REPORT Jack L. Koenig Department of Macromolecular Science Case Western Reserve University Cleveland, Ohio 44106
The science of polymers and polymerization is a relatively new field. In spite of limited scientific insights, the technology associated with the production, fabrication, and use of polymers has grown to a $100 billion-a-year industry involving 400,000 jobs. Last year, 1.2 trillion cubic inches of plastic was produced in the United States—nearly double the combined output of steel, aluminum, and copper. Plastics are ubiquitous today, present in everything from drinking cups to helmets to artificial hearts. For better or for worse, plastics have revolutionized our lifestyles. The history of plastics in this country dates back to the 1860s, when a synthetic substitute for ivory pool balls was being sought. It took the needs of World War II to spur large-scale production of a synthetic polymer (rubber) as a substitute for natural rubber, which was in short supply. As early as the 1930s, polymer chemists had the vision that synthetic macromolecules, obtained by addition or condensation polymerization, would one day become
useful commercial products. To manufacture these molecules on a large scale, it was necessary to develop new technologies, design suitable equipment, and discover effective additives as processing aids. For practical utilization of such products, it was also necessary to invent novel fabricating techniques. Finally, it was necessary to convince the public that plastics were not all bad and that they could be of great service to humanity. Polymers are no longer curiosity materials but are established commodity, engineering, and specialty plastics. Unfortunately, technological advances have outpaced the growth of basic scientific knowledge in polymer science, resulting in well-known problems in design, processing, quality control, and durability of polymers. Today the polymer industry is facing new challenges in emerging fields such as microelectronics, aerospace, biomedicine, and genetic engineering. Success in these new areas will depend on the knowledge base of polymer scientists and their ability to modify polymers to fill the needs of these new technologies. Polymer chemists are becoming quite advanced in their ability to engineer the molecular structures. Scientists can produce desirable properties by modifying backbone sequences with various monomers and new functionalities and by modifying the chain structure through chain branching, blocking, and grafting of chains with different lengths and distributions. In
Spectroscopic Characterization of Polymers
0003-2700/87/A359-1141 /$01.50/0 © 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987 · 1141 A
addition to molecular design, composi tional design of multipolymer systems such as alloys, blends, composites, and laminates has developed. All of these new polymeric materials require chem ical characterization to correlate the physical and mechanical properties and to define quality control stan dards. Challenge of polymer characterization
Polymer characterization has present ed major difficulties to the analytical chemist, who has had to develop tech niques to cope with the challenge. Even the elementary problem of measuring molecular weight is not easy. Yet such measurements are essential, because the physical, mechanical, and flow properties depend on the length of the polymer chain. Because of the limited solubility and high viscosity of poly mers, many classical techniques have been of little use or have had to be extensively modified to measure the molecular weight of polymers. For example, the usual colligative methods produce differential thermal or pressure effects so small that special techniques are required for their obser vation. In contrast to low-molecularweight substances, a polymer does not have a single molecular weight because the statistical nature of the polymer ization process produces molecules with a broad molecular weight distri bution. Therefore, with colligative property methods, one measures an av erage molecular weight (i.e., number average or weight average, depending on the method) with little insight into the molecular weight distribution. Size-exclusion chromatographic tech niques such as gel permeation have been developed to measure these mo lecular weight distributions. Special chromatographic instruments with a range of spectroscopic detectors (in cluding infrared and laser-light scat tering) have emerged commercially to aid the analytical chemist in the funda mental endeavor to measure the length of the polymer chain and its distribu tion (1). Determining the structure of the re peat unit of the polymer chain is com plicated by the presence of a number of isomeric species arising from multipath polymerizations, monomer isomerization prior to polymerization, and rotational isomerism of the polymer ized chain (2). A large portion of the commercial polymers are actually co polymers or terpolymers. For the 21 common vinyl and vinylidene mono mers available, there could be a total of 210 binary copolymers and 1330 terna ry polymers (3). In addition to these possible combinations, the monomer units can exhibit different sequence distributions on the backbone. These
Figure 1. FT-IR spectra of the monomer and the polymer of diacetylene. Adapted with permission from Reference 8.
Ο
100
200
300
400 Time (min)
Figure 2. Polymer conversion as a function of time. Adapted with permission from Reference 8.
