Thermally responsive hydrogels

Abstract

The present invention concerns new surface-active polyethylene glycol)-derived hydrogels that are crosslinked by using thiol-Michael addition and a crosslinker. The new concept of preparing thermally responsive hydrogels with a thin active surface layer relies on the forced emulsion formation and sedimentation from the aqueous prepolymer solution by using a crosslinker that is engineered to serve as an antisolvent for the prepolymer while at the same time exhibiting a suitable solubility profile in the sedimented hydrogel layer with respect to the supernatant aqueous phase. The solubility of the crosslinker is higher in the supernatant aqueous phase as compared to the sedimented hydrogel phase. This leads to different concentrations of crosslinker at the bottom and interfacial layers of the hydrogel, different degrees of crosslinking, as well as a higher equilibrium water concentration in the top active layer.

Description

[0001] Thermally Responsive Hydrogels

[0002] Field of the Invention

[0003] The invention concerns thermally responsive hydrogels which have application in connection with, for instance, smart windows.

[0004] Background to the Invention

[0005] Smart window devices have shown the potential to increase the energy efficiency and to reduce energy consumption for air conditioning and heating in buildings. In addition to the demand for energy saving solutions and energy efficiency, the importance of smart windows is also rising due to emerging environmental issues.

[0006] Many smart window concepts have been proposed and developed. The most promising and widely reported concepts are based on electrochromic, thermochromic and photochromic materials, as well as polymer dispersed liquid crystals and suspended particle devices. All of these materials have their own benefits and limitations such as high price, low electrochemical stability or stability towards UV radiation, limited number of possible cycles or the need for an external control circuits and energy supply.

[0007] The phase changing materials are an alternative solution for using in smart windows. Polymers that exhibit lower critical solution temperature (LCST) undergo transition from soluble to partially soluble state in response to increase in temperature, causing the formation of globular macromolecular structures which act as scattering centers for visible light. These kinds of polymers are called thermally responsive polymers and these can be reversibly switched between transparent and non-transparent state either in a solution or as hydrogels for a large number of cycles without losing performance.

[0008] The best-known class of thermally responsive phase-changing polymers are poly(N-alkyl acrylamide)s and, specifically, poly(N-isopropylacrylamide) (PNIPAm). PNIPAm was first synthesized in the 1950s and, more recently, its use for smart window applications has been explored, as well as its hybrids with nanoclay and different thermochromic oxides.

[0009] In addition to poly(N-a I kyl acrylamide)s, many other optically thermally responsive polymers and polymer systems have been proposed for the smart window devices. Polymers used in aqueous media for that application include poly(oligo ethylene glycol (meth)acrylate)s, poly(2-(dimethylamino)ethyl meth-acrylate), poly(2-oxazoline)s, poly( / V-vinylcaprolactam), various modified poly(vinyl ether)s, hydroxypropyl cellulose (HPC), HPC-acrylamide, HPC- polyacrylic acid, as well as hydroxypropyl methyl cellulose (HPMC)

[0010] Thus far, the transmittance change from the formation of macromolecular globular clusters that act as scattering centers has been observed in the bulk volume of the polymer solution or hydrogel. Although thin hydrogel layers in the range of 26-200 pm have been prepared by using suitable spacers for setting the thickness of the polymerizing solution between the glass panes, the thermally responsive effect fundamentally originates from the bulk volume and morphologically, the effect does not differ from the performance in thick hydrogel layers. To achieve sufficient luminous transmittance- and solar energy modulation, relatively thick layers of materials are typically needed. This is related to low refractive index contrast between the polymer and the liquid phase. The effect in bulk volume is also relatively slow due to thermal inertia and slow heat transfer by conduction. In many cases the external energy input by means of transparent Joule heaters are used to supply heat to the phasechanging active layer.

[0011] Statements of the Invention

[0012] The present invention concerns new surface-active polyethylene glycol)-derived hydrogels that are crosslinked by using thiol-Michael addition and a crosslinker. The new concept of preparing thermally responsive hydrogels with a thin active surface layer relies on the forced emulsion formation and sedimentation from the aqueous prepolymer solution by using a crosslinker that is engineered to serve as an antisolvent for the prepolymer while at the same time exhibiting a suitable solubility profile in the sedimented hydrogel layer with respect to the supernatant aqueous phase. The solubility of the crosslinker is higher in the supernatant aqueous phase as compared to the sedimented hydrogel phase. This leads to different concentrations of crosslinker at the bottom and interfacial layers of the hydrogel, different degrees of crosslinking, as well as a higher equilibrium water concentration in the top active layer.

[0013] Such surface-functional hydrogels are free from problems related to slow heat transfer into the bulk volume and consequent slow switching. The lack of mechanical degrees of freedom for efficient coil-to-globule transition, that is characteristic for the bulk volume of the crosslinked hydrogel, is less pronounced on the surface, enabling surface functionality between the bulk hydrogel and liquid solution while retaining significant mechanical robustness. Such morphology enables to achieve an exceptionally short response time in the order of 1 s for full switch between the opaque and transparent state. Integrated luminous transmittance TiUm of over 95%, luminous transmittance modulation ATium of 77.5% (wavelength range of 380-780 nm) and solar energy modulation ATSOi(wavelength range of 300-2500 nm) of 60% and higher.

[0014] According to the present invention, there is provided a method of making a surface-active thermally responsive hydrogel comprising reacting a prepolymer that is a polyethylene glycol derivative and a cross-linking agent which functionally includes a carboxyl group and an unsaturated C=C bond, wherein the polyethylene glycol derivative and the cross-linking agent are contacted under conditions providing emulsion sedimentation occurring in parallel to cross-linking in the deposited layer, at the interface of the emulsion droplets and at the propagating interface of the sedimented hydrogel phase and its supernatant liquid, the solubility of the crosslinker being higher in the supernatant liquid phase than in the sedimented hydrogel such that there is a differing concentration of the crosslinker in the hydrogel.

[0015] The polyethylene glycol derivative may be, for instance, a thiol-functional or an amine- functional derivative.

[0016] The cross-linking agent may also include, for example, an ester, ketone or amide functionality.

