Chemistry Reference
In-Depth Information
HIGH PRESSURE VESSEL HOUS NG
THE MEASURING CELL EQUIPPED
WITH THE VIBRATING W RE WHICH
SUSTAINS THE POLYMER SAMPLE
SQUARE MAGNET
ACROSS THE MEASURING CELL
ACTIVAT NG THE V BRATING WIRE
VIBRATING WIRE
SUSTAINING THE POLYMER
HOLDER
HIGH PRESSURE CALIBRAT NG
CELL, CONNECTED TO THE
MEASURING CELL
MASSIVE METAL THERMOSTAT
HOLDER W TH POLYMER SAMPLE
HIGH PRESSURE GAS RESERVO R
Fig. 1 The coupled VW
pVT
technique.
Left
: Photograph of the inside of the experimental setup
showing the three high pressure cells.
Right
: Equilibrium cell that houses the VW sensor and the
holder containing the polymer sample
polymer depends on the swollen volume of the polymer due to the gas sorption.
This VW sensor is essentially a high-pressure cell in which the polymer sample is
placed in a holder suspended by a thin tungsten wire (diameter 25
m
m, length 30
mm) in such a way that the wire is positioned in the middle of a high magnetic field
generated by a square magnet placed across the high-pressure cell. Through appro-
priate electric circuitry and electronic control, the tungsten wire is activated to
vibrate. The period of vibration, which can be accurately measured, is directly
related to the mass of the suspended sample. The working equation (
1
) for the VW
sensor relates the mass
m
sol
of gas absorbed (solubilized) in the polymer to the
change in volume
V
pol
of the polymer. The natural angular frequency of the wire,
through which the polymer sample holder is suspended, depends on the amount of
gas absorbed. The physical characteristics of the wire are accounted for in (
1
) as:
D
4
L
2
R
2
r
S
2
B
2
0
m
sol
¼ r
g
D
V
pol
þ
o
o
þ r V
C
þ
V
pol
:
(1)
p g
The volume of the degassed polymer is represented by
V
pol
, and
r
g
is the density
of the fluid. The terms
o
0
with
o
B
represent the natural (angular) frequencies of the
wire in vacuum and under pressure, respectively, and
V
C
is the volume of
the holder. The symbols
L
,
R,
and
r
s
are, respectively, the length, the radius, and
the density of the wire.
pVT
Method and Pressure Decay Measurements
For
pVT
measurements, the three-cell principle of Sato et al. [
14
] has been used
(Fig.
2
) to determine the amount of gas solubilized in the polymer. The experimen-
tal method consists of a series of successive transfers of the gas by connecting the
