1H/29Si cross-polarization NMR experiments of silica-reinforced polydimethylsiloxane elastomers:...

MAGNETIC RESONANCE IN CHEMISTRYMagn. Reson. Chem. 2006; 44: 375–384Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1763

1H/29Si cross-polarization NMR experiments ofsilica-reinforced polydimethylsiloxane elastomers:probing the polymer–filler interface†

Robbie Iuliucci,1∗ Craig Taylor2 and W. Kirk Hollis2

1 Department of Chemistry, Washington and Jefferson College, Washington, PA 15301, USA2 Los Alamos National Laboratory, Los Alamos, NM, 87545, USA

Received 2 September 2005; Revised 27 October 2005; Accepted 28 October 2005

Polydimethylsiloxane (PDMS) elastomers reinforced with fumed silica exhibit unusual strengthcharacteristics that are necessary for their designed applications. The microscopic details of the surfaceinteraction between the polymer and silica are not well characterized. 1H/29Si cross-polarization (cp)experiments are used to characterize cured and uncured samples of Dow Corning silastic 745. Changes tothe cp dynamics upon curing are evident by the variation in peak intensities in the variable contact-timespectra of the two samples. Estimates of the cp relaxation parameters are reported for the cured sample.Additional information can be obtained by expanding the 1H/29Si cp to a two-dimensional heteronuclearcorrelation experiment. Dramatic differences between the cured and uncured 1H/29Si HetCor spectra areobserved that are not visible in the 1D spectra. These changes can be rationalized as a dehydration of thesilica surface and an increased hardening of the polymer after the curing process. Furthermore, isolationof the NMR signal corresponding to nuclei at or near the polymer–filler interface may be achieved in the2D 1H/29Si HetCor spectrum. Published in 2006 by John Wiley & Sons, Ltd.

KEYWORDS: solid-state NMR; 1H/29Si cross-polarization; variable contact time; 2D heteronuclear correlation spectroscopy;polydimethylsiloxane; fumed silica; elastomer curing

INTRODUCTION

Cross-polarization (cp) dynamics has been commonly usedto characterize solid-phase materials such as polymers,1 sil-icate surfaces,2 – 6 and polydimethylsiloxane (PDMS)/silicacomposite materials.7 – 10 Potentially, one may obtain quan-titative information as well as a measure of heteronucleardipolar couplings, which can be related to molecular motionand/or nuclear distances, from the transfer profile. Theapproach recently has been applied toward exploring theinterface of organic-silicate composite material.11 – 16 In thisstudy, we wish to demonstrate the utility of cp spectra toprobe the polymer–filler interface in Dow Corning silas-tic 745, a polysiloxane containing inorganic silica fillers. Ifthe technique proves valid, it would be of great utility forthe engineering of fortified soft materials. However, the

†Dedicated to Professor David M. Grant of the Universityof Utah on the occasion of his 75th birthday, inrecognition of the outstanding contributions he has madeto the methodology of nuclear magnetic resonance and itsapplication to a wide range of chemical topics over a longperiod of time.

ŁCorrespondence to: Robbie Iuliucci, Department of Chemistry,Washington and Jefferson College, Washington, PA 15301, USA.E-mail: [email protected]/grant sponsor: National Science Foundation;Contract/grant number: CHE-0216075.

approach is challenged by the fact that the NMR signal ofthe bulk material must be distinguished from the minorsurface signal of interest. Otherwise, multicomponent mod-els must be employed for the nonlinear data analysis. Suchmodels cannot always be applied confidently. This is furthercomplicated in systems with polymer surface interfaces thatsuffer from poor signal-to-noise. In PDMS–silica filler sys-tems, however, a variation in the chemical shift exists for thedifferent components of the material. This variation allowsisolation of the surface interface in a two-dimensional het-eronuclear correlation (2D HetCor) spectrum. Polarizationdynamics can be studied in 2D HetCor experiments wherethe exchange mechanism is driven by cp.

The Dow Corning polysiloxane silastic 745, DC745, ishighly valued for its current military applications. A bettercharacterization of DC745’s uncured elastomer resin aswell as knowledge of its aging profile is of great interest.A more detailed understanding of the changes that takeplace during the shelf life of uncured elastomers is also ofgeneral interest to the silicone industry.17 In this regard,DC745 serves as a model silicone elastomer. DC745 hasalready been studied extensively and a series of pristine andartificially aged samples have been prepared.18 A test groupfor future experiments is available. DC745 consists primarilyof PDMS, where approximately 1 in 20 of the methyl groupsis substituted for phenyl rings and, to a lesser extent, themethyl is replaced by a vinyl group. Phenyl rings are found

This article is a US Government work and is in the public domain in the USA.

