Introduction to polymer spectroscopy

INTRODUCTION TO POLYMER SPECTROSCOPY A. H. FAWCETT

The Queen's University of Belfast

Historically there was a difficulty in dealing with macromolecules that was simply the realisation of their large size; the organic chemist's early painstaking methodology for isolating, studying and recognising the readily obtained small natural product molecule did not lend itself to the examination of many natural macromolecules such as cellulose and rubber. Such chemists, used to identifying their substances by the melting point complemented by similar studies on the derivatives and then the slow construction of the molecule by use of a developing repertoire of piecemeal reactions, were slow to accept how readily high polymers might be man-made by a simple but powerful repetitive process. Ancient practices and evolving technology might utilise materials such as wood, leather, silk and cotton, but the true macromolecular nature of these materials was not appreciated until about 60 years ago, and methods for exploring the large molecule and the development of appropriate concepts for a proper scientific enquiry took time to evolve. Spectroscopy has played a role in this process, light scattering in particular being used to show how high molecular weights might be, and NMR spectroscopy latterly being used to identify polymer structures. Now spectroscopy is at the heart of modern developments within polymer science, being used not only to characterise the microstructure of the chains, but also to monitor their dynamics, so important in determining the physical properties of interest to the materials scientist and engineer, and to explore the interesting properties that are being introduced in the search for special effects to be used in devices. Two developments have given us insight into polymers at the molecular level, the first being the spectroscopic techniques for recognising molecular components and the manner in which they are linked together, which is the topic of the first part of this book. Of course, the analytical problem of recognising a particular polymer is less severe to the man who chose the monomer and the polymerisation process (and any plasticiser or stabiliser) than it is to a would-be emulator, but the proper description of the microstructure of a macromolecule is as essential to the developmental chemist (Chapter 1) as it is to his competitor. For this purpose, NMR spectroscopy has now overtaken IR spectroscopy as the Polymer Spectroscopy. Edited by Allan H. Fawcett © 1996 John Wiley & Sons Ltd

analytical tool in general use. A second advance, much associated with Flory, was the development of statistical mechanical methods. These have provided insight into the equilibrium configurations of the isolated polymer chain and the manner in which modest thermal energies develop elaborate configurations within the backbones and any side chains, so that the calculations of the mean values of such quantities as dipole moment and end-to-end distance are complex, yet focus upon such readily visualised ideas as the potential surface for the conformations of each pair of adjacent bonds. NMR spectroscopic quantities such as chemical shift and coupling constant may be considered in just these terms, as Tonelli has described for us (Chapter 2). One has only to reflect on a subject area such as liquid crystals, where so often the description is formulated by the physicist in terms of unit cell properties, to realise how much closer workers with polymers routinely think in terms of molecular structure, and are able to link a certain molecular feature to an interesting property. Configurational elaborations are the prime characteristic of molecules rendered extremely long by the repetitive enchainment of a small number of simple residues: Ciardelli et al. describe the manner in whch stimuli such as light may induce changes in the structure of pendent groups and so in polymer-solvent interactions that are amplified by the connectivity of the system to cause profound changes in the equilibrium statistics of the single chain, and hence in its solution properties. Indeed, a group of chains may so be led to associate reversibly (Chapter 14). The manner in which light interacts with chromophores in bulk polymers, located either within the standard residues or merely within minor components such as end groups, is the subject of Phillips and Carey's contribution (Chapter 15). There are two interrelated factors to be disentangled—the manner in which light is absorbed, whether it is retained or migrates, and how the energy is eventually used, together with the dynamics of the moieties involved in this process. Excimer formation, luminescence, fluorescence and other photophysics processes are all subject to such factors as spacing constraints and the timescales of segmental motions, which in the bulk are not merely the property of a single molecule. Although the physical chemistry of the chain isolated in solution is well understood, the question of its performance within the bulk has thus become the subject of much study. Rapid movement between adjacent conformations ceases below the glass transition of an amorphous polymer, and in the crystalline state packing effects become significant and restrict configurations to a very few. The question of the location of the backbone is readily tackled: spectroscopic techniques for studying the configurations of the polymer in amorphous and crystalline phases within the bulk are well established; neutron scattering is a prime, if expensive, tool for the determination of molecular dimensions and for the study of dynamics (in a quasielastic scattering mode) and is now being developed as a method for studying surface structures (Chapter 13). The contrast is obtained by use of perdeuterated molecules. Light scattering is a more familiar

