Chemistry Reference
In-Depth Information
Fig. 23 Variation of
T
g
with pressure for the {CO
2
PS} system. Calculations have been made for
338.22, 362.50, 383.22, and 402.51 K.
Solid symbols
represent results for
z
= 1 and
open symbols
for
z
=2.
Inset
: Literature values are represented by
crosses
in a magnified section of the graph (the
same scale of temperature is kept).
Lines
are hand drawn through the points
function of pressure, the temperature of measurement plays a major role. If we do
not take into account this temperature, it is preferable to represent
D
T
g
as a function
of the mass fraction of the fluid in the polymer.
Compared to polar CO
2
and because of its non-polarizability, N
2
should be a
weaker plasticizing agent although, as shown in Fig.
24
, it induces significant shifts
of
T
g
with increasing pressures [
46
]. However, N
2
, which should also be a good
foaming agent, is not currently used in the foaming industry because of the need of
too high a pressure to attain the desired depression in
T
g
. Figure
25
shows the
scanning electronic microscopy (SEM) images of PS microstructures modified by
high pressure gases (CO
2
or N
2
) in the VW
pVT
technique instrument at a similar
temperature (
315 K) close to, but below,
T
g
(380 K). The modified PS exhibits
different patterns depending on the use of CO
2
or N
2
as blowing agents. For the
{N
2
PS} system, there is no appearance of a foam structure; the surface is only
damaged by the gas pressure. For the {CO
2
PS} system, a foam structure is
apparent. Further increase of temperature has shown that the observed microcel-
lular structure is highly temperature-dependent. Below
T
g
, the microcellular struc-
ture is obtained with perfect spherical bubbles, and the diameter of the bubbles
tends to increase with increasing
T
. At temperatures higher than
T
g
, this organized
structure disappears and the foam becomes more homogeneous.
The sorption of compressed gases in polymers can now be well documented. Our
results with CH
4
,CO
2
, and N
2
confirm earlier studies of Condo et al. [
81
,
82
] and
more recent investigations of Handa et al. [
83
,
84
] on retrograde vitrification of
polymers observed when a decrease of
T
g
is observed at gas pressures high enough
to overcome the purely hydrostatic effect.
