Glass temperature theoretical treatments

For the case that no convection stream is applied, the time, which elapses between the start of shear flow and the establishment of the temperature maximum in the center of the gap has been reported to be useful for the measurement. This measurement could be carried out within 10 sec (202). The possibility of such a procedure, however, follows from the theoretical treatment only for relatively wide gaps of d > 2 mms. For metal cylinders (199) as well as for glass cylinders (203) times of establishment can be calculated which are definitely too short for the application of this method, when smaller gap widths are used. 

There are many empirical correlations for the activation energy, as well as some theoretical treatments [19-21]. These treatments usually attempt to relate Ep to quantities such as the size of the penetrant molecule (usually expressed in terms of a penetrant diameter or the square of this diameter, and sometimes corrected for the penetrant shape if it deviates significantly from spherical), the glass transition temperature of the polymer, and whether the polymer is glassy or rubbery at the temperature of interest. The diffusivity and solubility components of the permeability are usually treated separately in such attempts, as described below. 

The two features are attributable to the same type of paraelectric center and correspond to its relaxation and resonance interactions with the alternating electric field. Pirc and Dick first gave a unified theoretical treatment of both peaks. Their treatment applied primarily to crystalline media and is qualitatively applicable to glasses if paraelectric centers are assumed to have a distribution of energy splittings and relaxation times instead of fixed values. In the case of glasses, the theory of low-energy excitations was initially formulated to explain their low-temperature thermal properties Pure vitreous silica apparently contains... 

After observing quite a few anomalous properties of optical transitions in glasses and attributing them to the dynamics of TLS [14], the tunneling model was adopted by Reinecke [15] to explain the low-temperature line widths of optical transitions in amorphous solids using the concept of spectral diffusion. This concept had originally been developed for the description of spin resonance experiments [16] and had already been applied to the theoretical treatment of the above mentioned ultrasonic properties of glasses [17]. Soon after this step, the possibility of a connection between thermal and optical properties of amorphous solids was supported by the observation of time dependence of spectral hole widths [18]. 

In the case of the glass transition, more complete experimental data are available, at least with respect to vinyl polymers. As will be discussed below, for certain types of structure a substantial effect of stereoregularity on the glass transition temperature, T, is found, and in addition a theoretical treatment is available which accounts for these effects. However, very little data is available with respect to the effect of stereoregularity on, for example, the configurational entropy S at T, on aC, the size of the heat capacity increment at T, 0 on other key thermodynamic parameters of the glassy state. ... 

In a 5-I. round-bottom flask (Pyrex) is placed a mixture of 500 g. of phthalic anhydride and 400 g. of 28 per cent ammonium hydroxide. The flask is fitted with an air condenser not less than 10 mm. in diameter and is then slowly heated with a free flame until the mixture is in a state of quiet fusion at a temperature of about 300°. It requires about one hour before all the water has gone and about one and a half to two hours before the temperature of the reaction mixture reaches 300° and the mixture is a homogeneous melt. It is advisable, during the heating, to shake the flask occasionally some material sublimes into the condenser and must be pushed down with a glass rod. The hot reaction mixture is now poured out into a crock, covered with a paper to prevent loss by sublimation, and allowed to cool. The product is practically pure without further treatment, and melts at 23 2-23 5 °. The yield is 470-480 g. (94-95 per cent of the theoretical amount). 

Recently, alternative theoretical expressions have been developed by using classical thermodynamic treatments to describe the compositional dependence of the glass transition temperature in miscible blends and further extended also to the epoxywater systems 2S,27). The studies carried out on DGEBA epoxy resins of relatively low glass transition have shown that the plasticization induced by water sorption can be described by theoretical predictions given by ... 

The latter reaction can form long chain phosphates, where n is theoretically infinite. Being formed by heat treatments, these phosphates are excellent candidates for high-temperature ceramics and glasses. Because the subject of this volume is low-temperature ceramics, we will not discuss the condensed phosphates in detail, except in one case in Chapter 15, where cements for geothermal wells are discussed with sodium metaphosphate. However, bear in mind that CBPCs can be precursors to high temperature phosphates and glasses. For this reason, as we have seen in the literamre survey presented in Chapter 3, early interest in CBPCs was the formation of refractory shapes at room temperature, which were then fired to produce the final refractory components. 

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