CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of priority of Singapore Patent Application No. 10201408804P, filed December 30, 2014, the contents of which being hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

The invention relates generally to smart windows. In particular, an improved window design is disclosed herein whereby a hydrogel bag having a large thermal mass and comprising a film or layer of a temperature responsive hydrogel encapsulated in a transparent plastic bag is applied to the window to enable automatic and reversible switching between transparent and opaque/translucent mode for energy consumption reduction.

BACKGROUND

Smart window refers to a window which automatically controls light transmission properties under the application of a voltage (electrochromism), light (photochromism) or heat (thermochromism).

Of particular interest is a thermally sensitive system that adjusts its optical properties by relying on temperature. Thermochromic material can be used in a passive and zero- energy input smart window, which can regulate solar transmission by temperature stimulus for energy consumption reduction. For example, when the outdoor temperature is higher than the transition temperature of the smart window, the transmission of solar light (250 nm~2500 nm) could be reduced to minimize the solar energy input and therefore, reduce indoor temperature to cut the electric energy consumption for air- conditioning. On the other hand, if the outdoor temperature drops to below the transition temperature of the smart window, the transmission of solar light could be increased to ensure maximum solar energy input. For ideal smart windows, a large disparity in solar transmission at temperatures above and below the transition temperature (T _c ) is desirable for a good solar energy modulation (Ar _so i). Meanwhile, the visible light (380 nm-780 nm) transmission (7i _um ) ideally should remain high, preferably larger than 70%, to ensure good indoor luminous condition. In addition, T _c of such desirable smart window should be within the range of 25 °C to 35 °C.

Vanadium dioxide (VO2) based inorganic materials are currently the most widely studied candidates for smart windows, although they are plagued with problems of high transition temperature (T _C -68 °C) and low 7i _um as well as low Ar _so i. The best reported results for VO2 based smart windows are 7i _um of -40% and Ar _so i of -20%.

Therefore, there remains an unmet need to provide for a smart window design that overcomes, or at least alleviates, the above drawbacks.

SUMMARY

According to a first aspect of the invention, there is provided a panel. The panel includes either (i) a hydrogel bag adhered onto a sheet of glass, transparent plastic, or a surface; or (ii) a hydrogel bag arranged in between two sheets of glass or transparent plastic, wherein the two sheets of glass or transparent plastic are spaced apart from each other. The hydrogel bag comprises a film of a temperature responsive hydrogel encapsulated in a transparent plastic bag, wherein the transparent plastic bag minimizes evaporative loss of water present in the temperature responsive hydrogel. The advantage of having such a panel design is that the large thermal mass of water in the hydrogel bag, where water is the major component in hydrogel, can reduce the temperature fluctuation as water can host a large amount of heat. It has been proven that hydrogel has the same thermal mass as water because hydrogel has more than 90% water by composition. Therefore, it could assist in warming up and cooling down an environment in winter and summer, respectively. This was not realized by any other known energy efficient windows. Water could also be used to retard fire propagation. A thick layer of hydrogel could have these two functions.

Another advantage is that a safety function can be provided with the hydrogel bag inserted or arranged in between the two sheets of glass as the hydrogel bag can be used to stop glass crack propagations.

In one embodiment, a 0.8 mm thick temperature responsive hydrogel film is encapsulated in a transparent plastic bag (see Fig. 1) which gives a four times higher Ar _so i compared with the best reported inorganic V0 ₂ results (see Table 1). The temperature responsive hydrogels are much easier to manufacture and their lower critical solution temperature (LCST) can be easily tuned according to specific usage conditions of smart (and safety) windows. According to a second aspect of the invention, there is provided a hydrogel bag including a film of a temperature responsive hydrogel encapsulated in a transparent plastic bag. The transparent plastic bag minimizes evaporative loss of water present in the temperature responsive hydrogel.