[0017] The method consists of three distinct steps: 1) Synthesizing, or otherwise providing, a crosslinker for a prepolymer together with the preparation of crosslinker solution with specific properties. 2) Preparation of aqueous prepolymer solution and 3) Combining the crosslinker and prepolymer solutions, leading to forced emulsion formation and start of the emulsion sedimentation. Both the substrate, i.e. transparent, non-functional hydrogel part at the bottom, and thermally responsive top layer are formed in the same process at different consecutive stages of emulsion sedimentation in a controlled manner. Preferably, the cross-linking agent is a dicarboxylic acid and either a diketone or a diamide, having unsaturated C=C bond functionality in each of its two ketone or amide branches. Particularly preferred crosslinking agents are:

[0018] 1) 4,ll-dioxa-7-hydroxy-Z2,Z12-tridecadienedioic acid (compound no. 1 on Figure 1) that contains two carboxyl groups, two unsaturated C=C bonds, two ketone linkages and one hydroxyl group;

[0019] 2) (2Z)-3-({3-[(2Z)-3-carboxyprop-2-enamido]-2-hydroxypropyl}carbamoyl)prop-2-enoic acid (compound no. 2 on Figure 1) that contains two carboxyl groups, two unsaturated C=C bonds, two amide linkages and one hydroxyl group;

[0020] 3) (2Z)-3-({3-[(2Z)-3-carboxy-N-methylprop-2-enamido]-2- hydroxypropyl}(methyl)carbamoyl)prop-2-enoic acid (compound no. 3 on Figure 1);

[0021] 4) (2Z)-3-({3-[(2Z)-3-carboxy-N-ethylprop-2-enamido]-2- hydroxypropyl}(ethyl)carbamoyl)prop-2-enoic acid (compound no. 4 on Figure 1); and

[0022] 5) (2Z)-3-({3-[(2Z)-3-carboxy-N-propylprop-2-enamido]-2- hydroxypropyl}(propyl)carbamoyl)prop-2-enoic acid (compound no. 5 on Figure 1).

[0023] In the less preferred case, the cross-linking agent is a dicarboxylic acid and a diester having unsaturated C=C bond functionality in each of its two ester branches, for example 2- butenedioic acid (2Z)-2-hydroxy-l,3-propanediyl ester (BAHPE) (compound no. 6 on Figure 1).

[0024] Additionally to the compounds listed above, the respective cis and trans isomers of these compounds can also be used, involving the rotations around the unsaturated C=C bonds in the molecules.

[0025] The crosslinking agent BAHPE is made by esterifying two hydroxyl groups of a glycerol (triol compound) with maleic anhydride in the presence of a tertiary amine catalyst (N,N- diisopropylethylamine) by using acetone as a solvent. Weight ratio of maleic anhydride, glycerol, DIPEA and acetone is 1:0.47:0.0607:x, where the value of x (weight ratio of acetone solvent with respect to other precursors) can be varied in the range of 1-5 and preferably is 2.21. The specific weight ratio between maleic anhydride and glycerol (1:0.47) corresponds the molar ratio of 2:1. Instead of triol a diol may be used, for instance 1,4-butanediol, 1,5- pentanediol, 1,6 hexanediol, 1,7-heptaneanediol or 1,8-octanediol. Also, a polyol may be used instead of triol, for instance, erythritol, xylitol, sorbitol or mannitol.

[0026] As an example of a suitable crosslinker synthesis together with the preparation of suitable solution, the BAHPE crosslinker synthesis is carried out as follows. A 4.1396 g quantity of maleic anhydride is dissolved in 9.726 mL of acetone, followed by the addition of 1.544 mL of glycerol and 0.973-2.919 mL of 1 M DIPEA in acetone (optimally 1.946 mL). The mixture reacts under stirring in a closed vial at 55 °C for 24 h. This results in crosslinker solution in acetone that additionally contains tertiary amine. This solution is used in its entirety in the process of forced emulsion formation as this solution is added into the prepolymer solution under mixing. Alternatively, the solution of BAHPE as well as other suitable crosslinkers discussed above can be prepared simply by mixing the pre-prepared crosslinker as a pure substance with acetone and tertiary amine (e.g. DIPEA) to obtain the solution with same composition and concentrations.

[0027] Examples of tertiary amines which may be used are triethylamine, tripropylamine, diethylisopropylamine and N,N-diisopropylethylamine.

[0028] Preferably, the thiol-functional polyethylene glycol derivative is ethoxylated trimethylolpropane tri(3-mercaptopropionate) (ETTMP) with the molecular weight in the range of 700-3000 (2 to 15 ethylene glycol units), most preferably 1300 (5 ethylene glycol units), or its chemical analogue with the molecular weight in the same range whereas the ester functionality is replaced either with an amide or ketone functionality.

[0029] Alternatively, a chemical analogue of ETTMP can be used where the thiol functionality is replaced with a primary amine functionality.

[0030] The formation of hydrogel at the bottom of the solution and the formation of clearly defined thermally responsive surface layer relies on the number of different complex processes that occur concomitantly. This involves the control of solubilities of the precursors by temperature and pH, forced emulsion formation and gradual sedimentation of the emulsion droplets in the gravitational field, emulsion coalescence and crosslinking. Preferably, the thiol-functional polyethylene glycol derivative and the cross-linking agent are contacted under conditions providing emulsion sedimentation occurring in parallel to cross-linking, through thiol-Michael click reaction in the deposited layer, at the interface of the emulsion droplets, as well as on the interface of deposited hydrogel and its supernatant liquid. In addition to the final stage of the emulsion sedimentation, the addition of prepolymer and crosslinker molecules directly from the supernatant solution phase onto the functional surface layer contributes to the growth of the functional surface layer as these molecules are present in the supernatant solution.

[0031] Preferably, the thiol-functional polyethylene glycol derivative is contacted with the crosslinking agent in the presence of a base. The base may be a primary or tertiary amine. Aliphatic primary amines which may be used include those up to decylamine, for example, ethylamine, propylamine and butylamine. A preferred amine is n-butylamine. Amino-alcohols such as 3- amino-l-propanol may also be used, as may ammonium hydroxide.

