Polystyrene standards,separation

Figure 5 SEEC calibration curves used for the determination of the pore flow ratio co. Polystyrene standards separated in DMF with 1 mM (a) or 10 mM (b) LiCI. Stationary phase Lichrosorb 100-10. or O, experimental values ---------, prediction using model -------, prediction for pressure-driven SEC (P).

PL 10 pM gel mixed B 500-10 Polymer MW distributions Polystyrene standard separation, polystyrene analysis, polymethyl methacrylate analysis polyethylene terephthalate, polymide, polyvinylchloride, pyrrolidone, fluoropolymei butyl rubber, polyethylene, copolymers... 

PL 5 pM gel mixed C 200-3 xW Rapid polymer MW distributions Polystyrene standard separation, polycarbonates, hydroxyethyl cellulose, polyether sulfone, polyurethane... 

PL 5 pM gel mixed D 200 - 0.4 X 10 Resins, condensation polymers Polystyrene standard separation, resins, epoxy resin, polybutadiene, polysiloxane, polycarbonate... 

PL gel 3 pM mixed D up to 30,000 Low MW resins Polystyrene standard separation, epoxy resins, prepolymers, novolak, polyesters, phenolic resins... 

Small particle size resins provide higher resolution, as demonstrated in Fig. 4.41. Low molecular weight polystyrene standards are better separated on a GIOOOHxl column packed with 5 /u,m resin than a GlOOOHg column packed with 10 /Ltm resin when compared in the same analysis time. Therefore, smaller particle size resins generally attain a better required resolution in a shorter time. In this context, SuperH columns are best, and Hhr and Hxl columns are second best. Most analyses have been carried out on these three series of H type columns. However, the performance of columns packed with smaller particle size resins is susceptible to some experimental conditions such as the sample concentration of solution, injection volume, and detector cell volume. They must be kept as low as possible to obtain the maximum resolution. Chain scissions of polymer molecules are also easier to occur in columns packed with smaller particle size resins. The flow rate should be kept low in order to prevent this problem, particularly in the analyses of high molecular weight polymers. 

Figures 13.8 and 13.9 show the separation of polystyrene standards using a typical mixed-bed column and its calibration plot, respectively. The major advantages of using a large i.d. 10-mm column are low hack pressure and relatively short run times. As seen in Fig. 13.8,10 standards from toluene thru 8.4 X 10 MW can be resolved in a mere 21 min. Because of the large 10-mm i.d. columns, 1.5-ml/min flow rates give a linear velocity equivalent to that of only 0.9 ml/min using a 7.6-mm i.d. column. Also, the gel volume contained in one 10 mm i.d. X 500 mm column is 39.3 ml, whereas a 7.6 mm i.d. X 300 mm column contains only 13.6 ml of gel volume. This bulk volume factor, combined with the large pore volumes of gels, obtains essentially the same resolution as that obtained on three standard 7.6 X 300-mm columns in series, but in about one-half the usual time required using the smaller columns.

Figures 13.25-13.28 show the ultrahigh resolution separations in chloroform of polystyrene standards, polytetramethylene glycol, urethanes and isocyanates, and epoxy resins, respectively. Multiple column sets of anywhere from two to six columns in series have been used for well over a year with no apparent loss of efficiency. The 500- and 10 -A gels can easily tolerate 15,000 psi or more. In fact, the limiting factor in the number of columns that can be used in series is generally the pump or injector in the FIPLC system. A pump capable of 10,000 psi operation should allow the use of a column bank of 10-12 50-cm columns with a total plate count of 500,000 or more.

FIGURE 13.25 Using chloroform as the solvent, a mixture of polystyrene standards were nicely separated on the 3-m set of columns. Run times here were 160 min. Plate count for toluene was calculated at 240,000 plates. The 500 MW Standard is separated nicely into its oligomers. 

Traditionally, column efficiency or plate counts in column chromatography were used to quantify how well a column was performing. This does not tell the entire story for GPC, however, because the ability of a column set to separate peaks is dependent on the molecular weight of the molecules one is trying to separate. We, therefore, chose both column efficiency and a parameter that we simply refer to as D a, where Di is the slope of the relationship between the log of the molecular weight of the narrow molecular weight polystyrene standards and the elution volume, and tris simply the band-broadening parameter (4), i.e., the square root of the peak variance. 

