CHAIN STRUCTURE AND CONFIGURATION
2.2 THEORY AND INSTRUMENTS
Koenig (3) defines the microstructure of a polymer in terms of its conforma-tion and configuraconforma-tion. The term conformaconforma-tion has taken on two separate meanings: (a) the long-range shape of the entire chain, which is discussed in Chapter 5, and (b) the several possibilities of rotating atoms or short segments of chain relative to one another, to be discussed later. The term configuration includes its composition, sequence distribution, steric configuration, geomet-ric and substitutional isomerism, and so on, and is the major concern of this chapter.
The several aspects of polymer chain microstructure have been studied by both chemical and physical methods. Koenig (3) describes several of these methods, which are summarized in Tables 2.1 and 2.2.
2.2.1 Chemical Methods of Determining Microstructure
The most basic method of characterizing any material uses elemental analysis (Table 2.1). Elemental analysis helps identify unknowns, confirms new syn-theses, and yields information on the purity of the polymer.
Functional group analysis relates to those reactions that polymers undergo, either intentionally or accidently. Selective degradation refers to those chem-ical reactions that a polymer undergoes which cut particular bonds. These may
C C
t t t g+tg–
C
C C C C C
C
C C
C
2.2 THEORY AND INSTRUMENTS 31
Table 2.1 Chemical methods of determining polymer chain microstructure (3)
Method Application Reference
Elemental analysis Gross composition of polymers and copolymers, (a) yielding the percent composition of each
element; C, H, N, O, S, and so on.
Functional group Reaction of a specific group with a known (b, c) analysis reagent. Acids, bases, and oxidizing and
reducing agents are common. Example: titration of carboxyl groups.
Selective Selective scissions of particular bonds, frequently (d) degradation by oxidation or hydrolysis. Example: ozonalysis
of polymers containing double bonds.
Cyclization Sequence analysis through formation of lactones, (e) reactions lactams, imides, a-tetralenes, and endone rings.
Cooperative Sequence analysis using reactions of one group (f) reactions with a neighboring group.
References: (a) F. E. Critchfield and D. P. Johnson, Anal. Chem., 33, 1834 (1961). (b) S. Siggia, Quantitative Organic Analysis via Functional Groups, 3rd ed., Wiley, New York, 1963. (c) N.
Bikales, Characterization of Polymers, Encyclopedia of Polymer Science and Technology, Wiley-Interscience, New York, 1971, p. 91 (d) R. Hill, J. R. Lewis, and J. Simonsen, Trans. Faraday Soc., 35, 1073 (1939). (e) M. Tanaka, F. Nishimura, and T. Shono, Anal. Chim. Acta, 74, 119 (1975). (f) J. J. Gonzales and P. C. Hammer, Polym. Lett., 14, 645 (1976).
be main chain or side chain. Similarly, cyclization reactions and cooperative reactions enable particular sequences to be identified. It must be emphasized that all these methods of characterization are widely used throughout the field of chemistry for big and little molecules alike. This last statement holds also for the physical methods.
2.2.2 General Physical Methods
The more important physical methods of characterizing the microstructure of a polymer are summarized in Table 2.2. Nuclear magnetic resonance, infrared, and Raman spectroscopy are considered in the following sections (4).
Ultraviolet and visible light spectroscopy makes use of the quantized nature of the electronic structure of molecules. One example that is commonly observed by eye is the yellow color of polymeric materials that have been slightly degraded by heat or oxidation. Frequently this is due to the appear-ance of conjugated double bonds (5). For example, the 10-polyene conjugated structure absorbs light at 473 nm in the blue region.
Mass spectroscopy makes use of polymer degradation, and particular masses emerging are identified. For example, polymers having higher alkane side groups usually have a mass peak at 43 g/mol, oxygen as alcohol or ether at 31, 45, or 59 g/mol, and so on (6). Mass spectrometry also provides a pow-erful method of identifying residual volatile chemicals, which is becoming increasingly important in reducing air pollutants. Mass spectrometry further provides a newer method of determining polymer molecular weights; see
Section 3.10. Electron spectroscopy for chemical applications (ESCA) is a rel-atively new method useful for surface analysis of polymers; see Section 12.3.
X-ray (7) and electron diffraction methods are most useful for determining the structure of polymers in the crystalline state and are discussed in Chapter 6. These methods do, however, provide a wealth of information relative to the inter- and intramolecular spacings, which can be interpreted in terms of con-formations and configurations.
2.2.3 Infrared and Raman Spectroscopic Characterization
The total energy of a molecule, consists of contributions from the rotational, vibrational, electronic, and electromagnetic spin energies. These states define the temperature of the system. Specific energies may be increased or decreased by interaction with electromagnetic radiation of a specified
wave-2.2 THEORY AND INSTRUMENTS 33
Table 2.2 Physical methods of determining polymer chain microstructure (3)
Method Application Reference
Nuclear magnetic Determination of steric configuration in (a–c) resonance homopolymers; composition of copolymers,
including proteins; chemical functionality, including oxidation products; determination of structural and geometric and
substitutional isomerism, conformation, and copolymer microstructure.
Infrared and Raman Molecular identification: determination of (d, e) spectroscopy chemical functionality; chain and sequence
(considered length; quantitative analysis; stereochemical together) configuration; chain conformation.
Ultraviolet and Sequence length; conformation and spatial (f) visible light analysis.
spectroscopy
Mass spectroscopy Polymer degradation mechanisms; order and (g) randomness of block copolymers, side
groups, impurities.
Electron spectroscopy Microstructure of polymers, particularly (h)
(ESCA) surfaces.
