Preferential Adsorption of Poly(ethylene glycol) on Hectorite Clay and

Dec 8, 2009 - Poly(N-isopropylacrylamide)/Laponite nanocomposite hydrogel (NC gel) was synthesized via in situ polymerization in the Laponite suspensi...
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Preferential Adsorption of Poly(ethylene glycol) on Hectorite Clay and Effects on Poly(N-isopropylacrylamide)/Hectorite Nanocomposite Hydrogels Xiaobo Hu, Tao Wang, Lijun Xiong, Chaoyang Wang, Xinxing Liu, and Zhen Tong* Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China Received September 2, 2009. Revised Manuscript Received November 18, 2009 Poly(N-isopropylacrylamide)/Laponite nanocomposite hydrogel (NC gel) was synthesized via in situ polymerization in the Laponite suspension containing PEG. The adsorption of PEG on Laponite platelets was characterized by zetapotential, which decreased with the PEG adsorption. The tensile strength decreased and elongation at break increased with increasing PEG concentration. The effective network chain density of PNIPAm/Laponite NC gels determined from the equilibrium modulus Ge decreased upon adsorption of PEG on the Laponite. All of these results revealed the preferential adsorption of PEG on the Laponite platelets occupying the active sites for the PNIPAm chain anchoring, which hindered their cross-linking effect in the NC gels. However, the temperature sensitive swelling behavior still remained in the PNIPAm/Laponite NC gels containing PEG with higher swelling volume below the LCST due to the lower cross-linker density. By adjusting the amount of added PEG, we can easily control the properties of the PNIPAm/ Laponite NC gels.

Polymer hydrogels with high mechanical properties have been extensively studied owing to the potential applications in the human body as articular cartilage, semilunar cartilage, tendon, and ligament. However, hydrogels cross-linked by organic crosslinkers (OR gels), for example, N,N0 -methylenebisacrylamide (BIS), showed low mechanical strength and elongation due to the inhomogeneous polymer network structure.1 Within this century, several hydrogels with high mechanical performance have been developed, including the topological gel with sliding cross-linkers,2 the nanocomposite hydrogel (NC gel) cross-linked with hectorite clay,3 the double-network hydrogel containing two interpenetrating networks,4 the macromolecular microsphere composite gel,5 and the tetra-PEG gel from tetrahedron-like macromonomers.6 Among them, polymer/hectorite nanocomposite hydrogel has been widely studied because of its facile fabrication and tentative application in cell culture.7 It is obtained through in situ polymerization of N-isopropylacrylamide (NIPAm) in aqueous suspensions of hectorite clay Laponite XLG without any chemical crosslinkers. The Laponite platelets act as multifunctional cross-linkers in the NC gel and the resultant hydrogel exhibits extraordinarily high mechanical properties with high transparency, for example, a tensile strength 10 times larger than that of the chemically crosslinked hydrogel and an elongation of more than 1000%.3 Some other extraordinary properties of the NC gels have been reported, *To whom correspondence should be addressed. Tel: (86)-20-87112886. Fax: (86)-20-87110273. E-mail: [email protected]. (1) Shibayama, M. Macromol. Chem. Phys. 1998, 199, 1. (2) Okumura, Y.; Ito, K. Adv. Mater. 2001, 13, 485. (3) Haraguchi, K.; Takehisa, T. Adv. Mater. 2002, 14, 1120. (4) Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. Adv. Mater. 2003, 15, 1155. (5) Huang, T.; Xu, H. G.; Jiao, K. X.; Zhu, L. P.; Brown, H. R.; Wang, H. L. Adv. Mater. 2007, 9, 1622. (6) Sakai, T.; Matsunaga, T.; Yamamoto, Y.; Ito, C.; Yoshida, R.; Suzuki, S.; Sasaki, N.; Shibayama, M.; Chung, U. I. Macromolecules 2008, 41, 5379. (7) Haraguchi, K.; Takehisa, T.; Ebato, M. Biomacromolecules 2006, 7, 3267. (8) Haraguchi, K.; Matsuda, K. Chem. Mater. 2005, 17, 931. (9) Haraguchi, K.; Li, H. J.; Okumura, N. Macromolecules 2007, 40, 2299.

