Recent Advances in Organosiloxane Copolymers - ACS Symposium

Chapter 14

Recent Advances in Organosiloxane Copolymers 1

2

J. D. Summers, C. S. Elsbernd, P. M. Sormani , P. J. A. Brandt , C. A. Arnold, I. Yilgor , J. S. Riffle, S. Kilic, and J. E. McGrath

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: January 7, 1988 | doi: 10.1021/bk-1988-0360.ch014

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Department of Chemistry and Polymer Materials and Interfaces Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Organosiloxane copolymers have been of great interest over the past several years due to the somewhat unique c h a r a c t e r i s t i c s imparted by the siloxane blocks. The excellent UV and oxidative resistance of these structures together with t h e i r outstanding thermal s t a b i l i t y and wide temperature use range make them candidates as modifiers for a multitude of applications. Organosiloxane materials have now been prepared which demonstrate a variety of interesting properties such as atomic oxygen resistance, biocompatibility, high gas permeabilities, and hydrophobic surfaces. In t h i s paper, the synthesis and properties of functional polysiloxane oligomers which are the precursors for the copolymers are discussed. A d d i t i o n a l l y , a summary and review of the preparation and c h a r a c t e r i s t i c s of the organosiloxane copolymers is given. The general theme of our research focuses on multiphase copolymer systems obtained v i a l i v i n g polymers, engineering polymers (both thermoplastics and thermosets) and polysiloxane systems. Organosiloxane copolymers have been of i n t e r e s t f o r a number of years due to the unusual c h a r a c t e r i s t i c s of polysiloxanes, such as their thermal and UV s t a b i l i t y , low glass t r a n s i t i o n temperature, very high gas permeability and, e s p e c i a l l y , their low surface energy character i s t i c s ( 1 - 2 ) . In the polydimethylsiloxane chain, 1, many workers have demonstrated that the methyl groups are oriented toward the a i r or vacuum surface. This provides a hydrophobic, low surface energy c h a r a c t e r i s t i c , which has a number of secondary but important applications including enhanced biocompatibility and more recently, resistance to atomic oxygen and oxygen plasmas. Our current work on 1

Current address: Experimental Station, Ε. I. du Pont de Nemours and Company, Wilmington, DE 19898 Current address: Specialty Chemical Division, 3M Center, St. Paul, MN 55144 Current address: Mercor, Inc., Berkeley, CA 94710 Correspondence should be addressed to this author. 0097-6156/88/0360-0180$06.00/0 © 1988 American Chemical Society

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Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Organosiloxane Copolymers

siloxane chemistry includes the preparation of functional oligomers both by e q u i l i b r a t i o n processes employing the commonly available c y c l i c tetramer, Ό^, and to some extent, l i v i n g polymerizations whic u t i l i z e the lithium siloxanolate i n i t i a t e d polymerization of the c y c l i c trimer, D (1-4). The novel oligomers are then incorporated into high performance block, segmented and g r a f t copolymers. 3

~Si-0'

I CH„


Functional Polysiloxane Oligomers For e q u i l i b r a t i o n processes, one must synthesize both oligomers and what are termed dimers, or disiloxanes. Our primary i n t e r e s t is in the u t i l i z a t i o n of these functional oligomers for the synthesis of both l i n e a r block or segmented copolymers, and also surface modified, oughened networks such as the epoxy and imide systems (3-27). The generalized structure of the oligomers of i n t e r e s t is shown in Scheme 1.

CH.

Y

I I

CH,

I

X-R-Si-0-(-Si-0-)-Si-R-X CH

Y

3

CH

Variables:

X, R, Υ, η

3

Scheme 1. General Structure of an α,ω-Difunctional Polysiloxane.

