Frozen “Tofu” Effect: Engineered Pores of Hydrophilic Nanoporous

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Frozen “Tofu” Effect: Engineered Pores of Hydrophilic Nanoporous Materials Dengxin Ji,† Haomin Song,† Borui Chen,† Feng Zhang,‡ Alec R. Cheney,† Nan Zhang,† Xie Zeng,† John D. Atkinson,§ Chi Zhou,‡ Alexander N. Cartwright,*,† and Qiaoqiang Gan*,† †

Department of Electrical Engineering, ‡Department of Industrial and Systems Engineering, and §Department of Civil, Structural and Environmental Engineering, The State University of New York at Buffalo, Buffalo, New York 14260, United States S Supporting Information *

ABSTRACT: Frozen tofu is a famous Asian food made by freezing soft bean curds, which are naturally porous to store flavor and nutrients. When the narrow pores of the soft bean curd are saturated with water and then frozen, pore widths expand to generate a completely new porous structurefrozen tofu has visibly wider pores than the initial bean curd. Intriguingly, this principle can be generalized and applied to manipulate micro/ nanopores of functional porous materials. In this work, we will manipulate the pore size of nanoporous polymeric photonic crystals based on the phase change between water and ice. Wetdrying and freeze-drying methods were applied to shrink or expand the pore size intentionally. This principle is validated by directly observing the optical reflection peak shift of the material. Owing to the change in pore size, the reflection peak of the polymeric photonic crystal structure can be permanently, and intentionally, tuned. This simple but elegant mechanism is promising for the development of smart materials/devices for applications ranging from oil/water membrane separations, health monitoring, and medical diagnostics to environmental monitoring, anticounterfeiting, and smart windows.

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INTRODUCTION Nanoporous materials are widely used for water and air pollution control, environmental catalysis, industrial separations, mechanical buffering, thermal and acoustic insulation, implantable bioscaffolds, supercapacitors, and batteries.1−5 For each of these applications, the pore width of the nanoporous materials is of the utmost importance, often for controlling their performance. Porous materials are also established as optical elements for colorimetric humidity and/or chemical sensors.6−8 For instance, micro-/nanoporous photonic crystals (PhCs) have been developed using self-assembly9−11 and holographic lithography12−14 methods to spatially tune the distribution of polymer-rich (P-rich) and void-rich (V-rich) regions within subwavelength dimensions. Therefore, the created photonic bandgaps can result in controlled reflection at selected wavelengths.15−17 Our previously reported spatially graded polymeric PhC structure is an example.18−20 For a given porous material, however, pore size distributions cannot be easily tailored or intentionally modified, especially postpreparation. This is due to low Young’s moduli and thermal expansion coefficients of the materials. A method providing intentional pore width manipulation has not been developed. This is a grand challenge for nanomanufacturing and materials engineering, at both fundamental and applied levels. Inspired by the frozen tofu mechanism, in this work, we report a method to address this technical challenge for © 2017 American Chemical Society

nanoporous material engineering. This method exploits the volume change of liquids during a phase change. As long as the liquid can diffuse into the pores of hydrophilic materials, one can manipulate the system temperature to control liquid-tosolid phase changes and introduce a volume change of micro-/ nanopores. This technology is developed to tune the microscopic pore size of hydrophilic porous materials. Using liquid mixtures with different volume change properties, the desired volume change can therefore be controlled, enabling accurate control of resulting pore sizes. Figure 1a shows the morphology change when going from bean curd to frozen tofu. This process is graphically illustrated by the cross-sectional images in Figure 1b. To demonstrate its feasibility, in this work, we select nanoporous polymer PhC gratings15,18−20 as a test bed. Specifically, we will fill these hydrophilic porous polymeric materials with water and control its adjacent environment temperature to change the volume of ice in the nano-/ micropores of the material. Owing to the phase change volume expansion of water droplets isolated in nanopores, nanopore widths are permanently increased by ∼10%/phase change cycles.21 As a result, the pore size can be intentionally manipulated, causing a reflection color change for the Received: July 3, 2017 Accepted: August 14, 2017 Published: August 23, 2017 4838

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Figure 1. (a) Photograph of the bean curd and frozen tofu showing different pore sizes because of the volume change from water to ice. (b) Conceptual illustration of pore size manipulation of nano/microporous materials using water-to-ice phase change.