According to a third aspect of the invention, there is provided a method for forming a hydrogel bag according to the second aspect. The method includes:

(a) dissolving monomers of a temperature responsive hydrogel in deionized water;

(b) adding a polymerization catalyst and a polymerization initiator to the solution of (a) to form a hydrogel solution;

(c) depositing the hydrogel solution of (b) onto a first transparent plastic sheet to form a film of hydrogel solution thereon;

(d) allowing the polymerization reaction to complete to obtain a film of a temperature responsive hydrogel;

(e) placing a second transparent plastic sheet over the film of temperature responsive hydrogel such that the film of temperature responsive hydrogel is arranged in between the first and second transparent plastic sheets; and

(f) sealing the edges of the first and second transparent plastic sheets to thereby encapsulate the film of temperature responsive hydrogel therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.

Fig. 1 shows a schematic diagram of a laminated safety smart glass according to one embodiment. The hydrogel bag is laminated in between two sheets of glass. In an alternative embodiment which is not illustrated, the hydrogel bag may be attached onto a sheet of glass, for example, by sticking via an adhesive onto the sheet of glass.

Fig. 2 shows temperature dependence of optical transparency of a sample of 0.8 mm poly(N-isopropylacrylamide) hydrogel film laminated in between two sheets of polyethylene terephthalate according to one embodiment.

Fig. 3A shows (left) a photograph of a poly(N-isopropylacrylamide) hydrogel bag laminated onto a sheet of glass taken at 9:30 am, indoor, 20 °C; (right) transparency of the laminated sheet of glass according to one embodiment.

Fig. 3B shows (left) a photograph of a poly(N-isopropylacrylamide) hydrogel bag laminated onto a sheet of glass taken at 3:30 pm, outdoor, 36 °C; (right) transparency of the laminated sheet of glass according to one embodiment.

Fig. 3C shows (left) a photograph of a poly(N-isopropylacrylamide) hydrogel bag laminated onto a sheet of glass taken at 8:30 pm, indoor, 20 °C; (right) transparency of the laminated sheet of glass according to one embodiment. Fig. 4A shows temperature variations of three glass houses. Glass house A is a bare glass house. Glass house B is a glass house laminated with a hydrogel bag encapsulating 0.8 mm poly(N-isopropylacrylamide) hydrogel. Glass house C is a glass house laminated with a hydrogel bag encapsulating 0.8 mm poly(N-isopropylacrylamide) hydrogel and a water bag with 2 cm thickness which is adhered to the hydrogel bag.

Fig. 4B shows a photograph of (from left to right) glass house C, glass house B, and glass house A at 9:30 am, indoor, 25 °C. Fig. 4C shows a photograph of (from left to right) glass house C, glass house B, and glass house A at 1:30 pm, outdoor, 35 °C.

Fig. 5 shows a photograph of three glass houses in a setup used for the cold weather test. Glass house D is a bare glass house. Glass house E is a glass house laminated with a 1 cm water bag. Glass house F is a glass house laminated with a 2 cm water bag.

Fig. 6 shows the results of a cold weather test (temperature versus time) for glass house D, glass house E and glass house F. Fig. 7 shows thermal durability tests of poly(N-isopropylacrylamide) hydrogel contained in bottles 1-4 (from left to right): (a) hydrogel before heating bottle 1 (monomer, 0.8 mol/100 ml, no poly(vinylalcohol) (PVA)), bottle 2 (monomer, 1.0 mol/100 ml, no PVA), bottle 3 (monomer, 1.2 mol/100 ml, no PVA), and bottle 4 (monomer, 0.8 mol/100 ml, with PVA); (b) hydrogels during heating up on a hotplate at 150 °C; (c) close-up photograph of the hydrogels being heated at 150 °C; (d) hydrogels after 3 hours of heating at 150 °C; (e) heat-treated hydrogels after cooling; and (f) close- up photograph of cooled hydrogels in bottles 1 and 2. In bottle 4, PVA provides channels for water to flow from the inside of the hydrogel to the surface. PVA is not used in bottles 1-3 as PVA would cause shrinkage of the hydrogel during heating up to 100 °C, rendering the hydrogel mixed with PVA unsuitable for the manufacture of safety glass.

