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Temperature-Triggered Gelation of Aqueous Laponite Dispersions Containing a Cationic Poly(N-isopropyl acrylamide) Graft Copolymer R. Liu,†,‡ N. Tirelli,*,§ F. Cellesi,§ and B. R. Saunders*,† Polymer Science and Technology Group, The School of Materials, The UniVersity of Manchester, GrosVenor Street, M1 7HS, U.K., School of Material and Chemical Engineering, Zhengzhou UniVersity of Light Industry, Zhengzhou, 450002, P.R. China, and Laboratory of Polymers and Biomaterials, School of Pharmacy, The UniVersity of Manchester, Oxford Road, M13 9PT, U.K. ReceiVed September 7, 2008. ReVised Manuscript ReceiVed NoVember 1, 2008 In this work, temperature-triggered gelation of aqueous laponite dispersions containing a cationic poly(Nisopropylacrylamide) (PNIPAm) graft copolymer was investigated. The copolymer used was PDMA+30-g-(PNIPAm210)14 [Liu et al. Langmuir 2008, 24, 7099]. DMA+ is quarternarized N,N-dimethylaminoethyl methacrylate. The presence of small concentrations of laponite enabled temperature-triggered gel formation to occur at low copolymer concentrations (e.g., 1 wt %). Dynamic rheological measurements of the gels showed that they had storage modulus values of up to 400 Pa when the total solid volume fraction (polymer and laponite) was only about 0.02. The storage modulus was dependent on both the temperature and the composition of the dispersion used for preparation. The key component that provided the temperature-triggered gels with their elasticity was found to be self-assembled nanocomposite (NC) sheets. These NC sheets spontaneously formed at room temperature upon addition of laponite to the copolymer solution. The NC sheets had lateral dimensions on the order of hundreds of micrometers and a thickness of a few micrometers. The NC sheets were present within the temperature-triggered gels and formed elastically effective chains. The NC sheets exhibited temperature-triggered contraction with a contraction onset temperature of 27 °C. A conceptual model is proposed to qualitatively explain the relationship between gel elasticity and dispersion composition.
Introduction Copolymers that reversibly form macromicelles continue to be an interesting and active area of research.1-5 In previous work from our group, we investigated the temperature-responsive solution behavior of a cationic graft copolymer,3 PDMA+x-g-(PNIPAmn)y, where DMA+ is quarternarized N,N-dimethylaminoethyl methacrylate, and NIPAm is N-isopropylacrylamide. The dilute copolymer solutions exhibited temperature-triggered association to give nanoparticles, while temperature-triggered gelation occurred at higher concentrations.3 There has been considerable interest in the literature concerning nanocomposite (NC) hydrogels prepared using freeradical polymerization of NIPAm6-10 in the presence of laponite. However, there are no examples to our knowledge involving (a) temperature-triggered formation of gels using laponite (or other anisotropic clay) dispersions or (b) self-assembly of laponite platelets with preformed temperature-responsive polymers. The present study focuses on laponite dispersions containing PDMA+30-g(PNIPAm210)14. This copolymer was selected here because it has been well studied, and its solution behavior is understood.3 A central point of this study is the role that added laponite plays in temperaturetriggered gel formation. We also report, for the first time, that self* Corresponding author. † The School of Materials, The University of Manchester. ‡ Zhengzhou University of Light Industry. § School of Pharmacy, The University of Manchester. (1) Alexander, C.; Renuka, P.; Sivanand, S.; King, S.; Heenan, R. K. Biomacromolecules 2008, 9, 1170. (2) Liu, B.; Perrier, S J. Polym. Sci. A, Polym. Chem. 2005, 43, 3643. (3) Liu, R.; De Leonardis, P.; Cellesi, F.; Tirelli, N.; Saunders, B. R. Langmuir 2008, 24, 7099. (4) Webber, G. B.; Wanless, E. J.; Buetuen, V.; Armes, S. P.; Biggs, S. Nano Lett. 2002, 2, 1307. (5) Yu, L.; Ding, J. Chem. Soc. ReV. 2008, 37, 1473. (6) Ferse, B.; Richter, S.; Arndt, K.-F.; Richter, A. Macromol. Symp. 2007, 254, 378. (7) Haraguchi, K.; Takehisa, T. AdV. Mater. 2002, 14, 1120. (8) Miyazaki, S.; Endo, H.; Karino, T.; Haraguchi, K.; Shibayama, M. Macromolecules 2007, 40, 4287. (9) Abdurrahmanoglu, S.; Can, V.; Okay, O. J. Appl. Polym. Sci. 2008, 109, 3714. (10) Murata, K.; Haraguchi, K. J. Mater. Chem. 2007, 17, 3385.
