Reconcentration of diluted polymer solutions by full adsorption/desorption procedure - 1. Eluent switching approach studied by size exclusion chromatography

A Full Adsorption-Desorption (FAD) procedure is proposed for the controlled reconcentration of diluted polymer solutions. In the first step, macromolecules are quantitatively trapped from their diluted solution onto an appropriate adsorbent. In the second step, polymer is released by a small amount of desorption promoting liquid. The efficiency of this reconcentration process was tested with poly(methyl methacrylate) (PMMA), polytetrahydrofuran (PTHF), poly(ethylene oxide) (PEO) and polystyrene (PS). Bare nonporous silica packed into FAD minicolumns was used for reconcentration of dilute solutions of PMMA, PTHF and PEO in toluene and chloroform. Nonporous silica bonded with aliphatic C-18 groups was applied to reconcentrate PS solutions from dimethylformamide (DMF). The desorbing Liquid was tetrahydrofuran for PMMA and PTHF, DMF for PEO and toluene for PS. The efficiency of reconcentration was tested by size exclusion chromatography that was coupled on-line with the FAD minicolumn. The efficiency of reconcentration was found to be very high in the case of medium polar and polar polymers PTHF, PMMA and PEG: full recovery of macromolecules was demonstrated by the SEC peak sizes and shapes. Consequently, agreement was found between the values of mean molar mass and polydispersity obtained for a series of polymer samples before and after the reconcentration procedure if the size of the FAD column was optimized. On the other hand, it was more difficult to effectively reconcentrate polystyrene which was not fully adsorbed on bare silica from the most common solvents. Agreement was obtained for molecular characteristics of PS before and after reconcentration from dimethylformamide on C-18 bonded silica for higher molar masses above ca. 9 x 10(4) g mol(-1). Very diluted solutions containing 5 mg of polymer per litre of solution could be easily treated in this way and no problems are anticipated even with more diluted systems. The FAD procedure also allows ready exchange of the sample solvent and it can be used in various analytical methods, especially in multidimensional liquid chromatography of macromolecules where effluent from a column separating in the 'first dimension' is further separated in the 'second- and higher dimension' columns. The reconcentration of polymer mixtures including the selective reconcentration of one blend component and thus the preseparation of blends is also feasible. Preparative reconcentration based on the controlled full adsorption-desorption processes is also expected to be possible. In any case, the appropriate adsorbent and both adsorbing and desorbing liquid must be carefully chosen. (C) 1998 Elsevier Science Ltd. All rights reserved.