Quasi-static glass transition temperatur

In the present study, we have made X-ray diffraction, neutron diffraction with isotopic substitution, and quasi-elastic neutron scattering measurements on highly concentrated aqueous solutions of lithium halides in a wide temperature range from room temperature to below glass transition temperature, from which the microscopic behaviors of the static structure and dynamic properties of the solutions are revealed with lowering temperature. The results obtained are discussed in connection with ice nucleation, anisotropic motion of water, crystallization, and the partial recovery of hydrogen bonds. 

The in-plane mechanical, viscoelastic and thermal properties of a satin weave carbon fabric impregnated with an amine cured epoxy resin were studied by Abot and co-workers [74]. The in-plane quasi-static behaviour including the failure modes under tension, compression and shear and all the mechanical properties including elastic moduli and strengths were determined. The viscoelastic properties including the glass transition temperature were also measured as well as the coefficients of thermal expansion. These measured properties for the fabric composites were also compared with their corresponding ones for a unidirectional composite with the same fibre and matrix. 

For quasi-static measurements such as illustrated in Figure 8.2, the glass transition temperature, Tg, is often taken at the maximum rate of turndown of the modulus at the elbow, where E = lO Pa. Often the glass transition temperature is defined as the temperature where the thermal expansion coefficient (Section 8.3) undergoes a discontinuity. (Enthalpic and dynamic definitions are given in Section 8.2.9. Other, more precise definitions are given in Section 8.5.)... 

Unlike crystalline melting, the glass transition temperature is a relaxation transition. This means that it is dependent on the effective frequency of the measureirtent. This frequency is found by dynamic mechanical (DMA), dielectric relaxation, and pulsed nuclear magnetic methods. Quasi-static methods, such as dilatometry, differential scanning calorimetry (DSC), and thermomechanical analysis (TMA), show that the effective frequency depends on the rate of temperature scan. This is one of the reasons why the glass transition temperatures reported for various amorphous materials appear so diverse. 

The absence of yield-point maxima on stress-strain plots obtained at room temperature and the early mechanical failures observed with all samples studied were typical of the stretching of brittle glassy polymers far below their glass transition temperatures, T. As expected, the quasi-static Young s moduli E (Table 1) were always lower (by about 20-30%) than the dynamic moduli E nevertheless, their composition dependence was essentially similar. The stresses, 0), and strains, at break for the PNC (Table 1) also turned out comparable to (albeit slightly smaller than) those for the pristine... 

Before entering into a detailed discussion of the glass transition, the five regions of viscoelastic behavior are briefly discussed to provide a broader picture of the temperature dependence of polymer properties. In the following, quasi-static measurements of the modulus at constant time, perhaps 10 or 100 s, and the temperature being raised l°C/min will be assumed. 

---