How does ethanol dissolve acrylic polymer?

Suresh Ahuja

H2O- and ethanol concentration-responsive polymer/gel inverse opal photonic crystal

Author links open overlay panel

, , , , ,

https://doi.org/10.1016/j.jcis.2021.07.112Get rights and content

Under a Creative Commons license

open access

Abstract

Responsive photonic crystals have attracted much attention due to their strong capability to manipulate the propagation of light in the visible region, but it is still a big challenge to invisibility and mechanical stability. Here, the novel Poly(ether sulfone)/Poly(acrylic acid) inverse opal photonic crystals, which have high mechanical stability and can release visible patterns after wetting with water, are discussed. The Poly(ether sulfone)/Poly(acrylic acid) inverse opal photonic crystals are also responsive to the concentration of ethanol, and the structural color response times increase with increasing ethanol concentration. This design uses the selective infiltration, hydrogen bonding and capillary action of solvent to realize the spectral diversity of reflectance. Owing to the high polarity and hydrogen bonding ability of carboxyl groups, water molecules are adsorbed easily by the poly(acrylic acid) gel. Subsequently, the encrypted information is decrypted due to the redshift of the structural color. Because of its lower polarity and hydrogen bonding ability relative to water, ethanol can impede the absorption of solvent by gel. Therefore, the ethanol concentration can be identified based on the structural color response time. Furthermore, reliable information decryption methods make Poly(ether sulfone)/Poly(acrylic acid) inverse opal photonic crystals potentially uesful as trusted encryption devices.

Graphical abstract

📷Download : Download high-res image (152KB) Download : Download full-size image

Keywords

Photonic crystal

Structural color

Capillary action

Hydrogen bonding

1. Introduction Photonic crystals (PCs) have attracted much attention because of their photonic bandgap (PBG) properties owing to the periodic variation of the lattice constant in regular arrays [1], [2], [3], [4], [5]. When the light energy is in the bandgap, it cannot propagate through such structures. Thus, corresponding structural colors are displayed when the bandgap is located within the visible region due to the reflection of light by PCs [6], [7], [8], [9]. In contrast to chemical dyes, structural colors have nonphotobleaching characteristics and stimulus-responsive properties because they are generated by the diffraction or reflection of light from physical structures [10].Recently, responsive PC-based encryption has attracted significant interest in the chemical sensing and anti-counterfeiting fields because the structural colors can be adjusted by external stimuli such as solvent, temperature, pressure, strain, and magnetic field [10], [11], [12], [13], [14], [15]. In practical application, the responsive PCs’ mechanism is that the patterns remain invisible under normal conditions, but they can be visualized by the alteration of periodic refractive index contrast which originates from the additional stimuli. For an ideal PC information encryption structure, the color of the pattern should be consistent with the background under normal conditions and display different visible structural colors under the influence of external stimuli [16]. Moreover, both the hiding and displaying of structural color must be realized easily. Additionally, for convenient application, the invisible pattern should be incorporated into a free-standing PC structure. Furthermore, the PC material should possess excellent mechanical stability, as this is beneficial for the durability of the structural colors [10].To date, a variety of responsive PCs with invisible patterns have been developed. For example, Zhong’s group used oxygen plasma etching to prepare a responsive hollow silica PC on a substrate, which resulted in a pattern with zero contrast compared with the background [16]. Importantly, the invisible pattern was only revealed by a dynamic humid flow rather than under static humidity, even up to 97.5%. In addition, Zhang’s group designed an ethanol-responsive free-standing bilayer polymer PC film without substrate by filling oligomer mixtures into a SiO2 template, and then subjecting them to photopolymerization and etching processes [17], [18]. This strategy introduced more response modes, while the preparation of bilayer PC structures required templated materials and more steps. In addition, the applications of responsive hydrogel PCs have also been studied extensively [12], [19], [20], [21]. However, hydrogel materials must be kept in a humid environment to maintain their activity, and poor mechanical stability also limits their application. Thus, more research is still needed still required to develop ideal responsive PC films.Herein, a novel responsive PC with an invisible pattern is fabricated by incorporating hydrophilic gels into an polyethersulfone (PES) inverse opal PC structure with superior chemical and physical stability. Furthermore, when compared with other polymeric materials, such as PVDF (n = 1.42) and PMMA (n = 1.50), PES has a higher refractive index (n = 1.68) which contributes to the formation of PCs with bright structural colors [10], [22], [23], [24]. Polyacrylic acid (PAA), which was chosen as the pore-filling gel to modify the PES film is prepared by means of cross-linking free radical polymerization, as cross-linking has the advantage of anchoring materials. In the presence of the PAA, the invisible pattern can be revealed easily by wetting due to the infiltration of water molecules in the PES/PAA region. However, in the absence of PAA, the wetting process is impeded by reentrant curvature in inverse opal PC, meaning that the PES region keeps the original structural color when it is wetted by water. Furthermore, the structural color response time of PES/PAA region can also be tuned by varying the ethanol concentration in aqueous solution, owing to its low polarity and the hydrogen bonding ability between carboxyl groups in PAA and ethanol molecules. A novel strategy used to achieve the invisible pattern is the construction of PC patterns with hydrogel in a polymer inverse opal PC. In the following discussion, the synthesis, structures, and mechanism of the solvent response of PES/PAA film will be investigated in detail. The results indicate that our responsive PCs is a potential candidate for anti-counterfeiting and sensing of the ethanol concentration.

