Recently, attention has been focused on the properties of glasses which have been prepared from the melt under p r e s~u r e .~~'~~~'~~~ Glasses made in this manner actually 67 0077-8923/81/0371-0067 $1.75/1 0 1981, NYAS powder^'^.'^ and deformed glasses,I9 which were associated with either the relaxation
The conformational and enthalpic changes that occur in poly(vinyl chloride) (PVC) glasses that have been vitrified from the melt under pressure have been examined by Fourier transform infrared spectroscopy and quantitative differential scanning calorimetry. It is shown that these pressures freeze in the high energy states that are characteristic of the vitrification temperature and increase the apparent glass transition temperature of the polymer. In addition, pressures in excess of the vitrification pressure, cause intermolecular effects that can be relaxed out below Tg. Both of these processes create characteristic endothermic and exothermic changes in the apparent heat capacity of the glass that appear over a period of time and are sensitive functions of the glass formation. processes as well as the subsequent annealing history. The endothermic events are interpreted as the stress perturbed volumetric relaxation process while the exotherms are associated with the release of the frozen in stresses.
SUMMARY: Fast, effective methods to alter the concentration of dissolved oxygen in tissue culture media were desired to study the effect of oxygen tension on the behavior of cells in culture. This report compares the efficacy of two such methods, with the goal to achieve defined oxygen tensions in the liquid phase. In one method, a slight overpressure of defined oxygen tension promoted diffusion from the gas phase to the liquid phase (normal diffusion). In the other method, a vacuum was used to remove dissolved oxygen from the media before exposure of samples to a defined oxygen overpressure (vacuum-assisted diffusion). With normal diffusion, the dissolved oxygen tensions became fixed within 90 min and never reached the target oxygen concentrations. However, with vacuum-assisted diffusion, target oxygen concentrations were reached within 7 min, demonstrating this method to be faster and more complete for altering the dissolved oxygen content in culture media.
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