The present invention relates to phase changing polymers and methods of making the same.
(Note: This application references a number of different publications as indicated throughout the specification. Each of these publications is incorporated by reference herein.)
The global issues of climate change and the rapidly escalating energy consumption have inspired developments for improved efficiency of energy usage. Stimuli-responsive materials can change their optical properties when exposed to external stimuli such as temperature, 1,2 pressure,3 light,4 magnetic field5 or electric field6 or when exposed to chemicals.7 In recent years, smart responsive materials have yielded impressive progress in smart windows,8 sensors,9 and other on-demand devices.10) Smart windows have tunable opacity to regulate solar-irradiation into buildings and residences, and thus can reduce the overall demand for air conditioning and heating. Thermochromic materials exhibit different colors or optical transmittance triggered by temperature changes, which could occur automatically through seasonal weather changes. Such autonomous modulation simplifies the structure and installation. The nonuse of electrical energy inputs makes it easy to achieve net energy savings.11
Several kinds of thermochromic materials have been extensively studied, including vanadium dioxide (VO2),12-16 ionic liquids,17 perovskites,18 liquid crystals19-21and hydrogels.22-24 The reversible metal-insulator phase transition of VO2 at the critical temperature (Tc) of 68° C. leads to a sharp change in the near-infrared spectrum. However, the high Tc and relatively low visible light transmittance limit the development of VO2 based smart devices. Both ionic liquids and perovskites have been investigated for photochromic switching based on their crystalline phase transition, but their switchable bandwidth in the solar flux range is narrow. Polymer-dispersed liquid crystals (PDLCs) can switch transmittance with a reversible transition of liquid crystal orientation via thermal or electric stimulation.25 The complexity of the system leads to high cost and low durability. Hydrogel have attracted a lot of interests in recent years thanks to the impressive transparency at ambient temperature and high opacity above its lower critical solution temperature.22,26 However, critical issues such as hydrostatic pressure, syneresis over time, and water leakage are not easy to resolve for hydrogel based systems. Therefore, there is a need to explore new solid-state thermochromic materials that have a transition temperature which can be naturally obtained for autonomous modulation, and the modulated spectrum is broadband in the solar flux range.
Phase-changing polymer (PCP) shows reversible phase transition between amorphous and semicrystalline state. Specifically, when the temperature is below the melting temperature (Tm) of a PCP, the PCP is at semicrystalline state. Once the temperature is above the Tm of PCP, namely crystals melt, PCP will absorb heat and be amorphous. PCP and its copolymers have been investigated in drug release,27 actuator,28 and thermal energy storage.29 We recently introduced a thermochromic solid-state phase-changing polymer film for smart windows.30 This system has the benefits of promising transmittance modulation and cycle stability, however, the regulation law is contrary to the common regulation of smart window as it is opaque at room temperature and transparent at elevated temperatures. In addition, the transition temperature was 46° C.; a Joule heater is required to control the transition, while the ideal thermochromic material should be triggered at 28-32° C. Therefore, new PCPs are therefore desired which are transparent at room temperature allowing sunlight to pass through, and turn opaque at high temperature to block solar radiation. The present invention satisfies this need.
The present disclosure describes a composition of matter useful as a thermochromic film in a smart window. The film can be embodied in many ways including, but not limited to, the following.
1. A solid state thermochromic polymer film comprising:
2. The solid state thermochromic polymer film of example 1, wherein the film:
3. The solid state thermochromic polymer film of example 2, wherein the opacity transition temperature is greater than 28° C. and less than 50° C.
4. The solid state thermochromic polymer film of any of the examples 1-3, wherein:
5 The solid state thermochromic polymer film of any of the examples 1-4, wherein:
6. The solid state thermochromic polymer film of any of the examples 1-5, wherein the at least one phase undergoing the crystal melting comprises:
7. The solid state thermochromic polymer film of example 6, wherein the polymer chain segments comprise at least one of an ethoxylated acrylate, an ethoxylated trimethylolpropane triacrylate, a poly(ethylene glycol) diacrylate, an ethoxylated methacrylate, an ethoxylated trimethylolpropane trimethacrylate, a poly(ethylene glycol) dimethacrylate, a propoxylated acrylate, a propoxylated diacrylate, a propoxylated trimethylolpropane triacrylate, a propoxylated methacrylate, a propoxylated dimethacrylate, or a propoxylated trimethylolpropane trimethacrylate.
8. The solid state thermochromic polymer film of any of the examples 1-7, wherein at least one of the separated solid phases does not undergo a phase change around the threshold temperature.
9. The solid state thermochromic polymer film of example 8, wherein the at least one of the separated solid phases which does not undergo the phase change around the threshold temperature comprises one or more first compounds comprising at least one of a hydroxyl, a cyano, a carboxylic acid, an amine, an alkylamine, an amide, a ketone, or an ether.
