Viscometric techniques

Objections were raised to other results of these authors derived by viscometric techniques. Thus the viscometric technique led to the erroneous value 119) of 1, instead of 4, as required by symmetry, for the equilibrium constant of the athermal exchange l20> ... 

In the polystyrene case, carefully handled conventional viscometric techniques were used to obtain [t]] values and subsequently the needed K and a values. The data are illustrated in Figure 2. The log-log plot of the reported molecular weights of the standards and the measured [n] values (plotted as [n] meas.) gives the expected straight line relationship in the molecular weight range of interest. A K value of 1.25 x 10 dl/g and an a value of 0.72, which define the solid line shown in the figure, were then used with the SEC... 

It should also be noted that the suggestion has been made that, at extremely low concentrations (<0.1 g./lOO ml. of solution), a negative slope occurs in the ij P/c versus c curves.129 If this is a general phenomenon, a radical alteration in viscometric technique may be necessary. 

Parenthetically, we wish to note that the initial association state results obtained from viscometric technique were declared some time ago to be invalid as a result of undefined technical difficulties. Suffice it to mention that the author of this erroneous assessment (reference to my book CARBANI0NS,... 

It should also be noted that the viscometric technique can detect the presence of star-shaped aggregates, having the ionic active centers. The addition of ethylene oxide to hydrocarbon solutions of poly(isoprenyl)lithium leads to a nearly two-fold increase in viscosity144). Conversely, this results in an approximately twenty-fold decrease in solution viscosity, after termination by the addition of trimethylchloro-silane. This change in solution viscosity is reflected in the gelation which occurs when difunctional chains are converted to the ionic alkoxy active centers 140,145,146). Branched structures have also been detected 147> by viscometry for the thiolate-lithium active center of polypropylene sulfide) in tetrahydrofuran. 

It should be mentioned that the viscometric technique yields a weight-average degree of association159), Nw, while a number-average (mole fraction) degree of association is actually required for the calculation of equilibrium constants associated with processes such as shown in Eq. (11). However, as is shown in the Appendix the near-monodisperse nature of these polymer systems permits the use of Nw in the calculation of these equilibrium constants. 

The viscometric technique has been used to determine equilibrium constants for systems such as ... 

The main advantages of the viscometric technique are (a) the simplicity of the method, and (b) the range of molecular weights that can be measured. The main disadvantage is the fact that the method must be calibrated. 

Photon correlation spectroscopy, carried out under very dilute conditions, has unambiguously demonstrated the expansion of carboxylic emulsion polymers at high pH, but it may not always be useful in predicting properties of practical interest. Of special concern is the apparent decrease in the intrinsic ionization constant of surface carboxyls at very low concentration. Since most uses of emulsion polymer occur at high concentrations, the measurement of particle-particle interactions is of great practical importance (21J. It has been found that the sedimentation and viscometric techniques closely reflect viscosity changes in latexes at much higher solids. Extension of the PCS approach to more concentrated systems is underway but not without problems (22). 

The influence of molecular structures and substituents on the antiozonant properties of a series of related aromatic diamine compounds was studied. The relative effectiveness of the compounds was determined by viscometric techniques and by comparison of the rate of degradation of protected vuicanizates. Results indicate that unsymmetrical p-phenylenediamine derivatives are less effective than analogous symmetrical compounds as antiozonants. The protective capacity of the antiozonants decreases as the size or number of the N-hydrogen substituents, or the distance between the amine groups, increases. The comparative stability of the free radicals of aryl diamines, in terms of the theory of resonance, is utilized to explain the relative inhibiting properties of the chemicals examined. 

A simple, rapid, and reliable viscometric technique for evaluating the protective capacity of commercially available materials as inhibitors of ozone-induced polymer chain scission has been described (1). This work included the results of an evaluation of several chemicals such as A,A -di-sec-butyl-p-phenylenediamine, nickel dibutyl di-thiocarbamate, l-(m-aminophenyl)-2,5-dimethylpyrrole, and 2,6-di-tert-butyl-4-methyl-phenol as antiozonants A,A -di-sec-butyl-p-phenylenediamine exhibited superior inhibiting characteristics. However, because of the potential toxic effects and relatively high vapor pressure of this chemical, its use is considered impractical. 

Quantitative shear stress—shear rate data can be obtained with agitators having complex geometries assuming that the shear rate is directly proportional to the rotational speed of the agitator and if the flow behavior of the fluid can be described by the power law model. The mixer viscometric technique is based on the assumption that for complex geometries one can assume an effective shear rate that is dependent on the RPM and on the design characteristic of the impeller (22, Z , 21, 28., 22, 20.) The procedure has been described in detail by Rao (27 ) and will be considered only in brief here. 

As mentioned earlier, viscometric manipulations are simplified considerably by using a capillary instrument of the Ubbelohde type, as modified by Davis and Elliott, rather than one of the Ostwald or Fenske t3q)e. This viscometer has a side arm at the base of the capillary which breaks the liquid flow to form a suspended level, and also reduces kinetic-energy corrections. The latter are very important, but can be made negligible by careful viscometer design. Kinetic-energy corrections are inversely proportional to the flow time, and so the viscometer should be designed so that solvent flow-times of 150-200 seconds are achieved. Full details of viscometric techniques can be found elsewhere. ... 

The opinion has also been expressed that the viscometric technique will overestimate the degree of dissociation in systems where the dissociation constants involving the influence of ethers are studied. This claim can be examined by considering a simple example. Bywater and Worsfold and Meier studied the influence of tetrahydrofuran on the propagation rate of styrene in benzene. Their kinetic results can be interpreted as showing that at an ether/active center ratio of about 10, the poly(styryl)li ium dimers were largely disrupted by solvation with the ether for the process shown in Eq. (11). This joint conclusionis identical to that reached by Morton via the viscometric technique. Thus, at least for the case of poly(styryl)lithium, the viscometric procedure does not appear to overestimate the extent of dimer dissociation. 

As stated earlier, polymer rheology is not confined to the study of liquid polymers. However, this section is focused on the analysis of polymer melts, since these materials have a great relevance in polymer processing. The viscometric techniques to be discussed in this section may apply not only to polymer melts but also to other polymeric liquid systems, such as solutions and suspensions. 

Wong WS (2012) Application of high temperature chromatographic and viscometric techniques for the characterization of highly isotactic polypropylene samples. In Proceedings 4th international conference on polyolefin characterization, Houston, October 2012... 

Various other polyamides have been studied. The polymerization of a-methyl-cr-butyl-/ff-propiolactam and octanelactam have both been described. A viscometric technique has also been used for investigation of the polymerization... 

Light scattering and viscometric techniques have been applied to the study of the solution properties of sodium amylose sulphate in salt solutions of different ionic strengths and at several temperatures. The molecular weight is correlated with the radius of gyration and the second virial coefficient. The stiffness parameters of the polymer chains were determined and compared with those of other polymers. 

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