Space environmentally stable polyimides and copolyimides derived from [2,4-bis(3-aminophenoxy)phenyl] diphenylphosphine oxide

Article abstract of DOI:10.1021/ma0201779

Polyimides with a combination of properties including low solar absorptivity (α) in thin films, atomic oxygen (AO), and ultraviolet (UV) radiation resistance, solubility in organic solvents in the imide form, high glass transition (T_g) temperatures, and high thermal stability have been prepared and characterized. The polymers were prepared by reacting a novel aromatic diamine, [2,4-bis(3-aminophenoxy)phenyl]diphenylphosphine oxide (2), with aromatic dianhydrides in a polar aprotic solvent. The diamine was prepared from commercially available starting materials in a two-step reaction in relatively high yield. Copoly(amide acid)s and copolyimides were prepared using 2 in combination with other commercially available aromatic diamines. The chemistry as well as physical and mechanical properties of the polymers and films will be discussed.

Full text of DOI:10.1021/ma0201779

4
968
Macromolecules 2002, 35, 4968-4974
Space Environmentally Stable Polyimides and Copolyimides Derived
from [2,4-Bis(3-aminophenoxy)phenyl]diphenylphosphine Oxide
†
‡
Ken t A. Wa tson , F r a n k L. P a lm ier i, a n d J oh n W. Con n ell*
National Aeronautics and Space Administration (NASA) Langley Research Center (LaRC),
Hampton, Virginia 23681-2199
Received February 4, 2002; Revised Manuscript Received April 8, 2002
ABSTRACT: Polyimides with a combination of properties including low solar absorptivity (R) in thin
films, atomic oxygen (AO), and ultraviolet (UV) radiation resistance, solubility in organic solvents in the
imide form, high glass transition (T ) temperatures, and high thermal stability have been prepared and
g
characterized. The polymers were prepared by reacting a novel aromatic diamine, [2,4-bis(3-aminophen-
oxy)phenyl]diphenylphosphine oxide (2), with aromatic dianhydrides in a polar aprotic solvent. The
diamine was prepared from commercially available starting materials in a two-step reaction in relatively
high yield. Copoly(amide acid)s and copolyimides were prepared using 2 in combination with other
commercially available aromatic diamines. The chemistry as well as physical and mechanical properties
of the polymers and films will be discussed.
In tr od u ction
and electronic interactions between polyimide chains
due to CTC formation. By minimizing or eliminating
CTC formation, low color or virtually colorless polyimide
films with improved optical transparency can be ob-
Organic polymers are subject to degradation when
exposed to AO, UV, and vacuum-ultraviolet (VUV)
radiation that are present in low Earth orbit (LEO) at
altitudes ranging from 160 to 800 km. At higher
altitudes AO becomes less prevalent, but UV, VUV,
electrons, protons, and other particulate radiation are
of concern. These species are present at sufficient
concentration and energy levels in LEO to cause organic
bond cleavage. The resulting bond cleavage can give rise
to reactions that result in chain scission and/or cross-
linking that can cause adverse physical, mechanical,
and optical property changes in the polymer. Perfluori-
nated materials such as copoly(fluoroethylene propy-
lene) exhibit good resistance to AO, but dramatic
increases in the erosion rate (mass loss) is observed
when the polymer is exposed simultaneously to AO and
4
,5
tained.
Currently there are no commercially available poly-
imides that are inherently resistant to degradation due
to AO exposure. Commercially available polymers used
in LEO are typically protected from AO by a protective
coating. Present methods for the elimination or reduc-
tion of erosion due to AO involve coating the polymer
with aluminum oxide, silicon dioxide, chromium oxide,
and fluorinated polymers. Usually this coating method
requires a specialized facility and a separate processing
step. In addition, there are limitations on the shape and
size of film that can be protected via coating. Other
problems associated with the use of coatings include
difficulty in uniformly coating complex shapes and the
risk of damage to the coating from manufacturing,
handling, storage, and space-based particle impact.
Another problem that arises is when there is a mis-
match between the coefficient of thermal expansion of
the substrate polymer and the coating, which upon
thermal cycling can cause spalling, cracking, or dis-
bonding. Once damage occurs to the protective surface
coating, the underlying polymer becomes exposed to AO,
and erosion proceeds that can lead to loss of mechanical
and optical properties. In addition, in applications where
the film must be folded for stowage prior to launch, the
use of a coating may not be viable as it will likely be
damaged due to folding and unfolding of the coated film.
1
,2
UV radiation.
