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Analysis of macromolecular structures by pulsed NMR
Radtat. Phys. Chem. Vol. 39, No. I, pp. 45-51, 1992 J. Radiat. Appl. htmum., Part C
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ANALYSIS
OF MACROMOLECULAR BY PULSED NMR A.
STRUCTURES
&ARLBSBY
Silver Spring, Watchfield, Swindon SN6 8TF, U.K. Abatrac-The 7” spin-spin relaxation curves obtained by pulsed NMR techniques can readily be used to study important features of macromolecular systems quite distinct from their chemical structure. Such features refer to more physical properties such as molecular size, flexibility and mobility, the influence of solvent and temperature on this motion (which is related to viscosity), crystalline fraction and the rate of crystallisation, polymerisation and other chemical reactions where there is a considerable change in dimensions etc. It can also serve to determine the degree of crosslinking, where this forms a partial or complete network. However it appears to indicate the presence of a network even when no permanent network is revealed by alternative and well-established techniques such as solubility and swelling which require much longer times. This difference is ascribed to the presence of some intermolecular binding somewhat akin to permanent crosslinks, but of a very shortlived dynamic nature, and this is referred to as due to entanglements between adjacent macromolecules. The T2 measurements reveal their presence if the life-time of these entanglements is comparable or longer than the period of measurement by pulsed NMR. The usual formulae used to determine network formation by permanent crosslinks can be applied to
such systems with entanglements or with entanglements plus crosslinks, so that the elastic properties can be determined by NMR r, measurements. Over a long time only the permanent crosslinks will provide elastic recovery but for sufficiently short times the entanglements provide an additional restoring force and this may be taken to be the cause of the rheological property referred to as creep and viscosity. Since the entanglements but not the permanent crosslinks depend on temperature, many of these physical properties and their variation with temperature can be related directly to the effect of these entanglements as determined by these r, measurements and derived from pulsed NMR. Another feature which emerges from these investigations is their dependence on solvent where present. The total variation can be ascribed to molecular dimensions and the free volume available for their motion (and hence their freedom to tecome disentangled). This free volume is influenced by temperature and concentration of solvent where present. The meaning of these T2 responses have been deduced from the changes in pulsed NMR responses to a series of macromolecular systems whose properties have been modified to known extents by known radiation doses. The interpretation of the r2 relaxation patterns obtained from other macromolecular structures now becomes possible. We can therefore hope to see this technique used not only for polymers but especially for biological systems where considerable changes of molecular behaviour such as conformation and motion can result from very minute chemical modifications. Such sensitive features might be for example molecular entanglements and concentration, radiation or chemically-induced crosslinking or degradation (scission), disruption of a regular refolding sequence etc. This T2 technique is particularly suitable for following such changes.
NMR SPIN-SPIN RELAXATION; LOW MOLECULAR WEIGHT FLEXIBLE POLYMER
Within the overall limits as described below, this NMR method offers a very simple, rapid and non-
destructive method for the determination of M, and requiring very small amounts of sample. In the relation between T2 and M, the constant C need only be determined once for each polymer, within wide limits as described below. The information this technique provides is in some respects analogous to that obtained from viscosity measurements and there appears to exist a relation between T2 and bulk vicosity. However C does vary with temperature and/or the presence of a solvent. This has been shown to be due, at least in part, to the increased free volume they can provide, allowing easier motion of each macromolecule, and it therefore appears equivalent to a reduction in molecular weight and a decreased viscosity. This increased mobility is revealed by a corresponding increase in T2. Increased free volume due to solvent and temperature may also intervene in
The spin-spin relaxation pattern obtained from simple flexible macromolecules below a certain molecular weight (typically 104), often consists of a simple exponential decay, character&d by a single time constant T2.
AWIA(O)= exp(-tlT2) where A(t) is the signal strength at time r after the pulse. A fuller examination of a series of similar polymers, differing only in their linear molecular weight M (conveniently obtained by exposure of a scissioning polymer such as PiB, polyisobutylene to increasing doses of y radiation), shows that there exists a simple relation between T2 and MO.’ over an approximately hundred-fold change in M. This relation can be expressed in the form T2 .
MQ.5=
C
45
A. CHARLESBY
46
providing higher mobility and higher T2 by allowing less hindrance and reducing interaction between adjacent macromolecular chains. This enhanced mobility due to solvent and temperature may also intervene by reducing the average lifetime of the adjacent molecular entanglements so that fewer would be observed over the measurement time. This influence of entanglements is discussed below. This quantitative assessment of polymer mobility can be used to determine and even account for many of the rheological properties of these polymeric systems on a molecular scale and under a variety of conditions. Using these NMR data one can consider such questions as the influence of solvent concentration and of temperature. This approach also provides an explanation of the physical behaviour of these macromolecular systems in terms of their mobility which is measured on a molecular scale, and assess its influence on such macroscopic behaviour as viscosity, as well as flow or ease of solution. One might hope to see this correlation extended further for a range of polymeric and other macromolecular systems, where a quantitative assessment of their ease of motion in various media can apparently be readily assessed quantitatively from simple T2 measurements. PARTIALLY OR FULLY CROSSLINKED LOWER MOLECULAR WEIGHT POLYMER
One of the great advantages of linear or branched polymers is that in many cases, their properties can be very greatly altered by a crosslinking reaction which involves only little chemical change. One of the earliest and most widespread of such modification of important physical properties is readily seen in the sulphur vulcanisation of rubber. In many polymers, notably polyethylene, cis-polyisoprene and polydimethyl silicone (PDMS) this crosslinking reaction can also result from the exposure to high energy radiation, the number of such reactions being proportional to total dose. This provides the possibility of a quantitative comparison of physical properties resulting from known degrees of crosslinking. Up to a certain dose (gelation dose r8) linear molecules become increasingly branched and converted to a star-like pattern but this increase in molecular weight has little effect on molecular dimensions, and there is only a small change in viscosity or NMR pattern. Beyond the gel point and at higher crosslink density two distinct structures emerge: one, a threedimensional network, is theoretically one large molecule (gel) which is insoluble; the other represents a soluble residue of unmodified or branched molecules (sol). The theoretical relation between crosslink density and sol (3) or gel (g = 1 - s), network fractions has been evaluated for various initial molecular weight distributions, assuming always that crosslinks occur at random. The dependence of the sol fraction s on crosslink density also depends on the initial