Fig. 8A shows the burning of a pure polyethylene terephthalate (PET) film (left film) and a hydrogel-laminated PET (right film) over a fire. Fig. 8B shows the results after a period of burning: the pure PET film burns with a hole after 3 seconds (left film), while the hydrogel-laminated PET stays intact after 50 seconds (right film).

Fig. 9 shows the results of the hydrogel bag-laminated glass panel after smashing. DESCRIPTION

The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized and structural and material changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. According to a first aspect of the invention illustrated in Fig. 1, there is provided a panel (100), which is suitable for use in windows or any partition structure separating a first environment from a second environment such as a facade of a building. The panel (100) includes two sheets (10, 20) of glass or transparent plastic spaced apart from each other. In present context, the term "transparent" (or associated terms such as transparency) refers to having the property of transmitting light without appreciable scattering so that bodies lying beyond are seen clearly. In a broader interpretation, a transparent article as mentioned herein allows the passage of a specified form of radiation, such as infrared (solar) or ultraviolet light.

With reference to Fig. 1, the two sheets (10, 20) of glass or transparent plastic are shown as flat and essentially parallel to each other. It is to be understood that the scope is not to be limited as such. For example, the two sheets of glass or transparent plastic need not be flat, such as both sheets are concave or convex and are parallel to each other. Alternatively, the first sheet may be concave or convex while the second sheet may be flat, and therefore not parallel to each other.

In present context, the term "sheet" refers to a sufficiently thin glass or transparent plastic such that the overall panel structure is transparent for light or other radiation to pass through when desired. Similarly, the spacing between the two sheets of glass or transparent plastic is sufficiently small for light or other radiation to pass through when desired. In various embodiments, the first and second sheets of the panel are both glass. In alternative embodiments, the first and second sheets of the panel are both transparent plastic. In yet further other embodiments, the first sheet may be glass and the second sheet may be a transparent plastic.

The panel (100) further includes a hydrogel bag (40) arranged in between the two sheets (10, 20) of glass or transparent plastic. In present context, the arrangement of the hydrogel bag (40) is such that it is located, positioned, or disposed in between the two sheets (10, 20) to form a sandwich structure. It is to be understood that the arrangement of the hydrogel bag (40) may be such that it is directly contacting (e.g. laminated) the two sheets (10, 20), or it is indirectly contacting (e.g. via an adhesive layer) the two sheets (10, 20). The hydrogel bag (40) may be contacting (whether directly or indirectly) 100% of the surfaces of the two sheets (10, 20) or less. In other words, there may be empty space or vacuum in between the two sheets (10, 20).

Advantageously, the hydrogel bag (40) is able to conform to the configuration of the spacing in between the two sheets (10, 20). For example, in the illustration shown in Fig. 1 where the two sheets (10, 20) are flat and essentially parallel to each other, the hydrogel bag (40) may have a corresponding sheet-like configuration. Other configurations of the hydrogel bag (40) are also possible.

The hydrogel bag (40) comprises a film of a temperature responsive hydrogel (30) encapsulated in a transparent plastic bag, wherein the transparent plastic bag minimizes evaporative loss of water present in the temperature responsive hydrogel. Alternatively, instead of the two-sheet panel design, the panel may include a hydrogel bag adhered onto a sheet of glass, transparent plastic, or a surface such as a wall (i.e. single panel design). In the case where an adaptive solar reflection is desired, the hydrogel bag may be adhered to an opaque surface such as a wall, so that the temperature responsive hydrogel turns opaque (or white) and reflect light, thereby lowering the temperature of the wall.

Accordingly, the hydrogel bag can be applied to any surface such as windows, walls and metal surfaces to modulate solar light smartly.

