Article pubs.acs.org/Langmuir
Reversible Recovery of Nanoimprinted Polymer Structures Tanu 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 microand nanometer-scale surface textures using a nanoimprint process. Highly ordered and well-defined micro- and nanometer surface textures, for example, high aspect ratio (∼5) micropillars, form the permanent shape memory of the Nafion films. When damaged, these permanently shaped micro- and nanostructures possess repair ability through a heat treatment. Reversible recoveries of the damages caused by mechanical and irradiation exposure have been demonstrated. The recovery retains above 90% of the structural fidelity, which is comparable to the shape recovery in bulk film.
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INTRODUCTION Physical texturing has been used as a method to modify properties of materials, such as wettability1 and adhesion forces,2 as well as to impart optical effects.3 Traditionally, physical textures were mostly achieved through etching techniques, for example, plasma etching and chemical and mechanical etching.4 Etching techniques are limited to produce random surface textures. While microembossing is capable of producing well-defined surfaces, it has not been widely used as a surface-texturing technique primarily because of its limitation in pattern resolution. With the advancement in the nanoembossing technique, commonly referred to as nanoimprinting,5 high-resolution and well-defined surface textures can be controllably fabricated. The development of nanoimprint technologies has spurred interests in surface texturing, in particular, surfaces that mimic nature. Owing to the recent developments of nanoscale engineering in physical sciences, biomimetics, an old science that is inspired by design and processes occurring naturally, has been developing rapidly in the past decade. A variety of biomimetic surface textures and the corresponding surface functionalities have been achieved through various nanofabrication technologies, such as electron beam lithography,6 casting,7 nanoimprint lithography,8,9 and a range of soft-lithography and self-assembly patterning techniques.10−13 Although these advanced nanofabrication methods have proven excellent in reproducing the structures and the corresponding biomimetic functionalities, there remains a challenge in dealing with the phenomena of wear in synthetic structures. The applications of these synthetic structures are commonly subjected to physical degradation © 2013 American Chemical Society
because of environmental and handling conditions. Once damaged, the surface/film will lose its functionality, thus compromising the durability of biomimetic functional products. While natural systems contain a high level of elegance and sophistication in their self-healing or self-repair strategy,14 there is an obvious potential benefit to the progress of biomimetic research if the same concept of repair ability can be implemented 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 be processed into a temporary shape and revert to their permanent shape upon exposure to an external stimulus, such as heat, light, moisture, or magnetic field. Furthermore, they have a high capacity for elastic deformation, tunable application temperatures, easy processing, and potential biocompatibility and biodegradability.16 Such properties have enabled shape memory polymers to be exploited in various applications, including biomedical devices,17,18 fasteners,19 smart dry adhesives,20 and adaptive optical devices.21 Switchable function using shape memory material has been reported.22 The progress of shape memory polymers can be found in a recent review.23 Although the shape memory effect in bulk film is relatively well-studied, it is only recently that the shape memory effects in the micro- and nanoscale geometries have been reported.24−26 In the studies of surface texturing, the durability of surface structures is mostly Received: May 2, 2013 Revised: July 9, 2013 Published: July 10, 2013 10498
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Scheme 1. Experimental Procedures To Investigate the Shape Memory Effect of Nafion Film in the Micro- and Nanometer Scalea
Experiment 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 flat Nafion film was restored after heat treatment. Experiment 2: (A) Permanent shape of Nafion film was reset by imprinting micro- and nanostructures at its melting temperature. (B) New permanent shape, featuring arrays of micro- and nanometer structures, was obtained after cooling and moldremoval processes. (C) Deformation of micro- and nanometer structures was introduced by applying a shear. (D) Restoration of the deformed structures was obtained after heat treatment. a
microscopy (SEM) chamber without dismounting it from the fixture and heated to around 140 °C in an oven to induce the shape recovery, restoring the permanent structures of the undamaged micro- and nanostructures. Characterization. Thermal behavior of Nafion 117 films was investigated by thermogravimetric analysis (TGA Q500) and differential scanning calorimetry (2920 Modulated DSC). For the first experiment, atomic force microscopy (AFM) imaging was carried out in a tapping mode with a commercial Multimode AFM, Veeco, equipped with a high aspect ratio tip silicon cantilever. For the second experiment, high-resolution SEM imaging of the micro- and nanostructures was carried out with a JEOL LV SEM 6360 and JEOL FESEM JSM-6700F, respectively. Prior to the SEM imaging, the sample was gold-sputtered using JEOL JFC-1200 fine coater, resulting in about a 10 nm thick Au layer on the top surface of the micro- and nanostructures. The sample was then gold-sputtered once more (∼10 nm thick Au) to image the sidewall surface of the collapsed structures under SEM. The indentation test was conducted on an Agilent G200 nanoindenter system (Agilent Technologies, Santa Clara, CA) with a built-in heating setup. The sample was fixed on a thin stainless-steel plate using high-temperature epoxy. The heating element with a heat capacity up to 350 °C (accuracy of ±0.1 °C) is mounted directly beneath the stainless-steel plate. The individual pillar was pressed at room temperature with a diamond Berkovich indenter (three-face pyramid stylus) at a constant strain rate of 0.05 s−1. The maximum load applied was 10 mN, which was held constant for 10 s before the indenter was withdrawn from the sample surface.
