Multidimensional Spectroscopy of Polymers - American Chemical

Chapter 11

Multidimensional NMR Spectroscopy of Polymers Section Overview

Downloaded by COLUMBIA UNIV on March 27, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch011

Klaus Schmidt-Rohr Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003

Nuclear Magnetic Resonance (NMR) provides powerful techniques for characterizing molecular and supermolecular properties of polymers. It uses the magnetic and electric interactions of nuclear spins to analyze structure, orientation, and mobility of well-defined chemical moieties. The nuclear magnetic moments are excellent probes that do not affect the state of the system, as NMR energies are only 0.1 kJ/mol or less. Since many polymers are insoluble, or exhibit their most interesting properties in the solid state, NMR research of polymers is dominated by solid-state NMR studies. Nevertheless, solution NMR methods play an important complementary role by determining composition and configuration (1). The following provides a brief introductory overview of the large field of NMR of polymers, including multidimensional techniques. More complete treatments can be found in recent monographs (2-6). Basics of Pulsed NMR The nuclear magnetization (which is proportional the expectation value of the nuclear spin) is aligned with the external B field in thermal equilibrium. It can be manipulated, e.g. rotated to be transverse to the B field, by radio frequency(rf)pulses. These are applied to the sample by a coil 4-20 mm in diameter which is part of a resonant circuit. The coil generates B fields, perpendicular to the B field, which oscillate with a frequency that is resonant with the nuclear Larmor frequency 0

0

1

0

ωL = -γB (1) 0

Here, γ is the magnetogyric ratio of the specific nucleus. With typicalB field strengths of 5-18 Tesla, proton resonance frequencies are 200-750 MHz. Those of other nuclei of interest in polymers are 2.5 to 10 times smaller. The transverse nuclear magnetization precesses around the B field with a frequency ωL, inducing a voltage in the NMR coil. The resulting free induction decay (FID) is detected in a phase-sensitive detector and the radio frequency is subtractedfromthe signal. This is equivalent to the transition into 0

0

0097-6156/95/0598-0184$12.00/0 © 1995 American Chemical Society Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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Multidimensional NMR Spectroscopy of Polymers 185


the rotating frame, which is a general concept in magnetic resonance that greatly simplifies the analysis of the action of rf pulses and the behavior of the magnetization. Fourier transformation of the digitized FID produces the ID NMR spectrum. Usually, between tens and thousands of FIDs (scans) are added in the computer memory to improve the signal-to-noise ratio, since the noise in different scans partially cancels. In spite of the significant improvements by signal averaging and the Fourier technique, the sensitivity of NMR is still a limiting factor and makes it difficult to obtain spectra from isolated thin films or from surfaces in low-surface-area materials. Dipolar Decoupling. In solids, due to dipolar interactions between nuclear spins, dipolar decoupling is a prerequisite for obtaining highly resolved NMR spectra. High-power irradiation of the proton resonance achieves decoupling of heteronuclear dipolar interactions of protons with the nuclear spins of interest. This heteronuclear, as well as the more complex multiple-pulse homonuclear decoupling, can be explained stringently by the averageHamiltonian theory, which considers the average Hamiltonian in an interaction frame "toggling" under the action of the radio-frequency pulses (7,2). The homonuclear dipolar couplings of protons, generating line broadenings of 3075 kHz, are so strong that even with advanced multiple-pulse decoupling sequences (7), *H spectra of solid polymers attain a resolution of only 0.3 - 2 ppm, much less than the chemical-shift resolution of solution NMR. Therefore, C rather than U spectroscopy is used most often in solid-state NMR. 1 3

l

NMR Interactions, Magic-Angle Spinning, Spin Diffusion and Cross Polarization. The versatility, but also the complexity, of solid-state NMR arises in part from the variety of interactions, or local fields, to which the nuclear spins are subject. The chemical shift, resulting from the shielding of the Bo field by the electrons in the molecule, is characteristic of the electronic, i.e. chemical, environment of the nucleus. Its orientation-independent part, the isotropic chemical shift, is used extensively in both solution and solid-state NMR spectroscopy for chemical identification. To obtain highly-resolved chemical-shift spectra in solids, dipolar decoupling must be applied and the broadening by the chemical-shift anisotropy has to be removed by fast magic angle sample spinning (MAS), around an axis making an angle of 54.74 , the root of 3cos 0 1, with the Bofield(5). In other cases, the anisotropy of NMR interactions, i.e. the dependence of the NMR frequency on the orientation of the molecular segment relative to the Bo field, can be of great value, since it allows to measure segmental orientations and reorientations. The quadrupole coupling of deuterons has proven a particularly useful 'molecular protractor , which is also sensitive to segmental reorientations (9). The quadrupolar coupling probes the angle θ between the C- H bond direction and the Bofield,.accordingto e

2

r

1

2

β

2

ω(θ)=ω(0 ) (3cos 0-l)/2.

