Chapter 2
Overview of NMR of Bulk Polymers Downloaded by STANFORD UNIV GREEN LIBR on June 19, 2012 | http://pubs.acs.org Publication Date (Web): October 14, 2011 | doi: 10.1021/bk-2011-1077.ch002
Hans Wolfgang Spiess* Max-Planck-Institute for Polymer Research, P. O. Box 3148, D-55021 Mainz, Germany *E-mail:
[email protected]
The favorable properties of bulk polymers are primarily due to their molecular structure but also heavily depend on their organization in bulk and their molecular dynamics. Solid state NMR techniques provide unique information on the local conformation of macromolecules as well as time scale and amplitude of rotational motions in bulk. Moreover, translational motions of the chains on mesoscopic length scales can be elucidated by advanced NMR techniques. In order to obtain this information, homo- and heteronuclear dipole-dipole couplings, involving 1H and heteronuclei such as 13C and 15N, isotropic as well as anisotropic chemical shifts, in particular of 13C, and quadrupole coupling of 2H are employed. Experiments can be performed on static samples and under magic angle spinning (MAS). Double quantum and multidimensional NMR techniques are especially informative. The methods will be briefly introduced and illustrated by recent experimental examples from different fields of polymer science, such as chain microstructure, defects and dynamics, chain packing in polymers for plastic electronics, proton motion in proton conductors and packing of macrocycles in nanotubes.
Introduction Precise knowledge of structure and dynamics of macromolecules of well-defined architectures is key when tailoring them for specific functions. For instance, such diverse technological challenges as efficient fuel cells, photonic sensors and devices or gene delivery systems all require transport of electrones, holes, protons or other ions. Likewise, the properties of conventional polymers © 2011 American Chemical Society In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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can be substantially improved by controlling, e.g., their microstructure and the processing conditions. Properties of macromolecular systems critically depend on the arrangement of the building blocks relative to each other and their mobility on different length- and time scales. Therefore, success in polymer science requires development of characterization techniques that are able to provide information on these aspects. Scattering of light, X-rays and neutrons, the various forms of microscopy as well as mechanical and dielectric relaxation are well-established in polymer science (1). NMR spectroscopy, on the other hand, is often considered just as an analytical tool, monitoring the various synthetic steps, which eventually lead to the new polymers or supramolecular systems. High-resolution NMR in one- and two dimensions (2) provides a number of important parameters, which are specific to macromolecules. Examples include stereochemical configuration, geometrical isomerism, and regioregularity. More recently, the rapid development of solid state NMR has substantially broadened the application of NMR to polymers (3). In particular, molecular and collective motions can be characterized in unprecedented detail. The new NMR methods use concepts well established in X-ray and neutron scattering, yet on much longer timescales. These techniques provide structural information with atomic resolution even for systems that lack crystalline order in the traditional sense (4). Moreover, solid state NMR concepts can also be applied to partially mobile systems such as liquid crystals. Further examples are highly viscous materials such as polymer melts in the vicinity of the glass transition or elastomers. This chapter is intended as an overview rather than a detailed description of the different possibilities NMR provides. It is based on previous reviews of the author (1, 4); for further details we refer to recent reviews of the subject (5–7).
NMR Background NMR spectroscopy is remarkably versatile and, therefore, is widely applied in many fields of science, in particular physics, chemistry, biology, materials science, and in medicine. This form of spectroscopy combines a number of features that makes it an almost ideal tool: NMR is non-destructive, sample preparation is easy, the possibility to observe different nuclei and isotopes provides extreme structural selectivity. Moreover, dynamic features can be studied over many decades of characteristic times, from picoseconds to minutes, and length scales from interatomic distances in the 100 pm range up to a meter or so in NMR imaging. The wealth of information accessible by NMR spectroscopy results from the fact that a variety of interactions of the nuclear spins with their surroundings can be exploited. The structure of polymers and other supramolecular systems can be elucidated along several routes. The chemical shift provides the basis of site selectivity. Geometric parameters such as internuclear distances as well as dihedral angles are encoded in the dipole-dipole and the J-coupling. Of the great variety of molecular motions possible in polymers, rotations have the most pronounced effects on NMR spectra and relaxation parameters, because the spin interactions 18 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
are angular-dependent. But conformational dynamics and translational motions can be tackled as well. The anisotropic nuclear spin interactions also offer a means to probe the alignment of residues in partially ordered systems such as drawn fibers or oriented liquid crystals. Therefore, a brief outline of these anisotropic spin interactions is given here.
