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2. A System of Chlorinated Hydrocarbon and Ethanol as the
Among several techniques for LAC with silica gel as the stationary phase, a system of chlo-roform and ethanol or 1,2-dichloroethane (DCE)-ethanol used as the mobile phase is explained rather in detail in this section.
-----Silica gel with a pore diameter of 30 A and a mean particle size of 5 /ëò was packed in
4.6-mm id x 50 mm in length stainless-steel tubing and was thermostated at a specified temperature. Gradient elution was performed with a mixture of chloroform and ethanol or DCE and ethanol. An example of gradient conditions is as follows: the initial mobile phase (A) was a mixture of chloroform and ethanol (99:1), the final mobile phase (B) was a mixture of chloroform and ethanol (93:7), and the composition of the mobile phase was changed from 100% A to 100% Â in 30 min. At column temperature 40-70°C, styrene copolymers of acrylates and metacrylates were separated in the order of decreasing styrene content in the copolymers . Ethanol was used as a displacer.
DCE was transparent at wavelengths over 230 nm and methacrylate and acrylate homo-polymers and copolymers as well as their styrene copolymers could be monitored with a UV detector at about 230 nm . An example is shown in Fig. 7 .
Styrene copolymers of acrylates and methacrylates as well as acrylate and methacrylate homopolymers and their copolymers were retained in the column with pure chloroform and DCE. Only PS could be eluted from the column. By adding ethanol in the mobile phase, the copolymers were eluted from the column in the order of decreasing styrene content in the copolymers. Copolymers having much acrylate or methacrylate required much ethanol in the mobile phase to elute from the column [56,59]. The copolymers tended to absorb on the
Retention Volume, mL
Figure 7 LAC chromatogram of polyethyl methacrylate (e), poly-n-butyl methacrylate (a) and their copolymers. Copolymer: (b) poly(ethyl methacrylate-n-butyl methacrylate), (25:75); (c) (50: 50); (d) (75:25); column temperature: 60°C; detector: UV at 233 nm; gradient condition: 100% DCE-ethanol (99:1) to 100% DCE-ethanol (90:10) in 20 min; flow rate: 0.5 mL min *. (Reprinted with permission from Ref. 58. Copyright (1990). American Chemical Society.)
surface of silica gel in a column at higher column temperature, and those having much acrylate or methacrylate required a lower column temperature for elution. At the same gradient elution condition, the copolymers were more retained and retarded at higher column temperature. The results are shown in Fig. 8 .
The main interaction of the copolymers with the stationary phase is hydrogen bonding of carbonyl groups in the copolymers to silanol groups of the silica surface. Ethanol is feasible to form hydrogen bonding to silanol groups and can control the content of the free silanol groups on the silica surface . Free silanol groups decrease in proportion to the ethanol content in the mobile phase. At higher column temperature, ethanol which formed hydrogen bonding to silanol groups desorbs; consequently, free silanol groups on the silica surface increase. Lower ethanol content in the mobile phase and higher column temperature result in the same phenomenon: free silanol groups on the silica surface increase and the copolymers tend to adsorb on the silica surface.
Plots of the relations between the styrene content and retention volume for the styrene copolymers of acrylate and methacrylate having the same ester groups [e.g., poly-(styrene-methy methacrylate) and poly(styrene-methyl acrylate)] lie roughly on the same line, indicating that a pair of styrene copolymers having the same ester groups and the same styrene content could not be separated . On the other hand, styrene copolymers of methyl acrylate, ethyl acrylate, «-butyl acrylate (or methacrylate) having the same styrene content could be separated and the elution was in the order of «-butyl, ethyl, and methyl acrylates (or meth-acrylates). The example is shown in Fig. 9 .
To determine the molecular weight dependence of retention volume, the copolymers were fractionated by SEC, and fractions were subjected to LAC. The example is shown in Fig. 10 . Peak retention volume of each fraction was almost the same, explaining negligible molecular weight dependence. Fractionation by SEC followed by LAC gave information on both MWD and CCD .
The method presented in this section can be applied to other styrene copolymers for which hydrogen bonding to the silica gel is feasible. Styrene-methyl methacrylate block copolymers  and styrene-vinyl acetate block copolymers  were characterized by SEC-LAC.
M M A , wt %
Figure 8 Relation of retention volume versus MMA content of P(S-MMA) at different column temperatures. (•) 80°C, (¦) 60UC, (î) 50°C, (Ä) 40°C, (?) 30°C; gradient condition: 100% chloroform-ethanol (99:1) to 100% chloroform-ethanol (95.5:4.5) in 15 min; flow rate: 0.5 mL min-1. (Reprinted with permission from Ref. 60. Copyright (1987) John Wiley & Sons.)