Light-Modulated Surface Micropatterns with Multifunctional Surface

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Light-Modulated Surface Micropatterns with Multifunctional Surface Properties on Photodegradable Polymer Films Juanjuan Wang, Jixun Xie, Chuanyong Zong, Xue Han, Jingxin Zhao, Shichun Jiang, Yanping Cao, Andreas Fery, and Conghua Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10573 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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ACS Applied Materials & Interfaces

Light-Modulated Surface Micropatterns with Multifunctional Surface Properties on Photodegradable Polymer Films Juanjuan Wang †, Jixun Xie†, Chuanyong Zong†, Xue Han†, Jingxin Zhao†, Shichun Jiang*,† ,Yanping Cao‡, Andreas Fery§, and Conghua Lu*,†

†

School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, P.

R. China

‡

AML, Department of Engineering Mechanics, Tsinghua University, Beijing, 100084,

P. R. China

§

Institute of Physical Chemistry and Polymer Physics, Leibniz Institute of Polymer

Research Dresden e.V., D-01069 Dresden, Germany

KEYWORDS: photodegradation, surface wrinkling, stress release, dynamic pattern, multifunctional surface

ABSTRACT: Photodegradable polymers constitute an emerging class of materials that are expected to possess advances in the areas of micro/nano- and bio-technology. Herein, we report a green and effective strategy to fabricate light-responsive surface 1

ACS Paragon Plus Environment


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micropatterns by taking advantage of photodegradation chemistry. Thanks to the molecular chain breakage during the photolysis process, the stress field of photodegradable polymer-based wrinkling systems undergoes continuous disturbance, leading to the release/reorganization of the internal stress. Revealed by systematic experiments, the light-induced stress release mechanism enables the dynamic adaption of not only thermal-induced labyrinth wrinkles, but uniaxially oriented wrinkle microstructures induced by mechanical straining. This method paves the way for their diverse applications, for example, in optical information display and storage, and the smart fabrication of multifunctional surfaces as demonstrated here.

1. INTRODUCTION As a kind of stimulus-sensitive materials, photodegradable polymers have gained considerable attention lately, because light can be spatiotemporally localized and controlled in a simple, green, and noncontact way.1-7 Various photo-labile chromophores have been introduced either into the polymer side-chain as pendants groups or along the polymer backbone that can exhibit photo-triggered chain scission.6-9 So far, there are three main classes of main-chain photodegradable polymers, i.e., polyacrylates, polytriazines, and polyacetals, which have found important applications in broad fields from photolithography, bio-patterning, to tissue engineering and drug delivery.1,10,11 Besides, novel photodegradable polymers bearing o-nitrobenzyloxy unit have recently been developed as positive photoresists to generate reactive micropatterns.12-14 Upon irradiation, main-chain photodegradable 2


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polymers

can

break

down

into

low

molecular

weight

molecules.

This

photofragmentation process usually creates a dramatic difference in the solubility between the exposed and unexposed regions. The exposed regions can be removed by solvents, which makes them a potentially useful candidate for photo-patterning. Surprisingly, although providing a robust top-down means to micro/nano-fabrication, there have been few reports that extend the concept of photo-cleavage to stimulus-adaptive dynamic patterns, which is of importance both from scientific and technological viewpoints. On the other hand, surface wrinkling is ubiquitous in both nature and engineering.15-18 As one of mechanical instabilities, surface wrinkling frequently occurs in a rigid/soft bilayer system with a thin stiff film resting on a compliant foundation, once the compressive strain (ε) exceeds the bilayer-defined critical value (εc). In the case of a small strain applied, the as-formed critical wrinkle wavelength λ, wrinkle amplitude A, and the required critical wrinkling strain εc are determined by: 17-19

ߣ=

భ

ாത య 2ߨℎ୤ ቀଷாത౜ ቁ ౩ ఌ

(1)

‫ = ܣ‬ℎ୤ ට ఌ − 1

(2)

ଵ ଷாത౩ ଶ/ଷ ) ସ ாത౜

(3)

ౙ

ߝୡ = (

where hf is the film thickness, ‫ܧ‬ത = ‫ܧ‬/(1 − ߥ ଶ ) is the plane-strain modulus (E is the elastic modulus, ν is the Poisson’s ratio), and the subscripts of f and s denote the film and substrate, respectively. This establishes surface wrinkling as an inexpensive and universal non-lithographic technique to fabricate micro/nanostructured surfaces over 3



