The effect on selected polyimide properties when pendant alkyl groups were attached to the polymer backbone was investigated. A series of polymers were prepared using benzophenone tetracarboxylic acid dianhydride (BTDA) and seven different p-alkyl-m,p'-diaminobenzophenone monomers. The alkyl groups varied in length from C(1) (methyl) to C(9) (nonyl). The polyimide prepared from BTDA and m,p'-diaminobenzophenone was included as a control. All polymers were characterized by various chromatographic, spectroscopic, thermal, and mechanical techniques. Increasing the length of the pendant alkyl group resulted in a systematic decrease in glass transition temperature (Tg) for vacuum cured films. A 70 C decrease in Tg to 193 C was observed for the nonyl polymer compared to the Tg for the control. A corresponding systematic increase in Tg indicative of crosslinking, was observed for air cured films. Thermogravimetric analysis revealed a slight sacrifice in thermal stability with increasing alkyl length. No improvement in film toughness was observed

Jensen, B. J.

Young, P. R.

English

NASA Technical Reports Server

,'W',4".=._Y,_/-pz/_-"
NASA Technical Memorandum 84575 NASA-TM-84575 19830006003
POLYIMIDESWITHPENDANTALKYLGROUPS
BRIAN J. JENSENAND PHILIPR. YOUNG
NOVEMBER1982
NationalAeronautics and
SpaceAdministration
LangleyResearchGentor
Hampton,Virginia23665
https://ntrs.nasa.gov/search.jsp?R=19830006003 2020-03-21T04:53:53+00:00Z

\
\
INTRODUCTION
Aromatic polyimldes have potential for extended use in vari-
ous aerospace applications provided they can be reliably processed
into useful articles. However, inherent problems with solubility,
, h_gh glass transition temperature (Tg), and the evolution of
volatiles during cure must be solved-before this potential can be
fully exploited. Several approaches have been taken to alleviate
some of the difficulties associated with processabillty. Addition
polyimides are being developed which are processed as ollgomerlc
segments and then thermally cured through reactive end groups
without the evolution of volatiles 1-3. Stereoisomeric varia-
tions have been built into the polymer backbone and the effect on
polymer properties determlned 4. Pendant phenyl groups were
added to the polymer backbone in an effort to improve solubil-
ity 5. Numerous other structure-property relationships have been
explored.
The objective of the present research was to investigate the
effect on selected properties when alkyl groups of various lengths
were attached pendant to the polyimlde backbone. The eight varia-
tions studied are shown in Figure 1. The polyimide with no pen-
dant group was included as a control. Although the data obtained
0 o o o
" " " F 0 H 0 0 0 -_
< " " ""..,_ H2.O: C :0-- C-i'd'r"N-- C_ C-f_C- N-I-_J"R HO'C"_ "_C-OHIH2N
0 0 _ " -/n0
DIAMINE BIDA POLY(AMIC}ACID
WHERE DESIGNATION A "H20
R= H CONIROL n O O O
CH3 C] l- v " ,, . "I
" C
C N" C C.N
R .% Lv o -In
ICHzI6CH) C? POLYMIDE
'c"z'zc", c,
(CHzIsCH! c9
CIH"CH2"CH3 C4B
CH)
Figure I. Polyimlde synthetic scheme.
in this study was somewhat limited by the amount of sample avail-
able, fundamental information was gained on how particular var£a-
tlons in molecular structure affected such properties as Tg, ^
thermal stability, and toughness.
4
EXPERIMENTAL
Characterization
Analytical chromatographic separations were obtained on a
Waters Associates ALC/GPC-244 HPLC. Analyses were performed at
254 nm on either a Waters _Porasil or C18 - _Bondapak column
(3.9 mm id x 30 cm). Preparative separations were obtained on a
Waters Prep LC/System 500 using a Prep PAK-500/Silica column and a
refractive index detector. Gel permeation chromatography was per-
formed on a 106 , 105 , 104 , 103 A _Styragel column bank.
The mobile phase for all separations was prepared from chromato-
graphic grade methylene chloride, methanol, l-propanol, water, or
N,N'-dlmethylacetamide (DMAc). Poly(amlc) acid inherent viscosi-
ties (qinh) were measured at 0.5% concentration in D_c at 35"C
with a Cannon-Ubbelohde viscometer.
