Anal. Chern. 1986, 58, 2242-2247
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Determination of Nonionic and Partially Hydrolyzed Polyacrylamide Molecular Weight Distributions Using Hydrodynamic Chromatography Martin A. Langhorst,*’ Frederick W.Stanley, Jr.,’ Sergio S.Cuti6,’ Jeffrey H. Sugarman,2Larry R. Wilson,] David A. H ~ a g l a n da,n~d Robert K. Prud’homme2
T h e Dow Chemical Company, Midland, Michigan 48667, and Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544
Polyacrylamlde and its copolymers of acrylic acM have wlde application as flocculants and moblllty control fluids In enhanced oll recovery. Detalled knowledge of the molecular welght and molecular weight dlstrlbutlon has not been avaliable for these polymers because thelr exceptionally hlgh molecular weight exceeds the capabilities of tradltlonal techniques, such as size excluslon chromatography (SEC). The COmMnatkn of hydrodynamic chromatography ( H E ) and low-angle laser llght scatterlng (LALLS) detection can be used to measure the molecular welght dlstrlbution of this group of polymers. Chromatographic conditions can be chosen so that the elution of the polymer Is prharlly a function of molecular weight and Mepemht of the composltlon of the polymer. Weight-average molecular weights determined by this technique agree well with those calculated from lntrlnslc vlscoslty measurements.
Polyacrylamide and its copolymers of acrylic acid have wide commercial application as flocculants and mobility control fluids in enhanced oil recovery. The key to understanding their behavior in solution is a detailed knowledge of their molecular weight and molecular weight distribution. In general, this information has not been available for these polymers because of their exceptionally high molecular weight. SEC, the most common method for obtaining distribution information, has an exclusion limit below the range of commercial poly(acry1amide acrylic acid) copolymers ((8-10) X IO6). Seright et al. ( I ) employed ultracentrifugation with fluorescence detection to measure the molecular weight distribution of commercially available polyacrylamides. They observed that these polymers contain fractions having molecular weights in excess of 50 X lo6. Light scattering techniques have been used to obtain weight-average molecular weights of these polymers. However, no distribution information is available, and the presence of “gels” requires extensive sample fitration with the potential of molecular weight degradation. Intrinsic viscosity is the most commonly used method to measure the molecular weight, but again with no distribution information available. Routine measurement of molecular weight distribution could be used to optimize polymerization kinetics of products with improved end-use performance, as well as contribute to a fundamental understanding of the behavior of these polymers in solution. Hydrodynamic chromatography (HDC) is a technique originally developed for sizing latex particles. The separation in HDC arises from the spacial variation of fluid velocity in a packed column. Large particles (or polymers) are sterically excluded from slow moving fluid near the packing material and are thus convected through the column faster than small particles (or IThe Dow Chemical Co. *Princeton University. University of Massachusetts at Amherst.
molecules). In an HDC experiment, one measures the location of the particle peak and compares it to the location of low molecular weight marker species. A retention factor, R,, is defined as the marker elution volume divided by the particle elution volume. The R, is always greater than unity. Recently, Prud’homme and co-workers have shown that hydrodynamic chromatography (HDC) offers considerable promise for the analysis of a variety of high molecular weight water-soluble macromolecules (2),including three important polymer conformations: rigid rod (xanthan polysaccharides), flexibile linear-chain coil (partially hydrolyzed polyacrylamides), and flexible branched-chain coil (dextrans). This technique has been extended to biological polymers, e.g., tobacco mosaic virus, linear DNA, and superhelical DNA (3). These previous characterizations of water-soluble polymers by HDC have related the elution position of the polymer to an effective diameter determined using a calibration obtained with latex spheres. In the work described in this paper, the molecular weight of the polymer that elutes from the HDC column will be measured using a low-angle laser light scattering (LALLS) detector. EXPERIMENTAL SECTION A schematic of the chromatographicsystem used in this work is illustrated in Figure 1. Note the presence of a sub-micrometer filter present between the column outlet and detector system inlet. This filter was necessary to reduce the amount of particulate material in the eluent, since particulates cause a large light scattering signal as they pass through the LALLS detector. The polymer concentration was detected with a Kratos 773 variable wavelength UY detector at 200 nm. The eluent used in these experiments was 0.1 M NaH2P04, 0.1% Brij 35 (nonionic surfactant),and 0.1% HCHO (preservative) at pH 4 except where noted. The columns used in this work were 10 mm i.d. X 25 cm length packed with nominal 15-pm styrene/divinylbenzene beads. Eluent was delivered to the system by use of a Waters M-45 pump with a microflow controller. Injection and switching valves were manufactured by Valco Instruments Co, Inc. (Houston, TX). A delayed marker injection technique was used to deliver the polymer and marker solutions to the column and has been previously discussed (4). The polymer concentration injected onto the column ranged from 75 to 100 ppm. The injection volume was 100 pL. A Nelson Analytical Model 761 A/D converter, using an HP-IB interface to an HP-85 computer was used to collect data from the detectors as well as control the valve operation. Concentrationand scattering intensity measurements were taken every 10 s, resulting in a total of 800 pairs of data points for each experiment. The PUSHER 500, 700, and 1000 samples analyzed in this study are commercially available partially hydrolyzed polyacrylamides with nominal weight average molecular weights of 5 x lo6, 7 X lo6, and 9 X lo6, respectively. Also analyzed were polymers A and B, which were prepared in the laboratory. These nonionic polymers have nominal weight average molecular weights of 9 x lo6 and 7 X lo6,respectively,and were hydrolyzed by using known amounts of NaOH. Polymer SF-210 was also analyzed, which is -10% hydrolyzed and has a nominal weight average molecular weight of (12-15) X lo6.
