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Langmuir 2008, 24, 12032-12039
Composites of Kaolin and Polydimethylsiloxane Yan Zhang,† David I. Gittins,‡ David Skuse,‡ Terence Cosgrove,† and Jeroen S. van Duijneveldt*,† School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K., and Performance Minerals, New Technology Group, IMERYS Minerals Ltd., Par Moor Centre, St. Austell, PL24 2SQ, U.K. ReceiVed June 11, 2008. ReVised Manuscript ReceiVed August 14, 2008 Kaolin particles were surface-treated with isobutyltrimethoxysilane (IBTMS), hydrogenated tallow (HT), and a polyisobutyl chain-based stabilizer (SAP) to make composites with polydimethylsiloxane (PDMS). IBTMS did not cover the strong acid sites on the kaolin surface and as a result a cross-linking reaction occurred for silanol-terminated PDMS. The polyisobutyl chain of SAP was found to be incompatible with PDMS and this caused aggregation of the kaolin particles. HT was the most effective at dispersing the particles into silanol-terminated PDMS. The aggregation state of the composites was characterized using rheology and microscopy. Both showed the HT-treated particles were well-dispersed in low molecular weight silanol-terminated PDMS, and they were weakly flocculated in higher molecular weight silanol-terminated PDMS. However, the same particles aggregated when dispersed in methyl-terminated PDMS. It appears the silanol-terminated PDMS acted as costabilizer through interaction with the kaolin surface. Transverse relaxation NMR was used to probe mobility of the PDMS chains in the composites. This showed little dependence on surface treatment, aggregation state, or polymer end groups. For all samples, chain mobility decreased with increasing kaolin concentration.
Introduction Clay-polymer composites have been studied widely because the incorporation of clay particles can lead to desirable changes in the material properties of the polymers, such as an improved mechanical response or a lowered gas permeability.1 Good dispersion of the filler in a polymer matrix results in a uniform stress distribution.2 A number of studies have been carried out on composites of clays and solid polymers, such as Nylon3 or polyolefins.4 The general method to make these composites is to modify the clay surface with an organic compound such as a silane or an amine and then mix them under high shear at a temperature above the melting point of the polymer. These studies mostly focused on characterization of the improved properties of the composites; less attention has been paid on how to achieve a uniform dispersion of clay. As these polymers and the resulting composites are solid, it is difficult to characterize the quality of the dispersion. If the composite were liquid at room temperature, both their preparation and characterization would be greatly simplified. There are many kinds of polydimethylsiloxane (PDMS) derivatives available commercially which are liquid over a wide range of molecular weights. They also have a variety of end groups, such as silanol end groups, which can interact with the clay surface strongly, and methyl end groups, which are less likely to bind to the clay surface. Recent work5 has been devoted to investigating the degree of dispersion of kaolin particles in nonpolar solvents, such as heptane or toluene. It was found that surface modification plays an important role in stabilizing the particles and a higher adsorbed * To whom correspondence should be addressed. † University of Bristol. ‡ IMERYS Minerals Ltd.
(1) Sinha Ray, S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539. (2) Moniruzzaman, M.; Chattopadhyay, J.; Billups, W. E.; Winey, K. I. Nano Lett. 2007, 7, 1178. (3) Buggy, M.; Bradley, G.; Sullivan, A. Composites Part A 2005, 36, 437. (4) Su, S.; Jiang, D. D.; Wilkie, C. A. Polym. Degrad. Stab. 2004, 84, 279. (5) Zhang, Y.; Gittins, D. I.; Skuse, D.; Cosgrove, T.; van Duijneveldt, J. S. Langmuir 2007, 23, 3424.
amount of stabilizer resulted in a better dispersion. In a nonpolar solvent, stabilization of particles is mainly caused by steric repulsion of the adsorbed stabilizer in a good solvent medium.6 However, the adsorbed stabilizer takes a less strongly stretched conformation in melts than the counterpart in a good solvent; hence, steric stabilization in a polymer melt becomes less straightforward7 and requires mixing of the stabilizer chain and the host polymer to be favorable.8 The conformation of attached polymers in a polymer melt has been calculated by de Gennes.9 At low grafting density the chains mix with the mobile chains and are not stretched, but they must stretch at a critical grafting density, and when the density is high enough, the grafted chains are completely stretched and segregate from the melt. Densely grafted polymer brushes are predicted not to be wetted by a melt of the same molecular weight and hence steric stabilization does not occur. However, when a polymer is grafted at a lower density, steric repulsion in the corresponding melt can be obtained.8 The wetting of a silicon wafer grafted with a layer of polystyrene10 by a polystyrene melt has been studied and complete wetting was observed at intermediate grafting densities as expected. Under favorable circumstances (small particles and strongly adsorbing polymer), it is even possible to achieve good dispersions without any further stabilizing layer: this was found with silica particles in a poly(ethylene glycol) melt up to Mw ) 8000 g/mol.11 Another work12 studied the stabilization of silica particles chemically grafted with a layer of PDMS in a PDMS melt. It was found that as the molecular weight of the PDMS melt increased, the grafting density needed to be raised to stabilize the particles. PDMS has also been found to adsorb onto silica (6) Cosgrove, T., Ed. Colloid Science; Blackwell Publishing Ltd: Oxford, 2005. (7) Jones, R. A. L.; Richards, R. W. Polymers at surfaces and interfaces; Cambridge University Press, Cambridge, 1999. (8) Borukhov, I.; Leibler, L. Macromolecules 2002, 35, 5171. (9) de Gennes, P. G. Macromolecules 1980, 13, 1069. (10) Maas, J. H.; Leermakers, F. A. M.; Fleer, G. J.; Cohen Stuart, M. A. Macromol. Symp. 2003, 191, 69. (11) Anderson, B. J.; Zukoski, C. F. Macromolecules 2007, 40, 5133. (12) Green, D. L.; Mewis, J. Langmuir 2006, 22, 9546.
