Future Directions for Silicon-Based Polymers - ACS Publications

42 Future Directions for Silicon-Based Polymers Downloaded by FUDAN UNIV on January 11, 2017 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch042

Donald R. Weyenberg and Thomas H. Lane Dow Corning Corporation, Midland, MI 48686-0994 Continued development of the field of polymer science requires an understanding of the structure, synthesis, and behavior of silicon -based polymers at four hierarchical levels of complexity: atomic, segmental, network, and domain. The inherent reactivity of monomers and their structure must be uncovered by physical and chemical studies of the behavior of polymer segments, networks, and domains and, finally, the complete system. The role of molecular modeling in bridging the gulf between polymer backbone design and physical behavior is discussed in this chapter. The commercial development and continued expansion of silicone applications are used to illustrate the dynamics of the cyclic process of science, invention, and innovation and to provide the basis for the further advancement of this exciting field of polymer chemistry.

SILICON-BASED POLYMERS

have been of technological importance throughout recorded history. Glass and ceramic materials were an integral part of early civilizations, and pottery relics are still used to gain insight into the extent of the technological advancement of early societies. As human needs expanded beyond simple utensils and tools for survival, glass and ceramic technologies grew in response to the changing and demanding requirements of an evolving world. The new sciences of optics and electronics followed, with the most recent example of this synergy being opticalfibertechnology.

Significant Events That Shaped Silicon Chemistry Two significant events during this century resulted in multiple-step changes in this technological area. Thefirstevent was the discovery and development 0065-2393/90/0224-0753$06.00/0 © 1990 American Chemical Society

Zeigler and Fearon; Silicon-Based Polymer Science Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

Downloaded by FUDAN UNIV on January 11, 2017 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch042

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SILICON-BASED POLYMER SCIENCE: A COMPREHENSIVE RESOURCE

of silicones or poly(organosiloxanes). These exciting and novel materials were first described by Robinson and Kipping (I) during the first two decades of this century. Kipping's pioneering work in the use of the Grignard reagent for the formation of silicon-carbon bonds (2) allowed these materials to become commercially viable products. Corning Glass Works first realized the potential importance of silicones and developed the technology for these materials in the 1930s. Later, this technology was launched commercially, through the newly formed joint venture, the Dow Corning Corporation, in 1943. The second key event was the semiconductor revolution. Silicon rapidly became the essential raw material for new electronic devices. Silicon chemistry grew as it became an integral part in the design and production of these electronic devices, which are housed on a thin chip of silicon. The objective of this chapter is to describe some of the key trends, driving forces, and challenges that will set the future direction of the rapidly advancing field of silicon-based materials. In the process of identifying these key trends, two assumptions were made. The first assumption is that commercial applications will be a major impetus for the development of siliconbased polymers. The response of the late Philip Handler, former head of the National Academy of Science, to a question on science spending corroborates this assumption. Handlers reply (3) was Science

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The second assumption is that historical information can provide useful insight into the discovery cycle. Therefore, the history of silicone commercialization will be used to explain and illustrate some of the most important emerging trends. Silicones and the Cycle of Science, Invention,

and Innovation

The word silicones, first used by Kipping (4), has become a generic term for materials based on organosilicon chemistry and the poly(siloxane)s in general. These polymers with Si-O-Si backbones are the hydrolysis and polymerization products of organochlorosilanes, which are in turn prepared ultimately from silica. Silica is reduced via a carbothermic process to silicon, which is converted to a variety of chlorosilanes. The major monomer, dimethyldichlorosilane, is produced in well over a billion pounds per year by several basic producers



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Future Directions for Silicon-Based

Polymers

755

of silicones. Along with other organoehlorosilanes, dimethyldichlorosilane is the basis of dozens of different siloxanes, which are formulated into hundreds of materials for thousands of applications in nearly every industry. The history of silicones has been summarized several times (5,6); only pertinent historical developments will be considered in this chapter. Silicones were commercially introduced in the United States in 1943. Initial commercialization in any new field of science is a seminal event, because it sets in motion an autocatalytic cycle of science, invention, and innovation (Figure 1). Responsible product development and production demand more precise information, which in turn calls for a deeper and a more critical scientific understanding of materials and processes. This fundamental understanding builds a knowledge base, which allows more invention and greater commercial innovation. This cycle accelerates the demand for more and better science. Knowledge Base

Science

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Figure 1. The cycle of science, invention, and innovation.