Thermal analyses were performed on a DuPont Model 990 Thermal
Analyzer in combination with a standard differential scanning
calorimetry (DSC) cell. Thermomechanical analyses (TMA) were per-
formed on films in combination with a DuPont Model 940 Thermo-
mechanical Analyzer. Torsional Braid Analyses (TBA) were per-
formed using an in-house system 6 on glass braids impregnated
with a 5% poly(amlc) acid solution in DMAc and cured in vacuum at
100°C, 200"C, and 275"C for one hour each. Thermogravimetric
analyses (TGA) were obtained on approximately 2 mg film samples
with a Perkin-Elmer TGS-2 instrument in 15 cm3/min flowing air.
Infrared spectra of monomers were obtained on KBr pellets
with a Perkin-Elmer Model 297 Spectrophotometer. Spectra of poly-
meric films were recorded on a Nicolet 3600A FTIR System. Elemen-
tal analyses of monomers, shown in Table I, were performed by
Galbraith Laboratories, Knoxville, Tennessee.
Mechanical properties were measured on an Instron Tensile
Tester at a crosshead speed of 0.2 in/min and a gauge length of
3.0 in. Five one-inch wide and nominally one-mil thick film
specimens were tested for each sample.
Monomers
The monomer 3,3',4,4'-benzophenone tetracarboxylic acid dlan-
hydride (BTDA) was obtained from commercial sources and vacuum
sublimed prior to use.
Table I.
ELEMENTAL ANALYSIS OF DIAMINE MONOMERS
0
U
zF..hIPERICAt PENDANT
FORMULA GROUP 'kC _H _N
CALC. FOUND CALC. FOUND CALC. FOU/_.____DD
CI3HI2N20 H 73.57 73.44 5.70 5.71 13.20 13.01
C17H20N20 C4 76.09 75.92 7.51 7.61 10.44 10.39
C17H20N20 C4B 76.09 76.24 ?.51 7.64 10.44 10.60
CI9H24N20 C6 76.99 77.07 8.16 8.21 9.45 9.51
C20Hz6N20 C7 77.38 71.54 8.44 8.22 9.02 9.G0
C21H28N20 C8 77.74 77.87 8.70 8.83 8.63 8.64
C22H30N20 C9 78.05 78.24 8.95 9.00 8.28 8.29
Diamlnes were obtained from several sources. The m,p'-
diaminobenzophenone (control) was synthesized by a previously re-
ported method 4 and purified by recrystallization from benzene,
toluene, and finally benzene. The p-rnethyl-m,p'-diaminobenzophen-
one monomer (CI) was synthesized by the scheme shown in Figure
2. This synthetic scheme was typical for all diamlnes included in
the study. The acid chloride I was reacted via a Freldel-Crafts
acylation with toluene and the resulting compound II nitrated in
mixed nitric and sulfuric acids. The dinitro compound III was
reduced by catalytic hydrogenation at 50 psi with a 5% platinum on
carbon catalyst yielding 35-40% of compound IV. The remaining
product formed appeared to be a hydrate. Compound IV was purified
by preparative liquid chromatography after unsuccessful recrystal-
llzation attempts. The p-sec-butyl-m,p'-diaminobenzophenone
(C4B) was prepared on a grant 7 with the Mississippi University
for Women, Dr. J. Richard Pratt, Principle Investigator. It was
purified by recrystallizing twice from benzene/cyclohexane. Five
diamines were prepared on a grant 8 with the University of
Southern Mississippi, Dr. Shelby F. Thames, Principle Investiga-
tor. Butyl (C4) and nonyl (C9) monomers were polymer grade as
received while the other three (C6, C7, C8) were purified by
preparative liquid chromatography.
3
: 0
_-OH +soc,2. " - .r"_'_c-c,
ozN_,..._ o=,-'..._I
0 0
,,_rc__,__% "03 ,,oz,-_) _T...,-c,3 oz" %
nl II
H2/Pt'_ 0
H2N-L_ I_/"l--CH 3
"IV
Figure 2. Synthetic scheme for C1 diamine (R=CH3).
Polymers and Film Preparation
Polymers were synthesized by solution polymerization in DMAc
distilled over calcium hydride and stored under nitrogen. Poly-
merizations were run for 24 hr at 25"C and 15% solids content in a
dry serum bottle equipped with a rubber stopper and a magnetic
stirring bar. No attempts were made to increase viscosity other
than by monomer purification. Solutions were stored at approxi-
mately -10"C until use.