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Anisotropic Spin Interactions As noted above, the information that can be extracted from solid state NMR spectra is encoded via spin interactions such as the chemical shift, the quadrupolar interaction, homo- and heteronuclear dipolar interactions, as well as J-couplings. For convenience, these couplings and the information that is accessible through them are listed in Table 1. A common characteristic of the relevant spin interactions is that they are anisotropic and can be described by second-rank tensors. The resulting orientation-dependent NMR frequency for each of the interactions is alike and of the following form:
Here ωL is the Larmor frequency, ωiso is the isotropic chemical shift and the other terms reflect the deviations due to angular-dependent contributions. The strength of the anisotropic interaction is specified by δ, and η (0≤η≤ 1) is the asymmetry parameter which describes the deviation from axial symmetry. The polar angles α, β, relate the orientation of the principal axes system of the interaction tensor with the external magnetic field. The most important interaction with respect to chemical information is the chemical shift. It results from the shielding of the magnetic field at the position of the nucleus by the electrons. Major advances have been achieved recently in quantum chemical calculations of this parameter in bulk at different levels of precision and calculational cost (8). The NMR frequency is thus shifted by an isotropic contribution ωiso and by angular dependent terms. Assuming an equal probability for all directions the powder average can be calculated and a broad powder spectrum is obtained, which reflects the chemical-shift anisotropy. For the case of an axially symmetric chemical-shift tensor, η is zero and the angulardependent term simplifies to δ (3cos2β-1)/2. Usually the chemical shift in polymers spans about 10 ppm for 1H, 200 ppm for 13C, and 400 ppm for 15N nuclei, see Fig. 1.
19 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Table 1. Interactions of nuclei in NMR providing access to structure and dynamics of polymers. Interaction
Electronic Structure
Geometry
Nuclei
Structure
Dynamics
Chemical shift
Yes
Intrinsic and orientation
1H, 13C,
Conformation, throughspace proximities
Conformational transitions, rotational motions
Throughspace distances
Translational and rotational motions
Conformation and intergroup binding
Conformational transitions, rotational motions
Electronic environment, chemical bonding
Rotational motions
15N, 19F,
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29Si, 31P
Dipoledipole coupling
No
J-Coupling
Yes
Quadrupole Coupling
Yes
Internuclear distance and orientation
1H, 13C,
Intrinsic, internuclear distance and orientation
1H, 13C,
Intrinsic and orientation
15N, 19F, 29Si, 31P
15N, 19F, 29Si, 31P
2H, 14N, 17O, 23Na, 27Al
For abundant nuclei with spin ½, the spectrum is often dominated by heteronuclear or homonuclear dipole-dipole interactions, i.e. the interactions between the magnetic moments of two neighboring spins. In this case there is no isotropic contribution and the interaction is axially symmetric, η=0, so that Eq. (1) simplifies correspondingly. For a two-spin system the interaction Hamiltonian, which defines the frequencies, reads
where r12 is the magnitude of the vector connecting the two spins, θ is the angle of this vector to the magnetic field. The Ii are spin operators and the γi (i=1,2) are the magnetogyric ratios of the spins. That is, the strength of the resulting line splitting depends strongly on the distance between the two spins, so that distance information can be extracted from such spectra. Homonuclear interaction (equivalent spins with γ1 = γ2 = γ and heteronuclear interaction (non-equivalent spins with γ1 ≠ γ2) have to be distinguished. In the latter case, the flip-flop term which is part of the product I1 I2 in Eq. (2) can be neglected. For a powder sample one has again to take into account all angles β and thus obtains the so-called Pake spectrum with a considerable anisotropic line-broadening of up to 50 kHz for homonuclear 1H-1H and up to 25 kHz for heteronuclear 13C-1H or 15N-1H dipolar interaction. Since the dipole-dipole coupling is a through-space interaction, however, we have in principle to evaluate the sum over all possible 20 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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pair interactions. This and the presence of molecular motions lead to considerable complications. Therefore, in practice a relatively structureless line-shape rather than a Pake spectrum but is observed.