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large areas.17,20-23 More importantly, owing to the intrinsic stress-relaxation nature of surface wrinkling, it provides a promising route to develop dynamic patterning systems in which surface patterns can be dynamically tuned through modulation of the stress state of the wrinkling system. Till now, much progress has been made in the fabrication of wrinkling patterns with extended applications from smart surface adhesion, tunable wettability, microlens arrays, optical metasurfaces, to responsive microfluidic channels, and stretchable devices.24-36 Among them, the elaborate manipulation of the stress/strain field through the system-matched stimuli (e.g., mechanical strain, pH, humidity, solvent, and temperature), is a desired strategy to tune the wrinkling patterns.29-36 For example, Crosby and his co-authors reported the fabrication of solvent-responsive surfaces with hierarchical morphology via osmotically driven wrinkling.33 Similarly, Zeng et al. utilized a series of moisture-sensitive film–substrate bilayers to achieve unique moisture-activated wrinkling dynamics.34 Jiang and his coworkers demonstrated temperature-controlled wrinkling patterns with reversible morphology and multifunctional properties via the dynamic Diels-Alder chemical reaction.36 Despite the merits in these works for preparing dynamic surface patterns, adaptive wrinkling systems controlled by light stimulus inherently possess distinct advantages of wireless, scalable and spatiotemporally selective capabilities over other stimuli, which is highly desirable and received increasing attention recently.37-41 Note that Yoon et al. reported the light-induced switching of surface creases based on a photothermal

effect

caused

by

iron

oxide

nanoparticles

4


embedded

in

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poly(N-isopropylacrylamide) gels.37 Very recently, several groups have harnessed the azobenzene-containing polymers to develop dynamic wrinkling systems by taking advantage of the photosoftening and/or stress release/reorganization arising from the trans/cis photo-isomerization of azo-components.38-41 In this paper, we present a green and effective strategy to fabricate photo-tunable wrinkling micropatterns on a photodegradable polymer film attached to a poly(dimethylsiloxane) (PDMS) substrate, which just involves a facile, optical step without wet chemistry. Herein, two kinds of typical commercial available photodegradable polymers, poly(methyl methacrylate) (PMMA) and poly(lactic acid) (PLA), were used as two representative models. In particular, PLA is one of biocompatible and bio-degradable polymers, which has been widely applied in the biomedical and ecological fields.42,43 Thanks to the molecular chain breakage during the photolysis of PMMA and PLA upon ultraviolet (UV) light irradiation, the stress field in the as-wrinkled PMMA and PLA-based film/substrate system undergoes continuous disturbance. This leads to the release of the internal stress/strain, which decreases the wrinkle amplitude and can finally erase the wrinkles. In the case of selective exposure, except the wrinkle erasure in the exposed region, the wrinkles around the unexposed region can be dynamically tailored and oriented towards the exposed boundary. This enables the fabrication of hierarchical patterns with multi-scale topography features, and has been demonstrated here for the repeatable optical writing and erasure of information. Moreover, we extend the photo-tunable patterning scheme to mechanical strain-induced wrinkles at a considerably large 5



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compressive strain (e.g., 50%), which enlightens the potential application of the present strategy in preparing smart surface of adjustable optical and wettability properties. It is pointed out that different from the traditional utilization of alteration of macroscopic properties like the solubility, to our best knowledge, it’s the first time that the “micromechanical effect” of the photodegradation process in molecular level is subtly exploited to construct stimulus-responsive microstructures in an environment-friendly

and

effective

way.

Notably,

compared

with

former

light-responsive systems, 37-41 this photodegradation-based patterning strategy using commercial polymers is more facile as it does not require new chemistry. Additionally, it provides novel insights into stimulus-adaptive structured surfaces, not only by extending the material systems under study and involved micro-mechanisms to a broader horizon, but also by opening up new avenues for their applications in multifunctional smart windows.

2. EXPERIMENTAL SECTION Materials. Poly(dimethylsiloxane) (PDMS) (Sylgard 184, Dow Corning), poly(methyl methacrylate) (PMMA, Acros Organics), poly(L-lactic acid) (PLLA), poly(D-lactic acid) (PDLA) (Zhejiang Hisun Biomaterials Co., Ltd, China), and poly(styrene) (PS, Mn = 250,000 g mol-1, Mw/Mn = 1.05, Acros Organics) were used as received. Different meshes/holes of copper grids were purchased from KYKY Technology 6


