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To separate copolymers by composition, interactive LC must be employed. SEC is one type of HPLC in which solute molecules are separated by the size-exclusion mechanism. The separation mechanisms of HPLC except SEC are based on interactions between the stationary phase and the solute molecules. Besides these interactions, the solubility difference of copol-ymers in the mobile phases is also one of the separation mechanisms for the copolymers. Besides these two mechanisms, there are others for the separation of copolymers according to composition. Therefore, NELC is defined as all kinds of HPLC except SEC for the separation of copolymers according to composition. Hyphenated techniques, such as SEC-NELC are required to characterize both MWD and CCD of copolymers in detail and accurately.
B. Separation Mechanisms by Composition
Liquid adsorption chromatography (LAC) is one mode of interactive LC, and silica gel is mostly used as the stationary phase. Hydrogen bonding between silanol groups at the silica surface and appropriate groups on the copolymer molecules is the main mechanism for ad-sorption. The initial mobile phase is a good solvent for the copolymer molecules that have to first adsorb on the surface of silica gel. Gradient elution is performed to increase the content of a displacer in the mobile phase, which decreases the adsorption power of the stationary phase for the absorbed copolymer molecules.
Solubility difference of copolymers in the mobile phase plays the important role for separation of the copolymers, and NELC used in this category is defined as liquid precipitation chromatography (LPC). The initial mobile phase is a nonsolvent for sample copolymers, so that the copolymers injected into a column precipitate on the inlet of the column. Subsequent gradient elution is performed by increasing the content of a good solvent for the copolymers in the mobile phase, so that the copolymers precipitated in the inlet of the column are redis-solved into the mobile phase when it reaches a specific composition and elute from the column. Solubility of the copolymers in the mobile phase governs the separation of the copolymers. The influence of the types of the stationary phase on the separation is small.
In LAC, the mobile phases during elution from the beginning until the complete elution of the copolymers must be good solvents for the copolymers. In LPC, the copolymers injected into the column must precipitate at first in the inlet of the column and redissolve in the mobile phase by increasing the amount of a good solvent in the mobile phase. If the separation mechanism is unclear, or if no adsorption to the stationary phase or no precipitation on the column are expected at the beginning of the sample elution, then the techniques should be classified by other separation modes. They are normal-phase (NPLC) and reversed-phase
(RPLC) liquid chromatography depending on the polarity of the mobile phase and the stationary phase.
Besides these four different separation modes, there are two other techniques to separate copolymers according to composition. They are explained in Sec. III.F.
C. Separation by Liquid Adsorption Chromatography
1. Survey of the Literature
Nonexclusion LC was first applied for the separation of styrene-methyl acrylate copolymers according to composition by Teramachi in 1979 . Silica gel was the stationary phase, and a gradient elution from 7% methyl acetate in carbon tetrachloride to 31% methyl acetate in carbon tetrachloride was applied. Styrene-methyl methacrylate copolymers, P(S-MMA), were separated by a gradient elution method from 3% THF in dichloroethane to 20% THF with silica gel as the stationary phase . Polyacrylates, polymethacrylates, and their copolymers were separated with a combination of silica gel-toluene-methyl ethyl ketone (MEK) . An evaporative light-scattering detector was used to monitor the polymers. A gradient elution condition was 2% MEK in toluene to 100% MEK (for polyacrylates, polymethacrylates, and their copolymers) or 50% MEK (for styrene copolymers of acrylates and methacrylates).
_____In LAC, the mobile phases including the initial and the final mobile phases in gradient
elution should be good solvents for the sample copolymers. All solvents described in the foregoing are good solvents for these polymers and copolymers. At least they can dissolve them. When carbon tetrachloride, dichloroethane. and toluene, including a small amount of displacers, are used as the initial mobile phases, these copolymers adsorb on the surface of silica gel, and methyl acetate, THF, and MEK are used as displacers. Both mobile phase and stationary phase play important roles for the separation of the copolymers according to composition. Hydrogen bonding between carbonyl groups of the copolymers (or acrylate and methacrylate polymers) and silanol groups of silica surface is the main interaction. Elution of these styrene copolymers is in the order of decreasing styrene content and that of polyacrylates and polymethacrylates is in the order of butyl, ethyl, and methyl acrylates and methacrylates.