VIBRATIONAL SPECTROSCOPY Introduction One of the most valuable realizations of a polymer scientist dealing with structural characterization is that use of many different techniques is required to fully understand polymer structures of different scales. Gel permeation chromatography, light scattering, and mass spectrometry (qv), particularly matrix assisted laser desorption mass spectrometry (MALDI), characterize overall molecular weight. Nuclear magnetic resonance can elucidate polymer chemical composition. Transmission electron microscopy is used to characterize density fluctuation within the sample to obtain detailed morphological features such as crystallite form and degree of phase separation in compatible systems. For information regarding atomic placement, X-ray and electron diffraction methods are generally utilized. Differential thermal calorimetric measurements provide insight regarding structural changes as functions of temperature and processing history. Vibrational spectroscopy is one of the oldest and most powerful technique for analyzing polymer composition and structure. In addition, the principal advantage for use of vibrational spectroscopy as a morphological tool is that it is extremely selective. Both crystalline and amorphous polymer structures can be characterized. Conformational order ranging from the shortest chemical repeat to several hundred angstroms can be measured. Unlike diffraction techniques which depend on long-range order or disorder, vibrational spectroscopy is characteristic of chain conformation at a more localized level. Although numerous techniques address polymer structure in the crystalline state, few provide information on the disordered state, a subject capable of being studied using vibrational spectroscopy. In fact, vibrational spectroscopy is routinely utilized to characterize polymer composition, segmental orientation, chain conformation, and intermolecular 311 Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
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interactions. Because of significant advances in instrumentation and simulation software, most laboratories possesses instrumentation capable of producing excellent data. Fibers, films, solutions, and melts can be analyzed with great confidence. The sample under examination need not be static in nature. Time-dependent phenomenon can be measured with time resolution as short as microseconds. It is also possible to measure samples in the range of micrometers or smaller with dedicated accessories. Vibrational spectroscopy provides information complementary to other characterization techniques and has proven of enormous value to studies in polymer science and engineering. Applications of vibrational spectroscopy of polymers are generally divided into two subsets: infrared absorption spectroscopy and Raman scattering. Each offers particular advantages. The physical origin of the infrared and Raman effects are quite different, however. One depends on the dipole change and the other on changes in polarizability tensor (1). Different vibrational modes are therefore active in Raman and infrared. The more symmetric the molecule, the greater the difference between Raman and infrared spectra (1). The two techniques are truly complementary, however. For both cases, although more so for infrared, substantial literature exists regarding their utility for polymer characterization (2–8). Infrared spectroscopy is more appropriate for studies of chemical composition and side groups. Raman scattering is more effective for structural characterization since intense Raman scattering is often associated with the significant polarizability changes of the all-carbon backbone of the polymer chain and can be used much more readily to analyze chain conformation. They can be implemented in a broad spectrum of configurations including remote sensing and for studying solids, liquids, and gases. Intractable samples can be studied without much preparation. The power of vibrational spectroscopy, ie its selectivity and sensitivity, cannot be overestimated. Most research laboratories possess an excellent Fourier transform infrared and/or Raman spectrometer. These instruments are accurate, sensitive, durable, versatile, easy to use, and inexpensive to purchase. Extensive software is available for instrument control and to correlate spectroscopic features to the possible molecular functional groups from which they originate.
