Subglass processes

As usual, the relaxation strength of the subglass processes increases as the frequency decreases. The relaxation strength of the y relaxation in P4THPMA is close to the corresponding to PCHMA [30] and significantly lower than that of the corresponding to PDMA [38,104],... 

In contrast to the a relaxation both the shape and location of the subglass process are insensitive to the degree of crystallization. Dielectric studies [17] yield the same conclusion. The process is therefore consistent with localized molecular motions, in contrast with the restrained long-range segmental motions involved in the glass-rubber a relaxation. 

The ultimate (fracture) properties of a wholly amorphous polymer are strongly dependent on temperature. At low temperatures, in the elastic region, the fracture is predominantly brittle and the fracture toughness is low. A considerable increase in the fracture toughness accompanies the onset of the subglass process when approaching the anelastic region. 

Amorphous polymers always show a glass transition process (a) and also one or more so-called subglass process(es), referred to as P, y, S, etc. In a... 

The a process is clearly the glass transition. It is present in all polymethacrylates. It obeys WLF temperature dependence. The high-temperature subglass process P is present in all polymethacrylates. It shows both mechanical and dielectric activity and it is assigned to rotation of the side group. The P process shows Arrhenius temperature dependence. 

The low-temperature subglass process (y) is not present in PMMA and PEMA. It appears in PwPMA and longer alkyl homologues. It obeys Arrhenius temperature dependence and the activation energy is the same for all the higher polymethacrylates. It is assigned to motions in the flexible methylene sequence. It was concluded by Willboum that a... 

Two or three relaxation processes occur in semicrystalline polymers. The low-temperature (y or P) process is a subglass process occurring in the amorphous phase. The medium or high temperature process (p or a,) is associated with the glass—rubber transition of the amorphous component. The glass transition is very weak, and in many cases difficult to find, in highly crystalline polymers like linear polyethylene. A certain class of polymers shows a high-temperature relaxation process denoted a, which is a combined crystalline and amorphous process. Reorientation of the chain by a 180° twist of the molecule in the crystals and a certain axial... 

DMTA measures stress and strain in a periodically deformed sample at different loading frequencies and temperatures. It provides information about relaxation processes in polymers, specifically the glass transition and subglass processes. The theoretical background is presented in section 5.5.1. It should be noted that the strains involved in the measurements should be small... 

Fig. 10.14). Subglass processes display an Arrhenius temperature dependence,... 

Dielectric measurements are carried out on PPOA and PPODG. The dielectric spectrum of PPOA in the bulk presents a prominent glass-rubber relaxation followed by a subglass absorption. The low-molecular-weight compound only exhibits a prominent glass-liquid absorption followed by a diffuse and weak subglass relaxation. This behaviour cannot be explained in terms of only intramolecular interactions, and therefore intermolecular interactions must play an important role in this process. 

The results reported for these systems, indicate that the subglass relaxation in P3M2NBM is indeed a broad and weak relaxational process [95], The low strength of the observed secondary processes in these systems, has been attributed to the bulkiness of the side chain. In one case, the low strength of the secondary relaxation prevents the analysis of this peak in terms of an empirical relaxation. The effect of the methyl substituent on the norbornyl ring is to lower the position of the value of the peak of the a -relaxation about 50 K. Another effect that can be observed in the low frequency side of the spectra in both polymers is the conductive one. 

Figure 2.40 show the loss permittivity of P4THPMA where three subglass absorptions, labeled as 5, y and j3 relaxations, centered at 140, 190, and 260 K at 1 Hz respectively, followed in the increasing order of temperatures by glass-rubber process or a relaxation located at 420 K at the same frequency. 

This process is partially overlapped with the next process, the j3 relaxation. To analyze the loss permittivity in the subglass zone in a more detailed way, the fitting of the loss factor permittivity by means of usual equations is a good way to get confidence about this process [69], Following procedures described above Fig. 2.42 represent the lost factor data and deconvolution in two Fuoss Kirwood [69] as function of temperature at 10.3 Hz for P4THPMA. In Fig. 2.43 show the y and relaxations that result from the application of the multiple nonlinear regression analysis to the loss factor against temperature. The sum of the two calculated relaxations is very close to that in the experimental curve. 

Therefore the comparison of the dielectric spectra of P25DFBM with those of P24DFBM and P26DFBM leads to conclude that the motions about the C31—CH2 bonds are involved in the development of the subglass y relaxation. The overlapping of the a and the relaxations that partially or totally mask the latter process... 

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... 

During the isothermal ciystallization of PET at 97.5 ° C, that is, about 20 °C above its calorimetric glass transition temperature, the peak of the a-process is positioned at 104 Hz, and the shape of the dielectric loss curve is asymmetric as it is typically observed for the a-process of amorphous polymets. Hgure 15 shows contributions from the DC conductivity on the lower frequency side and a tail of the subglass p-process, whose peak exists above 1 MHz. As time passes, the peak value of the a-process deaeases and another peak appears at 10-100 Hz. The shape of the dielectric loss curve of the oo-process is quite different from that of the a-process the peak width is much broader than that of the a-process, and the peak shape is symmetric. Figure 16 shows that Ae of the a-process deaeases with time and eventually... 

The subglass relaxation processes dealt with in section 5.5.2 obey Arrhenius temperature dependence. 

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