# Preparation and Properties of Thermoresponsive P(N-Isopropylacrylamide-co-butylacrylate) Hydrogel Materials for Smart Windows

Authors: Jae-Hyung Park; Ji-Won Jang; Jae-Hak Sim; Il-Jin Kim; Dong-Jin Lee; Young-Hee Lee; Sang-Hui Park; Han-Do Kim
Journal: International Journal of Polymer Science
 (2019)

---

## Abstract

Thermoresponsive polymers that exhibit phase transition in response to temperature change can be used as material for smart windows because they can control solar light transmission depending on the outside temperature. The development of thermoresponsive polymers for a smart window that can be used over a wide temperature range is required. Therefore, to obtain smart window materials that can be used at various temperatures, three-dimensional thermoresponsive P(NIPAm-co-BA) hydrogels were prepared by free radical polymerization from main monomer N-isopropylacrylamide, comonomer butyl acrylate, and crosslinking agent N,N′-methylenebisacrylamide (MBAm) in this study. This study examined the effect of BA content on the lower critical solution temperature (LCST) and the solar light transmittance of crosslinked P(NIPAm-co-BA) hydrogel films. The LCST of hydrogel films was found to be significantly decreased from 34.3 to 29.5°C with increasing BA content from 0 to 20 mol%. It was found that the transparent films at T=25°C (T<LCST) were converted to translucent films at a higher temperature (T=45°C) (T>LCST). These results suggested that the crosslinked P(NIPAm-co-BA) hydrogel materials prepared in this study could have high potential for application in smart window materials.


---

## Body

## 1. Introduction

In order to block sunlight, devices such as curtains and blinds have been used, but smart window technology is emerging that can adjust light freely according to ambient temperature. Generally, smart windows are being studied with liquid crystal displays (LCDs) and electrochromic (EC) devices, but they are difficult to commercialize because they require additional power and expensive installation costs. However, vanadium dioxide (VO2), which is a typical material of thermochromic (TC) compounds, is advantageous because it does not require power supply, but it has difficulty in commercialization due to difficulty in synthesis due to various crystal structures and low transmittance [1–4]. However, when substances sensitive to stimuli such as heat are used in smart windows, the light transmittance can be adjusted only by the temperature of sunlight, thereby reducing energy consumption [5].Stimuli-responsive polymers are characterized by the reversibility of their chemical or physical properties due to external stimuli such as temperature, light, pH, and current [6, 7]. A thermoresponsive (temperature-responsive) polymer with a temperature stimulus source is characterized by reversible sol-gel phase transition or volumetric phase transition at a specific temperature and is used in various fields such as drug delivery systems, sensors, and separation membrane [8–11]. The specific temperature at which the phase transition occurs is called the lower critical solution temperature (LCST).It is well known that poly(N-isopropylacrylamide) (PNIPAm) prepared by polymerization of monomer N-isopropylacrylamide (NIPAm) has a reversible transparent-to-opaque phase transition and is relatively insensitive to pH and has the advantage of being able to produce thermoresponsive polymer products at low cost [12–15]. The LCST of PNIPAm is about 34°C [16]. Therefore, at high temperatures such as in the summer (T>34°C), the polymer is aggregated due to the hydrophobic group interaction of PNIPAm and the polymer is separated from the solvent and becomes opaque. At low temperatures (T<34°C), such as normal room temperature, the hydrogen bonds between high polarity amide groups and high polarity water molecules cause the PNIPAm to dissolve in water, resulting in a transparent phase [17–19]. Therefore, when the thermally sensitive material is applied to the smart window, it is opaque when the outdoor temperature is higher than the LCST so that the solar energy (300~2500 nm) entering the room can be reduced and the solar energy to be injected into the interior can be minimized. At this time, when the light transmittance difference (ΔT) of the transparent/opaque phase is 70% or more, the energy saving effect can be obtained.Control of the LCST of thermoresponsive polymers is needed to develop materials for smart windows that can be used in a variety of environments such as cold polar regions and hot tropical regions. Various studies have been reported in which solvents, salts, and pH are applied as methods for controlling LCST. Several studies have controlled LCST by controlling hydrophilicity and hydrophobicity in polymeric structures [20–23].In the case of copolymers using comonomers, there have been reports of controlling LCST as a control of the hydrophilic and hydrophobic groups of monomers [23–25]. However, in these reports, the difference in light transmittance (ΔT) between light transmission and light interruption is relatively low, about 60%. In order to apply a thermoresponsive material to an effective smart window, a ΔT of 80% or more is required. When monomers with hydrophobic groups are applied, emulsifiers are needed to form micelles in aqueous solutions because of their low solubility in water [26].In this study, a series of thermoresponsive copolymer (poly(N-isopropylacrylamide-co-butyl acrylate), P(NIPAm-co-BA)) hydrogels were prepared by free radical polymerization using NIPAm as a main monomer, butyl acrylate (BA) as comonomer, N,N′-methylenebisacrylamide as a crosslinking agent, ammonium persulfate as an initiator, N,N,N′N′-tetramethylethylene diamine as a catalyst, sodium dodecyl sulfate as an emulsifier, and water as a solvent. This study focused on the effect of BA content on the LCST and light transmittance difference (ΔT) of the transparent/opaque phase of copolymer hydrogel and confirmed the applicability of synthetic copolymer hydrogel to a smart window.


