American Polymer Standards
Corporation
8680 Tyler Blvd.
Mentor, OH 44060
October 29, 1993
Validation of Gel Permeation Chromatographic Method
for determination
of Mn, Mw, Mv, Mp, Mz, MWD and Intrinsic Viscosity of
Polystyrene and
other THF soluble Polymers.
Objective:
To validate a method to characterize the molecular
weight distribution of
THF soluble polymers.
References:
"A differential Visometer", Part II, American
Laboratory April 1985. By
Dr. Max Haney, president of Viscotek Corporation.
"Absolute Molecular Weight Distribution of Polymer
by SEC-Viscometry"
By Dr. Dennis J. Nagy, Air Products and Chemicals, Inc. First
International GPC/Viscometry Symposium April 1991, Houston, TX.
"Principles of Polymer Chemistry", Cornell
University Press, Ithaca, NY.,
1953, P.J. Flory.
Analytical Methodology :
Operating Parameter:
Equipment: Waters M-150-C ALC/GPC chromatograph equipped
with the
Viscotek Differential Viscometer, Model 100
Columns: Styrene-DVB GPC columns: 2 X Linear + 500
Mobil Phase: THF (0.025% BHT)
Flow Rate: 1.0ml/min
Injection Volume: 150ul
Reagents and Standards:
THF (0.025% BHT) Chempure, CMS Corporation
Polystryene standards, narrow molecular weight
distribution with known
Mp and Intrinsic Viscosity values (minimum number 6 standards; average
number
12 standards). Complete list of available standards ID enclosed.
On broad MWD standard for checking out the validity of the
calibration
curve and the value of the Intrinsic Viscosity.
Preparation Of Standards:
10 to 25 mgs of each standard is dissolved at room
temperature in the
mobile phase in cleat borosilicate scintillation vials.
For example, 50.0 mgs of raw material are dissolved in 20.0
ml of mobile
phase to obtain 0.25% solution. Once the sample is in solution it is filtered
using 0.45 micron disk filter.
Analysis:
Each standard and each sample is chromatographed three
times in order to
assure reproducibility of results and to guard against unexpected
instrumental upsets.
Systems Suitability Requirements:
To insure that the instrument is operating correctly
a number of check are
performed. The differential refractometer should have a reading of
3.0
millivolts on the detector output readout, differential transducer readout
on the
viscometer should be set close to zero, systems backpressure
should be below 1,000psi and monodisperse low molecular weight
standard peak should be symmetrical (0.9 to 1.0
skew), total number of
columns plates above 20,000 plts/bank.
Calculations:
Unical GPC software, version 4.05 (7-6-92) form
Viscotek Corporation,
Houston,TX is used to do all the calculations. The software manual
describes in minute detail all the formulas, algorithms and convolute
intergrals used for
the calculations.
First, peak parameters are calculated in order to
determine the viscometer
offset fromthe refractometer, peak broadening and skewing corrections.
Second, the Universal Calibration table and plot are
calculated. Log
(mp X IV*) is plotted versus Elution Volume. IV* is the intrinsic
viscosity
of each standard under the operating conditions of the system.
Third, the unknown samples are calculated using the
Universal Calibration
method.
For each sample the following values are determined.
Mn
Number average molecular weight
Mp
Peak average molecular weight
Mv
Viscosity average molecular weight
Mw
Weight average molecular weight
Mz
Z-average molecular weight
IV*
Intrinsic Viscosity (dl/gm)
Validation of the Method:
Molecular weight is a weight in grams of Avogadro's number
(6 X 10^23)
of molecules of a substance.
A polymer sample does not have a single molecular weight.
Since polymers are mixtures of molecules having different
molecular sizes
and molecular weights they cannot be characterized by a single molecular
weight. All of the different molecules within the polymer sample are
measured and
averaged, the results is a molecular weight average.
The kind of molecular weight average is obtained
depends upon type of
measurement used;
Number Average Molecular Weight (Mn) is
obtained by counting number
of molecules per unit weight of sample. In this type of
measurement all
molecules are treated equally regardless of their weight or size. Osmotic
pressure, vapor pressure, freezing point depression, boiling point
elevation, hydroxyl
number, carboxyl equivalent --- all of these
measurements result in the average called the
number-average molecular
weight.
Number average molecular weight is very sensitive to
changes in the
weight fractions of low molecular weight species, because for small
molecules a small weight of material represents a large number of
molecules.
Another technique that is used to measure molecular
weights of polymer
is light scattering. A light shining through a very dilute
solution of a
polymer will be scattered by the polymer molecules. The scattering
intensity at
any gives angle is a function of the second power of the
molecular weight.
Because of this square function, large molecules
contribute much more to
the molecular weight than small molecules. This "weighted"
average is
called weight average molecular weight and is very sensitive to
changes in
the number of large molecules in a given sample of a polymer.
Viscosity of average molecular weight is obtained
from measurements of
dilute solutions of a polymer and gives us information about
the molecular
sizes or volume of the polymer molecules in the solution. Intrinsic
Viscosity is related to the viscosity average molecular weight. Large
molecules contribute
more to the viscosity of the polymer sample.
If a dilute solution of a polymer is subjected to a
centrifugal field at a fairly
low speed, a thermodynamic equilibrium is established where
the
molecules become distributed according to their sizes. The molecular
weight is obtained from this experiment is called Z-average molecular
weight. Vary
large molecules settle most in the gravitational field in the
centrifuge and
are even more important in the case of the Z-average that
the weight-average or
the viscosity-average molecular weight.
Different molecular weight averages are significant
in terms of processing
a polymer and in terms of the properties of a polymer
end-product.
All of the above described molecular weight averages
can e obtained by
Gel Permeation Chromatography (GPC) in a single run in
about 30 to 45
minutes.
Flory showed theoretically that hydrodynamic volume
of a polymer
molecule in solution is proportional to the molecular weight times
intrinsic
viscosity.
Benoit demonstrated Flory's theories by showing that
polymers of
different chemical structures will fall on the same calibration plot if
the
parameter of intrinsic viscosity is available.
Since GPC separates
polymers by hydrodynamic volume an d on-line
differential viscometer provides the value of intrinsic
viscosity over entire
molecular weight distribution, it became possible and
practical to calibrate
GPC systems with known standards and get absolute
molecular weights of
unknown polymers.
Reproducibility of Results:
The chromatography system
is calibrated each time samples are run to
insure good reproducibility on results.
In general the
reproducibility of results under ideal conditions is +/- 1.0%;
reproducibility month to month can realistically be +/-
2.5%.
Conclusion:
The assay system described
above is accurate and reproducible for the
characterization of the molecular weight distribution
of THF soluble
polymers. It directly measures all the polymer and does
not rely on
published Mark-Houwink constants of make theoretical
assumptions to
derive their values.
Written By: John E. Armonas/President & Polymer Chemist (10-29-1993)
Reviewed: May 2002