Binary Solvent Colloids of Thermosensitive Poly(N ... - ACS Publications

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Binary Solvent Colloids of Thermosensitive Poly(N‑isopropylacrylamide) Microgel for Smart Windows Mi Wang,†,‡ Yanfeng Gao,*,‡,§ Chuanxiang Cao,‡ Kaimin Chen,*,∥ Yicun Wen,† Dingye Fang,† Li Li,† and Xuhong Guo*,†,⊥ †

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ Shanghai Institute of Ceramics, 1295 Dingxi Road, Shanghai 200050, China § School of Materials Science and Engineering, Shanghai University, 99 Shangda Rd., Baoshan, Shanghai 200444, China ∥ College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China ⊥ Key Laboratory of Materials-Oriented Chemical Engineering of Xinjiang Uygur Autonomous Region, Shihezi University, Xinjiang 832000, PR China ABSTRACT: Thermosensitive poly(N-isopropylacrylamide) (PNIPAAm) microgel colloids were prepared by using water and high-boiling alcohol as binary solvent. Their thermosensitive behavior and solar modulation ability were studied by differential scanning calorimetery, ultraviolet−visible−near-infrared spectrophotometery, dynamic light scattering, and rheology. Effects of alcohol content and cross-linker dose on their microstructures and optical properties were investigated. A model house was constructed to test their energy-saving performance in smart windows. It was found that the solar modulation ability of PNIPAAm microgel colloids decreased with increasing N,N′-methylenebis(acrylamide) (BIS) dose or alcohol content. Compared to glycol, glycerol showed better compatibility with PNIPAAm hydrogels, inducing less deterioration of the solar modulation ability. With 0.1 wt % (of NIPA) BIS, when glycerol was added as a cosolvent, the prepared PNIPAAm microgel colloids exhibited spherical morphology, controllable LCST, short response time, suitable viscosity, low freezing point, restrained evaporation rate, and excellent energy-saving performance, which makes them much better candidates for application in smart windows than those using a single solvent.

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INTRODUCTION The term “smart window” refers to a system that can intelligently control the amount of transmitted light and heat in response to external stimuli, such as electricity (electrochromic),1 gas (gasochromic),2 heat (thermochromic),3,4 and light (photochromic).5 Thermochromic smart windows have attracted much attention because solar energy itself is used as a promoter against solar heat.6 Thermosensitive water-soluble polymers have recently attracted intensive investigation because of their thermochromic performance at near-room temperatures. The majority of this kind of polymers exhibit a lower critical solution temperature (LCST), below which the polymer is soluble and above which the polymer becomes insoluble.7,8 Consequently, the optical appearance of these polymers shifts between opaque and transparent upon heating or cooling, which makes them promising candidates for solar control glazing.9 As one of the typical thermosensitive polymers, poly(Nisopropylacrylamide) (PNIPAAm) features a LCST at about 32 °C, which is close to body temperature, and is relatively insensitive to concentration, pH, and ionic strength.10,11 Because of the reversible abrupt transparent−opaque transition,12 strong resistance to UV radiation,13 and low-cost preparation,14 PNIPAAm hydrogels have great potential application in smart windows. Smart windows based on thermosensitive PNIPAAm hydró et al.17−19 gels have been reported in the literature.15,16 Zrinyi © 2014 American Chemical Society

prepared thermotropic windows based on PNIPAAm hydrogels, whose transparency could be modulated not only by direct heat treatment but also by audio frequency electrical stimuli. Thermotropic PNIPAAm hydrogel layers place high demands on the sealing of the glazing system. If it is not sealed properly, the layer will dry out and cannot reverse autonomously with decreasing temperature. Watanabe20 constructed a PNIPAAmbased “affinity intelligent window” with an area of 1 m2, which was successfully operated in outdoor testing over a period of two years. Gao and co-workers21 designed a thermochromic system, which consisted of a glass cell filled with aqueous solution of PNIPAAm and showed a maximum reflection ratio of 33%, which led to a decrease of the cell temperature by 6.0 °C compared with that without PNIPAAm. However, it was a significant challenge for thermosensitive hydrogels to achieve both fast response and structural integrity during the repeated volume changes until the exploration of microgel.22 Nowadays, widespread interest has been concentrated on microgels, miniature hydrogels with sizes ranging from tens of nanometers to several microns, which exhibit many advantages over macrogels.23 One major advantage is that microgels undergo a rapid phase transition in response to external stimuli. Received: Revised: Accepted: Published: 18462

July 15, 2014 November 10, 2014 November 14, 2014 November 14, 2014 dx.doi.org/10.1021/ie502828b | Ind. Eng. Chem. Res. 2014, 53, 18462−18472

Industrial & Engineering Chemistry Research

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methylenebis(acrylamide) (BIS, 99%) and potassium persulfate (KPS, 99%) were bought from J&K Chemical Company (Shanghai, China). Poly(vinylpyrrolidone) (PVP, K30, Mw = 58 000 g/mol), glycol, and glycerol were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. All the agents were used without further purification. Water used in all syntheses was distilled and purified using osmosis and subsequent ion exchange (Millipore Mill-Q). Synthesis of PNIPAAm Microgel Colloids. Components of 0.05 g of PVP, 2.5 g of NIPA, and a certain amount of BIS (0.0025−0.025 g, 0.1−1 wt % of NIPA) were added in 100 g of water−glycol or water−glycerol solvent, for which the alcohol content was calculated by the weight percent. The mixture was stirred at 50 °C until a clear, transparent solution was formed to ensure complete dissolution. Then the solution was transferred to a three-necked 250 mL flask equipped with a stirrer (300 rpm), a nitrogen inlet, and a dropping funnel of constant pressure, and the flask was placed in an oil bath. After that, the reaction mixture was heated to 60 °C under a stream of nitrogen over the course of 1 h. Following the addition of 5 mL of KPS solution with the concentration of 0.01 g/mL (2 wt % of NIPA), the polymerization was carried out at 60 °C for 12 h under a nitrogen atmosphere. The final product was naturally cooled to room temperature and collected into transparent glass bottles. There was no prior treatment of samples before characterization, and the final polymer concentration was around 2.5 wt %. Characterization. The optical transmittances were monitored in a glass cuvette on the UV−vis−NIR spectrophotometer (Hitachi U-4100, Japan), shown in Figure 1. Products