assembled NC sheets form at room temperature, and investigate their temperature-triggered structural changes. Laponite is a synthetic smectite clay that forms structured gels in concentrated dispersions. The platelets have an average diameter and thickness of 20.4 and 1.2 nm, respectively.11 The particles have a negative face charge. They also have a positive or negative edge charge at low or high pH values, respectively. Laponite dispersions can form thixotropic gels at particle concentrations of ca. 1 wt %. Depending on the pH and ionic strength, these can occur by attractive (house of cards) or repulsive interactions.12 In the present work, PDMA+30-g-(PNIPAm210)14 was mixed with the laponite dispersions. The copolymer obstructs the normal gel formation mechanism for laponite and confers temperature-responsive behavior to the dispersions. The interactions between solution polymer and laponite dispersions have also attracted attention. The Cosgrove group has shown clearly that poly(ethylene oxide) (PEO) adsorbs strongly to laponite particles using small-angle neutron scattering.13 The rheological and microstructure of concentrated PEO/laponite dispersions have also been investigated.14,15 Gel formation of laponite dispersions at room temperature in the presence of sodium poly(acrylate) has also been reported.16 In that work, much higher laponite concentrations were required to form gels at room temperature compared to those required for triggered gel formation in the present work. It is well established that NIPAm can be polymerized in the presence of laponite to form NC hydrogels with good mechanical properties.7,8,17 The laponite particles appear to act as cross-linking points and provide additional strength to the hydrogels. The approach (11) Balnois, E.; Durand-Vidal, S.; Levitz, P. Langmuir 2003, 19, 6633. (12) Saunders, J. M.; Goodwin, J. W.; Richardson, R. M.; Vincent, B. J. Phys. Chem. B. 1999, 103, 9211. (13) Nelson, A.; Cosgrove, T. Langmuir 2005, 21, 9176. (14) Loizou, E.; Butler, P.; Porcar, L.; Kesselman, E.; Talmon, Y.; Dundigalla, A.; Schmidt, G. Macromolecules 2005, 38, 2047. (15) Mogondry, P.; Nicolai, T.; Tassin, J.-F. J. Colloid Interface Sci. 2004, 275, 191. (16) Labanda, J.; Llorens, J. J. Colloid Interface Sci. 2005, 289, 86. (17) Liu, Y.; Zhu, M.; Liu, X.; Jiang, Y. M.; Ma, Y.; Qin, Z. Y.; Kuckling, D.; Adler, H.-J. P. Macromol. Symp. 2007, 254, 353.
10.1021/la802941h CCC: $40.75 2009 American Chemical Society Published on Web 12/08/2008
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Scheme 1. Route to Preparation of MI2-PNIPAm20k and Formation of Temperature-Responsive Laponite/MI2-PNIPAm20k Dispersionsa
a
The laponite particle edges have a pH-dependent charge and are negative at high pH values and positive at low pH values.