2. Materials and methods 2.1. Materials and Chemicals. Absolute ethanol, ethyl orthosilicate, HF acid (40 wt%), and ammonia solution (25 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. 1-Methyl-2-pyrrolidinone (NMP), N-propyl alcohol, N,N-methylenebis acrylamide (MBAA) and AA were purchased from Fuyu Chemical Reagent Factory. Darocur 1173 was purchased from Aladdin. PES was obtained from BASF Corporation. 2.2. Preparation of monodispersed SiO2 spheres and SiO2 photonic crystals SiO2 spheres with different sizes were prepared by a modified Stöber method [10]. The diameters of SiO2 spheres were controlled by tuning the amount of aqueous ammonia. The as-prepared samples were washed and centrifuged three times with ethanol. Finally, the samples were dried at 80 °C. Then, SiO2 suspension (8 wt%) was prepared for further use by dispersing spheres in absolute ethanol via sonicating for 8 h. Subsequently, SiO2 photonic crystal templates were fabricated through the dip-coating method. 2.3. Preparation of PES film Firstly, PES (10 wt%, in NMP) and carbon black (CB, 0.1 wt%, in NMP) dispersion was sonicated for 12 h to obtain uniform solution. Then, PES/CB/NMP solution (0.1 mL/cm−1) was dropped on the surface of SiO2 template and placed in the oven horizontally. After heating 3 h at 80 °C, the NMP was evaporated. Then, the composite film was placed in the diluted HF acid (5 wt%) for 2 h to remove the SiO2 template, and the PES film with inverse opal was prepared. 2.4. Preparation of PES/PAA film Firstly, a mixture of AA (0.5 g), MBAA (0.05 g), and Darocur 1173 (0.02 g) was fabricated in 4 mL solution consisting of water and n-propyl alcohol (Vw: Vn = 1: 1). Then, 0.02 mL mixed solution mentioned above was dropped on the surface of PES film and then a clean glass slide was adhered to the film. To obtain a invisible pattern, an opaque mask was further placed on the glass slide. Thus, the photopolymerization of AA will occur under the radiation of UV light (9 mW/cm−2, 50 s). After washing with water and drying 40 °C, the PES/PAA film was obtained. 2.5. Characterization The morphologies of the samples were determined by using a scanning electron microscope (Nova Nano SEM 450). FT-IR spectra of the samples were characterized using the FT-IR method (NICOLET 6700, Thermo SCIENTIFIC). The static reflectance spectra of the colored films were obtained by using UV–vis spectrophotometer (Hitachi U-4100) and a Fiber Optic Spectrometer (EQ 2000). The photographs and video of different samples were obtained by a smartphone of Huawei P30. The tensile strength tests were performed with MTS (SANS) CMT 8502 Electromechanical Universal Testing Machine from USA. The bending test was performed by a homemade equipment. The contact angles of water and ethanol were obtained via JC2000D1 contact-angle system (Powereach, China).

3. Results and discussion 3.1. Preparation of PES film with PES/PAA pattern The fabrication process used to introduce invisible patterns into PCs is shown in Scheme 1. First, monodisperse SiO2 spheres with different diameters (225, 256, 286, 300 and 372 nm) were fabricated to prepare 3D ordered opal templates by the dip-coating method (i, ii, see Figure S1). The results of the particle size distributions are shown in Figure S2 in the Supporting Information. Based on the Bragg diffraction, the theoretical particle sizes of the SiO2 spheres were close to the actual values, in terms of the PBG peaks and structural colors (see Figure S3, Table S1). To infiltrate PES into the template, we dissolved the PES polymer in NMP solvent, and added a small amount of carbon black to increase the saturation of the structural color. After the PES polymer solution was dropped onto the surface of the SiO2 PC, the capillary forces allowed for the solution to permeate into the interstices between SiO2 spheres. During the process of heating at 80 °C, a PES/SiO2 composite film was formed gradually with the evaporation of the NMP solvent (iii). The refractive spectra of the PES/SiO2 PCs (Figure S4) indicated that the reflection peaks show redshift after the introduction of PES polymer because of the higher refractive index (n = 1.68) than air (n = 1.0). After removing the templates with HF solution (5%), a 3D PES inverse opal film with a bright structural color was obtained (iv). Next, to introduce an invisible pattern in the PES film, we employed a couple of monomers, including AA, in the presence of MBAA to work as the cross-linker, and employed Darocur 1173 as the photoinitiator. When a certain amount of precursor solution had filled the gap in the sandwich structure formed by the PES film and the mask, it quickly permeated into the holes. After photopolymerization induced by UV irradiation, a hydrophilic pattern was formed in the PES film (v). Under normal environmental conditions, the structural color of the PES/PAA composite film will be the same as that of the PES film (vi). 📷Download : Download high-res image (271KB) Download : Download full-size image Scheme 1. Schematic illustration of the routine to fabricate PES 3D inverse opal PC containing the invisible pattern formed in PES/PAA region: i) SiO2 3D PCs fabricated by dip-coating process; ii) SiO2 3D PCs; iii) PES/SiO2 fabricated by the driven forces including capillary force; iv) PES film was prepared after HF acid corrosion and washed with water; v) the incorporation of PAA under the irradiation of UV light; vi) PES film with invisible pattern formed by PES/PAA. 3.2. Characterization of PES and PES/PAA film Using the template method mentioned above, different colored PES films were fabricated (see Fig. 1a-e). The top surface (see Fig. 1f, g and Figure S5a-d) and cross-section (see Fig. 1h, i and Figure S5e-h) scanning electron microscopy (SEM) images demonstrate the presence of highly ordered apertures with face-centered cubic (red hexagons) closely packed structures. Fig. 1j shows the reflectance peaks of different colored PES films (452, 516, 560, 597, and 714 nm); the maximum reflectivity were larger than ∼ 40% owing to the high refractive index of the PES material. To express the structural colors in a more standardized way, the Commission Internationale de L’Eclairage (CIE) chromaticity values of the different samples shown in Fig. 1j are depicted in Fig. 1k. The resulting CIE chromaticity diagram demonstrates the preparation of a full color range of PES films. Fig. 1l further shows that the positions of the reflectance peaks blue shift gradually with increasing viewing angle, which is in accord with the properties of PCs [25]. 📷Download : Download high-res image (870KB) Download : Download full-size image Fig. 1. a–e) Digital photos of five colored PES films with blue, cyan, green, yellow, and red; f) typical SEM image of the top view of PES film; g) FCC arrays of pores on the surface of PES film; h) typical SEM image of the cross-sectional of PES film; i) the magnification SEM image of PES film; j) reflectance spectra of five colored PES films with different pore sizes; k) corresponding positions of (j) in the CIE chromaticity diagram; l) reflectance spectra of PES film under the detection angle ranging from 5 to 60°.The value of

is the average refractive index, which is calculated with the formula , where fpes and fo represent the volume rates of the framework and the material filling the pores [26]. For an opal structure, the volume rates of spheres and air are 74 and 26%, respectively. However, for an inverse opal, the volume rate of the framework often exceeds 26% [27]. To obtain the

for the inverse opal PES film, a simplified diagram was used to calculate the volume rate of air, according to the SEM image of inverse opals in Figure S6. Furthermore, the calculated volume rates for each section are shown in Table S2. Moreover, the theoretical values of PBG positions are close to the actual values, which also demonstrates the integrity of the 3D porous structure of PES films.