High molecular weight aromatic polyimide films
generally exhibit good toughness, flexibility, solvent
resistance, high glass transition temperatures (Tg), and
3
good thermal stability. In many cases these materials,
due to their aromatic structure, exhibit good UV radia-
tion resistance; however, polyimide films are typically
yellow to amber in color and consequently have high
solar absorptivities. There are exceptions and the
preparation of low color polyimides that are UV resis-
tant has been reported.4
The color in polyimide films may originate from
chromophoric units, impurities within the starting
material, or side reactions (isoimides). However, studies
have shown that charge-transfer complex (CTC) forma-
tion is the primary cause of color arising in polyimides.
Bulky side groups and meta-substitution of the aromatic
diamine portion have been shown to reduce conjugation
AO resistant polymer development has recently fo-
cused on incorporating phenylphosphine oxide (PPO)
groups into the backbone of aromatic polymers.6
X-ray photoelectron spectroscopy studies have shown
4
,5
-12
6
-11
that polymers containing chemically bound PPO units
as well as phosphazene groups
1
3,14
†
have significantly
ICASE, Mail Stop 132/C, NASA Langley Research Center,
Hampton, VA 23681-0001. This research was supported in part
by NASA under NASA Contract NAS1-97046 while the author was
in residence at ICASE, NASA Langley Research Center, Hampton,
VA 23681-2199.
increased AO and oxygen plasma resistance. More
recent work using X-ray adsorption near edge structure
spectroscopy has shown that a polyphosphate surface
15
‡
layer forms upon exposure of these materials to AO,
NASA Langley Research Center, Aerospace Research Summer
subsequently inhibiting erosion of the underlying poly-
mer. Reports of polyimides containing phosphine oxide
Scholar.
*
To whom correspondence should be addressed.
1
0.1021/ma0201779 This article not subject to U.S. Copyright. Published 2002 by the American Chemical Society
Published on Web 05/17/2002

Macromolecules, Vol. 35, No. 13, 2002
Polyimides and Copolyimides 4969
units have appeared previously;16,17 however, all of these
polyimides have been highly colored. This implies that
the PPO group when part of the polyimide backbone is
not sufficiently bulky to prevent CTC formation.
Recently, we reported the preparation and charac-
testing was 0.51 cm/min. Solar absorptivities were measured
on an AZ Technology model LPSR-300 spectroreflectometer
with measurements taken between 250 and 2800 nm with a
vapor-deposited aluminum on Kapton film (first surface mir-
ror) as a reflective reference for air mass 0 per ASTM E903.
An AZ Technology Temp 2000A infrared reflectometer was
used to measure thermal emissivities (ꢀ). Atomic oxygen
exposure studies were done at NASA Marshall Space Flight
Center, Atomic Oxygen Beam Facility.
1
2
terization of a novel diamine containing a PPO group.
Polyimides prepared from this diamine were resistant
to AO and UV radiation and formed films with low solar
absorptivities. However, films formed from these poly-
mers were relatively brittle and exhibited poor thin film
mechanical properties. More recently, work has focused
P r ep a r a tion of 2,4-Diflu or op h en yld ip h en ylp h osp h in e
Oxid e (1). Into a 1 L three-neck round-bottomed flask
equipped with a mechanical stirrer, thermometer, nitrogen gas
inlet, pressure equalizing addition funnel, and reflux condenser
with drying tube were placed magnesium turnings (13.5 g, 0.55
mol) and anhydrous tetrahydrofuran (THF, 50 mL). The
mixture was cooled to ∼5 °C using an ice-water bath. A
solution of 1-bromo-2,4-difluorobenzene (107.2 g, 0.55 mol, 20%
molar excess) in THF (200 mL) was placed in the pressure
equalizing addition funnel and added dropwise over a 1.5 h
period. After addition was complete, the mixture was allowed
to slowly warm to room temperature and stirred for 3 h, and
the flask was subsequently placed in an ice-water bath to cool
the mixture to ∼5 °C. A solution of freshly distilled diphen-
ylphosphinic chloride (107.9 g, 0.46 mol) in THF (100 mL) was
added dropwise over a 1 h period. The reaction mixture was
allowed to slowly warm to room temperature and then stirred
under nitrogen for 15 h. The resulting brown solution was
poured into an aqueous solution (700 mL) of concentrated
ammonium chloride to give two layers. The organic layer was
isolated and washed successively with water, 5% aqueous
sodium bicarbonate, and water and then dried over magnesium
sulfate. The THF was removed via rotoevaporation to give a
dark red viscous liquid (142.8 g, 98% crude yield). The crude
product was vacuum-distilled (at ∼20 mm Hg) from 178 to 181
°C, and a light-yellow semisolid was collected (118.6 g, 81%
yield). The material was subsequently dissolved in 120 mL of
hot toluene, treated which charcoal, and filtered while hot
through Celite. To the partially cooled toluene solution was
carefully added 120 mL of hexanes. The solution was allowed
to cool with stirring overnight, yielding 1 as a white crystalline
solid: yield 100.9 g, 69%; mp 111.1-112.3 °C as determined
on improving thin film properties without detracting
from AO or UV resistance.18 In addition, other proper-
ties of interest such as electrical conductivity to mitigate
charge buildup are of interest, and preliminary studies
involving the incorporation of carbon nanotubes into
1
9
these polymers have yielded promising results. The
current work is focused on developing space environ-
mentally durable polymers with specific combinations
2
0
of properties for applications on Gossamer spacecraft.
