Resins for Aerospace - American Chemical Society

16 Molecular Composites

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 26, 2015 | http://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch016

Rodlike Polymer Reinforcing an Amorphous Polymer Matrix G. HUSMAN, T. HELMINIAK, and W. ADAMS Air Force Materials Laboratory, Nonmetallic Materials Division, Wright-Patterson Air Force Base, OH 45433 D. WIFF and C. BENNER University of Dayton Research Institute, Dayton, OH 45469 Recent developments i n the synthesis of rodlike aromatic heterocyclic polymers have generated a great amount of interest i n the development of these polymers as structural materials. A large effort i s currently being expended to characterize these polymers and to develop them into useful product forms, such as fibers, films or sheets. The A i r Force Materials Laboratory and the A i r Force Office of S c i e n t i f i c Research are engaged i n a research and development program directed toward the preparation and processing of very high strength, environmentally resistant polymers for use as structural materials i n aerospace vehicles. The objective i s the attainment of mechanical properties for a structural material comparable with those currently obtained with fiber reinforced composites, but with s i g n i f i c a n t l y higher environmental resistance and without the use of a fiber reinforcement. The materials chosen for this effort are the r i g i d rod, extended chain, aromatic-heterocyclic polymers whose physical and chemical properties show promise for achievement of the program objectives. However, these materials present special processing problems because of the extended chain, r i g i d rod structural character of molecules. Present processing requires strong mineral or organic acid solvents and there is little opportunity to influence the polymer morphology once the material i s i n the s o l i d state. One potential concept for the u t i l i z a t i o n of the rodlike polymers i s molecular composites. This concept consists of blending a rodlike aromatic heterocyclic polymer with a c o i l l i k e aromatic heterocyclic polymer. The intent i s to reinforce the c o i l - l i k e or amorphous polymer with the rodlike polymer, thus forming a composite on the molecular l e v e l analogous to chopped fiber reinforced composites. The subject of this paper i s a study to demonstrate the f e a s i b i l i t y of this concept. Materials and Processing A variety of rodlike and amorphous polymers were studied i n this investigation. The chemical structures of the various This chapter not subject to U.S. copyright. Published 1980 American Chemical Society In Resins for Aerospace; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

204

RESINS

FOR

AEROSPACE

TABLE I POLYMERS STUDIED CHEMICAL STRUCTURE

ACRONYM


Rod-Like Polymers

PDIAB

ι H

5 - © ~

PBO

PBT

Coil-Like

Polymers

-Ki§r©cH» I H

M-PBI

H

AB-PBI

PEPBO

3.

PPBT

In Resins for Aerospace; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


16.

HUSMAN E T A L .