376 R. Iuliucci, C. Taylor and W. K. Hollis

to help improve creep resistance by stabilizing the siloxanebond and decreasing the rate of chain scission reactions.The vinyl serves as the cross-linking group in the polymerwhen peroxide cross-linking agents are added. In DC745,PDMS is mixed with a high-surface-area fumed silicate thatcontains quartz. Upon curing, the material transforms froma puttylike consistency to a firm rubber.

It is believed that the material’s unique strength char-acteristic stems from the surface interaction between thepolymer and the fumed silica filler.19 – 22 A cartoon represen-tation of the surface interface is displayed in Fig. 1. Physicalforces, hydrogen bonding, and van der Waals forces dom-inate polymer–filler interactions for hydrophilic surfaces.Covalent bonding, however, may be involved with cur-ing. Understanding this polymer–filler interaction on themolecular level serves as the primary objective of this study.

Fillers can be a way to improve the physical andmechanical properties of polymers.23 For example, in plasticsexposed to mechanical loads and high-heat service, glassfibers or mineral fillers can add rigidity and creep resistance.In general, the addition of inorganic fillers to elastomersleads to the mechanical reinforcement of polymers and toreduced swelling, hence rendering them mechanically andchemically more stable. For silica and zeolites in PDMS,such reinforcement has been explained by adsorption of thepolymer chains on the silanol groups of the filler surface.When the filler interacts well with the polymer, the effectivecross-linking density increases and the swelling degreeof the polymer network decreases. Finely divided fillerparticles have been used for many decades as reinforcingagents in PDMS elastomers and their effect on the resultingmacroscopic elastic properties is well documented.24 – 26 Itis also well known that the mechanical properties of filledsystems (elastic modulus, tear strength, etc.) are sensitiveto the surface treatment applied to the filler. Less wellunderstood, however, is the effect of the polymer–fillerinteraction on the local distribution of polymer around thefiller on a submicroscopic length scale. It is also not known

Silica-filled polydimethylsiloxane elastomer

Free polymer

Bound polymer

silica

[-SiO]x[-SiO]y [-SiO]z

CH3

CH3

CH3

C6H5

CH3

C2H3

[-SiO]x[-SiO]y [-SiO]z

CH3

CH3

CH3

C6H5

CH3

C2H3

Dphenyl DvinylD

polymer

Si

O

OO

O

Q#Si

Q4 Q3 or Q 2

Figure 1. A cartoon depicting the siloxane polymer silica fillersystem. Hydrogen bonding between hydroxyl sites on thesurface and polymer are assumed as the primary mechanismfor the surface interaction.

how these interactions change during the lifetime of thematerial. During storage, Crepe-hardening can result foruncross-linked siloxane filled with fumed silica.17

Macroscopic measurements on filled gels are usefulindicators of the reinforcing effect, but are incapableof providing information on the strength of the localinteractions that govern the underlying mechanism. Clearly,adsorption forces must play a central role in the adhesionof the polymer to the filler and these ultimately control themacroscopic mechanical behavior. These phenomena occurin a thin layer around the silica particles on a distancescale of the order of the size of a polymer molecule, i.e.10–100 A. Such length scales are ideally suited for NMR cpinvestigations that depend upon short length scale dipolarinteractions.

The employment of heteronuclear dipolar coupling formagnetization transfer limits cp to motionally constrainedand proximate spin pairs. In composite materials thatcontain regions with varying degree of motion, cp canselect the confined spin pairs from those undergoing rapidmovement, which eliminates their heteronuclear couplings.In DC745, the overwhelming bulk signal of flexible PDMSthat dominates a 29Si single-pulse excitation spectrum isremoved from the 1H/29Si cp spectrum. Cross-polarization,in 1D NMR, cannot confidently probe the PDMS–fillerinterface. Physical entanglements and cross-links reducepolymer motion in PDMS, resulting in additional polarizablesilicone. The silica that is insulated from the polymer canalso be cross-polarized from surface hydroxyl groups andadsorbed water. Although the dipolar coupling environmentof these sites may be different from that of the PDMS–silicainterface, the chemical shifts will not vary significantly. Thiscomplicates the cp dynamics as the transferred 29Si signalbecomes a superposition of all possibilities. Biexponentialcurve fitting has been applied to study the cp dynamics offumed silica.6 Here, however, one would have to extend thenonlinear curve fitting beyond two components, where thesolutions are rather fortuitous.