The chemistry and properties of PPO containing poly-
imides are described herein.
Exp er im en ta l Section
Sta r tin g Ma ter ia ls. The following materials were used as
received: 1,3-bis(3-aminophenoxy)benzene (APB, melting point
(
mp) 107-108.5 °C, Mitsui Chemicals America, Inc.), 2,2-bis-
(4-aminophenyl)hexafluoropropane (BIS-A-AF, mp 195-197
°
C, Central Glass Co., Ltd.), 2,2′-bis(trifluoromethyl)benzidine
(
(
ABL-21, mp 179.5-182 °C, Kriskev Company, Inc.), 9,9-bis-
4-aminophenyl)fluorene (9,9-FDA, mp 253-255 °C, TCI
America), 4,4′-bis(4-aminophenoxy)biphenyl (BAPB, mp 196-
1
97.5 °C, Kriskev Co., Inc.), 3,3′,4,4′-biphenyltetracarboxylic
dianhydride (s-BPDA, mp 297-298 °C, Ube Industries, Ltd.),
and 3,4′-oxydianiline (3,4′-ODA, 82-84 °C, Hyvoco, Inc.).
Oxydiphthalic dianhydride (ODPA, mp 224-225.5 °C, Imitec,
Inc.), pyromellitic dianhydride (PMDA, mp 284-286 °C, Allco
Chemical Corp.), 4,4′-perfluoroisopropylidiene dianhydride
-
1
(6FDA, mp 241-243 °C, Hoechst Celanese Inc.), and 4-ami-
by DSC. IR (KBr pellet, cm ): 1196.5 (νPdO). Anal. Calcd for
OP: C, 68.79%; H, 4.17%; F, 12.09%; P, 9.86%.
nophenol (mp 186-187 °C, Aldrich Chemical Co.) were purified
by vacuum sublimation prior to use. 4,4′-Biphenoxy dianhy-
dride (BPODA, mp 284-286 °C, Oxychem, Inc.) was recrystal-
lized from 1,2,4-trichlorobenzene prior to use. 2,2-Bis[4-(4-
aminophenoxy)phenyl]hexafluoropropane (4-BDAF, 159-161
18 13 2
C H F
Found: C, 68.38%; H, 4.26%; F, 11.17%; P, 10.28%.
P r ep a r a t ion of [2,4-Bis(3-a m in op h en oxy)p h en yl]d i-
p h en ylp h osp h in e Oxid e (2). Into a 500 mL three-neck
round-bottom flask equipped with a mechanical stirrer, ther-
mometer, and a Dean-Stark trap equipped with a drying tube
were placed 1 (39.50 g, 0.1257 mol), 3-aminophenol (28.80 g,
0.2639 mol), potassium carbonate (45.58 g, 0.3298 mol), NMP
(200 mL), and toluene (115 mL). The mixture was heated with
stirring to 135 °C while removing water via azeotropic distil-
lation. After 16 h, the toluene was removed from the reaction,
and the resulting solution was heated at 160 °C for 4 h. The
reaction mixture was cooled to room temperature and then
poured into 1 L of 5% aqueous acetic acid with vigorous
stirring. A tan gum formed that eventually solidified with
stirring. The crude solid was collected via filtration, washed
with excess water, and dried at 110 °C to afford 59.1 g of 2
(95% crude yield). The solid melted from 179.7 to 187.6 °C as
determined by DSC. The solid was recrystallized twice from
°C, Central Glass Co., Ltd.) was recrystallized from 2-propanol
prior to use. Diphenylphosphinic chloride (Zeneca) was vacuum-
distilled (163 °C at ∼20 mm), and 1-bromo-2,4-difluorobenzene
(Lancaster Synthesis, Inc.) was distilled (165 °C) prior to use.