Another advantage is that particles with narrow size distribution can be formed after phase transition of microgels, resulting in a uniformly distributed scattering state through the whole area without appearance of streaks. Therefore, thermosensitive microgels were chosen here as functional materials for smart windows to overcome the shortcomings in typical macrogels. Depending on the chemical composition and structure at both the molecular and the macroscopic levels, PNIPAAm may show significant differences in thermosensitive properties.24 The LCST of PNIPAAm can be affected by the nature of substituent groups, chain architectures, and additives.25 Over the past two decades, considerable research has been performed to tune the LCST of PNIPAAm, such as copolymerizing with more hydrophilic (higher LCST) or more hydrophobic (lower LCST) comonomers,26−29 controlling the chain architectures,30 and adding cosolvents.31 Among these methods, copolymerizing with other monomers usually induces some deterioration of the thermochromic performance of PNIPAAm, while controlling the chain architectures of PNIPAAm often makes the synthetic route more complicated and hard to manipulate.24,32 Kim and Cho33 prepared thermosensitive hydrogels based on interpenetrating polymer networks of PNIPAAm and polyurethane and found that the light control ability of products decreased with increasing polyurethane content. Kamigaito and co-workers34 found that by copolymerizing with amidemonomer or ester-monomer the LCST of PNIPAAm hydrogels was tunable in the range of 24.5−39.6 °C, but the hysteresis between the LCSTs during the heating−cooling cycle became larger at a higher comonomer content. Comparatively, adding cosolvent is more facile to scale-up, and the existing results explicitly elucidate that cosolvents can essentially change the LCST of PNIPAAm hydrogels.35 Nevertheless, the reported cosolvents include methanol,36 ethanol,37 acetone,38 2-propanol,39 tetrahydrofuran,40 dimethyl sulfoxide,41 benzaldehyde,42 and dimethylformamide,43 which are more volatile than water or toxic to humans, both resulting in higher requirements on the sealing of smart windows. Additionally, the influence of cosolvents on the other properties of products, such as response rate, viscosity, freezing tolerance, and recycling performance, which are all important parameters for industrialization, have been scarcely studied up to now. We report a thermochromic system for smart windows based on PNIPAAm microgel colloids, which were prepared using a binary solvent of water−alcohol mixture via suspension polymerization. Glycol and glycerol, both showing boiling points much higher than that of water, were adopted in the experiments and N,N-methylenebis(acrylamide) (BIS) was used as the cross-linker. Effects of solvent composition and cross-linker dose on the phase transition behavior of microgels were investigated using differential scanning calorimetery (DSC), ultraviolet−visible−near-infrared (UV−vis−NIR) spectrophotometery, dynamic light scattering (DLS), and rheology. The solar modulation abilities of the PNIPAAm microgel colloids were calculated based on the solar irradiance spectrum, and the energy-saving efficiencies of PNIPAAm-based smart windows were evaluated at bench scale. The influences of cosolvent on the structure, thermosensitivity, viscosity, freezing tolerance, and volatility of PNIPAAm microgel colloids were also investigated.

Figure 1. Testing device for the optical properties of PNIPAAm microgel colloids.

with a thickness of 2 mm were sandwiched between two pieces of float glass with a thickness of 4 mm and heated by circulating water. A thermocouple was used for temperature feedback. The morphology and size of microgels were characterized by transmission electron microscopy (TEM, JEM2010, JEOL, Japan). A drop of PNIPAAm microgel colloids was deposited on the carbon-coated copper grids and dried by infrared radiation before TEM analysis. The hydrodynamic diameter (Dh) of microgels was measured by DLS with a Particle Sizing System PSS 380 lighting scattering goniometry, where a He− Ne laser with wavelength 633 nm was used. The phase transition and the freezing properties of the resulting products were measured by DSC (DSC200F3, NETZSCH) in nitrogen flow over the temperature range of −50 to 60 °C with a heating rate of 2 or 10 °C/min. The samples applied for DSC

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EXPERIMENTAL SECTION Materials. N-isopropylacrylamide (NIPA, 98%) was obtained from AR, Aladdin Reagent (China) Co., Ltd. N,N′18463

dx.doi.org/10.1021/ie502828b | Ind. Eng. Chem. Res. 2014, 53, 18462−18472


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of water at 15 and 50 °C (left panel in Figure 2) indicates that the thermochromic behavior of the products comes from PNIPAAm. At 15 °C, PNIPAAm microgel colloids exhibited really high transparency, close to that of water, which decreased sharply with increasing temperature and became very low at 50 °C (right panels in Figure 2). The thermochromic performance of PNIPAAm microgel colloids prepared with appropriate amount of glycerol or glycol is almost the same as that without cosolvent. However, with a large amount of glycerol or glycol, the transparency of PNIPAAm microgel colloids below LCST will be sharply decreased and inhomogeneity will appear above LCST. Figure 3 shows the transmittances of PNIPAAm microgel colloids prepared with a binary solvent with different BIS doses and alcohol types. To avoid precipitation of polymers at temperatures above LCST, only a small amount of cosolvent, 5 wt % of total solvent, were added. It is elucidated that all products possess thermochromic properties, exhibiting transmittance at the low temperature much higher than that at the high temperature. Under the same BIS dose, the transmittance at 15 °C of the water−glycerol system was comparatively higher than that of the water−glycol system, especially for the PNIPAAm microgel colloids with 0.5 wt % (of NIPA) BIS. However, with 0.1 wt % (of NIPA) BIS, the water−glycol system presented slightly higher transmittance at 50 °C than the water−glycerol system. The LCST behavior of PNIPAAm hydrogels results from a balance between hydration (hydrogen bonds between water and polymers) and hydrophobic aggregation of PNIPAAm chains.44 PNIPAAm consists of both hydrophilic groups (−C O and −N−H) and hydrophobic moiety (hydrocarbon backbone and isopropyl groups). For PNIPAAm hydrogels at temperatures below LCST, the hydrogen bonds between the hydrophilic groups and solvent (−CO···H−O−H and H2O···H−N−) predominate over polymer−polymer interaction (−CO···H−N−) to stabilize a chain conformation of PNIPAAm, and the isopropyl groups were surrounded by “water cages”.45 The entropic gain of these ordered water molecules becomes larger with increasing temperature, and once it overcomes the enthalpy of hydrogen bonds between PNIPAAm hydrophilic groups and water molecules, PNIPAAm undergoes a sharp, well-defined, coil-to-globule transition, resulting in polymer self-association and phase separation.46 The demixing behavior of PNIPAAm hydrogels/water system is mainly dominated by the hydrophobic interaction, which can be regulated by adding cosolvents. Take methanol as an example. For PNIPAAm gels prepared using water− methanol mixtures, water is preferably attached to the oxygen

characterization were without any post-treatment and with original polymer concentration of 2.5 wt %. The rheological measurements were performed on a Physica MCR501 (Anton Paar) rheometer equipped with a Peltier device for temperature control. Parallel-plate geometry was used. The plate was made of stainless steel with a diameter of 25 mm. The upper plate was set at a distance of 1 mm, and a solvent trap was adopted to minimize water evaporation.