dependent upon the inherent properties of the materials used. Here, we aim to demonstrate that micro- and nanoscale surface textures possess self-repair capability when a shape memory material is used.
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EXPERIMENTAL SECTION
Materials. 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 first experiment (experiment 1 in Scheme 1) feature an array of 2 μm wide lines (with ∼4 μm pitch and a height of ∼2 μm) and ∼250 nm wide lines (with ∼500 nm pitch and a height of ∼200 nm), respectively. The molds used to obtain the micro- and nanostructures in the second experiment (experiment 2 in Scheme 1) feature a square array of microholes (∼5 μm diameter, ∼25 μm deep, and ∼12 μm spacing) and nanoholes (∼500 nm diameter, ∼2 μm deep, and ∼500 nm spacing), 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 put in a clean oven at 100 °C for 1 h. The molds were exposed to oxygen plasma for 10 min in RIE I Etcher, Sirus (Trion), operated at 200 mTorr oxygen pressure, 10 standard cubic centimeters per minute (sccm) oxygen flow rate, and a power of 100 W. The molds were further treated with a fluorosilane release agent through an overnight vapor deposition of 1H,1H,2H,2H-perfluorodecyltrichlorosilane selfassembled monolayer. For the first experiment, the imprinting was performed using an Obducat nanoimprinter (Sweden) at 140 °C and 6 MPa for 10 min, after which the sample was demolded at 25 °C. The recovery-annealing step was performed in a vacuum oven at 140 °C for 2 h. For the second experiment, the imprint process was performed using a SPECAC hydraulic press at 310 °C and a pressure of 100 kg for 10 min, after which the sample was demolded at 25 °C. Shearing of the microstructures was performed by rubbing an index finger on the surface of the sample with an applied force of approximately 10 N, resulting in the collapse of the microstructures. Deformation of the nanostructures was conducted by focusing electron beams in JEOL FESEM JSM-6700F until collapse of the nanostructures was clearly observed. The sample was then removed from the scanning electron
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RESULTS AND DISCUSSION The shape memory polymer used in this study was Nafion (DuPont), a commercially available thermoplastic polymer with a polytetrafluoroethylene backbone and perfluoroether sulfonic acid side chains. Nafion has been reported to exhibit excellent shape memory properties with more than 95% shape recovery as bulk films.27 Scheme 1 outlines the two experimental procedures, performed to investigate the shape memory behavior of Nafion film in the micro- and nanometer scales. The as-received Nafion film was annealed at 140 °C for 2 h 10499
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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-treatment recovery step (some shallow line-and-space patterns were still observed because of irreversible plastic deformation of Nafion).
prior to use. The purpose of annealing the film is to remove prior processing history and, thus, define the permanent memory of the film. After annealing, Nafion film exhibited a dark color and a moderate shrinkage of about 5%.
The first experiment (experiment 1 in Scheme 1) aimed to demonstrate that an initially flat Nafion film was able to restore its original flat surface after receiving temporary shape in the form of micro- and nanostructures on its surface. The 10500
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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 pillar structures after shearing, and (c) nearly recovered 5 μm pillar structures after heat treatment.
