Elastomers and Rubber Elasticity - American Chemical Society

25 Some Comments on Thermoplastic Elastomers

Downloaded by IOWA STATE UNIV on February 16, 2017 | http://pubs.acs.org Publication Date: July 19, 1982 | doi: 10.1021/bk-1982-0193.ch025

ROBERT E . COHEN Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, M A 02139

A brief review is given of the important quali tative features of thermoplastic elastomers. Parti cular emphasis is given to the molecular structure, bulk morphology and interfacial character of these materials. Both equilibrium and nonequilibrium structures are discussed

A large proportion of the papers in this symposium on "Elastomers and Rubbery Elasticity" have been based on research on materials which may be called "conventional elastomers", which are part of the larger category of materials known as thermosets. In a conventional elastomer the long flexible macromolecules are chemically linked together to form a single interconnected macromolecular network. Many of the advantageous properties of these materials can be related to the details of the structure of the covalent network, as discussed in detail in other papers in this volume. For completeness, it is worthwhile to include a brief review of another category of materials known as "thermoplastic elastomers". This relatively new class of materials(commercially available since around 1960), was developed in response to certain disadvantages inherent in the thermoset character of conventional elastomers. In particular, thermoplastic elastomers exhibit thermal- or solvent-induced reversibility in their network structures. This allows recycling of off-grade or scrap materials after processing. Furthermore many of the high-rate processing operations developed for engineering thermoplastics are readily adaptable for use with thermoplastic elastomers. The paragraphs which follow will provide a very brief review of this broad class of materials; emphasis will be given to the strong relationship between molecular structure, bulk morphology and the rubbery mechanical properties of these materials. For more complete details, the reader is referred to the excellent reviews which have already appeard on this topic(l-7).

0097-6156/82/0193-0483$06.00/0 © 1982 American Chemical Society

Mark and Lal; Elastomers and Rubber Elasticity ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Structure and Properties To explain the unique properties of thermoplastic elastomers, i t is necessary to appreciate the fact that these materials genera l l y exhibit phase-separated morphologies in the bulk state. The long f l e x i b l e chains of the rubbery network are held together at multifunctional junctions which act as physical, rather than chemical, linkages. In one major catagory of thermoplastic elastomers, these physical linkages are discrete domains(e.g. spheres of ca. 20 nm diameter) of a r i g i d glassy polymer embedded in a rubbery matrix. Because many f l e x i b l e rubbery chains originate in a given domain and terminate in neighboring domains, a continuous network structure i s obtained. At temperatures above the softening p o i n t ( i n the present example, the glass transition temperature, Tg) of the domains, the material can be made to flow l i k e a thermoplastic polymer melt; i t is important to note, however, that the two-phase domain structure may persist in the melt state giving rise to very complicated rheological behavior(8). After the melt processing i s complete, the material is cooled below the domain Tg, the fixed network structure i s re-established, and elastomeric benavior i s observed. Thus, the upper l i m i t of temperature for u t i l i z i n g a given thermoplastic elastomer as a rubbery component is dictated by the softening temperature of the dispersed domains. The lower bound on u t i l i z a t i o n temperature i s , as in conventional elastomers, governed by the value of T g ( o r the c r y s t a l l i z a t i o n temperature, T ) of the continuous rubbery phase of the material. Variations of the glassy/rubbery structure described above are also found in commercial thermoplastic elastomers. In addition to dispersed glassy domains, multifunctional linkages can be formed by regions of c r y s t a l l i n i t y , ionic attraction or strong hydrogen bonding. In many cases the characteristic dimensions of the domains are smaller than the wavelength of v i s i b l e l i g h t so that, even though a phase-separated morphology e x i s t s , optical transparency i s observed. The fact that these domains are, however, much larger than the typical tetrafunctional chemical l i n k age of a conventional elastomer means that some account must be made of the f i l l e r effect imparted by the domains. Often this f i l l e r effect has a favorable, reinforcing influence on the largestrain mechanical history. Another aspect of the submicron domain size characteristic of thermoplastic elastomers is the fact that for a given volume fraction of r i g i d phase, the surface-tovolume ratio is r e l a t i v e l y large(9,10) compared to that of a corresponding polymer blend in which very much l a r g e r ( t e n s or hundreds of microns) domains are found; furthermore the nature of the connectivity of chains across the interfaces in thermoplastic elastomers has a covalent bond character whereas relatively weak forces act across the interfaces in conventional blends. Thus in thermoplastic elastomers knowledge and control of the details of the i n t e r f a c i a l structure are l i k e l y to be especially important in the understanding of physical properties(11,12). m



