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Author ProofAuthor ProofFeature Article: Magic-angle spinningNMR spectroscopy provides a great dealof information about semicrystalline poly-mers.Akeyaspect of suchwork is the abilityto tailor pulse sequences so as to selectsubspectra appropriate to amorphous andcrystalline domains separately. This articlediscusses the basis of such work (whichinvolves differences in molecular-level mo-bility), describes relevant sequences, andillustrates their use for the special case offluorine-19 NMR of hydrogen-containingfluoropolymers (see Figure). & pleasecheck&
Selective NMR Pulse Sequences for the Studyof Solid Hydrogen-ContainingFluoropolymers
S. Ando, R. K. Harris,* P. Hazendonk, P.Wormald
Macromol. Rapid Commun. 2005, 26, 345–356
marc.200400517C
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Author ProofAuthor Proof
Selective NMR Pulse Sequences for the Study of
Solid Hydrogen-Containing Fluoropolymersa
Shinji Ando,1 Robin K. Harris,*2 Paul Hazendonk,3 Philip Wormald4
1Department of Organic and PolymericMaterials, Tokyo Institute of Technology, Ookayama,Meguro-ku, Tokyo 152-8552, Japan2Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UKFax: (þ44) 191-384-4737; E-mail: r.k.harris@durham.ac.uk
3Department of Chemistry and Biochemistry, University of Lethbridge, 4401 University Drive W, Lethbridge,Alberta T1K 3M4, Canada
4School of Chemistry, University of St. Andrews, Purdie Building, St. Andrews, Fife KY16 9ST, UK
Received: October 26, 2004; Revised: December 17, 2004; Accepted: December 20, 2004; DOI: 10.1002/marc.200400517
Keywords: domain structure; fluoropolymers; NMR; pulse sequences; relaxation
Introduction
Synthetic polymers form an extremely important class of
materials, which has been extensively studied by NMR
methods, applied to both solutions and solids. The first
experiments combining cross-polarization (CP), magic-
angle spinning (MAS), and high-powered proton decou-
pling (HPPD) were applied to obtain high-resolution 13C
spectra of three solid polymers.[1] Indeed, in the first decade
of the use of theCP/MAS/HPPDsuite of techniques,[2] solid
polymers formed a high proportion of the samples studied,
Summary: Fluorine-19 NMR spectra of solids have somespecial features, which are discussed in this article. Inparticular, they generally contain two abundant spin baths(protons and fluorine nuclei). This situation throws up somespecial operational requirements, as does the study of hetero-geneous samples. The relaxation characteristics of heteroge-neous systems, which are briefly described herein, frequentlypermit the use of specific pulse sequences to obtain subspec-tra for individual components. Various possible selectivesequences for use in fluorinated heterogeneous organicsolids are listed and their actions rationalized on the basisof molecular mobility. Semicrystalline hydrogen-containingfluoropolymers form especially suitable systems for suchoperations, and in order to understand their domain structuresit is essential to obtain subspectra of the amorphous andcrystalline domains. Examples are given of the use of selec-tive pulse sequences for studying fluoropolymers, especiallyfor poly(vinylidene fluoride) (PVDF) and the copolymerP(VDF75/TrFE25) (TrFE¼ trifluoroethylene).
DIVAM/CP spectra of the vinylidene fluoride/trifluorethy-lene copolymer as a function of the minipulse angle used.Top: Unfiltered spectrum. Middle: the amorphous domain.Bottom: the crystalline domain.&Q1 please check legend&
Macromol. Rapid Commun. 2005, 26, 345–356 DOI: 10.1002/marc.200400517 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
marc.200400517
Feature Article 345
a Based on a lecture given at the MACRO 2004 conference inParis, June 2004, and the SPSJ meeting in Sapporo, September2004.
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Author ProofAuthor ProofShinji Ando (homepage: http://www.op.titech.ac.jp/polymer/lab/sando/index.htm) received his B.E.,M.E., and Ph.D. degrees in polymer science and engineering from the Tokyo Institute of Technology in1984, 1986, and 1989, respectively. His work for the Ph.D was an investigation of the conformationdependence and the hydrogen-bond length dependence of 13C chemical shifts in oligo- and poly-peptides. He served as a research scientist at the Nippon Telegraph and Telephone Corporation from1989 to 1995, where he was engaged in analysis of optical properties and the development of thermallystable fluoropolymers applicable to optical micro-components and planar lightwave circuits. In 1995,he transferred to the department of organic and polymeric materials at the Tokyo Institute of Technologyas an associate professor. He commenced structural analysis and developments of novel fluoropolymersfor electronic and optical applications using solid-state NMR spectroscopy, optical spectroscopies, andcomputational chemistry. He stayed at the University of Durham, UK, as a visiting scientist from 1998 to1999, working with Prof. R. K. Harris. He analyzed the cross-polarization dynamics between 1H and 19Fnuclei in semicrystalline and amorphous fluoropolymers. He also investigated the morphology andmolecular motions of semicrystalline fluoropolymers such as PVDF, PVF, ETFE and so on. Recently,based on knowledge from 19F NMR spectroscopy, density functional calculations, and spectroscopies, hedeveloped highly fluorescent fluorinated polyimides. His research interests include the analysis andprediction of optical properties and electronic structures of thermally stable and functionalfluoropolymers and the structural analysis of semicrystalline and amorphous fluoropolymers usingsolid state 19F NMR spectroscopy.
