DOI:
10.1039/C6RA18738B
(Paper)
RSC Adv., 2016,
6, 85885-85890
Fabrication of PAM/PMAA inverse opal photonic crystal hydrogels by a “sandwich” method and their pH and solvent responses†
Received
23rd July 2016
, Accepted 12th August 2016
First published on
2nd September 2016
Abstract
This paper presents fabrication of polyacrylamide (PAM) and PAM/poly(methacrylic acid) (PMAA) inverse opal photonic crystal hydrogels (IOHs) based on poly(styrene-acrylic acid) (PSA) template by a “Sandwich” method. SEM images showed the obtained IOHs had a three-dimensional ordered structure and extremely uniform pore size without over-layers and defects. The IOHs exhibited brilliant structural color by controlling the PSA particle size, MAA and cross-linker compositions. The color and Bragg diffraction peak of the IOHs responded to external stimuli such as pH, methanol and ethanol. The reflectance was red-shifted in low pH and blue-shifted in high pH and in the presence of methanol and ethanol. The effects of the MAA and cross-linker composition on the response properties of the IOHs were investigated. The PMAA in the IOHs provided –COOH groups that are apt to form hydrogen bonds with water, enhancing the response sensitivity.
Introduction
Photonic crystals (PCs) have attracted much attention because of their abilities of periodically modulating refractive indices in space and possessing tunable photonic bandgaps, which could induce tunable color.1–4 The structure's color naturally offers a signal of the lattice structure of the photonic crystals for specific materials.5 Polymer hydrogels are well-known to change volume reversibly in response to external conditions,6 such as solvent,7 pH,8 temperature,9,10 ionic concentration,11,12 and light.13,14 Constructing hydrogels with phonic crystal structures, the swelling and shrinkage of the hydrogel photonic crystal would promote it to be an optical sensor with alterable colors or optical diffraction.15 The changes in structural color and diffraction wavelength in response to environmental variations provided a signal that can be quickly synchronized, and thus the materials have potential applications as sensors.16–19 Generally, the sensor materials are requested to the rapid response to the external environment and good mechanical stability. Inverse opal hydrogels (IOHs) based on the closest-packing crystal have their distinct structural features with three-dimensionally periodic arrays of air pores within a matrix of some functional materials.20–22 It proved that IOHs could be satisfied to the requests of rapid response and mechanical stability.23,24
At present, a variety of methods to prepare IOHs have already been developed. Hatton group25 has developed the co-assembly method for the preparation of silica inverse opal with crack-free and high order over centimeter length scales. Gao et al.21 prepared multi-responsive IOHs by vertical deposition–evaporation method. Our group reported to fabricate polyacrylamide (PAM) inverse opal based on poly(styrene-acrylic acid) (PSA) template for the first time by co-deposition method.26,27 All these different methods could construct the IOHs with few defects. Since colloid crystals and close degree of the packed colloid crystals might be especially important to create various inverse-opal structures and influence to the rapid response, it is necessary to select a useful infiltration method to avoid to cracks, domain boundaries, spherical vacancies, and other defects.
In the previous work, we prepared PAM IOH for pH sensing by co-deposition method. However, the sensitivity was low. For improving the sensitivity, we want to seek an appropriate method of fabrication of IOHs. In this work, we fabricated two IOHs of PAM or PAM/PMAA (polymethacrylic acid) hydrogels through “Sandwich” method28 using poly(styrene-co-acrylic acid) microsphere (PSA) (Fig. 1). They exhibited well mechanical stability and bright structural colors through well-ordered photonic crystal microstructure. In addition, response to various external stimuli such as solvent and pH was investigated.
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| Fig. 1 Preparation of PAM and PAM/PMAA IOHs by “sandwich” method. | |
Experimental section
Materials
Acrylamide (AM), acrylic acid (AA), methacrylic acid (MAA), ammonium persulfate (APS), N,N,N′,N′-tetramethylethylenediamine (TEMED) and styrene were purchased from Sinopharm Chemical Reagent Co., Ltd. N,N′-Methylene bisacrylamide (BIS) was obtained from Aladdin Chemistry Co., Ltd. (China). Styrene and AA were purified by distillation under vacuum. All the other affiliated chemicals not mentioned here were used without further purification. The glass slides (76.2 × 24.5 × 1 mm) for the colloidal crystal growth were well-cleaned using acetone, anhydrous ethanol and double-distilled water in an ultrasonic bath.
