Photoresponsive Surface Wrinkle Morphologies in Liquid Crystalline

Article pubs.acs.org/Macromolecules

Photoresponsive Surface Wrinkle Morphologies in Liquid Crystalline Polymer Films Takahiro Takeshima,† Wan-yu Liao,† Yuki Nagashima,† Koichiro Beppu,† Mitsuo Hara,† Shusaku Nagano,*,‡ and Takahiro Seki*,† †

Department of Molecular Design and Engineering, Graduate School of Engineering, and ‡Venture Business Laboratory, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 31, 2015 | http://pubs.acs.org Publication Date (Web): August 31, 2015 | doi: 10.1021/acs.macromol.5b01577

S Supporting Information *

ABSTRACT: Laminated bilayer films comprised of photoresponsive azobenzene-containing liquid crystalline polymer skin layers adhered on a thick elastomeric substrate were prepared, and the wrinkle formation upon uniaxial compression was investigated. Irradiation with UV light at 365 nm led to the disappearance of the wrinkle or reduction of wrinkle wavelength, depending on the content of azobenzene unit in the polymer. Such photoresponsive modulations of wrinkle formation could be well correlated with the photoinduced changes in the Young’s modulus determined by indentation− retraction force curve measurements using an AFM cantilever. Additionally, the thickness modulation of the skin layer caused by the photoinduced mass migration can also be applied to modify the wrinkle wavelength. These photoresponsive surface wrinkle modulations are anticipated to offer new possibilities for the surface microfabrication technology.

1. INTRODUCTION Fabrication and patterning of micro- and nanostructured surfaces are becoming increasingly important from the technological viewpoints in wetting, adhesion, optical, biological, and mechanical activities and lithographic technologies.1,2 Surface wrinkles are ubiquitous in nature and provide versatile and inexpensive tools for microscale surface fabrication.2−8 Surface wrinkles are driven by mechanical instabilities upon lateral compression beyond the buckling stress and typically fabricated with laminated bilayer films possessing a hard skin layer attached or deposited onto a soft elastomeric substrate. The wrinkle wavelength (λ) is given by the equation ⎛ Ef ⎞1/3 λ = 2πh⎜ ⎟ ⎝ Es ⎠

represents a particularly attractive stimulus because it enables rapid changes with high spatial resolution in noncontact ways. Yoon et al.13 studied light-triggered erasure of creased hydrogel surfaces utilizing a photothermal effect caused by iron oxide nanoparticles embedded in poly(N-isopropylacrylamide) gel sheets. Self-wrinkling by UV-initiated cross-linking (curing) of polymer thin films has been investigated in radical polymerization14 and thiol−ene condensation15 systems. Azobenzene (Az) is a widely used photochromic unit for modification and switching of material properties. Actually, photoinduced surface wrinkling has been observed in a glassy nematic film16 and molecular amorphous glass17 systems. It is anticipated that when the Az units are combined with liquid crystalline (LC) polymers, even more drastic mechanical effects are expected because cooperative motions and amplifications of molecular orientation are involved.18−28 We report herein photoinduced modulations of wrinkle morphologies in bilayer systems comprising Az-containing LC polymers as the surface skin layer and cross-linked poly(dimethylsiloxane) (PDMS) as the base substrate. The chemical structures of the LC polymers are displayed in Scheme 1 together with their abbreviations. Az-containing and cyanobiphenyl (CB)-containing LC homopolymers and LC copolymers of the two components were employed in this work. According to the eq 1 two strategies can be supposed to

(1)

where Ef and Es denote Young’s moduli of the hard skin layer and of the soft substrate, respectively, and h the thickness of the hard layer. For the surface skin layers, metal deposited films such as Au, Pt, and Ag layers, amorphous polymers such as polystyrene, and oxidized surface of elastomer films have been frequently used.4−6 The above relationship allows facile evaluation of the Young’s modulus of the materials. It has been shown that the wrinkling process provides useful information on the physical properties of thin films.5,9,10 Recent efforts have also been devoted to stimuli-responsive wrinkle formation,11,12 which are expected to provide a new direction in the research of stimuli-responsive materials. Light © XXXX American Chemical Society

