Polymers for Fibers and Elastomers - American Chemical Society

26 Tailoring P o l y m e r M o l e c u l e s for Specific F i b e r a n d R i b b o n Properties THADDEUS E. HELMINIAK and ROBERT C. EVERS

Downloaded by AUBURN UNIV on December 26, 2017 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch026

Air Force Wright Aeronautical Laboratories, Materials Laboratory, Wright-Patterson Air Force Base, OH 45433 Based on the unique film forming characteristics of certain rigid-rod aromatic-heterocyclic polymers capable of forming liquid crystalline solutions, a research and development effort to explore the potential of these "ordered polymers" as structural materials was initiated. Both poly (p-phenylenebenzobisoxazole) (PBO) and poly(p-phenylenebenzobisthiazole) (PBT) exhibited outstanding thermooxidative stability and formed nematic solutions in methanesulfonic acid or polyphosphoric acid. Attempts to modify polymer structure to permit polymer solubility and processing from noncorrosive solvents were unsuccessful. PBT and PBO were processed into fiber specimens which exhibited a high degree of order. In the case of PBT, modulus and tenacity values as high as 2600gpd (53.3Msi) and 30gpd (615Ksi), respectively, were obtained after appropriate heat treatment. Uniaxially oriented ribbons of PBT were also extruded which after heat treatment exhibited a tensile modulus of 1800gpd (36.9Msi) and a tenacity of 25gpd (512Ksi). Efforts were directed toward an improved understanding of the morphology of these "ordered polymers," PBO and PBT, either neat or in "composite" form with flexible matrix polymers. The Nonmetallic Materials Division of the Materials Laboratory has long been interested in aromatic-heterocyclic polymers as potential structural materials because of their high thermooxidative stability and environmental resistance. More recently, certain of these polymers have been intensely investigated because of their unique film forming characteristics and ability to form liquid crystalline solutions in strong acids. Their resultant potential as high performance structural materials has consequently been demonstrated in the fabrication of high strength-high modulus fibers, ribbons, and molecular composites. This paper describes early research leading to the discovery of these "ordered polymers" and the joint This chapter not subject to U.S. copyright. Published 1984, American Chemical Society

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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research and development program by A i r Force Wright Aeronautical Laboratories (AFWAL) and A i r Force Office of S c i e n t i f i c Research (AFOSR) to exploit the potential of r i g i d - r o d aromatic-heterocyclic polymers as high strength fibers and ribbons through modification of polymer structure and morphology.


Background Extensive research carried out within and under the sponsorship of the Materials Laboratory during the l a s t decade sought improvement in the thermooxidative s t a b i l i t y and environmental resistance of polymeric materials. These efforts focused on aromatic-heterocyclic structures and included consideration of ladder polymer structures which ultimately led to the observation of a unique f i l m forming phenomena. The f i r s t observation of t h i s was for the ladder polymer BBL.QJ

BBL When BBL was dried from a nonsolvent precipitated s l u r r y , a precipitated f i l m , possessing excellent mechanical properties, was formed by the coalescence of discrete p a r t i c l e s of s o l i d matter without going through a melt phase. Although further research considering the relationship between the macromolecular structure and t h i s f i l m forming phenomenon showed that the molecular geometry of the polymer chain was the c r i t i c a l factor,(2J the f i l m forming phenomenon was only observed f o r aromatic-heterocyclic ladder polymers. Concurrent work within the Materials Laboratory attempting to improve the molecular ordering of r i g i d - r o d aromatic-heterocyclic polymers led to the observation that certain r i g i d - r o d , aromatic-heterocyclic polymers capable of forming l i q u i d c r y s t a l l i n e solutions also formed precipitated films comparable to those previously observed only f o r highly extended ladder polymer molecules. Furthermore, i t was proposed that the high strength of these unique precipitated films stemmed from t h e i r "composite" character.(3_,4_) They were considered "composites" because the precipitated films were formed by the aggregation and coalescence of individual microscopic sheets of precipitated polymer. The high strength was attributed to the inherent strength of the microscopic sheets due to t h e i r high degree of molecular order. These observations and other considerations led to the hypothesis that a nonreinforced composite with useful mechanical properties might be achieved with appropriate polymer chain geometry and suitable processing. The key factor f o r success would be the a b i l i t y to achieve a high degree of molecular order. Further, the tendencies of those r i g i d - r o d polymers to form l i q u i d - c r y s t a l l i n e solutions strongly suggested the p o s s i b i l i t y of highly anisotropic f i b e r and ribbon formation. The Materials Laboratory with the additional


