10544
J. Phys. Chem. B 2008, 112, 10544–10551
Structure and Dynamics of Surfactant Interfaces in Organically Modified Clays Peter A. Mirau,*,† Jennifer L. Serres,† David Jacobs,‡ Patrick H. Garrett,‡ and Richard A. Vaia† Nanostructured and Biological Materials Branch, Air Force Research Laboratories, Dayton, Ohio 45433, and Department of Electrical Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45221 ReceiVed: February 19, 2008; ReVised Manuscript ReceiVed: May 21, 2008
Organic modification of clays with surfactants is required for the preparation of polymer-clay nanocomposites for a variety of applications. We have studied the structure and dynamics of interfaces in synthetic clays modified with phosphonium surfactants. The chemical shifts, line widths, and relaxation times measured by 31P, 13C, and 1H NMR and the relaxation times measured by impedance spectroscopy allow us to monitor the dynamics over a wide range of time scales. The results show that the phosphonium headgroup is most restricted and that the mobility increases with increasing separation from the clay surface. The carbon chemical shifts show that the 16-carbon and 12-carbon surfactant tails of hexadecyltributyl phosphonium and dodecytriphenyl phosphonium are disordered at the interface and experience mobility over a range of time scales. The dynamics depend most strongly on the structure of the surfactant headgroup, and tributylphosphoniums are more mobile than the triphenylphosphoniums. Two dimensional chemical shift anisotropy spin exchange experiments show that the phosphorus atoms in the triphenylphosphonium surfactant are immobile on the clay surface on a 1 s time scale. The dynamics measured by impedance spectroscopy show a similar dependence on headgroup structure, even though the processes occur on very different time scales and length scales. The relationship between the structure and dynamics of the interface and the properties of composites are considered. Introduction Polymer-clay nanocomposites have been extensively studied following the initial reports that composites with improved heat distortion temperatures could be prepared using low (1-5%) clay loadings.1 It has since been shown that composites with improved mechanical, optical and electrical properties can be simply fabricated from a wide variety of polymers while retaining the inherent advantages of polymers, including low cost and processability.2 It is commonly thought that low concentrations of nanoparticles can be used to modify the materials properties because they have very high surface areas and high concentrations of interfaces. While nanocomposites have been fabricated for many applications, important challenges remain for the production of composites with improved properties, and for understanding how the structure and dynamics at the organic-inorganic interface ultimately affects the materials properties. The properties of nanocomposites are often improved as the clay is more efficiently dispersed and exfoliated into the polymer matrix.2 In most cases the clay particles are not exfoliated, but rather form aggregated structures (tactoids) that are not uniformly dispersed in the polymer matrix. One motivation for our current research is to understand the behavior of surfactants at the clay interface in order to develop strategies for more efficient dispersion. With the exception of water-soluble polymers, most polymers of interest do not readily mix with clay nanoparticles.2 The dispersion can be improved by replacing the naturally occurring sodium ions at the clay surface with surfactants containing a * To whom correspondence should be addressed. E-mail: Peter.Mirau@ wpafb.af.mil. † Air Force Research Laboratories. ‡ University of Cincinnati.
positively charged headgroup and a hydrocarbon tail that promotes mixing with hydrophobic polymers. A large number of surfactants, including those with amine, trimethyl amines, and phosphonium head groups, have been explored as a means to increase dispersion and exfoliation.2 Since the dispersion and property improvements are related to the surface or interfacial behavior, the structure and dynamics of surfactants at the interface are of great interest. The structure and dynamics of organically modified clays have been studied by a number of methods, including X-ray diffraction,3 Fourier transform IR (FTIR),4 and NMR,5–10 electron paramagnetic resonance (EPR),11,12 and impedance spectroscopy.13,14 The results show that the properties of organically modified silicates depend on the concentration of surfactant binding sites (the cation equivalent concentration (CEC)), the surfactant density and the chemical structure of the surfactant, including the hydrocarbon tail length.3 NMR studies have shown that the dynamics of the hydrocarbon tail in organically modified silicates are heterogeneous,8–11 both in the organically modified clay and the polymer nanocomposites, with mobility increasing with distance from the inorganic surface.9,15 The dynamics of the headgroup has not been extensively studied.15 In these studies, we have used NMR and impedance spectroscopy to probe the structure and dynamics of phosphonium surfactants at the interface in organically modified clays. The phosphonium surfactants are of interest for their high temperature stability in nanocomposites,16 and they are advantageous for NMR studies because 31P is a spin-1/2 nucleus with good sensitivity.11 The phosphonium head groups are positively charged and bound within a few ångstroms of the clay surface, making them an excellent reporter group for the structure and dynamics of surfactants at the interface. Carbon and proton NMR provide information about the structure and dynamics of