10.1021/la8018259 CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
Composites of Kaolin and Polydimethylsiloxane Table 1. Overview of Materials Used samples
details
IBTMS-kaolin
kaolin particles treated with isobutyltrimethoxysilane (IBTMS) kaolin particles treated with SAP230 kaolin particles treated with hydrogenated tallow (HT) hydroxyl-terminated PDMS methyl-terminated PDMS
SAP-kaolin HT-kaolin OH-PDMS Me-PDMS
surfaces from its melt.13 As the surface chemistry of kaolin is similar to that of silica,5 PDMS might also physically adsorb onto the kaolin surface if the stabilizer did not cover the surface completely. The aim of the present work is to investigate the effect of both surface modification of kaolin and the end groups of PDMS on dispersing kaolin into PDMS melts and to characterize how kaolin particles affect the properties of PDMS and the resulting composites. Previously, suspensions of IBTMS- and HT-treated kaolin were studied.5 These surface treatments did not afford an effective stabilization, despite the particles having a refractive index (nD ) 1.55-1.56)14 close to that of toluene (nD25 ) 1.49),14 which means the van der Waals attraction would be nearly at a minimum. Dispersion into PDMS (nD25 ) 1.40),15 which has a lower refractive index than toluene, was therefore expected to be even less successful without the effect of adsorbed PDMS. Silanol-terminated PDMS was therefore chosen to compare these two surface modifications. In contrast to the other surface modifications, SAP stabilized kaolin very well in solvents such as heptane. The behavior of SAP- and HT-treated kaolin in silanolterminated PDMS was therefore compared. As HT was found to be the most suitable of these three stabilizers, both methyland silanol-terminated PDMS were finally investigated together with HT-treated kaolin to establish the effect of the end groups of PDMS.
Experimental Section a. Materials. Kaolin-grade “Polsperse 10” was obtained from Imerys Minerals Ltd.; its SEM image and the resulting size distribution can be found in ref 5. The surface modification with isobutyltrimethoxysilane (IBTMS) and SAP230 has been described previously5 and the modified particles are labeled IBTMS-kaolin and SAP-kaolin, respectively. The clay surface was also modified with 1.0 wt % hydrogenated tallow amine (HT) using a dry mixing route and this product was labeled HT-kaolin. Table 1 gives an overview of materials and abbreviations used in the text. HT was obtained from Akzo Nobel Chemicals (trade name ARMEEN HT); the hydrogenated tallow moieties consist of alkyl chains with around 65% C18, 30% C16, and 5% C14. Little amine was found to be washed off by Soxhlet extraction with toluene as the solvent, so the treated clay was used as-received. With use of the BET area of kaolin (13.3 m2/g) and an average molecular weight of 256.4 g/mL, the surface coverage of HT was found to be 2.9 µmol/m2, in agreement with the plateau adsorption reported in ref 5. This corresponds to an area per HT molecule of 0.57 nm2. Comparing this value with that obtained at high coverage on mica of 0.23 nm2, it shows that there is indeed space left at the kaolin surface for more molecules to adsorb to.16 Table 2 lists the properties of PDMS samples used: the molecular weights were obtained by GPC with polystyrene as the standards; the viscosity was measured by a controlled stress Bohlin CVO instrument; the radius of gyration (Rg) was calculated based on a freely rotating chain model with C∞ equal to 5.28 and the Si-O bond length equal to 1.64 Å.15 Methyl-terminated PDMS (referred (13) Cosgrove, T.; Turner, M. J.; Thomas, D. R. Polymer 1997, 38, 3885. (14) CRC Handbook of Chemistry and Physics, 89th Ed.; Lide, D. R., Ed.; CRC Press, Boca Raton, FL, 2008. (15) Mark, J. E., Ed. Polymer data handbook; Oxford University Press: Oxford, 1999. (16) Lee, Y.-L. Langmuir 1999, 15, 1796.