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such as characteristic layered morphologies8 and hydrophobic surface.9 Laponite belongs to the 2:1 phyllosilicates: an octahedral layer is surrounded by two tetrahedral layers. Upon dispersion in water, the lamellar crystal is swollen and gradually cleaved into discrete platelets with thickness of about 1 nm and diameter of about 30 nm. The suspension is stable within a definite concentration window due to the charges on the platelet surface and rim.10 Haraguchi et al.11 found that NIPAm monomers played an important role in preventing gelation of the clay suspension. The initiator potassium peroxydisulfate (KPS) and N,N,N0 ,N0 -tetramethylethylenediamine (TEMED) were located near the Laponite surface through ionic interaction in the suspension. They suggested that unique Laponite-brush particles, which were constructed by the exfoliated Laponite platelets with numbers of grafted polymer chains, were formed in the very early stage of the polymerization. Shibayama et al.12 investigated the gelation of NC gels with dynamic light scattering (DLS) and contrastvariation small-angle neutron scattering (SANS) and reported an ergode-nonergode transition, but the size of the clusters at the gelation threshold was found much larger than that of the OR gels. They also reported that there was a thin polymer layer surrounding the clay platelet with a thickness of ca. 1 nm due to the interaction between PNIPAm chains and Laponite platelets. They also revealed the “plane cross-linking” effect for the Laponite platelets in deformed NC gels.13 To observe the gelation at different stages independently, Ferse et al.14 used a photoinitiator to initiate the polymerization, which was feasible to start or terminate the reaction by switching on/off the UV light. They (10) Nicolai, T.; Cocard, S. Langmuir 2000, 16, 8189. (11) Haraguchi, K.; Li, H. J.; Matsuda, K.; Takehisa, T.; Elliott, E. Macromolecules 2005, 38, 3482. (12) Miyazaki, S.; Endo, H.; Karino, T.; Haraguchi, K.; Shibayama, M. Macromolecules 2007, 40, 4287. (13) Shibayama, M.; Karino, T.; Miyazaki, S.; Okabe, S.; Takehisa, T.; Haraguchi, K. Macromolecules 2005, 38, 10772. (14) Ferse, B.; Richter, S.; Eckert, F.; Kulkarni, A.; Papadakis, C. M.; Arndt, K. F. Langmuir 2008, 24, 12627.

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concluded that the PNIPAm chains were closely attached to the Laponite surface at the early stage of the polymerization. Recently, through the dynamic mechanical measurements, we found that the effective network chain density and the relaxation rate were very important to produce acrylamide NC gels with a high tensile strength and an elongation of higher than 4000%.15 Various monomers, such as acrylamide,15-17 and N,N0 -dimethylacrylamide,18,19 were introduced to develop new polymer/ Laponite NC gels. By dexterously using the stable window of the Laponite suspensions containing ionic monomers, we have successfully fabricated homogeneous transparent ultratensible NC gels with pH response and dual response of pH and temperature.20,21 Because of the cross-linking function of Laponite platelets, the surface modification will alter the mechanical and swelling properties of the NC gels. This surface modification also provides us a new way to study the cross-linking effect of the Laponite platelets. However, to our knowledge, no one has reported the preparation and properties of NC gels with surface-modified Laponite platelets. In this work, we allowed the Laponite platelets to adsorb poly(ethylene glycol) (PEG) and then investigated the properties of PNIPAm/Laponite NC gels prepared by in situ polymerization of NIPAm in the suspension of PEG-modified Laponite platelets. The reason for choosing PEG is that PEG chains have a high affinity to the clay particles and can be adsorbed by Laponite platelets.22-26 According to the adsorbed amount of PEG, bare, starved and saturated Laponite platelets can be prepared.27 The PNIPAm/Laponite NC gel was reported to be destructed in the PEG solution because the interaction between PEG and Laponite was stronger than that of PNIPAm and Laponite.28

