Anal. Chem. 1900, 60, 2695-2699
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High-Temperature Solid-state Nuclear Magnetic Resonance Using a Poly(amide-imide) Spinning System Richard C. Crosby and James F. Haw* Department of Chemistry, Texas A&M University, College Station, Texas 77843
David H. Lewis Chemagnetics, Inc., 208 Commerce Drive, Fort Collins, Colorado 80524
The deslgn of a maglc-angle splnnlng (MAS) system based on the high-temperature englneerlng therrnoplastlc Torlon (a pdy(amldelnide) resln produced by Amoco Chemical Corp.) Is descrlbed. Thls splnnlng system Is useful over a temperature range of 77-540 K. Torlon contalns carbon, and slgnHlcant background slgnals are observed In “C spectra that can, however, be substantlally suppressed by a modlfkatlon of the cross polarlratlon experlment. Torlon can be used for sdid-state NMR studles of %I, #INa, “B, and many other nuclel without a spectral background from the rotor. Thls Is an Important feature of the Torlon system relatlve to analogous deslgns based on ceramlcs. No 27AI NMR slgnal was detected at 2.35 T, but neutron actlvatlon revealed an alumlnum content of 600 (k30)ppm. A further advantage of the Torlon system Is that H Is not prone to catastrophic failure as are some ceramic splnnlng systems. The Torlon splnnlng system has been used In ”P MAS NMR experlments to observe the solld-state dehydratlon reactlon of the a-hopeite phase of Znt( P04)2.4H20 and the reversible removal of zeolitk water from hexagonal SmP04.xH20( x = 0-2.5). We also report the study of a morphologlcal transltlon In the Inorganic polymer poly[bls(3-methylphenoxy)phosphazene].
Variable-temperature (VT) solid-state NMR with magicangle spinning (MAS) is a relatively new technique ( 1 , 2 )for the study of structure, morphology, reactivity, dynamics, and magnetic and electronic properties of materials. Most of the VT MAS NMR studies in the literature deal with subambient temperatures (3-7), while very little work has been reported at temperatures appreciably above ambient (8-10). The reason for this difference is quite simple: many of the materials used to fabricate MAS spinning systems and sample rotors have good mechanical properties a t low temperatures but fail at high temperatures. Kel-F, for example, has been used to construct many different spinning syystem designs that work a t temperatures as low as 8 K (4), but MAS rotors machined from this material deform and fail at temperatures as low as 323 K. Many ceramics have excellent thermal properties, but they are not without problems. Although several materials such as boron nitride and Macor are machinable with standard techniques, aluminum oxide and zirconium oxide, the most commonly used ceramics in MAS probe construction, require very specialized machining techniques and tools that are not commonly available outside of specialty ceramics shops. Ceramics also show large NMR signals from nuclei present in their major structural components,and smaller background signals due to impurities or additives can also be encountered. Furthermore, when a ceramic component fails, it fails cats-
* Author to whom correspondence should be addressed. 0003-2700/88/0380-2695$01.50/0
strophically. Ceramic MAS rotor “explosions” frequently damage the coil and occasionally destroy the entire spinning system. The above disadvantages of ceramic-based spinning systems have motivated us to consider the use of high-temperature engineering thermoplastics for the construction of MAS systems and rotors. In this contribution, we report the design of a Torlon spinning system that reliably spins between 77 and 540 K. This spinning system is much more robust than ceramic-based systems, and it is free of many of the background signals observed with ceramic spinning systems. Furthermore, high-temperature thermoplastics are relatively easy to machine using standard techniques. Torlon’s major disadvantage is its 13Cand 15Nbackground signals, which can be minimized by exploiting differences in ‘H spin-lattice relaxation rates between the sample and rotor (vide infra). Another material that could prove useful for high-temperature MAS applications is Vespel (a polyimide thermosetting resin produced by Du Pont), whose properties are similar to those of Torlon. The use of the Torlon spinning system for high-temperature MAS NMR is illustrated by studies of two solid-state dehydration reactions (one reversible and one irreversible) and a morphological transition in a polymer.
