Glass transition temperature measurement using dielectric methods

Part of the work performed on a sample will be converted irreversibly into random thermal motion by movement of the molecules or molecule segments. This loss passes through a maximum at the appropriate transition temperature or relaxation frequency in the associated alternating mechanical field (torsion pendulum test). A similar effect is obtained by the delayed response of the dipoles with dielectric measurements. Therefore, dielectric measurements can be made only on polar polymers. According to the frequency used, the glass-transition temperatures measured with dynamic methods lie higher than those obtained by quasistatic methods (see Section 10.5.2). 

For thin polystyrene films annealed for 12 hours at 150 °C in high vacuum (10-6 mbar) and measured in a pure nitrogen atmosphere the dynamic glass transition was characterized using two experimental techniques capacitive scanning dilatometry and Broadband Dielectric Spectroscopy. Data from the first method are presented in Fig. 15a, showing the real part of the complex capacity at 1 MHz as a function of temperature for a thin PS film of 33 nm. 

Water absorption of the resultant cured materials was measured after immersion for 24 hrs at room temperature and reported as % weight gain. The modulus was measured according to ASTM D 2240, and the dielectric constant and dissipation factor were measured using a method similar to ASTM D 150. Glass and melting transitions were measured by differential scanning calorimetry (DSC). The thermal stability was determined in a forced air oven... 

Various methods, including dilferential scanning calorimetry (DSC), dielectric relaxation spectrometry, and NMR, are known to be useful to determine molecular mobility of freeze-dried formulations [16,17]. The glass transition temperature (Tg) has been used as a measure of molecular mobility of lyophilized formulations, since it indicates the critical temperature of a-relaxation for amorphous polymer materials. Freeze-dried formulations containing polymer excipients can be considered to exhibit low molecular mobility without a-relaxation at temperatures below Tg. 

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... 

Most of the physical properties of the polymer (heat capacity, expansion coefficient, storage modulus, gas permeability, refractive index, etc.) undergo a discontinuous variation at the glass transition. The most frequently used methods to determine Tg are differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical thermal analysis (DMTA). But several other techniques may be also employed, such as the measurement of the complex dielectric permittivity as a function of temperature. The shape of variation of corresponding properties is shown in Fig. 4.1. 

Study of dielectric parameters by the AC method is one of the earliest and commonly used techniques. The measurement of dielectric loss as a funetkm of temperature at fixed frequencies or vice versa shows maxima at a characteristic telaxatioo temperature/ frequency of the polymer. The dielectric constant shows dispershm regions corresponding to various types of polarization processes. Glass transition phenomena in several polymers have bwn investigated by AC dielectric studies [326327]. 

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