NMR Applied to Materials Analysis

focal point BY TOM M. APPLE

DEPARTMENT OF CHEMISTRY RENSSELAER POLYTECHNIC INSTITUTE

TROY, NEW YORK "J 2180-3590

NMR Applied to Materials Analysis

I N T R O D U C T I O N

T he need for high-performance ma te r i a l s in indust r ia l set- tings, in the home, and in mil-

itary applications has led to an ex- plosion of research directed towards the creation of new materials. Sci- entists are being asked to develop materials that can withstand hostile environments such as high temperature, mechanical stress, or the presence of corrosive agents. Other ef- forts have been focused on creating catalysts to direct reactions selective- ly and give products in high yield. In some cases, substances must display high strength and modulus, yet retain a light weight. Still other applications require unique optical or elec- trical properties, such as high con- duct iv i ty , insu la t ing capac i ty , or large nonlinearity in the optical re- sponse . These p rope r t i e s depend upon the microstructure of the material. Of particular importance are chemica l composi t ion , preferential orientation of molecules, size of domains , s ample he te rogene i ty , and motional properties of the substance or molecules contained within.

Nuclear magnetic resonance (NMR) is an extremely powerful tool for an- alyzing many of the characteristics alluded to above. The sensitivity of the resonance position to the chemical bonding has made N M R unpar- a l le led as a tool for e luc ida t ing

chemica l s t ructure . Fu r the rmore , NMR analysis does not require long- range order as does X-ray diffrac- tion. These benefits makes amorphous and semicrystalline materials amenable to study by NMR. While the shift of the nucleus is affected by short-range interactions, the degree to which order exists in a material may be derived from a study of the N M R line shapes. Highly crystalline materials yield sharp NMR resonances when examined with resolu- t ion-enhancing techniques, because every nucleus in a chemical structure experiences exactly the same envi- ronment. In an amorphous material there are changes in topology, bond

angles, and bond lengths, all of which modify the NMR chemical shift, and therefore broaden the resonance. By studying the anisotropy of shift interactions, one can identify the degree of preferential al ignment in drawn or ordered systems

One of the great advantages of N M R in the study of materials is the ability to quantitate the percentage of amorphous and crystalline compo- nents in polymers and ceramics. In many materials there is an intimate relationship between structure and m o l e c u l a r mot ion . For e x a m p l e , h igh- f requency molecu la r mot ions are often much more prevalent in amorphous domains than in crystal-

I I

~_L CYll chemical shift

FIG. I. A solid-state NMR shift pattern showing intensity vs. chemical shift for a powder sample of a single species. The positions ~ and ~ri represent the shifts of species having their unique shielding axis oriented parallel and perpendicular to the magnetic field, respectively.

12A Volume 49, Number 6, 1995

FIG. 2. raC NMR magic-angle spinning spectrum of hexamethyl benzene. Spin- ning sidebands are marked with an as- terisk (*).

line regions. NMR techniques exploit this property to distinguish between regions. NMR is unique in its ability to provide information on molecular motions over a frequency range of from 10 2 to 10 9 Hz. Anal- ysis of line shapes influenced by chemical-shift effects and quadrupolar interactions reveals details of motions at very low frequencies. These motions may include jumps, rotations, or chemical exchange. The frequencies as well as the angles of the motions or jumps can be determined.

Higher frequency motions can be probed by measuring NMR relaxation times. The relaxation time of the nuclear spins in the presence of ir- radiation (the Tip experiment) is sensitive to motions in the kHz range. Relaxation in the laboratory field, (the T~ experiment) gives information on rapid motions on the order of the NMR resonance frequency, typically 108--109 Hz. In addition to these powerful techniques, NMR is also capable of measuring transla- tional diffusion through the use of pulsed-field gradient experiments. ~ This method is of particular importance in the study of catalytic materials and the permeation of small molecules in polymers.