Infrared, Raman, and Normal Mode Molecular spectroscopy is now a mature field of study. It is, however, difficult to find references superior to the classic treatise written by Herzberg nearly 50 years ago (1). The origin of vibrational spectra is usually considered in terms of mechanical oscillations associated with mass of the nuclei and interconnecting “springs” (9). Vibrational spectroscopy considers the frequency, shape, and intensity of internuclear motions due to incident electromagnetic fields. In the harmonic approximation, the vibrational bands are associated with transitions between nearest vibrational states. When higher order transitions, resonance, and coupling between vibrational motions require analysis, quantum mechanical treatment is mandated (1). Improvements and advancements in polymer spectroscopy are driven by the many problems of interest in the polymer community. Infrared Spectroscopy. Infrared absorption spectroscopy is among the oldest and most widely used characterization techniques available to polymer
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scientists. Without exaggeration, nearly every laboratory has an excellent Fourier transform infrared spectrometer. Sample preparation does not require significant effort. Polymers mixed with potassium bromide and then pressed into pellets or films prepared from melt or cast from solution can be easily studied. For bulk samples or powders, or if a concentration profile is needed for a film, the reflectance technique is perhaps more suitable (10). With the recent improvements in instrumentation, quantitative spectroscopic analysis of polymer solutions is not difficult to achieve. The availability of small yet powerful computers attached to instruments has provided new and powerful analysis routines for structural characterization. Normal vibrations related to a change in dipole moment are infrared active. Groups with large dipole moments, such as C O and N H, typically have strong infrared absorptions. The majority of reported spectroscopic studies by infrared spectroscopy focus on determination of polymer molecular composition by analysis of characteristic vibrations of functional groups. The power of vibrational spectroscopy, ie its selectivity and sensitivity, cannot be overestimated. With accurately defined band assignment, particularly if the transition dipoles are well established, quantitative analysis of sample anisotropy in terms of segmental orientation can be accurately established. Virtually all instruments employ an interferometer for infrared spectroscopy (11,12). Extremely accurate positions, intensity, and shape can be obtained. An interferometer generates information from the entire range of a given spectrum during each scan. The instruments, which are routine and extremely reliable, collect large amounts of energy at high resolution. Data are obtained in a relatively short time scale. Time resolution of milliseconds can be carried out with ease. With additional modifications, high temporal resolution as short as microseconds can also be accomplished with commercial instrumentation. Instrument improvements make far- and near-infrared spectroscopy feasible for structural characterization. Tremendous advancements in a number of applications have been made possible by infrared spectroscopy. Specialized accessories facilitate surface studies. Developments in imaging techniques make high spatial resolution spectroscopy possible (13). High temporal resolution to follow kinetic developments can be accomplished with great precision and signal-to-noise ratio. Specialized techniques such as time-resolved spectroscopy and two-dimensional spectroscopy have contributed significantly to the understanding of molecular dynamics (14–16). Structural variations such as degree of crystallinity, segmental orientation, and composition at surface or bulk, can all be analyzed with the use of different types of spectroscopies. The development of photoacoustic spectroscopy should also be noted (17). This technique has contributed significantly to analysis of polymer structure, both surface and bulk, with sensitivity and speed. These application-driven developments are discussed below in greater detail. Raman Scattering. Although the Raman effect was first reported in 1928 (18,19), its utility as a characterization tool was not realized until the advent of lasers in the early 1960s (20). With the recent advances in lasers, detection systems, and spectrometer design, a tremendous potential exists for use of the Raman technique as a characterization technique. Since it is a scattering technique, samples such as powder, film, solid, filament, or solution can all be studied
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with little sample preparation. Biological samples can also be characterized since scattering from water is extremely weak. As virtually all polymers used commercially are carbon based, the change in polarizability along the backbone is large. Raman scattering is therefore particularly suitable for characterization of polymer backbone conformation. Confocal Raman microscopes are available with spatial resolution approaching the diffraction limit of the focused laser beam. The lasers generally used for Raman scattering produce highly polarized excitation radiation. The polarized or depolarized scattered radiation collected are especially suitable for analysis of segmental orientation. In fact, various Raman techniques have been used on-line to elucidate effects of changing processing parameters on morphology development in uniaxial and biaxial polymers. Despite its capability for delivering detailed structural information, Raman scattering has not yet achieved equal status with other characterization techniques. Its development has been slow because of the high initial investment and maintenance costs (high power lasers, sophisticated spectrometers, sensitive detectors, etc). The Raman effect is extremely weak, eg, only 1 in 104 or many fewer incident photons result in inelastic scattering (21). Intense excitation sources are needed which may also cause fluorescence that overwhelms the Raman signals. The earliest Raman studies on polystyrene and poly(methyl methacrylate) were possible only because the samples were