PAA chains can be covalently bonded to the PES film by the photopolymerization of AA molecules by reacting with the vinyl double bond each other in the presence of free radicals generated by UV irradiation [28]. Given the prominent IR bands for 📷COOH groups in the AA monomer, we used FT-IR spectroscopy to assess the changes in chemical composition after PAA grafting. As demonstrated in Fig. 2a, the peaks at 1575 cm−1 corresponding to the vibration of the aromatic rings in PES are observed for all samples [29]. After the formation of PAA, the typical peak at 2950 cm−1 ascribed to stretching vibrations for methylene (CH2) of the long alkyl chain of PAA appears [30]. In addition, a broad strong band at 3100–3500 cm−1, corresponding to the stretching vibration of O📷H in a 📷COOH group, is also observed [31], demonstrating successful incorporation of PAA into the PES inverse opal structure.

📷Download : Download high-res image (467KB) Download : Download full-size image Fig. 2. a) FT-IR spectra of PES and PES/PAA films; b) TG and DTG of the PES and PES/PAA films; c) the tensile curves of the pure PES film and PES/PAA films before and after immersing in NaOH (0.1 M) or HCl (0.1 M) solution 12 h; d) reflectance spectra of PES/PAA film before and after 24 h in an environment with 97.5% RH.

The position of PBG is closely related to the average refractive index of PC structure [32]. The PES inverse opal structure is composed of PES material and air, and their refractive indexes are 1.68 and 1, respectively. After the introduction of PAA whose refractive index is near 1.5 [33], the value of

should be changed. However, as shown in Figure S7, there is almost no change in the PBG peaks of 3D inverse opals before and after the introduction of PAA (from 452 nm to 455 nm). This can be explained by the low PAA content which is not sufficient to influence the value of

of the PES film. Hence, thermogravimetry (TG) was also performed to study the content of the PAA in the PES/PAA film. As shown in Fig. 2b, PES and PES/PAA films show similar losses at 35–300 °C, indicating the loss of moisture and/or the residual NMP solvent (its boiling point is 203 °C) within the films [34]. Furthermore, both TG curves exhibit appreciable weight losses occurring between 420 °C and 570 °C due to the degradation of PES, indicating that PES exhibits very high thermostability [35]. It should be noted that the weight loss of PES and PES/PAA films are very close between 35 and 800 °C which further suggests that only a small amount of PAA is anchored on the PES material (the difference in weight loss was less than 1 wt%). Further, the theoretical calculation and FDTD simulation also demonstrate the theoretical reflection peak positions of PES/PAA films are close to the actual results (Table S3, Figure S12).The tensile tests were designed to investigate the mechanical stability of PES/PAA films. As shown in Fig. 2c, when the tension is increased to 30 MPa, the tensile deformation of the PES/PAA film is only 0.032, a value very close to that of the PES film (∼0.03). Thus, it can be concluded that the introduction of PAA gels did not influence the mechanical stability of the PES material. The breaking stress of the PES/PAA film (∼50 MPa) is also similar to that of the PES film (∼51 MPa). Moreover, the breaking stress of the PES/PAA films did not change significantly even after immersion in HCl (0.1 M) or NaOH solution (0.1 M) for 24 h. Additionally, the reflection peak positions of the PES/PAA films before and after wetting by water remain unchanged (see Figure S8a, b), indicating that the PAA molecules are firmly anchored on the PES materials. Compressive tests of the PES/PAA films were also conducted, and the results are shown in Figure S9. When the pressure was increased to 16.2 MPa, the reflectivity of PES/PAA also maintained ∼ 30% of its initial value. Moreover, after 100 bending tests, the PES/PAA film also maintain the original nanostructure and brilliant color (see Figure S10). Thus, the above results prove that the PES/PAA film has good mechanical stability.For ideal encryption of information based on structural color, the encrypted information region should be invisible which means that the contrast between the encrypted information and the surrounding background should be as small as possible under normal conditions. Due to the poor hydrophilicity of PES, the incorporation of PAA which has excellent hydrophilic properties, not only changed the surface chemistry composition but also improved the hydrophilicity of the PES film. The existence of carboxylic acid groups of PAA and capillary action contributed to enhancing the water wetting behavior of the PES/PAA film by improving its water adsorption capacity [36], [37]. Thus, it is crucial to investigate whether hidden information can still be encrypted in the presence of different environmental humidities. Fig. 2d indicates that the encrypted information still remains invisible with the small PBG shift even under high relative static humidity (97.5% RH), which indicates that only a small quantity of water is introduced in the voids of the PES/PAA film. As previously reported, vapor condensation in porous PCs is mainly attributed to capillary condensation, and then the liquid forms a meniscus under static humidity conditions [16]. However, the large pore size often leads to condensed water remaining isolated without forming capillary bridges [38]. Hence, condensed water cannot fill the voids of the PES/PAA film completely. Thus, invisible information on the PES/PAA film was never revealed, even at 97.5% RH.

3.3. Water response for PES/PAA film Under normal conditions, the encrypted information was hidden due to the extremely low color contrast between the PES/PAA region and the surrounding background (see Fig. 3a and Figure S11a, b). Furthermore, no difference in the microstructure could be observed between the microstructure in the patterned and non-patterned regions, even with the assistance of an optical microscope (see Fig. 3b). After immersion in water, the PES/PAA regions changed color remarkably, and the hidden apple pattern (see Fig. 3a) and “DMU” letters (see Figure S11a) were immediately revealed in green, orange, and red colors (see Fig. 3c and Figure S11 c, d) from the corresponding photo images. However, the PES background still retained the original colors (see Fig. 3c). Thus, a clear boundary between the patterned and non-patterned regions could be observed in the microscopic image (see Fig. 3d). Furthermore, after the evaporation of water, those patterns reverted to their original invisible state quickly (see Movie S1). Thus, the invisible patterns were decoded depending on the redshift of the structural colors and this wetting response mode was realized by the selective infiltration of the liquid medium. This variation in structural colors suggests that the 3D periodic structure was changed when the film was wetted by water. Owing to the similar positions of the reflection peaks, we assume that the volume fractions of air (