Molecular

Composites

205

p o l y m e r s a r e shown i n T a b l e I . The p o l y m e r b l e n d s s t u d i e d a n d t h e i r weight percents a r e l i s t e d i n Table I I . The p o l y m e r b l e n d s s t u d i e d w e r e p r o c e s s e d a s t h i n f i l m s b y vacuum c a s t i n g f r o m d i l u t e s o l u t i o n s . The g e n e r a l p r o c e d u r e f o l l o w e d was t o p r e p a r e a 1-2% p o l y m e r s o l u t i o n i n methane s u l f o n i c a c i d and p u t t h e s o l u t i o n i n a s p e c i a l l y f a b r i c a t e d c i r c u l a r f l a t bottomed c a s t i n g d i s h . The d i s h was t h e n p l a c e d a n d l e v e l e d i n t h e b o t t o m o f a s u b l i m a t o r . The c o l d f i n g e r o f t h e s u b l i m a t o r was m a i n t a i n e d a t 25°C a n d t h e s u b l i m a t o r was c o n t i n u o u s l y e v a c u a t e d a n d h e a t e d t o 60°C t o f a c i l i t a t e t h e r e m o v a l o f t h e methane s u l f o n i c a c i d . A f t e r t h e f i l m s were formed and removed f r o m t h e c a s t i n g d i s h , t h e y w e r e g e n e r a l l y d r i e d a t 100°C i n a vacuum o v e n f o r 24 t o 48 h o u r s . The f i l m s p r o d u c e d w e r e a p p r o x i m a t e l y 5 cm i n d i a m e t e r a n d v a r i e d f r o m 1.3 χ 10 t o 16.5 χ 10~^cm i n t h i c k n e s s . M o s t o f t h e f i l m s r e t a i n e d a p p r o x i m a t e l y 20 t o 3 0 % r e s i d u a l s o l v e n t . Specimen P r e p a r a t i o n and T e s t i n g The f i l m s w e r e c u t w i t h a r a z o r b l a d e i n t o .635 cm s t r i p s . S t r i p s a t l e a s t 2.54 cm i n l e n g t h were u s e d f o r t e s t i n g , w h i l e s h o r t e r p i e c e s were used f o r a s c a s t m o r p h o l o g i c a l s t u d i e s . T e s t s were p e r f o r m e d o n a n I n s t r o n u n i v e r s a l t e s t m a c h i n e a t a c r o s s h e a d s p e e d o f .02 i n c h e s p e r m i n u t e . A f t e r i n i t i a l s p e c i m e n b r e a k s o c c u r r e d , r e m a i n i n g p i e c e s were r e t e s t e d u n t i l t h e l e n g t h became t o o s h o r t t o r e a s o n a b l y g r i p a n d t e s t ( a p p r o x i m a t e l y 1.5 cm). This provided not only as-cast data, but a l s o mechanically stretched data. I n a d d i t i o n , some s p e c i m e n s were p l a s t i c i z e d w i t h m e t h a n o l t o p e r m i t l a r g e r amounts o f s t r e t c h i n g . After t h e s e specimens were d r i e d o f t h e m e t h a n o l , they were mechani c a l l y t e s t e d t o determine t h e e f f e c t o f the s t r e t c h i n g . The r e s u l t s o f t h e t e s t i n g o f t h e AB-PBI/PDIAB b l e n d s a r e p r e s e n t e d i n T a b l e I I I . T h e s e b l e n d s w e r e s t u d i e d i n t h e most d e t a i l b e c a u s e t h e y a p p e a r e d t o be t h e most c o m p a t i b l e p o l y m e r b l e n d s a n d gave t h e most i n t e r e s t i n g r e s u l t s . Several general o b s e r v a t i o n s c a n b e made a b o u t t h e d a t a . I n t h e a s - c a s t (no s t r e t c h i n g ) polymer b l e n d s , t h e r o d - l i k e polymer appears t o a c t more a s a f i l l e r t h a n a r e i n f o r c e m e n t . However, s t r e t c h i n g ( b o t h m e c h a n i c a l a n d s o l v e n t ) a p p e a r s t o p r o v i d e some o r i e n t a t i o n a n d demonstrates a r e a l r e i n f o r c i n g e f f e c t ( s t r e n g t h and modulus i n creases). The r e i n f o r c i n g e f f e c t , h o w e v e r , d o e s n o t f o l l o w a r u l e - o f - m i x t u r e s b e h a v i o r , t h e 10 p e r c e n t b l e n d b e i n g p r o p o r t i o n a l l y b e t t e r t h a n t h e 20 p e r c e n t o r 30 p e r c e n t b l e n d s . The 57 p e r c e n t a n d 75 p e r c e n t b l e n d s c o u l d n o t b e s t r e t c h e d b e c a u s e o f t h e i r l o w s t r a i n - t o - f a i l u r e a n d no s i g n i f i c a n t r e i n f o r c i n g was observed. S i m i l a r t r e n d s were observed w i t h o t h e r polymer b l e n d s s t u d i e d ; h o w e v e r , t h e d a t a p r e s e n t e d r e p r e s e n t s t h e most s i g n i ficant results.


RESINS FOR A E R O S P A C E


206

TABLE I I POLYMER BLENDS STUDIED Weight P e r c e n t s o f Reinforeement Reinforcements

Matrix 1. 2. 3. 4. 5.