In composite materials, where the chemical shifts of thecomponents are resolved, the dynamics can be determinedindependently in a two-dimension spectrum. This is the casein siloxane polymers reinforced with inorganic silica. The29Si chemical shift of silicate, �80 to �115 ppm, are clearlyresolved from siloxane polymer shifts, �10 to �40 ppm.Silanol proton shifts in silica, ¾5 ppm, are well separatedfrom the methyl proton shifts of siloxane polymers, ¾0 ppm.The combination of 1H and 29Si shifts confidently isolatesthe signal corresponding to the elastomers surface in a2D 1H/29Si HetCor spectrum. Potentially, the magnetizationtransfer dynamics of the surface interface can be studied in a2D 1H/29Si HetCor spectra when cp mediates the exchange.

EXPERIMENTAL

Samples of DC745, cured and uncured gum resin, wereprovided by LANL. DC745 is cured by mill rolling a 99.5%gum resin and 0.5% catalyst blend by weight. A 50% activeLuperco 101XL (2,5-dimethyl-2,5-di(t-butylperoxy)hexane)was used as the catalyst. The milled pads were cured at

Published in 2006 by John Wiley & Sons, Ltd. Magn. Reson. Chem. 2006; 44: 375–384

1H/29Si cross-polarization NMR experiments of silica-reinforced polydimethylsiloxane elastomers 377

170 °C for ¾35 min and postcured at 150 °C for 1 h followedby 8 h at 250 °C. The cured rubber was sliced into smallsquare pieces (¾1–2 mm in length) with a razor blade andpacked into the zirconia rotors. The puttylike uncured DC745was placed directly into the rotor.

All NMR spectra were acquired on a Varian Inovaspectrometer and a wide bore 9.4 Tesla Oxford magnet withoperational frequencies for 1H and 29Si of 399.80 MHz and79.426 MHz respectively. Varian Chemagnetics T3 doubleresonance probes, 4.0 mm and 7.5 mm, were used. Magic-angle spinning with an automated spin controller wasemployed at speeds between 2.0 and 5.0 kHz for 29Si spectra.The 1H spectrum was acquired at 15 kHz, spinning with thehigh-speed-spinning-assisted WaHuHa pulse sequence.27

A 90° pulse width of 4.0 and 5.5 µs for 1H and 29Siwas employed respectively. For the 29Si spectra, relaxationdelays of 2–120 s were used. The signal was detectedunder 27 kHz of continuous wave decoupling, 0.05 msacquisition time, and a dwell time that produced a 250-ppmspectral window. For cp experiments, the duration of theHartmann–Hahn match was arrayed from 0.1–20 ms forvariable 1H/29Si contact-time data. A spin-lock field of 47 kHzwas used during the Hartmann–Hahn match condition. The2D HetCor experiments28 incorporated frequency-switchedLee–Goldburg29 homonuclear decoupling during the f1

period. A mixing time of 10 ms was used during cp in the2D HetCor experiment. All spectra were acquired at roomtemperature. The spectra were referenced externally to liquidTMS. The spectra were processed with the Varian VNMRJsoftware. The peak intensities from variable contact-timedata were analyzed in a curve-fitting tool of Matlab.30

RESULTS AND DISCUSSION

1D spectra of bulk materialThe single-pulse excitation (SPE) 29Si and multiple-pulsehigh-speed spinning 1H spectra of DC745 (Fig. 2) revealthe mobile polymer as the dominant component in theelastomer. The overpowering polymer signals, 0 ppm and�20.5 ppm in the 1H and 29Si spectra respectively, render theminor, but informative, inorganic silicate and phenyl-/vinyl-substituted polymer signal almost useless. NMR peakscan be assigned to their chemical sites based on knownliterature values.5,6,15,31 The phenyl-substituted siloxanepolymer (Dphenyl) appears at �32 ppm, while the inorganicfumed silicate shows up as broad peaks in the �80 to�115 ppm region of the 29Si spectrum. Geminal silanols,single silanol, and siloxane, designated by Q2, Q3, and Q4,have 29Si shifts of �89, �99, and �109 ppm respectively.The sharp peak at �110 arises from crystalline quartz. Thehydrogen-bonded silanol sites of fumed silicate and aromaticprotons of the phenyl rings account for the downfield1H peaks. A broad chemical shift range of 1–8 ppm hasbeen reported in silanol protons in fumed silica. This widechemical shift range is due to the varying degree of hydrogenbonding that the silanols can be involved with. An increasein hydrogen bonding results in a decreased shielding. Thecenter of the hydrogen-bonded silanol shift is around 5 ppmand isolated silanol protons have a chemical shift of 2.0 ppm.6

Water protons adsorbed to the silica surface appear in the3–5 ppm range. Water physisorbed to the silica has a 1Hchemical shift of 3.5 ppm. A liquidlike 1H chemical shift forwater of 4.9 ppm appears in humidified fumed silica. Waterexchange between the liquid and physisorbed environmentsis observed at an intermediate shift of 4.1 ppm.6

