Absolute polymer retention

Absolute polymer retention values showed an almost linear dependency upon polymer concentration. The effect of polymer slug size on absolute polymer retention is also discussed. 

The absolute values of polymer retention were found to be not as great as was earlier reported [6,10]. 

Absolute permeability Min. approx. 20 md No max. To avoid excessive polymer retention... 

Experimental variables such as temperature, flow rate, sample concentration and mobile phase composition can cause changes in the elution volume of a polymer [439,457,460-464]. Chromatographic measurements made with modem equipment are limited more by the errors in the absolute methods used to characterize the molecular weight of the calibration standards than any errors Inherent in the measurements themselves, since the determination of molecular weights by SEC is not an absolute method and is dependent on calibration [462]. The Influence of temperature on retention in SEC is not very great, since no strong sorptive interactions are involved in the retention mechanism. Temperature differences between the column and solvent delivery... 

The significant intrinsic limitation of SEC is the dependence of retention volumes of polymer species on their molecular sizes in solution and thus only indirectly on their molar masses. As known (Sections 16.2.2 and 16.3.2), the size of macromolecnles dissolved in certain solvent depends not only on their molar masses but also on their chemical structure and physical architecture. Consequently, the Vr values of polymer species directly reflect their molar masses only for linear homopolymers and this holds only in absence of side effects within SEC column (Sections 16.4.1 and 16.4.2). In other words, macromolecnles of different molar masses, compositions and architectures may co-elute and in that case the molar mass values directly calculated from the SEC chromatograms would be wrong. This is schematically depicted in Figure 16.10. The problem of simultaneous effects of two or more molecular characteristics on the retention volumes of complex polymer systems is further amplifled by the detection problems (Section 16.9.1) the detector response may not reflect the actual sample concentration. This is the reason why the molar masses of complex polymers directly determined by SEC are only semi-quantitative, reflecting the tendencies rather than the absolute values. To obtain the quantitative molar mass data of complex polymer systems, the coupled (Section 16.5) and two (or multi-) dimensional (Section 16.7) polymer HPLC techniques must be engaged. 

Because HIC supports are designed for macromolecules, they either possess pore diameters of at least 300 A to allow inclusion or are nonporous. Both silica and polymer matrices are used because the hydrophilic polymeric coating minimizes or ehminates most matrix-based effects. The absolute retention and selectivity of an HIC support may be affected by the specific composition of the bonded phase, as well as the ligand. For example, protein mixtures have shown distinct selectivity on different HIC columns which have propyl functional groups [5]. 

Though the absolute methods for the determination of molecular weights are well established, both theoretically and experimentally, the absolute measurements are difficult to carry out, are time-consuming, and often require expensive apparatus. For these reasons, for routine determinations of moleculcu- weight, the much faster secondary methods, such as solution viscosity and gel permeation chromatography, are commonly used. These methods require prior establishment of empirical relationships that relate the molecular weight to the viscosity of the polymer solution or to the retention times in a gel-permeation column. Once such calibration has been done, the secondary methods provide a fast, simple, and accurate... 

Universal Calibration In the conventional calibration (described above), there is a problem when a sample that is chemically different from the standards used to calibrate the column is analyzed. However, this is a common situation for instance, a polyethylene sample is run by GPC while the calibration curve is constructed with polystyrene standards. In this case, the MW obtained with the conventional calibration is a MW related to polystyrene, not to polyethylene. On the other hand, it is very expensive to constmct calibration curves of every polymer that is analyzed by GPC. In order to solve this problem, a universal calibration technique, based on the concept of hydrodynamic volume, is used. As mentioned before, the basic principle behind GPC/SEC is that macromolecules are separated on the basis of their hydrodynamic radius or volume. Therefore, in the universal calibration a relationship is made between the hydrodynamic volume and the retention (or, more properly, elution volume) volume, instead of the relationship between MW and elution volume used in the conventional calibration. The universal calibration theory assumes that two different macromolecules will have the same elution volume if they have the same hydrodynamic volume when they are in the same solvent and at the same temperature. Using this principle and the constants K and a from the Mark-Houwink-Sakurada equation (Eq. 17.18), it is possible to obtain the absolute MW of an unknown polymer. The universal calibration principle works well with linear polymers however, it is not applicable to branched polymers. 

Often, size exclusion chromatograms (SEC) (compare section 11.7, Size Exclusion Chromatography) of polymers under study are expressed as differential representations of molar mass dispersity. The chromatographic retention volumes are directly transformed into the molar masses. This approach renders useful immediate information about tendencies of molar mass evolution in the course of building or decomposition polyreactions but the absolute values of molar mass can be only rarely extracted from it. As a rale, polystyrene calibrations are applied for molar mass calculation so that one deals with the polystyrene equivalent molar masses, not with the absolute values. The resulting dispersity (distribution) functions may be heavily skewed because the linear part of the calibration dependence for the polymer under study may exhibit well different slope compared with the polystyrene calibration, which was employed for the transformation of retention volumes into molar masses. 

In the low permeability sand the absolute retention versus applied concentration curves show a leveling oif section. This fact is related to the mechanism of polymer entrapment in small pores. In very small pores (173 md sand), less polymer concentration is needed to effectively restrict a continuous polymer buildup in these pores. 

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