United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-86/016 May 1986
Project  Summary
Synthesis of  Aromatic
Polyimides  for  Use  as Solid
Sorbents
R. A. Markle
  High surface area, high sorbent, poly-
imide powders designed to replace or
complement Tenaxฎ GC were obtained
in high yield from  pyromellitic di-
anhydride (PMDA)  and  4,4'-
diaminodiphenylsutfone (DADS) by pre-
cipitation polymerization  in
tetrahydrofuran  (THF) solvent. The  in-
termediate polyamide-acid powders
were cyclodehydrated to high BET sur-
face area (25-50 m2/g) polyimide
powders by slowly heating to 230-240 C
in an evacuated rotary  evaporator.
About 40 weight percent of the powder
was in the 200-500 urn particle diame-
ter range desired for sorption sampling
of vapor-phase organics. The polyimide
powders are obtained as 0.01-1.0 u.m
diameter, loosely  agglomerated,
platelets and rod-like particles. Prelimi-
nary evaluation of the powder sorption/
desorption characteristics with ben-
zene vapor shows them to have greater
capacity than Tenax  but to have less
desirable breakthrough characteristics
due to slower sorption  kinetics as-
cribed to inadequate  connecting pore
volume. However, desorption occurs
smoothly and quantitatively at 100 C.

  This Project Summary was  devel-
oped by EPA's Environmental Monitor-
ing Systems Laboratory, Research Tri-
angle Park,  NC, to announce key
findings of the research project that is
fully documented in a separate  report
of the same title (see Project Report
ordering information at back).

Introduction
  Hazardous organic  vapors may  be
present in  air at  concentrations as low
as a few nanograms  per cubic  meter
and still present a potential biological
threat. Preconcentration of such vapors
is thus necessary in many cases to pro-
vide sufficient sample for accurate air
analysis. The available methods for pre-
concentrating vapor-phase organics in-
clude the use of sorbent  cartridges,
cryogenic systems, and solvent in-
pingers. Preconcentration using porous
polymeric sorbents has been the most
successful and useful procedure. Tenax
GC is the most used and most versatile
sorbent commercially available  at
present.
  However, Tenax  GC (porous powder
poly-2,6-diphenyl-para-phenylene
oxide) has  several performance defi-
ciencies that would be very desirable to
correct. These include low retention and
breakthrough volumes for high volatil-
ity organic compounds. In fact, the
more polar volatile organics such as
acrylonitrile and acetonitrile, vinyl chlo-
ride, methylene chloride, acrolein, and
propylene oxide cannot be adequately
analyzed using Tenax GC. In addition,
Tenax GC is subject to high background,
or column bleed, due to slow release of
prior adsorbed compounds or slow
thermal breakdown of the polymer
structure. Also, the elution or artifact
peaks is a particular problem.
  Consequently, the intent of this work
was to  prepare new porous polymeric
sorbent powders for use in determining
air concentrations of potentially haz-
ardous vapor-phase organic com-
pounds. New sorbent materials that are
superior to Tenax GC needed to  be de-
veloped with as many of the following
properties as possible:
  • Greater adsorption capacity for the