Experimental Section Materials. N-Isopropylacrylamide (NIPAm, Acros, 1% stabilizer), poly(ethylene glycol) (PEG, Uni-Chem) with molecular weight Mw of 2000 g/mol (PEG2K), 4000 g/mol (PEG4K), 10000 g/mol (PEG10K), and 20000 g/mol (PEG20K), and poly(ethylene glycol methyl ether methacrylate) with Mw of 2000 g/mol (PEGMA2K, Aldrich) were used as received. Potassium peroxydisulfate (K2S2O8) was recrystallized from deionized water before use. Synthetic hectorite Laponite XLG (Rockwood Ltd., [Mg5.34Li0.66Si8O20(OH)4]Na0.66) was used after dried at 100 °C for 2 h. N,N,N0 ,N0 -Tetramethylethylenediamine (TEMED, Sinopharm Group Chemical Reagent Co., Ltd.) was used as received. Water was purified by deionization and filtration with a Millipore purification apparatus (resistivity >18.2 MΩ cm) and bubbled with argon for more than 1 h prior to use. (15) Xiong, L.; Hu, X.; Liu, X.; Tong, Z. Polymer 2008, 49, 5064. (16) Zhu, M. F.; Liu, Y.; Sun, B.; Zhang, W.; Liu, X. L.; Yu, H.; Zhang, Y.; Kuckling, D.; Adler, H. J. P. Macromol. Rapid Commun. 2006, 27, 1023. (17) Zhang, W.; Liu, Y.; Zhu, M. F.; Zhang, Y.; Liu, X. L.; Yu, H.; Jiang, Y. M.; Chen, Y. M.; Kuckling, D.; Adler, H. J. P. J. Polym. Sci. A 2006, 44, 6640. (18) Haraguchi, K.; Farnworth, R.; Ohbayashi, A.; Takehisa, T. Macromolecules 2003, 36, 5732. (19) Can, V.; Abdurrahmanoglu, S.; Okay, O. Polymer 2007, 48, 5016. (20) Xiong, L.; Zhu, M.; Hu, X.; Liu, X.; Tong, Z. Macromolecules 2009, 42, 3811. (21) Hu, X.; Xiong, L.; Wang, T.; Lin, Z.; Liu, X.; Tong, Z. Polymer 2009, 50, 1933. (22) Nelson, A.; Cosgrove, T. Langmuir 2004, 20, 10382. (23) Lisi, R. D.; Gradzielski, M.; Lazzara, G.; Milioto, S.; Muratore, N.; Prevost, S. J. Phys. Chem. B 2008, 112, 9328. (24) Loyens, W.; Jannasch, P.; Maurer, F. H. J. Polymer 2005, 46, 915. (25) Baghdadi, H. A.; Sardinha, H.; Bhatia, S. R. J. Polym. Sci. B 2005, 43, 233. (26) Loizou, E.; Butler, P.; Porcar, L.; Schmidt, G. Macromolecules 2006, 39, 1614. (27) Loiseau, A.; Tassin, J. F. Macromolecules 2006, 39, 9185. (28) Abdurrahmanoglu, S.; Can, V.; Okay, O. J. Appl. Polym. Sci. 2008, 109, 3714.