Robin K. Harris obtained his Ph.D. (and, later, a Sc.D.) from the University of Cambridge (U.K.) andthen spent two years as a postdoctoral fellow in independent research at the Mellon Institute, Pittsburgh.On his return to the U.K. he was appointed to a lectureship at the new University of East Anglia(Norwich). He spent 20 years there, rising to a full professorship, before transferring to the University ofDurham (U.K.) in 1984, where he is now an Emeritus Professor. His interests in the first 20 years of hisresearch work were concentrated on solution-state NMR spectroscopy and its applications to manychemical topics (inter alia to silicones and silicates using 29Si NMR spectroscopy), including the use ofspectral analysis and the development of relevant computer programs. This began to change in the late1970s, when he set up, in collaboration with Professor K. J. Packer, the first CPMAS spectrometer inEurope. Gradually his research evolved to concentrate on solid-state NMR methods and their use in awide range of chemical areas, from ceramics and synthetic polymers to pharmaceutical polymorphism.He has been especially involved with the analysis of spinning sidebands and second-order effects inspin-½ spectra. In the 1990s he pioneered the use of 19F CPMAS NMR spectroscopy with high-powerproton decoupling, which led to applications to fluoropolymers. He has published around 500 scientificpapers, reviews, and also a textbook on NMR spectroscopy. He co-edited a monograph on ‘‘NMR andthe Periodic Table’’ in 1978. More recently, he has been co-editor-in-chief for the 9-volumeEncyclopedia of NMR spectroscopy.
Paul Hazendonk obtained his undergraduate degree in Chemistry from the University of Winnipeg,Canada, in 1993. He started his graduate studies in High-Resolution Nuclear Magnetic ResonanceSpectroscopy with T. Schaefer at the University of Manitoba, Winnipeg, obtaining his M.Sc. in 1995. Hecontinued his research with A. D. Bain, at McMaster University, Hamilton, Ontario, obtaining his Ph.D.in 2000. His thesis work was concerned primarily with dynamic NMR spectroscopy in solution andsolid-state. Over the next two years he served as a postdoctoral research fellow at the University ofDurham, Durham, U.K., with Professor R. K. Harris, working on solid-state NMR spectroscopy offluoropolymers. He investigated cross-polarization dynamics between multiple abundant spin systemssuch as occur between 19F and 1H nuclei in non-perfluorinated polymers. He is now an AssistantProfessor at the University of Lethbridge, Canada, where his research focuses on solid-state NMRspectroscopy of fluorine-containing materials such as organic and inorganic fluoropolymers andinorganic fluorides. One of the main objectives of his research is to provide a mechanistic understandingof the macroscopic properties of these materials at a molecular level, and to develop new solid-statefluorine NMR experiments to study these materials. He has authored and coauthored 25 peer-reviewedjournal articles and has presented 30 papers at national and international conferences.
346 S. Ando, R. K. Harris, P. Hazendonk, P. Wormald
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Author ProofAuthor Proof
and they continue to be highly investigated by MAS
NMR.[3] There are a number of reasons for this situation, in
particular: (i) the ability of NMR to obtain detailed chem-
ical information from amorphous as well as crystalline
materials; (ii) the remarkable versatility of NMR, exempli-
fied by the wide range of pulse sequences which can be
chosen to produce specific results.[2]
Most MAS work on polymers has, naturally, concen-
trated on the ubiquitous 13C nucleus,[3,4] but of course, a
number of different NMR-active nuclides are present in
particular polymeric systems,[3,5] for instance 15N, 29Si, and31P. These nuclei are also amenable to the CP/MAS/HPPD
combination of techniques. For 1H high-resolution spectra
of solids, one must generally use either very fast MAS[6]
or else multiple-pulse operation combined with MAS
(‘‘CRAMPS’’)[7–9] in order to overcome the strong homo-
nuclear (1H, 1H) dipolar interactions. The 19F nuclide is also
a special case. In several ways it is very suitable for high-
resolution NMR spectroscopy. Thus as it exists in 100%
natural abundance and has a high magnetic moment (in fact
the third highest of spin-½ nuclides after 3H and 1H), it has a
very high receptivity (0.834 of that of 1H, 4.90� 103 times
that of 13C).[10] Unlike 1H, however, it has a large chemical
shift range and so the spectra can be highly resolved (and
therefore, chemically informative). Arguably, then, 19F
NMR is to be preferred to either 1H or 13CNMR for suitable
cases, though clearly the three nuclides can be studied
together. Therefore, one would expect high-resolution 19F
NMR spectroscopy of solids to be very popular. However,
the same properties that give advantages also confer pro-
blems. For instance, for perfluorinated systems, the strong
(19F, 19F) dipolar interactions have (at least until recently)
required the use of CRAMPS,[11] which is technically
demanding. Moreover, for fluorinated materials that also
contain protons (to which the rest of this article is dedi-
cated), high-power proton decoupling has been considered
necessary (again, at least until very recently), and this is not
entirely straightforward because of the proximity of 1H
and 19F resonance frequencies (differing by only ca. 6%).
However, in the mid-1990s commercial probes capable of19F-{1H} double resonance, involving high powers in the
proton channel but with efficient filtering, became avail-
able, so that work in this area began.[12–19] There is a
residual oddity in that, at relatively low applied magnetic
fields, high-power proton decoupling results in the appea-
rance of the Bloch–Siegert effect,[20,21] which causes an
apparent chemical shift on 19F resonances,[13,18] as shown
in Figure 1 and expressed in Equation (1).