EXPERIMENTAL SECTION Torlon Spinning System. Figure 1 is a drawing of the Torlon magic-angle spinning system. This spinning system is similar in construction to a standard Chemagnetics 7.5-mm double-bearing unit (11)with all Kel-F and Delrin parts machined from Torlon 4203 (Regal Plastics). The NMR transmitter-receiver coil (8) is threaded into a coil former (7, Torlon), which in turn fits into the coil-housing body (6, Torlon). The top air-bearing assembly, consisting of the bearing (9, brass) and its retainer (11,Torlon) fits on top of the coil former (7) and is held in place by the top cover (10, Torlon) and six brass screws. The stator (3, brass) fits into the bottom of the coil-housing body and is held in place by the bottom cover (2, Torlon) and six brass screws. Two separate air streams are brought to the coil-housing body midway along its length via hollow axles and are channeled internally to the stator- and bearing-air chambers. A bracket (5, Torlon) attached to the rear of the coil-housingbody and accessed via a threaded push rod (not shown) allows adjustment of the spinning angle around the two air-supply axles. Spinning speeds up to 4.5 kHz are routinely achieved by using the device in Figure 1 with air or nitrogen as the spinning gas. The spinning system was tested to destruction and found to spin smoothly at temperatures from 77 to 560 K, at which point it failed due to softening of the coil housing body. Neutron activation was performed on a Torlon sample to check for the presence of silicon or aluminum. No silicon was detected, but an aluminum content of 600 (*30)ppm was determined. We speculate that the duminum was introduced with a polymerization catalyst. NMR Spectroscopy. All spectra were obtained on a Chemagnetics M-100s solid-state NMR spectrometer equipped with a 2.35-T superconducting magnet with a 70-mm air bore and a 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER 15, 1988
?
r u
r-7
TORLON
10
9
@
_ -
I
8
2.54cm
_ 11
Drawing of the Torion magicangle spinning system: (1) Screw (brass);(2) capspinner bottom cover (Torion);(3) stator (brass); (4)screw (brass):(5) magimngle bracket (Torbn):(6) coiChousing body (Torlon); (7) coil former (Torion); (8)coil: (9) bearing (brass): (10) cap-spinner top cover (Torlon);(11) air-bearing retainer ring (Torion). Flgure 1.
home-built variable-temperature cross polarization (CP)/MAS probe. Torlon and Teflon are used in place of Delrin and other standard plastics in the fabrication of some of the structural components and insulators in this probe. Observation frequencies between 25 MHz (13C)and 40 M H z (31P)are obtained by replacing the tuning capacitor of the observe channel resonant circuit. The diameter of the probe is 60 mm; since the magnet is not equipped with room-temperatureshims,this leaves a 5-mm air gap between the probe and the bore surface of the magnet. During prolonged high-temperatureoperation a stream of cool nitrogen gas is passed through this air gap to minimize heating of the magnet. A purge stream of cool nitrogen gas is also used to maintain the probe electronics near room temperature; this keeps probe retuning to a minimum. The CP/MAS NMR probe has two independent spinning-gas streams: the drive gas, which is used to propel the rotor, and the bearing gas, which is used to stabilize the rotor. For high-temperature operation the gas streams are heated in two stages. The first stage uses independent heaters and controllers for the two gas channels while the second stage is provided by a single aluminum block heater. This arrangement allows the temperature of the bearing- and drive-gas streams to be independently controlled and is important for accurate sample temperature measurement and control (5). Reagents. The poly[bis(3-methylphenoxy)phosphazene] (PBMP) was furnished by the Shin Nisso Kako Co., Ltd.,of Japan, and was used as received. SmP04.nHz0(n = 0-2.5) and Zn3(PO4).4Hz0 were obtained from Strem Chemical Co. and used without further purification. Poly(methy1methacrylate) (PMMA) was obtained from Aldrich and used as received.
RESULTS AND DISCUSSION On examination of the structure of Torlon (see Figure 2) (12) it is obvious that there will be a very significant 13C background due to the high carbon content of the polymer. This background will preclude Torlon from being the material of choice for 13C spectroscopy, but since 13C is an important NMR nucleus, it is appropriate to characterize the background and assess the extent to which it can be suppressed through suitable spectroscopic manipulations. Figure 2a is the 13C