Imprinting the Nafion film at 140 °C only resulted in a temporary shape change. When the sample is heated above the glass transition temperature, the temporary shape change can be erased. It is noted here that the glass transition temperature of Nafion is between 60 and 130 °C, while the melting temperature is between 300 and 330 °C. Panels a and c of Figure 2 show that the permanent shape of an initially flat Nafion film was almost recovered completely. The height of the micrometer-scale lines was reduced from ∼1.9 to ∼0.8 μm, and that 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 calculating the percentage of their structural height recovery R = (Hbef − Haft)/Hbef × 100%, where Hbef represents the height of the lines in the deformed/temporary state and Haft represents the height of the lines after the heat-treatment process. For these two particular examples, it corresponded to ∼95 and ∼93% recovery for Nafion film that featured micro- and nanometer structures, respectively. The above results are in agreement with a typical dual-shape memory effect on bulk Nafion film, where the fixing temperature is the same as the recovery temperature.27 The primary interest in this study is to make the micro- and nanostructures the permanent memory of Nafion. The second experiment (experiment 2 in Scheme 1) was designed to realize this objective. The experimental procedures involved (A) pressing a hard silicon mold that contained micro- and nanometer-scale surface-relief features into the Nafion film at its melting temperature, (B) cooling and mold-removal processes to obtain permanent structures on the surface of the Nafion film, (C) damaging micro- and nanometer structures via shearing or focused electron beams, and finally, (D) heating the damaged sample to its recovery temperature to restore the permanent shape of undamaged, structured Nafion film. Figure 3a shows the reproduction of high aspect ratio pillar structures on the Nafion film with feature sizes of ∼5 μm in
experimental procedures involved (a) pressing a hard silicon mold that contained micro- and nanometer-scale surface-relief features into a Nafion film at its glass transition temperature, (b) cooling and mold-removal processes to obtain temporary structures on the surface of the Nafion film, and finally, (c) heating the deformed sample to its recovery temperature to restore the permanent shape of an initially flat Nafion film. Panels a and c of Figure 1 show an example of ∼2 μm wide lines (with ∼4 μm pitch and a height of ∼2 μm) and ∼250 nm wide lines (with ∼500 nm pitch and a height of ∼200 nm) imprinted into Nafion at 140 °C and 6 MPa for 10 min. The replication of ∼2 μm lines was identical to the structures of the original master, and the tops of the lines were flat, which indicated full polymer filling of the cavities of the mold during imprinting. The actual height of the structures was ∼1.9 μm, as indicated in the sectional analysis of the AFM image in Figure 1b. The replication of ∼250 nm wide lines, however, was not identical to the structures of the mold. As shown in Figure 1d, the actual height of the structures was ∼157 nm, which could be associated with the fact that there was incomplete filling of the mold during imprinting. This observation was further substantiated by examining the sectional analysis of the AFM image, in which the tops of the structures were in the form of sharp peaks instead of flat surfaces. Nanoimprinting of the Nafion film becomes more challenging at a critical dimension of 250 nm and below. Attempts to nanoimprint Nafion with 100 nm feature size resulted in severe incomplete filling (not reported here). This is due to the considerably high molecular weight of Nafion as well as the complex intra- and intermolecular forces governing the thermomechanical properties,28 making the imprint process optimization non-trivial. While this problem may be solved, it is not the focus of this paper. Nevertheless, the yield of pattern transfer for both 2 μm and 250 nm line-and-space structures into Nafion was nearly 100%, and they can be used to investigate the feasibility in restoring the initially flat Nafion film after being structured through the imprinting process. 10501
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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 pillar structures via focused electron beams (the squared region indicates the area that is severely damaged), and (c) partially recovered 500 nm pillar structures 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 images for 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 pillar after the heating treatment.