Preparation of opal template
The monodispersed poly(styrene-co-acrylic acid) (PSA) colloids were synthesized by emulsifier-free emulsion polymerization according to our previous report.26,27
Colloid crystals of the PSA microspheres were fabricated by a vertical deposition method. A glass slide was vertically placed into the 1 wt% of the pre-prepared PSA microspheres suspension. Then, the glass slide with the PSA suspension was located in a vacuum drying oven at a constant temperature of 60 °C with the humidity of 40%. After drying the sediment, water in the colloidal suspension was evaporated and a solid structure of well-ordered PSA opal template on glass slide was obtained.
Preparation of inverse opal hydrogels (IOH) by “Sandwich” method
PAM and PAM/PMAA IOHs were prepared by “Sandwich” method. The “Sandwich” is the method that firstly covered the above obtained PSA microspheres with another glass slide, and the PSA microspheres were placed between two glass slides to form the “Sandwich-like” structure, as Fig. 1. AM and MAA with different molar ratios, BIS cross-linker, and APS were dissolved in water. Then, the PSA photonic crystals between two glass slides was immersed into the above mixture, and the solution filled into the photonic crystals by capillary forces. Once the infiltration was finished, PAM and PAM/PMAA were polymerized over a period of 7 h by placed the formed sample into an oven at 60 °C and evaporated the solvent slowly. Subsequently, the composite opal film could be easily peeled off from the substrate through immersion in water. Finally, high quality PAM and PAM/PMAA IOHs (the size 2.4 × 1.5 cm of the film) with bright colors were obtained after selectively removal of the PSA template by dissolving in dimethylbenzene for 48 h.
Characterization
The particle size of PSA microsphere was determined by dynamic light scattering on a Malvern laser particle sizes analyzer (Nano-S, Malvern, England). Surface morphologies of the composite opal films and IOH films were characterized by field emission scanning electron microscopy (SEM, Hitachi, S-4800). The pH and solvent dependent Bragg diffraction wavelengths of IOH were recorded by a miniature fiber optic spectrometer (FLA 4000+, China) from one spot by perpendicular to the IOH film. The corresponding color changes were observed by a digital camera under a daylight lamp.
Results and discussion
Fabrication of PAM and PAM/MAA IOHs
PSA microspheres with different sizes were obtained by emulsifier-free emulsion polymerization. DLS results showed that the sizes were 195, 226, and 293 nm. The PSA opal templates with three kinds of sizes were used to construct the colloid crystals on the treated glass slide. The different sizes of PSA resulted in the blue, green and pink colors of the opal templates (Fig. S1A†). It can be seen that the PSA microspheres were close-packed orderly from SEM images (Fig. S1B†). Each microsphere neighboured six others in a layer, which arranged to face-centered cubic (FCC) structure. Moreover, the sizes did not affect the structures of the PSA colloid crystals expect of the photonic band gap. Sharp single reflection peaks of the PSA opal templates were observed in reflection spectrum in Fig. S2a,† proving the single-crystalline of the PSA opal template. When the size of the PSA particles increased from 195 to 293 nm, the maximum Bragg diffraction peak (λmax) was red-shifted from 460, 537 to 692 nm, respectively (Fig. S2b†). These values were well agreed with the calculated peaks by the Bragg equation, depending on the Bragg diffraction of light waves. The increasing of the microsphere size induced the red-shift of reflection peak, due to the changing of the photonic band gap position.
PAM and PAM/PMAA photonic crystals were prepared by the “sandwich” method after immersed the PSA opal template between two glasses into monomer solution and then polymerization. Owing to the hydrogen effects between AM, MAA and the PSA particles, the monomers could surround the PSA microspheres and penetrated into the interstitial spaces between the PSA colloids.26 After the removal of PSA colloids free-standing IOH film was obtained. The thickness of IOH films can be modulated by changing the mass fraction of colloids suspended in the suspension solution. The higher the mass fraction of the colloids, the thicker the film would be acquired. Fig. 2 shows the typical SEM images of the resultant PAM and PAM/PMAA IOH films. Three-dimensional, highly ordered and interconnected macroporous arrays were successfully fabricated. Moreover, the observed domain kept an ordered FCC structure, indicating that the original arrangement of the colloidal crystal template was kept well during the evaporation and polymerization. These results indicated that the “sandwich” method is a suitable way for the fabrication of nearly well-ordered free-standing IOH film without over-layer and cracks.