Langmuir, Vol. 24, No. 20, 2008 12033 to as Me-PDMS) were obtained from Dow Corning (DC 200 fluid series). Silanol-terminated PDMS (referred to as OH-PDMS) were obtained from Fluorochem with the functionality content (the content of silanol groups) of 0.8-0.9 and 0.09 wt %, respectively. Toluene was obtained from Fisher Scientific. b. Sample Preparation. 1. Preparation at EleVated Temperature. IBTMS-kaolin or HT-kaolin were dispersed in toluene to form a 2% v/v suspension and ultrasound was applied to the suspension for about 5 min until the particles were completely wetted by toluene. Then a 50 wt % OH-PDMS or Me-PDMS solution in toluene was added to this suspension under stirring. The mixtures were stirred, heated in an oil bath, and refluxed overnight. Finally, the majority of toluene was distilled off. While the mixture was still hot, it was transferred into a smaller bottle and allowed to cool down. The residual toluene was evaporated off in a vacuum oven until the weight of the sample reached a constant value and no toluene odor could be detected. As a control sample, the bare kaolin without any surface treatment underwent the same procedure. 2. Preparation at Room Temperature. SAP-kaolin and HT-kaolin were dispersed in toluene and mixed with PDMS as above. Toluene was allowed to evaporate off at room temperature in a fume cupboard with good air flow for about 8-12 h until the weight of the mixture reached a constant value and no toluene odor could be detected. NMR relaxation measurements showed no slowly decaying component due to toluene. Magnetic stirring was applied during the process to avoid settling down of particles. The composites were then tumbled for at least 48 h to reach equilibrium and mixed by a whirl mixer before characterization. Care was taken to make the samples experience the same shear history. The mixtures were then investigated by microscopy, rheology, and relaxation NMR. The same procedure was applied to obtain composites of HT-kaolin in Me-PDMS and OH-PDMS. c. Characterization Methods. 1. Polarizing Light Microscopy (PLM). A 1 cm diameter hole was drilled through one microscope slide before the slide was glued to a second intact slide. The glue was applied around the hole and both slides were then aligned and pressed firmly together. Care was taken to make sure the surrounding area of the hole was sealed completely to avoid sample leaking. The sample containing 1% v/v kaolin was then dropped into the hole and left to rest for about 10 min to allow for any shear-induced structure to dissipate. PLM observation was carried out on a Nikon Optiphot-2 polarizing microscope. Pictures were obtained digitally through a JVC TM I500PS camera and Mitsubishi CP color video copy processor, linked to a personal computer. The clay aggregates were not visible under crossed polarizers, which implies that the particles did not form aligned (nematic) domains. However, a useful contrast was obtained by slightly uncrossing the polarizers. 2. Rheology. All rheology experiments were carried out at 25 °C using a controlled stress Bohlin CVO instrument. The cone and plate geometry (CP 4/40) was used. The samples were allowed to rest for up to 60 min after being loaded onto the plate depending on the concentration of the particles. In viscometry measurements, the shear stress was varied from 0.07 to 1000 Pa to achieve a shear rate up to 1000 s-1. A delay time of 20 s and an integration time of 60 s were allowed at each shear stress value. After the delay time, most samples reached a steadystate condition, with the exception of some samples under low shear. Increasing stress series were applied. The relative shear viscosity ηr is calculated according to
ηr )
η ηs
(1)
where η is the apparent shear viscosity of the composites and ηs is the viscosity of the suspending medium. The Bingham model,5,17 σ ) σby + ηplγ˙ , was used to fit the data at high shear rates, giving the Bingham yield stress and plastic (17) Larson, R. G. The structure and rheology of complex fluids; Oxford University Press: New York, 1999.