Science, invention, and innovation are separate but highly interdependent activities. Science is the knowledge base upon which all understanding is built. Invention is the discovery of new and useful things and their applications. Innovation involves the application of technology to a specific need. With innovation, new and useful things become available commercially. This iterative, cyclic process accelerates the pace of all three activities. Good scientists, inventors, and innovators are equally ill at ease with defined boundaries, and so the triangles in Figure 1 tend to grow in scope and



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rapidly multiply. This dynamic cyclic process is central to the health, growth, and vitality of any new technology and is a major contributor to and stimulus for the development of the underlying scientific base. Publications, patents, and sales are reasonable measures of the science, invention, and innovation supporting a given technology. Figure 2 summarizes the history of publication, excluding patent citations, in the area of organosilicon chemistry for each 5-year period over the last 50 years. The number of Chemical Abstracts citations per year increased from only a handful in the 1930s to over 4000 at present. The curve takes on an exponential character, which coincides with the first commercialization of silicones in the 1940s. A similar curve has been generated for the number of U.S. patents dealing with organosilicon chemistry or materials over the same 50-year period. The results were much as anticipated: few patents dealing with silicones in the 1920s through the mid-1950s and a sudden growth in the number of issued patents to its current level of nearly 4000 (U.S. patents) per year. Sales are a reasonable measure of commercialization, but reliable globalfiguresare not available. However, our best estimates show the same type of exponential growth, which has yielded the current multibillion-dollar industry.

The First Commercial Era of Silicones, 1943-1960.

The 45-year

commercial history of silicones is easily divided into several eras. The period from 1943 to 1960 was dominated by the classical silicones, poly(dimethylsiloxane) fluids, elastomers, and simple methyl and phenyl resins. Basic monomer processes were developed during this period. Poly(dimethylsiloxane) was the backbone of this growing industry. The unprecedented inertness of this polymer under thermal, chemical, and biological environments, coupled with its unique physical behavior, led to a myriad of commercial applications. Its uses include mechanical applications, in which stability in hostile environments was the desired attribute; surface treatments, in which low surface energy was the important feature; and cosmetic and biomedical applications, in which the biological inertness of the material was exploited. During this period, this new and healthy industry was expanding in all directions. Second-Generation Silicones, 1960-1980. The next era was dominated by second-generation silicones. The explosive growth during this period was fueled by several advances, including the development of fluorosilicones, silicone-polyether surfactants, and silanes with organic functional groups. However, our focus on this era will be limited to the tailoring of siloxane structures, because of the relevance of this development to this volume.


Future Directions for Silicon-Based

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Network Formation. The ability to tailor networks catalyzed the growth of this field. To make a strong net out of some long pieces of string, the challenge is to tie the strings together at some regular interval in a manner that would give the net strength. However, making this same net out of short strings presents a distinctly different problem. Now, all the ends of the short strings must be connected precisely in a reliable and repeatable fashion. The same challenge is presented by polymers. Tying low-molecular-weight, and thereforeflowable,molecules together into strong elastomers requires precise chemical manipulation of end groups. Siloxane polymerization techniques led to a variety of end groups, and the chemistry for tying these ends into networks was developed. The chemistry of network formation is generally one of three types: (1) a terminal silanol displacing an acyloxy, amide, or alkoxy group from a polyfunctional silane; (2) a polyfunctional acyloxy-, amide-, or alkoxy-siloxy-ended polymer reacting with water; or (3) a platinum-catalyzed coupling of hydride and vinylfunctionalized polysiloxanes. Liquid prepolymers could be converted in place to strong elastomers by any one of several techniques: by adding a catalyst, by heating, or by exposure to moisture. The technology of network formation had a dramatic impact on the industry, because it offered the product designer an opportunity to design simultaneously the ultimate material property and the application. This technology marked the beginning of a trend in specialty materials that continues today. Specialty Materials. The spectacular growth of silicone use in the construction industry is an excellent example of this new trend in specialty materials (5). Beginning with their initial use as weather-proofing sealants, all these silicone applications take advantage of their exceptional durability. Thus silicones are used in structures as sealants, adhesives, and coating materials. However, the ability of silicones to be cured in place and on site was the property that gave architects and construction engineers new degrees of freedom. The use of silicone adhesives for structural glazing allowed the all-glass exterior that is increasingly dominating our city skylines. After a few demonstrations in the 1960s, silicone sealant adhesives became the method of choice for securing exterior glass windows to high-rise structures. Silicone coatings are also gaining acceptance as premium roof membranes. Applied as a moisture-curing one- or two-part system, elastomeric silicone coatings are often the final outer weather-proofing membrane in a roofing system that incorporates layers of polyurethane foam for insulation. Silicones are also playing an important role in the increasingly popular fabric roof systems. Sealants have been used for difficult building joints with high movement, as well as for highways, where joint failure is a major problem and a tougher



