Incompatible polymers, viscosity

The flow behavior of the polymer blends is quite complex, influenced by the equilibrium thermodynamic, dynamics of phase separation, morphology, and flow geometry [2]. The flow properties of a two phase blend of incompatible polymers are determined by the properties of the component, that is the continuous phase while adding a low-viscosity component to a high-viscosity component melt. As long as the latter forms a continuous phase, the viscosity of the blend remains high. As soon as the phase inversion [2] occurs, the viscosity of the blend falls sharply, even with a relatively low content of low-viscosity component. Therefore, the S-shaped concentration dependence of the viscosity of blend of incompatible polymers is an indication of phase inversion. The temperature dependence of the viscosity of blends is determined by the viscous flow of the dispersion medium, which is affected by the presence of a second component. 

Adhesives and Sealants. Dominated by copolymers, adhesives and sealants remain somewhat of a specialty market. These polymers usually contain high copolymer content and low viscosity, and often require blending with oilier compounds prior to final application. Their uses are numerous, e.g., as seals for bottled drinks, as tie layers between incompatible polymers, or as automotive adhesives... 

To understand the mechanism of polyblending, experiments have been carried out with polymeric solution. W. Borchard and G. Rehage mixed two partially miscible polymer solutions, measured the temperature dependence of the viscosity, and determined the critical point of precipitation. When two incompatible polymers, dissolved in a common solvent, are intimately mixed, a polymeric oil-in-oil emulsion is formed. Droplet size of the dispersed phase and its surface chemistry, along with viscosity of the continuous phase, determine the stability of the emulsion. Droplet deformation arising from agitation has been measured on a dispersion of a polyurethane solution with a polyacrylonitrile solution by H. L. Doppert and W. S. Overdiep, who calculated the relationship between viscosity and composition. 

The viscosity of emulsions obtained from two mutually incompatible polymers dissolved in a common solvent was studied by a falling ball viscometer, a cone-plate viscometer, and a capillary viscometer. The two polymers are polyacrylonitrile and polyurethane, and the solvent is N-methyl-pyrrolidone. The measurements are compared with theory, and a model is proposed for the development of a stationary pressure flow of an emulsion in a capillary. 

The flow behavior of blends depends on the compatibility of their components. The viscosity of a compatible blend, for example polystyrene and polyoxyethylene, is the average of the viscosity of the components (52). Blends made up of high viscosity incompatible polymers may have lower viscosity than either component (53) (see Fig. 13.29). This effect may be the... 

Figure 13.29 High viscosity blends prepared from incompatible polymers at...

As mentioned above, the macromolecules develop into more extended conformations in a good solvent or plasticizer. This increases the viscosity of dilute solutions of polymer. Viscosity changes are used for comparative evaluation of polymer-plasticizer interaction. The plasticizer may also decrease the interaction between polymeric chains and this reduces the viscosity of concentrated polymeric solutions. Incompatible plasticizers, on the other hand, may considerably reduce the viscosity of polymer compositions even in small quantities (less than 1%) by acting as internal lubricants. 

Certain thermoplastic polyimides possess excellent resistances to high temperatures and chemicals, with Tgs ranging from 217 to 371 °C. Certain polyimides also exhibit excellent toughness and dielectric properties. The melt blending process of polyimides with other thermoplastic polymers is difficult due to polyimides high Tg, high melt viscosities, and incompatibility. A solution process is used, therefore, to achieve a semi-interpenetrating polyimide network... 

These assumptions are continuity of stress and velocity across the interfaces since stress discontinuities owing to high interfacial tensions increase the viscosity (3), to explain the unexpected low viscosities we must doubt the validity of the velocity condition at the interfaces. This is supported by the fact that the two polymers are incompatible, which implies a relatively low concentration or even complete absence of entanglements at the interface. A very thin layer of solute-free solvent around the droplets would result. Such a low viscosity layer might be the cause of the anomalous behavior. 

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