totally clear and fluorescence free. Raman spectra can today be readily obtained from most polymers. Recrystallization or extraction has been effective in removing impurities that cause fluorescence. Another alternative is to use long wavelength excitation (1.06 µm) as in most Fourier transform instruments. Nevertheless, the Raman technique yields tremendous structural information and effectively complements other characterization techniques. Many Raman instruments are designed around a dispersive spectrometer. In comparison to infrared instruments, this type of spectroscopy is much slower. There are, however, distinct advantages. These are (1) Since many excitation sources are in the visible range, the optical elements are generally much simpler to deal with in comparison to infrared. (2) The photomultiplier exhibits much flatter quantum efficiency. (3) Lower frequency vibrations can be studied with ease. (4) Because of the shorter excitation wavelength, high spatial resolution can be achieved for imaging studies. In addition, with the tremendous improvements in multichannel charge-coupled device (CCD) detectors, high quality Raman data can be obtained efficiently. The Raman effect, an absorption and emission phenomenon, can be described in the following fashion. Imagine placing a molecule in an electric field. This molecule will be polarized by the electric field; in other words a dipole will be induced in the molecule such that the electrons are distorted away from the direction of the field. If the electric field is oscillating (with frequency ω0 ), the induced dipole will oscillate with the same frequency, ω0 ; this is typical Rayleigh scattering. Now imagine the case where the molecule is already vibrating with some other frequency ωi (for example, bond stretching). The interaction between the oscillating
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molecule and the incident radiation will result in both elastic (Rayleigh) and inelastic (Raman) scattering. The polarizability of the oscillating molecule is described by α = α0 +
i = 3N − 6
∂α Qi + · · · = α0 + ∂ Qi
i=1
i = 3N − 6
αi Qi + · · ·
(1)
i=1
where Qi describes the vibration of the molecule. The interaction of the oscillating molecule with the incident radiation can be expressed as Iscattered = α E =
α0 +
i = 3N − 6 i=1
∂α Qi + · · · E ∂ Qi
(2)
The incident radiation is described by E = E0 cos ω0 t and the vibrational mode is described by Qi = Qi0 cos ωi t. After substituting these two expressions into the above equation, we get a description for scattered light by an oscillating molecule. Iscattered = α E =
α0 +
i = 3N − 6 i=1
= α0 E0 cos ω0 t +
i = 3N − 6 i=1
= α0 E0 cos ω0 t +
i = 3N − 6 i=1
∂α Qi + · · · E ∂ Qi
∂α 0 Q E0 cos ωi t cos ω0 t ∂ Qi i
(3)
∂α 0 Q E0 [cos(ω0 + ωi )t + cos(ω0 + ωi )t] ∂ Qi i
The scattered light includes both elastically scattered radiation (ie Rayleigh scattering) and inelastic scattering (Stokes and anti-Stokes Raman scattering), is dependent on the change in polarizability and is described by a tensor. In general, the scattering tensor (real and symmetric) is defined as
α11 α12 α13 αmol = α21 α22 α23 α31 α32 α31
(4)
Six independent elements need be determined. In this case, we designate α ij ’s to represent the change in polarizability elements in respect to the displacement relative to the equilibrium positions. The elements depend on the molecular symmetry, functional groups, and degree of orientational order in the sample. When the symmetry of the translational repeat of polymers is known, analysis of polarized Raman scattering yields significant structural information. The chain segment orientation distribution, the parameters needed to achieve the degree of anisotropy, and assignment of particular bands can all be obtained (22–27). Unless the sample is an oriented macroscopic single crystal (almost never the case), some polarization information in the scattered light is lost. Partially
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oriented systems can be either uniaxially oriented such as in fibers or planarly (biaxially) oriented films. Here the relationship between the molecular and laboratory frames is partially lost. Thus, specific information about each of the six elements is lost. Instead the tensor elements combine forming a new set of four elements, each being a linear combination of the previous six elements (24). These are useful for structural analysis and band assignment. The molecular orientation with respect to the laboratory frame can be completely random as in liquids or melts. Each scattering center is randomly oriented relative to the laboratory frame such that the scattering tensor is reduced to the relationship between the isotropic and anisotropic portions. Even under these circumstances, information about the scattering tensor elements associated with various modes can be obtained through measurements of the depolarization ratio ρ, defined to be the ratio of the scattered light intensity obtained when the scattered and incident beams are linearly polarized perpendicular to each other to that obtained when they are polarized parallel to each other (20). Careful analysis of polarized Raman spectra can still yield information regarding the local chain conformation, interaction with solvents, degree of chain extension, or amorphous portions of semicrystalline polymers or glassy solids (28–34). Since most polymers of interest involve carbon backbones, the differential polarizability tensor elements have large isotropic elements. In many cases, only the isotropic spectrum is used for structural characterization, thus eliminating vibrations unrelated to conformational analysis (35–38). Nature of Normal Vibrations. The vibrations observed using either infrared or Raman deal with oscillatory movements of nuclei vibrating about their equilibrium positions. This is based on the fact that in the presence of an exciting electromagnetic radiation, electronic and nuclei motions can be considered separately (1) and is generally referred as the Born Oppenheimer approximation (1). Quantum mechanics and associated wavefunctions need be considered in order to analyze the intensity of vibrational transitions. In addition, coupling of vibrations and other second-order effects can