X-ray and electron Identification of repeat unit in crystalline (i) diffraction polymers; inter- and intramolecular spacings;
(considered chain conformation and configuration.
together)
References: (a) F. Bovey, High Resolution NMR of Macromolecules, Academic Press, New York, 1972. (b) C. C. McDonald, W. D. Phillips, and J. D. Glickson, J. Am. Chem. Soc., 93, 235 (1971). (c) J. C. Randall, J. Polym. Sci. Polym. Phys. Ed., 13, 889 (1975). (d) J. Haslam, H. A. Willis, and M.
Squirrell, Identification and Analysis of Plastics, 2nd ed., Ileffe, London, 1972. (e) J. L. Koenig, Appl. Spectrosc. Rev., 4, 233 (1971). (f) Y. C. Wang and M. A. Winnik, Macromolecules, 23, 4731 (1990). (g) J. L. Koenig, Spectroscopy of Polymers, 2nd ed., Elsevier, Amsterdam, 1999. (h) D. T.
Clark and W. J. Feast, J. Macromol. Sci., C12, 192 (1975). (i) G. Natta, Makromol. Chem., 35, 94 (1960).
length. In the following discussion, it is important to remember that all such interactions are quantized; that is, only specific energy levels are permitted.
Infrared spectra are obtained by passing infrared radiation through the sample of interest and observing the wavelength of absorption peaks. These peaks are caused by the absorption of the electromagnetic radiation and its conversion into specific molecular motions, such as C—H stretching.
The older, conventional instruments are known as dispersive spectrometers, where the infrared radiation is divided into frequency elements by the use of a monochromator and slit system. Although these instruments are still in use today, the recent introduction of Fourier transform infrared (FT-IR) spec-trometers has revitalized the field (4). The FT-IR system is based on the Michelson interferometer. The total spectral information is contained in an interferogram from a single scan of a movable mirror. There are no slits, and the amount of infrared energy falling on the detector is greatly enhanced.
Together with the use of modern computer techniques, an entirely new breed of instrument has been created.
Raman spectra (8) are obtained by a variation of a light-scattering tech-nique whereby visible light is passed into the sample. In addition to light of the same wavelength being scattered, there is an inelastic component. The physical cause involves the light’s exchanging energy with the molecule. This inelastic scattering causes light of slightly longer or shorter wavelengths to be scattered. As above, there is an increase or decrease in a specific molecular motion.
Raman and infrared spectroscopy are complementary because they are governed by different selection rules (4,7,9). In order for Raman scattering to occur, the electric field of the light must induce a dipole moment by changing the polarizability of the molecule. By contrast, infrared requires an intrinsic dipole moment to exist, which must change with molecular vibration.
The fields have advanced way beyond the simple determination of spectra and correlating particular bands with particular chemical groups. Today, specific motions are calculated. For an example, see Figure 2.1 (4). Here, two conformational displacements of polystyrene are shown—one near 550 cm-1in the infrared spectrum, and one near 225 cm-1in the Raman spectrum.
These motions illustrate a degree of coupling between the ring and backbone vibrations.
2.2.4 Nuclear Magnetic Resonance Methods
Although X-ray (7), Raman spectroscopy (8), and infrared methods (9) are at the disposal of the polymer scientist for structural analyses, by far the most powerful method is nuclear magnetic resonance (NMR). Briefly, when the spin quantum number of a nucleus is–12 or greater, it possesses a magnetic moment. A proton has a spin of –12and is widely used in NMR studies. When placed in a magnetic field H0, it can occupy either of two energy levels, which corresponds to its magnetic moment, m, being aligned with or against
the field. The energy differences in the two orientations are given by Bovey et al. (10):
(2.5) The quantity DE indicates the energy that must be absorbed to raise the nuclei in the lower state up to the higher level and is emitted in the reverse process.
The separation of energy levels is proportional to the magnetic field strength.
In a field of 9400 gauss, the resonant frequency for protons is about 40 MHz.
For field strengths of the order of 10,000 gauss and up, the frequency, v, is in the microwave region.
In a molecule containing many atoms, the field on any one of these is altered by the presence of the others:
(2.6) Where HLis the local field with a strength of 5 to 10 gauss. It is these changes that are important in NMR characterization.
Other nuclei besides hydrogen (1H) that have a spin of–12or greater, and are used in NMR studies, include deuterium (2H), fluorine (19F), carbon-13 (13C), nitrogen-14 and -15 (14N and 15N), and phosphorus-31 (13P). Much higher resolutions are often possible with these nuclei, allowing exact sequences of structures to be determined along the chain.
A new technique for 13C NMR is the so-called magic angle method, which uses oriented specimens spun around an axis at q = 54.7° to reduce line broadening due to anisotropic contributions. This particular angle arises because the broadening component is proportional to the quantity 3 cos2q - 1, where q is the angle between the line connecting the nuclei and the direction of the magnetic field in isotropic compositions. At 54.7° this quantity is zero.
DE=2m(H0+HL) DE=h 2= mH0
2.2 THEORY AND INSTRUMENTS 35
Figure 2.1 Motions associated with the 567 cm-1infrared peak of polystyrene. Quite different motions are associated with the Raman peak at 225 cm-1, not shown.
While the fundamental unit for determining shifts in NMR peaks is the change in frequency in hertz, an important practical scale is based on the posi-tion of the tetramethylsilane peak, leading to the t scale (see Figure 2.6). This scale is now outmoded, and being replaced by scales based on shifts of parts per million, ppm (see Figure 2.8).
2.3 STEREOCHEMISTRY OF REPEATING UNITS