10. The solid state thermochromic polymer film of claim 1 prepared by copolymerization of a mixture comprising at least hexadecyl acrylate and 2-hydroxyethyl acrylate.
11. The solid state thermochromic polymer film of any of the examples 1-9 prepared by copolymerization of a mixture comprising at least hexadecyl methacrylate and 2-hydroxyethyl methacrylate.
12. The solid state thermochromic polymer film of example 10, example 11, or example 10 and example 11, wherein the mixture further comprises one or more multifunctional monomers comprising at least one of a diacrylate, a dimethacrylate, a triacrylate, a trimethacrylate, an oligoacrylate, or a oligomethacrylate.
13. The solid state thermochromic polymer film of any of the examples 10-12, wherein the copolymerization is by a means of ultraviolet (UV) exposure or heating
14. The solid state thermochromic polymer film of any of the examples 1-13, comprising a first polymer interspersed with a second polymer; wherein:
15. The solid state thermochromic polymer film of example 14 wherein the first polymer comprises one or more hydrocarbon groups comprising at least one of a dodecyl, a tetradecyl, a hexadecyl, or an octadecyl.
16. The solid state thermochromic polymer film of example 14 or 15 wherein the second polymer comprises one or more polar groups comprising at least one of a hydroxyl, a cyano, a carboxylic acid, an amine, an alkylamine, an amide, a ketone, or an ether.
17. The solid state thermochromic polymer film of any of the examples 14-16 wherein the first polymer is blended with the second polymer.
18. The solid state thermochromic polymer film of any of the examples 14-17 wherein the solid state thermochromic polymer film is prepared by partially reacting said first polymer with the second polymer.
19. The solid state thermochromic polymer film of any of the examples 1-18, wherein at least two of the separated solid phases comprise phase grains having a largest dimension larger than 1 micrometer.
20. The solid state thermochromic polymer film of any of the examples 1-19, wherein the two separated solid phases:
21. The solid state thermochromic polymer film of any of the examples 1-20, having the transmittance of at least 70% (e.g., 70% ≤ Transmittance ≤ 100%) below the threshold temperature due to the matching refractive indices of the two different phases and a lower transmittance of no more than 50% (e.g., 0% ≤ Transmittance ≤ 50%) above the threshold temperature due to the mismatching refractive indices of the two different phases.
22. The solid state thermochromic polymer film of example 21, wherein a thickness T of the solid state thermochromic polymer film is 1 micrometer ≤ T ≤ 1 millimeter.
23. The solid state thermochromic polymer film of example 22, wherein the thickness T is 10 micrometers ≤ T ≤ 500 micrometers.
24. A smart window comprising:
25. The smart window of example 24, wherein the transparent conductive layer comprises a transparent conductive material including at least one of a metal coating, metal nanowires, a metal grid, carbon nanotubes, graphene, or indium tin oxide.
26. The smart window of any of the example 24 and/or example 25, wherein the smart window:
27. The smart window or solid state thermochromic polymer film of any of the examples 1-26 wherein:
28. A thermochromic material exhibiting switchable optical transmittance via temperature change, wherein if the required temperature change is within seasonal weather changes, the transmittance change would consume low energy or be autonomous.
29. The material of example 28, comprising a solid-state thermochromic phase-changing copolymer film (TPCC) with a large transmittance modulation between room and hot temperatures. The polymer film comprises a hydrophilic poly(hydroxyethyl acrylate) (HEA) crosslinked with a hydrophobic phase-changing poly(hexadecyl acrylate-co-tetradecyl acrylate) (HDA-TA). The TPCC was designed such that the HEA and HDA-TA moieties produce µm-scale phase separation, the HDA-TA moiety undergoes reversible crystalline-to-amorphous transition at 28-32° C., and the refractive indices of the hydrophilic and hydrophobic phases are matched at ambient temperature but are mismatched when temperature is above the transition. The TPCC film showed high Δ T1um, Δ Tsolar and Δ TIR of 68.8%, 62.7% and 55.8%, respectively. The opacity switching was reversible without any decay even after 1000 heating-cooling cycles. The TPCC film was investigated for autonomous and climate-adaptable solar modulation window application.
Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
In one or more examples, a smart window (SW) is a glass, glazing, or film whose light transmission properties are altered as a result of temperature change, exposure to certain electromagnetic radiation, application of voltage, and/or other stimuli. The change of transmission properties can result from a change of reflectance, optical absorption, and/or light scattering.