[0032] Mixing of the pre-polymer solution with the crosslinker solution is preferably carried out for from 5s to 60 min, more preferably from 5 to 10 s, at temperature in the range of 17-25 °C, most preferably at 22 °C. During this process, the pH is dropping, small droplets are forming, creating an emulsion, and the prepolymer starts to crosslink. A period of from 2 to 6 days may be allowed for complete sedimentation and crosslinking at of 17-25 °C, most preferably at 22 °C.

[0033] As the crosslinker solution is added into prepolymer aqueous solution under mixing, a cloud of fine white emulsion forms abruptly. Significant part of the ETTMP phase separates out of the solution at this stage and fine emulsion is forced to form due the crosslinker being an antisolvent for ETTMP in the aqueous solution. The antisolvent effect specifically originates from the crosslinker and not from the acetone and an amine that are also contained in the solution since ETTMP exhibits full solubility in acetone. Adding the crosslinker solution into the prepolymer solution is also accompanied by the reduction of pH down to 2-3, but this contributes to the phase separation only to a small extent.

[0034] Metal nanoparticles (e.g. gold, silver, platinum, palladium) may be added into an active functional layer to provide color due to plasmon resonance. Coil-to-globule transition brings the nanoparticles close together and strong interparticle plasmon coupling arises, resulting in color change. Gold- and other noble metal nanoparticles have been used in conjunction with ultrathin stimuli-responsive polymer films for transduction of changes in the solution pH into an optical signal. At the pH at which the polymer chains exist as extended linear chains, gold NPs adsorbed on the polymer layer exist primarily as individual nanoparticles and at higher pH at which the polymer chains transition from coil to globule state, nanoparticles are brought closer together and strong interparticle plasmon coupling arises. Similar effect has been described by using thin stimuli-responsive polymer film to separate plasmonic nanostructures on the surface of the substrate and on top of the polymer film. These works represent some level of proof of concept for using stimuli-responsive polymers for active plasmonics. The use of active plasmonic effects for creating an efficient and visible color change in conjunction with coil-to-globule transition has not been demonstrated. The interfacial morphology and the unprecedented efficient and fast thermally responsive effect at the surface of the hydrogel enables to use coil-to-globule transition for active plasmonics effects to achieve color change. This is not possible with conventional thermally responsive hydrogels that utilize the coil-to-globule transition in the bulk volume of the material.

[0035] Preferably, the surface-functionalized metal nanoparticles are added into the supernatant solution sometime (l-6h) after the start of emulsion sedimentation. In case the thiol groups are created on the nanoparticle surfaces during the surface functionalization the nanoparticles bond to prepolymer via its thiol functionality, effectively participating the crosslinking reactions.

[0036] The present invention also provides a thermally responsive hydrogel having a thin top surface layer exhibiting reversible rapid switching of light scattering in the visible and near-infrared spectral range, said hydrogel being formed by a click-type reaction between a polyethylene glycol derivative having a crosslinkable moiety and a cross-linking agent which functionally includes a carboxylic acid group and an unsaturated C=C bond, wherein the polyethylene glycol derivative and the cross-linking agent are contacted under conditions providing emulsion sedimentation occurring in parallel to cross-linking in the deposited layer, at the interface of the emulsion droplets and at the interface of the sedimented hydrogel phase and its supernatant liquid, the solubility of the crosslinker being higher in the supernatant liquid phase than in the sedimented hydrogel such that the hydrogel has a differing concentration of crosslinking from its surface layer to its substrate layer. Preferably, the hydrogel comprises a cross-linked thiol-functional polyethylene glycol derivative. The thiol-functional polyethylene glycol derivative is preferably ethoxylated trimethylolpropane tri(3-mercaptopropionate).

[0037] In other similar prepolymers which 7 2 used, three of the branches of the molecule have on average ten ethylene glycol mor mits, typically ranging from five to twenty ethylene glycol units. In other prepolymers, the ethyl group at the centre of the molecule is not present and the ester linkage at the ends of the branches are replaced by ketone or amide linkages, making the hydrogel more stable against decomposition by hydrolysis.

[0038] The hydrogels of the invention, with their thermally responsive surface layers, exhibit outstanding temperature-induced switching from transparent to opaque state at temperature range of 15-35 °C. The switching temperature can be increased to 40-45 °C by decreasing the pH and by adding acetone into the aqueous operating solution of the hydrogel. The method of the invention makes it possible to deposit a relatively strong and more crosslinked substrate layer and thinner and less crosslinked thermally responsive surface layer. The resulting material exhibits switching in light scattering in the visible and nearinfrared spectral range with integrated luminous transmittance Tiumof over 95%, luminous transmittance modulation ATium of 77.5% (wavelength range of 380-780 nm) and solar energy modulation ATSOi(wavelength range of 300-2500 nm) of 60% and higher.

[0039] The responsive surface may be provided on, for instance, a ~6 mm thick gel sample. However, the same functionality can be provided on thinner gel layers or over large surface areas, more suitable for smart window devices.

[0040] Although many thermally responsive polymers are known, the unique thin layer on the surface of the hydrogel with excellent mechanical robustness, fast and outstandingly strong and fast response at the practically relevant temperature range makes this material particularly suitable for use with smart windows. No external energy supply is required and there is a passive response to the changes in the ambient temperature.

[0041] Hydrogels of the invention are also useful as thermo-optical switches; their optical transmittance being switched by changing the temperature with the parallel-bean transmittance change approaching nearly 100%. The optical switch application becomes possible due to switching time of 1 s or less and is usable in applications where such switching time is suitable. The optical switching effect can be obtained either as a response to changes in the ambient temperature or by using electrical heating by employing transparent Joule heaters.