Received: July 16, 2015 Revised: August 24, 2015

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DOI: 10.1021/acs.macromol.5b01577 Macromolecules XXXX, XXX, XXX−XXX

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water at room temperature for 10 h. This procedure released the LC film from the silicon wafer and transferred it to the PDMS film. The thickness of the surface LC layer was evaluated by atomic force microscopy (AFM) using a MPF-3D system (Asylum Research, Oxford Instruments). The lateral uniaxial compression of the bilayer film was performed with a Hoffman-type pinchcock to a 10% length reduction for all samples (Figure S2). 2.3. Measurements and Methods. UV−vis absorption spectra were taken on an Agilent 8453 spectrometer. The morphology of surface was observed with an optical microscope (BX51-P, Olympus) and a white light interferometric microscope (BW-S501, Nikon Instruments). The wrinkle wavelength was evaluated from the microscopic image via fast Fourier transformation using ImageJ software provided from National Institutes of Health. UV light (365 nm) irradiation was performed with a Supercure E203 (San-ei electronics) passing through a combination of glass filters of UV-35/UV35D (Toshiba). The intensity was 1.0 mW cm−2. The evaluation of Young’s modulus was achieved by indentation of AFM cantilever. A target film was prepared on a VUV-treated silicon wafer surface by spin-casting. Force curve profiles were obtained in the indentation and retraction motions of an Al-coated cantilever (SDSphere-NCH-S-10, NANOSENSORS; tip radius: 800 nm; force constant: 42 N m−1) attached to an AFM apparatus (MFP-3D, Asylum Research, Oxford Instruments). In one measurement, 256 force curves were obtained in a 2 μm × 2 μm area (16 × 16 points) at a lateral scanning speed of 100 nm s−1. To evaluate the modulus of a target polymer sample, three sets of above procedures were carried out. The spring constant was calibrated in the measurement for a mica surface. The contact angle of water droplet was measured with a CA-XP (Kyowa Interface). Averaged values were obtained after at least five measurements for one sample.


Scheme 1. Chemical Structure of Polymers

modify the wrinkle wavelength: (i) photoinduced change in Young’s modulus of the hard layer (Ef) and (ii) photoinduced modulation of thickness of the film (h). Both parameters can be controlled by photoirradiation. The involvement of the cis-Az isomer softens the film and further leads to a phase transition to the isotropic state when the cis content exceed a critical cis/ trans ratio.18 Also, patterned irradiation by interference laser beams or irradiation through a photomask can induce mass migration to generate surface relief structures.26,29 This process can be utilized to modify the thickness of skin layer. Regarding the photoresponsive wrinkle system using a Azcontaining LC film, Monobe and Ohzono et al.30 reported the liquid filament formation due to photoinduced wettability change. In this case, the wrinkle wavelength is fixed by an underlying polyimide film. As a related LC system, we further note that Agrawal et al.31 reported thermally induced change in the wrinkle formation in polystyrene/LC elastomer films ascribed to the LC to isotropic thermal phase transition.

3. RESULTS AND DISCUSSION 3.1. Polymer Synthesis. The molecular mass data of LC polymers used in this study are listed in Table 1. The molecular weight (Mn) ranged (1.8−7.6) × 104 with the polydispersity ranging 1.13−1.61.