M-PBI AB-PBI PPBT PEPBO AB-PBI

PDIAB PDIAB PBT PBO PBO

o, o, o, o, 0,

20, 10, 25, 25, 10,

50, 20, 50, 50, 20,

75 30, 57, 75 6 0 , 75 30

TABLE I I I MECHANICAL PROPERTIES - AB-PBI/PDIAB BLENDS % ROD

MODULUS

POLYMER

STRETCH/ % AREA REDUCTION

0 0 0 10 10 10 20 20 20 30 30 30 57 75

None Mech./20 Solvent/60 None Mech./37 Solvent/57 None Mech./37 Solvent/70 None Mech./20 Solvent/60 None None

1.03 2.00 3.37 2.00 4.60 6.86 1.58 2.38 7.17 1.25 2.16 8.96 1.34 1.51

(G P a )

STRENGTH (M P a ) 79.92 134.36 105.42 70.28 161.92 315.56 44.10 82.68 253.55 36.52 71.66 189.48 28.73 22.32


STRAIN ( % ) 98 43 12 46 28 14 26 15 9 14 22 4 5 4

16.

HUSMAN E T A L .

Molecular

Composites

207


Morphological Studies The m o r p h o l o g y o f t h e f i l m s h a s b e e n s t u d i e d u s i n g s c a n n i n g e l e c t r o n microscopy and x-ray d i f f r a c t i o n . A c a r e f u l study o f SEM p h o t o g r a p h s e x p l a i n s many o f t h e o b s e r v e d t e s t r e s u l t s . Fig u r e s 1-4 show SEM p h o t o g r a p h s o f t h e s u r f a c e s o f t h e 0%, 1 0 % , 20%, a n d 3 0 % r o d f i l m s r e s p e c t i v e l y . The m o r p h o l o g i c a l c h a n g e s are obvious. F i g u r e s 5-8 show edge v i e w s o f l i q u i d n i t r o g e n f r a c t u r e s o f t h e same f o u r f i l m s . S e v e r a l o b s e r v a t i o n s have been made f r o m t h e s t u d y o f t h e SEM p h o t o g r a p h s . A second phase o r conglomerate i s p r e s e n t i n t h e b l e n d s . These conglomerates i n t h e a s - c a s t ( n o t s t r e t c h e d ) f i l m s a p p e a r t o be s y m m e t r i c a n d s a u c e r shaped w i t h a n a s p e c t r a t i o ( l e n g t h / t h i c k n e s s ) o f 2-3. The a b s o l u t e s i z e o f the conglomerates i n c r e a s e s w i t h i n c r e a s i n g rod con tent. The v o l u m e c o n t e n t o f t h e c o n g l o m e r a t e s i n t h e f i l m i s g r e a t e r than the volume c o n t e n t o f r o d - l i k e polymer, i n d i c a t i n g t h a t t h e c o n g l o m e r a t e s c o n t a i n b o t h r o d - l i k e p o l y m e r a n d amorphous polymer. S t r e t c h i n g t h e f i l m s changes t h e shape o f t h e conglom e r a t e s , i n c r e a s i n g t h e l e n g t h and d e c r e a s i n g t h e w i d t h and t h i c k ness. T h i s c a n b e s e e n i n F i g u r e s 9 a n d 10 w h i c h show edges p e r p e n d i c u l a r t o and p a r a l l e l t o the s t r e t c h d i r e c t i o n o f a 30% r o d , solvent stretched film. A t a b u l a t i o n o f some o f t h e m o r p h o l o g i c a l phenomena i s p r e s e n t e d i n T a b l e I V . A l t h o u g h t h e measurements made w e r e r e l a t i v e l y c r u d e , t h e m a g n i t u d e s o f t h e measurements a n d p h e n o m e n o l o g i c a l t r e n d s c a n b e d e r i v e d f r o m t h i s d a t a . One i m p o r t a n t o b s e r v a t i o n i s t h a t volume c a l c u l a t i o n s i n d i c a t e t h a t the percent r o d l i k e p o l y m e r i n t h e c o n g l o m e r a t e s i s c o n s t a n t , a p p r o x i m a t e l y 57%. T h i s assumes t h a t a l l o f t h e r o d i s i n t h e s e c o n d p h a s e . I f t h i s i s t r u e , i t i n d i c a t e s t h a t 57% may b e a n e q u i l i b r i u m m i x c o n d i t i o n f o r t h e two p o l y m e r s . To v e r i f y t h i s , a 57% PDIAB/43% A B - P B I f i l m was p r e p a r e d . A s c a n b e s e e n i n F i g u r e 1 1 , no s e c o n d p h a s e was observed. R e s u l t s o f the x-ray d i f f r a c t i o n s t u d i e s are s t i l l b e i n g a n a l y z e d ; however, d e f i n i t e s i g n s o f o r d e r i n g and o r i e n t a t i o n have been observed. Analysis In order t o develop a b e t t e r understanding o f the r e s u l t s obtained, a n a l y s i s procedures developed f o r composite m a t e r i a l s were u t i l i z e d f o r d e t e r m i n i n g the e f f e c t i v e m o d u l i o f the conglom erates. The H a l p i n - T s a i e q u a t i o n s ( 1 ) u s e d a r e shown b e l o w : Halpin-Tsai

