Enthalpy relaxation spectra

The thermally stimulated current study of amorphous polymers shows the existence of two modes for the enthalpy relaxation spectra. The same result was found for the volume relaxation spectra. 

While borrowing from the classical models, these phenomenological approaches have also helped to clarify and refine the concepts of free-volume and configurational entropy and have focused attention on the nature of the relaxation spectrum (7-10,109,110). When a hquid in equiUbrium at a temperature Ti is suddenly cooled to a temperature, T2, its structure has no time to adjust and its properties (such as volume V, enthalpy H, or index of refraction n) exhibit instantaneous, solidlike changes characteristic of the glassy state as illustrated in Figure 9. For example, the instantaneous change in the enthalpy H may be written as follows ... 

Tanaka, Y., Yamamoto, T. Enthalpy relaxation of comb-like polymer analysed by combining activation energy spectrum and tmn models. J. Non-Cryst. Solids 358(14), 1687-1698 (2012)... 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

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