Chemistry Reference
In-Depth Information
Spheres
Cylinders
Lamellae
Cylinders
Spheres
f
A
>> f
B
f
A
<< f
B
Fig. 18 Possible ordered structures of AB diblock copolymer, depending on the respective volume
fractions
f
A
and
f
B
(by courtesy of Prof. H. Yoshida)
PMA(Az)
Fig. 19 Ordered periodic
hexagonal packed PEO
cylinders formed in the PMA
domain while CO
2
penetrates
the interface
CO
2
PEO
corresponding volume
V
tr
. The role of the nature of the pressurizing fluid on the
transition thermodynamics was also evaluated therefrom. For this, the rigorous
Clapeyron equation was advantageously used to document the pressure effect
because this equation relates the slope (d
T
/d
p
) of the phase boundary on the
p T
surface to the changes in volume
D
D
V
tr
and enthalpy
D
H
tr
at the transition, as given
by (
14
):
ð
d
p
=
d
T
Þ
tr
¼ D
H
tr
=
T
tr
D
V
tr
¼ D
S
tr
=D
V
tr
;
(14)
where
S
tr
is the change of entropy during the transition at temperature
T
tr
.
Remarkably, the transition entropy
D
S
tr
decreases with increasing pressure when
the pressurizing fluid is Hg; this is typically the manifestation of a pure hydrostatic
effect, which restricts molecular motions under inert Hg. In complete contrast,
D
S
tr
increases when the pressure is exerted by N
2
and CO
2
. In this respect, as observed
previously, N
2
is a “neutral” fluid as compared to “chemically active” CO
2
and,
consequently, the large increase in
D
S
tr
shows that the organization of nanostruc-
tures is easiest the more “active” is the fluid, in particular when the fluid is in
supercritical state [
2
,
59
,
60
]. The influence of the pressure-transmitting fluid on the
transition temperature
T
tr
is well illustrated (see Fig.
20
) in the case of PEO
114
-
b
-
PMA(Az)
40
copolymer by the increase of the Clapeyron slope (d
p
/d
T
)
tr
in the
D
