Sequential Actuation of Shape-Memory Polymers through Wavelength

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Letter Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Sequential Actuation of Shape-Memory Polymers through Wavelength-Selective Photothermal Heating of Gold Nanospheres and Nanorods Sumeet R. Mishra and Joseph B. Tracy* Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States

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S Supporting Information *

ABSTRACT: Photothermal triggering of shape-memory polymers is an appealing noncontact mode of actuation for responsive materials and soft robotics. Wavelength-selective photothermal triggering of shape recovery is reported in thermoplastic polyurethane shape-memory polymers with embedded gold (Au) nanospheres and nanorods. Lightemitting diodes with wavelengths of 530 and 860 nm matched to the surface plasmon resonances drive selective shape recovery. Wavelength-selective shape recovery enables sequential actuation, as demonstrated in a wavelength-controlled stage with optically controlled height and tilt angle using legs of shape-memory-polymer films with embedded Au nanospheres and nanorods. KEYWORDS: gold, nanoparticles, nanorods, surface plasmon resonance, photothermal heating, shape-memory polymer, soft robotics, sequential actuation

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actuation. There is significant interest in sequential actuation for applications in soft robotics, which is possible using light sources matched for the selective actuation of different parts of a structure.18,19 GNPs, especially GNRs, are well suited for sequential actuation because of their tunable optical absorbance spectrum and high extinction coefficients. Sequential actuation in a SMP structure with different shapes of GNPs embedded in different parts of the structure requires matching the SPR of the GNPs and the excitation wavelength; for two (or more) wavelengths of light, each wavelength must drive shape recovery of the part of the structure with the matching shape of GNPs while not affecting the parts of the structure embedded with other shapes of GNPs. Related reports of photothermal triggering of shape recovery in SMPs driven by laser excitation of GNPs of different shapes14,17 do not meet this requirement for sequential actuation. We demonstrate sequential actuation of a wavelength-controlled stage, whose tilt angle is determined by the sequence of illumination. Several studies have investigated the dynamics of photothermal heating using NPs.20−22 The surface of NPs heats quickly, within a few picoseconds of illumination.23,24 As the excitation dissipates via phonons, the environment or matrix surrounding the NPs is heated.25 Photothermal heating has been used for applications spanning multiple length scales. For example, triggered drug delivery utilizes microscale photo-

he surface plasmon resonance (SPR) of metal nanoparticles (NPs), which causes enhanced absorption and scattering of light due to coherent oscillations of metallic free electrons, is useful for many applications, including photothermal heating.1 There has been long-standing interest in dispersing NPs into polymers to make polymer composites, which has been especially focused on the dispersion of NPs (and orientation, in the case of nanorods) and how they affect or are affected by the morphology of the polymer matrix.2−4 There is also significant interest in exploiting the functional properties of NPs embedded in polymers. Photothermal heating is an appealing technique for obtaining remote and localized heating using noble-metal NPs, which has applications in therapy,5 drug delivery,6−8 and polymer curing9 and processing.10 In shape-memory-polymer (SMP)11,12 composites containing embedded plasmonic NPs, photothermal triggering of shape recovery is of significant interest.13−18 The SPR wavelength and intensity depend on the NP shape and the chemical environment. Gold (Au) nanorods (GNRs) exhibit a weak transverse SPR at 530 nm, overlapping with the SPR of Au nanospheres (GNSs), and a more intense, red-shifted longitudinal SPR. The longitudinal SPR or transverse SPR is excited by light polarized along the length or orthogonal to the length of the GNR, respectively. The wavelength of the longitudinal SPR is longer than 530 nm and depends linearly on the aspect ratio of GNRs. Here, we report wavelength-selective photothermal triggering of shape recovery in films of a commercially available SMP using embedded Au NPs (GNPs) with different shapes and light-emitting diodes (LEDs) for remote and sequential © XXXX American Chemical Society