## 2. Material and Methods

### 2.1. Materials

The main monomer of thermoresponsive polymer (P(NIPAm)) N-isopropylacrylamide (NIPAm, TCI, Japan) and comonomer butyl acrylate (≥99%, BA, Daejung, Korea) were used as received. N,N′-methylenebisacrylamide (≥99%, MBAm, Sigma-Aldrich, USA) as a crosslinking agent, ammonium persulfate (APS, Sigma-Aldrich, USA) as the initiator, N,N,N′,N′-tetramethylethylenediamine (TEMED, Sigma-Aldrich, USA) as the catalyst, and sodium dodecyl sulfate (≥99.0%, SDS, Sigma-Aldrich, USA) as the emulsifying agent were used without further purification.


### 2.2. Preparation of Thermoresponsive Polymers

The preparation procedure of thermoresponsive P(NIPAm-co-BA) is shown in Scheme1. First, SDS (0.05 g), an anionic emulsifier, is dissolved in water at 80°C under a nitrogen stream. After sufficient emulsification, the monomer NIPAm (2.5 g)/BA (5, 10, 15, and 20 mol%) and the crosslinking agent MBAm (25 mg) are added. When the mixture is completely dispersed and becomes a transparent liquid and the temperature drops to room temperature, then initiator APS (0.7 mL) and catalyst TEMED (0.4 mL) are added and dispersed using a vibrator. Here, APS of 5 wt% aqueous solution was used. Since the comonomer BA has low solubility in water, the polymerization was carried out using an SDS emulsifier. SDS used as an emulsifier forms a micelle in an aqueous solution, and hydrophobic BA is dissolved in the micelle. The polymerization is proceeded by arranging a large hydrophilic NIPAm on the outside of the micelle then the BA. The reaction was conducted at room temperature for 24 h to make a thermoresponsive copolymer (poly(N-isopropylacrylamide-co-butyl acrylate), P(NIPAm-co-BA)) hydrogel and coatings. The sample designation and composition are shown in Table 1.Scheme 1
Synthesis process of the thermoresponsive P(NIPAm-co-BA) gel materials.Table 1
Sample designation and composition of thermoresponsive P(NIPAm-co-BA) gel materials.
Sample designation
Composition
NIPAm (mol/g)
BA∗ (mol%/g)
MBAm (g)
SDS (g)
APS (g)
TEMED (g)
Water (g)
P-B-0
0.022/2.50
0/0
0.025
—
0.035
0.4
100
P-B-5
0.022/2.50
5/0.14
0.025
0.05
0.035
0.4
100
P-B-10
0.022/2.50
10/0.25
0.025
0.05
0.035
0.4
100
P-B-15
0.022/2.50
15/0.42
0.025
0.05
0.035
0.4
100
P-B-20
0.022/2.50
20/0.57
0.025
0.05
0.035
0.4
100
BA content (BA∗): BA mol% based on NIPAm.


