystyrene having a numberaverage molecular weight of 650,000 g/mol.

(b) Branched and atactic polyvinyl chloride with a weight-average molecular weight of 100,000 g/mol; linear and isotactic polyvinyl chloride having a

weight-average molecular weight of 75,000 g/mol.

(c) Random styrene-butadiene copolymer with 5% of possible sites crosslinked; block styrene-butadiene copolymer with 10% of possible sites crosslinked. (d) Branched polyethylene with a numberaverage molecular weight of 100,000 g/mol; atactic polypropylene with a number-average molecular weight of 150,000 g/mol.

8.50* For each of the following pairs of polymers, plot and label schematic

stress–strain curves on the same graph (i.e., make separate plots for parts a, b, c, and d).

(a) Isotactic and linear polypropylene having a weight-average molecular weight of 120,000 g/mol; atactic and linear polypropylene having a weight-average molecular weight of 100,000 g/mol.

(b) Branched polyvinyl chloride having a number-average degree of

polymerization of 2000; heavily crosslinked polyvinyl chloride having a

number-average degree of polymerization of 2000.

(c) Poly (styrene-butadiene) random copolymer having a number-average

molecular weight of 100,000 g/mol and 10%of the available sites crosslinked and tested at 20_C; poly(styrene-butadiene) random copolymer having a

number-average molecular weight of 120,000 g/mol and 15% of the available sites crosslinked and tested at _85_C. Hint: poly(styrene-butadiene) copolymers may exhibit elastomeric behavior.

(d) Polyisoprene, molecular weight of 100,000 g/mol having 10% of available sites crosslinked; polyisoprene, molecular weight of 100,000 g/mol having 20% of available sites crosslinked. Hint: polyisoprene is a natural rubber that may display elastomeric behavior.

8.51 List the two molecular characteristics that are essential for elastomers.

8.52 Which of the following would you expect to be elastomers and which