Polymer Science and Engineering-Facts and Trends Herman Ma* Polytechnic University of New York, 333 Jay Street, Brooklyn, NY 11201
Accomplished Facts The science of macromolecules had its origin in the study of important and indispensable natural materials: cotton, wool, silk, leather, keratin, rubber, and wood resins. At the end of the 1920's numerous classical theoretical and practical contributions and a prolonged and vigorous exchange of opinions firmly established the concept of linear chains with molecular weiehts of more than hundred thousands. I t immediately encouraged attempts to prepare compounds of similar size and structure and resulted within a few years in the existence of a variety of synthetic polymers, polyvinyls, polydienes, polyesters, and polyamides. Naturally these early synthetics were put to practical uses that were similar to those of their natural models and precursors-fibers. films. rubbers. and dastics. Ravon and cellulose acetate, the firit commkrcial"manmad~ fibers still used cellulose as their ~olvmeric base, but soon polvamides, . polyesters, and several addition polymers were dkveloped into an enormous variety of "fully" synthetic fibers that dominate today's textile industry and provide for additional improvements and expansions in the foreseeable future. Cellulose nitrate, cellulose acetate, and cellophane were the classical precursors of film-forming materials for photography, graphics, and packaging; they were soon followed by a plurality of polyolefius, vinyls, and polyesters, which gave these industries new and unexpected dimensions with numerous options for the future. Natural rubber served as an exceedingly successful model for an entire generation of
Thermoplastic resins have independent
svnthetic elastomers that. todav. extend the range of ruhberiness from 200 "C down to -76 ocand exceed 3000
10%
1.1 X lo6 X lo5 X lo5
22300 -500 -500
-1.0 -1.0
uo to
0.7 X lo1
120
Figure 3. Polyacatylene is polyconlugated.
Electrical and Optical Behavior
Flgure 2. Polyaromatic themosening system known as "Bakelite".
Table 2 presents the moduli, tensile strengths, and softening ranges of several modern polymeric fibers showing that their values overlao with those of metal and elass. Durine the last two decades their availability encoureged researc; and develonment of hieh-oerformance comoosites of low - . specific gravity, which were ahle, in many instances, to perform favorahlv in comnarison to the much heavier and much less corrosion-resistant metals. Polyaromatic thermosetting systems such as polyimides, polystyrylpyridenes, and others are reinforced with aramides or carbon fibers and are manufactured into a large variety of objects-rods, plates, pipes, wheels, sheets, and bolts-by such new processing techniques as filament winding (FW), prepregging, pultrusion, and reaction injection molding (RIM). The polyaromatric system that exists in "Bakelite" is shown in Figure 2. These new materials were first tailored and tried for space vehicles, rockets, and military aircraft where they found an adequate and demanding proving ground for their design and uses. Presently they invade, on a large scale, the construction of airplanes. boats. ships, railroad cars, buses, trucks, and automdhiles together wjth home and factory buildings. Compared with metals and ceramics the conditions under which h e y are produced and processed, and also those under which they may he recycled, require much less energy and time, resulting in a much lower insult on the environment. The weight reductions in comparison to the "all metal" orecursors-uo to 50% in tracked and untracked vehicles and up to 15Y in airplanes-permits either a higher payload or a considerable savine in fuel which. in turn, is fa\,orahle for the environment. The introduction of hiah-nerformance lightweight composites is still in its infancy; design and pror&sing; in many cases, arestill tentatkeand there exists,as yet, little reliable information on such phenomena as aging and fatiguing by mechanical or chemical processes under characteristic environmental conditions. However, the general experience is, a t present, highly encouraging; many additional simplifications and refinements are within reach. and it is hiehlv probable that these new composites willbecome a nent familv of huildine materials with a very wide ranee of useful applicability. ~~~~
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Traditionally organic substances, including organic polymers, are insulators: they may be nonpolar such as gasoline or polyethylene, polar such as sugar or nylon, or polarizable such as benzene or polystyrene, hut they do not possess any "free" electrons, which could act as carriers for an electric current. There exist two systems of organic compounds that exhibit considerable electronic conductivity: polyconjugated chain molecules (Fig. 3) and electron donor-acceptor comolexes. The conductivitv of intrinsicallv insulatine -.nolvmers such as polyacetylene, polyparapbenylene poly-paranhenvlene sulfide.. nolvovrrol. . ..- and several of their derivaiivescan be enhanced by about 10 to 15 orders of magnitude r i ~ h into t the metallic or semiconductine ranee bv the addition-doping-of electron donors (alkalrmet& A d organic bases) or acceptors (inorganic and organic acids such as AsFe Fg and acetic acid). These complexes are presently of great interest because they offer the promise of combining metallic and semiconducting characteristics with the plastic and elastic properties of organic polymers. This could, in principle, lead to sequences of N-R junctions along the length of very thin and flexible filaments and to paper-thin electrodes of rechargeablestorage batteries with a variety of voltages and with a power-to-weight efficiency of a t least 10 times that of the commercial lead accumulators. The realistic attainment of these intriquing prospects is presently pursued by a large number (probably several hundreds) of scientists and engineers in several of the leading industrial and university laboratories. There exist, right now, two factors that arhPreventing smooth and rapid progress in this highly attractive and exritina field. One is the missing, or a t best, incomplete theoreticaiunderstanding of the phenomenon, that is the mobility of current carrying electrons of "holes" in an oreanic chemical environment. The other is the relative instakility of the various "doped" oolvconiuaated svstems. Well-oreanized and vieorous ef. . . .. forth are now under way to make l;&resl in both directions, hut it would be difiicult toestimate how soon or how late one may expect that practical applications of these phenomena will be forthcoming. A second, differentway to arrive at metallic conductivity of organic substances is through the stacking of electrondonating and electron-accepting compounds.Organic electron donors are mainly ammonium, sulfonium, and phosphonium bases that contain condensed aromatic systems such as quinolinium or thiazonium; they are flat, diskshaped molecules with strong polarizability and having one or more polar group-CO, OH, NH, etc. (see Fig. 4). Electron
