Reversible Recovery of Nanoimprinted Polymer StructuresTanu Suryadi Kustandi,† Wei Wei Loh,† Lu Shen,† and Hong Yee Low*,†,‡

†Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 3 Research Link,Singapore 117602, Singapore‡Singapore University of Technology and Design, 20 Dover Drive, Singapore 138682, Singapore

ABSTRACT: A shape memory polymer, Nafion, has its shape memory simultaneously programmed and patterned with micro-and nanometer-scale surface textures using a nanoimprint process. Highly ordered and well-defined micro- and nanometersurface textures, for example, high aspect ratio (∼5) micropillars, form the permanent shape memory of the Nafion films. Whendamaged, these permanently shaped micro- and nanostructures possess repair ability through a heat treatment. Reversiblerecoveries of the damages caused by mechanical and irradiation exposure have been demonstrated. The recovery retains above90% of the structural fidelity, which is comparable to the shape recovery in bulk film.

■ INTRODUCTION

Physical texturing has been used as a method to modifyproperties of materials, such as wettability1 and adhesionforces,2 as well as to impart optical effects.3 Traditionally,physical textures were mostly achieved through etchingtechniques, for example, plasma etching and chemical andmechanical etching.4 Etching techniques are limited to producerandom surface textures. While microembossing is capable ofproducing well-defined surfaces, it has not been widely used asa surface-texturing technique primarily because of its limitationin pattern resolution. With the advancement in the nano-embossing technique, commonly referred to as nanoimprint-ing,5 high-resolution and well-defined surface textures can becontrollably fabricated. The development of nanoimprinttechnologies has spurred interests in surface texturing, inparticular, surfaces that mimic nature.Owing to the recent developments of nanoscale engineering

in physical sciences, biomimetics, an old science that is inspiredby design and processes occurring naturally, has beendeveloping rapidly in the past decade. A variety of biomimeticsurface textures and the corresponding surface functionalitieshave been achieved through various nanofabrication technol-ogies, such as electron beam lithography,6 casting,7 nanoimprintlithography,8,9 and a range of soft-lithography and self-assemblypatterning techniques.10−13 Although these advanced nano-fabrication methods have proven excellent in reproducing thestructures and the corresponding biomimetic functionalities,there remains a challenge in dealing with the phenomena ofwear in synthetic structures. The applications of these syntheticstructures are commonly subjected to physical degradation

because of environmental and handling conditions. Oncedamaged, the surface/film will lose its functionality, thuscompromising the durability of biomimetic functional products.While natural systems contain a high level of elegance andsophistication in their self-healing or self-repair strategy,14 thereis an obvious potential benefit to the progress of biomimeticresearch if the same concept of repair ability can beimplemented into the synthetic structures.One strategy to achieve repair ability in the patterned

structure is to use stimulus−responsive materials, in particular,shape memory polymers.15 Shape memory polymers can beprocessed into a temporary shape and revert to their permanentshape upon exposure to an external stimulus, such as heat, light,moisture, or magnetic field. Furthermore, they have a highcapacity for elastic deformation, tunable application temper-atures, easy processing, and potential biocompatibility andbiodegradability.16 Such properties have enabled shape memorypolymers to be exploited in various applications, includingbiomedical devices,17,18 fasteners,19 smart dry adhesives,20 andadaptive optical devices.21 Switchable function using shapememory material has been reported.22 The progress of shapememory polymers can be found in a recent review.23 Althoughthe shape memory effect in bulk film is relatively well-studied, itis only recently that the shape memory effects in the micro- andnanoscale geometries have been reported.24−26 In the studies ofsurface texturing, the durability of surface structures is mostly

Received: May 2, 2013Revised: July 9, 2013Published: July 10, 2013

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dependent upon the inherent properties of the materials used.Here, we aim to demonstrate that micro- and nanoscale surfacetextures possess self-repair capability when a shape memorymaterial is used.

