Research Article www.acsami.org
Transitional Suspensions Containing Thermosensitive Dispersant for Three-Dimensional Printing Xiaofeng Wang,† Yuehua Sun,† Chaoqun Peng,† Hang Luo,‡ Richu Wang,*,† and Dou Zhang*,‡ †
School of Materials Science and Engineering, Central South University, Changsha 410083, China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
‡
ABSTRACT: Tailoring the rheology of suspensions is an essential and persistent issue form many applications, especially three-dimensional (3D) printing. Colloidal suspensions of ceramic powder (Al2O3) dispersed by a special thermosensitive dispersant (poly(acrylic acid)−poly(N-isopropylacrylamide), PAA−PNIPAM) were designed, which underwent a remarkable fluid-gel transition in response to thermal stimulus due to the phase transition of the graft chains (-PNIPAM). 3D periodic structures with a fine size of 100 μm were assembled by 3D printing. KEYWORDS: suspensions, dispersant, rheology, thermosensitive, 3D printing
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INTRODUCTION Colloidal suspensions are used in various industries, including ceramic processing,1 paints,2 optoelectronic engineering,3,4 pharmaceutical engineering,5 metamaterials,6 and photonic materials.7 In order to meet the demands of these applications, colloidal suspensions can be prepared as stable fluids, gels, or colloidal crystals by tailoring the interactions between colloidal particles to engineer the desired degree of colloidal stability. These interactions stem from the balance between ubiquitous long-range, attractive van der Waals forces and Coulombic, steric, and other repulsive interactions, which are traditionally engineered by changing the ionic strength (charge),8 pH value,9 and particle volume fraction,10 as well as by other methods.11 Exploring approaches for controlling particle motion in suspensions and tailoring the rheology are persistent issues.12 Nature provides an interesting example of colloidal suspensions, which are composed of colloidal particles with “head−tails” structure in the form of octopus (or multitail tadpole). Octopuses flock or scatter in water by driving their tails. The sophisticated natural conformation and assembly of octopuses inspires a novel route to tailor the behavior of colloidal suspensions. Provided the conformations of the tails are regulated by some external stimuli, aggregation or dispersion of octopus-shaped colloidal particles should be easily tailored. To the best of our knowledge, octopus-shaped colloidal particles universally exist in colloidal suspensions, in which particles that behave as heads are absorbed by some species (i.e., dispersants), which function as tails. The traditional approach employs the optimal molecular architecture and characterization of polyelectrolytes, for example, poly(acrylic acid) (PAA),13 and a comb polymer, poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO),14 to enhance repulsive forces and steric stabilization.1 However, these polymers are insensitive to external stimuli (e.g., thermal field). Assuming © XXXX American Chemical Society
that a specific dispersant is utilized in response to a certain external stimulus, the dispersant fragments, that is, the tails, will become controllable, and the interactions among colloidal particles will be tailored conveniently by varying the external conditions. Herein, we design and demonstrate aqueous colloidal suspensions composed of alumina (Al2O3) microparticles and a thermosensitive dispersant, poly(acrylic acid)-poly(N-isopropylacrylamide) (PAA−PNIPAM), which undergo a remarkable fluid-gel transition in response to a thermal stimulus due to the phase transition of the graft chains (−PNIPAM). To illustrate the ability of tailoring rheological behavior, concentrated alumina suspensions were used for three-dimensional (3D) periodic structures with a fine size of 100 μm by 3D printing.
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EXPERIMENTAL SECTION
Synthesis of PAA−PNIPAM. The thermosensitive dispersants, PAA−PNIPAM, were prepared as follows.15−17 A semitelechelic poly(N-isopropylacrylamide) (PIPAAm) with a terminal amino end group was synthesized by radical telomerisation of the Nisopropylacrylamide (IPAAm) monomer using 2-aminoethanethiol hydrochloride (AET-HCl) as a chain transfer agent (Figure 1a). Purified IPAAm (2.5 mmol/L, Aladdin Reagent Co., Shanghai, China), AET-HCl (0.2 mmol/L, Tokyo Chemical Industry Co., Japan), and N,N′-azobis(isobutyronitrile) (0.05 mmol/L, AIBN, Shanghai Chemical Reagent Co., China) were dissolved in dry tetrahydrofuran (THF, Shanghai Chemical Reagent Co., China). The solution was treated by three freeze−thaw cycles to remove the dissolved oxygen, and then the mixture was polymerized at 60 °C for 15 h with vigorous stirring. Potassium hydroxide (KOH, Shanghai Chemical Reagent Co., China) dissolved in methanol (Shanghai Chemical Reagent Co., China) was Received: August 25, 2015 Accepted: November 10, 2015
A