molecular weight distribution and is best expressed in terms of the crosslinking coefficient 6, the average number of crosslinked units per weight average molecule. A simple general rule is that for any initial distribution of molecular weight the incipient gel first starts to appear (gel point) when 6 = 1 and grows rapidly as 6 increases above 1. This gel point represents a dramatic change in behaviour which can be readily followed not only by the formation of an insoluble fraction but (as has now been found) also by the spin-spin relaxation NMR. For such partiallylinked polymer, the NMR consists of a relaxation curve which is the sum of two components of different T, with T, being usually far shorter than T2,. The signal decreases as the sum of two exponentials: A(t)/A(O)=fexp(-tlT2,,}
+(1 -fiexp{-tlT2,)
where f corresponds to the soluble fraction s and (1 -f) to the gel, network fraction g = 1 -s. T,, is found to be proportional to the average molecular weight between successive crosslinks MC while at first T2, would be expected to measure the average molecular weight of the residual non-network polymer, but this has not been fully confirmed as yet. We might indeed expect that for polymer molecules not forming part of the network but nevertheless affected by its presence, e.g. being trapped within the pores of the crosslinked lattice, a very different value of T,, might emerge. Its mobility would certainly be influenced by the presence of the three-dimensional lattice, and this NMR technique might indeed provide separate information on the mobility of certain other macromolecules such as those trapped near the surfaces etc. Its use for measuring molecular mobility in such complex polymeric and even biopolymeric systems deserves fuller attention. It therefore appears that this NMR type of analysis can provide quantitative information on the characteristic parameters of a partially crosslinked network at least up to a molecular weight typically of less than about lo4 and well above the crystallisation or glass temperature, since it requires a polymer flexible at the temperature of measurement. One would also expect the limiting figure referred to above as about lo4 to vary with temperature and even trace solvent. PULSED
NMR RESPONSES TO HIGHER POLYMER
MOLECULAR
For flexible macromolecules of much higher molecular weight the T2 relaxation curve differs markedly from that obtained from lower molecular weight polymer of identical chemical structure. Indeed the relaxation curve usually consists of the superposition of two exponential decays, just as if a permanently partially crosslinked system were involved, although it is completely soluble. This is ascribed to some form of intermolecular binding of a non-permanent dynamic nature which is best described as intermolecular entanglements. These are
Analysis of macromolecular structures by pulsed NMR continuously forming and disappearing, but if their lifetime is longer than that measured by pulsed NMR they will appear as effective crosslinks, although they will not be found in solubility measurements since these require far longer times. A comparison of T2 NMR and solubility data can therefore serve to separate temporary, dynamic entanglements and permanent covalent crosslinks. In the absence of permanent crosslinks the usual relation between crosslink density and sol fraction still applies, except that crosslink density is replaced by entanglement density and sol fraction s by the relevant fraction f of the total initial amplitude. In particular the concentration at which this second entangled structure (with a second component TzF) is first found corresponds to one entangled unit per weight average molecule. This may be written as 6, = 1 and the figure of 104corresponds to a situation at which there is one entangled unit per weight average molecule (i.e. about 1 per 100 units). This molecular weight will differ according to chain flexibility, concentration of solvent where present and also temperature. All these influence the free volume in which the molecules can move and hence the lifetime of the entanglement and therefore the molecular weight at which this second component T, is first seen. It has been confirmed that for a low molecular weight polymer such as PDMS a sufficient density of crosslinks can give a specimen with any desired value of sol fraction s and hence off. For a polymer of identical chemical structure but of much higher molecular weight, pulsed NMR can provide the same f value (i.e. the same apparent network fraction) though this has been obtained with no permanent crosslinks but is due to the presence of these temporary entanglements, which on a sufficiently short time scale of measurement appear as equivalent to permanent crosslinks. However in this latter case the f value can be increased by a rise in temperature, which reduces the lifetime and hence effective number of these entanglements. With a sufficient temperature rise f tends to unity (no network, permanent or dynamic) and this temperature corresponds to the point at which the number of these entangled units has sunk to S, = 1. Once f reaches unity i.e. one entangled unit per weight average molecule the pulsed NMR shows a simple exponential decay in spin-spin decay. Thus from a simple knowledge of weight average molecular weight of a flexible polymer and the f/temperature curve the basic characteristics of entanglements can be derived quantitatively and the importance of the relevant parameters evaluated. It may be accepted that these entanglements of a well-defined lifetime can serve to explain such physical properties as creep of polymers under stress. It is not felt that adequate use has been made of this pulsed NMR technique to trace and evaluate in a quantitative manner the mechanical behaviour of important polymeric systems, despite its more academic
41
and quantitative aspects and especially of its very high industrial and commercial interest. Perhaps experts in Nuclear Magnetic Resonance and in Rheology do not mix. They should! The importance of these entanglements has long been known and utihsed. One simple example is native rubber which behaves as a liquid due to its initial dilution, so that the lifetime or entanglement density is too low (S, < 1) for a network to form. Its subsequent manipulation provides a green strength rubber where the entanglement density rises so that d, > 1 and a dynamic network and shape is conserved on a temporary basis during processing. The final shape is retained on an elastic basis by the introduction, by chemical or radiation means, of permanent bond crosslinks so that a stable network is formed with required elastic properties. However the full understanding on a quantitative basis, and the assessment, by means of pulsed NMR T2 measurements, of these most important entanglements for the physical properties of macromolecules has not been fully explored. A further direct relation between entanglement density and the formation of a dynamically evolving network structure can be followed by the changes in spin-spin relaxation in a polymer which does not crosslink, but whose average molecular weight is reduced by increasing radiation doses (polyisobutylene PiB) above the glass or crystalline temperature. An initially very high molecular weight gives a pulsed NMR T2 pattern which is almost completely T, in nature, since essentially all polymer molecules take part in the dynamic network; this polymer will show almost complete elastic behaviour, at least for very short periods of time. A reduction in average molecular weight such as by exposure to radiation results in a two component spin-spin pattern, with an increasing fraction f of the longer time component T, due to molecules not incorporated by entanglements into the network, and therefore no longer able to participate in carrying some of any imposed external stress. The point is reached at which only a T2 component remains. This point corresponds to 6, = 1 and there is no network present. At this stage one can only refer to a PiB of this low molecular weight as a liquid under the conditions of measurement, especially of time of stress and of temperature. Here again we can hope for future assessment of such basic properties as solid-liquid differences in terms of molecular weight of such polymeric structures, since this will determine whether we are dealing with a three-dimensional system of short life or with a true viscous liquid. This is quite distinct from any differences based on crystalline or glassy state and on true liquids. Indeed one may enquire as to the correct description of a system of a very flexible and elastic network formed by entanglements or crosslinks so that it may be considered as a single but gigantic molecule. If we ignore minor components should we evaluate a car tyre comprising a single three-dimensional molecule as a solid, liquid or even a gas?
A.