■ EXPERIMENTAL SECTIONMaterials. Perfluorosulfonic acid ionomer (Nafion 117, equivalent

weight of 1100 and 0.18 mm thick) was obtained from Sigma Aldrich.The molds used to obtain the micro- and nanostructures in the firstexperiment (experiment 1 in Scheme 1) feature an array of 2 μm widelines (with ∼4 μm pitch and a height of ∼2 μm) and ∼250 nm widelines (with ∼500 nm pitch and a height of ∼200 nm), respectively.The molds used to obtain the micro- and nanostructures in the secondexperiment (experiment 2 in Scheme 1) feature a square array ofmicroholes (∼5 μm diameter, ∼25 μm deep, and ∼12 μm spacing)and nanoholes (∼500 nm diameter, ∼2 μm deep, and ∼500 nmspacing), respectively.Micro- and Nanofabrication: Shape Memory Investigation.

Silicon molds were cleaned in a Piranha solution (a 3:1 mixture of 96%sulfuric acid with 30% hydrogen peroxide) at 120 °C for 30 min,rinsed with deionized water, dried in a stream of dry nitrogen, and putin a clean oven at 100 °C for 1 h. The molds were exposed to oxygenplasma for 10 min in RIE I Etcher, Sirus (Trion), operated at 200mTorr oxygen pressure, 10 standard cubic centimeters per minute(sccm) oxygen flow rate, and a power of 100 W. The molds werefurther treated with a fluorosilane release agent through an overnightvapor deposition of 1H,1H,2H,2H-perfluorodecyltrichlorosilane self-assembled monolayer. For the first experiment, the imprinting wasperformed using an Obducat nanoimprinter (Sweden) at 140 °C and 6MPa for 10 min, after which the sample was demolded at 25 °C. Therecovery-annealing step was performed in a vacuum oven at 140 °C for2 h. For the second experiment, the imprint process was performedusing a SPECAC hydraulic press at 310 °C and a pressure of 100 kgfor 10 min, after which the sample was demolded at 25 °C. Shearing ofthe microstructures was performed by rubbing an index finger on thesurface of the sample with an applied force of approximately 10 N,resulting in the collapse of the microstructures. Deformation of thenanostructures was conducted by focusing electron beams in JEOLFESEM JSM-6700F until collapse of the nanostructures was clearlyobserved. The sample was then removed from the scanning electron

microscopy (SEM) chamber without dismounting it from the fixtureand heated to around 140 °C in an oven to induce the shape recovery,restoring the permanent structures of the undamaged micro- andnanostructures.

Characterization. Thermal behavior of Nafion 117 films wasinvestigated by thermogravimetric analysis (TGA Q500) and differ-ential scanning calorimetry (2920 Modulated DSC). For the firstexperiment, atomic force microscopy (AFM) imaging was carried outin a tapping mode with a commercial Multimode AFM, Veeco,equipped with a high aspect ratio tip silicon cantilever. For the secondexperiment, high-resolution SEM imaging of the micro- andnanostructures was carried out with a JEOL LV SEM 6360 andJEOL FESEM JSM-6700F, respectively. Prior to the SEM imaging, thesample was gold-sputtered using JEOL JFC-1200 fine coater, resultingin about a 10 nm thick Au layer on the top surface of the micro- andnanostructures. The sample was then gold-sputtered once more (∼10nm thick Au) to image the sidewall surface of the collapsed structuresunder SEM.

The indentation test was conducted on an Agilent G200nanoindenter system (Agilent Technologies, Santa Clara, CA) with abuilt-in heating setup. The sample was fixed on a thin stainless-steelplate using high-temperature epoxy. The heating element with a heatcapacity up to 350 °C (accuracy of ±0.1 °C) is mounted directlybeneath the stainless-steel plate. The individual pillar was pressed atroom temperature with a diamond Berkovich indenter (three-facepyramid stylus) at a constant strain rate of 0.05 s−1. The maximumload applied was 10 mN, which was held constant for 10 s before theindenter was withdrawn from the sample surface.