In present context, the temperature responsive hydrogel (30) is an intelligent material that undergoes phase transition with homogeneous reversibility by utilizing solar (heat) energy and sensing environmental temperature changes. In particular, the temperature sensitive hydrogel (30) undergoes a hydrophilic-to-hydrophobic transition at its lower critical solution temperature (LCST). In one illustrated embodiment, poly(N- isopropylacrylamide) (PNIPAm) is chosen as a candidate since it is the most typical temperature responsive hydrogel known and its LCST (-32 °C) lies between the range of 25-35 °C, making it suitable for smart windows application. By tuning the thickness of the hydrogel and designing suitable panel setups, the reversibly tunable transparency of PNIPAm hydrogels can be utilized in the smart windows application. As an example, a 0.8 mm thick PNIPAm hydrogel film encapsulated in a polyethylene terephthalate bag gives a four times higher Ar _so i compared with the best reported inorganic V0 ₂ results (see Table 1). In present context, by "encapsulated", "encapsulation" or associated term is meant that the hydrogel is completely enclosed within or completely surrounded by the transparent plastic bag. In other words, contact of the hydrogel (30) with the exterior environment (i.e. other than the transparent plastic bag) is avoided. The advantage of providing such an encapsulation is to avoid leakage issue of the hydrogel. In this connection, the transparent plastic bag encapsulating the hydrogel (30) is formed of a material that minimizes evaporative loss of water present in the hydrogel (30). As one can appreciate, minimizing evaporative loss of water present in the hydrogel (30) would translate to optimal performance of the hydrogel since, as mentioned in earlier paragraphs, temperature dependent phase transition property of the hydrogel is being made use of in present invention to render feasible and practical implementation of smart windows.

In various embodiments, the temperature responsive hydrogel has a lower critical solution temperature (LCST) in a range of between 15 °C and 60 °C, such as 25 °C and 35 °C. Accordingly, suitable temperature responsive hydrogels include, but are not limited to, poly(vinylmethylether), poly(vinylcaprolactam), hydroxypropylcellulose, poly(N-isopropylacrylamide) (PNIPAm), or a mixture thereof. It is to be appreciated that more than one type of temperature responsive hydrogel may be encapsulated in the transparent plastic bag.

In one embodiment, the temperature responsive hydrogel is poly(N- isopropylacrylamide) .

As mentioned in earlier paragraphs, the transparent plastic bag is formed of a material that minimizes evaporative loss of water present in the temperature responsive hydrogel. Accordingly, suitable materials for the transparent plastic bag include, but are not limited to, polyethylene terephthalate (PET), polyvinyl butyral (PVB), ethylene-vinyl acetate polymer, thermoplastic polyurethane (TPU), polypropylene (PP), polycarbonate (PC), poly(methyl methacrylate) (PMMA), polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), thermoplastic polyolefins (TPOs), methacrylate-butadiene- styrene (MBS), polyvinyl chloride (PVC), and a mixture thereof.

In one embodiment, the transparent plastic bag is polyethylene terephthalate. In various embodiments, the thickness of the film of temperature responsive hydrogel is between 50 μιη and 5 cm. For example, the thickness may be 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1.0 mm, and up to 5 cm, including any value in between. In the present context, the thickness of the transparent plastic bag is taken to be negligible compared to the thickness of the film of temperature responsive hydrogel. In other words, it is to be understood that reference to the thickness of the film of temperature responsive hydrogel is analogous to reference to thickness of the hydrogel bag, and vice versa.

In one embodiment, the thickness of the film of temperature responsive hydrogel is 0.8 mm.

In various embodiments, the hydrogel bag is permanently adhered to at least one of the two sheets of glass or transparent plastic. For example, the hydrogel bag may be laminated onto at least one of the two sheets of glass or transparent plastic. In alternative embodiments, the hydrogel bag is removably arranged in between the two sheets of glass or transparent plastic. For example, the hydrogel bag may act as free pieces which can be taken in and out of the sandwich structure for easy maintenance or replacement.