48 CRYSTALLINITY
AND CRYSTALLISATION WITHT,NMR
CHARLESBY
RATE
At sufficiently low temperatures the spin-spin relaxation pattern changes in many cases to include a quadratic component with a time constant Tze of the order of 10 ps; A(t)/A(O) = C exp( - t*/T:,) + . . . Here C represents the fraction of the molecular system with greatly reduced mobility due to being held in a rigid lattice, crystalline or glassy. The remainder of the spin-spin relaxation is a simple exponential decay of much longer lifetime, or two such decays as above. By comparing the relative contributions to this more complex spin-spin relaxation curve the degree of crystallinity may be deduced. A further step may be taken by measuring the rate of crystallisation at any temperature. From well above the melting point a polymer specimen is rapidly cooled to any required temperature at which it is then kept and the change in the T,, component followed in the relaxation curve. From a series of such curves the crystallisation rate can be evaluated for each desired temperature and over a time from seconds to days. The results compare well with those obtained by more conventional means but extend over a far longer time scale. They permit one to accept or reject alternative theories of crystallisation. It is generally accepted that in many polymers crystalline regions form and are held to each other by molecular chains which pass through two or more crystalline regions but part of whose length is in the intervening amorphous volume. In addition there may be molecular chains only partially within a crystal but with a loose free end in this amorphous volume, or even entirely within it. The pulsed NMR spin-spin relaxative curve comprises both a quadratic t* as well as one or two linear t components in the exponential decay. The former represents the crystalline fraction, while the latter gives the amorphous, linking fraction etc. The value of T, from the latter is a direct measure of the length of these linking chains. As the temperature is raised both T,, and Tt increase with thermal expansion and more free volume. At a point close to the melting point the magnitude C of the T,, component decreases rapidly as the smaller crystals melt first, and there is a corresponding increase in T, as the average length of these linking chains increases. At the temperature beyond which all crystalline regions and the quadratic component have vanished, the T, value, which has perhaps increased a hundredfold during the short melting range, shows only a small further increase, due to thermal expansion and reduction in density of entanglements of adequate lifetime. This would correspond to the mobility of entangled chains as described above. If T, may be taken as a measure of the average molecular weight MC between entanglements and/or crosslinks these measurements may serve as a quantitative measure of the number and length of these linking chains between crystals. Since
these are responsible for many of the macroscopic mechanical properties of such polymeric materials, this assessment on a molecular scale of the binding characteristics could permit a far more detailed and quantitative measure of the causes of physical deformation and recovery. These lengths of molecular chains spanning two crystalline regions can often be severed e.g. by radiation. The polymer would be weakened by the loss of some links but the crystallinity could rise as some of these severed chain lengths become incorporated into the crystals. However the increase in crystalline fraction is relatively small. A far greater increase in crystallinity is obtained if the irradiated polymer is reheated; this allows the fragmented ends to be incorporated into the new crystalline regions. This reformulation of the severed chains into the parent crystals and the far greater recrystallisation following melting can be traced quantitatively by pulsed T, measurements. It is clear that the use of pulsed NMR T2 data can provide a new and powerful method of measuring on a molecular scale a number of the parameters which intervene in fixing the mechanical and dynamic properties of these partially crystalline polymers. Their further study, in collaboration with macroscopic studies could result in a fuller understanding of the processess. One could hope to see this method further extended and also applied to the transition from the glassy to the liquid or more elastic state. RADIATION
CHEMISTRY AND THE INFLUENCE OF ADDITIVES
Many radiation-induced changes result in a marked change in molecular dimensions or shape and these can be greatly modified by the presence of additives. The effect is most pronounced with macromolecules where it can be readily evaluated by pulsed NMR determination of spin-spin relaxation. A simple example is irradiation of a simple polymer in the presence of oxygen. This can result in main chain scission with the formation of much shorter chains and of course a deterioration in mechanical strength. The formation of these shorter chains is readily seen in the spin-spin relaxation curve, where it appears as an additional component. Its longer time T,, is a measure of the reduced molecular weight due to scission, while its initial amplitude represents the fraction of the original material thus affected. No doubt the effect of antioxidants on this reaction, as on others, could be quantitatively assessed in this manner. For many polymers which crosslink under irradiation, there is a more rapid spin-spin relaxation decay as crosslinking reduces the length of the chains between successive links. For polymers irradiated in an aqueous solution the enhanced crosslinking reaction is readily demonstrated as due to both the direct effect (energy absorbed directly by the polymer) and
Analysis of macromolecular structures by pulsed NMR indirectly by the water when the radiolytic products react with dissolved polymer. The influence of concentration, dose etc is readily followed quantitatively by spin-spin relaxation which shows the enhanced degree of crosslinking as the solution becomes weaker and macromolecules are further apart. This of course is due to the greater indirect effect shared among fewer macromolecules. At very low concentrations the macroscopic network becomes more difficult to form, due to the preferential internal linking leading to microgel particles. So far no NMR results have appeared on this microgel structure of separate particles. Where an additive is present in the solution its effect on the NMR pattern is readily seen and the changes interpreted quantitatively. For example bovine serum albumin in solution shows a simple exponential decay with time constant T2 which decreases rapidly with radiation. However in the presence of a very low concentration of thiourea this decrease becomes very small. Here again one would like to see these measurements extended to determine the effect of concentration of macromolecule and of dose. Does the thiourea additive lose its effect beyond a certain dose, i.e. is it used up or does it continue to function with equal efficiency at very high doses? Here again the method shows great promise for an extension to macromolecules of biological interest. FILLERSAND
SURFACE REACTIONS
Many polymers show improved mechanical properties in the presence of solid particle fillers; examples are carbon black in rubber and silica in silicones. The question arises as to whether this is due to an additional chemical bonding effect, to physical reorientation of forces around each particle so that tension along a molecular chain is diverted in part to a solid particle, or to an alternative surface reaction. Another possibility is that the modulus of an elastic network is proportional to the number of effective, load-carrying segments within the lattice so that splitting of the stress in one polymer segment by interposition of a solid particle effectively doubles the number of these load-carrying segments. Use of this pulsed NMR technique for T2 which measures many of the basic properties of a network, with either permanent crosslinks or entangled chains, may serve to clarify the situation. It might also provide evidence of new surface reactions since the mobility of chains on or near such surfaces would be very different from free chains in the amorphous region or in a network system, forming part of a network, or in a crystalline or glassy region. There is some evidence of a further component in some polymeric NMR spin-spin relaxation curves but the origin of this has not yet been investigated. CHAIN REACTIONS; POLYESTER CURING, POLYMERISATION AND GRAFTING
A number of radiation-induced reactions involve a rapid chain reaction in which radiation only serves
49
as the initiating step. Polymerisation can serve as the simplest example and the course of the reaction has been followed by pulsed NMR. Two components are seen, the longer time T2 being due to the residual monomer, the second, far shorter to the polymer being formed. However as in certain liquid systems, a third component also appears (about 20% of total) with an intermediate time scale T2 whose origin is unknown. A somewhat similar component has also been observed in some molten polymer and one may enquire whether this is due to some unsuspected structure, possibly of a dynamic nature, in such high-molecular weight fluids. The curing of unsaturated polyester systems has also been followed by pulsed NMR of polyesterstyrene mixtures. The spin-spin relaxation of the unirradiated mixture shows a long-term component Tu due to the soluble fraction and at partial curing there is also the expected much shorter-term component due to the gel, glassy structure but here again there is some evidence of an additional, non-allocated component. DISCUSSION
For a number of decades a very considerable academic research effort has been devoted to the basic study of radiation induced chemical reactions in low molecular weight systems to discover the products and later the mechanisms and kinetics of the reactions. Advanced methods including pulse radiolysis and electron spin resonance have been utilised to determine characteristics of the various stages in the processes involved leading to the final radiation products. This work has led to important advances in our understanding of chemical reaction processes whether or not induced by radiation, but this aspect need not be considered here. The situation with high molecular weight materials is rather different, in that major physical or biological changes can result from extremely small amounts of absorbed radiation energy, even without a chain reaction. The doses involved may only result in one chemical change in a molecular weight of lo6 to 10”. The importance of such small doses resides in the fact that it may be the morphology or mobility of the systems which is profoundly a&&d by these minute chemical changes, and for such systems the powerful analytical technique represented by NMR T, measurements appears most promising. As an example one can consider the biological effects of very low doses of radiation. Unless there is a chain reaction, the chemical change may represent only one bond or less in lo’, yet this can result in a major biological effect. The question is then, are there so many of these vulnerable bonds in the molecule, such that radiation-induced damage to any one is lethal; or is the absorbed energy transferred selectively over considerable distances before causing a change to some vital bond. Or alternatively is the nature of the change far less selective, but its importance arises
50
A.