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Synthesis of (a) the semitelechelic PIPAAm, (b) the macromonomer of PIPAAm, and (c) the PAA−PNIPAM. used to remove the hydrochloric acid byproduct. The reactant was poured into a large amount of diethyl ether to precipitate the NH2− PIPAAm. The oligomer product was collected by filtration and purified by repeated precipitation in diethyl ether from THF. The number-average molecular weight of the resultant oligomer was 1100, as determined by gel permeation chromatography (GPC). For the second step of the PIPAAm macromonomer preparation, a polymerizable end group was introduced into the semitelechelic PIPAAm (4 mmol/L) using an amide condensation reaction between PIPAAm and excessive N-acryloxysuccinimide (12 mmol/L, NAS, Tokyo Chemical Industry Co., Japan) in THF at 4 °C for 36 h (Figure 1b). The reactant was poured into a large amount of diethyl ether to precipitate the product. The product was also collected by filtration and purified by repeated precipitation in diethyl ether from THF. The graft copolymers of PNIPAAm chains grafted to the PAAc backbone were prepared by copolymerization of the PIPAAm (0.58 g) with acrylic acid (2.66 g, AAc, Shanghai Chemical Reagent Co., China) in 10 mL of methanol at 60 °C for 15 h using AIBN as an initiator (Figure 1c). This solution was also degassed by three freeze−thaw cycles prior to the reaction. The reactant was dropped into methyl ethyl ketone (MEK) to precipitate the copolymer and to remove the unreacted PIPAAm macromonomer and AAc in the supernatant. The products were further purified by redissolving in methanol and reprecipitating in THF. The number-average molecular weight of the copolymers was 13 000, as determined by GPC. The copolymer compositions were determined by back-titration2 and were found to contain 76 wt % of AAc. Preparation of Suspensions. Alumina suspensions containing various microparticle volume fractions (ϕ = 0.1, 0.2, 0.3, and 0.4) were prepared by adding an appropriate amount of alumina powder (AKP50, D50 = 0.2, sizes of 0.1−0.4 μm, and specific surface area of 10.2 m2/ g, Sumitomo Chemical Co., Japan) to premixed aqueous solutions consisting of PAA−PNIPAM (0.8 mg/g Al2O3) and deionized water. The suspension pH values were controlled at 10 using 0.1 mol/L nitric acid and ammonia solution (Shanghai Chemical Reagent Co., China). To generate well-dispersed and stable suspensions, these mixtures were ball-milled using 10 mm diameter, high-purity zirconia balls in high-density poly(ethylene) bottles of a certain volume for 24 h at room temperature (∼22 °C). For direct-write assembly, the suspension (ϕ = 0.4) was filtered through an 800 mesh screen to
eliminate any large agglomerates. An aliquot of cellulose (Shanghai Chemical Reagent Co., China) stock solution was added to yield a final cellulose concentration of 10 mg/mL, and the suspension was once again homogenized by ball-milling for 3 h. Cellulose served as a plasticizer in our experiments, thereby guaranteeing the rigid core-fluid shell architecture of the deposited filaments to facilitate rod−rod adhesion during assembly.4,18 The typical composition of the suspensions (ϕ = 0.4) for 3D printing were 10 mL of deionized water, 26.5 g of alumina powder, 21.2 mg of PAA−PNIPAM, and 100 mg of cellulose. Characterization. Adsorption isotherm measurements were conducted using a total organic carbon (TOC) analyzer (TOC-L, Shimadzu, Kyoto, Japan), which provided a quantitative measure of the nonadsorbed fraction of PAA−PNIPAM in solution. Al2O3 suspensions (ϕ = 0.10, pH = 10) were prepared as described earlier, and then centrifuged at 3000 rpm for 10 min to separate the solids and supernatant. The supernatant was immediately decanted and diluted with deionized water for the TOC measurement. Several aliquots were taken from each sample and measured, and an average value was reported based on standard calibration curves (correlation coefficient, R = 0.99) obtained for pure dispersant solutions of varying concentrations. This experiment was performed at ambient temperature (22 °C). Zeta-potential (ζ) measurements (Zetasizer4; Malvern Instruments, Ltd., Malvern, U.K.) were carried out on alumina suspensions (ϕ = 0.0001) as a function of pH in the presence and absence of thermosensitive dispersant PAA−PNIPAM. The rheology of the fresh suspension was measured using a stresscontrolled rheometer (AR2000, TA Instruments, New Castle, DE) with a parallel plate (40 mm in diameter). Two modes were employed, namely, steady state flow and oscillatory mode. The viscosity measurements were performed in flow temperature ramp mode with a steady increase from 25 to 50 °C, at a shear rate of 10 1/s. An oscillatory temperature ramp mode was used to study the gelling behaviors of the aqueous solutions, that is, the storage modulus representing the degree of solidity was monitored with increasing temperature. The frequency was 1 Hz, and the train was controlled at 0.4%. The gap between parallel plates was 1000 μm. To minimize the thermal lag, we performed all measurements at a slow temperature ramp rate (1 °C/min). B
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces 3D Printing. 3D periodic structures were fabricated using a modified deposition apparatus based on a dispersing system (DR2203, EFD Inc., East Province, RI) consisting of four parts: a three-axis moving stage, a suspension delivery system, a fluid dispenser, and a temperature-controlling coagulation reservoir. The delivery system, connected to an air-powdered fluid dispenser (Performus V, EFD, Inc.), was mounted on the three-axis stage controlled by a computer regulator. The reservoir underlying the deposition nozzle of the delivery system was filled with a nonwetting oil and mounted on an x-y stage. The suspension (40 vol % microparticles) housed in a 10 mL syringe (EFD Inc.) was delivered through the nozzle at room temperature (22 °C) and coagulated to form self-supporting filaments when deposited in the heated oil coagulation reservoir (T > Tcritical), i.e., it was consolidated via the rheological transition and water elapsing, and robotically deposited onto the moving x−y stage. After a given layer was generated, the stage was inclined in the z direction (0.9 diameter of filament to ensure suitable adhesion to the underlying layers) and another layer was deposited. This process was repeated to yield 3D structures.5,9 The suspension flowed through a cylindrical nozzle at the volumetric flow rate required to maintain a constant speed (v) of 1 mm/s. The nozzle with a diameter of 100 μm was utilized to create the 3D periodic structure. The temperature of the nonwetting oil was maintained at 45 °C during printing.