The o v e r a l l synthesis of f u n c t i o n a l l y terminated oligomers involves e q u i l i b r a t i o n of the c y c l i c tetramer in the presence of a functional disiloxane as i l l u s t r a t e d in Scheme 2. In this case, one u t i l i z e s a c a t a l y s t which can be e i t h e r an a c i d i c or basic moiety. A basic c a t a l y s t (for example) attacks the s i l i c o n of the tetramer and i n i t i a t e s the ring-opening polymerization. However, in the presence of the disiloxane, the polymerization undergoes what is e f f e c t i v e l y a very useful chain transfer reaction. This occurs because, l i k e the monomer and growing chain, the silicon-oxygen-silicon bond of the disiloxane is s u f f i c i e n t l y polar that i t is attacked by the active ionic species and, thus, takes part in the e q u i l i b r a t i o n process. On the other hand, one takes advantage of the fact that the s i l i c o n a l k y l (or aryl) bond is more covalent in nature and, thus, stable during the polymerization. The e q u i l i b r a t i o n process is generalized in Scheme 3 wherein D " refers to the c y c l i c , tetrameric s t a r t i n g material and "MM" denotes a disiloxane with a terminal group associated with each "M". This shorthand nomenclature system is one t r a d i t i o n a l l y u t i l i z e d by siloxane chemists. As one can note from Scheme 3, the dimer w i l l be incorporated into the chain and the chain ends w i l l be i d e n t i c a l with the terminal units of "MM" or, as we have c a l l e d i t , the disiloxane. This entire concept is well-known and, in n


182

INORGANIC AND ORGANOMETALLIC POLYMERS

I 4-Si-O-}I Y +

"Cyclics" χ = 3 or 4 Y = CH , C H , CH = CH , or CH CH CF

X

CH.

!

CH

I

3

3

6

5

2

2

2

3

0

3

X-R-Si-O-Si-R-X II CH CH

"End-Blocker" R = a l k y l , a r y l , aralkyl or a chemical bond X = -NH , -N(CH ) r "OH, " C l , 2


Catalyst, heat

3

2

Λ -CH-CH

|

2

CH.

Y

CH

I I

I

X-R-Si-O-f-Si-O-4—Si-R-X

I C H

I Y

3

Ί C H

3

+

Scheme 2.

Cyclics General Synthesis of "End-Blocked" Polysiloxanes v i a E q u i l i b r a t i o n Processes.

x D

(x + 4)

4

+ MM

• MD M χ

χ cat. MD M + MD M x y Scheme 3.

• MD.

, .M + MD. .M (x+w) (y-w)

E q u i l i b r a t i o n Polymerization Processes (Shown u t i l i z i n g base catalysis)*

f a c t , methyl terminated polydimethylsiloxane f l u i d s and o i l s have been prepared in this way f o r some time. However, the additional feature which we have focused on is to design the endgroups to be organofunctional. Many oligomers with various f u n c t i o n a l i t i e s have been prepared in our laboratory ( 4 ) . However, the amino terminated species has been studied most extensively due to i t s wide u t i l i t y as a component of a large number of segmented copolymers (e.g. imides, amides, ureas, e t c . ) . In order to prepare functional oligomers of t h i s type, one must f i r s t prepare the disiloxane. One route to t h i s was pointed out some years ago by Saam and Speier (28). They showed that i t was possible to react allylamine with a protecting reagent such as hexa-



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methyldisilazane to produce the protected amine material, plus ammonia which could be removed by d i s t i l l a t i o n . The protected allyamine group undergoes h y d r o s i l y l a t i o n with dimethylchlorosilane in the presence of various platinum c a t a l y s t s (such as c h l o r o p l a t i n i c acid) to produce mostly the chlorosilane intermediate. Purification of t h i s material by vacuum d i s t i l l a t i o n is desirable. In order to prepare the disiloxane, one may hydrolyze the chlorosilane to produce f i r s t a s i l a n o l , which r e a d i l y undergoes dehydration to produce a stable siloxane bond. At the same time, water is able to e a s i l y hydrolyze the l a b i l e s i l i c o n - n i t r o g e n bond to regenerate the desired primary amine group. Many other methods have been reported for the preparation of analogous disiloxanes. For example, unsaturated n i t r i l e s can be hydrosilylated and subsequently reduced to produce an amine group. A l t e r n a t i v e l y , allylamine can also be protected v i a imine formation. The general area is of great interest now since i t has been demonstrated that the available l i n e a r siloxane segmented copolymers have a variety of interesting properties. A number of other reactive endgroups have been prepared such as secondary amines, aromatic amines, silylamines, phenolic hydroxyls, epoxides, alkenes, and silanes (4). In addition to the dimethylsiloxane repeat unit in the backbone, i t is possible to produce a number of other important structures, such as methyl-vinyl, diphenyl, trifluoropropyl-methyl, and methyl-silane. A l l of these have been demonstrated to be useful for various reasons in our laboratory and elsewhere. The synthetic process for oligomeric species is to react the aminopropyl-functional disiloxane with the c y c l i c tetramer as i l l u s t r a t e d in Scheme 4. (CH )

