Glass transition calorimetric methods

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. 

There are various methods of the glass transition temperature evaluation based on temperature dependence of polymer physical properties in the interval of glass transition 1) specific volume of polymer at slow cooling (dilatometric method) 2) heat capacity (calorimetric method),3) refraction index (refractometric method) 4) mechanical properties 5) electrical properties (temperature dependence of electric conductivity) or maximum of dielectric loss 6) NMR ° 7) electronic paramagnetic resonance, etc. 

One criterion to distinguish the miscibility of blends is the glass transition temperature (Tg) that can be measured with different calorimetric methods [95]. Tg is the characteristic transition of the amorphous phase in polymers. Below Tg, polymer chains are fixed by intermolecular interactions, no diffusion is possible, and the polymer is rigid. At temperatures higher than Tg, kinetic forces are stronger than molecular interactions and polymer chain diffusion is likely. In binary or multi-component miscible one-phase systems, macromolecules are statistically distributed on a molecular level. Therefore, only one glass transition occurs, which normally lies between the glass transition temperatures of the pure components. 

Deuteron NMR provides an excellent, albeit expensive, method for the determination of the glass transition, Tg. The quadrupole echo spectrum disappears (due to the echo distortion caused by isotropic motion) when the correlation time for chain motion approaches the quadrupole coupling frequency, typically about 30° above the calorimetric Tg. At higher temperatures, a narrow line appears due to fast isotropic motion. Early on, deuteron NMR and the spin alignment experiment (Figure 8.2(b)) were used to characterize slow motions of polystyrene [3]. More recently, deuteron NMR has been used to characterize... 

Calorimetric Methods.—Differential Scanning Calorimetry (d.s.c.). The use of commercially available d.s.c. apparatusto study phase separation in polymer and copolymer solutions is described. In this context perhaps the main advantage of the method is to distinguish between liquid-liquid phase separation, where the enthalpy change is usually small, and crystallization of polymer (see Chapter 12). The method is much used to study the glass transition in polymer mixtures (see p. 320). 

The enthalpy change can be measured using calorimetric methods, such as differential scanning calorimetry, which is a common means to locate the glass transition temperature. However, it turns out that the... 

The glass transition temperature (Tg) of blends of fully amorphous polymers is frequently measured to assess miscibility. There are a number of experimental techniques calorimetric (DSC/DTA), dilatometric, dynamic mechanical and dielectric. The resolution of the Tg methods has been the object of some discussion and the proposed resolution limit for domains should range from 2 to 15 nm. The presence of two Tg s in a binary system indicates phase separation on a level greater than this minimum domain size. Figure 4.16 shows schematically the recorded glass transition temperature(s) as a function of composition for case a (complete miscibility), case b (partial miscibility) and case c (complete immiscibility). 

The number of phenomena which can be directly studied by thermal analysis (DSC (DTA), TG, TMA, DMTA, TOA and DETA) is impressive. Typical of these methods is that only small amounts of sample (a few milligrams) are required for the analysis. Calorimetric methods record exo- and endothermic processes, e.g. melting, crystallization, liquid-crystalline phase transitions, and chemical reactions, e.g. polymerization, curing, depolymerization and degradation. Second-order transitions, e.g. glass transitions, are readily revealed by the calorimetric methods. Thermodynamic quantities, e.g. specific heat, are sensitively determined. TG is a valuable tool for the determination of the content of volatile species and fillers in polymeric materials and also for studies of polymer degradation. The majority of the aforementioned physical transitions can also be monitored by TMA (dilatometry). DMTA and DETA... 

The presence of the pendant branches does, however, lead to a decrease in the glass transition temperatme for the copolymer networks as measmed by both dynamic mechanical and calorimetric methods (refer to Table 35.2), as the nomeactive chain ends introduce... 

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