This invention describes a solid state thermochromic polymer film for smart window applications. The polymer film is transparent below a threshold temperature (transition temperature) and becomes opaque above the threshold temperature. The polymer film comprises two or more separated solid phases with matching refractive indices, and appears optically transparent. One of the phases is (semi)crystalline and undergoes a crystal melting above the threshold temperature with a large reduction of its refractive index. The polymer film thus becomes optically opaque above the threshold temperature. This opacity change is reversible and may be repeated for numerous cycles. The crystal melting and its reverse process, recrystallization, are also called a phase change.
In materials science, “separated solid phases” means multiple immiscible domains in a material, each of the separated solid phases with its own distinctive composition and properties. The domains, also called grains, can be distinguished under optical or electronic microscopes.
A smart window or coating based on the said thermochromic polymer film allows light to transmit across the film below the threshold temperature, and becomes less transparent, or opaque, when its temperature rises above the threshold temperature. The threshold temperature is dependent on the crystal melting temperature, and can be varied based on the chemical composition of the (semi)crystalline domain. Melting usually occurs in a temperature range, from as narrow as ± 1° C. to as much as ± 5° C. or even wider. A narrow melting temperature range is generally desired.
In one or more examples, the thermochromic polymer film can be made of or comprise:
The temperature change to induce the opacity change can be caused by environmental temperature change, such as solar flux heating, the presence of a nearby heat source such as an oven or a lamp, or electrical heating. The electrical heating may be administered with the use of a transparent conductor. For instance, a transparent conductive coating of indium tin oxide (ITO) in thermal contact with the thermochromic film may induce uniform heating of the polymer film and consequently uniform change of opacity of the polymer film.
In one specific example, the thermochromic polymer film is transparent at ambient temperature with a parallel transmittance >70%. It becomes translucent or opaque above a threshold temperatures with a parallel transmittance <50%. The threshold temperature of the polymer film in one example is 30° C., and its opacity change occurs from a temperature of 29° C. to a temperature of 32° C.
The thickness of the SW film can be selected from 2 µm up to several millimeters. A Thinner film generally leads to a smaller opacity change.
In this example, a synthetic polymer film comprising a hydrophilic poly (hydroxyethyl acrylate) (HEA) was crosslinked with a hydrophobic phase-changing poly (hexadecyl acrylate-tetradecyl acrylate) (HDA-TA) to produce an adaptive, broadband, and efficient thermochromic phase-changing copolymer (TPCC) film. This new thermochromic film was designed such that the HEA and HDA-TA moieties produce µm-scale phase separation. The refractive indices of the hydrophilic and hydrophobic phases are matched at ambient temperature. At 28-32° C. and above, the HDA-TA moiety undergoes a reversible crystalline-to-amorphous transition resulting in mismatched refractive indices which strongly scatters light at moiety boundaries. The resulting thermochromic film exhibits high luminous, infrared and solar transmittance modulation with a low transition temperature of 28-32° C.
The polymer film is prepared from a mixture of three acrylate compounds and cured under UV light. The precursor solution consists of acrylate compounds, a cross-linker, a photo-initiator and a surfactant. Hexadecyl acrylate (HDA) with long alkyl chains is selected for the phase-changing component due to its sharp and reversible crystalline-to-amorphous transition. However, the cured poly(HDA) film has a Tm around 40° C. as determined by differential scanning calorimetry (see
The tunable light management mechanism is dictated by the refractive index match and mismatch. Poly(HDA-TA) is a phase-changing polymer that shows large change of refractive index during the semicrystalline-to-amorphous transition. At T<Tm, amorphous poly(HEA) phase shows comparable refractive index with the semicrystalline poly(HDA-TA) phase, which allows the TPCC film to have a high transmittance. At T>Tm, the poly(HDA-TA) domain melts, and the amorphous poly(HDA-TA) phase has a lower refractive index than the amorphous poly(HEA). This results in significant light scattering. Furthermore, the difference of hydrophilicity between the two constituent polymers can cause phase separation and produce µm-sized surface patterns, increasing the overall opacity of the TPCC film in the opaque state.
To understand the transparency modulation mechanism, we used scanning electron microscopy (SEM) to reveal the phase separation of poly(HDA-TA) and poly(HEA). Poly(HDA-TA) and poly(HEA) were shown to have a uniform and smooth surface (
Since the TPCC film’s tunability revolves around the refractive indices of its components, we measured the refractive indices of the neat poly(HEA) and the neat poly(HDA-TA) films with increasing temperature using a prism coupler.