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issue. High-elongation, low-modulus, high-adhesion, eure-in-place sealants are finding widespread application in both new highway constructions and in the repair of failed joints. The use of newer, sophisticated, and expensive silicones to replace less expensive materials, in this case asphalts, is now becoming standard practice, because the total cost over the lifetime of the system is dramatically reduced. Liquid silicone rubber is a popular product with broad industrial use. These materials are pumpable liquids that can be injected into low-pressure molds for curing in a few seconds to several minutes at 100-200 °C. Liquid silicone rubbers are formed by hydrosilation. Network formation is by a platinum-catalyzed coupling of a siloxane hydride with a vinylsiloxane. The complete system contains reinforcing fillers and pigments to give an elastomer with properties similar to those of materials prepared from highmolecular-weight polymers. The network-forming chemistry comprises a subtle sequence of reactions that must take place in the same way and to the same extent every time the mold is filled. This step certainly involves one of the most demanding industrial uses of transition-metal catalysts. The network-forming chemistry has been the subject of some excellent studies (7-9), which both explain the system and demonstrate its use in preparing model networks. The range of silicones for these form-in-place applications continues to grow (5). A variety of silicone foams is particularly useful in fire-resistant penetration seals. A promising new silicone is a latex that deposits a coherent and fully cured elastomer on loss of water. These new forms offer the convenience of silicones from water-based systems and are appearing as easyto-use, water-based silicone caulks and as high-performance exterior-coating systems. The protection of electronic devices has been a key application for specialty silicones, and this application continues to keep pace with the rate of device development (5). Silicones are used in various ways, ranging from resinous circuit board coatings to encapsulants, with the silicone gels representing a unique solution to a difficult problem, stress relief. These dielectric gels are prepared by hydrosilation and are lightly cross-linked poly(dimethylsiloxane)s. Their modulus is extremely low, but they are elastic in their behavior. They have the stress-relief characteristic of a liquid but the nonflow property of an elastomer. These jellylike materials maintain their physical profile over the broad temperature range of-80 to 200 ° C . These examples have been selected to emphasize a point: There has been and continues to be a very central, dominant, and underlying direction for this seemingly random explosive growth of second-generation silicones, and this direction is consistently toward more-specialized materials intended as integral parts of a system and optimized to enhance performance and reduce the cost of a specific system.


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The Current Era of Silicon-Based Materials, 1980-

. The

exciting research frontiers presented in this book suggest several common themes and challenges. First, the commercial visions that will provide the sustained impetus for the advancing boundaries of this field are very much in place. In addition to the excitement that surrounds new science, a similar excitement surrounds a vision for new commercial applications. The various thrusts in this field share a pattern, which is characteristic of modern materials science and which is converging with the fundamental demands of the modern marketplace: the movement of materials science toward the understanding of increasingly higher levels of aggregation in asymmetric systems. Increasingly, structures beyond the molecular level are being described and understood, and the information from these activities is rapidly allowing design and synthesis at the supramolecular level. In the marketplace, the inexorable trend toward the synthesis of systems builds upon and converges with the increasing sophistication of materials. This convergence is consistent with a thought-provoking article by Wrighton (II), which suggests that system synthesis is one of the key challenges in chemistry.