I CH

II

CH 3

1

CH

2

1

CH

s

2

Si

/

H-NH-CH.-^Si-O-Si-fCH-^r-NH- + (CH ) - S i 2 23. ι 2 3 2 0

3

\

0 \«.' Si

Si"(CH )

I { C E

(DSX)

CH.

I J

3

+ J catalyst heat CH.

I J

0

CH Θ Θ Θ Θ

3

( D

4

)

3

H N-fCH ) '•( "Si-0-^-Si-fCH -hr-NH 2 2 3 η Ζ ό Ζ 0

]

3 2

CH

o

0

+

Cyclics

3

C y c l i c s are removed v i a vacuum stripping Catalyst must be neutralized for maximum s t a b i l i t y Mn is controlled at equilibrium v i a i n i t i a l [D 1/[DSX] r a t i o Other endgroups possible with appropriate DSX and catalyst 4

Scheme 4.

Synthesis of Difunctional Bis-(Aminopropyl) Polydimethylsiloxane Via An E q u i l i b r a t i o n Polymerization Process,



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Here one may choose a suitable catalyst, t y p i c a l l y a potassium or tetramethylammonium siloxanolate. The process then goes through a series of e q u i l i b r a t i o n steps (as discussed e a r l i e r ) to produce the ring-chain equilibrium between the linear oligomer and the c y c l i c species. C y c l i c species are predominantly the tetrameric c y c l i c , which can be removed by vacuum d i s t i l l a t i o n . In the case of the preferred quaternary ammonium catalysts, one may decompose the catalyst by simply heating the reaction mixture b r i e f l y to about 150°C. This w i l l deactivate the catalyst, as indeed was pointed out a number of years ago by G i l b e r t and Kantor (29). E f f e c t i v e l y , i t is possible to obtain a stable, l i n e a r , functionalized oligomer free of c y c l i c s . This can be v e r i f i e d v i a chromatography (e.g. GPC and/or HPLC). In the case of a potassium siloxanolate catalyst, one must neutralize the equilibrated product (for example, with ion exchange systems) to achieve maximum s t a b i l i t y . We have already demonstrated that i t is possible to synthesize a variety of molecular weights. The number average molecular weight at equilibrium is governed by the i n i t i a l molar r a t i o of the c y c l i c tetramer to the disiloxane. In general, the DSX incorporates more readily using the quaternary ammonium catalyst. Some c h a r a c t e r i s t i c s of aminopropyl terminated polydimethylsiloxane oligomers are i l l u s t r a t e d in Scheme 5.

Mn (q/mole) 600 1000 1800 2400 3800 CH. i 3 H

2 N

Tq°C

6 11 22 30 50

-115 -118 -121 -123 -123

CHι 3

-eCH t3-f-Si-0-^Si-eCH +3-NH 2

2

CH

Scheme 5.

η

3

CH

2

3

Characteristics of Aminopropyl Terminated Polydimethylsiloxane Oligomers Synthesized by a Base Catalyzed E q u i l i b r a t i o n Process,

The number average molecular weights of the oligomers can be obtained yia t i t r a t i o n of the amine endgroups. They can also be estimated by H-NMR up to reasonably high molecular weights, by comparing the integrated r a t i o of the methylene protons at the end of the chain to the s i l i c o n methyls in the backbone. The t i t r a t i o n values for the endgroups and the number average molecular weight determined by proton NMR agree quite nicely in the case of the polydimethylsiloxane system described bearing α,ω-aminopropyl groups. Hydroxyalkyl terminated oligomers are not so simple. Problems can occur i f the hydroxyl group interacts with the growing species to produce a silicon-oxygen-carbon bond. In this case, one may f i n d e f f e c t i v e l y


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lower -OH concentrations than anticipated. In f a c t , i t may be necessary to also block the hydroxyl group during the e q u i l i b r a t i o n .