has been extended to prepare NIPAm-based microgel particles.18 In the present work, we prepare reVersible NC hydrogels simply by heating mixtures of laponite and PDMA+30-g-(PNIPAm210)14. The abbreviation of3 MI2-PNIPAm20k is used for this copolymer from this point onward. The number “2” signifies that there are approximately two positive charges per PNIPAm side chain.3 The number-average molar mass for each PNIPAm side chain3 is ca. 20 kg mol-1. Associating copolymer solutions are capable of forming gels when the copolymer concentration exceeds a critical value (Cp*). Transient network theory19,20 was developed a number of years ago19 to describe gel structure-property relationships for these systems. The shear modulus of the network is an increasing function of the number density of junctions and also temperature. These ideas have been applied to the temperature-triggered formation of PNIPAm gels.21 Below a critical temperature (association temperature), the copolymer solution behaves as a fluid. At temperatures greater than this temperature, junctions form and gelation occurs. This mechanism has also been successfully used to explain temperature-triggered gelation of emulsions and latexes.22,23 A defining feature of dispersions that exhibit temperature-triggered gelation22,24,25 is that the Cp* value is significantly smaller than that for the parent polymer solution. This is due to attractive interactions between the polymer and the dispersed phase. In the mixed laponite/ copolymer systems considered in this work, it will be shown that a decrease of Cp* to low levels occurs, which is attributed to attraction between the copolymer chains and dispersed laponite particles.
Experimental Details Materials. The synthesis and characterization of the macroinitiator (MI2, Scheme 1) and the PNIPAm copolymer used in this work were described fully earlier.3 Briefly, MI2 is a statistical copolymer,26 prepared by ATRP (atom transfer radical polymerization), and contains an average of two positive charges per isobutyrate side chain.3 Its number average molar mass (Mn) was 6,480 g/mol and the polydipsersity was 1.4. MI2-PNIPAm20k was prepared by aqueous ATRP. Based on GPC and1H analysis the copolymer had an average composition3 of PDMA+30-g-(PNIPAm210)14 and an estimated Mn value of 348,000 g/mol. Laponite RD was a gift from (18) Zhang, Q.; Tang, Y.; Zha, L.; Ma, J.; Liang, B Eur. Polym. J. 2008, 44, 1358. (19) Green, M. S.; Tobolsky, A. V. J. Phys. Chem. 1946, 14, 80. (20) Indei, T. J. Chem. Phys. 2007, 127, 144904. (21) Durand, A.; Hourdet, D. Polymer 2000, 41, 545. (22) Alava, C.; Saunders, B. R. Langmuir 2004, 20, 3107. (23) Alava, C.; Saunders, B. R. J. Colloid Interface Sci. 2006, 293, 93. (24) Koh, A. Ph.D. Thesis, Adelaide University, Australia, 2002. (25) Koh, A.; Heenan, R. K.; Saunders, B. R. Phys. Chem. Chem. Phys. 2003, 5, 2417. (26) Chen, X. Y.; Armes, S. P. AdV. Mater. 2003, 15, 1558.
Rockwood Additives UK and was used as received. All other reagents were purchased from Aldrich and used without further purification. Water was of Milli-Q quality. Preparation of Mixed Laponite/Copolymer Dispersions. Scheme 1 depicts the general procedure use to prepare the laponite/ copolymer NC sheets. Laponite was dispersed in water for at least 1 day and then mixed with an appropriate concentration of MI2PNIPAm20k solution. The concentrations for each component within the dispersions considered in this work are given below. The method used to prepare the mixed dispersions is illustrated by the following example. The dispersion containing 0.5 wt % laponite and 2.0 wt % copolymer was prepared by adding dropwise 0.5 mL of a 2.0 wt % laponite dispersion into 1.5 mL of an aqueous solution of 2.67 wt % MI2-PNIPAm20k with vigorous stirring. The dispersion was allowed to equilibrate for at least 1 day prior to further measurements. All measurements were made within 5 days of mixture preparation, and no aging effects were evident within this time period. When required, the NC sheets were separated from the supernatant using high-speed centrifugation and purified by repeated rinsing, centrifugation, and redispersion in water. Physical Measurements. The determination of the cloud point temperature (Tcp) of the MI2-PNIPAm20k solutions was conducted with a Hitachi U-1800 spectrophotometer using a wavelength of 400 nm and thermostatic control. The turbidity, τ, was calculated from the