[0042] Description of the Drawings

[0043] The accompanying drawings are as follows:

[0044] Figure 1 shows the chemical structures of the range of suitable crosslinkers;

[0045] Figure 2 shows the structure and synthesis of the crosslinker and ETTMP referred to in the Detailed Description of the Invention;

[0046] Figure 3 shows a hydrogel of the invention, 7 cm in diameter, in cold (approximately 15°C) and warm state (approximately 35°C) with a thermally responsive top layer of a thickness of about 260pm, the inset showing the photo of the cross-section of the hydrogel;

[0047] Figure 4 shows a) time-lapse of the emulsion sedimentation and hydrogel formation at the bottom of a vial, b) optical microscope image of emulsion droplets at the last phase of emulsion sedimentation, c) micro computed tomography image of the section of hydrogel surface in the warm state where the thermally responsive layer is indicated by a double arrow and d) profilometry measurement results of the hydrogel surface in cold and warm state; and

[0048] Figure 5 shows a) FTIR spectra of ETTMP, crosslinker and the hydrogel, both from the inert bottom layer and the thermally responsive top layer, b) parallel-beam transmittance depending on temperature at different wavelengths measured in pure water, c) parallelbeam transmittance spectra at different temperatures and d) transition temperature for different wavelengths as the hydrogel operates in pure water. Detailed Description of the Invention

[0049] An embodiment of the present invention will now be described, by way of example only.

[0050] 1) Experimental section la) Materials

[0051] Acetone (>99.5 %), n-butylamine (>99.5 %) and N,N-diisopropylethylamine (DIPEA, >99 %) were supplied by Sigma-Aldrich. Maleic anhydride (>98 %) and glycerol (>99.5 %) were supplied by Alfa Aesar. Ethoxylated trimethylolpropane tri(3-mercaptopropionate) (Thiocure ETTMP 1300) prepolymer was supplied by Bruno Bock Chemische Fabrik GmbH & Co. KG. All chemicals were used as received. lb) Preparation of hydrogels with thermally responsive surface layer

[0052] Hydrogel was synthesized in 3 steps. The first step involved the synthesis of a crosslinker for ETTMP. In this step, two hydroxyl groups of glycerol were esterified by reaction with maleic anhydride by using a tertiary amine as a catalyst. For that 4.1396 g of maleic anhydride was dissolved in 9.726 mL of acetone in a 20 mL glass vial, followed by the addition of 1.544 mL of glycerol and 0.973 - 2.919 mL of 1 M DIPEA in acetone (typically 1.946 mL). The mixture reacted under stirring in a closed vial at 55 °C for 24 h. The exposure to the ambient humidity was carefully avoided in this step. Glycerol does not dissolve in acetone and forms a coarse emulsion upon addition. As tertiary amine catalyst is added, homogeneous solution is obtained within minutes, indicating fast reaction. This is also accompanied by a colour change from colourless to light yellow. Thus, the reaction time of 24 h is excessive and ensures the reaction proceeds to completion. This step yields homogeneous solution of crosslinker 2- butenedioic acid (2Z)-2-hydroxy-l,3-propanediyl ester (BAHPE) in acetone while tertiary amine catalyst is also retained in the solution. The structure and synthesis of crosslinker (A) and the structure of ETTMP (B) are in Figure 2.

[0053] As a second step 16.6 mL of Thiocure ETTMP 1300 was dispersed in 140 mL of deionized water (13.44 wt.%) in a separate container, followed by addition of 0 - 240 pL (typically 120 pL, 7.83 mM solution) n-butylamine and mixing overnight on a magnetic stirrer at room temperature (22 °C). Thiocure ETTMP 1300 does not fully dissolve in water at a given concentration at room temperature and forms a slightly turbid emulsion.

[0054] As a final step, crosslinker solution from the step 1 was added into the prepolymer solution of the second step and mixed for 10 s on a magnetic stirrer, followed by filling into glass containers with a flat bottom at the liquid column height of ~4 cm. The solution was left undisturbed in a closed container for 6 days. The emulsion fully sediments in about 24 h and ~6 mm thick layer of hydrogel forms at the bottom of the container.

[0055] Desirably, the solution of prepolymer ETTMP should stay in a partially soluble state or close to or slightly above the solubility limit at room temperature (22 °C) at around 5-30 wt. %, preferably about 13.5 %. This is obtained by controlling the temperature and concentration of the solution and the addition of base (preferably primary amine) that at the later stage of adding the crosslinker solution also plays a different role, being a catalyst for crosslinking. A base (that has no capacity to catalyze the crosslinking reaction) like ammonium hydroxide, sodium hydroxide, potassium hydroxide or similar, as well as 3-amino-l-propanol can be added to adjust the solubility of the ETTMP prepolymer in the stage of preparing the prepolymer solution.

[0056] The tertiary amine catalyzes the reaction of maleic anhydride with glycerol and the primary amine is a catalyst for crosslinking (addition reaction between the thiol in the prepolymer and C=C bond in the crosslinker). In principle, only the tertiary amine may be used but not the other way around. A higher quantity of tertiary amine can be added in the crosslinker synthesis and then primary amine in the prepolymer solution is not needed. However, using different catalysts with optimal efficiency for different reactions may reduce the likelihood failure although in this particular example the primary amine can be replaced by tertiary amine with almost no problem.

[0057] The first function of the tertiary amine is to act as catalyst. The second function relates to the formation of the active layer. The total amount of amine affects the thickness of the thermally responsive layer and is not very important for controlling the solubilities.

[0058] The tertiary amine and primary amine are interchangeable for the crosslinking step, meaning that the concentration of tertiary amine, such as DIPEA, in the crosslinker solution could be increased and the concentration of primary amine in the pre-polymer solution could be reduced accordingly.