Figure 1. Chemical shift ranges of organic compounds. (3)
In the case of a deuterated sample (2H, spin I=1), the spectra are usually dominated by the quadrupole interaction, that is, the coupling of the nuclear quadrupole moment with the electric field gradient of the C-2H bond. For deuterons in C-2H bonds this can lead to a splitting of about 250 kHz. As in the case of the dipole-dipole interaction, a Pake spectrum is obtained for a powder sample. The z-principal axis of the quadrupolar interaction is oriented along the bond axis. This makes deuteron NMR particularly useful for studies of segmental orientations and molecular dynamics (reorientation), where the line shapes strongly depend on the orientation of an oriented sample such as a polymer fiber, or on the geometry of rotational motions generating averaged spectra, Fig. 2a. It is important to note that in the fast limit where the averaging is complete, the anisotropic interaction can still be described by an expression as Eq. (1) with averaged coupling parameters and angles. Even if an interaction is axially symmetric in the static case, it can become asymmetric upon motional averaging (3) and Fig. 2a. In sufficiently mobile (i.e. liquid-like) systems where the residues undergo rapid isotropic thermal motions, the anisotropy is typically averaged out completely, leaving only the isotropic contributions. 21 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Manipulation of Spin Interactions
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The rich information content of solid state spectra makes them difficult to handle, in particular if more than one of the interactions introduced above has to be taken into account. NMR methodology, however, provides the possibility to decouple and recouple spin interactions nearly as desired (see (9) for a comprehensive introduction). Moreover, the different sources of information can be separated and correlated using the two-dimensional techniques discussed below.
Figure 2. Solid state NMR spectra; a) static, b) under MAS. The most prominent example of a technique for decoupling or line-narrowing is magic-angle spinning (MAS). Here, the angular dependent part of the interactions is modulated by rapidly spinning the sample around an axis inclined at an angle Θ to the magnetic field. If the spinning axis is chosen along the so-called magic-angle Θm=54.7°, the relevant scaling factor (3cos2Θ-1)/2 becomes zero and the anisotropic part of the interaction vanishes, see Fig. 1. Today, fast MAS with rotation frequencies reaching 70 kHz is possible. This paves the way for easy access to high spectral resolution in solid state 1H-NMR and, more important, largely simplifies the dipolar coupled network that prevents the dipole-dipole coupling between 1H from being used for specific structural investigations in non-spinning samples. In addition, it is possible to manipulate the spin interactions by pulse techniques (5–9) These act on the spin operators in the corresponding Hamiltonians rather than on the geometric part. Depending on the applied pulse sequence, a given spin interaction can be switched on and off in order to 22 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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discriminate the different contributions to the desired information. For instance, heteronuclear dipole-dipole couplings are averaged out by MAS, yet a simple train of 1800 pulses properly synchronized with the rotor phase reintroduces the coupling for a well-defined period of time. This so-called Rotational-Echo Double-Resonance (REDOR) technique can be seen as the basis of many more sophisticated ways of reintroducing homo- and heteronuclear dipole-dipole as well as anisotropic chemical shift interactions under MAS. Pictorially speaking the mechanical rotation by MAS in real space is partially offset by counter rotations in spin space. Magic angle spinning modulates the spin interactions periodically. This means it generates rotational echoes and data acquisition can be performed in two ways: First, if only the echo-height is monitored (rotor synchronized acquisition) a single line results in the spectrum for each resolved site and information about the anisotropic coupling is lost. Secondly, if the whole echo-train is monitored, a side band pattern results, which retains information about the anisotropic coupling, yet with spectral resolution of the different sites. This is important for precise structural elucidation, based on the dipole-dipole coupling as well as using this interaction to study molecular dynamics resulting in reduction of the coupling, Fig. 2b.
