Chemistry Reference
In-Depth Information
In the present work, polystyrene-based nanospheres with functional groups on
the surface were synthesized and used as templates for PI and then to prepare PI
nanocomposite membranes for gas separation. PI nanofoams, preferably having a
closed cell structure which is important for minimizing water absorption, were
prepared by the thermal decomposition of the PS-based nanospheres. It was ex-
pected that the special interaction of the functional groups on the surface with the
carboxyl groups of the poly(amic acid) will improve the dispersion homogeneity
of the nanospheres in the PI matrix.
2. EXPERIMENTAL
2.1. Materials
Pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA) were purchased
from J&K-ACROS (USA) and sublimed twice prior to use. Styrene was purified
by distillation under reduced pressure. 4-Vinylpyridine (MERCK - Schuchardt,
Germany) was distilled before use. The cross-linking agents, ethylene glycol di-
methacrylate (EGDMA, Aldrich) and divinylbenzene, were used as supplied. The
solvent and dispersion medium, N,N'-dimethyl acetamide (DMAc), was dried
over CaH
2
, distilled under vacuum, purged with nitrogen, and used immediately.
Deionized water was purified by double distillation. Other reagents were com-
mercially available and used as received.
2.2. Synthesis of polystyrene (PS) and poly(styrene-co-4-vinylpyridine) (PSVP)
nanospheres
Standard microemulsion polymerization technique was used. The reactions were
carried out in three-neck flasks at 70
C equipped with a condenser, me-
chanical stirrer, and inlets for nitrogen. 80 g water, 2 g sodium dodecyl sulfonate
(SDS), 20 g styrene, 0.5 g EGDMA (or divinylbenzene) and 0.172 g potassium
persulfate were fed into the reaction vessel to form the monomer emulsion. Prior
to polymerization, the reaction mixture was purged with nitrogen, and a small
positive pressure of nitrogen was maintained during the synthesis. The mixture
was then stirred vigorously and heated at 70
±
0.1
°
C for 5 hrs. Partially cross-linked
polystyrene (PS) nanospheres produced by this procedure yielded particles with
average size 35 nm. PSVP nanospheres were prepared by the same process using
a mixture of 4-vinylpyridine (5 g) and styrene (15 g) as the monomers. All the re-
sulting nanospheres were freeze dried before further use.
°
2.3. Dispersion of PS and PSVP nanospheres in poly(amic acid) solution
PMDA-ODA poly(amic acid) (PAA) solutions containing PS and/or PSVP nano-
spheres were prepared by an in-situ condensation polymerization approach. The
predetermined amounts of nanospheres were mixed with 60 ml of DMAc in a 100
ml three-neck flask fitted with a nitrogen inlet and an anhydrous calcium chloride
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