_JL *Ά

=

Equations

(1 + ζ

η

(1 - η

v ) f

v ) f


RESINS F O R A E R O S P A C E


208

Figure 1.

100% AB-PBI surface (1500X)

Figure 2.

9 0 % AB-PBI/10% surface (ISOOX)

PDIAB

Figure 3.

8 0 % AB-PBI/20% surface (1500X)

PDIAB

Figure 4.

70% ΑΒ-ΡΒΙ/30Ψο surface (1500χ)

PDIAB



HUSMAN E T A L .

Molecuhr

209

Composites

Figure 5.

100% AB-PBI

edge

(2000χ)

Figure 6.

90%> AB-PB1/10% edge(2000x)

PDIAB

Figure 7.

80% AB-PBÎ/20% edge(2000X)

PDIAB

Figure 8.

70% ΑΒ-ΡΒΙ/30Ψο edge(2000X)

PDIAB




210

Figure 9. 70Ψο AB-PBI/30% solvent-stretched edge ± to (2500X)

PDIAB stretch

Figure 10. 70% AB-PBI/30% solvent-stretched edge || to (2500X)

PDIAB stretch

TABLE I V MORPHOLOGY OF AB-PBI/PDIAB BLENDS Conglomerate

90/10

80/20

70/30

1.78 2.54 3.81

3.81 5.08 5.08

5.84 7.62 6.35

.635 .508 .254

1.27 .889 .508

1.91 1.27 .762

1.78 1.52 1.02

3.81 3.30 1.14

5.84 5.08 1.27

4

Length (cm χ 10 ) Initial Mechanically Stretched Solvent Stretched -4 Thickness (cm χ 10 ) Initial Mechanical Stretched Solvent Stretched Width (cm χ 1 θ " ) Initial Mechanically Stretched Solvent Stretched 4

Aspect Ratio (L/T) Initial Mechanically Stretched Solvent Stretched

2.8 5.0 15

3.0 5.7 10

3.1 6.0 8

Volume % i n F i l m

18%

35%

53%

Volume % i n Conglomerate

56%

57%

57%


16.

HUSMAN E T A L .

Molecular

(

E

Composites

f

-

211

1)

η = _Jh ( E

f

+

ζ)


Λ where, Ε E^

= Composite

o r f i l m modulus.

= C o r r e s p o n d i n g m a t r i x modulus. = Volume f r a c t i o n o f r e i n f o r c e m e n t .

E^

= Corresponding e f f e c t i v e r e i n f o r c e m e n t modulus.

ζ

= Measure o f r e i n f o r c e m e n t dependent on boundary conditions ( f o r these c a l c u l a t i o n s , taken as ζ = 2 ( a / ) . b