optical density (OD) values using the equation τ ) 2.303(OD/L), where L was the path length. Gelation temperatures were determined using the tube inversion method. The internal diameter of the tubes was ca. 17 mm. Dynamic oscillatory measurements were conducted using a strain-controlled rheometer (Gemini Advanced Rheometer, Bohlin Instruments). A parallel plate geometry was used, with an upper plate diameter and gap of 20 mm and 0.1 mm, respectively. A frequency range of 0.1-15.9 Hz was employed at a strain of 10%. The operating temperature was varied in the range from 30 to 50 °C. A thin layer of dodecane was used as a liquid seal to prevent water evaporation during measurement. Initial measurements of modulus versus strain established that the measurements were conducted in the linear viscoelastic region. A Leica DM2500 microscope operating in transmitted light mode with a temperature-controlled Mettler stage was used to obtain the phase contrast images. Scanning electron microscopy (SEM) measurements were obtained using a Philips FEGSEM instrument. Thermogravimetic analysis (TGA) experiments were conducted using a TA Instruments Q5000 IR model. The NC sheets were separated from the supernatant using high-speed centrifugation and purified by repeated rinsing, centrifugation, and redispersion in water. The NC sheets were dried in oven at 70 °C before TGA measurements. TGA experiments were conducted in nitrogen over the range of 25-800 °C using a 20 °C/min heating rate.
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Figure 1. Gelation phase diagram for MI2-PNIPAm20k solutions containing laponite. The laponite concentrations (in weight percent) are given. Data for pure MI2-PNIPAm20k solutions are also shown for comparison. Note that Cp is the copolymer concentration present within the dispersion in weight percent. The error bars are the uncertainties in the gelation temperature measurements.
Results and Discussion Temperature-Triggered Gelation of Laponite/MI2-PNIPAm20k Dispersions. Mixing laponite dispersions with MI2-PNIPAm20k solutions was a straightforward method for preparing temperatureresponsive laponite dispersions (see Scheme 1). Temperaturetriggered gelation of as-prepared laponite/MI2-PNIPAm20k dispersions was investigated using tube inversion measurements. A gel was considered to be present when tube inversion exhibited a self-supporting fluid that did not flow. Tube inversion gives the points at which gels are formed that resist gravity. As will be discussed below, these points occur at higher temperature or concentration than the true gel points. The as-prepared dispersions gave gels that exhibited good stability with time at elevated temperatures. Figure 1 shows a phase map for dispersions containing three different laponite concentrations. Data for the pure MI2-PNIPAm20k solutions are also shown for comparison. Gels were observed for the laponite/MI2-PNIPAm20k dispersions at temperatures and Cp values at which the equivalent pure MI2PNIPAm20k solutions did not form gels. The addition of laponite strongly decreased both the gelation temperature, Tgel, and the critical value of Cp for gelation. Importantly, a gel formed at 45 °C for a dispersion containing a CL (laponite concentration) of 0.7 wt % when Cp was only 1.0 wt % (Figure 1). The total solid volume fraction (φsolid) was only 0.013 (i.e., 98.7 vol % water). This is suggestive of an attractive interaction between laponite and MI2-PNIPAm20k. It is important to note that the temperature-triggered gels that form for laponite/MI2-PNIPAm20k systems are different from those that form for pure laponite dispersions.12 Those thixotropic gels can be formed at CL values greater than ca. 1 wt % and form slowly over a period of several hours at room temperature. The gels for the laponite/MI2-PNIPAm20k dispersions form rapidly at temperatures above Tgel. They are also reversible and flow upon tube inversion when cooled to below Tgel. The viscoelastic properties of the laponite/MI2-PNIPAm20k dispersions were investigated using oscillatory rheology measurements as a function of temperature (Figure 2). The data obtained by heating from room temperature to the temperature shown are considered first (top four panels). These data show a crossover (tan δ ) 1) is present at 30 °C only at the high oscillation frequency (ω) portion of the graph. (Note that tan δ ) G′′/G′, where G′′ and G′ are the loss and storage modulus,
