Polymer glass transition point

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...

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. 

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. 

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. 

Toluene also reacts with ethylene to produce -ethyltoluene [622-96-8] or j -methylethylbenzene, which can be dehydrogenated to give -methylstyrene. The polymer (PMS), has a high glass-transition point and better flow properties, and has gained significant commercial importance in recent years. 

If a sample of an amorphous polymer is heated to a temperature above its glass transition point and then subjected to a tensile stress, the molecules will tend to align themselves in the general direction of the stress. If the mass is then cooled below its transition temperature while the molecule is still under stress, the molecules will become frozen in an oriented state. Such an orientation can have significant effects on the properties of the polymer mass. The polymer is thus anisotropic. 

Creep properties are very much dependent upon temperature. Well below the glass transition point very little creep will take place, even after long periods of time. As the temperature is raised, the rate of creep increases. In the glass-transition region the creep properties become extremely temperature-dependent. In many polymers the creep rate goes through a maximum near the glass-transition point. 

We have seen already (Sect. 13.4.7) that every amorphous material (including that in semi-crystalline polymers) becomes brittle when cooled below the first secondary transition temperature (Tp) and becomes ductile when heated above the glass transition point (Tg). Between these two temperatures the behaviour - brittle or ductile - is mainly determined by the combination of temperature and rate of deformation. 

Orientation is generally accomplished by deforming a polymer at or above its glass transition point. Fixation of the orientation takes place if the stretched polymer is cooled to below its glass transition temperature before the molecules have had a chance to return to this random orientation. By heating above the Tg the oriented polymer will tend to retract in amorphous polymers the retractive force is even a direct measure of the degree of orientation obtained. 

Uniaxial orientation is accomplished by stretching a thread, strip or bar in one direction. Usually this process is carried out at a temperature just above the glass transition point. The polymer chains tend to line up parallel to the direction of stretching, although in reality only a small fraction of the chain segments becomes perfectly oriented. 

At the melting point (Tm) and at the glass transition point (Tg) its value is nearly zero in the intermediate region a maximum (vmax) is observed at a temperature Tk. Gandica and Magill (1972) have derived a master curve, valid for all "normal" polymers, in which the ratio v/vmax is plotted vs. a dimensionless crystallisation temperature ... 

If the reaction temperature is below the polymer glass transition temperature and the amount of monomer in the particle decreases far enough, the glass effect may become important. The polymerization rate virtually goes to zero because the particle becomes so internally viscous, essentially glasslike, that the diffusion of monomer to the radicals is limited. The glass transition point varies for different polymers. This effect has also been studied by several authors [34,39,40]. 

We will furthermore present data showing the effect of polymerization temperature on limiting conversion. With polymerizations below the glass transition point of the polymer, the monomer-polymer solution reaches its glass transition point at a conversion <100%. At this point reactions involving small molecules, such as propagation, become diffusion controlled. This causes the rate of polymerization to fall to virtually zero in the normal polymerization time scale, i.e. in practice the reaction rate reaches a limiting conversion <100%. 

In conclusion we may say that a conversion near 100% can only be reached within reasonable time if the polymerization temperature is above the glass transition point of the polymer. This conclusion holds for bulk, emulsion, and suspension polymerization but, of course, not for solution polymerization where the solvent-polymer mixture usually has a glass transition point which is well below ordinary polymerization temperatures. 

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