Polymer HPLC exclusion-interaction effects

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). 

In this case, enthalpic interactions within the HPLC system exceed the exclusion effects (Eigure 16.3b). The retention volumes of polymer species as a rule exponentially increase with their molar masses. The limitations of the resulting procedures were elucidated in Section 16.3 the retention of (high)polymers is usually so large that these do not elute from the column (Section 16.6). Therefore, the majority of enthalpy controlled HPLC procedures is applicable only to oligomers—up to... 

In this case, the enthalpic interactions within the HPLC system exceed the exclusion effects (see Figure 3(e)). The retention volumes of polymer species as a rale exponentially increase with their molar masses. The important limitation of the resulting procedures was presented in section 11.5.2.3. The retention of (high) polymers is usually so intense that the latter do not elute from the column any more. Therefore, the majority of enthalpy controlled HPLC procedures is applicable only to ohgomers - up to molar mass of few thousands g.mol. Still, the reduced sample recovery may affect results of separation even in case of oligomers. The selectivity of enthalpy driven HPLC separation is much higher than in the case of SEC but, naturally, the sequence of molar masses eluted from the column is reversed. If the effect of enthalpy is reduced, problems with sample recovery are mitigated - but at the same time the separation selectivity is reduced. 

As expected, a small molar mass dependence of the H PLC separation is indicated by a drift of the peaks for components of similar chemical composition (e.g., with peaks nos. 1, 5, 9, and 13). This kind of behavior is normal for polymers, because pores in the HPLC stationary phase lead to size-exclusion effects, and these overlap tvith the enthalpic interactions at the surface of the stationary phase. The 2D experiment is sufficiently sensitive to reveal influences of this type. The amount of butadiene in each peak could be qmntitatively determined through an appropriate calibration with samples of known composition. The molar masses could be calculated based on a conventional molar mass calibration of the second dimension. 

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