Double Quantum NMR The basic concept for using the distance- and angular dependent dipole-dipole coupling for structural studies is described in extended reviews (5, 7). In a twodimensional experiment, double quantum coherence (DQ) is created, e.g. between two 1H with like or different chemical shifts Fig. 3a. During excitation of the DQ coherence, the dipole-dipole coupling between the two spins, which is largely reduced by MAS, is recoupled by a suitable pulse sequence. In the evolution time of DQ coherence recoupling is turned off, such that the different residues can be distinguished by their different chemical shifts. In the subsequent reconversion to single quantum coherence needed for signal detection, recoupling is again applied. Thus, a 2D spectrum, as shown in Fig. 3b is recorded, in which information about internuclear distances is, first of all, encoded in the strength of the DQ peaks. It should be appreciated that the technique is based on analyzing signals resulting from a coherence originating from two spatially separated nuclei. Thus the basic physics is analogous to coherent X-ray or neutron scattering, where the coherent superposition of scattered waves is exploited to deduce the distance between the scattering centers. In NMR, however, no translational symmetry is necessary to determine distances in the range of 0.5 nm. In fact, proximities of 1H in the same or different moieties can be probed by DQ NMR by analyzing the intensities of a so-called rotor-synchronized spectrum, which can typically be recorded in about 10 min for 10 mg as-synthesized samples, without the need of isotopic labeling. If the dipole-dipole coupling needs to be determined more accurately, for more precise determination of internuclear distances or molecular dynamics, see above, DQ sideband spectra, as displayed in Fig. 3c are recorded. The measurement time is then considerably longer, typically overnight. 23 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Figure 3. Principle of double quantum NMR. a) pulse sequence; b) double quantum NMR spectra; c) sideband patterns. Such DQ spectra can be recorded for both homonuclear 1H-1H, or heteronuclear 1H-13C, 1H-15N coherences, exploiting the much higher site selectivity of 13C and 15N chemical shifts. In the heteronuclear case polarization transfer and recoupling take advantage of the popular REDOR technique introduced above usually applied to isotopically labeled samples.
Two-Dimensional NMR Spectroscopy Most of the advanced NMR techniques make use of two-dimensional (2D) spectroscopy, because of the superb increase of resolution and ease of information encoding. There is not enough space to explain these techniques in detail here, but comprehensive books are available (3, 10) and only the basics are described here. In general, a 2D NMR experiment is divided in several time periods that follow each other. In order to record a 2D NMR spectrum, a two-dimensional data set is acquired as a function of two time variables t1 and t2 as shown schematically in Fig. 3a for the specific case of double quantum NMR. This is preceded by the so-called preparation period in which coherences are excited by a suitable pulse sequence, which in the simplest case is only one radio-frequency pulse. Unlike conventional (1D) NMR spectroscopy, the excited signal is not directly acquired, but is allowed to evolve in the so-called evolution period under influence of the relevant spin interactions. The evolution time t1 is incremented in subsequent experiments and provides the first time dimension of the experiment. After the evolution period (and an optional mixing time), the remaining signal is directly detected in the detection period for each time increment thus generating a 2D data set. Two-dimensional Fourier transformation then gives the 2D spectrum. Optionally, a so-called mixing period of length tm can be inserted between the evolution and detection periods. During tm changes in the system can occur, 24 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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for instance, by molecular motions, spin interactions, relaxation or by spin manipulation (for instance recoupling). The different aspects of 2D NMR spectroscopy are reflected in the different variants that can be distinguished. One variant, separation spectroscopy, is used to separate different interactions by taking advantage of the spin manipulation techniques For instance, during the evolution period the spin manipulation can be made such that only the isotropic chemical shift is acquired while in the detection period the full spectrum is acquired. This leaves the anisotropy to be studied site-selectively. Other 2D NMR techniques, so-called correlation techniques, aim at obtaining new information by