) and PES/PAA () in a dry state are the same as those of the PES film (see Figure S7). Thus, the average refractive index () of the PES/PAA region can be described as 2 = 2 + (1- (/( + ) + /( + ))2. Hence, the maximum reflected wavelength () can be obtained by the Bragg diffraction formula:  = 1.633D(2 − sin2θ)1/2, where D is the pore size, (≈1.33) is the refractive index of the water, (1.00) is the refractive index of air, represents the volume of air and represents the volume of water. Even if part of the air in the pores is replaced by water, the higher would lead to an increase of . As shown in the reflectance spectra (see Fig. 3e, Figure S11d) and Table S4, we found that the theoretical maximum reflected wavelengths of PES/PAA films after wetting by water were close to the actual values when the pores were filled with water (

= 0). This supports the notion that the micropores are completely filled with water. Moreover, the reflection peaks still maintained their original optical properties after cycles of water response and air-drying (see Fig. 3f). 📷Download : Download high-res image (784KB) Download : Download full-size image Fig. 3. a) PES film with PES/PAA invisible pattern; b) PES/PAA pattern in PES film; c) reflectance spectra of the no-wetting region and wetting region during the water response; d, e) microscope images of the boundary between pattern and no-pattern regions, proving the high invisibility in the dry state and the sharp contrast after immersing in water; f) reflection cycles of the PES/PAA pattern (apple area) of the spectra before and after water wetting.In an inverse opal PC, the infiltration ability of different solvents can be judged by the relationship of the intrinsic contact angle (θc) and the reentrant curvature exhibited by interpore necks [39]. This reentrant curvature can be exhibited by neck angle, the tangent angle that the necks form with the pore wall. The total free-energy change, which is associated with the decent of the liquid front from the top of the pore down to an azimuthal angle, is given by: ΔG = γlaπ(D/2)2([(sin2 φ) - (sin2 φ0)] − 2(cos θc)(cos φ0 - cos φ)), where φ is the central angle of the position of the solid–liquid–gas junction point at any moment and γla represents the liquid–air surface tension and θc is the intrinsic contact angle. Based on previous literatures, the non-wetting state occurs when (dΔG)/(dφ)| φ =φ0 greater than 0 and θc > φ0. Therefore, the non-wetting state transforms into the wetting state when θc = φ0. As shown in Fig. 4a, for the blue PES film, the intrinsic contact angle of the PES film is 69°. The reentrant curvature, which can be expressed by the tangent angle (φ0), was also measured and shown to be 19° (Fig. 4b, φ0 = arcsin(D/D0), where D and D0 represent the diameters of the necks and the pores respectively) [40]. Thus, in the PES region, the contact angle of water on the film is θc > φ0, indicating a nonwetting state [39], [40]. In other words, when water is dropped on the PES film, it stays on the tops of the pores and stops infiltrating due to the increase in liquid–air surface tension at the minute area below the pore (see Fig. 4e). Thus, the microscopic images of the non-patternrd region, before and after water wetting, did not change (see Fig. 3d, e). As a result, the structural colors of PES films still maintained their original states (see the non-patterned areas in Fig. 3b and Figure S11a, c). 📷Download : Download high-res image (743KB) Download : Download full-size image Fig. 4. Contact angles of water on the PES region (a) and (c) PES/PAA region; SEM images of the pores and necks of PES (b) and PES/PAA (d) films; schematic illustrating the relationship between the neck angle (φ0) and the contact angle (θc) of water under e) no-wetting state and h) wetting state; f, g) cross-section SEM images of the PES/PAA inverse opal; i) schematic diagram of possible (i) electrostatic repulsion and (ii) hydrogen bond forms during the water wetting process in PES/PAA region; SEM images of PAA gel obtained via j) heating at 60 °C and k) freeze drying; l) display of the invisible information realized by the water response.After the introduction of PAA, the contact angle decreased to 52° (Fig. 4c), indicating a small improvement in the hydrophilicity of the PES film. The cross-section SEM images of the PES/PAA film demonstrated that although the pore sizes (D) had not changed compared with those of the PES film, almost all the necks (D0) between the two pores were blocked due to the cross-linking of PAA (see Fig. 4f, g). The sizes of the remaining unblocked necks were also smaller than those in the PES film. For example, in the PES/PAA region, the sizes of some necks were reduced to ≈ 40 nm (see Fig. 4d), and the corresponding tangent angle was calculated to be ≈ 11°. Hence, in the PES/PAA region, the contact angle of water on the film is θc > φ0, also indicating a nonwetting state. However, the water molecules can infiltrate into the PES/PAA regions easily but not into the PES areas. This water response mode of structural colors can be explained by the infiltration response mechanism due to the change in nave when the air in the inverse opal structure is replaced by water.In terms of Flory equation [41], for a hydrophilic polymer material with 3D cross-linking network structure, the water molecules can be absorbed due to the capillary action of the interconnected void structures and the formation of hydrogen bonding between hydrophilic groups and targeted molecules [37], [41], [42], [43]. PAA gel, which is formed by the crosslinking polymerization of AA monomers, is hydrophilic due to the formation of hydrogen bonds between carboxyl groups and H2O molecules. Thus, we speculated that the different permeabilities of water for the PES and PES/PAA regions were caused by the water absorption of PAA gel due to the capillary action of interconnected pore structures and the formation of hydrogen bonds between carboxyl groups and H2O molecules [37], [42]. Hence, it is possible that water molecules entered the gel through the capillary action in the voids in the gel and then interacted with hydrophilic groups of PAA. Subsequently, the hydrophilic groups (📷COOH) began to dissociate. With hydrophilic group dissociation, the number of anions (📷COO−) increased. Then the gel network expanded due to the increasing electrostatic repulsion among anions (see Fig. 4i). To maintain electrical neutrality, cations cannot diffuse to the outside, resulting in an increasing concentration of cations in the gel network. As a result, water molecules further infiltrated into the gel due to the osmotic pressure formed between the inside and outside areas of the network [37]. Therefore, a number of voids in the PAA gel were formed after immersion in water, due to the expansion of crosslinking network compared to that of PAA gel without water (Fig. 4j, k). Thus, although the expansion ability of PAA is limited due to the existence of the PES skeleton, the water molecules can overcome the resistance of reentrant curvature to infiltrate into inverse opal structures (see Fig. 4h) owing to the capillary action of the interconnected pore structures and the formation of hydrogen bonds when PAA is anchored in the PES structure (see Fig. 4l). Therefore, a significant structural color difference is observed between PES region and PEA/PAA pattern after wetting with water. 