Figure 2. (a) Reflection spectra of the PhC sample with freeze-wet-drying treatments. The dashed lines and numbers on the top panel indicate the peak wavelength position for each step. (b) Photographs of the original sample (upper panel) and the sample after four cycles of freeze-wet-drying treatments (lower panel). (c) Cross-sectional scanning electron microscopy (SEM) image of reflection PhC grating and (d) transmission electron microscopy (TEM) image of PhC grating. (e) Extracted thicknesses of P-rich region (tp), V-rich region (tv), and period (P). (f) Change ratio of tv, tp, and P after each cycle.

PhC sample used in this experiment relies on phase separation materials.15,18 During the photopatterning process, the prepolymer syrup was exposed to interference patterns introduced by the incident laser beam. In the constructive interference regions, the chain extension (polymerization) between the monomers will occur, thus repelling the solvent to the destructive interference regions. Postcure using an UV lamp, the solvent will evaporate upon removing the cover glass slide. The constructive interference regions are called P-rich regions because they are mainly filled with polymer materials, and the destructive interference regions are called V-rich regions because more voids are located in these regions. Owing to the spatially manipulated refractive index distribution of Vrich and P-rich regions within subwavelength dimensions of porous polymer PhC structures, one can control their reflection

polymeric PhC structure. By monitoring this reflection peak wavelength shift during freezing−melting cycles, one can track changes to the pore size of the material. Importantly, this mechanism is general, only requiring water adsorption in pores. Previously viewed as challenging, straightforward control of water and temperature enables accurate engineering of micro-/ nanopores. Pores with selected dimensions pave the way toward the development of smart materials/devices for applications ranging from water/oil membrane separation and biomedical sensing to environmental monitoring, anticounterfeiting, and smart windows.

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RESULTS Frozen Tofu Manipulation of Nanoporous Polymer PhC Structures under Wet-Drying. The porous polymeric 4839

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Table 1. Geometric Parameter Extraction of the PhC Structure Processed with Frozen-Wet-Drying λnorm (nm)

region

origin

548.8 ± 0.62

WD cycle 1

546.9 ± 0.46

WD cycle 2

544.7 ± 0.71

WD cycle 3

542.5 ± 0.54

WD cycle 4

541.1 ± 0.60

P-rich V-rich P-rich V-rich P-rich V-rich P-rich V-rich P-rich V-rich

status

n 1.48 1.21 1.49 1.22 1.5 1.23 1.51 1.25 1.51 1.26

± ± ± ± ± ± ± ± ± ±

0.0016 0.012 0.0010 0.009 0.0019 0.015 0.0014 0.011 0.0016 0.013

t (nm)

P (nm)

σp (%)

σv (%)

Uσ (%)