[0059] The resulting polymer has a top layer that has scattering properties. This can be explained by structural differences between the top and bottom layers, the top layer being slightly less crosslinked while, at the same time, containing higher concentration of crosslinker and an increased water content compared to the bottom layer. Higher crosslinker concentration and lower crosslinking degree at the same time simply means that ETTMP prepolymer branches are terminated with higher probability with the crosslinker molecules without further branching. The top layer may be from 0 to 300 micrometer (or more but no advantage if thicker). An ideal thickness is about 250 micrometers, the blocking of the light decreasing below this figure. lc) Characterization of samples

[0060] Measurement of thermo-optical performance. Two different measurement setups were used to characterize the optical properties of the hydrogels thoroughly. Parallel beam transmittance dependence on temperature was measured in the wavelength range of ~380- 950 nm in different aqueous solvents to deduce the hydrogel transition temperatures / LCST values. The measurements were carried out using a white light source Ocean Optics LS-1, spectrometer Ocean Optics HR2000 + ES and a custom-built optical setup, enabling the measurement of transmittance and optical haze according to the ASTM standard D1003. With this setup, the light that deviates less than 2.5 degrees from the direction of the incident beam after passing the sample was detected as transmitted light, yielding parallel beam transmittance. Stainless steel sample holder was custom built, designed to fit into a 40 mL glass cuvette (Hellma), allowing measurements to be carried out in the water. Baseline correction was made by using a sample cell filled with the same aqueous solution as in the case with measurement of the hydrogel. This excludes the influence of reflections from the glass surfaces from the measured transmittance values. Temperature was measured using a PtlOO resistance temperature probe, built into the sample holder. Temperature of the sample was controlled by externally heating and cooling the measurement chamber. The accuracy of all temperature measurements was 0.1 °C. Integrated luminous transmittance TiUm (380-780 nm), luminous transmittance modulation ATium, solar transmittance Tsoi(300-2500 nm) and solar transmittance modulation ATSOiwere measured and calculated in a standard manner, as described in the previous works.20-36-41For that, transmittance spectra of the hydrogels were measured above and below the LCST by using Agilent Cary 5000 UV-Vis-NIR spectrophotometer, equipped with an integrating sphere. Using an integrating sphere, both specular and diffuse transmittance was correctly included in the transmittance measurement. Baseline correction was made by using an unobstructed light path, i.e. light reflections from the measurement glass cuvette are not excluded from these measurements. The light path length in the cuvette was 10 mm, and the glass thickness was 2.5 mm on both sides.

[0061] Tium and Tsoiwere calculated by using equations 1 and 2: where T(A.) denotes the measured transmittance, (DiUm(A.) is the standard luminous efficiency function for photopic vision and 0SOi(A.) represents the solar irradiance spectrum for an air mass of 1.5. Luminous- and solar transmittance modulation were calculated by using equations 3 and 4: Tlum= Tlumat < LCST - Tlum(at > LCST) (3) Tsol= Tsol(at < LCST) - Tsol(at > LCST) (4)

[0062] Optical performance in different aqueous solvents. Thermo-optical performance of hydrogels was measured in 5 different aqueous environments: 1) pH 6 phosphate buffer, 2) pH 2.6 citric acid-Na2HPO4 buffer, containing 20 vol. % acetone, 3) pH 2.6 citric acid-Na2HPO4 buffer with 25 vol. % acetone added, 4) milli-Q water, containing 10.46 vol. % of crosslinker solution (obtained in Step 1 as described in section 2.2) and 18 mM butylamine, resulting in pH 1.84, 5) pH 1.8 with milli-Q water containing 2.66 vol. % of crosslinker solution and 6 vol. % acetone. These aqueous environments will be denoted as Solutions 1-5. Additionally, in solution no. 5, the hydrogel went through 50 heating and 50 cooling cycles between 35 °C and 4 °C to investigate the performance's stability.

[0063] Micro computed tomography analysis (pCT) was carried out on pCT50 scanner (SCANCO Medical AG, Switzerland). X-ray source scan parameters were as follows: 45 kVp, 88 pA, voxel size 2.5 pm. The sample was placed in a polypropylene container, filled with filtered tap water and sealed. The sample temperature was maintained at 35 °C during the scan period.

[0064] Fourier-transform infrared spectra of the hydrogels, crosslinker and prepolymer were obtained using Bruker Vertex 70 FT-IR spectrometer with an attenuated total reflection (ATR) accessory.

[0065] Profilometry measurements of the hydrogel surfaces were carried out with the sample immersed in water both in cold and warm state by using Bruker DektakXT with the stylus force of 1 mg and stylus radius of 12.5 pm.

[0066] Optical microscope images were obtained by using Olympos BX51 optical microscope. pH of the solutions was measured by using Mettler Toledo FiveEasy pH analyzer with precalibrated LE409 pH electrode.

[0067] 2) Results and Discussion

[0068] 2.1) General considerations for choosing prepolymer and crosslinker combination and catalysts for hydrogel preparation

[0069] In the first step of the synthesis, a crosslinker (compound A in Scheme 1) is synthesized for the prepolymer (ETTMP) that is functionally a dicarboxylic acid, a diester, and having unsaturated C=C bond functionality in two of the branches, enabling crosslinking through the unsaturated bonds. As maleic anhydride reacts with glycerol in a 2:1 molar ratio, on average, two of the hydroxyl groups in each glycerol molecule are esterified and one remains unreacted. The tertiary amine that catalyzes this reaction not only has an important role in the synthesis of the crosslinker, but its concentration has a strong influence on the formation and properties of the thermally responsive layer as well as the entire crosslinker synthesis solution is used as an input for the functional hydrogel preparation. Tertiary amine can also be expected to deprotonate some of the carboxylic acid that is functionally present in the crosslinker and form carboxylate salt. This does not seem to hinder its effect on the consequent preparation steps, though.

[0070] Prepolymer Thiocure ETTMP 1300 is a 3-branched thiol-functional polyethylene glycol (PEG) derivative with ~10 ethylene glycol monomer units in each branch, terminated by thiopropionate (compound B in Scheme 1). Ethylene glycol-derived polymers are known to commonly exhibit concentration-dependent LCST. Thus, it could be expected that ETTMP also exhibits LCST and scarce data in few studies remotely suggest this to be the case, as ETTMP has exhibited strong non-linear concentration- and temperature-dependent solubility. To the best of our knowledge, no studies focus on investigating the thermoresponsive properties of ETTMP either as a pure substance in a solution or in a crosslinked hydrogel form.