All other materials were used as received.
Ch a r a cter iza tion . Inherent viscosities (ηinh) were obtained
on 0.5% (w/v) polyimide solutions in N-methylpyrrolidinone
(NMP) at 25 °C. Differential scanning calorimetry (DSC) was
conducted on a Shimadzu DSC-50 thermal analyzer. Melting
points ranges were determined by either DSC (10 °C/min
recorded at the onset to melt and the endothermic peak) or
visually determined on a Thomas-Hoover capillary melting
point apparatus (uncorrected). T ’s were determined at 20 °C/
g
min and were taken as the inflection point of the ∆T vs
temperature curve. Dynamic thermogravimetric analyses
ethanol with charcoal treatment to afford 2: yield 60%; mp
1
(TGA) were performed on a Seiko model 200/220 instrument
195.2-196.5 °C (DSC). H NMR (CDCl
3
, 300 MHz, ppm): δ
on film or powder samples at a heating rate of 2.5 °C/min in
7.93 (dd, J ab ) 8.5 Hz, J bc ) 8.5 Hz, 1H); 7.40-7.48 (m, 4H);
7.08 (t, J ) 8.1 Hz, 1H); 6.94 (t, J ) 8.0, 1H); 6.68-6.72 (m,
1H); 6.32-6.50 (m, 5H); 6.01-6.04 (m, 1H); 5.93 (t, J ) 2.2
3
air and nitrogen at a flow rate of 15 cm /min. Elemental
analyses were performed by Galbraith Laboratories, Inc.,
Knoxville, TN. UV/vis spectra were obtained on thin films
using a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer.
Infrared spectra were obtained on a Perkin-Elmer 1600 series
Hz, 1H); 3.62 (br, 4H). 31P NMR (acetone-d
), 121 MHz, ppm):
δ 27.46 (singlet). IR (KBr pellet, cm ): 1180.2 (νPdO). Anal.
Calcd for C30 P: C, 73.16%; H, 5.12%; N, 5.69%; P,
6.29%. Found: C, 72.85%; H, 5.00%; N, 5.69%; P, 6.22%. Mass
6
-
1
25 2 3
H N O
1
31
FTIR. A Bruker 300 was used to obtain H and P NMR
spectra. Thin-film tensile properties were determined accord-
ing to ASTM D882 using either four or five specimens (0.51
cm wide) per test conditions using a Eaton model 3397-139
+
+
spectroscopic analysis: M 492 [C30
25 2 3
H N O P ].
Gen er a l P r oced u r e for th e P r ep a r a tion of P olyim id es.
Phenylphosphine oxide-containing polyimides were prepared
by reacting stoichiometric quantities of 2 with various dian-
hydrides. The following procedure is for the preparation of P 1
1
1.4 kg load cell on a Sintech 2 test frame. The test specimen
gauge length was 5.1 cm, and the crosshead speed for film

4
970 Watson et al.
Macromolecules, Vol. 35, No. 13, 2002
Sch em e 1. Syn th esis of
2,4-Bis(3-a m in op h en oxy)p h en yl]d ip h en ylp h osp h in e
Oxid e (2)
Sch em e 2. Syn th esis of P h en ylp h osp h in e Oxid e
Con ta in in g P olyim id es
[
difficult since phosphine oxide compounds, in general,
are soluble in many organic solvents, and purification
via crystallization is often difficult. Purification of the
crude Grignard product (1), typically obtained as viscous
red oil, was accomplished by vacuum distillation to yield
a light yellow semisolid. The semisolid was subsequently
crystallized from a 50/50 mixture of toluene/hexanes to
achieve a white crystalline solid in 69% overall yield.
The Grignard adduct was subsequently reacted with 2
molar equivalents of 3-aminophenol in the presence of
potassium carbonate in NMP to yield 2. Two recrystal-
lizations of diamine 2 from ethanol afforded polymer
grade monomer in 60% overall yield. Characterization
by mass spectrometry, H and P nuclear magnetic
spectroscopy (NMR), and elemental analysis indicated
high purity with the data being consistent with the
expected chemical structure. The structure of the di-
amine was designed to impart several desirable proper-
ties into polyimides. The polymers formed were antici-
pated to have AO resistance due to the presence of the
PPO functional group, UV resistance due to the aro-
matic imide ring, and low color due to the presence of
meta linkages and bulky pendant groups (PPO group).