Organosiloxane

Copolymers

During the past few years (4, 15-27, 30-36), we have prepared block or segmented copolymers from a variety of hard segments. In p a r t i c u l a r , we have focused on polyureas, polyurethanes, polyamides and polyimides. In addition, we have u t i l i z e d these oligomers to modify epoxy networks. Thus, the e q u i l i b r a t i o n procedure works extremely well from a synthetic point of view. Although r e l a t i v e l y l i t t l e (1,7,37) has been reported with respect to the detailed k i n e t i c s and mechanisms involved in these o v e r a l l processes, we have begun to explore this important aspect and some of our results have been recently reported (38,39). In this paper, a review of the u t i l i z a t i o n of polysiloxane oligomers for the preparation of soluble, high performance segmented polyimide-siloxane copolymers and various elastomeric copolymers w i l l be included. Much of the early e f f o r t on siloxane systems has been summarized in reference 9. In the synthesis of organosiloxane copolymers i t should be noted that one has the choice of introducing either a silicon-oxygen-carbon link or a silicon-carbon l i n k between the two d i s s i m i l a r segments. In small molecules, quite d i s t i n c t l y d i f f e r e n t hydrolytic s t a b i l i t i e s are observed. For example, one often uses silicon-oxygen-carbon bonds as protecting groups which are stable under anhydrous neutral or basic conditions, but which quickly revert when treated with aqueous a c i d i c environments. The benefit of the silicon-carbon bond is that i t is s i g n i f i c a n t l y more h y d r o l y t i c a l l y stable than the silicon-oxygen-carbon bond. However, Noshay, Matzner and coworkers (9) showed that the silicon-oxygen-carbon bond in hydrophobic high molecular weight copolymers was much more stable than would have been predicted from small molecule considerations. Apparently the hydrophobic environment protects the p o t e n t i a l l y hydrolyzable group. The chemistry for synthesizing functional polysiloxane oligomers appropriate for producing block copolymers with silicon-carbon links between the blocks has been reviewed e a r l i e r in this monograph. Representative structures have been depicted in Scheme 1. One generally prepares the Si-C linked f u n c t i o n a l i t i e s by conducting h y d r o s i l y l a t i o n reactions between Si-Η groups and the corresponding unsaturated, functional endgroup. In some cases, as previously described, the endgroup must either f i r s t be protected or subsequently further reacted after h y d r o s i l y l a t i o n , in order to f i n a l l y produce the desired functionality. Polysiloxanes, 2, appropriate for producing Si-O-C units between the blocks in copolymers are shown below. The copolymerization reaction involves 0 II

X = N(CH ) , OCH CH , CI, 0"C-CH 3

2

2

3

3

2 Y = CHl

CH

3

Y

CH, 3

3'

CH=CH2' CH CH CF 2

2

3


186


attack of a nucleophile on the terminal s i l i c o n atom of the siloxane with either concerted or subsequent elimination of the functional group "X". Noshay and coworkers developed an interesting route for the preparation of p e r f e c t l y alternating siloxane poly(arylene ether sulfone) copolymers v i a reaction of hydroxyl groups with silylamines (X - N(CH ) in 2) which is reviewed in reference 9. A variety of d i f f e r e n t hard segments were incorporated into the resulting copolymers. These workers observed that, in fact, the p r o c e s s i b i l i t y of these copolymers was very dependent upon the d i f f e r e n t i a l s o l u b i l i t y parameter between the hard segment and the non-polar polydimethylsiloxane block (14). In some cases, the microphase separation was apparently so well developed that the materials developed an extremely high v i s c o s i t y in the melt, rendering them e s s e n t i a l l y non-processible. This phenomenon occurred even though the materials were l i n e a r , and indeed, solvent-castable from a variety of appropriate organic l i q u i d s . Although this v i s c o s i t y e f f e c t is general with microphase separated systems, i t is p a r t i c u l a r l y pronounced f o r the organosiloxane materials (14). Despite some of the melt fabrication problems, the organosiloxane systems produced by the silylamine-hydroxyl reaction (9) produced interesting, perfectly alternating copolymers.


