Chemistry Reference
In-Depth Information
The ideal permselectivity of the films toward gas A relative to another gas B
can be calculated from the individual pure gas permeabilities:
A/B
)
id
= P
A
/P
B
(3)
where P
A
and P
B
are the corresponding permeability coefficients of gases A and
B, respectively.
The error in the absolute values of the permeability coefficients is estimated to
be about
(
α
10%, due to uncertainties in the determination of the downstream vol-
ume and the effective membrane area and thickness, while the reproducibility was
better than
±
±
4%.
3. RESULTS AND DISCUSSION
3.1. Synthesis of PS and PSVP nanospheres and fabrication of PMDA-ODA PI/PS
or PSVP-based nanospheres composite films
Figure 1 depicts the procedure for the preparation of nanocomposite films and
nanofoams studied in this work. The first stage involves the synthesis of the PS or
PSVP nanospheres of nanometer size. The ability to tailor the characteristics and
structures of emulsion and suspension polymer particles is a direct result of the
extensive research efforts that have been carried out over the past 75 years. Nu-
merous techniques have been developed during this time to modify and to control
the particle size, degree and type of functionality, as well as the shape and internal
structure of emulsion polymer particles.
In this process, the dimensions of the nanosphere templates play an important
role in the preparation of the nanofoam materials. The criteria for the selection of
template include the dimension of particles, particle monodispersity and the dis-
persion properties in the PI matrix. An ionic surfactant, SDS, was used in the
emulsion polymerization. The cross-linking agent, EGDMA (or divinylbenzene),
was added to the system at the maximum concentration of 2 mol% that provided
partial cross-linking in the nanospheres. A larger amount of the cross-linking
agent led to partial aggregation resulting in the difficulty of thermolysis. Figure 2
shows the TEM micrographs of the nanospheres which were synthesized using
the procedure described above. It was found that the average diameter of the
spheres was 35 nm, while these nanospheres were nearly monodispersed. The de-
composition temperature of the nanospheres is critical for our purpose; it should
be high enough to permit standard film preparation, solvent removal and imidiza-
tion below the T
g
of the PI matrix to avoid foam collapse. The TG curves of typi-
cal PS and PSVP nanospheres samples in nitrogen are shown in Figure 3. It can
be seen that the polystyrene-based nanospheres decomposed rapidly between 300
and 400
C. The XPS spectra of these nanospheres are shown in Figure 4. It can be
seen that nitrogen element, which is derived from pyridinyl group, is character-
ized in the PSVP nanospheres by the peak at approximately 402 eV.
°
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