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respectively.) For the majority of the frequency range G′ < G′′ and the dispersion can be considered as a fluid at this temperature. However, the rheological behavior transformed to that of a gel, G′ > G′′, for all ω values, at temperatures greater than or equal to 35 °C. The data obtained at higher temperatures (42 and 50 °C) show a general increase in G′ and G′′ with temperature. These rheological features are consistent with temperaturetriggered gel formation. We investigated the reversibility of the fluid-to-gel transitions by cooling two samples from 50 °C and measuring the viscoelastic spectra at 35 and 30 °C (bottom of Figure 2). Hysteresis was evident because the values for G′ were larger than those obtained during the heating cycle. Furthermore, it can be seen that a weak gel remained at 30 °C. The hysteresis is attributed to entanglements. The subject of reversibility of the temperature-triggered gelation will be the subject of future work and is not considered further here. Data discussed from this point onward were obtained during heating cycles. Figure 3 shows the variation of G′ and G′′ with temperature for the laponite/MI2-PNIPAm20k dispersions from Figure 2. Linearity of G′ and also G′′ with temperature is evident for temperatures greater than or equal to 35 °C. The data can be explained using transient network theory,19 which shows that the high-frequency shear modulus is proportional to temperature. This implicitly assumes that the number density of elastically effective chains does not change significantly over the temperature range of these experiments; however, a contribution of a temperature-dependent dehydration (which would increase the number of elastically effective chains) can not be completely ruled out. Extrapolation of the linear fits to the G′ and G′′ versus temperature data to the point of intersection (i.e., tan δ ) 1) gives a value for the gelation temperature, as deduced by rheology. This value is 34.5 °C, and is close to the Tcp value of 32.5 °C determined from the turbidity data (below). The Tcp value for MI2-PNIPAm20k corresponds to the temperature at which chain contraction and association begins.3 It is reasonable to expect that gel formation would begin once the temperature reached Tcp. The gels that form at Tcp are very weak. A study of the variation of G′ and G′′ with Cp at 42 °C was conducted when CL was fixed at 0.5 wt % (see Figure 4). It can be seen that G′ passes through a maximum value when Cp is 2.5 wt %, indicating that an optimum ratio range of Cp to CL exists. It can also be seen from the data that the G′ and G′′ values for Cp ) 5.0 wt % at 42 °C are much greater than those for the pure MI2-PNIPAm20k solution at a Cp value of 5 wt % (see Figure 4). These data show the ability of a minor amount of laponite to substantially increase the elasticity within the gels. We investigated briefly the effect of added electrolyte on the viscoelastic properties of the mixtures by preparing a mixture containing laponite (0.5 wt %) and MI2-PNIPAm20k (2.0 wt %) in the presence of 0.10 M NaNO3. It can be seen from Figure 4 that this resulted in a major decrease in both G′ and G′′. This is attributed to electrostatic screening, which reduces the attractive interactions between cationic copolymer chains and the anionic laponite particles. The remainder of the work considers dispersions without any added electrolyte. The data shown in Figure 4 were obtained using a fairly wide range of φsolid values (from 0.012 to 0.052). A series of measurements were conducted in order to illustrate further the role laponite plays in the elasticity of the temperature-triggered gels. In this case a narrower range of φsolid was used (from 0.021 to 0.022). The MI2-PNIPAm20k concentration was fixed at 2.0 wt %, and the laponite concentration was varied between 0.2 and 0.5 wt % The data are shown in Figure 5. A maximum for G′
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Figure 2. Storage and loss modulus as a function of oscillation frequency for laponite/MI2-PNIPAm20k dispersions at selected temperatures. Cp and CL (laponite concentration) were 2.0 and 0.5 wt %, respectively. The data were obtained by heating the dispersions to the temperature shown unless otherwise stated. The two panels at the bottom were obtained by cooling a mixture from 50 °C to the temperatures shown.