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RESULTS AND DISCUSSION Effect of Alcohol Type. To lower the sealing requirement of smart glazing and to avoid introducing toxicity, glycerol and glycol were adopted as cosolvents for the synthesis of PNIPAAm microgels. As the most commonly used high-boiling alcohols, glycerol and glycol show atmospheric boiling points of 290.9 and 197.3 °C, respectively, which are much higher than that of water. As is known, glycerol−water and glycol−water mixtures are frequently applied as antifreeze solutions, which may bring lower freezing points to extend the working area of PNIPAAm-based smart windows. Transmission curves over the whole solar spectrum (380− 2600 nm) of PNIPAAm microgel colloids were recorded to calculate the energy-saving performance of PNIPAAm-based smart windows. As the LCST of PNIPAAm hydrogels is around 32 °C, the optical properties of PNIPAAm microgel colloids before and after the phase transition were measured at 15 and 50 °C, respectively. Figure 2 presents the transmittance curves

Figure 2. Transmittance curves of water (left panel) and photographs of PNIPAAm microgel colloids prepared with pure water and 0.1 wt % (of NIPA) BIS at 15 and 50 °C (right panels).

of water and the typical photographs of the resulting products at temperatures below and above LCST. It is seen that thermochromic PNIPAAm microgel colloids were obtained, and the negligible difference between the transmittance curves

Figure 3. Transmittance curves of PNIPAAm microgel colloids prepared with various doses (0.1−1 wt % of NIPA) of BIS and binary solvent: (a) water−glycerol and (b) water−glycol. 18464



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Figure 4. TEM images of PNIPAAm microgels prepared with 0.1 wt % (of NIPA) BIS and different glycerol contents: (a) 10 wt % and (b) 30 wt % (of total solvent).

Figure 5. Particle size distribution of PNIPAAm microgels determined (a, b) from TEM images in Figure 4 and (c, d) by DLS at 15 and 50 °C.

°C) and glycol (ε = 41.4 at 20 °C) are much lower than that of water (ε = 80.1 at 20 °C). Thus, the addition of glycerol or glycol renders solvent lower polarity. Adding glycerol or glycol can reduce the hydrophilic interaction below LCST because of its hydrophobic glyceryl or ethylidene groups, and reduces the hydrophobic interaction above LCST because of its polyhydroxy structure. Because the polarity of glycerol is higher than that of glycol, glycerol showed a relatively smaller deterioration on the swelling−deswelling behavior of PNIPAAm microgels. The glycerol−water system showed higher solubility of swollen PNIPAAm microgels and lower solubility of deswollen PNIPAAm microgels, compared to the that of the glycol−water system. This results in higher transmittance below LCST, lower transmittance above LCST, and better thermochromic performance. It can be concluded that compared to glycol, glycerol showed better compatibility to PNIPAAm hydrogels, inducing smaller deterioration of their swelling− deswelling behavior and solar modulation ability. Effect of BIS Dose. The structure and the optical properties of PNIPAAm hydrogels can be influenced significantly by the degree of cross-linking. Although self-cross-linked PNIPAAm

atom and methanol to the hydrogen atom of the amide group.47 As the hydrogen atom is further away from the backbone of the PNIPAAm chain than the oxygen atom, methanol resides preferably in the outer regions of the solvation shell with the methyl group pointing toward the bulk solvent. Therefore, the PNIPAAm chain-solvation shell entity has a hydrophobic methyl-rich surface, resulting in higher hydrophobicity and lower solubility of swollen PNIPAAm networks. After phase transition, the hydrogen bonds between polymers and solvents are deteriorated. According to the principle of “like dissolves like”, the methyl groups of methanol are more likely to reside around the isopropyl groups of PNIPAAm, with the hydroxyl groups pointing toward the bulk solvent. In addition, there will be a hydroxyl-rich shell for dehydrated PNIPAAm networks, resulting in their lower hydrophobicity and higher solubility. It is concluded from the above that the swelling− deswelling behavior of PNIPAAm microgels is restrained by cosolvent. The effect of cosolvent type on the thermochromic properties of PNIPAAm microgel colloids can be explicated by polarity. The dielectric constants of glycerol (ε = 46.5 at 20 18465



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Figure 6. DSC analysis for the LCSTs of PNIPAAm microgel colloids prepared with 0.1 wt % (of NIPA) BIS and 0−35 wt % (of total solvent) glycerol: (a) heating stage and (b) cooling stage. (c) LCST as a function of glycerol content (lines are linear fitting of data); (d) absolute ΔH as a function of glycerol content (lines are to guide the eye). Error bars are smaller than data points.

PNIPAAm microgels changed a little, while the size distribution became broader. Because the TEM measurement was performed at room temperature, which is lower than the LCST of PNIPAAm, the average size (450 and 440 nm with 10 and 30 wt % glycerol, respectively) and size distribution of PNIPAAm microgels determined by TEM agreed well with those (490 and 470 nm with 10 and 30 wt % glycerol, respectively) determined by DLS (Figure 5). It is worth noting that the size distribution of PNIPAAm microgels was widened both by increasing glycerol content from 10 to 30 wt % and increasing temperature from 15 to 50 °C. Also, the size decrease when temperature increased from 15 to 50 °C altered from 230 to 200 nm upon increasing the content of glycerol from 10 to 30 wt %. This is probably caused by the decreasing polarity of solvent and the consequently restrained swelling−deswelling behavior of PNIPAAm microgels. Effect of Glycerol on LCST. The LCST and thermal properties around LCST of the prepared PNIPAAm microgel colloids were manifested via DSC as shown in Figure 6. The heating rate was fixed at 2 °C/min to achieve thermodynamic equilibrium. With the increase of glycerol content from 0 to 35 wt % (of total solvent), the phase transition temperatures, referred to the peak temperature, reduced from 32.2 to 20.4 °C in heating operation and 30.3 to 18.2 °C for cooling operation, respectively (Figure 6a,b). This indicates that adding glycerol is an effective way to regulate the LCST of PNIPAAm hydrogels, which can greatly expand their application fields. LCSTs in both heating and cooling processes shifted down almost linearly with increasing glycerol content, and there was a hysteresis of about 2 °C in the heating−cooling process for each sample (Figure 6c), which may be due to the nonequilibrium process of DSC or the formation of some additional hydrogen bonds in its collapsed state.51 When the glycerol content increased from 0 to 35 wt % (of total solvent), the absolute enthalpy changes