Several NMR methods have been developed which allow very detailed short-range structural information to be obtained on materials, z-7 These methods exploit the dipolar interactions between nuclear spins, both he t e ronuc lea r and homonuclear . They are valuable not only for de-

FIG. 3. =PSi NMR spectra of silicon oxy- carbides derived from various sol-gel preparations of (CH30)3Si-CH2-Si(OCH3)3. {Top) MeOH solvent, acid catalyzed. {Mid- dle) THF solvent, acid catalyzed. {Bottom) MeOH solvent, no catalyst.

300 200 ] O0 -0 -1 O0 -200 -300

APPLIED SPECTROSCOPY 13A

focal point

! . . . . ! ,

50 -0

ppm , , , i , ,

-50

ppm

200 100 -0 - 100 -200 -300

FtG. 4. (Bottom) 29Si NMR spectra of polymethylvinylsilane and elemental silicon--a precursor of SiC. (Top) pyrolyzed product--l~-SiC.

termining internuclear distances but, by extension, can probe reactivity at certain sites by exploring the change of the dipolar interactions between nuclei following a reaction. 8,9 Dipo- lar interactions also provide a basis for establishing connectivities over longer ranges, t° They can be used to determine domain sizes in materials and the intimacy of mixing between these domains. N M R experiments which make use of these long-range e f fec t s o f d ipo la r in te rac t ions ,

known as spin-diffusion, are of particular importance in the study of semicrystall ine polymers and block copolymers.

In this article we shall focus on the use of N M R (1) to determine chemical identity, (2) to establish the a m o u n t o f c rys t a l l i ne and amorphous phases in materials, (3) to probe motions of molecules, and (4) to determine short-range structural information such as internuclear distances. A number of review articles

TABLE I. Comparison of NMR spectral properties of crystalline and amorphous materials.

Crystalline NMR property region Amorphous region

~H NMR linewidth Broad ~H Tip" Short MASb linewidth Narrow (13C, 29Si, iSN) cross-polarization rate Fast T~ c Long

Narrower than crystalline Longer than crystalline Broader than crystalline Lower than crystalline Shorter than crystalline

" Spin-lattice relaxation time in the rotating frame. The time constant for the approach to equi- l ibrium of the magnetizat ion when it is maintained along the radio-frequency field in a reference frame which rotates about the laboratory magnet ic field.

h Magic-angle spinning. Spin-lattice relaxation time. The time constant for restoration o f the equilibrium magnetizat ion along the laboratory magnetic field.

and monographs are available for a more in-depth look at the application of N M R to solids, lj,~2 catalysts, ~3-t(' ceramics, ~7 glasses] 8 and polymers.~9-2J

D E T E R M I N A T I O N O F C H E M I C A L S T R U C T U R E IN M A T E R I A L S

To analyze solid materials we require high resolution and the ability to carry out analyses in a quantitative manner. In examining the NMR spectrum of solid materials, one is confronted by several interactions which may degrade reso lu t ion . These interactions are averaged o u t in liquids by rapid isotropic motion. They include the dipole-dipole coupling among the N M R active nuclei, the anisotropy of the chemical shift, and, for nuclei with spin greater than 1/2, the interaction of the nuclear quadrupole moment of the nucleus with the e lec t r ic - f ie ld gradient . Whi le these in te rac t ions m a y be viewed as a hindrance to obtaining high-quality spectra, we shall see that a great deal of information may be obtained from them. For now, however, let us consider ways in which these interactions can be over- come to give high-resolution spectra in a quantitative way, so that chemical structure may be determined.