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RESULTS AND DISCUSSION Figure 2 shows the interaction between Al2O3 microparticles and poly(acrylic acid)-poly(N-isopropylacrylamide) (PAA− PNIPAM). The dispersant is a comb-like polyelectrolyte that contains graft chains (-PNIPAM) along its backbone of carboxylate groups (−PAA). The graft chains show a sharp phase transition from hydrophilic to hydrophobic in response to temperature.16,19 Therefore, the polyelectrolyte is a thermosensitive species. This thermosensitive dispersant strongly interacts with microparticles. PAA−PNIPAM becomes negatively charged because of its carboxyl groups (−COOH), which are fully ionized under the suspension conditions (pH > 9).20 In contrast, Al2O3 microparticles are positively charged under these conditions (Figure 2a).20 Due to strong electrostatic attractions between oppositely charged species, the polyelectrolyte firmly adsorbs onto Al2O3 microparticles to generate a dramatic charge reversal. The ζ curves shown in Figure 2a indicate that the PAA−PNIPAM species plays the same role in the zeta potential as poly(acrylic acid) ,20 which decreases the isoelectric point of alumina particles (pH = 4.2) at ambient temperature. The data shown in Figure 2a also suggest that alumina particles suspended near pH = 10 adopt an effective charge of approximately −45 mV in solution with increasing alkalinity. Because the electrophoretic mobility of particles depends upon the surface charges (the viscosity is very low and not considered), the particles adsorbed with thermosensitive dispersant species generate the observed charge build-up. The adsorption between particles and ionized species is further supported by the data shown in Figure 2b, which reflects the difference in the content of ionized PAA− PNIPAM species in the bulk of the solution. The absence of ionized species in the bulk solution, especially at the lowest ionized species concentrations studied, indicates that adsorption of the ionized species onto particles occurs at room temperature. The plateau value of adsorption, which occurs at 0.8 mg/g, reveals an adequate amount of PAA−PNIPAM for complete surface coverage of the particles. In our approach, we first generated well-dispersed alumina suspensions (pH = 10) with thermosensitive dispersant, PAA− PNIPAM. We then tailored the fluid-gel transition of the suspensions by raising the temperature above the critical value
Figure 2. (a) Zeta potential (ζ) of Al2O3 particles with or without thermosensitive dispersant, PAA−PNIPAM, as a function of pH values. (b) Adsorption of PAA−PNIPAM onto Al2O3 (solids loading ϕ = 0.1, pH = 10). Measurements were performed at ambient temperature (∼22 °C). The dotted line in panel b was calculated, representing the PAA−PNIPAM that was completely adsorbed by alumina particles.
(Tcritical ∼ 32.5 °C,19 which is the critical hydrophilic− hydrophobic transition of graft chains, − PNIPAM) to generate the desired rheological response. A rise in the viscosity of the suspensions accompanies this transition (Figure 3a). The traditional view is that the viscosity depends on the effective volume fraction, which is described by the modified empirical expressions as follows.1,21 ηr = (1 − ϕeff /ϕmax(eff))−n
(1)
ϕeff = (Vsolid + Vpolymer)/Vtotal
(2)
where ϕmax(eff) is the effective maximum volume fraction, n is the scaling exponent, Vsolid is the volume of powder (solidstatus phase), and Vpolymer is the volume of the polymer layer containing the anchored solvent. The flow properties of suspensions are governed by the volume fraction, ϕeff, which is the effective volume fraction of microparticles, and ϕmax(eff), C
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
attribute this to the greater contribution of the high volume fraction of microparticles on viscosity compared with that of nanospheres. In the oscillatory model of the plate−plate rheometer, the elastic properties dramatically rise upon exceeding the critical temperature (T > Tcritical), especially for high microparticle volume fractions (Figure 3b). The elastic modulus increases by 2 orders of magnitude (or more) due to strengthened interparticle attractions,1 as given by the following scaling relationship.24 G′ = G0(ϕ/ϕgel − 1)s
(3)
where G′ is the elastic modulus, G0 is a constant, ϕgel is the volume fraction of microparticles at the gel point, and s is the scaling exponent (∼2.5). The equilibrium mechanical properties of microparticle gels are governed by two parameters: ϕ, which is proportional to the bond density, and ϕgel, which scales inversely with bond strength. Our observations reflect that interparticle attractions intensify with increasing temperature, and microparticle gels experience significant increases in their elastic properties (Figure 3). At low temperature (T < Tcritical), the polyelectrolyte dispersant, PAA−PNIPAM (Figure 4), coated on the particles is soluble and charged. Long-range
Figure 3. Rheological behavior of alumina colloidal suspensions containing thermosensitive dispersant PAA−PNIPAM: (a) viscosity and (b) storage modulus.
which accounts for the approach to infinite viscosity (solid body) at this critical effective volume fraction. In our systems, the thermosensitive dispersant is dissolved at a low temperature (T < Tcritical), and the relationship between viscosity and the volume fraction agrees with eqs 1 and 2. However, above Tcritical, the situation changes from hydrophilic to hydrophobic due to the phase transition of grafted chains (−PNIPAM).22,23 At a high temperature (T > Tcritical), the system changes to a biphasic mixture comprised of alumina microparticles and nanospheres, which results from the phase transition of the graft chains. The Vpolymer in eq 2 changes to Vpolymer′, consisting of adsorbed and unadsorbed thermosensitive polymers, which enhance the effective volume fraction of the suspension.22 In addition, under such conditions, the interaction forces (e.g., electrostatic and steric forces) between microparticles and nanospheres are weakened, which induces flocculation of biphasic particles. Furthermore, aggregation among organic nanospheres is another reason for the rheological transition of suspensions. The volume fraction of microparticles plays an essential role in fluidity. At lower microparticle volume fractions ( Tcritical) (Figure 2a). However, at higher microparticle volume fractions (>0.2), the viscosity is almost independent of the microparticle volume fraction. We
Figure 4. Schematic illustration of (a) the molecular architectures of the thermosensitive dispersant PAA−PNIPAM, (b) alumina-coated PAA−PNIPAM, and (c) the suspension comprised of alumina powder and the PAA−PNIPAM. The graft chains adsorbed on alumina microparticles have extending and soluble tails at low temperatures (T < Tcritical), which turn into globules and solid-status nanospheres at high temperatures (T > Tcritical).