In theory, the proton-decoupled 29Si SPE spectral intensi-ties are quantifiable. However, 29Si quartz sites are noto-riously known for their long T1 values, and intensitiesare underestimated. Thus, one can only approximate theamounts of the DC745 components based on signal intensityand make relative qualitative comparisons between simi-lar materials. The 29Si SPE spectra of uncured and curedDC745 are shown in Fig. 3, where some interesting differ-ences are noted. The broadness of the �20-ppm D signal inthe uncured sample obscures the Dphenyl signal at �32 ppm.The different silicon Q sites are more pronounced in thecured spectrum. The spinning sidebands, a manifestation ofmagic-angle spinning, in the uncured sample do not appearabove the noise of the spectrum. However, sidebands havebeen observed in spectra with slower spinning speeds (notshown). The differences in the spinning sideband intensitymay indicate a change in the chemical shift anisotropy aftercuring.

Cross-polarizationTypically, the use of cp, the heteronuclear transfer ofmagnetization from an abundant to a dilute spin system, is toachieve an increase in signal-to-noise. Here, cp is employedas a means to edit or filter the 29Si spectrum by decreasingsignal that corresponds to the mobile polymer. The filteringprocess exploits the angular and distance dependence of thedipolar interaction, which serves as the transfer mechanismduring cp. This is evident in the spatial factors of theheteronuclear Hamiltonian,

ωijHSi D(

� 12

)�oh8�2

(�H�Si

r3ij

)�3 cos2 �ij � 1� �1�

where the usual definitions are assumed.32 The interaction isscaled by the inverse cubic distance between spins i and j, r3

ij,which confines the spatial transfer of magnetization to thenearest spin pair. Owing to the averaging of the angular term,molecular motion reduces or in the presence of fast isotropicmotion can even eliminate the dipolar coupling. Abovethe glass-transition temperature (�120 °C), PDMS exhibitsa relatively significant motion. However, the completeaveraging of the dipolar interactions is prevented by thepolymer chain entanglements, cross-links, and adhesion tothe silicate filler and quartz. Residual dipolar couplingsabove the glass-transition temperature have been studied inDC745.33

In the absence of homonuclear decoupling, spin diffusion,the homonuclear transfer of magnetization throughout theabundant spin network, can transpire during cp. Thisindirectly spreads the magnetization beyond the nearestspin pair. Spin diffusion can span distances of 20–200 nmin solids, depending on the proton spin density and thestrength of homonuclear dipolar coupling.32 Given the weak



(a)

(b)

Figure 2. Single-pulse excitation 1H (a) and 29Si (b) spectra of DC745. The polymer signal dominates both spectra. The blown-upspectrum reveals features in the baseline.

homonuclear dipolar couplings found in DC745, smallerlength scales are presumed here.

The cp spectra enhance the appearance of the subtlepeaks corresponding to the smaller amounts of inorganicand substituted polymer components (Fig. 4). Both thecured and uncured spectra are similar, with the primary

difference being the larger amount of rigid polymer signalin the cured spectrum. The broadness of the Dphenyl andthe inorganic silicate peaks become apparent. This chemicalshift dispersion can be attributed to a variation in localstructure. The PDMS signal remains relatively narrow,suggesting either a homogenous local structure and/or



(a) (b)

Figure 3. Single-pulse excitation 29Si spectra of uncured (a) and cured (b) DC745. Three silicon polymer signals are observed:PDMS (D), phenyl mono-substituted (Dphenyl), and vinyl mono-substituted (Dvinyl) polymer. There exist four types of inorganic 29Sinuclei: tetrahedral silica where all four peripheral oxygen are bonded to another silicate (Q4), or where one or two peripheral silicatesare replaced by a hydroxyl group (Q3 or Q2, respectively). The Q4 can either be crystalline (sharp resonance) or disordered (broadresonance).

(a)

(b)

Figure 4. The cp/mas 29Si spectra of uncured (a) and cured (b) DC745. Two inorganic silica sites Q3/Q4 are readily observed. Notethe increased intensity of the polymer signal in the cured spectrum.

residual motion. Two inorganic silicate sites (Q3 and Q4)are evident. Some indication of the presence of Q2 is seenwith a small shoulder at ¾85 ppm. The lack of chemicallybonded protons to the Q4 silicon would challenge cp to thissite. It is suspected that this observed cp signal corresponds

to sites that are at or near the surface of the fumedsilicate, where, through space, dipolar couplings wouldexist. Unlike the Q4 in fumed silicate, the narrow quartzpeak observed in the SPE spectra is not cross-polarizable anddisappears.



