Chemistry Reference
In-Depth Information
which does not curve away from the
c
axis at the high concentrations. At very
low concentrations, such plots may also curve upward. This effect is thought to
be due to absorption of polymer on the capillary walls and can be eliminated by
avoiding such high dilutions.
EXAMPLE 3-3
The tables below give the mean flow times (t) in a suspended-level viscometer recorded for
solutions of two of five monodisperse samples of polystyrene at various concentrations (c) in
cyclohexane at 34
C. Under these conditions, the mean flow time (t
0
) for cyclohexane is
151.8 s.
Sample B
c, 310
3
g/cm
3
t,s
1.586
158.5
3.172
166.5
Sample E
c, 310
3
g/cm
3
t,s
1.040
176.1
2.080
209.20
Determine the intrinsic viscosities of these samples. The intrinsic viscosities of the other
three polystyrene samples were evaluated under the same conditions and are given in the fol-
lowing table together with M
w
values determined by light scattering.
[
η
], cm
3
/g
Polystyrene sample
M
w
, g/mol
A
37,000
15.77
B
102,000
C
269,000
42.56
D
690,000
68.12
E
2,402,000
Using these data together with the calculated values of intrinsic viscosities for samples B
and E, evaluate the constants of the MarkHouwinkSakurada equation for polystyrene in
cyclohexane at 34
C. What can you deduce about the conformation of the polystyrene chains
under the conditions of the viscosity determinations?
Solution
Sample B: t
0
5151:8s;
t
1
5166:5s;
t
1
5158:5s
2
½η 5
2
166
:
5
1
4
ð
158
:
5
Þ 2
3
ð
151
:
8
Þ
3
10
23
:
172
3
3
151
:
8
525:13 cm
3
=g
