Normalized molecular weight viscoelasticity

These normal stresses are more pronounced for polymers with a very broad molecular weight distribution. Viscosities and viscoelastic behavior decrease with increasing temperature. In some cases a marked viscosity decrease with time is observed in solutions stored at constant temperature and 2ero shear. The decrease may be due to changes in polymer conformation. The rheological behavior of pure polyacrylamides over wide concentration ranges has been reviewed (5). 

Polyolefin melts have a high degree of viscoelastic memory or elasticity. First normal stress differences of polyolefins, a rheological measure of melt elasticity, are shown in Figure 9 (30). At a fixed molecular weight and shear rate, the first normal stress difference increases as MJM increases. The high shear rate obtained in fine capillaries, typically on the order of 10 , coupled with the viscoelastic memory, causes the filament to swell (die swell or... 

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. 

Endo,H., Fujimoto.T., Nagasawa,M. Normal stress and shear stress in a viscoelastic liquid under steady state flow Effect of molecular weight heterogeneity. J. Polymer Sci. Pt.A-2 9,345-362 (1971). 

The effects of coupling of the DTO and RB units in not only one- but also three-dimensional arrays are discussed below and molecular weight trends illustrated. A fundamental connection between relaxation times and normal mode frequencies, shown to hold in all dimensions, allows the rapid derivation of the common viscoelastic and dielectric response functions from a knowledge of the appropriate lattice vibration spectra. It is found that the time and frequency dispersion behavior is much sharper when the oscillator elements are established in three-dimensional quasi-lattices as in the case of organic glasses. 

This relaxation has been discovered in some non-vulcanized amorphous polymers and copolymers. It tends to fall at approximately 1.2Tg. Because it appears to be connected with the change from the viscoelastic to the normal viscous state it will also depend on the molecular weight and accordingly increases with Mw. 

Furthermore, in the line-shape analyses of these viscoelastic spectra, the molecular-weight distributions of P7, FIO, F35, and F80 included in the calculations are identical, respectively, to those extracted from the line-shape analyses of the spectra of the pure melt systems, some of which have been shown in Chapter 10. Thus, the close agreements between theory and experiment in the line-shape analyses have been achieved under the consistency of maintaining the same molecular-weight distributions of the samples between the pure-melt and blend-solution systems. Thus, in a quantitative way, the universality of viscoelastic spectrum is shown extending over the melt and blend-solution systems in accordance with Eq. (11.5) with the molecular weight normalized with respect to Mg for the melt and Mg for the blend solution. 

The unusually high viscosity is not the only surprise that polymeric fluids offer. Another interesting and maybe even more important property is the viscoelasticity. Depending on how rapidly the external force changes, pol nneric fluids can behave either hke normal, low molecular weight liquids, albeit very viscous, or like elastic sohds. 

Both natural and synthetic rubbers normally have a gel component, which is a part that remains undissolved in a solvent (61,62). The gel component is probably produced by chain branching during the polymerization process or by slight cross-linking when handling rubbers. The most common example is seen in unmilled natural rubbers. When such a component is present, SEC analysis affords only the molecular weight data on the soluble fraction, excepting the gel fraction. In this case, to understand the viscoelastic properties of the rubbers connected with the SEC data is not appropriate because the gel contributes to these properties. Studies of the influence of the gel fraction on the mechanical properties of natural... 

---