correlating different interactions. For instance, the 13C chemical shift anisotropy can be correlated with the heteronuclear dipolar powder pattern in order to obtain information on the relative orientation of the two tensors. Moreover, since no signal is detected during the evolution time, it can conveniently be used to provide the basis for recording a double quantum spectrum which correlates single and double quantum coherences. Considering the manifold of spin manipulation techniques there is a wealth of such 2D NMR techniques that can be derived for different purposes. Finally, introducing a mixing time tm, 2D exchange spectroscopy can be performed, Fig. 4. The most important application of such exchange techniques, with respect to polymer investigations, is the study of slow molecular dynamics. In these experiments, reorientations due to molecular dynamics are allowed to take place during the mixing time tm and lead to characteristic off-diagonal patterns in the resulting 2D spectra. If the mixing time is increased in a series of 2D experiments, slow dynamics in the range of milliseconds to seconds can be investigated in detail. For instance, rotation of molecules by a well-defined angle leads to an elliptical exchange ridge for a powder. This can be viewed as a Lissajous-figure, from which the angle, by which the molecules have rotated, can directly be read off by a ruler. Measuring time can be dramatically reduced in a 1D variant under MAS. Likewise, exchange of magnetization can also occur by cross relaxation or mutual spin-flips, the latter being designated as spin-diffusion. It can be described by a diffusion equation and provides access to domain dimensions in phase separated systems.
Applications The chapters in this book describe NMR studies of polymers or supramolecular structures in the solid-, the liquid crystalline-, or the liquid state. Often one or the other aspect of the techniques introduced above is used. Therefore, in this overview I have refrained from trying to describe a comprehensive set of NMR applications in polymer science. Instead, I have just included a few recent examples of our own work.
25 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Figure 4. Principles of 2D exchange NMR for studying molecular motions. Chain Microstructure, Defects, and Dynamics The macroscopic properties of polyolefins strongly depend on the chain microstructure. In recent years, new single site catalysts have enabled much greater synthetic control over the polydispersity, type of branch, and branch content. In polyethylene, the physical properties of both the solid and the melt can be tuned by the presence of branches of various lengths in the polymer backbone. Short-chain branches (SCB) can be introduced by copolymerization of ethylene with short alkenes, or by isomerisation reactions during ethylene homopolymerization Such branches form structural defects during crystallization and thus strongly affect crystallization rates, ultimate crystallinity, and other bulk mechanical properties. Similarly, long-chain branches (LCB) are formed by macromonomer incorporation during polymerization. With these branches typically being longer than the entanglement molecular weight their presence strongly affects the processability of the bulk polymer. Long-chain branching is known to influence the zero-shear viscosity even at concentrations of 2 LCB per 100 000 CH2 groups. Thus, it is very important to quantify the degree of chain branching and 13C NMR in solution seems to be the method of choice as the chemical shifts of branch points as well as adjacent carbon positions can be distinguished from the backbone resonances (2). However, when applied to polyethylene a number of problems arise. The most important being the low solubility of polyethylene, even at high temperatures. The low concentration of 13C nuclei results in low NMR signals requiring extended measurement times, especially for the quantification of very low levels of branching. In specific cases up to two million transients had to be acquired at high field in order to determine branch contents of 3–8 branches per 100 000 CH2. Another inherent limitation of solution-state NMR is the lack of access to crosslinked and other non-soluble fractions of polyethylene. Solid state 26 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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type NMR, under magic-angle spinning (MAS), on the other hand, can be used to overcome these limitations, despite the substantial loss of spectral resolution as compared to solution NMR. Under optimal conditions time savings of about two orders of magnitude are achieved, allowing quantification of 7-9 branches in 100 000 CH2 groups within 13 h (11), see Fig. 5a. In commercial polymer samples the irregular