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corresponds to a validation of the two previous modes through the thermal II
differential comparative mode. This allows direct comparison of the response and
behavior of two polymer samples, MDPE and PVDF, in similar supercritical
conditions. A MDPE polymer sample was placed in the measuring vessel while a
PVDF polymer sample of equal size and volume was placed in the reference vessel.
Both cells were connected to the gas line. The calorimetric signal, i.e., the differen-
tial heat flux, was thus directly proportional to the thermal effect due to the
difference in the gas polymer interactions between the two polymers interacting
with the same gas. In that case, the differential heat flux between the measuring and
the reference vessels is small, because calorimetric signals of {gas-MDPE} and
{gas-PVDF} systems have relatively close amplitudes; the detection sensitivity of
the apparatus was then optimal. For each thermal II differential with reference
sample and thermal II differential comparative mode, the data were corrected
through a blank standard calibration. Under identical conditions of
T
and
p
, and
under the assumption that there were no interactions between the stainless steel rod
and the gas, blank experiments were performed in which the polymer samples were
replaced by a metal sample of identical dimensions.
Investigations of polymer behavior [
4
] consist typically of measuring the physi-
cochemical properties in the solid state, i.e., at temperatures between
T
g
and
T
m
.
MDPE and PVDF were submitted to gas pressure of either CO
2
or N
2
at different
temperatures between 333 and 403 K, under pressure steps or scans in the range
between 0.1 and 100 MPa. The polymer samples were extruded MDPE (reference
Finathene 3802) and PVDF (reference Kynar 50HD, polymer without additives like
plasticizers or elastomers). Their transitions temperatures
T
g
and
T
m
were, respec-
tively, 163.0 K and 400.0 K for MDPE, and 235.0 and 440.9 K for PVDF. The two
polymers had degrees of crystallinity
X
c
, of 49% and 48%, respectively. The masses
of samples were about 2 5 mg, and thermograms were obtained under a continuous
flow of N
2
at 15 mL min
1
. Measurements were performed on cylindrical rod
samples (75.0 mm in height, 4.4 mm in diameter) having a relatively small mass,
i.e., about 1.0 g for the MDPE sample and 1.9 g for the PVDF sample; measure-
ments were taken from 352.38 to 401.50 K. For each investigation, a new sample
was used. More details are given elsewhere [
4
]. Using the thermal II differential
mode with reference sample, pressure changes of CO
2
and N
2
were performed on
MDPE and PVDF samples at 352 and 372 K under pressure jumps of 6 28 MPa in
the pressure range between 0.1 and 100 MPa. The CO
2
-pressurizing pressure jumps
manifest themselves by exothermic heat fluxes [
29
,
45
], whereas CO
2
-depressuri-
zation pressure jumps exhibit endothermic heat fluxes, both passing through a
minimum around 20 MPa (see Fig.
11
).
Interestingly, the heat flux minimum is reflected in the isotherms of
a
pol-g-int
coefficients of the fluid-saturated polymers plotted as functions of the feed pressure.
The global cubic thermal expansion coefficients
a
pol-g-int
of saturated polymer were
obtained through the procedure previously described [
45
]. Comparison of these
coefficients for both polymers (MDPE and PVDF) under CO
2
and N
2
, i.e., the
corresponding curves for the {CO
2
-MDPE}, {CO
2
-PVDF} and {N
2
-PVDF} sys-
tems, show a clear difference (Fig.
11
). Additional investigations of {Hg-MDPE}
