Several studies have been carried out on a polyurethane elastomer (Solithane 113). At atmospheric pressure, dielectric methods were used to study E" over the temperature range from -180 to +20·C. Three peaks in E" were observed: an a peak associated with the glass transition and two low temperatures peaks (P and r). Mechanical tests were peformed to osberve the effect oft~e pressure-induced glass transition P g on the Young's modulus. The glass transition was studied of pressure from 1 bar to 6.5 k bar by observing the shift as a function ofthe dielectric a peak with pressure and also by volumetric methods. A densification of the glass and a resultant shift in the glass-transition temperature could be achieved by forming the glass at high pressures (Tg) as opposed to pressuring the glass formed at 1 bar (T ;). It was found that AP / Aa > dT g / dP, but AP / Aa-;::;;dT; /dP. At high pressures, dT g /dP reached a limiting value of 10.4 ·Clkbar.
It is known that application of hydrostatic pressure causes an increase in Tg at low pressures; however, some question has arisen as to whether the increase will continue indefinitely. Two possibilities exists: (i) Tg may level off to a constant value at high pressures, or (ii) Tg may continue to increase at all pressures. The former is suggested by thermodynamic theories of the glass transition; the latter by free volume theories. In this study dielectric methods have been used to measure the pressure dependence of the α-transition temperature, Tα, for a number of amorphous polymers and to relate this to the pressured dependence of Tg. The results indicate that at low pressures dTα/dP may decrease with increasing pressure (i.e., Tα may appear to be levelling off), but at higher pressures (greater than ∼2 kbar) dTα/dP reaches a constant, non-zero value (i.e., Tα increases linearly with pressure). In addition, the high pressure limiting value of dTg/dP appears to be roughly proportional to the atmospheric pressure Tg of the polymer tested.
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