### 2.3. Characterization

#### 2.3.1. FT-IR Analysis

The chemical structure of thermoresponsive copolymer P(NIPAm-co-BA) hydrogel was confirmed by infrared spectroscopy (FT/IR-6200, JASCO). FT-IR spectra were recorded at 32 scans and a resolution of 16 in the wavenumber range of 4000-400 cm-1.


#### 2.3.2. Thermal Characterization Analysis

The LCST of P(NIPAm-co-BA) was measured under a nitrogen atmosphere using a differential scanning calorimeter (DSC Q25, TA instrument, USA). The heating rate was 1°C min-1. The temperature at which phase transition occurs, LCST, was measured as peak temperature causing exothermic heat change.


#### 2.3.3. Transmittance Analysis

The transmittance of the P(NIPAm-co-BA) sample was measured in the range of 300-2500 nm using an ultraviolet/visible/near-infrared spectrophotometer (UV-VIS-NIR spectrometer, V-770, JASCO, Japan). The range of 300-2500 nm is the wavelength range of sunlight. In order to compare the transmittance from room temperature to high temperature, a temperature control device was installed and the transmittance at 25, 33, and 45°C was measured.


## 2.1. Materials

The main monomer of thermoresponsive polymer (P(NIPAm)) N-isopropylacrylamide (NIPAm, TCI, Japan) and comonomer butyl acrylate (≥99%, BA, Daejung, Korea) were used as received. N,N′-methylenebisacrylamide (≥99%, MBAm, Sigma-Aldrich, USA) as a crosslinking agent, ammonium persulfate (APS, Sigma-Aldrich, USA) as the initiator, N,N,N′,N′-tetramethylethylenediamine (TEMED, Sigma-Aldrich, USA) as the catalyst, and sodium dodecyl sulfate (≥99.0%, SDS, Sigma-Aldrich, USA) as the emulsifying agent were used without further purification.


## 2.2. Preparation of Thermoresponsive Polymers

The preparation procedure of thermoresponsive P(NIPAm-co-BA) is shown in Scheme1. First, SDS (0.05 g), an anionic emulsifier, is dissolved in water at 80°C under a nitrogen stream. After sufficient emulsification, the monomer NIPAm (2.5 g)/BA (5, 10, 15, and 20 mol%) and the crosslinking agent MBAm (25 mg) are added. When the mixture is completely dispersed and becomes a transparent liquid and the temperature drops to room temperature, then initiator APS (0.7 mL) and catalyst TEMED (0.4 mL) are added and dispersed using a vibrator. Here, APS of 5 wt% aqueous solution was used. Since the comonomer BA has low solubility in water, the polymerization was carried out using an SDS emulsifier. SDS used as an emulsifier forms a micelle in an aqueous solution, and hydrophobic BA is dissolved in the micelle. The polymerization is proceeded by arranging a large hydrophilic NIPAm on the outside of the micelle then the BA. The reaction was conducted at room temperature for 24 h to make a thermoresponsive copolymer (poly(N-isopropylacrylamide-co-butyl acrylate), P(NIPAm-co-BA)) hydrogel and coatings. The sample designation and composition are shown in Table 1.Scheme 1
Synthesis process of the thermoresponsive P(NIPAm-co-BA) gel materials.Table 1
Sample designation and composition of thermoresponsive P(NIPAm-co-BA) gel materials.
Sample designation
Composition
NIPAm (mol/g)
BA∗ (mol%/g)
MBAm (g)
SDS (g)
APS (g)
TEMED (g)
Water (g)
P-B-0
0.022/2.50
0/0
0.025
—
0.035
0.4
100
P-B-5
0.022/2.50
5/0.14
0.025
0.05
0.035
0.4
100
P-B-10
0.022/2.50
10/0.25
0.025
0.05
0.035
0.4
100
P-B-15
0.022/2.50
15/0.42
0.025
0.05
0.035
0.4
100
P-B-20
0.022/2.50
20/0.57
0.025
0.05
0.035
0.4
100
BA content (BA∗): BA mol% based on NIPAm.