Poly(amlc) acid solutions were spread on glass plates using a
doctor blade set at 15 mil. Films were dried overnight in a
flowing-air dry box at 25"C. Each film was then cured in vacuum
for one hour each at IO0"C, 200"C, and 280"C. Three other films
were cured in air for one hour each at 100"C, 200"C, and 300"C.
Resultant films were approximately l-mil thick.
4
RESULTSAND DISCUSSION
The primary concern in the polymer synthesis was whether
sufficiently high molecular weight polymer could be obtained to
cast films. Therefore, high purity monomers were essential to
eliminate the possibility of impurities limiting polymer molecular
weight. Since the pendant groups were adjacent to the amine
reactive site, steric hlnderence was also considered a potential
problem. The C4B (branched) diamine was included to further
investigate this area of concern.
Recrystalllzatlon was the first method employed to purify the
dlamlne monomers. If this was unsuccessful, vacuum sublimation
was utilized. Monomers that were still unacceptable were further
purified by preparative liquid chromatography. Monomers were sub-
jected to both normal and reverse phase chromatography to test for
purity. Figure 3 is an example of normal phase chromatography on
the C6 diamine, as received and after purification by prepara-
tive chromatography. Infrared spectra were consistent with the
corresponding structures and were essentially identical except for
increasing absorption with increasing alkyl group length from 2930
cm -I to 2850 cm-I.
NORh'ALPHASE
SOLVENT:2.0_.1-PrOH/CH2CI2
FLOW:].0 ml/min [1n II
C NH2
ABSORBANCE II 2 t 215 3 II
I II (o61 ,,,,
B£FORE II AFTER II
PUR'F'CAt'ONJI PURIFICATIONIl
IMPURITYII Jl
,m[ct , -LJL_... ,' :nJ___r___r_J L_ , ,
0 2.5 5.0 7.5 10.012.50 2.5 5.0 7.5 10.012.5
RETENTIONT;:.I£. rain
Figure 3. High pressure liquid chromatography of C6 dlamlne
before and after purification.
5
r
After obtaining the purest monomers possible, polymerizations
were run on approximately one gram of each monomer. Although the
control dlamlne was the only monomer less than 99% pure, it pro-
duced the highest viscos£ty polymer. The C4B diamine produced
polymer with inherent viscosity (nlnh) of 0.19 dl/g and the C1
diamine produced polymer of only 0.31 dl/g viscosity. The C4B
polymer was possibly limited by steric hlnderence. This did not
apply to the CI polymer. The latter polymerlzaton was repeated
with the same results. Neither of these two polymers could be in-
cluded in much of the testing since they would not form flexible
films. However, one polymerization of the C9 diamine which re-
sulted in polymer with qinh = 0.28 dl/g did form a usable film.
Figure 4 shows gel permeation chromatograms of the poly(amic)
acids along with corresponding nln_. As expected, a decrease in
elutlon time with increasing qinh is observed.
Characterization of the polymers in the cured imide form in-
cluded infrared analysis on film samples which exhibited the same
trend as the monomers, increased absorption in the 2930 cm-I -
2850 cm-I region as the pendant groups increased in length.
This data also confirmed that imidization had taken place during
cure by the appearance of characterlst£c imide absorptions at 1780
cm-I, 1730 cm-I, ]380 cm-I. and 720 cm-I. No significant
difference in the IR spectra for air cured and vacuum cured films
of the same polymer was noted.
COLUMNS:106'10S'10%03.S YRAm
SOLVENT:DMAC
FLOW:1.5 mllmin
_ D_ECIOR:REFRACIIVEINDEX
CTIVE _..--..-,,',_. 4]
.36
I ! l I J
10 15 20 Z5 3O
REID'_TIO_TIME.min
Figure 4. Gel permeation chromatography of poly(amic) acid
solutions.
One objective of this project was to study the effect the
variation in diamlne structure had on polymer Tg. Therefore,
Tg was measured by several techniques. Torsional braid analyses
data are summarized in Figure 5. The damping peak maximum and the
corresponding loss of rigidity are indicative of polymer T_.
Vacuum cured polyimldes showed a decrease in Tg as the alk_l
group increased in length. For the C9 polylmlae, a decrease of
approximately 60"C from that of the control (Tg = 274"C) was
noted. Further evidence for the decrease in T8 is shown in
Figure 6 by DSC thermograms of vacuum cured films. Figure 7 gives
the same trends by thermomechanical analysis. The opposite trend
in Tg was observed for air cured samples. Although the effect
of azr cure was not as dramatic, a 20"C increase in Tg for the
c9 polylmide over the control was noted. This data xs summar-
ized in Figure 8. An increase in Tg is probably indicative of
air induced crossllnklng. Apparently, the longer pendant group
polyimides crosslinked to a greater extent than the shorter pen-
dant chain polymers. This data suggests the possibility of vary-
ing the Tg of a polyimide over a wide temperature range by vary-
ing the length of an alkyl group pendant to the backbone and by
choice of cure atmosphere.
