Viscosity measurement

A method and apparatus for determining the solution viscosity of polymers using gel partition chromatography (GPC). The method comprises obtaining a GPC analysis of the solution and measuring the pressure drop at a selected point in the GPC flow path. The GPC analysis provides a measure of the polymer concentration in the solution. The viscosity can be determined from the pressure drop in combination with the known polymer concentration.

BACKGROUND OF THE INVENTION 
The present invention pertains to a method and apparatus for determining 
the solution viscosity of polymers. The solution viscosity of polymers is 
very important in the manufacturing of various polymers. It is used to 
determine if the molecular weight of the polymer is correct and that the 
distribution of various molecular weight components within the polymer are 
correct. The molecular weight of the polymer is important in the control 
of the production of the polymer. 
In the past, various methods have been suggested for determining the 
solution viscosity of polymers. One approach has been the drying of a 
polymer sample, dissolving a known amount of the sample in a selected 
solvent and determining the viscosity of the solution. This requires 
precise temperature equilibrium of the solution and the elimination of any 
gas bubbles or incompletely / dissolved polymer in the solvent. Also, 
attempts have been made to estimate the viscosity from the pressure drop 
produced by the introduction of a sample of the polymer into a solvent 
stream. These attempts are only approximate unless it is possible to 
simultaneously measure the polymer concentration to ensure that the 
polymer concentration in the solution remains constant. In the absence of 
measurements of the polymer concentration in the solvent it is impossible 
to determine the solution viscosity. 
BRIEF SUMMARY OF THE INVENTION 
The present invention solves the above problems by providing a method for 
determining the polymer concentration in the solvent and at the same time 
measuring the pressure drop due to the presence of the polymer. In 
particular, the invention utilizes a liquid gel partitioned chromatograph 
to measure the polymer concentration in the solvent. At the same time the 
pressure drop caused by the presence of the polymer in the solvent is 
measured in the waste disposal line of the chromatograph. Normally, it is 
desirable to have a chromatograph record of the polymer and using the 
present invention the solution viscosity can be obtained at the same time 
without additional effort or time. 
The chromatograph of the invention is designed to permit the dissolving of 
a polymer sample in a suitable solvent while controlling the flow rate and 
mixing of the dissolved polymer so that an accurate chromatographic record 
of the polymer may be obtained. A particularly suitable chromatograph is 
one manufactured by Applied Automation Company of Bartlesville, Okla. This 
chromatograph is provided with means for on-line dilution and filtration 
of polymer samples prior to the chromatographic analysis. In particular, 
the instrument is provided with accurate flow controllers to obtain 
constant flow rates and suitable mixing chambers to mix the polymer sample 
with the solvent. The chromatograph has been modified to provide a 
pressure transducer in the discharge line of the diluted polymer on the 
upstream side of the flow control capillary in the discharge line. The 
signal from the pressure transducer and from the chromatograph analysis 
are recorded on the same chart record and to the same time base so that an 
accurate correlation can be made between the polymer solids present in the 
sample and the pressure drop resulting from the presence of the polymer. 
From the correlation, one can then determine the solution viscosity of the 
polymer.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown a conventional liquid gel partition 
chromatograph of the type manufactured by Applied Automation, Inc. The 
chromatograph is designed to take a sample from a process stream through a 
sample line 10 and return it to the process stream through line 11. A 
process loop pump is used to remove the sample from the stream and provide 
sufficient pressure to return it to the process stream. A supply of 
solvent is provided in a reservoir 13 and supplied to the chromatograph by 
pump 14. The solvent pressure is controlled by a pressure regulator 15 and 
a flow control 16. Both the flow controller and pressure regulator are 
located in a constant temperature compartment so that an accurate flow of 
solvent at a constant is maintained. It is a necessary requirement of this 
invention that each component following the regulators 15 and 42 be at a 
constant temperature, although different temperatures can be maintained 
for different components if desired. The solvent flows through line 17 
which is connected to the two position valve 12. The solvent flows from 
the valve 12 through line 21 to a mixing chamber 22 and then by line 23 to 
a second two position valve 24. From the valve 24 the solvent flows 
through a filter 25 and then back to the valve 24 through a line 26 to a 
third two position valve 27. All of the valves 12, 24 and 27 are 
conventional chromatography valves which have multiple positions so that 
flow streams can be directed to various portions of the equipment or 
samples contained in sample loops can be injected into the equipment by 
directing a flow of suitable fluid through the valve. From valve 27, the 
solvent flows through a line 30 to a waste disposal 31. Mounted in the 
line 30 before the waste disposal is a capillary 32 and a pressure 
transducer 33. The capillary serves to further control the flow rate of 
the solvent while the pressure transducer measures the pressure in the 
solvent line and particularly the pressure drop caused by the passage of a 
polymer sample. 