microgels can be prepared by cross-linker-free polymerization routes, it is difficult to control the particle size and the thermosensitive properties.48 The stable particle formation of self-cross-linking chain networks requires that the local polymer chain concentration be high enough, which is easy to cause irreversible aggregates of PNIPAAm, resulting in bad transparency below LCST or precipitation above LCST.49 Chemical cross-linkers are usually incorporated to ensure the intact particle nature of the PNIPAAm microgels, but their swelling degree decrease with increasing cross-linking density,50 which probably induces a lower solar modulation ability of PNIPAAm microgel colloids. In this work, a small amount of BIS (0.1−1 wt % of NIPA) was added to make sure that PNIPAAm microgel colloids were obtained. For both glycerol and glycol, the transmittance at 15 °C largely decreased with increasing BIS content, while the transmittance at 50 °C reduced slightly and kept extremely low values (Figure 3). This indicates that with a small amount of alcohol, the solubility of PNIPAAm decreases with increasing BIS dose, which is probably attributed to higher cross-linking density and increased hydrophobicity of PNIPAAm networks. Thus, PNIPAAm microgel colloids with 0.1 wt % (of NIPA) BIS and varying glycerol content were prepared for further study. Effect of Glycerol on Gel Size. The morphology, size, and size distribution of PNIPAAm microgels were characterized by TEM and DLS (Figures 4 and 5). As shown by TEM images in Figure 4, well-defined spherical PNIPAAm microgels with a size range of 250−650 nm were prepared. Figure 5a,b shows the particle size distribution of the prepared PNIPAAm microgels obtained from measuring the size of at least 100 individual particles from the TEM images. Figure 5c,d shows the particle size distribution of the prepared PNIPAAm microgels determined by DLS below and above LCST. Upon increasing glycerol content, the particle size of 18466



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Figure 7. Transmittance and temperature as a function of time upon heating or cooling across LCST for PNIPAAm microgel colloids prepared with 0.1 wt % (of NIPA) BIS and different glycerol content (of total solvent): (a) 0 wt %, (b) 10 wt %, (c) 20 wt %, and (d) 30 wt %. First-order derivatives of transmittance versus time (dTr/dt) are shown as the insets.

(ΔH) during the endothermic and exothermic period decreased from 1.12 to 0.15 J/g and 1.02 to 0.13 J/g, respectively (Figure 6d). It is also worth noting that at high glycerol content the endothermic peak (Figure 6a) becomes broad and weak, but the exothermic peak (Figure 6b) is still narrow with a sharp onset. This implies that increasing glycerol content maintains the structure better for the swelling of polymers than for the deswelling of hydrogels. The effect of glycerol on thermal properties and LCST could be explicated by the hydrogen bonds between PNIPAAm and solvents and the hydrophobic aggregations between PNIPAAm chains. Both the hydrogen bonds below LCST and the hydrophobic interaction above LCST can be weakened by the increase of glycerol content, resulting in a gradually restrained swelling−deswelling transition of PNIPAAm microgels. Thus, lower energy is needed for the dehydration or hydration of polymer networks, inducing a subsequent decrease of the LCST and absolute ΔH for both heating and cooling processes. Additionally, as the hydrogen bonds between polymer and solvents are more stable than those between polymer and polymer,52 more energy is needed to break the polymer−solvent hydrogen bonds than the polymer−polymer ones. That is to say, it is easier to swell PNIPAAm networks than to deswell them. Therefore, the absolute ΔH is relatively smaller in the cooling process than in heating (Figure 6d). Effect of Glycerol on Response Speed. The response speed of PNIPAAm hydrogels can be influenced by many factors, such as size, structure, and heating rate. PNIPAAm microgels with small size exhibit response much faster than that of large ones. The deswelling time of conventional bulk PNIPAAm hydrogels was about 40 min;53 however, this was reduced to 15 s when their size was reduced to 200 μm with a porous structure.54 For PNIPAAm nanofibers with a diameter in the range of 700−900 nm, the response time to the temperature change was only 2−3 s.24 Individual NIPA colloids with a diameter of 350 nm showed evidence for nanosecond to microsecond volume phase transitions upon laser T-jumps.55

Here, the effect of cosolvent glycerol on the response speed of PNIPAAm microgels was explored. To investigate the thermosensitive response performance of the prepared PNIPAAm microgel colloids, light transmittances at 1100 nm and temperature as a function of time during the switching process across LCST are shown in Figure 7. In the heating process, a sample at 15 °C was quickly transformed to a water bath at 50 °C and its transmittance was recorded using a spectrophotometer. In the cooling process, a sample at 50 °C was transferred to a water bath at 15 °C. Both cooling and heating process in all cases were kept the same and within a very short time. As shown in Figure 7, all samples show reversible LCST behavior, and abrupt transmittance changes, refer to phase transitions, occur during the switching process. The duration time, defined by the time region within which transmittance change between neighboring points is larger than 2%, of transparent−opaque transition increased from 50 to 150 s with increasing glycerol content from 0 to 30 wt % (of total solvent). The dTr/dt plots inserted in Figure 7 illustrate that the longer transition time is mainly due to the extended trailing edge. This indicates that the phase transition of PNIPAAm hydrogels includes more than one step, especially for system with high glycerol content. At low glycerol contents (0 to 10 wt % of total solvent), the phase transition spent almost the same time upon heating and cooling. But, at higher glycerol contents (20 and 30 wt % of total solvent), the transparency−opaque transition spent much longer time than the opaque−transparency transition, which was consistent with the DSC data in Figure 6a,b. The microdynamics during the deswelling process of the PNIPAAm hydrogels upon heating can be described as follows:56 The isopropyl groups of PNIPAAm first undergo dehydration. The main chains follow by diffusion and aggregation; hydrophilic groups then give up their hydrogen bonds with water. Finally, hydrogen bonds between the hydrophilic groups of PNIPAAm form to stabilize the polymer chains, which can be mainly divided into two parts: dehydration 18467



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Figure 8. Viscosity of PNIPAAm microgel colloids prepared with 0.1 wt % (of NIPA) BIS and 0−35 wt % (of total solvent) glycerol. (a) Viscosity as a function of shear rate at 10 °C; (b) viscosity as a function of temperature with a shear rate of 10 s−1 and a heating rate of 8 °C/min.

