Glass transition point

To erase information by the transition amorphous — crystalline, the amorphous phase of the selected area must be crystallized by annealing. This is effected by illumination with a low power laser beam (6—15 mW, compared to 15—50 mW for writing/melting), thus crystallizing the area. This crystallization temperature is above the glass-transition point, but below the melting point of the material concerned (Eig. 15, Erase). 

Table 3.1 Influence of experimental time scale on the glass transition point of a polyoxacyclobutane ...

The size of the group attached to the main chain carbon atom can influence the glass transition point. For example, in polytetrafluoroethylene, which differs from polyethylene in having fluorine instead of hydrogen atoms attached to the backbone, the size of the fluorine atoms requires the molecule to take up a twisted zigzag configuration with the fluorine atoms packed tightly around the chain. In this case steric factors affect the inherent flexibility of the chain. 

Figure 6.4. Power factor-temperature curves for three polar polymers whose polar groups are integral with or directly attached to the main chain. The rise in power factor above the glass transition point is clearly seen in these three examples...

In addition to the normal methylene linkage formation involved in polymerization with both resoles and novolaes, other, usually less desirable, eondensation by-products are also seen in novolac synthesis. Among these are benzodioxanes and dibenzyl ethers. The reaction pH has significant effect on the relative amounts produced. Fig. 15 shows typical structures for these by-products. When such byproducts are present, the meaning of the molar ratio changes and variability with respect to molecular weight development, glass transition point, and solubility may be seen. They also lead to poor raw material utilization. 

In calculation the authors of the model assume that the cube material possesses the complex modulus EX and mechanical loss tangent tg dA which are functions of temperature T. The layer of thickness d is composed of material characterized by a complex modulus Eg = f(T + AT) and tg <5B = f(T + AT). The temperature dependences of Eg and tg SB are similar to those of EX and tg <5A, but are shifted towards higher or lower temperatures by a preset value AT which is equivalent to the change of the glass transition point. By prescibing the structural parameters a and d one simulates the dimensions of the inclusions and the interlayers, and by varying AT one can imitate the relationship between their respective mechanical parameters. 

However, we believe that Eqs. (15) and (16) can prove quantitatively correct only for experimental data at melt temperatures that are far from the glass-transition point,... 

Another transition appeared at about 130 °C on the rescanning after rapid cooling of the molten sample. It seems to be a glass transition point. The polyamide prepared in bulk at 70 °C also melted sharply at 250—260 °C. On the other hand, the crosslinked polymer, which was prepared in bulk at 100 °C in a high conversion, has nothing but a broad endothermic curve up to near 300 °C as shown in Fig. 9. 

It is appropriate to differentiate between polymerizations occuring at temperatures above and below the glass transition point(Tg) of the polymer being produced. For polymerizations below Tg the diffusion coefficients of even small monomer molecules can fall appreciably and as a consequence even relatively slow reactions involving monomer molecules can become diffusion controlled complicating the mechanism of polymerization even further. For polymerizations above Tg one can reasonably assume that reactions involving small molecules are not diffusion controlled, except perhaps for extremely fast reactions such as those involving termination of small radicals. 

The alignment of a lamellar microstructure by electric fields has been reported [64], The electric fields were applied across a melt of a PS-PMMA block copolymer and were maintained throughout cooling down to below the glass transition point. SAXS studies show persuasive evidence that the microstructure was aligned by an electric field. 

Thus the quantum yield for acid production from triphenylsulfonium salts is 0.8 in solution and about 0.3 in the polymer 2 matrix. The difference between acid generating efficiencies in solution and film may be due in part to the large component of resin absorption. Resin excited state energy may not be efficiently transferred to the sulfonium salt. Furthermore a reduction in quantum yield is generally expected for a radical process carried out in a polymer matrix due to cage effects which prevent the escape of initially formed radicals and result in recombination (IS). However there are cases where little or no difference in quantum efficiency is noted for radical reactions in various media. Photodissociation of diacylperoxides is nearly as efficient in polystyrene below the glass transition point as in fluid solution (12). This case is similar to that of the present study since the dissociation involves a small molecule dispersed in a glassy polymer. 

The Tg of the neat thermoset was undetected. The triazlne network showed a small damping peak between -60 to -80 C. This was attributed to a beta transition due to rotation of the blsphenol-A unit. Other workers had reported glass transition points for the cured material at 2U °C. The conversions of those reactions were lower than encountered here. These results may indicate mobility of branch points ending with unreacted cyanates. 

Figure 3.1-3 Phase diagram for[EMIM]CI/AICl3 (o) melting and freezing points ( ) glass transition points.

Dense, spherical particles with smooth surfaces and diameters of 20 p,m or greater provide the optimal flow behavior. The presence of more than 5% fines (particles less than about 20 fim in diameter) may prohibit flow altogether. Binders and other additives can inhibit proper flow when the temperature exceeds the glass transition point of the additive, and relative humidity cau affect the flow behavior of water-soluble binders. 

In a polymeric solid it was found that the line width AH of the absorption curve decreases with increasing temperature with a distinct reduction occurring near the glass transition point. This is shown in Figure 4 for polystyrene and polymethylmethacrylate where the line width is plotted vs. temperature. 

These temperatures have only a comparative value as they are more or less dependent on the method of measurement. They are also dependent on crystallinity. When dealing with unoriented, amorphous films, the softening temperatures correspond more or less to the glass transition temperatures. When the films are crystalline, softening temperatures range from the glass transition point to the crystalline melting point of the polymers. 

The Iso-Free-Volume State at the Glass-Transition Point.77... 

---