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Zhang et al. Table 2. Properties of PDMS Samples
samples
viscosity (mPa s)
Mw (g/mol)
Mn (g/mol)
polydispersity index
Rg (nm)
7k OH-PDMS 30k OH-PDMS
99 770
7240 29900
3580 12000
2.1 2.5
2.5 4.5
9k Me-PDMS 34k Me-PDMS
97 920
9460 33700
4860 11500
1.9 2.9
2.9 4.4
viscosity. The Bingham yield stress was used to evaluate the extent of aggregation. In oscillation experiments, an amplitude sweep with the shear stress starting at 0.05 Pa was carried out at a frequency of 1 Hz to find the linear region, and then a frequency sweep between 1 and 100 Hz was applied at a fixed stress in the linear region. The storage and loss moduli were used to evaluate the dispersion state of the samples. Each sample was measured three times at the same stress and the data points agreed within experimental error. This confirms the samples had reached equilibrium before the measurements started. 3. Proton TransVerse Relaxation NMR. Proton transverse relaxation NMR was carried out on a Bruker MSL 300 MHz NMR spectrometer. The CPMG18 pulse sequence was applied; the 180° pulse spacing was 500 µs and the recycle delay was 5 s. Two thousand and forty-eight data points were collected for each decay and 32 decays were collected. The CPMG sequence removes the effect of magnetic field inhomogeneity and chemical shift (i.e., information of protons with different electronic environments) at the same time. All mobile protons in the system under study should be detected. However, if the protons are in the solid state material and they relax faster by strong dipole-dipole coupling, they cannot be refocused by the 180° pulse, and as a result, no signal is observable. Indeed, no signal was detected for pure kaolin with the experimental parameters chosen above. The observed decays should arise from mobile PDMS segments only. In theory, the polymer in a filler-polymer system should be composed of three parts: immobile, disturbed, and free. According to earlier work on silica and PDMS composites,19 the magnetization intensity was found to be significantly lower than expected if all PDMS were detected, taking into account the volume occupied by the silica. This could be ascribed to the lost magnetization of the immobile chains in the experiments. Hence, only the disturbed and free parts are recorded. A similar decrease in overall intensity was not found for the kaolin composites studied here. This can be understood as the specific surface area of the kaolin is low compared to the particles studied in that work. A bound layer of PDMS of 0.5 mg/m2 (as derived previously)19 would correspond to well below 1% of the total PDMS present, even at the highest kaolin volume fraction of 20% studied here. Models to simulate the transverse relaxation decay in these multiphase systems are still under debate.19-22 The Brereton model23,24 can be used to analyze NMR relaxation in polymer melts. For entangled systems at high molecular weight deviations from single exponential decay arise. The inclusion of clay particles, studied here, introduces further decay modes. Whereas the pure polymer data could be fitted by a sum of two exponentials, three or even four were needed to achieve a satisfactory description for the clay-polymer composites. We are not aware of a theoretical approach that encompasses the complicated dynamics in such a system. Instead, the decay is characterized below by t1/e, the time taken for the measured magnetization to reach 1/e of its initial intensity. Because a higher relaxation rate corresponds to a lower relaxation time and (18) Farrar, T. C.; Becker, E. D. Pulse and Fourier transform NMR; Academic Press: London, 1971. (19) Cosgrove, T.; Roberts, C.; Garasanin, T.; Schmidt, R. G.; Gordon, G. V. Langmuir 2002, 18, 10080. (20) Cosgrove, T.; Turner, M. J.; Griffiths, P. C.; Hollingshurst, J.; Shenton, M. J.; Semlyen, J. A. Polymer 1996, 37, 1535. (21) Prunelet, A.; Fleury, M.; Cohen-Addad, J.-P. C. R. Chim. 2004, 7, 283. (22) Ghose, S.; Isayev, A. I.; von Meerwall, E. Polymer 2004, 45, 3709. (23) Brereton, M. G.; Ward, I. M.; Boden, N.; Wright, P. Macromolecules 1991, 24, 2068. (24) Ries, M. E.; Brereton, M. G.; Ward, I. M.; Cail, J. I.; Stepto, R. F. T. Macromolecules 2002, 35, 5665.
a less mobile chain, the chain mobility of the polymers can be compared qualitatively based on these times.
