Chemistry Reference
In-Depth Information
of a magnetic coupling to transmit the weight to a balance [
7
,
8
] has permitted the
pressure range to be extended up to 35 MPa.
With vibrating or oscillating techniques, the change of mass of a polymer sample
is calculated from the resonance characteristics of a vibrating support, either a
piezoelectric crystal [
9
,
10
] or a metal reed [
11
], to which the polymer sample is
fixed (very often this support is a spherical quartz resonator on which a thin polymer
film is wrapped). Depending on the type of oscillator, the maximum pressure can be
between 15 and 30 MPa.
With the
pVT
techniques based on the pressure decay method [
12
,
13
], a polymer
sample is seated in a container of known volume acting as equilibrium cell; the
quantity of gas initially introduced in this cell is evaluated by
pVT
measurements in
a calibrated cell from which the gas is transferred into the equilibrium cell in a
series of isothermal expansions. The pressure decay in the equilibrium cell during
sorption permits evaluation of the amount of gas penetrating into the polymer. The
pressure decay principle allows a sensitivity of few hundredths of milligram of
absorbed gas per gram of polymer [
14
].
With the glass flow techniques, the solubility of gases in polymers is obtained
from gas flow measurements by inverse gas chromatography [
15
]. In this proce-
dure, the polymer sample (glassy or molten) acts as the chromatographic stationary
phase to measure retention times.
2.1.2 Coupled VW-pVT Technique
In all the techniques where the polymer sample is immerged in the penetrating gas,
the associated swelling of the polymer due to the gas sorption is an important
phenomenon that needs to be accurately evaluated. Swelling can affect the buoyancy
force exerted by the gas on the polymer sample in the case of gravimetric measure-
ments, as well as the internal volume in the case of
pVT
measurements. Usually,
swelling is determined separately by techniques using direct visual observation and
estimation of the volume change and is in the order of 0.3% of the volume of the
initially degassed polymer [
16
]. Alternatively, swelling has been estimated using a
theoretical model like the Sanchez Lacombe molecular theory [
17
].
Recently, Hilic et al. [
18
,
19
] designed an original technique to evaluate the gas
solubility in polymers that permits simultaneous determination in situ of the amount
of gas penetrating the polymer and the concomitant change in volume of the
polymer due to gas sorption. This technique associates a VW force sensor, acting
as gravimetric device, and a pressure decay installation to evaluate the amount of
gas penetrating into the polymer.
Vibrating-Wire Sensor
The VW sensor (Fig.
1
) is employed as a force sensor to weigh the polymer sample
during the sorption: the buoyancy force exerted by the pressurized fluid on the
