Thermoplastic polyimides are a class of promising high temperature polymers for aerospace applications. NASA-developed LARC-TPI is a prominent member of this family of polymers. Its physical characteristics have been measured as a function of its curing schedule. The results and their possible interpretations are discussed

Singh, Jag J.

Stoakley, Diane M.

English

NASA Technical Reports Server

NASA Technical Memorandum 100530 
AN INVESTIGATION OF PHYSICAL PROPERTIES 
OF THERMOPLASTIC POLYIMIDES 
JAG J, SINGH AND DIANE M, STOAKLEY 
(aAS&-TH-l00530) A I IIVESTIGBTIOH OP 
PHYSICAL PROPERTIES OF TBEBHOPLASTIC N88-14151 
POLYIIEIIDES (HASA) 13 p CSCL 110 
U n c l a s  
G3/24 01 I6685 
DECEMBER 1987 
National Aeronautics and 
Space Administration 
Langley Resmrch center 
Hampton, Virginia 23665-5225 
https://ntrs.nasa.gov/search.jsp?R=19880004769 2020-03-20T09:01:07+00:00Z
ABSTRACT 
. 
I -  
Thermoplastic polyimides are a class of promising high 
temperature polymers for aerospace applications. NASA- 
developed LARC-TPI is a prominent member of this family of 
polymers. We have measured its physical characteristics as a 
function of its curing schedule. 
sible interpretations are discussed in this report. 
The results and their pos- 
INTRODUCTION 
Langley Research Center Thermoplastic Polyimide (LARC-TPI) 
is one of the most promising thermoplastic polyimides devel- 
oped at Langley. It has high mechanical strength and retains 
it at elevated temperatures (over 250OC). It also absorbs a 
very low amount of moisture. 
flow properties that are significantly superior to those of 
conventional polyimides, further improvements in its ease of 
processability are desirable to expand its use as a high per- 
formance matrix resin for aerospace applications. 
undertaken a study to determine the effect of the imidization 
cure schedule on the processability and resulting polymer 
properties of LARC-TPI. It is hoped that by varying the cure 
temperatures and processi-ng techniques, the rheological char- 
acteristics of the resins can be optimized. 
in this direction, we have investigated the effects of dif- 
ferent curing temperatures/cycles on their properties. 
Although LARC-TPI possesses 
We have 
As a first step 
The test samples were prepared according to two different 
(No measurements of the molecular weights of cure schedules. 
the final products could, however, be made because of the 
insolubility of the polyimides.) 
in detail for their physical characteristics. The results 
are summarized in the following sections. 
These samples were studied 
EXPERIMENTAL PROCEDURE AND RESULTS 
The test sample discs were prepared according to the fol- 
lowing curing/molding prescriptions. 
(a) Preparation of Thermoplastic Polvimide (TPII Moldinq 
Powders 
LARC-TPI polyamic acid resin, 29.1% solids in 
2-methoxyethyl ether (diglyme), was obtained from Mitsui 
Toatsu Company of Japan. 
percent solids by addition of dimethylacetamide (DMAC). The 
This resin was diluted to 15 weight 
polymer was precipitated by slowly dripping the resin into a 
blender of distilled water. The precipitate was filtered and 
washed three times with additional distilled water. The 
resulting powder was dried in a vacuum oven at room tempera- 
ture overnight. 
The powder was imidized according to two different cure 
schedules: (1) The TPI polyamic acid powder was heated in a 
forced air oven for 1 1/2 hours at 100°C, 1 hour at 18OoC and 
1 hour at 22OoC; (2) The powder was heated as above, with an 
additional curing for 1 hour at 30OoC. 
(b) Preparation of TPI Molded Discs 
Three molded discs each were prepared from the TPI 
powders which had been imidized to 22OoC and 30OoC. For each 
disc, 2.00 grams of the imide powder were placed in a 
2.86 cm diameter steel mold. The molding conditions used for 
the 220°C-cured powder were as follows: 
(a) Maximum Temperature = 349% 
(b) Time at Maximum Temperature = 15 minutes 
(c) Maximum Pressure = 2500 psi 
Molding pressure was increased to 5000 psi for the 30OoC- 
cured powder, the maximum temperature and time at maximum 
temperature remaining the same. 
These conditions yielded 2.86 cm diameter, 0 . 2 4  cm thick 
The increased pressure required to obtain satisfact- discs. 
orily molded discs of the 300°C-cured powder is indicative of 
less melt flow in the higher temperature-cured polyimide. 
X-ray diffraction studies with Cu-K, x-rays showed that both 
types of discs were completely amorphous, i.e., had no mea- 
surable crystallinity in them. Figure l shows the nominal 
chemical structure of LARC-TPI polymer. 
(c) Saturation Moisture Contents 
The Itas receivedtt sample discs were first desiccated 
in a vacuum oven at 100°C. They were then immersed in dis- 
tilled water at 90°C for several weeks until their weights 