3

2

Polycarbonate (15-18) and aromatic polyester (19,20) copolymers produced using this same general route have been investigated in our laboratories. The chemistry is i l l u s t r a t e d in Scheme 6. The perfectly alternating copolymers developed very uniform morphology (17) reminiscent of the t r i b l o c k styrene-diene materials. Typical s t r e s s - s t r a i n and dynamic mechanical behavior is shown in Figures 1 and 2. By contrast, i f the copolymers were prepared by either a randomly coupled route or v i a an in-situ generation of the hard block (16,17), the morphology was not regular and, indeed, the physical properties of both elastomeric and r i g i d compositions were quite d i f f e r e n t from those of the perfectly alternating systems. The aromatic polyester/siloxane copolymers synthesized in our laboratories included a series of siloxanes which were themselves comprised of "blocky" structures. Both diphenylsiloxane sequences as well as trifluoropropyl-methylsiloxane units were combined with dimethylsiloxane units. These copolymers are of considerable i n t e r e s t as multiphase damping materials (19) due to the fact that two, three or even more relaxations can be designed into these copolymers as a function of block length and composition. Indeed, s i g n i f i c a n t loss peaks ranging from -120 to well over 200°C have been achieved (19,20). Recent work has focused on a variety of thermoplastic elastomers and modified thermoplastic polyimides based on the aminopropyl end f u n c t i o n a l i t y present in suitably equilibrated polydimethylsiloxanes. Characteristic of these are the urea linked materials described in references 22-25. The chemistry is summarized in Scheme 7. A c h a r a c t e r i s t i c s t r e s s - s t r a i n curve and dynamic mechanical behavior for the urea linked systems in provided in Figures 3 and 4. I t was of interest to note that the ultimate properties of the soluble, processible, urea linked copolymers were equivalent to some of the best s i l i c a reinforced, chemically crosslinked, s i l i c o n e rubber


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m a t e r i a l s d e s c r i b e d in t h e l i t e r a t u r e . Various other fundamentally o r i e n t e d s t u d i e s which d e s c r i b e t h e k i n e t i c s and mechanisms of t h e s y n t h e s i s o f t h e s e o l i g o m e r s a r e p r o v i d e d in t h e r e f e r e n c e s h e r e i n . A number of o t h e r h a r d segments such as t h e i m i d e s and amides have a l s o been i n v e s t i g a t e d . One was a b l e t o show a good c o r r e l a t i o n

STRAIN.%

Figure 1. Stress-strain curves of polycarbonate-polydimethylsiloxane block copolymers (Crosshead Speed: 5 cm/min). (Reproduced from Refs. 1 5 , 18. Copyright 1980, American Chemical Society.)



188


Figure 2. Storage moduli and loss tangents versus temperature f o r three t y p i c a l "block copolymers; Frequency: 11 Hz; IYL of blocks expressed in g/mole. (Reproduced from Refs. 15» 18. Copyright 1980, 198^ American Chemical Society.)


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A B C D Ε


PSX-1150-HMDI-81 PSX-2740-HMDI-91 PSX-3740-HMDI-96 Filled high structure silica Filled low structure silica

Modulus (ΜΡα) 110.4 56 41

Figure 3 . Stress versus % elongation behavior f o r siloxane-urea segmented copolymers from H-MDI as a function of molecular weight of oligomer used and the hard segment content a t 25 °C. (Reproduced with permission from Ref. 2 5 . Copyright 1984 IPC Business Press, Ltd.)