In certain embodiments, it may be advantageous to separately include a water bag in addition to the hydrogel bag such that the overall thickness (or amount) of water in the panel design is increased. The advantage of having such a panel design is that the large thermal mass of water in the hydrogel bag, where water is the major component in hydrogel, and water in the water bag, can further reduce the temperature fluctuation as water can host a large amount of heat. Therefore, it could assist in warming up and cooling down an environment in winter and summer, respectively (to be illustrated in the example section given in later paragraphs). This was not realized by any other known energy efficient windows. Water could also be used to retard fire propagation.

The water bag consists of water encapsulated in a transparent bag as defined above for the hydrogel bag. Advantageously, the water bag is thicker than the hydrogel bag.

Fig. 8A shows the burning of pure PET film (left film) and a hydrogel-laminated PET (right film) over a fire. Fig. 8B shows the results after a period of burning: the pure PET film (left film) burns with a hole after 3 seconds, while the hydrogel-laminated PET (right film) stays intact after 50 seconds. Another advantage is that a safety function can be provided with the hydrogel bag inserted or arranged in between the glasses as the hydrogel bag can be used to stop glass crack propagations. Fig. 9 shows the results of the hydrogel bag-laminated glass panel after smashing.

In a second aspect of the invention, a hydrogel bag comprising a film of a temperature responsive hydrogel encapsulated in a transparent plastic bag is disclosed. The transparent plastic bag minimizes evaporative loss of water present in the temperature responsive hydrogel.

Properties, requirements and materials of the hydrogel bag, temperature responsive hydrogel, and transparent plastic bag have been discussed in earlier paragraphs and are not repeated hereinafter for brevity.

According to a third aspect of the invention, a method for forming a hydrogel bag of the second aspect is disclosed. The method comprises:

(a) dissolving monomers of a temperature responsive hydrogel in deionized water;

(b) adding a polymerization catalyst and a polymerization initiator to the solution of (a) to form a hydrogel solution;

(c) depositing the hydrogel solution of (b) onto a first transparent plastic sheet to form a film of hydrogel solution thereon;

(d) allowing the polymerization reaction to complete to obtain a film of a temperature responsive hydrogel; (e) placing a second transparent plastic sheet over the film of temperature responsive hydrogel such that the film of temperature responsive hydrogel is arranged in between the first and second transparent plastic sheets; and

(f) sealing the edges of the first and second transparent plastic sheets to thereby encapsulate the film of temperature responsive hydrogel therebetween.

In order that the invention may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non-limiting examples.

Examples

Experimental

Materials

Chemicals used in the examples were N-isopropylacrylamine (NIP Am, > 98%, Wako Pure Chemical Industries Ltd), N,N'-methylenebis(acrylamide) (> 99%, crosslinker, Sigma- Aldrich), polyvinylalcohol (PVA, M _w = 61,000, Sigma- Aldrich) and Ν,Ν,Ν',Ν'- tetramethylethylenediamine (TEMED, catalyst, 99%, Sigma- Aldrich), ammonium persulfate (APS, initiator, 98%, Alfa Aesar) and multipurpose sealant (Selleys All Clear). Deionized (DI) water (18.2 ΜΩ) was used throughout the experiments. All were used as received without any further purification.

Preparation of the sandwiched hydrogel structure

PNIPAm hydrogel was synthesized by in-situ polymerization of monomer in deionized (DI) water. 11.3 g (0.1 mol) NIPAm and 308 mg (0.002 mol) Ν,Ν'- methylenebis(acrylamide) were dissolved in 70 °C DI water to make a homogenous 100 ml aqueous solution. After the homogeneous and transparent solution was obtained, the temperature was reduced to 25 °C and this solution was purged with nitrogen gas (N ₂ ) for 30 minutes. 520.0 μΐ _^ of this stock solution was pipetted into one 1 mL centrifuge tube, and then 7.0 μΐ _^ TEMED (catalyst) and 16.0 μΐ _^ APS (initiator) were added in sequence to the centrifuge tube with vigorous vibration on vortex for 10 seconds. The solution was dropped onto the surface of a sheet of polyethylene terephthalate (PET) and covered by another sheet of PET to make a sandwich structure after the polymerization reaction was complete at room temperature after 24 hours. A sealant was applied at the edges of the two PET sheets to prevent mass exchange with outside environment. When the hydrogel is below T _c , the transparency allows large transmission of solar radiation, while above T _c the phase separation induced scattering center partially blocks radiation, thereby reducing the solar radiation transmission.