DdðBSÞ ¼ ðgFB1HÞ2=ðo2F � o2
HÞ ð1Þ
where gF is the magnetogyric ratio of 19F, B1H is the proton
radiofrequency magnetic field strength, oF is the 19F
resonance frequency, and oH is the 1H ‘‘decoupling’’ fre-
quency. However, once this is recognized, it poses no
difficulties for chemical shift measurement. All that is
required is the use of the same 1H radiofrequency (RF)
Philip Wormald obtained his undergraduate degree in Chemistry at the University of Stockholm,Sweden in 1989, after which, he worked on solid-state nuclear magnetic resonance of ligno-cellulosicmaterials at the Swedish Paper and Pulp Research Institute and the Royal Swedish Institute ofTechnology. In 1998 he moved to the University of Durham, UK, were he worked in the Solid-StateNMR Research Service for other UK universities (funded by the Engineering and Physical SciencesResearch Council) and for industry. During this time he continued research with Professor R. K. Harris,obtaining his Ph.D. in 2005. His thesis work was primarily concerned with 19F and 1H relaxation invinylidenefluoride-based polymers and the relationship of their macroscopic properties to themolecular level. Since 2002 he has been a senior research fellow in solid-state NMR spectroscopy at theUniversity of St Andrews, Scotland, and was made an honorary lecturer there in 2004. His researchinvolves: solid-state NMR spectroscopy of novel microporous and battery materials, as well asfluoropolymers. In the former case, the main objectives are the development of Multiple-QuantumMagic-Angle Spinning (MQMAS) methodologies and the structural determination of microporousmaterials such as zeolites and catalysts. For fluoropolymers, the work involves high-resolution solid-and solution-state NMR spectra of modifiable low-molecular-weight systems for structuraldetermination and to further understand macroscopic properties at the molecular level of thesematerials. He has authored and coauthored 13 peer-reviewed journal articles and contributed to twobooks in solid-state NMR spectroscopy.
Figure 1. 188MHz 19F CPMAS spectra without and with high-power proton decoupling, showing the Bloch–Siegert shift(ca., 2.8 ppm) example.
Selective NMR Pulse Sequences for the Study of Solid Hydrogen-Containing Fluoropolymers 347
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Author ProofAuthor Proofpower during 19F observation of the reference sample (e.g.,
liquid C6F6) as when the sample of interest is examined.[22]
The Bloch–Siegert effect is avoided by 19F CRAMPS
operation with synchronized p pulses on the proton channelproviding the heteronuclear decoupling.[19] Moreover, the
effect is not significant for spectrometers operating at 7.1 T
and above. However, for observation of 19F spectra of
proton-containing fluoropolymers, increasing B0 provides
no advantages in dispersion and requires higher spin rates to
minimize the occurrence of spinning sidebands.
There are some other residual problems. For instance,
geminal (19F, 19F) isotropic indirect (i.e., scalar) couplings
are significant (ca. 280 Hz) for CF2 groups, for example, in
P(VDF/TrFE)[23] (VDF¼ vinylidene fluoride, TrFE¼ tri-
fluoroethylene) and in principle cause splittings in spectra.
Moreover, partly for this reason and partly not fully
understood, linewidths in 19FMAS spectra of solids remain
relatively high (e.g., hundreds of Hz)[15] even for well-
crystallized samples. In addition, the large 19F shielding
anisotropies can make some specialized pulse sequences
inefficient or complex.[19]
With the relatively recent advent of high-speed
(>20 kHz) MAS, HPPD appears to be no longer essential
for obtaining high-resolution 19F spectra[24–26] of some
fluorinated solids that also contain protons. However, such
spin rates can cause substantially higher increases in sample
temperature unless controlled. Moreover, there remains a
number of advantages of using CP from protons, and this is
generally more efficient at somewhat lower spin rates (e.g.,
ca. 15 kHz), which then also requires HPPD during signal
acquisition.Whereas the gain in sensitivity relative to direct
polarization is rather small (gH/gF¼ 1.062),[10] CP dis-
criminates against compounds in a heterogeneous sample
which are perfluorinated. This also applies to probe com-
ponents, which frequently involve poly(tetrafluoroethene)
(PTFE) and consequently lead to background signals (albeit
broad) for MAS-only operation for some spectrometer/
probe combinations (see Figure 2).[13,16] More importantly,
CP is involved in a number of specialized pulse sequences,
as described below, including some two-dimensional
experiments.
All these considerations clearly apply to NMR studies of
hydrogen-containing fluoropolymers.[12,13] Such synthetic
macromolecules are important industrially because of their
excellent stability against chemical degradation under a
variety of conditions and because of their special properties
(e.g., piezoelectricity, ferroelectricity, pyroelectricity,
etc.).[27] Moreover, they are frequently complex in physical
structure and are thus of intrinsic interest. They are gene-
rally semicrystalline so their domain structures require
study. Several of them exhibit polymorphism of their
crystalline domains so that characterization methods are
necessary and their phase transition behavior becomes
important in practical usage. Such polymorphism is fre-
quently of the conformational type, and variable conforma-
tion also characterizes amorphous domains. As with many
polymers, especially those produced commercially, the
nature of the end-groups is relevant, as are any chemical
defects in the polymer chains. Such defects, arising from an
occasional reversal in the ordering of a monomer unit in a
chain, are common[28–31] in samples of poly(vinylidene
fluoride), PVDF. Finally, mobility at the molecular level, as
always for polymers, conveys distinctive properties, which
are temperature dependent. These motions can be analyzed
by measurement of NMR relaxation times.
