Biomedical Engineering Reference
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10 0
80
60
One-phase solution
40
20
0
Two-phase solution
Glass
-20
Solution + glass
-40
0
0.2
0.4
0.6
0.8
1. 0
φ
2
Phase diagram of aPS-TD obtained during cooling: by visual determination of the turbidity (
○
);
demixing onset temperature measured by DSC (
●
); glass transition temperature determined by
DSC (
▲
). Adapted from Arnauts et al.(
1993
) with permission from John Wiley & Sons.
Figure 8.6
Atactic poly(styrene) (aPS) has no order with respect to the side of the chain on which
the phenyl groups are attached. Moderately concentrated solutions of aPS in trans-
decalin (TD) solidify on cooling. This solidi
cation results from the interference between
-
liquid
liquid demixing (LL demixing) and a glass transition (T
g
). In a
rst step, the LL
demixing transforms the solution into an opaque, complex,
finely dispersed two-phase
system. This initial, metastable situation has to evolve towards a macroscopic two-phase
system in which two solutions with different concentrations are in equilibrium with each
other (Arnauts et al.,
1993
).
The phase diagram in
Figure 8.6
was established by optical and calorimetric techni-
ques, using a narrow molecular mass distribution sample.
No demixing was observed for c > 60%w/w. Calorimetric investigations measured T
g
,
which increased with polymer content. Details concerning the spinodal line in the phase
separated system are given in
Figure 8.7
, and this locus was determined by light
scattering techniques. Solutions of aPS-TD exhibited an upper critical solution temper-
ature (UCST) at 15°C and the critical concentration appeared to be situated around
c = 6% w/w.
The region below the binodal in the phase diagram of a two-phase system exhibiting an
UCST can be divided into two parts, namely a metastable region between the binodal and
the spinodal and an unstable region below the spinodal (
Chapter 10
). In the metastable
region, minimization of the free energy of the system requires that large changes in
concentration occur about the average in neighbouring regions and in the phases. In this
region, phase separation proceeds through a nucleation and growth mechanism
(Wellinghoff et al.,
1979
). Isolated particles nucleate and continue to grow with the
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