Viscosity, molecular weight determination

Figure 2.11 Fluids, resins, and elastomers. Silicone fluids are linear chains where molecular weight determines viscosity, silicone resins are branched polymers and thus glass-like solids and silicone elastomers are composed of long, linear polysiloxane chains reinforced with an inorganic filler and cross-linked. (From Plastics Handbook, Modern Plastics Magazine, McGraw-Hill, New York, 1994, p. 86.)...

The solubility of PVP in water-acetone systems varies considerably with the K value of the polymer. Addition of acetone to an aqueous PVP solution K value of 100) may be used to fractionate the polymer in order to study the molecular weight distribution. The phase relation of the PVP-acetone-water system is beyond the scope of the present chapter. Interested readers are referred to Fikentscher and Herrle [1] for a more detailed discussion of the topic. Along with this, ref. [4-8 and 50] should be reviewed for their discussion of molecular weight determinations, viscosity phenomena, and solubility characteristics. 

This chapter contains one of the more diverse assortments of topics of any chapter in the volume. In it we discuss the viscosity of polymer solutions, especially the intrinsic viscosity the diffusion and sedimentation behavior of polymers, including the equilibrium between the two and the analysis of polymers by gel permeation chromatography (GPC). At first glance these seem to be rather unrelated topics, but features they all share are a dependence on the spatial extension of the molecules in solution and applicability to molecular weight determination. 

Hven fractionated polymer samples are generally polydisperse, which means that the molecular weight determined from intrinsic viscosity experiments is an average value. The average obtained is the viscosity average as defined by Eqs. (1.20) and (2.40) as seen by the following argument ... 

The formaldehyde-to-phenol molar ratios of most novolacs lie somewhere between 0.30 and 0.99. This is a very broad range in the eyes of a novolac chemist. Novolacs are extremely sensitive to molar ratio variation and they are usually specified to the nearest 0.001 molar ratio unit or less. Also unlike resoles, one does not have the option of selecting the development of molecular weight through viscosity control. The molecular weight and viscosity are largely determined by... 

Recent advances have greatly decreased the difficulties raised by these cautionary points. In particular, on-line viscosimetry and multi-angle lightscattering make it possible to determine the molecular weight and viscosity of samples as a function of elution volume. With such detectors, effects of solute-column packing interactions become unimportant, since the properties of narrow fractions can be measured. These detectors will be discussed in greater detail below. 

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. 

In support of the association theory, colloid chemists cited non-reproduceable cryoscopic molecular weight determinations (which were eventually shown to be caused by errors in technique) and claimed that the ordinary laws of chemistry were not applicable to matter in the colloid state. The latter claim was based, not completely without merit, on the ascerta-tion that the colloid particles are large aggregates of molecules, and thus not accessible to chemical reactants. After all many natural colloids were shown to form double electrical layers and adsorb ions, thus they were "autoregulative" by action of their "surface field" (29). Furthermore, colloidal solutions were known to have abnormally high solution viscosities and abnormally low osmotic pressures. 

Staudinger was probably the first to recognize this, for in 1928 he proposed that synthetic macromolecules were poly-disperse and their molecular weights would have to be expressed as average values (93). He also recognized the dependence of physical properties on molecular weight, and pursued this dependence as a measure of molecular weight determination. The result was his application of solution viscosity. 

For most polymers there is a definite relationship between molecular weight and solution viscosity. The viscosity method of molecular-weight determination... 

The insecticidal efficiency under laboratory conditions of various members of two series of narrow boiling petroleum fractions was determined for adult female California red scale, Aonidiella auranfii (Mask.) and eggs of the citrus red mite, Paratetranychus citri (McG.). Correlations between efficiency and various properties of the fractions such as structural character, molecular weight, and viscosity were observed. These correlations Indicate that conventional spray oils used on citrus in California may contain appreciable amounts of hydrocarbons of low insecticidal activity. The evidence indicates that the insecticidal efficiency of spray oils against citrus insect pests may be considerably improved by proper selection with respect to structural character and molecular size. 

Stimulated by these new data, Fujita et al. (8i) undertook a third independent measurement of [tf] as a function of Mw with PBLG in DMF. The results stood in close agreement with those of Spach et al. (80). Independently, Tsvetkov et al. (82) had reported that plots of log[ j] versus log MSD (MSD means the molecular weight determined by the sedimentation-diffusion method) for PBLG in DMF and EDC were fitted approximately by a straight line of the even lower slope 1.4. Very recently, Norisuye (S3) again attempted to determine the viscosity-... 

In principle, it only remains to show that the non-draining case is actually prevailing in solutions of coil molecules. For this purpose, reference is made to the experiences gathered in the field of molecular weight determinations with the aid of intrinsic viscosity measurements. From eq. (3.37) one obtains, when eq. (3.58) is used for the relaxation... 

The GPC analysis of block copolymers is handicapped by the difficulty in obtaining a calibration curve. A method has recently been suggested to circumvent this difficulty by using the calibration curves of homopolymers. This method has been extended so that the calibration curves of block copolymers of various compositions can be constructed from the calibration curve of one-component homopolymers and Mark-Houwink parameters. The intrinsic viscosity data on styrene-butadiene and styrene-methyl methacrylate block polymers were used for verification. The average molecular weight determined by this method is in excellent agreement with osmometry data while the molecular weight distribution is considerably narrower than what is implied by the polydispersity index calculated from the GPC curve in the customary manner. 

In contrast to these thermodynamic methods, the viscosity molecular weight determination depends on the interference in the flow of the solvent caused by the dissolved molecules. In contrast to osmometry and light scattering, it has not been possible to develop the viscosity effect into an absolute measure of molecular weight. Rather, it must be calibrated, preferably by light scattering measurements. 

Physical Measurements. Molecular Weight. Intrinsic viscosities were determined using dilute solution viscometry (Cannon-Ubbelohde viscometers). For the poly (methyl methacrylate) polymer the following empirical expressions were used to obtain molecular weights (4) ... 

This substance has a long history.245 Its structure was established by Staudinger on the basis of decomposition products and molecular weight determinations through viscosity.246 A paper on the early development of the Polish rubber industry stresses the difficulties faced in the 19th century, the existence of two factories before 1914, and the rapid growth after 1918.247... 

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