Figure 3. Variation of G′ (filled squares) and G′′ (open diamonds) with temperature for a laponite (0.5 wt %)/MI2-PNIPAm20k (2.0 wt %) dispersion. These data were obtained using a value for ω of 12 rad s-1.
is also evident for these data, again suggesting an optimum composition ratio range for achieving high gel elasticity. The optimum ratio for CL to Cp for these data is 0.13. We investigated the effect of temperature on the optimum ratio using temperatures in the range of 35 to 46 °C. The position for the maximum G′ did not change. Thus, we conclude that the optimum ratio of CL/Cp ) 0.13 is not temperature dependent within this temperatures range. What is the nature of the species responsible for the gelation behavior of these laponite/MI2-PNIPAm20k dispersions? The
Figure 4. Variation of G′ (filled squares) and G′′ (open squares) with MI2-PNIPAm20k concentration for laponite (0.5 wt %)/MI2-PNIPAm20k dispersions at 42 °C. A value for ω of 12 rad s-1 was used. The filled and open circles are, respectively, G′ and G′′ values obtained in the presence of 0.10 M NaNO3 (see text). The filled and open triangles are, respectively, G′ and G′′ values for a pure MI2-PNIPAm20k gel (without added laponite) and are shown for comparison.
morphology of the as-prepared laponite/MI2-PNIPAm20k mixtures was investigated using SEM. Samples were dried either at 25 °C (fluid) or 50 °C (gel) (see Figure 6). This process removes water and would have caused some collapse of the hydrated structures initially present. Interestingly, it can be seen from Figure 6a,b that sheets are evident. Some of the sheets are ribbonlike. The edges of the sheets can be seen, and it is suggested that
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Figure 5. Variation of G′ (filled square) and G′′ (open squares) with laponite concentration for laponite/MI2-PNIPAm20k (2.0 wt %) dispersions at 42 °C. A ω value of 12 rad s-1 was used.
they have a micrometer thickness (in the dried state). These sheet morphologies appear to be rather smooth (Figure 6b) and are unprecedented for laponite/polymer mixtures to our knowledge. The image for the sample dried at 50 °C (Figure 6c) shows a more fibrous structure (presumably due to some degree of sheet contraction). Higher magnification (Figure 6d) shows that an extended three-dimensional sheet morphology is also present. A high degree of porosity is evident, which is expected for the space-filling gel phase of these systems. We used variable-temperature phase contrast microscopy to examine the temperature-triggered changes of the sheets in the as-prepared dispersions. Figure 7 shows a sequence of images obtained during heating for a mixture containing 0.5 wt % laponite and 2.0 wt % MI2-PNIPAm20k. Sheet-like structures with lateral dimensions of several hundreds of micrometers are clearly evident. Despite some corrugation, the homogeneity in brightness in the phase contrast pictures suggests them to be distinctively flat at 27 °C, with a thickness in the region of a few micrometers. This is consistent with the SEM data (Figure 6a,b). TGA analysis of
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the dried sheets showed that they contained about 37 wt % of laponite. They are, therefore, NC sheets. Control experiments using laponite or MI2-PNIPAm20k alone did not show the presence of sheets. The NC sheets originate from the combination of laponite and MI2-PNIPAm20k. Centrifugation was used to collect the NC sheets and it was determined gravimetrically that they accounted for about 75 wt % of the total mass of laponite and MI2-PNIPAm20k used. About 25 wt % of the initial material remained in the supernatant. A mass balance calculation reveals that the material that remained in the supernatant contained negligible laponite and can be considered as solution MI2-PNIPAm20k. This conclusion was supported by quantitative Fourier transform infrared (FTIR) analysis of the sheet and supernatant phases. The NC sheets were temperature-responsive (Figure 7). An increase in temperature caused extensive NC sheet contraction. This is apparent from the higher variation in brightness of the phase contrast pictures. The