 |
| Fig. 2 SEM images of PAM (a) and PAM/PMAA (b) IOHs. The scale bar is 500 nm. | |
Fig. 3 shows the photograph of PAM IOH films based on PSA template with different sizes and their reflectance spectra. The PAM IOH films had variable colors depending on the size of the PSA microspheres (Fig. 3A). Reflectance spectra displayed a sharp single reflection peak (Fig. 3B), which reveals the high crystalline quality of the inverse opal film. The measured strongest reflection peaks red-shifted from 436, 508, to 613 nm for the sizes of the PSA particles increasing from 195, 226 to 293 nm, respectively. This result was similar to the PSA opal template, indicating the successful construction of the PAM IOH. In addition, PAM/PMAA IOHs also showed the same results. By changing the cross-linker content, the color and reflectance spectra of the PAM and PAM/PMAA IOHs were no significant changes, as shown in Fig. S3.† With increasing the cross-linker, the crosslinking degree of the IOHs was enhanced, which was reduced the swelling degree. Thus, the reflection peaks of IOHs were changed to blue-shift (Fig. S3c†). However, the blue-shift was low (just from 478 to 496 nm), resulting in no significantly differences by naked eyes. Accordingly, the cross-linker did not affect the PAM and PAM/PMAA IOHs.
 |
| Fig. 3 (A) Photograph of PAM IOHs prepared by PSA template with different diameters of (a) 195 nm, (b) 226 nm, and (c) 293 nm. Reflectance spectra (B and C) of PAM IOHs. | |
The effect of MAA composition on the color and reflectance spectra of the PAM and PAM/PMAA IOHs (4% of BIS and 195 nm of PSA) was also investigated, and the results are shown in Fig. 4. Increasing the MAA molar ratio from 0, 0.05, 0.1, to 0.2, the color of the IOHs was changed from blue to green-blue. The results from reflectance spectra showed the red-shift of reflection peaks from 435, 480, 505, to 560 nm. With the increment of MAA, the repelling force between the polymer chains became strong, which induced the strong of the ionic strength. Thus, the pores of IOHs became big, resulting in the red-shift of reflection peaks. Accordingly, the color and reflection peaks could be adjusted by the MAA composition.
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| Fig. 4 Photographs (A) and reflection spectra (B) of PAM/PMAA IOHs with different molar ratios of 0.05MAA (a), 0.1MAA (b) and 0.2MAA (c). | |
pH response of PAM and PAM/MAA IOHs
pH response of the PAM and PAM/PMAA IOHs (4% of BIS, 226 nm of PSA and 0.1MAA) was investigated. Fig. 5 shows the reflection spectra and photographs of the IOH films at different pH values. The color of the PAM and PAM/PMAA IOHs changed from yellow-green to red in the solutions from low to high pH values. In various pH solutions, both kinds of IOHs showed the single peak in the reflectance spectra. It can be clearly seen that the Bragg diffraction peak of the PAM IOHs shifted markedly from 600 to 720 nm (difference of peaks Δλmax = 120 nm) by increasing pH from acid to weak alkalinity, which was larger than the PAM IOH prepared by co-deposition method and other reports.27,29 Although the exact reason for the advantage of the “Sandwich” method is still unknown, we speculated that the PSA particles were fixed during infiltration of monomer and closer structure was further formed between the layers.
 |
| Fig. 5 Reflection spectra and photographs of PAM (a) and PAM/PMAA (0.1MAA) (b) IOHs at different pH values. (c) Reflectance peak value of two IOHs at different pH values. The cross-linker content is 4%, and the size of PSA is 226 nm. | |
Introducing MAA into the IOHs, the maximum Bragg diffraction peak of the PAM/PMAA IOHs changed from 555 to 635 nm during pH value of around 2.0 to 8.0. When pH further increased to above 8.5, strong alkalinity, the peaks of IOHs changed to blue-shift. These resulted from the electrostatic repulsion among the carboxylate anions on the polymer chains and the increase in osmotic pressure. Increasing pH value from acid to weak alkalinity (∼8.5), ionization of the carboxyl groups increased, leading an increase in the carboxylate anions. The carboxylate anions on the polymer chains brought about an electrostatic repulsion, tending to expand the polymer network. Thus, the IOH swelled and the Bragg photonic band gap increased, resulting in the red-shift of the reflection peak (Fig. S4b†). At higher pH, the IOH was hydrolyzed and some amide groups hydrolyzed to carboxyl groups, which ionized as a function of pH. However, when PMAA was introduced into PAM, the hydrolysis of PAM was inhibited because the presented carboxylic groups decreased the hydrolysis reaction for the equilibrium of the hydrolysis. So, the pH response and red-shift of the reflection wavelength of the PAM/PMAA was slower than that of the PAM IOHs. Since ionization was complete at pH 9.0, further increases in pH only increases the ionic strength that decreases the osmotic pressure and makes the hydrogel shrink, causing a blue-shift.
In addition, by changing the content of cross-linker, there was no significant difference of the PAM and PAM/PMAA IOHs (Fig. S4†), as discussed in above (Fig. S3†).
