Viscometers online

To avoid these artefacts, it was proposed that 0.3 M acetic acid/0.2 M sodium acetate (pH = 4.5) be used as solvent since there was no evidence for aggregation in this mixture [50]. At that time, it was demonstrated that the value of DA modifies the K and a parameters. In this solvent, the absolute M values were obtained from steric exclusion chromatography (SEC) equipped with online viscometer and light-scattering detector, allowing the Mark-Houwink parameters to be determined without fractionation and also the relation between the radius of gyration and the molecular weight. 

Table 5 shows the given (determined by LALLS) and intrinsic viscosities determined from an online viscometer (Viscotek Model 110) and the measured determined by different calibration techniques for six samples. The deviations [(measured -given )/(given )] x 100 between... 

The necessary conditions for application of the universal calibration method and for calculation of molecular weights through Eq. 6 is the knowledge of the [r/],- values, which are obtained from the Mark-Houwink equations when the pertinent values of K and a constants are known. An alternative way is to make a continuous measurement of [r/], at the different elution volumes with an online viscometer detector coupled to the usual concentration detector system. 

If an online viscometer (VISC) is used instead of a MALS detector in SEC (again, in conjunction with a concentration-sensitive detector), the ratio, g, of the intrinsic viscosities of the branched molecule ([ j]b) and of the linear standard ([t/]l), at the same molar mass (M), has been used for the branching calculations ... 

The opportunity to measure the dilute polymer solution viscosity in GPC came with the continuous capillary-type viscometers (single capillary or differential multicapillary detectors) coupled to the traditional chromatographic system before or after a concentration detector in series (see Viscometric Detection in GPC/SEC, p. 2411). Because liquid continuously flows through the capillary tube, the detected pressure drop across the capillary provides the measure for the fluid viscosity according to the Poiseuille s equation for laminar flow of incompressible liquids.Most commercial online viscometers provide either relative or specific viscosities measured continuously across the entire polymer peak. These measurements produce a viscometry elution profile (chromatogram). Combined with a concentration-detector chromatogram (the concentration versus retention volume elution curve), this profile allows one to calculate the instantaneous intrinsic viscosity [rj], of a polymer solution at each data point i (time slice) of a polymer distribution. Thus, if the differential refractometer is used as a concentration detector, then for each sample slice i. 

There are several commercially available online MALS detectors. Wyatt Instruments (Santa Barbara, California, U.S.A.) commercializes two MALS detectors Dawn-DSP (18 angles) and mini-Dawn (3 angles 45°, 90°, and 135°). Precision Detectors (Franklin, Massachusetts, U.S.A.) commercializes a double-angle LS 15° and 90°. SEC online measurement of (s ) could be performed by other techniques. Viscotek Co. (Houston, Texas, U.S.A.) commercializes an alternative three-detector SEC system, the SEC . The SEC system consists of (1) an online viscometer (Vise), (2) an online rightangle LS detector (RALS), and (3) a concentration detector. This three-detector SEC system also measures More exactly, is calculated,... 

Online viscometers (single, dual, or four capillary type viscometers with a symmetrical or asymmetrical bridge) These measure the pressure difference between a sample path and a reference path filled with pure solvent. Viscometers are used to measure specific and intrinsic viscosity, molar masses based on Benoit s universal calibration approach [6] and Mark-Houwink coefficients. Mass spectrometry (MS) detectors (see Chapter 10) Different MS methods have been used in macromolec-ular analysis. They are used to determine absolute molar masses for homo- and copolymers and to detect polymer structures. Matrix-assisted laser desorption ionizationtime of flight (MALDI-ToF) and electrospray ionization (ESI) are the most common instruments used in combination with SEC (Section 9.4.2.4). A recent application summary is available [12]. 

An independent task of copolymer analysis is the determination of meaningful copolymer molar masses. Obviously, results based on a single molar mass calibration generated with standards will not give accurate results, as the calibration will depend on the local composition of the species. A way to overcome this calibration dilemma is the use of online viscometers. An empirical method for samples with few hetero-contacts (e.g., block copolymers and graft copolymers) is using the multiple concentration detector approach. 

