UV-RI dual detector system

When one of the constituents A or B of a copolymer A-B has an ultraviolet (UV) absorption and the other does not, a UV-RI dual-detector system can be used for the determination of chemical heterogeneity of the copolymer. A point-to-point composition with respect to retention volume is calculated from two chromatograms and a variation of composition is plotted as a function of molecular mass. The response factors of the two components in the two detectors must be calibrated first. 

Let A be a constituent which has UV absorption. and are defined as the response factors of an RI detector for A and B constituents, and as that 

The weight fraction Wa.i of the A constituent at each retention volume i of the chromatograms for the copolymer is given by 

Additivity of the refractive index increments (dn/dC) or the response factors of homopolymers is valid for copolymers, and eqn (5.49) is applicable [8, 41]. When the refractive index increments or the response factors of one or two homopolymers which compose a copolymer cannot be measured because of insolubility of the homopolymer(s), then the extrapolation of the plot of RI response factors of the copolymers of known compositions can be employed [42]. This was the case for PS AN, where the ratio of K(styrene) to K(acrylonitrile) in chloroform was 1.244. 

The values of these response factors are dependent on the interval used to sample chromatograms, attenuation of the detectors, flow rate, and recorder chart speed, but the ratio of was almost constant in the same 

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Quinones and Hydroqulnones. In the analysis of quin-ones and hydroqulnones, the use of two different dual detector systems was required. The retention data for hydroqulnones shows the normal behavior of hydroxyl groups associating with the solvent, THF. Thus octyl quinone and hydroquinone elute almost together. Similarly dioctylquinone and octyl hydroquinone elute together (Figure 7). The UV/RI response ratio for benzoquinone is 3.75. Hydroquinone and dioctylquinone show similar disparities in the UV/RI responses. This information provides a very good method for detecting impurities in dioctyl hydroquinone. 

Let A be a constituent that has UV absorption. and are defined as the response factors of an RI detector for the A and B constituents, and as the response of the UV detector for A. These response factors are calculated by injecting known amounts of homopolymers A and B into the SEC dual-detector system, calculating the areas of the corresponding chromatograms, and dividing the areas by the weights of homopolymers injected as... 

Multiple detection applied to the SEC characterization of copolymers is attractive because it yields both CCD and MWD information. A dual detection system based on two concentration detectors, for example, RI and UV, is useful where narrow standards of the homopolymers are available and where both homopolymers obey universal calibration. However, in other copolymer systems the addition of a third detector, LALLS, can offer the advantage of on-line determination of molecular weight for each eluting species. The triple detection approach gave similar values to the dual detector approach for a model copolymer system (PS-PMMA) studied. It was also applicable to a more difficult copolymer system (PS-PEO), although it appeared that where one homopolymer was present in very small quantities, an average p value gave more consistent results than correction for pi across the distribution. 

For polymer systems without UV activity the combination of a RI detector with a density (D) detector can be used. The working principle of the density detector is based on the mechanical oscillator method. Since this detector yields a signal for every polymer, provided that its density is different from the density of the mobile phase, this detector can be regarded as universal [29,30,36]. The separation of mixtures of polystyrene and polybutadiene by SEC with dual den-sity-RI detection is presented in Figs. 7 and 8. In a first set of experiments, the response factors of both polymers in both detectors have to be determined. Then from the intensity of each slice of the elution curves in both detectors, the mass distribution of both polymers across the elution volume axis can be calculated. As can be seen in Fig. 7, a separation into the component peaks is obtained due to the fact that the molar masses of PS and PB are sufficiently different. For both components the individual elution profiles can be determined and using corresponding calibration curves for PS and PB the individual MMDs can be calculated. The same information can be extracted from an experiment where the molar masses of the components are similar and SEC separation does not work (see Fig. 8). Again the individual mass distributions are obtained and the MMDs for PS and PB can be determined. 

The application of dual detection [UV and refractive index (RI)] to the SEC analysis of polystyrene-poly(methyl methacrylate) (PS-PMMA) has already been studied in this laboratory (2). Both MWD and CCD were determined using a methodology outlined by Runyon et al. (3). This approach relies on SEC column calibration with narrow polydis-persity standards for each of the homopolymers as well as a measure of the detector response factors for each homopolymer to produce a copolymer MWD. In the case of PS and PMMA this is feasible, but in other block copolymer systems the availability of suitable molecular weight standards may be more limited. In addition, this procedure does rely on true SEC and is not valid for block copolymers for which the universal calibration does not hold true for both blocks in a given solvent system. 

The p-dicumyl chloride/BCl3/isobutylene inifer system has been thoroughly investigated. Detailed characterization research including various chemical techniques [3,8,10],1H NMR spectroscopy [1,3,8], GPC equipped by dual RI and UV detectors [1], end-group determination by dehydrochlorination [9], and kinetic studies [8] proved that the structure of the product is as shown by formula I in Scheme I and that the molecule is perfectly bifunctional (Ern = 2.0). 

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