Subject polymers, flow characteristics

The most widely used thermoplastic polymer is the ethylene—vinyl acetate copolymer, which is obtainable in a wide range of molecular weights as well as in a variety of compositions. Often flexibilizers or plasticizers are added in order to improve both the mechanical shock resistance and the thermal properties of the adhesive. Polybutenes, phthalates, and tricresyl phosphate have been used as plasticizers. Tackifying agents can also be added. Because hot-melt adhesives are frequendy ethylene-based, they are subject to oxidation if, as in a typical situation, the adhesive sits in an applicator for long periods before use. Thus, antioxidants such as hindered phenols are often used, as are fillers. Fillers are added to opacify or to modify the adhesive s flow characteristics, as well as to reduce cost. Wax is also a very important component. Wax alters surface characteristics by decreasing both the liquid adhesive s surface tension and its viscosity in the melt. Upon solidification, however, the wax acts to increase the strength of the adhesive. Both paraffin and microcrystalline wax are used (see Waxes). 

Kirkland, and Ely (24) have discussed the SEC technique. Barth (25) has reported a practical approach to steric exclusion chromatography of water-soluble polymers. However, SEC is not easily carried out for the subject polymers because of the high molecular weight (10 -10 g/mole) and the poly-electrolyte characteristics of the charged polymers. In order to obtain meaningful SEC data, the columns, mobile phase, concentration of polymer solution, sample preparation method, flow rate, and shear degradation of the polymer should be considered in an SEC experiment. 

Transient molecular deformation and orientation in the systems subjected to flow deformation results in transient and orientation dependent crystal nucleation. Quasi steady-state kinetic theory of crystal nucleation is proposed for the polymer systems exhibiting transient molecular deformation controlled by the chain relaxation time. Access time of individual kinetic elements taking part in the nucleation process is much shorter than the chain relaxation time, and a quasi steady-state distribution of clusters is considered. TVansient term of the continuity equations for the distribution of the clusters scales with much shorter characteristic time of an individual segment motion, and the distribution approaches quasi steady state at any moment of the time scaled with the chain relaxation time. Quasi steady-state kinetic theory of nucleation in transient polymer systems can be used for elongation rates in a wide range 0 < esT C N. ... 

Polymers are used for mobility control in chemical flooding processes such as micellar-polymer and caustic-polymer flooding and in polymer augmented waterflooding. Selection of a polymer for mobility control is a complex process because it is not possible to predict the behavior of a polymer in porous rock from rheological measurements such as viscosity/ shear rate curves. Polymers used for mobility control are non-Newtonian fluids. Flow characteristics are controlled by the shear field to which the polymer is subjected. Properties of polymers can be measured under steady shear in rheometers. However, in porous rock, it is difficult to define the shear environment a polymer experiences as it flows through tortuous pores. 

Many materials of practical interest (such as polymer solutions and melts, foodstuffs, and biological fluids) exhibit viscoelastic characteristics they have some ability to store and recover shear energy and therefore show some of the properties of both a solid and a liquid. Thus a solid may be subject to creep and a fluid may exhibit elastic properties. Several phenomena ascribed to fluid elasticity including die swell, rod climbing (Weissenberg effect), the tubeless siphon, bouncing of a sphere, and the development of secondary flow patterns at low Reynolds numbers, have recently been illustrated in an excellent photographic study(18). Two common and easily observable examples of viscoelastic behaviour in a liquid are ... 

Clearly the macroporous polymers are the only solids that have the physical characteristics useful for gravity flow columns. This conclusion is frequently not considered or discussed. With respect to the polymers listed in Table I, the most useful are those with the highest surface areas however, these are subject to the caveat about pore size discussed next. 

Increase in the flow of materials under acoustic treatment is conditioned by different factors. Investigation into rheological and molecular-mass characteristics of polymers having been subjected to acoustic treatment has revealed that, in the case of a low-intensity treatment, the effect is of a reversible (thixotropic) character. However, at high intensities of acoustic treatment, rheological characteristics of the material are not restored completely after the vibration effect is terminated and the... 

Methods available to control non-Newtonian flows are the same as for Newtonian flows, including pressure-driven flows via syringe pumps, capillary-driven flows, and electrokinetic flows. These methods and others are covered in more detail in other entries. In many cases the behavior of polymer solutions subject to a driving force is similar to that of the corresponding Newtonian solvent as long as the characteristic strain rate remains below the coil-stretch transition or De <2- Above this critical value, shear thinning and elasticity are important. 

The Brownian motion of a polymer chain for self-diffusion is carried out by the integration of Brownian motions of monomers. Therefore, the entropic elasticity of chain conformation in a random coil allows a large-scale deformation, with its extent subject to the external stress for polymer deformatiOTi and flow, and hence exposes the characteristic feature of a mbber state in a temperature window between the glass state and the fluid state. 

While the above characteristics and advantages have been discussed specifically in reference to thermal FFF, much the same can be said in regard to the other FFF subtechniques, including sedimentation FFF, electrical FFF, and flow FFF. All of these are flexible techniques subject to ready optimization. Their application to polymers will be described after establishing the necessary theoretical framework for FFF. 

The formation of banded textures in thin-film samples of solutions of hquid crystalline polymers (LCPs), subjected to shear, has been reported in the literature since 1979 [15]. Because of the symmetrical properties of the liquid crystal solutions, large domains of weU-oriented polymer chains are formed during shear flow, while defects are squeezed into small regions. The shear accounts for an additional energy stored in the solution. When the shear is stopped, the system will first relax with a characteristic time fb to a transient state. In this state the distortion energy is minimized, and the orientational order is kept, resulting in a banded stmcture. This behavior is observed only if two conditions are fulfilled [16] ... 

DMA methods are widely used by thermal analysts to determine the viscoelastic properties of pol5uners for a number of purposes (see Viscoelasticity). The primary application of these techniques to the study of polymeric solids and melts is well documented. Excellent general discussions covering the subject are provided in References 70-72. Linear Amorphous Polymers (qv) exist in a number of characteristic physical states depending on the time scale and temperature of measurement. These are illustrated in Figure 31 in terms of an arbitrary modulus fimction and are classified as glassy, leathery, rubbery, rubbery flow, and viscous (73). All linear amorphous polymers exhibit these five physical states when they... 

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