■ RESULTS AND DISCUSSION

The shape memory polymer used in this study was Nafion(DuPont), a commercially available thermoplastic polymer witha polytetrafluoroethylene backbone and perfluoroether sulfonicacid side chains. Nafion has been reported to exhibit excellentshape memory properties with more than 95% shape recoveryas bulk films.27 Scheme 1 outlines the two experimentalprocedures, performed to investigate the shape memorybehavior of Nafion film in the micro- and nanometer scales.The as-received Nafion film was annealed at 140 °C for 2 h

Scheme 1. Experimental Procedures To Investigate the Shape Memory Effect of Nafion Film in the Micro- and NanometerScalea

aExperiment 1: (a) Annealed Nafion film was imprinted at its glass transition temperature to introduce temporary structures on its surface. (b)Arrays of micro- and nanometer structures were obtained on the surface of Nafion film after cooling and mold-removal processes. (c) An initially flatNafion film was restored after heat treatment. Experiment 2: (A) Permanent shape of Nafion film was reset by imprinting micro- and nanostructuresat its melting temperature. (B) New permanent shape, featuring arrays of micro- and nanometer structures, was obtained after cooling and mold-removal processes. (C) Deformation of micro- and nanometer structures was introduced by applying a shear. (D) Restoration of the deformedstructures was obtained after heat treatment.

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prior to use. The purpose of annealing the film is to removeprior processing history and, thus, define the permanentmemory of the film. After annealing, Nafion film exhibited adark color and a moderate shrinkage of about 5%.

The first experiment (experiment 1 in Scheme 1) aimed todemonstrate that an initially flat Nafion film was able to restoreits original flat surface after receiving temporary shape in theform of micro- and nanostructures on its surface. The

Figure 1. AFM images and section analysis of (a and b) 2 μm and (c and d) 250 nm line-and-space structures imprinted on annealed Nafion film.

Figure 2. AFM images and section analysis of imprinted (a and b) 2 μm and (c and d) 250 nm line-and-space structures after the heat-treatmentrecovery step (some shallow line-and-space patterns were still observed because of irreversible plastic deformation of Nafion).

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experimental procedures involved (a) pressing a hard siliconmold that contained micro- and nanometer-scale surface-relieffeatures into a Nafion film at its glass transition temperature,(b) cooling and mold-removal processes to obtain temporarystructures on the surface of the Nafion film, and finally, (c)heating the deformed sample to its recovery temperature torestore the permanent shape of an initially flat Nafion film.Panels a and c of Figure 1 show an example of ∼2 μm widelines (with ∼4 μm pitch and a height of ∼2 μm) and ∼250 nmwide lines (with ∼500 nm pitch and a height of ∼200 nm)imprinted into Nafion at 140 °C and 6 MPa for 10 min. Thereplication of ∼2 μm lines was identical to the structures of theoriginal master, and the tops of the lines were flat, whichindicated full polymer filling of the cavities of the mold duringimprinting. The actual height of the structures was ∼1.9 μm, asindicated in the sectional analysis of the AFM image in Figure1b. The replication of ∼250 nm wide lines, however, was notidentical to the structures of the mold. As shown in Figure 1d,the actual height of the structures was ∼157 nm, which couldbe associated with the fact that there was incomplete filling ofthe mold during imprinting. This observation was furthersubstantiated by examining the sectional analysis of the AFMimage, in which the tops of the structures were in the form ofsharp peaks instead of flat surfaces. Nanoimprinting of theNafion film becomes more challenging at a critical dimension of250 nm and below. Attempts to nanoimprint Nafion with 100nm feature size resulted in severe incomplete filling (notreported here). This is due to the considerably high molecularweight of Nafion as well as the complex intra- andintermolecular forces governing the thermomechanical proper-ties,28 making the imprint process optimization non-trivial.While this problem may be solved, it is not the focus of thispaper. Nevertheless, the yield of pattern transfer for both 2 μmand 250 nm line-and-space structures into Nafion was nearly100%, and they can be used to investigate the feasibility inrestoring the initially flat Nafion film after being structuredthrough the imprinting process.

