THEORETICAL ANALYSIS OF THE RELATIONSHIP BETWEEN SOLVATED POLYMER PEAK AND SOLVENT PEAK IN CHROMATOGRAMS FROM SIZE-EXCLUSION CHROMATOGRAPHY WITH MIXED ELUENTS

Abstract

Each size-exclusion chromatography (SEC) curve generally exhibits two peaks when a solvent mixture is employed as the eluent.The first peak is the solvated-polymer peak and the second is the free-solvent peak.The area of the solvated-polymer peak Aeff_3 can be regarded as the sum of contributions from the naked-polymer A_3 and from the extra polymer-bound solvent A_1.The free-solvent peak,however,is caused by deficiency of the same solvent that enriches itself around polymer chains.Theoretical analysis shows that the size of free-solvent peak A_2 and that of the extra polymer-bound solvent A_1 are equal in magnitude but opposite in sign.This relationship suggests that the preferential adsorption coefficient λ_V can also be determined from the analysis of the polymer peak.Polystyrene(3)-benzene(1)-methanol(2) is the system taken to test the correctness of the theoretical analysis.The polystyrene (PS) sample was purchased from Krauer Company,with M_w=9.26×104,and M_w/M_n=1.1. Solvents (analytical grade purity) were purified before use.Benzene was purified by repeatedly shaking with concentrated sulphuric acid (15 vol% benzene) in a separating funnel until the acid phase remained colorless.The benzene phase was then washed with water,aqueous sodium hydroxide (10 wt%),again with water,dried with anhydrous calcium chloride,and finally distilled. Methanol was fractionally distilled,and the middle cut was used.The eluents were benzene-methanol mixtures of different compositions at a flow rate of 0.4 cm3/min.The mixtures were made by volume and equilibrated in the reservoir of the GPC system.The injected sample solutions were prepared using solvent mixtures from the solvent reservoir.The range of volume fraction of benzene in the eluent U_10 was from 1.00 to 0.80. All measurements were conducted at room temperature with a Waters Associates model 244DS liquid chromatograph,equipped with a model R401 differential refractometer and a model 730 microcomputer.A single column packed with Styragel (nominal porosity 50 nm,length 30 cm) was employed.When a solvent mixture is used as eluent,two measurements can be carried out,with and without column in place.When there is a column in place,the polymer-bound solvent will get separate from the free solvent.The selective binding of solvent to the polymer causes polymer-bound solvent signal and the polymer signal to merge.The measured peak area is Aeff_3.In the experiment without column in place,there is a direct connection between the outlet of the injector and the detector.In this situation,the chromatographic system cannot separate the components.The free solvent,polymer-bound solvent,and the solute will elute at the same time.The composition deviations of polymer-bound solvent and free solvent to the original solvent mixture caused by preferential solvation can generate signals,but,since they elute at the same time,the contributions from them to the refractive index will cancel because of their opposite response signals.In this case the measured peak area is A_3.When Aeff_3 and A_3 are available,the preferential adsorption coefficient λ_V can be obtained. For the system consisting polystyrene (3) plus benzene (1)-methanol (2) solvent mixtures,the values of λ_V from the present work are plotted against (1-u_10),together with the results for the same system reported by Hert from measurements of dialysis equilibrium and light scattering technique.The agreement of the present results with those from previous work shows that the proposed method is valid.The advantages of studying the solvated polymer peak are discussed. This method provides an alternate way to determine specific refractive index increment at constant potential (dn/dc)_μ for polymer/mixed solvent systems.