Liquid chromatography at the critical point

As has been pointed out, both entropic and enthalpic interactions affect the chromatographic behavior of macromolecules. They are adjusted to the required type of separation by selecting appropriate stationary and mobile phases. In a third mode of liquid chromatography of polymers, liquid chromatography at the critical condition (LCCC) (Entelis etal., 1985,1986 Pasch, 1997), the adsorptive interactions are fully compensated by entropic interactions. This mode is also referred to as liquid chromatography at the critical point of adsorption. Hence, TAS is equal to AH and therefore, AG becomes zero. K is 1 irrespective of molar mass and, consequently, homopolymer molecules of different molar masses coelute in one chromatographic... 

In the case of heterogeneous polymers the experimental methods need to be refined. In order to analyze those polymers it is necessary to determine a set of functions / (M), which describe the distribution for each kind of heterogeneity i This could be the mass distributions of the blocks in a diblock copolymer. The standard SEC methods fail here and one needs to refine the method, e.g., by performing liquid chromatography at the critical point of adsorption [59] or combine SEC with methods, which are, for instance, sensitive to the chemical structure, e.g., high-pressure liquid chromatography (HPLC), infrared (IR), or nuclear magnetic resonance spectroscopy (NMR) [57],... 

Coupling of Liquid Chromatography at the Critical Point of Adsorption and SEC... 

The application of liquid chromatography at the critical point of adsorption to block copolymers is based on the consideration that Gibbs free energy AGab of a block copolymer AnBm is the sum of the contributions of block A and block B, AGa and AGB respectively. 

The use of liquid chromatography at the critical point of adsorption (CC, critical chromatography) for the determination of the functionality type distribution of telechelics was demonstrated by Evreinov and co-workers [2,11-15]. Meanwhile, a significant number of investigations on functional polyolefines, oligoethers, polyesters, and epoxy resins were conducted showing the usefulness of this new technique. 

Fig. 14. Schematic representation of different chromatographic situations in liquid chromatography at the critical point of adsorption of block copolymers. For explanation of 1-4 see text...

According to Gorbunov and Skvortsov [18], triblock copolymers of the ABA type may be analyzed by liquid chromatography at the critical point of adsorption similar to the analysis of diblock copolymers. The two possible cases for this type of investigation, i.e. (a) the analysis with respect to the inner block B using the critical conditions of the outer block A, and (b) the analysis of the outer block A using the critical conditions of the inner block B, will be discussed briefly. 

As heterogeneous polymers are distributed in more than one molecular parameter, more than one chromatographic separation technique must be used. For functional homopolymers evidence is first obtained that the optimum separation protocol includes liquid chromatography at the critical point of adsorption as the first dimension of separation, yielding fractions which are homogeneous in functionality. When these fractions are subjected to any molar mass sensitive separation technique, MMD for each functionality fraction, and therefore the complete FTD-MMD relationship, is obtained. Two-dimensional separations of this type are very much susceptible to automation, as has been shown by Much et al. [88] and Kilz and coworkers [89-91]. 

Separation of a technical polyethylene oxide (PEG) by liquid chromatography at the critical point of adsorption and analysis of fraction by MALDl-MS. Peak assignment indicates degree of polymerization (n). Column Nucleosil 100 RP-18 (126 x 4 mm I.D.) eluent acetonitrile-water (70 30 v/v). (Reprinted with permission from Ref. 176)... 

Pasch, H., Liquid Chromatography at the Critical Point of Adsorption— A New Technique for Polymer Characterization, Macromol Symp., 110,107,1996. Pasch, H., Hyphenated Techniques in Liquid Chromatography of Polymers, Advan. Polym. Sci., 150,1, 2000. 

Pasch H. Liquid chromatography at the critical point of adsorption—a new technique for polymer characterization. Macromol Symp 1996 110 107-20. 

Spriestersbach KH, Rode K, Pasch H. Matrix-assisted laser desorption/ionization mass spectrometry of synthetic polymers, 7. Analysis of fatty alcohol ethoxylates by coupled liquid chromatography at the critical point of adsorption and MALDI-TOF mass spectrometry. Macromol Symp 2003 193 129-41. 

Macromolecular Materials and Engineering 286, No.3, 30th March 2001, p.161-7 MATRIX-ASSISTED LASER DESORPTION/ IONIZATION MASS SPECTROMETRY OF SYNTHETIC POLYMERS, 5A. ANALYSIS OF POLY(PROPYLENE OXIDE)S BY COUPLED LIQUID CHROMATOGRAPHY AT THE CRITICAL POINT OF ADSORPTION AND MALDI-TOF MASS SPECTROMETRY Keil C Esser E Pasch H Darmstadt,Deutsches Kunststoff-Institut... 

Much more important than GC are the different techniques of LG. Using SEC, LAC or liquid chromatography at the critical point of adsorption (LCCC) polymers can be fractionated with respect to different aspects of molecular heterogeneity, including molar mass, funcdonality, and chemical composition. As will be shown in the next applications, LC can be efficiently... 

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