van der Waals interactions dominate the bonding between microparticles in the suspensions. However, at high temperature (T > Tcritical), the graft chains of the thermosensitive dispersant still covering the microparticles precipitate from solvent and form solid-status nanospheres (Figure 4). The interparticle bonding intensifies because of the decrease of the D
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces aforementioned interaction force in the biphasic mixture. Note that the curves shown in Figure 2 should level off when the temperature is above the critical value (T > Tcritical), which might be higher than that in practice due to solvent evaporation. Interestingly, the Al2O3 suspensions containing various microparticle volume fractions (ϕ = 0.1−0.4) exhibit significant differences in elasticity due to the changing temperature (Figure 3). When the critical temperature is exceeded (T > Tcritical), the elastic modulus responds faster in suspensions containing high microparticle volume fractions compared to those with low microparticle volume fractions (ϕ < 0.1). The differences reveal interactions between oppositely charged nanospheres and PAA−PNIPAM-coated Al2O3 microparticles at higher temperatures. Because the microparticles are closer to each other in suspensions containing higher microparticle volume fractions,21 the bonding is much easier to generate. To illustrate the ability of tailoring rheological behavior, we used concentrated alumina suspensions for 3D printing of 3D periodic structures. The suspension (ϕ = 0.4) was utilized because its elasticity can be sharply tailored by raising the temperature (Figure 3). This suspension possesses the following advantages: (1) its viscosity at room temperature is low enough to enable smooth flow through the deposition nozzle without nozzle clogging; (2) at high temperatures (T > Tcritical), it generates a microparticle gel that exceeds the minimum elasticity (G′ = ∼ 104 Pa) required to print the 3D stuctures,9 and (3) the microparticle volume fraction is high enough to minimize drying-induced shrinkage of the assembled structure.25,26 The 3D periodic structure (five layers, Figure 5) was assembled with the modified apparatus by squeezing the suspension through a cylindrical nozzle. Optical images (Figure 5a,b) show a tetragonal symmetry for 3D lattices, which reveals that rods deposited on the top and underlying layers exhibit excellent height uniformity, even across spanning distances
approaching 0.5 mm. Higher magnification views of the structure (Figure 5c,d) illustrate the cylindrical nature of the rods, their smooth surface, and high degree of regularity, which are preserved during printing and drying.
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CONCLUSIONS
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AUTHOR INFORMATION
In summary, we have developed an interesting avenue to tailor the rheological behaviors of suspensions, from stable fluid to colloidal gel, with increasing temperature, which is favorable for 3D printing. The thermosensitive polyelectrolyte, poly(acrylic acid)-poly(N-isopropylacrylamide), was synthesized and utilized as a dispersant to stabilize the alumina microparticles, and the suspension rheology was conveniently regulated by modulating the temperature. Equally important, our suspension design and patterning approach can be extended to other stimuli-responsive polyelectrolytes through control under fields, such as thermal, electric, and magnetic fields. For example, functional colloidal suspensions are engineered with the comblike dispersant containing stimulus graft chains, that is, thermoresponsive (e.g., poly(ethylene oxide)27) or electroresponsive (e.g., polythiophene28 and polypyrrole29). This broad palette of suspensions will enable a myriad of technological applications to be pursued, including metamaterials, photonic crystals, drug delivery materials, tissue engineering materials, and precursors for novel coatings and composites.
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grant No. 51202296), and supported by State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. We thank Prof. Haipu Li and Dr. Guipeng Yu (School of Chemistry and Chemical Engineering, Central South University) for fruitful discussions and synthesis of poly(acrylic acid)-poly(N-isopropylacrylamide).
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REFERENCES
(1) Lewis, J. A. Colloidal Processing of Ceramics. J. Am. Ceram. Soc. 2000, 83, 2341−2359. (2) Pichumani, M.; Bagheri, P.; Poduska, K. M.; González-Viñas, W.; Yethiraj, A. Dynamics, Crystallization and Ctructures in Colloid Spin Coating. Soft Matter 2013, 9, 3220−3229. (3) Lewis, J. A.; Ahn, B. Y. Device Fabrication: Three-Dimensional Printed Electronics. Nature 2015, 518, 42−43. (4) García-Tuñon, E.; Barg, S.; Franco, J.; Bell, R.; Eslava, S.; D'Elia, E.; Maher, R. C.; Guitian, F.; Saiz, E. Printing in Three Dimensions with Graphene. Adv. Mater. 2015, 27, 1688−1693. (5) Wu, L.; Zhang, J.; Watanabe, W. Physical and Chemical Stability of Drug Nanoparticles. Adv. Drug Delivery Rev. 2011, 63, 456−469. (6) Stebe, K. J.; Lewandowski, E.; Ghosh, M. Materials science: Oriented Assembly of Metamaterials. Science 2009, 325, 159−160. (7) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Photonic Crystals: Putting a New Twist on Light. Nature 1997, 386, 143−149. (8) Li, Q.; Lewis, J. A. Nanoparticle Inks for Directed Assembly of Three-Dimensional Periodic Structures. Adv. Mater. 2003, 15, 1639− 1643.
Figure 5. Optical images of 3D periodic lattice (five layers) assembled from thermosensitive alumina suspensions deposited through a 100 μm nozzle. (a) Top view, (b−d) higher magnification images of the 3D structure shown in panel a. The deposited rods maintain their cylindrical shape, smooth surface, and high degree of regularity upon assembly and drying. E