The rapid decline of refractive index of poly(HDA-TA) is related to semicrystalline-to-amorphous transition. Variable temperature X-ray diffraction (XRD) analysis was performed to observe the phase transition (
To further monitor the phase transition property, differential scanning calorimetry (DSC) curves of poly(HDA-TA) and TPCC samples at a heating/cooling rate of 15° C. min-1 from 0 to 60° C. was examined (Figure S1). The DSC diagrams show a characteristic melting peak during heating and a recrystallization peak during cooling for each sample. The melting transition temperature (Tm) as defined by the peak value is 40.0° C. for poly(HDA) and 25.1° C. for Poly(TA). The difference in Tm is because HDA has a longer alkane chain than TA, so the melting process requires more energy. The Tm of poly(HDA) is tuned down by copolymerizing with poly(TA) with a shorter alkane chain. The Tm of poly(HDA-TA) copolymers are optimized at 36.3° C. with HDA:TA weight ratio of 4:1, and the resulting Tm of the TPCC is 33.4° C. The specific melting enthalpy (ΔHm) was estimated by integrating the heat flow peak divided by the mass of each sample (Table 1). Note that TPCC is composed of 6:1 HEA: (HDA-TA), So here, the DSC curve of TPCC is multiplied by 7 for comparison with other samples. The ΔHm value is 30.9, 19.7, 29.3 and 17.9 J g-1 for poly(HDA), poly(TA), poly(HDA-TA) and TPCC, respectively. To further compare the degree of crystallinity, the ΔHm of per mol of alkyl side chain was calculated because the crystallization of these phase changing polymer occurs mainly in the long n-alkyl side chain.33 The calculated ΔHm per mol of side chain value is 0.180, 0.136, 0.176 and 0.108 J mol-1 for poly(HDA), poly(TA), poly(HDA-TA) and TPCC, respectively. The lower AHm per mol of alkyl chain for poly(TA) in comparison with poly(HDA) could be attributed to less side chain carbons are able to crystallize in TA than that in HDA. The low ΔHm per mol of side chain of the TPCC may be explained by the constrained crystallization of the alkyl chains due to chemical crosslinks and confined domain size in the TPCC.
In order to observe the transition temperature of TPCC more intuitively, the TPCC film was photographed during temperature increase as shown in
A neat poly(HEA) film is always transparent due to the amorphous state and cannot provide transmittance modulation throughout the whole solar spectrum (
The TPCC film is transparent at low temperature before the phase transition of the HDA-TA moiety and opaque at high temperature after the phase transition. The HEA and HDA-TA domains are phase separated on the µm-scale. The refractive index matching of the HEA and HDA-TA domains below Tm leads to high transparency, while the refractive index mismatch above Tm enhances Mie scattering effect. The measured transmittance spectra of the TPCC device (300-2500 nm) with a TPCC film thickness of 170 µm are showed in
To demonstrate the reversibility of the transparency switching, 1000 cycles of heating-cooling test were carried out on the TPCC film (
The hydrophilic HEA and hydrophobic HDA-TA are phase separated in the TPCC film. The refractive indices of the two phase domains are matched at ambient temperature, leading to a transparent film. The HDA-TA moiety undergoes reversible crystalline-to-amorphous transition at 32° C., and its refractive index at the amorphous state above 32° C. is smaller than the HEA phase, resulting in high opacity. The Tlum of the TPCC film is 80.9% in the cold state and drops to 12.1% above 32° C. The TPCC film showed high ΔTlum, ΔTsolar and ΔTIR of 68.8%, 62.7%, and 55.8%, respectively. The opacity switching was maintained even after 1000 heating-cooling cycles. There is room to further modify the phase separated domain sizes and refine the refractive indices to further improve both the Tlum and broadband transmittance modulation to meet the requirements of a variety of applications including smart windows for energy savings, optical modulators and display technologies.
Materials: Hydroxyethyl acrylate (HEA), 2,2-Dimethoxy-2-phentlacetophenone (DMPA), Triton X-100 and trimethylolpropane trimethacrylate (TMPTA) were purchased from Sigma-Aldrich. Hexadecyl acrylate (HDA) and tetradecyl acrylate (TA) were purchased from TCI.
Fabrication of TPCCfilms: Hydroxyethyl acrylate (HEA), hexadecyl acrylate (HDA), tetradecyl acrylate (TA) were mixed with 1.714, 0.223 and 0.058 g, then 0.02 g (1 wt%) TMPTA was added as cross-linker, then 0.02 g (1 wt%) Triton X-100 was added as surfactant and 0.02 g (1 wt%) DMPA was added in as a photo-initiator. The whole mixture was placed on a hot plate and heated to 60° C. to render a clear solution, and followed with thorough sonication for 20 min. The TPCC film was fabricated by injecting the clear solution between two glass slides separated by spacers with thickness of 170 µm and then cured under UV light for 3 mins. For comparison, poly(HEA) and poly(HDA) were prepared through similar method with only HEA or HDA monomer and photo-initiator. Poly(HDA-TA) was prepared with the weight ratio of HDA and TA of 4:1.
