Homopolymers, coupled column

Cortes et al. [18] have quantitatively determined polymer additives in a polycarbonate homopolymer and an ABS terpolymer. In that case, a multidimensional system consisting of a microcolumn SEC was coupled on-line to either capillary GC or a conventional LC system. The results show losses of certain additives when using the conventional precipitation approach. An at-column GC procedure has been developed for rapid determination (27 min) of high-MW additives (ca. 1200Da) at low concentrations (lOOppm) in 500- xL SEC fractions in DCM for on-line quality control (RSD of 2-7%) [36], Also, SEC-NPLC has been used for the analysis of additives in dissolution of polymeric... 

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. 

One or more detectors is attached to the output of the columns. For routine analysis of linear homopolymers, this is most often a Differential Refractive Index (DRI) or a UV detector. For branched or copolymers, however, it is necessary to have at least two sequential detectors to determine molecular weight accurately. Branched polymers can be analyzed using a DRI detector coupled with a "molecular weight sensitive" detector such as an on-line viscometer (VIS) or a low-angle laser light scattering (LALLS) detector. 

SEC is routinely used to produce narrow distribution samples by fractionation. However, even with preparative SEC columns the procedure is not particularly effective, and many repeated fractionations are required to produce more than milligram samples. Nevertheless, such samples are invaluable in crystallization-rate [70] and other studies where molecular mass exclusion is very important. Coupling molecular mass with temperature-rising elution fractionation, in which molecular species separate by composition, in copolymers, or by degree of branching and tacticity, in homopolymers, makes this a most important method for separating molecular mass and chemical effects in crystallization studies [17]. 

HPLC equipment dedicated to high-osmotic-pressure chromatography is used for the fractionation of narrow polymer fractions from broad distribution samples. This technique, which employs columns that are packed with a control pore glass of very narrow pore distribution, separates polymers by molecular weight as a function of osmotic pressure. When this approach is coupled with a fraction collector the technique can generate polymer fractions in significant quantities for further study by nuclear magnetic resonance, (NMR), FTIR, or other spectroscopic techniques. This technique can offer superior resolution to the previously mentioned preparative GPC. This technique has been applied to the characterization of both copolymers and homopolymers. 

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