189


190


Figure 4. Dynamic m e c h a n i c a l b e h a v i o r o f s i l o x a n e - u r e a copolymers p r e p a r e d from H-MDI. Curve A: PSX-1 50-HMDI-81; c u r v e Β : PSX1770-HMDI-87; c u r v e C: PSX-770-HMDI-91; c u r v e D: PSX-3680-HMDI94. (Reproduced w i t h p e r m i s s i o n from R e f . 2 5 . C o p y r i g h t 1984 IPC B u s i n e s s P r e s s , L t d . )


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I

CH, 3

0 „

CH, I 3

H-f-O-C^H—C-C^H—0-C-^-0-CH—C-C^HT-OH 6 4 J 6 4 a 6 4 | 6 4 L

J

CH

CH

3

(A)

CH.

CH, 3 (CH, ) ,Ν-hsi-cHrSi-N ( CH )

I j

J Z

D

CH Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: January 7, 1988 | doi: 10.1021/bk-1988-0360.ch014

I

3

J

ο

CH (B)

Δ

3

Solvent, (e.g., Chlorobenzene) Heat

—f—A-Β—]— n 1

Scheme 6.

+ HN(CH,) 3 2

J

Perfectly Alternating Polycarbonate-Polydimethylsiloxane Block Copolymers.

I

j

CH, 3

CH, 3

H N- ( CH ) J-hsi-0-}^Si-(CH ) Ν Η 2

2

2

CH

Γ

2

CH,

0

OCN-R-NCO

(aromatic or cycloaliphatic)

+

H N-R -NH

(chain extender, optional)

J H

? 3

-f-rUREA HARD^j-t—Si-0 ) J ^ SEGMENT j fe

J

n

a

Scheme 7.



192


between the i n t e n s i t y of the hydrogen bond formation and the ultimate mechanical properties at room temperature. In t h i s regard, the urea linked systems were preferred. On the other hand, thermal s t a b i l i t y was best for the polyimide systems. Brief discussion of the novel surface properties in these elastomers is appropriate. The bulk microphase separation characteristics of thermoplastic elastomers has been studied in great d e t a i l and is very important. However, less information is known about the surface structure in these two-phase materials. A number of investigators have been interested in these areas. Some of the f i r s t work, described in reference 26, focused on the (at that time) somewhat unique siloxane enhancement at the air/vacuum surface in pure copolymers and even in homopolymer-block copolymer blends. E s s e n t i a l l y , the polydimethylsiloxane displays very low surface energy which provides a thermodynamic d r i v i n g force for migration to the a i r or vacuum i n t e r f a c e . In contrast to the homopolymer, the siloxane segment is chemically linked and cannot macroscopically phase separate. X-ray photoelectron spectroscopy studies (XPS or ESCA) conducted demonstrated that the surface structure was predominantly siloxane, even when the bulk siloxane compositions were r e l a t i v e l y low. An important c r i t e r i o n , though, was the development of microphase structure in the bulk, which apparently frees the siloxane microphase to more e a s i l y migrate to the a i r or vacuum interface. Many other studies of this phenomenon have appeared in the l i t e r a t u r e since that time. In the cases of the siloxane modified polyimides, discussed b r i e f l y below, i t is of great interest in both the area of oxygen plasma processing and atomic oxygen resistance in outer space applications. In the presence of atomic oxygen, apparently the siloxane structure on the surface is converted to an organosilicate-type, ceramic-like material which provides protection during the etching process. Further discussion of t h i s phenomenon is provided in references 32 and 40-43. Although most of the multi-block or segmented thermoplastic elastomer studies have u t i l i z e d the readily available c y c l i c tetramer (D^) structure as a starting monomer (4), there has also been i n t e r e s t in materials derived from the c y c l i c trimer ί ^). c y c l i c trimer has the advantage that one can produce predictable molecular weights with narrow d i s t r i b u t i o n s , due to the a b i l i t y for the c y c l i c trimer to produce l i v i n g lithium siloxanolate chain ends. This has been described by a number of authors (9). Recently, polymers based upon t-butylstyrene and (44) have been studied. These materials, again, r e l y somewhat upon the lower s o l u b i l i t y parameter associated with the t-butylstyrene group r e l a t i v e to styrène. As a r e s u l t , the subsequent block polymers that were prepared process e a s i l y and may lead to a t t r a c t i v e specialty elastomeric and r i g i d copolymers. As mentioned e a r l i e r , siloxanes impart a number of b e n e f i c i a l properties to polymeric systems into which they are incorporated, including enhanced s o l u b i l i t y , resistance to degradation in aggressive oxygen environments, impact resistance and modified surface properties. These p a r t i c u l a r advantages render polysiloxanemodified polyimides a t t r a c t i v e for aerospace, microelectronic and other high performance applications (40-43). 0



