82
A. Kushwaha et al. / Polymer 78 (2015) 81e93
systematic comparative studies between the PHAePBO and API-TR
routes have yet to be done on comparable API and PHA precursor
structures to explore the potential of PHA cyclodehydration as an
alternative energy-efficient route to produce high performance
PBO-based gas separation membranes. Moreover, systematic
studies investigating the influence of the composition of the
contiguous atmosphere on the PHA cyclodehydration process may
provide insights into the practicality of this approach.
(a)
In this work, we compare API thermal rearrangement and PHA
cyclodehydration with an emphasis on elucidating the chemical
changes that occur during thermal treatment. Additionally,
morphological changes are evaluated by comparing transport
(
b)
properties in the PHA cyclodehydration route in both N
2
and air
atmospheres. The systems considered are 6FAP-6FDA-API and
Fig. 1. (a) API-TR route; (b) PHA cyclodehydration route to form PBOs.
6
6
FAP-6FC-PHA precursors, since the TR polymers derived from
FAP-6FDA-API showed the most promising separation perfor-
mance [7]. Moreover, these two precursors are very comparable in
their chemical structures, which should produce nearly identical
PBO structures upon thermal treatment. A general comparison of
these two routes is shown in Table 1.
2
to form PBOs by releasing two H O molecules per repeat unit, as
shown in Fig. 1(b) [11,12]. The second major challenge associated
with the API-TR route is the thermo-oxidative stability of pre-
cursors when exposed to high TR temperatures, which requires
strict inert atmosphere for successful TR process. It was proposed
that the presence of oxygen generates free radicals for polyimides
at elevated temperatures, and this type of chemistry can substan-
tially degrade mechanical properties and the efficiency of the TR
conversion [15]. Interestingly, cyclodehydration of PHAs appears to
have good oxidative stability that does not require inert atmo-
sphere [15]. As such, formation of PBO membranes via PHA cyclo-
dehydration seems to be a very promising route to address the
challenges faced by the API-TR route, which also defines the
research scope of this paper.
2. Experimental
2.1. Materials
Aromatic diamine 2,2-bis(3-amino-4-hydroxyphenyl)hexa-
0
fluoropropane (6FAP, ꢁ98.5%) and aromatic dianhydride 2,2 -
bis(3,4-dicarboxy-phenyl) hexafluoropropane dianhydride (6FDA,
ꢁ99%), purchased from Akron Polymer Systems, were dried under
ꢀ
ꢀ
vacuum overnight at 65 C and 170 C, respectively. Aromatic diacid
0
2,2 -bis (4-carboxyphenyl)hexafluoropropane (6FAC, ꢁ98%) was
A few recent reports from several research groups have inves-
tigated the physical and transport properties of PBO membranes
prepared via the PHA cyclodehydration route using various PHA
precursors [11,12,15e17]. Studies by Calle et al. [11] demonstrated
that PHA thermally treated in argon had essentially the same PBO
structure as those prepared from polyhydroxyimide (API) pre-
cursors. Han et al. [12] reported a series of PHAePBO systems
showing excellent separation properties and ascribed the perfor-
mance to the optimized cavity size distribution in the thermally
converted films. In this study, the PHA-to-PBO conversion tem-
purchased from TCI Chemicals and used as received. Anhydrous N-
methylpyrrolidone (NMP, ꢁ98%), trimethylsilylchloride (TMSC,
ꢁ98%), thionyl chloride (ꢁ99%), pyridine (anhydrous, ꢁ99.8%) and
o-dichlorobenzene (o-DCB, ꢁ99%) were purchased from Sigma
Aldrich and used as received. Methanol and hexanes were pur-
chased from BDH (VWR) and EMD Chemicals, respectively.
2.2. Synthesis of 6FAP-6FDA API precursor
API precursor was synthesized from the 6FAP diamine and 6FDA
dianhydride via polycondensation, following the conventional two-
step solution imidization. Since 6FAP-6FDA polyimide synthesis is
known in the literature, the synthesis details are provided in the
ꢀ
perature reportedly began at 230e250 C, which is significantly
lower than the temperature needed for the API analogs. More
recently, Smith et al. reported solid-state NMR studies that clearly
confirmed the formation of PBOs via cyclodehydration of PHAs [17].
Although comparable systems were reported in these studies, they
did not focus on thermo-oxidative stability (atmosphere effect) or
1
Supporting information (Scheme S1). H NMR (500 MHz, DMSO-
d
6
)):
d
7.04e7.06 (d, J ¼ 8.30 Hz, 2H), 7.18e7.19 (d, J ¼ 6.60 Hz, 2H),
7.47 (s, 2H), 7.71 (s, 2H), 7.91e7.92 (d, J ¼ 5.10 Hz, 2H), 8.10e8.11 (d,
ꢂ1
transport properties for CO
2
/CH
4
and O
2
/N
2
for partially converted
J ¼ 7.55 Hz, 2H), 10.39 (s, 2H). ATR-FTIR (membrane,
n
, cm ): 3393
PHAePBO membranes. Studies by Wang and Chung [16] revealed
(hydroxy eOH str), 1788 (imide sym C]O str), 1720 (imide asym
C]O str), 1300 (imide eCeN), 724 (imide ring deformation).
ꢀ
that PHA membranes treated in the range of 300e425 C had
similar PBO conversion, but enhanced separation performance was
only obtained for higher temperature treatments. The same group
later reported that nearly identical PBO structure and gas perme-
2.3. Synthesis of 6FAP-6FC-PHA precursor
ꢀ
0
abilities were obtained for PHA films converted at 300 C and
2.3.1. Monomer 2,2 -bis(4-carboxyphenyl)hexafluoropropane (6FC)
ꢀ
4
25 C, regardless of air or N
2
environment [15]. However, their
synthesis
work investigated API and PHA precursors with different chemical
compositions (e.g., a biphenyl diamine for the API series versus a
hexafluoroisopropylidene-based diamine for the PHA series). The
nucleophilicity of the hydroxyl group within the PHA or API has
been shown to strongly effect thermal reactivity for these systems
The diacid chloride monomer, 6FC, used for the synthesis of the
poly(hydroxy-amide) precursor was synthesized by the chlorina-
tion of diacid 2,2 -bis(4-carboxyphenyl)hexafluoropropane (6FAC)
0
with thionyl chloride, following a previously reported procedure
[19]. 6FAC was treated in 35% w/v solution in thionyl chloride for
ꢀ
[18], so modification of the diamine makes direct structure/prop-
14 h at 90 C in a single neck round bottom flask equipped with a
erty comparisons difficult. Moreover, the final TR polymer struc-
tures are different, not only from the standpoint of meta/para
backbone connectivity [17], but also from the difference in the
oxazole heterocycle location in the polymer backbone. Therefore,
magnetic stirrer and reflux condenser. The pale-yellow product
obtained was dried by evaporating the thionyl chloride in a rotovap,
ꢀ
followed by vacuum drying at about 65 C for 6 h. The dry product
was purified by recrystallization using hexanes. The final 6FC