Characterization

The transmittance spectra in 250 nm - 2500 nm wavelength range were collected on a UV-Vis-NIR spectrophotometer (Cary 5000, Agilent, USA) at normal incidence. The spectrophotometer is equipped with a heating and cooling stage (PE120, Linkam, UK).

The integral luminous transmittance 7i _um (380 nm-780 nm), IR transmittance 7½ (780 nm -2500 nm) and solar transmittance T _s0 \ (250 nm-2500 nm) were calculated by using equation 1:

where Γ(λ) denotes spectral transmittance, φ _1απι (Λ) is the standard luminous efficiency function of photopic vision in the wavelength range of 380 nm-780 nm, φ _Ά (Λ) and <P _sol (A) are the IR/solar irradiance spectrum for air mass 1.5 (corresponding to the sun standing 37° above the horizon). A7ium/iR/soi is obtained by A7i _U m/iR/soi = r _lum /iR/ _S oi, 20 °c - T lum/IR/sol, 40 °C

It is herein for the first time reported a temperature responsive hydrogel film encapsulated in a transparent plastic bag which gave a four times higher Ar _so i compared with traditional inorganic V0 ₂ . The PNIPAm hydrogel bags are much easier to manufacture with different thickness and their LCST can be easily tuned according to specific usage conditions of smart windows. Results and Discussion

Fig. 2 shows the solar light transmittance (250 nm-2500 nm) profiles of PNIPAm hydrogel films each with a thickness of 0.8 mm laminated in between two sheets of PET. According to Fig. 2, two transmittance reductions happen at around 1430 nm and 1930 nm wavelength, which is due to the absorption of water at these two wavelengths. The absorption peak of water at around 1430 nm is due to the O-H stretch in the water molecule, while at around 1930 nm water has a unique peak due to a combination of O- H stretch and H-O-H bending.

For the PNIPAm hydrogel film with 0.8 mm thickness, it maintains transparency below the LCST with a T\ _am of 88.5 % at 20 °C. It turns into milky white at 40 °C with a dramatically diminished 7i _um of 0.6% (Fig. 2, Table 1). The sharp decline of T\ _um suggests that the LCST occurs between 30 °C and 35 °C. The blocking effect around the LCST was not only observed for the luminance range but also for the infrared region, thereby leading to soared Ar _so i. Therefore, the effective blocking of both IR and visible light at temperature above LCST accounts for the overall solar modulating capability of PNIPAm hydrogel (78% at 40 °C), which is nearly four times higher than the best reported V0 ₂ sample.

Table 1: Thermochromic properties of PNIPAm hydrogel film with 0.8 mm thickness

Tlum Tlum Tlum ATsoi

Sample (20 °C) (80 °C) (avera;

(%) (%) (%) (%)

Hydrogel 0.6 42.8 78.3

Best reported data [19] 44.8 50 20

Theoretical calculation 42 46.5 20 data [6]

To demonstrate the reversibility of the optical properties of the hydrogels, an outdoor test of a hydrogel bag with 0.8 mm PNIPAm hydrogel was done in Singapore at atmospheric temperature of 35 °C on a sunny day from 10:00 am to 8:30 pm. As shown in Fig. 3A, when the hydrogel bag was indoors at a temperature of around 25 °C, the hydrogel bag was clearly transparent. When the hydrogel bag was moved outdoor at more than 35 °C, it became totally opaque (Fig. 3B). When the hydrogel bag was later moved back indoor at around 8:30 pm, transparency was recovered (Fig. 3C). This experiment demonstrates the ability of the temperature responsive hydrogels to reverse its optical properties in tandem with temperature changes.

