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 5100 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 186Os, when a synthetic substitute for ivory pool balls was being sought. It took the needs of World War I1 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 1930%polymer chemists had the vision that synthetic macromolecules, obtained by addition or condensation polymerization, would one day become
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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, i t 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
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- 0003-2?00187/A359-1141/S01.50/0 @ 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1. 1987 * 1141 A
addition to molecular design, compositional design of multipolymer systems such as alloys, blends, composites, and laminates has developed. All of these new polymeric materials require chemical characterization t o correlate the physical and mechanical properties and to define quality control standards. Challenge of polymer characterization
Polymer characterization has presented major difficulties to the analytical chemist, who has had to develop techniques 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 polymers, 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 observation. In contrast to low-molecularweight substances, a polymer does not have a single molecular weight because the statistical nature of the polymerization process produces molecules with a broad molecular weight distribution. Therefore, with colligative property methods, one measures an average molecular weight (i.e., number average or weight average, depending on the method) with little insight into the molecular weight distribution. Size-exclusion chromatographic techniques such as gel permeation have been developed to measure these molecular weight distributions. Special chromatographic instruments with a range of spectroscopic detectors (including infrared and laser-light scattering) have emerged commercially to aid the analytical chemist in the fundamental endeavor to measure the length of the polymer chain and its distribution (I). Determining the structure of the repeat unit of the polymer chain is complicated by the presence of a number of isomeric species arising from multipath polymerizations, monomer isomerization prior to polymerization, and rotational isomerism of the polymerized chain (2). A large portion of the commercial polymers are actually copolymers or terpolymers. For the 21 common vinyl and vinylidene monomers available, there could he a total of 210 binary copolymers and 1330 ternary polymers (3). In addition to these possible combinations, the monomer units can exhibit different sequence distributions on the backbone. These 1142A
Flgure 2. Polymer wnversion as a function of time. Adapted with wrmisim from Reference8.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19. OCTOBER 1. 1987
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Flgure 3. Structure of the cross-linked diacetylene polymer. Adapted with permission from Reference 8
comonomer units (A and B) can be distributed on the polymer chain as blocks (AAAAABBBBBB), in random order (AABABBBAAA),or in alternating sequence (ABABABABAB). The comonomer composition can be the same in all three cases, but the sequence distribution leads to substantially different physical and mechanical properties. For purposes of polymer microstructure characterization, only those spectroscopic techniques that exhibit sensitivity to differences in monomer sequences (i.e., AAA, AAB, ABB, ABA, BBB) can be used (2). The process of characterizing polymers is complex. The analytical chemist is faced with a complex mixture, even when attempting to determine fundamental molecular structure of the polymer chain. These polymer chains exhibit conformational variations, 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 he achieved only with quality instrumentation, extensive experience, and a knowledge of the advantages and disadvantages of various spectroscopic techniques. Spectral simulation methods, as well as a series of chemometric techniques, are utilized to aid in the interpretation of polymer spectra. FT-IR spectroscopy of polymers The advent of Fourier transform infrared (FT-IR) spectroscopy has yielded considerable improvement in both the quality and the interpretation of poly1144A
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 polymer spectra and the recording of spectra with the full-scale sensitivity of a milliabsorbance unit. Additionally, FT-IR spectroscopy makes it unnecessary to use only the direct transmission sampling technique to record an IR spectrum of a polymer. Several 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).Finally, for analysis of impurities and microsamples, IR microscopes are commercially available with a spatial resolution that is on the order of the wavelength of the light. At 400 cm-', this amounts to a field of view of 25 pm2. Two-dimensional absorbance maps of samples can be obtained by computer-controlled scanning techniques. A viewing area mapped with a 100 X 100 grid at 16 cm-l can be collected in less than 10 min; data may be presented with the absorbance changes color-coded for easy inspection and interpretation. 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 immediately on the instrumental microcomputer or transmitted to a mainframe computer. Prior to interpretation and analysis, one can use suitable spectral stripping methods to remove instrumental artifacts by ratioing reference spectra. Sampling technique perturbations 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 multivariate least-squares techniques for quantitative analysis of multicomponent systems such as polymers using the entire spectrum (3600 points each) for as many as eight reference spectra (7).