Imprinting the Nafion film at 140 °C only resulted in atemporary shape change. When the sample is heated above theglass transition temperature, the temporary shape change canbe erased. It is noted here that the glass transition temperatureof Nafion is between 60 and 130 °C, while the meltingtemperature is between 300 and 330 °C. Panels a and c ofFigure 2 show that the permanent shape of an initially flatNafion film was almost recovered completely. The height of themicrometer-scale lines was reduced from ∼1.9 to ∼0.8 μm, andthat of the nanometer-scale lines was reduced from ∼158 to∼10 nm, as shown in panels b and d of Figure 2, respectively.Their shape memory effect can also be quantified by calculatingthe percentage of their structural height recovery R = (Hbef −Haft)/Hbef × 100%, where Hbef represents the height of the linesin the deformed/temporary state and Haft represents the heightof the lines after the heat-treatment process. For these twoparticular examples, it corresponded to ∼95 and ∼93%recovery for Nafion film that featured micro- and nanometerstructures, respectively. The above results are in agreement witha typical dual-shape memory effect on bulk Nafion film, wherethe fixing temperature is the same as the recovery temper-ature.27

The primary interest in this study is to make the micro- andnanostructures the permanent memory of Nafion. The secondexperiment (experiment 2 in Scheme 1) was designed to realizethis objective. The experimental procedures involved (A)pressing a hard silicon mold that contained micro- andnanometer-scale surface-relief features into the Nafion film atits melting temperature, (B) cooling and mold-removalprocesses to obtain permanent structures on the surface ofthe Nafion film, (C) damaging micro- and nanometerstructures via shearing or focused electron beams, and finally,(D) heating the damaged sample to its recovery temperature torestore the permanent shape of undamaged, structured Nafionfilm.Figure 3a shows the reproduction of high aspect ratio pillar

structures on the Nafion film with feature sizes of ∼5 μm in

Figure 3. Representative SEM images of (a) 5 μm pillar structures (aspect ratio of 1:5) imprinted on annealed Nafion film, (b) deformed 5 μm pillarstructures after shearing, and (c) nearly recovered 5 μm pillar structures after heat treatment.

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diameter, ∼25 μm in height, and ∼12 μm in spacing. Bylooking at the top view of the sample, it is evident that the pillarstructures were vertically oriented from the substrate withminimal distortion. These structures served as the newpermanent shape of the Nafion film. Subsequently, theintended deformation and physical damage was introduced tothe microstructures by applying shear through a finger rubbingaction on the surface of the structured film. This deformationwas evident from the collapse of the microstructures, as clearlyshown in Figure 3b. Upon reheating the sample to the recoverytemperature at 140 °C, the damaged structures recover to theirpermanent structures that were set earlier during the imprintingprocess. Figure 3c shows the SEM images of the near-completerestored microstructures after the heat-treatment process. It canbe seen that the collapsed micropillars were back into thestanding position, almost vertically from the substrate. Theobserved incomplete recovery could be attributed to the ∼10nm thick Au coating on the top and sidewall surfaces of thepillar structures, which was required for the SEM imaging. Itshould be noted that the experiments were all performed in acontrolled condition; i.e., the samples were not removed fromthe SEM studs throughout the whole process of imaging,damaging, and heat treatment. The experiments were alsoconducted more than 5 times, and the experimental analysiswas carried out across different areas of the samples to ensureintegrity and repeatability of the results.Excellent replication of ∼500 nm wide pillars was also

achieved by imprinting Nafion at 310 °C. Figure 4a shows thenew permanent shape of Nafion film, featuring sub-micrometerpillar structures (aspect ratio of 1:4) on its surface. Thedeformation was consequently induced into the structures byfocused electron beams, resulting in the collapse of nanopillars,as clearly shown in Figure 4b. In this case, radiation heat causesNafion to soften and the high aspect ratio pillars collapsed.Although incomplete recovery of the collapsed nanopillars was

observed after the heat-treatment process, it is obvious that thesidewall exposure of the nanopillars in Figure 4c is less thanthat in Figure 4b, when the sample was viewed from the topunder SEM. Apart from the fact that Au coating may influencethe recovery of the collapsed nanopillars, it is important toemphasize that their recovery was also affected by the degree ofdamage made to the structures. It can be clearly seen fromFigure 4c that the recovery of the damaged structures in theseverely damaged area (post-imaging color was incorporatedinto the pre-identified region to highlight the pillars for claritypurposes) was not as good as that in other areas. Those pillarsthat are located outside the focused electron beam area (non-colored pillars) were seen to be well-recovering to theirpermanent structures, standing straight vertically from thesubstrate.We further investigated the cycling effects on the recovery of