± ± ± ± ± ± ± ± ± ±

189.3 ± 0.42

89.9 39.1 91.7 41.1 93.6 42.9 95.4 46.7 95.4 48.6

10.1 60.9 8.3 58.9 6.4 57.1 4.6 53.3 4.6 51.4

±0.30 ±2.3 ±0.19 ±1.7 ±0.36 ±2.8 ±0.26 ±2.1 ±0.30 ±2.4

164.6 24.7 163.6 23.6 162 23.1 160.4 22.5 160.3 21.9

0.19 0.23 0.11 0.14 0.22 0.26 0.15 0.18 0.17 0.20

187.2 ± 0.25 185.1 ± 0.48 182.9 ± 0.33 182.2 ± 0.37

rich-modulated structure with 15−150 nm pores. On the basis of this TEM image, approximately 30% of the total area is occupied by pores. However, because of its random and rough nature, it is difficult to extract an overall change in period, pore size, or porosity of a bulk sample. It is also questionable to conclude that a feature size is consistently changing based on a single 190 nm slice because external stimulation (e.g., cutting, milling, etc.) may impact the morphology of the sample. To the greatest extent possible, optical techniques provide an indirect and nondestructive way to characterize porous materials. High wavelength resolution (∼0.1 nm) is helpful for discerning small changes in average pore size of a thin film over small or large areas, as observed here and discussed below. According to previously reported matrix analyses on the optical properties of PhC structures,23,24 one can extract averaged geometric parameters based on optical spectra (see section 3 in the Supporting Information for details). Following this procedure, thicknesses (t) and effective refractive indices (n) for P-rich and V-rich regions before and after each freezewet-drying cycle were determined (columns “n” and “t” in Table 1). Importantly, the extracted data accurately measures geometric property changes within the P-rich and V-rich regions of a bulk sample [column “t (nm)” in Table 1] over a large area. This cannot currently be accomplished using microscopic characterization methods. The extracted period of the grating is changed from 189.3 to 187.2 nm (cycle 1), 185.1 nm (cycle 2), and 182.9 nm (cycle 3) as shown in column “P (nm)” in Table 1. The structure is nearly saturated (182.2 nm) in cycle 4, which is consistent with observations of sample cracking because of mechanical limitations (lower panel in Figure 2b). To quantify geometric parameter changes, thicknesses of the P-rich and V-rich regions (tp and tv, respectively) and the period (P = tp + tv) are plotted in Figure 2e, showing that the polymer grating shrinks after each freezewet-drying cycle. On the basis of change ratios for these regions (Figure 2f), it is clear that the V-rich region is more impacted (the red curve), confirming that the pore size change is the major physical mechanism. To further validate this conclusion, component volume fractions of P-rich and V-rich regions are extracted using the two-component Bruggeman model25

peak, which is an indirect indication of pore size manipulation. Importantly, if the dimension of nanopores can be manipulated, the effective refractive index of each layer can be changed and will result in a spectral position shift of the reflection peak. To validate this prediction, we employed the holographic lithography method18,20 to fabricate a nanoporous polymeric PhC structure (see section 1 in the Supporting Information for fabrication details). Its initial reflection peak is located at 548.8 nm (curve 1 in Figure 2a for the normalized reflection spectrum; the raw data are shown in Figure S1). The sample was placed on top of a thermoelectric cooler for in situ temperature control. When a drop of water was placed on top of the sample, it immediately dispersed into the film, filling the pores. As a result, the reflection peak shifted up to 573.1 nm because the refractive index of the voids changed (curve 2 in Figure 2a). Subsequently, the sample was cooled to −5 °C; ice formation was observed with an optical microscope (see Movie S1). In this case, the reflection peak shifted to 574.2 nm (curve 3 in Figure 2a), indicating a change of the effective refractive index in the V-rich region. When the sample was warmed, the ice melted and its reflection peak further shifted to 577.7 nm (curve 4 in Figure 2a) because the newly expanded pores are filled with water, which has a larger refractive index than ice. More intriguingly, when the sample is dried, the reflection peak shifted to 546.9 nm (curve 5 in Figure 2a), which is smaller than the original reflection peak (548.8 nm), indicating a permanent change to the porous PhC structure. With cycling, further shifts toward the two ends compared with the previous cycle were observed (i.e., curves 6−9 for cycle 2 in Figure 2a and curves 10−17 in Figure S2 for cycles three and four). For clarity, peak wavelengths are highlighted in the top panel. Results confirm that pore size can be manipulated by repeating the ice-expansion process, until the sample cracks after four cycles because of mechanical limitations of the porous network of the polymer (Figure 2b). It should be noted that if the sample is never frozen, its optical reflection spectrum can only be tuned between curves 1 and 2 (i.e., from 548.8 to 573.1 nm). No further shifts are observed if the nanopores do not expand. Next, we analyze these optical signals in detail to interpret relationships between the reflection peak shift and the geometric features of PhC materials. Quantitative Analysis. Owing to randomness in the structure of many porous polymeric materials, it is challenging to characterize the pores using direct measurements (e.g., SEM, TEM, etc.). As shown in Figure 2c, SEM analysis of a clipped sample shows that the period of the as-produced polymer grating is ∼200 nm. Following established procedures,22 an ∼190 nm thick polymer grating slice is prepared for TEM characterization (Figure 2d), showing a periodic V-rich- and P-

σpGp + σvGv = 0

(1)

Here, σp, and σv are volume fractions of polymer and voids, respectively. Gp =

n p2 − n2 n p 2 + 2n 2

, Gv =

n v 2 − n2 n v 2 + 2n 2

; np, and nv are the

refractive constants for polymer and voids, respectively (i.e., np = 1.51 and nv = 1).15 n is the effective index of the porous polymeric material extracted in the previous paragraph. As 4840

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Figure 3. (a) Reflection spectra of the PhC sample with FD treatment. The dashed lines and numbers on the top panel indicate the peak wavelength position for each step. (b) Photographs of the original sample (left panel) and the sample after 4 cycles of frozen-wet-drying processes (right panel). (c) Extracted thicknesses of P-rich region (tp), V-rich region (tv), and period (P), corresponding to the shifted peak wavelengths. (d) Change ratio of tv, tp, and P after each FD cycle.