Figure 4. The 75-MHz 13C NMR spectrum of PVB. DBtails Of the spectral parameters can be found in the Original reference. Adapted with wrmlssion from Reference 11.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1. 1987
I
In LC sample injection, “just right”is all wrong. ne explains why Why not load a 20-pL sample into a 20-pl sample loop? Seems just right. But it‘s all wrong. It’s wrong because loading a volume of sample equal to the volume of the loop produces the worst accuracy and precision. Much more sample-or much less-is better: This is one of several important facts about LC sample injection not
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an investigation. Figure 3 shows the proposed structure of the polymer.
Figure 5. Structures for various steree sequences 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 analysis. In most practical cases for polymers, however, curve-fitting techniques for quantitative analysis are limited because there are no suitable reference spectra. As a result, it is necessary to generate such reference spectra from the spectra of the mixtures to he analyzed. Factor analysis, or major component analysis, has successfully been used for this purpose (7).For example, this technique was used to study the curing reaction of the hightemperature diacetylene adhesives heing considered for use in the aerospace industry. Pure spectra of the crosslinked resin and the pure monomer were generated. (The monomer is always partially polymerized by thermal energy.) The degree of cure was determined as a function of temperature and time without prior knowledge of the structure of the polymer (8).The spectra of the purified monomer and the polymer are shown in Figure 1. The degree of cure can he determined by curve fitting these two spectra to that of the polymer mixture a t any time of cure; Figure 2 shows the results of such 1146A
NMR specbcseopy ot polymers Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful tools for the investigation of chemical 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 occurred 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 chemical 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 enchainment of asymmetric monomers, and to studies of chemical and geometric isomers and their sequence distribution (10).However, the proton spectra of polymers with severe multiplicity of line splittings show extensive overlap of resonances, making resonancestructure assignments difficult. Developments in Fourier transform techniques with pulsed NMR made i t possible to obtain NMR spectra of the :arbon backbone of the polymer chain. Dne could measure the ‘3C 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 compared with the 10-ppm range of the proton, but the sensitivity of the 13C is much lower than for proton NMR. However, proton-decoupled I3C NMR spectra generally show only single resonances for chemically inequivalent carbons. 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 proton spectra based on our knowledge of the 13C spectra. Two-dimensional (ZD) correlated NMR experiments provide an experimental hasis for making proton line assignments in a straightforward manner (19,ZO). An example ofan application of the 2D experiments involves poly(vinylbutyrd)(PvB), which is the interlayer in modem automobile windshields. P W is prepared by condensing butyraldehyde on poly(viny1 alcohol)(PVA) to form BA rings. These BA rings may have a meso or racemic configuration, depending on whether the hydroxyl groups are in the meso or racemic stereochemicalconfwation. Because of the presence of unreaded PVA hydroxyls, PVB is a copolymer of BA rings and vinyl alcohoL Consequently, PW has a
Figure 8. The 30O-Mi-k ’H NMR speclrum of a 2 % solution of PVB in Me@-& at 100 o c . Details 01 Uw specbal parameters can be found in the Original reference. Adapted with pemisskm fmm Reference 11.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19. OCTOBER 1. 1987
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Use stored raw dn
opftmize methods with
paDhrc took
1Add, subtradordtuufe chmtograms
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complex microstructure. The 75-MHz I3C NMR spectrum of PVB has several lines that arise from the variety of structures in the polymer (Figure 4). Some of the l3C resonances can be assigned based on previous assignments 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 multiplicity of the various carbon resonances was determined using the Distortionless Enhancement by Polarization Transfer (DEPT) experiment. This technique can be used to generate subspectra associated with methine, methylene, and methyl signals. The line assignments in the 13C spectrum are transferred to the proton spectrum using the 2D I3C-lH correlated spectrum, which is a map of each 13C resonance related to the corresponding proton resonances of all directly attached protons. The ‘H spectrum of PVB a t 100 “C is shown in Figure 6. Figure 7 shows the 2D 13C-1H correlated spectrum. The 13C chemical shift of each peak is indicated on the horizontal axis, and the corresponding 1H 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 corresponding to methyl 1maps to the upfield triplet in the proton spectrum. Hence, the triplet at 0.9 ppm is assigned to the BA methyl 1protons. In a similar fashion, other lines in the ‘H spectrum can be assigned. Verification of the assignments is possible using another 2D technique termed homonuclear 2D correlated spectroscopy (COSY) (11). The COSY
5
.