tool for investigating polymers; the method was introduced originally by such luminaries as Debye, and has developed, with the availability of lasers, in the quasielastic mode, not just for chains isolated in solution but also for gels, when various modes of motion may be inferred from treatments of the fluctuations of the intensity of the light scattered. The technique is now applied to studying phase separating mixtures and events within polymers upon surfaces (Chapter 12). Richards also covers the small angle light scattering method as used to investigate semi-crystalline polymers. IR and Raman spectroscopy characterise the high frequency vibrations of the skeleton and pendent atoms of the macromolecule, and so immediately tell us what groups are present; they have a useful analytical capacity to distinguish, for example, a poly(methyl methacrylate) (PMMA) from a PVC or a polyolefin. Vibration modes extend over several simple oscillators (such as bonds and bond angles); in the crystalline state they reflect the arrangement adopted within the unit cell, from which IR bands and Raman shifts follow conventional symmetryrelated selection rules. They may be used to measure crystallinity, as such. In the amorphous state conformational elaborations are not averaged out on the timescale of the vibration. Observed bands are thus composite and relatively broad, and although they may indicate whether in a rubber a double bond is cis or trans, and may measure the presence of methyl groups in low density polyethylene, band frequencies are not as sensitive as solution NMR spectroscopy to microstructure details extending over several residues. The fine structure observed in the shifts of linear polymers is itself a topic of careful consideration, as Tonelli and Howarth et al. have described (Chapters 2 and 3). The conformational origin within vinyl polymers of the patterns displayed in 13 C shifts is now well established, and provides the best source of information on tacticity and residue sequence, so that one might attempt to discriminate between mechanisms for propagation, such as those of the Bernoullian and Markov type, those involving charge-transfer complexes, and mechanisms involving catalysts derived from metal complexes (Chapter 1). Once one has evidence on the reaction mechanism, one may proceed to the design of new and better catalysts. Like vibration spectroscopy, NMR in the solid state, made feasible by the cross polarisation-magic angle spinning dipole decoupling method, is similarly rather insensitive to microstructural issues within the crystalline and amorphous states, but interesting results may be obtained when carefully chosen systems are compared: Harris presents the cases of the 4/1 helix of syndiotactic polypropylene and the 3/1 helix of isotactic polypropylene, the former clearly displaying sensitivity to the helix structure through the gamma-gauche effect so that internal and external methylenes are distinguished, and the latter displaying some sensitivity to the helix sense of the neighbouring chains (Chapter 4). The solid state NMR method is capable also of sensing inhomogeneities such as arise from microcrystals within a homopolymer such as polyethylene, and within blends of two different and only partly compatible polymers (Chapters 4 and 5), an area