In addition, enhanced processability of the fully imidized
polyimides is expected due to the improved solubility
of the polymers resulting from the presence of a bulky
pendant PPO group.
P olym er Syn th esis. Polyimides containing PPO
groups were prepared from 2 and various aromatic
dianhydrides in NMP or DMAc under a nitrogen atmo-
sphere as shown in Scheme 2. Typically, the poly(amide
acid) was prepared by dissolving the diamine and then
adding a molar equivalent of dianhydride as a solid.
Solvent was then added to make a 20% (w/w) solution.
The reaction mixture was stirred at room temperature
for 24 h followed by chemical imidization/cyclodehydra-
tion using a mixture of acetic anhydride and pyridine.
The resulting polymers were isolated in >95% yield. The
chemical structures of the aromatic dianhydrides used
in this study are shown in Figure 1.
and is representative for the preparation of all polymers. Into
a 100 mL three-neck round-bottom flask equipped with a
mechanical stirrer, nitrogen gas inlet, and drying tube filled
-
3
with calcium sulfate were placed 2 (2.3768 g, 4.8258 × 10
mol) and NMP (7.0 mL). Once dissolved, oxydiphthalic dian-
-
3
hydride (1.4970 g, 4.8258 × 10
mol) was added with
additional NMP (8.0 mL) to give a solution with a concentra-
tion of 20% (w/w) solids. The resulting light yellow solution
was stirred overnight at room temperature under a nitrogen
atmosphere. A 0.5% (w/v) solution in NMP at 25 °C exhibited
an ηinh of 1.01 dL/g. The remaining polymer solution was
chemically imidized by the addition of 1.49 g of acetic
anhydride and 1.15 g of pyridine (3 molar excess). The solution
was stirred at room temperature overnight. The polymer was
precipitated in a blender containing a 50/50 mixture of water/
methanol, filtered, washed with excess water, and dried in a
vacuum oven at 110 °C overnight to afford a white, fibrous
material. Films were cast as described below. Copolymers were
prepared in a similar manner using 75 mol % of 2, 25 mol %
of another diamine, and a stoichiometric amount of dianhy-
dride.
1
31
F ilm P r ep a r a tion . Thin films were cast from polyimide
solutions in N,N-dimethylacetamide (DMAc), NMP, or chloro-
form (typically 10-20% w/w solids). The polymer solutions
were centrifuged, and the decantate was doctored onto clean,
dry plate glass and dried to a tack-free form in a low-humidity
chamber. The films then were stage-dried in a forced air oven
to 250 °C and held for 1 h.
Resu lts a n d Discu ssion
P r ep a r a tion of[2,4-Bis(3-a m in op h en oxy)p h en yl]-
d ip h en ylp h osp h in e Oxid e (2). This novel diamine
was synthesized in two steps as shown in Scheme 1.
The first step involved a Grignard reaction between
1
-bromo-2,4-difluorobenzene (20% molar excess) with
diphenylphosphinic chloride. Since diphenylphosphinic
chloride readily hydrolyzes to diphenylphosphinic acid,
it was vacuum-distilled prior to use and stored under a
nitrogen atmosphere. The crude product was obtained
in 81% yield. Initial purification of this material was
P olym er Ch a r a cter iza tion . Characterization of
polymers prepared from 2 and various dianhydrides is
presented in Table 1. The ηinh’s ranged from 0.53 to 1.01

Macromolecules, Vol. 35, No. 13, 2002
Polyimides and Copolyimides 4971
Ta ble 1. Ch a r a cter iza tion of P olyim id es P r ep a r ed fr om 2
5
% weight loss (°C)
tensile
tensile
polymer
dianhydride
ηinh (dL/g)
Tg (°C)
strength (MPa)
modulus (GPa)
elong (%)
air
nitrogen
P 1
P 2
P 3
P 4
P 5
ODPA
BPODA
6FDA
s-BPDA
PMDA
1.01
0.81
0.66
0.97
0.53
212
227
230
234
251
97
82
87
90
2.8
2.5
2.9
2.8
4.7
4.0
3.3
3.1
461
464
453
455
421
478
494
465
471
430
Ta ble 2. Op tica l P r op er ties of F ilm s P r ep a r ed fr om 2
transparency
at 500 nm
(%)
film
thickness
(cm)
dianhy-
dride
polymer
R
ꢀ
R/ꢀ
-
3
3
3
3
P 1
P 2
P 3
P 4
ODPA
BPODA
6FDA
85
80
87
76
4.06 × 10
5.08 × 10
4.32 × 10
6.10 × 10
0.06 0.56 0.10
0.09 0.68 0.13
0.05 0.55 0.09
0.10 0.62 0.15
-
-
-
s-BPDA
8
2 to 97 MPa and 2.5 to 2.9 GPa, respectively. The
elongations to break ranged from 3.1 to 4.7%.
