Thermodynamic Characterization of Low Molecular Weight Materials in the Glassy State.

Heats of solutions ((DELTA)h(,soln)) of Aroclor 5460 and atactic polystyrene glasses are measured by techniques of solution microcalorimetry isothermally and (VBAR)(DELTA)h(,soln)(VBAR) was found to decrease linearly with temperature until the transition temperature is reached and above that remained constant. The slopes of these straight lines are identical irrespective of the thermal histories as long as the histories remained unchanged for both Aroclor 5460 and polystyrene. In the case of polystyrene, the slope is reproduced within experimental error irrespective of the molecular weight. The slopes for Aroclor 5460 and polystyrene are -0.054 and -0.09 cal g('-1) K('-1) respectively. Long annealing of both supercooled and superheated samples yielded the equilibrium isothermal enthalpies at various temperatures. These values are not equal to the heats of mixing at temperatures studied below T(,2)('e). The value decreased linearly, proving that the equilibrium state of a glass is not the extrapolation of the amorphous liquid state. The equilibrium enthalpies have slopes of -0.008 and -0.04 cal g('-1) K('-1), as compared to -0.054 and -0.09 cal g('-1) K('-1) for Aroclor 5460 and polystyrene respectively in the non-equilibrium state. The heats of dilution measurements are obtained to calculate the heats of mixing for the systems Aroclor 5460-acetophenone and polystyrene-toluene. The heats of mixing for Aroclor 5460 with acetophenone are identical to the heats of solution above the transition temperatures, indicating that the assumption that above the transition temperature the glass is in its amorphous liquid state is accurate. The heat of mixing remained constant over the whole range of temperatures. In contrast, the heat of mixing of the latter system increased with temperature and tends to be identical to the heat of solution above the transition temperature. Present data is compared with the literature data. Physical aging or annealing has been studied for quenched glasses at different temperatures by following the changes in isothermal heats of solution enthalpies and specific volumes. These data do not follow a simple kinetic behavior with a single retardation time. Nonequilibrium thermodynamics should be used to treat this phenomenon with multiple retardation times. The data fit an Avrami type equation, (x(,t) - x(,(INFIN)))/(x(,0) - x(,(INFIN))) = e('-kt('n)), where x is any parameter at time, t(x(,t)) or at the initial value (x(,0)) or at infinite time (x(,(INFIN))); k is a temperature-dependent constant; and n is a constant called the exponent. Aroclor 5460 yields a value of n equal to 0.5 irrespective of temperature or the type of parameter. However, the polystyrene data include two sets of values depending on the temperature. Above 65(DEGREES)C the exponent is almost double that of below the temperature, indicating some kind of transition in polystyrene at that temperature (65(DEGREES)C). Isothermal enthalpies have been obtained for polystyrene (390,000) glasses prepared under different conditions of applied pressure and cooling rates. The (VBAR)(DELTA)h(,soln)(VBAR) increases with increasing pressure and cooling rates. The equation that determines the value of a given sample of known conditions at vitrification is given. The thermograms obtained for Aroclor 5460 and polystyrene have been integrated by using different methods of defining the behavior of amorphous liquid curve below the T(,g). A good set of data compared to the isothermal data is not obtained. A better fit is obtained by integration of specific heat data for Aroclor 5460 and not for polystyrene.PhDPhysical chemistryUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/158306/1/8116293.pd