diameter, ∼25 μm in height, and ∼12 μm in spacing. By looking at the top view of the sample, it is evident that the pillar structures were vertically oriented from the substrate with minimal distortion. These structures served as the new permanent shape of the Nafion film. Subsequently, the intended deformation and physical damage was introduced to the microstructures by applying shear through a finger rubbing action on the surface of the structured film. This deformation was evident from the collapse of the microstructures, as clearly shown in Figure 3b. Upon reheating the sample to the recovery temperature at 140 °C, the damaged structures recover to their permanent structures that were set earlier during the imprinting process. Figure 3c shows the SEM images of the near-complete restored microstructures after the heat-treatment process. It can be seen that the collapsed micropillars were back into the standing position, almost vertically from the substrate. The observed incomplete recovery could be attributed to the ∼10 nm thick Au coating on the top and sidewall surfaces of the pillar structures, which was required for the SEM imaging. It should be noted that the experiments were all performed in a controlled condition; i.e., the samples were not removed from the SEM studs throughout the whole process of imaging, damaging, and heat treatment. The experiments were also conducted more than 5 times, and the experimental analysis was carried out across different areas of the samples to ensure integrity and repeatability of the results. Excellent replication of ∼500 nm wide pillars was also achieved by imprinting Nafion at 310 °C. Figure 4a shows the new permanent shape of Nafion film, featuring sub-micrometer pillar structures (aspect ratio of 1:4) on its surface. The deformation was consequently induced into the structures by focused electron beams, resulting in the collapse of nanopillars, as clearly shown in Figure 4b. In this case, radiation heat causes Nafion 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 the sidewall exposure of the nanopillars in Figure 4c is less than that in Figure 4b, when the sample was viewed from the top under SEM. Apart from the fact that Au coating may influence the recovery of the collapsed nanopillars, it is important to emphasize that their recovery was also affected by the degree of damage made to the structures. It can be clearly seen from Figure 4c that the recovery of the damaged structures in the severely damaged area (post-imaging color was incorporated into the pre-identified region to highlight the pillars for clarity purposes) was not as good as that in other areas. Those pillars that are located outside the focused electron beam area (noncolored pillars) were seen to be well-recovering to their permanent structures, standing straight vertically from the substrate. We further investigated the cycling effects on the recovery of the nanoimprinted Nafion film. For this purpose, nanoindentation experiments were conducted using AFM, which is equipped with a heating stage. Using an AFM-based nanoindentation technique, the mechanical loading and the heattreated recovery can be monitored on a single micropillar. This technique allowed us to examine the cycling effect by focusing on an individual pillar. Because of the resolution limit of the microscope, this experiment was carried out on the 5 μm diameter pillars. The individual pillar was indented at room temperature with a diamond Berkovich indenter (three-face pyramid stylus) at a constant strain rate of 0.05 s−1. The maximum load applied was 10 mN, which was held constant for 10 s before the indenter was withdrawn from the sample surface. The consecutive loading and unloading was repeated for a number of times corresponding to the number of cycles. At the end of the desired number of cycles and without removing the sample from the stage, heating was introduced at 140 °C. Upon cooling, the same pillar was imaged. Figure 5 shows a series of images of the 5 μm pillars through 25 heating 10502
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demonstrated an approach to address the durability of microand nanoscale surface textures. With the fabrication of microand nanoscale surface textures onto suitable shape memory polymers, the surface functionalities, such as those studied in biomimetic surfaces, will now carry self-repair function.
and cooling cycles. Qualitatively, the AFM images show that the mechanical loading and heating cycles have not resulted in permanent change to the pillar. We further measured the hardness and modulus of this pillar. Hardness and modulus data are extracted using the established Sneddon equation.29 The hardness and modulus are plotted in Figure 6. The cycling
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 6. Hardness and modulus of 5 μm pillar as a function of the indent and heating cycle.
did not significantly change the modulus and hardness of the micropillars. It is worth mentioning that both the hardness and modulus 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 Nafion film were measured to be 0.25 ± 0.005 and 0.02 ± 0.004 GPa, respectively. A typical Nafion film has a Young’s modulus of less than 1 GPa.30 A flat Nafion film was subjected to a “simulated” imprinting process using a flat Si wafer as the imprinting mold; this is referred to as “conditioned” Nafion film. The “conditioned” film provides a reference for the modulus and hardness of the Nafion film without any surface feature. The modulus and hardness of the conditioned Nafion film are 0.44 ± 0.007 and 0.038 ± 0.0005 GPa, respectively. The higher modulus and hardness of the conditioned Nafion film are expected because the high-temperature heating is similar to the annealing effect. The significantly higher modulus and hardness observed in the imprinted feature is likely due to the realignment of the polymer chain during the imprint process flow; to fully understand this phenomenon would be a subject of a future report. For the purpose of this report, the AFM nanoindentation results show that, while the imprinting process results in plastic deformation of the Nafion film, the indentation results show that the induced mechanical damage results in recoverable elastic deformation.
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CONCLUSION A commercially available shape memory polymer, Nafion, was programmed with permanent memory made up of micro- and nanoscale surface textures. Using a nanoimprint process and selection of the imprint temperature at the shape fixation temperature (usually the melting point), the new permanent memory has been achieved. Two types of deformations were induced in the microstructured Nafion film. Mechanical rubbing and radiation exposures simulated the typical damages caused by user handling and wear. Upon heat treatment near the glass transition temperature, the deformed microtextures recovered to their permanent shapes. This work has 10503
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