Chemistry Reference
In-Depth Information
made; and
p
is the variation of gas-pressure change under investigation at constant
temperature
T
. Volumes
V
SS
and
V
pol
are those of the stainless steel inert reference
and of the polymer sample, respectively. In (
13
), it was assumed for simplicity
faute
de mieux
that the volume of the polymer did not change significantly upon gas
sorption. This assumption may be justified in the sense that the pressure is much
higher (
D
100 MPa) in calorimetric measurements than in the VW
pVT
technique
(
40 MPa); the hydrostatic pressure must probably compensate for a large part of
the swelling effect due to gas sorption, as a result of the equilibrium between the
plasticization effect and the hydrostatic effect.
Three differential modes were investigated, taking into account the differential
principle of the instrument (Fig.
10
): thermal I differential without reference
sample, thermal II differential with reference sample, and thermal II differential
comparative mode. With the thermal I differential mode, in an initial experiment
the polymer sample is placed in the measuring cell, which is connected to the gas
line. The reference cell, not connected to the gas line, acts as a thermal reference.
An additional blank experiment (under identical conditions) is performed in which
the polymer sample is replaced by an inert-metal (stainless steel) sample of similar
volume. Then, the difference in the heat effects between polymer and blank
experiments allow quantification of the thermal effect due to the gas polymer
interactions. In the thermal II differential mode, the polymer sample is placed in
the measuring cell while an inert-metal sample of equal dimensions is seated in the
reference cell, both cells being connected to the gas line which serves to pressurize.
Then, under gas pressure, the calorimetric differential signal is proportional to the
thermal effect due to the gas polymer interactions. The third and last mode
a
b
c
M
R
M
R
M
R
P
O
L
Y
M
E
R
P
O
L
Y
M
E
R
S
T
E
E
L
M
D
P
E
P
V
D
F
{fluid-polymer}
INTERACTIONS
THERMODYNAMIC II
DIFFERENTIAL
{fluid-polymer}
INTERACTIONS
THERMODYNAMIC II
DIFFERENTIAL COMPARATIVE
{fluid-polymer}
INTERACTIONS
THERMAL I DIFFERENTIAL
Fig. 10 Three differential modes of scanning transitiometry according to the differential principle
of the calorimetric detector, taking into account the respective roles of the measuring (M) and
reference (R) vessels and the content of the reference vessel. (a) Thermal I differential without
reference sample mode. (b) Thermal II differential with reference sample mode. (c) Thermal II
differential comparative mode: in this case a direct comparison between two polymers (MDPE and
PVDF) samples is possible