[0071] ETTMP can be crosslinked to form hydrogels by using its thiol functionality. For example, Pritchard et al. and Khan et al. prepared ETTMP-derived hydrogels for biomedical applications, crosslinked by polyethylene glycol diacrylate using Michael addition. Precipitation of the polymer from the solution in some of the experiments was also reported by Pritchard et al., but thermally responsive properties were not described. Wang et al. reported the synthesis of degradable PEG analogues with the LCST in the range of 10-50 °C, derived from dithiols and PEG diacrylates / di(meth)acrylates and thermally responsive hydrogels for biomedical application by using Michael addition for crosslinking.46These are somewhat remote chemical analogues to the hydrogels reported in the current study.

[0072] In the current application, ETTMP reacts with unsaturated C=C bonds of the crosslinker by base-catalyzed thiol-Michael addition. Primary amines are known to be more efficient catalysts for this reaction than tertiary amines, although both can be used for this purpose.The base abstracts a proton from thiol, resulting in the thiolate anion, a strong nucleophile and highly reactive towards the unsaturated C=C bond. In the current application, the tertiary amine from step 1 and primary amine from step 2 end up in the final solution from which the hydrogel forms. In terms of thiol-Michael addition, these amines similarly influence the hydrogel formation in the final mixed solution. As reaction time of hours and days is used in each stage, rather than minutes and seconds, the higher catalytic efficiency of primary amine is not clearly pronounced. 2.2) Operational range of precursor concentrations

[0073] Amine catalysts. Different amine catalysts are present in crosslinker and prepolymer solutions which are combined to form a solution from which the hydrogel is deposited by emulsion sedimentation. The total combined concentration of amines in the solution, where the hydrogel forms, is a critical process parameter. An optimal concentration of amines exists where hydrogel formation with a thermally responsive surface layer is possible. The optimal concentration of DIPEA and n-butylamine in the final mixed solution was found to be at around 11.2 and 7.05 mM, respectively. This yields mechanically strong hydrogel with ~260 pm thick thermally responsive surface layer with excellent performance (Figure. 1) and good resistance to mechanical manipulation. Material can be bent, and mechanical flexing and rubbing of the surface can be applied by fingers without any harm to the surface nor the thermo-optical response of the material (see Supplementary Video). Thus, the material can be bent to cover curved shapes.

[0074] The absence of primary amine in the prepolymer solution or a two-fold decrease of the concentration of tertiary amine in the crosslinker solution, both yields mechanically weaker and more flexible hydrogel. Sticky and viscous surface partially washes away on handling and slow thermally induced light scattering effect exists in most of the hydrogel volume. Decrease in the concentration of base catalysts from the optimal value gradually leads to the formation of viscous liquid layer at the bottom of the container instead of a hydrogel. On the other hand, two-fold increase in the concentration of n-butylamine or DIPEA from the optimal value yields mechanically stiffer hydrogel with a significantly weaker thermally induced light scattering effect and a thinner thermally responsive layer. The further increase in amine concentration eliminates the thermally responsive active layer completely. Thus, the method enables us to easily obtain thermally responsive surface layer thickness in the range of 0-300 pm simply by adjusting the amine concentration. A ~250 pm thick active layer results in an excellent thermo-optical performance, and there is little to no need to prepare thicker functional layers with this specific polymer system. Thermo-optical properties of the hydrogel are discussed in detail in section 3.4.3.

[0075] At the optimal concentration of the base catalysts, the pH of the final mixed solution is at around 2.8, with carboxylic acid functionality clearly dominating over the influence of base catalysts in terms of pH. This is expected based on the molar ratios of precursors. Also, DIPEA is a weak base similar to triethylamine with p / Caat around 11. The pH of the solution above the hydrogel changes very little during the emulsion sedimentation and hydrogel formation.

[0076] Replacing n-butylamine with an equimolar amount of dodecylamine or 3-amino-l-propanol yields hydrogels with similar properties despite very different hydrophilic-lipophilic balance of these compounds. This shows that the nature and the length of the alkyl chain in the primary amine is not a critical parameterforthe process, and only primary amine functionality is relevant. The use of dodecylamine leads to reduced quality in terms of somewhat inhomogeneous bottom layer, though. Shorter primary amines, as compared to dodecylamine, that are liquid at room temperature offer more convenient handling, as well as higher solubility in water. Thus n-butylamine is a preferred choice.

[0077] ETTMP prepolymer and crosslinker concentrations. ETTMP is not fully soluble in water at room temperature at the used concentration of 13.44 wt. %, forming a slightly turbid emulsion in preparation of the prepolymer solution by mixing on a magnetic stirrer. To achieve full solubility, the mixture can be cooled to nearly 0 °C or the pH raised by increasing the concentration of n-butylamine from 7.83 mM to ~0.1 M. Still, these conditions do not result in a thermally responsive hydrogel. The concentration of ETTMP can be varied around 13-14 wt. %, chosen in the partial solubility range at room temperature.

[0078] The molar ratio of prepolymer and crosslinker is roughly 1 to 0.87. Exact molar ratio can't be given as ETTMP exhibits a range of molecular weight values at around 1300 g / mol. Also, stoichiometric chemical reaction is not pursued here. By intention, the concentration of the crosslinker will be different in the different hydrogel layers, resulting from the complex formation mechanism. The requirements for the properties of the crosslinker are discussed in Section 3.3.

[0079] 2.3) Formation of the thermally responsive layer (see Figure 2)

[0080] The formation of hydrogel at the bottom of the solution and the formation of clearly defined thermally responsive surface layer relies on the number of different complex processes that occur concomitantly. This involves the control of solubilities of the precursors by temperature and pH, forced emulsion formation and gradual sedimentation of the emulsion droplets in the gravitational field, emulsion coalescence and crosslinking both in the sedimented layer and at the interface of the emulsion droplets, as well as on the interface of deposited hydrogel and its supernatant liquid.

[0081] As the crosslinker solution is added into ETTMP prepolymer aqueous solution under mixing, a cloud of fine white emulsion forms abruptly with the emulsion droplet diameter of ~5 pm (Figure. 2b). Significant part of the ETTMP phase separates out of the solution at this stage and fine emulsion is forced to form due the crosslinker being an antisolvent for ETTMP in the aqueous solution. The antisolvent effect specifically originates from the crosslinker and not from the acetone and tertiary amine that are also contained in the solution since ETTMP exhibits full solubility in acetone. Adding the crosslinker solution into the prepolymer solution is also accompanied by the reduction of pH down to 2.8, but this contributes to the phase separation only to a marginal extent.