&4llLi%BY
from the considerably modified morphology or mobility of the macromolecule induced by this very small change? For example even in the simplest macromolecule of molecular weight exceeding lo6 a simple link averaging one per molecule anywhere along its length can profoundly affect its physical performance in well-known ways (even though the precise chemical mechanisms involved have not been settled despite very extensive studies for a number of decades). Similarly with some biological systems involving pairs of macromolecules, they may become linked together anywhere along their length and this may render them no longer capable of separating and continuing their primary function. Main chain scission may be another such reaction. The use of pulsed NMR T2 appears of great promise for such studies. The physical properties of most polymers are of very great importance, and are the subject of detailed investigations on such properties as deformation and creep, recovery etc. The use of T2 techniques is expected to provide much relevant information on behaviour on a molecular scale, allowing our understanding and manipulation of these parameters to be more effectively used and controlled. Some preliminary but important information has become available on the transition from solid, crystalline or glassy, to liquid or entangled. The extension of our conventional concepts, based on experience with low molecular weight systems, to linear, branched or even network structures requires considerable more information and here again we might utilise much of the data obtained from NMR spin-spin relaxation measurements. The number and lifetime of the so-called entanglements is one such topic. For example the NMR response of a partially crystalline polymer obtained from the melt may be dependent on its prior state even prior to melting, a memory conferred in it in a previous semi-crystalline state. How long does this memory last in the liquid or fluid state; and would it depend on prior orientation? In partly crosslinked systems we can envisage obtaining data on the mobility of chemically free macromolecules trapped within a network, formed by permanent links or short-lived entanglements. This may be of practical interest in the behaviour of membranes and of other chemical processes.
information to other systems, irradiated or not. In this respect radiation treatment, because of its quantitative aspect, readily quantised and reproduced and its enormous range of intensities and times serves as an excellent means of providing specimens for the calibration and interpretation of such NMR spectra, even those from other sources. Of special importance is the knowledge we can derive on a molecular scale on many basic properties not readily obtained in other ways. The motion of molecules confined within a lattice, the nature of the melting process, the composite structure involving both crystalline and amorphous regions, and the temporary memory imparted by the presence of entanglements in high molecular weight polymer may be considered as only four aspects which merit further attention. The extension of these and related properties concerned with molecular configuration and motion to biological systems is especially promising.
REFERENCES
General
AC-Use of pulsed NMR in measurement of radiation effects in polymers. Radial. Phys. Chem. 14, 919 (1979). AC,RF-Use of pulsed NMR to follow radiation effects in long-chain polymers. Rudiat. Phys. Chem. 15,393 (1980). AC,RF-Pulsed NMR technique for studying radiation effect in macromolecules. 6th Znt. Congress Radiat. Res., Tokyo, p. 336 (1979). AC--Characterisation of polymers using pulsed NMR. Tihany 5th Symposium, Akad Kiado, Budapest, p. 843 (1982). AC-Use of pulsed NMR to study radiation effects. Z. FI. Mitt. 97, 24 (1984). AC-Radiation effects in macromolecules; their determination with pulsed NMR. Radiat. Phys. Chem. 26 (5), 463 (1985). AC-Use of pulsed NMR to determine morphology of macromolecules. J. Radioanal. Nucl. Chem. 101 (2). 401 (1986). Polyethylene
DWM,DCD,EWA-.J. Polym. Sci. 59, 301 (1962). CONCLUSION
Although information has been obtained mainly on only a limited number of simple polymers, sufficient data have been accumulated to show clearly the type of information which we can expect from the spin-spin relaxation curves and the resultant T2 values. These relate less to chemical structure but rather to morphology and mobility of these systems. By studying a series of polymeric specimens, irradiated and therefore modified to a known extent, we can interpret the various NMR patterns and apply this
AC,PX,RF-Very high molecular weight polyethylene. Radiat. Phys. Chem. 11, 83 (1978). BJB,AC,RF-Crystallisation kinetics from melt and solution. Proc. R. Sot. Land. A367, 343 (1979). RF,AC-Effect of previous history on NMR relaxation of polyethylene. Eur. Polym. J. 15, 953 (1979). RF,AC-Entanglement effects on NMR spinspin relaxation. J. Polym. Sci. 16, 339 (1978). IK,AC-NMR relaxation in solid and molten polyethylene structures. J. Polym. Sci. 19, 803 (1981).
Analysis of macromolecular structures by pulsed NMR Polystyrene AC,EMJ-Entanglement and network formation in polystyrene, viscoelastic behaviow from pulsed NMR. Eur. Polym. J. 21, 55 (1985). Polydimethyi siloxane (PDMS) RF,AC-Radiation-induced crosslinking and gel formation in linear PDMS. Rudiat. Phys. Chem. 8, 555 (1976). RF,AC-Crosslinking and entanglements in HMW linear PDMS. Radiat. Phys. Chem. 10, 61 (1977). RF.JHS.AC--J. Polvm. Sci. (Pub. Phw. Edn) 16, 1041 , (1978):
.
_
AC,RF-Pulsed NMR studies of irradiated PDMS. Tihanv 4th SvmD. Radiat. Chem., Akad Kiado, Budap&;p. 335 (1977). AC,RFJHS-Analysis of crosslinked and entangled polymer networks. Proc. R. Sot. Land. A335,189 (1977). Polyisoprene NMR of cis-polyisoprene 1. Polymer M, 207 (1979). RF,AC-Pulsed NMR of cir-polyisoprene 2. Polymer 20, 211 (1979). AC,BJI&Pulsed NMR of cis-polyisoprene solution T, and T, relaxation, free volume, viscosity relationships. Eur. Polym. J. 17, 645 (1981). RF,AC-Pulsed
Polykobutylene (PiB) AC,JHS-Molecular weight determination of irradiated PiB by NMR. Radiat. Phys. Chem. 8,719 (1976). AC,BJB-Entanglement density in irradiated PiB. Rudiat. Phys. Gem. 19, 497 (1976). Polymerisation TK-Pulsed NMR investigation on polymerisation I. Methyl methacrylate. Pofym. J. (Japan) 18 (I l), 859 (1986).
51
TK-Pulsed NMR investigation on polymerisation II. Bulk polymerlsation of methacrylic acid. Polym. J. (Jopan) 19 (2), 285<1987). - TK,TK,MN,MK-Pulsed NMR study bulk polymerisation of nethacrylic acid. Mem. Nat. Defense Acad. 28 (l), 53 (1988). HS,MI,TK,KT,AN-PO/. J. (Japcm) 14, 149 (1982). Polyester curing MA,ZV,FR,PH-Radiation-induced crosslinking in polyester-styrene systems. Polymer 30, 1498 (1989). Authors MA-M. Andreis; BJB-Barbara Bridges; AC-A. Charlesby; RF-R. Folland; PH-P. He&g; EMJ-E. M. Jaroszkiewicx; IK-I. Kamel; PK-P. Kafer; TK-T. Kurotu; TK-Takakaxu; MN-Makoto Nagai; MKMotomu Kasagi; DWM-D. W. McCall; FR-F. Ranogajec; HS-H. Serizawa; JHS-Judith S. Steven; ZV-Zorika Veksli.
SOME MISCELLANEOUS PAPERS ON NMB OF POLYMERS
Belousova M. V. et al., Akad. Nauk. Inst. Khim. Fiz. 70-72 (1985). Draghicescu P. and Grosescu R., NMR Relaxation in some irradiated polymers; Magnetic Resonance etc., pp. 222-234. North-Holland, Amsterdam (1973). Kadomatsu Y. et al. Molecular motions in gammairradiated PTFE. Rep. Prog. Poly. Phys. Jpn 17, 51 I-514 (1975). Peffley W. M., Honnold V. R. and Binder D., Xray and NMR on irradiated PTFE and PClFr . J. Polym. Sci. A J 4, 977-983 (1966).