High-Resolution NMR in Solids. The chemical shift of a nucleus de- pends upon its orientation in the magnetic field. This is known as the anisotropy of the chemical shift. An- isotropy is due to the nonspherical nature of the electronic distribution in molecules. I f the shielding envi- ronment surrounding a nucleus has axial symmetry , one observes broad lines in a powdered sample, due to the presence of many orientations (Fig. 1). A nucleus having its long shielding axis along the magnetic field resonates at the position ~ll, while a nucleus oriented perpendicular to that orientation resonates at the value ~l- Nuclei with intermedi- ate orientations fall between these two extremes. The chemical-shift anisotropy can be removed by rapid spinning of the sample at an angle of


54.7 ° relative to the magnetic field. 22 This technique, referred to as magic- angle spinning (MAS), provides the resolution required to separate the peaks in a material's NMR spectrum. In addition, the apparent signal-to- noise ratio is enhanced, because the spinning concentrates the intensity of the signal into narrow peaks. The spinning speed of the sample must be greater than the width of the spectrum (in Hz) in order to obtain all of the intensity in the central resonance. With the advent of higher magnetic- field strengths, it has become in- creasingly difficult to meet this cri- terion, because the anisotropy scales with the magnet ic - f ie ld strength. Failure to spin the sample rapidly enough causes the appearance of spinning sidebands spaced about the central resonance position at inter- vals of the spinning frequency. This effect is shown in Fig. 2. A ~3C NMR spectrum of hexamethyl benzene is shown in which the aromatic resonances are split into s idebands (marked by asterisks). While the intensity of these sidebands can be used to obtain information on the anisotropy of the chemical shift, sidebands can also make interpretation of the NMR spectrum difficult. This consideration has led to techniques to e l iminate s idebands f rom the spectrum of samples spun at lower s p e e d s . 23-26

The chemical shift can be used to ascertain the identity of a particular material. For example, Fig. 3 shows the 298i NMR spectra obtained by pyrolyzing differing sol-gel polymer prepara t ions of (CH30)3S i -CH 2- Si(OCH3)3 to form silicon oxycar- bides. 27 The three preparations in- volved either methanol or tetrahy- drofuran as a solvent and were poly- merized both with, and without, an acid catalyst. The resonances observed correspond to Si with four ( - 1 1 0 ppm), three ( - 6 5 ppm), and two ( - 2 0 ppm) oxygen neares t neighbors. In some instances, second nearest-neighbor distributions can be revealed as well. Another example of using magic-angle sinning for studying the conversion of polymers to ceramic materials is shown in Fig. 4.

FIG. 5.

I I ~ . . _ ~ - - _ -80 ° C i i

;! .6o.c

ppm ' ' ' ' ' ' ' ' ' I I

40 20 0 -20 -40

~H NMR spectra of polysilaethylene as a function of temperature.

The bottom spectrum shows the 298i NMR of the ceramic precursor mixture of polymethylvinylsilane ( - 18 ppm) and elemental silicon ( - 8 2 ppm). The top spectrum shows the same mixture following pyrolysis at 1000°C. This top spectrum is indic- ative of the formation of [3-Sic.

Quantifying Spectra. In obtain-

ing an NMR spectrum, one averages the signals obtained from many experiments. These experiments typically involve displacing the nuclear spin magnetization to a position perpendicular to its equilibrium position along the magnetic field. The dephasing of the magnetization is observed in the plane perpendicular to

I I J I I I I I I I I I I I I I I J 1 I ] I I I

] 0 -0 -10 -20 -30

p p m

I I I I

FIG. 6. (Top) Low-temperature ~3C NMR magic-angle spinning spectrum of polysilaethylene. (Bottom) Spectrum of sample obtained under identical conditions, but warmed to room temperature.


focal point

0 .9

0 .8

0 .7

0 .6

0 .5

0 .4

0 .3

0 .2

0.1

0

2

m

m

. . . . ; . . . . ; . . . . ; . . . . ; . . . . ; . . . . ; . . . . ; . . . . 1

2.5 3 3 .5 4 4 .5 5 5 .5 6

I O 0 0 / T

FIG. 7. 'H spin-lattice relaxation time (Ti) measurements as a function of temperature (T) for polysilaethylene.