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Development Ltd. (Beijing, China). Specifically, the copper grids with 50 mesh-sized hexagonal hole (H50), 200 mesh-sized round hole (R200), 400 mesh-sized round hole (R400), 500 mesh-sized round hole (R500), 100 mesh-sized square hole (S100), 200 mesh-sized square hole (S200), and 300 mesh-sized square hole (S300) were employed. Fabrication of PDMS sheets. PDMS sheets with a thickness of ～2 mm were prepared by mixing the base/curing agent at a 10:1 weight ratio. After degassing for 1 h, the mixed base/curing agent was poured into a culture dish and heated at 70 °C for 4 h. The cured PDMS sheet was cut into 2 cm × 2 cm for the thermal-induced surface wrinkling or 6 cm × 2 cm for the mechanical strain-induced surface wrinkling. Thermal-induced surface wrinkling on PLA/PDMS and PMMA/PDMS bilayers. PLA/PDMS and PMMA/PDMS bilayers were prepared by spin-coating PLA and PMMA solution in chloroform onto a PDMS substrate, respectively. Therein, the stereocomplexation between PLLA and PDLA (named as PLLA-PDLA) was obtained by mixing the equimolar solutions of PLLA and PDLA. Before spin coating, the PDMS sheet was treated with oxygen plasma (OP; Harrick PDC 32G) at a pressure of 0.02 mbar with a medium power (10.5 W) for 30 s. The PLA or PMMA film thickness was adjusted by varying the spin-coating speed (e.g., 1000, 2000, and 3000 rpm) or solution concentration (e.g., 0.5 wt %, 1 wt %, and 2 wt %). The obtained bilayer was degassed for 2 h to remove the residual solvent completely. The above-processed sample was heated at 90 °C (for PLA/PDMS bilayer) or 100 °C (for PMMA/PDMS bilayer) for 0.5 h, and then cooled to room temperature gradually. The 7



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resulting samples were kept in dark for the subsequent experiment. Mechanical strain-induced surface wrinkling on PLLA/PDMS bilayer. The PLLA/PDMS bilayer was prepared by transferring PPLA film onto a pre-stretched PDMS substrate, according to the method described by Stafford et al.19 Here the PLLA film was spin-coated on a glass slide at first. Meantime, the PDMS sheet was mechanically pre-stretched to a fixed uniaxial pre-strain (e.g., 2.5%, 10%, 20%, and 50%) by a home-made tensile holder. Then, the PLLA-coated side of the glass slide was brought into contact with the PDMS sheet, after an OP treatment (30 s at 10.5 W) for both. Next, the samples were immersed into deionized water, and then the PLLA film was detached from the glass substrate slowly to obtain the PLLA/PDMS bilayer. Finally, the pre-strain was slowly released once the bilayer was dried. Exposure of the as-wrinkled PLA/PDMS and PMMA/PDMS bilayers to ultraviolet (UV) light. A 500 W high-pressure Hg lamp was used to irradiate the above wrinkled samples at the optical power density of ~ 65 mW cm-2. In order to realize the selective exposure to UV light, a fresh copper grid or photomask was conformally covered on the wrinkled surface. After the selective exposure was performed for a designed duration, the copper grid or photomask was removed carefully. Characterization. UV/Vis absorption spectra of PLLA and PMMA films were recorded

on

a

T6

NEW CENTURY spectrophotometer.

Gel

Permeation

Chromatography (GPC) was examined on Waters 1515 (USA) equipped with three columns (Styragel HT3、Styragel HT4、Styragel HT/5) and a Waters 2414 refractive 8


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index detector using polystyrene standards. Optical images were recorded using an inverted Observer A1 microscope (Zeiss, Germany) equipped with a charge coupled device camera. Atomic force microscope (AFM) images were obtained in tapping mode on an Agilent 5500 AFM/SPM microscope with silicon cantilevers (NC-W, the typical frequency of 285 kHz). The Young’ s moduli of the PDMS substrate and the PMMA/PDMS and PLLA/PDMS bilayers were measured by a Linkam TST-350 tensile stress tester (Linkam Scientific Instruments, Ltd., U.K.). The contact angle experiments were performed using a Powereach contact angle goniometer (JC2000D), equipped with a CCD camera. The optical power density was measured with the optical power meter (zhongjiao Aulight, Beijing, China).

3. RESULTS AND DISCUSSION Despite the vast developments in the fields of synthesis of photo-sensitive degradable chromophores, the most commercially successful chromophore thereof is carbonyl group.8,9 The photochemical properties of polyacrylates and polyesters, which bear carbonyl groups on the side-chain and backbone respectively, have been intensively studied.44-49 It is well accepted that Norrish reactions predominate the photodegradation of both polyacrylates and polyesters.9,44,46,48 Specifically, in the case of polyacrylates, one of the best-known polymers in the photopatterning field is poly(methyl methacrylate) (PMMA). Upon exposure to UV light, main scission of the PMMA backbone initiated by photoinduced side-chain scission takes place.44 Likewise, as one of most attractive biodegradable aliphatic polyesters, poly(L-lactic 9