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Sample Preparation. Laponite suspension was prepared by gradually adding required amount of Laponite powder into a known volume of water under continuous and vigorous stirring. As the stability of the Laponite suspension is sensitive to the dispersion process,10 every suspension was prepared following the same procedure. The suspension was left for about 2 h with stirring to allow the Laponite platelets to be completely exfoliated and evenly distributed. Aqueous PEG stock solution was left for 1 day to ensure complete dissolution. A known volume of the PEG solution was dropwise added into the Laponite suspension and stirred for 4 h to allow the Laponite platelets to adsorb PEG chains until equilibrium. The pH of the final solution was measured to be 9.5. In-situ free radical polymerization was carried out to fabricate the nanocomposite hydrogel (NC gel) in a way similar to that reported by Haraguchi et al.3 The monomer NIPAm, catalyst TEMED, and aqueous solution of initiator K2S2O8 (20 mg/mL) were subsequently added into the Laponite suspension under agitation. In all cases, the mole ratio of monomer/initiator/ catalyst was kept at 100:0.426:0.735. The Laponite and monomer concentrations were fixed at 2 w/v % and 10 w/v %, respectively. The reaction was carried out at 20 °C for 24 h. The NC gels were in two shapes: a disk of 25 mm diameter  1.5 mm thickness for rheology measurement and a rod of 6.0 mm diameter  40 mm length for tensile measurement. In the present paper, the NC gel samples were coded as G2-m-nK, where G2 stood for 2 w/v % of Laponite XLG, m for 100  PEG/clay (w/w) and n for Mw/1000 of PEG. For an example, G2-20-10K means that a NC gel contains 2 w/v % (clay-to-water) of Laponite XLG and the mass ratio of PEG10K (Mw=10000 g/mol) to Laponite is 0.2. Characterization. The zeta-potential of Laponite suspensions containing PEG was measured with a Zetasizer Nano-ZS90 (Malvern) at 25 °C. The Laponite suspension of 2.0 w/v % with desired amount of PEG was filtered into the cell through a Millipore filter of 0.22 μm pore size. The zeta-potential value was the average of at least five successive measurements. Tensile properties were measured on the as-prepared NC gel of 6.0 mm diameter  40 mm length with a Zwick Roell testing system at 25 °C. The sample length between the jaws was 20 mm and the crosshead speed was 100 mm/min. The tensile strain was taken as the length change related to the original length and the tensile strength was estimated on the cross section of the original sample. Rheology measurements were carried out on the as-prepared NC gel with a strain-controlled rheometer ARES-RFS using the parallel plates of diameter of 25 mm. Silicone oil was laid on the edge of the fixture plates to prevent solvent evaporation. First, the dynamic strain sweep was carried out at angular frequency of 1 rad/s to determine the linear viscoelasticity region. Then, the frequency sweep was performed over the range of 0.001-100 rad/s within the linear viscoelasticity region. All rheology measurements were performed at 20 ( 0.1 °C controlled by a Peltier plate. Swelling experiments were performed on the NC gels (initial size 6 mm diameter  20 mm long) after being immersed in a large excess of water for approximately 200 h to attain equilibrium at 20 °C with daily refreshing of water. Then, the NC gel sample was moved into a water bath at desired temperature for at least 72 h before measurement. Swelling ratio was represented by the diameter ratio for the swollen gel d to the as-prepared gel d0.

Results and Discussion Adsorption of PEG on Laponite Platelets. The adsorption of PEG chains on the Laponite platelets has been extensively studied using dynamic light scattering22 and small-angle neutron scattering.29 Since zeta-potential measurement is an effective way to test the stability of a colloid, we adopted it to evaluate the (29) Nelson, A.; Cosgrove, T. Langmuir 2004, 20, 2298.

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Figure 1. Zeta-potential of the Laponite XLG suspension as a function of PEG concentration.

adsorption effect of PEG on the Laponite. Figure 1 shows the zeta-potential of Laponite suspensions of 2.0 w/v % as a function of PEG concentration. The viscosity of corresponding PEG/ Laponite solutions was used in calculating the zeta-potential instead of water viscosity. On being dispersed in water, the Laponite platelets carry net negative charge due to the negative charges on the surface beyond the positive charges at the rim. The zeta-potential of pure Laponite platelets is about -85 mV when the viscosity of Laponite suspension is used, which makes them completely dispersed and stable in aqueous suspension because of the strong electrostatic repulsion. With the addition of PEG, the absolute zeta-potential starts to decrease. When an equal-weight of PEG is added, the zeta-potential of the suspension becomes ca. -40 mV. This reduction in absolute zeta-potential induced by PEG addition is due to the adsorption of PEG chains on the Laponite platelets. The PEG adsorption alters with the chain length because the chain number changes with PEG molecular weight at a given PEG weight concentration and the adsorbed chain conformation depends on the chain length. As seen from Figure 1, PEG10K seems to be more efficient in reducing the absolute zetapotential than PEG20K. On the basis of the assumption of complete adsorption of all the PEG chains by the Laponite platelets, the number of adsorbed PEG chains on one Laponite platelet n can be evaluated as n ¼