sheets crumple and become more extended in the direction perpendicular to the image plane. The aqueous phase begins to phase separate at 33 °C (shown by the arrows), which is close to the Tcp value of 32.5 °C for MI2PNIPAm20k (below). The NC sheets continue to contract up to a temperature of ca. 40 °C. The images in Figure 7 show clear evidence of two distinct phases. The NC sheets are dispersed in a solution containing MI2-PNIPAm20k. In order to investigate the temperature-triggered contraction of the NC sheets in the absence of solution copolymer, they were separated from the solution phase using centrifugation and washed repeatedly with water. The temperature-triggered contraction was investigated using phase-contrast microscopy. (Micrographs at selected temperatures and also a video of the NC sheet contraction are shown in the Supporting Information.) A lateral contraction ratio was measured by taking the ratio of the lateral distance across a segment of NC sheet at a given temperature and dividing that by the value measured at 24 °C. These data were obtained from several sheet regions and averaged (see Figure 8). The contraction ratio does not correspond to a linear swelling ratio because crumpling of the sheets occurred. Nevertheless, it
Figure 6. SEM for mixed laponite/MI2-PNIPAm20k dispersions. The samples were prepared using a mixture containing 0.5 wt % laponite and 2.0 wt % MI2-PNIPAm20k and dried at 25 °C (a,b) or 50 °C (c,d).
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Figure 7. Phase contrast micrographs for as-prepared laponite/MI2-PNIPAm20k (0.5 wt %/2 wt %) dispersions at various temperatures. The temperatures are shown for each image. The arrows indicate the incipient phase separation of the copolymer solution (see text).
Figure 8. Contraction ratio (see text) as a function of temperature for purified NC sheets prepared using 0.5 wt % laponite and 2.0 wt % MI2-PNIPAm20k. The error bars cover the minimum and maximum values obtained for different sheet regions (see text).
is used here because it shows clearly that temperature-triggered contraction of the sheets begins at temperatures of 27 °C. This is much lower than the phase separation temperature of solution MI2-PNIPAm20k (ca. 33 °C) and suggests that MI2-PNIPAm20k adsorbed to the laponite particles has a significantly smaller lower critical solution temperature (LCST) than solution MI2PNIPAm20k. PNIPAm chains adsorbed to interfaces can have a decreased LCST.27 We used variable-temperature turbidity measurements to further probe the temperature-dependent properties of laponite/ MI2-PNIPAm20k dispersions (see Figure 9). The data show a broad increase in turbidity for the dispersions containing laponite. The turbidity transition for pure MI2-PNIPAm20k solution is much sharper. The Tcp value for MI2-PNIPAm20k is estimated as 33.5 °C. It can be seen that the presence of laponite decreases the Tcp value to 32.0-32.5 °C. This slight decrease is attributed
Figure 9. Variation of turbidity with temperature for laponite/MI2PNIPAm20k dispersions. The concentration of MI2-PNIPAm20k used was 0.20 wt %. The concentration of laponite used (in wt %) is shown in the caption. The Tcp values were estimated from the points of inflection.
to electrostatic screening. Laponite provides a significant source of ions when dispersed in water.28 Closer scrutiny of the data shown in Figure 9 reveals that shoulders are present at temperatures between 29 to 32 °C for each laponite concentration used. They are not present for the data obtained using pure MI2-PNIPAm20k solution. Therefore, it is proposed that the shoulder corresponds to a collapse of interfacial MI2-PNIPAm20k chains that are adsorbed to the laponite particles. Therefore, two types of MI2-PNIPAm20k can be identified from the data shown in Figure 9: nonadsorbed and adsorbed chains. The adsorbed MI2-PNIPAm20k chains appear to have an onset temperature for collapse of 29 °C, which is 3.5 °C lower than the Tcp for the solution copolymer. This is consistent with the observation of sheet crumpling (Figure 8) at temperatures significantly lower than the LCST for the solution copolymer.