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support of AFOSR undertook the task of exploring the potential of t h i s "ordered polymer" approach to structural materials for aerospace a p p l i c a t i o n s . The principal efforts of t h i s program i n i t i a l l y have been concerned with the choice of the macromolecular chemical structure while concurrent studies, theoretical and experimental, have been considering the problems of characterizat i o n , evaluation and processing into high strength fibers and ribbons.


Discussion Materials. On the basis of the observations made above, the choice of molecular design has centered on that of an extended chain, r i g i d - r o d molecule for reasons of molecular ordering and an aromatic-heterocyclic structure for thermal and oxidative s t a b i l i t y . Three such polymers have been synthesized, PDIAB,(j>) PB0,(3,j>,_7) and PBT.(8) H I

-OQ&-©-K®>®-

—in

PBO

PBT The PBO polymer system offers improved thermooxidative s t a b i l i t y over PDIAB and does not pick up the moisture that the benzimidazole structure does, whereas PBT provides the best thermal and oxidative s t a b i l i t y of the three systems (weight retention of>50% after 200 hours at 371°C in c i r c u l a t i n g a i r ) . Although these three materials have been successfully synthesized, they have presented special processing problems because of the extended chain, r i g i d - r o d structural character of the molecules. Present processing requires strong mineral or organic acid solvents such as methanesulfonic acid or polyphosphoric a c i d . This problem of s o l u b i l i t y has during the course of the research manifest i t s e l f in several ways; the limited number and type of solvents, r e l a t i v e l y low s o l u b i l i t y and d i f f i c u l t y in polymerization to high molecular weight due to apparent i n s o l u b i l i t y . This problem has been addressed with two approaches. The f i r s t attack of the problem has been an extensive s o l u b i l i t y study to discover noncorrosive organic solvents or to determine organic chemical structures that could be expected to dissolve these polymers.(9) The results have not provided a pure organic single solvent nor do the results hold s i g n i f i c a n t promise for a new single processing solvent.(10) A second approach to the improvement of s o l u b i l i t y


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consists of polymer chain structural modifications either in the form of pendant additions or the introduction of swivel j o i n t s into the main chain forming an a r t i c u l a t e d molecular structure. The former has been accomplished f o r PB0(6) and PBT(8) to give the modified PBO and PBT structures shown below.

