Molecular weight for a thermoplastic
1. Calculate the weight average molecular weight for a thermoplastic that has the following weight fractions
for the molecular weight ranges listed. Hint: consult pages 17 and 18 of lecture file 210426.
Molecular Weight Range
(g/mol)
Weight Fraction Molecular Weight Range
(g/mol)
Weight
Fraction
0 – 5,000 0.01 20,000 – 25,000 0.19
5,000 – 10,000 0.04 25,000 – 30,000 0.21
10,000 – 15,000 0.16 30,000 – 35,000 0.15
15,000 – 20,000 0.17 35,000 – 40,000 0.07
2. The average molecular weight of a random poly(isobutylene-isoprene) copolymer is known to be
77,630 g/mol with a degree of polymerization of 1250. (a) Determine the number fraction of each
of the two components. (b) Which type(s) of copolymer(s) will this copolymer be, considering the
following possibilities: random, alternating, graft, and block? Why?
3. Compute the weight percent sulfur that must be added to completely crosslink an alternating
acrylonitrile-butadiene copolymer, assuming that four sulfur atoms participate in each crosslink.
4. A polymer material, subjected to a constant strain of 0.5, undergoes stress relaxation from 3.5 MPa
to 0.5 MPa in 30 seconds. What is its modulus of elasticity at 10 second?
5. Consider a unidirectional Kevlar 49-fiber-epoxy composite material containing 63 percent by volume of
Kevlar 49 fibers. Calculate (a) density, (b) modulus of elasticity under isostrain condition, and (c) modulus of
elasticity under isostress condition.
Material Density (g/cm3
) Modulus of Elasticity (GPa)
Kevlar 49 1.44 207
Epoxy Resin 1.20 4.50
6. A unidirectional carbon-fiber-epoxy-resin composite contains 65 percent by volume of carbon fiber and 35
percent epoxy resin. Calculate for the composite (a) the average density, (b) the weight percentages of
carbon fibers and epoxy, (c) the maximum possible tensile modulus of elasticity, (d) the minimum tensile
modulus of elasticity, (e) the maximum tensile strength, and (f) the fraction of the tensile load carried by the
carbon fibers.
Material Density (g/cm3
) Modulus of Elasticity (GPa) Tensile Strength (MPa)
Carbon Fiber 1.79 340 2140
Epoxy Resin 1.20 4.50 64