Molecular shape viscosity, effect

The above discussion easily motivates the notion that reorientation times will become long as the liquid is cooled towards the glass transition, but it does not explain the shape of the observed relaxation function. Part of the shear viscosity in fluids is due to coupling to molecular reorientation. This effect has been studied in detail in alkane liquids26,27. At low viscosities the shear modulus can be described by... 

Molecules in the transition area of molecular weight (2000-4000 Da depending on molecular shape, rigidity, and solvent viscosity) show little or no NOE. For these molecules an alternative experiment called ROES Y (rotating-frame Overhauser effect spectroscopy, Chapters 8 and 10) is effective. 

Starch acetate solutions exhibit viscosity effects characteristic of dissolved high polymers. Since the viscosity of a starch solution varies in relation to the size of the starch molecules, viscosity measurements have afforded a rapid means for following molecular breakdown, and, in addition, have provided information on the molecular weight and shape of the dissolved molecules. 

A characteristic feature of a dilute polymer solution is that its viscosity is considerably higher than that of either the pure solvent or similarly dilute solutions of small molecules. This arises because of the large differences in size between polymer and solvent molecules, and the magnitude of the viscosity increase is related to the dimensions of the polymer molecules in solution. Therefore, measurements of the viscosities of dilute polymer solutions can be used to provide information concerning the effects upon chain dimensions of polymer structure (chemical and skeletal), molecular shape, degree of polymerization (hence molar mass) and polymer-solvent interactions. Most commonly, however, such measurements are used to determine the molar mass of a polymer. 

Lower effective viscosity at a liquid/liquid interface has been correlated experimentally with spherical molecular shapes" and theoretically with high surface tension. " A local dip in density is also a feature of liquids that partially wet hydrophobic surfaces." These observations may explain the reduced effective viscosity experienced by the nonpolar solute at the water/CCl4 interface, but, this remains an open issue, as the X-ray reflectivity measurements... 

Note that, apart from the filler particle shape and size, the molecular mass of the base polymer may also have a marked effect on the viscosity of molten composites [182,183]. The higher the MM of the matrix the less apparent are the variations of relative viscosity with varying filler content. In Fig. 2, borrowed from [183], one can see that the effect of the matrix MM on the viscosity of filled systems decreases with the increasing filler activity. In the quoted reference it has also been shown that the lg r 0 — lg (MM)W relationships for filled and unfilled systems may intersect. The more branches the polymer has, the stronger is the filler effect on its viscosity. The data for filled high- (HDPE) and low-density polyethylene (LDPE) [164,182] may serve as an example the decrease of the molecular mass of LDPE causes a more rapid increase of the relative viscosity of filled systems than in case of HDPE. When the values (MM)W and (MM)W (MM) 1 are close, the increased degree of branching results in increase of the relative viscosity of filled system [184]. 

Diffusion and sedimentation measurements on dilute solutions of flexible chain molecules could be used to determine the molecular extension or the expansion factor a. However, the same information may be obtained with greater precision and with far less labor from viscosity measurements alone. For anisometric particles such as are common among proteins, on the other hand, sedimentation velocity measurements used in conjunction with the intrinsic viscosity may yield important information on the effective particle size and shape. ... 

Recently, we explored the effect of molecular weight on the pattern and employed post-dewetting processes to alter the shape of the dewetted polymer droplets. Since the viscosity of a polymer solution is nonlinear with respect to concentration and also strongly dependent on polymer weight, we expected a drastic effect. Figure 11.4... 

Their low melt viscosity permits molding of thin sections and complex shapes. However, their tendency to form ordered structures causes LC materials to be particularly susceptible to molecular orientation effects during processing. 

The Vel data as a function of flow rate, Q, are shown for a 10 g/mol molecular weight polystyrene in Figure A. Both the Ubbelohde viscometric data and the membrane viscometer data are platted on the same graph for a 0.6 urn pore membrane at a low concentration of 100 ppm. The flow is Newtonian. The actual agreement of the capillary and membrane viscosities at low flow rates is always excellent when << Dj., and the concentration is extremely low. At small pore size, high concentrations, and high shear rates the flow can become non-Newtonian. The latter effects are only briefly discussed in this paper, but it is this effect that offers an oportunity to characterize the shape rather than the overall size. Even for a relatively large pore (0.6, Hi , membrane the shear rates vary from 100 s at E mi/Hr to 10 s at 200... 

Polystyrene latexes were similarly prepared by Ruckenstein and Kim [157]. Highly concentrated emulsions of styrene in aqueous solutions of sodium dodecylsulphate, on polymerisation, yielded uncrosslinked polystyrene particles, polyhedral in shape and of relative size monodispersity. Interestingly, Ruckenstein and coworker found that both conversions and molecular weights were higher compared to bulk polymerisation. This was attributed to a gel effect, where the mobility of the growing polymer chains inside the droplets is reduced, due to increased viscosity. Therefore, the termination rate decreases. 

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