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Volume 65
Number 4
April 1988
335
H+
H'
ce-
ce-
Quinolinium
Pyridinium
Figure 4. Structures of quinolium and pyridinium
acceptors are highly conjugated condensed aromatic molecules that offer to a donated electron many energy levels through dislocation and resonance. Since these donors are also flat and disk-like systems, the complex formations lead to quasi-cylindrical stacks that form small but well develo ~ e crvstals. d usuallv deeolv . . colored. It was found that these crystals have an extremelv anisutropic conductivity \,cry hiah tuoto IO~recinroral~~hmcmr in thedirectionoftheaxis oflhe s'tack and very low (down to 10-lo ohm cm-9 perpendicular thereto. Anv individual donor molecule D in such a column is flanked by two acceptor units; to one of them it is I~ondedbv relativelv strong: com~lexincpolar bonds, hydrogen bridges, and resonating groups; as result the "intracomplex"distances are between 0.23 and 0.26 nm. The other acceptor belongs to the next "sandwich", its distance from the donor D is larger (above 0.28 nm up to 0.35 nm). It has been found that any factor that diminishes this intercomplex distance, external pressure, or appropriate molecular structure of the two partners of the complex increases the conductivity. The ideal case would apparently he if the intercornplex distance would be equal to the intracomplex distance; the donor-acceptor column would then possess optimal electron-transfer conditions and the highest conductivity. In fact typical metallic character and even some type of supraconductivity have been observed in some of these systems. Interesting optical properties of certain polymers have been known since the days of Polaroid, when it was found that highly oriented linear macromolecules (e.g., polyvinyl alcohol) are caoahle of eivine.. com~lexeswith iodine and organicdyestuf?$that are strongly b&fringent and dichroic. These com~oundshave found wide applications as ~ o l a r i e .ers in microscopes, cameras, and sunglasses and are calling for permanent improvements concerning the degree of anisotropy and environmental stability. Another optical use of highly transparent, soft, and flexible polymeric gels (e.g., slightly crosslinked polymethacrylates) has led to the development of very useful contact
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*
lenses that are convenient to wear, cannot crack or fracture,, and represent an important contribution of macromolecules to medicine and health care. Efforts for further improvements and simplifications are still occurring. I t aooears that there will he another use of svnthetic polymers, particularly homo- and copolymers of acrylic and methacrylic systems, in a new field of technology, namely in fiber optics and light telephony. Fibers or wires of a highly ~ u r i f i e dsilica com~ositionare alreadv in use for signal transfer-telephone and TV-over large distances sue; as Boston-Washington and Sacramento-San Diego. In these optical cables the "core" has to be covered with a "cladding" that reduces the losses of the message-carrying light beam and permits covering of large distances with the aid of this new method. This method offers many important advantages over the classical signal transfer with electric currents. I t appears that, for the composition of these claddings, synthetic polymers of high transparency and adequate refractive index are of interest, and it was also found that for shorter distances (e.e.. inside of a citv or of a larae building such as a bank, school; or hospital) even the coreof the lighi cable could he made of an appropriate synthetic polymer.
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Other Trends There are a ftw other pnmisiny trends, but they are still somuch in their infancy thar i r would bedifficult at this time to forecast their growth and their ultimate commercial success. One is the design of radiation-resistant thin films made of poly-p-xylene, polyimides, or polyphenylhydrazides for the construction of solar sails and solar space power stations. Another is the develo~mentof films with unusual barrier properties for oxygen such as poly vinyls and copolymers of ethylene and vinyl acetate. They could be of interest for the tendencies to replace metals and glass to produce impermeable cans and other containers for food packaging. General References1 Mechanical and Thermal Delmonte,Jahn. Technolog).of Carbon Fiber Composites;Van Nostrand: New York. 1481
Zaehsrisdes,A. E.;Porter,R.Strangthond SfiffnessofPo1ymars;DekkecNew York, 1983.
Electrical and Ootical
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Barrier Properties Ryder. Leanard B. Ploatiea Ens. 1985. (May). 4148.
'All comprehensive treatises mentioned contain extensive references to the original literature.
Workshop for High School Chemistry Teachers Hope College in Holland, Michigan, will host a five-week Summer Workshop and Leadership Activities Project of national scope from June 27 to July 29,1988. This program, supported by the National Science Foundation, is designed for teachers of Advanced Placement and second-year or honors high school chemistry. Sir semester hours of graduate credit will be awarded. The project will include intensive training in selected topics in chemistry with a strong emphasis on laboratory work and will equip participants to share knowledge in outreach activities directed to other teachers. Five key topics, Chemical Equilibrium,Oxidation-Reduction and Electrochemical Cells, ChemicalThermodynamics,Chemical Kinetics,and Chemical Bonding and the Geometry of Molecules, will be presented by five guest lectures. Stipends and allowance support for books, room, board, and travel will be provided, contingent on Hope College's receipt of the official grant letter, which is expected in the near future from the NSF Grants Office. Application forms and brochures describing the program may be obtained from Eugene C. Jekel, Director of NSF Summer Project, Hope College, Holland, MI 49423. Telephone: (616) 394-7631.
336
Journal of Chemical Education