Ar

Ar


Modified PBO where R and R' can be H or Ar P

P

Ar

Ar

Modified PBT where R and R can be H or Ar 1

In neither case were pendant phenyl groups of s u f f i c i e n t influence to s i g n i f i c a n t l y improve the s o l u b i l i t y or offer processing solvent alternatives of practical value. The synthesis of a r t i c u l a t e d molecular structures was based on theoretical considerations of the phase e q u i l i b r i a of rodlike polymers.(11) Articulated PBO, PBT, and PDIAB polymers which contained diphenoxybenzene "swivels" were synthesized(12) as were PBT and PBO structures with thermooxidatively stable biphenyl and bipyridyl "swivels".Q3) Although differences from PBO, PBT and PDIAB i n film-forming properties and solution behavior were observed,(14) the s o l u b i l i t i e s were not s u f f i c i e n t l y altered to afford processing alternatives. Characterization. Extensive characterization has been carried out on PBO and PBT establishing that the polymers exhibit r o d - l i k e behavior in solution and can e x i s t in either o p t i c a l l y i s o t r o p i c or anisotropic states depending on temperature, concentration and molecular weight.(15-17) These data show that PBT i s r o d l i k e , protonated in strong acids used as i t s solvent and i s of a f a i r l y high degree of polymerization. A weight average degree of polymerization of approximately seventy i s readily attainable and i n t r i n s i c v i s c o s i t y values indicating much higher molecular weights have been recorded.(18) Phase e q u i l i b r i a data indicate PBT can form stable solutions in the nematic state as can PBO. However, PBO and PDIAB, as measured by i n t r i n s i c v i s c o s i t y , have never been polymerized to as high a molecular weight as PBT (fy]=47dl/gm). (JL8) Concerns regarding the kinetics and mechanism for the polymerization of extended chain r i g i d - r o d macromolecules to high molecular weight have prompted a study to address t h i s problem.(19) Processing. Processing of the "ordered polymers" into f i b e r specimens was carried out i n order to assess the potential of these polymers for applications requiring high modulus and strength. Through t h i s approach, i t was possible to minimize the sample



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quantities required and take advantage of developed technology to provide the requisite specimens for evaluation of mechanical properties and morphology. I n i t i a l l y , PBO of moderate molecular weight (M=3.88dl/g) was wet spun from methanesulfonic acid dopes into f i b e r specimens exhibiting t e n s i l e modulii of approximately 700gpd (14 Msi) after heat treatment.(7J Low tenacity values of 5gpd (102Ksi) were recorded, attributable to the limited polymer molecular weight and/or processing defects. The processing defects arose from incursions into the f i b e r surface which occurred during coagulation, resulting in a reduction of cross-sectional area having well oriented molecules and, thus, a lower tenacity. With the subsequent a v a i l a b i l i t y of higher concentration dopes of high molecular weight PBT, dry-jet wet spinning of these methanesulfonic acid and polyphosphoric acid dopes led to PBT fibers with as-spun modulus and tenacity as high as 2050gpd (42Msi) and 20gpd (400Ksi), respectively.(20,21) Through appropriate heat treatment of the PBT f i b e r , these values were increased to 2600gpd (53Msi) and 30gpd (615Ksi), respectively, with an elongation of 1.1 percent being recorded. Efforts are continuing to improve on the spinning and coagulation process to eliminate or minimize incursion defects and to optimize f i b e r properties. Efforts have also been made to process both PBO and PBT into a continuous ribbon. While i n i t i a l attempts to extrude PBO dopes were not p a r t i c u l a r l y promising, (_7) subsequent efforts with PBT dopes have yielded u n i a x i a l l y oriented ribbons ( l . O i n . x 0.5mil) with as spun properties of 650gpd (13Msi) modulus, 13gpd (250Ksi) tenacity, and 3.6 percent elongation.(22) Appropriate heat treatment of the ribbons increased the modulus and tenacity values to 1800gpd (37Msi) and 25gpd (512Ksi), respectively, and decreased the elongation to one percent. As with the PBO and PBT f i b e r s , investigations of the f i l m morphology indicated a high degree of o r i e n t a t i o n . Efforts are currently underway to obtain wider ribbons as well as b i a x i a l l y oriented specimens. Concurrent to these efforts to process the neat polymers, the r i g i d - r o d molecules were used as reinforcement in f l e x i b l e aromaticheterocyclic polymer and thermoplastic polymer matrices to provide composites at the molecular level which are analogous to chopped f i b e r composites.(23) I n i t i a l investigations revealed that these molecular composites exhibited s i g n i f i c a n t increases in t e n s i l e modulus and strength with only ten percent of r i g i d - r o d polymer in the matrix.(24) Subsequent morphology studies f o r vacuum cast and shear quenched films showed that maximum dispersion and orientation of r o d - l i k e PBT molecules in a poly-2,5(6)-benzimidazole matrix i s achieved from processing from solution at or near i t s c r i t i c a l concentration.(25) These studies are continuing. Future efforts are being directed toward an improved understanding of the morphology of the "ordered polymers" PBO and PBT, neat or in "composites" form with f l e x i b l e matrix polymers. Emphasis w i l l continue to be placed on learning to process the extended chain, r i g i d - r o d aromatic-heterocyclic polymers into useful forms for structural applications.


