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Capillary electrophoresis for the characterization of synthetic polymers
Oudhoff, K.A.
Publication date
2004
Link to publication
Citation for published version (APA):
Oudhoff, K. A. (2004). Capillary electrophoresis for the characterization of synthetic polymers.
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Summary y
Inn the research described in this thesis the possibility of using capillary electrophoresis (CE) for efficientt and specific separations of synthetic polymers was investigated.
Chapterr 1 is a general introduction to the research described in this thesis. In sections 1.1 and 1.2 brieff outlines are presented of the fields of synthetic polymers and of modern separation techniques thatt can be used to characterize these. Section 1.3 contains an overview of the literature on applying CEE for the characterisation of synthetic macromolecules. Electrophoretic techniques such as capillaryy gel electrophoresis (CGE) and capillary zone electrophoresis (CZE) are useful tools for the determinationn of the average molar masses and molar-mass distributions of polyelectrolytes and of polymerss with charged end-groups. Neutral polymers can be separated according to size by size-exclusionn electrochromatography (SEEC) or micellar electrokinetic chromatography (MEKC), or byy CZE after derivatization. CZE is also useful for the determination of the average charge density andd the charge-density distribution of polyelectrolytes and charged copolymers. The last section of thiss chapter describes the scope of the thesis.
Inn Chapter 2 the use of two CE techniques (CZE and MEKC) for the separation of poly(ethylene glycol)) (PEG) and polypropylene glycol) (PPG) is discussed. With both techniques prior derivatizationn of the PEGs and PPGs was required. For CZE two types of derivatization reagents weree tested. Derivatization with 1,2,4-benzenetricarboxylic anhydride (BTA) yielded three isomeric derivativess for each polymer chain. CZE allowed separation of the isomers, but the electropherogramss were very complex due to the occurrence of multiple peaks for each PEG macromolecule.. With phthalic anhydride (PhAH) as reagent the PEGs and PPGs were modified to polymerr derivatives with a charge of -2. Using a borate buffer containing 30% (7V) acetonitrile it
wass possible to separate PEG derivatives up to an average molar mass of 4000 Da. Penta-ethylene glycoll was added to the sample solution as internal standard to provide information on the degree of polymerisationn of the polymers. An approximately linear relationship was found between the reciprocall of the mobility and the molar mass of the PEG and PPG derivatives. From this, it could bee concluded that the charged polymer derivatives were separated according to the basic CZE separationn principle: differences in migration velocities were the result of differences in charge-to-sizee ratios of the analytes.
Summary Summary
Priorr to the MEKC analysis the PEGs and PPGs were derivatized with phenyl isocyanate. From the experimentall results it could be concluded that the MEKC separation mechanism for PEG derivativess was based on an approximately constant interaction of the phenyl isocyanate end-groups withh SDS aggregates in combination with a friction coefficient that depended on the polymer chain length.. This separation mechanism allowed for the separation of PEGs with slightly longer chains (MMM < 5000 Da) than was possible with CZE. In the MEKC separations of PPG derivatives it was observedd that their hydrophobic chain effected the interaction between the polymer derivative and SDSS aggregates. Therefore, PPG polymers with longer chains migrated with a higher mobility those withh than shorter chains, which was an opposite elution order in comparison with PEGs. It was foundd that the PPG derivatives did not migrate according to the conventional MEKC retention model.. It seems that the PPGs and SDS ions form aggregates in solution, the charge-to-size ratio of whichh depends on the PPG chain length. Due to the vastly different separation mechanisms for PEGss and PPGs, separation of samples of these two types of polymers with similar chain lengths couldd be obtained.
Chapterr 3 describes research performed on the characterisation of technical glycerin-based polyols. Thesee are macromolecules with glycerol as starting compound, to which chains of ethylene oxide (EO)) and propylene oxide (PO) had been polymerised in a batch process. In this chapter the determinationn of the sizes of the macromolecules is first discussed. To use the CZE method describedd in Chapter 2 for the characterisation of glycerin-based polyols, it appeared necessary to increasee the concentration of acetonitrile (to 50% Vv) in the buffer, so as to minimize the influence
off the EO-PO distribution of the polymers. The linear relationship between the inverse electrophoreticc mobility and the molar mass of the glycerin-based polymers was used as calibration curvee for the determination of their mass-polydispersity. All technical glycerin-polyols had polydispersitiess below 1.03.