01465724/92
ht.
s5.00 + 0.00
Copyright Q 1992 Pergamoa Press plc
Printed in Great Britain. All rights reserved
ANALYSIS
OF MACROMOLECULAR BY PULSED NMR A.
STRUCTURES
&ARLBSBY
Silver Spring, Watchfield, Swindon SN6 8TF, U.K. Abatrac-The 7” spin-spin relaxation curves obtained by pulsed NMR techniques can readily be used to study important features of macromolecular systems quite distinct from their chemical structure. Such features refer to more physical properties such as molecular size, flexibility and mobility, the influence of solvent and temperature on this motion (which is related to viscosity), crystalline fraction and the rate of crystallisation, polymerisation and other chemical reactions where there is a considerable change in dimensions etc. It can also serve to determine the degree of crosslinking, where this forms a partial or complete network. However it appears to indicate the presence of a network even when no permanent network is revealed by alternative and well-established techniques such as solubility and swelling which require much longer times. This difference is ascribed to the presence of some intermolecular binding somewhat akin to permanent crosslinks, but of a very shortlived dynamic nature, and this is referred to as due to entanglements between adjacent macromolecules. The T2 measurements reveal their presence if the life-time of these entanglements is comparable or longer than the period of measurement by pulsed NMR. The usual formulae used to determine network formation by permanent crosslinks can be applied to
such systems with entanglements or with entanglements plus crosslinks, so that the elastic properties can be determined by NMR r, measurements. Over a long time only the permanent crosslinks will provide elastic recovery but for sufficiently short times the entanglements provide an additional restoring force and this may be taken to be the cause of the rheological property referred to as creep and viscosity. Since the entanglements but not the permanent crosslinks depend on temperature, many of these physical properties and their variation with temperature can be related directly to the effect of these entanglements as determined by these r, measurements and derived from pulsed NMR. Another feature which emerges from these investigations is their dependence on solvent where present. The total variation can be ascribed to molecular dimensions and the free volume available for their motion (and hence their freedom to tecome disentangled). This free volume is influenced by temperature and concentration of solvent where present. The meaning of these T2 responses have been deduced from the changes in pulsed NMR responses to a series of macromolecular systems whose properties have been modified to known extents by known radiation doses. The interpretation of the r2 relaxation patterns obtained from other macromolecular structures now becomes possible. We can therefore hope to see this technique used not only for polymers but especially for biological systems where considerable changes of molecular behaviour such as conformation and motion can result from very minute chemical modifications. Such sensitive features might be for example molecular entanglements and concentration, radiation or chemically-induced crosslinking or degradation (scission), disruption of a regular refolding sequence etc. This T2 technique is particularly suitable for following such changes.
NMR SPIN-SPIN RELAXATION; LOW MOLECULAR WEIGHT FLEXIBLE POLYMER
Within the overall limits as described below, this NMR method offers a very simple, rapid and non-
destructive method for the determination of M, and requiring very small amounts of sample. In the relation between T2 and M, the constant C need only be determined once for each polymer, within wide limits as described below. The information this technique provides is in some respects analogous to that obtained from viscosity measurements and there appears to exist a relation between T2 and bulk vicosity. However C does vary with temperature and/or the presence of a solvent. This has been shown to be due, at least in part, to the increased free volume they can provide, allowing easier motion of each macromolecule, and it therefore appears equivalent to a reduction in molecular weight and a decreased viscosity. This increased mobility is revealed by a corresponding increase in T2. Increased free volume due to solvent and temperature may also intervene in
The spin-spin relaxation pattern obtained from simple flexible macromolecules below a certain molecular weight (typically 104), often consists of a simple exponential decay, character&d by a single time constant T2.
AWIA(O)= exp(-tlT2) where A(t) is the signal strength at time r after the pulse. A fuller examination of a series of similar polymers, differing only in their linear molecular weight M (conveniently obtained by exposure of a scissioning polymer such as PiB, polyisobutylene to increasing doses of y radiation), shows that there exists a simple relation between T2 and MO.’ over an approximately hundred-fold change in M. This relation can be expressed in the form T2 .
MQ.5=
C
45
A. CHARLESBY
46
providing higher mobility and higher T2 by allowing less hindrance and reducing interaction between adjacent macromolecular chains. This enhanced mobility due to solvent and temperature may also intervene by reducing the average lifetime of the adjacent molecular entanglements so that fewer would be observed over the measurement time. This influence of entanglements is discussed below. This quantitative assessment of polymer mobility can be used to determine and even account for many of the rheological properties of these polymeric systems on a molecular scale and under a variety of conditions. Using these NMR data one can consider such questions as the influence of solvent concentration and of temperature. This approach also provides an explanation of the physical behaviour of these macromolecular systems in terms of their mobility which is measured on a molecular scale, and assess its influence on such macroscopic behaviour as viscosity, as well as flow or ease of solution. One might hope to see this correlation extended further for a range of polymeric and other macromolecular systems, where a quantitative assessment of their ease of motion in various media can apparently be readily assessed quantitatively from simple T2 measurements. PARTIALLY OR FULLY CROSSLINKED LOWER MOLECULAR WEIGHT POLYMER
One of the great advantages of linear or branched polymers is that in many cases, their properties can be very greatly altered by a crosslinking reaction which involves only little chemical change. One of the earliest and most widespread of such modification of important physical properties is readily seen in the sulphur vulcanisation of rubber. In many polymers, notably polyethylene, cis-polyisoprene and polydimethyl silicone (PDMS) this crosslinking reaction can also result from the exposure to high energy radiation, the number of such reactions being proportional to total dose. This provides the possibility of a quantitative comparison of physical properties resulting from known degrees of crosslinking. Up to a certain dose (gelation dose r8) linear molecules become increasingly branched and converted to a star-like pattern but this increase in molecular weight has little effect on molecular dimensions, and there is only a small change in viscosity or NMR pattern. Beyond the gel point and at higher crosslink density two distinct structures emerge: one, a threedimensional network, is theoretically one large molecule (gel) which is insoluble; the other represents a soluble residue of unmodified or branched molecules (sol). The theoretical relation between crosslink density and sol (3) or gel (g = 1 - s), network fractions has been evaluated for various initial molecular weight distributions, assuming always that crosslinks occur at random. The dependence of the sol fraction s on crosslink density also depends on the initial
molecular weight distribution and is best expressed in terms of the crosslinking coefficient 6, the average number of crosslinked units per weight average molecule. A simple general rule is that for any initial distribution of molecular weight the incipient gel first starts to appear (gel point) when 6 = 1 and grows rapidly as 6 increases above 1. This gel point represents a dramatic change in behaviour which can be readily followed not only by the formation of an insoluble fraction but (as has now been found) also by the spin-spin relaxation NMR. For such partiallylinked polymer, the NMR consists of a relaxation curve which is the sum of two components of different T, with T, being usually far shorter than T2,. The signal decreases as the sum of two exponentials: A(t)/A(O)=fexp(-tlT2,,}