2. EXPERIMENTAL SECTION

Table 1. Syntheses and Characterizations of Polymers

2.1. Polymer Synthesis. Synthesis procedures of monomers and homopolymers of PAz32,33 and PCB34 were reported previously. The polymers were synthesized by the atom transfer radical polymerization (ATRP) method. The polymerization conditions for the random copolymers containing both mesogens, P(Az50-CB50) and P(Az25CB75), are described in the Supporting Information. The number-average molecular weight (Mn) was estimated by 1H NMR using a JNM-GSX270 (JEOL). The polydispersity index (Mw/ Mn) was evaluated by gel permeation chromatography composed of a high performance liquid chromatography (DS-4/UV-41, Shodex) connected with columns of KF-803L and KF-804L (Shodex). A UV detector at 254 nm was used for the measurements. The calibration curve was made using TSK polystyrene standards. Thermal phase transition behavior was evaluated by differential scanning calorimeter (DSC) on a Q200 (TA Instruments) under nitrogen gas atmosphere. The temperature scanning ratio was 2 °C min−1. 2.2. Film Preparations. Elastomeric PDMS films were prepared using Sylgard184 silicone elastomer kit (Dow Corning Toray). The base and curing agents were mixed at 10:1 ratio by weight and stirred at room temperature under reduced pressure. This precursor material was sandwiched between two Teflon sheets using a 4 mm gap spacer and kept at 75 °C for 4 h to yield a cross-linked PDMS elastomer film. Adhesion of a thin LC films onto a PDMS substrate film was achieved as follows.35 A silicon wafer was exposed to vacuum UV light (VUV, 172 nm) at 1.0 × 103 MPa using a UER20-172 V (Ushio Inc.) to make the surface hydrophilic.36 A LC polymer film was prepared by spincasting from a chloroform solution. A centimeter-sized wafer piece was cleaved from the spin-coated wafer and placed onto the PDMS film with the film side down. The laminated film was immersed into

polymer

feeding monomer molar ratio (Az:CB)

unit number of obtained polymer (m, n)d

Mne

Mw/Mnf

100:0a 50:50b

(154, 0) (60, 60)

7.6 × 104 5.2 × 104

1.46 1.61

25:75b

(34, 104)

5.5 × 104

1.54

0:100c

(0, 142)

1.8 × 104

1.13

PAz P(Az50CB50) P(Az25CB75) PCB a

b

Reference 33. Polymerization conditions are given in the Supporting Information. cReference 34. dSee the chemical structure in Scheme 1. e Estimated from 1H NMR. fEstimated from GPC.

The thermal phase transition properties of the polymers are summarized in Table 2. The phase characterization was made by DSC and polarized optical microscopic observations. The Table 2. Thermal Phase Transition Behavior on the Cooling Process polymer PAz P(Az50-CB50) P(Az25-CB75) PCB B

phase change behavior glass−59 °C−smectic isotropic glass−50 °C−smectic glass−46 °C−smectic glass−48 °C−smectic

C−92 °C−smectic A−117 °C− A−134 °C−isotropic A−123 °C−isotropic A−103 °C−isotropic DOI: 10.1021/acs.macromol.5b01577 Macromolecules XXXX, XXX, XXX−XXX

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due to the photoinduced smectic LC to isotropic phase transition of the PAz thin film as previously confirmed.29 In the cis-Az rich state, further compressions to 20−30% reduction did not lead to the wrinkle formation, indicating that the inability to form wrinkles is due to sufficient softening of the PAz layer upon UV light irradiation and not due to a shift in critical buckling stress. This will be quantitatively discussed in the section 3.3. UV light spot exposure was carried out also under the compressed condition for a PAz (80 nm)/PDMS (4 mm) bilayer film. The existing wrinkle before irradiation (c) disappeared in the local UV-irradiated region (d). In this way, area selective wrinkle formation became available in this photoresponsive bilayer system. Once the wrinkle was erased by UV irradiation, the wrinkle was not formed even after successive exposure to visible light that recovers the trans-Az form. This can be explained by assuming a segmental relaxation of polymer chains in the photosoftened film without crosslinking (see section 3.3). Next, UV Irradiation was performed through a line and space (20 μm for each) photomask onto a PAz (80 nm)/PDMS (4 mm) bilayer film, and successively the film was uniaxially compressed. The optical microscopic images showing the surface morphologies are shown in Figure 2. When the uniaxial compression was made parallel and perpendicular to the line


PAz polymer adopted both smectic C and smectic A in the LC temperature range.37 The other polymers indicated only smectic A between the glass transition and isotropization temperature range. The wrinkle formation was always achieved at room temperature, i.e., commonly in the glassy state of the polymers before UV irradiation. 3.2. Wrinkle Formation and UV Irradiation. First, a bilayer film composed of PAz and PDMS was examined. The surface morphologies were observed by optical microscope (Figure 1). A PAz film of 170 nm thickness was transferred to

Figure 1. Optical microscopic images of wrinkle morphologies after 10% uniaxial compression before (a) and after (b) UV irradiation at 500 mJ cm−2 for PAz (170 nm)/PDMS (4 mm) bilayer film. In (c) and (d), images obtained in the same procedures before (c) and after spot exposure to UV light for PAz (80 nm)/PDMS (4 mm) bilayer film.