Relaxation in Homogeneous andHeterogeneous Samples
One of the most powerful attributes of solid-state NMR
spectroscopy is its ability to address heterogeneous systems
and, in particular, to separately obtain subspectra relating
to different components by use of specialized pulse
sequences. This ability is clearly of value not only in cases
where heterogeneities correspond to different chemical
components but also, as in the situations considered here,
for samples that are chemically uniform but physically
diverse, that is, for domain structures of pure polymers. This
property of NMR renders it very unusual, if not unique,
among characterization methods, especially as it extends to
amorphous as well as crystalline domains (in contrast to all
diffraction tools). NMR discrimination methods rely on the
versatility of NMR as expressed in the immense range of
pulse sequences that are possible. These can be tailored to
give specific results. Their effective operation in terms of
selectivity must rely on differences in the properties of the
various physical domains that are under investigation.
The property that readily distinguishes solid samples
even of the same material but in different physical form (or
of domains of samples of chemically homogeneous mate-
rials) is molecular-level mobility. This is, of course, a
complex property. Its extent shows a strong dependence on
the motional frequency considered, and it varies greatly
Figure 2. Fluorine-19 MAS spectra[16] of a physical mixture ofPTFE (95%) and PVDF (5%). (a) Direct polarization. (b)1H! 19F cross polarization. The PVDF is severely discriminatedagainst in (b).
348 S. Ando, R. K. Harris, P. Hazendonk, P. Wormald
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Author ProofAuthor Proofwith temperature as well as with molecular environment in
the solid state. In particular, amorphous domains, which are
unordered and, therefore, generally less tightly packed than
the ordered crystalline domains, are usually the more mo-
bile. Theirmobility normally increases significantlywhen a
sample is heated through the glass-transition temperature.
Any molecular motion causes modulation (leading to
partial averaging) of nuclear dipole–dipole interactions and
hence affects relaxation times and related parameters.
Relaxation can also be caused by re-orientation of the
shielding tensor, the anisotropy ofwhich is often substantial
for 19F. A number of different relaxation times may be
measured by NMR spectroscopy and influence the opera-
tion of NMR experiments in different ways. In order to
provide a reasonable background to the present review
article, the relevant parameters are described briefly below,
but for further detail the reader is referred to references
[2,32,33]. The relaxation parameters in question include:
(i) Spin-lattice (also known as longitudinal) relaxation.
The characteristic time T1 is generally of the order of
seconds for abundant spins such as 1H or 19F in solids
(though it can be significantly longer). It governs the
return of magnetization in the direction of the applied
field (Bo) to its equilibrium value following a
perturbation and responds to motions in the region of
the NMR frequency, that is, hundreds of MHz. It
therefore influences the delay between repetition of
pulse cycles in the NMR experiment.
(ii) Spin-lattice relaxation in the rotating frame. The
characteristic time is designated T1r and usually lies in
the tens of ms range for solids. It governs return of
magnetization perpendicular to Bo but under the
influence of a radiofrequency field B1, to equilibrium
following establishment of a significantmagnitude. Its
value is influenced by motions at the nutation
frequency related to B1 (i.e., to gB1/2p), which is
usually in the tens of kHz range. It influences the
operation of the CP experiment.
(iii) Spin–spin relaxation, perhaps better denoted as
transverse relaxation. The characteristic time T2relates to the return of magnetization perpendicular
to Bo to zero (in the absence of B1) following the
establishment of a non-zero value. Its magnitude is
influenced by low-frequency motions, and for rela-
tively rigid solids (static samples) it is generally a few
tens of ms. It governs the observed free-induction
decay and hence the linewidth of resonances. It may
have a complicated dependence on MAS rate.
(iv) Cross-polarization rate (characteristic time THF). In
the CP experiment (Figure 3), the contact time, t, is avariable. The magnetization in the observed nucleus
first increases as a function of t at a rate governed by
THF and then decays as it leaks to the lattice by a T1rprocesses. The value of THF is generally in the region
of tens or hundreds of ms.
Whereas linewidths tend to decrease, and transverse rela-
xation rates tend to increase, monotonically as increasing
temperature promotes more molecular mobility, T1 and T1rpass through one or more minima. Hence the effect of
temperature change on T1 and T1r is sometimes difficult to
predict and measurements at a single temperature can be
misleading. Of course, for amorphous polymer domains,
passing through the glass-transition temperature induces
substantial changes in molecular mobility and, hence, in
relaxation times.
The typical times mentioned above are those appropriate
for abundant spins such as 1H and 19F. The behavior of spin-
lattice relaxation, both in the laboratory and the rotating
frame, is generally single-exponential for a homogeneous
sample, but transverse relaxation is a more complex pheno-
menon, which may be difficult to fit mathematically.[34,35]
The time T2 may therefore have a meaning that depends on
the circumstances. For a heterogeneous sample with large
domain sizes, spin-lattice relaxation may be treated inde-
pendently for the various domains (i.e., the total magnetiza-
tion will relax as the sum of two exponentials, if there are
two domains, with coefficients appropriate to the domain
concentrations), as may spin-lattice relaxation in the rotat-
ing frame and transverse relaxation. However, the pheno-
menon of spin diffusion spreadsmagnetization in a random-
walk fashion such that the degree of spread is governed by
time. Hence, in typical times, themagnetization of different
Figure 3. (a) Standard CP sequence. (b) Delayed-acquisition CP(dipolar dephasing/non-quaternary suppression) sequence. (c)Delayed CP sequence.