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acid) (PLLA) bearing carbonyl groups in its backbone mainly undergoes main-chain scission via the Norrish II type photo cleavage.48 Moreover, neat poly(D-lactic acid) (PDLA, the enantiomer of PLLA) and stereocomplexation between PLLA and PDLA (named as PLLA-PDLA) could be photodegraded with UV light exposure as well.49 The latter has been usually utilized to improve thermal/mechanical properties and hydrolysis-resistant of PLA-based materials. In our experiments, UV/Vis spectra of both PMMA and PLLA films that were exposed to UV light exhibit a pronounced increase of the absorbance in 250-400 nm (Supporting Information: Figure S1). This suggests the possible formation of an unsaturated group in combination with carbonyl products, and also supports the previously proposed Norrish reactions mechanism.44,45,47 In other words, the UV exposure leads to the photolysis and the decrease in the molecular weight in the PLLA and PMMA films, which was verified by GPC analysis of the as-irradiated films (Supporting Information: Table S1). Like the reversible trans/cis cycling in azo-containing films,40,41 the continuous molecular chain breakage during the polymer photolysis might also have a significant influence on the local stress state of as-wrinkled films in terms of mechanics. Thus we envisage that those polymers could be used as top-layer films in the film/substrate system to dynamically tune surface wrinkling patterns through a system-matched light stimulus. First, thermal-induced wrinkling systems were examined. A film/substrate system of PMMA/PDMS bilayer was heated to 100 °C, and labyrinth wrinkles were generated upon cooling to room temperature (Figure 1a and Supporting Information: 10


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Figure S2a). When exposed to UV light, the wrinkling patterns were gradually erased (Figure 1b-e and Supporting Information: Figure S2b-d). From the ex situ AFM characterization (Figure 1a-e) and in situ optical observation (Supporting Information: Figure S2), it can be seen that the wrinkle wavelength λ does not change significantly while the wrinkle amplitude A progressively decreases with the exposure time t (Figure 1f). The optical dose (De) required to erase the surface wrinkle patterns is estimated to be 8.5 J cm-2 (exposure time t ~ 130 s, see Supporting Information: Figure S2). Then the evolution of the thermal-induced wrinkling morphologies of PLLA/PDMS (Figure 2 and Supporting Information: Figure S3), PDLA/PDMS (Supporting Information: Figure S4) and (PLLA-PDLA)/PDMS bilayers (Supporting Information: Figure S5) with UV light irradiation were also investigated. Basically, the wrinkle evolution with the UV exposure time t is similar to that of PMMA/PDMS bilayer, in which De is estimated to be 11.7 J cm-2 (exposure time t ~ 180 s, see Supporting Information: Figure S3-S5), implying that this photo-tunable wrinkling pattern evolution is independent of the optical activity of the applied PLA molecule. For comparison, another common non-photodegradable polymer of poly(styrene) (PS) was examined (Supporting Information: Figure S6). The thermal-induced wrinkles in the PS/PDMS bilayer, in which PS can absorb UV light while does not degrade, are not eliminated by UV exposure. This result reveals that the degradation process rather than thermal effect caused by the UV light absorption leads to the wrinkle erasure. Furthermore, owing to the convenient spatiotemporal control of light, the selective exposure of the wrinkled bilayers can be easily realized such as through copper grids, 11



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where the exposed part is defined as D1, and the unexposed part is defined as D2 (Supporting Information: Figure S7). The labyrinth wrinkles in the wrinkled PMMA/PDMS (Figure 3a,b and Supporting Information: Figure S8) and PLLA/PDMS bilayers (Figure 3c,d and Supporting Information: Figure S9) in the exposed D1 region have been erased after the UV light irradiation, in accord with those observed during the blanket exposure (Figures 1 and 2, and Supporting Information: Figures S2-S5). Meanwhile, the initially disordered wrinkles in the unexposed D2 region have dynamically evolved into highly ordered wrinkles with the orientation perpendicular to the D1/D2 boundary, regardless of the initial wrinkle wavelength (Supporting Information: Figure S8a-c for PMMA/PDMS bilayer and Figure S9a-c for PLLA/PDMS bilayer), the geometries and meshes of the applied copper grids (Supporting Information: Figure S8d-f for PMMA/PDMS bilayer and Figure S9d-f for PLLA/PDMS bilayer). This wrinkle reorientation in the unexposed D2 region is attributed to the boundary effect 15, 50-52 of the formed D1/D2 boundary by the selective UV exposure. Meantime this process requires the bilayer system to locally deform and thus an energy barrier between the initial labyrinth wrinkles and the succeeding oriented wrinkles needs to be surmounted. It is expected that the stress reorganization