(2)

Dipolar spin-pair couplings, which probe the direction of the internuclear vector and the Bo field, can provide similar orientational (70) and dynamical (77) information. In addition, they can establish couplings between spins,

Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS

which is useful for probing proximities of segments (72,73), as well as for characterizing domain sizes by a multi-step magnetization-transfer process known as spin diffusion (14-16). Cross polarization from protons to rare nuclei, a technique widely used for signal enhancement in solids, also relies on dipolar couplings, which bring about heteronuclear magnetization transfer in the presence of radio-frequency fields of matched strength (77). Dipolar decoupling and cross polarization require double-resonance equipment for simultaneous rf irradiation on protons and the rare nuclei of interest In recent years, an increasing number of tripleresonance experiments has also been developed, which are applied to systems containing fluoropolymers ^ H - ^ F - ^ C ) , partially deuterated polymers ^ H C- H) (72), or Copolymers OH-13C-15N, i H - U C ^ P ) (75). Several of the following papers give aflavorof these developments. Downloaded by COLUMBIA UNIV on March 27, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch011

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Two- and Multi-Dimensional NMR The introduction of two-dimensional (2D) NMR (18,19) has greatly enhanced the potential of NMR and helped to make it one of the most versatile and powerful spectroscopic techniques available today. In such 2D NMR spectra, a correlation between peaks can be established that provide detailed structural or dynamical information; spectral overlap can be eliminated by separation of spectral patterns in the second dimension; and two-dimensional exchange intensity patterns can be obtained that characterize segmental dynamics in unprecented detail. Figure 1 shows the basic building blocks of a 2D experiment. By systematic incrementation of the evolution time ti and acquistion of the NMR signal during the detection period t2, a two-dimensional time signal s(ti,t2) is obtained. The evolution of the magnetization or coherence of the spin system with a frequency ωι is thus recorded indirectly through the modulation of the amplitude or phase of the actually detected NMR signal oscillating with the frequency o>2. The 2D spectrum, which is obtained by Fourier transformation of the 2D time signal s(ti,t2), will then exhibit a signal at (0)1,02). In a 2D exchange spectrum, off-diagonal intensity, at Û)2*CÛI, provides detailed information about the mixing process, which may be chemical exchange, molecular reorientation, or dipolar exchange. In 2D separation spectra, one dimension separates the signals by their well-resolved isotropic chemical shifts, while a broadline spectrum of the same site, reflecting dipolar couplings, chemical-shift anisotropy, quadrupolar couplings, or the like, is displayed along the other dimension. Three-dimensional (3D) NMR is a logical extension of the concept of 2D NMR. The third dimension can be used to separate overlapping 2D spectra (20,21), to correlate a further anisotropic interaction (22), or to study details of a complex exchange process (23,2). Due to the large size of the 3D data fields, the development of 3D NMR has been coupled with the increased availability of sufficient computer storage capacity. NMR in Structural Studies of Polymers. In the last two decades, NMR has seen the steady development of new or improved methods, many of them twodimensional, for the investigation of polymer structure on many levels, from the AngstrOm to the centimeter length scale:



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Multidimensional NMR Spectroscopy of Polymers