1142 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
500
Figure 3. Structure of the cross-linked diacetylene polymer. Adapted with permission from Reference 8.
comonomer units (A and B) can be dis tributed on the polymer chain as blocks (AAAAABBBBBB), in random order (AABABBBAAA), or in alternating se quence (ABABABABAB). The comon omer composition can be the same in all three cases, but the sequence distri bution leads to substantially different physical and mechanical properties. For purposes of polymer microstruc ture characterization, only those spec troscopic techniques that exhibit sensi tivity to differences in monomer se quences (i.e., AAA, AAB, ABB, ABA, BBB) can be used (2). The process of characterizing poly mers is complex. The analytical chem ist is faced with a complex mixture, even when attempting to determine fundamental molecular structure of the polymer chain. These polymer chains exhibit conformational varia tions, different crystalline phases, and a disordered or amorphous component, in addition to process-induced chain orientation. It is not surprising that polymer samples produce weak, broad, badly overlapped spectral bands with variable backgrounds. Success can be achieved only with quality instrumen tation, extensive experience, and a knowledge of the advantages and dis advantages of various spectroscopic techniques. Spectral simulation meth ods, as well as a series of chemometric techniques, are utilized to aid in the interpretation of polymer spectra.
mer IR spectra. Single scans through an empty sample compartment achieve a signal-to-noise (S/N) ratio of 1000/1 or better at a resolution of 2-4 cm" ' (4). This high S/N ratio is critical to the isolation of small features of the poly mer spectra and the recording of spec tra with the full-scale sensitivity of a milliabsorbance unit. Additionally, FT-IR spectroscopy makes it unnecessary to use only the di rect transmission sampling technique to record an IR spectrum of a polymer. Sev eral new sampling techniques allow the
polymer to be examined in its fabricated state; that is, as a powder or fiber (diffuse reflectance), film (reflectance absorbance), coating (specular reflectance), or bulk sample (photoacoustic) (5). Fi nally, for analysis of impurities and microsamples, IR microscopes are com mercially available with a spatial reso lution that is on the order of the wavelength of the light. At 400 cm - 1 , this amounts to a field of view of 25 μτη2. Two-dimensional absorbance maps of samples can be obtained by computer-controlled scanning tech niques. A viewing area mapped with a 100 X 100 grid at 16 cm" 1 can be col lected in less than 10 min; data may be presented with the absorbance changes color-coded for easy inspection and in terpretation. To reap the maximum benefit from the higher sensitivity of these FT-IR spectra, data-processing techniques are used to obtain all available information (6). Fortunately, the spectral data are in digital form and may be processed im mediately on the instrumental micro computer or transmitted to a main frame computer. Prior to interpretation and analysis, one can use suitable spec tral stripping methods to remove in strumental artifacts by ratioing refer ence spectra. Sampling technique per turbations can be removed with baseline and scattering corrections, and known impurities (e.g., residual solvent, monomers, and additives) can also be removed. It is possible to use multivari ate least-squares techniques for quanti tative analysis of multicomponent sys tems such as polymers using the entire spectrum (3600 points each) for as many as eight reference spectra (7).
FT-IR spectroscopy of polymers
The advent of Fourier transform infra red (FT-IR) spectroscopy has yielded considerable improvement in both the quality and the interpretation of poly
Figure 4. The 75-MHz
13
C NMR spectrum of PVB.
Details of the spectral parameters can be found in the original reference. Adapted with permission from Reference 11.
1144 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
an investigation. Figure 3 shows the proposed structure of the polymer. NMR spectroscopy of polymers
Figure 5. Structures for various stereosequences in PVB. Adapted with permission from Reference 11.