only be addressed using quantum mechanical formalisms. The classical picture involving masses and spring constants is the norm, however. In this case, the equilibrium geometry of molecules and polymers is the basis for studies on the optical activity of various infrared or Raman active modes (9,39). Both infrared and typical Raman spectra characterize the absorption of energy from the incident beam by the relative motions of nuclei associated with a polymer. Vibrations are considered in terms of the classical expressions governing motion of nuclei vibrating about their equilibrium positions with a simple harmonic motion (40). The potential and kinetic potential energies of molecules are defined in terms of the coordinates most appropriate to the molecular structures. All relative motions of atoms about the center of mass (vibrations) are linear combinations of a set of coordinates, known as normal coordinates. For every normal mode of vibration, all coordinates vary periodically with the same frequency and pass through equilibrium simultaneously. Application of vibrational spectroscopy depends extensively on characteristic vibrations associated with functional groups (5). For composition analysis, the software associated with various spectrometers is capable of searching the extensive reference library to yield the best match for the measured spectra. For more
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quantitative analysis of sample composition, changes in chain conformation, or packing, it is often necessary to consider the relative band intensity or bandwidth. A normal coordinate analysis (described below) is helpful in band assignment and interpretation of observed features. For a nonlinear small molecule of N nuclei, there are 3N-6 vibrations. The 6 degrees of freedom associated with the overall molecular translations and rotations cannot be considered as vibrations. However, the spectra obtained for polymers are not as complicated as might be imagined considering the extremely high molecular weight of most polymers of interest. In fact, for infinite polymers of well-defined conformation, there are 3N-4 optically active vibrations, where N now refers to the number of nuclei per translationally equivalent structural unit (8,39,41). In comparison to small molecules, 2 additional vibrational transitions exist because only the rotation about the chain axis can occur without being drawn back toward the equilibrium position. The optical activity of expected vibrations can be analyzed based on the helical chain structure (8,39,41). For polyethylene, the translational equivalent repeat is CH2 CH2 ; N is 6. For polypropylene N is 27, since there are nine atoms per chemical repeat and three units form a translationally equivalent repeat unit. For highly ordered polymers or oligomers, symmetry analysis or group theory is a powerful tool for determining the number of expected vibrations and their optical activities (40). The infrared and Raman spectra obtained for a highly crystalline polyethylene film are shown in Figure 1. Numerous studies have been carried out to elucidate the vibrational features of polyethylene or n-alkanes (42–44). Note that in Figure 1, none of the bands present in the infrared are Raman active. This mutual exclusion reflects the fact that the polyethylene chain conformation has inversion symmetry (1,6,8,45–47). Most polymer solids contain both crystalline and amorphous regions. The number of vibrations observed is generally higher than expected purely from symmetry considerations of well-defined chain conformation. In polyethylene, for example, by measuring a series of samples containing different degrees of crystallinity, vibrational transitions associated with irregular chain conformations, presumably in the amorphous regions, can be found in the 1300 cm − 1 region (Fig. 2). Since the structures are irregular, transition assignments cannot be made from symmetry considerations and numerical analysis is required as demonstrated below. Coupled Oscillator Model. The theory that vibrations couple (such as crystal field splitting in ordered polymers) is generally accepted. Individual chemical repeats may vibrate independent of each other. As polymer spectra do not significantly differ from the constituent monomers, this is the basis for analysis of composition analysis using “fingerprint” features. Information regarding chain conformation, configuration, or packing in the condensed state may reside in the band splitting, band shape or position of the observed vibrations. The degeneracy of the vibrations of functional groups in each chemical repeat may be lifted due to the covalent interconnecting bonds. This strong interaction may introduce additional features expected from consideration of a single monomer unit. The presence of secondary intermolecular interactions may introduce additional features (7). This can be demonstrated most clearly in spectra obtained for an oligomer with well-defined chain conformations arising from the connectivity of the chain of repeat units. These additional spectroscopic features, found in both infrared and
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Fig. 1. Infrared and Raman spectra for polyethylene.
Raman spectra of oligomers, have been explained in terms of a coupled oscillator model (42,48). Detailed understanding of the vibrations found for oligomers yields considerable insight into vibrational spectra obtained for polymers. The idea that vibrations can couple is quite general. For each vibration with frequency ω0 found for the individual chemical repeat units, a series of vibrations arising from the backbone coupling may shift from the unperturbed value. For a chain with N chemical repeat units or N oscillators, instead of a set of N degenerate vibrations of frequency ω0 , a series or progression of vibrations may exist. Representative infrared and Raman spectra for C20 H42 are shown in Figure 3. The frequency separation of the progression modes is inversely proportional to the unperturbed frequency ω0 . The localized vibration, such as the CH2 stretching
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Fig. 2. Amorphous bands found for polyethylene: (a) polyethylene film quenched from melt; (b) same film after annealing.
