Article pubs.acs.org/Langmuir
Drying Mechanism of Poly(N-isopropylacrylamide) Microgel Dispersions Koji Horigome† and Daisuke Suzuki*,†,‡ †
Graduate School of Textile Science & Technology, Shinshu University, 3-15-1, Tokida, Ueda 386-8567, Japan International Young Researchers Empowerment Center, Shinshu University, 3-15-1, Tokida, Ueda 386-8567, Japan
‡
S Supporting Information *
ABSTRACT: The drying mechanism of poly(N-isopropylacrylamide) (pNIPAm) microgel dispersions was investigated. The microgels were synthesized by temperature-programmed aqueous free radical precipitation polymerization using NIPAm, N,N′-methylenebis(acrylamide), and water-soluble initiator. Drying processes of the microgel dispersions were observed with a digital camera and an optical microscope, and the resultant dried structures were observed by scanning electron microscopy. We found that the presence of the microgels changed the behavior of the drying process of water. In particular, the microgels were adsorbed at the air/water interface selectively within a few minutes irrespective of the microgel concentration. The relationship between the drying mechanism and structure of the resultant microgel thin film has been clarified by changing the microgel concentration of the dispersions.
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of particle films. These studies may find many applications in photonic devices,10,12,14,34,35 inkjet printing,36 DNA chip,37 and coating.38 Hydrogel particles (microgels) are spherical, cross-linked polymeric networks swollen by a good solvent.39−41 One of the most extensively studied stimulus-responsive microgels is composed of poly(N-isopropylacrylamide) (pNIPAm), which is a thermosensitive polymer and exhibits a lower critical solution temperature (LCST) around ∼31 °C in aqueous media.42,43 Microgels composed of pNIPAm show a volume phase transition temperature (VPTT) around the LCST.44 In 1986, Pelton and Chibante first observed drying structure of the pNIPAm microgels by transmission electron microscopy.45 When the microgel dispersions were dried, non-close-packed ordered structure of microgels were assembled on a glass surface. Tsuji and Kawaguchi reported that dried structure of the microgels produced iridescent colors due to diffraction.34 The color was depended on incident light and observation angle. Okubo et al. investigated that drying dissipative structure of the pNIPAm microgels at high concentration (2.52−0.038 wt %).46 They revealed that the flickering (or flame-like) spoke lines were observed especially at low temperatures and at high concentrations. Quint et al. presented a simple approach for the fabrication of two-dimensional non-close-packed arrays with exceptional long-range order which result from the self-healing properties of the microgels.47 Recently, we reported that drying
INTRODUCTION When a droplet of colloidal dispersions dries on a solid surface, a thin ring-shaped stain is typically left, wherein most of the solid material is deposited after evaporation. This phenomenon is known as the coffee-ring effect,1−5 which is produced by radial capillary flows from the drop center to its edge. Then, suspended or dissolved solutes assemble to the perimeter. The first theory of evaporation of a droplet in gaseous media was presented in the 19th century. Deegan et al. formulated a theoretical basis for the convection flow responsible for particle transport in the droplets.1,6,7 Hu and Larson improved upon the model of Deegan et al. to include the effect of Marangoni flow due to thermal gradients along the meniscus surface.8 The evaporation mechanisms have been exploited as a means of particle self-assembly. For example, one- or two-dimensional structures of colloidal particles have been formed by capillary flow and evaporative flux driven assembly on a solid surface.9−13 McGrath et al. have demonstrated three-dimensional ordered structures were formed by simple solvent evaporation.14 Harris et al. showed that honeycomb and parallel pattern formations of the particles were formed by controlling of convection.15 In addition, there were many studies of drying phenomena: the studies focusing on capillary force,16,17 the deposition environment,18 kinds of particles such as core− shell,19,20 ellipsoid particles,21 and kinds of solid substrates22 in order to control the properties of the resultant films. Subsequent work has further expanded these knowledge to include an understanding of how temperature,23 pH,24−26 charge,19 electric or magnetic field,27,28 particle size,29 and hydrophilic or hydrophobic properties30−33 affect the formation © 2012 American Chemical Society
Received: June 18, 2012 Revised: August 1, 2012 Published: August 23, 2012 12962
dx.doi.org/10.1021/la302465w | Langmuir 2012, 28, 12962−12970
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droplet was taken with a digital camera (Canon, EOS kiss ×4). Then, interfaces of the air/water and the solid/water and droplets of the microgel dispersions were observed with the optical microscope as a function of time. The center-to-center distances of microgels were measured from optical microscope images. The averages of the centerto-center distances were calculated from 20 measurements. The width and height of the droplets were measured with a calliper. The contact angles were analyzed by Image J ver.1.45 m (Wayne Rasband (NIH), free software). The microgels dried on the substrate were observed by scanning electron microscopy (SEM; Hitachi Ltd., S-3000N). The samples were sputtered with Pt/Pd before the observation.