that is similarly tractable by modern two-dimensional methods that are being developed within IR spectroscopy (Chapter 7). Both chemical shift and IR vibration frequency of one chain are sensitive to the nature of the neighbouring chains, particularly if an interaction such as a hydrogen bond is possible. The timescale of magnetic polarisation decay is capable of being linked to the size of the inhomogeneities. Mobility as measured by proton or carbon NMR relaxation times is a property of matter, including polymeric materials and any permeated liquid, that may be sensed by a scanning technique and displayed in an image form, usually in two dimensions. Koenig surveys for us the various applications he has made, the images providing an interesting comparison with the more conventional light and electron microscope viewing methods (Chapter 6). Vibration spectroscopy is sensitive, as Hendra and Maddams describe (Chapter 7), to such factors as anisotropy within such samples as uniaxially drawn rods and biaxially drawn films, allowing their properties to be optimised from an understanding of the molecular process. Such well established use of IR spectroscopy is now being succeeded by dynamic dichroic methods, to reveal how the backbones and side chains separately respond to imposed cyclic stresses. This provides a fascinating account of the manner in which different modes of motion come into play. A development of Raman spectroscopy described by Young is the response of certain vibrations in the spectrum to a progressive strain imposed upon the material, a technique that may exploit recent instrumental developments such as charge coupled device cameras and the confocal Raman microscope (Chapter 8). For a composite material, the technique allows us to answer a question such as the manner of the distribution of strain along a polyaramid fibre within a matrix that initially bears the imposed stress; the particular interest is the length of fibre required to take up the strain. The timescale of the response of a polymer to a stimulus ranges from the high frequencies of IR radiation through to the low frequencies or long time scales of diffusion of the whole molecule by the reptation mechanism, a process that is amenable to study by dielectric relaxation spectroscopy, as in studies on cispolyisoprene by Adachi. The dielectric response is present only from polar units, and is governed by the location of the dipoles, whether within side chains or backbones, in the geometry of the dipole itself and the geometry and flexibility of the neighbouring segments. For the chain in solution, simple and satisfactory accounts are available in these terms, and only in special cases do the dipoles themselves mutually organise to control the response. For the bulk material, whether in crystalline, amorphous or liquid crystalline form, cooperations between chains may be significant. For example, the alpha relaxation of crystalline polyethylene is a progression of a kink in one chain within a crystalline region, as computer simulations have modelled: it is the linear all-trans neighbours that define the tube within which the single chain performs (Chapter 11). Distributions of correlation times may be extremely wide in an amorphous material, but how

much this derives from variations in local conformations and orientations of the dipole within the chain in question, and how much from intra-chain influences (which may themselves have a response) is, as they say, a very good question! The same issues arise when studying the dynamic mechanical behaviour of polymers, a method closer to the concerns of the polymer engineers. Perhaps the developing power of NMR spectroscopy to measure correlation functions and the magnitude of the orientational jump and to identify the pathways of the motion will help provide an answer to these questions (Chapter 5). As Spiess describes, the NMR method might measure the angle of displacement, as well as its frequency, for poly(oxymethylene), displaying helical jump dynamics. Two-dimensional and three-dimensional experiments are now being performed to measure motions and to determine order within oriented solids (Chapter 5). The use of a paramagnetic probe coupled with electron spin resonance (ESR) monitoring provides information, within the timescale range of 10"3 s to 10"7 s, of a complementary nature, for by sensing the mode of rotation of the radical within the polymeric matrix, it measures the behaviour of the "holes", the packets of free volume, that facilitate the movements of the chains and play a vital role in the glass transition, Tg, phenomena. Locating the radical on the chain or at its end allows one to sense the extra degree of freedom at a polymer chain end (Chapter 9). The ESR technique in this book is applied to a second issue, monitoring the radicals actually responsible for a polymerisation of pure monomer plus a certain amount of crosslinker, the interest lying in the changes that take place to create a new regime when the gel effect operates, during which termination reactions are much retarded by the immobilisation of the radicals, as they are also in the final period, when the development of a glass is the cause of onset of a third regime (Chapter 10). O'Donnell's work monitors the radicals by ESR and the unreacted groups by near-IR spectroscopy, to reveal new insight into the kinetics during these periods. This study of the chemistry of free radical polymerisation is succeeded by a discussion of an equally important topic, as far as industrial use is concerned, the detailed chemistry of degradation by ionising radiation of polystyrene and poly(methyl methacrylate): following such training, O'Donnell's previous students helped develop microlithography. This book records the principal lectures given at a Conference in Grasmere organised by the Macro Group. The proceedings of two of the previous conferences with this subject area and sponsorship have also been published [1, 2] and provide a useful indication of the developments that have occurred over recent years in the practice and value of polymer spectroscopy.

REFERENCES [1] KJ. Ivin (Ed.), Structural Studies of Macromolecules by Spectroscopic Methods, John Wiley & Sons, London, 1976. [2] A.H. Fawcett, Br. Polym. /., 1987,219,97 and following papers.