10.1021/jp801479h CCC: $40.75 2008 American Chemical Society Published on Web 07/31/2008
Structure and Dynamics in Clay Nanocomposites the hydrocarbon tail in the galleries between the clay platelets, which can be directly compared with previous studies of ammonium surfactants.5,9,10 A preliminary report of the NMR of phosphonium surfactants at the clay interfaces has been published.17 Like NMR, impedance spectroscopy is sensitive to the structure and dynamics over a range of time scales and length scales.18–21 Impedance spectroscopy is ideal for measuring bulk material features, including percolative charge transport at low frequencies and long length scales, heterogeneous interfacial relaxation related to microstructure, and molecular fluctuations that occur at high frequencies and short length scales. It has been demonstrated in bulk polymers that the dielectric transitions detected by impedance spectroscopy can in favorable cases be directly related to the low frequency dynamics observed by NMR.22,23 Methods and Materials The synthetic clays Laponite B (Na+0.8[(Si8Mg5.2Li0.8)O20(OH)2F2]-0.8, CEC ) 105 meq/100 g) and Laponite RD (Na+0.42[(Si8Mg5.58Li0.42)O20(OH)4]-0.42, CEC ) 55 meq/100 g) were obtained from Southern Clay Products, Inc. The phosphonium surfactants hexadecyltributylphosphonium bromide and dodecyltriphenylphosphonium bromide were obtained from Aldrich and used without further purification. The composites were prepared by dispersing the clays in distilled water for 24 h followed by the slow addition of a surfactant solution. A 50% excess of surfactant relative to the CEC was added to ensure complete coverage of the clay surface. The precipitate was collected and Soxlet extracted with water and then ethanol for 24 h to remove any excess surfactant. Chemical analysis of the purified Laponite/surfactant samples showed that the residual bromine level was less than 0.07%, demonstrating that all free surfactant has been removed by Soxlet extraction. The X-ray diffraction spectra were measured on a Bruker AXS D8 Discover and a Molecular Metrology SAXS in transmission mode. Solid-state NMR spectra were acquired with cross polarization and magic-angle sample spinning on Varian Unity spectrometer at 400 MHz for protons, 100 MHz for carbons, and 162 MHz for phosphorus. A 90° pulse width of 5 µs and a cross polarization time of 1 ms was used in all experiments. The proton T1 and T1F and the carbon or phosphorus relaxation times were measured using pulse sequences reported in the literature.24–26 Wide line correlation spectra (WISE)27 were acquired using the 90(H)-t1-CP-Acq pulse sequence where CP and Acq represent the times for cross polarization and carbon or phosphorus signal acquisition. High power proton decoupling with two-pulse phase modulation (TPPM) was used during acquisition.28 The WISE spectra were acquired with a proton sweep width of 250 kHz using time-proportional phase incrementation (TPPI) for quadrature detection.29 The phosphorus 2D spin exchange spectra were acquired with the 90(H)-CPt1-90(P)-τm-90(P)-Acq pulse sequence26,30,31 using TPPM decoupling during the t1 and acquisition periods, and TPPI for quadrature detection. Isothermal impedance spectra were collected using Novocontrol’s Alpha impedance analyzer in the frequency range of 10-2 to 106 Hz. Novocontrol’s QUATRO Cryosytem was used for temperature control (0 to 150 °C, with 10 °C increments) and complex spectra were acquired in a nitrogen atmosphere. Laponite powders were compressed (4.5 t, for ∼60 s) into cylindrical disks having a thickness of ∼2 mm and a diameter of 22 mm, and the samples were dried under vacuum (1 mBar)
J. Phys. Chem. B, Vol. 112, No. 34, 2008 10545 SCHEME 1: The Structure of Hexadecyltributyl Phosphonium (C16P+(C4H5)3) and Dodecyltriphenyl Phosphonium (C12P+(C6H5)3) Surfactants