The effect of the hydrogel bags on reducing temperature fluctuations was examined. The actual temperature variations of glass houses was recorded in Table 2 and plotted in Fig. 4A. Glass house A is a bare glass house. Glass house B is a glass house laminated with a hydrogel bag encapsulating 0.8 mm poly(N-isopropylacrylamide) hydrogel (hydrogel/PET). Glass house C is a glass house laminated with a hydrogel bag encapsulating 0.8 mm poly(N-isopropylacrylamide) hydrogel and a water bag with a 2 cm thickness which is adhered to the hydrogel bag. The water bag, which is thicker than the hydrogel bag, is adhered to the hydrogel bag which is in turn adhered to the glass. Actual photographs of glass houses A, B, and C are shown in Fig. 4B and 4C. As the hydrogels have more than 90% water by composition, water is used to simulate the large thermal mass effect of hydrogel. It can be seen that from 10:00 am to 4:30 pm, there is a smaller increase in temperature of both glass houses B and C compared with glass house A. For glass house B, the average reduced temperature is ~ 5 °C while for glass house C, due to the large thermal mass of water, the average reduced temperature could be up to 10 °C. However, after 4:30 pm, there is a slightly higher temperature of 3-5 °C for glass house C compared with glass house A, which is expected as water has smoothed out the temperature variation. As a useful and practical application, the setup used in glass house C may be simulated in a business or office setting when there is minimal or no usage of the office building usually after, for example, 5:30 pm in Singapore, in countries having four seasons, and in countries with large variations in daily temperature such as those in the Middle East. This clearly demonstrates the advantage of utilizing the large thermal mass of water in helping to keep temperature fluctuations in a room or building small.

Table 2: Recorded temperature of three glass house during a day. A is a bare glass house, B is a glass ouse with 0.8 mm hydrogel/PET and C is a glass house with 0.8 mm hydrogel PET with 2 cm thick water bas.

The performance of the hydrogel bag in winter conditions was also simulated and evaluated. Three glass houses were placed in an environment with a temperature of -10 °C. 100 W light bulbs inside the glass houses were used as a heating source. There were three setups, an example of which is shown in Fig. 5: glass house D is a bare glass house, glass house E is a glass house laminated with a 1 cm water bag, and glass house F is a glass house laminated with a 2 cm water bag.

A warming effect was observed in winter conditions. The average temperature in the bare glass tank (i.e. glass house D) was 24 °C. For glass house E, the average temperature increased to 27 °C. For glass house F, the average temperature increased to 30 °C (Fig. 6). As hydrogels have more than 90% water by composition, this test shows that hydrogel bags will be effective in keeping an environment warm in winter. Overall, the hydrogel bags of the present invention will be useful in countries with four seasons to help keep an environment warm during winter time and cool during summer time.

Table 3: Temperature variations in winter condition tests.

