Page 279 - Gas Adsorption Equilibria

5. Oscillometry 265

The swelling isotherm i. e. the (specific) data of the volume of the PC-
pellets as a function of the gas pressure (and temperature) exhibits two almost
linear branches intersecting at about 4 MPa. At lower pressures (p < 4 MPa)
the specific volume (Vas/(ms + ma)) increases only slightly with the
At higher pressures (p > 4 MPa) an increase of the steepness of the
isotherm can be observed, this possibly indicating a phase change transition
(glass transition) due to sorption of The sorption isotherm
determined from the indicates a simple Langmuir type
behavior. Equilibration times ranged from 4 hours at low gas pressures
(p < 1 MPa) up to 24 hours at high pressures ( p = 6 MPa) approaching
saturation of the subcritical gas. Relative uncertainties of all data are
about 3 %. More accurate measurements seem to be possible but will be
laborious and very sensitive to disturbances.

4. OSCILLOMETRIC – MANOMETRIC
MEASUREMENTS OF GAS ABSORPTION IN
SWELLING MATERIALS

4.1 Introductory Remarks

In this section we will discuss in brief another method to measure gas
solubilities in swelling materials like polymers or resins. It consists of a
combination of oscillometric measurements using a rotational pendulum and a
gas expansion experiment, i. e. volumetric or manometric measurements of
the sorptive gas. Hence this procedure can be called the oscillometric-
manometric method [5.27]. The main advantage of this method compared to
oscillometric-gravimetric measurements is that it does not need a
microbalance which often includes sophisticated mechanical and electronic
elements. Instead only a storage vessel for the sorptive gas is needed, the
pressure vessel surrounding the oscillating disk of the pendulum serving also
as sorption vessel, cp. Fig. 5.13 and also Fig. 2.1. Hence measurements of this
type may be easily automated. However, they only will lead to less accurate
results compared to those of oscillometric-gravimetric measurements, as
manometric measurements are less accurate than gravimetric [5.16, 5.27].
Still, results can be very useful for industrial purposes, namely providing
basic data for processes like pre-swelling, dying, or impregnating polymers
with supercritical gases like carbon dioxide [5.31, 5.32].