+(1 -fiexp{-tlT2,)
where f corresponds to the soluble fraction s and (1 -f) to the gel, network fraction g = 1 -s. T,, is found to be proportional to the average molecular weight between successive crosslinks MC while at first T2, would be expected to measure the average molecular weight of the residual non-network polymer, but this has not been fully confirmed as yet. We might indeed expect that for polymer molecules not forming part of the network but nevertheless affected by its presence, e.g. being trapped within the pores of the crosslinked lattice, a very different value of T,, might emerge. Its mobility would certainly be influenced by the presence of the three-dimensional lattice, and this NMR technique might indeed provide separate information on the mobility of certain other macromolecules such as those trapped near the surfaces etc. Its use for measuring molecular mobility in such complex polymeric and even biopolymeric systems deserves fuller attention. It therefore appears that this NMR type of analysis can provide quantitative information on the characteristic parameters of a partially crosslinked network at least up to a molecular weight typically of less than about lo4 and well above the crystallisation or glass temperature, since it requires a polymer flexible at the temperature of measurement. One would also expect the limiting figure referred to above as about lo4 to vary with temperature and even trace solvent. PULSED
NMR RESPONSES TO HIGHER POLYMER
MOLECULAR
For flexible macromolecules of much higher molecular weight the T2 relaxation curve differs markedly from that obtained from lower molecular weight polymer of identical chemical structure. Indeed the relaxation curve usually consists of the superposition of two exponential decays, just as if a permanently partially crosslinked system were involved, although it is completely soluble. This is ascribed to some form of intermolecular binding of a non-permanent dynamic nature which is best described as intermolecular entanglements. These are
Analysis of macromolecular structures by pulsed NMR continuously forming and disappearing, but if their lifetime is longer than that measured by pulsed NMR they will appear as effective crosslinks, although they will not be found in solubility measurements since these require far longer times. A comparison of T2 NMR and solubility data can therefore serve to separate temporary, dynamic entanglements and permanent covalent crosslinks. In the absence of permanent crosslinks the usual relation between crosslink density and sol fraction still applies, except that crosslink density is replaced by entanglement density and sol fraction s by the relevant fraction f of the total initial amplitude. In particular the concentration at which this second entangled structure (with a second component TzF) is first found corresponds to one entangled unit per weight average molecule. This may be written as 6, = 1 and the figure of 104corresponds to a situation at which there is one entangled unit per weight average molecule (i.e. about 1 per 100 units). This molecular weight will differ according to chain flexibility, concentration of solvent where present and also temperature. All these influence the free volume in which the molecules can move and hence the lifetime of the entanglement and therefore the molecular weight at which this second component T, is first seen. It has been confirmed that for a low molecular weight polymer such as PDMS a sufficient density of crosslinks can give a specimen with any desired value of sol fraction s and hence off. For a polymer of identical chemical structure but of much higher molecular weight, pulsed NMR can provide the same f value (i.e. the same apparent network fraction) though this has been obtained with no permanent crosslinks but is due to the presence of these temporary entanglements, which on a sufficiently short time scale of measurement appear as equivalent to permanent crosslinks. However in this latter case the f value can be increased by a rise in temperature, which reduces the lifetime and hence effective number of these entanglements. With a sufficient temperature rise f tends to unity (no network, permanent or dynamic) and this temperature corresponds to the point at which the number of these entangled units has sunk to S, = 1. Once f reaches unity i.e. one entangled unit per weight average molecule the pulsed NMR shows a simple exponential decay in spin-spin decay. Thus from a simple knowledge of weight average molecular weight of a flexible polymer and the f/temperature curve the basic characteristics of entanglements can be derived quantitatively and the importance of the relevant parameters evaluated. It may be accepted that these entanglements of a well-defined lifetime can serve to explain such physical properties as creep of polymers under stress. It is not felt that adequate use has been made of this pulsed NMR technique to trace and evaluate in a quantitative manner the mechanical behaviour of important polymeric systems, despite its more academic
41
and quantitative aspects and especially of its very high industrial and commercial interest. Perhaps experts in Nuclear Magnetic Resonance and in Rheology do not mix. They should! The importance of these entanglements has long been known and utihsed. One simple example is native rubber which behaves as a liquid due to its initial dilution, so that the lifetime or entanglement density is too low (S, < 1) for a network to form. Its subsequent manipulation provides a green strength rubber where the entanglement density rises so that d, > 1 and a dynamic network and shape is conserved on a temporary basis during processing. The final shape is retained on an elastic basis by the introduction, by chemical or radiation means, of permanent bond crosslinks so that a stable network is formed with required elastic properties. However the full understanding on a quantitative basis, and the assessment, by means of pulsed NMR T2 measurements, of these most important entanglements for the physical properties of macromolecules has not been fully explored. A further direct relation between entanglement density and the formation of a dynamically evolving network structure can be followed by the changes in spin-spin relaxation in a polymer which does not crosslink, but whose average molecular weight is reduced by increasing radiation doses (polyisobutylene PiB) above the glass or crystalline temperature. An initially very high molecular weight gives a pulsed NMR T2 pattern which is almost completely T, in nature, since essentially all polymer molecules take part in the dynamic network; this polymer will show almost complete elastic behaviour, at least for very short periods of time. A reduction in average molecular weight such as by exposure to radiation results in a two component spin-spin pattern, with an increasing fraction f of the longer time component T, due to molecules not incorporated by entanglements into the network, and therefore no longer able to participate in carrying some of any imposed external stress. The point is reached at which only a T2 component remains. This point corresponds to 6, = 1 and there is no network present. At this stage one can only refer to a PiB of this low molecular weight as a liquid under the conditions of measurement, especially of time of stress and of temperature. Here again we can hope for future assessment of such basic properties as solid-liquid differences in terms of molecular weight of such polymeric structures, since this will determine whether we are dealing with a three-dimensional system of short life or with a true viscous liquid. This is quite distinct from any differences based on crystalline or glassy state and on true liquids. Indeed one may enquire as to the correct description of a system of a very flexible and elastic network formed by entanglements or crosslinks so that it may be considered as a single but gigantic molecule. If we ignore minor components should we evaluate a car tyre comprising a single three-dimensional molecule as a solid, liquid or even a gas?
A.
