Temperature dependence melt viscosity measurements

If the entire temperature dependence of viscosity is to be measured, it is necessary to use several methods based on different principles. In the viscosity range 10 —10 dPa s, use is mostly made of rotary viscometers. A platinum cylinder rotates around its axis in the glass melt in a crucible, and the force required for revolving the cylinder at a certain speed is measured. In another arrangement, the external crucible is rotated while the internal one is suspended on a torsion wire. Within the same viscosity region, it is possible to measure with a counterbalanced sphere viscometer a plat inum sphere suspended on a thin wire from the balance arm is immersed in the glass melt in a crucible. The other balance arm is loaded and the speed at which the sphere is withdrawn from the melt is measured. 

Another early work in this area is that of DeMeuse and Jaffe from Celanese who used several techniques such as rheology. X-ray diffraction, and calorimetry to demonstrate that a model system consisting of blends of two LCP polyesters of differing HBA/HNA ratios were in fact not miscible. This is in contrast to low molecular weight liquid crystals in which two liquid crystals which form the same type are expected to be miscible. This paper also demonstrated that transesterification did not occur at an appreciable rate under the conditions used for melt rheology measurements. They also discuss the concept that for copolymers in which the distribution of copolymer ratio is extremely wide, there could be phase separation within a nominally homogeneous copolymer. This is precisely the mechanism invoked to explain the anomalous temperature dependence of viscosity of HIQ LCP. ... 

Generally, the rheology of polymer melts depends strongly on the temperature at which the measurement is carried out. It is well known that for thermorheological simplicity, isotherms of storage modulus (G (co)), loss modulus (G"(complex viscosity (r (co)) can be superimposed by horizontal shifts along the frequency axis ... 

Polyethylene is, depending on the molecular weight, a waxy or solid, crystalline substance. Following the above-mentioned procedure, a high molecular crystalline product with a melting range around 130 C is obtained. At room temperature it is insoluble in all solvents. At higher temperatures (100-150 °C) it can be dissolved in aliphatic and aromatic hydrocarbons.Viscosity measurements can be performed in xylene,tetralin or... 

Yield stress values can depend strongly on filler concentration, the size and shape of the particles and the nature of the polymer medium. However, in filled polymer melts yield stress is generally considered to be independent of temperature and polymer molecular mass [1]. The method of determining yield stress from flow curves, for example from dynamic characterization undertaken at low frequency, or extrapolation of shear viscosity measurements to zero shear rate, may lead to differences in the magnitude of yield stress determined [35]. 

An important difference between the PS-gas systems (Kwag et al., 1999) and the PDMS-C02 system (Gerhardt et al., 1997) is that the viscosity measurements of the PS-gas systems are conducted at temperatures within 75 °C of T of PS, whereas the PDMS-C02 measurements were performed nearly 200 °C above Tg of PDMS. The difference between these two thermal regimes leads to several differences in the observed rheological behavior. The viscosity reductions relative to the pure polymer are much greater for PS-gas systems than for PDMS-C02 systems at similar dissolved gas compositions, and the dependence of ac on temperature is much more pronounced for the PS-gas systems. These trends are consistent with the observations of Gerhardt et al. (1997, 1998) that the effect of dissolved gas on polymer melt viscosity occurs primarily through a free-volume mechanism. 

The ratio of viscosity and density is the kinematic viscosity, which is directly measured in gravity-driven flows. The kinematic viscosity has the same temperature dependence as the friction coefficient. The density of polymer melts weakly decreases as temperature is raised, imparting a weak temperature dependence to the modulus at any relaxation time r. The temperature dependence of the viscosity of polymer melts is dominated by the dependence of the friction coefficient. Near the glass... 

The study of polymer melts and especially their elasticity was one of the areas that drove the development of commercial DMAs. Although a decrease in the melt viscosity is seen with temperature increases, the DMA is most commonly used to measure the frequency dependence of the molten polymer as well as its elasticity. The latter property, especially when expressed as the normal forces, is very important in polymer processing. 

Figure 13.6 shows the influence of temperature on specific volume (reciprocal specific gravity). The exact form of the curve is somewhat dependent on the crystallinity and the rate of temperature change. A small transition is observed at about 19°C and a first order transition (melting) at about 327°C. Above this temperature the material does not exhibit true flow but is rubbery. A melt viscosity of 10 °-10 poises has been measured at about 350°C. A slow rate of decomposition may be detected at the melting point and this increases with a further increase in temperature. Processing temperatures, except possibly in the case of extrusion, are, however, rarely above 380°C. 

The formation of S o in sulfur melts is a slow reaction, and it takes about 1 h at 160 °C to establish the equilibrium concentration [24, 58]. From the temperature dependence of the polymer content, from the heat capacity Cp of the melt [29] as well as from calorimetric measurements [56, 58] it was concluded that the reaction Ss Sqo is endothermic with an estimated activation energy of ca. 120 kj mor (Ss) [58]. The same value was derived from DSC measurements of liquid sulfur [58]. In this context it was observed that the sudden viscosity increase of liquid sulfur takes place at exactly 159 only if the heating rate approaches zero. If the heating rate is varied between 1.25 and 40 K min higher transition temperatures are observed as the data in Table 1 show [58]. 

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