for four days at 65 °C to minimize the presence of bound water. Data analysis and model fitting with interactive parameter adjustment were performed using an in-house complex nonlinear least-squares software tool that allows us to simultaneously fit the real and imaginary components of the impedance spectra.26 Results The addition of nanometer-sized clay particles to polymers has been shown to improve many properties of polymers, including thermomechanical, optical, degradation, barrier, and dielectric durability.1,2 Since hydrophilic aluminosilicates are not miscible with most polymers, the inorganic particles are usually coated with surfactants to improve the dispersion. Natural Montmorillonite is most commonly used in nanocomposites,4 but it contains paramagnetics that can affect the NMR analysis.32 For these studies we have chosen synthetic Laponites that are free of paramagnetics, but crystallographically similar to Montmorillonite. Both Laponite and Montmorillonite are 2:1 phyllosilicates consisting of stacks of platelets, each comprised of an alumino (dioctohedral Montomorillonite) or magneso (trioctohendral Laponite) central layer sandwiched between two silica tetrahedral layers. Isomorphic substitution within the central octahedral layer creates a net negative charge on the platelet surface that is more diffuse than created when isomorphic substitution occurs within the tetrahedral layers, such as in the 2:1 phyllosilicates mineral Saponite.33 Thus, to the first order the electrostatic interaction between the inorganic platelet and interlayer cations is comparable for Laponite and Montmorillonite and the impact of surface charge density will be reflected by the cation exchange capacity. Here in, the two Laponites examined have exchange capacities of 55 and 105 meq/100 g, which bracket the cation exchange capacities of commonly used Montmorillonites. Also, consistent with synthetic magnesosilicates, Laponite has a smaller average lateral platelet dimension relative to Montmorillonites (25 nm versus 100-150 nm). This smaller platelet size decreases the extent of vertical ordering of the platelets and facilitates dispersion. Since the NMR response is dominated by local interactions (typically less than 2 nm) the difference in the lateral dimensions is not expected to influence the generality of our conclusions to other octohedrally substituted 2:1 phyllosilicates. The Laponites in these studies were modified with hexadecyltributyl phosphonium bromide (C16P+(C4H5)3) or dodecytriphenyl phosphonium bromide (C16P+(C4H5)3) (Scheme 1). X-ray Characterization. X-ray scattering is frequently used to characterize organically modified silicates and nanocomposites because the diffraction peak associated with spacing between the platelet layers can be easily observed in intercalated structures and tactoids.2,34 Table 1 lists the layer spacings for Laponite B and Laponite RD, and the clays modified with the C12P+(C6H5)3 and C16P+(C4H5)3 surfactants. The layer spacings
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TABLE 1: The Layer Spacings for Laponite and Organically Modified Laponite Composites Measured by X-ray Diffraction clay Laponite B Laponite B Laponite B Laponite RD Laponite RD Laponite RD Montmorillonite
surfactant C16P+(C4H5)3 C12P+(C6H5)3 C16P+(C4H5)3 C12P+(C6H5)3 C16P+(C4H5)3 C12P+(C6H5)3
d (nm) 1.00 2.12 2.12 0.99 1.93 1.97 0.96 2.10 1.85
for organically modified Montmorillonite are also included in Table 1. These data show that the layer spacings for Laponites modified with the phosphonium surfactants are similar to those observed for Montmorillonite, suggesting that the synthetic Laponite clays are a good model system for organically modified silicates. X-ray studies of Montmorillonite as a function of surfactant chain length show that short chain alkyl ammonium surfactants form monolayers that give rise to 1.4 nm layer spacings, while the longer surfactants form double layers with spacings on the order of 1.8-2.1 nm.4 The X-ray data for the C16P+(C4H5)3 and C12P+(C6H5)3 modified Laponites are consistent with the formation of an organic bilayer separating the clay sheets. 13C NMR Studies. The solid-state carbon chemical shifts detected with cross polarization and magic-angle sample spinning can be used to monitor the conformation of the hydrocarbon tail.9 Methylene carbons in the ordered all-trans conformation appear at 33 ppm, while methylene carbons with a greater concentration of gauche conformations appear near 30 ppm.35 Figure 1 compares the solid-state carbon spectra obtained with cross polarization for the semicrystalline C16P+(C4H5)3 and the organically modified Laponite B. The bulk phosphonium surfactant shows a peak at 33 ppm, which is assigned to methylene carbons in an all-trans crystalline conformation, while the methylene chemical shifts in the Laponite composites range between 30.2 and 31.2 ppm (Table 2). The chemical shift data shows that the hydrocarbon tails are conformationally disordered at ambient temperature in the organically modified Laponites. These conclusions are supported by differential scanning calorimetry studies (not shown) which do not show the order-disorder transition observed for clay composites with long hydrocarbon tails.36
Figure 1. The 100 MHz carbon spectra for (a) bulk C16P+(C4H5)3 and (b) the composite with Laponite B obtained with cross polarization and magic-angle sample spinning.