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces (9) Smay, J. E.; Cesarano, J., III; Lewis, J. A. Colloidal Inks for Directed Assembly of 3-D Periodic Structures. Langmuir 2002, 18, 5429−5437. (10) Trappe, V.; Prasad, V.; Cipelletti, L.; Segrè, P. N.; Weitz, D. A. Jamming Phase Diagram for Attractive Particles. Nature 2001, 411, 772−775. (11) Rao, R. B.; Krafcik, K. L.; Morales, A. M.; Lewis, J. A. Microfabricated Deposition Nozzles for Direct-write Assembly of Three-Dimensional Periodic Structures. Adv. Mater. 2005, 17, 289− 293. (12) Bonacucina, G.; Cespi, M.; Misici-Falzi, M.; Palmieri, G. F. Colloidal Soft Matter as Drug Delivery System. J. Pharm. Sci. 2009, 98, 1−42. (13) Loiseau, J.; Doërr, N.; Suau, J. M.; Egraz, J. B.; Llauro, M. F.; Ladavière, C.; Claverie, J. Synthesis and Characterization of Poly (acrylic acid) Produced by RAFT Polymerization. Application as A Very Efficient Dispersant of CaCO3, Kaolin, and TiO2. Macromolecules 2003, 36, 3066−3077. (14) Kirby, G. H.; Lewis, J. A. Comb Polymer Architecture Effects on the Rheological Property Evolution of Concentrated Cement Suspensions. J. Am. Ceram. Soc. 2004, 87, 1643−1652. (15) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi, A.; Sakurai, Y.; Okano, T. Comb-type Grafted Hydrogels with Rapid Deswelling Response to Temperature Changes. Nature 1995, 374, 240− 242. (16) Chen, G.; Hoffman, A. S. Temperature-Induced Phase Transition Behaviors of Random vs. Graft Copolymers of Nisopropylacrylamide and Acrylic Acid. Macromol. Rapid Commun. 1995, 16, 175−182. (17) Wang, X.; Wang, R.; Feng, Y.; Zhang, D.; Peng, C. Postcasting Contraction: Improving the Density of Gelcast Nanoparticle Green Bodies with Heated Liquid Desiccants. J. Am. Ceram. Soc. 2015, 98, 1706−1710. (18) Smay, J. E.; Gratson, G. M.; Shepherd, R. F.; Cesarano, J., III; Lewis, J. A. Directed Colloidal Assembly of 3D Periodic Structures. Adv. Mater. 2002, 14, 1279−1283. (19) Schild, H. G. Poly (N-isopropylacrylamide): Experiment, Theory and Application. Prog. Polym. Sci. 1992, 17, 163−249. (20) Cesarano, J., III; Aksay, I. A.; Bleier, A. Stability of Aqueous αAl2O3 Suspensions with Poly(methacrylic acid) Polyelectrolyte. J. Am. Ceram. Soc. 1988, 71, 250−255. (21) Bergström, L. Rheological Properties of Concentrated, Nonaqueous Silicon Nitride Suspensions. J. Am. Ceram. Soc. 1996, 79, 3033−3040. (22) Wang, X.; Sun, Y.; Peng, C.; Zhang, D.; Chen, Y.; Wang, R. Colloidal Processing of ZnO Using Thermosensitive Poly(Nisopropylacrylamide) as a Coagulating Agent. Ceram. Int. 2015, 41, 9163−9167. (23) Shiraga, K.; Naito, H.; Suzuki, T.; Kondo, N.; Ogawa, Y. Hydration and Hydrogen Bond Network of Water during the Coil-toGlobule Transition in Poly (N-isopropylacrylamide) Aqueous Solution at Cloud Point Temperature. J. Phys. Chem. B 2015, 119, 5576−5587. (24) Rueb, C. J.; Zukoski, C. F. Viscoelastic Properties of Colloidal Gels. J. Rheol. 1997, 41, 197−218. (25) Calvert, P.; Crockett, R. Chemical Solid Free-form Fabrication: Making Shapes without Molds. Chem. Mater. 1997, 9, 650−663. (26) Lewis, J. A. Direct Ink Writing of 3D Functional Materials. Adv. Funct. Mater. 2006, 16, 2193−2204. (27) Wang, X.; Wang, R.; Peng, C.; Li, H.; Liu, B.; Wang, Z. Thermoresponsive Gelcasting: Improved Drying of Gelcast Bodies. J. Am. Ceram. Soc. 2011, 94, 1679−1682. (28) Mawad, D.; Molino, P. J.; Gambhir, S.; Locke, J. M.; Officer, D. L.; Wallace, G. G. Electrically Induced Disassembly of Electroactive Multilayer Films Fabricated From Water Soluble Polythiophenes. Adv. Funct. Mater. 2012, 22, 5020−5027. (29) Schmidt, C. E.; Shastri, V. R.; Vacanti, J. P.; Langer, R. Stimulation of Neurite Outgrowth Using an Electrically Conducting Polymer. Proc. Natl. Acad. Sci. U. S. A. 1997, 94, 8948−8953.
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DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Transitional Suspensions Containing Thermosensitive Dispersant for Three-Dimensional Printing Xiaofeng Wang,† Yuehua Sun,† Chaoqun Peng,† Hang Luo,‡ Richu Wang,*,† and Dou Zhang*,‡ †
School of Materials Science and Engineering, Central South University, Changsha 410083, China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
‡
ABSTRACT: Tailoring the rheology of suspensions is an essential and persistent issue form many applications, especially three-dimensional (3D) printing. Colloidal suspensions of ceramic powder (Al2O3) dispersed by a special thermosensitive dispersant (poly(acrylic acid)−poly(N-isopropylacrylamide), PAA−PNIPAM) were designed, which underwent a remarkable fluid-gel transition in response to thermal stimulus due to the phase transition of the graft chains (-PNIPAM). 3D periodic structures with a fine size of 100 μm were assembled by 3D printing. KEYWORDS: suspensions, dispersant, rheology, thermosensitive, 3D printing
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INTRODUCTION Colloidal suspensions are used in various industries, including ceramic processing,1 paints,2 optoelectronic engineering,3,4 pharmaceutical engineering,5 metamaterials,6 and photonic materials.7 In order to meet the demands of these applications, colloidal suspensions can be prepared as stable fluids, gels, or colloidal crystals by tailoring the interactions between colloidal particles to engineer the desired degree of colloidal stability. These interactions stem from the balance between ubiquitous long-range, attractive van der Waals forces and Coulombic, steric, and other repulsive interactions, which are traditionally engineered by changing the ionic strength (charge),8 pH value,9 and particle volume fraction,10 as well as by other methods.11 Exploring approaches for controlling particle motion in suspensions and tailoring the rheology are persistent issues.12 Nature provides an interesting example of colloidal suspensions, which are composed of colloidal particles with “head−tails” structure in the form of octopus (or multitail tadpole). Octopuses flock or scatter in water by driving their tails. The sophisticated natural conformation and assembly of octopuses inspires a novel route to tailor the behavior of colloidal suspensions. Provided the conformations of the tails are regulated by some external stimuli, aggregation or dispersion of octopus-shaped colloidal particles should be easily tailored. To the best of our knowledge, octopus-shaped colloidal particles universally exist in colloidal suspensions, in which particles that behave as heads are absorbed by some species (i.e., dispersants), which function as tails. The traditional approach employs the optimal molecular architecture and characterization of polyelectrolytes, for example, poly(acrylic acid) (PAA),13 and a comb polymer, poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO),14 to enhance repulsive forces and steric stabilization.1 However, these polymers are insensitive to external stimuli (e.g., thermal field). Assuming © XXXX American Chemical Society
that a specific dispersant is utilized in response to a certain external stimulus, the dispersant fragments, that is, the tails, will become controllable, and the interactions among colloidal particles will be tailored conveniently by varying the external conditions. Herein, we design and demonstrate aqueous colloidal suspensions composed of alumina (Al2O3) microparticles and a thermosensitive dispersant, poly(acrylic acid)-poly(N-isopropylacrylamide) (PAA−PNIPAM), which undergo a remarkable fluid-gel transition in response to a thermal stimulus due to the phase transition of the graft chains (−PNIPAM). To illustrate the ability of tailoring rheological behavior, concentrated alumina suspensions were used for three-dimensional (3D) periodic structures with a fine size of 100 μm by 3D printing.