ðmPEG =MPEG ÞNA A mL S

ð1Þ

Here, mPEG and mL are the mass concentration of PEG and Laponite, respectively; S and A are the specific surface area of Laponite and the surface area of one Laponite platelet, respectively; MPEG is the molecular weight of PEG and NA the Avogadro constant. The available specific surface area from the supplier is 370 m2/g.30 A is about 1500 nm2 calculated from the diameter of Laponite platelet. Accordingly, when PEG is 0.20 of Laponite in weight, one Laponite platelet adsorbs ca. 50 of PEG10K chains or 25 of PEG20K chains. To investigate the adsorption of PEG chains and NIPAm monomers by the Laponite platelets, we carried out in situ freeradical polymerization of NIPAm (0.05 mol/L) in the Laponite suspension (0.1 w/v %) with or without PEG10K (0.1 w/v %). No bulk gel was formed at such a low concentration and the system (30) Laponite Product Bulletin L-XLG-06i.http://www.rockwoodadditives. com/product_bulletins_eu/PB%20Laponite%20XLG.pdf

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after reaction was still a suspension. The hydrodynamic diameter of the clay particles was roughly estimated with dynamic light scattering (DLS).22,31 The diameter of the clay particles with polymerized NIPAm but without PEG was 5-6 nm larger than that of the net Laponite platelet. However, this diameter was only 1-2 nm larger than that of the net Laponite platelet if PEG was added. This seems that the preferential adsorption of PEG chains obstructs the anchoring of propagating PNIPAm chains on the Laponite platelets, leading to a smaller size of the platelets. We cannot go further to the adsorbed PEG layer thickness with DLS because of the disk shape of the Laponite platelet and relaxation distribution.22 Mechanical Properties of PNIPAm/Laponite NC Gels with PEG. The properties of bulk PNIPAm-Laponite NC gels containing PEG become interesting to understand the crosslinking mechanism of Laponite. As illustrated in Figure 2A, transparent and uniform NC gels were successfully fabricated when the PEG10K/Laponite concentration ratio was below 0.3. With increasing PEG10K content in the Laponite suspension, the appearance of the NC gel obviously changed. The sample G2-3010K became very sticky and soft (data are not shown here) and the sample G2-50-10K was translucent (Figure 2B) and flowed at 25 °C (Figure 2C). When these NC gels were transferred to 40 °C, all of them became opaque white, indicating that the PNIPAm chain was formed. The preferential adsorption of PEG on Laponite platelets will occupy the active sites on the Laponite surface and reduce the anchoring of PNIPAm chains, resulting in a decrease in the cross-linking density of the PNIPAm network. When the PEG concentration is high, the Laponite cross-linked PNIPAm network becomes too weak to support its weight. This is the reason that the G2-50-10K gel flows at 25 °C. The preferential adsorption of PEG can be also recognized from the tensile properties of these NC gels. Figure 3 summarizes the strength and elongation at break as a function of the weight ratio of PEG10K/Laponite. (The stress-strain curves of the NC gels are demonstrated in Figure S1 of the Supporting Information with an inset for the curves at strain lower than 200%.) With increasing PEG10K content in the Laponite suspension, the tensile strength decreases and the elongation at break increases even above 2700% as the PEG10K concentration reaches 0.6 w/v%. In the NC gels, the Laponite platelet plays the role of a multifunctional cross-linker and the PNIPAm chains are anchored on the Laponite platelet surface.11 Haraguchi et al. reported that several tens of PNIPAm chains were grafted on one Laponite platelet in the NC gel.32 The present change in the mechanical properties is induced by the reduction of the binding sites on Laponite surface for PNIPAm chain anchoring due to the PEG adsorption. Adsorption of PEG corresponds to decrease of the Laponite content in the NC gel when considering the cross-linking effect. For an example, adding PEG of 0.2 of Laponite decreases 70.4% of the strength and increases the elongation at break to 1.32 times of the origin. According to the literature data for the similar NC gels,32 the Laponite content should be decreased from 5.94 to 3.3 w/v % for the same strength reduction and from 5.94 to 0.66 w/v % for the same elongation reduction. PEG adsorption appears more effective than reduction of the Laponite content in the NC gels for the mechanical properties. We measured tensile properties of gels containing PEG of different molecular weights (Figure S2 in the Supporting (31) Agrawal, S. K.; Sanabria-DeLong, N.; Tew, G. N.; Bhatia, S. R. Langmuir 2008, 24, 13148. (32) Haraguchi, K.; Takehisa, T.; Fan, S. Macromolecules 2002, 35, 10162.

