Chemistry Reference
In-Depth Information
3-9 The Mark
Houwink relation for polypropylene in
o
-dichlorobenzene at
130
C was calibrated as follows. A series of sharp fractions of the poly-
mer was obtained by fractionation, and the molecular weight of each frac-
tion was determined by membrane osmometry in toluene at 90
C. The
samples were then dissolved in
o
-dichlorobenzene at 130
C and their
intrinsic viscosities ([
η
]) were measured. The resulting data fitted an
expression of the form
ln
½η 5
ln
K
a
ln
M
n
1
where
K
and
a
are the desired Mark
Houwink constants. It was concluded
from this that intrinsic viscosities of all polypropylene polymers in the
appropriate molecular weight range could be represented by
KM
n
½η 5
Is this conclusion correct? Justify your answer very briefly.
3-10 What molecular weight measurement methods could be used practically to
dete
rm
ine the following?
(a)
M
n
of a soluble polymer from reaction of glycol, phthalic anhydride,
and acetic acid. The approximate molecular weight of this sample is
kn
own to be about 1200.
(b)
M
w
of a polystyrene with molecular weight about 500,000.
(c)
M
n
of high-molecular-weight styrene-methyl methacrylate copolymer
with uncertain styrene content.
3-11
In an ideal membrane osmometry experiment a plot of
π
/cRT
against
c
is
a straight
line with intercept 1
/M
. Similarly, an ideal
light-scattering
experiment at zero viewing angle yields a straight
τ
against
c
with intercept 1
/M
. For a given polymer sample, solvent, and
temperature,
(a) Are the
M
values the same from osmometry and light scattering?
(b) Are the slopes of the straight-line plots the same?
Explain your answers briefly.
line plot of
Hc/
3-12 A sample of poly(hexamethylene adipamide) weighs 4.26 g and is found
to contain 4
10
2
3
m
ol
COOH groups by titration with alcoholic KOH.
From this information
M
n
of the polymer is calculated to be 2100. What
assumption(s) is (are) made in this calculation?
3-13 A dilute polymer solution has a turbidity of 0.0100 cm
2
1
. Assuming that
the solute molecules are small compared to the wavelength of the incident
light, calculate the ratio of the scattered to incident light intensities at a
90
angle to the incident beam and 20 cm from 2 mL of solution. Assume
that all the solution is irradiated.
3