The chemical shifts of the cured and uncured DC745are nearly identical. Although the physical change withcuring is self-evident as the puttylike material transformsinto a rubbery material, there is no NMR evidence ofsignificant chemical changes upon curing. This suggeststhat the surface-binding mechanism is not chemical andconstitutes hydrogen bonding and van der Waal forces. It isbelieved that the siloxane polymer chemically bound to thesilicate surface would result in a 29Si chemical shift in the �50to �70 ppm region. Aminopropylalkoxysilanes deposited onsilicate surfaces resulted in shifts of �49 to �54 ppm formonodentate bonding and �57 to �60 ppm for bidentate.34

Tridentate bonding showed shifts of �66 to �68 ppm. DC745subjected to high temperatures for extensive periods of timeproduces a new resonance in the 29Si spectrum in the �65to �70 ppm region.35 Under those conditions, however, thechemical changes are obvious. The material undergoes acomplete degradation of the polymer where the elasticity islost and the rubber decomposes into a chalklike solid.

Variable contact-time cross-polarization spectraWhile the signal-enhancing feature of cp is advantageousin itself, here we wish to characterize the materials basedon their cp by observing the cp dynamics. Under cp, themagnetization (M) of the 29Si dilute spin builds up with timeaccording to the following expression:36

M�t� D M1(1 � THSi

T1�

)1 � e

�(

1� THSiT1�

)t

THSi

e

�tT1� �2�

where THSi and T1� are the proton–silicon cross-relaxationtime and the longitudinal relaxation of the protons in therotating frame respectively. This expression is valid only inthe fast cp regime where THSi << T1� and the decay of thesignal due to the longitudinal relaxation of the silicones in therotating frame is ignored. The three parameters of Eqn (2),M1, THSi, and T1�, can be determined by fitting the peakintensities in a series of spectra where the cp time is varied.The variable contact-time spectral array for the cured anduncured DC745 are shown in Fig. 5. The exponential fits forthe variable contact experiment are shown in Fig. 6 and thecorresponding cp parameters are reported in Table 1.

Changes to the relaxation dynamics in the two DC745samples are clearly evident from the spectral intensitydifferences. In the cured sample, the buildup of D is fasterand a clear maximum point is observed. Above 8 ms, all ofthe peaks including the polymer begin to decay. The buildupcurve of the uncured D takes place at a slower rate and amaximum transfer is never observed within the time frameprobed. The decay of signal in the uncured Q sites is lesspronounced.

The extraction of three parameters from nonlinear curvefitting can be daunting. The curve fitting becomes furtherchallenged with spectra lacking desirable signal-to-noise.The reliability of the parameters reported in Table 1 istherefore questionable. A mathematically unique solutioncannot be determined with confidence for D, Q3, and Q4

sites in the uncured sample. The PDMS signal shows a linear

20 18 10 0.114 0.51.02.03.04.05.06.08.01216 2.5 1.5

18 10 0.114 0.51.02.03.04.05.06.08.01216

(a)

(b)

Figure 5. An array of cp/mas 29Si spectra for uncured (a) andcured (b) DC745 where the contact time is varied from 0.1 ms(right) to 20 ms (left).

Table 1. Cross-polarization parameters DC745

T1� (ms) THSi (ms) M1�a.u.�

CuredD 13 š 3 13 š 3 361 š 43Dphenyl 15 š 2 1.5 š 0.3 73 š 6Q3 14 š 4 2.8 š 0.7 66 š 10Q4 13 š 3 4.2 š 0.9 100 š 16

UncuredD 10 000 77 700Dphenyl 63 š 40 1.3 š 0.3 30 š 3Q3 10 000 1.6 17Q4 10 000 1.7 18

magnetization transfer. Without observing a maximum, M1

for D is underestimated. The lack of a measurable decayin the signal at longer contact periods prevents a realisticmeasurement of T1�. The nonlinear analysis reports theupper-bound limit for this parameter in the cases of D,Q3, and Q4.

Other considerations bring into question the cp valuesreported here. A fast cp regime cannot be determined



(b)

(a)

Figure 6. Buildup curves for the peak intensities of the cp/mas 29Si spectra of uncured and cured DC745. The curves are fit for thecp parameters according to expression 2. Plot (a) contains the polymer peaks, while the peaks corresponding to the Q sites areshown in plot (b).

from the appearance of the transfer curve alone and theslow cp regime, THSi > T1�, has been reported for silicatesystems.37 – 39 It should be emphasized that the cured Dsolution finds THSi D T1�, which violates the fast cp regimeassumption. Complimentary information is necessary, suchas the independent measurement of T1�, to determine thetype of cp regime. Secondly, the above analysis considers

only a monoexponential polarization transfer, which is toosimple a model to explain a composite material. A singlespectral resonance can be described by multiple species withseparate transfer dynamics. This situation is not uncommonfor Q3 sites and typically two separate species are reported.6

With the presence of PDMS, it would not be unrealistic toexpect more than two Q3 species in DC745.