the field following this displacement. This process is then repeated after the magnetization has realigned with the magnetic field. I f this process is done correctly, the area of an N M R resonance peak is proportional to the amount of that species present in the sample. Problems arise when the time required for the reestablishment of equilibrium is prohibitively long. This is often the case for rigid solids containing few protons, a condition often encountered in ceramic materials. In systems containing more than one phase, such as semicrystalline polymers or ceramic materials having amorphous and crystalline regions, the time required for equil- ibration of the two regions can differ dramatically. Thus, if one uses typical relaxation delays of seconds or tens of seconds in the N M R experiment, one sample region is fully recovered while the other region (usu- ally the rigid, crystalline region) is not recovered. The N M R spectrum

thereby under-emphasizes the crystalline region, leading to inaccurate de te rmina t ions of crys ta l l in i ty . In these cases the safest approach is to perturb the magnetization to a small- er extent away from its equilibrium position. For example, if the magnetization is rotated only 30 ° from the magnetic field, one obtains half the signal size, but the displacement f rom equilibrium is only 1/7th that of the typical displacement. Thus, whi le one sacr i f ices some signal magnitude, both portions of the sample are evenly represented, since the m a g n e t i z a t i o n need r e c o v e r only slightly.

The spectra of nuclei such as 13C, 29Si, and ~SN are often obtained by c ros s -po l a r i za t i on , a me thod by which magnetization is transferred from abundant protons to the nucleus to be observed. Cross-polarization can greatly enhance the signals ob- ta ined f r o m mate r i a l s con ta in ing protons, because the magnetization

derives f rom the protons which are more highly polarized and which tend to relax faster than the rarer nuclei. Caution must be used to obtain quantitative data when cross-polarization is employed. Cross-polarization rates depend upon the strength of the dipolar coupling between protons and the insens i t ive nucleus. Thus, cross-polarization is efficient if protons are directly attached to the nucleus to be enhanced, but less so if the proton is distant. Motion par- tially averages the dipolar interaction and can, therefore, diminish the rate of cross-polarization. The cross-polarization mixing time is limited by the loss of magnetization of the protons during the experiment (the ~H T~o). Using cross-polarization to obtain the spectrum of a semicrystalline polymer may result in the en- hancement of a rigid crystalline region and the diminution of the signal f rom an amorphous mobile region. In the attempt to quantify spectra obtained with cross-polarization, the contact time, the t ime that the cross- po l a r i za t ion is a l l owed to occur, should be varied from experiment to experiment. Methods are then applied to correct the intensities. 28

I D E N T I F Y I N G A N D Q U A N T I F Y I N G R E G I O N S O R P H A S E S OF M A T E R I A L S

There are several tools available to the N M R spectroscopist to analyze the regions of a sample. Most methods depend upon the disparity of molecular motions in different regions of the sample. Table I com- pares the N M R properties of crys- ta l l ine vs. a m o r p h o u s reg ions of polymers.

Most of the properties of the crystalline regions originate from the strong dipolar interactions between nuclei. Figure 5 shows ~H N M R spectra of a sample of polysilaethylene as a function of temperature. 29 The sample begins solidifying and crystallizing below 0°C. The crystalline region contains strong p ro ton- proton dipolar couplings and con- tributes to a very broad line which is barely observable above the base-


.1

C

.05

.001

10

.'Wx.~,,~e,W'w',

I I I I I I I I 500 400 300 200 100 0 -100 -200

C~11 (Yiso (Y33 Chemical shift (ppm)

Fro. 8. (Right) NMR spectra of rhodium dicarbonyl species included in Y zeolites acquired at (a) 298 K, (b) 323 K, and (c) 423 K. (Left) Simulations of the NMR spectra of rhodium dicarbonyl species undergoing 180 o flips about their C2 axis as a function of the ratio of the flipping rate to the anisotropy of the chemical shift.

line. The mobile portion of the sample gives the narrower resonances, which can be assigned to protons attached to carbon (upfield) and protons attached to silicon (downfield). When the sample has been complete- ly solidified, it still contains both c rys ta l l ine and amorphous glassy regions. By obtaining the 13C spec- t rum at low tempera tu res under cross-polarization and magic-angle spinning conditions (Fig. 6, top), one can distinguish the crystalline region ( - 6 ppm) from the amorphous region ( - 9 ppm). The crystalline region adopts the all-trans configura- tion, while the amorphous region is motionally averaged between trans and gauche c o n f o r m a t i o n s ? ° The

gauche con fo rma t ion causes the well-known shift of the resonance position upfield by several ppm. 31 Thus, by studying this spectrum as a function of cross-polarization time, one can quantitatively determine the percentage of crystalline and amorphous polymer. Upon warming of the sample, all polymer chains sample both trans and gauche confor- mations, and the spectrum shifts upfield (Fig. 6, bottom).

