Biomedical Engineering Reference
In-Depth Information
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Figure 13.2
The optical fiber interface with the cell. (Reproduced from Winoto
et al.
13
with permission from the American Chemical Society.)
without changing the mixture composition. It also has a borescope for visual
observation of the phase transitions, and transmitted- and scattered-light
intensity probes, arranged as shown in Figure 13.2. The apparatus is also
equipped with a data-acquisition system that allows for measurements at
constant pressure and temperature and for increasing and decreasing the
pressure and temperature at constant rate. The CP is detected by a
transmitted-light intensity (TLI) probe, which measures light at 180u from
the laser. The MP is detected using a scattered-light intensity (SLI) probe,
which is measured at 90u from the laser.
The CP at a given temperature is the specific pressure at which the capacity of
the solvent is just enough to allow complete dissolution of the polymer. Above
the CP, the polymer is dissolved. Below the CP, the polymer forms a distinct
separate phase, at the onset of which the solution becomes opaque, or ''cloudy.''
Similarly, for block copolymers the MP at a given temperature is the specific
pressure at which solvent selectivity forces the less-soluble block to aggregate as
the micelle cores. In the micellar region, below the MP but above the CP, a
micellar solution exists as a result of nano, rather than bulk, phase separation.
At pressures above the MP, the solvent is no longer selective enough to induce
solvent-phobic block aggregation, so the micelles cease to exist.
In a typical experiment, a known amount of copolymer is loaded into the cell
and pressurized with a known amount of solvent, both obtained by weighing
the cell. Following weighing, the cell is brought to and maintained at a desired
pressure and temperature to dissolve the copolymer completely and left,
stirred, in the one-phase region for about 90 min to reach equilibrium.
To obtain a CP data point, the pressure is lowered slowly until the solution
becomes cloudy at the onset of bulk phase separation, which is indicated by a
drop in TLI. Pressure drop rates as low as 15 bar min
21
are used to obtain a
reproducible CP, to within 3 bar. The bulk phase boundary can also be