The Synthesis of Systems Materials, components, and systems are often regarded as separate and sequential levels of integration. Materials are said to be synthesized, and systems are said to be assembled. The verb most commonly used with a component is to form. The old paradigm of "synthesize, form, and assemble" is no longer valid for the development of advanced materials; that distinction is disappearing. The old hierarchy is meaningless for microelectronic devices, and modern composite technology is challenging this distinction in larger structural systems. The commercial opportunities and implications of this blurring distinction are immense for the materials technologist and for materials science. The materials technologist becomes an active partner in restructuring the design and manufacture of systems. The value of a specialty material can become very large if it eliminates the formation and assembly steps. In-fact, system synthesis is the logical extension of that trend observed with specialty silicones. This exciting development will have an increasing effect in the field of silicon-based polymers. Hierarchical Levels. The hierarchical structure in polymers is well recognized. The implications for new materials have been discussed recently by Bernent (12) and Baer et al. (13), who emphasized four levels of structure based on dimension: molecular, nanometer, micrometer, and maeromolecular levels. These authors (12, 13) illustrated these concepts with both syn-


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thetic and natural materials. Injection-molded liquid-crystal polymers were used as examples of a synthetic system, and a tendon was used as an example of a natural system. In each case, the unique physical properties are due to a hierarchy of highly anisotropic structures ranging from the smallest oriented molecular segment, which gives the structure its modulus and which extends over no more than 50 A, to the microphase structures responsible for distribution of load, which are measured in hundreds of micrometers. Atoms, Segments, Networks, and Domains. A related and simplified hierarchy seems applicable to the materials concepts described in this volume. Four levels make up the hierarchy of a system: atoms, segments, networks, and domains. At the atomic level, the design of all materials begins with an understanding of the basic interatomic interactions. These bonding and nonbonding interactions set the framework for the material and define its physical and thermodynamic properties. At the segment level, the cooperative motions of polymer segments that allow chain movement are the key determinant of the dynamic behavior of a polymer under mechanical stress. The concept of glass transition rests on these segmental motions, which reflect the steric inhibition of backbone conformational relaxation. The impact of backbone conformation on electronic and optical properties is becoming better understood. At the network level, the ultimate mechanical properties of a homogeneous polymeric matrix are determined by the overall network of polymer chains that prevents the molecules from reacting independently to a stress. The network can be formed by the simple entanglement of glassy polymer segments, as in thermoplastics, or by the alignment of crystalline polymer segments. However, chemical network formation is being used increasingly either as an alternative to or as a complement of plastic formation. The mechanistic richness of substitution at silicon gives these polymer systems a clear advantage when chemical networking is desired. At the domain level, controlled heterogeneity on the micrometer scale is, in fact, responsible for the unique properties of many, if not most, of our advanced structural materials. Fiber reinforcement, rubber toughening, crystallite reinforcement, and particulate reinforcement are familiar techniques. In each case, with proper adhesion at the interface, the resulting material provides the combination of strength, modulus, and toughness for the intended application. Nature's materials, like wood or collagen, make beautiful use of these separate and very different domains. The final system, whether a composite airplane wing or a synthetic tendon, relies on all of these hierarchical levels for proper functioning. Its design requires a fundamental understanding at each level, as well as the ability to synthesize the desired molecular identities. Each emerging discipline discussed in this volume presents a unique


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set of scientific challenges, but among each set are usually some obstacles that limit the pace of advancement. Atomic Structure. The control of atomic structure is fundamental to any system, and an incomplete understanding of atomic structure can limit advancement. For example, our understanding of preceramic polymers, up through the formation of networks, is improving; but the full exploitation of this chemistry is still limited by the lack of detailed knowledge of the structure of the resulting ceramic at the atomic level. Even with more familiar silicone polymer systems, synthetic barriers are encountered as polymers other than poly(dimethylsiloxane) are used. Stereochemical control is inadequate in the polymerization of unsymmetrical cyclic siloxanes to yield novel linear materials. Reliable synthetic routes to model ladder systems are insufficient. Segmental Behavior. The understanding of segmental behavior is clearly a critical factor in many of the frontiers discussed, both because synthetic capability has broadened the potential array of new materials and because this understanding is a vital key in predicting the physical behavior and uses of polymeric materials. With computer modeling, significant improvements in this area can be achieved. Molecular Modeling of Segmental Behavior. Ab initio calculations of both monomer and small siloxane chain segments have clarified the electrostatic interactions between the silicon and oxygen atoms of the siloxane chain. Theflexibilityof the siloxane bond is due to charge transfer from the lone pairs at oxygen to the covalent region between silicon and oxygen. This charge transfer destroys the sp hybridization geometry at oxygen and shortens the Si-O bond, as shown by Grigoras and Lane (Chapter 7 of this volume). These data, in conjunction with skeletal, torsional, bending, and stretching information, give a more accurate description of polymer conformation. In addition, the balance between the steric and electrostatic interactions, and not the van der Waals interactions, controls the conformation of linear siloxane. This finding means that the backbone conformation is neither helical nor hydrocarbonlike in nature (trans-trans). Rather, the backbone has a cis-trans conformation and forms randomly oriented planar segments (14). Chainflexibilityhas been studied as a function of the substituent groups on silicon. These studies are a significant step towards a truly quantitative model for the prediction of siloxane chain flexibility. These early attempts to predict segmental behavior fromfirstprinciples are a milestone and suggest that the computational and synthetic chemists are becoming true partners in designing new materials. This partnership is 3