stabilized. The saturation moisture uptakes of the various 
discs are summarized in the table below. Within the accuracy 
of the measurements, the average saturation moisture contents 
of the discs are approximately equal. 
- . __ 
No. 
1 
2 
3 
1 
2 
3 
Table I 
Summary of Saturation Moisture Contents 
of the Various LARC-TPI Discs 
---_- -- -. . _ _ _ ~  
Sample Classification 
(22O0C-Cured Samples) 1 
(300°C-Cured Samples) 1 
Saturation Moisture Content 
(Weight %) 
2.94 
2.87 
2.89 
AV = (2.90+0.03) - 
3.14 
2.94 
2.85 
Av = (2.98+0.11) - 
(d) Density Measurements 
The sample densities were calculated on the basis of 
The results are sum- their measured dimensions and weights. 
marized in Table I1 below. 
samples cured at 22OoC is lower than density of the samples 
cured at 30OoC. 
Clearly, the density of the 
Table I1 
Summary of Densities of Various 
LARC-TPI Discs 
-- I Sample Classification 
No* i 
___ -- 
. 
1.112 
1.170 
1.081 
Av = (1.121+0.045) - 
1.328 
1.347 
1.357 
Av = (1.344+0.014) - 
3 
(e) Shear Stress Measurements 
No. 
Short bar shear stress measurements were made for two 
samples of each type. Three small bars were cut from each 
sample and subjected to known loads. The results are summa- 
rized in Table I11 below. The shear stress value for the 
30o0C-cured samples is considerably higher than that for the 
220OC-cured samples. 
Sample Classification Shear Stress (psi) 
1 
2 
(300°C-Cured Samples) 3246+1325 
3 2 95z52 2 I 1 Av = (3271f712) 
Infrared SDectroscopv Measurements 
Fourier transform spectra of the two types of TPI 
powders were measured and are shown in figure 2. Although 
both spectra contain an anhydride peak at -1856 cm'l, the 
greater absorption in the amic acid-OH region 
(3000-3500 cm'l) and the presence of amide-I1 band 
(1540 cm'l in the 220°C-cured powder indicate that the 
lower-temperature-cured TPI powder is not fully imidized. 
The presence of un-imidized polyamic acid in the 22O0C-cured 
powder may have contributed to its increaseed moldability. 
However, the water of further imidization generated during 
the molding process may have resulted in void formation. The 
remaining polyamic acid component would also have contributed 
to lowering the mechanical strength of the 220°C-cured discs. 
. 
Photographs of the fracture surfaces of the two types of 
discs showed small voids in the specimens molded from the 
22O0C-cured powder that were not present in the 300°C-cured 
discs (see figure 3). These voids, of course, would also  
lead to lower densities observed in the discs prepared from 
the 220OC-imidized powder. 
(9) Positron Lifetime Measurements 
Sample Classification 
220°C-Cured Samples 
300°C-Cured Samples 
. 
Positron Lifetime. Measurements 
T1 I1 ' ?2 I2 
(PSI ( % I  (PSI ( % I  
(Short Lifetime Parameters) (Long Lifetime Para- 
meter s) 
1660+200 1.1+0.5 
1628+219 1.2+0.6 
381+2 - - - - 
3 7 4 + 2  - - 
98.9+0.5 
98.8+0.6 - - 
Positron lifetime measurements were made in each 
type of samples, using standard fast-fast coincidence life- 
time measurement techniques. 
as measured with a Co60 source, was approximately 300 pico- 
seconds. Figure 4 shows a typical lifetime spectrum in 
LARC-TPI samples. The lifetime spectra were analyzed using 
the RESOLUTION program(') which automatically calculates the 
lifetimes and the relative intensities of the two components 
after subtracting a flat background from the raw spectrum. 
The results are summarized in Table IV below. 
that the lifetimes (both short and long) are slightly smaller 
in the higher-temperature-cured samples, though the errors on 
the longlife values are too large to permit a definitive 
statement about them. 
The lifetime system resolution, 
It appears 
Table IV 
Summary of Positron Lifetime Measurements 
in LaRC-TPI Samples 
. 
5 
DISCUSSION 
On the basis of the data summarized in Tables 1, 2 and 4, 
it appears that the samples cured at 3OO0C have slightly 
higher saturation moisture content, higher density, but 
approximately comparable positron annihilation characteris- 
tics. Table 3 shows that the bar shear stress value in the 
samples cured at 3OO0C is over six times larger than in the 
220°C-cured samples! The saturation moisture content data, 
though inconclusive, are somewhat surprising in view of the 
results shown in figures 2 and 3. If the lower-temperature- 
cured discs have higher free volume, they are also expected 
to have higher saturation moisture content as well as higher 
positron lifetime. Even though the mean ~2-values in the 
220°C-cured samples are slightly higher than the correspond- 
ing values in the higher-temperature cured samples, the 
errors on the individual values are so large that no definite 
conclusion can be drawn about their true values. 
If we associate higher density and higher strength with 