PDMS thick film (4 mm) (hereafter the bilayer films are denoted such as PAz (170 nm)/PDMS (4 mm)) was compressed uniaxially to 10% reduction in length. The resulting wrinkle pattern was shown in Figure 1a. The same procedure was achieved after irradiation with UV light at 500 mJ cm−2. In this irradiation condition, the PAz film reached to the photostationary state with cis-isomer content >90% (Figure S3). After the UV irradiation, the wrinkle was not formed. This fact indicates that the Young’s modulus of PAz in the cis-rich state became comparable with that of the PDMS substrate. This discriminating on/off change of wrinkle formation should be

Figure 2. Optical microscopic images of wrinkle morphologies obtained after 10% uniaxial compression applied parallel (a) and perpendicular (b) to the line pattern (20 μm) of UV irradiation for a PAz (80 nm)/PDMS (4 mm) bilayer film. C


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indented to a depth ranging 10−50 nm from the film surface. Therefore, LC polymer films of 150 nm thickness were used to avoid the effect of solid substrate. The results are summarized in Table 3. The Young’s modulus of PDMS film of 1 mm

and space pattern, the wrinkles in the shaded (trans-Az) regions were generated perpendicular (a) and parallel (b) to the irradiation pattern, respectively. Thus, different direction modes of wrinkling patterns were obtained by utilizing the on/off photoswitching. As indicated above, the homopolymer PAz film led to the erasure of the wrinkles upon UV irradiation. Here, LC copolymers (P(Az50-CB50) and P(Az50-CB50)) with reduced content of Az mesogen by random copolymerization with the nonphotoreactive CB-containing monomer were examined. In these random LC copolymers, the birefringent character was retained after UV irradiation (Figure S4), indicating that unlike PAz, the photoinduced smectic LC to isotropic phase transition did not occur. LC polymer (80 nm)/PDMS (4 mm) bilayer films were prepared, and the wrinkle wavelength was compared for the films before and after UV irradiation. Optical microscopic images are displayed in Figure 3. After UV

Table 3. Young Modulus (E) Obtained by AFM Indentation Measurements and the Wrinkle Wavelength (λ) wrinkle wavelength (μm)

polymer


PAz P(Az50CB50) P(Az25CB75) PCB PDMS

wrinkle wavelength (μm)

calcd

E(cis-Az)a after UV irradiation (MPa)

obsd

calcd

1.7 1.9

4.0 ± 2.0 32 ± 2

0 1.3

0.8 1.6

2.5

2.1

62 ± 1

2.3

2.0

2.8 n/a

2.2 n/a

n/a n/a

n/a n/a

n/a n/a

E before irradiation (MPa)

obsd

40 ± 5 55 ± 1

1.5 1.9

66 ± 2 80 ± 5 1.3 ± 0.1

a

The photostationary state by irradiation with 365 nm UV light at 500 mJ cm−2.

thickness (corresponding to Es) under investigation was found to be 1.3 ± 0.1 MPa, which agrees well with the literature data, 1.0−2.5 MPa, as estimated by tension tests.38 The E value of PAz before irradiation was 40 ± 5 MPa and increased along with increasing the CB unit content. E of PCB gave a 2-fold value (80 ± 5 MPa) compared with that of PAz. The CB mesogen possesses a polar cyano group at the terminal, which promotes antiparallel aggregation of the mesogens.39 This strengthened interaction is likely to increase the elastic modulus. Additionally, these values are larger than those obtained for a typical amorphous Az polymer (PDR1) studied for photoinduced surface relief grating formation (E = 220−340 kPa) as reported by Yager and Barrett.40 The oriented molecular packing in the glassy state of LC polymers is possibly reflected to the larger elastic modulus compared with the amorphous PDR1. However, an attention needs to be paid for the direct comparison because the shape and diameter of cantilever tips were different. Yager and Barrett40 used a tip with 20−60 nm radii, while we used one with much larger diameter (800 nm). In Table 3, the calculated and observed wavelengths are compared. The wavelengths were calculated by the equation ⎛ (1 − vs)Ef ⎞1/3 λ = 2πh⎜ ⎟ ⎝ (1 − vf )Es ⎠

Figure 3. Optical microscopic images of wrinkle morphologies after 10% uniaxial compression of P(Az50-CB50) (80 nm)/PDMS (4 mm) before and after UV irradiation. Images obtained in the same procedures for P(Az25-CB75) (80 nm)/PDMS (4 mm) are shown in (b).