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Conceptual Model of Triggered Gelation of Laponite/MI2PNIPAm20k Dispersions. The 0.5 wt % laponite/2.0 wt % MI2PNIPAm20k mixtures contain mostly NC sheets and a minority of solution polymer. Upon heating the NC sheets appear to aggregate and begin to form a space-filling network via hydrophobic cross-links. This is probably assisted by the residual copolymer chains in solution which associate at Tcp, presumably adsorbing onto sheets, and reinforce the network structure. The mechanism up to this point is comparable to that proposed earlier for temperature-triggered aggregation of PNIPAm copolymercoated emulsions and latexes.23 However, in the present case, self-assembled NC sheets are present, which appear to play a significant role in producing gels at much lower values of φsolid (ca. 0.013). If there is too much or too little copolymer, then the G′ values are less than the maximum value (Figures 4 and 5). The gel structures are suggested to vary from NC sheets separated by relatively weak copolymer network linkages (for high ratios of Cp to CL) to laponite particles coated with relatively thin layers of MI2-PNIPAm20k or small sheets (for low ratios of Cp to CL). The optimum G′ value is suggested to originate from a structure that has the best combination of elastic contributions from interconnected NC sheets and adsorbed solution copolymer. The above discussion assumes that it is the NC sheets that are primarily responsible for elasticity of the gels. This was tested by removing the sheets from the supernatant by centrifugation, redispersing them in water after several rinsing cycles, and measuring G′ and G′′ versus ω at 50 °C. The data showed that the G′ values for a 0.8 wt % dispersion of sheets at 50 °C were approximately an order of magnitude higher than those for the supernatant (see Supporting Information). Some comments about NC sheet formation are warranted. Evidence has been presented for an attractive interaction between laponite and MI2-PNIPAm20k, and this probably involves electrostatic interactions as well as hydrogen bonding. It is noteworthy that electrostatic attraction is not a prerequisite for polymer adsorption onto silicates.13,14 The particle faces of laponite contain oxygen, and the edges contain both oxygen and (27) Zhu, P. W.; Napper, D. H. J. Colloid Interface Sci. 1994, 168, 380. (28) Lair, V.; Turmine, M.; Peyre, V.; Letellier, P. Langmuir 2003, 19, 10157.
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hydroxyl groups. The possibility of hydrogen bonding with the amide groups of the copolymer chains exists. The results presented above are consistent with the view that adsorption of MI2PNIPAm20k onto laponite is assisted by the opposite charges of the particles and copolymer. The anisotropy present within the NC sheet structures is very interesting. It is tempting to suggest that this originates from self-assembly of anisotropic laponite particles in a regular face-to-face arrangement. More work, however, is required to decide this question, and this will be the subject of a future publication.
Conclusion In this work we have investigated temperature-triggered gelation of dispersions containing laponite and MI2-PNIPAm20k. The interactions between both components results in the formation of NC sheets. The NC sheet structure may be related to the anisotropic structure of laponite. In addition, laponite decreases Tgel for concentrated dispersions and increases the elasticity within the gels as judged by G′. Gels with high G′ values have been achieved using very low φsolid values because of the strong attraction between the two components. The work has shown that the elasticity of the laponite/MI2-PNIPAm20k gels is maximized at a CL-to-Cp ratio of 0.13 when φsolid is ca. 0.02. The laponite/MI2-PNIPAm20k mixtures investigated here are the first examples to our knowledge of (a) temperature-responsive gelation of a laponite dispersion and (b) self-assembly of laponite with a temperature-responsive copolymer to form micrometersized NC sheets. It is possible that these new systems may have future application as actuators. Acknowledgment. B.R.S. gratefully acknowledges the EPSRC (EP/E001319/1) for funding this work. We thank Mr. Dave Hui for technical assistance with SEM. Supporting Information Available: Images and a video of the purified sheet contraction with temperature. A figure showing the rheological data for the purified sheets compared to the supernatant is also shown. This material is available free of charge via the Internet at http://pubs.acs.org. LA802941H