distribution of the branching sites along the main chain results in uncertainty of the branching influence on morphology, chain dynamics, and other physical properties. Here polyethylene samples with regularly spaced methyl branches obtained by Acyclic Diene METathesis (ADMET) polycondensation (12) provide much more specific information. By deuteration of the methyl branches, the molecular dynamics of the defect site can be studied selectively via 2H NMR, whereas molecular reorientations of the polymer chain between the branching sites can be monitored separately via 13C chemical shift anisotropy. Combining these results with studies of local conformations, a twist motion was identified, which is centered at the branching sites for a spacing of more than twenty CH2 units between subsequent branching points. In methyl branched PE samples with shorter spacing, e.g. 14 CH2 units between subsequent methyl branches, a collective dynamic process emerges from this twist motion, which ultimately leads to the formation of a rotator phase as indicated in Fig. 5b. Thus, localized and collective mobility induced by the defects can clearly be distinguished. The superb spectral resolution of a 850 MHz solid state NMR spectrometer was essential for obtaining these results. Dynamics in Polymeric Proton Conductors The growing necessity for clean energy sources to substitute fossil energy has created high demands for batteries and fuel cells. Therefore, various approaches have been proposed aiming at developing new classes of proton conducting membranes for high temperature applications (13). One promising approach to the development of such a material is to combine the functions of the protogenic group and the proton solvent in a single molecule. Such molecules must be amphoteric in the sense that they behave as both a proton donor (acid) and proton acceptor (base), and they must form dynamical hydrogen bond networks. The first leads to the formation of a high concentration of intrinsic protonic defects as a result of self-dissociation, and the latter may promote a high mobility of these protonic charge carriers (excess and deficient protons). The mobility of intrinsic defects is generally higher than that of extrinsic defects, which may be introduced by acid or basic doping disturbing the local symmetry of the hydrogen bond network. Typical amphoteric liquids include phosphonic acid and diverse heterocycles such as imidazole, pyrazole, benzimidazole and triazole. In the liquid state, they all show relatively high proton conductivities with significant contributions from structure diffusion, i.e. the motion of protonic defects (excess or deficient protons) via intermolecular proton transfer, coupled to hydrogen bond breaking and forming processes. A promising approach to eliminate the liquid electrolyte is to attach the protic groups of the liquid electrolyte to the backbone of the polymer, 27 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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such that only chemical decomposition would result in a loss of ion carriers, a prominent example being poly (vinyl phosphonic acid). From 1H, 2H, 13C, and 31P NMR combined with computer simulation, detailed information on the proton mobility, water content, and the unwanted condensation of the phosphonic acid groups can be obtained. The protons are highly mobile, but on the same time scale no mobility associated with reorientation of the phosphonic acid groups or the polymer backbone is observed. The 1H chemical shifts of P-OH protons provide evidence for the presence of a complex hydrogen bond network, see Fig. 6, which allows for proton transport via a Grotthus-type mechanism along a given chain as well as between adjacent chains. The MD simulations further show that proton vacancies can be trapped in the anhydride defect produced by the condensation. More important, as shown in Fig. 6, the highest intrinsic proton mobility with essentially isotropic geometry arises in systems like poly(vinyl phosphonic acids), where the hydrogen bonded network is highly disordered, characteristic of a hopping mechanism for proton conductivity (14).
Figure 5. a) Quantification of low branch content in optimized melt state under MA. b) Local and Collective Motions in precise polyethylene with CD3 Branches spaced every 15th and every 21st CH2 group.
13C-NMR
28 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Figure 6. Left: Distribution of the CP-OH angle within a given phosphonic acid group of poly(vinyl phosphonic acid) PVPA, predicted from first-principles molecular dynamics simulations. Right: Experimental 2H NMR lineshapes as a function of temperature, illustrating motional narrowing due to the hydrogen bond dynamics, exhibiting an effectively isotropic motion.