[0082] Functional hydrogel is deposited at the bottom of the solution from slow sedimentation of the fine emulsion. The sedimentation stage starts right after the mixing of the emulsion is stopped. Sedimentation from emulsion is a critical factor, occurring parallel with the crosslinking reactions. Approximately the last 3 % of the sedimented emulsion forms the active surface layer, with sedimentation after approximately 12-24 hours, during which the crosslinking reactions have already occurred. In addition to the final stage of the emulsion sedimentation, the addition of prepolymer and crosslinker molecules directly from the supernatant solution phase onto the functional surface layer can be expected to contribute to the growth of the functional layer as these molecules are present in the supernatant solution. Crosslinking of the hydrogel is, in the most part, completed in 2-3 days, forming both the substrate, i.e. transparent, non-functional hydrogel part at the bottom, and thermally responsive top layer in the same process at different consecutive stages of emulsion sedimentation.

[0083] The photographs showing the time-lapse of the emulsion sedimentation and the formation of the hydrogel layer at the bottom of the vial are presented in Figure. 2a. Importantly for the process, the crosslinker exhibits some solubility in the sedimented layer, sufficient for crosslinking while its solubility in the supernatant aqueous phase is higher than in the sedimented hydrogel phase. This leads to different concentrations of crosslinker at the bottom and interfacial layers of the hydrogel, which is the underlying mechanism behind forming a thin active, thermally responsive surface layer. This interpretation is confirmed by the FTIR measurements of the top and bottom layers of the hydrogel (see below). This solubility profile results from the specific chemical structure of the compound.

[0084] When glycerol is replaced with 1,6-hexanediol or 1,5-pentanediol in the crosslinker synthesis, resulting in the absence of the residual OH-group in the crosslinker, an opposite distribution of the crosslinker results - a hydrogel is formed with thermally responsive effect in bulk volume in bottom layer and almost no thermally responsive effect in the top layer. In that case, the bottom layer is also mechanically softer, while the top layer is stiffer. This is a definitive proof of the underlying mechanism. This opens up a route for synthesizing a wide variety of functional hydrogels by using a variety of crosslinkers based on the plethora of different diols and polyols with different pendant alkyl and other functional groups, as well as chemical modifications to the ETTMP prepolymer.

[0085] Emulsion sedimentation is also achieved when water is replaced with ethylene glycol as the reaction medium, but in that case, a liquid layer is obtained without functional properties. Water specifically is necessary as a protic solvent for crosslinking by thiol-Michael addition to occur in the investigated system. Using other solvents instead of water where all of the precursors are fully soluble (e.g. dimethyl adipate), neither hydrogel nor thermally responsive material is obtained.

[0086] 2.4) Properties of the hydrogels

[0087] 2.4.1) Morphology

[0088] Morphology. The observed thermally induced optical effect results from coil-to-globule transition of the molecular chains near the surface. The hydrogel is optically homogeneous and transparent in the cold state, and no light scattering is detectable. An intense Mie-type light scattering and bright white appearance is induced in the warm state, which results from the formation of globular scattering centres and refractive index contrast. Tomography measurements (Figure. 2c) clearly show approximately 260 pm thick layer on the top surface of the gel, consisting of globules in size range of 15-20 pm at temperature above the transition temperature. Some porosity is also visible below the active layer, but the switchable optical response results almost exclusively from the thin top layer. This is confirmed by visually observing the cut cross-sections of the hydrogels (Figure. 1, inset). The porosity in the bottom layer can be practically eliminated by reducing the mixing time after the emulsion formation and allowing the sedimentation to start quickly.

[0089] In addition to tomography measurements, profilometry measurements of the hydrogel surface in the opaque and transparent state were carried out. Interestingly, no statistical difference in surface roughness can be measured in the transparent and opaque state (Figure. 2d). Within the margin of error, the RMS surface roughness is the same in the transparent and opaque state (1.049 pm and 0.995 pm respectively). No meaningful difference in surface features is detectable with the profilometer, showing a relatively smooth surface in both states. Water-immersed AFM studies were unsuccessful due to insufficient density contrast between the water and hydrogel top surface to obtain images of single globules. It is also nearly impossible to obtain useful optical microscope images in the opaque state due to intense optical scattering, posing a very difficult challenge for characterization. This leaves X- ray computed tomography as the main usable tool.

[0090] Using a relatively thin functional surface layer on the hydrogel largely eliminates the problems related to slow heat transfer into the bulk volume and consequent slow switching. Additionally, the lack of sufficient mechanical degrees of freedom for efficient coil-to-globule transition in crosslinked bulk hydrogel volume is less pronounced in the surface layer, enabling surface functionality between the bulk hydrogel and liquid solution while retaining mechanical robustness.

[0091] 2.4.2) FTIR results

[0092] The analysis of FTIR measurements (see Figure 3) enables it to be concluded that the ratio of ETTMP and crosslinker differs in the top and bottom layer. The thermally responsive surface layer of the hydrogel contains more crosslinker and at the same time is less crosslinked. As the crosslinker is difunctional in terms of C=C double bonds, statistically higher percent of the branches in ETTMP molecules in the top layer are terminated by crosslinker molecule that has been added to thiol through C=C bond by click reaction without further crosslinking through the second C=C bond. This also explains the higher water content in the top layer as the crosslinker is more hydrophilic than ETTMP. Higher water content is also a factor that contributes to the possibility of coil-to-globule transition. The direct measurement of water concentration shows that the water content in the bottom and top layer is 43 and 56 wt. %, respectively.

[0093] The equilibrium water content is typically about 13% higher in the top layer but can be in the range of 5-20%. It is nearly impossible to obtain the similarly precise number for the difference in the crosslinker concentration. From the FTIR data, an approximate difference in the number of C=C bonds can be estimated. The top layer is about 3.75% less crosslinked and this is sufficient to create a large difference between the bottom and top layers. The difference in crosslinking is likely to be in the range 1-10%. Beyond that the top layer is so loosely bonded that it might wash away during handling.

