LIQUID-CHROMATOGRAPHY OF MACROMOLECULES AT THE CRITICAL ADSORPTION POINT - II - ROLE OF COLUMN PACKING - BARE SILICA-GEL

Liquid chromatography of macromolecules at the critical adsorption poi nt (LC CAP) presents a potentially very powerful method for molecular characterization of complex polymers. However, LC CAP applicability is limited due to various experimental problems. The pore sizes and surf ace chemistry of the column packings belong to the most important weak points of the method. The LC CAP behavior of poly( methyl methacrylat e)s was investigated using bare silica gels of 6, 12, and 100 nm pore sizes and with various amounts of surface silanols. Tetrahydrofuran as the adsorption suppressing liquid and toluene as the adsorption promo ting liquid were mixed to form the ''nearly critical'' eluents. Both p ore size and surface chemistry of silica were found to strongly influe nce the retentive characteristics of the system in the critical adsorp tion area. Macromolecules that were large enough to be excluded from t he packing pores hardly followed the LC CAP rules: their retention vol umes changed irregularly with the polymer molar mass and their recover y dropped sharply. The narrow pore silica gel-packed column governed t he elution patterns of the whole column set composed of silica gels wi th different pore sizes. This makes the conventional LC CAP characteri zation of common polymers with broader molar mass distribution impract ical and even not feasible. A hybrid column system was proposed contai ning narrow pore nonadsorptive column added in series to the meso-and macroporous LC CAP silica gels. This narrow pore column would allow se paration of gas, impurities, and system peaks from the polymer peaks. The possible successive changes of the surface of silica gel, e.g., du e to formation of silanols by hydrolysis or due to irreversible adsorp tion of some admixtures from the sample or eluent may make the LC CAF irrepeatable. Pronounced peak broadening was observed in the critical adsorption area and this effect increased strongly with the polymer mo lar mass. (C) 1998 John Wiley & Sons, Inc.