Results a. Comparison of IBTMS- and HT-Treated Kaolin. Aluminosilicates can catalyze chemical reactions due to the strong acid sites on their surface caused by substitution of Si atoms by Al atoms.25 In earlier work, it was found that the surface of kaolin particles is composed of a layer of aluminosilicate.5 A Hammett indicator (dimethyl yellow)5,25 showed HT reacted with the acid sites, but IBTMS did not, so the property of kaolin as a catalyst remained after its surface was modified by IBTMS.5 Figure 1 shows the composites containing 10% v/v particles prepared at elevated temperatures. Sample (a) is made from 7k OH-PDMS and IBTMS-kaolin; sample (b) is from 7k OH-PDMS and HT-kaolin; and sample (c) is from 9k Me-PDMS and IBTMS-kaolin. Sample (a) was a rubber-like material, but (b) and (c) still flowed. Both samples (a) and (c) were made using IBTMS-treated particles, so the difference of the resulting composites is ascribed to the different end groups of the two polymers since their backbone structures are the same. Hence, the change in sample (a) is probably due to further polymerization of the OH-PDMS catalyzed by the kaolin particles, and possibly also cross-linking with these particles. Another control experiment was done dispersing untreated kaolin into 7k OH-PDMS; this resulted in a rubber-like composite as well. This further demonstrates the catalytic effect of the strong acid groups on the reaction of the silanol groups of OH-PDMS. As a result, IBTMS was not investigated further in the following work. As HT-kaolin cannot catalyze these reactions, the resulted polymer composite was a fluid. b. Comparison of SAP- and HT-Treated Kaolin in OH-PDMS. 1. Polarizing Microscope Photos. The kaolin particles have a diameter around half a micrometer,5 so they might not be seen under a light microscope if the particles were suspended individually. Particle aggregates, however, could be observed. Figure 2 shows the polarizing microscope photographs of 1.0% v/v HT-treated (left) and SAP-treated (right) kaolin in 7k OH-PDMS. The SAP-kaolin sample aggregated strongly in this OH-PDMS whereas the same surface treatment provided a good stabilization in solvents such as heptane.5 SAP did not mix well with 7k OH-PDMS and phase separation was observed in their mixture. It is likely that the adsorbed polyisobutene chain of SAP was incompatible with OH-PDMS, resulting in aggregation. In contrast, the dispersion with HT-kaolin looked more uniform, although the same particles flocculated strongly in heptane. The reason for this will be discussed further below. 2. Rheology. Figure 3a shows the relative shear viscosity against shear rate for HT- and SAP-treated kaolin in 7k OH-PDMS with different particle concentrations. At a given clay concentration, the shear viscosity of the composites with SAP-kaolin is consistently much higher than that with HT-kaolin, reflecting stronger particle interactions in the SAP-kaolin samples. (25) Solomon, D. H.; Hawthorne, D. G. Chemistry of pigments and fillers; Krieger Publishing Company: Malabar, FL, 1991.
Composites of Kaolin and Polydimethylsiloxane
Langmuir, Vol. 24, No. 20, 2008 12035
Figure 1. Digital photograph of the mixtures containing 10% v/v kaolin particles prepared at elevated temperature: (a) 7k OH-PDMS and IBTMS-treated kaolin; (b) 7k OH-PDMS and HT-treated kaolin; (c) 9k Me-PDMS and IBTMS-treated kaolin.
Figure 2. Microscope photographs of 1.0% v/v HT-treated (left) and SAP-treated (right) kaolin in 7k OH-PDMS.
Figure 3b shows the same series of data plotted as shear stress against shear rate. The Bingham model5 was used to fit the data at high shear rates and it gave a Bingham yield stress which can be used to compare the extent of aggregation. Figure 4 shows the log-log plot of the derived Bingham yield stress as a function of the volume fraction of kaolin. The Bingham yield stress showed a power law dependence on the volume fraction of kaolin, with an exponent of 1.6 and 1.7 for HT and SAP, respectively. These values are somewhat lower than the theoretical value of 2 obtained theoretically for the yield stress associated with breakup of pairs of spherical particles.17 The SAP samples show a higher yield stress than the HT ones at all kaolin concentrations, in agreement with the larger extent of aggregation in the SAP samples seen in the microscope images. At volume fractions of kaolin higher than 10%, viscometry experiments became difficult because the composites were very viscous. However, the storage and loss moduli of a composite can reveal the extent of aggregation in a filler-polymer composite as well: the higher the modulus, the more severe the particle aggregation.26 Hence, small amplitude oscillatory rheology was carried out to characterize these concentrated composites. Figure 5 shows the storage modulus (G′) and loss modulus (G′′) against frequency for HT- and SAP-treated kaolin dispersed in the same OH-PDMS. As the kaolin content is increased, the composite has a higher modulus for the same surface treatment as expected. Furthermore, the modulus of the SAP samples is about an order of magnitude higher than that of the HT ones at the same kaolin content. This is ascribed to stronger particle interactions in the SAP samples and agrees with the results obtained from rheology (a higher viscosity as shown in Figure 3). (26) Cassagnau, P. Polymer 2008, 49, 2183.
Figure 3. Viscometry of HT- and SAP-treated kaolin in 7k OH-PDMS as a function of particle concentration: (a) Relative viscosity against shear rate; (b) shear stress against shear rate. The solid lines are Bingham fits to the data at higher shear rates: (b) 1.0% HT; (2) 5.0% HT; (9) 10.0% HT; (O) 1.0% SAP; (4) 5.0% SAP; (0) 10.0% SAP.
Figure 4. Bingham yield stress against volume fraction for HT- and SAP-treated kaolin in 7k OH-PDMS.
For both SAP- and HT-treated kaolin at 20%, both moduli show power law behavior with an exponent around 0.5 across the frequency range studied. This behavior was also observed in the celebrated work of Winter and Chambon on cross-linking in PDMS melts27 and shows these samples are at the gel point. (27) Winter, H. H.; Chambon, F. J. Rheol. 1986, 30, 367.
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Zhang et al.
Figure 7. Comparison of magnetization decays for pure 7k OH-PDMS and two pairs of its mixtures with 5.0% v/v and 15.0% v/v HT- and SAP-treated kaolin.