SEM images were obtained on scanning electron microscope. (FEI Nova Nano 230) Optical microscope images were observed using a Zeiss microscope. Water contact angles were performed on an APPR telescope-goniometer. Transition temperature were characterized by PerkinElmer differential scanning calorimeter (DSC 8000) under nitrogen atmosphere from 0 to 60° C. at a heating or cooling rate of 15° C. min-1. Refractive indices were obtained on a Metricon refractometer (2010/M) and were tested at 632 nm. The IR thermal images were recorded via an infrared camera (ICI 9320P). The digital photographs of TPCC at different temperatures were taken in an oven with a thermocouple connected to the TPCC to read the temperature. Temperature-dependent XRD analysis were measured from 25 to 45° C. at a heating/cooling rate of 10° C. min-1 using a Rigaku SmartLab diffractometer with CuKα (1.5418 Å) radiation. The average crystal size from XRD results was calculated by Equation 1:
where D is the average crystal size, λ is X-ray wavelength, β is line broadening in radius and θ is Bragg angle. The UV lamp used for the stability test of TPCC under UV exposure is UVP Blak-Ray™ B-100AP High-Intensity UV Inspection Lamps. Transmittance spectra of the samples were measured using an UV-Vis-NIR spectrophotometer with tungsten halogen and deuterium lamps. (Shimadzu UC-3101PC)
The optical modulation ability of the TPCC film is characterized by the integral transmittance in the solar, luminous and infrared wavelength ranges. Tsolar (300-2500 nm), Tlum (390-780 nm), and TIR(780-2500 nm), are shown as follows:
T(λ) denotes the recorded transmittance at a particular wavelength, For Tsolar/IR,Ψ(λ) is the solar irradiance spectrum for air mass 1.5; and for T1um, Ψ(λ) is the CIE “physiologically-relevant” luminous efficiency function.
The solar energy shielding test was conducted on a model styrofoam chamber with a dimension of 8×6.5×5 inch3 with wall thickness of 1 inch. The 2 ×3 inch2 window devices made by double-glass slides or the TPCC device were assembled on the model chamber. A standard solar simulator (100 mW cm-2, Oriel Sol3A, Newport) was calibrated to air mass 1.5 on the top side of the window. Pico TC-08 USB Thermocouple Data Logger with PicoLog Data Logging Software for temperature measurement was employed to collect data.
Hydroxyethyl acrylate (HEA) and stearyl acrylate (SA) were mixed at different ratios. Trimethylolpropane triacrylate (TMPTMA, a crosslinker) and 2,2-dimethoxy-2-phenylacetophenone (DMPA, a photoinitiator) were at added at approximately 1 wt% each of the total weight. The mixture was heated at 65° C. and stirred to render a clear solution. The solution was injected while hot between two glass slides (separated with spacers of 170 µm thickness), and then cured under UV light for 3 minutes. The resulting film sandwich was heated to 65° C. and then cooled to room temperature. The heating and cooling were repeated for 5 cycles, and the transmittance of the film is shown in
The HEA:SA weight ratio was varied from 1:2 to 6:1. All films showed high transparency at the unheated state and high opacity at the heated state.
Hydroxyethyl acrylate (HEA, 1.7 grams), stearyl acrylate (SA, 0.28 grams), and 2-butanone (0.25 mL) were mixed, heated and stirred to obtain a clear solution. Trimethylolpropane triacrylate (TMPTMA, a crosslinker) and 2,2-dimethoxy-2-phenylacetophenone (DMPA, a photoinitiator) were at added at approximately 1 wt% each of the total weight. The solution was injected while hot between two glass slides (separated with spacers of 170 µm thickness), and then cured under UV light for 3 minutes. The resulting film sandwich was heated to 65° C. and then cooled to room temperature. The heating and cooling were repeated for 2 cycles, and the transmittance of the film is shown in
Hydroxyethyl acrylate (HEA, 1.7 grams) and hexadecyl acrylate acrylate (HDA, 0.28 grams) were mixed, heated and stirred to obtain a clear solution. Trimethylolpropane triacrylate and 2,2-dimethoxy-2-phenylacetophenone were at added at approximately 1 wt% each of the total weight. The solution was injected while hot between two glass slides (separated with spacers of 170 µm thickness), and then cured under UV light for 3 minutes. The resulting film sandwich was heated to 55° C. and then cooled to room temperature. The heating and cooling were repeated for 2 cycles, and the transmittance of the film is shown in
A copolymer film with HEA:HDA weight ratio of 4:1 was similarly prepared. It showed similarly high transparency at the unheated state and high opacity at the heated state (see
Block 1100 represents combining (e.g., mixing) a first composition comprising one or more first polymers and/or one or more first co-monomers and/or one or more first monomers with a second composition comprising one or more second polymers, one or more second co-monomers, or one or more second co-monomers so as to form a mixture. In one or more examples, the weight percentage of the first composition and the second composition are each greater than 10 wt% of the total weight of the mixture. Other compounds may also be mixed together, such as a volatile solvent that dissolves one or more of the other compositions, a polymerization initiator, a colorant, a UV stabilizer, a flame retardant additive, and/or an adhesion promotion agent. The mixture may be treated using one or more heating, stirring, or sonication steps, to form a uniform liquid mixture.