40

due to the dynamic evolution of the stress field in the adjacent

exposed area could provide the required driving force. Those results above are very similar to the phenomena in the azo-containing polymer-based wrinkling system reported in our previous study recently, where the surface wrinkles can be dynamically tuned and/or erased with visible light.40 12


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Thereinto, experimental results and theoretical analyses have revealed that the light-induced photoisomerization of azobenzene moieties in the wrinkled film led to the continuous variation of the stress field and thus brought the on-demand evolution of the wrinkle morphologies.40 Here, regarding the blanket exposure, the wrinkle amplitude is increasingly reduced when the film compressive strain/stress is gradually released, which is well consistent with Equation 2. When the applied strain ε lowers than the critical value εc, the wrinkles would be fully erased. Furthermore, in the case of the selective exposure, the ordered orientation in the unexposed regions shows intuitively the dynamic evolution of the stress field coupled with the boundary effect of the selective exposure-induced D1/D2 boundary. In short, we speculate that the wrinkle evolution in this case might be attributed to the modulation of the local stress field as well. As has been noted, the polymer degradation induced by the UV light exposure leads to the wrinkle erasure, in which the continuous molecular chain breakage takes place. Considering that the localized force caused by the reversible trans/cis photoisomerization of azo component results in the variation of the stress field in the wrinkled film,40,41 the photolysis process in the photodegradation film/substrate system may also have a significant influence on the local stress state. Thus the continuous disturbance and the resultant release of the localized stress in the wrinkled PMMA/PDMS and PLA/PDMS bilayers may can be ascribed to the photo-fragmentation of PMMA and PLA chains during the UV irradiation, respectively. Here the light-induced stress-release mechanism is further confirmed by the following experiments using the PLLA/PDMS bilayer as an example. 13



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As a typical bilayer buckling system, there are three characteristic parameters of λ, A, and εc describing the key features of wrinkling patterns. Besides, the magnitude of the applied overstrain relative to the critical wrinkling value (i.e., ε/εc) is also crucial to the onset of surface wrinkling and the wrinkling pattern evolution. In the case of thermal-induced surface wrinkling with a small applied ε resulting from the different thermal expansion coefficients between PLLA film and PDMS substrate, λ and A of the resulting wrinkles can be defined as Equations 1 and 2, respectively. When cooling the system which had been heated to a specific temperature (e.g., 90 °C), surface wrinkling is induced (Supporting Information: Figure S10a-f). As expected from Equation 1, the thermal-induced wrinkle wavelength λ linearly depends on the PLLA film thickness hf (Supporting Information: Figure S10g). From Equation 1 and 3, we can have గ௛౜

ߣ=ଶ

ඥఌౙ

(4)

Thus, the critical wrinkling strain εc of the PLLA/PDMS bilayer can be estimated to be ~ 0.55% according to Figure S10g (Supporting Information) and Equation 4. Additionally, the wrinkle amplitude A changes simultaneously with λ (Supporting Information: Figure S10h), and the aspect ratio (namely A divided by λ) of the wrinkles is basically constant and equals to the curve slope. From Equation 2 and 4, we can have ஺ ఒ

=

ଶඥఌିఌౙ గ

(5)

So when the PLLA/PDMS bilayer was heated at 90 °C and then cooled to room temperature for surface wrinkling, the exerted compressive strain ε is calculated to be 14


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around 3% according to Equation 5 and Figure S10h (Supporting Information). Here the wrinkling pattern evolution with the blanket UV light exposure of different initial wrinkle wavelengths λ of the wrinkled PLLA/PDMS bilayers (5-30 µm) was further investigated (Supporting Information: Figure S11). The results indicate that the optical erasure dose De is roughly independent of the initial wrinkle wavelength λ. Even for the optical erasure of the sub-micron scale wrinkles (Supporting Information: Figure S12), the required De is almost equal to that of the above micron-scale wrinkles (Supporting Information: Figure S11). Considering the fact that the exerted ε in the PLLA/PDMS bilayer is constant in those situations (i.e., heating at 90 °C and then cooling to room temperature), the independence of De on λ is rational under our light-induced stress-release mechanism. In addition to the labyrinth wrinkles induced by thermal processing, the mechanical strain-induced wrinkles having periodic striped structures could be tuned and erased by UV light exposure similarly, even at a considerably large pre-strain (εpre), e.g., 50% (Figure 4 and Supporting Information: Figure S13). Benefiting from the precisely control over the εpre, this mechanical strain-induced surface wrinkling provides an effective way to quantitatively study the effect of ε on De. Here εpre can be regarded as ε when the pre-strain is fully released. From Figure 4d, we clearly see that De increases with εpre. This direct connection between De and ε further verifies that the photo-tunable evolution of surface wrinkles is intimately related to the photo-degradation-induced stress/strain release in the wrinkled film/substrate system. Except the chain breakage in the molecular levels, the photodegradation of PMMA 15