Received: March 11, 2018 Accepted: May 30, 2018

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DOI: 10.1021/acsanm.8b00394 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Nano Materials thermal heating,6−8 while macroscale photothermal heating is used in soft robotics.26 Photothermal heating has several advantages over conventional thermal heating: (1) If the matrix is sufficiently transparent for light to penetrate and reach the NPs, the matrix can be heated in a noncontact mode. (2) Heating can be precisely controlled by the intensity, spectrum, timing, and spatial profile of illumination.27,28 For example, melting or decomposition of polymers in close proximity to GNPs can be driven with spatial control using a laser.29 (3) Placement of the GNPs also allows for highly localized photothermal heating because the GNPs locally transduce light into heat. Spatially controlled photothermal heating based on the placement of GNRs has been used for polymer processing10,30 and for triggering thermoresponsive polymers.6−8 (4) If GNPs with anisotropic shapes and optical properties are used, such as GNRs, they provide additional means for controlling the selectivity of photothermal heating. GNRs can be aligned through shear, pressure, and electric fields.28,30−36 When using a light source matched to a particular longitudinal SPR wavelength in conjunction with a polarizer, polarizationselective photothermal heating can be achieved in a sample containing aligned GNRs.28,35 A typical two-stage, thermally activated SMP has hard and soft components. SMPs can be engineered with a wide range of molecular structures and morphologies, such as two different polymers, amorphous or crystalline phases, or chemically different segments of the same polymer chain. The hard component has a higher modulus and a higher glass transition or melting temperature than the soft component, allowing the hard component to serve as the backbone of the SMP and hold its shape. The mechanical properties of the soft component, however, change dramatically at lower temperatures.37,38 Once heated above the transition temperature, which can be the glass transition or melting temperature of the soft component, the SMP can be deformed into a temporary shape that is preserved at room temperature. Heating the sample above the transition temperature drives recovery of the initial, permanent shape. The behavior of a sample of GNPs dispersed in a SMP during photothermal heating is determined by many factors, including the properties of the illumination, shape geometry, spectral overlap of the illumination with the GNPs, and placement and loading of the GNPs. These variables provide rich opportunities for complex behaviors, but they must also be well controlled to purposefully program a desired behavior. For example, illuminating a three-dimensional object with spatially uniform, collimated light gives nonuniform intensities because the intensity of the light depends on the angle of illumination, which varies when the object has topography. Depending on the application, this effect may be undesired or could be harnessed to allow a shape change in a controlled pattern and sequence, which is especially appealing for soft robotics. Polarization control of shape recovery is also possible when using GNRs.28 We report the use of two shapes of GNPs with distinct optical properties for obtaining selective, wavelength-controlled photothermal heating and shape recovery of SMPs. Wavelength and polarization control could potentially be combined for more complex, simultaneous control. In comparison with polarization control, however, wavelength control does not require the alignment of GNRs. A series of GNRs with different aspect ratios would also allow for independent control of shape recovery, provided their longitudinal SPR absorbance bands do

not substantially overlap and they are excited with wavelengthmatched light sources. GNSs stabilized by oleylamine with an average diameter of 14.1 nm and GNRs stabilized by cetyltrimethylammonium bromide with average dimensions of 81.0 nm × 24.0 nm and an average aspect ratio of 3.6 were synthesized according to established procedures.39,40 Details of these syntheses and of composite sample preparation are provided in the Supporting Information. Poly(ethylene glycol) thiol (PEG-thiol) was used to PEGylate the GNRs41 for dispersion in Diaplex, a thermoplastic polyurethane (TPU) SMP with a transition temperature37 of 55 °C. Optical characterization of the NPs and thin films is reported as optical absorbance spectra, which are the same as the extinction spectra and were obtained directly from Beer’s law. It should be noted that the absorbance includes both absorption and scattering processes,42 but absorption is expected to be the predominant mechanism of extinction for these sizes of GNSs and GNRs.43 Optical absorbance maxima for dispersions of GNSs and PEGylated GNRs in tetrahydrofuran occurred at 521 and 799 nm, respectively (Figure 1). Dispersing the PEGylated GNRs in

