Upper glass transitions

Values of the upper glass transition temperatures of the siloxane modified polyimides were found to be a function of both the level of incorporated siloxane as well as the siloxane molecular weight (Table II). The upper transition temperature of the solution... 

Table II. Upper Glass Transitions of Polydmide Siloxane) Segmented Copolymers...

In some semi-crystalline polymers, two glass transitions can be distinguished a lower glass transition, Tg(L) and an upper glass transition, Tg(U). It may be assumed that Tg(L) arises from purely amorphous material, while Tg(U) arises from amorphous material which is under restraint due to the vicinity of crystallites. Frequently Tg(U) increases with the degree of crystallization. Some general rules are ... 

Upper critical solution temperature (UCST), 211 Upper glass transition, 171 Use or performance properties, 877 UV absorbers, 784... 

Fig. 21. Entropy versus log-temperature diagram for the hard-sphere model. The solid curves give the computer simulation values for the supercooled fluid, glass, and crystal. The dashed curves have the following bases (a) a calculation from the virial equation using the known first seven coefficients and higher coefficients obtained from the conjectured closure (the plot corresponds quite closely with that calculated from the Camahan-Starling equation ) and (i>) an extrapolation of higher temperature behavior such as that used by Gordon et al., which implies a maximum in the series of virial coefficients. The entropy is defined in excess of that for the ideal gas at the same temperature and pressure. Some characteristic temperatures are identified 7, fusion point 7 , upper glass transition temperature T/, Kauzmann isoentropic point according to closure virial equation.

A plot of peak height intensity vs. temperature for the 720 cm" band is shown in Figure 23. In this figure and all similar ones, the data is normalized to the lowest temperature measurement and is offset for clarity. The slow-crystallized sample shows a continual rise of intensity starting at the yn relaxation, ca. 110 K. This increase ends at approximately Tg(U) or 240 K. The isopentane-quenched sample, however, remains essentially constant in intensity until rg(U), whereupon it also shows a decrease. After the quenched sample is annealed, the intensity has minor fluctuations at yn and in the upper glass transition region. 

Polyvinyl fluoride has a number of transitions below the melting temperature, the values of which depend on the measurement techniques. The lower glass transition occurs at -15 to -20°C and is believed to relate to relaxation free from restraint by crystallites. The upper glass transition ranges from 40 to 50°C, apparently due to amorphous regions under restraint by crystalhtes.1 1 Yet another transition occurs at -80°C because of short-chain amorphous relaxation and another at 150°C associated with premelting intracrystalline relaxation. 

Figure 3.7 Schematic representation of the three amorphous transitions in bulk crystallised, semi-crystalline PE and their dependence on crystallinity. T (U) the upper glass transition TJiL) the lower glass transition Ty, the local mode (crankshaft process) [23]...

G.S.Y. Yeh (University of Michigan, Ann Arbor, Michigan) You talked about crystallization at the lower glass transition temperature. Does this mean that at that point you see mostly very small crystals or nodules, and then when you get to the upper glass transition temperature, do you see mote, larger lamellae ... 

P.H. Geil Yes, that s what I m saying in a sense. I can, therefore, constrain the molecules between the nodules in those samples and that does not relax out again until I get to the upper glass transition, which I think mote or less agrees with Boyer s concpet [R.F. Boyer, Macromolecules, 6, 288-298 (1973) J. Macromol. ScL, Phys., B8, 503-537 (1973)], even though it wasn t applied directly to this kind of a sample. 

Reactive compatibilization can also be accomplished by co-vulcanization at the interface of the component particles resulting in obliteration of phase boundary. For example, when cA-polybutadiene is blended with SBR (23.5% styrene), the two glass transition temperatures merge into one after vulcanization. Co-vulcanization may take place in two steps, namely generation of a block or graft copolymer during vulcanization at the phase interface and compatibilization of the components by thickening of the interface. However, this can only happen if the temperature of co-vulcanization is above the order-disorder transition and is between the upper and lower critical solution temperature (LCST) of the blend [20]. 

Figure 5. Logarithm of the retractive force at 49% strain (lower curve) and sample temperature (upper curve) plotted against logarithm of time reduced to 263 K. Cross-links are introduced at log t/aT is 3 in the glassy state where the spike on the force curve is due to thermal contraction upon cooling below the glass transition temperature. Equilibrium force at 263 K after cross-linking is feQ. (Reproduced, with permission, from Ref. 27. Copyright 1981, Journal of Chemical Physics.)...

Except for a lew thermoset materials, most plastics soften at some temperatures, At the softening or heat distortion temperature, plastics become easily deformahle and tend to lose their shape and deform quickly under a Load. Above the heat distortion temperature, rigid amorphous plastics become useless as structural materials. Thus the heat distortion test, which defines The approximate upper temperature at which the material can be Safely used, is an important test (4,5.7.24). As expected, lor amorphous materials the heat distortion temperature is closely related to the glass transition temperature, hut tor highly crystalline polymers the heat distortion temperature is generally considerably higher than the glass transition temperature. Fillers also often raise the heat distortion test well above... 

Among the spectrum of melt-spinnable fibers such as polyolefins and nylons, PET stands at the upper end in terms of crystalline melt temperature and glass transition temperature. This provides superior dimensional stability for applications where moderately elevated temperatures are encountered, e.g. in automobile tires or in home laundering and drying of garments. The high thermal stability results from the aromatic rings that hinder the mobility of the polymer chain. 

G. Biroli and J. P. Bouchaud, Diverging length scale and upper critical dimension in the mode coupling theory of the glass transition. Europhys. Lett. 67, 21 (2004). 

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