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193

The thermal-mechanical properties of siloxane-modified polyimides are a function of the weight f r a c t i o n of incorporated siloxane, molecular weight of the siloxane blocks and chemical architecture of both the siloxane and imide segments. Thus, copolymers with high concentrations of incorporated siloxane (>50%), where the siloxane is the continuous phase, behave as thermoplastic elastomers, whereas lower siloxane concentrations r e s u l t in more r i g i d materials which behave e s s e n t i a l l y as modified polyimides. At high polyimide compositions, the upper glass t r a n s i t i o n value and most mechanical properties approach those of the unmodified controls. However, the surface structure can s t i l l be strongly dominated by the low surface energy polysiloxane microphase. The great difference in s o l u b i l i t y parameters of the siloxane and imide segments is a d r i v i n g force for microphase separation, p a r t i c u l a r l y when higher molecular weight siloxane oligomers are incorporated. A d d i t i o n a l l y , because the siloxane component possesses a r e l a t i v e l y low surface energy, i t w i l l migrate to the a i r or vacuum i n t e r f a c e , y i e l d i n g a surface dominated by the siloxane component. This e f f e c t is observed even for low (~5 weight %) levels of siloxane incorporation. In aggressive oxygen environments, the surface siloxane segments convert to a ceramic-like s i l i c a t e (SiO ) which provides a protective outer coat to the bulk material. The conversion of polysiloxane to s i l i c o n dioxide in oxygen plasma has been documented within the l i t e r a t u r e of the electronics industry as well as in our laboratory. The copolymers investigated were largely based upon 3 , 3 ' , 4 , 4 ' benzophenone tetracarboxylic dianhydride (BTDA), the meta-substituted diamine 3,3'-diaminodiphenyl sulfone (DDS) and aminopropyl-terminated polydimethylsiloxane oligomers of various molecular weights in the range of 950 to 10,000 grams per mole. The incorporated siloxane oligomer has been varied from 5 to 70 weight percent. Conversion of the poly(amic acid) to the f u l l y imidized polyimide was accomplished by two d i f f e r e n t techniques. The f i r s t , and most common, route was via bulk thermal imidization with the loss of water. In t h i s case, temperatures near the glass t r a n s i t i o n temperature of the f u l l y imidized product must be employed in order to achieve complete imidization. In the second method, imidization was accomplished at lower temperatures in the range of 150 to 170°C by means of a solution imidization procedure ( 4 2 , 4 3 ) . Although in both cases, tough, transparent, f l e x i b l e and soluble films were obtained. The choice of imidization method a f f e c t s material properties. The solution imidized materials are markedly more soluble than the bulk imidized systems, f o r example, but their thermal and mechanical properties do not vary s i g n i f i c a n t l y . The polysiloxane-amic acid intermediates were prepared as previously described (42,43) then converted to the f u l l y imidized segmented copolymers by either the bulk or solution imidization route. The bulk-thermal imidization technique involved casting the amic acid solutions onto a substrate, then removing solvent in a vacuum oven at 100°C f o r several hours. The films were then thermally cycled at τ 0 0 , 200 and 300°C for an hour at each temperature in a forced a i r convection oven to complete the c y c l i z a t i o n . The solution imidization procedure has been further described by Summers, et. a l . ( 4 2 , 4 3 ) . A representative structure for the poly(imide siloxane) copolymers is depicted in Figure 5.



Figure 5.