The r e s u l t s o f t h e a n a l y s i s a r e shown i n T a b l e V. S e v e r a l o b s e r v a t i o n s c a n be made f r o m t h e s e c a l c u l a t i o n s . The h i g h e r e f f e c t i v e modulus o f t h e conglomerates i n t h e s t r e t c h e d 10% r o d f i l m i n d i c a t e s a h i g h e r degree o f o r i e n t a t i o n i n these conglomerates. T h i s i n c r e a s e d o r i e n t a t i o n and c o r r e s p o n d i n g h i g h e r a s p e c t r a t i o of t h e s e conglomerates a c c o u n t s f o r t h e h i g h e r r e l a t i v e modulus observed i n t h e 10% r o d f i l m s . The m a g n i t u d e s o f t h e c o n g l o m e r a t e m o d u l i a r e h i g h i n d i c a t i n g r e l a t i v e l y good t r a n s l a t i o n o f r o d properties. E v e n i n t h e most h i g h l y s t r e t c h e d 1 0 % r o d f i l m , t h e conglomerate aspect r a t i o and degree o f o r i e n t a t i o n a r e n o t nearly sufficient to obtain desired properties. Therefore, the conglomeration i s c o n s i d e r e d u n d e s i r a b l e and complete d i s p e r s i o n o f t h e r o d s p l u s t h e a b i l i t y t o o r i e n t them i n t h e f i l m i s t h e desired goal.

Processing Studies In an attempt t o a c h i e v e d i s p e r s i o n o f t h e r o d s , v a r i o u s p r o c e s s i n g t e c h n i q u e s were s t u d i e d . I n s t e a d o f vacuum c a s t i n g the f i l m s , a technique o f p r e c i p i t a t i n g from d i l u t e s o l u t i o n i n a h i g h h u m i d i t y e n v i r o n m e n t was d e v e l o p e d . A 1 0 % PDIAB/90% AB-PBI f i l m made b y t h i s p r o c e s s i s shown i n F i g u r e 1 2 . A s c a n b e s e e n , no v i s i b l e s e c o n d p h a s e i s p r e s e n t . M e c h a n i c a l p r o p e r t i e s o f t h i s f i l m a r e g i v e n i n T a b l e V I . The p r o p e r t i e s a r e much b e t t e r t h a n t h o s e o b t a i n e d f r o m t h e vacuum c a s t f i l m s i n d i c a t i n g e x c e l lent translation of rod properties.




212

Figure 11.

43% ΑΒ-ΡΒΙ/57Ψο edge (1500X)

PDIAB

TABLE V CALCULATED CONGLOMERATE MODULUS - AB-PBI/PDIAB BLENDS 90/10

AB-PBI/PDIAB ( I n i t i a l )

Ê

f

- 16.15 G P a

90/10

AB-PBI/PDIAB (Mech.

Ë

f

- 33.35 G P a

90/10

AB-PBI/PDIAB ( S o l v e n t

Stretched)

80/20


Stretched)

70/30


Stretched)

Stretched)

\ \ ï

f

«

=

26.23 G P a 15.43 G P a

- 14.94 G P a



16.

HUSMAN ET A L .

Molecular

Composites

213

Figure 12. 90 % AB-PBI/ΙΟΨο PDIAB water-precipitated edge (1500χ)

TABLE V I

STRETCH % AREA REDUCTION

MECHANICAL PROPERTIES - 9 0 % A B - P B I / 1 0 % PDIAB (PRECIPITATED) STRAIN STRENGTH MODULUS (G P a )

(M P a )

3.08

92.39

15

Mech./5

4.00

122.09

13

Solvent/55

9.65

243.95

3

None


(%)

RESINS FOR

214

AEROSPACE


Summary The c o n c e p t o f m o l e c u l a r c o m p o s i t e s has b e e n d e m o n s t r a t e d . Although the s t u d i e s to date are v e r y p r e l i m i n a r y , the concept a p p e a r s t o be v e r y p r o m i s i n g . F u t u r e w o r k w i l l be o r i e n t e d t o w a r d c h a r a c t e r i z i n g t h e s o l u t i o n b e h a v i o r o f t h e p o l y m e r b l e n d s and d e v e l o p i n g p r o c e s s i n g t e c h n i q u e s t o a c h i e v e b e t t e r morphology cont r o l as w e l l as o r i e n t a t i o n c o n t r o l . Literature Cited 1. H a l p i n , J. C., Composite Materials RECEIVED

February

6,

Tsai, S. W., " E n v i r o n m e n t a l D e s i g n , " AFML-TR-67-423.

Factors

in

1980.


Resins for Aerospace - American Chemical Society

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