Op t ica l Tr a n sp a r en cy. Diamine 2 contains meta
linkages and a bulky pendant group and thus provides
a highly irregular structure to the polyimide. This was
expected to disrupt the formation of charge transfer
complexes and lead to low color polyimide films. De-
pending on the dianhydride used, virtually colorless
polyimide films were prepared. P 1 and P 3 were nearly
colorless, P 2 was light yellow, P 4 was medium yellow,
and P 5 was orange in color. Thin films were measured
for optical transparency using UV/vis spectroscopy with
the percent transmission at 500 nm (the solar maxi-
mum) reported in Table 2. Film thicknesses ranged from
-
3
-3
4
.06 × 10 to 6.10 × 10 cm. Polymers P 1 and P 3
exhibited optical transparency as high as 87%. P 3,
which contains trifluoromethyl groups, exhibited the
highest transparency. The film from P 5 was too brittle
for transparency measurements, but the orange color
of the film indicates lower percent transmission values
compared to those of other films.
Sola r Absor p tivities a n d Th er m a l Em issivities.
Two of several important properties of materials for
space applications are R and ꢀ. Solar absorptivity
pertains to the fraction of incoming solar energy that is
absorbed by the film, and ꢀ is a measure of the films
ability to radiate energy from the film surface. Typically,
a low colored film exhibits a low R value. Thin films were
measured for R and ꢀ, and the results are shown in
Table 2. R values ranged from 0.05 to 0.10, and ꢀ values
ranged from 0.56 to 0.68. Consistent with the optical
transparency measurements, P 1 and P 3 films exhibited
the lowest R values.
F igu r e 1. Dianhydrides used for polyimide synthesis.
dL/g, indicating medium to high molecular weight
polymer formation. The Tg’s ranged from 212 to 251 °C.
All of the polymers exhibited solubility in DMAc and
NMP in the imide form; polymers P 1 and P 3 were also
soluble in chloroform. The polymers exhibited good
solubility presumably due to the bulky pendant PPO
group. P 3’s high solubility can also be attributed to the
trifluoromethyl groups. P 5, despite having an inherent
viscosity of 0.53 dL/g, formed only a brittle film, and
thus tensile properties and other characterization tests
were not performed. The polymers exhibited tempera-
tures of 5% weight loss ranging from 421 to 464 °C in
air and 430 to 494 °C in nitrogen. The typical thermal
cycle experienced by spacecraft in LEO is (100 °C; thus,
the thermal properties of these polymers are sufficient
for use in LEO.
Atom ic Oxygen Exp osu r e. Ground-based AO ex-
posure was performed on three thin film samples of P 1
at the Marshall Space Flight Center fast AO facility.
2
The exposed films had an area of 3.6575 cm , the
-
3
thickness was 2.54 × 10 cm, and the weights of the
-
2
-2
films ranged from 1.1817 × 10 to 1.844 × 10 g. The
films were exposed to AO fluences ranging from 1.71 ×
20
20
2
1
0
to 2.13 × 10 AO/cm , and the effects on R, ꢀ, and
20
mass loss were determined. Exposure of 2.0 × 10 AO/
2
cm is roughly equivalent to 6 months exposure to AO
Th in F ilm Ten sile P r op er ties. Thin films were cast
from polyimide solutions (NMP, DMAc, or chloroform)
and thermally exposed in a forced air oven to remove
residual solvent. All of the films prepared except for P 5
were tough and creasable and exhibited low color. Thin
film mechanical properties are presented in Table 1. The
films exhibited room tensile strengths and moduli from
in a LEO environment. The results are presented in
Table 3. The ability of a material to undergo minimal
changes in R, ꢀ, and other properties upon exposure to
radiation in space is of significant importance. The films
exhibited a slight initial increase in R and negligible
changes in ꢀ after initial exposure to AO. The sample
2
0
2
exposed to 2.13 × 10 AO/cm exhibited a weight loss

4
972 Watson et al.
Macromolecules, Vol. 35, No. 13, 2002
F igu r e 2. Transmission spectrum of P 1 before and after AO
exposure.