The use of the complete spectra for curve fitting increases the precision of the measurement relative to a singlepoint Beer's law measurement by the square root of the number of data points used in the multivariate analy sis. In most practical cases for poly mers, however, curve-fitting tech niques for quantitative analysis are limited because there are no suitable reference spectra. As a result, it is nec essary to generate such reference spec tra from the spectra of the mixtures to be analyzed. Factor analysis, or major component analysis, has successfully been used for this purpose (7). For ex ample, this technique was used to study the curing reaction of the hightemperature diacetylene adhesives be ing considered for use in the aerospace industry. Pure spectra of the crosslinked resin and the pure monomer were generated. (The monomer is al ways partially polymerized by thermal energy.) The degree of cure was deter mined as a function of temperature and time without prior knowledge of the structure of the polymer (8). The spec tra of the purified monomer and the polymer are shown in Figure 1. The degree of cure can be determined by curve fitting these two spectra to that of the polymer mixture at any time of cure; Figure 2 shows the results of such
Nuclear magnetic resonance (NMR) spectroscopy is one of the most power ful tools for the investigation of chemi cal structures. It should be no surprise that much of our basic knowledge of polymer microstructure is based on the use of NMR. The initial advance oc curred in the 1960s, when Bovey and Tiers recognized the steric content of the proton NMR spectra of poly(methyl methacrylate) (9). They found that the proton spectra showed chemi cal shifts and homonuclear coupling constants that were sensitive to chain configuration and conformation. The NMR technique was quickly applied to copolymer sequence measurements, to determination of the direction of en chainment of asymmetric monomers, and to studies of chemical and geomet ric isomers and their sequence distri bution (10). However, the proton spec tra of polymers with severe multiplicity of line splittings show extensive over lap of resonances, making resonancestructure assignments difficult. Developments in Fourier transform techniques with pulsed NMR made it possible to obtain NMR spectra of the carbon backbone of the polymer chain. One could measure the 13C nucleus in spite of its lower magnetogyric ratio and lower natural abundance. The pro
ton-decoupled 13C NMR spectra reveal more structural detail because the chemical shift range is 250 ppm com pared with the 10-ppm range of the proton, but the sensitivity of the 13C is much lower than for proton NMR. However, proton-decoupled 13C NMR spectra generally show only single reso nances for chemically inequivalent car bons. Structural assignments of the spectra are simplified using theoretical chemical shift calculations, spectral catalogs, and model compounds. The ideal NMR polymer analysis would utilize the high sensitivity of the proton NMR spectrum and the high specificity of the 13C NMR spectrum. Our goal is to unravel the complex pro ton spectra based on our knowledge of the 13C spectra. Two-dimensional (2D) correlated NMR experiments provide an experimental basis for making proton line assignments in a straightforward manner (19,20). An example of an appli cation of the 2D experiments involves poly(vinylbutyral)(PVB), which is the interlayer in modern automobile wind shields. PVB is prepared by condensing butyraldehyde on polyvinyl alco hol) (P VA) to form BA rings. These BA rings may have a meso or racemic config uration, depending on whether the hydroxyl groups are in the meso or racemic stereochemical configuration. Because of the presence of unreacted PVA hydroxyls, PVB is a copolymer of BA rings and vinyl alcohol. Consequently, PVB has a
Figure 6. The 300-MHz λΗ NMR spectrum of a 2% solution of PVB in Me2SO-d6 at 100 °C. Details of the spectral parameters can be found in the original reference. Adapted with permission from Reference 11.
1146 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
complex microstructure. The 75-MHz 13C NMR spectrum of PVB has several lines that arise from the variety of structures in the polymer (Figure 4). Some of the 13C resonances can be assigned based on previous as signments of PVA; the BA rings can be assigned on the basis of chemical shifts. The labels are related to the carbons shown in Figure 5 (11). The multiplic ity of the various carbon resonances was determined using the Distortion less Enhancement by Polarization Transfer (DEPT) experiment. This technique can be used to generate subspectra associated with methine, meth ylene, and methyl signals. The line as signments in the 13C spectrum are transferred to the proton spectrum us ing the 2D 1 3 C-'H correlated spectrum, which is a map of each 13C resonance related to the corresponding proton resonances of all directly attached pro tons. The Ή spectrum of PVB at 100 °C is shown in Figure 6. Figure 7 shows the 2D 13C-XH correlated spec trum. The 13C chemical shift of each peak is indicated on the horizontal axis, and the corresponding 1 H chemical shifts of all directly attached protons are indicated on the vertical axis. This map can be used to determine specific assignments in the ! H spectrum. For example, the carbon resonance corre sponding to methyl 1 maps to the upfield triplet in the proton spectrum. Hence, the triplet at 0.9 ppm is as signed to the BA methyl 1 protons. In a similar fashion, other lines in the XH spectrum can be assigned. Verification of the assignments is possible using another 2D technique termed homonuclear 2D correlated spectroscopy (COSY) (11). The COSY
Figure 7. 2D 1 3 C - 1 H correlated s p e c t r u m of PVB at 100 ° C . The 13C chemical shift of each peak is indicated on the horizontal axis. Corresponding Ή chemical shifts of all directly attached protons are indicated on the vertical axis. Adapted with permission from Reference 11.
