Chemistry Reference
In-Depth Information
CO
2
PVDF interactions, and the global
a
pol-g-int
for the {CO
2
-MDPE} system is
higher than the global
a
pol-g-int
for the {CO
2
-PVDF} system. As shown by Fig.
11
,
in the case of PVDF, N
2
acts as a “relatively neutral” fluid like Hg, but with
stronger interactions. The values of
a
pol-g-int
with N
2
are smaller than those with
CO
2
[
a
pol-g-int
{N
2
-PVDF}
< a
pol-g-int
{CO
2-
PVDF}], demonstrating that interac-
tions of PVDF with N
2
are weaker than with CO
2
. With N
2
(a relatively neutral
fluid) the heat effects reflect the sorption under pressure and parallel the remarkable
plasticization efficiency of N
2
in PS, particularly at elevated pressure [
46
,
47
] (see
Sect.
3.4.3
). The PVDF values during decompression under N
2
and/or CO
2
are
similar, which is satisfactory as regards the reversibility of the sorption/desorption
phenomena. The minimum of
a
pol-g-int
observed with {CO
2
-MDPE} and {CO
2
-
PVDF} systems at about 15 MPa corresponds to the supercritical domain of CO
2
.
The dependency of
a
pol-g-int
coefficients on the nature of the pure gas (i.e., a
minimum corresponds in a mirror-image to the maximum in the temperature
dependence of
a
p
for pure CO
2
gas) is a striking feature of previous studies [
45
].
This clearly shows the influence of supercritical sorption on the thermophysical
properties of the polymers. With the semicrystalline polymers, low pressures most
probably induce a first adsorption of CO
2
in the amorphous part and in some
interstitial sites of the crystalline part, with the possible formation of a microorga-
nized domain generated in the amorphous phase of the polymer [
44
] (see also Sect.
3.1.1
). High pressures favor the absorption into the whole polymer matrix (i.e., deep
inside the interstitial or other voids in the polymer) with a mechanical distension,
the CO
2
-saturated polymer behaving as a pseudohomogeneous state [
45
]. Further-
more, the minimum would mean that supercritical gas polymer interactions are
favored. The lowering of molecular polymer polymer interactions is concomitantly
associated with the ease of CO
2
dissolution into the polymer matrix, thus inducing
an increase of free volume together with an increase in polymer chain mobility [
48
].
This plasticization effect is shown by the minimum of
a
pol-g-int
as a function of
pressure. Quantitatively, this is confirmed by the net increase of gas sorption into
the polymer and the swelling of the polymer due to the sorption around 15 MPa (as
investigated by the gravimetric volumetric VW
pVT
method) [
49
,
50
]. As a matter
of fact, around this pressure there is compensation between plasticization and
hydrostatic pressure effects upon high CO
2
-pressure sorption into the polymer.
The supercritical hydrostatic pressure corresponding to the minimum for MDPE
is slightly smaller than that for PVDF.
The thermal II differential comparative mode is conveniently adapted to com-
pare two different polymer samples submitted to the same gas under pressure. This
mode was used to measure the differential heat flux obtained when a MDPE and a
PVDF sample (of identical size and volume, each placed in one of the two calori-
metric vessels) were simultaneously submitted to the same gas pressure at an
identical temperature (372.59 K). The experimental signal, the differential heat
flux d
Q
{MDPE-PVDF}
, compares directly the interactions of the two polymers in the
same gas/supercritical environment at constant temperature. The calorimetric
responses were collected during pressure jumps and during continuous volume