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Figure 2. Photos of the PNIPAm NC gels with PEG10K: (A) as-prepared G2, G2-10-10K, and G2-20-10K gels; (B) as-prepared G2-50-10K gel; (C) flow of G2-50-10K gel at 25 °C.

Figure 3. Tensile strength (A) and elongation at break (B) as a function of PEG concentration in the gels.

Information). All the samples have lower strength and higher elongation at break when compared with the corresponding data of the NC gels without PEG. No unambiguous conclusion can be drawn for the molecular weight dependence due to the scattering of the data. But it seems that the strength tends to decrease and elongation to increase with increasing PEG molecular weight, except PEG20K. The conformation of the adsorbed PEG chains, depending on their molecular weight, will disrupt the anchoring of PNIPAm chains on the Laponite platelets in a different way. To deeply understand the adsorption of PEG chains on the Laponite platelet, poly(ethylene glycol methyl ether methacrylate) (PEGMA) was chosen to replace PEG. The PEG moiety in PEGMA can be adsorbed on the Laponite platelet and the methacrylate end group of PEGMA can be copolymerized into the network to form PEG grafted PNIPAm. Figure 4 represents the tensile strength and elongation at break for the NC gels containing PEGMA2K, comparing with those containing PEG2K. At the same concentration, the NC gel containing PEGMA2K always has higher tensile strength and lower elongation than that containing PEG2K. This is due to the additional junction effect contributed by the adsorbed PEGMA2K to the PNIPAm network in the gel. Effective Cross-linking Density. We also tried to quantify the effective cross-linking density in the gels with rheology 4236 DOI: 10.1021/la903298n

Figure 4. Comparison of tensile strength (A) and elongation at break (B) for the gels containing PEG2K and PEGMA2K.

measurements.15,20 The rheological measurements also provide the shear elastic modulus, useful for soft tissue engineering as an example, which sometimes is not directly comparable to the compression modulus.25,31,33 The linear viscoelasticity region of shear strain γ from 0.1 to 10% was determined from the γ independency of the complex module G* for all samples. Thus, the rheology measurements were carried out at γ = 1% to ensure availability of the linear viscoelasticity and enough sensitivity. Figure 5 depicts angular frequency ω dependence of the storage modulus G0 and loss modulus G00 for the gels with different contents of PEG10K. G0 is always higher than G00 and plateaus appear at low frequency range for all samples, indicating a crosslinked network in each sample. Moreover, the G0 value in the low frequency region declines following the sequence of G2, G2-1010K, and G2-20-10K, suggesting that the preferential adsorption of PEG reduces the cross-linking density in the gel. The effective network chain density N in gels can be evaluated from the equilibrium modulus Ge as Ge =NRT.15 Here, R and T are the gas constant and absolute temperature, respectively. Ge was taken from the plateau value of G0 . On the other hand, the network chain density N* can also be estimated from (33) Okay, O.; Oppermann, W. Macromolecules 2007, 40, 3378.