Frequently a triple-detector SEC technology is used, where three online detectors are used together in a single SEC system. In addition to the concentration detector, an online viscometer and a MALLS instmment are coupled to the SEC (TriSEC). With TriSEC, absolute molar mass determination is possible for polymers that are very different in chemical composition and molecular conformation. The usefulness of the TriSEC approach has been demonstrated in a number of applica-tions. " ° ... 

To address the molecular heterogeneity of complex polymers, fractionation methods quite frequently must be hyphenated with information-rich detectors, as has been highlighted in Section 2.03.4. For SEC, multidetector setups are required to analyze distributed properties like branching or molecular shape in addition to MMD. In such cases, light-scattering detectors or online viscometers are used. These couplings, however, do not provide information on the chemical composition or the microstmcture as a function of molar mass. 

The size exclusion chromatography for this study was done in the routine manner execept for the inclusion of an online viscosity detector called a Differential Viscometer <3> (Viscotek Corp., Porter, Texas, USAl. This instrument together with an RI concentration detector permits the calculation of intrinsic viscosities across the chromatogram. An IBM PC data system with software is also provided (5). The software acquires data from both detectors, and performs calculations of intrinsic viscosity and molecular weight distributions using the Universal Calibration Method. 

This type of measurements can very elegantly be realized online by coupling several detectors at the end of the SEC column such as a concentration detector (refractive index detector, spectrophotometric detector, etc.) and an absolute detector measuring the molar mass or related property of the separated species such as laser light scattering detector or capillary viscometer detector. These modern sophisticated separation systems allow not only the separation of the analyzed species but also their very detailed analysis and characterization as concerns the MMD or PSD, as well as other structural and compositional characteristics of simple polymers, co-polymers, etc. A schematic representation of a procedure of SEC data treatment from an experimental chromatogram to the final MMD or PSD data is shown in Figure 8. 

Rotational (torque measurements) or capillary process viscometers, as well as ultrasound and tube oscillations [58] can be used to measure viscosity online during a polymerization reaction. The first two techniques have been widely applied in the monitoring of both chain growth polymerizations and step-growth polymerizations. 

Refractive index detectors are used to measure concentration profiles and (if used alone) molar masses based on a calibration. In addition, they are used to measure the slice concentration when working with online light scattering detectors, viscometers, or online mass spectrometry. They are also used to measure the refractive index increment, dnidc, in a single point experiment when light scattering or triple detection is used. In combination with other concentration detec-... 

FIGURE 9.16 Mark-Houwink plot for a sample with long-chain branching. The intrinsic viscosity of the sample and the molar mass are measured online using a viscometer and an RI detector. The intrinsic viscosity of the linear counterpart is calculated using the Mark-Houwink coefficients. 

The physical meaning of viscosity was discussed in Chapter 5. In the online monitoring context, a very simple and robust means of measuring total dilute solution viscosity, rj, is with a single capillary viscometer. With a measurement of the pure solvent viscosity, and knowledge of the polymer concentration, c, the reduced viscosity, is given by... 

Park and Rhee [64] used a learning-based NLMPC to control semibatch copolymerization. The purpose was to linearize a nonlinear model based on previous batch data. In this way, the prediction is a function of the increment of inputs between two consecutive batches. They used an online densitometer to obtain conversion and a viscometer to... 

Obviously, temperature trajectories are not anployed in continuous reactors. Instead, the various vessels in a series of polymerization reactors may operate at different tanperatures. Then, the polymerizing mixture will see diffeoit tanpera-tures as it passes from one vessel to the next Likewise, monomer trajectories are replaced in continuous systans with intermediate injection of a more reactive monomer between polymerization vessels. This strategy can be exploited to adjust the CCD. Monomer conversion can be adjusted by manipulating the feed rate of initiator or catalyst. If online MWD is available, initiator flow rate or reactor temperature can be manipulated to adjust MWD. Polymer quality and end-use property control are hampered as in batch polymerization case by infrequenL ofif-Une measurements, hi addition, online measurements may be severely delayed due to the constraints of the process flow sheet. For example, even if online viscometry (via melt index) is available every 1-5 min, the viscometer may be situated at the outlet of an extruder downstream of the polymerization reactor. The transportation delay between the reactor where the develops and the viscometer where the is measured (or inferred) may be several hours. Thus, even with frequent sampling, the data are old. 

---