Molecular dimensions viscosity

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

Thin film lubrication (TFL), as the lubrication regime between elastohydrodynamic lubrication (EHL) and boundary lubrication, has been proposed from 1996 [3,4], The lubrication phenomena in such a regime are different from those in elastohydrodynamic lubrication (EHL) in which the film thickness is strongly related to the speed, viscosity of lubricant, etc., and also are different from that in boundary lubrication in which the film thickness is mainly determined by molecular dimension and characteristics of the lubricant molecules. 

Being compared to conventional Reynolds equations, /12 can be regarded as a modification coefficient of the micropolar effects on viscosity, and its effects are shown in Fig. 8. This shows that the microstructure and microrotation will add an increase in lubricant viscosity. When the ratio hH increases, the viscosity enhancement decreases further increasing the ratio, the modiflcation approaches unit. Because I is related to the molecular size, and h is the film gap, this means that if the problem scale is much larger than the molecular dimension, microrotation and the microstructure of particles will contribute msignrhcantly to the macroscopic properties. The larger N is, the more the increase is, as also evidenced by Fig. 8. 

The effective viscosity is also affected by the microrotation of the rigid particles. If the gap is much larger than the molecular dimensions, the boundary walls will have little influence on the microrotation motion. This means that if the gap between the solid walls is sufficiently large, the micropolarity can be reasonably taken out of consideration without losing precision. The microrotation in thin film lubrication will result in viscosity-enhancements and consequently higher film thicknesses, which contribute to a better performance of lubrication. 

When the water film is squeezed out, the thick water layer is removed and the surfaces are separated by lubricant film of only molecular dimensions. Under these conditions, which are referred to as BL conditions, the very thin film of water is bonded to the substrate by very strong molecular adhesion forces and it has obviously lost its bulk fluid properties. The bulk viscosity of the water plays little or no part in the frictional behavior, which is influenced by the nature of the underlying surface. By comparing with the friction force of an elastomer sliding on a rigid surface in a dry state, Moore was able to conclude that for an elastomer sliding on a rigid surface under BL conditions, one can expect ... 

Parameter relating the intrinsic viscosity to the molecular dimension /r2 (Chaps. VII and XIV). 

If the branching density is sufficiently high to hinder segmental flexibility and impose strong excluded volume and even steric interactions, molecular dimensions become rigid. Measurements of solution and melt viscosity showed that the properties of dendritic molecules approached that of solid spheres as the... 

The interface between two fluids is in reality a thin layer, typically a few molecular dimensions thick. The thickness is not well defined since physical properties vary continuously from the values of one bulk phase to that of the other. In practice, however, the interface is generally treated as if it were infinitesimally thin, i.e., as if there were a sharp discontinuity between two bulk phases (LI). Of special importance is the surface or interfacial tension, a, which is best viewed as the surface free energy per unit area at constant temperature. Many workers have used other properties, such as surface viscosity (see Chapter 3) to describe the interface. 

Figure 4.2 is a plot of log(cr) versus log(viscosity) constructed from dielectric data of Figure 4.1 and measurements on a dynamic rheometer. The figure shows that at a viscosity less than 1 Pas (10P), a is proportioned to l/tj because the slope of log(n) versus log( j) is approximately — 1. The gel point of the polymerization reaction occurs at 90 min based on the crossover of G and G" measured at 40rad/s. This is very close to the time at which rj achieves 100 Pas, which is also often associated with gel. The region of gel marks the onset of a much more rapid change in viscosity than with a. This is undoubtedly due to the fact that as gel occurs the viscoelastic properties of the resin involve the cooperative motion of many chains, whereas the translational diffusion of the ions continues to involve motions over much smaller molecular dimensions. 

The main topics of interest are the molecular dimensions in solution, and their relation to intrinsic viscosity, and the thermodynamic properties of solutions. 

At this point, it is convenient to recall Figure 7.13 and the discussion of it. In that context we observed that there is generally a variation of properties in the vicinity of an interface from the values that characterize one of the adjoining phases to those that characterize the other. This variation occurs over a distance r measured perpendicular to the interface. In the present discussion viscosity is the property of interest and the surface of shear —rather than the interface per se —is the boundary of interest. The model we have considered until now has implied an infinite jump in viscosity, occurring so sharply that r is essentially zero. From a molecular point of view such an abrupt transition is highly unrealistic. A gradual variation in rj over a distance comparable to molecular dimensions is a far more realistic model. 

Topographical features comprise a significant portion of the interphase. In general, the epoxy matrix will conform to the topographical features of the substrate down to the molecular dimensions of the resin molecule. Since most epoxies are applied as a liquid of moderate to low viscosity, intimate contact between epoxy and substrate is achieved. Two aspects of the topographical features of the substrate must be considered as to their effect on the interphase structure of the epoxy. 

Rankine and Smith 3 have employed the method of determination of the molecular dimensions of gaseous molecules from viscosity measurements to decide the constitution of the sulphur dioxide molecule. From... 

There are presently two main difficulties which handicap attempts at exact calculation. The first concerns the intermolecular potential, and the hazards of extrapolation from models derived from viscosity measurements have been discussed. Furthermore, such a method is of dubious validity for polyatomic molecules, because the intermolecular repulsive potential will generally appear to become progressively shallower with increasing molecular dimensions if the viscosity data are cast, for example, in the Lennard-Jones form. Energy transfer depends... 

The lubricant film between the two surfaces is no longer a liquid layer, instead the lubrication two surfaces are separated by film of only molecular dimensions and may contact each other. The properties of lubricant other than viscosity. 

This expression is known as the Stokes-Einstein equation. This formula correctly relates diffusivity to molecular dimensions and viscosity for cases in which Stokes law is applicable. 

The conformational state may be expressed in molecular dimensions (e.g. the mean square end-to-end distance of a polymer molecule) or in the limiting viscosity number (intrinsic viscosity). 

If the interaction forces between polymer and solvent molecules can be neglected (the so-called -solution) the polymer molecule is in an unperturbed conformational state. In this situation, the molecular dimensions and the limiting viscosity number can be predicted rather accurately. For a normal dilute polymer solution, however, only approximate values of these quantities can be estimated. 

Rankine, A.O. and C.J. Smith. 1921. On the viscosities and molecular dimensions of methane, sulphuretted hydrogen, and cyanogen. Phil. Mag. 62 615-620. 

When the film thickness h is sufficiently large, one observes the rheological behavior typical of bulk fluids [201,202]. Flow can be described by the bulk viscosity pg and a shp length 5 at each wall. As in simulations, typical values of S are comparable to molecular dimensions and would be irrelevant at the macroscopic scale. However, a few systems show extremely large slip lengths, particularly at high shear rates [203,204]. 

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. The magnitude of the viscosity increase is related to the dimensions of the polymer molecules and to the polymer-solvent interactions. Viscosity measurements thus provide a simple means of determining polymer molecular dimensions and thermodynamic parameters of interactions between polymer and solvents. These aspects will also be considered in a later part of this chapter. 

Determination of Polymer Molecular Dimensions Grom Viscosity... 

In terms of chemistry, it is important to note that the intrinsic viscosity, [ ], or some other measure of molecular dimensions in solution, is the driving force in thickening efficiency, not the molecular weight per se. In other words, it should not be expected that two chemically different polymers of the same molecular weight will thicken in the same way. Indeed, if chemical structures are radically different, it is virtually certain that they will not. 

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