Gradient Analysis of Polymer Blends, Copolymers, and Additives with ELSD and PDA Detection Peter G. Alden, Michael Woodman | ||||||||||||||||
Abstract
In recent years there has been increased interest in using gradient HPLC techniques, such as Gradient Polymer Elution Chromatography (GPEC), with polymers for determining the compositional drift of copolymers, the composition of polymer blends, or for the analysis of polymer additives. Depending upon the gradient conditions and columns selected for analysis, separations may be obtained dependent on molecular weight or based upon precipitation, or adsorption mechanisms. The use of an Evaporative Light Scattering Detector (ELSD) allows one to perform solvent gradients with a universal mass detector and observe both UV absorbing and non-UV absorbing polymer samples without baseline disturbances from the solvent gradient. The addition of a Photodiode Array Detector (PDA) allows for compositional analysis across the molecular weight distribution of many copolymers, can be useful for the identification of components in a polymer blend, and also is invaluable for the quantitation of polymer additives and other small molecules in traditional reverse phase separations. This paper demonstrates the advantages of gradient analysis of polymers as compared to results obtainable with Gel Permeation Chromatography. The instrumentation used to carry out this work is described and examples of this technique for the analysis of polymer blends are shown. The effects of column functionality and solvent composition on the separation of polystyrene standards and samples is described and the best conditions observed are used to analyze various copolymers for monomer composition. Finally, the traditional use of gradient separations with the same instrumentation for the analysis of several types of polymer additives is also shown. The most common chromatographic method for the analysis of polymers is Gel Permeation Chromatography (GPC) where the separation is based upon the size of the polymer sample in solution, or the hydrodynamic volume of the polymer solution. Figure 1 shows the chromatograms obtained using GPC for a polystyrene sample, polystyrene-acrylonitrile copolymer (25% acrylonitrile) and a polystyrene-butadiene rubber (50% styrene) analyzed separately. Even though the samples are of different molecular weight, the hydrodynamic volumes are similar enough that the polymer peaks are observed at nearly the same retention time. The chromatograms obtained for the GPC analysis of a blend of approximately the same concentration of each of the polystyrene, polystyrene-acrylonitrile, and the polystyrene-butadiene samples are also shown in Figure 1.
However, when this same polymer blend is analyzed in a gradient mode, the three components can easily be baseline resolved as demonstrated in Figure 2 which shows the overlay of two replicate injections of the polymer blend run on a prototype divinylbenzene-vinylpyrolidone column with a gradient from 100% Acetonitrile (ACN) to 100% Tetrahydrofuran (THF) over 20 minutes. Using this technique, the samples are dissolved in THF and then injected into the chromatographic system running 100% ACN.
All gradient work was carried out using the following system configuration unless otherwise noted.
The most commonly used detector for GPC is the Refractive Index (RI) detector; however, the sensitivity of the RI to changes in mobile phase composition makes it unacceptable as a detector for Gradient Polymer Analysis.
Using this system, a wide variety of different types of polymers, polymer blends and copolymers can be analyzed. Figure 4 shows an overlay of chromatograms obtained for many types of polymers run on a
When using this technique for the analysis of polymer blends or copolymers, it is necessary that the separation be independent of molecular weight so that the polymers are separated only by composition. Unfortunately, since this is primarily a precipitation/redissolution mechanism, some molecular weight dependence is inevitable, but it can be minimized through the judicious selection of columns, mobile phases, and gradient conditions. Figure 5 shows an overlay of chromatograms obtained from a series of narrow polystyrene standards run on
By changing to a Nova-Pak?? C18 column (3.9mm x 30cm) and using a 30 minute gradient, the chromatograms shown in figure 7 were obtained. Using these conditions, the molecular weight dependence for the 43,900 MW and higher polystyrene standards is nearly eliminated. As expected the lower MW standards that are soluble in ACN are eluted earlier in the chromatogram, however, the low MW oligomers are being split into three peaks, indicating that they are being separated by their differing end groups.
The choice of mobile phase used as the non-solvent can have significant effects on the separations obtained from gradient analysis of polymers.
This non-solvent effect can also be seen when analyzing broad MW polymer samples.
Figure 9
Gradient analysis is a powerful tool for evaluating copolymer materials. A series of random styrene-butadiene rubbers (SBR) were run using this 100% ACN to 100% THF gradient on a prototype DVB/Vinylpyrolidone column (3.9mm x 15cm) in 20 minutes. Five different SBRs with composition ranging from 50% styrene to 5.2% styrene were injected along with a narrow polystyrene standard (355K MW) and a narrow polybutadiene standard (330K MW). An overlay of the resulting chromatograms is shown in
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figure 10. The different SBRs are easily separated by their relative amounts of styrene and butadiene. These SBRs were previously analyzed by traditional GPC to be sure that the molecular weights were high enough that molecular weight dependence would be insignificant, and the molecular weights were all found to be approximately 200,000 to 300,000 by relative calibration with polystyrene.
Using the gradient results, a calibration curve was constructed to determine % styrene vs retention time and is shown in figure 11. The plot exhibits a good correlation between % styrene and retention time so that this method could be used to determine the approximate composition of an unknown SBR. The UV data from the PDA could also be used to crosscheck the results from the ELSD.
In a similar manner, figure 12 shows the chromatograms obtained for a series of block styrene-butadiene copolymers with a similar separation as the random SBRs.
The data is plotted in figure 13 showing a calibration curve similar to the one obtained for the random SBRs. Using this gradient method, species with only slight differences in structure can easily be separated.
Figure 14 shows an overlay of individual injections of polymethylmethacrylate, polymethylmethacrylate, poly-n-butylmethacrylate, poly-n-hexylmethacrylate, and poly-laurelmethacrylate run on a Nova-Pak?? C18 (3.9mm x 15cm) with a gradient of 100% ACN to 100% THF in 30 minutes. The chromatograms show excellent separation between each component in the homologous series of methacrylates and could easily be resolved with a faster gradient.
The chromatogram in figure 15 shows the separation of the same methacrylates injected as a mixture and run under identical conditions demonstrating an identical separation when the components are run in a mixture.
This same method using identical conditions also has utility for analyzing lower molecular weight compounds.
Low molecular weight polymer additives can be analyzed with this method by the traditional reverse phase mechanism. Many types of polymer additives will be shown using the following conditions that were chosen to be compatible with a Mass Spectrophotometer:
Figure 17 shows the separation of Tinuvin 440, Tinuvin 900, and Tinuvin 328 that are UV stabilizers commonly used in polyolefin resins.
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Figure 19 shows the chromatograms for the slip agents oleamide and erucamide and the antistat stearic acid.
Figure 21 shows 12 overlays of a separation of 10 common antioxidants run using a modified version of the approved ASTM method for the analysis of additives in polyolefins.
The use of gradient methods for the analysis of polymers allows for separations that are essentially independent of molecular weight. Individual polymers in blends having the same molecular weight distribution can easily be separated and copolymers can be separated by their monomer ratios. Using the same instrumentation, mostcommon polymer additives may also be analyzed. The Evaporative Light Scattering Detector is a universal detector which is unaffected by changes in mobile phase gradient composition and the Photodiode Array Detector allows for positive identification of many compounds and compositional analysis of copolymers. These gradient methods are highly reproducible techniques and are extremely well suited for deformulation applications. |