Viscous transition

When heating an amorphous polymer it eventually reaches a temperature designated as its flow temperature. This is a very viscous transition state and further heating leads to a viscous melt. 

The results are shown in Fig. 3.5. According to Fig. 3.5b, mechanochemical yield per unit of consumed energy appeared to be a constant only starting from some threshold power per 1 g of the mixture under treatment. This threshold is about 5 W/g. The existence of threshold effect in solids is connected with fragile-viscous transition the latter occurs only when a definite level of energy input to a solid is exceeded. 

Film mass-transfer theory. The film theory, which is the simplest and most elementary theory, assumes the presence of a fictitious laminar film next to the boundary. This film, where only molecular diffusion is assumed to be occurring, has the same resistance to mass transfer as actually exists in the viscous, transition, and turbulent core regions. Then the actual mass transfer coefficient A is related to this film thickness <5y by... 

The flow of slurry in a pipeline is much different from the flow of a single-phase liquid. Theoretically, a single-phase liquid of low absolute (or dynamic) viscosity can be allowed to flow at slow speeds from a laminar flow to a turbulent flow. However, a two-phase mixture, such as slurry, must overcome a deposition critical velocity or a viscous transition critical velocity. The analogy can be made here in terms of an airplane if the speed drops excessively, the airplane stalls and stops flying. If the slurry s speed of flow is not sufficiently high, the particles will not be maintained in suspension. On the other hand, in the case of highly viscous mixtures, if the shear rate in the pipeline is excessively low, the mixture will be too viscous and will resist flow. 

When absolute (or dynamic) viscosity is an important factor, such as in clayish slurries or homogeneous flows, another parameter, the viscous transition critical velocity Kj-must be determined. There are two regimes for flow of homogeneous mixtures. Flow at speeds less than Fj-is associated with laminar flows, whereas flows above Vj-are characteristic of turbulent flows. 

Demand for temperature controlled troughs came from the material scientists who worked witli large molecules and polymers tliat establish viscous films. Such troughs allow a deeper understanding of tire distinct phases and tire transitions in LB films and give more complete pressure-area isotlienns (see d) below). 

Irregularities such as branch points, comonomer units, and cross-links lead to amorphous polymers. They do not have true melting points but instead have glass transition temperatures at which the rigid and glasslike material becomes a viscous liquid as the temperature is raised. 

Viscous Transport. Low velocity viscous laminar dow ia gas pipes is commonplace. Practical gas dow can be based on pressure drops of <50% for low velocity laminar dow ia pipes whose length-to-diameter ratio may be as high as several thousand. Under laminar dow, bends and fittings add to the frictional loss, as do abmpt transitions. 

Colloidal State. The principal outcome of many of the composition studies has been the delineation of the asphalt system as a colloidal system at ambient or normal service conditions. This particular concept was proposed in 1924 and described the system as an oil medium in which the asphaltene fraction was dispersed. The transition from a coUoid to a Newtonian Hquid is dependent on temperature, hardness, shear rate, chemical nature, etc. At normal service temperatures asphalt is viscoelastic, and viscous at higher temperatures. The disperse phase is a micelle composed of the molecular species that make up the asphaltenes and the higher molecular weight aromatic components of the petrolenes or the maltenes (ie, the nonasphaltene components). Complete peptization of the micelle seems probable if the system contains sufficient aromatic constituents, in relation to the concentration of asphaltenes, to allow the asphaltenes to remain in the dispersed phase. 

The Knudsen number Kn is the ratio of the mean free path to the channel dimension. For pipe flow, Kn = X/D. Molecular flow is characterized by Kn > 1.0 continuum viscous (laminar or turbulent) flow is characterized by Kn < 0.01. Transition or slip flow applies over the range 0.01 < Kn < 1.0. 

Slip Flow In the transition region between molecular flow and continuum viscous flow, the conductance for fully developed pipe flow is most easily obtained by the method of Brown, et al. (J. Appl. Phys., 17, 802-813 [1946]), which uses the parameter... 

Often, a pilot plant will operate in the viscous region while the commercial unit will operate in the transition region, or alternatively, the pilot plant may be in the transition region and the commercial unit in the turbulent region. Some experience is required to estimate the difference in performance to be expected upon scale-up. 

Transition from Thermal Activation to Viscous Drag... 

Transition from thermal activation to viscous drag occurs when... 

J.N. Johnson and D.L. Tonks, Dynamic Plasticity in Transition from Thermal Activation to Viscous Drag, in Shock Compression of Condensed Matter— 1991 (edited by S.C. Schmidt, R.D. Dick, J.W. Forbes, and D.G. Tasker), Elsevier Science, Amsterdam, 1992, pp. 371-378. 

Fig. 23.7. A modulus diagram for PMMA. It shows the glassy regime, the gloss-rubber transition, the rubbery regime and the regime of viscous flow. The diagram is typical of linear-amorphous polymers.

It should be pointed out that the view of the glass transition temperature described above is not universally accepted. In essence the concept that at the glass transition temperature the polymers have a certain molecular orientation time is an iso-elastic approach while other theories are based on iso-viscous. 

Transition region or state in which an amorphous polymer changed from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. Transition occurs over a narrow temperature region similar to solidification of a glassy state. This transformation causes hardness, brittleness, thermal expansibility, specific heat and other properties to change dramatically. 

Reynolds number A dimensionless parameter that represents the ratio of the inertia forces to the viscous forces in a flow. Its magnitude denotes the actual flow regime, such as streamline (laminar), transitional, or turbulent. 

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