Selective NMR Pulse Sequences for the Study of Solid Hydrogen-Containing Fluoropolymers 349
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Author ProofAuthor Proofdomains can be averaged if the domains are small enough.
In such cases, single values of T1 (and of T1r) will be
observed even for heterogeneous samples. The critical
domain size for these events is of the order of nano-
meters.[36] Moreover, T1 is more readily averaged than T1r.
Relaxation behavior in the critical region is complex[34,35]
and, whilst sums of exponentials may be used, the various
T1 values will be distorted from those intrinsic to the various
domains and the coefficients will not correspond to the
concentrations of the domains. Transverse relaxation is
virtually unaffected by spin diffusion. The effects of spin-
diffusion on T1r in heterogeneous systems can be
minimized[37,38] by spin-locking at the magic angle.[39,40]
Discriminating Experiments in19F Solid-State NMR Spectroscopy
As stated above, pulse sequences can be devised to discri-
minate between various domains in a heterogeneous poly-
mer. These are primarily based on differences in mobility,
which cause changes in the various relaxation times by both
dipolar and shielding anisotropy mechanisms. Since the
relationship between mobility and relaxation is not simple,
a variety of responses to the NMR experiment in question
can occur. Therefore, whereas in some cases T1 may differ
greatly between two domains (and an experiment based on
T1 differentiation will work well), in others it will prove to
be better to discriminate by T1r or T2. Moreover, such a
choice will be temperature dependent. In addition, whereas
sometimes the relaxation of 19F itself may provide a good
opportunity for selectivity, in other cases it will prove to be
better to use differentiation based on 1H relaxation, byCP to19F. Hence there are many possibilities, and therefore, a
range of appropriate pulse sequences is described briefly
here.
Firstly, the cross-polarization pulse sequence (Figure 3)
is itself selective. This is because the 19F magnetization
variation with contact time (Figure 4) has the functional
form[41] of Equation (2):
MFðtÞ / � expð�t=THF*Þ þ expð�t=T1r*Þ ð2Þ
where THF* and T1r* are themselves functions of THF (the
cross-polarization time), T1r(H) and T1r(F). (The cross-
polarization dynamicswill bemore complexwhen there are
resolved 19F resonances for a given domain such that there
are several distinct fluorine spin baths.[42] In addition, dipo-
lar oscillations are often seen at short contact times.[43–45])
The values of both T�HF and T
�1r will depend on local mole-
cular mobility. CP rates depend on the strength of (H,F)
dipolar interactions which are weakened by molecular
mobility. Therefore, CP with short contact times generally
favors crystalline domains. However, the same molecular
mobility also frequently causes T1r(H) for amorphous
domains to be significantly shorter, which means that
crystalline domains may also be selected by long contact
times. Figure 4 shows schematically the contact-time
dependence for a system containing two domains (labelled
A and B) with equal amounts of the observed nuclide but
with CP and relaxation characteristics of crystalline (A) and
amorphous (B) materials. It can be seen that, in general, CP
favors crystalline domains, especially at short contact times
(when CP toA is efficient and the signal is large) and at very
long times (when domain Bmay be undetectable because of
the noise level).
Discriminating CP experiments, which depend on T1(H)
or T1(F), involve an inversion-recovery component, of 1H
magnetization pre-contact or of 19F magnetization post-
contact, respectively (Figure 5). In each case, the recovery
time can be set so as to nullify the magnetization of one
domain, resulting in only the signal of the other domain
being observed. The null condition for a simple systemwith
relaxation time T1 requires the recovery time, t, to be T1ln
2¼ 0.693 � T1. For the inversion-recovery experiment on19F, the magnetization after the CP must first be placed in
Figure 4. Schematic plot of variable-contact-time CP intensity,S, for two domains with very different CP rates and effective T1r.The detectability level would be determined by the noise.
Figure 5. (a) Pre-CP inversion-recovery sequence. (b) Post-CPinversion-recovery sequence.
350 S. Ando, R. K. Harris, P. Hazendonk, P. Wormald
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Author ProofAuthor Proofthe –z direction by a 90x
8 pulse and then brought back to the y
direction (to be measured) after the relevant nulling time.
Success for this experiment depends on T1 for the two
domains differing substantially. However, there is a
complication with these experiments since, in principle,
spin-lattice relaxation for a coupled heteronuclear system is
not single exponential unless the two types of spin are
decoupled during the time allowed for relaxation. Decou-
pling would, therefore, be desirable for significant periods
of time, which could cause probe damage. The method will
often work without decoupling but the appropriate condi-
tions are not readily determined. However, spin diffusion is
relatively efficient at averaging longitudinal relaxation
rates,[32] so discrimination based on T1 is not often
implemented for fluoropolymers.