0003-2700/86/0358-2242$01.50/00 1986 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986 2243 Prttcolumn Filter
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Flgwe 3. Typical molecular weight distribution of a partlally hydrolyzed polyacrylamkle (PUSHER 700) calculated from the HDCILALLS experiment.
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Flgure 1. Schematic of the hydrodynamlc chromatograph with low angle laser light scattering (LALLS) detection
Flgure 4. Composite chromatograms of a typical ultrahigh molecular weight partially hydrolyzed polyaaylamkle (polymer A) at three dlfferent flow rates: 500 pL/min (. -), 100 pL/mln (-), and 25 pLImln (- - -): 0.1 M NaH,PO, pH 4 eluant containing 0.1 % Brij 35.
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(Spectrum 12000-14000 molecular weight cutoffl and then placed overnight in a stirred beaker of pure eluent. This "batch" dialysis permits complete recovery of the polymer solution from the bag. In Figure 2, a typical composite chromatogram is shown. The smooth curve is the UV detector response, displaying two peaks, the smaller of which is a low molecular weight marker (Crz0?-). The jagged curve is the laser photometer output, which has been offset by the time delay between the two detectors. Note that the light scattering peak precedes the concentration peak. This is due to the fact that the intensity of scattered light is a function of the product of molecular weight and concentration. In HDC, as in SEC, higher molecular weight material always elutes faster than lower molecular weight material. From this collection of data, the molecular weight distribution can be calculated (5) and plotted, as shown in Figure 3.
Detector R m n s e
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4 5 6 7 S Q 10 Data Points Collected IHundreds) 13 14 Elution Volume (mls)
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Flgure 2. Typical composite chromatogram Illustrating UV and LAUS
responses for a nonionic polyacrylamkle on the HDC. The intensity of scattered light was measured in a Chromatix KMX-6 laser photometer. From the measured intensity of scattered light and the concentration (as measured by the UV detector), the molecular weight of a particular fraction was calculated. The W response-concentration relationship was linear up to at least 50 ppm solution concentration and was observed to be independentof the degree of hydrolysis and molecular weight of the polymer at 200-210 nm. For these calculations, it is necessary to know the refractive index increment (dnldc), and second virial coefficient of the polymer solution, and the eluent refractive index. The dn/dc value of 0.163 was determined with a KMX-16 laser photometer and was not found to be a function of the composition of the copolymer up to 30% acrylic acid. For an accurate dn/dc measurement, it is necessary that the polymer solution be in dialytic equilibrium with the eluent used in the reference cell. In addition, the polymer concentrationmust be known in order to determine dnldc. For dialysis, a known volume of a polymer solution was placed in dialysis tubing
Influence of Flow Rate on Elution Position. The flow rate dependence on the elution position of a given polymer is illustrated in Figure 4. The peak maximum shifts to larger apparent diameter as a flow rate is decreased from -0.5 mL/min to 0.1 mL/min, while the peak shape remains basically unchanged. Further reduction in flow rate from 0.1 mL/min to 0.025 mL/min does not substantially alter the peak maximum but significantly alters the peak shape of the eluting polymer. While there appears to be a distinctive flow-rate dependence for the ultrahigh molecular weight polymers, this same dependence does not appear for lower molecular weight polyacrylamides. Figure 5 illustrates the chromatograms of a 2 X lo6 molecular weight standard at 0.5 and 0.1 mL/min. The elution positions and peak shape are almost identical. The change in the elution position of these ultrahigh molecular weight water-soluble polymers as a function of flow rate can be explained by deformation of the polymer during the chromatographic analysis (6). If a polymer molecule is deformed (stretched out in the direction of flow) during passage through a porous bed, we would expect the apparent size of the molecule to be smaller than an undeformed molecule, since in HDC it is the dimension transverse to the direction of flow that is measured. The deformed molecules, as compared to undeformed molecules, have a slower average
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986 -
Polymer
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Deformation And Degradation
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Figure 5. Composite chromatograms of a 2 X 10’ nonionic polyacrylamide standard at two different flow rates: 500 pL/min (-) and 100 pL/min (--): 0.1 M NaH,PO, pH 4 eluant containing 0.1 % Brij 35.
velocity due to their ability to sample a wide range of transverse locations (and flow velocities) within the bed. As a result, high molecular weight deformed molecules may elute at the same velocity as lower molecular weight undeformed molecules, and the chromatogram will not accurately reflect the “actual” molecular weight distribution of the polymer. Durst and co-workers (7,8)in their investigations of the flow behavior of ultrahigh molecular weight partially hydrolyzed polyacrylamidesfound that highly dilute (50-200 ppm) solutions of these polymers possess only slightly higher shear viscosity than pure solvents; but in porous media flow, they can exhibit extensionalviscosities that are d r a m a t i d y higher than those of pure solvents. Specifically, at low Reynolds number (Re) (0.5, the polymer will be deformed in the flow from its equilibrium (relaxed) random coil shape. If De