O O O O
II II II 7
t SA_IPLE:CONTROL.VACUUM
RELATIVE CURE.275o O O
RIGIDITY
I AI_OSPHERE-NITROGEN
_=dl3OC/mln _ Ig.°C
I I I CONTROL 274
C4 266
C6 245
C7 226
1 csRELATIVE 222
DAMPING _ C9
I 212
t',
I . I I I I I
-200 -lO0 O lO0 200 300 400
" IEA_PERAIUREOc
Figure 5. Polylmide Tg by torsional braid analysis.
7
ls9o. o
SAMPLE:FILM.VACUUMCURUZS0°C
ATMOSPHER_AIR
dT= 20oClminEXO dt
l C48"
_T
] c,C 0CONTROL O O Or II It II II "1
Z6Z° O O
! ! ! ! I150 Z00 230 3O0 350
TE/VIPERATURF.,,°1_
Figure 6. Differential scanning calorimetry of polyimide films.
C_
SAMPLE:VACUUM
ELONGATION CUREJ2g0OC
ATMOSPHERE;AIR
dT .o..
_'_ = :_ Imln
I I ! I
v t_ zoo _o _oo
T_PERATUR_°C
Figure 7. Thermomechanlcal analyses of vacuum cured polyimide
films.
8
Figure 8. Thermomechanical analyses of air cured polyimlde films.
Thermogravimetric analyses were run to determine the effect
of structural variations on thermal stability. Figure 9 shows TGA
curves for the control and C9 polyimldes. No noticeable weight
loss is observed below 300"C for any of the polymers. Above
300"C, the C9 polyimide was the least thermally stable and the
control the most stable. The polymers with intermediate alkyl
group length fell in order between these two extremes. Air cured
films exhibited slightly better thermal stability than correspond-
ing vacuum cured films. Depending on the potential application,
the detrimental decrease in thermal stability with increasing
alkyl group length may be offset by a corresponding beneficial
lowering in Tg. Also, processing in vacuum but post curing in
air could result in an increase in Tg of about 100°C.
Vacuum cured films were also subjected to tensile testing.
The resultant data are presented in Table 2. Inherent viscosity is
included to give an indication of polymer quality and relative
molecular weight. The toughness index, the ratio of sample
stress-strain curve area to control stress-strain curve area,
gives an indication of film toughness. No improvement in tough-
ness was observed by this index. One trend apparent from the
table is the relationship between film properties and nin h. If
all polymers had the same approximate molecular weight as the con-
trol, an increase in toughness might have been observed.
100_
g0
ZC 9
6O
WEIGHT O 0 O O
_F iI II II II L,,'_ C C-NRETENTION, C'_N-c_ "_C-
• 40- -_ R mm " -Jno o
SAMPLE:FILM, VACUU/',_CURFJ?80°C20
AThlOSPHERE:AIR. 15cm31min(IT
I _" = 5°C/min! ! I !
o ]oo 2oo 3oo 4oo 5oo 6oo 7oo
l_IPEP,ATURE,°C
Figure 9. Thermogravimetric analyses of vacuum cured polylmlde
fiIms o
Table 2.
MECHANICAL PROPERTIES OF VACUUM CURED POLYIMIDE FILMS (al
qinh.(bl TENSILEk| IOUGHNESSEl!
SAMPLE dl/g STRF,.NGTH.MPa ELONGATION._k) INDEX
CONTROL .48 106.9 4.55 1.00
C1 .31 (e} (e) (e}
C4(fl .36 71.6 3.39 0.44
C4B ,19 (e) (el (el
C6 .37 86.3 4.33 0.75
C7 .41 85.8 4.% 0.93
C8 .43 87.2 5.22 0.95
,?
C9 .38 82.9 4.90 0.82
(alCUREDAT2_°C _--
(hi0.5_ SOLIDSIN DMAcAT35°C
k) AVERAGEOFFIVESPECIMENS
(d)RATIOOFAREAUNDERSIRESS-SIRAINCURVES
(e)WOULDNOTFORMFILA!