Similar, but somewhat simpler pulse sequences are
available (Figure 6) for selectivity based on T1r(H) or
T1r(F). In the former case a 908(H) pulse is followed by a
spin-lock period that lies between the values of T1r(H) for
the two domains. A CP contact time follows immediately
the spin-lock period ends, no additional 908 pulse being
required. For the sequence based on T1r(F), the spin-lock
period (which must be between the values of T1r(F) for the
two domains) follows the CP contact time, and acquisition
of the signal occurs immediately the spin-locking ends. In
these cases, decoupling during the relaxation (spin-lock)
periods is scarcely feasible since it is essential to avoid CP
during these times, so the relaxation may, in practice, be
complex.
There are several pulse sequences based on discrimina-
tion by linewidths (i.e., transverse relaxation). The simplest
is the dipolar dephasing sequence (Figure 3b),[46] also
known as non-quaternary suppression[47,48] because of its
use for selecting 13C signals for quaternary carbons while
eliminating those fromCH andCH2 carbons. This sequence
involves a (non-observed) free induction decay period
during a decoupling window immediately after CP. Under
these conditions, 19F signals from crystalline domains
correspond to broad lines (because the polymer chains are
rigid, giving rise to strong heteronuclear and homonuclear
dipolar interactions) and so they decay quickly, leaving sig-
nals from amorphous domains (corresponding to relatively
sharp lines) to be detected following the end of the de-
coupling window. In fact, a post-CP delayed-acquisition
sequence with proton decoupling during the delay would
also give some discrimination because of the differences in
(F,F) homonuclear dipolar interactions. Another alterna-
tive, which often gives similar results for semicrystalline
polymers, is the delayed CP sequence (Figure 3c), which
relies on differences in proton bandwidths. In both the
delayed-acquisition and delayed-CP cases (especially the
former), it may be advantageous to introduce a p pulse into
the middle of the delay period, with rotor synchronization,
to refocus chemical shifts.
A further selective method based on differentiation
between strong and weak (i.e., partially averaged) homo-
nuclear dipolar interactions is the so-called dipolar filter
(DF) pulse sequence (Figure 7a).[49] This consists of a series
of 908 pulses with phase cycling. Weak dipolar couplings
are refocussed, but strong dipolar interactions are affected
less efficiently and consequently magnetization from rigid
regions tends to be eliminated by this pulse sequence.
Recently, a new method for obtaining selective
spectra based on proton transverse relaxation has been
Figure 6. (a) Pre-CP spin-lock sequence. (b) Post-CP spin-locksequence.
Figure 7. (a) Dipolar filter sequence. (b) DIVAM/CP sequence.(c) Direct DIVAM sequence.
Selective NMR Pulse Sequences for the Study of Solid Hydrogen-Containing Fluoropolymers 351
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Author ProofAuthor Proofreported. This pulse sequence is called Discrimination
Induced by Variable Amplitude Minipulses (DIVAM)
(Figure 7b)[44,45,50] and consists of a series of (usually 12)
minipulses of constant phase but with pulse (nutation)
angles chosen to eliminate the magnetization from either
rigid or relatively mobile domains. During the minipulse
intervals My decays relatively slowly for the latter, so that
the net pulse angle gradually increases over the 12 pulses
whereasMy for the rigid regions is essentially lost between
the minipulses (but a substantial Mz is retained if the
minipulse angle is small). When the minipulse angle and
interval for 1H are set so as to give a net nutation of 908 forthe mobile domain magnetization, a final 908 pulse fol-
lowed by a CP contact time will give the 19F spectrum
selectively for the rigid domain. Alternatively, a CP contact
time immediately following the 12 minipulses (but without
the extra 908pulse)will give the 19F spectrummainly for the
mobile domain. This may also be obtained by use of larger
minipulse angles leading to, say, a net nutation angle of
3608 forM(mobile), by which timeM(rigid) may have been
lost (see Figure 8), so that CP (908 pulse plus contact time)
will yield a spectrum of the mobile region only. It will be
seen that this ability to obtain spectra of both rigid and
mobile domains selectively is an advantage of this method.
The DF method can only give selective spectra of mobile
regions, as does dipolar dephasing, and T1r filter methods
only yield spectra of the regionwith the longer T1r. It is true
that subtraction of a single selected spectrum from the full
spectrum can yield the complementary spectrum, but this is
often unsatisfactory in practice.
Several of the above discriminationmethods can, alterna-
tively, proceed using 19F direct polarization (DP) instead of1H! 19F cross polarization. Hence DP can involve inver-
sion-recovery (use of T1(F)), spin-locking (use of T1r(F)),
dipolar dephasing (which simply becomes delayed acquisi-
tion), and dipolar filter components in the pulse sequence.
Recently, direct DIVAM (Figure 7c) has also been
utilized.[51] However, the direct DIVAM pulse sequence
appears to be more complicated in operation than CP/
DIVAM. The effects depend on offset and shielding aniso-
tropy (both ofwhich aremuch larger than the corresponding
parameters for protonNMR) but not significantly on dipolar
interactions. Gerstein et al. have shown[52] that significantly
delayed acquisition proton spectra can reveal the existence
of mobile moieties in a solid system even at a very low
concentration, though the cause of this effect is subject to
controversy.[53,54] DP methods are, naturally, available for
perfluorinated systems also. For fluoropolymers containing
protons, direct detection of their 1H spectra is also possible
(as well as 19F! 1H CP). However, 19F spectra contain the
big advantage of better resolution. Methods involving1H! 19F CP are often preferred because of the elimination
of background signals from fully protonated or fully
fluorinated components of the probe (and rotor caps).