Figure 9. (a) DSC curves and (b) calculated freezing points and absolute ΔH of PNIPAAm microgel colloids. Samples were prepared with 0.1 wt % (of NIPA) BIS and 0−30 wt % (of total solvent) glycerol.

by deteriorating hydrogen bonds between solvent and polymers and polymer shrinking by forming new hydrogen bonds between polymers. Here, in the heating stage, the abrupt decrease of transmittance can be attributed to dehydration of PNIPAAm chains, while the lagging end can be attributed to the polymer shrinking to form stabilized status. At the end of dehydration, the interactions between the glyceryl groups of glycerol and the isopropyl groups of PNIPAAm delayed the hydrophobic aggregation, resulting in a reduced shrinking speed. However, the opaque−transparent process transformed in the opposite direction during cooling: hydrogel expanded by breaking hydrogen bonds between polymers and forming hydrogen bonds between solvent and the hydrophilic groups of PNIPAAm. Effect of Glycerol on Viscosity. The rheological properties of the prepared PNIPAAm microgel colloids were illustrated in Figure 8. Their apparent viscosity decreased monotonically, and the shear-thinning phenomenon tended to disappear upon replacing more water by glycerol, which indicated that the viscosity was dominated by the interaction between solvent and polymer as well as the interaction among PNIPAAm microgel particles. The polarity of glycerol and its interaction with PNIPAAm are weaker compared to that of water, although its viscosity is much higher than that of water. To further investigate the phase transition of PNIPAAm microgel colloids, the apparent viscosity as a function of temperature for samples with different glycerol content are summarized in Figure 8b. Interestingly, the apparent viscosity increased abruptly upon increasing temperature to LCST, and the LCST decreased with increasing glycerol content, which was fully consistent with the DSC results in Figure 6c. Before and after the LCST, the viscosity decreased gradually with increasing temperature. The sudden enhancement of viscosity may be attributed to the new generation of hydrophobic interactions among PNIPAAm polymer chains during the phase

separation at LCST. On the other hand, the microgel dimension change also contributes to the viscosity variation during the increase of temperature. Effect of Glycerol on the Freezing Point and Volatility of Water. Mixtures of water and glycerol are usually used as antifreeze for industrial applications. The freezing point of the mixture gradually reduces from 0 to −12.2 °C as the glycerol content increased from 0 to 35 wt % (of total solvent).57 In this work, the freezing points of the prepared PNIPAAm microgel colloids were manifested via DSC with a heating rate of 10 °C/ min (Figure 9). With increasing glycerol content from 0 \to 35 wt % (of total solvent), the freezing temperature and the absolute freezing enthalpy (ΔH) decreased from −18.1 to −32 °C and 257.4 to 111.7 J/g, respectively. This indicates that adding glycerol is an effective way to reduce the freezing point of PNIPAAm hydrogels, which will largely expand their applications. The freezing point of the prepared PNIPAAm microgel colloids is much lower than that of the water−glycerol mixture with the same glycerol content. The absolute freezing enthalpy of the PNIPAAm microgel colloids without glycerol is much smaller than that of bulk water (333.5 J/g at around 0 °C), indicating that the polymer− water hydrogen bonds sharply restrain the freezing of water. For a water-rich glycerol−water system, the extra water is free from the glycerol hydrogen bond network and forms water cooperative domain, which leads to the freezing of water upon cooling. With the formation of the frozen water state, some interfacial water presents between the ice core and mesoscopic glycerol−water domain, in which glycerol concentration was enriched up to the critical value of 40 mol %.58 Here, the decrease in ΔH can be attributed to the decreasing amount of extra water. The evaporation of water in the synthesized PNIPAAm microgel colloids is shown in Figure 10. PNIPAAm microgel colloids (20 g) were poured into glass bottles of 25 mL, sealed 18468



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where T(λ) denotes the transmittance obtained by spectrophotometer at wavelength λ and Eλ(λ) is the solar irradiance spectrum for air mass 1.5 corresponding to the sun standing 37° above the horizon. The visible region and the infrared region of sunlight are the wavelength ranges of 380−760 nm and 760−2600 nm, respectively. The light modulation ability (ΔT) for the solar energy, visible light, and infrared light were obtained by the equation ΔT = Tint(L) − Tint(H), in which, Tint(L) is the integral transmittance at a low temperature and Tint(H) is the integral transmittance at a high temperature. Figure 11a shows the transmittance curves of PNIPAAm microgel colloids prepared using water−glycerol solvent with various glycerol contents. All the samples displayed thermochromic property, which is suitable for application in smart windows. At the low temperature (15 °C), the samples were clear and exhibited extremely high transmittances for sunlight. At the high temperature (50 °C), the samples became milky white and blocked most of the sunlight. Figure 11b illustrates the Tint of solar energy before and after LCST. With increasing glycerol content from 0 wt % to 35 wt % (of total solvent), the Tint at 15 °C kept above 80% without significant change, while the Tint at 50 °C increased from 8% to 21%. The gradually deteriorated solar modulation ability with more glycerol is probably due to the lower light blockage after phase transition. The increasing solubility of hydrophobic PNIPAAm chains may be caused by the polarity of glycerol being lower than that of water. Figure 11c presents the light modulation ability of PNIPAAm microgel colloids with various glycerol contents. Both the visible and infrared light modulation abilities were reduced by increasing glycerol content, resulting in a subsequent decrease of solar modulation ability form 76% to 60%. It is worth noting that each sample showed modulation ability in the visible region much higher than that in the infrared region, which can

Figure 10. Evaporation curves at 60 °C for PNIPAAm microgel colloids prepared using binary solvent with 0−30 wt % (of NIPA) glycerol. A photograph taken at the beginning of the test is shown in the inset.

and placed in a drying oven at 60 °C. The samples were heated for 30 min to achieve thermodynamic equilibrium before removing the bottle caps (inset of Figure 10). Then, the mass loss (percentage of the initial weight) at 60 °C for each sample was recorded as a function of time. The mass loss rate of the PNIPAAm microgel colloids decreased gradually with increasing glycerol content, indicating that adding glycerol is an effective method for reducing the water evaporation of PNIPAAm microgel colloids, which may lower the sealing requirement of PNIPAAm-based smart windows. Effect of Glycerol on Energy-Saving Performance. To quantitatively investigate the solar modulation abilities of the prepared PNIPAAm microgel colloids, the integral transmittance (Tint) was calculated based on the measured spectra according to the following equation: Tint =

∫ T(λ) Eλ(λ) dλ/∫ Eλ(λ) dλ

(1)

