Polymer HPLC macromolecules exclusion

FIGURE 16.3 Dependences of the polymer retention volume on the logarithm of its molar mass M or hydrodynamic volume log M [T ] (Section 16.2.2). (a) Idealized dependence with a long linear part in absence of enthalpic interactions. Vq is the interstitial volume in the column packed with porous particles, is the total volume of liquid in the column and is the excluded molar mass, (b) log M vs. dependences for the polymer HPLC systems, in which the enthalpic interaction between macromolecules and column packing exceed entropic (exclusion) effects (1-3). Fully retained polymer molar masses are marked with an empty circle. For comparison, the ideal SEC dependence (Figure 16.3a) is shown (4). (c) log M vs. dependences for the polymer HPLC systems, in which the enthalpic interactions are present but the exclusion effects dominate (1), or in which the full (2) or partial (3,4) compensation of enthalpy and entropy appears. For comparison, the ideal SEC dependence (Figure 16.3a) is shown (5). (d) log M vs. dependences for the polymer HPLC systems, in which the enthalpic interactions affect the exclusion based courses. This leads to the enthalpy assisted SEC behavior especially in the vicinity of For comparison, the ideal SEC dependence (Eigure 16.3a) is shown (4). 

The solubility of macromolecules as a rule improves with the rising temperature. Solvent - polymer mixtures usually exhibit the upper consolute temperature or upper critical solution temperature, UCST, with a maximum on the plot of system concentration versus temperature. Above the critical solution temperature, polymer is fully soluble at any concentration. For practical work, the systems with UCST below ambient temperature are welcome. There are, however numerous polymer - solvent systems, in which the solvent quality decreases with increasing temperature. The plot of system concentration versus temperature exhibits a minimum. The phenomenon is called lower consolute temperature or lower critical solution temperature, LCST Polymer is only partially soluble or even insoluble above lower critical solution temperature. This unexpected behavior can be explained by the dominating effect of entropy in case of the stiff polymer chains or by the strong solvent - solvent interactions. The possible adverse effect of rising temperature on polymer solubility must be kept in mind when woiking with low solubility polymers and with multicomponent mobile phases. It may lead to the unforeseen results especially in the polymer HPLC techniques that combine exclusion and interaction retention mechanisms, in coupled methods of polymer HPLC (see section 11.8, Coupled Methods of Polymer HPLC). 

Separation of macromolecules in polymer HPLC is based on difference in their size or enthalpic interactivity. In the former case, the entropy of exclusion... 

Size exclusion chromatography is undoubtedly the most important method of polymer HPLC. Therefore, it is inappropriate to speak about HPLC and SEC. The reason for above uncertainty in terminology may be the fact that the exclusion based separations are rare in the case of low-molecular substances except mainly the situation when macromolecules are to be (pre)sepaiated, or just removed from the complex sample. The terms HPLC of low-molecular substances and polymer HPLC are employed in present chapter as general designations to differerrtiate the two groups of liquid chrorrratography methods. 

The presently most popular approach to two-dimensional polymer HPLC avails partial or preferably full suppression of the molar mass effect in the Id column so that the complex polymer or complex polymer system is separated mainly or even exclusively according to chemical structure or physical architecture of macromolecules occuring in sample. Appropriate coupled methods of polymer HPLC are to be applied to this purpose (compare section 11.8). In the 2d separation column - it is usually SEC - the fractions from the Id column are further discriminated according to their molecular size. In other words, fractions obtained in the first-dimension column are separated in the self-existent second-dimension column, which applies distinct separation mechanism(s). Only exceptionally SEC... 

Size exclusion chromatography (SEC) is a method by which molecules can be separated according to their size in solution, thus relating indirectly to their molecular masses. To achieve this, stationary phases contain pores through which compounds are able to diffuse to a certain extent. Although the efficiency of separation can never attain that observed with HPLC, SEC has become an irreplaceable tool to separate natural macromolecules in order to study the distribution of synthetic polymer masses. Though the separation of compounds according to their sizes is not the most efficient process for small and medium molecules, this approach remains very useful in industry where the products are most often mixtures of compounds of very different masses. The instrumentation is comparable to that used in HPLC. 

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