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beginning at a time when the availability and capability of supercomputers are growing tremendously. Clearly, the direction will be toward modeling to guide a synthetic program. Networks and Domains. The synthesis and understanding of networks and discrete domains are well-illustrated in many chapters in this volume, especially for combinations of relatively well-studied homopolymer systems to provide new capabilities. The block copolymers of poly(dimethylsiloxane) with organic systems or the inorganic "eeramer" composites are the classic examples. However, most of the future structural systems, whether sol-gel materials, structural adhesives, or ceramic-in-ceramic composites, will require a detailed understanding at the network and domain levels. The overall challenge is to move our understanding and our ability to synthesize upward in this hierarchy, toward these higher aggregate levels. The Challenge. The challenge is to continue the present movement toward the true merging of polymer chemistry and materials science. Historically, these disciplines have been moving together. Materials science, with its roots in engineering and its goal of relating structure and processing to property and use, has been moving down this hierarchical scale to the aggregation level in search of fundamentals. Polymer science, with its roots in chemistry and synthesis, has been moving up the hierarchical scale. This movement is a key challenge for the entire field of advanced materials, but it is a particularly exciting challenge for silicon-based polymers. From the point of view of materials, silicon-based polymers span the three traditional domains: plastics, ceramics, and metals. Potential applications are equally diverse. Silicon-based polymers range from structural materials, to optoelectronic devices, and to speciality materials for biomedical applications. We are in a unique position to capture the benefits of this merger of materials and polymer science.

Summary We have attempted to focus on the key forces that will shape our future challenges. We have tried to demonstrate the importance of commercialization as a driving force, not only for the technology, but for the underlying science. We have shown how this commercial push toward system synthesis is accelerating and encouraging the move toward higher levels of aggregation and toward understanding, modeling, and synthesizing more-complex aggregate levels of structure. Finally, although commercial innovation is a critical stimulus, particularly for individuals, science propels the system. The really important new inventions and innovationsflowfrom science, not the other way around.



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Acknowledgments We thank John Zeigler and Gordon Fearon for organizing the symposium on which this book is based. References


1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Robison, R.; Kipping, F. S. J. Chem. Soc. 1908, 93, 439. Kipping, F. S. J. Chem. Soc. 1907, 91, 209. Science, 1979, 204(4392), 474-479. Kipping, F.; Lloyd, L. J. Chem. Soc. 1901, 79, 449. Weyenberg, D. R. "Silicones: Past, Present, and Future" Presented at Inter national Organosilicon Symposium, St. Louis, MO, June 1987; in press. Liebhatsky, S. S. Silicones Under the Monogram; Wiley: New York, 1978. Chandra, G.; Lo, P.Y.;Hitchcock, P. B.; Lappert, M . F. Organometallics 1987, 6, 191. Macosko, C. W.; Benjamin, G. S. Pure Appl. Chem. 1981, 53(8), 1505-1518. Mark, J. E.; Ning, Y. P. Polym. Eng. Sci. 1985, 25(13), 824-827. Cush, R. J. Plastic Rubber Int. 1984, 9(3), 14-17.

11. Wrighton, M . S. Comments Inorg. Chem. 1985, 4(5), 269. 12. Bernent, A . L., Jr. Metall. Trans., A 1987, 18A, 363.

13. Baer, E.; Hiltner, Α.; Keith, H . D. Science 1987, 235, 1015. 14. Grigoras, S.; Lane, T. H . J. Comput. Chem. 1988, 25, 9.

RECEIVED for review May 27, 1988. ACCEPTED revised manuscript October 28, 1988.


Future Directions for Silicon-Based Polymers - ACS Publications

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