Thermal analysis and dilatometry of glasses formed under elevated pressure

Abstract

Densified glasses, formed from the liquid state by cooling at 5°C/hr under elevated pressures of up to 517 MN/m2, were studied by thermal analysis and dilatometry at ambient pressure. The materials studied were polystyrene, poly(methyl methacrylate), phenolphthalein, sucrose, and a 1 : 1 mixture by weight of potassium and calcium nitrate. Enthalpies of the densified glasses were found to be up to 2–6 J/g greater than the enthalpies of the glasses formed at atmospheric pressure. The major enthalpy and volume relaxation on heating the densified glasses was found to be not correlated The upper temperature limit of the glass‐transition region remained constant for all glasses cooled at various pressures, while the lower temperature limit of the polymeric glasses decreased up to 55°C with increasing pressure. The interpretation of the data suggests that an additional, significant volume relaxation occurred on depressurization at room temperature for all glasses analyzed.

Thermodynamics of the densification process for polymer glasses

“…On the other hand, the data in the row above on the same polymer reveal good agreement including that with dT gldP (PVT) of ref. [9]. In all cases agreement is very good for polystyrene.…”

“…For polystyrene [8], the data have been evaluated as MI = H(densified) -H(normal) first decreasing slightly and then going back to zero at about 800 bar, followed by an increase with nearly constant slope up to the maximum pressure. Weitz and Wunderlich [9] have also observed this behavior and interpreted it in terms of two opposing mechanisms arising from holes and rotational isomers. It is not clear, however, that the apparent negative values of MI obtained by experiment are significant.…”

“…Note that Tg (in li eu a nd Tt(p , P'). 9 of Tl) applies here, since the trans ition is observ ed at the formation pressure (P = PI) in both cases. The isobaric extension of VB with increasing T (see das hed line ) to its intersection with the liquid line yields the fic ti ve te mperature…”

“…Also poor agreement is noted for a-PMMA of ref. [9], where both quantities are obtained from the same sample. On the other hand, the data in the row above on the same polymer reveal good agreement including that with dT gldP (PVT) of ref.…”

“…[6], [9] and [55] on polystyre ne, and [56] on PaMS. Yourtee and Cooper [6] observe a velY slight decrease in the densification rate with formation pressure for polystyrene over a 6 kbar range.…”