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EXPERIMENTAL SECTION
Synthesis of PAA−PNIPAM. The thermosensitive dispersants, PAA−PNIPAM, were prepared as follows.15−17 A semitelechelic poly(N-isopropylacrylamide) (PIPAAm) with a terminal amino end group was synthesized by radical telomerisation of the Nisopropylacrylamide (IPAAm) monomer using 2-aminoethanethiol hydrochloride (AET-HCl) as a chain transfer agent (Figure 1a). Purified IPAAm (2.5 mmol/L, Aladdin Reagent Co., Shanghai, China), AET-HCl (0.2 mmol/L, Tokyo Chemical Industry Co., Japan), and N,N′-azobis(isobutyronitrile) (0.05 mmol/L, AIBN, Shanghai Chemical Reagent Co., China) were dissolved in dry tetrahydrofuran (THF, Shanghai Chemical Reagent Co., China). The solution was treated by three freeze−thaw cycles to remove the dissolved oxygen, and then the mixture was polymerized at 60 °C for 15 h with vigorous stirring. Potassium hydroxide (KOH, Shanghai Chemical Reagent Co., China) dissolved in methanol (Shanghai Chemical Reagent Co., China) was Received: August 25, 2015 Accepted: November 10, 2015
A
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. Synthesis of (a) the semitelechelic PIPAAm, (b) the macromonomer of PIPAAm, and (c) the PAA−PNIPAM. used to remove the hydrochloric acid byproduct. The reactant was poured into a large amount of diethyl ether to precipitate the NH2− PIPAAm. The oligomer product was collected by filtration and purified by repeated precipitation in diethyl ether from THF. The number-average molecular weight of the resultant oligomer was 1100, as determined by gel permeation chromatography (GPC). For the second step of the PIPAAm macromonomer preparation, a polymerizable end group was introduced into the semitelechelic PIPAAm (4 mmol/L) using an amide condensation reaction between PIPAAm and excessive N-acryloxysuccinimide (12 mmol/L, NAS, Tokyo Chemical Industry Co., Japan) in THF at 4 °C for 36 h (Figure 1b). The reactant was poured into a large amount of diethyl ether to precipitate the product. The product was also collected by filtration and purified by repeated precipitation in diethyl ether from THF. The graft copolymers of PNIPAAm chains grafted to the PAAc backbone were prepared by copolymerization of the PIPAAm (0.58 g) with acrylic acid (2.66 g, AAc, Shanghai Chemical Reagent Co., China) in 10 mL of methanol at 60 °C for 15 h using AIBN as an initiator (Figure 1c). This solution was also degassed by three freeze−thaw cycles prior to the reaction. The reactant was dropped into methyl ethyl ketone (MEK) to precipitate the copolymer and to remove the unreacted PIPAAm macromonomer and AAc in the supernatant. The products were further purified by redissolving in methanol and reprecipitating in THF. The number-average molecular weight of the copolymers was 13 000, as determined by GPC. The copolymer compositions were determined by back-titration2 and were found to contain 76 wt % of AAc. Preparation of Suspensions. Alumina suspensions containing various microparticle volume fractions (ϕ = 0.1, 0.2, 0.3, and 0.4) were prepared by adding an appropriate amount of alumina powder (AKP50, D50 = 0.2, sizes of 0.1−0.4 μm, and specific surface area of 10.2 m2/ g, Sumitomo Chemical Co., Japan) to premixed aqueous solutions consisting of PAA−PNIPAM (0.8 mg/g Al2O3) and deionized water. The suspension pH values were controlled at 10 using 0.1 mol/L nitric acid and ammonia solution (Shanghai Chemical Reagent Co., China). To generate well-dispersed and stable suspensions, these mixtures were ball-milled using 10 mm diameter, high-purity zirconia balls in high-density poly(ethylene) bottles of a certain volume for 24 h at room temperature (∼22 °C). For direct-write assembly, the suspension (ϕ = 0.4) was filtered through an 800 mesh screen to
eliminate any large agglomerates. An aliquot of cellulose (Shanghai Chemical Reagent Co., China) stock solution was added to yield a final cellulose concentration of 10 mg/mL, and the suspension was once again homogenized by ball-milling for 3 h. Cellulose served as a plasticizer in our experiments, thereby guaranteeing the rigid core-fluid shell architecture of the deposited filaments to facilitate rod−rod adhesion during assembly.4,18 The typical composition of the suspensions (ϕ = 0.4) for 3D printing were 10 mL of deionized water, 26.5 g of alumina powder, 21.2 mg of PAA−PNIPAM, and 100 mg of cellulose. Characterization. Adsorption isotherm measurements were conducted using a total organic carbon (TOC) analyzer (TOC-L, Shimadzu, Kyoto, Japan), which provided a quantitative measure of the nonadsorbed fraction of PAA−PNIPAM in solution. Al2O3 suspensions (ϕ = 0.10, pH = 10) were prepared as described earlier, and then centrifuged at 3000 rpm for 10 min to separate the solids and supernatant. The supernatant was immediately decanted and diluted with deionized water for the TOC measurement. Several aliquots were taken from each sample and measured, and an average value was reported based on standard calibration curves (correlation coefficient, R = 0.99) obtained for pure dispersant solutions of varying concentrations. This experiment was performed at ambient temperature (22 °C). Zeta-potential (ζ) measurements (Zetasizer4; Malvern Instruments, Ltd., Malvern, U.K.) were carried out on alumina suspensions (ϕ = 0.0001) as a function of pH in the presence and absence of thermosensitive dispersant PAA−PNIPAM. The rheology of the fresh suspension was measured using a stresscontrolled rheometer (AR2000, TA Instruments, New Castle, DE) with a parallel plate (40 mm in diameter). Two modes were employed, namely, steady state flow and oscillatory mode. The viscosity measurements were performed in flow temperature ramp mode with a steady increase from 25 to 50 °C, at a shear rate of 10 1/s. An oscillatory temperature ramp mode was used to study the gelling behaviors of the aqueous solutions, that is, the storage modulus representing the degree of solidity was monitored with increasing temperature. The frequency was 1 Hz, and the train was controlled at 0.4%. The gap between parallel plates was 1000 μm. To minimize the thermal lag, we performed all measurements at a slow temperature ramp rate (1 °C/min). B