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The microphase-separated morphologies which give rise to the characteristic behavior of thermoplastic elastomers i s often a result of a structure at the molecular level which is of a block copolymer n a t u r e ( 2 ) . Considerable progress has been made in the f i e l d of thermodynamics in predicting the complete details of the microphase morphology(domain s i z e , shape, spatial arrangement) from knowledge of the block copolymer molecular structure and composition(13-15). Some theoretical work has also been done on the nature of the interfaces in block copolymer thermoplastic elastomers(16-17). Although careful experimental work has shown that the predictive c a p a b i l i t i e s of thermodynamic theories are quite good(18), this is only true for samples which have been carefully prepared so as to achieve, as nearly as possible, an equilibrium structure in the phase separated morphologies. In actual practice, kinetic effects w i l l strongly influence the morphology, and therefore the properties, of a processed thermop l a s t i c elastomer a r t i c l e ; strong thermal history effects have been found(e.g. changes in properties on annealing(19) or i s o thermal ageing(20)) and remarkably strong processing history effects are seen in solvent casting operations(nature of the s o l v e n t ( 2 J ) , or rate of solvent evaporation(22)). Nonequilibrium structures in the i n t e r f a c i a l regions of thermoplastic e l a s tomers can also be envisioned with a corresponding p o s s i b i l i t y for influencing properties. On the one hand, these strong nonequilibrium effects add greatly to the d i f f i c u l t y of establishing structure/property relationships for thermoplastic elastomers at the same level as has been achieved for conventional elastomers. On the other hand, however, nonequilibrium effects may be used to advantage in achieving various levels of performance for a single thermoplastic elastomer composition. Methods of Synthesis The methods of synthesis of thermoplastic elastomers vary widely as may be expected from the wide variety of molecular structures which can be used to provide the desired morphologies and p r o p e r t i e s ( ] ) . One p a r t i c u l a r l y versatile synthetic method is homogeneous anionic polymerization(23,24) which can be used to obtain an essentially unlimited variety of block copolymers. Using this technique i t i s possible to control the essential det a i l s of the chain structure, including t a c t i c i t y , microstructure, molecular weight and i t s d i s t r i b u t i o n , in each block sequence of the copolymer. Condensation polymerizations involving monomers and prepolymers, heterogeneous(Ziegler-type) c a t a l y t i c polymerization, end-or g r a f t - l i n k i n g of functionalized polymers, and mechano-chemistry can also be used to provide compositions which exhibit the properties of thermoplastic elastomers. With such a variety of powerful synthetic methods available i t appears certain that new and improved types of thermoplastic


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elastomers w i l l be developed in the future and that further understanding of the relationships between the molecular structure of these materials and their properties w i l l be obtained. At the present time the anionically polymerized styrene-diene block copolymers and their hydrogenated analogs hold the largest share of the thermoplastic elastomer market(T). Segmented polyurethane block copolymers account for another large fraction of the current market for these materials.


Summary Reversible network structure i s the single most important characteristic of a thermoplastic elastomer. This novel property generally arises from the presence of a phase-separated morphology in the bulk material which in turn i s dictated by the molecular structure, often of a block copolymer nature. A wide variety of synthetic methods can, in p r i n c i p l e , produce endless varieties of thermoplastic elastomers; this fact coupled with the advantageous processing characteristics of these materials suggest that the use of thermoplastic elastomers w i l l continue to grow in the 1980's. Acknowledgements The author wishes to acknowledge the hospitality and support of Istituto Guido Donegani, Novara, Italy, during the 1981-82 academic year. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Dreyfus, P.; Fetters, L.J.; Hansen, D.R. Rubber Chemistry and Technology 1980, 53(3), 728. Noshay, A; McGrath, J. "Block Copolymers: Overview and Critical Survey"; Academic Press, NY, 1977. Manson, J.A.; Sperling, L.H. "Polymer Blends and Composites"; Plenum Press, NY 1976. Holden, G.; Moacanin, J.; Tschoegl, N.W., eds. "Block Copoly mers"; J. Polymer Science, 1969, C-26. Sperling, L.H., ed. "Recent Advances in Polymer Blends, Grafts and Blocks"; Plenum Press, NY 1974. Allport, D.G.; Janes, W.H., eds. "Block Copolymers"; Wiley, NY, 1973. Aggarwal, S.L. ed. "Block Copolymers"; Plenum Press, NY, 1970. Gouinlock, E.; Porter, R. Polymer Eng. Sci. 1977, 17, 535. Meier. D.J. Polymer Preprints 1970,11,400. Meier, D.J. Polymer Preprints 1974, 15, 171. Chen, Y.D.M.; Cohen, R.E. J. Appl. Polymer Sci. 1977, 21, 629. Leary, D.F.; Williams, M.C. J. Polymer Sci., Phys. 1973, 11, 345.


25. 13. 14. 15. 16. 17. 18.


19. 20. 21. 22. 23. 24.

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Meier, D.J. J. Polymer Sci. 1969, C-26, 81. Helfand, E.; Wasserman, Z. Macromolecules 1978, 11, 960. Leibler, L. Macromolecules 1980, 13, 1602. Helfand, E. Macromolecules 1975, 8, 552. Meier, D.J. Prepr. Polym. Colloq. Soc. Polym. Sci. Japan 1977, 83. Hashimoto, T.; Shibayama, M.; Kawai, H. Macromolecules 1980, 13, 1237. Cohen, R.S.; Tschoegl, N.W. Intern. J. Polymeric Mater 1972, 2, 49. Tant, M.R.; Wilkes, G.L. Polymer Eng. Sci. 1981, 21, 325. Beamish, Α.; Goldberg, R.A.; Hourston, D.J. Polymer 1977, 18, 49. Bates, F.S.; Cohen, R.E. J. Polymer Sci, Phys. 1980, 18, 2143. Fetters, L.J. J. Polymer Sci. 1977, C-26, 1. Szwarc, M. "Carbanions, Living Polymers, and Electron Trans fer Processes"; Interscience, NY, 1968.

RECEIVED January 26,

1982.


Elastomers and Rubber Elasticity - American Chemical Society

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