Isoviscous state

In the light of the above results the concept of iso-free-volume at Tg cannot be considered proved, and indeed, it is in contradiction with many experimental data. This is why some authors41,44 have developed new ideas according to which liquids and polymers at Tg are in a state of isoviscosity (isoviscous state), not necessarily in the free-volume state. In connection with this we should like to note that some improvements to the SB rule were proposed55 in the form ... 

FIGURE 8.11 State diagram for food materials, showing the Tg curve and isoviscous states above Tg. Maximally freeze-concentrated solids with a solute concentration of C g have Tg at T g. Ice melting within maximally freeze-concentrated materials occurs at T m. The equilibrium melting curve shows the equilibrium melting point Tm as a function of concentration. (From Roos, Y.H. and Karel, M., Food Technol., 45, 66, 1991b.)... 

Several phenomenological models have been used to provide an understanding of Tg. One of them is an isoviscous state. As a polymer is cooled from its melt state, the viscosity increases rapidly to a common maximum value, approximately 10 Pa-s (10 poise) at Tg for all glassy materials - both low molecular weight and polymeric. Another view is that the Tg represents a state of iso free volume. Free volume, Vf, is defined as the difference between the specific or actual volume V of the polymer at a given temperature and its equilibrium volume at absolute zero Vo, Vf = V-Vg. Each term is temperature dependent [5, 8, 9]. 

Figure 5 was designed to illustrate a common pattern for Tg and T// in contrast to the behavior, all with respect to zero shear melt viscosity. Tj represents an isoviscous state over the entire molecular weight range, with the value of log t]q at Tj- dependent on the heating rate or frequency used to determine Tf. By contrast, Tg and T are essentially isoviscous up to Mg., above which log t]q data increases without limit. 

Fig. 2. The three stages of the fusion and flow process (initial movements, smoothing Tgi, globule formation and TJ) in anionic PS samples as a function of reciprocal M . For background discussion on the use of the terms isoviscous state, iso surface tension state, and annealing, see ref. 3.

C.P. Bosnyak (Dow Chemical Company, Midland, Michigan) I think another interesting point is that it appears as a very sharp transition effect, a very sharp exotherm, and this is then obviously specific to a temperature. I don t know whether this refers to a Ty temperature, or a sort of isoviscous state, or where chains just simply reach a certain energy level where they re able to revert back to their unoriented state. Do you know the answer to this ... 

Collisions of non-isoviscous droplets occur, e.g., in circulation zones inside spray towers, when droplets with different drying state collide. Such collisions can also be produced on purpose, e.g., to achieve encapsulation. The study of collision of non-isoviscous droplets in this work is done in cooperation with the Institute of... 

We first noted this fact about Tg in 1963. The plot for T/ is shown here for the first time using Til values from Fig. 3 and log tjq data used to estimate Tn in ref. 7. Fig. 5 dramatizes the essential difference between Tn from quasi-equilibrium methods (namely, an iso free volume state. Til just as with Tg, whereas Tf is isoviscous. (See Fig. 36 of ref. 1 for more elaboration.)... 

---