- Solution NMR and solid-state magic-angle spinning experiments characterize polymers in terms of chemical groups (0.1-1 nm). (7,5) - The nuclear Overhauser effect in solution and dipolar splittings in solids yield inter- and intramolecular distances (0.1-1 nm). (1,12,13,24) - Anisotropic interactions measure orientation distributions of various segments in both crystalline and amorphous regions of polymer fibers and films (0.2-2 nm). (9,22) - Conformation dependent chemical shifts, in solids for instance based on the γ-gauche effect, provide information on torsion angles along a chain (0.30.9 nm). (7,5) - Packing-induced chemical-shift differences confirm occurrence of various sites in the crystal structure (0.3-0.7 nm). (5) - Solution NMR yields information about polymer configurational statistics (0.3-1.5 nm). (74) - Relaxation times and lineshapes characterize molecular mobility in various phases (0.5-500 nm). (9,25) - Spin and noble-gas diffusion allow for estimating the sizes of supramolecular domains in semicrystalline polymers, polymer blends, and block copolymers (0.5-5000 nm). (14,15,26,27,2) - Magnetic-resonance imaging techniques can provide spatially resolved spectroscopic information (20 μτη-Ι cm). (28) Polymer Dynamics and Multidimensional NMR. Next to structural studies, the second large area of interest for NMR in polymer science is the investigation of molecular dynamics. By relaxation measurements, line-shape studies, and two-dimensional exchange experiments, correlation times between 10-iO-io-2 10-5-10- , and ΙΟ^-ΙΟ seconds can be determined, respectively. While structural NMR studies often have to compete with similarly powerful scattering techniques, multidimensional exchange NMR in solids is without rival in providing details about polymer dynamics on a molecular level. Two-dimensional exchange spectra without MAS exhibitridgepatterns which are characteristic of the geometry of the reorientation occurring during the mixing time. In H 2D NMR, a reorientation by a specific jump angle β givesriseto a pair of identical elliptical patterns (29,2). The dependence of the exchange patterns on β is shown in Figure 2. The ratio of the lengths a and b of the semi-axes of each ellipse is related to the jump angle β according to 1

2

j

2

Itan3l = b/a.

(3)

Diffusive or ill-defined motions, with a wide reorientation-angle distribution, exhibit featureless 2D exchange patterns (29,30). 3D exchange spectroscopy probes the orientation of a molecular segment three times, making it possible to investigate orientational memory and to determine the number of energy minima that are available to a reorienting molecular segment (2,25.). A reduced 4D experiment has proved useful for distinguishing heterogeneous and homogeneous distributions of correlation times (30). The contributions to the following NMR section include a variety of dynamical studies. They concern the nature of the α relaxations in semicrystalline and amorphous polymers, the β relaxation in poly(methacrylates), sidegroup dynamics, phenyl ring libration, motion in


188



evolution

mixing

detection

Figure 1. Schematic representation of the generic two-dimensional NMR experiment. During the evolution time ti, the spin system evolves with frequency ωΐ- After the mixing period, in detection time t2, the signal with frequency 002 is detected. The two-dimensional time signal obtained by systematically incrementing ti is Fourier transformed to yield the twodimensional NMR spectrum.

β

0°

10°

20°

30°

(180°)

(170°)

(160°)

(150°)

70° (110°)

60° (120°)

90°

80° (100°)

2

Figure 2. Contour plots of exchange patterns in H 2D NMR spectra. The figure shows the dependence on the reorientation angle β, which is the angle between the C - H bond orientations before and after the molecular reorientation. The spectra exhibit ellipticalridgeswhich are characteristic of β. 2


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Multidimensional NMR Spectroscopy of Polymers 189

liquid-crystalline polymers, and the diffusion of small molecules in a polymer solution. Segment Identification by NMR One of the great strengths of NMR is the assignment of structural or dynamical features to specific moieties or phases in the material. As mentioned above, for complex polymers this is achieved in 2D or 3D experiments that separate the signals of different groups according to the corresponding isotropic chemical shifts. An alternative, chemical, route to obtaining high structural resolution is provided by selective isotopic labelling. While most routine applications of NMR observe the signals of C , H P , S i , etc. in natural abundance, many specific NMR studies rely on synthetic chemistry for isotopic labelling. In the following articles, systematic deuteration as well as C labelling are encountered. For NMR studies of proteins, N labelling by biosynthetic methods is also employed. In addition to achieving selectivity, isotopic enrichment also serves to overcome the sensitivity problem of NMR, and of solid-state NMR in particular. While rare nuclei or spin pairs can often be observed without enrichment in solution NMR, they are not detectable in the solid state because the peak heights in solids are smaller, as the linewidths are larger by orders of magnitude. As NMR techniques and technologies are developed further, problems currently solved by isotopic enrichment will become accessible in unlabelled samples, with the necessary segment identification achieved by chemical-shift separation in a multi-dimensional experiment. Much of the technical progress wiïl rely on enhancements of the signal-to-noise ratio, for instance by further increases in magnetic field strengths and by line-narrowing techniques. Even then, many interesting, ever more complex, scientific questions will be accessible only by isotopic enrichment. 1 3