2D HetCor experimentsTo help alleviate the issue of a single resonance represent-ing multiple species, we resort to 2D NMR. All neighboringprotons participate in cp, creating multiple pathways formagnetization transfer. Given favorable resolution, the 1Hchemical shift of the different polarization sources can be dis-tinguished in a 2D experiment. When cp drives the exchangemechanism, the 2D 1H/29Si HetCor spectrum (Fig. 7) cor-relates the 1H/29Si chemical shifts where dipolar couplingsexist. The projection along either dimension is identical to a1D cp/mas spectrum. A 1D cp/mas 1H spectrum differs froma 1D 1H WaHuHa high-speed MAS spectrum in that only theproton signal participating in cp is observed. Note the subtledifferences in the 1H projections to that shown in Fig. 7(a).The 1H projection clearly shows the increased polymer sig-nal with curing. The uncured 1H projections show the strongpresence of a unique peak at 5.7 ppm. On the basis of the pro-ton shifts of fumed silica, this peak is attributed to hydrogen-bonded silanols. It should be emphasized that a lack of signalin the 3–5 ppm proton shift range does not exclude the pres-ence of additional water near the silica surface. Through thehydrogen bonding with the silanol groups, silica can havemany layers of adsorbed water. However, these outer layershave liquidlike properties and are not cross-polarizable. Aspinning sideband appears at 5 ppm in the 1H dimensionof the cured spectrum. With Lee–Goldburg homonucleardecoupling being applied during the 1H evolution, the Lar-mor precession of the protons becomes scaled by a factorof 1.73. Adjusting the 1H chemical shift axis for this scalingartificially places the 3.5 kHz spinning sideband at 5.0 ppm.

The advantage of the 2D spectrum, however, is the factthat the correlated spectrum gives more information byproviding the proton source of transferred magnetizationduring cp. The multiple cp pathways of DC745 are evidentin the HetCor spectrum. For example, the phenyl-substitutedsiloxane has a 29Si shift of �30 ppm. Two peaks are seen alongthe �30 ppm 29Si axis. The peak that has a 1H shift at 0 ppmcorresponds to magnetization transferred by methyl protons,while the down field peaks stem from aromatic protons. Ina similar fashion, the inorganic silicate along the �85 to�115 ppm 29Si axis has three potential proton polarizationsources – hydroxyl groups of the Q2/Q3 sites, water on thesilica surface, and methyl protons of the polymer. Becauseof the chemical shift separation in the two dimensions, fourdistinct regions of peaks appear in the 2D HetCor spectrum.The top left quadrant is the 29Si-detected silicate that is cross-polarized by either water or the Q2/Q3 hydroxyl groups.Owing to the low occurrence of the phenyl rings, significantcontributions of detected silicate signal from aromaticprotons is unlikely. The top right quadrant is 29Si-detectedsilicate cross-polarized from the protons in the methyl orvinyl group of the polymer. It is in this region that thesignal confidently corresponds to the surface interface sincethe transferred proton source can only come from polymernuclei. The bottom left quadrant is the 29Si-detected polymersilicone that has been cross-polarized by either the aromaticprotons or by hydroxyl protons on the silicate surface. Thelast quadrant, bottom right, is the silicone polymer cross-polarized by methyl protons. This quadrant gives a measure

of the degree to which the polymer can be polarized. But thisquadrant does not specify if the ability to polarize is due toentanglements, cross-linking, or the silica interface.

Pronounced changes to the HetCor spectrum areobserved with sample curing. There is significant inten-sity loss to the hydroxyl-silicate region that suggests that thecuring process dehydrates the material. The 5.7 ppm peakin the uncured sample disappears. Two mechanisms couldbe proposed: the direct removal of adsorbed water or indi-rectly from a condensation reaction between the neighboringsilanol site. Temperatures used in the curing and postcur-ing of DC745 could drive adsorbed water from the silicasurface. The proton peak at 5.7 ppm could correspond to asilanol hydrogen bonded to water, and it is this water that isremoved during the curing/postcure period. However, eventemperatures of 190 °C can allow siloxane bonds to formbetween two neighboring surface silanols.40 A stiffening ofthe polymer also takes place as suggested by the growthof polarizable polymer signal and the enhancement of thespinning sideband. The formation of cross-links during cur-ing would result in an increase of cross-polarizable PDMS.However, cross-linking cannot explain this signal increasealone since the polymer’s spinning sideband also appearswith the 29Si silicate shift. This suggests that the polymer isin close proximity with the filler.