Figure 11. (a) Transmittance curves, (b) Tint(c) light modulation ability calculated from panel a, (c) light modulation ability, and (d) recycling performance of PNIPAAm microgel colloids. Samples were prepared with 0.1 wt % (of NIPA) BIS and 0−35 wt % (of total solvent) glycerol. 18469



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Figure 12. (a) Photographic illustration of the model house: 1, temperature monitor; 2, temperature probe; 3, bath lamp; 4, smart window; and 5, reference window. (b) Temperature as a function of irradiation time for smart windows containing PNIPAAm microgel colloids prepared with 0−30 wt % (of total solvent) glycerol and reference windows with pure glass or sealed water.

glycerol content of 20 and 30 wt %, temperature growth slopes changed little during the whole process. With glycerol content increasing from 0 to 30 wt %, the transparent−opaque transition temperature and enthalpy change of PNIPAAm microgel colloids decreased from 32.2 to 22 °C and 1.12 to 0.15 J/g, respectively (Figure 6). Therefore, it is easier for the samples with high glycerol content to dehydrate than those with low glycerol content, resulting in a slower increase rate of temperature. However, after 3 min of irradiation, all the samples had accomplished the transparent− opaque transition, and the temperature increase afterward is mainly dominated by the light-blocking ability. As the transmittance of the dehydrated sample increased upon increasing glycerol content (Figure 11), PNIPAAm microgel colloids without glycerol exhibited the best heat-insulation performance.

be attributed to the light absorption of water in the infrared region. Figure 11d exhibits the recycling performance of the prepared thermosensitive PNIPAAm microgel colloids. After six cycles, the solar modulation ability of the samples kept almost the same. It is demonstrated that the obtained PNIPAAm hydrogels possessed excellent reversible phase transition behavior upon heating and cooling. All the samples were sealed in transparent glass bottles and placed outdoors for a duration test in Shanghai, China. There were no precipitate or stratification after 1 year of exposure, and the samples displayed thermosensitive properties and solar modulation ability that were the same as those of the fresh samples (data not shown). These results indicate good stability of the obtained PNIPAAm microgel colloids with 0.1 wt % BIS, which can be applied as efficient thermosensitive materials for smart windows. Model House Test. To test the applied performance of PNIPAAm-based smart windows, a model house was made of wooden boards 1.5 cm thick (Figure 12a). The thickness of the board separating the two rooms was 2 cm. These boards were painted on both sides for thermal insulation. Each of the separated spaces had a volume of 34 × 27 × 29 cm3 (2.6 × 104 cm3) with a window of 25 × 30 cm2. The prepared PNIPAAm microgel colloid was sealed in a cuvette made of float glass 4 mm thick to form the smart glass. Windows of the two rooms were made of the smart glass and a reference glass, and the rooms were sealed during the testing process. Two bath lamps (PHILIPS, R125 IR R150 W) whose wavelength covers the main heating range of solar radiation, were used to irradiate the rooms through the window, while two thermoelectric couples were employed to monitor the temperature change in each room. The testing results (Figure 12b) illustrate that after irradiation for 30 min, the application of PNIPAAm microgel colloids causes a temperature reduction of more than 20 °C compared to that of the blank glass cuvette, and a temperature reduction of 8−10 °C compared to that of the glass cuvette with water. The temperature reduction caused by water can be attributed to the optical and heat absorption of water because of the small space of the tested room. This suggests that the prepared PNIPAAm microgel colloids are ideal materials for smart windows. At low glycerol content, the slope of the temperature increase curve changed from high to low at ca. 3 min. Upon increasing the glycerol content from 0 wt % to 20 wt %, the temperature enhancement slope decreased very fast at the beginning while increased after 3 min. For the samples with

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CONCLUSION

Thermosensitive PNIPAAm microgel colloids were prepared using water−alcohol binary solvents. Both the glycol and glycerol were used to partly replace water. Compared to water− glycol system, PNIPAAm microgel colloids prepared with water−glycerol exhibited a better compatibility with the thermosensitive hydrogels, resulting in smaller deterioration of the solar modulation ability. The solar modulation ability of the PNIPAAm solution decreased with increasing BIS dose. Well-defined spherical PNIPAAm microgels with a size of 200− 700 nm were prepared, and their size distribution was widened by increasing glycerol content. Upon increasing the glycerol content, the LCST, transition enthalpy, response speed, and deswelling ratio during the switching process all decreased because of the lower polarity of glycerol compared to that of water and thus the weakening hydrogen bonds between polymer and solvent while enhancing among polymers. The viscosity of PNIPAAm microgel colloids was dominated by the polymer−solvent interactions and decreased with increasing glycerol content. They exhibited excellent heat-insulation performance, showing a temperature reduction of more than 20 °C compared to that of float glasses. The freezing point and the evaporation rate of PNIPAAm microgel colloids also decreased with increasing glycerol content, which is beneficial for expanding their applications. We demonstrated that the colloids of thermosensitive PNIPAAm microgels prepared in water−glycerol binary solvent were much more suitable candidates for smart window materials than those in a single solvent. 18470