TABLE 2: The 13C NMR Chemical Shifts and Relaxation Times for the Methylene Carbons in Surfactants and Organically Modified Laponites clay
surfactant +
C16P (C4H5)3 Laponite B C16P+(C4H5)3 Laponite RD C16P+(C4H5)3 C12P+(C6H5)3 Laponite B C12P+(C6H5)3 Laponite RD C12P+(C6H5)3
δC (ppm) T1(H) (s) T1F (ms) T1(C) (s) 33.0 31.2 31.2 33.0 30.2 30.2
1.5 0.2 0.3 3.1 0.3 0.3
72.9 1.9 2.0 7.7 2.7 3.0
2.0 0.5 0.5 2.4 0.5 0.5
TABLE 3: The31P NMR Chemical Shifts and Relaxation for the Surfactants and Laponite Composites clay
surfactant
C16P+(C4H5)3 Laponite B C16P+(C4H5)3 Laponite RD C16P+(C4H5)3 C12P+(C6H5)3 Laponite B C12P+(C6H5)3 Laponite RD C12P+(C6H5)3
δP (ppm) T1(H) (s) T1F(H) (ms) T1(P) (s) 34.5,33.1 34.8 34.8 24.3, 21.9 24.6 25.6
1.5 0.2 0.2 3.0 0.5 0.3
105.0 1.5 1.8 12.3 2.4 3.0
6.2 1.9 1.3 74.0 8.6 15.9
31P NMR Studies. Solid-state phosphorus NMR is an excellent tool for the study of surfactant dynamics, since 31P NMR has a high sensitivity the phosphorus atoms are close to the clay surface.5,6,11 The 162 MHz 31P NMR spectra of the C16P+(C4H5)3 surfactant in the bulk and the Laponite B composite show a very high signal-to-noise ratio after only a few scans (Figure S1, Supporting Information). Two sharp peaks are observed for the bulk C16P+(C4H5)3 and C12P+(C6H5)3 surfactants at 34.5 and 33.1 ppm and 24.3 and 21.9 ppm, respectively. These peaks are assigned to nonequivalent phosphorus atoms in the crystalline unit cell. The Laponite composites modified with the C16P+(C4H5)3 and C12P+(C6H5)3 surfactants show peaks at 34.8 and 24.6 ppm (Table 3). Carbon NMR Relaxation Studies. The solid-state NMR relaxation times and line widths can be measured in several experiments that allow us to probe the dynamics over a range of time scales. The relaxation times are sensitive to fluctuations in the vicinity of the observed nucleus, and can be used to distinguish between the dynamics in hydrocarbon tail (detected by carbon NMR) and headgroup (detected by phosphorus NMR). The proton relaxation, which we measure through cross polarization, is dominated by proton spin diffusion,24 which tends to average the relaxation times for nearby protons. Large differences in the proton and carbon relaxation times are observed for surfactants in the bulk and at the clay surface (Table 2). The proton T1’s for the methylene carbons in the C16P+(C4H5)3 surfactant decrease from 1.5 s for the bulk sample to 0.2-0.3 s in the Laponite composites, while the proton T1F relaxation times decrease from 72 ms in the bulk to 2 ms in the composite. The carbon T1 relaxation times for the bulk surfactants (2.0-2.4 s) are also reduced (∼0.5 s) at the clay surface. No significant differences in the proton or carbon relaxation times are observed for the composites with 12-carbon versus 16-carbon chains, and no significant differences are observed as a function of the concentration of surfactant binding sites (CEC ) 105 versus 55 meq/100 g). Phosphorus NMR Relaxation Studies. The phosphorus and proton relaxation times measured through {1H-31P} cross polarization (Table 3) provide information about the molecular dynamics in the proximity of the headgroup. As with the relaxation times measured for the hydrocarbon tail, the general trend is that the relaxation times are much longer in the bulk surfactants compared to those at the Laponite surface. However, in contrast to the carbon relaxation times, there are significant
Structure and Dynamics in Clay Nanocomposites
J. Phys. Chem. B, Vol. 112, No. 34, 2008 10547
TABLE 4: The Proton Dipolar Line Widths Measured by {1H-31P} and {1H-13C} NMR for the Surfactants and Laponite Composites ∆υ1/2 (kHz) clay Laponite Laponite – Laponite Laponite
{1
surfactant B RD B RD
C16P+(C4H5)3 C16P+(C4H5)3 C16P+(C4H5)3 C12P+(C6H5)3 C12P+(C6H5)3 C12P+(C6H5)3
31
H- P}
{1H-13C}
43.8 8.7 8.6 30.4 17.5 22.8
35.0 15.2 12.2 33.9 16.3 16.3
TABLE 5: The31P Line Width Perpendicular to the Diagonal In31P-31P 2D Exchange Experiments As a Function of Temperature and Spin Exchange Time ∆υ1/2 (Hz) temperature (°C)
τm ) 1 ms
τm ) 1 s
33 93
504 584
935 1052
differences in the spin-lattice relaxation times for the surfactants with triphenyl and tributyl phosphonium head groups. The proton T1 relaxation times for the head groups are on the order of 1.5-3.0 s in the bulk surfactants and 0.2-0.5 s in the Laponite composites. The proton T1F values are very different (105 versus 12.3 ms) for the bulk tributyl and triphenyl surfactants, reflecting a difference in headgroup dynamics in the semicrystalline state. The T1F relaxation times are reduced for the Laponite composites to 1.5-1.8 ms and 2.4-3.0 ms for the C16P+(C4H5)3 and C12P+(C6H5)3 surfactants, respectively. Large differences in the phosphorus T1’s are observed for the C16P+(C4H5)3 and C12P+(C6H5)3 surfactants, both in the bulk (6.2 and 74 s) and at the clay surface (1.3-1.9 and 8.6-15.9 s). These results show that the tributyl surfactant is much more mobile in the vicinity of the headgroup than the triphenyl one, even though the tributyl surfactant has a longer hydrocarbon tail. We also note significant differences in the 31P relaxation times for the C12P+(C6H5)3 surfactants in composites with a higher (Laponite B) and lower (Laponite RD) density of surfactant binding sites. Proton Dipolar Line Widths. The