The most widely used molecular weight characterization method has been GPC, which separates compounds based on hydrodynamic volume. State-of-the-art GPC instruments are equipped with a concentration detector (e.g., differential refractometer, UV, and/or IR) in combination with viscosity or light scattering. A viscosity detector provides in-line solution viscosity data at each elution volume, which in combination with a concentration measurement can be converted to specific viscosity. Since the polymer concentration at each elution volume is quite dilute, the specific viscosity is considered a reasonable approximation for the dilute solution s intrinsic viscosity. The plot of log[r]]M versus elution volume (where [) ] is the intrinsic viscosity) provides a universal calibration curve from which absolute molecular weights of a variety of polymers can be obtained. Unfortunately, many reported analyses for phenolic oligomers and resins are simply based on polystyrene standards and only provide relative molecular weights instead of absolute numbers. 

The Separation of Some Polystyrene Standards by Exclusion Chromatography on Silica Gel... 

Figure 8. HDC separation of a synthetic biomodal mixture of 380 A and 1760 A polystyrene standards at 220 nm and 254 nm wavelength (weight ratio is 1.00/120)...

The molecular-weight distributions were measured using a Waters GPC in the dual-detector mode (DRI and UV). The UV detector was operated at 254 nm. The samples were prepared by dissolving 2 mg of polymer in 10 ml of THE The injection volume was 200 pi. Separations were effected using two Polymer Labs 10-gm PL mixed-B columns. THF was used as the mobile phase. The molecular-weight distributions were calculated relative to narrow polystyrene standards ranging from 102 to 4 x 106 M ,... 

Fig. 14 a, b. Effect of gradient steepness on the very fast separation of polystyrene standards in a molded monolithic poly(styrene-co-divinylbenzene) column (Reprinted with permission from [121]. Copyright 1996 Elsevier). Conditions column, 50 mm x8 mm i.d., mobile phase, linear gradient from 100% methanol to 100% tetrahydrofuran within a 1 min b 12 s, flow rate, 20 ml/min, peaks represent polystyrene standards with molecular weights of 9200,34,000 and 980,000 (order of elution), 3 mg/ml of each standard in tetrahydrofuran, injection volume 20 pi, UV detection, 254 nm... 

Figure 3. GPC separation of polystyrene standards at different flow rates. Column Perkin-Elmer/PL gel 10- mixed.

The GPC of a local crude (Bryan, Texas) sample spiked with a known mixture of n-alkanes and aromatics is shown in Figure 5 and the GPC of the crude is shown in Figure 6. The hydrocarbon mixture is used to calibrate the length of the species which separates as a function of retention volume. Ttie molecular length is expressed as n-alkane carboa units although n-alkanes represent only a fraction of the hydrocarbons in the crude. In addition to n-alkanes, petroleum crude is composed of major classes of hydrocarbons such as branched and cyclic alkanes, branched and cyclic olefins and various aromatics and nonvolatiles namely asphaltenes. Almost all of the known aromatics without side chains elute after n-hexane (Cg). If the aromatics have long side chains, the linear molecular size increases and the retention volume is reduced. Cyclic alkanes have retention volumes similar to those of aromatics. GPC separates crude on the basis of linear molecular size and the species are spread over 10 to 20 ml retention volume range and almost all of the species are smaller than the polystyrene standard (37A). In other words, the crude has very little asphaltenes. The linear... 

Gel permeation chromatography is to be used to separate a mixture of four polystyrene standards of molecular mass 9200, 76000, 1.1 x 106 and 3 x 106 daltons. Three columns are available for this exercise. They are prepacked with gel with the following fractionation ranges for molecular weights ... 

Fig. 5. Transition from the exclusion to the adsorption separation mode through critical conditions for polystyrene standards at a varying composition of the binary eluent (CCI4—CHC13). (Column Si-300, flow rate u = 0.5 rnl/min, volume of.santple 10 pi, UV detector, >, = 275 nm, t = 27 °C)...

Fig. 3. Isocratic separation of oligomers from a polystyrene standard (M = 2100 g/mol) on a silica column (250 x 4 mm do = 6 nm dp = 5 pm). Injection m0 = 0.3 mg in 10 pi eluent n-pentane — tetrahydrofuran (87 13, v/v) flow rate 1 ml/inin UV detection at 254 nm. (From Ref.14) with permission)...

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