Page 271 - Gas Adsorption Equilibria

5. Oscillometry 257

since the volume of the material needed for buoyancy or dead space
corrections respectively, depends not only on pressure and temperature of the
sorptive gas but also on the amount of gas absorbed. Today there are several
techniques described in literature for measuring polymeric swelling
phenomena [5.21-23], usually determining the change in one or two
dimensions of a polymer sample. However, these methods require a specific
shape or state of the polymeric material, i. e. a thin film or a liquid drop and,
despite the geometry, often assume isotropic swelling of the polymer sample.

By combining oscillometric and gravimetric measurements it is possible to
simultaneously determine the mass of gas absorbed in a polymeric
material of mass and the volume (V) of the sample material in the
absorption state thus providing together with (p) and (T) the basic data for the
thermal equation of state of the polymer loaded with gas [5.8], [5.26]. The
polymer samples which can be investigated in this way can be either highly
viscous liquids or dense solid materials formed as cylindrical rings, pulverous
flakes or pellets. Isotropic swelling is not mandatory for the method to be
applied.

In this section we first will describe the experimental device, Sect. 3.2,
then provide formulas to calculate and of a polymer sample from
measured data Sect. 3.3, and finally present an example namely sorption of
in (swelling) polycarbonate (Makrolon 2400), Sect. 3.4.

3.2 Experimental

An instrument for combined oscillometric-gravimetric measurements of
sorption equilibria in swelling (polymeric) materials has been designed and
built at IFT during 1997-2000, [5.2, 5.7, 5.8]. It mainly consists of the
rotational pendulum already described in Sect. 2.1 and a microbalance
installation as presented in Chap. 3, Sect. 2.1.1. A schematic diagram of the
instrument is given in Figure 5.9 followed by photos of the ring slit of the
pendulum filled with polycarbonate pellets, Fig. 5.10 and a front view of the
installation in our laboratory (PB-A 0126/1) in 1998, Fig. 5.11. Detailed
information of the instrument including specifications of vessels, tubes,
valves, and materials are given in the literature [5.2, 5.7] and references cited,
cp. [Tom94], [Sch99a].