Chemistry Reference
In-Depth Information
can be applied. For t
his
group
(i
n which
M
w
5
2
M
n
), whole polymers can be used
for calibration, and
M
w
and
M
n
can be obtained from solution viscosities. The
most important examples of this exceptional class of polymers are commercial
fiber-forming nylons and poly(ethylene terephthalates).
The procedure described for calibration of
K
and
a
is laborious because of the
required fractionation process. The two constants are derived as described from
the intercept and slope of a linear least squares fit to [
M
values for a series of
fractionated polymers. Experimentally,
K
and
a
are found to be inversely corre-
lated. If different laboratories determine these MHS constants for the same poly-
mer
η
]
solvent comb
ina
tion, the data set yielding the higher
K
value will produce
the lower
a
. Thus,
M
v
from
Eq. (3-44)
is often essentially the same for different
K
and
a
values, provided the molecular weight ranges of the samples used in the
two calibration processes overlap.
We have assumed so far that
K
and
a
are fixed for a given polymer type and
solvent and do not vary with polymer molecular weight. This is not strictly true.
Oligomers (less than about 100 repeating units in most vinyl polymers) often con-
form to
KM
0
:
5
v
½η 5
(3-86)
with
K
and the exponent
a
independent of the solvent. The MHS constants deter-
mined for higher-molecular-weight species may depend on the molecular weight
range, however. Tabulations of such constants therefore usually list the molecular
weights of the fractions for which the particular
K
and
a
values were determined.
Table 3.1
presents such a list of some common systems of more general interest.
An alternative procedure for determining the MHS constants from gel perme-
ation chromatography is given in
Section 3.4.3
, after the latter technique has been
described.
3.3.3
Measurement of Intrinsic Viscosity
Laboratory devices are available to measure intrinsic viscosities without human
intervention. These are useful where many measurements must be made. The
basic principles involved are the same as those in the glass viscometers which
have long been used for this determination. An example of the latter is the
Ubbelohde suspended level viscometer shown in
Fig. 3.7
. In this viscometer a
given volume of polymer solution with known concentration is delivered into
bulb B through stem A. This solution is transferred into bulb C by applying a
pressure on A with column D closed off. When the pressure is released, any
excess solution drains back into bulb B and the end of the capillary remains free
of liquid. The solution flows from C through the capillary and drains around the
sides of the bulb E. The volume of fluid in B exerts no effect on the rate of flow
through the capillary because there is no back pressure on the liquid emerging
from the capillary. The flow time
t
is taken as the time for the solution meniscus
to pass from mark
a
to mark
b
in bulb C above the capillary. The solution in D
