Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
Scanning electron micrographs of dried aPS-TD samples with various concentrations: (a) c =4.4
wt% (white bar is 1 μm); (b) c = 6.7 wt% (bar is 1 μm); (c) c = 15.9 wt% (bar is 10 μm).
Reproduced from Arnauts et al.( 1993 ) with permission from John Wiley & Sons.
Figure 8.8
Figure 8.8c was obtained in two steps (c = 16% w/w). First, the solution was cooled into
the binodal region and left there overnight (Cl). Under these conditions, the concentrated
phase formed a continuous matrix. In the second step, the solution was quenched to 9°C
(C2), which induced spinodal demixing in the concentrated phase. Then the concentrated
phase was vitri
ed (C3) and the solvent eliminated, and the image shows a complex,
porous material.
8.3.2
Atactic poly(styrene) in carbon disulphide: gels induced by
conformational changes
Wellinghoff et al.( 1979 ) investigated aPS in carbon disulphide (CS 2 ) and determined the
melting temperatures visually by gelling the solutions in tilted test tubes, increasing
temperature and observing when the gel in the non-vertical test tubes began to
ow under
its own weight and formed a level meniscus. Gel
'
melting
'
points (gel to
'
mobile
transition
) of aPS in CS 2 were determined for various molecular masses: as the molec-
ular mass increased from 4000 to 670 000 g mol 1 , the gel melting points increased from
'
25°C to +5°C. The authors then considered the gelation mechanism(s) of these sol-
utions. Crystallization was de
nitely excluded in the aPS-CS 2 systems since high-
sensitivity DSC measurements on these solutions revealed neither endothermic nor
exothermic peaks. Additionally, the IR spectra of aPS-CS 2 solutions showed no signifi-
-
cant change upon cooling to
78°C. The remaining possibility is that quenching the
solution into the unstable region yielded a
fine dispersion of polymer-rich regions whose
T g was suf
ciently low to pin different polymer chain ends in the same glassy particle.
The small size of these domains (<100 nm, as indicated by the gel clarity) and the rapidity
of their formation (<1 min) suggested, according to (Wellinghoff et al., 1979 ), that they
developed by the spinodal mechanism.
Tan et al.( 1983 ) prepared aPS gels in various solvents by dissolving the polymer at
room temperature at the desired concentration in sealed tubes and cooling them slowly.
Among other solvents, they also investigated CS 2 ; here the (Flory)
θ
-temperature was
estimated to be around
73°C. They performed DSC measurements and used two simple
mechanical methods to determine the sol
-
gel transition: the tilting-tube and falling-ball
methods.
 
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