On-line SEC-LALLS

VPO gives suitable results for Mn if adequate thermistor beads are used. Reliable Mw values are obtained by LALLS if anisotropy, fluorescence and light absorption are taken into account. Preliminary experiments by on-line SEC-LALLS are promising but need more investigation. 

Figure 2. Recorder traces of on-line SEC-LALLS experiment.

Aqueous SEC/LALLS circumvents the problems of SEC calibration for PVA because it measures absolute molecular weight on-line. SEC/LALLS was successfully used to measure the absolute molecular weight of fully and partially hydrolyzed PVA over a wide range of molecular weights by this author (25,26). This was the first reported use of aqueous SEC/LALLS for absolute molecular... 

Lecacheux et al.240 have examined xanthan samples from various suppliers using SEC and refractive index detection coupled with on-line LALLS (Table 5, XCPS-2). Figure 13 shows LALLS and refractive index chromatograms... 

This section on the contraction factor may be concluded with an example of a comb macromolecule [136]. Due to the route of preparing this comb, unattached side chains also occurred in the system. Figure 27 shows the result of the molar mass dependence of [rj] which was obtained from a SEC/LALLS/VISC fractionation. One observes at low molar masses a straight line with an exponent of 11 =0 JO that coincides with the exponents of linear side chains. It follows a... 

This paper deals with a detailed analysis concerning the determination of M , Mw and MWD of lignins using respectively VPO, low angle laser light scattering (LALLS) and SEC. In addition, recent results obtained by on-line LALLS-SEC are presented. 

Due to the problems encountered with SEC-LALLS and SEC-viscometry, a triple-detector SEC technology has been developed, where three on-line detectors are used together in a single SEC system. In addition to the concentration detector, an on-line viscometer and a LALLS instrument are coupled to the SEC... 

Details of the chromatographic system with on-line LALLS were described previously (8, 9). After a series arrangement of four SEC columns (300 X 7 mm PLgeL, 10 fim. Polymer Laboratories, Church Stretton,... 

LALLS Detection. Consideration of the treatment of light scattering for heterogeneous copolymers (6) permits the dependence of the apparent molar mass M at the ith elution volume interval in a SEC-on-line LALLS experiment to be represented in terms of Pi and Qi by... 

Representative results for a blend and a diblock copolymer are shown in Table I. It is evident for both samples that the on-line infrared and RI concentration detectors provide excellent estimates of overall composition, component molar masses, and the heterogeneity parameter H, Data for blends in Table I were obtained to assess the proposed methods. Further evidence for the capability of the SEC-LALLS procedure can be seen in Figure 2, which shows plots of data for Ws and H computed across the MMD for blend sample 4. In this figure the expected results... 

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. 

As an alternative to universal calibration, one can measure the molecular weight of the eluting species on-line using a detector that responds continuously to the size of the solute. The two most common detectors used for this purpose are the LALLS detector and the CV detector. Various designs of both detectors have been reported (8-14). It should be noted first that these detectors obviate the necessity for universal calibration to translate the SEC retention time into molecular weight, but they do not remove problems that may exist with regard to the response of the concentration detector, which, as mentioned, is usually a DRI or an infrared instrument. 

---