Fig. 3. Progression bands observed for C20 H42 : (a) infrared spectrum; (b) Raman spectrum.
vibration in the 3000 cm − 1 region, is hardly perturbed by the interaction along the chain. In contrast, low frequency components such as the CH2 rocking vibration at 700 cm − 1 , can be perturbed significantly. The frequency of individual vibrations can be expressed as ωi2 = ω02 + ωi2
(5)
where ω0 is the unperturbed frequency of an oscillator in the chemical repeat. The ωi is the frequency of the ith mode in the progression. ωi2 is the perturbation dependent on geometric parameters (conformation) and interactions between adjacent units. Seldom is there a need to consider interactions beyond adjacent units. When molecular weight increases, the intensity is concentrated at the unperturbed position with an asymmetric band shape (7). For chains with well-defined conformation, the perturbing effects of coupling are clear and can be analyzed. Spectroscopic features can therefore be used to analyze chain conformation. For disordered structures, the vibrational spectra observed may be complex owing to
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Fig. 4. Crystalline field splitting observed for polyethylene: (a) film quenched from melt; (b) film annealed at 80◦ C.
changes in both band frequency and shape. In this case, the vibrational mode can be quite complex because contributions of various internal coordinates to the vibration (character of the vibration) may change significantly as compared to an ordered structure. When understood, changing frequency and character of several vibrations provide tremendous insight into the structure of polymers in solution or melt. Additional features are observed for highly crystalline polymers. The crystalline structure of polyethylene has been well characterized (45). There are two chains per crystalline unit cell. Just as strong coupling exists between vibrations along the polymer chain, coupling can also occur for equivalent vibrations in the crystalline unit cell. The nonbonded intermolecular coupling between the two chains effectively removes the degeneracy between the equivalent oscillators associated with the chemical units of the two individual chains. Instead of one component near 720 cm − 1 , a doublet at 721 and 730 cm − 1 is observed (Fig. 4). The magnitude of the splitting and the relative intensity of the two components depends on the specificity and magnitude of the nonbonded van der Waals interactions between the two chains in the unit cell. In trans-1,4-polybutadienes, the splitting, particularly at low temperatures, seen at 240 cm − 1 is consistent with predictions from theoretical analysis (49). Similar observations have been found for crystalline structures of proteins and model polypeptides. There the hydrogen bonded amide units exhibit well-defined spectroscopic features due to transition dipole coupling (50,51). Similar effects in poly(lactic acid) have recently been observed (52,53). Even the isolated C O stretching vibration is sufficiently perturbed to have at least three components. In this case, strong dipoles of the ester groups affect both chain conformation and chain packing. The nature of intermolecular interactions can also be studied by analyzing the frequency and intensity of the external vibrations, or lattice modes, of the unit cell (49,54,55). The coupling of chains can be characterized by the whole chain segment movements which generally occur in the very low frequency region, ie, typically 2000) of finite chains are considered.
Structural Characterization The use of vibrational spectroscopy to identify molecular functional groups of polymers has been historically successful. New types of problems and specific requirements demand that vibrational spectroscopy move beyond composition analysis to specific areas such as surface characterization, imaging at specific areas of interest, kinetics of phase transformation, and others. The examples presented below demonstrate the developments in the most basic structural characterization. Chain Configuration. Infrared spectroscopy has many fingerprint bands characteristic of functional groups. Extensive collections of spectra have been published. In addition, libraries and efficient search routines can match experimental data to those in the collection and identify those present. Raman spectroscopy, unlike the infrared technique, has not been extensively utilized for composition analysis. For many functional groups, Raman scattering can, in fact, make composition analysis a relatively easy task. For example, cis–trans structures of polybutadienes, vinyl C C stretching, compounds containing sulfurs, and perchlorates, all exhibit strong, easily identified Raman bands for analysis. Aromatic units typically exhibit strong Raman scattering. Unique applications employ both types of spectroscopies for structural analysis. The complementary nature of infrared and Raman effects can be seen in the structure determination of 1:1 hexafluoroisobutylene and vinylidene fluoride copolymer. This problem is virtually impossible to solve with other techniques. The two monomers can be linked in two ways during polymerization, by formation of head-to-tail (normal linking) or head-to-head linkages. By comparing infrared and Raman spectroscopic data, it was concluded that the polymers formed are indeed alternating in nature (66). It was additionally concluded that only head-to-tail