(2)

where νs and νf are Poisson coefficients of the PDMS elastomer and transferred polymer thin films, respectively. The values of νs and νf were assumed to be 0.33 and 0.50 for the elastomer and polymer materials, respectively.35 As indicated in Table 3, good coincidences were observed for the calculated and observed wavelengths. This fact indicates that evaluation of Young’s modulus by the indentation experiment was performed adequately. In Figure 4, the wrinkle wavelength (λ) (a) and (Ef/Es)1/3 in eq 1 (b) are plotted against Az content for the four polymers. The film thickness of the transferred layer was consistently prepared as 80 nm. As seen, good correlations were obtained between the two plots. The increase in Az content led to nearly proportional decreases in the wrinkle wavelength and (Ef/

irradiation at 500 mJ cm−2 (see Figure S3 for the spectral change), the wrinkles were not diminished, but the wavelength was slightly reduced: 1.9 μm → 1.3 μm for P(Az50-CB50) (a) and 2.5 μm → 2.3 μm for P(Az25-CB75) (b). These wavelength reductions indicate modest decreases in Young’s modulus (softning) caused by the UV irradiation. 3.3. Evaluations of Young’s Modulus. Actual Young’s moduli were evaluated quantitatively by force−distance measurements in the repeated indentation and retraction processes onto the polymer films using an AFM cantilever tip (Figure S5). In these experiments, the AFM cantilever was D


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proceeded to a level that the lowest region reached to the substrate surface; thereby, the wrinkle was not formed upon compression. On the other hand, for P(Az25-CB75), the migration hardly occurred, and the surface undulation was negligible. Therefore, P(Az50-CB50) (100 nm)/PDMS (4 mm) was selected as the suitable sample. Figure 5 shows the relief structure inscribed by the patterned light irradiation. The lower and higher areas corresponded to

Figure 4. Wrinkle wavelength obtained for LC polymer (80 nm)/ PDMS (4 mm) films (a) and Young’s modulus obtained for 150 nm thickness films of LC polymers (b) as a function of Az content of the LC polymers.

Figure 5. AFM image of surface relief structure of P(Az50-CB50) (100 nm)/PDMS (4 mm) bilayer film obtained after line and space patterned (20 μm) UV irradiation (a). A thickness profile corresponding to the white line in the AFM image is shown in (b).

Es)1/3. Upon UV irradiation, the Young’s modulus was decreased depending on the Az content. The extent of decrease became more prominent as the Az content increased. E of PAz film in the cis-rich state decreased to 4.0 ± 0.2 MPa, a comparable value with the PDMS substrate (1.3 ± 0.2 MPa), which reasonably explains the fact that the wrinkle was not generated upon compression. The significant photosoftening due to the involvement of cis-Az isomer has also been quantitatively evaluated by Shimamura et al.41 for Az-containing cross-linked elastomeric films. 3.4. Winkle Formation on a Thickness-Modulated Surface. When patterned UV irradiation was made onto Azcontaining LC polymer films, surface relief structures are formed via mass migrating motions.26,29 The three polymer films under study (PAz, P(Az50-CB50, and P(Az25-CB25)) were subjected to UV exposure (1.0 J mJ cm−2) through a 20 μm line and space photomask at 90 °C.29 The mass migration efficiently is known to occur when polymer films are at the smectic state; therefore, the above procedure was achieved at the elevated temperature. As expected, the most efficient migration occurred in the PAz film, and relief formation was highly suppressed with the decrease of Az mesogen content (Figure S6). After the relief formation, the bilayer films were uniaxially compressed. For the PAz film, the efficient migration

