Size exclusion chromatographic measurements

Wyatt, P. J., Hicks, D. L., Jackson, C., and Wyatt, G. K., Absolute gel permeation chromatographic determination of molecular weight and sizes. Part 2. Incorporation of gel permeation chromatography-size exclusion chromatographic measurements, Am. Lab., 20, 108, 1988. 

The size exclusion chromatographic measurements with tracers of different molecular diameters give information on the differential porosity, i.e. on the pore diameter distribution /4/. The evaluation of the retention times tR.i yields the pore volume... 

Anionic pol5Tnerizations make the molecular weight standards that are used to calibrate size-exclusion chromatographs. Equation (13.38) predicts PD = 1.001 at In = 1000. Actual measurements give about 1.05. The difference is attributed to impurities in the feed that cause terminations and thus short chains. Also, the chromatograph has internal dispersion so that a truly monodisperse sample would show some spread. Even so, a PD of E05 is extremely narrow by pol5Tner standards. This does not mean it is narrow in an... 

The validity of a practical method of direct viscosity calculation from size exclusion chromatographic analysis is demonstrated. The method is convenient to use and is not limited by the availability of narrow MWD standards. It is possible to accurately measure polymer Mark-Houwink constants using the suggested broad standard SEC-[n] and SEC-MW calibration procedure. 

The calibration standards included sodium form polystyrene sulfonates obtained from Pressure Chemical Co., Pittsburgh, Pa., and sodium toluene sulfonate. Measurements were taken at 0.5 to I.Oml/mln flow rates. The logarithm of the molecular weight of the standards was linear it suggests a framework for approaching an interpretion of the structure of the scission products. This application of size exclusion chromatography measurements must be viewed as a first approximation because of the unmeasured differences between the chromatographic behavior of the linear standards and the expected branched structure of the scission products. 

Experimental. The differential refractive indexes of polymer solutions were measured at 25°C with a Waters Scientific R-403 differential refractometer connected on-line with a size exclusion chromatograph. The refractometer was calibrated to refractive index units (Riu) with benzene/carbon tetrachloride solutions. The rationale behind using the refractometer on-line with the chromatograph is the elimination of impurities in the sample (water, residual monomer etc.) which affect the refractive index measurements particularly at low polymer concentrations and to calibrate the detectors at the flow conditions at which they were normally operated. Polymer solutions of several concentrations (0.015-0.0025 wt %) were injected repeatedly to verify the reproducibility of the measurements, which was typically An 0.5 x 10-6 for replicates on the same solutions. 

Experimental. Absorbances of selected polymer and copolymer samples were measured with a Waters 440 ultraviolet absorbence detector equipped with a 254 nm filter, and a Beckman 25 ultraviolet/visible spectrophotometer. The Water s instrument was used on-line with the size exclusion chromatograph (16). The Beckman 25 was used to calibrate and standardize the Water s 440 UV detector. THF was used as standard solvent. The extinction coefficients were estimated from absorbance measurements at several polymer concentrations. The standard deviation for the absorbance measurements was typically . 001 Au for replicates on the same sample. All measurements were done at room temperature. 

Size exclusion chromatographic analyses in tetrahydrofuran were performed with a Waters 150C GPC with six p-Styragel columns having a continuous porosity range of 10 - 10 k and also with the Perkin- Elmer Model 601 HPLC with three p-Styragel columns (10 , lO", 10 A) after calibration with standard polystyrene samples. Intrinsic viscosities were measured In chloroform at 30.0 C and in tetrahydrofuran at 40°C using an Ubbelohde type viscometer. 

K12 Kim, S., Cotts, P.M., and Volksen, W., On-line measurement of the rms radius of gyration and molecular weight of polyimide precursor fractions eluting from a size-exclusion chromatograph, J. Polym. ScL PartB Polym. Phys., 30, 177, 1992. 

The IR spectra of the polymers were measured on a Perkin Elmer 16 PC FTIR spectrophotometer. The NMR analyses were performed on a Bruker ARX 300 NMR spectrometer in deuterated chloroform using TMS as internal standard. The UV spectra were recorded on a Milton Roy Spectronic 300 Array spectrophotometer and the molar absorptivity (Emu ) was calculated on the basis of the repeat unit of the polymer. The relative molecular weights of the polymers were estimated using a size-exclusion chromatograph (SEC) system equipped with a Waters 510 HPLC pump, ultrastyragel... 

Size exclusion chromatography measures directly this excluded-volume. Silica beads used in SEC (Fig. 2-c) have been claimed to be "fractal. We analyse chromatographic data using this picture and shall see in section III that in many cases, it is possible to characterize a chromatographic column by a fractal dimension Dp... 

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