Anal. Chem. 2001, 73, 3781-3783
Fractionation of Moderate Molecular Weight Polysiloxanes by Centrifugal TLC Shashi K. Gupta, Jonathan R. Sargent, and William P. Weber*
K. B. and D. P. Loker Hydrocarbon Institute, Department of Chemistry, University of Southern California, Los Angeles, California 90089-1661
Fractionation of polydispersed polysiloxanes (1-2 g) into narrow molecular weight fractions has been achieved by a rapid (30-60 min) convenient process, using the Cyclograph Centrifugal Chromatography System. These narrow molecular weight poly(dimethylsiloxane) fractions can be used as secondary standards for GPC. Polymer synthesis often leads to polydispersed materials. The extent of polydispersity depends on the mechanism of the polymerization reaction. It is desirable to have homogeneous narrow molecular weight materials to determine the effect of molecular weight on polymer properties. For this reason, fractionation of polymers is quite important. Fractionation of a polymer sample is its separation into different molecular weight fractions that are narrow or homogeneous in molecular weight distribution and composition. Previous work has been done in this field and numerous methods have been reported. Among these are column chromatography,1 thin-layer chromatography,2 supercritical fluid extraction,3 supercritical fluid chromatography,4 field flow fractionation,5,6 high-pressure liquid chromatography,7 gel permeation chromatography (GPC),8,9,10 liquid-liquid chromatography,11 and gel centrifugation chromatography.12,13 All of these techniques have specific limitations. In the laboratory, the common practice is to precipitate a polymer from a solvent-nonsolvent combination. By careful adjustment of solvent composition, this process can be used to fractionate a polymer sample to obtain lower polydispersed fractions. Determination of a suitable solvent system can be time* Corresponding author: (e-mail)
[email protected]. (1) Barrall, E. M. II; Johnson, J. F.; Cooper, A. R. Fractionation of Synthetic Polymers; Tung, L. H., Ed.; Marcel Dekker: New York, 1977; pp 267-344. (2) Inagaki, H. Fractionation of Synthetic Polymers; Tung, L. H., Ed.; Marcel Dekker: New York, 1977; pp 649-344. (3) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice; Buttersworths: Boston, 1986; pp 143-180. (4) Ute, K. Chromatogr. Sci. Ser. 1998, 75, 349-368. (5) Colfen, H.; Antonietti, M. Adv. Polym. Sci. 2000, 150, 67-187. (6) Giddings, J. C. Pure Appl. Chem. 1979, 51 (7), 1459-71. (7) Chang, T.; Lee, W.; Lee, H. C. Annu. Technol. Conf.-Soc. Plast. Eng. 1998, 56 (2), 2101-2105. (8) Moore, J. C. J. Polym. Sci., Part A: Polym. Chem. 1996, 34 (10), 18331842. (9) Nakano, M.; Goto, Y. Jpn. Kokai Tokkyo Koho Patent JP 55030620, 1980. (10) Peyrouset, A.; Panaris, R. Bull. Soc. Chim. Fr. 1974, 11 (Pt. 1), 22792284. (11) Muelle, W. Ger. Offen. Patent DE 19840126, 1984. (12) Andersen, K. B.; Vaughan, M. H. J. Chromatogr. 1982, 240 (1), 1-8. (13) Yoshida, T.; Hatano, T. Analusis 1997, 25 (8), M20-M22. 10.1021/ac010219b CCC: $20.00 Published on Web 07/03/2001
© 2001 American Chemical Society
consuming and tedious.14 GPC and column chromatography are two of the common techniques employed for polymer fractionation. Preparative GPC (based on size exclusion chromatography), though effective, is able to handle only small amounts of material. Preparative-scale GPC columns are quite expensive. Column chromatography, on the other hand, can handle larger amounts of sample. A lot of work has been done in this field,1 but it is still a complicated, tedious, and relatively expensive procedure. Large amounts of solid support and redistilled solvents, which have no residue on evaporation, are required. Sample preparation by careful deposition on a suitable substrate has been shown to be of importance to successful fractionation.15,16 We report a new method for fractionation of polysiloxanes, which is convenient and fast. A few years ago, Analtech, Inc. introduced the Cyclograph Centrifugal Chromatography System17 for preparative thin-layer chromatographic (TLC) separation of small molecules. This instrument has been used for the separation and purification of small molecules in various fields,18 such as pharmaceutical,19 natural product synthesis,20 fossil hydrocarbon analysis,21 and routine organic synthesis.22 It has not previously been applied to the fractionation of polymers. We have used this instrument to fractionate poly(dimethylsiloxanes) with broad polydispersity to obtain narrow-dispersed fractions. These fractions can serve as secondary standards for siloxane polymers. EXPERIMENTAL SECTION GPC. The molecular weight distribution of the polymers was determined on a Waters system comprised of a U6K injector, a 510 HPLC pump, a R401 refractive index detector, and a Millenium 2.15 Session Manager Control System. Two 7.8 mm × 300 mm Styragel columns packed with