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    more volatile vapor-phase organics
  • Greater selectivity in  adsorption
    and thermal release of multiple pol-
    lutants
  • Improved thermal properties (i.e.,
    minimal background or bieed con-
    tribution to eluted  volatiles  for
    analysis)
  • Minimal artifacts formed during
    collection of reactive organic com-
    pounds
  • Low water vapor retention
  • Resistance to reactive inorganic
    gases such as ozone, NOX and SOX.
Procedures
  Straightforward, practical procedures
for purifying to 99.9+ percent were de-
veloped for all four monomers—PMDA,
DADS, DADM, DCPDA. (DADM is 4,4'-
diaminodiphenylmethane and DCPDA
is 2,6-dichloro-p-phenylenediamine.)
These monomers are usable to prepare
reproducibly the three desired poly-
imides as their fine-particle-precipitated
polyamide-acid precursors in tetrahy-
drofuran. Total  polymer yields, as de-
sired, met or exceeded 100 grams from
three polymerizations of each monomer
pair  (from  polymerizations of  DADS/
PMDA). The molecular weights (Mv) of
the polyamide-acid intermediate poly-
mers were consistent with those re-
ported by  prior workers. The DADS
polymers had an Mv of 7,000 with the
Mv for DCPDA and DADM polymers
about 3,000 and 10,700 respectively.
  The polyamide-acids were readily cy-
clodehydrated in dry powder form to
the polyimides. Based on TGA results,
the polyimides are all stable to —250 C
with  only minor weight losses of 1-3
weight percent, which are attributable
to physical losses of water and ad-
sorbed solvents.  Above -250  C the
weight losses continue at an  acceler-
ated  rate,  indicating that chemical
changes are occurring.
  The  polyimides are recovered after
the cyclodehydration as powders which
in two cases (DADS and DCPDA poly-
imides) are recovered with appreciable
fractions in the desired  particle size
range of 200-500 jim. It is likely that the
percentage of this particle size fraction
can be increased by manipulating cer-
tain  polymerization variables, such as
the PMDA addition rate and the reaction
temperature/time cycles and by stand-
ardizing  and controlling the  powder
handling and size classification proce-
dures.
Results and Discussion
  Approximately 100 grams or more of
three  experimental polyimide powder
sorbents based on pyromellitic di-
anhydride (PMDA) reaction with one of
three  carefully  selected aromatic di-
amines including 4,4'-diaminodiphenyl-
sulfone (DADS), 2,6-dichloro-p-
phenylinediamine (DCPDA), and  4-4'-
diaminodiphenylmethane  (DADM)
were prepared in tetrahydrofuran (THF)
solvent and characterized. These poly-
imides were selected for scale-up stud-
ies on the present program as the most
promising sorbents of 61  polyimides
synthesized and studied in a prior EPA
program.
  The DADS/PMDA polyimide powders
were found to be reproducibly obtain-
able products with excellent potential
sorbent characteristics. These poly-
imides appear to have promise either as
replacements for Tenax GC or as com-
plementary sorbents used in tandem
with Tenax.  The DCPDA and DADM
polyimides, on the other hand, are not
promising candidates based on a pre-
liminary assessment of their properties.
  The polyimides were prepared in
three approximately equalized batches
to obtain a measure of the reproducibil-
ity of the processes and their products.
The procedures used in the prior work
were followed as closely as possible. In
this convenient precipitation  polymer-
ization procedure, the monomers, puri-
fied to 99.9+ percent purity, are reacted
in tetrahydrofuran, which is a solvent
for the monomers but a nonsolvent for
the intermediate polymer products
(polyamide-acids). The polymers are
thus obtained directly as finely divided
solids of approximately the desired
mesh size when dried. These powders
are then converted chemically to the de-
sired polyimides by a thermal  cyclode-
hydration process which involves heat-
ing under vacuum to 230 C in a rotating
reaction chamber.
  How reproducible these processes
are and whether the products obtained
have  sorption  properties that make
them  suitable for field or commercial
use as reusable adsorption resins for
collection and analysis of vapor-phase
organics in ambient air needed to be
determined.  The important polymer
properties that  might  affect their  ulti-
mate sorbent performance were:

  • Polymer  yield
  • Molecular weight
  • Powder particle size
  • Powder surface area (BET)
  •  Thermal stability
  •  Sorption characteristics

  All the polyimides were obtained i
>90 percent yields. The promisin
DADS polyimides could in fact be ok
tained in  nearly quantitative overa
yield. The important question not yc
answered is how much of the product
can be prepared  in the desired macrc
scopic (200-500 |xm) particle size rang
that is most practical for actual use ii
packing columns for air pollution moni
toring. In the present work about 3C
40 percent of the DADS products wen
in this range, while the DCPDA product
were with one exception 50-67 percen
in this range. The DADM products wen
only 3-5 percent  in the  desired range
with the bulk of the products  {85-91 per
cent) in the range of <105 ^m.
  The surface areas were determine*
by the BET nitrogen adsorption tech
nique (Table 1). This method  is the mos
accurate technique available for mea
suring absolute  surface areas,  espe
cially in the range >10 m2/g. The valui
found for Tenax  (25 m2/g) is in agree
ment with the manufacturer's and re
ported values. The surface areas of th<
DADS polyimides (with two exceptions
were quite reproducible in the range 26
38 m2/g. One value of 50 m2/g  was de
termined (Polyimide  No.  3, <212
105 M,m particle size).  Interestingly
another sample (>212 |xm particle size
of the product measured 34 m2/g. This is
very close to both the mean and the me
dian value for all the DADS Polyimide
surface areas (32 m2/g). These value;
may be greater than those found for the
standard commercial product Tenax GC
(24  m2/g). However, the experimenta
error in measurement may be  as grea
as ฑ50 percent so that 34 ฑ 17 m2/g anc
25 ฑ 12 m2/g is perhaps a more correc
way to express the results, showing tha
they do not differ from Tenax to a statis
tically significant degree. It is concludec
that the DADS polyimide surface areas
are  at least as high as Tenax and thai
values  significantly  greater thar
32 m2/g, can probably be obtained  b\
optimizing the DADS polymerizatior
and product recovery procedures.
  The polyimide made from PMDA
DADA in the THF had the best proper-
ties, including the following:
  • Polymer yield - 97 to 99 percent o1
    theory
  • Thermal stability - stable to 250 C
  • Particle size  distribution - 40 per
    cent of product 200-500 n-m
  • Surface area - 26 to 50 m2/gm