spectrum contains the normal proton spectrum along the diagonal, and spinspin coupling parameters give rise to pairs of cross peaks or off-diagonal
Figure 8. The 25-MHz 13C spectrum of branched polyethylene in 1,2,4-trichlorobenzeneat 110 °C. Adapted with permission from Reference 13. 1148 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
peaks connecting the coupled protons. The COSY spectrum is a map of the complete homonuclear coupling net work. As a result of carrying out this sequence of NMR experiments, it is possible to make a complete assign ment of the Ή and 13C NMR spectra and to use them for characterization of polymer samples. Recently these modern techniques have been used to characterize epoxide resins used in graphite fiber-reinforced composites for aerospace applications (12). The complete elucidation of the proton NMR spectrum permits the use of this technique in the study of the aging reactions of the composite in the low-conversion or prepreg stage. Another instrumental development has been the availability of NMR in struments with higher field strengths. These higher fields increase the resolu tion of the chemical shift resonances. Consequently, longer sequence lengths can be detected. Perhaps the crowning achievement of high-resolution 13C NMR is the detection and identifica tion of the short and long chain branches in polyethylene. Figure 8 shows the high-resolution 13C NMR spectrum of branched polyethylene with the resonance assignments indi-
Figure 9. Stereoscopic drawings of the three crystalline polymorphs of isotactic poly(1-butène): (a) form I, 3^ helix; (b) form II, 11 3 helix; (c) form III, ΑΛ helix. Adapted with permission from Reference 15.
cated. NMR measurement of polyeth ylene branches requires the sensitivity to be 1 unit per 1000 carbon atoms or less. Although high-resolution NMR is extremely important to the character ization of polymers, it has one severe limitation: Polymer samples must be dissolved or melted to obtain the spec tra. In 1976 this limitation was re moved when Schaefer and Stejskal re ported the observation of the solidstate 13C NMR spectra of cellulose in wood and collagen in ivory (14). A new era in the NMR of polymers resulted, because now the polymers could be ex amined in their engineering state as solids. Early investigations established that the isotropic chemical shifts ob tained from solids generally are close to those measured in solution, so the en tire catalog of NMR spectra could be utilized for structure studies. An inter esting new aspect is the loss of dynamic averaging (which occurs in solution) in the solid state. This means that in the freezing of the free rotation of the substituents, fixed conformations are formed. Solid-state 13C NMR can de tect this conformational isomerism in the solid state. A number of examples are available. Isotactic poly(l-butene) assumes a 3i helical crystalline conformation at room temperature. At 90 °C and above,
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From A Through H, Just Some of the Compounds You Can Measure At low Picogram Levels îHÈzr4 IM
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this polymer transforms to a tetragonal form, called form II, in which the chain conformation is an 11 3 helix. Poly(lbutene) also has a third polymorph, form III, which is orthorhombic crystalline and has 4X-helical conformation. The stereoscopic drawings of the three crystalline polymorphs are shown in Figure 9 (15). The high-resolution NMR spectra of all three polymorphs have been observed as shown in Figure 10 (25). Form I gives a well-resolved spectrum, whereas the resonances of form II are much broader and are deshielded relative to form I. The spectrum of form III is well resolved, and all resonances are deshielded relative to those of form I. It is obvious from the spectra of poly(l-butene) that the solid-state 13C spectra are sensitive to small conformational changes in the solid state. It soon became apparent that not only could the static structure of solid polymers be determined, but also the dynamic states could be found by mea-
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987 · 1151 A
suring molecular motion and correla tion times through relaxation times (76). These measured correlation times reflect the frequency of the motion. Be cause the performance of polymers is determined almost entirely by their temperature-transition behavior and energy dissipation properties (brittle, tough, rubbery, etc.), it is critical to be able to relate these dynamic properties to segmental motion in the polymer. Most engineering plastics, such as polyethylene, polypropylene, and oth ers, are semicrystalline. The differ ences in the relative mobility of the crystalline (rigid) and amorphous (flexible) components allow the NMR spectroscopists to isolate these reso nances from each other using "motion al domain" techniques. In fact, NMR techniques allow examination of the motion over 5 decades of frequency. Recent studies on polycarbonate of bisphenol A (BPA) suggest that the shear dynamical mechanical loss and the di electric loss result from the reorienta tion of the carbonate group. The bulk, shear, and dielectric losses all occur co incident in time, because the same co operative motion results from the translation of the large BPA unit dur ing the conformational interchange (17). One of the more interesting new tex tile fibers, poly(butylene terephthai ate), exhibits an unusual crystal-crys tal transition, which is induced by strain. This strain-induced transition is remarkable in that it is reversible—it transforms back when the strain is re duced. This property makes it an ideal material for producing the currently
popular stretch jeans. Detailed NMR studies using proton, 13C, and deuteri um NMR have mapped the transition al behavior of the various segments of the polymer (Figure 11). From these studies, the reversibility of the crystalcrystal transition is believed to arise from the unique chain packing of the aromatic rings and the low barrier to their slippage past each other under strain. The interaction between the ar omatic rings behaves as a pseudocross-link that brings the rings back to their original state of packing when the stress is removed (18).