Table 2. Geometric Parameter Extraction of the PhC Structure Processed with FD λnorm (nm)

region

origin

622.65 ± 0.43

FD cycle 1

631.14 ± 0.81

FD cycle 2

641.97 ± 1.26

FD cycle 3

654.32 ± 0.65

FD cycle 4

667.65 ± 0.72

P-rich V-rich P-rich V-rich P-rich V-rich P-rich V-rich P-rich V-rich

status

n 1.49 1.23 1.47 1.22 1.46 1.21 1.44 1.19 1.43 1.17

± ± ± ± ± ± ± ± ± ±

0.0008 0.0063 0.0018 0.014 0.0029 0.021 0.0014 0.010 0.0016 0.011

t (nm)

P (nm)

σp (%)

σv (%)

Uσ (%)

± ± ± ± ± ± ± ± ± ±

213.0 ± 0.24

91.7 42.9 88.1 41.1 86.2 39.1 82.4 35.4 80.6 31.6

8.3 57.1 11.9 58.9 13.8 60.9 17.6 64.6 19.4 68.4

±0.15 ±1.2 ±0.34 ±2.7 ±0.54 ±4.0 ±0.26 ±1.9 ±0.30 ±2.1

185.9 27.1 190.7 28.1 194.5 29.8 201.0 30.9 206.0 32.7

shown in Table 1, the volume fraction of voids, σv, also decreases after each wet-drying (WD) cycle, attributed to the surface tension of water during evaporation. Although nanopores expand when water freezes, their diameters ultimately decrease because of surface-tension-induced collapse.26 Therefore, the effective refractive index of the V-rich region increases and its effective thickness decreases after the freeze-wet-drying process, resulting in a blue shift of the resonant reflection wavelength (i.e., peak 1−5−9 in Figure 2a). Understanding this freeze-wet-drying process provides an opportunity for selective manipulation of pore dimensions in nanoporous materials. If surface tension-induced shrinking is avoided, pore widths can be permanently increased, countering the shrinking observed in previous results. In the case of expanded pores, the reflection peak shift would be the opposite of what is observed in Figure 2a. Accordingly, this technique is expected to provide opportunities to intentionally increase or decrease pore widths, and the described optical method allows us to quantify these changes. To validate this hypothesis, in the next section, we employed freeze-drying (FD, i.e., ice removal that bypasses the liquid phase to prevent surface tensioninduced pore collapse) and then characterized the reflection peak shift of the polymer PhC grating. Freeze-Dried Porous PhCs: Pore Expansion. FD (also known as lyophilization or cryodesiccation) is a dehydration process based on sublimation of liquid below its freezing

0.11 0.13 0.24 0.29 0.40 0.48 0.20 0.25 0.23 0.28

218.8 ± 0.53 224.3 ± 0.88 231.9 ± 0.45 238.7 ± 0.51

point.27 It has been used for the manipulation of nanoporous materials via hydrogel expansion.28−31 Because ice in the nanopores will not melt before drying, the expanded pores should not collapse and the effective refractive index in the Vrich region of the polymer PhCs should decrease. In this case, the reflection peak should shift to longer wavelengths. To validate this hypothesis, we selected a polymer PhC grating with reflection peak at 622.7 nm (curve 1 in Figure 3a). When submerged in water, the reflection peak shifts to 671.7 nm (curve 2 in Figure 3a). This wet sample was then freeze-dried for 24 h (see Methods for details). As a result, its reflection peak shifted to 631.1 nm (curve 3 in Figure 3a). This shift to a longer wavelength is the opposite of what was observed for the freeze-wet-drying sample in Figure 2a. This supports our hypothesis: nanopores were permanently expanded by ice, with no shrinkage during sublimation. Importantly, this process can be repeated to further expand the nanopores, by ∼10% per cycle (curves 4−6 in Figure 3a). The sample cracked during cycles 3−4 because of the mechanical expansion limit of this polymer (Figure 3b). To better interpret the observed optical reflection peak shift of the freeze-dried porous PhC sample, we again performed geometric data extraction using the aforementioned processing procedure (Table 2). Similarly, extracted thicknesses of the P-rich region (tp), V-rich region (tv), and period (P) are plotted in Figure 3c, showing increases after each FD cycle. Void volume fractions in both P-rich and V-rich 4841

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Figure 4. (a) Reflection spectra of the PhC sample with water condensation freeze-wet-drying treatment. The dashed lines and numbers on the top panel indicate the peak wavelength position for each step. (b,c) Microscope images of the PhC sample under (b) 25 and (c) 2 °C for ∼6 min.