proton dipolar line widths contain valuable information about the molecular dynamics of surfactants on a kHz frequency scale. The proton line widths in rigid solids are on the order of 50 kHz, but can be averaged by molecular motion with a correlation time faster than the inverse of the line width. The proton line widths tend to overlap and are difficult to measure directly, but they can be measured indirectly using 2D wide-line correlation spectroscopy (WISE),27 which correlates the carbon or phosphorus chemical shift with the proton line width (Figure S2, Supporting Information). The dipolar line widths for the bulk surfactants and the modified Laponite measured by 2D {1H-13C} correlation for the hydrocarbon tail and by {1H-13P} correlation for the head groups are compiled in Table 4. The hydrocarbon tails show line widths on the order of 34-35 kHz in the bulk surfactants that are averaged to 15-16 kHz at the Laponite surface. As with the carbon T1’s, the proton line widths for the hydrocarbon tails are insensitive to the hydrocarbon tail length, the structure of the headgroup, or the density of surfactant binding sites. The results for the protons in the vicinity of the phosphonium show that the dynamics are more restricted near the headgroup. The line widths for the bulk surfactants are broad, as expected for an immobile semicrystalline material. Molecular motions of the protons nearest the phosphonium partially average the
dipolar couplings in the Laponite composites, but the averaging is much more efficient for the C16P+(C4H5)3 (∼8 kHz) than the C12P+(C6H5)3 surfactant (17-22 kHz). These data support the conclusion that the headgroup dynamics are sensitive to phosphonium headgroup structure. 2D Chemical Shift Anisotropy (CSA) Exchange NMR. While the proton, phosphorus, and carbon relaxation times are sensitive to fluctuations in the internuclear vectors near the nuclei of interest, the reorientation of the principal axis system of the phosphorus atoms at the Laponite surface is a more direct measure of the headgroup dynamics.25,26 The CSA’s for phosphorus atoms are on the order of several kHz and depend on the structure of the atoms bonded to the phosphorus. The CSA line shapes are effectively averaged by magic-angle sample spinning, but can be directly observed in the static spectra. Figure 2 compares the phosphorus line shapes for the bulk surfactants with the Laponite composites in the absence of magic-angle sample spinning. The tributyl surfactant has a small anisotropy (2.6 kHz), as might be expected for a phosphorus atom bonded to four methylene carbons, while the CSA is much larger for the triphenyl surfactant. The complex line shape for the bulk C12P+(C6H5)3 surfactant (Figure 2c) is a consequence of nonequivalent molecules in the unit cell (Figure S1, Supporting Information). The spectrum for the C12P+(C6H5)3/ Laponite B complex shows a broad line with a width of 4.2 kHz. This shows that molecular motion at the clay surface is insufficient to average the CSA line shape, making it possible to study the molecular dynamics of phosphorus atoms at the surface using 2D spin exchange spectroscopy.37 2D spin exchange experiments have been used to study the ultraslow dynamics (ms-s) in polymers and other systems.37 This experiment takes advantage of the fact the lines are inhomogeneously broadened and the chemical shifts are associated with particular orientations of the principal axis of the phosphorus CSA tensor relative to the magnetic field and as given by
δ ω ) ωiso + (3 cos2 θ - 1 - η sin2 θ cos φ) 2
(1)
where ω is the chemical shift, ωiso is the isotropic chemical shift, δ is the coupling constant, and θ and φ relate the orientation of the principal axis of the chemical shift to the magnetic field direction. If a phosphorus atom reorients during the spin exchange period, then the spin changes frequency and off-diagonal intensity is observed in the 2D spectrum.37 Magnetization confined to the diagonal is indicative of atoms that do not change their orientation during the mixing time. Figure 3 shows the phosphorus 2D spin exchange spectrum for the C12P+(C6H5)3/Laponite B composite obtained with mixing times of 1 ms and 1 s. The magnetization is confined to the diagonal in the spectrum acquired with a 1 ms spin exchange time, showing that there is no significant reorientation of the phosphorus atom on the ms time scale. With a mixing time of 1 s, Figure 3b shows that there is nearly complete exchange leading to nearly equal line shapes in both dimensions. Spectra acquired with mixing times of 10 and 100 ms also showed signals mainly confined to the diagonal (not shown). The CSA for the C16P+(C4H5)3 is too small to permit accurate measurements of the spin exchange. There are two processes that that can lead to change in chemical shift changes during the exchange time, reorientation on the surface leading to a change in chemical shift as given by eq 1, or phosphorus-phosphorus spin exchange between neighboring phosphorus atoms on the Laponite surface.26 The
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Figure 2. The 162 MHz solid-state phosphorus spectra in the absence of magic-angle sample spinning for (a) C16P+(C4H5)3, (b) C16P+(C4H5)3/ Laponite B, (c) C12P+(C6H5)3, and (d) C12P+(C6H5)3/Laponite B.