linkages were observed. The proposed chemical structure is shown below. (C(CF3 )2 CH2 CF2 CH2 )n The CH2 units are decoupled by the presence of a rather bulky CF2 unit between them. If the copolymers were formed in a head-to-head linkage, considerable coupling of the stretching vibrations would be expected. In fact, in both infrared and Raman spectra, the CH2 symmetric and antisymmetric vibrations were observed at 3040 and 2985 cm − 1 . This coincidence provides proof that high frequency CH2 vibrations are decoupled and thus degenerate. If the polymer were formed in a head-to-head linkage, mutual exclusion would be expected, contrary to observation. Similar configurational analysis can be extended to other polymers formed with substituted monomers. Because of significant advances in catalyst design (67), synthesis of highly ordered polyolefins with fewer defects and improved control of defect distribution has become possible. These advances in chemistry have raised an even greater interest
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in the study of polyolefin structure and development of more diverse applications. In a head–tail alternating copolymer, the possibility exists that a head–head or tail–tail defect linkage can occur. Specific spectroscopic features have been identified for analysis of chain–chain configuration in syndiotactic polypropylenes (sPPs) containing varying racemic content (37). The earliest method for characterization of sPP configuration was, in fact, infrared spectroscopy. That study suggested a structural parameter referred to as “syndiotacticity index” (68,69). Raman features sensitive to tacticity distribution were subsequently found. The Raman active skeletal bending modes in the 300–400 cm − 1 region (Fig. 5) were sufficiently sensitive to differentiate between racemic and meso sequences for characterization of the overall chain configuration (37,70). Based on theoretical normal coordinate analysis, the syndiotacticity content of imperfect polypropylenes can be directly measured using the relative intensity of Raman active bands in the 300 and 400 cm − 1 region (Fig. 5). The syndiotacticity index, which provides a fast and extremely easy parameter for determination of chain configuration, is shown in Figure 6. It was also demonstrated that those bands are sensitive to differences in chain configuration and insensitive to chain conformation (64). Chain Conformation—Ordered. For sPP, the number of seemingly stable chain conformations and packing is intriguing. The relative volume fraction of each state is highly dependent on thermal history, time, processing, and, in many cases, type and number of configurational defects. At least three different crystalline forms are associated with sPP. The helical form I, with ggtt chain conformation, is the most common (71,72). Form II with an all-trans sequence can be obtained by quenching the polymer from the melt, followed by stretching (72). Form III, with conformational features of both forms I and II, has also been suggested. This form appears to have the conformation (g2 t2 g2 t6 )n (73–75). It was recently found that the planar zigzag structure forms spontaneously over an extended length of time by quenching from melt into ice water and storing at or below 0◦ C (76,77). Form III has also been observed when a drawn sample of form II is exposed to benzene, toluene, or xylene vapor (73,74). Both forms II and III are stable at room temperature, and then revert to form I spontaneously with heating. Raman spectra obtained from polypropylenes of different conformational distributions can be simulated accurately (37). Identification of specific features such as those centered at 800 and 400 cm − 1 enabled understanding of the change of polypropylene from a completely disordered state to a more ordered state or states including crystallization. Although the bands found in the 800 cm − 1 region (Fig. 7) are generally referred to as regularity bands, they have only recently been accurately assigned (37). The bands at approximately 826 and 845 are suggested to be associated with form I and the amorphous chains, respectively (70,78). Based on simulation results, the 830 cm − 1 band can only be associated with long helical ggtt chain conformations. All extremely short helical segments situated between either conformational or configurational defects contribute to the intensity in the middle 845 cm − 1 region. The broad band at 845 cm − 1 is the dominant feature of a polypropylene melt. With increasing configurational disorder, the 826 and 865 cm − 1 bands decrease in intensity relative to the 845 cm − 1 peak. The high frequency component is then assigned to an all-trans structure of fairly long length. Based on these experimental and simulated Raman features, it is possible to deduce that sPP crystallizes into a helical structure at crystallization temperatures
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Fig. 5. Fourier transform Raman spectra of various syndiotactic polypropylenes with known configurational defects: (a) experimental; (b) theoretical.
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Fig. 6. Syndiotacticity index for syndiotactic polypropylene developed based on the relative intensity of the Raman active 300 and 400 cm − 1 bands. Solid; trans SPP–orinted; 75% racemic–aging time; • Flory RIS calculations; spp melt; —∇— calculated curve; + syndiotacticity index.