Future HetCor studiesIn order to extract quantifiable information from the 2D spec-tra, the polarization dynamics must be studied. A series of2D HetCor spectra where the contact time is varied wouldprovide buildup curves similar to the 1D data. This 3D dataset (1H chemical shift – 29Si chemical shift – contact time)would distinguish the cp characteristics of the different pro-ton sites. In principal, different cp parameters would beobserved for the 29Si because of the different magnetizationsources. Alternatively, one could exploit the separation of theproton chemical shifts. An array of 1D cp experiments withselective 1H excitation would provide similar informationto the 2D. The polarization dynamics would be assessed inconjunction with an independent measurement of T1�. Spindiffusion, homonuclear proton magnetization transfer, takesplace during cp. This spin diffusion extends the length scalemonitored by polarization by means of the proton–protonnetwork. The signal in the upper-right quadrant that corre-sponds to the surface interface must be at or near the surfacedue to the distance constraint of the dipolar interaction.However, the act of spin diffusion extends the probed regionbeyond the vicinity of the polymer–silicate interface. The sig-nal buildup according to the proton–silicon cross-relaxationtime would not reflect the directly coupled 1H and 29Si spins.Homonuclear proton decoupling, i.e. Lee–Goldburg, wouldquench the proton–proton spin diffusion. Cross-polarizationunder these conditions41 would be limited to directly coupledproton–silicon spins. Without homonuclear decoupling, onecannot assume with confidence that only directly coupledproton–silicon interactions are observed. At this point intime, the modified pulse sequence has not been implemented.



(a)

(b)

Figure 7. The 2D HetCor 1H/29Si spectra for uncured (a) and cured (b) DC745. The 1H chemical shift runs horizontally, while the 29Sishift is vertical. On the basis of 1H/29Si chemical shift differences, the peaks can be assigned to the different material components.

CONCLUSIONS

Little is known about the microscopic details relating to theshelf life and age profile of reinforced siloxane elastomers.If NMR signals corresponding to nuclei at or near the poly-mer–filler interface can be isolated, one may characterize

this interaction by its cp dynamics. In addition, the use ofcp enhances the spectral appearance of a composite materialthat contains regions with varying degrees of motion. Here,the techniques of cp are explored in a cured and uncuredpolysiloxane reinforced with fumed silica and DC745 thatserves as an example of siloxane elastomer. Although no 29Si



chemical shift changes are observed between the two sam-ples, their transfer profiles are dramatically different. Thisindicates a variation in the cp dynamics due to chemicalchanges that take place with curing.

The polarization parameters may prove sensitive tochemical changes in siloxane materials associated with aging.However, care must be taken with the extraction of theseparameters before conclusive remarks can be made withconfidence. The type of cp regime, fast or slow, describingthe transfer must be identified by complementary informa-tion. This can be supplied by separate T1� measurements andby the TORQUE experiment.37 Secondly, the transfer pro-file should represent only NMR signals stemming from thepolymer–filler interface. The influence of multiple pathwaysfor magnetization transfer to each 29Si site can complicatethe physical interpretation of the cp parameters. This com-plication can be eliminated by going to 2D NMR. The added1H chemical shift dimension allows one to assign regionsof the 2D spectrum to different components of the material.In this regard, one can isolate NMR signals that correspondto the silicate surface and the polymer–silicate filler inter-face. A series of 2D HetCor spectra where the contact timeis varied in increments would display the cp dynamics of29Si signal due to its proton source. The implementation ofLee–Goldburg homonuclear decoupling during cp wouldquench spin diffusion. This would restrict magnetizationtransfer to directly coupled 1H/29Si spins. Under such con-ditions, internuclear distances may be referred from theobserved proton–silicon cross-relaxation time. These dis-tances may be used to refine atomistic models of the surfaceinteraction.20

Qualitatively, the 2D HetCor spectra display changes toDC745 due to the curing process where notable changes to thesignal intensity are observed. These changes are consistent,with the curing process dehydrating the silicate surfaceas well as increasing the polymer’s rigidness. 2D 1H/29SiHetCor spectra on artificially aged DC745 may reveal otherchanges. Further quantitative information may be providedby exploring the polarization dynamics in the 2D 1H/29SiHetCor or by 1D 1H selective excitation cp experiments.

AcknowledgementsThe NMR spectrometer was purchased through the acquisitionof funds from the National Science Foundation’s Major ResearchInstrumentation Program (Award No. CHE-0216075).

