Molecular weight determination viscometry

The viscosity is measured with the help of a viscometer and the method is known as viscometry. Molecular weight Determination by Viscometry... 

For Molecular weight determination by viscometry we do not need absolute h value, viscosity measurements may be carried out in simple Ostwald Viscometer. Because of (the non-Newtonian behaviour of most macromolecular solutions at high velocity gradients in the capillary, the viscometer dimensions are chosen in such a manner that the viscosity gradient is the smallest possible. 

Viscosity measurements alone cannot be directly used in the Mark-Houwink-Sakurada equation to relate absolute viscosity and polymer molecular weight, since additional unknowns, K and a must be determined. Therefore, viscometry does not yield absolute molecular weight values it rather gives only a relative measure of polymer s molecular weight. Viscosity measurements based on the principle of mechanical shearing are also employed, most commonly with concentrated polymer solutions or undiluted polymer these methods, however, are more applicable to flow properties of polymers, not molecular weight determinations. 

A detailed structure characterization of isobutylene and 0-pinene copolymers has been carried out including homogeneity studies (by GPC), quantitative composition and sequence analysis (by PMR and reactivity ratios) and molecular weight determinations (by osmometry and viscometry). Analysis of our data leads us to conclude that isobutylene and 0-pinene can be readily copolymerized to reasonably high molecular weight materials and that the products are perfectly random, statistical copolymers showing no detectable tendency for blockiness . 

It is possible to add a second, molecular weight-sensitive detector to an SEC system to provide a direct means of absolute molecular weight calibration without the need to resort to external standards. These detectors represent refinements in classic techniques, such as light-scattering photometry, capillary viscometry (for intrinsic viscosity), and membrane osmometry for on-line molecular weight determination. Yau recently published a review of this subject with comparisons of the properties and benefits of the principal detectors currently in use (22). The present discussion is restricted to lightscattering and viscometry detectors because Yau s osmometry detector is not yet commercially available. The reader is referred to Chapter 4 for a comprehensive discussion of molecular weight-sensitive detectors. 

Table 3 Comparison of the Molecular Weights Determined by SEC-Viscometry and Suppliers Data ... 

A relationship is shown to exist in viscometry experiments between particle size or molecular size and the viscosity of dispersions of inorganic colloids or the viscosity of macromolecular solutions. It is therefore possible to determine the molecular weight from the viscosity of dilute macromolecular solutions. Since this experiment can be rapidly performed with simple equipment, it is, in practice, the most important molecular-weight-determination method. However, the method is not an absolute one, since the viscosity depends on other molecular properties (for example, on the shape of the molecule), as well as on the molecular weight. 

A third way to determine MW is via the viscosity average molecular weight, determined by dilute solution viscometry, the intrinsic viscosity of a polymer solution [4]. 

Molecular weight determination of the oligomer prepared by telomerization was carried out by viscometry. For such low molecular weights extremely clean equipment was essential. Intrinsic viscosity in benzene solution was determined using an Ostwald viscometer at 30 0.05°C the relationship [n] = 1 04 x 10 v was used to calculate the viscosity average molecular weights. ... 

Spatorico, A.L. and Coulter, B. (1973) Molecular-weight determinations by gel-permeation chromatography and viscometry. Journal of Polymer Science Part B-Polymer Physics, 11,1139-1150. 

---