Hybridizing Poly(ε-caprolactone) and Plasmonic Titanium Nitride

Feb 18, 2016 - ABSTRACT: Plasmonic nanoparticles can confine light in nanoscale and locally heat the surrounding. Here we use titanium nitride (TiN) ...
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Hybridizing Poly(ε-caprolactone) and Plasmonic Titanium Nitride Nanoparticles for Broadband Photoresponsive Shape Memory Films Satoshi Ishii,*,†,‡,# Koichiro Uto,†,§,# Eri Niiyama,†,∥ Mitsuhiro Ebara,† and Tadaaki Nagao†,‡ †

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan ‡ CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan § Department of Bioengineering, University of Washington, Seattle, Washington 98195, United States ∥ Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan S Supporting Information *

ABSTRACT: Plasmonic nanoparticles can confine light in nanoscale and locally heat the surrounding. Here we use titanium nitride (TiN) nanoparticles as broadband plasmonic light absorbers and synthesized a highly photoresponsive hybrid cross-linked polymer from shape memory polymer poly(εcaprolactone) (PCL). The TiN−PCL hybrid is responsive to sunlight and the threshold irradiance was among the lowest when compared with other photoresponsive shape memory polymers studied previously. Sunlight heating with TiN NPs can be applied to other heat responsive smart polymers, thereby contributing to energy-saving smart polymers research for a sustainable society. KEYWORDS: shape memory polymer, photoresponsive polymer, photothermal effect, plasmonic resonance, nanoparticle, sunlight

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INTRODUCTION The propagation of light is affected by nanoparticles (NPs) in a non-negligible manner even though the size of the NP is much smaller than the wavelength of light. A common example is the blue sky, where the color comes from preferential scattering of blue light compared to other longer wavelength. Interaction of light by NPs are dramatically enhanced at their resonances because NPs effectively work as optical antennas and collect light from the area larger than their geometrical cross sections.1 The best known resonance for NPs is localized surface plasmon resonance that can be excited in metallic NPs. Conventionally, gold and silver NPs have been used intensively as plasmonic NPs. However, metal NPs are not the only class of materials that excite surface plasmons. Recently, studies have shown that highly doped transparent conductive oxides2 and transition metal nitrides,3,4 such as titanium nitride (TiN) and zirconium nitride, also show plasmonic resonances. The optical absorption loss of TiN is larger than that of gold or silver, hence the line width of its plasmonic resonance is broader. Quite recently, we demonstrated analytically and experimentally that the broad resonance of TiN NPs is advantageous for sunlight absorption. 5 The absorption efficiency of TiN NP was better than gold NPs6−8 or carbon NPs9−11 at identical concentrations which have been conventionally used for light absorbing NPs. Additionally, since the resonance peak of TiN NPs is around 700 nm, the plasmonenhanced absorption covers near-infrared (NIR).12 Note that spherical or cubic NPs made of gold or silver resonate at around 400−600 nm and the resonant peaks are narrow. The shape of gold or silver NPs needs to be either nanoshells, © XXXX American Chemical Society

nanorods, or another less symmetric structures to shift their resonances to NIR, which is sometimes difficult to fabricate. Together with the chemical stability and inexpensive price, TiN NPs are quite attractive as light absorbers for sunlight and NIR light. Scattering and absorption of light by NPs is not limited to NPs in atmosphere or liquid but also in polymers.13 Nanoparticles have been successively incorporated into wide range of polymers including polyelectrolyte (PE),14 polystyrene (PS),15 poly(methyl methacrylate) (PMMA),16 lead dimethacrylate,17 and poly(ethylene-co-vinyl alcohol) (EVOH),18 to name a few. Among them, light absorbing NPs hybridized into shape memory polymers19−23 has attracted substantial interests. Instead of thermally driven shape memory polymer either by a heater in contact or putting it in hot water, such hybrid shape memory polymers can be heated remotely by shining light. In addition, unlike a light-responsive shape memory polymers using photoresponsive molecules such as cinnamoyl group,24,25 the hybridized system has advantages of wide tunable nature of light absorption property. Gold26−28 and carbon NPs including carbon nanotubes29−32 have been mixed with shape memory polymers and their photoinduced shape memory functions have been demonstrated.33 Metal−ligand complexes34 and dyes35,36 have been also used for photoresponsive shape memory polymers. Received: December 25, 2015 Accepted: February 8, 2016

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DOI: 10.1021/acsami.5b12658 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

(XES-40S1, San-Ei Electronic) at different irradiances for a few minutes and its length was recorded afterward. For the surface patterning, to study photoinduced surface shape memory effect, a poly(urethane acrylate) (PUA) mold, which has a periodic grooves of 800 nm periodicity prepared by capillary force lithography,44,45 was used to create surface texture on the 20 wt % TiN−PCL hybrid film. Then the sample was irradiated by a 805 nm laser diode equipped with current controller (LDC 210C) and temperature controller (TED 200C), and temperature controlled laser diode mount (TCLDM9) (all from Thorlabs Ltd.) for 10 s at the irradiance of 1000 mW/cm2. The surface morphologies before and after the irradiation were recorded by an atomic force microscope (AFM) in noncontact mode using Si3N4 cantilever (spring constant; 42 N m−1) (SPM-9500J3, Shimadzu Co.). All the photothermal experiments were performed at room temperature (21−22 °C).