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Future prospects
The polymers of the future will utilize unified approaches that blend a knowl edge of macromolecular design with synthetic methods to achieve new ma terials having unique and controllable properties for a wide range of new ap plications. The role of analytical chem istry is to make the necessary structure and dynamic measurements with skill and dispatch. The wealth of ideas and the ingenuity that have characterized the polymer industry in the past will create a sound foundation for dealing with future problems—problems that will be considered insignificant when compared with the opportunities that lie ahead. References
(1) Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Size-Exclusion Chromatography; Wiley-Interscience: New York, 1979. (2) Koenig, J. L. Chemical Microstructure of Polymer Chains; Wiley: New York, 1982. (3) Cheng, H. N.; Bennett, M. Anal. Chem. 1984, 56, 2320.
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Send for Ihe Coulochem* brochure and a bibliography of current applications.
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Figure 11. Schematic representation of the motional dynamics of various carbons of the segmented poly(butylene terephthaiate) copolymer at ca. 25 °C.
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987 · 1153 A
CONJURE UP ALL THE DATA YOU NEED
(4) McDonald, R. Anal. Chem. 1986, 58, 1906. (5) Culler, S. R.; Ishida, H.; Koenig, J. L. Ann. Rev. Mater. Sci. 1983,13, 363-86. (6) Gillett, P. C ; Lando, J. B.; Koenig, J. L. In Fourier Transform Infrared Spectros copy; Ferraro, J. R.; Basile, L. J., Eds.; Academic: New York, 1985; Vol. 4, p. 1. (7) Haaland, D. M.; Easterling, R. G.; Vopicka, D. A. Appl. Spectrosc. 1985,39, 73. (8) Koenig, J. L.; Shields, C. M. J. Polym. Sci., Polym. Phys. Ed. 1985, 2, 845. (9) Bovey, F. Α.; Tiers, G.V.D. J. Polym. Sci. 1960,44,173. (10) Bovey, F. A. High Resolution NMR of Macromolecules; Academic: New York, 1972. (11) Bruch, M. D.; Bonesteel, J. K. Macromolecules 1986,19,1622. (12) Herring, F. G.; Jagannathan, N. R.; Luoma, G. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 1649. (13) Bovey, F. Α.; Schilling, F. C.; McCracking, F. L.; Wagner, H-L. Macromolecules 1976, 9, 76. (14) Schaefer, J.; Stejskal, E. 0 . J. Am. Chem. Soc. 1976, 98,1031. (15) Belfiore, L. Α.; Schilling, F. C ; Tonelli, A. E.; Lovinger, A. J; Bovey, F. A. Macro molecules 1984,27,2561. (16) NMR and Macromolecules Sequence, Dynamic and Domain Structure; Ran dall, J. C , Jr., Ed.; ACS Symposium Se ries 247; American Chemical Society: Washington, D. C , 1984. (17) Jones, A. A. Macromolecules 1985,18, 902. (18) Jelinski, L. W.; Dumais, J. J.; Engel, A. K. Macromolecules 1983,16, 403. (19) Benn, R.; Gunther, H. Angew. Chem. Int. Ed. Engl. 1983,22, 350. (20) Derome, A. E. Modern NMR Techniques for Chemistry Research; Pergamon: Ox ford, 1987.
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CHEMISTRY
Jack L. Koenig received his Ph.D. in theoretical spectroscopy from the University of Nebraska in 1959. He is currently a professor in the Depart ment of Macromolecular Science at Case Western Reserve University and the principal investigator for the Na tional Science Foundation-sponsored Materials Research Group involved with the theory of polymer transitions and dynamics. Koenig has received numerous awards and honors, includ ing the Pittsburgh Society Spectrosco py Award {1984) and the ACS Money Medal (1986). His research interests include IR and Raman spectroscopy of polymers and solid-state NMR.
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