## 2.3. Characterization

### 2.3.1. FT-IR Analysis

The chemical structure of thermoresponsive copolymer P(NIPAm-co-BA) hydrogel was confirmed by infrared spectroscopy (FT/IR-6200, JASCO). FT-IR spectra were recorded at 32 scans and a resolution of 16 in the wavenumber range of 4000-400 cm-1.


### 2.3.2. Thermal Characterization Analysis

The LCST of P(NIPAm-co-BA) was measured under a nitrogen atmosphere using a differential scanning calorimeter (DSC Q25, TA instrument, USA). The heating rate was 1°C min-1. The temperature at which phase transition occurs, LCST, was measured as peak temperature causing exothermic heat change.


### 2.3.3. Transmittance Analysis

The transmittance of the P(NIPAm-co-BA) sample was measured in the range of 300-2500 nm using an ultraviolet/visible/near-infrared spectrophotometer (UV-VIS-NIR spectrometer, V-770, JASCO, Japan). The range of 300-2500 nm is the wavelength range of sunlight. In order to compare the transmittance from room temperature to high temperature, a temperature control device was installed and the transmittance at 25, 33, and 45°C was measured.


## 2.3.1. FT-IR Analysis

The chemical structure of thermoresponsive copolymer P(NIPAm-co-BA) hydrogel was confirmed by infrared spectroscopy (FT/IR-6200, JASCO). FT-IR spectra were recorded at 32 scans and a resolution of 16 in the wavenumber range of 4000-400 cm-1.


## 2.3.2. Thermal Characterization Analysis

The LCST of P(NIPAm-co-BA) was measured under a nitrogen atmosphere using a differential scanning calorimeter (DSC Q25, TA instrument, USA). The heating rate was 1°C min-1. The temperature at which phase transition occurs, LCST, was measured as peak temperature causing exothermic heat change.


## 2.3.3. Transmittance Analysis

The transmittance of the P(NIPAm-co-BA) sample was measured in the range of 300-2500 nm using an ultraviolet/visible/near-infrared spectrophotometer (UV-VIS-NIR spectrometer, V-770, JASCO, Japan). The range of 300-2500 nm is the wavelength range of sunlight. In order to compare the transmittance from room temperature to high temperature, a temperature control device was installed and the transmittance at 25, 33, and 45°C was measured.


## 3. Results and Discussion

### 3.1. Synthesis and Characterization of Thermoresponsive Polymers

Figure1 shows the FT-IR spectra of PNIPAm and a typical P(NIPAm-co-BA) (P-B-20) to confirm the synthesis. Figure 1(a) shows the FT-IR spectrum of the sample P-B-0 (PNIPAm) prepared using only NIPAm as the monomer. The synthesis (polymerization) of monomer NIPAm was confirmed by the disappearance of the C=C stretching peak at 1630 cm-1. The NH stretching peak at 3300 cm-1, C=O stretching (amide I) peak at 1650 cm-1, and NH bending (amide II) peak at 1540 cm-1 also confirmed the poly(N-isopropylacrylamide) structure. Figure 1(b) shows the FT-IR spectrum of sample P-B-20 prepared using BA as a comonomer. Copolymerization was confirmed by adding a C=O (saturated ester) absorption peak at 1735 cm-1 and a C-O stretching absorption peak at 1260 cm-1 due to the BA component to the spectrum of Figure 1(a). The synthesis of the copolymer was also confirmed by the disappearance of the C=C stretching peak at 1630 cm-1.Figure 1
FT-IR spectra of (a) PNIPAm and (b) a typical P(NIPAm-co-BA) (P-B-20).


