Glass transition temperature dielectric thermal analysis

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. 

The glass transition temperatures ofthe polyimides are 195-250 °C their 10% weight loss temperamres (dynamic thermogravimetric analysis, air, AT = 4.5 °C/min) are 390-422 °C. Of particular interest are the dielectric constants of these polyimides. At a relative humidity of 50% these constants are 2.70-2.90 and are comparable with constants of the best fluorinated polyimides [21, 50-55]. The lowest dielectric constant (2.70) was observed for polyimide based on 6F dianhydride, containing the highest amount of fluorine. Thermal treatment of this polymer film at 280-290 °C for 1 hour led to a decrease (2.45) of dielectric constant due to the possible formation of nanofoams [56]. 

The numerical value of the glass-transition temperature depends on the rate of measurement (see Section 10.1.2). The techniques are therefore subdivided into static and dynamic measurements. The static methods include determinations of heat capacities (including differential thermal analysis), volume change, and, as a consequence of the Lorentz-Lorenz volume-refractive index relationship, the change in refractive index as a function of temperature. Dynamic methods are represented by techniques such as broad-line nuclear magnetic resonance, mechanical loss, and dielectric-loss measurements. Static and dynamic glass transition temperatures can be interconverted. The probability p of segmental mobility increases as the free volume fraction / Lp increases (see also Section 5.5.1). For /wlf = of necessity, p = 0. For / Lp oo, it follows that p = 1. The functionality is consequently... 

The dielectric thermal analysis technique normally obtains data from thermal scans at constant impressed frequency. The glass transition temperature at which molecular motions become faster than the impressed timescale are recorded as peaks in e" and tan 8. 

Differential scanning calorimetry, thermomechanical analysis, dynamic mechanical analysis, differential thermal analysis, dielectric thermal analysis, infrared and NMR spectroscopy, are some of the instrumental techniques that have been applied to the determination of glass transition and other transition temperatures in polymers (Chapter 13). 

Effect of nano particles of Al Oj on conventional SPE films have been examined by FTIR, DSC and B-G spectroscopy. The dispersal of Al O nano particles to the SPEs shows dechnation in the glass transition and melting temperature as established from DSC analysis. The FUR spectra show possible interactions between Al O nano particles and host SPE films. The optimum room temperature ionic conductivity of the order of 7 x 10 S/cm having minimum activation energy (E 0.22eV) is observed for NCPE films. This shows one order increment in the conductivity over the conventional SPE films. The temperature dependent conductivity shows Arrhenius type thermally activated behavior before as well as after glass transition temperature. Maximum value of ion transference number is found to be 0.96 which is indicative of predominant ionic (protonic) transport in the SPE and NCPE thin films. It has been observed that dielectric constant for SPE and NCPEs increases with temperature while it decreases with frequency. 

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). 

With a direct measurement of cooperativity of the thin films via dielectric spectroscopy not attainable, attention was turned to probing the system indirectly. As discussed previously, changes in the glass transition temperature can indicate changing cooperativity. Thermal mechanical analysis (TMA) was used to survey the Tg of the system. Due to the favorable interactions of the PMMA side chains and the native oxide layer of silicon, the thinner films were expected to increase in cooperativity and therefore show an increase in Tg. The tests were started with the thickest films, 900 nm. 

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. 

Changes in physical state may be observed from changes in thermodynamic quantities, which can be measured by calorimetric techniques, dilatometry, and thermal analysis. Spectroscopic methods are also available for the determination of changes in molecular mobility around transition temperatures. In addition to the changes in thermodynamic quantities and molecular mobility, a glass transition has significant effects on mechanical and dielectric properties. 

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