Viscometer trace

Figure 3. Effect of the commercial hydraulic filters on the baseline noise of the viscometer trace (Mobile Phase THF, Flow Rate 0.5 ml/min. Temperature 30 C)...

Figure 5. Viscometer trace before and after the column screen replacement.

Figure 10 shows DRI and viscometer traces for the NBS 706 polystyrene standard. Based on the information from these two chromatograms in conjunction with the universal calibration curve, one can calculate the intrinsic viscosity EnJCv) and molecular weight M(v) at each retention volume as shown in... 

Figure 8. FFT Smoothed Viscometer Trace for a PMMA Sample (Eastman 6041).

Figure 16. DRI and Viscometer Traces of an Unfractionated Star-shaped Polystyrene.

Figure 7. Typical RRIM viscometer traces for pressure and volume throughout.

In operation, the viscometer of Figure 2 will generate two separate signal detector traces for recording. The differential log-anplifier will generate a viscosity (In hi) trace vdiile the concentration detector will generate a concentration (c) trace. 

Figure 6. Typical viscometer Output Signal Traces. (Diffepential-ppessupe Capillapy Viscometep)...

Figure 11 illustrates an SEC separation of a sample of 3-conponent polystyrene mixture with the dual concentration and viscosity detectors of Figure 10. The top trace shows the concentration elution profile of the SEC separation as detected by a uv-photometer. The bottom trace records the same SEC separation, except with the viscometer signal from the log-amplifier output.

Whatever numerical errors are incurred in calculating percentage concentrations from detector responses, it is clear from fig. 6 that the viscometer detector gives a much better qualitative indication of the presence of high-MW species in a sample than the DRI. This is particularly noticeable in fig. 6a where the presence of 1.5% high-MW polymer is barely discernible from noise in the DRI trace, whilst a substantial peak is obtained from the VISC. 

Figure 16 shows the viscometer and DRI traces of another star-branched polystyrene. This sample contained about 12% of the starting linear arm precursor which eluted at retention volume ca. 52 ml. The kinetic molecular weight of the linear precursor was 260,000. The results obtained for the individual peak through the SEC/Viscosity methodology are summarized in Table 7. It is seen that the measured of the linear arm is very closed to the kinetic value. The average functionality of this star polymer is calculated to be f = 10. 

Pass dry filtered air through the viscometer to remove final traces of solvent. Mount viscometer in water bath. 

Fig. 3-8. Typical GPC raw data. The unils of the vcitical axis depend on the detectors used, while those on the horizontal axis are elapsed time. In this case tlie lower curve is that of the differential refractometer, while the upper curve is the trace produced by a continuous viscometer (which is described briefly in Section 3.4.4). The citrve proceeds from left to right.

Figure 2A. Detector output tracings from the viscometer (-------) placed...

Another valuable technique for study of the physical form of the active centres is viscometry. It is reasonably simple to add a vacuum viscometer to the sealed polymerization vessel and to measure the viscosity of the solution after polymerization is complete but while active centres still exist [31]. These values can be compared with the viscosity after destruction of the centres by traces of alcohol. As the solution viscosity is sensitive to the apparent total molecular weight of the polymer any association of polymer molecules via their active ends will be readily apparent. This technique was used [32] in very early experiments to show that in hydrocarbon solvents extensive association of polymer chains often occurs and has an important effect on the polymerization kinetics. Light scattering techniques can be employed in a similar manner [33] enabling the apparent molecular weight to be directly determined. 

Detectors must be sensitive and must have a wide linear range in order to respond to both trace amounts and large quantities of material, if necessary. They must be nondestructive to the eluting components if they are to be collected for further analysis. There are different types of detectors for GPC, the most common ones being the refractive index (RI) detector, the UV detector, viscometer detector, as well as light scattering and infrared detectors. 

Hyphenated methods use SEC apparatus equipped with two or three detectors, mounted in parallel or in series. In the SEC/IntrinsicViscosity method, one places a viscometer after the SEC apparatus and records two traces, namely, the RI response and the viscometer response. This technique encounters some problems. For instance, the pistons of the solvent delivery system cause systematic pressure drops that disturb the measurement. Furthermore, it cannot be applied to samples with broad MMD (say with a polydispersity index MwIMn larger than 1.8-2.0). In fact, it has been shown by computer simulation that for broad MMD samples, the time-lag between the two detectors is overestimated, which implies that there is a discrepancy, AC, given by... 

Figure 2 shows the viscometer and refractometer tracings as a function of elution volume for a mixture of equal amounts of three nearly monodisperse polystyrene standards. Note that the refractometer is proportional to concentration c the signal from the viscometer is proportional to [t ]c. By dividing the viscometer output by the refractometer signal, we can then determine [13] at each elution volume increment. 

Fig. 6.74. Hysteresis/structure-related effects in a a Couette viscometer, b a clutch in on-off DC operation, and c a valve experiencing choking phenomena at nominally constant flow rate, where the uppermost trace is ongoing DC voltage and the lower is valve pressure drop versus time...

Between successive determinations of kinematic viscosity, clean the viscometer thoroughly by several rinsings with the sample solvent, followed by the diying solvent (see 7.3). Dry the tube by passing a slow stream of filtered dry air through the viscometer for 2 min or until the last trace of solvent is removed. 

Thus, by simply measuring t, the kinematic viscosity v = q/p of any low-viscosity Newtonian liquid can be determined. It is important to note that because the lower bulb contains liquid during the measnrement, h depends on the liquid level in both arms of the viscometer. Calibration experiments must therefore be performed with the same initial head of liquid as the actual measuranents to yield accurate results. In practice, Equation 7.17 leads to small but systanatic errors in viscosity data, particularly for low viscosity liquids. The source of errors can be traced to omission of the energy required to accelerate the liquid as it enters and leaves the capillary. When this effect is taken into account, Equation 7.17 reads [30]... 

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