Ta ble 3. Effect of F a st AO Exp osu r e on P 1
exposure (AO/cm2)
R
ꢀ
mass loss (g) mass loss (%)
none
0.187 0.74
0.221 0.74
0.218 0.73
0.227 0.74
0
0
2
2
2
0
0
0
-4
-4
-4
1
2
2
.71 × 10
.00 × 10
.13 × 10
4.1 × 10
3.2 × 10
5.0 × 10
2.24
1.74
2.73
of 2.7%. This calculates to a reaction efficiency of 4.8 ×
-
25
3
1
0
cm /AO. For comparison purposes the reaction
-24
3
efficiency of Kapton HN is 3.0 × 10
cm /AO. However,
since phosphine oxide containing polyimides do not
exhibit linear erosion rates, this comparison is not
quantitative. Other phosphine oxide-containing poly-
imides have exhibited nonlinear erosion rates after
longer duration AO exposure.12 The results suggest that
AO reacts indiscriminately with the polymer, resulting
in off-gassing of volatile organic species and eventual
buildup of a polyphosphate layer which subsequently
retards additional erosion. This is analogous to what
has been reported to occur with silicon-containing
polymers.21 The preliminary results observed with the
polyimide tested herein are consistent with previous
results.
The transmission spectrum of P 1 before and after AO
exposure is shown in Figure 2. The sample exposed to
AO exhibited a slight decrease in transmission from
around 500-700 nm. At 500 nm, the transmission of
the control sample was 85%. For the AO-irradiated
sample, the transmission decreased to 81%. The slight
lowering in transmission is likely due to the formation
of a polyphosphate surface layer on the film. This is
consistent with similar tests performed on this same
polymer by a private company.22
F igu r e 3. Diamines used in copolymer synthesis.
structures of the diamines used for copolymer synthesis
are shown in Figure 3. Characterization of the copoly-
mers is summarized in Table 4. Inherent viscosities
ranged from 0.26 to 1.07 dL/g and Tg’s ranged from 203
to 249 °C. Copolymers prepared from ODPA typically
had the lowest Tg’s, while films prepared from s-BPDA
typically had the highest Tg’s.
Th in F ilm Ten sile P r op er ties, Cop olym er s. Table
4 lists thin film mechanical properties of the copolymers
at room temperature. Tensile strengths ranged from 79
to 113 MPa and moduli ranged from 2.7 to 3.5 GPa.
Elongations to break ranged from 3.1 to 9.7%. P 14, with
an inherent viscosity of 0.34 dL/g, gave a brittle film;
thus, no tensile properties were obtained.
Op tica l P r op er ties, Cop olym er s. Optical proper-
ties of the copolymers were measured using UV/vis
spectroscopy on thin films. The optical properties are
listed in Table 5. Transparencies of thin films at 500
nm ranged from 80 to 89%, and most of the films were
colorless or nearly colorless. Film color was partially
dependent upon which dianhydride and diamine were
used to prepare the copolymer as well as thickness of
Although exposure of these polyimides to UV radia-
tion was not conducted as part of this study, they are
anticipated to exhibit good resistance based on previous
2
3,24
studies of polyimides of a similar chemical structure.
Other radiation effects on these polyimides have been
studied as well.25,26 In general, these polyimides have
exhibited good stability with only slight changes in
optical properties as a result of UV exposure.