ppm
I
300 Figure 2.
I
230
180
80
20
- 50
Cross polarization spectra that illustrate the I3C background of the Torion spinning system and rotor: (a) I3C CP spectrum of the Torlon spinning system alone; (b) I3C CPIMAS spectrum of a Torion rotor; (c) 13C CP/MAS spectrum of PMMA in a Torion rotor obtained by using the Torion spinning system: (d) I3C CP/MAS NMR spectrum of PMMA obtained by using the background-suppression experiment described in the text. All spectra were obtained using a 2-ms contact time and 4000 transients. cross-polarization (CP) spectrum of the Torlon spinning system. No rotor was present during this experiment. The broad, almost featureless line shape observed in Figure 2a is due to overlapping chemical-shift anisotropy powder patterns from the various unique carbon sites in the polymer. These patterns me further broadened by incomplete 'H decoupling due to reduced radio-frequency power levels outside of the coil volume. Figure 2b shows the 13CCP/MAS spectrum of a Torlon rotor. The resonances between 116 and 139 ppm are due to aromatic carbons while the asymmetric doublet centered at 160 ppm is consistent with the amide linkages expected in this type of polymer (13Cresonances due to carbons adjacent to 14N usually show this splitting pattern in low-field 13C MAS experiments) (13). The approximate magnitude of the Torlon background signal for a typical sample can be appreciated in Figure 2c, which shows the 13C CP/MAS spectrum of PMMA (the weight of the PMMA is 0.15 g and the weight of the rotor is 2.18 9). Although most of the 13Cresonances of PMMA are (fortuitously) in "clear" regions of the Torlon spectrum, the upfield methyl signal is almost completely obscured by the accidental overlap of a spinning sideband from the Torlon aromatic carbon signals. One could attempt to minimize the Torlon background by spectral subtraction, but this requires that the background spectrum be obtained under identical conditions of spinning speed and decoupling power, and these are very difficult requirements to meet for V T experiments performed at several temperatures. A more satisfying way to address the background problem is to exploit differences in the relaxation properties of Torlon and the sample of interest. Since the protons in typical rigid solids have common values of TI, it is possible
ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER 15, 1988
to use a proton inversion-recovery sequence prior to the cross polarization pulse and select the waiting period between the proton pulses to satisfy the null condition (0.69 TIunder ideal conditions) for the Torlon proton magnetization (14). The 'H Tlvalues of Torlon and PMMA were found to be 620 and 220 ms, respectively, at 298 K and a field strength of 2.35 T. A delay period of 334 ms was empirically found to be most effective at reducing the Torlon rotor background when a 1-s recycle delay was used (Figure 2d). A modest amount of background from the spinning system is observed in Figure 2d, apparently a result of radio-frequency inhomogeneity. Although not perfect, the results of this experiment do demonstrate that it is possible to perform 13CCP/MAS NMR with a Torlon rotor and spinning system, provided the signal from the sample is moderately strong and the proton Tlvalues of the sample and rotor differ. This suppression technique can, in principle, be used for 15Nspectroscopy, although we have not assessed its effectiveness for that nucleus. Since enriched samples are frequently used in lSNCP/MAS NMR, the rotor background might, in any event, be minimal. The most promising applications of the Torlon spinning system involve nuclei other than 13C,15N,or 'H. Since several of the chemical problem areas in our research require 31PMAS NMR, we will illustrate the high-temperature capabilities of the Torlon spinning syetems with this nucleus. We have also performed 27AlMAS studies at temperatures as high as 508 K by using the spinning system in Figure 1. In spite of the fact that aluminum was detected by neutron activation analysis, we have not observed an nAl MAS NMR signal from Torlon at a field of 2.35 T. We cannot, however, rule out the possibility of observing an 27Albackground at higher field strengths, especially in experiments that require a large number of transients. We are using 31PMAS NMR to study the morphology of polyphosphazenes, which are inorganic polymers based on chains of alternating phosphorus and nitrogen atoms. Many poly(organophosphazenes) of the general structure
gNi OR
where OR is an alkoxy or aryloxy group, are semicrystalline and undergo two first-order phase transitions. The first transition, which occurs at a temperature designated T(l), is a conversion to a mesomorphic phase that appears to involve a pseudohexagonal structure (15). Depending on the type of side group present this mesomorphic state can persist over a temperature range of 150-250 K (16). The second transition is the true melting point (Tm). Alexander and co-workers have used wide-line NMR to study the mesomorphic transition in poly[bis(2,2,2-trifluoroethoxy)phosphazene]and have concluded that the polymer backbone is undergoing large amplitude motions a t temperatures above T(1)(17). Using differential scanning calorimetery Singler and co-workers have observed this type of transition behavior for PBMF (18). Figure 3 reports 31PCP/MAS spectra of PBMP obtained at temperatures ranging from 77 to 453 K. The spectra of PBMP obtained at 77,112, and 223 K are virtually identical (spinning sidebands are observed in the 77 and 112 K spectra due to a decrease in spinning speed) and consist of a 600-Hz-wide resonance with a maximum at -13 ppm and hints of two shoulders at -8 and -18 ppm. We have recently reported that the 31P MAS NMR spectra of the elastomeric polymers poly[bis(methoxy)phosphazene] and poly[bis(ethoxy)phosphazene] broaden significantly at temperatures below the glass transitions due to dipolar coupling to 14N (19). In those spectra, broadening from the 31P-31Pdipolar interaction and the 31Pchemical shift interaction was considered negligible
I
50
.