Solvent response of PAM and PAM/MAA IOHs
To know solvent response of the IOHs, methanol was used as a model solvent. Fig. 6 shows photographs and reflection spectra of the PAM and PAM/PMAA (0.2MAA) IOHs at different volume ratios of methanol and water. For the case of the PAM IOHs, increasing the water content (or decreasing the methanol content), the color of the IOHs was clearly different, and red-shifted from green-blue, green-orange, green-red to light red (Fig. 6a). This meant that the increasing of methanol in water induced the blue-shift of reflectance peak position. The different value between reflectance peaks (Δλmax) from 10% to 100% of water was 170 nm, which was higher than some reports.21,27
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| Fig. 6 Photographs and reflection spectra of PAM (a) and PAM/PMAA (b) IOHs at different volume ratios of methanol and water. Reflectance peak value (c) of IOHs. The cross-linker content is 4%, and the size of PSA is 226 nm. | |
After introduced MAA into the PAM IOH, the reflectance response of the PAM/PMAA IOHs to methanol also showed the red-shift with decreasing the methanol concentration, which was similar to the PAM IOH (Fig. 6b). While the color of the PAM/PMAA IOHs changed from green, dark green, yellow, pink to light red during the methanol concentration decreasing, which were easier to be distinguished in naked eyes, compared with the PAM IOHs. Especially, Δλmax (∼215 nm) of PAM/PMAA was higher than that of PAM (Fig. 6c), suggesting the higher methanol sensitivity of the PAM/PMAA IOHs. The reason for the phenomenon might be attributed to the hydrogen bonds among PAM, PMAA and water, leading the swelling of the IOHs. Existing methanol in water might induce the shrinkage of the polymer chains (Fig. S5†), special for PAM. With an increase in methanol concentration, the polymer chains in the IOHs were shrunk much, leading the pore among the IOHs became smaller and decreasing the Bragg diffraction wavelengths. On opposite, with increasing the water composition, IOHs could swell to form large pores, resulting in the high Bragg diffraction. Thus, the PAM/PMAA IOHs showed high response property to methanol.
The MAA composition influenced on the solvent response of IOHs was also detected, as shown in Fig. 7. All the IOHs with various PMAA compositions showed the distinct color change depending on the methanol concentration (Fig. 7a–c). For the higher MAA composition, the color change and Δλmax of the IOHs were larger (Fig. 7d). Increasing the PMAA chains, the interactions between MAA and water became larger, leading the swell of the PAM/PMAA IOHs. Therefore, the addition of the MAA in the IOHs could enhance the solvent sensitivity.
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| Fig. 7 Photographs (a–c) and reflectance peak value (d) of PAM/PMAA IOHs with different MAA compositions of 0.05MAA (a), 0.1MAA (b) and 0.2MAA (c) in methanol/water mixed solvent with different volume ratios of water and methanol. The values of 10% to 100% from left to right mean the content of water. | |
Moreover, the PAM/PMAA IOHs also showed the changes of the color and reflectance peak value by immersed the IOHs into the ethanol/water mixed solvent (Fig. S6†), which showed similar properties to the IOHs in methanol/water mixed solvent. These results indicated that the prepared PAM and PAM/PMAA IOHs could response to several external stimuli.
Conclusions
In summary, “Sandwich” method is a suitable way to prepare the PAM and PAM/PMAA IOHs using PSA as opal template. The obtained IOHs were a three-dimensional ordered structure and extremely uniform pore size without over-layers and defects and had optically tunable property. The structural color of the IOHs could be controlled in the visible light range by adjusting the synthesis conditions of the hydrogels such as the PSA particle size and MAA content, while independing on the cross-linker compositions. The both kinds of IOHs responded to various external stimuli such as pH, methanol and ethanol. The Bragg diffraction peak was red-shifted in the range of pH at 2.0–8.5, while blue-shifted in pH above 8.5 and in high composition of methanol and ethanol, due to various swell degrees in different media. Moreover, the response to the solvents of the IOHs was enhanced (Δλmax = ∼215 nm) by the introduced MAA into the IOHs. The obtained the PAM and PAM/PMAA IOHs had the potential application as the sensors, photo-catalysts, and optical switches.
Acknowledgements
This study was supported by the National Nature Science Foundation of China (No. 21571084), the Fundamental Research Funds for the Central Universities (JUSRP51408B) and MOE & SAFEA for the 111 Project (B13025).
Notes and references
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Footnote |
† Electronic supplementary information (ESI) available: Characterization of PSA microsphere, cross-linker effect of the IOHs and swelling properties of the IOHs. See DOI: 10.1039/c6ra18738b |
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