89 Advances in Polymer Science Polymer Characterization/ Polymer Solutions With Contributions by M. Andreis, H. Gräger, J. L. Koenig, M.Kötter,W.-M.Kulicke With 89 Figures and 10 Tables Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo ISBN-3-540-50473-7 Springer-Verlag Berlin Heidelberg NewYork ISBN-0-387-50473-7 Springer-Verlag NewYork Berlin Heidelberg Library of Congress Catalog Card Number 61-642 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illus- trations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. Duplication of this publication or parts thereof is only permitted under the provisions of the German Copyright Law of September 9, 1965, in its version of June 24, 1985, and a copyright free must always be paid. 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Prof. John K. Stille, Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, U.S.A. Table of Contents Drag Reduction Phenomenon with Special Emphasis on Homogeneous Polymer Solutions W.-M. Kulicke, M. Kötter, and H. Gräger 1 Application of NMR to Crosslinked Polymer Systems M. Andreis and J. L. Koenig 69 Author Index Volume 1-89 161 Subject Index 175 Application of NMR to Crosslinked Polymer Systems Mladen Andreis1 and Jack L. Koenig Department of Macromolecular Science, Case Western Reserve University Cleveland, OH 44106, USA Approximately 80% of synthetic polymers for commercial use are crosslinked. Yet, there are very few techniques which reveal the nature of the crosslink structure of these polymers. This article reviews the utility of NMR for the characterization of the type, number and distribution of crosslinks in polymeric materials. It discusses the theory and experimental ramifications of this unusual application of NMR. This is the first article directed specifically towards developing an under- standing of this aspect of NMR. It summarizes all aspects of solution and solid state NMR including chemical shift and relaxation measurements for the study of crosslinking. Due to the recent develop- ments in solid state NMR and deuterium NMR, our understanding of the crosslinked structure of polymers has been substantially increased through NMR. It is important that an appreciation of the developing potential of high resolution solid state NMR be recognized so it can be used in the future. 1 Introduction 71 2 Principles of NMR Spectroscopy 72 3 Solid State NMR 73 3.1 Broad Line NMR 73 3.2 Pulsed NMR 81 3.2.1 Spin-Lattice Relaxation Times 82 3.2.2 Spin-Lattice Relaxation in the Rotating Frame 89 3.2.3 Spin-Spin Relaxation 91 4 High Resolution NMR in Solutions 104 4.1 High Resolution *H NMR 105 4.2 High Resolution 13C NMR 112 4.2.1 Characterization of Polymer Networks 112 4.2.2 Relaxation in Polymers 115 4.3 High Resolution NMR of Other Nuclei 118 1 Permanent address: Ruder Boskovic Institute, Zagreb, Yugoslavia Advances in Polymer Science 89 70 M. Andreis and J. L. Koenig 5 High Resolution Solid State NMR 118 5.1 High Resolution Solid State'H NMR 120 5.2 High Resolution Solid State 13C NMR 124 5.2.1 Gated High Power Decoupling 125 5.2.2 Spin-Lattice Relaxation 130 5.2.3 Cross-Polarization 132 5.2.4 Spin-Lattice Relaxation in the Rotating Frame 145 5.2.5 Dipolar Dephasing (Interrupted Decoupling) 148 5.2.6 Distortionless Enhancement by Polarization Transfer 150 5.3 High Resolution Solid State NMR of Other Nuclei 152 5.3.1 29SiNMR 152 5.3.2 15N NMR 153 6 References 155 Application of NMR to Crosslinked Polymer Systems 71 1 Introduction The application of nuclear magnetic resonance (NMR) spectroscopy to polymer systems has contributed to significant advances in understanding of their structure and dynamical properties at the molecular level. From the analytical point of view, NMR spectroscopy is particularly suitable for a determination of the polymer structure by direct observation of the protons and carbons in different structural moieties. However, until the mid-1970s the application of this technique was limited to polymer solutions and to some elastomers in the solid state with a relatively high degree of the molecular mobility which allows the observation of the motionally narrowed absorption signals. The importance of crosslinked polymers, since the discovery of cured phenolic formaldehyde resins and vulcanized rubber, has significantly grown. Simultaneously, the understanding of the mechanism of network formation, the chemical structure of crosslinked systems and the motional properties at the molecular level, which are responsible for the macroscopic physical and mechanical properties, did not accompany the rapid growth of their commercial production. The insolubility of polymer networks made impossible the structural analysis by NMR techniques, although some studies had been made on the swollen crosslinked polymers. In the early stages of the development of NMR techniques (1950s-mid 1970s), the studies of polymers could be classified into two major domains : broad line NMR of the solid state and the high resolution NMR of polymer liquids and solutions. In this period, crosslinked polymers were investigated by the broad line and pulsed NMR techniques, respectively. These studies in the solid state yield information primarily on macromolecular dynamics, and indirectly on the network structure. With the development of Fourier transform (FT) techniques in NMR spectroscopy (early 1970s), the first major advance in the NMR technology was made. A significant increase in the sensitivity, as compared to the conventional continuous wave method, resulted in the NMR spectroscopy of rare nuclei, particularly 13C NMR, which is essential for polymer studies. The 13C NMR analysis of swollen lightly crosslinked polymers was made possible. The relaxation measurements, based on the different pulse sequences, provided additional information on the network dynamics. A direct method for the determination of the chemical structures of crosslinked polymers became available in the mid-1970s with the second major advance in NMR