3.4. Ethanol concentration response for PES/PAA film Similarly to the infiltration response mode caused by pure water, the structural color response of the PES/PAA film can also be triggered by a mixture of water and ethanol. Moreover, the infiltration process was influenced by the ethanol concentration. As shown in Figure 5a–j, new reflection peaks for PES/PAA films appeared after wiping with the mixture of water and ethanol. The variation in the reflection peaks can also be explained by the infiltration response mechanism. 📷Download : Download high-res image (1MB) Download : Download full-size image Fig. 5. PES/PAA films response to ethanol concentration (a-j, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 99%) in reflection spectra; (k) contact angles of aqueous solution with different ethanol concentration on the surface of PES/PAA film; (l) linear correlation between the logarithm of structural color response time and the ethanol concentration. Response time of the PES/PAA film wiped by aqueous solution with different concentrations of ethanol is shown in the inset figure.Water − ethanol hydrogen bonds are formed when ethanol molecules are added in water, and these increase with the concentration of ethanol [44]. When the PES/PAA film is wetted by this water and ethanol mixture, the water molecules in water - ethanol and water-water hydrogen bonds can be absorbed by PAA. Alongside these water molecules, the ethanol molecules in water–ethanol hydrogen bonds can also be adsorbed by the PAA gel. When the concentration of ethanol was below 50% in our study, the number of water molecules in the water – water hydrogen bonds decreased with the increase in ethanol concentration. [44] Meanwhile, the number of water molecules in the water–ethanol hydrogen bonds increases gradually. Thus, the content of ethanol, which has a higher refractive index (∼1.36) than the water infiltrating into the PES/PAA region, gradually increased. As a result, the new reflection peak positions were redshift (from 525 nm to 546 nm) by the increases in ethanol concentration (Figure 5a–e).When the concentration of ethanol was greater than 50%, the dominant type of hydrogen bond was the water - ethanol hydrogen bond [44]. The solvent simultaneously infiltrated in the form of water–ethanol hydrogen bonds. The content of the ethanol and water infiltrating into the PES/PAA region no longer changes. Hence, the new reflection peak positions did not change (∼552 nm).With increasing ethanol concentration, the structural color response time continuously increased (see Figure 5a-j). Additionally, ethanol has a lower surface tension than water. Thus, the contact angles of the mixed solution on the surface of the PES/PAA film decreased with the increasing ethanol concentration (see Figure S13 and Figure 5 k), which is a trend that opposes that of the structural color response time. For example, when the concentration of ethanol was 10%, the redshift of the structural color was observed for one second. When the concentration of ethanol increased to 99%, the structural color response time increased to 45 s. According to the Flory theory, the polymer–solvent affinity and the osmotic pressure inside and outside the polymer network are both closely related to absorption capacity [41]. Thus, the difference in structural color response time may be caused by the change in polymer–solvent affinity and the osmotic pressure inside and outside the polymer network when ethanol is introduced in the aqueous solution.In the PES/PAA film, almost all the necks between two adjacent pores were blocked by PAA gels. Thus, due to this disappearance of necks, the physical structure of this porous material was detrimental to the spontaneous infiltration of the solvent. Furthermore, the contact angles decreased when ethanol was introduced into the aqueous solution. With regards to PAA gel, water molecules can be absorbed easily, owing to their strong hydrogen bonding ability with carboxylic acids, which results in a high polymer–solvent affinity. Furthermore, its high polarity is beneficial to the dissociation of carboxyl groups, which contributes to the increase in osmotic pressure both inside and outside the polymer. Hence, water molecules infiltrated the PES/PAA film immediately as a result of the PAA gel that was grafted to the PES film.Compared to water, the hydrogen-bonding ability between carboxylic acids and ethanol is reduced [45]. Therefore, it was harder to drive ethanol molecules into the PES/PAA films than it was to drive water. Furthermore, the dissociation of hydrophilic groups (📷COOH) was also hindered by the low polarity of ethanol [46]. Therefore, when water molecules were partially substituted with ethanol, the expansion of the cross-linking network was restricted because of the decreased electrostatic repulsion. Consequently, as a result of the reduced osmotic pressure between the inside and outside of the network, the infiltration rate decreased. Figure 5l shows the reflection spectra shifting time of the PES/PAA film, which were recorded by a fiber optic spectrometer after wiping with various aqueous solutions with different ethanol concentrations. Obviously, there is a linear correlation between the logarithm of the structural color response time and the ethanol concentration at the settled range. Thus, the concentration of ethanol in the aqueous solution can be directly investigated alongside the structural color response time. Additionally, the feature of adjustable color response time makes the information decryption methods more diverse and complex which contributes to their use as potentially high-security encryption devices in the anti-counterfeiting.