structure of mixture of cationic and anionic microgels in various conditions.48 Even through the cationic and anionic microgels were flocculated, resultant microgel thin films were not multiple layer but monolayer. These drying mechanisms of microgels are very interesting. However, to the best our knowledge few works have been reported on drying mechanism of the microgel dispersions in detail. In this present work, we report on the behavior of microgels through drying of the dispersions. The microgels of pNIPAm cross-linked N,N′-methylenebis(acrylamide) (BIS) were selected since the microgels have been investigated extensively, and their properties can be tuned precisely. Herein, the size of the pNIPAm microgels was controlled in order to observe the microgels clearly with an optical microscope. Different concentrations of microgel dispersions were monitored with a digital camera, an optical microscope, and a scanning electron microscope.
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RESULTS AND DISCUSSION Microgel Synthesis and Characterization. The synthesis of the pNIPAm microgels was carried out by aqueous free radical precipitation polymerization. In this case, to observe the microgels clearly with the optical microscope, micrometersized, large pNIPAm microgels were synthesized by using the temperature-programmed method.49 The resultant microgels were characterized using DLS, laser Doppler velocimetry, and optical microscopy: Table 1 summarizes the hydrodynamic
EXPERIMENTAL DETAILS
Materials. All reagents were purchased from Wako Pure Chemical Industries and were used as received. Water for all reactions, solution preparation, and polymer purification was first distilled and then ionexchanged (EYELA, SA-2100E1). Polystyrene substrate (Iwaki, 60 mm/Nontreated Dish, Asahi Glass Co., Ltd.) was used as received. Preparation of pNIPAm Microgels. Microgels were prepared by temperature-programmed precipitation polymerization reported previously.49 A mixture of N-isopropylacrylamide (NIPAm, 3.361 g, 99 mol %), N,N′-methylenebis(acrylamide) (BIS, 0.046 g, 1.0 mol %), and water (195 mL) was poured into a 300 mL three-neck, roundbottom flask equipped with a mechanical stirrer, a condenser, and nitrogen gas inlet. The total monomer concentration was fixed at 150 mM. The monomer solution was bubbled for 30 min with nitrogen gas to purge oxygen at 45 °C. Under a stream of nitrogen and with constant stirring at 200 rpm, the initiator potassium peroxodisulfate (KPS: 0.108 g) dissolved in 5 mL of water was injected to the flask to start the polymerization. After 15 min, a temperature was increased from 45 to 70 °C at a rate of ∼4.5 °C/min. Then the polymerization continued for 4 h. After the polymerization, the microgel dispersion was cooled to room temperature. The microgels were purified by centrifugation/redispersion with water twice using a relative centrifugal force (RCF) of 50000g and by means of dialysis for 3 days. Characterization of pNIPAm Microgels. The hydrodynamic diameters of the microgels were characterized by dynamic light scattering (DLS: Malvern Instruments Ltd., ZetasizerNanoS). DLS data were an average of 15 measurements with 30 s acquisition times. The microgels were analyzed at a concentration of ∼0.005 wt %. NaCl was then used to adjust to 1 mM total salt concentration. The samples were allowed to equilibrate thermally at the desired temperature for 10 min before the measurements. Scattered light was collected at 173°. The hydrodynamic diameters of the microgels were determined from the measured diffusion coefficients by using the Stokes−Einstein equation (Zetasizer software v6.12). The microgels in an aqueous solution were observed with an optical microscope (BX51, Olympus) equipped with a digital camera (ImageX Earth Type S-2.0 M Ver.3.0.5, Kikuchi-Optical Co., Ltd.). The microgels were transferred into Vitrotube borosilicate rectangular capillaries (0.1 × 2.0 mm) by capillary action. Herein in order to observe the microgel clearly, colloidal crystals of the microgels were formed through the thermal annealing process.50,51 The electrophoretic mobility (EPM) of the microgels was measured with a ZetasizerNanoZS (Malvern, Zetasizer software Ver. 4.20). The samples were allowed to equilibrate thermally at the desired temperature for 10 min before the measurements. The microgels were analyzed at a concentration of ∼0.005 wt %. NaCl was used to adjust to 1 mM total salt concentration. Observation of the Drying Process. Dispersions of the microgels (50 μL) was put on the polystyrene substrate and dried at room temperature (25 ± 2 °C, humidity