Structural Representation of Poly(Imide-Siloxane) Segmented Copolymers (PSX = Polydimethylsiloxane),


M

§

MM

r

S3

Μ

ζ ο

>

Ζ

ο ο > ζ > ζ ο

s

14.

SUMMERS ET AL.

195


One of the major goals of t h i s endeavor was to s o l u b i l i z e the normally intractable polyimides by the incorporation of siloxane segments, and, optionally, by solution imidization. S o l u b i l i t i e s of a series of siloxane-modified polyimide copolymers were evaluated in a variety of solvents as indicated in Table I. Copolymer s o l u b i l i t y was found to be a function of the siloxane oligomer concentration

Table I.

S o l u b i l i t i e s of Bulk (B) and Solution (S) Imidized Poly(Imide Siloxane) Segmented Copolymers


Solvent

Control

20% PSX

40% PSX

60% PSX

B

B

B

B

S

S

S

S

N-methylprrolidinone (NMP)

S

S

M S

M S

M S

S

S

Dimethylacetamide (DMAc)

I

S

M S

Tetrahydrofuran (THF)

I

I

I

I

I

S

S

S

Methylene Chloride (CH C1 )

I

I

I

I

I

S

S

S

2

Key:

2

S = Soluble;

M » Marginally soluble;

I = Insoluble

as well as the method of imidization. The solution imidized copolymers and also the solution imidized control (no siloxane) were a l l soluble in dipolar, aprotic solvents such as Nmethylpyrollidinone (NMP) and dimethylacetamide (DMAc). The s o l u b i l i t y of the solution imidized copolymers and control in NMP or DMAc ranged from «»10 weight percent for the control t o «20 weight percent f o r the copolymers. At 40 weight percent siloxane, the solution imidized copolymers were soluble in a variety of alternative solvents, including tetrahydrofuran, methylene chloride and diglyme. The bulk imidized materials were less soluble than those which were solution imidized, requiring a t least 10 weight percent siloxane at s o l i d s concentrations of less than 5 percent to achieve s o l u b i l i t y in NMP and DMAc. The bulk imidized control was not soluble in d i p o l a r , aprotic solvents. I n t r i n s i c v i s c o s i t i e s of the copolymers ranged from 0.50 to 0.85 d l / g , i n d i c a t i n g that high molecular weight copolymers were obtained by both imidization techniques over the entire composition and segment molecular weight range. Values of the upper glass t r a n s i t i o n temperatures of the siloxane modified polyimides were found to be a function of both the l e v e l of incorporated siloxane as well as the siloxane molecular weight (Table I I ) . The upper t r a n s i t i o n temperature of the solution



196

Table I I . Upper Glass Transitions of Poly(Imide Siloxane) Segmented Copolymers

WT. % PSX

PSX Mn


CONTROL CONTROL

IMIDIZATION METHOD

[nl

(NMP

25°C)

BULK SOLN

1.36

Τ (°C) g 272 265

f

10 10

900 900

BULK SOLN

0.62 0.63

256 251

10 10

2100 2100

BULK SOLN

0.78 0.73

261 260

10

5000

BULK

0.71

264

10

10000

BULK

0.73

266

20 20

900 900

BULK SOLN

0.78 0.67

246 240

20 20

2100 2100

BULK SOLN

0.60 0.57

258 252

20

5000

BULK

0.51

262

40 40

900 900

BULK SOLN

0.55 0.58

225 218

imidized materials was found to be only s l i g h t l y depressed from i t s bulk imidized analogue. Generally, the upper t r a n s i t i o n temperature increased with greater s i l o x i n e oligomer molecular weight and with decreasing siloxane incorporation. In many cases, the copolymers' upper t r a n s i t i o n temperature was depressed only s l i g h t l y r e l a t i v e to that of the control, i n d i r e c t l y indicating that good microphase separation was achieved. The lower temperature siloxane t r a n s i t i o n is d i f f i c u l t to detect by DSC f o r low levels of siloxane incorporation, i . e . ,

Recent Advances in Organosiloxane Copolymers - ACS Symposium

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