higher molecular weight, we would again have expected lower 
positron lifetime as well as low saturation moisture content 
in the 300°C-cured samples. Merrigan et a1 ( 2 )  and Chuang et 
a1 ( 3 ,  have reported that the orthopositronium annihilation 
mean life in polymers goes down as the polymer molecular 
weight goes UJ. However, West et d4) report that the mean 
life value remains essentially unchanged when the molecular 
weight of the polymer (polystyrene) changes from lo3 to 2 x 
lo6 amu! Our own measurements in polystyrene also show no 
changes in orthopositronium annihilation rates when the 
molecular weight changes from 19,800 to 860,000 amu. Even 
though we cannot extrapolate the polystyrene data to the 
thermoplastic polyimides, it does appear that the observed 
positron lifetime data are not inconsistent with the assump- 
tion of higher molecular weights for the 300°C-cured samples. 
But the problem of slightly higher saturation moisture con- 
tent of 30O0C-cured discs remains somewhat surprising. It is 
possible that the water diffusion process in these discs is 
aided by an optimal molecular architecture of the 300°C-cured 
samples. 
CONCLUDING REMARKS 
The experimental results reported in this paper show that 
the polyimide samples cured at higher temperature have higher 
density and higher shear strength. Photographs of the frac- 
ture surfaces suggest that the higher temperature-cured discs 
have fewer microvoids and may therefore be expected to have 
lower free volume and correspondingly lower saturation mois- 
ture content. This is not consistent with the observed satu- 
, ’  
I .  
ration moisture contents in the two types of samples. The 
experimentally observed saturation moisture content in the 
220°C-cured samples is (2.90+0.03) w/O as opposed to 
(2.98+0.11) w/o for this 300°C-cured samples. Normally, posi- 
tron lifetime data can shed some light on the issue of free 
volume in polymers. Unfortunately, the probability of posi- 
tronium formation in polyimides is rather low, thereby neces- 
sitating inordinately long times for adequate data statis- 
tics. Such statistics are necessary to definitively resolve 
the issue of relative free volume in polyimides as a function 
of their curing temperatures. 
slightly higher saturation moisture content in the 30OoC- 
cured samples is due to its altered chemical architecture 
which allows additional water molecule bond sites on the 
polymer chains. 
appear that the polyimide discs cured at higher temperatures 
may well have higher molecular weights. 
It is possible that the 
In view of the foregoing discussion it would 
REFERENCES 
1. J. J. Singh, W. H. Holt and W. Mock, Jr.: Bulletin of 
American Physical Society (BAPS), Vol. 30, 4 2 ,  1985. 
2. J. A. Merrigan, J. H. Green and S. J. Tao: in Physical 
Methods of Chemistry, Pt. I11 D, Chapter VIII, edited by A. 
Weissberger and B. W. Rossiter. (Wiley Interscience, New 
York, 1972). 
3. S. Y. Chuang and S. J. Tao: J. Appl. Phys.; 44, 5171, 
1973. 
4 .  D. H. D. West, V. J. McBrierty and C .  F. G .  Delaney: 
Appl. Phys.; z, 171, 1975. 
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Report Documentation Page 
1. Report No. 
NASA TM- 100530 
2. Government Accession No. 
7. Author(s1 
Jag 3. Singh and Diane M. Stoakley 
19. Security Classif. (of this report) 
Unc lass i f ied  
9. Performing Organization Name and Address 
20. Security Classif. (of this page) 
Unc lass i f i ed  
NASA Langley Research Center 
Hampton, VA 23665-5225 
12. Sponsoring Agency Name and Address 
Nat ional  Aeronautics and Space Admin is t ra t ion  
Washington, DC 20546 
15. Supplementary Notes 
3. Recipient's Catalog No. 
5. Report Date 
December 1987 
6. Performing Organization Code 
8. Performing Organization Report No. 
10. Work Unit No. 
505-63-01 -01 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Techn i ca 1 Memorandum 
14. Sponsoring Agency Code 
6. Abstract 
Thermoplastic polyimides a re  a c lass  o f  promising h igh  temperature 
polymers f o r  aerospace app l ica t ions .  
member o f  t h i s  fam i l y  o f  polymers. We have measured i t s  phys ica l  c h a r a c t e r i s t i c s  
as a func t i on  of i t s  cur  ng schedule. 
p re ta t i ons  are  discussed i n  t h i s  repor t .  
NASA-developed LARC-TPI i s  a prominent 
The r e s u l t s  and t h e i r  poss ib le  i n t e r -  
17. Key Words (Suggested by Author(s)) I 18. Distribution Statement 
UNLIMITED - UNCLASSIFIED Thermoplastic Polyimides, Curing 
Temperature, Molecular Weight, I n f r a r e d  
Spectroscopy, Posi t ron L i fe t ime 
Subject Category 24 
21. No. of pages 22. Price 
E A  F O R M l a  OCT 86 


An investigation of physical properties of thermoplastic polyimides

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