The polymer sample is injected into the chromatograph system from the 
sample loop 20 through which the process stream normally flows. By 
properly positioning the valve 12, the sample contained in the sample loop 
20 can be isolated and then transported by the solvent in the line 17 to 
the mixing chamber 22. The process stream sample is mixed with a solvent 
in the chamber 22 and then transported through the valve 24 and filter 25 
to the valve 27. Valve 27 is also provided with a sample loop 51 through 
which the diluted process stream sample normally flows. Thus, the diluted 
sample of the polymer from the process stream will flow through the line 
30 and be disposed in the waste disposal 31. In passing to the waste 
disposal 31 the pressure increase caused by the polymer sample will be 
measured by the pressure transducer 33. 
The chromatograph is also supplied with an eluent in reservoir 40 having a 
pump 41, pressure regulator 42 and flow control 43. The eluent serves to 
transport a sample of the diluted polymer from the process stream to the 
chromatograph column and detector. In particular, the eluent from the flow 
control 43 is passed by a line 50 to the valve 27 where it can serve to 
transport the sample contained in the sample loop 51 via the line 52 to 
the column 53. Again, the valve 27 should be positioned so that the eluent 
can transport the measured sample to the column 53. Column 53 is packed 
with a suitable gel, for example, Waters Associates MicroBondagel column 
E-500. The column will separate the polymer according to the size of the 
molecules contained in the polymer sample and elute them from the bottom 
of the column to the detector 46. Detector 46 can be any type of detector 
but preferably an ultraviolet or refractive index detector. As shown in 
the drawings, the eluent is taken from the pressure regulator 42, passed 
through a line 44 to a reference column 45 which is also connected to the 
detector 46. Both of the discharges from the detector are passed through 
capillaries 47 to the waste disposal 31. Capillaries 47 again serve to 
further control the flow of both the sample containing portion of the 
steam and the reference stream. 
While not shown in FIG. 1, the signal from the pressure transducer 33 and 
the detector 46 are supplied to a chart recorder having a pair of pens so 
that both the pressure of the solvent stream containing the process sample 
as well as the output from the chromatograph can be recorded in a 
correlatable manner. A representative recording is shown in FIG. 2 in 
which the curve 60 illustrates the pressure as measured by the pressure 
transducer while the curve 61 illustrates the signal from the detector 46. 
The chromatograph signal is shown as having four peaks 62-65 that 
represent the various molecular weights of the polymer contained in the 
sample. Also shown is a dilution solvent peak 66 at the right of the plot 
which goes off scale. By integrating the area under the peaks, one can 
obtain a representation of the total polymer contained in the sample. This 
representation can then be correlated with the pressure reduction caused 
by the passage of the polymer sample through the pressure transducer to 
obtain the solution viscosity of the polymer. This peak of pressure 
increase is represented by the upside-down peak in the pressure transducer 
recording 60. 
From the above description of the chromatograph apparatus used in the 
present invention, it is appreciated that the polymer sample concentration 
will reach a maximum and then slowly decay on an exponential curve as 
additional solvent is added to the mixing chamber. This relationship is 
clearly shown in the pressure curve 60 of FIG. 2 where there is a sudden 
increase in the pressure of the solvent stream as the initial portion of 
the polymer sample passes through the pressure transducer and then an 
exponential increase in pressure. It is apparent that the actual height of 
the peaks and the area under the peaks in the chromatograph curve of FIG. 
2 depend upon the time that elapses between the opening of the valve 12 to 
inject the sample into the mixing chamber 22 and the opening of the valve 
27 to inject the sample into the chromatograph column 53. This variable 
can be controlled accurately maintaining the same time interval between 
the opening of the two valves. 
In addition to the above, several modifications can be made to the 
equipment. For example, one could provide a continuous detector in the 
solution stream after the capillary 32 to provide a continuous measurement 
of the concentration of the polymer in the diluent stream. This provides 
an indication of variation in concentration of polymer with time. Of 
course, it would also be possible using this approach to eliminate the 
chromatograph column though normally, a composition analysis of a polymer 
is also desired. Also, instead of using the areas of the peaks to 
determine the concentration of polymer in the sample, one could use the 
peak heights although this would probably not be as accurate.