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 Table 1.
 No.
Polyimides made in THF—Particle Sizes and Surface Areas

                       Particle Size Distribution, Weight Percent
  Composition
                                                                                   Surface Area, m!/g
                                                 \i.m
                                                                                              \i,m
>212
                                 <212->105
                                                               <105
>212
                                                                              <212->105
<10S
Tenax9
DADS/PMDA
 la>Cyclodehydration to 300 C.
 (b'Cyclodehydration to 230 C.
 Product 9b.
 ld>N-phenyl imide end groups.
                                  34
                                                                                            25
                                       27
                                39
                                                                            26Pป
2
3
4
5
6
7
8
9a
9b
10
11
DADS/PMDA
DADS/PMDA
DADM/PMDA
DADM/PMDA
DADM/PMDA
DCPDA/PMDA
DCPDA/PMDA
DCPDA/PMDA
DCPDA/PMDA
DADS/PMDA
DADS/PMDA'd>
31
23
5
4
3
59
33
49
67
42
40
39
39
10
8
6
38
40
34
20
26
30
30
38
85
88
91
3
27
18
13
32
30
—
34
—
—
—
—
—
—
—
32
24
—
50
—
—
—
0.5
0.1
21
4. 1 lcl
—
—
38
—
2.4
4.6
2.1
—
—
—
—
—
—
  • Sorption characteristics - prelimi-
    nary tests with benzene  indicate
    substantially greater sorption ca-
    pacity than Tenax but poorer sorp-
    tion  kinetics,  resulting in early
    breakthrough
  • Desorption characteristics  - effi-
    cient (100 ฑ 3 percent) recovery of
    sorbed benzene at a moderate tem-
    perature (100 C)
  The DADS polyimides made in NMP
 (solution polymerization) show very
 low surface areas  (<1 m2/g). This ap-
 pears likely to be due to a combination
 of factors, including the nature  of the
 polyamide-acid precipitation  process
 from true solution in a polar solvent, the
 effect of relatively low molecular weight
 polymer chains on this process, and
 possible  retention of small amounts of
 NMP  by  the fine particle product after
 washing  and drying. Immediately after
 precipitation, the products qualitatively
 appeared to  have  high surface  areas.
 After the precipitates were carefully
 washed with acetone and/or water to re-
 move NMP, they were  either  vacuum
 dried at  room  temperature or freeze
 dried from a frozen water slurry. During
 this drying  process, a  considerable
 amount of agglomeration invariably oc-
 curred. Then, during cyclodehydration
 up to 230 C, the products further ag-
 glomerated  to very large particles  or
 even flake or sheet material.
  Decided improvement  was obtained
when  DADS/PMDA Product 10, made in
THF, was dissolved  in NMP and recov-
ered by precipitation  using the  same
                             procedures. Precipitation with acetone
                             or water yielded  polyimide powders
                             with surface areas of 2 and 4 m2/g re-
                             spectively when the products were not
                             heated above room temperature during
                             washing and were freeze-dried from a
                             frozen water slurry. It is likely that the
                             higher molecular weight of Product 10
                             is the reason for this improvement. Al-
                             though the actual surface  areas were
                             still disappointingly low, this does indi-
                             cate that if the desired higher molecular
                             weight polyamide-acids  can be
                             achieved by solution polymerization of
                             DADS and PMDA, further increases in
                             surface area can be expected.

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                                            ft. A. Markle is with Battelle Columbus Laboratories, Columbus. OH 43201 -2693.
                                            James D. Mulik is the EPA Project Officer (see below).
                                            The complete report, entitled "Synthesis of Aromatic Polyimides for Use as Solid
                                              Sorbents. "(Order No. PB86-176559/AS; Cost: $11.95, subject to change) will
                                              be available only from:
                                                    National Technical Information Service
                                                    5285 Port Royal Road
                                                    Springfield, VA22161
                                                    Telephone: 703-487-4650
                                            The EPA Project Officer can be contacted at:
                                                    Environmental Monitoring Systems Laboratory
                                                    U.S. Environmental Protection Agency
                                                    Research Triangle Park. NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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