Transitional specific volume

In addition the transition between upstream and downstream specific volumes may be smoothed, so that the transitional specific volume, v, used in equation (18.34) may be given by ... 

This formulation for transitional specific volume allows for a smooth transition at flow reversal, while giving a slightly greater weight to the accurate determination of forward flow. 

For a property which manifests a change at the glass transition (specific volume for example), the experimentally determined value below T is dependent on the sample thermal history. For example, for faster cooling rates, the glass is observed to have a greater specific volume. In addition, for... 

Polymeric materials are unique owing to the presence of a glass-transition temperature. At the glass-transition temperatures, the specific volume of the material and its rate of change changes, thus, affecting a multitude of physical properties. Numerous types of devices could be developed based on this type of stimuli—response behavior however, this technology is beyond the scope of this article. 

Transitions from one polymorphic form to another may be accompanied by changes in specific volume, which may lead to destmction of the crystal and containers in which the substance is stored. 

As the temperature is decreased, free-volume is lost. If the molecular shape or cross-linking prevent crystallisation, then the liquid structure is retained, and free-volume is not all lost immediately (Fig. 22.8c). As with the melt, flow can still occur, though naturally it is more difficult, so the viscosity increases. As the polymer is cooled further, more free volume is lost. There comes a point at which the volume, though sufficient to contain the molecules, is too small to allow them to move and rearrange. All the free volume is gone, and the curve of specific volume flattens out (Fig. 22.8c). This is the glass transition temperature, T . Below this temperature the polymer is a glass. 

Figure 13.6 shows the influence of temperature on specific volume (reciprocal specific gravity). The exaet form of the eurve 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 viseosity 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 inerease in temperature. Processing temperatures, exeept possibly in the case of extrusion, are, however, rarely above 380°C. 

Glass transition temperature (Tg), measured by means of dynamic mechanical analysis (DMA) of E-plastomers has been measured in binary blends of iPP and E-plastomer. These studies indicate some depression in the Tg in the binary, but incompatible, blends compared to the Tg of the corresponding neat E-plastomer. This is attributed to thermally induced internal stress resulting from differential volume contraction of the two phases during cooling from the melt. The temperature dependence of the specific volume of the blend components was determined by PVT measurement of temperatures between 30°C and 270°C and extrapolated to the elastomer Tg at —50°C. 

FIGURE 24.6 Specific volume as a function of temperature at the indicated pressures for 1,2-PB network (molecular weight between cross-hnks 8 kg/mol) [81]. The solid squares denote the glass transition defined from the intersection of the glassy and liquid data. 

Fig. 129.—Specific volume of pure poly-(N,N -sebacoyl piperazine) plotted against the temperature. 2" = 180-181 °C. A second-order transition appears also at Tg S2°C, (Flory, Mandelkern, and Hall. )...

Figure 3 Curves of specific volumes vs. temperature for poly(vinyl acetate) measured on cooling. Equilibrium values measured 0.02 h and 100 h after cooling, as indicated. Tg and Tg are glass transitions respectively at fast and slow cooling rate. Reproduced from Ref. [2] with permission of John Wiley Sons, Inc.

Finally, I would like to thank Dr. Anthony Wood and Hannah Young from Pfizer, Sandwich. Tony was Editor-in-Chief in Volume 41 and Hannah Young was his key Administrative Assistant. Their gracious help in the transition to Volume 42 was much appreciated. In Volume 42, I would specifically like to thank Ms. Catherine Hathaway, who was the key Administrative Assistant for the volume. 

Fig. 1 9 Determination of glass transition and crystalline melting temperatures by changes in specific volume.

Figure 17. Specific volume Vt and isothermal compressibility (at the glass transition temperature Tg) calculated from the LCT as a function of the inverse number l/M of united atom groups in single chains for constant pressure (P = I atm 0.101325 MPa) F-F and F-S polymer fluids. Both quantities are normahzed by the corresponding high molar mass limits (i.e., by...

A transition of this kind from metal to insulator will occur when some parameter, for instance the specific volume, the c/a ratio or the composition in an alloy, changes in such a way that two bands cease to overlap, producing a full valence band and an empty conduction band with an energy gap between them (see Fig. 4.1). A simple case is that due to the change in volume of a divalent metal. In any divalent metal, if the volume increases sufficiently, an s-like valence band will separate off from a p-like conduction band, the density of states going from the form of Fig. 1.13(b) to that of Fig. 1.13(c). The most favourable case is mercury,... 

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