4. Conclusions In conclusion, responsive PCs were prepared by incorporating PAA inside PES inverse opals. The use of PES material ensured good flexibility, excellent tensile strength and anti-compressive properties of the film. The capillary action and the formation of hydrogen bonds between water molecules and carboxyl groups in the PAA gel promoted the infiltration of water into the PES/PAA region. Hence, the decryption of information was triggered rapidly by the wetting process. Meanwhile, this decrypted information can recover to their hidden state upon the evaporation of water. The ethanol concentration in aqueous solution can be identified based on the structural color response time due to the low polymer–solvent affinity which impeded the rapid absorption of the solvent by the gel. Considering that the PES/PAA PC has these merits, it is believed that it will be highly useful in sensing and anti-counterfeiting fields.

CRediT authorship contribution statement Xia Hongbo: Investigation, Methodology, Formal analysis, Data curation, Writing – original draft. Li Dan: Supervision, Writing - review & editing. Wu Suli: Writing - review & editing, Supervision, Project administration. Feng Shuai: Writing - review & editing, Formal analysis, Data curation, Visualization. Meng Chao: Formal analysis, Data curation, Visualization. Dong Bin: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements This work is supported by the National Special Support Program for High-level Personnel Recruitment (W03020231), National Natural Science Foundation of China (Grant No. 11974069, 12004066), LiaoNing Revitalization Talents Program (XLYC1902113), Science and Technology Project of Liaoning Province (2020JH2/10100012), Science and Technique Foundation of Dalian (Grant No. 2017RD12).

Appendix A. Supplementary material

What’s this?

The following are the Supplementary data to this article:

Download : Download video (2MB)

Supplementary video 2.

Download : Download Word document (3MB)Supplementary data 1.

References [1]D.H. Kou, Y.C. Zhang, S.F. Zhang, S.L. Wu, W. MaHigh-sensitive and stable photonic crystal sensors for visual detection and discrimination of volatile aromatic hydrocarbon vaporsChem. Eng. J., 375 (2019), Article 121987

View PDFView articleView in ScopusGoogle Scholar[2]

J. Zeng, W. Fan, K.Y. Jia, S.H. Tu, L.M. Wu

Novel retroreflective structural color films based on total internal reflection interference

J Colloid and Interf Sci, 597 (2021), pp. 306-313

View PDFView articleView in ScopusGoogle Scholar[3]

H.Y. Tan, Q.Q. Lyu, Z.J. Xie, M.M. Li, K. Wang, K. Wang, B.J. Xiong, L.B. Zhang, J.T. Zhu

Metallosupramolecular photonic elastomers with self-Healing capability and angle-independent color

Adv. Mater., 31 (2019), pp. 1805496-1805502

View in Scopus

Google Scholar[4]

D.P. Yang, S.Y. Ye, J.P. Ge

Solvent wrapped metastable colloidal crystals: Highly mutable colloidal assemblies sensitive to weak external disturbance

J. Am. Chem. Soc., 135 (2013), pp. 18370-18376

CrossRef

View in ScopusGoogle Scholar[5]

V. Caligiuri, G. Tedeschi, M. Palei, M. Miscuglio, B. Martin-Garcia, S. Guzman-Puyol, M.K. Hedayati, A. Kristensen, A. Athanassiou, R. Cingolani, V.J. Sorger, M. Salerno, F. Bonaccorso, R. Krahne, J.A. Heredia-Guerrero

Biodegradable and insoluble cellulose photonic crystals and metasurfaces

ACS Nano, 14 (2020), pp. 9502-9511

CrossRef

View in ScopusGoogle Scholar[6]

X. Dong, P. Wu, C.G. Schaefer, L.W. Zhang, C.E. Finlayson, C.C. Wang

Solvatochromism based on structural color: smart polymer composites for sensing and security

Mater. Des., 160 (2018), pp. 417-426

View PDFView articleView in ScopusGoogle Scholar[7]

A.C. Arsenault, D.P. Puzzo, I. Manners, G.A. Ozin

Photonic-crystal full-colour displays

Nat. Photonics, 1 (2007), pp. 468-472

CrossRef

View in ScopusGoogle Scholar

[8]J.P. Ge, Y.D. Yin, Responsive photonic crystals, Angew Chem.-Int Edit, 50(2011) 1492-1522.Google Scholar

[9]

Q. Xiang, Y.W. Luo

Three-layer structured soft particles to construct photonic paper exhibiting responsive spatio-temporal color patterns

Chem. Eng. J., 380 (2020), Article 122494

View PDFView articleView in ScopusGoogle Scholar[10]

Y. Meng, F.F. Liu, M.M. Umair, B.Z. Ju, S.F. Zhang, B.Z. Tang

Patterned and iridescent plastics with 3D inverse opal structure for anticounterfeiting of the banknotes

Adv. Opt. Mater., 6 (2018), pp. 1701351-1701357

View in Scopus

Google Scholar[11]

Y.-J. Quan, Y.-G. Kim, M.-S. Kim, S.-H. Min, S.-H. Ahn

Stretchable biaxial and shear strain sensors using diffractive structural colors

ACS Nano, 14 (2020), pp. 5392-5399

CrossRef

View in ScopusGoogle Scholar[12]

Y.F. Yue, T. Kurokawa

Designing responsive photonic crystal patterns by using laser engraving

ACS Appl. Mater. Interfaces, 11 (2019), pp. 10841-10847

CrossRef

View in ScopusGoogle Scholar[13]

L.R. Shang, W.X. Zhang, K. Xu, Y.J. Zhao

Bio-inspired intelligent structural color materials

Mater Horizons, 6 (2019), pp. 945-958

CrossRef

View in ScopusGoogle Scholar[14]

L. Bai, Z.Y. Xie, W. Wang, C.W. Yuan, Y.J. Zhao, Z.D. Mu, Q.F. Zhong, Z.Z. Gu

Bio-inspired vapor-responsive colloidal photonic crystal patterns by inkjet printing

ACS Nano, 8 (2014), pp. 11094-11100

CrossRef

View in ScopusGoogle Scholar[15]

J.Y. Chen, L.R. Xu, M.J. Yang, X.C. Chen, X.D. Chen, W. Hong

Highly stretchable photonic crystal hydrogels for a sensitive mechanochromic sensor and direct ink writing

Chem. Mater., 31 (2019), pp. 8918-8926

CrossRef

Google Scholar[16]

K. Zhong, J.Q. Li, L.W. Liu, S. Van Cleuvenbergen, K. Song, K. Clays

Instantaneous, simple, and reversible revealing of invisible patterns encrypted in robust hollow sphere colloidal photonic crystals

Adv. Mater., 30 (2018), pp. 1707246-1707253

View in Scopus

Google Scholar

[17]Y. Qi, L. Chu, W.B. Niu, B.T. Tang, S.L. Wu, W. Ma,; S.F. Zhang, New encryption strategy of photonic crystals with bilayer inverse heterostructure guided from transparency response, Adv Funct Mater, 29(2019) 1903743-1903753.Google Scholar

[18]

Y. Qi, W. Niu, S. Zhang, S. Wu, L. Chu, W. Ma, B. Tang

Encoding and decoding of invisible complex information in a dual-response bilayer photonic crystal with tunable wettability