Figure 1. (a and b) Transmission electron micrographs and (c) optical absorbance spectra of GNSs and PEGylated GNRs dispersed in tetrahydrofuran.

the TPU caused a red shift in the longitudinal SPR of 71 nm, which was accounted for in selecting the wavelength of the LED for exciting the GNRs. There were smaller red shifts in the absorbance of the GNSs and of the transverse SPR for GNRs of 23 and 11 nm, respectively, when they were dispersed in the TPU. We attribute these red shifts to both the increased refractive index of the TPU films compared with tetrahydrofuran and clustering of the GNSs and GNRs, which is apparent in scanning electron microscopy (SEM) micrographs (Figure S1). A higher loading was chosen for the GNSs than for the GNRs to ensure wavelength-selective photothermal heating and shape recovery. Identical samples of GNS-TPU and GNR-TPU were cut from the films with dimensions of 15 mm × 6 mm × 100 μm to demonstrate wavelength-selective photothermal heating (FigB

DOI: 10.1021/acsanm.8b00394 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

the NPs with the light source is a key consideration, and selectivity is maximized by minimizing the spectral overlap of different GNP samples, such as could be obtained by exciting GNRs of different aspect ratios at their longitudinal SPRs. In the case of overlapping optical absorbance spectra, as observed here at 530 nm, wavelength selectivity can be obtained by adjusting the concentrations of GNPs in the samples. The GNR-TPU sample also undergoes shape recovery if illumination at 530 nm is prolonged. Furthermore, for high concentrations of GNPs, the light may be completely absorbed in the outer layer of the film, resulting in nonuniform heating and altered or inhibited shape recovery. A wavelength-controlled stage was designed to demonstrate mechanical coupling of selective photothermal heating of the GNS-TPU and GNR-TPU samples and sequential actuation. The tilt angle of the Al foil top was controlled by the sequence of illumination at 530 and 860 nm, which dictates the sequence of extension of two accordion-shaped strips of GNR-TPU and GNS-TPU that serve as legs to lift the Al foil top and were cut from the same films prepared earlier. Each leg is composed of a film with dimensions of 42 mm × 17 mm × 100 μm before folding by hand into accordion shapes at room temperature. One end of each leg was attached to the rectangular Al foil stage, and the other end was attached to a glass slide base. LEDs with wavelengths of 530 and 860 nm were positioned to illuminate the stage from the side, with the 530 nm LED near the GNS-TPU leg and the 860 nm LED near the GNR-TPU leg. Extension of the accordion legs and controlled lifting and tilting of the Al foil top were investigated by sequentially switching the 530 and 860 nm LEDs on and off. Because each LED is spectrally matched to cause selective shape recovery (Figure 2) of only one leg, the sequence of illumination determined the sequence of extension and the tilt angle of the stage in the intermediate state after illumination with the first LED (Figure 3 and Videos S3 and S4). Each leg extended independently, allowing control over the height and tilt of the stage. Several issues must be considered in

ure 2). Each sample was folded in half by hand and pinched to create a stressed crease at room temperature. Both folded

Figure 2. (a) Optical absorbance spectra of GNS-TPU and GNR-TPU thin films with an inset photograph of the films. (b and c) Photographs of the GNS-TPU and GNR-TPU samples in temporary tent shapes before and after illumination with (b) 530 and (c) 860 nm LEDs to drive wavelength-selective shape recovery.