Figure 5. (a) Storage modulus (G′); (b) loss modulus (G′′) against frequency for HT- and SAP-treated kaolin in 7k OH-PDMS with different concentrations of kaolin. Figure 8. Time for the measured magnetization to reach 1/e of its initial intensity as a function of particle concentration for composites of HTand SAP-treated kaolin and 7k OH-PDMS.
Figure 6. Normalized intensity of magnetization against time for HTtreated kaolin in 7k OH-PDMS for a series of particle concentrations (from right to left): 0.0%, 1.0%, 5.0%, 10.0%, 15.0%, and 20.0%.
The lower concentration samples at 10% show the moduli bending down at low frequency, as expected for samples that should still be considered to be fluid. 3. Proton Relaxation NMR. Figure 6 shows the normalized CPMG intensity as a function of time for HT-kaolin samples at different particle concentrations. It can be seen that the magnetization decayed faster with the addition of kaolin, which shows the chain mobility of PDMS was lowered as expected.
The intensity decays of SAP-kaolin samples are similar to those for HT-kaolin samples and can be found in the Supporting Information. Figure 7 compares two pairs of HT-kaolin and SAP-kaolin samples with the same particle concentrations (note the time axis is linear in Figure 6 and logarithmic in Figure 7 to improve clarity). It can be seen that, at the same kaolin content, the decays for HT samples are slightly faster than those for the SAP ones. Figure 8 gives the time of t1/e and the values for the SAP samples are slightly longer than that of the HT ones. This means the PDMS chains in the SAP samples are slightly more mobile than those in the HT ones. The reason will be discussed further below. It is also worth noticing that although the OH-PDMS chains were somewhat more mobile for SAP samples than for HT ones, the viscosity of the SAP samples was much higher than that of the HT ones. NMR probes the motion of the PDMS chain in the space between clay particles. An increase of clay content builds up more restriction for a polymer molecule to move, and this restriction translates into an increased decay rate. In rheology, however, both polymers and particles contribute to the overall viscosity and the particle-particle interaction might dominate in an aggregated system,26 which then has a much higher viscosity. The same particles were investigated in Me-PDMS and similar results were found. All these data showed that the HT treatment
Composites of Kaolin and Polydimethylsiloxane
Figure 9. Microscope photographs of 1.0% v/v HT-kaolin in (a) 9k Me-PDMS, (b) 7k OH-PDMS, (c) 34k Me-PDMS, and (d) 30k OH-PDMS.
resulted in the most homogeneous dispersion. The reason why HT-kaolin aggregated in a solvent due to its low surface coverage and formed homogeneous dispersions in PDMS is discussed further below. c. Comparison of HT-Kaolin in Methyl- and SilanolTerminated PDMS. 1. Microscope Pictures. Figure 9 shows microscope images of 1.0% v/v HT-kaolin in Me-PDMS and OH-PDMS. As can be seen, the dispersions were more homogeneous than those of SAP-kaolin, although the composites with methyl-terminated PDMS (Figure 9a,c) looked less uniform than those with silanol-terminated PDMS (Figure 9b,d). 2. Rheology. Figure 10 shows the relative shear viscosity against shear rate data for the four series of samples. All of the samples showed shear thinning behavior. This can be caused by either the alignment of kaolin plates along the shear direction, if they are well-stabilized, or breakup of aggregates if they are flocculated. For suspensions of platelike particles of aspect ratio p f 0, the ratio of high shear to low shear viscosity tends to 4.6 p.17 As discussed below, the particles studied here have an aspect ratio p ≈ 0.1, so shear thinning by a factor of 2 could be expected for well-stabilized platelets. It is difficult to measure the shear viscosity at low shear rate for these samples, but the samples in OH-PDMS show shear thinning of this order. However, a much more pronounced effect is seen for the samples in Me-PDMS. This indicates aggregation is occurring in these composites. The Bingham model was used to fit the data at high shear rates to evaluate the aggregation (see Figure 11) and the original shear stress against shear rate data are given in the Supporting Information. The Bingham yield stress values for the 7k OH-PDMS series are nearly zero at all concentrations investigated, which indicates that the particles were well-stabilized in these suspensions. The series of 30k OH-PDMS showed a small yield stress. In contrast, for the Me-PDMS samples, a significant yield stress developed on increasing the kaolin content. For the 34k Me-PDMS, this yield stress is already noticeable at a clay volume fraction of only 0.005. The rheology is very sensitive to the extent of aggregation that was observed in the micrographs discussed above (Figure 9). In the present work, OH-PDMS could be acting as a stabilizer due to the favorable interaction between the end silanol group and hydroxyl groups on the kaolin surface. This adsorption might