Adv. Funct. Mater., 29 (2019), pp. 1906799-2190809

View in Scopus

Google Scholar[19]

J.-P. Couturier, M. Sütterlin, A. Laschewsky, C. Hettrich, E. Wischerhoff

Responsive inverse opal hydrogels for the sensing of macromolecules

Angew Chem-Int Edit, 54 (2015), pp. 6641-6644

CrossRef

View in ScopusGoogle Scholar[20]

Y.P. Wang, W.B. Niu, C.-Y. Lo, Y.S. Zhao, X.M. He, G.R. Zhang, S.L. Wu, B.Z. Ju, S.F. Zhang

Interactively full-color changeable electronic fiber sensor with high stretchability and rapid response

Adv. Funct. Mater., 30 (2020), pp. 2000356-12000345

View in Scopus

Google Scholar[21]

Z. Cai, D.H. Kwak, D. Punihaole, Z. Hong, S.S. Velankar, X. Liu, S.A. Asher

A photonic crystal protein hydrogel sensor for candida albicans

Angew Chem-Int Edit, 54 (2015), pp. 13036-13040

CrossRef

View in ScopusGoogle Scholar[22]

Z.P. Meng, B.T. Huang, S.L. Wu, L. Li, S.F. Zhang

Bio-inspired transparent structural color film and its application in biomimetic camouflage

Nanoscale, 11 (2019), pp. 13377-13384

CrossRef

View in ScopusGoogle Scholar[23]

C.H. Park, J.G. Kim, S.-G. Jung, D.J. Lee, Y.W. Park, B.-K. Ju

Optical characteristics of refractive-index-matching diffusion layer in organic light-emitting diodes

Sci. Rep., 9 (2019), p. 8690

View in Scopus

Google Scholar[24]

D.K. Rhee, P.J. Yoo

Interconnected assembly of ZrO2@SiO2 nanoparticles with dimensional selectivity and refractive index tunability

J. Mater. Chem. C, 7 (2019), pp. 8176-8184

CrossRef

View in ScopusGoogle Scholar[25]

S.L. Wu, H.B. Xia, J.H. Xu, X.Q. Sun, X.G. Liu

Manipulating luminescence of light emitters by photonic crystals

Adv. Mater., 30 (2018), pp. 1803362-1803387

View in Scopus

Google Scholar[26]

H.L. Zhao, J.P. Gao, Z. Pan, G.B. Huang, X.Y. Xu, Y.H. Song, R.N. Xue, W. Hong, H.X. Qiu

Chemically responsive polymer inverse-opal photonic crystal films created by a self-assembly method

J. Phys. Chem. C, 120 (2016), pp. 11938-11946

CrossRef

View in ScopusGoogle Scholar[27]

F.F. Liu, B. Shan, S.F. Zhang, B.T. Tang

SnO2 Inverse opal composite film with low-angle-dependent structural color and enhanced mechanical strength

Langmuir, 34 (2018), pp. 3918-3924

CrossRef

View in ScopusGoogle Scholar[28]

B. Van der Bruggen

Chemical modification of polyethersulfone nanofiltration membranes: A review

J. Appl. Polym. Sci., 114 (2009), pp. 630-642

CrossRef

View in ScopusGoogle Scholar[29]

C. Klaysom, B.P. Ladewig, G.Q.M. Lu, L.Z. Wang

Preparation and characterization of sulfonated polyethersulfone for cation-exchange membranes

J. Membr. Sci., 368 (2011), pp. 48-53

View PDFView articleView in ScopusGoogle Scholar[30]

L.Y. Wang, Y. Zhang, Y.Y. Zhu

One-pot synthesis and strong near-infrared upconversion luminescence of poly(acrylic acid)-functionalized YF3:Yb3+/Er3+ nanocrystals

Nano Res., 3 (2010), pp. 317-325

CrossRef

View in ScopusGoogle Scholar[31]

T. Gao, X. Wang, L.Y. Yang, H. Huang, X.X. Ba, J. Zhao, F.L. Jiang, Y. Liu

Red, yellow, and blue luminescence by graphene quantum dots: Syntheses, mechanism, and cellular imaging

ACS Appl. Mater. Interf., 9 (2017), pp. 24846-24856

CrossRef

View in ScopusGoogle Scholar[32]

J.B. Kim, S.Y. Lee, J.M. Lee, et al.

Designing structural-color patterns composed of colloidal arrays

Appl. Mater. Interf., 11 (2019), pp. 14485-14509

CrossRef

View in ScopusGoogle Scholar[33]

P.C. Chen, L.S. Wan, Z.K. Xu

Bio-inspired CaCO3 coating for superhydrophilic hybrid membranes with high water permeability

J. Mater. Chem., 22 (2012), pp. 22727-22733

CrossRef

View in ScopusGoogle Scholar[34]

J. Huang, K.S. Zhang, K. Wang, Z.L. Xie, B. Ladewig, H.T. Wang

Fabrication of polyethersulfone-mesoporous silica nanocomposite ultrafiltration membranes with antifouling properties

J. Membr. Sci., 423–424 (2012), pp. 362-370

View PDFView articleView in ScopusGoogle Scholar[35]

X.-G. Li, H.-T. Shao, H. Bai, M.-R. Huang, W. Zhang

High-resolution thermogravimetry of polyethersulfone chips in four atmospheres

J. Appl. Polym. Sci., 90 (2003), pp. 3631-3637

View in Scopus

Google Scholar[36]

X. Chen, Y. He, C.C. Shi, W.G. Fu, S.Y. Bi, Z.Y. Wang, L. Chen

Temperature- and pH-responsive membranes based on poly (vinylidene fluoride) functionalized with microgels

J. Membr. Sci., 469 (2014), pp. 447-457

View PDFView articleView in ScopusGoogle Scholar[37]

S. Sharma, A. Dua, A. Malik

Polyaspartic acid based superabsorbent polymers

Eur. Polym. J., 59 (2014), pp. 363-376

View PDFView articleView in ScopusGoogle Scholar[38]