Inn the second part of this chapter the development of a CE method for the quantification of by-productss in glycerin-based polyols is discussed. It was essential that the elution of PPG-rich polymerss was reversed via interaction of the macromolecules with SDS aggregates in the buffer solution.. Using a background electrolyte of 10 mM SDS and 25% (7V) acetonitrile in borate buffer
thee byproducts and polyols were separated based on their number of end-groups, with a minimal influencee of the size of the polymers. From a validation study it was demonstrated that the proposed systemm is useful for the determination of the quantities of by-products in technical products.
Summarx Summarx
Chapterr 4 describes the development of a CZE method for the separation of carboxymethylcellulosess (CMCs) according to their degree of substitution (DS). Because of the low sensitivityy of UV detection for CMCs, fused-silica capillaries with an internal diameter of 75 urn weree used. Sample solutions of CMCs with different DS values and concentrations of 1 g l"1 could bee separated using a 10 mM borate buffer at a voltage of 10 kV and on-capillary detection of the UVV absorbance at 196 nm. With the use of an internal standard (phthalic acid), which migrated with aa higher mobility than the CMCs, the precision of the method was better than 0.1 %.
Inn CZE separations broad peaks were observed for the CMC samples. Experiments with indirect UVV detection with benzenesulfonate and picrate as monitoring ions indicated that the peak widths actuallyy reflected the variation in electrophoretic mobility, and thus in the DS value. From this it couldd be concluded that both the average DS and the degree-of-substitution distribution (DSD) for CMCss can be established. The relationship between the electrophoretic mobility and the average DS valuee of CMCs was used as a calibration curve to determine the polydispersity of technical products.. A single calibration curve could only be used within a specific CMC-molar-mass range. Thiss limitation of the method was identified by experiments carried out with CMC samples fractionatedd according to molecular size by SEC. These experiments showed a small, but definite effectt of the polymeric size on the mobility observed in the electrophoretic separation.
Chapterr 5 describes a non-aqueous CE method optimised for the characterisation of a new type of macromolecules,, viz. metallo ^(terpyridine) complexed diblock polymers. An important focus of thee method development was the influence of the background-electrolyte ions and organic solvents onn the direction and velocity of the electro-osmotic flow (EOF). It was observed that the positively chargedd polymers interacted with the negatively charged capillary wall, resulting in a low repeatabilityy of the method. The use of modified capillaries solved this problem. With the deactivatedd capillary and a background electrolyte of Ba(C104) in NMF at an ionic strength of 5 5
mMM it was possible to separate the metallo polymers with average molar masses up to 30,000 Da. Thee major target of this research was the determination of the molar-mass distribution of the macromolecules.. Similar to the CZE systems described in Chapters 2 and 3, the linear plot of the reciprocall mobility versus the molar mass of the polymers was used as a calibration curve. The polydispersitiess of the samples were in the range of 1.02 to 1.06. These values were in agreement withh the polydispersities of the polymer standards used in the synthesis of the metallo polymers. Inn some samples additional polymers (by-products) were detected that migrated with a higher
iss most likely that formation of mono complexes occurred. However, it remained unclear whether thee technical samples already contained mono complexes or that these were formed because of the organicc solvent applied.
Inn Chapter 6, the use of capacitively coupled contactless conductivity detection <C D) for the detectionn of macromolecules has been examined. Polystyrene standards used as model compounds weree separated with size-exclusion electrochromatography (SEEC) and simultaneously detected withh C4D and UV detection. The mass-calibration curves obtained with both detection methods had identicall shapes. Therefore, it could be concluded that the conductivity detector could be used to monitorr the elution of polystyrene. Furthermore, the linearity, repeatability and robustness of the detectionn mode were demonstrated.
Thee origin of the C4D signal for polystyrenes was unclear. Theoretically, the effect of the polymers onn the viscosity of the solution might be the basis of the detection signal. The experiments performedd with polystyrenes with different molar masses demonstrated that the size of the polymers,, and therefore the viscosity of the sample zone, did not correlate with the response of the conductivityy detector. Further research needs to be performed to achieve a complete understanding off the origin of the detection signal for polystyrene.