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result in extended brushes creating steric repulsion and reducing particle aggregation. This interpretation is supported by the results of AFM experiments on the interaction between end-functionalized polyisoprene and the surface of oxidized silicon. A strong interaction was found with hydroxyl-terminated polymer and a weak interaction was found with methyl-terminated chains.28 3. Proton Relaxation NMR. Figure 12 shows the normalized CPMG intensity against time for the composites with 9k Me-PDMS at different particle concentrations. The other decays for 7k OH-PDMS, 30k OH-PDMS, and 34k Me-PDMS are more or less similar to this decay and they are given in the Supporting Information. In all cases, the magnetization decayed faster with the addition of kaolin as expected. Figure 13 shows the t1/e as a function of kaolin content. It became shorter when the concentrations of the particles were increased and this means the chain mobility of PDMS was lowered. For the low Mw polymers (Figure 13a), t1/e is slightly less for OH-PDMS than for Me-PDMS at the same kaolin content. However, this appears to be due to a difference in the pure polymerssthe trend on increasing kaolin content is very similar. For the high Mw samples (Figure 13b), t1/e is very similar. Altogether, the NMR experiments do not show evidence that the impact of HT-kaolin on OH-PDMS is different from that on Me-PDMS. In rheology experiments, HT-kaolin was well-stabilized in OH-PDMS even though the same treated particles aggregated strongly in a solvent having a refractive index very close to the particles, which means HT does not provide enough steric stabilization to overcome the van der Waals attraction. Both OH-PDMS and Me-PDMS were found to adsorb onto the silica surface and OH-PDMS adsorbed more by taking a stretched conformation.29 Since kaolin has a surface chemistry similar to that of silica,5 the same result can be expected. The rheology data also showed the stabilization occurred in OH-PDMS and not in Me-PDMS, so it is deduced that the adsorbed layer of OH-PDMS on the kaolin surface is thicker than that of Me-PDMS. Hence, the conformation of adsorbed OH-PDMS chains may be a stretched brush caused by the preferential adsorption of the end silanol groups, which gives a thicker layer; the adsorbed Me-PDMS takes a train-loop-tail conformation, which gives a thinner layer as illustrated in Figure 14a,b. The reason why NMR did not show this difference is because the constrained segments are a small proportion of the total. The more stretched chains provide sufficient steric repulsion to make the kaolin particles better dispersed in OH-PDMS than in Me-PDMS as shown in Figure 14c,d.
Discussion and Conclusion Treatment with the silane (IBTMS) did not cover the acid sites on the kaolin surface and the strong acid groups on the kaolin surface could catalyze polymerization of OH-PDMS and possibly enable cross-linking. The OH-PDMS composite with IBTMS-kaolin was therefore a highly viscous, rubber-like material. Although SAP stabilized kaolin very well in nonpolar solvents, this approach did not work in PDMS. This is shown by microscope observations and a nonzero Bingham yield stress. When the clay concentration is higher than a critical value, the HT samples show a Bingham yield stress as well. In oscillatory rheology, the storage modulus of the SAP samples is much higher than that (28) Wang, J.; Stark, R.; Kappl, M.; Butt, H.-J. Macromolecules 2007, 40, 2520. (29) Patel, A.; Cosgrove, T.; Semlyen, J. A.; Webster, J. R. P.; Scheutjens, J. M. H. M. Colloid Surf. A-Physicochem. Eng. Asp. 1994, 87, 15.
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Figure 10. Relative viscosity against shear rates in different PDMS samples for a series of HT-kaolin content: (a) 7k OH-PDMS (b, 0.6%; O, 1.1%; 1, 2.0%; 4, 3.0%; 9, 3.9%); (b) 9k Me-PDMS (b, 0.5%; O, 1.0%; 1, 2.0%; 4, 3.0%; 9, 4.1%); (c) 30k OH-PDMS (b, 0.6%; O, 0.9%; 1, 2.0%; 4, 3.0%; 9, 3.7%); (d) 34k Me-PDMS (b, 0.5%; O, 0.9%; 1, 1.8%; 4, 2.8%; 9, 3.9%).
Figure 11. Bingham yield stress of the composites as a function of the volume fraction of HT-kaolin.
of the HT ones whereas their degrees of dispersion were less. The mobility of OH-PDMS chains is lowered with the addition of kaolin particles, in particular for the HT-kaolin samples in which the clay particles are dispersed most homogeneously. A treatment with HT only is not enough to stabilize kaolin in PDMS. Microscopy observation and rheology show the particles dispersed fairly homogeneously in OH-PDMS, but they aggregated in Me-PDMS. This is because the end silanol groups in OH-PDMS interacted more strongly with the kaolin surface,
Figure 12. Normalized intensity of magnetization against time for HTtreated kaolin in 9k Me-PDMS for a series of particle concentrations (from top to bottom: 0.0%; 0.5%; 1.0%; 2.0%; 3.0%; 4.1%).
and thus helped to stabilize the particles by taking up a stretched brush conformation. Neutron scattering experiments could be used to characterize such behavior quantitatively. Whereas the aggregation state of HT-kaolin is very different in the OH and Me-PDMS samples shown in rheology, the increase of NMR relaxation rate on introducing kaolin is very (30) Woodward, A.; Cosgrove, T.; Espidel, J.; Jenkins, P.; Shaw, N. Soft Matter 2007, 3, 627.