spectroscopy — high resolution of solids. The combination of different techniques, such as dipolar decoupling (DD), magic angle spinning (MAS) and cross-polarization (CP) makes possible the observation of the chemical structures of polymers in the solid state. With this improvement, the growth of solid state techniques, as well as their application to crosslinked polymers, became very rapid. A number of published NMR papers indicate the complexity of the crosslinking mechanism and the net- work structures and dynamics in the solid state. The importance of NMR spectroscopy in determining the polymer molecular structure and dynamics, as well as the rapid development of spectroscopic techniques, resulted in a number of review articles, which have appeared since the late 1950s *_8). The particular types of polymers, as well as the particular NMR rechniques are separately reviewed, such as the characterization of crosslinked polymers by high resolution solid state NMR 9). 72 M. Andreis and J. L. Koenig The scope of the paper is to review the application of different NMR techniques, particularly the high resolution solid state methods, to crosslinked polymers and the potential of each particular technique in the investigation of network structure and dynamics at the molecular level. 2 Principles of NMR Spectroscopy Nuclear magnetic resonance (NMR) is based on a phenomenon that nuclei which possess both magnetic and angular moments (i.e. have odd mass number or odd atomic number) interact with an applied magnetic field B yielding 21 + 1 (where 1 0 is the nuclear spin quantum number) energy levels with separation AE : AE = hoj = yhB (1) 0 where h is the Planck constant divided by 2TC, OJ is the Larmor frequency of nuclear precession and y is the gyromagnetic ratio. The interaction of a single spin with the magnetic field (in the range 106 to 108 Hz) is described by a Zeeman Hamiltonian : H = - hB I (2) z Y 0 z where I is the z-component of the spin angular momentum operator I (in the z direction of the applied field). Spectroscopy detection of these energy levels is possible when transitions between them are induced by an alternating magnetic field B(ojt) of the frequency a (perpendicular to the static field B ) which satisfy 1 0 the resonance condition a = yB . 0 The properties of multi-spin systems are determined by different types of inter- actions. They can be described by a Hamiltonian H: H = H + H + H + H, + H (3) z D Q 6 where Hamiltonian terms are described as follows: H = Zeeman interaction with the applied field; z H = direct dipole-dipole interaction with other nuclei; D H = quadrupolar interaction (for nuclei with I > 1/2); Q H = chemical shift interaction; g Hj = indirect (electron-coupled) spin-spin couplings to other nuclei. Contributions of the four last terms in Eq. (3), depend on the physical state. In the solid state, the strong dipolar and quadrupolar terms are usually dominant, and the weak interactions such as chemical shift and spin-spin coupling are obscured. In contrast, the dipolar and quadrupolar interactions in liquids are averaged to zero, giving rise to high resolution spectra in which chemical shifts and J-couplings can be observed. Furthermore, rapid motions in solutions average the above men- tioned tensors to scalar quantities. According to these observations, NMR studies Application of NMR to Crosslinked Polymer Systems 73 can be classified into two major domains : broad line (low resolution) studies of the solid state and sharp line (high resolution) studies in the liquid state. However, it is found that a combination of techniques, such as proton dipolar decoupling (removes the dipolar interactions), magic angle spinning (reduces the chemical shift tensor to the isotropic chemical shift value), and cross-polarization (increases the sensitivity of rare spins, like 13C) applied to a solid state material, results in sharp Unes for 13C nuclei in the solid state10). Thus, the observation of narrow lines or high resolution NMR in the solid state is possible. 3 Solid State NMR As previously observed, the dominant terms in the Hamiltonian which describe a spin system in the solid state are the dipolar and quadrupolar terms. In the case of nuclei with I = 1/2 (such as *H, 13C, 19F and 29Si) the quadrupolar interaction is zero. The dipolar Hamiltonian H (for a homonuclear spin system) has the general D form: 1,13 3(1^X1^)- H = y2h2£ (4) D r3. r?. L 1J IJ where y is the gyromagnetic ratio of the nuclei at the resonance; L and I. are the nuclear spin angular momentum operators, and r.. is the vector joining the ith and jth nuclei. Two types of low resolution solid state NMR techniques can be distinguished : a) broad üne NMR in which the absorption signal is obtained by sweeping the magnetic induction B in the vicinity of the resonance value B (Eq. (1)), and b) pulsed tech- 0 niques which are based on the possibility of rotating the magnetization under the influence of particular radiofrequency pulses, or pulse sequences. 3.1 Broad Line NMR The proton NMR absorption signal of a solid homogeneous polymer usually con- sists of a single broad peak. This is a consequence of a large number of interactions between the various nuclear magnetic moments which give rise to a local field, B , contributing to the single resonance field, B . Since almost every proton exists loc 0 in a slightly different magnetic environment, the resonance envelope consists of a superposition of numerous individual resonances generating a single broad absorp- tion line. Theoretical calculations of B, and their contributions to the shape of the oc resonance lines are possible only for the simplest molecules. Thus, the more suitable quantity, the moment of the absorption line, AM , based on a quantum mechanical n determination of the mean square local field in the rigid lattice, was introduced 11). The n-th moment is defined in terms of the magnetic field as : +f(B-B )nI(B)d(B-B ) 0 0 AM = "°° • (5) n +f 1(B) d(B - B ) 0