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Literature Cited 1. Arnold, F. E.; Van Deusen, R. L. J. Appl. Polymer Sci. 1971, 15, 2035. 2. Sicree, A. J.; Arnold, F. E.; Van Deusen, R. L. J. Polymer Sci., Polymer Chem. Ed. 1974, 12, 265. 3. Helminiak, T. E.; Arnold, F. E.; Benner, C. L. Am. Chem. Soc., Prepr. Polymer Div. 1975, 16 (2), 659. 4. Helminiak, T. E.; Arnold, F. E. U.S. Patent 4 051 108, 1977. 5. Kovar, R. F.: Arnold, F. E. J. Polymer Sci., Polymer Chem. Ed. 1976, 14, 2807. 6. Wolfe, J. F.; Arnold, F. E. Macromolecules 1981, 14, 909. 7. Choe, E. W.; Kim, S. N. Macromolecules 1981, 14, 920. 8. Wolfe, J. F.; Loo, B. H.; Arnold, F. E. Macromolecules 1981, 14, 915. 9. Bonner, D. C. Air Force Materials Laboratory Technical Report AFML-TR-76-91 1976. 10. Bonner, D. C. Air Force Materials Laboratory Technical Report AFML-TR-77-73 1977. 11. Flory, P. J. Macromolecules 1978, 11, 1141. 12. Evers, R. C.; Arnold, F. E.; Helminiak, T. E. Macromolecules 1981, 14, 925. 13. Evers, R. C. U.S. Patent 4 359 567, 1982. 14. Berry, 6. C., private communications, Carnegie-Mellon University (Jan 1983). 15. Berry, G. C.; Casassa, E. F.; Metzger, P.; Venkatraman, S. Air Force Materials Laboratory Technical Report AFML-TR-78-164 (Part 11, 1980. 16. Berry, G. C.; Casassa, E. F.; Lee, C. C.; Furukawa, R.; King, R. S.; Veukatraman, S. Air Force Wright Aeronautical Laboratories Technical Report AFWAL-TR-80-4099, 1980. 17. Chu, S. G.; Vemkatraman, S.; Berry, G. C.; Einaga, Y. Macromolecules 1981, 14, 939. 18. Wolfe, J. F . , private communications, SRI International (Jan 1983). 19. Cotts, D. B.; Berry, G. C. Macromolecules 1981, 14, 930. 20. Hwang, W-F.; Wiff, D. F . , private communications, University of Dayton Research Institute (Jan 1983). 21. Allen, S. R.; Filippov, A. G.; Farris, R. J.; Thomas, E. L . ; Wong, C. P.; Berry, G.; Chenevey, E. C. Macromolecules 1981, 14 1135. 22. Chenevey, E. C., private communications, Celanese Research Company (Jan 1983). 23. Helminiak, T. E.; Benner, C. L . ; Arnold, F. E.; Husman, G. U.S. Patent 4 207 407, 1980. 24. Husman, G.; Helminiak, T. E.; Adams, W. W.; Wiff, D. R.; Benner, C. L. Amer. Chem. Soc., Prepr. Org. Coat. Plast. Div. 1979, 40, 797. 25. Hwang, W-F.; Wiff, D. R.; Helminiak, T. E. Amer. Chem. Soc., Prepr. Org. Coat. Plast. Div., 1981, 44, 32. RECEIVED May 11, 1984


Polymers for Fibers and Elastomers - American Chemical Society

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