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(18) Gyenes, T.; Szilagyi, A.; Lohonyai, T.; Zrinyi, M. Electrically adjustable thermotropic windows based on polymer gels. Polym. Adv. Technol. 2003, 14, 757−762. (19) Szilagyi, A.; Gyenes, T.; Filipcsei, G.; Zrinyi, M. Thermotropic polymer gels: Smart gel glass. Macromol. Symp. 2005, 227, 357−366. (20) Watanabe, H. Intelligent window using a hydrogel layer for energy efficiency. Sol. Energy Mater. Sol. Cells 1998, 54, 203−211. (21) Ye, X. W.; Luo, Y. W.; Gao, X.; Zhu, S. P. Design and evaluation of a thermochromic roof system for energy saving based on poly(Nisopropylacrylamide) aqueous solution. Energy and Buildings 2012, 48, 175−179. (22) Rotzetter, A. C. C.; Schumacher, C. M.; Bubenhofer, S. B.; Grass, R. N.; Gerber, L. C.; Zeltner, M.; Stark, W. J. Thermoresponsive Polymer Induced Sweating Surfaces as an Efficient Way to Passively Cool Buildings. Adv. Mater. (Weinheim, Ger.) 2012, 24, 5352−5356. (23) Guan, Y.; Zhang, Y. J. PNIPAAmM microgels for biomedical applications: From dispersed particles to 3D assemblies. Soft Matter 2011, 7, 6375−6384. (24) Wang, J.; Sutti, A.; Wang, X.; Lin, T. Fast responsive and morphologically robust thermo-responsive hydrogel nanofibres from poly (N-isopropylacrylamide) and POSS crosslinker. Soft Matter 2011, 7, 4364−4369. (25) Liu, R. X.; Fraylich, M.; Saunders, B. R. Thermoresponsive copolymers: From fundamental studies to applications. Colloid Polym. Sci. 2009, 287, 627−643. (26) Yin, X.; Hoffman, A. S.; Stayton, P. S. Poly(N-isopropylacrylamide-co-propylacrylic acid) copolymers that respond sharply to temperature and pH. Biomacromolecules 2006, 7, 1381−1385. (27) Mu, B.; Liu, P.; Tang, Z.; Du, P.; Dong, Y. Temperature and pH dual-responsive cross-linked polymeric nanocapsules with controllable structures via surface-initiated atom transfer radical polymerization from templates. Nanomedicine (N.Y., NY, U.S.) 2011, 7, 789−796. (28) Luo, Y. L.; Yu, W.; Xu, F.; Zhang, L. L. Novel thermoresponsive self-assembly micelles from a double brush-shaped PNIPAM-g-(PA-b-PEG-b-PA)-g-PNIPAM block copolymer with PNIPAM polymers as side chains. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 2053−2067. (29) Luo, Y.; Liu, L.; Wang, X.; Shi, H.; Lv, W.; Li, J. Sugar-installed thermoresponsive micellar aggregates self-assembled from “coil-combcoil” triblock glycopolymers: Preparation and recognition with Concanavalin A. Soft Matter 2012, 8, 1634−1642. (30) Sun, S. T.; Wu, P. Y. Tailoring the morphology of branched poly(N-isopropylacrylamide) via self-condensing atom-transfer radical copolymerization and its unique self-assembly behavior in alcohol. Soft Matter 2011, 7, 7526−7531. (31) Takahashi, T.; Fukazawa, H.; Kawaguchi, H. Particle-forming precipitation polymerization under unusual conditions. Prog. Polym. Sci. 2004, 124, 164−167. (32) Han, Y.; Nishimura, T.; Kato, T. Morphology tuning in the formation of vaterite crystal thin films with thermoresponsive poly(Nisopropylacrylamide) brush matrices. CrystEngComm 2014, 16, 3540− 3547. (33) Cho, S.; Kim, B. Thermo-sensitive hydrogels based on interpenetrating polymer networks made of poly(N-isopropylacrylamide) and polyurethane. J. Biomater. Sci., Polym. Ed. 2010, 21, 1051− 1068. (34) Mizuntani, M.; Satoh, K.; Kamigaito, M. Degradable poly(Nisopropylacrylamide) with tunable thermosensitivity by simultaneous chain-and step-growth radical polymerization. Macromolecules 2011, 44, 2382−2386. (35) Jung, S. C.; Bae, Y. C. Molecular Thermodynamic Analysis for Assessing the Relationship between Reentrant Swelling Behavior and Ternary Liquid-Liquid Equilibrium for Poly(N-isopropylacrylamide) Nanometer-Sized Gel Particles in a Water-Tetrahydrofuran Cosolvent System. J. Phys. Chem. B 2012, 116, 2208−2215. (36) Kojima, H.; Tanaka, F.; Scherzinger, C.; Richtering, W. Temperature dependent phase behavior of PNIPAAmM microgels in mixed water/methanol solvents. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1100−1111.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Natural Science Foundation of China (51273063, 51003030, 21476143, and 11076002/A06), the Fundamental Research Funds for the Central Universities, the Higher School Specialized Research Fund for the Doctoral Program (20110074110003), and 111 Project Grant (B08021) for support of this work.

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REFERENCES

(1) Wang, S. M.; Liu, L.; Chen, W. L.; Zhang, Z. M.; Su, Z. M.; Wang, E. B. A new electrodeposition approach for preparing polyoxometalates-based electrochromic smart windows. J. Mater. Chem. A 2013, 1, 216−220. (2) Ranjbar, M.; Tahmasebi Garavand, N.; Mahdavi, S. Electroless plating of palladium on WO3 films for gasochromic applications. Sol. Energy Mater. Sol. Cells 2010, 94, 201−206. (3) Zhang, Z. T.; Gao, Y. F.; Luo, H. J.; Kang, L. T.; Chen, Z.; Du, J.; Kanehira, M.; Zhang, Y. Z.; Wang, Z. L. Solution-based fabrication of vanadium dioxide on F:SnO2 substrates with largely enhanced thermochromism and low-emissivity for energy-saving applications. Energy Environ. Sci. 2011, 4, 4290−4297. (4) Gao, Y. F.; Cao, C. X.; Dai, L.; Luo, H. J.; Kanehira, M.; Ding, Y.; Wang, Z. L. Phase and shape controlled VO2 nanostructures by antimony doping. Energy Environ. Sci. 2012, 5, 8708−8715. (5) Yoshimura, K.; Yamada, Y.; Okada, M. Optical switching of Mgrich Mg-Ni alloy thin films. Appl. Phys. Lett. 2002, 81, 4709−4711. (6) Seeboth, A.; Ruhmann, R.; Muhling, O. Thermotropic and Thermochromic Polymer Based Materials for Adaptive Solar Control. Materials 2010, 3, 5143−5168. (7) Maeda, Y.; Mochiduki, H.; Ikeda, I. Hydration changes during thermosensitive association of a block copolymer consisting of LCST and UCST blocks. Macromol. Rapid Commun. 2004, 25, 1330−1334. (8) Diehl, C.; Schlaad, H. Thermo-Responsive Polyoxazolines with Widely Tuneable LCST. Macromol. Biosci. 2009, 9, 157−161. (9) Szilágyi, A.; Gyenes, T.; Filipcsei, G.; Zrínyi, M. Thermotropic polymer gels: Smart gel glass. Macromol. Symp. 2005, 227, 357−366. (10) Lutz, J. F.; Akdemir, O.; Hoth, A. Point by point comparison of two thermosensitive polymers exhibiting a similar LCST: Is the age of poly(NIPAM) over? J. Am. Chem. Soc. 2006, 128, 13046−13047. (11) Rzaev, Z. M.; Dincer, S.; Pişkin, E. Functional copolymers of Nisopropylacrylamide for bioengineering applications. Prog. Polym. Sci. 2007, 32, 534−595. (12) Wang, Y. Q.; Zhang, Y. Y.; Zhang, F.; Li, W. Y. One-pot synthesis of thermal responsive QDs−PNIPAAmM hybrid fluorescent microspheres by controlling the polymerization temperature at two different polymerization stages. J. Mater. Chem. 2011, 21, 6556−6562. (13) Ionov, L.; Diez, S. Environment-friendly photolithography using poly(N-isopropylacrylamide)-based thermoresponsive photoresists. J. Am. Chem. Soc. 2009, 131, 13315−13319. (14) Jochum, F. D.; Theato, P. Temperature-and light-responsive smart polymer materials. Chem. Soc. Rev. 2013, 42, 7468−7483. (15) Seeboth, A.; Schneider, J.; Patzak, A. Materials for intelligent sun protecting glazing. Sol. Energy Mater. Sol. Cells 2000, 60, 263−277. (16) Inoue, T.; Ichinose, M.; Ichikawa, N. Thermotropic glass with active dimming control for solar shading and daylighting. Energy and Buildings 2008, 40, 385−393. (17) Zrinyi, M.; Szilagyi, A.; Filipcsei, G.; Feher, J.; Szalma, J.; Moczar, G. Smart gel-glass based on the responsive properties of polymer gels. Polym. Adv. Technol. 2001, 12, 501−505. 18471