### 3.2. Thermal Characterization of Thermoresponsive Polymer Hydrogels

Figure2 shows the DSC curves of PNIPAm (P-B-0) gel and P(NIPAm-co-BA) (P-B-0, P-B-10, P-B-15, and P-B-20) gels. The phase transition temperature, LCST, was taken as the peak point at which the heat flow changes during the heating process. In the case of PNIPAm (P-B-0) without BA, phase transition starts at about 34°C. It is well known that, at temperatures below LCST, the polar amide group of PNIPAm gel and solvent water form a hydrogen bond well, which results in a transparent phase, but when the temperature is above the LCST, the hydrophobic interaction of the isopropyl group of PNIPAm gel dominates and results in phase separation; as a result, the opaque phase is formed [12].Figure 2
DSC curves of PNIPAm (P-B-0) gel and P(NIPAm-co-BA) (P-B-0, P-B-10, P-B-15, and P-B-20) gels.In this study, comonomer BA with hydrophobicity greater than NIPAm was used for LCST control. The LCST values of the synthesized P(NIPAm-co-BA) gel materials are shown in Table2 and Figure 3. It was found that LCST decreased significantly with increasing BA content.Table 2
LCST of thermoresponsive P(NIPAm-co-BA) gel materials and transmittance% (T%) of thermoresponsive P(NIPAm-co-BA) gel materials at 550 nm and room temperature.
Sample designation
LCST (°C)
T%
P-B-0
34.3
92.1
P-B-5
32.4
90.4
P-B-10
31.8
89.1
P-B-15
30.6
87.8
P-B-20
29.5
84.2Figure 3
The LCST of thermoresponsive P(NIPAm-co-BA) gel materials.When the contents of comonomer BA were 0, 5, 10, 15, and 20 mol%, the LCSTs of P(NIPAm-co-BA) hydrogels were 34.3, 32.4, 31.8, 30.6, and 29.5°C, respectively. The LCST of sample P-B-20 (29.5°C) with a BA content of 20 mol% showed an LCST decrease of about 4.8°C compared with P-B-0 (34.3°C) without BA. The addition of comonomer BA indicates that it is an effective method to reduce the LCST of PNIPAm polymer materials, which makes it possible to apply a wider range of applications. This decrease in LCST can be seen to be an effective comonomer BA for controlling the LCST of the PNIPAm polymer gel. The decrease in LCST due to the addition of BA can be explained by the weakening of the hydrogen bond between the copolymer and the solvent water and the strengthening of the hydrophobic bond between the polymer chains. The reason for enhancing the hydrophobicity of PNIPAm hydrogel by BA is that the combination of the hydrophobic butyl group of BA and the isopropyl group of NIPAm causes the polymer to interfere with the dissolution of the aqueous phase, thereby increasing the hydrophobicity of the polymer and weakening the hydrogen bonding with water [21]. As a result, LCST decreased from 34.3 to 29.5°C with increasing BA content from 0 to 20 mol%.