Hydrogen Bonding and Side Chain Conformation in Rigid-Rod Copolymers In recent years the interest in polymer science has shifted considerably to include liquid crystals (15) and supramolecular structures (16). As a recent example, let us consider rod-coil copolymers such as oligo(p-benzamide)poly(ethylene glycol) (OPBA-PEG) copolymers with an oligomeric rod and flexible PEG chains. They aggregate on a nanometer length scale, which is important for many applications such as organic photovoltaics. However, this aggregation behavior and the driving forces such as hydrogen bonding and π-π interactions, as well as the role of the side groups, are not yet fully understood. These non-covalent interactions can be studied by advanced solid state NMR, combined with X-ray diffraction (XRD), differential scanning calorimetry (DSC), and polarization optical microscopy (POM) (17). As shown in Fig. 7, longer OPBAs form layered ß-sheet like aggregates stabilized by amide hydrogen bonds, similar to polyamides with flexible side groups attached to the repeat units rather than the end groups (18). These aggregates are remarkably stable and apparently represent an equilibrium structure in both unsubstituted OPBAs and OPBA-PEG rod coil copolymers. The binding of the PEG also introduces a liquid-crystalline phase and the local structural order is improved in the copolymer, if the sample is preorganized in that phase. Thus, by combining different methods of structural investigation a model of local aggregation and packing in both the liquid-crystalline and the solid state could be developed.
29 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Figure 7. Scheme packing of rod-coil copolymers. The rigid rods are held together by intermolecular hydrogen bonds and the space between the stacks is filled by the PEG side chains adopting a Gaussian coil-like conformation. Packing in Polymers with Ultrahigh Charge Carrier Mobility for Field-Effect Transistors Applications in plastic electronics like flexible displays or smart tags call for organic semiconductors that can be easily processed into thin films and show fieldeffect transistor (FET) charge carrier mobilities exceeding 0.1 cm2/Vs. To this end a copolymer consisting of an alternating arrangement of cyclopentadithiophene (CDT) as a donor and benzothiadiazole (BTZ) as an acceptor unit has recently been examined (19), Fig. 8. The planar arrangement of the conjugated backbone ensures a close π-stacking which is required for efficient inter-molecular charge carrier transport. Moreover, it was assumed that the close packing of the polymer chains might originate from assisting donor-acceptor interactions. Indeed, for the optimum molar mass, charge carrier motilities up to 3.3 cm2/Vs were achieved. For the first time such a macromolecular system was recently examined by solid state nuclear magnetic resonance (NMR) to provide a molecular scale picture of the packing, Fig. 8. The 1H 2D DQ NMR spectra clearly reveal the relevant packing contacts, confirming the expected π-π stacking for the polymer backbone. The packing of the donor and the acceptor groups, however, was found to be more delicate. Donor-acceptor groups are π-π stacked in a lamellar fashion and these groups are ordered in an alternating way as shown in Figure 8d. Thereby, the acceptor groups in one layer are located on top of the acceptor groups in adjacent layers, however, not always in the exact same position, leading to a heterogeneous packing. This model derived from NMR is consistent with the findings of X-ray scattering. It also allows for optimal packing of the side chains that in the case of long and bulky alkyl chains (C16) should be advantageous in order to avoid steric clash. Conclusively, solid state NMR does not reveal a donor-acceptor overlap within 0.4 nm. Thus, donor-acceptor interaction between the neighboring CDT and BTZ groups located a adjacent chains apparently contributes little, if any to the observed improvement in charge carrier mobility. NMR rather shows the complexity of this remarkable CDT-BTZ copolymer system. This implies that the enhanced packing order resulting in improved thin film crystallinity with 30 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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increasing Mn results primarily from π-stacking interactions of the planar extended conjugated backbones. (19).
Figure 8. Local packing and organization of the donor-acceptor groups in a CDT-PTZ copolymer. (a) 2D contour plot of the 1H-1H DQ NMR spectrum. (b) Color scheme used for assignments. (c) Expansion of the backbone region showing the contacts between donor and acceptor groups. (d) Schematic drawing illustrating the local packing of donor-acceptor groups in two neighboring CDT-BTZ copolymer chain.