Figure 3. The phosphorus 2D exchange spectrum for the C12P+(C6H5)3/Laponite B composite with spin exchange times of (a) 0.001 s and (b) 1s.
rate of phosphorus-phosphorus spin exchange depends on the dipolar coupling which is given by
( )
µ0 pγP2 4π D) r-3
(2)
where γP is the phosphorus magnetogyric ratio and r is the phosphorus-phosphorus separation. The magnitude of the dipolar coupling is such that spin exchange becomes possible for phosphorus atoms separated by a few nm or less. These two processes (reorientation versus spin diffusion) cannot be directly distinguished from the 2D exchange spectra, but can be distinguished by the temperature dependence of the exchange process. Reorientation on the surface is an activated process that is strongly temperature dependent while phosphorus-phosphorus spin exchange (spin diffusion) is not. We have evaluated the effect of temperature on the 2D exchange spectra by measuring the full-width at half-maximum perpendicular to the diagonal in spectra obtained with mixing times of 1 ms and 1 s at 33 and 93 °C. The results (Table 5) show that while the line widths perpendicular to the diagonal increase with mixing time, this process does not depend strongly on the temperature. These data demonstrate that the magnetization exchange in the 2D experiments with long mixing times is due predominantly to spin exchange between phosphorus atoms anchored on the clay surface, and that for exchange times as long as 1 s that the phosphorus atoms in the C12P+(C6H5)3/ Laponite B composite do not reorient to any significant degree. The rate of spin exchange calculated for surfactants separated
from each other by 1 nm is on the order of 1-2 s-1, which adequately accounts for the spin exchange observed in the 2D experiments. Impedance Spectroscopy. The impedance spectra have been measured for the Laponite/phosphonium samples for comparison with the NMR data and other organically modified clays.14 Figure 4 shows the dielectric loss spectrum for the Laponite B/C16P+(C4H5)3 as a function of frequency at 20 °C. The spectra were fit to a generalized impedance model with a constant phase angle to account for electrode polarization, DC conductivity, and low- and high-frequency Havriliak-Negami peaks as given by14
Z * (ω) )
[
σDC + iωεo ε∞ +
κo ∆εLF 1 + (iωτLF)RLF
+
∆εHF 1 + (iωτHF)RHF
]
+
ACPA (iω)γCPA
(3)
where κo is the cell constant, ACPA and γCPA are the amplitude and phase parameters in the constant phase model, σDC is the DC conductivity, ε0 is the permittivity of free space, τLF and τHF are the relaxation times for the low- and high-frequency relaxation modes, and ∆εLF and ∆εHF are the corresponding dielectric strengths. The peak shapes and fits are similar to those previously reported for organically modified Cloisite 20A,14 but significant differences are observed in the magnitudes of the relaxation parameters.
Structure and Dynamics in Clay Nanocomposites
J. Phys. Chem. B, Vol. 112, No. 34, 2008 10549 TABLE 6: The Low-Frequency (LF) and High-Frequency (HF) Dielectric Strengths and Relaxation Times for the C12P+(C6H5)3 and C16P+(C4H5)3 Laponite Composites C12P+(C6H5)3
C16P+(C4H5)3
parameter
LF
HF
LF
HF
∆ε τ (s)
74.5 80.5
3.4 0.0013
49.6 0.0044
8.2 0.000017
are in the range of 49-75 for the low frequency relaxation and 3.4-8.2 for the high frequency relaxation in the C12P+(C6H5)3 and C16P+(C4H5)3 surfactants at the Laponite surface. The dielectric strengths depend only weakly on temperature (not shown). Discussion
Figure 4. The dielectric loss spectra for the C16P+(C4H5)3/Laponite B composite at 20 °C. The contributions from DC conductivity and the low-frequency and high-frequency relaxation peaks are also shown.
Figure 5. The temperature dependence of the low-frequency (LF) and high-frequency (HF) relaxation times as a function of inverse temperature for the Laponite composites.