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces 3D Printing. 3D periodic structures were fabricated using a modified deposition apparatus based on a dispersing system (DR2203, EFD Inc., East Province, RI) consisting of four parts: a three-axis moving stage, a suspension delivery system, a fluid dispenser, and a temperature-controlling coagulation reservoir. The delivery system, connected to an air-powdered fluid dispenser (Performus V, EFD, Inc.), was mounted on the three-axis stage controlled by a computer regulator. The reservoir underlying the deposition nozzle of the delivery system was filled with a nonwetting oil and mounted on an x-y stage. The suspension (40 vol % microparticles) housed in a 10 mL syringe (EFD Inc.) was delivered through the nozzle at room temperature (22 °C) and coagulated to form self-supporting filaments when deposited in the heated oil coagulation reservoir (T > Tcritical), i.e., it was consolidated via the rheological transition and water elapsing, and robotically deposited onto the moving x−y stage. After a given layer was generated, the stage was inclined in the z direction (0.9 diameter of filament to ensure suitable adhesion to the underlying layers) and another layer was deposited. This process was repeated to yield 3D structures.5,9 The suspension flowed through a cylindrical nozzle at the volumetric flow rate required to maintain a constant speed (v) of 1 mm/s. The nozzle with a diameter of 100 μm was utilized to create the 3D periodic structure. The temperature of the nonwetting oil was maintained at 45 °C during printing.
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RESULTS AND DISCUSSION Figure 2 shows the interaction between Al2O3 microparticles and poly(acrylic acid)-poly(N-isopropylacrylamide) (PAA− PNIPAM). The dispersant is a comb-like polyelectrolyte that contains graft chains (-PNIPAM) along its backbone of carboxylate groups (−PAA). The graft chains show a sharp phase transition from hydrophilic to hydrophobic in response to temperature.16,19 Therefore, the polyelectrolyte is a thermosensitive species. This thermosensitive dispersant strongly interacts with microparticles. PAA−PNIPAM becomes negatively charged because of its carboxyl groups (−COOH), which are fully ionized under the suspension conditions (pH > 9).20 In contrast, Al2O3 microparticles are positively charged under these conditions (Figure 2a).20 Due to strong electrostatic attractions between oppositely charged species, the polyelectrolyte firmly adsorbs onto Al2O3 microparticles to generate a dramatic charge reversal. The ζ curves shown in Figure 2a indicate that the PAA−PNIPAM species plays the same role in the zeta potential as poly(acrylic acid) ,20 which decreases the isoelectric point of alumina particles (pH = 4.2) at ambient temperature. The data shown in Figure 2a also suggest that alumina particles suspended near pH = 10 adopt an effective charge of approximately −45 mV in solution with increasing alkalinity. Because the electrophoretic mobility of particles depends upon the surface charges (the viscosity is very low and not considered), the particles adsorbed with thermosensitive dispersant species generate the observed charge build-up. The adsorption between particles and ionized species is further supported by the data shown in Figure 2b, which reflects the difference in the content of ionized PAA− PNIPAM species in the bulk of the solution. The absence of ionized species in the bulk solution, especially at the lowest ionized species concentrations studied, indicates that adsorption of the ionized species onto particles occurs at room temperature. The plateau value of adsorption, which occurs at 0.8 mg/g, reveals an adequate amount of PAA−PNIPAM for complete surface coverage of the particles. In our approach, we first generated well-dispersed alumina suspensions (pH = 10) with thermosensitive dispersant, PAA− PNIPAM. We then tailored the fluid-gel transition of the suspensions by raising the temperature above the critical value
Figure 2. (a) Zeta potential (ζ) of Al2O3 particles with or without thermosensitive dispersant, PAA−PNIPAM, as a function of pH values. (b) Adsorption of PAA−PNIPAM onto Al2O3 (solids loading ϕ = 0.1, pH = 10). Measurements were performed at ambient temperature (∼22 °C). The dotted line in panel b was calculated, representing the PAA−PNIPAM that was completely adsorbed by alumina particles.
(Tcritical ∼ 32.5 °C,19 which is the critical hydrophilic− hydrophobic transition of graft chains, − PNIPAM) to generate the desired rheological response. A rise in the viscosity of the suspensions accompanies this transition (Figure 3a). The traditional view is that the viscosity depends on the effective volume fraction, which is described by the modified empirical expressions as follows.1,21 ηr = (1 − ϕeff /ϕmax(eff))−n
(1)
ϕeff = (Vsolid + Vpolymer)/Vtotal
(2)
where ϕmax(eff) is the effective maximum volume fraction, n is the scaling exponent, Vsolid is the volume of powder (solidstatus phase), and Vpolymer is the volume of the polymer layer containing the anchored solvent. The flow properties of suspensions are governed by the volume fraction, ϕeff, which is the effective volume fraction of microparticles, and ϕmax(eff), C
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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attribute this to the greater contribution of the high volume fraction of microparticles on viscosity compared with that of nanospheres. In the oscillatory model of the plate−plate rheometer, the elastic properties dramatically rise upon exceeding the critical temperature (T > Tcritical), especially for high microparticle volume fractions (Figure 3b). The elastic modulus increases by 2 orders of magnitude (or more) due to strengthened interparticle attractions,1 as given by the following scaling relationship.24 G′ = G0(ϕ/ϕgel − 1)s
(3)
where G′ is the elastic modulus, G0 is a constant, ϕgel is the volume fraction of microparticles at the gel point, and s is the scaling exponent (∼2.5). The equilibrium mechanical properties of microparticle gels are governed by two parameters: ϕ, which is proportional to the bond density, and ϕgel, which scales inversely with bond strength. Our observations reflect that interparticle attractions intensify with increasing temperature, and microparticle gels experience significant increases in their elastic properties (Figure 3). At low temperature (T < Tcritical), the polyelectrolyte dispersant, PAA−PNIPAM (Figure 4), coated on the particles is soluble and charged. Long-range
Figure 3. Rheological behavior of alumina colloidal suspensions containing thermosensitive dispersant PAA−PNIPAM: (a) viscosity and (b) storage modulus.