48 CRYSTALLINITY
AND CRYSTALLISATION WITHT,NMR
CHARLESBY
RATE
At sufficiently low temperatures the spin-spin relaxation pattern changes in many cases to include a quadratic component with a time constant Tze of the order of 10 ps; A(t)/A(O) = C exp( - t*/T:,) + . . . Here C represents the fraction of the molecular system with greatly reduced mobility due to being held in a rigid lattice, crystalline or glassy. The remainder of the spin-spin relaxation is a simple exponential decay of much longer lifetime, or two such decays as above. By comparing the relative contributions to this more complex spin-spin relaxation curve the degree of crystallinity may be deduced. A further step may be taken by measuring the rate of crystallisation at any temperature. From well above the melting point a polymer specimen is rapidly cooled to any required temperature at which it is then kept and the change in the T,, component followed in the relaxation curve. From a series of such curves the crystallisation rate can be evaluated for each desired temperature and over a time from seconds to days. The results compare well with those obtained by more conventional means but extend over a far longer time scale. They permit one to accept or reject alternative theories of crystallisation. It is generally accepted that in many polymers crystalline regions form and are held to each other by molecular chains which pass through two or more crystalline regions but part of whose length is in the intervening amorphous volume. In addition there may be molecular chains only partially within a crystal but with a loose free end in this amorphous volume, or even entirely within it. The pulsed NMR spin-spin relaxative curve comprises both a quadratic t* as well as one or two linear t components in the exponential decay. The former represents the crystalline fraction, while the latter gives the amorphous, linking fraction etc. The value of T, from the latter is a direct measure of the length of these linking chains. As the temperature is raised both T,, and Tt increase with thermal expansion and more free volume. At a point close to the melting point the magnitude C of the T,, component decreases rapidly as the smaller crystals melt first, and there is a corresponding increase in T, as the average length of these linking chains increases. At the temperature beyond which all crystalline regions and the quadratic component have vanished, the T, value, which has perhaps increased a hundredfold during the short melting range, shows only a small further increase, due to thermal expansion and reduction in density of entanglements of adequate lifetime. This would correspond to the mobility of entangled chains as described above. If T, may be taken as a measure of the average molecular weight MC between entanglements and/or crosslinks these measurements may serve as a quantitative measure of the number and length of these linking chains between crystals. Since
these are responsible for many of the macroscopic mechanical properties of such polymeric materials, this assessment on a molecular scale of the binding characteristics could permit a far more detailed and quantitative measure of the causes of physical deformation and recovery. These lengths of molecular chains spanning two crystalline regions can often be severed e.g. by radiation. The polymer would be weakened by the loss of some links but the crystallinity could rise as some of these severed chain lengths become incorporated into the crystals. However the increase in crystalline fraction is relatively small. A far greater increase in crystallinity is obtained if the irradiated polymer is reheated; this allows the fragmented ends to be incorporated into the new crystalline regions. This reformulation of the severed chains into the parent crystals and the far greater recrystallisation following melting can be traced quantitatively by pulsed T, measurements. It is clear that the use of pulsed NMR T2 data can provide a new and powerful method of measuring on a molecular scale a number of the parameters which intervene in fixing the mechanical and dynamic properties of these partially crystalline polymers. Their further study, in collaboration with macroscopic studies could result in a fuller understanding of the processess. One could hope to see this method further extended and also applied to the transition from the glassy to the liquid or more elastic state. RADIATION
CHEMISTRY AND THE INFLUENCE OF ADDITIVES
Many radiation-induced changes result in a marked change in molecular dimensions or shape and these can be greatly modified by the presence of additives. The effect is most pronounced with macromolecules where it can be readily evaluated by pulsed NMR determination of spin-spin relaxation. A simple example is irradiation of a simple polymer in the presence of oxygen. This can result in main chain scission with the formation of much shorter chains and of course a deterioration in mechanical strength. The formation of these shorter chains is readily seen in the spin-spin relaxation curve, where it appears as an additional component. Its longer time T,, is a measure of the reduced molecular weight due to scission, while its initial amplitude represents the fraction of the original material thus affected. No doubt the effect of antioxidants on this reaction, as on others, could be quantitatively assessed in this manner. For many polymers which crosslink under irradiation, there is a more rapid spin-spin relaxation decay as crosslinking reduces the length of the chains between successive links. For polymers irradiated in an aqueous solution the enhanced crosslinking reaction is readily demonstrated as due to both the direct effect (energy absorbed directly by the polymer) and
Analysis of macromolecular structures by pulsed NMR indirectly by the water when the radiolytic products react with dissolved polymer. The influence of concentration, dose etc is readily followed quantitatively by spin-spin relaxation which shows the enhanced degree of crosslinking as the solution becomes weaker and macromolecules are further apart. This of course is due to the greater indirect effect shared among fewer macromolecules. At very low concentrations the macroscopic network becomes more difficult to form, due to the preferential internal linking leading to microgel particles. So far no NMR results have appeared on this microgel structure of separate particles. Where an additive is present in the solution its effect on the NMR pattern is readily seen and the changes interpreted quantitatively. For example bovine serum albumin in solution shows a simple exponential decay with time constant T2 which decreases rapidly with radiation. However in the presence of a very low concentration of thiourea this decrease becomes very small. Here again one would like to see these measurements extended to determine the effect of concentration of macromolecule and of dose. Does the thiourea additive lose its effect beyond a certain dose, i.e. is it used up or does it continue to function with equal efficiency at very high doses? Here again the method shows great promise for an extension to macromolecules of biological interest. FILLERSAND
SURFACE REACTIONS
Many polymers show improved mechanical properties in the presence of solid particle fillers; examples are carbon black in rubber and silica in silicones. The question arises as to whether this is due to an additional chemical bonding effect, to physical reorientation of forces around each particle so that tension along a molecular chain is diverted in part to a solid particle, or to an alternative surface reaction. Another possibility is that the modulus of an elastic network is proportional to the number of effective, load-carrying segments within the lattice so that splitting of the stress in one polymer segment by interposition of a solid particle effectively doubles the number of these load-carrying segments. Use of this pulsed NMR technique for T2 which measures many of the basic properties of a network, with either permanent crosslinks or entangled chains, may serve to clarify the situation. It might also provide evidence of new surface reactions since the mobility of chains on or near such surfaces would be very different from free chains in the amorphous region or in a network system, forming part of a network, or in a crystalline or glassy region. There is some evidence of a further component in some polymeric NMR spin-spin relaxation curves but the origin of this has not yet been investigated. CHAIN REACTIONS; POLYESTER CURING, POLYMERISATION AND GRAFTING
A number of radiation-induced reactions involve a rapid chain reaction in which radiation only serves
49
as the initiating step. Polymerisation can serve as the simplest example and the course of the reaction has been followed by pulsed NMR. Two components are seen, the longer time T2 being due to the residual monomer, the second, far shorter to the polymer being formed. However as in certain liquid systems, a third component also appears (about 20% of total) with an intermediate time scale T2 whose origin is unknown. A somewhat similar component has also been observed in some molten polymer and one may enquire whether this is due to some unsuspected structure, possibly of a dynamic nature, in such high-molecular weight fluids. The curing of unsaturated polyester systems has also been followed by pulsed NMR of polyesterstyrene mixtures. The spin-spin relaxation of the unirradiated mixture shows a long-term component Tu due to the soluble fraction and at partial curing there is also the expected much shorter-term component due to the gel, glassy structure but here again there is some evidence of an additional, non-allocated component. DISCUSSION
For a number of decades a very considerable academic research effort has been devoted to the basic study of radiation induced chemical reactions in low molecular weight systems to discover the products and later the mechanisms and kinetics of the reactions. Advanced methods including pulse radiolysis and electron spin resonance have been utilised to determine characteristics of the various stages in the processes involved leading to the final radiation products. This work has led to important advances in our understanding of chemical reaction processes whether or not induced by radiation, but this aspect need not be considered here. The situation with high molecular weight materials is rather different, in that major physical or biological changes can result from extremely small amounts of absorbed radiation energy, even without a chain reaction. The doses involved may only result in one chemical change in a molecular weight of lo6 to 10”. The importance of such small doses resides in the fact that it may be the morphology or mobility of the systems which is profoundly a&&d by these minute chemical changes, and for such systems the powerful analytical technique represented by NMR T, measurements appears most promising. As an example one can consider the biological effects of very low doses of radiation. Unless there is a chain reaction, the chemical change may represent only one bond or less in lo’, yet this can result in a major biological effect. The question is then, are there so many of these vulnerable bonds in the molecule, such that radiation-induced damage to any one is lethal; or is the absorbed energy transferred selectively over considerable distances before causing a change to some vital bond. Or alternatively is the nature of the change far less selective, but its importance arises
50
A.
&4llLi%BY
from the considerably modified morphology or mobility of the macromolecule induced by this very small change? For example even in the simplest macromolecule of molecular weight exceeding lo6 a simple link averaging one per molecule anywhere along its length can profoundly affect its physical performance in well-known ways (even though the precise chemical mechanisms involved have not been settled despite very extensive studies for a number of decades). Similarly with some biological systems involving pairs of macromolecules, they may become linked together anywhere along their length and this may render them no longer capable of separating and continuing their primary function. Main chain scission may be another such reaction. The use of pulsed NMR T2 appears of great promise for such studies. The physical properties of most polymers are of very great importance, and are the subject of detailed investigations on such properties as deformation and creep, recovery etc. The use of T2 techniques is expected to provide much relevant information on behaviour on a molecular scale, allowing our understanding and manipulation of these parameters to be more effectively used and controlled. Some preliminary but important information has become available on the transition from solid, crystalline or glassy, to liquid or entangled. The extension of our conventional concepts, based on experience with low molecular weight systems, to linear, branched or even network structures requires considerable more information and here again we might utilise much of the data obtained from NMR spin-spin relaxation measurements. The number and lifetime of the so-called entanglements is one such topic. For example the NMR response of a partially crystalline polymer obtained from the melt may be dependent on its prior state even prior to melting, a memory conferred in it in a previous semi-crystalline state. How long does this memory last in the liquid or fluid state; and would it depend on prior orientation? In partly crosslinked systems we can envisage obtaining data on the mobility of chemically free macromolecules trapped within a network, formed by permanent links or short-lived entanglements. This may be of practical interest in the behaviour of membranes and of other chemical processes.
information to other systems, irradiated or not. In this respect radiation treatment, because of its quantitative aspect, readily quantised and reproduced and its enormous range of intensities and times serves as an excellent means of providing specimens for the calibration and interpretation of such NMR spectra, even those from other sources. Of special importance is the knowledge we can derive on a molecular scale on many basic properties not readily obtained in other ways. The motion of molecules confined within a lattice, the nature of the melting process, the composite structure involving both crystalline and amorphous regions, and the temporary memory imparted by the presence of entanglements in high molecular weight polymer may be considered as only four aspects which merit further attention. The extension of these and related properties concerned with molecular configuration and motion to biological systems is especially promising.
REFERENCES
General
AC-Use of pulsed NMR in measurement of radiation effects in polymers. Radial. Phys. Chem. 14, 919 (1979). AC,RF-Use of pulsed NMR to follow radiation effects in long-chain polymers. Rudiat. Phys. Chem. 15,393 (1980). AC,RF-Pulsed NMR technique for studying radiation effect in macromolecules. 6th Znt. Congress Radiat. Res., Tokyo, p. 336 (1979). AC--Characterisation of polymers using pulsed NMR. Tihany 5th Symposium, Akad Kiado, Budapest, p. 843 (1982). AC-Use of pulsed NMR to study radiation effects. Z. FI. Mitt. 97, 24 (1984). AC-Radiation effects in macromolecules; their determination with pulsed NMR. Radiat. Phys. Chem. 26 (5), 463 (1985). AC-Use of pulsed NMR to determine morphology of macromolecules. J. Radioanal. Nucl. Chem. 101 (2). 401 (1986). Polyethylene
DWM,DCD,EWA-.J. Polym. Sci. 59, 301 (1962). CONCLUSION
Although information has been obtained mainly on only a limited number of simple polymers, sufficient data have been accumulated to show clearly the type of information which we can expect from the spin-spin relaxation curves and the resultant T2 values. These relate less to chemical structure but rather to morphology and mobility of these systems. By studying a series of polymeric specimens, irradiated and therefore modified to a known extent, we can interpret the various NMR patterns and apply this
AC,PX,RF-Very high molecular weight polyethylene. Radiat. Phys. Chem. 11, 83 (1978). BJB,AC,RF-Crystallisation kinetics from melt and solution. Proc. R. Sot. Land. A367, 343 (1979). RF,AC-Effect of previous history on NMR relaxation of polyethylene. Eur. Polym. J. 15, 953 (1979). RF,AC-Entanglement effects on NMR spinspin relaxation. J. Polym. Sci. 16, 339 (1978). IK,AC-NMR relaxation in solid and molten polyethylene structures. J. Polym. Sci. 19, 803 (1981).
Analysis of macromolecular structures by pulsed NMR Polystyrene AC,EMJ-Entanglement and network formation in polystyrene, viscoelastic behaviow from pulsed NMR. Eur. Polym. J. 21, 55 (1985). Polydimethyi siloxane (PDMS) RF,AC-Radiation-induced crosslinking and gel formation in linear PDMS. Rudiat. Phys. Chem. 8, 555 (1976). RF,AC-Crosslinking and entanglements in HMW linear PDMS. Radiat. Phys. Chem. 10, 61 (1977). RF.JHS.AC--J. Polvm. Sci. (Pub. Phw. Edn) 16, 1041 , (1978):
.
_
AC,RF-Pulsed NMR studies of irradiated PDMS. Tihanv 4th SvmD. Radiat. Chem., Akad Kiado, Budap&;p. 335 (1977). AC,RFJHS-Analysis of crosslinked and entangled polymer networks. Proc. R. Sot. Land. A335,189 (1977). Polyisoprene NMR of cis-polyisoprene 1. Polymer M, 207 (1979). RF,AC-Pulsed NMR of cir-polyisoprene 2. Polymer 20, 211 (1979). AC,BJI&Pulsed NMR of cis-polyisoprene solution T, and T, relaxation, free volume, viscosity relationships. Eur. Polym. J. 17, 645 (1981). RF,AC-Pulsed
Polykobutylene (PiB) AC,JHS-Molecular weight determination of irradiated PiB by NMR. Radiat. Phys. Chem. 8,719 (1976). AC,BJB-Entanglement density in irradiated PiB. Rudiat. Phys. Gem. 19, 497 (1976). Polymerisation TK-Pulsed NMR investigation on polymerisation I. Methyl methacrylate. Pofym. J. (Japan) 18 (I l), 859 (1986).
51
TK-Pulsed NMR investigation on polymerisation II. Bulk polymerlsation of methacrylic acid. Polym. J. (Jopan) 19 (2), 285<1987). - TK,TK,MN,MK-Pulsed NMR study bulk polymerisation of nethacrylic acid. Mem. Nat. Defense Acad. 28 (l), 53 (1988). HS,MI,TK,KT,AN-PO/. J. (Japcm) 14, 149 (1982). Polyester curing MA,ZV,FR,PH-Radiation-induced crosslinking in polyester-styrene systems. Polymer 30, 1498 (1989). Authors MA-M. Andreis; BJB-Barbara Bridges; AC-A. Charlesby; RF-R. Folland; PH-P. He&g; EMJ-E. M. Jaroszkiewicx; IK-I. Kamel; PK-P. Kafer; TK-T. Kurotu; TK-Takakaxu; MN-Makoto Nagai; MKMotomu Kasagi; DWM-D. W. McCall; FR-F. Ranogajec; HS-H. Serizawa; JHS-Judith S. Steven; ZV-Zorika Veksli.
SOME MISCELLANEOUS PAPERS ON NMB OF POLYMERS
Belousova M. V. et al., Akad. Nauk. Inst. Khim. Fiz. 70-72 (1985). Draghicescu P. and Grosescu R., NMR Relaxation in some irradiated polymers; Magnetic Resonance etc., pp. 222-234. North-Holland, Amsterdam (1973). Kadomatsu Y. et al. Molecular motions in gammairradiated PTFE. Rep. Prog. Poly. Phys. Jpn 17, 51 I-514 (1975). Peffley W. M., Honnold V. R. and Binder D., Xray and NMR on irradiated PTFE and PClFr . J. Polym. Sci. A J 4, 977-983 (1966).