Dielectric relaxation spectroscopy, glass

In Chapter 7, Mano and Dionisio describe how electrical methods, and particularly dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarisation current (TSDS) techniques, play a major role as tools for e2q)loring molecular mobility. DRS enables molecular relaxational processes (both slow and fast) to be studied. For example, the localized motions of glass formers in the glassy state give rise to local fluctuations of the dipole vector that are the origin of the secondary relaxation processes detected by dielectric relaxation spectroscopy, while above, but near, the glass transition, cooperative motions result in a distinguishably different relaxation process (the a-relaxation). 

Dielectric relaxation spectroscopy (DER), e.g. [103-105], DER monitors the mobility of dipolar groups in the polymer and also of small dipolar molecules (e.g. water) that may be dissolved in the polymer system. Corresponding to mechanical measurements, the maxima of dissipated energy indicate phase transition processes. Dilatometry, pVT measurements, e.g. [50,106]. These measurements unequivocally show a first order transition by a step in V T) and a bend if there is a glass transition. The important partial derivatives isobaric expansivity and isothermal compressibility can be derived from the corresponding measurements. The method is, however, quite time consuming and not widely used. 

The a relaxation peak in dynamic mechanical spectroscopy and dielectric relaxation spectroscopy of non-crystalUne polymers also reflect the glass transition phenomenon ... 

A number of studies have been carried out of the glass transition process using dielectric relaxation spectroscopy. With the advent of computer-assisted measurements the DRS technique has increased in popularity and is now routinely used for the study of molecular mobility in polymeric materials. Chemical bonds may possess dipole moments as a consequence of the dilferences in... 

The glass transition (Tg) of the amorphous PVDF regions is in the range of -40 to -30°C, depending upon the sample and test method. Other sub-Tg transitions have been studied recently by dielectric relaxation spectroscopy (108). These studies also indicate correlations with other techniques and identify a 50°C molecular chain transition as probably related to the amorphous region at the surfaces of crystals (109). Permeation characteristics are very sensitive to these transitions as well as the usual environmental parameters (110). Water molecules trapped in the amorphous regions are monomeric, not associated and clustered as in the liquid state (111). 

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). 

The first data on polymer systems were collected via (laser-) light-scattering techniques [1] and turbidity measurements, further developed by Derham et al. [2,3]. Techniques based on the glass-transition of the polymer-blend constituents were also tested, such as DSC, Dynamic Mechanical Spectroscopy, and Dielectric relaxation [4]. Films made from solutions of... 

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