Bulk polymer viscosity, relation

II. Relation of Solution Properties and Bulk Polymer Viscosity... 

Extensional Viscosity. In addition to the shear viscosity Tj, two other rheological constants can be defined for fluids the bulk viscosity, iC, and the extensional or elongational viscosity, Tj (34,49,100—107). The bulk viscosity relates the hydrostatic pressure to the rate of deformation of volume, whereas the extensional viscosity relates the tensile stress to the rate of extensional deformation of the fluid. Extensional viscosity is important in a number of industrial processes and problems (34,100,108—110). Shear properties alone are insufficient for the characterization of many fluids, particularly polymer melts (101,107,111,112). 

Schreiber HP, Bagley EB, West DC (1963) Viscosity/molecular weight relation in bulk polymers-I. Polymer 4 355-374... 

From the weak dependence of ef on the surrounding medium viscosity, it was proposed that the activation energy for bond scission proceeds from the intramolecular friction between polymer segments rather than from the polymer-solvent interactions. Instead of the bulk viscosity, the rate of chain scission is now related to the internal viscosity of the molecular coil which is strain rate dependent and could reach a much higher value than r s during a fast transient deformation (Eqs. 17 and 18). This representation is similar to the large loops internal viscosity model proposed by de Gennes [38]. It fails, however, to predict the independence of the scission yield on solvent quality (if this proves to be correct). 

Although these examples demonstrate the feasibility of using calculated values as estimates, several constraints and assumptions must be kept in mind. First, the diffusant molecules are assumed to be in the dilute range where Henry s law applies. Thus, the diffusant molecules are presumed to be in the unassociated form. Furthermore, it is assumed that other materials, such as surfactants, are not present. Self-association or interaction with other molecules will tend to lower the diffusion coefficient. There may be differences in the diffusion coefficient for molecules in the neutral or charged state, which these equations do not account for. Finally, these equations only relate diffusion to the bulk viscosity. Therefore, they do not apply to polymer solutions where microenvironmental viscosity plays a role in diffusion. 

This increase in viscosity is all the more pronounced when the molecular weight of polymers is high. In Figure 6, is plotted against the concentration ratio, r, for the couple PAA-ISo OOO/PEO-750 000. In this system, when a < 3%, phase separation occurs, bulk solid particles appear and precipitate. So, no viscosity measurement is possible. For a 4% the specific viscosity of the mixture is very higher than the sum of viscosities of the two individtial polymer solutions. The comparison may be easier if we define another parameter the gain in viscosity, g, which is given by the relation ... 

In a better solution than that provided by a theta solvent the polymer coil will be more expanded. The radius of gyration will exceed the which is characteristic of the bulk amorphous state or a theta solution. If the polymer radius in a good solvent is times its unperturbed /-g, then the ratio of hydrodynamic volumes will be equal to a and its intrinsic viscosity will be related to [ /] by... 

The possibihty of being able to change easily and controllably the film thickness by varying the ccaicentration of the solution used for spin-coating opened up a whole field of research on questions related to the chain-like nature of these macromolecules. In particular, at that time it was unclear (and partially still is ) if polymer properties like viscosity, chain conformation (as expressed for example by the radius of gyration), chain orientation, and interdiffusion rate, or mechanical properties and Tg change once the thickness of the film decreases below the diameter of Gaussian polymer coils in bulk samples [1]. 

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