REFERENCES1. Bovey FA, Mirau PA. NMR of Polymers. Academic Press: San

Diego, 1996.

2. Maciel GE, Sindorf DW. J. Am. Chem. Soc. 1980; 102: 7607.3. Sindorf DW, Maciel GE. J. Phys. Chem. 1982; 86: 5208.4. Sindorf DW, Maciel GE. J. Am. Chem. Soc. 1983; 105: 3767.5. Chuang IS, Maciel GE. J. Am. Chem. Soc. 1996; 118: 401.6. Liu CC, Maciel GE. J. Am. Chem. Soc. 1996; 118: 5103.7. Litvinov VM, Spiesis HW. Makromol. Chem. 1991; 192: 3005.8. Legrand AP, Lecomte N, Vidal A, Haidar B, Papirer E. J. Appl.

Polym. Sci. 1992; 46: 2223.9. Ito M, Nakamura T, Tanaka K. J. Appl. Polym. Sci. 1985; 30: 3493.

10. Cosgrove T, Turner MJ, Thomas DR. Polymer 1997; 38: 3885.11. Huijgen TP, Gaur HA, Weeding TL, Jenneskens LW, Schu-

urs HEC, Huysmans WGB, Veeman WS. Macromolecules 1990;23: 3063.

12. Pfleiderer B, Albert K, Bayer E, van de Ven L, de Haan J,Cramers C. J. Phys. Chem. 1990; 94: 4189.

13. Gambogi JE, Blum FD. Macromolecules 1992; 25: 4526.14. Comotti A, Simonutti R, Sozzani P. Chem. Mater. 1999; 8: 2341.15. Simonutti R, Comotti A, Negroni F, Sozzani P. Chem. Mater.

1999; 11: 822.16. Simonutti R, Comotti A, Bracco S, Sozzani P. Chem. Mater. 2001;

13: 771.17. Osaheni JA, Truby KE, Silvi N. Macromol. Symp. 2001; 169: 261.18. Maxwell R, Chien A, Balazs B, LeMay J. ACS Polym. Mater. Sci.

Eng. 2000; 82: 183.19. Chahal RS, St. Pierre LE. Macromolecules 1969; 2: 193.20. Hanson DE. J. Chem. Phys. 2000; 113: 7656.21. Gee RH, Maxwell RS, Balazs B. Polymer 2004; 45: 3885.22. Polmanteer KE. Rubber Chem. Technol. 1981; 54: 1051.23. Litvinov VM, Zhadanov AA. Polym. Chem. USSR 1988; 30: 1000.24. Wagner MP. Rubber Chem. Technol. 1976; 49: 703.25. Bachmann JH, Sellers JW, Wagner MP, Wolf RF. Rubber Chem.

Technol. 1959; 32: 1286.26. Iler RK. The Chemistry of Silica. Wiley: New York, 1979.27. Hafner S, Spiess HW. Solid State NMR 1997; 8: 17.28. Caravtti P, Braunschweiler L, Ernst RR. Chem. Phys. Lett. 1983;

100: 305.29. Bielecki A, Kolbert AC, Levitt MH. Chem. Phys. Lett. 1989; 155:

341.30. Matlab Release 12.1. The MathWorks: Natick, 2001.31. Mayer BX, Zoellner P, Kaehlig H. J. Chromatogr., A 1999; 848:

251.32. Schmidt-Rohr K, Spiess HW. Multidimensional Solid-STate NMR

and Polymers. Academic Press: New York, 1994.33. Maxwell R, Balazs B. J. Phys. Chem. 2002; 116: 23, 10 492.34. Ek S, Iiskola EI, Niinistoe L, Vaittinen J, Pakkanen TT, Root A.

J. Phys. Chem. 2004; 108: 11 454.35. Iuliucci R. 1H/29Si Cross-polarization NMR spectrum of

thermally decomposed DC745 acquired at the University ofUtah, August 2000; unpublished results.

36. Mehring M. Principles of High resolution NMR in Solids. Springer:Berlin, 1983.

37. Klur I, Jacquinot JF, Brunet F, Charpentier T, Virlet J, Scheider C,Tekely P. J. Phys. Chem. B 2000; 104: 10 162.

38. Fyfe CA, Brouwer DH, Tekely P. J. Phys. Chem. A 2005; 109: 6187.39. Gardiennet C, Marica F, Fyfe C, Tekely P. J. Chem. Phys. 2005;

122: 054705.40. Zhuravlev LT. Pure Appl. Chem. 1986; 61: 1969.41. Fu R, Hu J, Cross TA. J. Magn. Reson. 2004; 168: 8.


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