Of course, any of the selective pulse sequences described
above may be used as a preliminary to further manipulation
of the magnetization of the selected domain. For instance,
they can be used in a Goldman–Shen[55] experiment to
utilize spin diffusion in order to obtain information on the
size of domains.[56] Combinationwith awideline separation
(WISE) pulse sequence[57] enables heteronuclear correla-
tion (two-dimensional) experiments to be carried out selec-
tively for crystalline and amorphous domains.[17]
All the pulse sequences described above involve ob-
servation of 19F subspectra. It is possible to use analogous
methods to obtain proton subspectra for amorphous and
Figure 8. Explanation of DIVAM, showing how magnetizations with short and longtransverse relaxation times behave during the operation of a succession of minipulsesof common phase. To obtain the pure subspectra, CP is used following the situationsshown at the bottom.Under the conditions illustrated, the ‘‘puremobile spectrum’’ (seebottom left) will usually contain a small contribution from the rigid domains unless asmall oppositely phased pulse is applied before the CP.
352 S. Ando, R. K. Harris, P. Hazendonk, P. Wormald
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
WILEY-VCH
Author ProofAuthor Proofcrystalline domains[44] (assuming the existence of two
abundant spin baths, 19F and 1H), but such procedures are
less attractive because fluorine spectra have a far superior
chemical shift dispersion than proton spectra.
Applications to Fluoropolymers
As far as we are aware, domain-selective measurements
based on T1 differences have not been implemented on
fluoropolymers, largely because typical domain sizes imply
substantial (usually complete) averaging caused by spin
diffusion. However, circumstances may occur for which
such methods are appropriate.
The first-reported domain-selective 19F-{1H} experi-
ments on a fluoropolymer (PVDF) involved using a T1r(H)
filter (i.e., a spin-locked delayed-contact CPMAS pulse
sequence) as an introduction to aWISE sequence.[17] Such a
filter is effective (Figure 9) at ambient probe temperature
because T1r(H) for the crystalline domains is significantly
longer than for the amorphous domains. Figure 9 shows
how this method can be used[12] to study the polymorphic
form of the crystalline domains in PVDF samples produced
by different processing methods. It should, however, be
noted that the precise values of the relevant relaxation
parameters are difficult to obtain accurately because of the
effects of spin diffusion and of cross-relaxation between
proton and fluorine spin baths. Moreover, it needs to be
recognized that CP dynamics are complicated when two
abundant spin nuclides are involved,[41,58] rendering the
variable-contact-time method for obtaining T1r difficult to
apply. In any case, all relaxation parameters will depend on
the nature of the sample (average molecular mass, disper-
sion, regio-irregularity, etc.) and on temperature. However,
the ratio of T1r(H) values for the crystalline and amorphous
domains for PVDF is found to be generally in the range of
2–3,[44,59,60] which suffices tomake the pre-CPproton spin-
lock an efficient tool to select the spectrumof the crystalline
domains. A similar situation applies to T1r(F), so that a
post-CP 19F spin-lock is equally effective in selecting such
domains. Figure 10[60] illustrates this fact and also shows
that the selectivity extends to the spinning sidebands. The
result suggests that some reverse units in PVDF occur in
rigid regions (possibly at the interface of amorphous and
crystalline domains). This has been confirmed by the use of
a post-CP 19F spin-lock as a preparation phase to a RFDR
experiment.[60] Experiments conducted at aorund 100 8Cshowed that T1r(F) values for both the crystalline and
amorphous domains of PVDFwere significantly lower than
those at approximately 60 8C but that the ratio remained
approximately the same, so that the T1r(F) filter selection
Figure 9. Fluorine-19 spectra (centreband signals only) of twosamples of PVDF, showing discrimination using a T1r(H) filter.The pre-CP proton spin-lock duration was 40 ms and a shortcontact time (50 ms) was used. The different polymorphic contentof the two samples is clearly shown.[13]
Figure 10. Fluorine-19 CPMAS spectra of a sample of PVDFusing the standard sequence (top) and a T1r(F) filter (bottom). Inthis case the full spectrum, including the spinning sidebands, isshown.
Figure 11. Fluorine-19 CPMAS spectra of PVDF showingselection of the amorphous subspectrum by dipolar dephasing.
Selective NMR Pulse Sequences for the Study of Solid Hydrogen-Containing Fluoropolymers 353
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Author ProofAuthor Proof
method was still viable at the higher temperature.[60] The19F DP/spin-lock method has been shown[61,62] to be
effective for selecting the subspectrum of the g-crystallineform of PVDF.
However, T1r filters only select for crystalline domains,
so they must be matched with other methods. The dipolar
dephasing pulse sequencewas the first one used to select the
spectrum of amorphous domains in PVDF (Figure 11),[59]
since it was found that the decay time of the magnetization
for these regions during the decoupling window was about
three times that for the crystalline domains. Figure 11[59]
illustrates this situation; the lower spectrum should be
Figure 12. Domain selection of PVDF in 19F direct-polariza-tion spectra.[60] Top: full spectrum. Middle: dipolar-filteredspectrum. Bottom: T1r(F)-filtered spectrum. In this case, T1r(F)for the principal amorphous peak is 3.5 ms, whereas for the high-frequency crystalline peak it is measured to be 9.5 ms, a sufficientdifference to give good selectivity, as shown in the bottomspectrum.