samples, in their temporary shapes, were placed together on the center of a stage. LEDs provided illumination from above. When both tent shapes were exposed to the 530 nm LED, only the GNS-TPU sample underwent shape recovery (Figure 2b and Video S1). The higher absorbance of the GNS-TPU film than the GNR-TPU film at 530 nm (Figure 2a) results in faster heat generation in the GNS-TPU sample to drive shape recovery. Absorption by the transverse SPR of the GNRs at 530 nm does not generate sufficient heat to drive shape recovery. In contrast, excitation with the 860 nm LED gives selective heating and shape recovery of the GNR-TPU film, while the GNS-TPU film retains its temporary shape (Figure 2c and Video S2). Therefore, wavelength-selective shape recovery of the GNS-TPU and GNR-TPU samples has been achieved using LEDs emitting at 530 and 860 nm, respectively. Lasers are more commonly used for photothermal heating than LEDs, but we show that LEDs are sufficient for triggering shape recovery within a few seconds. It should also be noted that if GNRs are used for triggering shape recovery of other SMPs with higher transition temperatures, the GNRs may be susceptible to thermal reshaping.44,45 Several coupled parameters need to be controlled to obtain wavelength-selective photothermal heating. Spectral overlap of

Figure 3. Photographs of a wavelength-controlled stage constructed from accordion legs of GNR-TPU (left leg) and GNS-TPU (right leg) with an Al foil top and a piece of red tape for visibility. The sequence of illumination determined the tilt angle. C

DOI: 10.1021/acsanm.8b00394 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation (NSF; Grant DMR-1056653) and the Research Triangle MRSEC (Grant DMR-1121107). We thank Charles Mooney for assistance with SEM and Michael Dickey for loaning us the 530 nm LED. This work was performed, in part, at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and NSF (Grant ECCS-1542015). The AIF is a member of the Research Triangle Nanotechnology Network, a site in the National Nanotechnology Coordinated Infrastructure.

the design of an operational stage. For example, opening one of the legs too far with the other leg closed can cause the stage to fall onto the closed leg. Prolonged illumination can also cause the stage to collapse because the increased flexibility can compromise the structural rigidity of a leg (Video S5). Prolonged exposure to either LED could also potentially heat both legs, even if the absorption of one leg is weak. This can be easily avoided by designing the system to give fast recovery, especially by controlling the relative concentrations of the different types of GNPs. The simple design reported here could be extended to utilize more than two wavelengths and more than two accordion legs. For example, the use of three or more legs containing GNRs with different longitudinal SPRs could tilt the stage in different directions, accompanied by raising or lowering the height of the stage. In conclusion, GNSs and GNRs with distinct SPRs dispersed in SMP thin films make up a simple system that provides wavelength-selective photothermal heating using LEDs. LEDs are inexpensive, available in many different wavelengths, and safer to use than lasers. Excitation with each LED selectively heats one type of GNP while not sufficiently heating the other type to drive shape changes. Sequential actuation is demonstrated by coupling the wavelength selectivity of GNSs and GNRs in a wavelength-controlled stage using folding SMP legs with embedded GNSs or GNRs. The height and tilt of the stage are remotely controlled by the sequence of illumination by LEDs with wavelengths of 530 and 860 nm. The concept of wavelength-controlled photothermal heating of GNPs with different SPRs could be extended to more than two wavelengths using a series of GNRs of different aspect ratios to impart more complex functions in soft robots while retaining simple device architectures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00394.

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Letter

Experimental details for the GNS and GNR synthesis, composite preparation, photothermal heating, and SEM images of the GNS-TPU and GNR-TPU samples (PDF) Wavelength-selective heating of Au nanosphere tent at 530 nm (Video S1) (MPG) Wavelength-selective heating of Au nanorod tent at 860 nm (Video S2) (MPG) Wavelength-controlled stage, illuminated first at 860 nm (Video S3) (MPG) Wavelength-controlled stage, illuminated first at 530 nm (Video S4) (MPG) Overexposure causing the stage to fall over (Video S5) (MPG)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Joseph B. Tracy: 0000-0002-3358-3703 Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acsanm.8b00394 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsanm.8b00394 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX