1

Feature article for Macromol. Rapid. Comm.

Selective NMR Pulse Sequences for the Study of

Solid Hydrogen-containing Fluoropolymers†

Shinji ANDO1, Robin K. HARRIS2*, Paul HAZENDONK3 & Philip WORMARD4

1Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo

152-8552, Japan.

2Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE.

3Department of Chemistry & Biochemistry, University of Lethbridge, 4401 University Drive W., Lethbridge,

Alberta, Canada, T1K 3M4.

4School of Chemistry, University of St. Andrews, Purdie Building, St. Andrews, Fife KY16 9ST, U.K.

*To whom correspondence should be addressed:

FAX +44-(0)191-384-4737; email r.k.harris@durham.ac.uk

Key Words: Fluoropolymers; domain structure; magic-angle spinning NMR; relaxation; pulse sequences

Abstract

Fluorine-19 NMR spectra of solids have some special features, which are discussed in this article.

In particular, they generally contain two abundant spin baths (protons and fluorine nuclei). This

situation throws up some special operational requirements, as does the study of heterogeneous

samples. The relaxation characteristics of heterogeneous systems, which are briefly described herein,

frequently permit the use of specific pulse sequences to obtain subspectra for individual components.

Various possible selective sequences for use in fluorinated heterogeneous organic solids are listed

and their actions rationalised on the basis of molecular mobility. Semi-crystalline hydrogen-

containing fluoropolymers form especially suitable systems for such operations, and in order to

understand their domain structures it is essential to obtain subspectra of the amorphous and crystalline

domains. Examples are given of the use of selective pulse sequences for studying fluoropolymers,

especially for PVDF and the copolymer P(VDF75/TrFE25).

Introduction

Synthetic polymers form an extremely important

class of materials which have been extensively studied

by NMR methods, applied to both solutions and solids.

The first experiments combining cross-polarisation

(CP), magic-angle spinning (MAS) and high-powered

proton decoupling (HPPD) were applied to obtain

high-resolution 13C spectra of three solid polymers.1

Indeed, in the first decade of the use of the CP/MAS/

HPPD suite of techniques,2 solid polymers formed a

high proportion of the samples studied, 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

chemical information from amorphous as well as

crystalline materials, and (ii) the remarkable versatility

of NMR exemplified by the wide range of pulse

sequences which can be chosen to produce specific

results.2

Most MAS work on polymers has, naturally,

concentrated 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 and 31P. 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 MAS6 or else multiple-pulse

operation combined with MAS (“CRAMPS”)7-9 in

2

order to overcome the strong homonuclear (1H, 1H)

dipolar interactions.The 19F nuclide is also a special

case. In several ways it is very suitable for high-

resolution NMR. Thus, because it exists in 100%

natural abundance and has a high magnetic moment

(in fact the third highest of spin-1/2 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 eith 1H or 13C NMR for suitable

cases, though clearly the three nuclides can be studied

together. One would therefore expect high-resolution

19F NMR of solids to be very popular. However, the

same properties that give advantages also confer

problems. 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 which also contain protons (to which the rest

of this article is dedicated), 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

of 19F-{19H} double resonance, involving high powers

in the proton channel but with efficient filtering,

became available, 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 appearance of the Bloch-Siegert

effect20,21, which causes an apparent chemical shift on

19F resonances13,18, as shown in figure 1 and expressed

in equation 1:

δ(BS) = (γF B1H)2 / (ωF 2 – ωH2) [1]

where γF is the magnetogyric ratio of 19F, B1H is

the proton radiofrequency magnetic field strength, ωFis the 19F resonance frequency and ωH is the 1H

“decoupling” frequency. However, once this is

recognised, it poses no difficulties for chemical shift

measurement. All that is required is the use of the

same 1H RF power 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 synchronised

π pulses on the proton channel providing 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 minimise the occurrence of

spinning sidebands.

with decoupling

without decoupling

Figure 1. 188 MHz 19F CPMAS spectra withoutand with high-power proton decoupling, showingthe Bloch-Siegert shift (ca 2.8 ppm) example.

Figure 2. Fluorine-19 MAS spectra16 of a physicalmixture of PTFE (95%) and PVDF (5%). (a) Directpolarisation. (b) 1H→19F cross polarisation. ThePVDF is severely discriminated against in (b).

SPE

CP

PTFE

PVDF

PTFE

3

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, e.g. in P(VDF/TrFE)23 and in principle cause

splittings in spectra. Moreover, partly for this reason

and partly not fully understood, linewidths in 19F MAS

spectra of solids remain relatively high (e.g. hundreds

of Hz) 15even for well-crystallised samples. In

addition, the large 19F shielding anisotropies can make

some specialised 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 spectra24-26

of some fluorinated solids which 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 polarisation is rather small (γH/γF = 1.062)10,

CP discr iminates agains t compounds in a

heterogeneous sample which are perfluorinated. This

applies also to probe components, which frequently

involve 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

specialised pulse sequences, as described below,

including some two-dimensional experiments.

All these considerations clearly apply to NMR

studies of hydrogen-containing fluoropolymers12,13.

Such synthetic macromolecules are important

industrially because of their excellent stability against

chemical degradation under a variety of conditions and

because o f the i r spec ia l p roper t i e s (e .g .

piezoelectricity, ferroelectricity, pyroelectricity etc.).27

Moreover they are frequently complex in physical

structure and are thus of intrinsic interest. They are

generally semi-crystalline 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

behaviour becomes important in practical usage. Such

polymorphism is frequently of the conformational

type, and variable conformation also characterises

amorphous domains. As with many polymers,

especially as produced commercially, the nature of the

end-groups is relevant, as are any chemical defects in

the polymer chains. Such defects, arising from

occasional reversal in the ordering of a monomer unit

in a chain, are common28-31 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 analysed by

measurement of NMR relaxation times.

Relaxation in Homogeneous and Heterogeneous

Samples

One of the most powerful attributes of solid-state

NMR is its ability to address heterogeneous systems

and, in particular, to obtain, separately, subspectra

relating to different components via use of specialised

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 which are

chemically uniform but physically diverse, i.e. 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 which 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 which are under investigation.

The property which readily distinguishes solid

samples even of the same material but in different

physical form (or of domains of samples of chemically

homogeneous materials) is molecular-level mobility.

4

This is, of course, a complex property. Its extent shows

a strong dependence on the motional frequency

considered and it varies greatly with 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 mobile. Their mobility normally increases

significantly when 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

of which is often substantial for 19F. A number of

different relaxation times may be measured by NMR

and influence the operation 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 (B0) to its equilibrium value following a

perturbation and responds to motions in the region of

the NMR frequency, i.e. 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 T1ρ and usually lies

in the tens of ms range for solids. It governs return of

magnetization perpendicular to B0 but under the

influence of a radiofrequency field B1, to equilibrium

following establishment of a significant magnitude.

Its value is influenced by motions at the nutation

frequency related to B1 (i.e. to γB1/2π), 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 T2 relates

to the return of magnetization perpendicular to B0 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 relatively rigid solids

(static samples) it is generally a few tens of µs. 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, τ,

is a variable. The magnetization in the observed

nucleus first rises as a function of τ at a rate governed

by THF then decays as it leaks to the lattice by T1ρ

processes. The value of THF is generally in the region

of tens or hundreds of µs.

Whereas linewidths tend to decrease and transverse

relaxation rates tend to increase monotonically as

increasing temperature promotes more molecular

mobility, T1 and T1ρ pass through one or more minima.

Thus the effect of temperature change on T1 and T1ρ is

sometimes difficult to predict and measure,ments at a

single temperature can be misleading. Of course, fo

amorphous polymer domains, passing through the

Spin Lock(SL)

Dipolar Decoupling

Contact(CP)

(DD)

F

H

π/2

tCP

SL

F

H

π/2

tCP

DD

b)

a)

CP

τDelay

SL

F

H

π/2

tCP

DD

c)

CP

τ

Delay

Figure 3. (a) Standard CP sequence, (b) delayed-acquisition CP (dipolar dephasing/non-quaternarysuppression) sequence, (c) delayed CP sequence.

5

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

behaviour 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 phenomenon which may

be difficult to fit mathematically.34,35 The time T2

may therefore have a meaning which depends on the

circumstances. For a heterogeneous sample with large

domain sizes, spin-lattice relaxation may be treated

independently for the various domains (i.e. the total

magnetization 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 rotating frame and

transverse relaxation. However, the phenomenon of

spin diffusion spreads magnetization in a random-walk

fashion such that the degree of spread is governed by

time. Thus, in typical times, the magnetization of

different domains can be averaged if the domains are

small enough. In such cases single values of T1 (and

of T1ρ) will be observed even for heterogeneous

samples. The critical domain size for these events is

of the order of nm. 36 Moreover, T1 is more readily

averaged than T1ρ. Relaxation behaviour in the critical

region is complex34,35 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 T1ρ in heterogeneous systems can be

minimised37,38 by spin-locking at the magic angle39,40

Discriminating Experiments in 19F Solid-state NMR

As stated above, pulse sequences can be devised

to discriminate between various domains in a

heterogeneous polymer. These are primarily based

on differences in mobility which cause changes in the

various relaxation times via both the dipolar and the

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

via T1ρ 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, via CP to 19F. Thus there are many

possibilities, and therefore a range of appropriate pulse

sequences is described briefly here.

Firstly, the cross-polarisation pulse sequence

(Figure 3) is itself selective. This is because the 19F

magnetization variation with contact time (Figure 4)

has the functional form41 of equation 2:

Contact time, tCP

B undetectable

detectability level

SBmax

SAmax

log

S

slope T1111ρρρρ(A)

slope T1111ρρρρ(B)

Figure 4. Schematic plot of variable-contact-timeCP intensity, S, for two domains with verydifferent CP rates and effective T1ρ. The

DD

CP

SL DD

F

H

CP

SL DD

F

H

τ

π/2

π

π/2π/2

π/2 τ

a)

b)

DDDD

Figure 5. (a) Pre-CP inversion-recovery sequence,(b) post-CP inversion-recovery sequence.

6

MF(t) ∝ exp(−τ/T*HF) + exp(−τ/T*1ρ) [2]

where T*HF and T*

1ρ are themselves functions of THF

(the cross-polarization time), T1ρ(H) and T1ρ(F). [The

cross-polarisation dynamics will be more complex

when there are resolved 19F resonances for a given

domain such that there are several distinct fluorine spin

baths.42 Also, dipolar oscillations are often seen at short

contact times.43,44] The values of both and will depend

on local molecular 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 favours crystalline

domains. However, the same molecular mobility also

frequently causes T1ρ(H) for amorphous domains to

be significantly shorter, which means that crystalline

domains may be selected by long contact times also.

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) material. It can be

seen that, in general, CP favours crystalline domains,

especially at short contact times (when CP to A is

efficient and the signal is large) and at very long times

(when domain B may 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 magnetisation pre-contact or of 19F

magnetisation post-contact, respectively (Figure 5). In

each case, the recovery time can be set so as to null

the magnetisation of one domain, resulting in only the

signal of the other domain being observed. The null

condition for a simple system with relaxation time T1

requires the recovery time, τ, to be T1ln2 = 0.693 T1.

For the inversion-recovery experiment on 19F, the

magnetisation after the CP must first be placed in the

–z direction by a 90x° 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. Decoupling

would therefore be desirable for significant periods

of time, which could cause probe damage. The method

will often work without decoupling but the appropriate

conditions are not readily determined. However, spin

diffusion is relatively efficient at averaging

longitudinal relaxation rates32, 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 T1ρ(H) or

T1ρ(F). In the former case a 90°(H) pulse is followed

by a spin-lock period which lies between the values

of T1ρ(H) for the two domains. A CP contact time

Spin Lock

CP

SL DD

F

H

tSL

Spin LockCP

SL DD

F

H

tSL

π/2

π/2

a)

b)

Figure 6. (a) Pre-CP spin-lock sequence, (b) post-CP spin-lock sequence.

CP

SL DD

F

H

π/2

DD

F

H

π/2

CP

SL DD

F

H

π/2

x x x x x x x x x x x x

x x x x x x x x x x x x

x -y x x -y x -x y -x -x y -x

a)

b)

c)

Figure 7. (a) Dipolar filter sequence, (b) DIVAM/CP sequence (c) direct DIVAM sequence.

7

follows immediately the spin-lock period ends, no

additional 90° pulse being required. For the sequence

based on T1ρ(F) the spin-lock period (which must be

between the values of T1ρ(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

discrimination via linewidths (i.e. transverse

relaxation). The simplest is the dipolar dephasing

sequence (Figure 3b),45 also known as non-quaternary

suppression46,47 because of its use for selecting 13C

signals for quaternary carbons while eliminating those

from CH and CH2 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 signals from amorphous domains

(corresponding to relatively sharp lines) to be detected

following the end of the decoupling 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 alternative,

which often gives similar results for semi-crystalline

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 π pulse into the middle of the delay period,

with rotor synchronisation, to refocus chemical shifts.

A further selective method based on differentiation

between strong and weak (i.e. partially averaged)

homonuclear dipolar interactions is the so-called

dipolar filter (DF) pulse sequence (Figure 7a). 48 This

consists of a series of 90° 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

reported. This pulse sequence is called Discrimination

Induced by Variable Amplitude Minipulses (DIVAM)

(Figure 7b) 44,49,50 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

Figure 8. Explanation of DIVAM, showing howmagnetisations with short and long transverse re-laxation times behave during the operation of asuccession of minipulses of common phase. Toget the pure subspectra. CP is used following thesituations shown at the bottom. Under the condi-tions illustrated, the “pure mobile spectrum” (seebottom left) will usually contain a small contribu-tion from the rigid domains unless a small oppo-sitely-phased pulse is applied before the CP.

–60 –80 –100 –120 δδδδF/ppm

melt-crystallisedfrom the film

(αααα−−−−form)

biaxially-drawn9 µµµµm film

(mostly ββββ−−−−form)residualαααα−−−−form

Figure 9. Fluorine-19 spectra (centreband only)of two samples of PVDF, showing discriminationusing a T1ρ(H) filter. The pre-CP proton spin-lockduration was 40 ms and a short contact time (50ms) was used. The different polymorphic contentof the two samples is clearly shown.13

8

increases over the 12 pulses whereas My for 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 90° for the mobile

domain magnetization, a final 90° pulse followed 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 90° pulse) will give the 19F spectrum mainly

for the mobile domain. This may also be obtained by

use of larger minipulse angles leading to, say, a net

nutation angle of 360° for M (mobile), by which time

M (rigid) may have been lost (see Figure 8), so that

CP (90° 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 T1ρ

filter methods only yield spectra of the region with

the longer T1ρ. 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 discrimination methods can,

alternatively, proceed using 19F direct polarisation (DP)

instead of 1H→19F cross polarisation. Thus DP can

involve inversion-recovery (use of T1(F)), spin-locking

(use of T1ρ(F)), dipolar dephasing (which simply

becomes delayed acquisition) and dipolar filter

components in the pulse sequence. Recently, direct

DIVAM (Figure 7c) has also been utilised.51 However,

the direct DIVAM pulse sequence appears to be more

complicated in operation.than CP/DIVAM. The effects

depend on offset, and shielding anisotropy (both of

which are much larger than the corresponding

parameters for proton NMR) but not significantly on

dipolar interactions. Gerstein et al. have shown52 that

significantly-delayed acquisition proton spectra can

reveal the existence of mobile moieties in a solid

system even in 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 1H spectra is also possible (as well

as 19F→1H CP) 19F spectra contain the big advantage

of better resolution. Methods involving 1H→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

50 0 –50 –100 –150 –200 –250 δδδδ////ppm

STANDARD

SPECTRUM

SPECTRUM

from POST-CP

T1_(F) FILTER

contact time 3 ms

reverse units

in crystalline ( _ rigid)

domains?

spinning sidebands spinning sidebands

-130-120-110-100-90-80-70-60

δF / ppm

G

F

E

D

C AB

-130-120-110-100-90-80-70-60

δF / ppm

G

F

E

D

C

AB

powder(αααα-form)

annealed film

(γγγγ-form)

a) Direct Polarisation b) After spin-lock for 20 ms

Figure 10. Fluorine-19 CPMAS spectra of asample of PVDF using the standard sequence (top)and a T1ρ(F) filter (bottom). In this case the fullspectrum, including the spinning sidebands, isshown.

normal spectrum

dephased spectrum

_-phase

amorphous

_-phase

reverse units

– 60 –120 _F / ppm

Figure 11. Fluorine-19 CPMAS spectra of PVDFshowing selection of the amorphous subspectrumby dipolar dephasing.

9

-250-200-150-100-500

δF / ppm

1

3

4

56

78

a) Direct polarisation

b) Delayed CP

(delay time = 0.5 ms

scans = 128)

c) Short CP (0.2 ms)

further manipulation of the magnetization of the

selected domain. For instance, they can be used in a

Goldman-Shen55 experiment to utilise spin diffusion

in order to obtain information on the size of domains.56

Combination with a wideline separation (WISE) pulse

sequence57 enables heteronuclear correlation (two-

dimensional) experiments to be carried out selectively

for crystalline and amorphous domains17.

All the pulse sequences described above involve

observation of 19F subspectra. It ispossible to use

analogous methods to obtain proton subspectra for

amorphous and crystalline domains44 (assuming the

existence of two abundant spin baths, 19F and 1H), but

such procedures are less attractive because fluorine

spectra have 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}

experiments on a fluoropolymer (PVDF) involved

using a T1ρ(H) filter (i.e. a spin-locked delayed-contact

CPMAS pulse sequence) as an introduction to a WISE

sequence. 17 Such a filter is effective (Figure 9) at

ambient probe temperature because T1ρ(H) for the

crystalline domains is significantly longer than for the

amorphous domains. Figure 9 shows how this method

can be used12 to study the polymorphic form of the

crystalline domains in PVDF samples produced by

– 75 – 100 – 125 δδδδF/ ppm

POWDER(αααα−−−−PHASE)

40

10

0.01

ττττ /ms

Figure 12. Domain selec-tion of PVDF in 19F direct-polarisation spectra60. Top:full spectrum. Middle: dipo-lar-filtered spectrum. Bot-tom: T1ρ(F)-filtered spec-trum. In this case, T1ρ(F) forthe principal amorphouspeak is 3.5 ms, whereas forthe high-frequency crystal-line peak it is measured tobe 9.5 ms – a sufficient dif-ference to give good selec-tivity, as shown in the bot-tom spectrum.

Figure 13. CPMAS spectra of PVDF, obtained us-ing a Goldman-Shen pulse sequence following se-lection of the crystalline subspectrum56. The de-lay time allowed for spin diffusion is given at theright-hand side. The signal for the amorphousphase can be seen to grow in as the delay timeincreases, analysis of which allows domain sizesto be determined.

Figure 14. 19F-{1H}M A S s p e c t r a o fP(VDF75/TrFE25) at68ºC, obtained by (a)direct polarisation, (b)delayed CP (delay time0.5 ms), and (c) short-contact CP (contacttime 0.1 ms).

Figure 15. DIVAM/CP spectra of the VDF/TrFEcopolymer as a function of the minipulse angleused. Top: Unfiltered spectrum. Middle: Selectionof the amorphous domain. Bottom: Selection ofthe crystalline domain. The inter-pulse spacing was6 ms, and the minipulse nutations angles used areindicated.

θθθθ= 0º

θθθθ= 30º

θθθθ= 7.3º

10

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 recognised that CP dynamics are

complicated when two abundant spin nuclides are

involved,41,58 rendering the variable contact time

method for obtaining T1ρ difficult to apply. In any

case, all relaxation parameters will depend on the

nature of the sample (average molecular mass,

dispersion, regio-irregularity etc.) and on temperature.

However, the ratio of T1ρ(H) values for the crystalline

and amorphous domains for PVDF is found to be

generally in the range 2-3,44,59,60 which suffices to

make the pre-CP proton spin-lock an efficient tool to

select the spectrum of the crystalline domains. A

similar situation applies to T1ρ(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 ca. 100°C showed that

T1ρ(F) values for both the crystalline and amorphous

domains of PVDF were significantly lower than those

at ca. 60°C but that the ratio remained approximately

the same, so that the T1ρ(F) filter selection method was

still viable at the higher temperature.60 The 19F DP/

Figure 16. Direct DIVAM measurements onPVDF. Left: normalised experimental results.Right: simulations. The value of the inter-pulsespacing was 6 ms.

spin-lock method has been shown61,62 to be effective

for selecting the subspectrum of the γ crystalline form

of PVDF.

However, T1ρ filters only select for crystalline

domains, so they must be matched with other methods.

The dipolar dephasing pulse sequence was 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 compared

with the upper one of Figure 9 for the crystalline

domains. Later,56 the dipolar filter (DF) sequence was

also used for this purpose (Figure 12, middle spectrum)

and proved to be very effective. The T1ρ(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-contact CP experiment, whereas it is largely

obscured in the DP expeiment because the resonances

are broad and the crystallinity of the sample is

relatively low.

The DIVAM/CP method was first used49 to select

for amorphous domains in the case of PVF and was

DELAY

ssb

CF2CHF

Figurs 17. Rotor-synchronised delayed-acquisitionspectra of P(VDF75/TrFE25)

Direct DIVAM for PVDF: Simulated data τ=6µs

Pulse Angle

0 20 40 60 80

No

rma

lize

d In

ten

sity

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Pulse Angle vs Amorpho

Pulse Angle vs Crystaline

Pulse Angle vs Defect

Direct DIVAM for PVDF: Experimental data τ=6µs

Pulse Angle

0 20 40 60 80

Nli

dI

tit

-1.0

-0.5

0.0

0.5

1.0Pulse Angle vs Normalized Amorphous

Pulse Angle vs Normalized Crystaline2

Pulse Angle vs Normalized defect2

(a)(b)

11

later applied44 to PVDF. Its action was explained in

terms of a simple dephasing process later50 and it was

shown to be also effective in selecting for crystalline

domains in a VDF/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

difference in shielding anisotropy between the fluorine

nuclei in the crystalline and amorphous regions51

(Figure 16) On the other hand, the signal for the

reverse units seems to be governed primarily by the

offset term. The effect of relaxation could not be

simulated together with the spin dynamics, so one

cannot rule out a role for transverse relaxation in the

selection process. Neither the T1ρ(H) filter nor the

dipolar dephasing method appeared to give43 any

significant selection for p(TrFE).

A combination of a T1ρ(F) filter with a short spin-

diffusion time gave60 a DP spectrum of PVDF at 100°C

which revealed the existence of a signal in the “reverse

unit” region which arose from a highly mobile group.

This signal was dramatically selected60 in a delayed-

acquisition spectrum obtained at 60°C. It has been

attributed to –CF2H end groups occurring in only ca.

0.013% concentration, as attested by solution-state

spectra of a telomer.63 The delayed acquisition

experiment has similarly revealed the existence of very

mobile groups in a VDF/TrFE copolymer (Figure

17).64

Conclusion

It is shown herein that there are many ways to

discriminate effectively between 19F spectra of

crystalline and amorphous domains for semi-

crystalline fluoropolymers. The pulse sequences

involved all rely on differences in magnetisation

relaxation between the domains. Since various

relaxation properties (linewidth, T1, T1ρ) may be

involved, and either 1H or 19F relaxation can be used,

the optimum experiment must be carefully chosen in

each case.

AcknowledgementsWe 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 copoly-mer, and N. Andres and T. Montina for their assistancein simulating the direct DIVAM results.

References1. Schaefer J, Stejskal EO 1976. J Am Chem Soc

98:1031.

2. Duer MJ. 2004. Introduction to Solid-State NMR

Spec t ro scopy. ed . , Oxfo rd : B lackwe l l

Publishing.1998

3. In Ando I, Asakura T, editors. Solid State NMR of

polymers, ed., New York: Elsevier.

4. McBrierty VJ, Packer KJ. 1993. Nuclear Magnetic

Resonance in Solid Polymers. ed.: Cambridge

University Press. (1991)

5. In Mathias LJ, editor Solid State NMR of Polymers,

ed., New York: Plenum Press.

6. Hafner S, Spiess HW 1998. Concepts Magn Reson

10:99.

7. Ryan LM, Taylor RE, Paff AJ, Gerstein BC 1980.

J Chem Phys 72:508.

8. Gerstein BC, Pembleton RG, Wilson RC, Ryan LM

1977. J Chem Phys 66:361.

9. Harris RK, Jackson P 1990. NATO ASI C 322:355.

10.Harris RK, Becker ED, Cabral de Menezes SM,

Goodfellow R, Granger P 2001. Pure Appl Chem

73:1795.

11. Harris RK, Jackson P 1991. Chem Rev 91:1427.

12.Har r i s RKM, G.A. ; Hols te in , P. 1998 .

Fluoropolymers. In Ando I, ; Asakura, T.;, editor

Solid state NMR of polymers, ed., New York:

Elsevier. p 667-712.

13.Harris RK, Monti GA, Holstein P. 1998. Fluorine-

19 NMR. In Ando I, ; Asakura, T.;, editor Solid

state NMR of polymers, ed., New York: Elsevier.

p 253-266.

14.Ando S, Harris RK, Scheler U. 2002. In Grant DM,

Harris RK, editors. Encyclopedia of NMR, ed.:

Wiley. p 531-550.

15.Carss SA, Harris RK, Holstein P, Say BJ, Fletton

RA 1994. J Chem Soc, Chem Comm:2407.

16.Harris RK, Carss SA, Chambers RD, Holstein P,

Minoja AP, Scheler U 1995. Bull Magn Reson

17:37.

17.Scheler U, Harris RK 1996. Solid State NMR 7:11.

18.Vierkotter S 1996. J Magn Reson A 118:84.

19.Scheler U, Harris RK 1996. Chem Phys Lett

262:137.

20.Bloch F, Siegert A 1940. Phys Rev 57:552.

12

21.Freeman R. 1988. A Handbook of Nuclear

Magnetic Resonance. ed., Harlow: Longman

Group.

22.Holstein P, Scheler U, Harris RK 1998. Polymer

20:4937.

23.Mabboux PY, Gleason KK 2002. J Fluorine Chem

113:27.

24.Scheler U 1999. Bull Magn Reson 19:52.

25.Scheler U 1998. Solid State NMR 12:9.

26.Sue T-W, Tzou D-LM 2000. Polymer 41:7289.

27.1997. In Scheirs J, editor Modern Fluoropolymers,

ed., Chichester: John Wiley and Sons.

28.Ferguson RC, Brame EG 1979. J Phys Chem

83:1397.

29.Tonelli AE, Schilling FC, Cais RE 1982. Macromol

15:849.

30.Cais RE, Kometani JM 1985. Macromol 18:1357.

31.Lovinger AJ, Davis DD, Cais RR, Kometani JM

1987. Polymer 28:617.

32.Kenwright AM, Say, B.J. 1993. Solid-state Proton

NMR of Polymers. In Ibbett RN, editor NMR

Spectroscopy of Polymers, ed., Glasgow: Blackie

Academic & Professional. p 231-274.

33.Harris RK. 1996. NMR Studies of Solid Polymers.

In Fawcett AH, editor Polymer Spectroscopy, ed.,

Chichester: John Wiley & Sons. p 117-133.

34.Kenwright AM, Packer KJ, Say BJ 1986. J Magn

Reson 69:426.

35.Geppi M, Harris RK, Kenwright AM, Say BJ 1998.

Solid State NMR 12:15.

36.S c h m i d t - R o h r K , S p i e s s H W. 1 9 9 4 .

Multidimensional Solid State NMR and Polymers.

ed., London: Academic Press.

37.Jones GP 1966. Phys Rev 148:332-335.

38.Tse D, Hartmann SR 1968. Phys Rev Lett 21:511-

514.

39.Goldburg WI, Lee M 1963. Phys Rev Lett 11:255-

258.

40.Lee M, Goldburg WI 1965. Phys Rev A 140:1261-

1271.

41.Ando S, Harris RK, Reinsberg SA 1999. J Magn

Reson 141:91.

42.Hazendonk P, Harris RK, Galli G, Pizzanelli S

2002. Phys Chem Chem Phys 4:507-513.

43.Reinsberg SA, Ando S, Harris RK 2000. Polymer

41:3729.

44.Ando S, Harris RK, Reinsberg SA 2002. Magn

Reson Chem 40:97.

45.Opella SJ, Frey MH 1979. J Am Chem Soc

101:5854.

46.Fyfe CA, Rudin A, Tchir WJ 1980. Macromol

12:1322.

47.Balimann GE, Groombridge CJ, Harris RK, Packer

KJ, Say BJ, Tanner SF 1981. Phil Trans R Soc Lond

A299:643.

48.Egger N, Schmidt-Rohr K, Blumich B, Domke

WD, Stapp B 1992. J Appl Polym Sci 44:289.

49.Ando S, Harris RK, Holstein P, Reinsberg SA,

Yamauchi K 2001. Polymer 42:8137.

50.Hazendonk P, Harris RK, Ando S, Avalle P 2003. J

Magn Reson 162:206.

51.Hazendonk P, Andres N, Nieboer J, Montina T in

preparation.

52.Gerstein BC, Hu JZ, Zhou J, Ye C, Solum M,

Pugmire R, Grant DM 1996. Solid State NMR 6:63.

53.Fung BM, Tong T-H, Dollase T, Magnuson ML

1996. J Magn Reson A 123:56-63.

54.Ding S, McDowell CA 1995. J Magn Reson A

117:171-178.

55.Goldman M, Shen L 1966. Phys Rev 144:322.

56.Holstein P, Monti GA, Harris RK 1999. Phys Chem

Chem Phys 1:3549.

57.Schmidt-Rohr K, Clauss J, Spiess HW 1992.

Macromol 25:3273.

58.Ando S, Harris RK, Monti GA, Reinsberg SA 1999.

Magn Reson Chem 37:709.

59.Holstein P, Harris RK, Say BJ 1997. Solid State

NMR 8:201.

60.Wormald P, Apperley DC, Beaume F, Harris RK

2003. Polymer 44:643.

61.Park J-W, Seo Y-A, Kim I, Aimi K, Ando S, Ha C-

S 2004. Macromol 37:429.

62. Koseki Y, Aimi K, Ando S unpublished work.

63. Wormald P. 2004. ed.: University of Durham.

64. Ando S, Avalle P, Harris RK unpublished work.