,
25
.
,
0
,
,
-25
.
,
-40
,
2687
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Flgure 3. Variable-temperature 31PCP/MAS NMR spectra of poly[bis(3-methylphenoxy)phosphazene]. Two-millisecond contact times were used and each spectrum is a result of 200 transients except for the 77, 112 and 223 K spectra, which are a result of 400 transients. Asterisk denotes spinning sideband.
because it has been shown that these interactions are effectively averaged in the MAS experiment when moderate spinning speeds are used (20). Broadening due to dipolar coupling to 14N is also operative here, and it is clear from Figure 3 that all of the PBMP is in a rigid state a t temperatures of 223 K and below, presumably a semicrystalline phase or a mixtute of semicrystalline and glassy phases. At higher temperatures, however, an additional, sharp resonance is observed at -18.4 ppm. Relaxation measurements (not shown) reveal this resonance to be due to a highly mobile component which is almost certainly the mesomorphic phase. The T(1) transition is essentially complete by 363 K, and no further changes are observed with further increases in temperature to 453 K. The T, of PBMP does not occur until 621 K and the polymer decomposes almost immediately after melting (18). No attempt was made to observe either of these processes using NMR. A second example of high-temperature MAS NMR using the Torlon spinning system is demonstrated in Figure 4, which shows a 31PCP/MAS NMR study of the solid-state dehydration reaction of Zn3(P04)2.4Hz0,which yields Zn3(P04)2.2H,0. The first stage of the dehydration reaction of the a-hopeite phase of Zn3(P04),.4H20is known to occur between 578 and 383 K (21,221. Studying this reaction by MAS NMR presents an interesting experimental challenge: since a gas is evolved, the position of equilibrium is strongly influenced by the partial pressure of water vapor. Indeed, when a Torlon rotor with a tightly fitting cap is used, no changes are observed in the 31P MAS spectrum until well above the reported transition temperature. In order to vent water vapor from the rotor, a l-mm hole was drilled in the center of the rotor cap. Loss of solids from the rotor was prevented by placing a small quantity of glass wool in the cap before packing the Zn3(P04)2.4Hz0. This arrangement allows water vapor to escape from the rotor, and the 31PCP/MAS spectra so obtained (Figure 4) reveal a transition at 378 K from Zn3(P04)2-4H20(4.4 ppm) to a new phase at 8.0 ppm, which was shown by X-ray powder diffraction to be Zn3(P04)2.2H20.The small peak that remains at 4.4 ppm at even the highest tem-
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER 15, 1988
lh
-A
378 K for 55 min
- A
24
'
a
-a
-d4
Flgure 4. Variable-temperature 31P CPIMAS NMR spectra of zinc phosphate tetrahydrate. Twmillisecond contact tknes were used and 400 transients were accumulated for each spectrum except for the 298 K spectrum, which Is a result of 40 transients.
perature examined (453 K) was identified by the following experiment. A sample of Zn3(P04)2+4H20 was degassed at 573 K for 48 h. A 31PMAS NMR spectrum (not shown) of the degassed sample shows only a single resonance at 3.8 ppm. Also, no 31Psignal can be obtained by cross polarization for this material. These data indicate that all H@ is absent from the heated sample and suggest that the peak seen at 4.4 ppm is due to another phase of Z~I~(PO,)~.~H~O, either the Bhopeite or parahopeite phase, both of which are reported to have transitions a t higher temperatures than that observed for a-hopeite (22). The reversibility of the dehydration reaction was assessed by acquiring a MAS spectrum (not shown) of a sample of Zn3(P04)2.4H20that was heated to 473 K, cooled, and allowed to remain in the atmosphere for 2 weeks. This spectrum is identical with the one acquired at 453 K, indicating that the reaction is either very slow or irreversible under these conditions. We have also used the Torlon spinning system to examine the dehydration of the lanthanide phosphate SmP04-xH20 ( x = 2.5). In the previous example, loss of water resulted in the formation of a different compound. In contrast, some lanthanide phosphates in the hexagonal modification are known to contain up to 3 mol of zeolitic water which can be reversibly removed without changing the lattice structure (23). Figure 5 reports 31PMAS spectra of SmPO, at various temperatures. The spectrum acquired at 296 K consists of three partially resolved resonances at 3.8,14.9, and 26.1 ppm. When the temperature is raised to 323 K, the three peaks shift to 6.0, 16.7, and 28.2 ppm, respectively. This downfield shift continues as the temperature is increased until at 523 K the center peak has shifted to 32.9 ppm. Temperature-dependent chemical shifts are often observed in NMR studies of paramagnetic compounds (24);however, in this case the contribution of paramagnetism to the observed chemical shifts appears to be minor (vide infra). When a room-temperature 31PMAS spectrum (not shown) was obtained of a sample of SmPOl that had been heated to 523 K and allowed to cool for 2 h in in the absence of air, it was found that the chemical shifts were quite different from those observed for unheated, commercial SmPO,. This indicates that factors other than paramagnetism contribute to the temperature-dependent chemical shifts observed for this compound. We have per-
44
00
-110
-44
Figure 5. Variable-temperature 31PMAS NMR spectra of SmPO,.xH,O. All spectra were obtained by using a 0.5srecycle time and 400 transients.
ti0
8b
do
6
-40
-sb
Flgure 6. Room-temperature 31PMAS spectra of a sample of SmPO,-xH,O, which was heated in an oven at 480 K for 24 h: (a) acquired immediately after removal from the oven; (b) sealed in an alrtight rotor and stored in an inert atmosphere at room temperature for 18 days: (c) left In the atmosphere at room temperature for 6 h ; (d) left in the atmosphere at room temperature for 18 days. Spectra a-c were acquired at room temperature using a 0.5s recycle time and 400 transients. Spemum d was acquired at room temperature using a 1-s recycle tlme and 400 transients.
formed a series of experiments to determine the nature of the temperature-dependent chemical shifts in SmPO1. A sample of commercial SmPO, was heated in an oven at 480 K for 24 h. This sample was divided into two portions, one portion was sealed in an airtight rotor and the other was left open to the atmosphere. A room-temperature 31PMAS spectrum of the sealed sample is shown in Figure 6a; it consists of a single resonance at 26.0 ppm, a value comparable to that obtained
ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER 15, 1988
in the variable-temperatureexperiment at 473 K. This sample was left in the rotor and stored in an inert atmosphere for 18 days, at which time its 31PMAS spectrum was again obtained (also at room temperature). That spectrum, which is reported in Figure 6b, is not appreciably different from that in Figure 6a. Also, no signal is observed in a 31PCP/MAS experiment on this sample, suggesting an absence of protons. If paramagnetism is responsible for the temperature-dependent chemical shifts, then these spectra should have the same chemical shifts as the unheated, commercial SmP04. As can be seen this is not the case. Figure 6c is a spectrum of the sample that was exposed to the atmosphere (in this case for 6 h). It is approximately a superposition of the spectra of heated and untreated samples, demonstrating that water has reentered some of the channels. Figure 6d is a spectrum of the same sample after expmure to the atmosphere for 18 days; it is virutally identical with that which is seen for untreated SmPO1. It is also possible to obtain a good-quality 31P CP/MAS spectrum of this material with a modest number of scans. These data indicate that SmP04 is losing H20 as it is heated and that H20 is slowly readsorbed from the atmosphere after the temperature is reduced. This high-temperature solid-state NMR study of zeolitic water in SmPOl suggests that future studies of the reaction of adsorbates on catalysts will be possible with the Torlon spinning system.
CONCLUSIONS The Torlon system reliably spins at temperatures between 77 and 540 K. The upper temperature limit possible with the design in Figure 1will likely increase as new thermoplastics with higher decomposition temperatures are developed. As MAS NMR experiments are extended to ever higher temperatures, there will, however, come a point at which even the best engineering thermoplastic will not be equal to the task. It is, therefore, essential that developmentof ceramic spinning systems continues. The advantages of the Torlon system relative to existing ceramic MAS spinning assemblies are its reliability, ease of manufacture and repair, and the small or nonexistent background signals from various nuclei that are found in many ceramics. The principal disadvantage of Torlon is its 13C and 16Nbackgrounds, although these can be dealt with in favorable cases. Using the Torlon spinning system, we have shown that high-temperature 31PMAS NMR can be used to probe the mesomorphic transition in a phosphazene inorganic polymer. For PBMP, the 31Pchemical shift is surprisingly sensitive to morphology. In addition, a solid-state dehydration reaction has been studied by use of the Torlon spinning system and a high-temperature, gas-permeable rotor. Further development of gas-permeable rotors for the study of reactions of the gas-solid interface is in progress. Finally, reversible removal
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of zeolitic water from SmP04has been shown to produce large changes in the 31PMAS NMR spectrum of that material. Although interesting in their own rights, the above studies have served to illustrate the potential of the Torlon spinning system in chemical investigations of solids at elevated temperatures. ACKNOWLEDGMENT The authors thank Victor J. Bartuska and David G. Dalbow of Chemagnetics, Inc., and Shawn Maynard of Texas A&M University for their assistance with this project. We also thank Shin Nisso Kako Co., Ltd., of Japan for supplying the PBMP. LITERATURE CITED (1) Lyeria, James, R.; Yannonl. Constantino. S.; Fyfe, Colin, A. Acc. Chem. Res. 1982. 15. 208-216. (2) Haw, James, F. A&/. 'Chem. 1988, 60, 559A-565A. (3) Macho, V.; Kendrick, R.; Yannoni, C. S. J . Magn. Reson. 1983, 52, 450-458. (4) Hackmann. A.; SeMei, H.; Kendrick, P. C.; Myre, P. C.; Yannoni, C. S. J . Magn. Reson. 1988. 79, 148-153. (5) Haw, James, F.; Campbell, Gordon, C.; Crosby, Richard, C. Anal. Chem. 1988, 58, 3172-3177. (8) Lyerla, J. R.; Fyfe, C. A.; Yannoni, C. S. J . Am. Chem. Soc. 1979, 101, 1351-1352. (7) Jelinski, L. W.; Dumais, J. J.; Engei, A. K. Macromolecules 1983, 76. 403-409. (8) SchneMer, B.; Doskociiova, J. B.; Ruzlcka, 2. J . Magn. Reson. 1980, 37, 41-47. (9) Geschke, Dieter; Quiilfeldt, Edgar J . Magn. Reson. 1985, 65, 326-331. (10) Doty, D. Presentation at 28th Rocky Mountain Conference, Denver, CO, 1988. (11) Bartuska, V. J.; Lewis, D. H.; Lewis, R. B.; Daibow, D. G. U S . Patent 4511041. . - . . - . ., 1985. .- - -. (12) Goodman, H. G. Handbook of T h e m e t Plesfics; Noyes: Park Rklge: NJ, 1986; p 295. (13) Hexem, J. G.; Frey, M. H.; Opella, S. J. J . Chem. Phys. 1982, 77, 3847-3856. (14) Zumbuiyadis, N. J . Magn. Reson. 1982, 49, 329-331. (15) Schnelder, N. S.; Desper, C. R.; Slngier. R. E.;Alexander, M. N.; Sagaiyn, P. L. In Orgenometellic folymefs; Carraher, c. E., Sheats, J. E., Pittman, C. U.,Eds.; Academic: New York. 1978. (16) Desper. C. R.; SchneMer, N. S.; Higgenbotham, E. J . Powm. Scl. Polym. Left.Ed. 1977, 15, 457-461. (17) Alexander, M. N.; Desper, C. R.; Sagaiyn, P. L.; Schneider, N. S. Macromolecules 1977. IO, 721-723. (18) Singier, R. E.; Schneider, N. S.; Hagneuer, G. L. Powm. €ng. Scl. 1975, 15, 321-338. (19) Crosby, R. C.; Haw, J. F. Macromo/ecules 1987. 20, 2324-2326. (20) Harris, R. K.; Jackson, P.; Phillips, J. W. J . Magn. Reson. 1987, 73, 178-183. (21) Shchegrov, L. N. Izv. Akad. Neuk SSSR. Neorg. Mater. 1988, 22(1), 157-159. (22) Internafional Crifical Tables of Numerical De&, Physics, Chemistry. and Technology; Washburn, E. W., Ed.: McGraw-Hill: New York. 1926. (23) Kuznetsov, V. 0.; Petushkova, S. M.; Tanananev. I. V. Zh. Neorg. Khim. 1969. 14. 1449-1451. (24) Campbei,-&rdon, C.-Crosby, Richard, C.; Haw, James, F. J . Magn. Reson. 1988, 69, 191-195.
RECEIVED for review April 21,1988. Accepted September 21, 1988. This work was supported by the National Science Foundation (Grant CHE-8700667).