M O T I O N A L P R O P E R T I E S

Figure 7 shows the spin-lattice relaxation times (TO measured for protons in the mobile region of a sample of polysilaethylene as a function of

temperature. While the analysis of the relaxation of polymers can be quite complex because of a distribution of correlation times and the anisotropy of motion, 19,2°,32 the correlation time for the motion could be estimated from the T~ data, and an ac t iva t ion ene rgy for the trans- gauche isomerization can be determined, as in the case shown here. The values match well those determined from Monte Carlo calculations. 29

Slow motions that have frequencies on the order of the NMR linewidth (typically in the 102-105 Hz range) can be examined with a great deal of detail. Figure 8 (right) shows NMR spectra of ~3C nuclei of rho-


focal point

30

40

50

0

10

20

60

70

80

90

FIG. 9. 'gF NMR MREV-8 spectra of stretched polytetrafluoroethylene as a function of/3, the angle in degrees between the stretch axis and the magnetic field. The y axis has been scaled to ac- commodate the increased intensily of the crystalline region at high values of the angle #.

dium dicarbonyl species included in a zeolite matrix as a function of temperature. 33 The ~3C NMR spectrum was broadened to a width of greater than 400 ppm by the anisotropy of the chemical shift. As the molecules underwent motion at higher temper- atures, their orientation relative to the magnetic field changed. This motion resulted in line-shape changes which could be modeled. The results of calculations of dicarbonyl moie- ties undergoing rapid flips about their C2 axis are shown in Fig. 8

(left) as a function of the ratio of the flipping rate to the anisotropy of the chemica l shift. These s imulat ions matched the data well and revealed that the flipping rate is several hun- dred Hz. By measuring the frequency of the flipping motion as a function of temperature, we determined the activation energy for the motion to be 12 kJ/mol.

Many studies of this nature have been carried out with the use of 2H NMR in polymeric and liquid crystalline systems. 34 In those cases the quadrupolar nature of the deuterium nucleus provides the anisotropy of the NMR signal. This anisotropy is typically about l0 s Hz and provides an excellent probe of slow motions in these systems.

NMR is also an excellent probe of t ranslat ional mot ion in materials. Application of the pulsed-field-gradient (PFG) method ~ allows the materials scientist to determine diffusion constants of molecules over ranges from l0 5 to 10 9 cm2/s. In the PFG experiment, the nuclear spins are subjected to radio-frequency pulses which are designed to re- focus the magnetization following its natural dephasing. Magnet ic- f ie ld gradient pulses are also applied. These pulses hinder the refocusing process if the nuclei move to a new position in the magnetic field during the experiment. By measuring the amount of refocused magnetization, one can determine the diffusion co- efficient of the molecule.

PFG studies of benzene adsorbed in co-cation exchanged zeolites with a concentration of benzene of two molecules per zeolite cage have been carried out. 35 It was observed that the diffusion constant was strongly re- lated to the number of ions residing in the zeolite supercages. The ions act as strong adsorption sites for the benzene. Substituting divalent Ca 2+ ions for Na + ions lowers the number of ions in the cages and increases the diffusivity. The temperature depen- dence of the diffusion constant allows a calculation of an activation energy for diffusion of 20.5 kJ/mol. The di f fus ion cons tant increases when the concentration of benzene

in the cages exceeds the number of ions in the cage.