Cop olym er Syn th esis. A series of copolymers were
prepared in order to further investigate effects of
polymer structure on the color, R, ꢀ, and other film
properties. Copolymers were prepared using 75 mol %
of 2, 25 mol % of a second commercially available
aromatic diamine, and 100 mol % of an aromatic
dianhydride. Three dianhydrides were chosen because
they tended to give polyimide films with low solar
absorptivities. The dianhydrides selected were ODPA,
-
3
6
FDA, and s-BPDA (structures shown in Figure 1). The
the film. Film thickness ranged from 2.0 × 10 to 5.3

Macromolecules, Vol. 35, No. 13, 2002
Polyimides and Copolyimides 4973
Ta ble 4. Cop olym er Ch a r a cter iza tion : 75% (2), 25% Cod ia m in e
tensile strength
(MPa)
tensile
modulus (GPa)
polymer
dianhydride
codiamine
ηinh (dL/g)
Tg (°C)
elong (%)
P 6
P 7
P 8
P 9
ODPA
ODPA
ODPA
ODPA
ODPA
6FDA
ODPA
ODPA
6FDA
6FDA
6FDA
s-BPDA
6FDA
6FDA
s-BPDA
s-BPDA
6FDA
APB
1.07
0.66
0.57
0.57
0.73
0.53
0.39
0.46
0.34
0.42
0.58
0.49
0.57
0.39
0.26
0.28
0.48
203
216
218
220
222
224
226
227
233
234
234
237
238
238
240
246
249
97
105
101
104
99
101
113
108
a
108
92
90
94
88
3.1
2.8
3.2
3.0
3.3
3.4
3.3
3.0
a
3.2
2.9
3.0
2.8
2.7
3.5
3.5
3.2
5.0
7.9
6.3
6.6
9.5
4.1
8.7
6.2
a
4.9
4.3
9.6
5.0
4.3
9.7
3.9
3.1
4-BDAF
3,4′-ODA
BIS-A-AF
BAPB
P 10
P 11
P 12
P 13
P 14
P 15
P 16
P 17
P 18
P 19
P 20
P 21
P 22
APB
9,9-FDA
ABL-21
ABL-21
3,4′-ODA
4-BDAF
4-BDAF
BAPB
BIS-A-AF
3,4′-ODA
9,9-FDA
9,9-FDA
102
90
79
a
Brittle film.
Ta ble 5. Cop olym er Op tica l P r op er ties: 75% (2), 25% Cod ia m in e
transparency at
polymer
dianhydride
codiamine
500 nm (%)
film thickness (cm)
R
ꢀ
R/ꢀ
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
P 6
P 7
P 8
P 9
ODPA
ODPA
ODPA
ODPA
ODPA
6FDA
ODPA
ODPA
6FDA
6FDA
6FDA
s-BPDA
6FDA
6FDA
s-BPDA
s-BPDA
6FDA
APB
83
80
88
85
82
86
87
81
89
88
87
85
88
89
80
85
86
3.1 × 10
2.5 × 10
2.0 × 10
3.6 × 10
2.8 × 10
4.3 × 10
3.8 × 10
3.6 × 10
2.0 × 10
3.3 × 10
5.3 × 10
2.8 × 10
4.1 × 10
2.5 × 10
3.6 × 10
3.3 × 10
3.8 × 10
4-BDAF
3,4′-ODA
BIS-A-AF
BAPB
0.07
0.06
0.60
0.38
0.12
0.16
P 10
P 11
P 12
P 13
P 14
P 15
P 16
P 17
P 18
P 19
P 20
P 21
P 22
APB
0.07
0.06
0.64
0.59
0.11
0.10
9,9-FDA
ABL-21
ABL-21
3,4′-ODA
4-BDAF
4-BDAF
BAPB
BIS-A-AF
3,4′-ODA
9,9-FDA
9,9-FDA
0.06
0.53
0.11
10- cm. Copolyimides prepared from 6FDA formed
3
color films. As part of the Materials International Space
Station Experiment (MISSE), several representative
samples have been placed on the international space
station and are being exposed to LEO for 1 year. The
samples will subsequently be returned to Earth and
analyzed.
×
films with the highest optical transparency with two
films (P 14 and P 19) having optical transparency as high
as 89%. It appears that the second diamine used had
little effect on optical transparency but that may be
expected using only 25 mol %. The R and ꢀ values of
selected copolymer films that had high optical transpar-
ency are shown in Table 5.
Ack n ow led gm en t. The authors gratefully acknowl-
edge Dr. Dave Edwards, Marshall Space Flight Center,
for performing the AO exposure test and Mr. Bob
Mojazza, Triton Systems, Inc., for his assistance in
obtaining additional AO exposure data.
Con clu sion s
A novel diamine, [2,4-bis(3-aminophenoxy)phenyl]-
diphenylphosphine oxide, was prepared, characterized,
and subsequently reacted with various dianhydrides to
prepare phenylphosphine oxide-containing polyimides.
A series of copolymers were prepared using 75 mol %
of 2 and 25 mol % of another aromatic diamine. Several
of the resulting polyimides and copolyimides exhibited
a combination of desirable properties such as low solar
absorptivity, high optical transparency, good tensile
properties, AO resistance, and enhanced solubility. The
PPO pendant to the polymer backbone not only is
responsible for imparting AO resistance to the polymer
films but also is also partially responsible for imparting
low color to the films by reducing charge transfer
complex formation. The bulky PPO group also serves
to increase the solubility of the fully imidized polymers.
The trifluoromethyl groups and meta linkages present
in some of the polymers also aid in forming low or no
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