Block 1102 represents film formation comprising processing of the liquid mixture into a thin film comprising two separated solid phases, comprising a first phase and a second phase. The processing may involve one or more of the techniques including coating, printing, lamination, and injection between two sheets, evaporation of volatile components, UV light exposure, visible light exposure, infrared light exposure, heating, heating and cooling treatment, and vacuum treatment. In one or more examples, at least 50% of the first phase comprises the one or more first polymers and/or one or more first co-monomers and/or one or more first monomers, and at least 50% of the second phase comprises the one or more second polymers, the one or more second co-monomers, or the one or more second co-monomers. In one or more examples the second polymers are cross-linked with the first polymers. In one or more examples the heating is to a temperature between 50 and 100 degrees and the cooling is to a temperature of less than 30 degrees. In one or more examples, the two separated phases comprise a blend.
Block 1104 represents the composition of matter.
The composition of matter can be embodied in many ways including, but not limited to, the following.
1. A solid state thermochromic polymer film 1000, 100 comprising two or more separated solid phases (e.g., a first phase 104 comprising first domains 1006 and a second phase 102 comprising second domains 1008), wherein the separated solid phases 102, 104 are transparent and comprise at least one phase 104 (or one or more phases) which undergoes a crystal melting. The at least one phase 104 which undergoes the crystal melting has a reduced refractive index above a threshold temperature, becomes opaque when the temperature of the at least one phase rises above the threshold temperature, and reverts to being transparent when the temperature lowers to an ambient temperature below said threshold temperature.
2 The solid state thermochromic polymer film of example 1, wherein the film:
3. The solid state thermochromic polymer film of example 2, wherein the opacity transition temperature is greater than 28° C. and less than 50° C.
4. The solid state thermochromic polymer film of any of the examples 1-3, wherein:
5. The solid state thermochromic polymer film of any of the examples 1-4, wherein:
the refractive index reduction is greater than 0.01 over a temperature range of +/-3° C. with respect to the threshold temperature.
6. The solid state thermochromic polymer film of any of the examples 1-5, wherein the one of the phases (first phase 104) undergoing the crystal melting comprises a crystal melting moiety comprising:
6b. The solid state thermochromic polymer film of any of the examples 1-5, wherein the at least one phase 104 undergoing the crystal melting comprises:
7. The solid state thermochromic polymer film of example 6 or 6b wherein the polymer chain segments can be selected from the group comprising ethoxylated acrylate, ethoxylated trimethylolpropane triacrylate, poly(ethylene glycol) diacrylate, ethoxylated methacrylate, ethoxylated trimethylolpropane trimethacrylate, or poly(ethylene glycol) dimethacrylate.
7b. The solid state thermochromic polymer film of example 6 or 6b, wherein the polymer chain segments 1002 comprise at least one of an ethoxylated acrylate, an ethoxylated trimethylolpropane triacrylate, a poly(ethylene glycol) diacrylate, an ethoxylated methacrylate, an ethoxylated trimethylolpropane trimethacrylate, a poly(ethylene glycol) dimethacrylate, a propoxylated acrylate, a propoxylated diacrylate, a propoxylated trimethylolpropane triacrylate, a propoxylated methacrylate, a propoxylated dimethacrylate, or a propoxylated trimethylolpropane trimethacrylate.
8. The solid state thermochromic polymer film of any of the examples 1-7b, wherein at least one of the separated solid phases (second phase 102) does not undergo phase change around the threshold temperature.
9. The solid state thermochromic polymer film of example 8, wherein the one of the solid phases (second phase 102) which does not undergo phase changing around the threshold temperature comprises at least one first compound 1004 selected from the group consisting or comprising of hydroxyl, cyano, carboxylic acid, amine, alkylamine, amide, ketone, ether.
9b. The solid state thermochromic polymer film of example 8, wherein the at least one of the separated solid phases 102 which does not undergo the phase change around the threshold temperature comprises one or more first compounds 1004 comprising at least one of a hydroxyl, a cyano, a carboxylic acid, an amine, an alkylamine, an amide, a ketone, or an ether.
10. The solid state thermochromic polymer film of any of the examples 1-9b prepared by copolymerization of a mixture containing at least hexadecyl acrylate and 2-hydroxyethyl acrylate.
11. The solid state thermochromic polymer film of any of the examples 1-9b prepared by copolymerization of a mixture containing at least hexadecyl methacrylate and 2-hydroxyethyl methacrylate.