above 0◦ C (Fig. 7). Conversely, for sPP crystallized at temperatures below 0◦ C, only all-trans structures are formed. Raman data are clear regarding the structures formed (37,64). The mechanisms responsible for such behavior are still unavailable. Vibrational spectroscopy can also determine chain packing and associated structural transitions in semicrystalline polymer solids. As mentioned above, intermolecular vibrations, lattice modes characteristic of whole chain segment movements, or delocalized skeletal modes of fully ordered crystalline states, are found in the low frequency region (49,54,55). For polymers with a well-defined crystalline state, the lattice modes are extremely sensitive to changes in the magnitude and specificity of intermolecular interactions (49). For highly crystalline trans-1,4-polybutadiene, the vibrations, which were observed below 200 cm − 1 in the infrared and Raman spectra, can all be assigned to lattice modes (Fig. 8) (49). This is indicated not only by the fact that the single-chain calculation predicts no modes below 200 cm − 1 (49,79), but also by temperature dependence. When
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Fig. 7. Raman spectra obtained for syndiotactic polypropylene as a function of time at room temperature. The initial sample is obtained by quenching a molten sample into liquid nitrogen.
the temperature is lowered, the four Raman bands at 48, 70, 104, and 118 cm − 1 all exhibit large shifts to higher frequencies. This is characteristic of interchain modes, since intermolecular interactions increase in magnitude as the distances diminish in the contracted unit cell. Crystal field splittings should also increase under these circumstances, as seen for the bands near 240 cm − 1 . In addition, if the TPBD (trans-1,4-polybutadiene) sample is heated beyond the crystal phase transition, the four Raman bands disappear. This is as expected, since not only do distances between chains increase but torsional oscillations within the chain increase. Both features diminish the magnitude and specificity of interchain interactions. Several studies have used TPBD as a model to investigate intermolecular forces and their influence on chain conformation. By incorporating intermolecular atom–atom interactions derived by other characterization techniques, these lattice modes and crystal field effects are all satisfactorily accounted for (49,79). By studying TPBD in the highly crystalline state as well as in single crystals, bands associated with noncrystalline conformations were also assigned. Chain Conformation—Disordered. An area of current interest is application of vibrational spectroscopy, particularly Raman, for analysis of disordered chains which may include polymers in solution, melt, or the solid state. These chains lack long-range order but may contain short ordered sequences. In addition, these disordered chains may adopt a specific conformational distribution,
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Fig. 8. Low frequency Raman spectrum of TPBD at 110 K, using an iodine absorption cell. Bandpass, 3 cm − 1 , laser power, 150 mW at 5145.42 ˚A.
depending on geometric constraints such as surfaces or interfaces. Other constraints may include the presence of junctions such as in networks. Long-range electrostatic forces along the chain may reasonably be expected to further perturb the structure. Polyelectrolytes (qv) or polymeric electrolytes are such examples. Many characterization techniques are capable of measuring ordered structure. Few, however, match the capability of vibrational spectroscopy to quantitatively describe disordered chains. The frequency and relative intensity of observed vibrational bands are dependent on the relative concentration of specific localized structures. For samples involving well-defined structures and dependable band assignments, the method works well. However, when a broad distribution of a large variety of conformations is involved, the method is difficult to apply with confidence. The bands of disordered structures are broad, weak, and often overlap. Vibrational spectroscopy, particularly Raman, is ideal for studies involving disordered structures as polarizability changes associated with different carbon–carbon backbone conformations are directly reflected in the obtained spectra. Because it is a scattering phenomena associated with a tensor, the depolarized data still contain considerable structural information for samples of partial disorder or even in melt or solution. Based on relative intensity and, particularly, bandshape of skeletal bands, it has become possible to obtain the relative energy difference between various rotational isomeric states. In the first study of this type, Snyder and co-workers analyzed low frequency (0–600 cm − 1 ) Raman spectra of n-alkanes in the liquid state (35). The method was subsequently applied to analysis of the low frequency Raman spectrum of molten state isotactic polypropylene. The vibrational spectroscopic analysis was successfully used to differentiate the correct model governing the chain (80) and has been extended to analysis of higher frequency vibrations (0–1500 cm − 1 ) of liquid n-alkanes (36). The utility of Raman spectroscopy has proven useful in the study of poly(ethylene oxide) (PEO) in both aqueous solutions and molten states (38). The
