Liquid adsorption chromatography composition distribution

Liquid adsorption chromatography (LAV) and GPC have been used to determine chemical composition distribution (CCD) and MWD of styrene-methyacrylate copolymers (230). 

Mori S. (1988), Determination of chemical composition and molecular weight distributions of high-conversion styrene-methyl methacrylate copolymers by liquid adsorption and size exclusion chromatography. Anal. Chem., 60,1125-1128. 

The theory of adsorption at porous adsorbents predicts the existence of a finite critical energy of adsorption e, where the macromolecule starts to adsorb at the stationary phase. Thus, at > the macromolecule is adsorbed, whereas at e < e the macromolecule remains unadsorbed. At e = Ec the transition from the unadsorbed to the adsorbed state takes place, corresponding to a transition from one to another separation mechanism. This transition is termed critical point of adsorption and relates to a situation, where the adsorption forces are exactly compensated by the entropy losses TAS = AH [2, 7]. Accordingly, at the critical point of adsorption the Gibbs free energy is constant (AG = 0) and the distribution coefficient is Kj = 1, irrespective of the molar mass of the macromolecules. The critical point of adsorption relates to a very narrow range between the size exclusion and adsorption modes of liquid chromatography. It is, therefore, very sensitive towards temperature and mobile phase composition. 

For a long time, the use of chromatography was mainly limited to the determination of molecular masses and to the qualitative estimation of the heterogeneity of the samples. At present, it is a highly efficient technique giving very accurate values [82-89]. Copolymers are complex macromolecular systems, characterized by two distributions in molecular mass and chemical composition liquid chromatography used at the critical point of adsorption allows the determination of the molecular heterogeneities (chemical and molecular mass distributions) [83]. 

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