Dielectric relaxation spectroscopy, glass transition temperature

At low temperature the material is in the glassy state and only small ampU-tude motions hke vibrations, short range rotations or secondary relaxations are possible. Below the glass transition temperature Tg the secondary /J-re-laxation as observed by dielectric spectroscopy and the methyl group rotations maybe observed. In addition, at high frequencies the vibrational dynamics, in particular the so called Boson peak, characterizes the dynamic behaviour of amorphous polyisoprene. The secondary relaxations cause the first small step in the dynamic modulus of such a polymer system. 

Abbreviations DEA, dielectric analysis >OC. degree of crystallinity DSC, di erential scanning calorimetry LM, local mobility (secondary relaxations) SR, structural relaxation 7g, determination of glass transition temperature TSDC. thermally stimulated depolarization current spectroscopy XRD, X-ray difTractometry. Source Adapted from Ref. 15. 

Does a shift of the glass transition temperature reflect a change in the relaxation time distribution or in its mean value only This makes a fundamental difference underlining the power of microscopic techniques like Broadband Dielectric Spectroscopy. 

Photophysical and photochemical processes in polymer solids are extremely important in that they relate directly to the functions of photoresists and other molecular functional devices. These processes are influenced significantly by the molecular structure of the polymer matrix and its motion. As already discussed in Section 2.1.3, the reactivity of functional groups in polymer solids changes markedly at the glass transition temperature (Tg) of the matrix. Their reactivity is also affected by the / transition temperature, Tp, which corresponds to the relaxation of local motion modes of the main chain and by Ty, the temperature corresponding to the onset of side chain rotation. These transition temperatures can be detected also by other experimental techniques, such as dynamic viscoelasticity measurements, dielectric dispersion, and NMR spectroscopy. The values obtained depend on the frequency of the measurement. Since photochemical and photophysical parameters are measures of the motion of a polymer chain, they provide means to estimate experimentally the values of Tp and Tr. In homogeneous solids, reactions are related to the free volume distribution. This important theoretical parameter can be discussed on the basis of photophysical processes. 

Similar to polyethylenes the morphology of these polymers is also described as a lamellar stack of crystalline and non-crystalline layers. This so-called two phase model is applied for the interpretation of X-ray diffraction data as well as for heat of fusion or density measurements. However, it is well known that several mechanical properties, as well as the relaxation strength at the glass transition temperature, cannot be described by such a simplistic two-phase approach, as discussed by Gupta [59]. Prom standard DSC measurements [60], dielectric spectroscopy, shear spectroscopy [61], NMR [62], and other techniques probing molecular dynamics at the glass transition (a-relaxation) temperature, the measured relaxation strength is always smaller than expected... 

Figure 9 Correlation of the segmental relaxation time ts measured in dilute solution and the glass transition temperature of the dense system. The data were taken from Adachi, K. Dielectric Spectroscopy of Polymeric Materials, American Chemical Society Washington, DC, p 261 and represent different chain structures. The line is a linear regression to the data.

In the literature, the DMA temperature of the loss factor peak is generally reported to be higher than the values that are measured using DSC. This divergence is due to differences between the measuring frequencies of the techniques and to analysis differences [88]. The glass transition temperature of semicrystalline polymer can be easily reveal by relaxation techniques, such as dynamic mechanical and dielectric spectroscopies, since DMA is a sensitive method (more sensitive then DSC) to evaluate T and viscoelastic properties of polymers [89]. is associated with a rapid decrease in storage... 

In the far infrared spectra of polymer as well as in the mid-infrared spectra, certain frequencies at which the resonance absorption of radiation by a system of oscillating atoms or molecules occurs are analyzed. However, in condensed matter, polymers in particular, at certain temperatures and frequences the processes are possible which also cause dissipation of energy of the electromagnetic field through configurational and other rearrangements. Various methods of dielectric or mechanical spectroscopy allow these processes to be detected in the form of the relaxation type absorption an a peak of losses immediately above Tg (the glass transition temperature) and the p, y and d peaks at temperatures below Tg [89,90]. 

The effect of solvent on molecular motions and the glass transition temperature may be studied using dielectric relaxation spectroscopy. Adachi et studied polystyrene/toluene, poly(vinyl... 

Because of the link between Tg and the mechanical and thermal properties, the dependence of the glass transition temperature on blend composition is of much interest and has been the subject of many experimental and theoretical papers. The subject is particularly vast since TgS can be determined using various experimental techniques, including differential scanning calorimetry, dynamic mechanical thermal analysis, and dielectric relaxation spectroscopy (DRS). 

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