We can gain insight into the dynamics of surfactants at the clay interface by examining the values of the relaxation times over the temperature range of 0 to 150 °C, as shown in Figure 5 for the low- and high-frequency relaxation times. Table 6 lists the values of the relaxation times and dielectric strengths at 30 °C for a more direct comparison with the NMR data. The dielectric properties of a number of polymers and composite materials have been previously reported,13,14,18 and it has been shown that the dielectric strength is related to the magnitude and concentration of the fluctuating dipole moments, which in hydrocarbon polymers is typically less than 1.19–21 The data in Table 6 show the dielectric strengths
We have used a NMR and impedance spectroscopy to measure the structure and dynamics of surfactants in organically modified layered silicates. The organic modification of the clays plays a critical role in the dispersion and therefore indirectly affects many of the properties. The more direct role that the surfactant may play in modulating the interface between the polymer and the clay is potentially very important, but not well understood. The methods were chosen to probe the dynamics of the headgroup, the hydrocarbon tail and the entire molecule over a wide time scale. To understand the NMR results it is first necessary to appreciate the sources of relaxation and how the time scales of molecular motion affect the relaxation times and dipolar line widths. As a general rule the spin-lattice relaxation times are sensitive to molecular motion near the inverse of the Larmor frequency (400 MHz for protons, 162 MHz for phosphorus, and 100 MHz for carbons), while the rotating-frame relaxation times are sensitive to molecular motions near the inverse of the spinlocking field (50 kHz). The proton relaxation times in solids are averaged by proton spin diffusion,26,38,39 and do not directly reflect the local dynamics. The proton dipolar line widths are averaged by large amplitude molecular motions that are fast compared to the inverse of the rigid line width (50 kHz), and the 2D spin exchange experiments are sensitive to molecular motions on a millisecond or slower time scale. The upper limit on the {31P-31P} 2D exchange experiments is determined by the phosphorus spin-lattice relaxation times and the rates of {31P-31P} spin diffusion.37 The 13C and 31P spin-lattice relaxation is due primarily to dipolar interactions between the nuclei of interest and the nearby protons. The relaxation times are extremely long in rigid systems, and it is the fluctuations in the {1H-13C} or {1H-13P} internuclear vectors that lead to shortened relaxation times. This makes it possible, for example, to have an immobile phosphorus atom with a shortened spin-lattice short relaxation time due to molecular motions of nearby methylene groups that modulate the {1H-13P} internuclear vector. The dipolar line widths measured by 2D wide line correlation NMR are determined by the degree to which molecular motions modulate the {1H-1H} internuclear vectors. These line widths are a measure of the dynamics of the protons that are within a few ångstroms of the carbon or phosphorus of interest and can be used to distinguish between molecular motions in the headgroup and hydrocarbon tail. Spin exchange in the {31P-31P} 2D experiments depends only on reorientation of the phosphorus atom or {31P-31P} spin diffusion. The NMR results for the phosphonium surfactants at the clay interface show a gradient in molecular dynamics with molecular
10550 J. Phys. Chem. B, Vol. 112, No. 34, 2008 motion increasing with increasing separation from the inorganic surface, as noted in some other clay/surfactant complexes.12 The dynamics of the phosphorus atom in the headgroup at the Laponite surface can be directly probed for the triphenyl surfactant with the 2D {31P-31P} exchange experiments. The results show that the phosphorus atoms are immobile on a one second time scale, and that the line broadening in the 2D exchange spectrum at long delay times can be assigned to {31P-31P} spin diffusion. The surface binding of the phosphonium headgroup is due to electrostatic interactions between the positively charged phosphorus atom and the negatively charged surface, as well as interaction of the phenyl groups with the clay surface. Further studies are required to determine if the slow headgroup dynamics are specific to the triphenyl phosphonium or a general property of surfactants bound to the clay surface. We can monitor the dynamics of the surfactant headgroup slightly further from the surface by measuring the {1H-31P} spin-lattice relaxation times and the {1H-1H} dipolar line widths, which are sensitive to molecular motions in the methylene groups closest to the phosphonium. The results show that the there are large amplitude molecular motions in the methylene groups that shorten the 31P T1 and decrease the 1H dipolar line width relative to bulk surfactants. Furthermore, we note that molecular motions in the C16P+(C4H5)3 surfactant are more efficient at promoting relaxation than they are in the triphenyl surfactant, suggesting that the dynamics at the surface are strongly dependent on the chemical structure of the headgroup. The {1H-13C} measurements, along with the chemical shifts, provide information about the structure and dynamics of hydrocarbon tail. The results show that the surfactants adopt an all-trans conformation in the bulk and a more disordered conformation at the clay surface. Very