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and PLLA may also bring changes of bulk properties (e.g., film thickness, Young’s modulus)49,53 and thus may have an effect on the evolution of the wrinkling morphologies upon light irradiation. However, there are no obvious changes in the film thickness for both the UV-exposed PMMA and PLLA films in our cases. Specifically, PMMA and PLLA films (hf ~ 220 nm) spin-coated on silicon wafers were selectively exposed to UV light via a 500 mesh-sized copper grid (Supporting Information: Figure S7d). Within the required doses for the wrinkle erasure (8.5 J cm-2 for thermal-induced wrinkles of PMMA/PDMS and 11.7 J cm-2 for PLLA/PDMS bilayers, respectively), the exposed regions cannot be distinguished from the un-exposed parts through AFM characterization (data not shown here). Additionally,

owing

to

the

quantitative

relation

of

the

wrinkle

wavelength/amplitude with the elastic modulus ratio of the bilayer, film thickness, and the imposed strain (shown in Equations 1-3), surface wrinkling has been widely applied for thin film metrology, characterization of residual stress in films, and so on.19,54-56 Here surface wrinkling was utilized to characterize the variation of mechanical properties of PMMA and PLLA films induced by UV light irradiation (Supporting Information: Figure S14). Firstly, thermal-induced surface wrinkling with disordered wrinkle patterns was performed on PMMA/PDMS and PLLA/PDMS bilayers (Supporting Information: Figure S14a,d), respectively. Then, the blanket exposure of the as-wrinkled bilayers to UV light leads to the optical erasure of the initial labyrinth patterns and the generation of de-wrinkled smooth surface. Subsequently, the 2nd thermal-induced surface 16


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wrinkling was imposed on the above dewrinkled bilayers as the foregoing process (Supporting Information: Figure S14b,e). The corresponding changes of the wrinkle wavelength λ and amplitude A were summarized in Figure S14c,f (Supporting Information). Evidently, in comparison with the 1st wrinkling, both λ of PMMA and PLLA-based wrinkling bilayer in the 2nd wrinkling do not show significant variation (0.7% and 1.3%, respectively). Since the film thickness of PMMA and PLLA films and the elastic modulus of PDMS substrate (Es, ~1.5 MPa) are roughly constant before and after UV irradiation, the elastic modulus of films Ef (PMMA) and Ef (PLLA) shouldn’t change significantly according to Equation 1. This speculation is further confirmed through the modulus measuring tests (Supporting Information: Tables S2,3 and Figure S15). Note that in the 2nd surface wrinkling, the wrinkle amplitude A of the PMMA/PDMS bilayer also undergoes a moderate change (7.4%) after UV exposure (Supporting Information: Figure S14c). With respect to the PLLA/PDMS bilayer, the wrinkle amplitude A exhibits a sharp decrease of 76.7% (Supporting Information: Figure S14f), which is unexpected according to Equation 2. Given that PLA films are susceptive to thermal effects,57,58 we speculate the thermal degradation during the 2nd wrinkling may play a major role in the unexpected decrease in the wrinkle amplitude A, which is corroborated by UV/Vis spectrum characterizations (Supporting Information: Figure S16). To exclude the thermal effect on the wrinkle formation, surface wrinkling induced by mechanical straining was implemented (Supporting Information: Figure S14g-i). For example, the PLLA film attached on a pre-stretched 17



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(εpre = 5%) PDMS sheet was selectively exposed to UV light for a dose of 11.7 J cm-2, and then the prestrain was released to induce surface wrinkling. As presented in Figure S14i (Supporting Information), λ and A of the exposed area decrease 4.5% and 20.2%, respectively, compared with those of the unexposed region. The change of A obtained from the mechanically wrinkled PLLA/PDMS system is comparative to that in the thermally wrinkled PMMA/PDMS system. The slightly larger variation of the former may be due to the generation of some cracks during the relaxation (as indicated by the white arrows in Supporting Information: Figure S14h), which could also release partial stress and reduce the amplitude of neighboring wrinkles.25 In light of the dynamic control over wrinkle micropatterns by simple photodegradation reactions, we highlight original applications to demonstrate the superior aspects of the current photo-responsive systems. Encouraged by the unique advantages of light stimulus (e.g., spatiotemporal control, non-contact and remote operation), first we show the use of dynamic wrinkling patterns in optical information display and storage by dint of the straightforward selective exposure. This patterning scheme has enabled the fabrication of highly ordered hierarchical patterns with well-defined microstructures (Figure 3, and Supporting Information: Figure S8 and S9). In the case of the large-area selective wrinkle erasure, representative examples are given in Figure 5. As presented in Figure 5a, the “TJU” logo is “written” through selective UV exposure of the as-wrinkled PMMA/PDMS bilayer with the corresponding photomask, which has a remarkable stability and can be maintained in the daylight for a long period (e.g., three months). The optical visibility is due to the 18