12. The solid state thermochromic polymer film of any of examples 10 and/or 11, wherein the mixture further comprises a multifunctional monomer selected from the group consisting of diacrylate, dimethacrylate, triacrylate, trimethacrylate, oligoacrylate, and oligomethacrylate.
12b. The solid state thermochromic polymer film of any of examples 10 and/or 11, wherein the mixture further comprises one or more multifunctional monomers comprising at least one of a diacrylate, a dimethacrylate, a triacrylate, a trimethacrylate, an oligoacrylate, or an oligomethacrylate.
13. The solid state thermochromic polymer film any of the examples 10-12b, wherein the copolymerization is by a means of ultraviolet (UV) exposure or heating.
14. The solid state thermochromic polymer film of any of the examples 1-13, comprising a first polymer 1002 interspersed with a second polymer 1004;
15. The solid state thermochromic polymer film of example 14 wherein the first polymer comprises hydrocarbon groups selected from the group including dodecyl, tetradecyl, hexadecyl, and octadecyl.
15b. The solid state thermochromic polymer film of example 14 wherein the first polymer 1002 comprises one or more hydrocarbon groups 1010 comprising at least one of a dodecyl, a tetradecyl, a hexadecyl, or a octadecyl.
16. The solid state thermochromic polymer film of any of the examples 14-15b wherein the second polymer comprises polar groups selected from the group consisting or comprising of hydroxyl, cyano, carboxylic acid, amine, alkylamine, amide, ketone, and ether.
16b. The solid state thermochromic polymer film of any of the examples 14-15b wherein the second polymer comprises one or more polar groups 1012 comprising at least one of a hydroxyl, a cyano, a carboxylic acid, an amine, an alkylamine, an amide, a ketone, or an ether.
17. The solid state thermochromic polymer film of any of the examples 14-16b wherein the first polymer 1002 is blended with the second polymer 1004.
18. The solid state thermochromic polymer film of any of the examples 14-17 wherein the solid state thermochromic polymer film 1000 is prepared by partially reacting said first polymer with second polymer.
19. The solid state thermochromic polymer film of any of the examples 1-18, wherein at least two of the separated phases comprise phase grains 1006, 1008 having a largest dimension 1014 larger than 1 micrometer.
20. The solid state thermochromic polymer film of any of the examples 1-19, wherein the two separated phases 102, 104:
21. The solid state thermochromic polymer film of any of the examples 1-20, having the transmittance of at least 70% (e.g., 70% ≤ Transmittance ≤ 100%), for one or more wavelengths λ or all the wavelengths λ wherein 400 nanometers ≤ λ ≤ 1400 nanometers, below the threshold temperature due to the matching refractive indices of the two different phases and a lower transmittance of no more than 50%, e.g., 0% ≤ Transmittance ≤ 50%, (for one or more wavelengths λ or all the wavelengths λ wherein 400 nanometers ≤ λ ≤ 1400 nanometers) above the transition temperature due to the mismatching refractive indices of the two different phases.
22. The solid state thermochromic polymer film of any of the examples 1-21, wherein a thickness T of the solid state thermochromic polymer film 1000 is 1 micrometer ≤ T ≤ 1 millimeter.
23. The solid state thermochromic polymer film of any of the examples 1-21, wherein a thickness T of the solid polymer film is 10 micrometers ≤ T ≤ 500 micrometers.
Block 1106 represents including the polymer film or composition of matter in a smart window. In one or more examples, the composition of matter or film is combined with a transparent heater. The smart window can be embodied in many ways including, but not limited to, the following.
24. A smart window comprising:
25. The smart window of example 24, wherein the transparent conductive layer comprises a transparent conductive material including at least one of a metal coating, one or more metal nanowires, a metal grid, one or more carbon nanotubes, graphene, or indium tin oxide.
26. The smart window of any of the examples 24 and/or 25, wherein the smart window:
27. The solid state thermochromic polymer film of any of the examples 1-26, wherein the polymers in the polymer film 1000 have a refractive index in a range of 1.3-1.8 or in a range of 1.4-1.6.
28. The solid state thermochromic polymer film 1000 of any of the examples 1-27, wherein:
29. The solid state thermochromic polymer film of any of the examples 1-28, wherein the transparency of at least one of the thermochromic film or the phase undergoing the crystal melting, at the temperature below the threshold temperature, is sufficient for use as an external window on a building, house, or vehicle and the opacity at the temperature above the threshold temperature is such that the transparency is reduced by at least 20%.
30. The solid state thermochromic polymer film of any of the examples 1-29, wherein the separated phases comprise a crosslinked polymers or crosslinked comonomers.