Another advantage of the present hydrogel bag is that the panel design is also useful as a safety glass in addition to being a smart window. In the case of a safety glass application, the hydrogel bag is preferably laminated in between two sheets of glass at around 130 °C. The thermal stability of PNIPAm hydrogel up to 150 °C has been tested. Fig. 7 shows the photographs of the thermal durability tests. Four hydrogels with different compositions, i.e. bottle 1 (monomer, 0.8 mol/100 ml, no polyvinylalcohol (PVA)); bottle 2 (monomer, 1.0 mol/100 ml, no PVA); bottle 3 (monomer, 1.2 mol/100 ml, no PVA); bottle 4 (monomer, 0.8 mol/100 ml, with PVA) are shown in Fig. 7(a) and a clear transparent solution in each bottle could be observed at room temperature. When the bottles were heated up on a hotplate at 150 °C, the hydrogels in all four bottles immediately had a milky white coloration (Fig 7(b) and (c)). After three hours of heating at 150 °C, the hydrogels in bottles 1 and 2 still maintained the uniformity in milky white coloration whereas for bottles 3 and 4, water started to separate out from the hydrogels (Fig. 7(d)). It is encouraging to note that after cooling down the four bottles, the initial transparency at room temperature could be observed (i.e. reversible switching between transparent and opaque mode) even for the hydrogels in bottles 3 and 4. Therefore, by tuning the composition of the hydrogel as demonstrated in bottles 1 and 2, the present hydrogel bags can withstand high temperatures up to 150 °C and maintain the uniformity in coloration at high temperature and good reversibility in terms of optical properties. PVA, which was previously used in the synthesis of PNIPAm hydrogels, was not used in this invention as PVA provides a channel for water to flow from the inside of the hydrogel to the surface at high temperatures. Hence, the hydrogels mixed with PVA are not able to withstand high temperatures (as demonstrated by the hydrogel in bottle 4) and are not suitable for the manufacture of safety glass. Comparative Example

The presently disclosed hydrogel bag design has many advantages over current market products as summarized in Table 4. None of the current commercial products such as double glazed glass, safety glass and low emissivity glass can respond to temperature automatically in modulating transparency, while the present design gives -80% Ar _so i, which means the largest possible savings in energy for air conditioning consumption. The present design could also be used as a safety glass since it can be easily laminated between glasses. A thick hydrogel lamination further offers large heat storage and fire resistance which could result in huge energy savings in countries with four seasons and large daily temperature variations. Table 4: Competitor analysis

In conclusion, uniqueness and advantages associated with the present panel design and hydrogel bag can be summarized as follows: -

- Encapsulation of a temperature responsive hydrogel in a transparent plastic bag solves the leakage issue of the hydrogel and offers the flexibility of applying "water bag" since hydrogel is more than 90% water by composition. If the hydrogel is simply laminated or sandwiched in between two sheets of glass or transparent plastic, the water in the hydrogel gets evaporated easily and the durability of the hydrogel is not lasting. One advantage of encapsulating the hydrogel is that water loss is minimized and the hydrogel can act as interlayer to provide safety functionality as well. The encapsulated hydrogel can act as smart window sheets which can be laminated onto glass or kept as free pieces which can be taken in or out freely for maintenance or replacement.

- The encapsulated hydrogel can be inserted as lamination used in safety glass, which offers smart and safety glass bi-functionality.

- The panel design gave a four times higher Ar _so i (78%) than best reported results. - The hydrogel bag encapsulating 0.8 mm poly(N-isopropylacrylamide) hydrogel could offer around 5 °C temperature reductions for more than 6 hours, which is very good for use in Singapore, for example.

- The hydrogel bag encapsulating 0.8 mm poly(N-isopropylacrylamide) hydrogel and had a water bag with 2 cm thickness adhered could offer around 10 °C temperature reduction for about 5 hours and the temperature fluctuation is smoothened out. Such panels may be applicable to countries experiencing four seasons or large daily temperature variations. In Singapore context for example, such panels may be used in office buildings where after work hours air- conditioning is not required.

- By tuning the hydrogel composition, the thermal stability of the hydrogel may reach up to 150 °C for three hours, which is important for safety glass applications. By "comprising" it is meant including, but not limited to, whatever follows the word "comprising". Thus, use of the term "comprising" indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present. By "consisting of is meant including, and limited to, whatever follows the phrase "consisting of. Thus, the phrase "consisting of indicates that the listed elements are required or mandatory, and that no other elements may be present.

The inventions illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

By "about" in relation to a given numerical value, such as for temperature and period of time, it is meant to include numerical values within 10% of the specified value.

The invention has been described broadly and generically herein. Each of the narrower species and sub-generic groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group. References

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