Glass transition point amorphous polymers

For cross-linking by y rays, see Section 21.2.1. Inspection of the Gr values shows that, although styrene can be graft-copolymerized onto poly(vinyl chloride), vinyl chloride cannot be grafted onto poly(styrene). In the first case, so many radicals are produced on the polymer chain that practically no homopolymerization occurs. Above the glass transition temperature (amorphous polymers) or the melting point (crystalline polymers), the Gr values of the polymer increase because of increased chain mobility. Thus different effects can be observed by varying the 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. 

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. 

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. 

In view of the multitude of observed deformation mechanisms it is useful at this point to examine the effects of external variables, especially that of ambient temperature, on the deformation behavior of semi-crystalline thermoplastics. At room temperature many of these polymers are above their glass transition point and owe their strength and stiffness to the crystalline phases. The first displacements start in the relatively soft amorphous layers, but the stress-strain curve is largely determined by the presence and arrangement of the crystals. Interlamellar slip has been identified as an important mechanism, but, in addition, crystalline deformation mechanisms occur at moderate strains The corresponding stress-strain curve shows an... 

V2 = 1). The transition (partial or complete) into the liquid crystalline state occurs only after the system is heated above the glass-transition point. For real polymeric systems with semiflexible chains, the liquid crystalline state in the initial solution often is not realized, so the formation of nonequilibrium amorphous polymer upon the introduction of a nonsolvent is quite probable. 

Polyethylene is a semicrystalline polymer. It means that at ambient temperatures the polymer consists of two rather distinct fractions, or phases—crystalline and amorphous. The amorphous part of polyethylene, which is a sort of rubbery at ambient temperatures, becomes a glass-like at a certain transition temperature, the so-called glass transition point. For polyethylene the glass transition point varies from very low to low (from -130 to 20°C), thus making the plastic ductile at common temperatures. The lower glass transition point (y-transition) is always present in the range of -130 to -100°C, the higher one (P-transition, at —20°C) is manifested not in all PE materials. To complicate the picture even more, we can notice that there is one more transition in polyethylenes, called a-transition, commonly found between 10 and 70°C, and it is associated with crystallinity of PE. For WPC the last two transitions (a- and P-) are of little importance. 

When oriented polymers are heated, they will try to reach their original high entropy, and shrinkage will occur as soon as the molecules can move sufficiently to recoil to their undisturbed dimensions. For amorphous polymers this will be the case when the glass transition point 7 is reached. For crystalline polymers the behaviour is more complicated. 

If the reference temperature is taken about 43 °K above the glass transition point Tg, the constants C, and C2 are essentially the same for a large number of amorphous polymers (C, = 8.86 and C2 = 101.6). This results in the following equation ... 

For a crystallized semicrystalline pol3uner, the web is heated to below, but near, the melting point. Amorphous polymers will only need to be heated to slightly above their glass-transition temperatures (Tg). 

One view of a gel is as a modified mbber. The properties of amorphous polymers change dramatically above the glass transition point where the chains become mobile. Since polymer chains are mobile in solution and we think of a simple gel as a cross-linked solution, we can regard a single phase gel as a dilute rubber. Dense polyacrylamide, for instance, is a glassy polymer. We do not want to compare the gel properties with this state but with the same polymer as a cross-linked mbber above its glass transition. 

Crystalline polymers possess a melting temperature T , below which it becomes a crystalline solid. In contrast, amorphous polymers do not exhibit a discrete melting temperature, but, instead, these polymers bring about two transitions at the flow or softening point and the glass-transition point Tg. 

In the field of polymer/additive analysis a rather limited number of other laboratory performance studies is available. Recently, the Swiss Eederal Laboratories for Materials Testing and Research (EMPA, St. Gallen) has organised a series of interlaboratory tests on polymeric materials, examining the glass transition point by DSC (amorphous thermoplastics), antioxidant content in polyolefins. 

Melting a polymer sample and cooling it quickly to a temperature lower than the glass transition point (Eg) produces an amorphous glass sample. The next section describes this cold crystallization process. 

Irregularities such as branch points, comonomer units, and cross-links lead to amorphous polymers. They do not have true melting points but instead have glass transition temperatures at which the rigid and glasslike material becomes a viscous liquid as the temperature is raised. 

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