4 min ± 2 s were used to obtain PhCs with partially filled pores. Samples were then transferred immediately (5 ± 0.5 s) into the freeze-drier at −28 °C. After 24 h, reflection peaks for the three samples shifted to 625.5 nm (2 min sample, curve 4) and 633.0 nm (4 min sample, curve 5). Because sample pores were manipulated within the mechanical expansion limit of the solid, this accurate and intentional pore width manipulation is repeatable. To demonstrate this tunability, we repeated this process with another three cycles. The data for cycle 2 are shown in Figure 4a (i.e., curves 7−8), and data for cycles 3 and 4 are shown in Figure S4 in the Supporting Information. On average, the WD process shifted the reflection peak back to 616.1 ± 1.3 nm (see curve 6 for cycle 2), and then partial porefilling followed by FD shifted the reflection peaks to 625.1 ± 0.8 nm (2 min, see curve 7 for cycle 2) and 633.3 ± 1.04 nm (4 min, see curve 8 for cycle 2). Importantly, after the four-cycle treatment, the sample did not crack, contrasting previous observation shown in Figures 2b and 3b. Therefore, this experimental data demonstrated that the optical property of the polymeric PhCs can be tuned multiple times using the partial filling strategy when staying within the mechanical expansion limit of the material. Intriguingly, although it has been recognized that there are rich new physics related to microscopic freezing, melting, and structure of water/ice in the nanoporous material,34,35 there are no nanomaterial engineering studies using water-to-ice phase change. In particular, our experimental results indicate a new research topic on dynamics regarding the permeation,36 nucleation, and growth37 of water/ice in partially filled nanopores. In conclusion, we developed a nanomanufacturing process to accurately manipulate pore sizes of nanoporous materials. Building upon our previous progress in holographic patterning of nanoporous polymers,18−20 this work used porous photonic crystal structures to demonstrate the feasibility of postproduction manipulation of nanopores. Reflection resonances of polymeric PhC structures were intentionally decreased or increased using WD and FD processes, respectively. Using matrix analysis of the optical reflection properties of the PhC structure, geometric features of the P-rich and V-rich regions were extracted to show the manipulation of spatial feature sizes. Owing to the volume change between water and ice in pores, the dimension of porous materials can be controlled.

regions (Table 2) show opposite trends compared to Table 1, supporting proposed mechanistic differences associated with WD and FD. As shown in Figure 3d, major void volume changes again occurred in the V-rich region (the red curve), demonstrating the ice-induced nanopore expansion. Although FD methods were widely used for the fabrication of graphene oxide porous monoliths,32,33 there is no attempt to intentionally tune the pore sizes after the material/structure fabrication. In particular, in these reports, the aqueous graphene oxide solution was frozen followed by FD to achieve the porous monoliths which is a replica of the ice crystal generated in the freezing process. Depending on the temperature, the size of the ice crystal varies from tens of microns to hundreds of microns. Therefore, the pore size of the final monolith is in the micronscale range. There is no literature manipulated nanometer scale pores, which will be demonstrated in this next section. Accurate Spectral Tunability Using Partially Filled Pores. On the basis of the aforementioned WD and FD processes, we have demonstrated methods to intentionally shrink and expand the pores of polymeric PhC films. The reflection peak wavelength was changed after each cycle, an indirect confirmation of the intended pore width manipulation. However, this peak shift is not continuous because of the given volume expansion (∼10% per cycle) during the water-to-ice phase change. In this section, we aim to demonstrate more accurate control over pore size changes using partially filled nanopores. To do this, we selected a polymer PhC grating with an original reflection peak at 621.0 nm (curve 1, Figure 4a). After filling with water and FD, its reflection peak shifted to 637.1 nm (curve 2). The now-dry sample then underwent a WD cycle, causing the reflection peak to shift back to 617.5 nm (curve 3). To obtain accurate tuning between these two states (i.e., curves 2 and 3), we then placed the sample on a thermoelectric cooler under ambient conditions (relative humidity of 22%). When the temperature dropped to 2 °C, the reflection color of the sample obviously changed (from Figure 4b to 4c, see also the first 4 min in Movie S2). Small droplets were observed on the surface of the sample after ∼5 min, indicating that the pores were saturated with moisture extracted from the air (Figure 4c shows the image after 6 min). Knowing the time required to completely saturate the pores of the sample (5−6 min, Movie S2), condensation times of 2 and 4842