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structures of this family of polymers, either homopolymer or part of a copolymer, determine properties in numerous applications including thermoelastomers (81– 83), polymeric electrolytes (84–87), and surfactants. Steric considerations alone suggest that the minimum energy conformer is all trans, designated ttt, in a notation that successively references backbone O C, C C and C O bonds. However, the most stable conformation in the crystal includes a gauche conformation and is designated tgt (62). Since the conformation present in the crystal must be the result of the interaction of a number of factors, there is no way to use the crystal structure to predict with certainty the dominant conformation present in the melt or solution. Chain structure departs from tgt and changes to ttt when PEO is blended with poly(methyl methacrylate) (88). The structure of adsorbed PEO chains in the air–solution interface was found to be very different from the disordered structure expected for the solution state (89–91). In fact, the vibrational bands for these adsorbed chains have characteristics similar to those in the crystalline state (90). Various theoretical studies have led to quite different results for chain conformations of PEO (62,92–98). Because of differences in the local dielectric environment, different sets of rotational isomeric states need be considered. Several liquid systems of PEO, both molten state and solution, have been investigated using vibrational spectroscopy (99). The Raman spectrum of PEO in the melt as well as chloroform and aqueous solutions reveal that the conformations in an aqueous solution retain the tgt conformation that occurs in the crystalline solid. The Raman spectrum of the chloroform solution, however, more closely resembles that of the melt. The conformational distribution was not deducible due to uncertainty in band assignments. The isotropic Raman spectra of PEO in the melt and aqueous solution shown in Figure 9 suggest new indicators for characterizing conformational distribution. Considerable differences are evident in band shape for the 850 cm − 1 band and the region near 365 cm − 1 (38). Although insignificant in the normal Raman spectrum, the shape and position of features in these two regions differ substantially in the isotropic spectrum. The strong bands in the 850 cm − 1 region are almost mirror images in the two experiments (Fig. 9). The most intense feature in the 800 cm − 1 region for the aqueous solution is located at 880 cm − 1 . In contrast, the intense feature for the melt centers at 830 cm − 1 . The shapes of the skeletal deformation in the 200 cm − 1 region are also quite different in frequency and shape. These are to be correlated with differences in conformational distributions in the melt and aqueous solution of PEO (Fig. 9). The relationship between the global chain conformation for polyelectrolytes, in terms of a persistence length and excluded volume parameter to chain conformation described by a rotational isomeric state model, is untested. Vibrational spectroscopy can partially close the gap by measuring conformational changes as chain stiffness is varied. Most previous solution characterizations of polyelectrolytes employed methods such as viscometry and light scattering which provide only global structural information. These studies were interpreted to show that chain stiffness increases with ionization. It has been suggested that in the extreme case the chain is fully extended. Polarized Raman spectroscopy is ideally suited to address issues of local structure and thus complement previous work (33,34). Information provided by polarization studies, which are sensitive to
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Expt. for melt gg′g 11%
ttt 14%
tgg′ 37%
Simulated
ttg 7%
tgt 27%
tgg 4% Expt. for aqueous solution tgg′ 13%
gg′g 0%
ttt 11%
tgg 16%
ttg 9%
tgt 51% Simulated
1000
800
600
Wave number, cm−1
Fig. 9. Simulated and observed isotropic Raman spectra of poly(ethylene oxide). Different conformational distributions for melt and aqueous solution are described in text.
changes in conformational distribution, ie changes in the distribution of torsional angles among the various rotational states permitted in a vinyl chain backbone, can yield substantial structural information. Similar approaches have been pursued to study conformational changes in neutral polymer solutions, for example to identify the θ condition or quantify the enthalpy or entropy of helix formation (28–32). A direct calculation of the persistence length from depolarization data has been hampered by the absence of polarizability derivative values for different conformations, even in small molecules. Other studies (100,101) make it possible, in conjunction with a simplified form of the rotational isomeric state model, to calculate the persistence length using the CH stretching depolarization ratio of poly(acrylic acid) (PAA) (33,34). Several interesting aspects of ionized structure were revealed by the Raman study (33,34). Based on polarization data, the E, C∞ , and Lt values can be calculated from the depolarization data. The energy difference between isomers grows by a surprisingly low value of approximately 8% as compared to the initial value of 6155 J/mol (1471 cal/mol) for the unionized polymer to 6627 J/mol
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(1584 cal/mol) for the fully ionized sample. In comparison, the energy change between the same isomers is 600 cal/mol for polyethylene at room temperature and reportedly 1100–2300 cal/mol for polytetrafluoroethylene (62). The small change in conformer energy upon PAA ionization indicates that the polymer does not stiffen appreciably upon ionization in a salt-free, semidilute solution. The lengths calculated from spectroscopic data fall within the range of persistence lengths determined via small-angle X-ray scattering (SAXS) (102). Such analysis demonstrates the potential for the Raman technique to contribute to the understanding of the interplay of local and nonlocal polyelectrolyte forces. Analysis of Structural Unit-Size. The strength of vibrational spectroscopy lies in its ability to characterize chemical composition and localized structure of polymers. The observation of the intense longitudinal acoustic mode (LAM) in the extremely low frequency region (