S T R U C T U R A L I N F O R M A T I O N

Ordering. We have seen how the anisotropy of the chemical shift can give us information about slow motions in materials. The patterns as- sociated with the chemical-shift anisotropy essentially tell us the number of molecules in the sample which are situated at a given orientation in the magnetic field. In systems which have preferential alignment, the intensity at certain points of the pattern will be enhanced, mirroring the presence of a large number of molecules at that particular orientation. This information can be used to determine the degree to which molecules are aligned in oriented liquid crystals or in poled or extruded polymers. Fig- ure 9 shows the 19F NMR spectra of semi-crystal l ine polytetrafluoroethylene which had been elongated in an Instron to 140% its original length. 36 The spectra were obtained by the homonuclear dipolar decou- pling method MREV-8. 37,38 The spectra are composed of two parts: a mobile, amorphous portion and a rigid crystalline region. The amorphous region exhibits a broad featu- reless resonance which is invariant to the sample position [3 (the spectra have been vertically scaled to allow for the greater intensity of the crystalline region at high values of 13). The spectrum from the crystalline region is sensitive to orientation because the crystallites are preferen- tially aligned along the stretch axis. By studying the changes in the cen- ter of mass of the line and its asso- ciated moments, 39 one may determine the degree to which the crystallites order along the stretch axis during and following elongation. Of course this method may be used to probe relaxation of aligned or poled materials as well.

Internuclear Distances. The strength o f d ipo l e -d ipo l e interactions between nuclei in solids de- pends upon the internuclear separa- tion as the inverse third power in static systems. Thus, a measurement


~ , x x x Data o

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2.58 oA

• , - - . 2.48 A

".\,

" \ ~ " , , / . - " " % v ' . . . • . . j - , , - . . . .

^ x

Data

calc. 2.58 A

I

s t

I I I I I I I I 0 1 2 3 4 0 0.5 1

Time (msec) Freq. (kHz)

FIG. 10. (Left) Dipolar oscillation between neighboring 13C nuclei in Rh(13C0)2 anchored to the zeolite lattice. Theoretical oscillations for various distances are also given for comparison. (Right) The Fourier transform of the dipolar oscillation.

of the d ipolar coup l ing o f nuclei can y i e l d the i n t e r n u c l e a r s e p a r a t i o n , Severa l s trategies are ava i lab le for the scient is t s tudying mater ia ls to de- t e rmine the strength o f the dipolar coupl ing . 2-7 F igure 10 shows the effects o f d ipolar coup l ing be tween two ~3C nuclei o f inc luded Rh(CO)2 in a Y zeoli te . 4° The d ipolar coup l ing causes an osc i l la t ion o f the N M R re- s p o n s e u n d e r a C a r r / P u r c e l l / M e i - b o o m / G i l l ( C P M G ) sequence . 4t The C P M G sequence r e m o v e s chemica l shift effects . Also shown for c o m - par ison are the s imula t ions for various in te rnuclear dis tances and the Four ie r t ransform of the data, which a l lows a more prec ise m e a s u r e m e n t o f the in te rnuclear distance. The expe r imen t showed that the Rh(CO)2 species was present as a square pla- nar Rh c o m p l e x with the two car- bonyl moie t ies and two zeol i t ic ox- ygens fo rming the l igands. Exper i - ments s imilar to this have been used to ident i fy reac t ion pa thways dur ing

po lymer i za t ion and ident i fy species adsorbed on surfaces. 8.9

C O N C L U S I O N

The utili ty with which the inter- act ions in N M R can be manipu la ted by the expe r im en t e r a l lows N M R to be appl ied to many systems in a mul- t i t ude o f ways , T h e i n f o r m a t i o n avai lab le is rich and varied. Grea t strides are con t inu ing to be made in areas o f magne t ic resonance imag- ing , 42-45 h i g h - r e s o l u t i o n N M R o f n o n i n t e g e r q u a d r u p o l a r nuc le i , 46-5° th rough-space cor re la t ion o f resonances, 3,5-7 and spectral edit ing. 5~.52 These advances p romise to make N M R an e v e n more impor tan t tool for the mater ia ls scientist .

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NMR Applied to Materials Analysis