### 3.3. Transmittance Analysis of Thermoresponsive PNIPAm Hydrogels

Figure4(a) shows the light transmittance of PNIPAm and P(NIPAm-co-BA) hydrogels at T=25°C (T<LCST) in the entire solar spectrum (300-2500 nm) region. The reason why the transmittance is low in the region of 1450 nm and 1900 nm is due to the absorption of solvent water. Absorption occurs at 1450 nm due to stretching vibration of O-H group of the instantaneous molecule, and absorption occurs at 1900 nm due to stretching of O-H group and bending of H-O-H group. The transmittance of PNIPAm and P(NIPAm-co-BA) at 25°C (T<LCST) in the visible region is shown in Figure 4(b). Figure 4(c) shows the transmittance in the entire solar spectrum (300-2500 nm) region at a temperature of 45°C (T>LCST) higher than LCST. At 25°C, which is lower than LCST, the light transmittance of visible light decreases slightly as BA content increases. The light transmittance of all samples, when measured at 45°C, which is higher than LCST, was almost 0% in all areas regardless of BA content (see Figure 4(c)). Table 2 shows the light transmittance measured at 550 nm in visible light. The transmittance decreased from 92.1 to 84.2% with increasing BA content from 0 to 20 mol%. The reason why the light transmittance is lowered at a temperature higher than LCST is that the hydrogen bond between the water and the polar group of polymer is broken and phase separation occurs due to the interaction of the hydrophobic group of P(NIPAm-co-BA). In the infrared region, there is almost no difference in transmittance regardless of the BA content. However, when the content of BA was 20 mol% or more, the transmittance was low even at room temperature. Therefore, the content of BA used in this study was 5–20 mol%.Figure 4
Transmittance of P(NIPAm-co-BA) hydrogels (a) atT=25°C (T<LCST) in the entire solar spectrum (300-2500 nm) region, (b) at 25°C (T<LCST) in the visible region, and (c) in the entire solar spectrum (300-2500 nm) region at a temperature of 45°C (T>LCST) higher than LCST.
(a)
(b)
(c)


### 3.4. Application of P(NIPAm-co-BA) Hydrogel for Smart Window

The obtained thermoresponsive PNIPAm and P(NIPAm-co-BA) gels were coated on a glass plate with a thickness of 5 mm. Then, a glass plate coated with P(NIPAm-co-BA) was placed on a logo-engraved paper, and the light transmission and blocking were examined by irradiating artificial sunlight at each temperature (25, 33, and 45°C) on a glass plate. The results of this experiment are shown in Figure5. The LCST of all samples was higher than 25°C, so the light irradiated on the glass plate at 25°C was penetrated and the letters became clear. Since the LCST of the thermoresponsive PNIPAm (P-B-0) was 34.5°C, it was found that the letters were clear at 33°C but not at 45°C. The samples P-B-5, P-B-10, P-B-15, and P-B-20 had the LCST of 33°C or less, so they could not transmit light at 33 and 45°C. Also, at 45°C, which is higher than LCST of all samples, all samples were opaque within 10 sec of light irradiation. These results confirmed that all samples synthesized in this study exhibited effective light blocking properties at different temperatures. Therefore, if P(NIPAm-co-BA) hydrogels prepared in this study are applied to a smart window, it would be possible to produce an environmentally friendly smart window that does not require a separate power device.Figure 5
Transparency/translucency of thermoresponsive P(NIPAm-co-BA) gel coated on glasses at 25, 33, and 45°C.


## 3.1. Synthesis and Characterization of Thermoresponsive Polymers

Figure1 shows the FT-IR spectra of PNIPAm and a typical P(NIPAm-co-BA) (P-B-20) to confirm the synthesis. Figure 1(a) shows the FT-IR spectrum of the sample P-B-0 (PNIPAm) prepared using only NIPAm as the monomer. The synthesis (polymerization) of monomer NIPAm was confirmed by the disappearance of the C=C stretching peak at 1630 cm-1. The NH stretching peak at 3300 cm-1, C=O stretching (amide I) peak at 1650 cm-1, and NH bending (amide II) peak at 1540 cm-1 also confirmed the poly(N-isopropylacrylamide) structure. Figure 1(b) shows the FT-IR spectrum of sample P-B-20 prepared using BA as a comonomer. Copolymerization was confirmed by adding a C=O (saturated ester) absorption peak at 1735 cm-1 and a C-O stretching absorption peak at 1260 cm-1 due to the BA component to the spectrum of Figure 1(a). The synthesis of the copolymer was also confirmed by the disappearance of the C=C stretching peak at 1630 cm-1.Figure 1
FT-IR spectra of (a) PNIPAm and (b) a typical P(NIPAm-co-BA) (P-B-20).