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Figure 5. Angular frequency ω dependence of storage modulus G0

(solid symbols) and loss modulus G00 (open symbols) for the gels with different PEG10K contents at 20 °C.

the tensile stress τ at small elongation of R = 2 (stain 100%) as τ = N*RT [R - (1/R)2].15 The N and N* values for the NC gels with PEG10K are plotted in Figure 6A against PEG content at constant Laponite concentration of 2 w/v %. The N* value is higher than the N value for the same gel possibly due to the departure from the affine deformation assumption and volume compression during elongation.15 As explained above, the effective network chain density decreases with increasing PEG content because the PEG adsorption reduces the active sites on the Laponite platelets available for the PNIPAm chain anchoring. This is equivalent to a decrease in the functionality of the cross-linker Laponite platelet. The decrease in N with increasing PEG content is observed for adsorbing other PEG samples, such as PEG2K, PEG4K, PEG20K, and PEGMA2K (Table S1 in the Supporting Information). The higher N value for the gels containing PEGMA2K compared with those containing PEG2K at the same PEG content is attributed to the adsorbed PEGMA2K, which is covalently jointed to the PNIPAm network, in consistence with the tensile results in Figure 4. Figure 6B shows the PEG molecular weight dependence of the effective network chain density N at two PEG concentrations. There is no obvious tendency on the PEG molecular weight, despite scattering of the data. This seems due to abundant conformations of the adsorbed PEG chains on the Laponite platelet, which obstruct the anchoring of propagating PNIPAm chains in different ways. Thermosensitivity of NC Gels. Figure 7 illustrates the equilibrium swelling ratio d/d0 for the gels containing PEG at different temperatures. As expected from the phase separation in PNIPAm aqueous solutions at ∼33 °C,34 these NC gels manifest a sudden volume contraction when heated beyond ∼35 °C, showing the volume phase transition. All of the samples are transparent at room temperature (Figure 2) and opaque white after shrinking at high temperature. The existence of PEG does not cease the appearance of the phase transition in the gels for the ratio of PEG/PNIPAm is small (1/50-2/50 in weight) and the PEG chains are not copolymerized to the PNIPAm network but just penetrated in it. By comparing the data at temperatures below the phase transition, the equilibrium swelling ratio is found to increase with increasing PEG content in the NC gels. This is due to the decrease in the effective cross-linking density (Figure 6A) rather than the hydrophilicity of the PEG, because the concentration of PEG is much low in the NC gels. (34) Tong, Z.; Zeng, F.; Zheng, X.; Sato, T. Macromolecules 1999, 32, 4488.

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Figure 6. The effective network chain density N determined from equilibrium modulus (square) and N* determined from tensile stress (circle) varying with PEG concentration (A) and N with PEG molecular weight MPEG (B).

Figure 7. Equilibrium swelling ratio d/d0 of the gels with indicated composition over the temperature range from 20 to 45 °C.

Conclusions In this study, we have found the preferential adsorption of PEG chains on the Laponite platelets in the NC gels. The adsorbed PEG chains obstruct the in situ propagation of PNIPAm chains from the Laponite platelets. The effective network chain density of the gels is reduced after adsorption of PEG chains on the Laponite platelets. Consequently, the mechanical properties and swelling behavior are accordingly changed for the PNIPAm/ Laponite NC gels. Therefore, addition of PEG into the Laponite suspension is a potential simple method to tune the properties and response of the NC gels. DOI: 10.1021/la903298n

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Acknowledgment. The financial support of the NSF of China (50773024 and 20534020) and the National High Technology Research and Development Program of China (2009AA03Z102) is gratefully acknowledged. We thank Mr. Weixiang Sun of SCUT for the viscosity measurements.

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Supporting Information Available: Figures of tensile stress-strain curves, PEG molecular weight dependence of tensile strength, and elongation at break for the NC gels; table of the effective network chain density. This material is available free of charge via the Internet at http://pubs.acs.org.

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