1
I 90
Flgun 7.2D ’%-‘H
I 70
I
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I
50
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30
10
correlated spectrum of PVB at 100 OC.
% ‘%chemical shlft of each peak is indicated on the hmllontal axis. Correapondi~‘H chsmlcal shifts ol all directly attached oro1ms are indicated on the VBnicBl axis. Adapted wim permlsslonhorn
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
Flgure 8. The 25-MHz ’% spectrum of branched polyethylene in 1.2,4-trichlorobenzene at 110 ‘C. Adapted with permission from Reference 13.
1148A * 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 network. As a result of carrying out this sequence of NMR experiments, it is possible to make a complete assignment of the ‘H 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 instruments with higher field strengths. These higher fields increase the resolution of the chemical shift resonances. Consequently, longer sequence lengths can be detected. Perhaps the crowning achievement of high-resolution ‘3C NMR is the detection and identification of the short and long chain branches in polyethylene. Figure 8 shows the high-resolution ‘3C NMR spectrum of branched polyethylene with the resonance assignments indi-
The sophisticated LC Gradient Pump with a simple controller When it comes to routine gradient separations, it pays to focus on the essentials. Why complicate programming with a variety of options that you’ll never use? Why pay extra for a lot of whistles and bells? Why compromise features that really do make a difference in the accuracy and reproducibility of your results? The new LC Gradient Pump combines all the best flow and mixing features to. gether with s e l f - c o n t i i d binary or ternary gradient control to ensure quality results. There’s no need for additional control components and their added costs. With the new LC Gradient Pump you get all you really need and more, for a lot less. Why not simply ask for a demonstration?
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1 rvdure9. Stereoscopic drawings of the three crystalline polymorphs of isotactic poly(1-butene): (a) form I, 31 helix; (b) form 11, 1Is helix: (c)fcfm 111, 41 helix. Adapted wllh permlsslon from Reference 15.
cated. NMR measurement of polyethylene branches requires the sensitivity to he 1 unit per 1000 carbon atoms or less. Although high-resolution NMR is extremely important to the characterization of polymers, it has one severe limitation: Polymer samples must be dissolved or melted to obtain the spectra. In 1976 this limitation was removed when Schaefer and Stejskal reported the observation of the solidstate l3C 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 examined in their engineering state as solids. Early investigations established that the isotropic chemical shifts obtained from solids generally are close to those measured in solution, so the entire catalog of NMR spectra could be utilized for structure studies. An interesting 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 suhstituents, fixed conformations are formed. Solid-state '3C NMR can detect this conformational isomerism in the solid state. A number of examples are available. Isotactic poly(1-butene) assumes a 31 helical crystalline conformation at room temperature. At 90 "C and above,
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Through H, Jnst Some of the Compounds ku Can Measure At low Picogram Levels
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Flgure IO. High-resolution solid-state 13C NMR spectra at 50.3 MHz of poly(1butene): (a) form I at 20 OC; (b) form II at -60 OC: (c) form 111 at -10 OC; (d) amorphous at 43 'C. The vertical dashed llnes represent the peak positions of form I.Details of the spectral parameters can be found in Uw original reference. Adapted With permission from Reference 15.