## 3.2. Thermal Characterization of Thermoresponsive Polymer Hydrogels

Figure2 shows the DSC curves of PNIPAm (P-B-0) gel and P(NIPAm-co-BA) (P-B-0, P-B-10, P-B-15, and P-B-20) gels. The phase transition temperature, LCST, was taken as the peak point at which the heat flow changes during the heating process. In the case of PNIPAm (P-B-0) without BA, phase transition starts at about 34°C. It is well known that, at temperatures below LCST, the polar amide group of PNIPAm gel and solvent water form a hydrogen bond well, which results in a transparent phase, but when the temperature is above the LCST, the hydrophobic interaction of the isopropyl group of PNIPAm gel dominates and results in phase separation; as a result, the opaque phase is formed [12].Figure 2
DSC curves of PNIPAm (P-B-0) gel and P(NIPAm-co-BA) (P-B-0, P-B-10, P-B-15, and P-B-20) gels.In this study, comonomer BA with hydrophobicity greater than NIPAm was used for LCST control. The LCST values of the synthesized P(NIPAm-co-BA) gel materials are shown in Table2 and Figure 3. It was found that LCST decreased significantly with increasing BA content.Table 2
LCST of thermoresponsive P(NIPAm-co-BA) gel materials and transmittance% (T%) of thermoresponsive P(NIPAm-co-BA) gel materials at 550 nm and room temperature.
Sample designation
LCST (°C)
T%
P-B-0
34.3
92.1
P-B-5
32.4
90.4
P-B-10
31.8
89.1
P-B-15
30.6
87.8
P-B-20
29.5
84.2Figure 3
The LCST of thermoresponsive P(NIPAm-co-BA) gel materials.When the contents of comonomer BA were 0, 5, 10, 15, and 20 mol%, the LCSTs of P(NIPAm-co-BA) hydrogels were 34.3, 32.4, 31.8, 30.6, and 29.5°C, respectively. The LCST of sample P-B-20 (29.5°C) with a BA content of 20 mol% showed an LCST decrease of about 4.8°C compared with P-B-0 (34.3°C) without BA. The addition of comonomer BA indicates that it is an effective method to reduce the LCST of PNIPAm polymer materials, which makes it possible to apply a wider range of applications. This decrease in LCST can be seen to be an effective comonomer BA for controlling the LCST of the PNIPAm polymer gel. The decrease in LCST due to the addition of BA can be explained by the weakening of the hydrogen bond between the copolymer and the solvent water and the strengthening of the hydrophobic bond between the polymer chains. The reason for enhancing the hydrophobicity of PNIPAm hydrogel by BA is that the combination of the hydrophobic butyl group of BA and the isopropyl group of NIPAm causes the polymer to interfere with the dissolution of the aqueous phase, thereby increasing the hydrophobicity of the polymer and weakening the hydrogen bonding with water [21]. As a result, LCST decreased from 34.3 to 29.5°C with increasing BA content from 0 to 20 mol%.