31. The solid state thermochromic film of any of the examples 1-29, wherein the at least one phase undergoing the crystal melting comprises a first polymer or first co-monomer and another of the separated solid phases comprises a second polymer or second co-monomer, wherein the first polymer is crosslinked to the second polymer and the first polymer or first co-monomer comprises larger side chains comprising a phase changing hydrocarbon as compared to the second polymer or second co-monomer.
32. The solid state thermochromic film of example 31 wherein the at least one phase undergoing the crystal melting comprises a crystalline phase and the other of the separated solid phases is amorphous.
33. The solid state thermochromic film of example 31 or 32 wherein the another of the separated solid phases does not undergo the crystal melting.
The global issues of climate change and the rapidly escalating energy consumption have inspired developments in the efficiency of energy usage. Utilizing smart windows’ tunable opacity to control both the timing and amount of light transmission would have a direct reduction in the overall demand for air conditioning and heating Smart windows can also be deployed in business and household rooms to improve privacy protection.
Three different technologies have been developed for smart windows: photochromic, electrochromic and thermochromic technologies. Currently available photochromic, electrochromic, and thermochromic smart window materials have limited bandwidth modulation, have short lifetimes, and/or must undergo complex production methods.
Photochromic-based smart windows are based on special compounds added into a film or coating. The compounds are light sensitive and exhibit changes to their optical absorption spectrum (US 6,446,402 B1). The opacity change is limited to a narrow optical wavelength range, which is not a desirable feature. The absorption at a certain wavelength range usually results in a fixed color for the opaque and/or transparent states, which is undesirable for general applications. The optical absorption also leads to heating of the window.
Electrochromic smart windows utilize the insertion and extraction of electrons in electrochemical redox reactions of the host materials to change colors. The switching speed depends on the active device area, diffusion length, and coefficient of electrolyte ions. The switching time of large-area electrochromic windows can take up to 10 minutes for practical usage. Electrochromic windows overall share the same problems of photochromic windows in that they have fixed colorations at either the transparent state, opaque state, or both. More critically, electrochromic windows have complicated structures, limited cycle lifetime, and excessive sealant due to the use of liquid electrolytes.
Thermochromic smart windows traditionally use VO2-based materials which change colors due to a metal-insulator transition at critical temperatures, but these materials have low transmittance at visible light range for the transparent state, low oxidation resistance, and high cost for fabrication. Hydrogel-based thermochromic smart windows take advantage of hydrogels’ phase separation property to enable a wide modulation wavelength range and high transmittance modulation contrast (Li, X.H., Liu, C., Feng, S.P. and Fang, N X, 2019. Broadband light management with thermochromic hydrogel microparticles for smart windows has been reported (see Joule, 3(1), pp.290-302). However, the inclusion of water in a hydrogel-based thermochromic smart window hinders the cyclic stability due to water evaporation. Thermochromic polymer-dispersed or polymerstabilized liquid crystals suffer from having a limited bandwidth due to their fixed pitches. Cholesteric liquid crystals or stacked liquid crystals with various pitches were used to enable large bandwidth switching, but they are expensive for large area applications in buildings.
Thermochromic smart windows based on solid state polymer films (US 6,362,303 B1) are opaque at ambient temperature due to light scattering, and turn transparent when heated above a threshold temperature. While useful for some applications, for smart windows applications for energy savings, the opacity change is in the wrong direction: when outdoor temperature is cool, this film is opaque and reflects sun light to lower the solar heating effect in the room. When it is hot outside, the film becomes transparent and allows the sunlight to penetrate through the window and heat inside the room.
Here we introduce a new thermochromic smart window material which is transparent when it is cold and allows sunlight to warm up the room. When it becomes hot (e.g., in the middle of the summer day) the window becomes opaque to block sunlight from entering the room. It is advantageous over the hydrogel-based thermochromic smart window because hydrogen can lose water over time, and the hydraulic pressure can be too large for large window areas. Embodiments of the present thermochromic films are based on all-solid polymer film, containing no water or any other volatile compounds.
We note that solar flux ranges from UV, to visible, and infrared. Much of the solar flux heat is transmitted in the visible and near infrared wavelength range. The present thermochromic polymer film changes its opacity in a broad wavelength range, covering most of the solar flux wavelength range.
The following references are incorporated by reference herein
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This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
This application claims the benefit under 35 U.S.C. Section 119(e) of copending and commonly-assigned U.S. Provisional Pat. Application Serial No. 63/050,602, filed on Jul. 10, 2020, by Qibing Pei “A PHASE-CHANGING POLYMER FILM FOR THERMOCHROMIC SMART WINDOWS APPLICATIONS,” Attorney Docket 30794.778USPl (UC Ref. 2020-944), which application is incorporated by reference herein.
Filing Document | Filing Date | Country | Kind |
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PCT/US2021/041275 | 7/12/2021 | WO |
Number | Date | Country | |
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63050602 | Jul 2020 | US |