F. Gallego-Gómez, A. Blanco, C. López

Exploration and exploitation of water in colloidal crystals

Adv. Mater., 27 (2015), pp. 2686-2714

CrossRef

View in ScopusGoogle Scholar

[39]I.B. Burgess, L. Mishchenko, B.D. Hatton, M. Kolle, M. Lončar, J. AizenbergEncoding complex wettability patterns in chemically functionalized 3D photonic crystalsJ. Am. Chem. Soc., 134 (2012)1374 1374Google Scholar

[40]

I.B. Burgess, N. Koay, K.P. Raymond, M. Kolle, M. Lončar, J. Aizenberg

Wetting in color: Colorimetric differentiation of organic liquids with high selectivity

ACS Nano, 6 (2012), pp. 1427-1437

CrossRef

View in ScopusGoogle Scholar

[41]P.J. FloryPrinciples of polymer chemistryCornell University Press, Ithaca (1953)Google Scholar

[42]

F.R. He, Q.F. Zhou, L.Z. Wang, G.B. Yu, J.C. Li, Y.H. Feng

Fabrication of a sustained release delivery system for pesticides using interpenetrating polyacrylamide/alginate/montmorillonite nanocomposite hydrogels

Appl. Clay Sci., 183 (2019), Article 105347

View PDFView articleView in ScopusGoogle Scholar[43]

Y. Sun, J.P. Gao, Y. Liu, H.Y. Kang, M.H. Xie, F.M. Wu, H.X. Qiu

Copper sulfide-macroporous polyacrylamide hydrogel for solar steam generation

Chem. Eng. Sci., 207 (2019), pp. 516-526

View PDFView articleView in ScopusGoogle Scholar[44]

A. Ghoufi, F. Artzner, P. Malfreyt

Physical properties and hydrogen-bonding network of water–ethanol mixtures from molecular dynamics simulations

J. Phys. Chem. B, 120 (2016), pp. 793-802

CrossRef

View in ScopusGoogle Scholar[45]

M. Yoshikawa, T. Yukoshi, K. Sanui, N. Ogata

Selective separation of water–ethanol mixture through synthetic polymer membranes having carboxylic acid as a functional group

J. Polym. Sci., Part A: Polym. Chem., 24 (1986), pp. 1585-1597

View in Scopus

Google Scholar[46]

G. Christensen, H. Younes, H.P. Hong, S. Pauline

Effects of solvent hydrogen bonding, viscosity, and polarity on the dispersion and alignment of nanofluids containing Fe2O3 nanoparticles

J. Appl. Phys., 118 (2015), Article 214302

View in Scopus

Google Scholar

Cited by (24) Smart colloidal photonic crystal sensors 2024, Advances in Colloid and Interface Science

Sensing blood components and cancer cells with photonic crystal resonator biosensor 2024, Results in Optics
Preparation of a new PVDF membrane with inverse opal structure for high-precision separation 2024, Journal of Industrial and Engineering Chemistry
Responsive photonic hydrogel for colorimetric detection of formaldehyde 2023, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
Portable colorimetric photonic indicator for ethanol concentration sensing 2023, Chemical Engineering JournalCitation Excerpt :The development of portable structurally colored colorimetric sensors with full ethanol concentration response remains a challenge. An excellent immersion PC sensor needs to consider the following: 1) constant swelling of polymer matrix should be formed under different ethanol concentrations (benefit for producing stable reflection spectrum),[23] 2) FWHM of reflection peak generated by porous structure should be as narrow as possible (producing a high resolution of color),[24] 3) reflections corresponding to different ethanol concentrations should cover as much visible spectrum as possible,[25] and 4) high response sensitivity of polymer should be provided.[26] The autonomic solvent control (reversible swelling/shrinking) of the polymer in response to external stimuli is the key to achieving the above goal, which is conducive to enlarging the wavelength difference corresponding to adjacent concentrations in a limited wavelength range.
Preparation of Janus structural color sheets with flexibility, stability and low angle dependence based on textile 2022, Colloids and Surfaces A: Physicochemical and Engineering AspectsCitation Excerpt :This color has the unique properties of high brightness, high saturation, never fading and environment friendly [1–4]. Photonic crystals (PCs) made from periodic arrangements of media with different refractive indices possess photonic forbidden bands that make it impossible for photons to propagate and produce structural colors [5,6]. Currently, photonic crystal structural color materials self-assembled from colloidal microspheres demonstrate promising application in the fields of sensors [7,8], displays [9–11] and anti-counterfeiting [12–15].

View all citing articles on Scopus

Fabrication of polyacrylamide inverse opal photonic crystal Films based on co-deposition Materials Letters, Volume 150, 2015, pp. 5-8Huanhuan Liu, …, Shirong Liu
Core-shell based responsive colloidal photonic crystals for facile, rapid, visual detection of acetone Reactive and Functional Polymers, Volume 158, 2021, Article 104779Meghana Mary Thomas, …, Saju Pillai
Polymer-infiltrated SiO2 inverse opal photonic crystals for colorimetrically selective detection of xylene vapors Sensors and Actuators B: Chemical, Volume 291, 2019, pp. 67-73Yuqi Zhang, …, Ji-Jiang Wang

Citations

Citation Indexes: 20

Captures

Readers: 7

View details📷

How can I use the cif data obtained from rietveld refinement extracted via gsas2, for microstructural analysis using ETEX software?

How to completely dissolve soy lecithin?

Have you heard about Technological Surveillance on Oceanographic Research Vessels?

What is the best way to get rid of unpleasant odour of acrylic emulsion beside redox post polymerization?

Why can't we get on top of this incubator infection (fungal)?

How to conduct bio accessibility analysis of Heavy metals?

I need to buy reducing agent Bruggolite FF6 M, how can I buy?

Can i use polystyrene as a matrix in a hybrid COMPOSITE?

About mineral exploration?

How to set the background of plot to white color in Silvaco Tonyplot?

Adhesion strength of coating?

How to increase simulation box size?

How to heat polymer for about 130° without oil bath ?

Polymer wear calculation?

For the moringa oleifera extract using ethanol as the solvent,what is the alternative method to concentrate the extract other than rotary evaporation?

How to explain the different interaction between soluble and insoluble polymer in water?

Why is it not common to see similar pairs in polymer sliding pairs?

HAs anyone used TGA to find activation enegy of water evaporation?

Comparison of Methanol and ethanol treatment on collagen based scaffolds?

What is the difference between coordination polymer, MOFs, MCOFs?