11!POORQUALIIYFILM
i0
CONCLUSIONS
An investigation was conducted to determine the affect on
polylmide properties when alkyl groups are attached pendant to the
polymer backbone. A control polymer with no pendant groups and
•_ seven different polymers with pendant groups ranging in length
from one carbon to nine Carbons were included in this study. The
polymers were characterized in both the poly(amic) acid and poly-
imide stages.
The data obtained in this study proves the feasibility of
varying Tg over a wide temperature range by attaching alkyl
groups pendant to the polylmide backbone. A 70"C decrease was
observed for the vacuum cured C9 polyimlde compared to the con-
trol as measured by TMA. Also, an increase in Tg, indicative of
crosslinking, was observed for air cured films as the alkyl group
increased in length. However, the incorporation of pendant alkyl
groups in the polymer chain led to a slight sacrifice in thermal
stability as_measured by TGA. Mechanical property data indicated
no improvement in film toughness with increasing pendant alkyl
group length.
REFERENCES
I. E. A. Burns; R. J. Jones; R. W. Vaughan; and W. P. Kendrlck:
TRW-II926-6013-RO-00, NASA CR-72633, January 1970.
2. T. T. Seraflni; P. Delvigs; and G. R. Lightsey: J. Appl.
Polym. Sci., 16, 905 (1972).
3. T. L. St. Clair; and R. A. Jewell: SAMPE Tech. Conf. Proceed-
ings, 8, 82 (1976).
4. V. L. Bell, Jr.; B. L. Stump; and H. Gager: J. Polym. Sci.,
14, 2275 (1976).
5. F. W. Harris; W. A. Feld; and L. H. Lanier: Appl. Polym.
Symp., 26, 421 (1975).
6. S. K. Dalal; G. L. Carl; A. T. Inge; and N. J. Johnston: Am.
Chem. Soc., Div. Polymer Chem., Preprlnts, vol. 15, 576
(1974).
7. NASA Grant NSG-1539, Mississippi University for Women,
Columbus, MS 39701.
8. NASA Grant NSG-25-005-008, University of Southern Mississippi,
Hattlesburg, MS 39401.
Ii
1. Report No. 2. GovernmentAccessionNo. 3. Recipient'sCatalogNo.
NASATM-84575
4. Title and Subtitle 5. Report Date
November 1982
Polyimideswith PendantAlkyl Groups 6. PerformingOrganizationCode
505-33-33-09
7. Author(s) 8. Performing Orgamzation Report No.
Brian J. Jensen and Philip R. Young
10. Work Unit No.
9. Performing Organization Name and Address r
NASALangley Research Center 11 Contractor GrantNo.
Hampton, VA 23665
13. Type of Report and Period Covered
12. Sponsoring Agency Name and Address TechnicalMemorandum
NationalAeronauticsand Space Administration 14SponsoringAgency Code
Washington,DC 20546
15. Supplementary Notes
Use of commercial products or names of manufacturers in this report does not consti-
tute official endorsement of such products or manufacturers, either expressed or
implied, by the National Aeronautics and Space Administration.
16. Abstract
An investigationwas conductedto determinethe
effect on selectedpolyimidepropertieswhen pendant
alkyl groups were attachedto the polymer backbone. A
series of polymerswere preparedusing benzophenone
tetracarboxylicacid dianhydride(BTDA) and seven
different p-alkyl-m,p'-diaminobenzophenonemonomers. The
alkyl groups varied in length from C1 (methyl)to C9
(nonyl). The polyimidepreparedfrom BTDA and
m,p'-diaminobenzophenonewas includedas a control.
All polymerswere characterizedby variouschromato-
graphic, spectroscopic,thermal, and mechanical
techniques. Increasingthe lengthof the pendant alkyl
group resulted in a systematicdecrease in glass
transitiontemperature(To! for vacuum cured films. A
70 C decrease in Ta to 193 C was observed for the nonyl
polymer compared t_ the Tg for the control. A
correspondingsystematicIncreasein Ta, indicativeof
crosslinking,was observedfor air cured films.
Thermogravimetricanalysis revealeda slight sacrificein
thermal stabilitywith increasingalkyl length. No
improvementin film toughnesswas observed.
17. Key Words (Sugg_ted by Author(s)) 18. Distribution Statement
polyimides
characterization Unclassified-Unlimited
pendant alkyl groups Subject Category 27
19. Security Oassif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price"
Unclassified Unclassified 12 A02
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Polyimides with pendant alkyl groups

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