Composites of Kaolin and Polydimethylsiloxane
Figure 13. Time to reach 1/e of the initial intensity (t1/e) as a function of particle volume fraction: (a) 7k OH-PDMS and 9k Me-PDMS; (b) 30k OH-PDMS and 34k Me-PDMS.
Figure 14. Illustration of the effect of Me-PDMS and OH-PDMS on dispersing HT-kaolin particles: (a) conformation of adsorbed Me-PDMS on the HT-kaolin surface; (b) conformation of adsorbed OH-PDMS on the HT-kaolin surface; (c) HT-kaolin in Me-PDMS melts; (d) HT-kaolin in OH-PDMS melts.
similar for both series of samples. It appears therefore that the NMR experiments mainly probe the confining effect of the kaolin, irrespective of the aggregation state of the particles. To gauge the extent to which motion of the bulk PDMS is influenced by the presence of the clay platelets, it is instructive to calculate the typical distance a PDMS molecule diffuses during the NMR experiment according to this equation: 〈l〉 ) (6D∆)1/2, where D is the self-diffusion coefficient and ∆ is the diffusion time.30
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Taking D ≈ 0.81 × 10-11 m2 s-1 for the low Mw polymer used here31 and equating the lag time ∆ to the 1/e relaxation time of 0.4 s, we obtain 〈l〉 ∼ 4.4 µm. Since the clay platelets have an average diameter d ) 630 nm and their BET surface area Asp ) 13.3 m2 g-1, their thickness can be estimated as L ) 2/AspF ) 58 nm, where F ) 2.6 g cm-3 is the density of kaolin,5 so the volume of a single platelet is V ≈ d2L ) 2.3 × 10-20 m3. In PDMS suspensions with a clay volume fraction φ g 0.01, assuming a homogeneous dispersion, the sample volume per platelet was at most V/0.01 ) 2.3 × 10-18 m3, equivalent to a sphere with radius a ≈ 0.8 µm. Even at this lowest concentration therefore, this size was less than the diffusive length scale 〈l〉 estimated above. This suggests all the PDMS experienced restricted diffusion in the presence of the clay platelets, which is reflected in a lowering of the NMR relaxation time compared to the pure polymer. The standard routes for stabilizing particles in dispersions are to use either low molecular weight stabilizers (e.g., stearyl coating of silica)32 or high molecular weight (polymeric) stabilizers, with a suitable end functionality (anchor group) to ensure a high surface coverage. In many cases, in particular with large particles, a low molecular weight stabilizer is not sufficient to counteract van der Waals interactions, and polymeric stabilizers offer the best result. For the kaolin particles used in the present work, a polymer stabilizer afforded good dispersions in a solvent (toluene).5 When dispersing particles in a polymer melt, polymeric stabilizers can still be used but it is vital to ensure the stabilizing polymer layer mixes well with (is wetted by) the melt surrounding it. In general this can be achieved by using the same polymer chemistry for the stabilizing layer and the melt. Even so, wetting is only obtained under suitable conditions of surface coverage and molecular weights.8-10 Note that, with a strongly adsorbing polymer, small particles can even be stabilized in a melt without an additional stabilization layer.11 The present work suggests an intermediate approach toward stabilization in melts for particles which have active functional groups such as strong acidic hydroxyl groups: a low molecular weight stabilizer is used to coat the particles, and this not only removes the effect of the active functional groups but also renders the hydrophilic surface hydrophobic; then a melt of end-functionalized polymer is used that is able to provide additional stabilization by adsorbing to the particles. Acknowledgment. We gratefully acknowledge assistance and advice by Dr. C. Flynn (rheology) and Dr. Y. Espidel (NMR). We thank Dr. J. Phipps for a critical reading of manuscript and one of the anonymous reviewers for suggestions regarding presentation of the rheological data. YZ thanks EPSRC and Imerys for funding through a Dorothy Hodgkin Postgraduate Award. Supporting Information Available: Further results from rheological and NMR measurements are given in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. LA8018259 (31) Roberts, C.; Cosgrove, T.; Schmidt, R. G.; Gordon, G. V. Macromolecules 2001, 34, 538. (32) Van Helden, A. K.; Jansen, J. W.; Vrij, A. J. Colloid Interface Sci. 1981, 81, 354.
