Liquid chromatography at the critical

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],... 

Phillips, S. L., and Olesik, S. V. (2003). Initial characterization of humic acids using liquid chromatography at the critical condition followed by size-exclusion chromatography and electrospray ionization mass spectrometry. Anal. Chem. 75,5544—5553. 

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]. 

In tile technique of liquid chromatography at the critical condition (LCCC), macromolecules of different sizes are eluted at the same time (Figure 10.19). ° This peculiar elution behavior is achieved making use of columns in which the macromolecules are at the adsorption-elution transition, and Figure 10.19 reports the calibration curve for LCCC along with the curves for adsorption and size exclusion chromatography, respectively. ... 

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. 

Kitayama, T., Janco, M., Ute, K., Niimi, R., Hafada, K., and Berek, D., Analysis of Poly(ethyl methacrylate)s by On-line Hyphenation of Liquid Chromatography at the Critical Adsorption Point and Nuclear Magnetic Resonance Spectroscopy, Anal Chem., 71, 1518, 2000. 

Liquid chromatography at the critical condition (LCCC) is performed at the elution-adsorption transition. It can be used to separate macromolecules with different functionalities such as chains with different chain ends or to separate linear chains from cycles. LCCC was used [77] to separate cycles from linear chains in poly(bisphenol-A-carbonate) PC. Figure 45.20 contains the LCCC trace. The trace is bimodal, with two bands, Z1 and Z2. The MALDI spectrum of Z1 displayed a large number of peaks, ranging approximately from 2.0 to 10 kDa, due to PC chains terminated with n-butyl on one side or on both sides. The MALDI spectrum of Z2 was far less crowded. It is made of cycles and one can note the systematic absence of linear chains. This implies that the LCCC separation is perfect. 

Kitayama, T. Janco, M. Ute, K. Niimi, R. Hatada, K. Analysis of poly(ethyl methacrylate)s by on-hne hyphenation of liquid chromatography at the critical adsorption point and nuclear magnetic resonance spectroscopy. Anal. Chem. 2000, 72, 1518-1522. 

Cho D, Park S, Kwon K, Chang T, Roovers J. Structural characterization of ring polystyrene by liquid chromatography at the critical condition and MALDl-TOF mass spectrometry. Macromol 2001 34 7570. 

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

Kitayama T, Janco M, Ute K, Niimi R, Hatada K, Berek D. Analysis of polyjethyl methacrylatejs by on line hyphenation of liquid chromatography at the critical... 

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