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31

29


9

1 3

1 5

Outlook In the future, multidimensional NMR will certainly continue to provide unprecedented insights into polymer dynamics. We may also expect more 2D NMR investigations of the chain structure in amorphous and crystalline phases of synthetic as well as biological polymers. Spin-diffusion NMR studies will be applied to all kinds of heterogeneous polymer systems. These developments will make NMR comparable and complementary to the scattering methods that have been applied to these problems so far. In summary, it is to be expected that NMR will establish itself firmly as a set of techniques for studying polymers not only on a segmental level, but actually on a wide range of length scales. Literature Cited (1) Bovey, F. A. Chain Structure and Conformation of Macromolecules; Academic Press: New York, 1982. (2) Schmidt-Rohr, K; Spiess, H. W. Multidimensional Solid-State NMR and Polymers; Academic Press: San Diego, 1994.



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(3) McBrierty, V. J.; Packer, K. J. Nuclear Magnetic Resonance in Solid Polymers; Cambridge University Press: Cambridge, 1993. (4) Koenig, J. L. Spectroscopy of Polymers; ACS Reference Book Series: Washington, D.C., 1992. (5) Tonelli, A. E. NMR Spectroscopy and Polymer Microstructure: The ConformationalConnection;VCH Publishers: New York, 1989. (6) Komoroski, R. Α., Ed. High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk; VCH Publishers: Deerfield Beach, Fla., 1986. (7) Haeberlen, U. Advances in Magnetic Resonance. Supplement 1; Academic Press: New York, 1976. (8) Schaefer, J.; Stejskal, E. O. J. Am. Chem. Soc. 1976, 98, 1031. (9) Spiess, H. W. Adv. Polym. Sci. 1985, 66, 23. (10) Opella, S. J. Ann. Rev. Phys. Chem. 1994, 45, 659. (11) Schaefer, J.; Stejskal, E. O.; McKay, R. Α.; Dixon, W. T. Macromolecules 1984, 17, 1749. (12) Pan, Y.; Gullion, T.; Schaefer, J. J. Magn. Reson. 1990, 90, 330. (13) Schmidt, Α.; Kowalewski, T.; Schaefer, J. Macromolecules 1993, 26, 1729. (14) Caravatti, P.; Neuenschwander, P.; Ernst, R. R. Macromolecules 1985, 18, 119. (15) VanderHart, D. L. J. Magn. Reson. 1987, 72, 13. (16) Clauss, J.; Schmidt-Rohr, K.; Spiess, H. W. Acta Polym. 1993, 44, 1. (17) Pines, Α.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (18) Jeener, J.; Meier, Β. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546. (19) Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principles of Nuclear Magnetic Resonance in One and Two Dimensions; Clarendon: Oxford, 1987. (20) Nakai, T.; Ashida, J.; Terao, T. J. Chem. Phys. 1988, 88, 6049. (27) Lee, Y. K.; Emsley, L.; Larsen, R. G.; Schmidt-Rohr, K.; Hong, M.; Frydman, L.; Chingas, G. C.; Pines, A. J. Chem. Phys. 1994, 101, 1852. (22) Chmelka, B. F.; Schmidt-Rohr, K.; Spiess, H. W. Macromolecules 1993, 26, 2282. (23) Schmidt-Rohr, K.; Kulik, A. S.; Beckham, H. W.; Ohlemacher, Α.; Pawelzik, U.; Boeffel, C.; Spiess, H. W. Macromolecules 1994, 27, 4733. (24) Raleigh, D. P.; Creuzet, F.; Das Gupta, S. K.; Levitt, M. H.; Griffin, R. G. J. Am.. Chem.. Soc. 1989, 111, 4502. (25) Schmidt-Rohr, K. Clauss, J.; Spiess, H. W. Macromolecules 1992, 25, 3273. (26) Colombo, M. G.; Meier, Β. H.; Ernst, R. R. Chem. Phys. Lett. 1988, 146, 189. (27) Tomaselli, M.; Meier, B. H.; Robyr, P. Suter, U. W., Ernst, R. R. Chem. Phys. Lett. 1993, 205, 145. (28) Blümich, B.; Kuhn, W., Eds. Magnetic Resonance Microscopy, VCH Publishers: Weinheim, 1992. (29) Schmidt, C.; Blümich, B.; Spiess, H. W. J. Magn. Reson. 1988, 79, 269. (30) Schmidt-Rohr, K.; Spiess, H. W. Phys. Rev. Lett. 1991, 66, 3020.

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Multidimensional Spectroscopy of Polymers - American Chemical

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