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lower transmittance and stronger scattering of the unexposed part with the reserved wrinkles compared with the exposed and de-wrinkled region. Notably, PMMA/PDMS bilayers have a good durability in the cycling of the thermal-induced wrinkling/UV-exposed de-wrinkling (Supporting Information: Figure S17). Thus, repeatable optical writing and erasure of information can be easily realized, as demonstrated here (Figure 5b). In the cycling, selective exposure upon a globally wrinkled surface is regarded as the step of “writing information”, and the blanket exposure and/or the thermal-induced wrinkling represents the process of erasing the information. Moreover, smart materials with adjustable physical properties have recently attracted significant attention because of their potential applications in various fields, such as biotechnology and biomedicine, sensors and actuators, and microfluidic.1,2,5 Here, we demonstrate that the PLLA-based adaptive wrinkling system induced by mechanical straining can serve well for the development of a new type of smart surfaces with variable optical and wettability properties, by dynamically tuning surface geometry in response to light stimulus (Figure 6). Figure 6a shows a representative result of the light transmittance T change measured by UV/Vis spectroscopy. For example, after releasing the prestrain to induce surface wrinkling, the sample of PLLA/PDMS bilayer first changes from optically transparent (line-v in Figure 6a, T ≈ 91.5%) to translucent (line-i in Figure 6a, T ≈ 25%) as a result of the light scattering by surface wrinkles. In general, periodic surface structures with micrometer sizes should be related to the scattering intensity, and affect the light 19



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transmittance of the films.33 When the wrinkled sheet was exposed to UV light, the wrinkle amplitude gradually decreases with the exposure time while the wrinkle wavelength maintains (Figures 2 and 4). This leads to the decreased wrinkle curvature (i.e., the aspect ratio), which renders the light to scatter to a smaller degree and in turn results in a larger transmittance (lines-ii and iii in Figure 6a). As the surface wrinkles were totally erased, the bilayer become transparent with T nearly equal to that of the non-wrinkled PLLA/PDMS counterpart (lines-iv and v in Figure 6a). Meantime, along with the UV light exposure, the corresponding bilayer displays not only tunable transparency (row I in Figure 6b), but controllable structure colors (row II in Figure 6b), which arises from the Bragg diffraction from the periodic wavy structures (Supporting Information: Figure S13). In parallel, the surface wettability of the resulting films was studied by measuring the static water contact angles perpendicular (θ⊥) and parallel (θ∥) to the direction defined by the wrinkle grooves (illustrated by the upper-right inset of Figure 6c). Initially, the wrinkled PLLA/PDMS surface exhibits an apparent wetting anisotropy (defined as θ⊥－θ∥) (i, Figure 6c) because water droplets typically spread along the grooved patterns.24 As the UV exposure time increases until the wrinkled PLLA film transforms to a smooth surface, θ⊥ drops sharply from 124 ° to 102 °, while θ∥ is slightly decreased from 107 ° to 102 ° that is equal to the value of corresponding θ⊥ (i →iv, Figure 6c), which is in agreement with the final isotropic smooth state. In this way, we can prepare surface coatings with photo-tailored optical and wettability properties, which might be used as multifunctional smart windows. 20


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4. CONCLUSIONS In summary, a facile and effective method based on modulating the local stress field with UV light exposure has been proposed for the first time to fabricate photo-tunable wrinkling micropatterns on commercially available photodegradable polymer films. Systematic experiments have been conducted to reveal the underlying light-induced stress-release mechanism, which is due to the continuous molecular chain breakage during the photodegradation process. Moreover, this patterning scheme is highly applicable to both thermal-induced labyrinth wrinkles both uniaxially oriented wrinkle structures induced by mechanical straining. As demonstrated here, the characteristics of our light-responsive composite film/substrate systems possess potentials in diverse on-demand applications including optical information display and storage, and smart fabrication of multifunctional surfaces with tunable optical and wettability properties. Considering the flourish of the synthesis of photo-sensitive degradable chromophores, this light control mechanism coupled with surface wrinkling can be extended to other functional photodegradable polymer-based systems with novel applications, such as in biotechnology and quality control during transport of UV-sensitive goods.