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Importantly, by controlling moisture diffusion and partially filled nanopores, porous structures were accurately and intentionally manipulated. Therefore, the freezing and melting phenomenon associated with water confined in nanopores provides a unique opportunity to manipulate different-sized pores, thereby providing a new approach to address grand challenges in nanomanufacturing and materials engineering. One can start from a given polymeric photonic crystal structure and manipulate its optical property using post-WD and postFD processing. In this case, we do not have to change the optical patterning system and run the fabrication for new samples. This property is extremely important for real applications, for example, adsorption of gas/aqueous phase pollutants using nanoporous materials. The efficiency highly depends on the pore size distribution because of the selective adsorption of nanoporous materials. Thus, intentionally tuned pores can significantly enhance the performance of gas/aqueous filters. This work also enables the development of smart materials that can be readily exploited in scalable devices for industrial oil/water/gas separations, health monitoring, medical diagnostics, environmental monitoring, anticounterfeiting, and smart windows, so the expected impact of the described methodology is expansive.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: anc@buffalo.edu (A.N.C.). *E-mail: qqgan@buffalo.edu (Q.G.). ORCID

Dengxin Ji: 0000-0002-3987-1407 Nan Zhang: 0000-0002-1517-333X John D. Atkinson: 0000-0003-1545-2213 Author Contributions

D.J. and H.S. contributed equally to this work. Q.G., A.N.C., and C.Z. conceived the idea and supervised the project. Q.G., A.N.C, C.Z., D.J., H.S., B.C., F.Y., A.R.C., F.Z., N.Z., X.Z., and J.D.A. designed, fabricated, and characterized the treated porous PhC structures. D.J., H.S., and Q.G. performed numerical simulation and data analysis. D.J., H.S., A.D.A., C.Z., A.N.C., and Q.G. wrote the manuscript. All reviewed the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Q.G. acknowledges funding support from the National Science Foundation (grant nos. CMMI1562057 and ECCS1507312). N.Z. acknowledges the financial support from Chinese Scholarship Council (CSC).

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METHODS FD Process. The as-prepared sample was placed in the freeze-dry system (FreeZone Triad model 74000 Series, Labconco) to remove the water by freezing and vacuum drying for around 24 h. Specifically, the sample was further frozen at −30 °C for 6 h under vacuum as low as 0.018 mbar; then, the temperature was ramped to −10 °C at a rate of 0.25 °C/min and held at this temperature for another 12 h; and finally, the chamber temperature was elevated to room temperature at a rate of 1 °C/min and held for 6 h to allow the solvent content to be completely sublimated. The vacuum of the system was maintained at 0.018−0.014 mBar during the entire process. Optical Characterization. The reflection spectra of polymeric PhCs at normal incidence were characterized using a microscopic Fourier transform infrared spectroscope (Bruker, VERTEX 70 + Hyperion 1000). The observation area for each sample was 500 μm × 500 μm. The angle-dependent spectra were measured by a portable spectrometer (Jaz, Ocean Optics). A stepping-motor-driven rotation stage (SGSP-120YAW, Sigma Koki Co.) was used to precisely control the incident angle of a broadband white light source (EQ-99-FC, Energetiq) assembled with a collimator (RC04FC-P01, Thorlabs).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00901. Fabrication of polymer porous photonic crystal; the normalized reflection spectra for third and fourth cycles of porous polymer PhCs using WD; detailed description of extraction of geometric features of polymer porous PhCs using matrix analysis; and reflection spectra of accurate spectral tuning of porous polymer PhCs using partially filled frozen “tofu” process (PDF) Ice formation on polymer porous photonic crystals (AVI) Color change in partially filled pores of polymer porous photonic crystals (AVI) 4843

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DOI: 10.1021/acsomega.7b00901 ACS Omega 2017, 2, 4838−4844