similar relaxation times and dipolar line widths are observed for the 16-carbon and 12carbon surfactants, demonstrating that the dynamics of the hydrocarbon tails are insensitive to the hydrocarbon chain length, the chemical structure of the headgroup or the density of surfactant binding sites. In contrast to the NMR, impedance spectroscopy is sensitive to the dynamics of ions and dipoles in an alternating electric field. In addition to the DC conductivity, we observe high- and low-frequency peaks in the dielectric spectra that are qualitatively similar to those observed in organically modified Montmorillonites.14 The low frequency peaks are assigned to the accumulation of charge carriers at the alumino-silicate layers and at the boundaries between crystallites. This peak is very sensitive to the presence of impurities (e.g., bound water), the porous microstructure of the pressed sample, and excess surfactant. These effects were minimized using Soxlet extraction and extensive sample drying. The high-frequency peaks have dielectric strengths greater than those typically observed in hydrocarbon polymers19–21 and are assigned to ionic fluctuations at the interface.26 The highly dispersive (non-Debye) shape of the high-frequency peak suggests a heterogeneous distribution of local mobilities and environments. While the relaxation times from dielectric spectroscopy cannot be directly correlated with the NMR results, both studies show that the mobility of the C12P+(C6H5)3 surfactant at the Laponite surface is greatly restricted relative to the C16P+(C4H5)3 surfactant. In summary, we have used NMR and impedance spectroscopy to characterize the structure and dynamics of surfactants at the inorganic interface in synthetic clay composites. The results show a dynamics gradient with mobility increasing
Mirau et al. with distance from the surface. The phosphonium head groups are strongly bound at the surface and show restricted mobility, and the dynamics depend strongly on the surfactant headgroup, but not on the density of binding sites or the hydrocarbon tail length. Supporting Information Available: The Supporting Information shows the phosphorus spectrum of C16P+(C4H5)3 in the bulk and the Laponite B composite and the 2D wide line correlation spectra for the C16P+(C4H5)3/Laponite B and C16P+(C6H5)3/ Laponite B composites. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Okada, A.; Dawasumi, M.; Usuki, A.; Kojima, Y.; Kurauchi, T.; Kamigaito, O. In Polymer Based Molecular Composites; Schaefer, D. W., Mark, J. E. Eds.; Materials Research Society: Pittsburgh, PA, 1990; Vol. 171, pp 45-50. (2) Ray, S. S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539–1641. (3) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 8000–8009. (4) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017–1022. (5) Kubies, D.; Jerome, R.; Grandjean, J. Langmuir 2002, 18, 6159– 6163. (6) Muller, R.; Hrobarikova, J.; Calberg, C.; Jerome, R.; Grandjean, J. Langmuir 2004, 20, 2982–2985. (7) Osman, M. A.; Ernst, M.; Meier, B. H.; Suter, U. W. J. Phys. Chem. B 2002, 106, 653–662. (8) Urbanczyk, L.; Hrobarikova, J.; Calberg, C.; Jerome, R.; Grandjean, J. Langmuir 2006, 22, 4818–4824. (9) Wang, L. Q.; Liu, J.; Bordia, R. J. Phys. Chem. B 2000, 104, 2810– 2816. (10) Zhu, J.; He, H.; Zhu, L.; Wen, X.; Deng, F. J. Colloid Interface Sci. 2005, 286, 239–244. (11) Panek, G.; Schleidt, S.; Mao, Q.; Wolkenhauer, M.; Spiess, H. W.; Jeschke, G. Macromolecules 2006, 39, 2191–2200. (12) Schleidt, S.; Spiess, H. W.; Jeschke, G. Colloid Polym. Sci. 2006, 284, 1211–1219. (13) Davis, R. D.; Bur, A. J.; McBrearty, M.; Lee, Y. H.; Gilman, J. W.; Start, P. R. Polymer 2004, 45, 6487–6493. (14) Jacobs, J. D.; Koerner, H.; Heinz, H.; Farmer, B. L.; Mirau, P. A.; Garrett, P. H.; Vaia, R. A. J. Phys. Chem. B 2006, 110, 20143–20157. (15) Mao, Q.; Schleidt, S.; Zimmermann, H.; Jeschke, G. Macromol. Chem. Phys. 2007, 208, 2145–2160. (16) Xie, W.; Xie, R.; Pan, W. P.; Hunter, D.; Koene, B.; Tan, L. S.; Vaia, R. A. Chem. Mater. 2002, 14, 4837–4845. (17) Mirau, P. A.; Vaia, R. A.; Garber, J. Polymer Preprints (American Chemical Society, DiVision of Polymer Chemistry) 2005, 46, 440–441. (18) McCrum, N.; Read, B.; Williams, G. Anelastic and Dielectric Effects in Polymeric Solids; Wiley: New York, 1967. (19) Scheidler, P.; Kob, P.; Binder, K. Eur. Phys. J. E 2003, 12, 5–9. (20) Schonhals, A.; Kremer, F.; Hoffman, A.; Fischer, E. W.; Schlosser, E. Phys. ReV. Lett. 1993, 70, 3459–3462. (21) Schonhals, A.; Kremer, F. Non-Cryst. Solids 1994, 172-174, 336– 343. (22) Pschorn, U.; Rossler, E.; Sillescu, H.; Kaufmann, S.; Schaefer, D.; Speiss, H. Macromolecules 1991, 24, 398–402. (23) Schmidt-Rohr, K.; Spiess, H. Macromolecules 1991, 24, 5288– 5293. (24) Komoroski, R. A. High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk; VCH Publishers Inc.: Dearfield Beach, FL, 1986; Vol. 7. (25) Mirau, P. A Practical Guide to the NMR of Polymers; John Wiley & Sons: Hoboken, NJ, 2004. (26) Schmidt-Rohr, K.; Speiss, H. W. Multidimensional Solid-State NMR and Polymers; Academic Press: New York, 1994. (27) Schmidt-Rohr, K.; Clauss, J.; Spiess, H. Macromolecules 1992, 25, 3273–3277. (28) Bennett, A. E.; Rienstra, C. M.; Auger, M.; Lakshimi, K. V.; Griffin, R. G. J. Chem. Phys. 1995, 103, 6951–6958. (29) Marion, D.; Wuthrich, K. Biochem. Biophys. Res. Commun. 1983, 113, 967. (30) Hagemeyer, A.; Schmidt-Rohr, K.; Spiess, H. AdV. Magn. Reson. 1989, 13, 85–130.
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