ASSOCIATED CONTENT Supporting Information UV/Vis spectra of PMMA and PLLA films after the blanket exposure to UV light 21



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(Figure S1), changes of the molecular weight in the UV-irradiated polymers (Table S1), optical observation of the wrinkle pattern evolution with the exposure time during the blanket exposure of the thermally wrinkled PMMA/PDMS (Figure S2), PLLA/PDMS (Figure S3), PDLA/PDMS (Figure S4), (PLLA-PDLA)/PDMS (Figure S5), and PS/PDMS (Figure S6) bilayers, optical images of different morphologies of copper grids (Figure S7), selective exposure of the wrinkled PMMA/PDMS (Figure S8) and PLLA/PDMS (Figure S9) bilayers through copper grids, dependence of λ and A on the PLLA film thickness and the wrinkle wavelength, respectively (Figure S10), dependence of the exposure dose on λ of the wrinkled PLLA/PDMS bilayer during the blanket exposure (Figure S11), AFM images of the wrinkled PLLA/PDMS bilayer with submicrometer wrinkle wavelength before and after UV light irradiation (Figure S12), optical observation of the wrinkle evolution with the exposure time during the blanket exposure of the mechanically wrinkled PLLA/PDMS (Figure S13), characterization of the variation of mechanical properties of PMMA and PLLA films induced by UV light irradiation through surface wrinkling (Figure S14) and tensile tests (Figure S15 and Tables S2,3), UV/Vis spectra of PMMA and PLLA films after the UV light exposure and subsequent heating (Figure S16), and the durability of the PMMA/PDMS bilayer after the cycling of thermal-induced wrinkling/UV-exposed de-wrinkling (Figure S17). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 22


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Corresponding Author *E-mail: [email protected] (C.L.). *E-mail: [email protected] (S.J.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21574099, 21374076, 51573131, 21374077, and 11572179).

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Figure 1. Wrinkling pattern evolution (ex-situ) of the wrinkled PMMA/PDMS bilayer during the blanket exposure to UV light with the exposure time t: 0 s (a), 30 s (b), 60 s (c), 90 s (d), 150 s (e). (a-e) AFM height images; (f) Dependence of the wrinkle wavelength λ and amplitude A on t, respectively. Scale bar: 20 µm.

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Figure 2. Wrinkling pattern evolution (ex-situ) of the wrinkled PLLA/PDMS bilayer during the blanket exposure to UV light with the exposure time t: 0 s (a), 70 s (b), 110 s (c), 180 s (d), 290 s (e). (a-e) AFM height images; (f) Dependence of the wrinkle wavelength λ and amplitude A on t, respectively. Scale bar: 10 µm.

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Figure 3. Optical images of the hierarchical patterns obtained when the wrinkled PMMA/PDMS bilayer (a, b) and PLLA/PDMS bilayer (c, d) were selectively exposed to UV light through the copper grids with 400 mesh-sized round hole (a), 300 mesh-sized square hole (b, d), and 200 mesh-sized round hole (c), respectively. Scale bar: 100 µm.

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Figure 4. In-situ optical observations of the wrinkle evolution on the mechanically wrinkled PLLA/PDMS bilayer during UV light irradiation with the exposure dose of 0 J cm-2 (a1-c1), 3.9 J cm-2 (a2), 7.8 J cm-2 (b2), 13 J cm-2 (b3), 19.5 J cm-2 (c2), and 31.2 J cm-2 (c3), respectively. The initial wrinkles were induced by mechanical straining with the pre-strain εpre of 1.5% (a1, a2), 10% (b1-b3), 20% (c1-c3). (d) Dependence of the optical erasure dose De on εpre. Scale bar: 100 µm.

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Figure 5. Application of the photo-tunable PMMA/PDMS wrinkling system for optical information display and storage (a) and repetitive writing/erasure of information by multiple cycling of selective UV exposure /thermal-induced wrinkling (b). Inset in (a) is the corresponding photomask.

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Figure 6. (a) Influence of UV exposure time t on the transmittance (T) of a mechanically wrinkled PLLA/PDMS bilayer from the εpre of 50%. (b) Corresponding digital photographs showing the transparency (row I) and structural color (row II). (c) Plots of water contact angles in two directions (θ⊥, θ∥ (illustrated by the upper-right inset)) on the wrinkled PLLA/PDMS bilayer after different UV exposure time. The as-wrinkled bilayer was exposed to UV light for t: 0 s (i), 220 s (ii), 450 s (iii), and 780 s (iv), respectively. For comparison, the initial non-wrinkled PLLA/PDMS bilayer with smooth surface was also provided (line-v in (a)).

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Table of contents

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Light-Modulated Surface Micropatterns with Multifunctional Surface

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