the shaded and UV-irradiated regions, respectively. The height difference was approximately 10 nm, corresponding to 10% height modulation of the total thickness. This corrugated film was kept at room temperature for 1 day, which resulted in the cis-to-trans thermal back-reaction of the Az unit almost fully, and thus the Young’s modulus of the film was constant throughout the film. Next, this sample was subjected to uniaxial compression perpendicular to the irradiation line pattern. The optical microscopic and white light interferometric microscopic images of the wrinkled film are shown in parts a and b of Figure 6, respectively. The observed averaged wrinkle wavelengths and the calculated ones are listed in Table 4. Here, the wavelength was also calculated by eq 2. As shown, we recognized reasonable agreements with the observed and calculated wavelengths. This means that patterned thickness change caused by the phototriggered mass migration can be also utilized for the modulation of wrinkle wavelength. As shown in Figure 6, hierarchical surface topographical undulations involving the wrinkles within the photogenerated surface relief structure were formed. As widely studied, hierarchical topographical surfaces provides interesting aspects in surface wettability behavior.30,42−45 Therefore, the wetting E


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Figure 7. Photographs of water droplets on the film sample shown in Figure 5 before (a, corresponding to the state in Figure 5) and after 10% compression (b, corresponding to the state in Figure 6).

Figure 6. Optical microscopic image (a) and white light interferometric microscopic image (b) after 10% uniaxial compression perpendicular with the line pattern for the sample shown in Figure 5. The surface height profile corresponding to the white line in part b is indicated in part c.

compressed elastomer films. Depending on the polymer composition, photoinduced erasure or wavelength modulations were attained. The surface relief formation generated via mass migration enables the modulation of the skin film thickness, which can be another strategy for the modulation of wrinkle morphology. From the fundamental interest, the importance of actual determination of the elastic modulus of Az-containing LC polymers is stressed. The photosoftening has always been assumed to explain the efficient phototriggered mass migration processes of LC polymer film systems.26,29,46,47 This work quantitatively elucidated the photoinduced change of the mechanical properties for the first time. The light-stimulus process allows noncontact and addressable manipulations without any modification of the substrate, which will offer great advantages in surface wrinkling technologies. Another advantage to use Az-containing LC polymers is that the mechanical procedures may be coupled with the anisotropic photoalignment of LC materials,48 which may offer another new direction. The present work has provided only primitive results of the photomodulated wrinkle structures; however, we expect that optimization of the polymer material and customization of processing will lead to precise microfabrications with satisfactory accuracy and structural regularity.

Table 4. Wrinkle Wavelength on Photopatterned Surface of P(Az50-CB50)/PDMS film thickness (nm)

observeda (μm)

calculatedb (μm)

110 (UV irradiated region) 100 (shaded region)

1.9 1.7

1.8 1.6

a

Obtained from the FFT analysis of optical microscopic images. bSee the text.

property for water droplet was examined before and after the wrinkle formation. The contact angle on the pattern irradiated film before compression was 89.1 ± 0.8°, a slightly smaller value than that of a PAz film (103.7 ± 0.8°) (Figure 7a).33 This should mean that the existence of CB mesogen with a more polar cyano group increases the wettability. The film was then compressed to a 10% reduction, and then a water droplet was placed again. In this case, the contact angle drastically increased to 130.3 ± 0.6°, as indicated in Figure 7b. The increased contact angle should be attributed to the existence of hierarchical surface micromorphologies.42−45 The increased contact angle reverted to the original level when another water droplet was placed without applying the compression. In this way, a large wettability control could be achieved by this mechanical procedure.

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4. CONCLUSION Recent research trends have demonstrated that surface wrinkle formation can be a versatile and low-cost powerful tool for surface engineering of polymer films, which is also suited for large-scale fabrications. The present work proposes new possibilities of photoresponsive wrinkle formation on laterally

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01577. F


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Synthesis procedures of polymers, DSC, UV−vis absorption spectra, POM, force curves, AFM images, and other materials (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.N.). *E-mail: [email protected] (T.S.). Notes

The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (S23225003 to T.S. and B25286025 to S.N.) and for Young Scientists (B25810117 to MH) from The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, the PRESTO program of Japan Science and Technology Agency to S.N. This work was also supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas “Photosynergetics” (No. 15H01084) from MEXT, Japan.

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