which accounts for the approach to infinite viscosity (solid body) at this critical effective volume fraction. In our systems, the thermosensitive dispersant is dissolved at a low temperature (T < Tcritical), and the relationship between viscosity and the volume fraction agrees with eqs 1 and 2. However, above Tcritical, the situation changes from hydrophilic to hydrophobic due to the phase transition of grafted chains (−PNIPAM).22,23 At a high temperature (T > Tcritical), the system changes to a biphasic mixture comprised of alumina microparticles and nanospheres, which results from the phase transition of the graft chains. The Vpolymer in eq 2 changes to Vpolymer′, consisting of adsorbed and unadsorbed thermosensitive polymers, which enhance the effective volume fraction of the suspension.22 In addition, under such conditions, the interaction forces (e.g., electrostatic and steric forces) between microparticles and nanospheres are weakened, which induces flocculation of biphasic particles. Furthermore, aggregation among organic nanospheres is another reason for the rheological transition of suspensions. The volume fraction of microparticles plays an essential role in fluidity. At lower microparticle volume fractions ( Tcritical) (Figure 2a). However, at higher microparticle volume fractions (>0.2), the viscosity is almost independent of the microparticle volume fraction. We
Figure 4. Schematic illustration of (a) the molecular architectures of the thermosensitive dispersant PAA−PNIPAM, (b) alumina-coated PAA−PNIPAM, and (c) the suspension comprised of alumina powder and the PAA−PNIPAM. The graft chains adsorbed on alumina microparticles have extending and soluble tails at low temperatures (T < Tcritical), which turn into globules and solid-status nanospheres at high temperatures (T > Tcritical).
van der Waals interactions dominate the bonding between microparticles in the suspensions. However, at high temperature (T > Tcritical), the graft chains of the thermosensitive dispersant still covering the microparticles precipitate from solvent and form solid-status nanospheres (Figure 4). The interparticle bonding intensifies because of the decrease of the D
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces aforementioned interaction force in the biphasic mixture. Note that the curves shown in Figure 2 should level off when the temperature is above the critical value (T > Tcritical), which might be higher than that in practice due to solvent evaporation. Interestingly, the Al2O3 suspensions containing various microparticle volume fractions (ϕ = 0.1−0.4) exhibit significant differences in elasticity due to the changing temperature (Figure 3). When the critical temperature is exceeded (T > Tcritical), the elastic modulus responds faster in suspensions containing high microparticle volume fractions compared to those with low microparticle volume fractions (ϕ < 0.1). The differences reveal interactions between oppositely charged nanospheres and PAA−PNIPAM-coated Al2O3 microparticles at higher temperatures. Because the microparticles are closer to each other in suspensions containing higher microparticle volume fractions,21 the bonding is much easier to generate. To illustrate the ability of tailoring rheological behavior, we used concentrated alumina suspensions for 3D printing of 3D periodic structures. The suspension (ϕ = 0.4) was utilized because its elasticity can be sharply tailored by raising the temperature (Figure 3). This suspension possesses the following advantages: (1) its viscosity at room temperature is low enough to enable smooth flow through the deposition nozzle without nozzle clogging; (2) at high temperatures (T > Tcritical), it generates a microparticle gel that exceeds the minimum elasticity (G′ = ∼ 104 Pa) required to print the 3D stuctures,9 and (3) the microparticle volume fraction is high enough to minimize drying-induced shrinkage of the assembled structure.25,26 The 3D periodic structure (five layers, Figure 5) was assembled with the modified apparatus by squeezing the suspension through a cylindrical nozzle. Optical images (Figure 5a,b) show a tetragonal symmetry for 3D lattices, which reveals that rods deposited on the top and underlying layers exhibit excellent height uniformity, even across spanning distances
approaching 0.5 mm. Higher magnification views of the structure (Figure 5c,d) illustrate the cylindrical nature of the rods, their smooth surface, and high degree of regularity, which are preserved during printing and drying.
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CONCLUSIONS
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AUTHOR INFORMATION
In summary, we have developed an interesting avenue to tailor the rheological behaviors of suspensions, from stable fluid to colloidal gel, with increasing temperature, which is favorable for 3D printing. The thermosensitive polyelectrolyte, poly(acrylic acid)-poly(N-isopropylacrylamide), was synthesized and utilized as a dispersant to stabilize the alumina microparticles, and the suspension rheology was conveniently regulated by modulating the temperature. Equally important, our suspension design and patterning approach can be extended to other stimuli-responsive polyelectrolytes through control under fields, such as thermal, electric, and magnetic fields. For example, functional colloidal suspensions are engineered with the comblike dispersant containing stimulus graft chains, that is, thermoresponsive (e.g., poly(ethylene oxide)27) or electroresponsive (e.g., polythiophene28 and polypyrrole29). This broad palette of suspensions will enable a myriad of technological applications to be pursued, including metamaterials, photonic crystals, drug delivery materials, tissue engineering materials, and precursors for novel coatings and composites.
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grant No. 51202296), and supported by State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. We thank Prof. Haipu Li and Dr. Guipeng Yu (School of Chemistry and Chemical Engineering, Central South University) for fruitful discussions and synthesis of poly(acrylic acid)-poly(N-isopropylacrylamide).
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REFERENCES
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Figure 5. Optical images of 3D periodic lattice (five layers) assembled from thermosensitive alumina suspensions deposited through a 100 μm nozzle. (a) Top view, (b−d) higher magnification images of the 3D structure shown in panel a. The deposited rods maintain their cylindrical shape, smooth surface, and high degree of regularity upon assembly and drying. E
DOI: 10.1021/acsami.5b07913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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