Figure 13. CPMAS spectra of PVDF, obtained using a Gold-man–Shen pulse sequence following selection of the crystallinesubspectrum.[56] The delay time allowed for spin diffusion is givenat the right-hand side. The signal for the amorphous phase can beseen to grow in as the delay time increases, analysis of whichallows domain sizes to be determined.
Figure 14. 19F–{1H} MAS spectra of P(VDF75/TrFE25) at68 8C, obtained by (a) direct polarization, (b) delayed CP(delay time 0.5 ms), and (c) short-contact CP (contact time0.1 ms).
Figure 15. DIVAM/CP spectra of the VDF/TrFE copolymer as afunction of the minipulse angle used. Top: Unfiltered spectrum.Middle: Selection of the amorphous domain. Bottom: Selection ofthe crystalline domain. The inter-pulse spacing was 6 ms, and theminipulse nutation angles used are indicated.
354 S. Ando, R. K. Harris, P. Hazendonk, P. Wormald
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Author ProofAuthor Proof
compared with the upper one of Figure 9 for the crystalline
domains. Later,[56] the dipolar filter (DF) sequencewas also
used for this purpose (Figure 12, middle spectrum) and
proved to be very effective. The T1r(H) and DF methods
have been inserted before a Goldman–Shen pulse sequence
in order to determine the lamellar sizes of the two domains,
as is shown in Figure 13.[56]
Delayed-CP and short-contact CP can be readily used to
select for amorphous and crystalline regions respectively, as
is illustrated for a sample of P(VDF75/TrFE25) in Figure 14.
This clearly shows that the sample (as received from a
commercial source) contains both mobile (amorphous) and
rigid (crystalline) domains. The spectrum of the latter is
clearly depicted by the short-contactCP experiment,where-
as it is largely obscured in the DP experiment because the
resonances are broad and the crystallinity of the sample is
relatively low.
The DIVAM/CP method was first used[45] to select for
amorphous domains in the case of PVF, and was later appli-
ed[44] to PVDF. Its action was later explained in terms of a
simple dephasing process[50] and it was shown to also be
effective in selecting for crystalline domains in aVDF/TrFE
copolymer.[50] Figure 15 illustrates its use for selecting both
amorphous and crystalline domains merely by varying the
minipulse nutation angle. Domain selectivity for PVDF by
the direct DIVAM pulse sequence has been successfully
simulated and appears to be largely dependent on the differ-
ence in shielding anisotropy between the fluorine nuclei
in the crystalline and amorphous regions[51] (Figure 16). On
the other hand, the signal for the reverse units seems to be
governed primarily by the offset term. The effect of relaxa-
tion could not be simulated togetherwith the spin dynamics,
so one cannot rule out a role for transverse relaxation in the
selection process.
Neither the T1r(H) filter nor the dipolar dephasing me-
thod appeared to give[43] any significant selection for
p(TrFE). A combination of a T1r(F) filter with a short spin-
diffusion time[60] gave a DP spectrum of PVDF at 100 8Cwhich revealed the existence of a signal in the ‘‘reverse
unit’’ region that arose from a highly mobile group. This
signal was dramatically selected[60] in a delayed-acquisi-
tion spectrum obtained at 60 8C. It has been attributed to
–CF2H end groups occurring in only approximately 0.013%
concentration, as attested by a solution-state spectra of a
telomer.[63] The delayed-acquisition experiment has simi-
larly revealed the existence of verymobile groups in aVDF/
TrFE copolymer (Figure 17).[64]
Conclusion
It is shown herein that there are many ways to discriminate
effectively between 19F spectra of crystalline and amor-
phous domains for semicrystalline fluoropolymers. The
pulse sequences involved all rely on differences in magne-
tization relaxation between the domains. Since various
relaxation properties (linewidth, T1, T1r) may be involved,
Figure 16. Direct DIVAM measurements on PVDF. Left: normalized experimental results. Right:simulations. The value of the inter-pulse spacing was 6 ms.
Figure 17. Rotor-synchronized delayed-acquisition spectra ofP(VDF75/TrFE25).
Selective NMR Pulse Sequences for the Study of Solid Hydrogen-Containing Fluoropolymers 355
Macromol. Rapid Commun. 2005, 26, 345–356 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
WILEY-VCH
Author ProofAuthor Proofand either 1H or 19F relaxation can be used, the optimum
experiment must be carefully chosen in each case.
Acknowledgements: We thank Dr. Paolo Avalle for Figure 16,Dr. David Apperley for much assistance with obtaining spectra,Dr. Keitaro Aimi for work on g-PVDF and the copolymer, andN. Andres and T. Montina for their assistance in simulating thedirect DIVAM results.
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6. When you have completed the corrections to your article, go to File/Export/Annotations (inAcrobat 4.0) or File/Export/Notes (in Acrobat 3.0). Save your NOTES file to a place on yourharddrive where you can easily locate it. Name your NOTES file with the article numberassigned to your article in the original softproofing e-mail message.
7. When closing your article PDF be sure NOT to save changes to original file.
8. To make changes to a NOTES file you have exported, simply re-open the original PDFproof file, go to File/Import/Notes and import the NOTES file you saved. Make changes and re-export NOTES file keeping the same file name.
9. When complete, attach your NOTES file to a reply e-mail message. Be sure to include yourname, the date, and the title of the journal your article will be printed in.
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