Glass-transition temperature, water effect

The highly polar nature of the TGMDA—DDS system results in high moisture absorption. The plasticization of epoxy matrices by absorbed water and its effect on composite properties have been well documented. As can be seen from Table 4, the TGMDA system can absorb as much as 6.5% (by weight) water (4). This absorbed water results in a dramatic drop in both the glass transition temperature and hot—wet flexural modulus (4—6). 

Partially aromatic PAs have a higher glass transition temperature and a higher melting temperature dian their aliphatic counterparts thus these polymers have a better dimensional stability.18 Due to the high Tg and the low water absorption as a result of die presence of die aromatic unit, the effect of water on the properties at room temperature is small. The barrier properties of partially aromatic PAs are also better. However, despite all these favorable properties, their commercial market is relatively small. 

Fluorinated poly(arylene edier)s are of special interest because of their low surface energy, remarkably low water absorption, and low dielectric constants. The bulk—CF3 group also serves to increase the free volume of the polymer, thereby improving various properties of polymers, including gas permeabilities and electrical insulating properties. The 6F group in the polymer backbone enhances polymer solubility (commonly referred to as the fluorine effect ) without forfeiture of die thermal stability. It also increases die glass transition temperature with concomitant decrease of crystallinity. 

The presence of a solvent, especially water, and/or other additives or impurities, often in nonstoichiometric proportions, may modify the physical properties of a solid, often through impurity defects, through changes in crystal habit (shape) or by lowering the glass transition temperature of an amorphous solid. The effects of water on the solid-state stability of proteins and peptides and the removal of water by lyophilization to produce materials of certain crystallinity are of great practical importance although still imperfectly understood. 

The effect of physical aging on the crystallization state and water vapor sorption behavior of amorphous non-solvated trehalose was studied [91]. It was found that annealing the amorphous substance at temperatures below the glass transition temperature caused nucleation in the sample that served to decrease the onset temperature of crystallization upon subsequent heating. Physical aging caused a decrease in the rate and extent of water vapor adsorption at low relative humidities, but water sorption could serve to remove the effects of physical aging due to a volume expansion that took place in conjunction with the adsorption process. 

As the temperature is lowered further, the viscosity of the unfrozen solution increases dramatically until molecular mobility effectively ceases. This unfrozen solution will contain the protein, as well as some excipients, and (at most) 50 per cent water. As molecular mobility has effectively stopped, chemical reactivity also all but ceases. The consistency of this solution is that of glass, and the temperature at which this is attained is called the glass transition temperature Tg-. For most protein solutions, Tg- values reside between -40 °C and -60 °C. The primary aim of the initial stages of the freeze-drying process is to decrease the product temperature below that of its Tg- value and as quickly as possible in order to minimize the potential negative effects described above. 

The typical differential scanning colorimetric (DSC) traces shown in Figure 9.2 compare the thermal transitions of similar low-DEG-content PEN and PET resins. The fact that the glass transition temperature (Tg) of PEN is 45-50 °C higher than that of PET has a major influence on the processing and performance of PEN applications. In addition, the fact that PEN S Tg is 20-25 °C above the boiling point of water has a significant effect on the thermal stability potential of many hot, aqueous exposure applications. 

The data in Fig. 13 show that the glass transition temperatures of all materials is reduced by the absorption of water. This seems to be due to the plasticizing effect of the water on the binder. There is a marked difference between the elastic states of the dressed and undressed foams, the latter becoming much more plastic after immersion in water. Increased plasticity is due to the loss of adhesion between the binder and the filler, indicating that water absorption by syntactic foams is multistaged. 

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