Article

(37) Hore, M. J.; Hammouda, B.; Li, Y.; Cheng, H. Co-Nonsolvency of Poly(n-isopropylacrylamide) in Deuterated Water/Ethanol Mixtures. Macromolecules 2013, 46, 7894−7901. (38) Munk, T.; Hietala, S.; Kalliomäki, K.; Nuopponen, M.; Tenhu, H.; Tian, F.; Rantanen, J.; Baldursdóttir, S. Behaviour of stereoblock poly(N-isopropyl acrylamide) in acetone−water mixtures. Polym. Bull. 2011, 67, 677−692. (39) Oh, S. Y.; Kim, H. J.; Bae, Y. C. Molecular thermodynamic analysis for phase transitions of linear and cross-linked poly(Nisopropylacrylamide) in water/2-propanol mixtures. Polymer 2013, 54, 6776−6784. (40) Hao, J.; Cheng, H.; Butler, P.; Zhang, L.; Han, C. C. Origin of cononsolvency, based on the structure of tetrahydrofuran-water mixture. J. Chem. Phys. 2010, 132, 154902. (41) Yamauchi, H.; Maeda, Y. LCST and UCST behavior of poly(Nisopropylacrylamide) in DMSO/water mixed solvents studied by IR and micro-Raman spectroscopy. J. Phys. Chem. B 2007, 111, 12964− 12968. (42) Hofmann, C.; Schönhoff, M. Do additives shift the LCST of poly (N-isopropylacrylamide) by solvent quality changes or by direct interactions? Colloid Polym. Sci. 2009, 287, 1369−1376. (43) Xia, Y.; Yin, X.; Burke, N. A.; Stöver, H. D. Thermal response of narrow-disperse poly(N-isopropylacrylamide) prepared by atom transfer radical polymerization. Macromolecules 2005, 38, 5937−5943. (44) Schild, H. Poly(N-isopropylacrylamide): Experiment, theory and application. Prog. Polym. Sci. 1992, 17, 163−249. (45) Li, D.; Zhang, X. Y.; Yao, J. F.; Simon, G. P.; Wang, H. T. Stimuli-responsive polymer hydrogels as a new class of draw agent for forward osmosis desalination. Chem. Commun. 2011, 47, 1710−1712. (46) Debord, J. D.; Lyon, L. A. Synthesis and characterization of pHresponsive copolymer microgels with tunable volume phase transition temperatures. Langmuir 2003, 19, 7662−7664. (47) Walter, J.; Sehrt, J.; Vrabec, J.; Hasse, H. Molecular dynamics and experimental study of conformation change of poly(Nisopropylacrylamide) hydrogels in mixtures of water and methanol. J. Phys. Chem. B 2012, 116, 5251−5259. (48) Gao, J.; Frisken, B. J. Influence of reaction conditions on the synthesis of self-cross-linked N-isopropylacrylamide microgels. Langmuir 2003, 19, 5217−5222. (49) Gao, J.; Frisken, B. J. Cross-linker-free N-isopropylacrylamide gel nanospheres. Langmuir 2003, 19, 5212−5216. (50) Varga, I.; Gilányi, T.; Meszaros, R.; Filipcsei, G.; Zrínyi, M. Effect of cross-link density on the internal structure of poly(Nisopropylacrylamide) microgels. J. Phys. Chem. B 2001, 105, 9071− 9076. (51) Cheng, H.; Shen, L.; Wu, C. LLS and FTIR studies on the hysteresis in association and dissociation of poly(N-isopropylacrylamide) chains in water. Macromolecules 2006, 39, 2325−2329. (52) Deshmukh, S. A.; Li, Z.; Kamath, G.; Suthar, K. J.; Sankaranarayanan, S. K. R. S.; Mancini, D. C. Atomistic insights into solvation dynamics and conformational transformation in thermosensitive and non-thermo-sensitive oligomers. Polymer 2013, 54, 210− 222. (53) Zhang, X. Z.; Yang, Y. Y.; Chung, T. S.; Ma, K. X. Preparation and characterization of fast response macroporous poly(N-isopropylacrylamide) hydrogels. Langmuir 2001, 17, 6094−6099. (54) Mou, C. L.; Ju, X. J.; Zhang, L.; Xie, R.; Wang, W.; Deng, N. N.; Wei, J.; Chen, Q.; Chu, L. Y. Monodisperse and Fast-Responsive Poly(N-isopropylacrylamide) Microgels with Open-Celled Porous Structure. Langmuir 2014, 30, 1455−1464. (55) Reese, C. E.; Mikhonin, A. V.; Kamenjicki, M.; Tikhonov, A.; Asher, S. A. Nanogel nanosecond photonic crystal optical switching. J. Am. Chem. Soc. 2004, 126, 1493−1496. (56) Sun, B. J.; Lin, Y. N.; Wu, P. Y.; Siesler, H. W. A FTIR and 2DIR spectroscopic study on the microdynamics phase separation mechanism of the poly(N-isopropylacrylamide) aqueous solution. Macromolecules 2008, 41, 1512−1520. (57) Lane, L. B. Freezing Points of Glycerol and Its Aqueous Solutions. J. Ind. Eng. Chem. 1925, 17, 924−924.

(58) Hayashi, Y.; Puzenko, A.; Feldman, Y. Ice nanocrystals in glycerol-water mixtures. J. Phys. Chem. B 2005, 109, 16979−16981.

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