## 3.3. Transmittance Analysis of Thermoresponsive PNIPAm Hydrogels

Figure4(a) shows the light transmittance of PNIPAm and P(NIPAm-co-BA) hydrogels at T=25°C (T<LCST) in the entire solar spectrum (300-2500 nm) region. The reason why the transmittance is low in the region of 1450 nm and 1900 nm is due to the absorption of solvent water. Absorption occurs at 1450 nm due to stretching vibration of O-H group of the instantaneous molecule, and absorption occurs at 1900 nm due to stretching of O-H group and bending of H-O-H group. The transmittance of PNIPAm and P(NIPAm-co-BA) at 25°C (T<LCST) in the visible region is shown in Figure 4(b). Figure 4(c) shows the transmittance in the entire solar spectrum (300-2500 nm) region at a temperature of 45°C (T>LCST) higher than LCST. At 25°C, which is lower than LCST, the light transmittance of visible light decreases slightly as BA content increases. The light transmittance of all samples, when measured at 45°C, which is higher than LCST, was almost 0% in all areas regardless of BA content (see Figure 4(c)). Table 2 shows the light transmittance measured at 550 nm in visible light. The transmittance decreased from 92.1 to 84.2% with increasing BA content from 0 to 20 mol%. The reason why the light transmittance is lowered at a temperature higher than LCST is that the hydrogen bond between the water and the polar group of polymer is broken and phase separation occurs due to the interaction of the hydrophobic group of P(NIPAm-co-BA). In the infrared region, there is almost no difference in transmittance regardless of the BA content. However, when the content of BA was 20 mol% or more, the transmittance was low even at room temperature. Therefore, the content of BA used in this study was 5–20 mol%.Figure 4
Transmittance of P(NIPAm-co-BA) hydrogels (a) atT=25°C (T<LCST) in the entire solar spectrum (300-2500 nm) region, (b) at 25°C (T<LCST) in the visible region, and (c) in the entire solar spectrum (300-2500 nm) region at a temperature of 45°C (T>LCST) higher than LCST.
(a)
(b)
(c)


## 3.4. Application of P(NIPAm-co-BA) Hydrogel for Smart Window

The obtained thermoresponsive PNIPAm and P(NIPAm-co-BA) gels were coated on a glass plate with a thickness of 5 mm. Then, a glass plate coated with P(NIPAm-co-BA) was placed on a logo-engraved paper, and the light transmission and blocking were examined by irradiating artificial sunlight at each temperature (25, 33, and 45°C) on a glass plate. The results of this experiment are shown in Figure5. The LCST of all samples was higher than 25°C, so the light irradiated on the glass plate at 25°C was penetrated and the letters became clear. Since the LCST of the thermoresponsive PNIPAm (P-B-0) was 34.5°C, it was found that the letters were clear at 33°C but not at 45°C. The samples P-B-5, P-B-10, P-B-15, and P-B-20 had the LCST of 33°C or less, so they could not transmit light at 33 and 45°C. Also, at 45°C, which is higher than LCST of all samples, all samples were opaque within 10 sec of light irradiation. These results confirmed that all samples synthesized in this study exhibited effective light blocking properties at different temperatures. Therefore, if P(NIPAm-co-BA) hydrogels prepared in this study are applied to a smart window, it would be possible to produce an environmentally friendly smart window that does not require a separate power device.Figure 5
Transparency/translucency of thermoresponsive P(NIPAm-co-BA) gel coated on glasses at 25, 33, and 45°C.


## 4. Conclusions

In modern architectural structures, the area of glass windows is increasing dramatically, and the importance of controlling the heat energy generated through the windows is emerging. Various smart windows are being developed in accordance with this trend. The conventional smart window has a disadvantage that an external electric power device is required. Therefore, a material which can freely control the transmittance of sunlight is getting attention. In this study, the properties of thermoresponsive P(NIPAm-co-BA) hydrogels polymerized using main monomer NIPAm and comonomer BA were investigated. The LCST of P(NIPAm-co-BA) decreased from 34.3 to 29.5°C with increasing BA content from 0 to 20 mol%. The solar transmittance showed a high transmittance atT=25°C (T<LCST) for all samples and a transmittance close to 0% at T=45°C (T>LCST). This result suggests that the application of BA as a comonomer in thermoresponsive copolymer P(NIPAm-co-BA) hydrogel synthesis is desirable for controlling LCST. From these results, it can be seen that the P(NIPAm-co-BA) hydrogels prepared in this study will be able to be used as eco-friendly materials for smart windows that do not require external power supply.



---

*Source: 3824207-2019-11-13.xml*