the nanoimprinted Nafion film. For this purpose, nano-indentation experiments were conducted using AFM, which isequipped with a heating stage. Using an AFM-based nano-indentation technique, the mechanical loading and the heat-treated recovery can be monitored on a single micropillar. Thistechnique allowed us to examine the cycling effect by focusingon an individual pillar. Because of the resolution limit of themicroscope, this experiment was carried out on the 5 μmdiameter pillars. The individual pillar was indented at roomtemperature with a diamond Berkovich indenter (three-facepyramid stylus) at a constant strain rate of 0.05 s−1. Themaximum load applied was 10 mN, which was held constant for10 s before the indenter was withdrawn from the samplesurface. The consecutive loading and unloading was repeatedfor a number of times corresponding to the number of cycles.At the end of the desired number of cycles and withoutremoving the sample from the stage, heating was introduced at140 °C. Upon cooling, the same pillar was imaged. Figure 5shows a series of images of the 5 μm pillars through 25 heating

Figure 4. SEM images of (a) 500 nm pillar structures (aspect ratio of 1:4) imprinted on the annealed Nafion film, (b) deformed 500 nm pillarstructures via focused electron beams (the squared region indicates the area that is severely damaged), and (c) partially recovered 500 nm pillarstructures after heat treatment (the colored pillar at the bottom-right corner was broken during the recovery process).

Figure 5. Microscope images of 2 μm pillars on the Nafion film. A specific pillar was selected next to a defected pillar and is circled in these imagesfor ease of identification. The images on the top row are the pillar with the indent, and the corresponding images on the bottom row are the pillarafter the heating treatment.

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and cooling cycles. Qualitatively, the AFM images show thatthe mechanical loading and heating cycles have not resulted inpermanent change to the pillar. We further measured thehardness and modulus of this pillar. Hardness and modulusdata are extracted using the established Sneddon equation.29

The hardness and modulus are plotted in Figure 6. The cycling

did not significantly change the modulus and hardness of themicropillars. It is worth mentioning that both the hardness andmodulus values are substantially higher than reported values.This is due to the higher indentation force use over a thin film.For confirmation, the hardness and modulus of pristine Nafionfilm were measured to be 0.25 ± 0.005 and 0.02 ± 0.004 GPa,respectively. A typical Nafion film has a Young’s modulus ofless than 1 GPa.30 A flat Nafion film was subjected to a“simulated” imprinting process using a flat Si wafer as theimprinting mold; this is referred to as “conditioned” Nafionfilm. The “conditioned” film provides a reference for themodulus and hardness of the Nafion film without any surfacefeature. The modulus and hardness of the conditioned Nafionfilm are 0.44 ± 0.007 and 0.038 ± 0.0005 GPa, respectively.The higher modulus and hardness of the conditioned Nafionfilm are expected because the high-temperature heating issimilar to the annealing effect. The significantly higher modulusand hardness observed in the imprinted feature is likely due tothe realignment of the polymer chain during the imprintprocess flow; to fully understand this phenomenon would be asubject of a future report. For the purpose of this report, theAFM nanoindentation results show that, while the imprintingprocess results in plastic deformation of the Nafion film, theindentation results show that the induced mechanical damageresults in recoverable elastic deformation.

■ CONCLUSIONA commercially available shape memory polymer, Nafion, wasprogrammed with permanent memory made up of micro- andnanoscale surface textures. Using a nanoimprint process andselection of the imprint temperature at the shape fixationtemperature (usually the melting point), the new permanentmemory has been achieved. Two types of deformations wereinduced in the microstructured Nafion film. Mechanicalrubbing and radiation exposures simulated the typical damagescaused by user handling and wear. Upon heat treatment nearthe glass transition temperature, the deformed microtexturesrecovered to their permanent shapes. This work has

demonstrated an approach to address the durability of micro-and nanoscale surface textures. With the fabrication of micro-and nanoscale surface textures onto suitable shape memorypolymers, the surface functionalities, such as those studied inbiomimetic surfaces, will now carry self-repair function.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: hongyee_low@sutd.edu.sg.

NotesThe authors declare no competing financial interest.

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Figure 6. Hardness and modulus of 5 μm pillar as a function of theindent and heating cycle.

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