Amorphous polymers relaxation processes

The average polymer melt relaxation times between the processing temperature Tp and the solidifying temperature (the Tg in amorphous polymers and somewhere between Tg and with polycrystalline polymers). 

The lattice models provide useful interpretations of spin relaxation in dissolved polymers and rubbery or amorphous bulk polymers. Very large data bases are required to distinguish the interpretive ability of lattice models from other models, but as yet no important distinction between the lattice models is apparent. In solution, the spectral density at several frequencies can be determined by observing both carbon-13 and proton relaxation processes. However, all the frequencies are rather hl unless T2 data are also included which then involves the prospect of systematic errors. It should be mentioned that only effective rotational motions of either very local or very long range nature are required to account for solution observations. The local... 

The high-temperature relaxation process is typical for amorphous polymers and can be assigned to the a-relaxation that appears in the whole frequency range and in the temperature interval from 50 to 100°C. This process is well observed for all samples. It corresponds to the glass-rubber transition of the amorphous phase. 

Another example of an application of Eq. (145) is on microcomposite polymer materials. We have performed dielectric measurements of the glass transition relaxation process in a nylon-6,6 sample quenched in amorphous (QN), a crystalline nylon-6,6 sample (CN), and a microcomposite sample (MCN), which is the same crystalline nylon-6,6 but with incorporated kevlar fibers [275,276],... 

The high-frequency (3, 7,. .. subsidiary peaks in amorphous polymers are characteristically very broad with a half-height width of several decades (compared with 1.14 decades for a single Debye relaxation process), although a good, linear Arrhenius plot is usually obtained, suggesting a non-co-operative mechanism. Figure 3.8 shows the Arrhenius plot for the /3-relaxation of poly(epichlorhydrin),... 

Given all these caveats, how fast are relaxation processes in amorphous polymers In low molecular weight liquids relaxations are very fast, occurring in time frames of the order of 10 10 secs. In a polymer melt (T > T)... 

In the preceding sections, we have looked at the various types of relaxation processes that occur in polymers, focusing predominantly on properties like stress relaxation and creep compliance in amorphous polymers. We have also seen that there is an equivalence between time (or frequency) and temperature behavior. In fact this relationship can be expressed formally in terms of a superposition principle. In the next few paragraphs we will consider this in more detail. First, keep in mind that there are a number of relaxation processes in polymers whose temperature dependence we should explore. These include ... 

Relaxation processes in amorphous polymers below the glass transition involve local... 

The relaxation behavior of selected semicrystalline ESI is depicted in Figure 26.3. It can be seen that the loss peak evident in the temperature range —50 to +50 °C shows increasing breadth of the relaxation process as the styrene content in ESI decreases. The relaxation processes associated with this loss peak are complex in nature. The relaxation behavior of semicrystalline polymers is fundamentally different from that of amorphous polymers. The long-range segmental motions associated with the Tg process become hindered owing to the restrictions imposed by the crystallites. 

The effect of diluents on the viscoelastic behavior of amorphous polymers is more complex at temperatures below T, i.e., in the range of secondary relaxation processes. Mechanical, dielectric and NMR measurements have been performed to study the molecular mobility of polymer-diluent systems in this temperature range (see e.g. From extensive studies on polymers such as polycarbonate, polysulfone and polyvinylchloride, it is well known that diluents may suppress secondary relaxation processes. Because of the resulting increase in stiffness, these diluents are called antiplasticizers . Jackson and Caldwell have discussed characteristic properties... 

Aliphatic polyesters may present a crystalline a process, and as a consequence the notations (3 and y are adopted for the glass-rubber and subglass relaxations, respectively. Although fully amorphous polymers cannot be achieved by quenching, it is possible to obtain polyesters with different degrees of crytallinity by copolymerization with a noncrystallizable diol. For example, the polyester of 1,6-hexanediol condensed with adipic acid is about 60% crystalline, while the polyester of this diacid with 2,5-hexanediol is completely amorphous. By varying the l,6-hexanediol/2,5-hexanediol... 

As indicated above, a relaxation is associated with molecular motions in which crytalline entities take part. However, the development of this process apparently requires the presence of an amorphous phase. Actually, as shown in Figure 12.32, the relaxation curves of polymethylenic waxes in which the crystallites are formed by totally extended chains (degree of crystallinity 100%) do not present a relaxation (41,42). Since neither totally crystalline nor totally amorphous polymers display a relaxation, one must conclude that this absorption is caused by molecular motions occurring in the crystalline-amorphous interphase. 

Most crystalline polymers with metylenic groups in their structure and with a degree of crystallinity below 50% present a sub-glass relaxation whose intensity and location scarcely differ from those observed for the amorphous polymer in the glassy state. The temperature dependence of this relaxation follows Arrhenius behavior, and its activation energy is of the same order as that found for secondary processes in amorphous polymers. 

Annealing promotes crystallite thickening at the expense of the crystalline-amorphous interphase and the amorphous phase. This process decreases the intensity of the glass-rubber relaxation and enhances that of the a relaxation if the crystalline polymer develops this absorption. 

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