[0094] 2.4.3) Thermo-optical performance

[0095] When the hydrogel is inside the aqueous solution with pH 2.8 that remains above the hydrogel from the synthesis, no scattering is visible below ~25-26 °C. The supernatant liquid contains some unreacted prepolymer, though and becomes slightly turbid when the temperature is raised, exhibiting cloud point at 26 °C - a good correlation with the induced scattering in the thermally responsive layer. Thus, the supernatant aqueous solution should be replaced for correct measurements and, ultimately, for practical operation. At no point is the hydrogel dried, just transferred into a different aqueous solution or, equivalently, the supernatant solution is replaced. Using the distilled water reduces LCST by about 5 °C, and some scattering becomes visible at room temperature. Thus, it is evident that the LCST of the hydrogel depends on pH.

Claims

CLAIMS1. A method of making a thermally responsive hydrogel having a thermally responsive surface layer that is thin in relation to a substrate, the method comprising contacting a polyethylene glycol derivative having a crosslinkable moiety to undergo a Thiol-Michael addition or amineacrylate Michael addition reaction with a cross-linking agent which functionally includes a carboxylic acid group and an unsaturated C=C bond, wherein the polyethylene glycol derivative and the cross-linking agent are contacted under conditions providing emulsion sedimentation occurring in parallel to cross-linking in the deposited layer, at the interface of the emulsion droplets and at the propagating interface of the sedimented hydrogel phase and its supernatant liquid, the solubility of the cross-linking agent being higher in the supernatant liquid phase than in the sedimented hydrogel such that there is differing concentration of the cross-linking agent in the hydrogel.

2. A method according to Claim 1, wherein the cross-linking agent is a dicarboxylic acid and a diester having unsaturated C=C bond functionality in each of its two ester branches.

3. A method according to Claim 1, wherein the cross-linking agent is a dicarboxylic acid and a diketone having unsaturated C=C bond functionality in each of its two ketone branches.

4. A method according to Claim 1, wherein the cross-linking agent is a dicarboxylic acid and a diamide having unsaturated C=C bond functionality in each of its two amide branches.

5. A method according to Claim 2, wherein the crosslinking agent is 2-butenedioic acid (2Z)- 2-hydroxy-l,3-propanediyl ester.

6. A method according to Claim 5, wherein the crosslinking agent is made by esterifying two hydroxyl groups of glycerol with maleic anhydride in the presence of a tertiary amine catalyst.

7. A method according to Claim 3, wherein the crosslinking agent is 4,ll-dioxa-7-hydroxy- Z2,Z12-tridecadienedioic acid.

8. A method according to Claim 4, wherein the crosslinking agent is (2Z)-3-({3-[(2Z)-3- carboxyprop-2-enamido]-2-hydroxypropyl}carbamoyl)prop-2-enoic acid.

9. A method according to Claim 4, wherein the crosslinking agent is (2Z)-3-({3-[(2Z)-3-carboxy- N-methylprop-2-enamido]-2-hydroxypropyl}(methyl)carbamoyl)prop-2-enoic acid.

10. A method according to Claim 4, wherein the crosslinking agent is (2Z)-3-({3-[(2Z)-3- carboxy-N-ethylprop-2-enamido]-2-hydroxypropyl}(ethyl)carbamoyl)prop-2-enoic acid.

11. A method according to Claim 4, wherein the crosslinking agent is (2Z)-3-({3-[(2Z)-3- carboxy-N-propylprop-2-enamido]-2-hydroxypropyl}(propyl)carbamoyl)prop-2-enoic acid.

12. A method according to any of the preceding claims, wherein the polyethylene glycol derivative is ethoxylated trimethylolpropane tri(3-mercaptopropionate).

13. A method according to any of the preceding claims, wherein the polyethylene glycol derivative is ethoxylated trimethylolpropane tri(3-aminopropionate).

14. A method according to any of the preceding claims, wherein the polyethylene glycol derivative is contacted with the cross-linking agent in the presence of an amine.

15. A method according to Claim 14, wherein the amine is n-butylamine.

16. A thermally responsive hydrogel having a thin top surface layer relative to a substrate layer, the top surface layer exhibiting reversible rapid switching of light scattering in the visible and near-infrared spectral range, and wherein the hydrogel has a differing concentration of cross-linker between its surface and the substrate layer.

17. A hydrogel according to claim 16, wherein said hydrogel is formed by a Thiol-Michael addition or amine-acrylate Michael addition reaction between a polyethylene glycol derivative having a cross-linkable moiety and a cross-linking agent which functionally includes a carboxylic acid group and an unsaturated C=C bond, wherein the polyethylene glycol derivative and the cross-linking agent are contacted under conditions providing emulsion sedimentation occurring in parallel to cross-linking in the deposited layer, at the interface of the emulsion droplets and at the interface of the sedimented hydrogel phase and its supernatant liquid, the solubility of the cross-linking agent being higher in the supernatant liquid phase than in the sedimented hydrogel such that there is a differing concentration of cross-linking agent in the hydrogel.

18. A hydrogel according to Claim 17, wherein the polyethylene glycol derivative is a crosslinked thiol-functional polyethylene glycol derivative.

19. A hydrogel according to Claim 18, wherein the polyethylene glycol derivative is ethoxylated trimethylolpropane tri(3-mercaptopropionate).

20. A hydrogel according to Claim 18, wherein the polyethylene glycol derivative is ethoxylated trimethylolpropane tri(3-aminopropionate).

21. A hydrogel according to any of Claims 17 to 20, wherein the top surface layer includes plasmonic metal nanoparticles for reversible change in color.

22. A smart window comprising a hydrogel as claimed in any of Claims 17 to 21 or as made by a method as claimed in any of Claims 1 to 15.

23. A thermo-optical switch comprising a hydrogel as claimed in any of Claims 17 to 19 or as made by a method as claimed in any of Claims 1 to 15.

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