Empty Helical Nano-Channels from Low-Symmetry Macrocycles Natural channel-forming structures are mandatory for connecting different compartments within a living organism. For instance, trans-membrane proteins function as ion-channels, transporters, or antibiotics. Recently, synthetic nanochannels as large as 1.3 nm in diameter in a tight tubular supramolecular superstructure have been generated from shape-persistent macrocycles. Solid state NMR combined with computer simulation was able to provide the details of the packing. The interplay of charge-transfer type interactions and steric effects due to the side chains were unraveled (20). Moreover, it was shown that the channels are empty and do not host solvent molecules or back-folded alkyl chains. Fig. 9b displays the 2D NMR 13C{1H} hetero-nuclear correlation (HETCOR) spectrum for the macrocycle in the LC phase. In this spectrum a remarkable 31 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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spectral resolution is observed, with 13C line widths in the order of ~100 Hz for the core of the macrocycle, implying a very high degree of local order. From the 2D NMR spectrum in Fig. 9b the complete assignment given in Fig. 9a is obtained. It includes only one fourth of the total number of 13C signals possible for the macrocycle indicating that the macrocycles are located in highly symmetric environments, which can be envisaged in a helical arrangement of the macrocycles in a column.
Figure 9. Spectroscopic fingerprints of the helical assembly in an empty nanochannel from solid state NMR. a) Assignment of the aromatic carbon and proton resonances. b) 2D 13C{1H} HETCOR NMR spectrum in the liquid crystalline phase. c)The helical packing with a pitch of 60° leads to intermolecular correlation peaks AB and BB observed in the 1H-1H DQ- NMR spectrum d). The packing environment is also reflected in the 1H chemical shifts for the core protons of the macrocycle. These differ substantially from each other (Fig. 9b), and also from the values found in solution, which is a clear indication of π-π stacking. The specific pitch angle can be determined by combining 2D NMR 1H-1H DQ-SQ (Double-Quantum Single-Quantum) correlation spectroscopy with ab initio calculations. This fixes the pitch angle between adjacent macrocycles to ~60° as illustrated by Fig. 9c and the inset of Fig. 9d. Within the stack every fourth molecule is eclipsed, i.e. related by translation, resulting in a helical arrangement (Fig. 10), as also observed in other self-assembled organic compounds. The pitch angle of 60° is supported by additional NMR experiments and ab initio calculations investigating the packing of pairs of macrocycles considering their energetics and the 1H chemical shifts of neighboring macrocycles. These show excellent agreement between experimental and calculated values. Thus, the internal order of the channels can be molecularly controlled and adjusted for future applications in recognition, stabilization, or organization of nanoparticles. 32 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Figure 10. a) Overall view of the helical packing arrangement of the macrocyclic channel with a pitch angle of 60°. b) Top view of the macrocycle. c) Packing arrangement for the side chains illustrating the stabilizing effect of the outer phenylene groups.
Conclusions and Outlook Advances in synthesizing, characterizing, as well as understanding macromolecular and supramolecular systems lead to an enormous variety and complexity in the field of polymer science (21). The traditional separation in terms of structure vs. dynamics, crystalline vs. amorphous, or experiment vs. theory is increasingly overcome (1). As far as characterization of such materials is concerned, no experimental or theoretical/simulation approach alone can provide full information. Instead, a combination of techniques is called for and conclusions should be backed by results provided by as many complementary methods as possible, see Table 2, where NMR and scattering are compared. Combining scattering or NMR spectroscopy with computer simulation is well established today in the study of structure and dynamics of biomacromolecules. The examples described here show the power of such an approach in the supramolecular field involving the combination of spectroscopy, scattering and computer simulation. Last, but not least, the development of NMR spectroscopy is far from complete. In particular, in order to meet the ever-increasing demands of miniaturization, the sensitivity of NMR spectroscopy has to be increased substantially and several approaches in response to that challenge are underway (22). 33 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Table 2. Comparison of scattering and NMR techniques as well as the information provided about structure and dynamics of materials. WAXS: Wide Angle X-ray scattering; SAXS: Small Angle X-ray Scattering; SANS: Small Angle Neutron Scattering.
Acknowledgments This paper is based on and concludes my work at the Max Planck Institute for Polymer Research for more than 25 years. It has provided me with a unique scientific environment, in which new ideas and approaches prospered. It gives me great pleasure to thank my colleagues and co-workers for all their contributions.
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