9
[A.1-4]
[ ]
η
η
i
sp i
i
C
≅
where
[ ]
η
= intrinsic viscosity
The set of data points {
C
i
, [
η
]
i
,
V
i
} is thus obtained across the entire chromatogram. The first
pair of this set constitutes the
Intrinsic Viscosity Distribution
(IVD) of the polymer sample. Note
that it is obtained directly from the detector responses of the detector.
DERIVING MOLECULAR WEIGHT DISTRIBUTION
The
Molecular Weight Distribution
(MWD) can be obtained at least 3 ways:
(1)
Conventional Calibration
Conventional calibration generally calibrates the molecular weight or intrinsic viscosity of
a given substance to the retention volume. The only case where this can be rigorously held true
is when there is no variation of structure (including branching, conformation, etc.) between the
standards and the unknowns. Therefore, for each different structure or substance, a new
calibration curve must be made to obtain a true molecular weight or intrinsic viscosity
distribution. “Apparent molecular weights” referenced to standards such as polystyrene are
commonly used; however, this leads to disproportionate information if there are major structural
differences within the sample such as branching. This type of error can cause gross
underestimation of sample polydispersity.
In conventional calibration techniques, the absolute concentration is not required for the
sample measurement. The overriding concern is the ability to measure relative concentrations
along the elution profile. Therefore, the RI detector is often used to determine relative mass.
(2) Universal Calibration
By calibrating the column with primary molecular weight standards, one can convert the
intrinsic viscosity distribution into the molecular weight distribution via the Universal Calibration
Curve. This is usually accomplished by measuring the retention volumes
V
i
and intrinsic
viscosities [
η
]
i
for a set of narrow distribution polymer standards. The product of intrinsic
viscosity and molecular weight for each of the standards is then plotted against the retention
volumes, obtaining a smooth curve.
R e te n tio n V o lu m e (m l)
U n iv e rs a l C a lib ra tio n
0 .8 6
2 .4 1
3 .9 5
5 .5 0
7 .0 5
8 .6 0
9 .3
1 1 .7
1 6 .5
1 8 .9
2 1 .3
1 4 .1
Figure 1 Typical Narrow Standard Universal Calibration Curve
Log
(IVxMW)