this polymer transforms to a tetragonal form, called form 11, in which the chain conformation is an 113 helix. Poly(1butene) also has a third polymorph, form 111, which is orthorhombic crystalline and has 4,-helical conformation. The stereoscopic drawings of the three crystalline polymorphs are shown in Figure 9 ( 1 5 ) . The high-resolution NMR spectra of all three polymorphs have been observed as shown in Figure 10 115). Form I gives a well-resolved spectrum, whereas the resonances of form I1 are much broader and are deshielded relative to form I. The spectrum of form 111is well resolved, and all resonances are deshielded relative to those of form I. It is obvious from the spectra of poly(1-butene) that the solid-state '3C spectra are sensitive to small conformational changes in the solid state. I t soon became apparent that not only could the static structure of solid polymers he determined, but also the dynamic states could be found by mea-
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1151A
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The essential handbook of safety practices and toxic effects for laboratories that handle dangerous chemicals! This new edition covers important changes in legislation, regulations, precautionary safety methods, and toxicity assessments. Also includes expanded coverage of legislation and safety practices in the United States. Contents: Introduction; Health and Safety at Work Act 1974; Safety Planning and Management; Fire Protection; Reactive Chemical Hazards; Chemical Hazards and Toxicology; Health Care and First Aid: Hazardous Chemicals; Precautions Against Radiations; An American View. L. Bretherick, Editor ISBN 0-85186-489-9 Plasticbound; 6 I 8 pages US & Canada $54.00
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Industrial Applications of Surfactants
A N A L Y T I C A L CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
suring molecular motion and correlation times through relaxation times (16).These measured correlation times reflect the frequency of the motion. Because 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 others, are semicrystalline. The differences in the relative mobility of the crystalline (rigid) and amorphous (flexible) components allow the NMR spectroscopists to isolate these resonances from each other using "motional 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 dielectric loss result from the reorientation of the carbonate group. The bulk, shear, and dielectric losses all occur coincident in time, because the same cooperative motion results from the translation of the large BPA unit during the conformational interchange (17). One of the more interesting new textile fibers, poly(huty1ene terephthalate), exhibits an unusual crystal-crystal transition, which is induced by strain. This strain-induced transition is remarkable in that it is reversible-it transforms back when the strain is reduced. This property makes it an ideal material for producing the currently
popular stretch jeans. Detailed NMR studies using proton, I3C, and deuterium NMR have mapped the transitional behavior of the various segments of the polymer (Figure 11). From these studies, the reversihility 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 aromatic rings behaves as a pseudocross-link that brings the rings back to their original state of packing when the stress is removed (18).
From I Through 2,
Future prospects The polymers of the future will utilize unified approaches that blend a knowledge of macromolecular design with synthetic methods to achieve new materials having unique and controllable properties for a wide range of new applications. The role of analytical chemistry 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) Yaw 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) Chew, H. N.; Bennett, M.Anal. Chem. 1984,56,2320.
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Flgure 11. Schematic representationof the motional dynamics of various carbons of the segmented poly(buly1ene terephthalate) copolymer at ca. 25 'C. Adapted wim permission from Reference 18. CIRCLE 41 ON READER SERVICE CARD
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Alsoin Ro2hiiieCenlre NY;Homsb): AusWk; Vkmne, Austria;Misksauga, h a d e ; W$tfom!EngIeM; Munich, Germany; MUm, taw; T&yo, Uveoht, TheN~iMr. lands; a n d G l e ~S, w M .
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(4) McDonald, R. Anal. Chem. 1986, 58, 1906.
(5) Culler, S. R.; Ishida, H.: Koenig, J. L. Ann. Reu. Mater. Scr. 1983,13,363-86. (6) Gillett, P. C.; Lando, J. B.; Koenig, J. L. In Fourier TransformInfrared Speetroscopy; Ferraro, J. R.; Basile, L. J., Eds.; Academic: New York, 1985; Vol. 4, p. 1. (7) Haaland, D. M.; Easterling, R. G.; Voplcka, D. A. Appl. Speetrose. 1985,39,73. (8) Koenig, J. L.; Shields, C. M. J. Polym. Sei., Polyrn. Phys. Ed. 1985,2,845. (9) Bavey, F. A.; Tiers, G.V.D. J. Polym. Sei. 1960.44, 173. (10) Bovey, F. A. High Resolution NMR of Macromolecules; Academic: New York, In.,"
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118) Jelinski, L. W.; Dumais, J .I.: Engel, A. K . Macromulerules 1983.16,403.
(191 Benn, R.: Cunther. H. Anpek'. Ckem. Int. Ed. E q l . 1983.22,350 (20)Uerome, A. E. Modern NMR Trc.hniyues /or Chemdsrr) Research: PerRamon: Ox. lord, 1987.
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Jack L. Koenig receiued h u Ph.U. In theoretical spectroscopy f r o m the University of Nebraska in 1959. He is currently a professor in the Department of Macromolecular Science at Case Western Reserve University and t h e principal investigator for the National Science Foundation-sponsored Materials Research Group incrolued with the theory of polymer transitions and dynamics. Koenig has received numerous awards and honors, including thePittsburgh Society Spectroscop y Award (1984) and the ACS Morley Medal (1986). His research interests include IR and Raman spectroscopy of polymers and solid-state NMR.
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