Polymer HPLC absorption

Sample detection in polymer HPLC is a large problem. Only few polymers bear chromophores that allow direct application of photometers, typical for HPLC of many low-molar-mass substances. The use of detectors that operate in the infrared region of spectrum is complicated by the absorptivity of eluents. Still, there is observed some revival in this area, especially in the SEC of polyolefins. 

Another enthalpy driven process in polymer HPLC is the distribution of solute molecules between the volume of the mobile phase and the volume of the chromatographic stationary phase. The accepted terminology designates the processes of this kind absorption or the partition, which is driven by enthalpy and therefore it is the enthalpic partition. To prevent confusion between processes of adsorption and absorption, the term enthalpic partition is preferred in present chapter - also as the opposite to entropic partition, exclusion. Surprisingly, in many works the processes of adsorption and enthalpic partition are confused though the principal differences in their qualities are evident. 

HPLC methods of determining the amounts of different additives in polymeric materials are preceded by an extraction process or dissolution of the polymer matrix. Although extraction-HPLC is often observed to be superior to the traditional spectroscopic techniques (UV and IR) in analysing additives, it is frequently difficult to obtain reproducible results in view of the variability of the extraction yield. On the other hand, it is equally difficult to obtain quantitative data in the dissolution/reprecipitation-HPLC method because of entrapment of analytes in the polymer precipitate and the potential for high absorption of the additives on the polymer surface. 

Also, there might be absorptions in the IR spectra which are characteristic for syn- or isotacticity (such as in PMMA) or for branching points of nonlinear polymer chains (such as in polyethylene). Using data pools and programs which simulate IR spectra it is possible nowadays to characterize nearly all kinds of polymers very quickly using IR spectroscopy with respect to their constitution and their composition. Also, IR spectroscopy can be coupled with polymer chromatography (SEC, HPLC). Then it provides detailed chemical information on each individual chromatographic fraction. 

HPLC is very useful in additive studies on polymer extracts. The technique can be used for both reverse phase and absorption columns and isocratic and gradient elution [44-48]. 

The IR spectra of the polymers were measured on a Perkin Elmer 16 PC FTIR spectrophotometer. The NMR analyses were performed on a Bruker ARX 300 NMR spectrometer in deuterated chloroform using TMS as internal standard. The UV spectra were recorded on a Milton Roy Spectronic 300 Array spectrophotometer and the molar absorptivity (Emu ) was calculated on the basis of the repeat unit of the polymer. The relative molecular weights of the polymers were estimated using a size-exclusion chromatograph (SEC) system equipped with a Waters 510 HPLC pump, ultrastyragel... 

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