In the current work, we synthesized the hybrid of TiN NPs and shape memory polymer to demonstrate that TiN NPs in a shape memory polymer is effective in absorbing sunlight and NIR to heat the polymer. Poly(ε-caprolactone) (PCL) was chosen as a shape memory polymer due to its wide usage in biomedical and environmental applications and its biocompatible and biodegradable properties.37−39 We have successfully demonstrated the excellent tunability of the melting temperature of PCL, which is a switching temperature to induce shape change, by tailoring the PCL’s nanoarchitectonics.38,40 In addition, we have also shown NIR photothermal actuation of the PCL through the embedded gold nanorods which responded to NIR light irradiation. Such hybrid shape memory PCL enables remote and spatio-temporal control of shape memory transitions that has been applied as novel cell culture substrate for dynamic control of cellular function.41 Unlike the gold-PCL hybrid, the TiN−PCL hybrids presented here respond to the entire spectrum of sunlight, thus exhibiting energy-saving as well as ubiquitous use in developing countries. Our proof of concept demonstration could pave a way toward the applications of TiN−PCL hybrid in biomedical and environmental field and goes beyond to open the possibility of incorporating TiN NPs into other smart polymers

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RESULTS AND DISCUSSION Figure 1 shows before and after the thermal heating of the TiN−PCL hybrids at 80 °C. The films with uniform black color

EXPERIMENTAL SECTION

Synthesis of TiN−PCL hybrids. TiN NPs were prepared by thermal plasma processing (Nisshin Engineering Inc.).42 Before mixing the PCL with the TiN NPs, the as-prepared TiN NPs was treated with hexanoic acid (caproic acid) to adjust the surface polarity. This process made the TiN NPs disperse well in xylene which is a nonpolar liquid (see Supporting Information (Figure S1)). Two-branched 20PCL macromonomer (2b20PCL-m) was synthesized by ring opening bulk polymerization using 1,4-butanediol as an initiator and thin octanoate as a catalyst followed by acrylation reaction of terminal hydroxyl groups in branched PCL chains using acryloyl chloride. Details of the procedure are given in our previous report.43 Then the 2b20PCL-m of 45 wt % in xylene was then mixed with the surface treated TiN NPs with hexanoic acid at four different concentrations (1, 5, 10, and 20 wt %). The aforementioned 2b20PCL-m solutions, containing modified TiN NPs and benzoyl peroxide (BPO) as an initiator for cross-linking, were drop casted on a glass plate using a Teflon sheet as a spacer to fabricate film. The mixed macromonomer solutions were then heated at 80 °C for 3 h to form a cross-linked composite PCL film. Characterization. Transmission electron microscopy and X-ray diffraction studies on the TiN NPs themselves were reported in our previous work5 and also presented in Figure S2. From those studies, the particle size is ∼40 nm on average and titanium nitride phase is the only phase observed by XRD. The actual content of TiN NPs in the hybrid films were evaluated by thermogravimetric analysis (EXSTAR6000 TG/DTA, SII Nanotechnology). The melting temperature as well as endothermic enthalpy change (ΔH) were determined by differential scanning calorimetry ((DSC 6100, Seiko Instruments). In addition, spatial distribution of the TiN NPs in PCL was observed by a scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) spectroscopy (SU8000, Hitachi) and an optical microscope (ME600L, Nikon). To examine the optical property of the hybridization of TiN NPs, the transmittance of the films were measured by a UV−vis spectrometer (V-570, JASCO). Analysis of Photoinduced Shape Memory Effect. The performance of the shape memory effects on the hybrid films were first confirmed by thermally heating the films. To study sunlightinduced shape memory effect in macroscopic scale, each film of ∼25 mm × ∼5 mm was manually pulled to the twice the length of its original. Then the elongated film was irradiated by a solar simulator

Figure 1. Photographs of cross-linked PCL containing 0, 1, 5, 10, and 20 wt % of TiN NPs before (top) and after (bottom) heating at 80 °C, which is well-above the melting temperature of PCL and TiN−PCL hybrids (45.5−46.6 °C).

originated from TiN NP were obtained even at 1 wt % TiN NP content, suggesting the TiN NPs were highly dispersed in PCL matrix. To achieve homogeneous incorporation of TiN NPs into cross-linked PCL matrix, the surface modification of TiN NPs with hexanoic acid was essential. In fact, the selection of adequate surface ligand of TiN NPs, hexanoic acid in this study, should be extremely important to fabricate the cross-linked hybrid film dispersed TiN homogeneously because the TiN NPs hardly dispersed in both of xylene and cross-linked PCL matrix without any modification (Figure S1). In addition, a SEM image, an EDX mapping and an optical microscope image are shown in Figure S3. These images confirm homogeneous distribution of the TiN NPs except for some aggregated particles. The appearance of the cross-linked PCL and their hybrid with TiN NPs changed from opaque/dark to transparent/shiny state due to phase transition. These changes in appearance visually confirm that all the films showed crystal-to-amorphous transition of cross-linked PCL and incorporated TiN NPs did not hinder the phase transitions of PCL matrix. Figure 2a shows the data directly obtained from the thermogravimetric analysis. The weight increase of TiN NPs at around 450 °C was most probably due to oxidation, B

DOI: 10.1021/acsami.5b12658 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Thermogravimetric analysis for the TiN−PCL hybrid films with different compositions. (b) Relationship between TiN NP contents in cross-linked PCL hybrid and their feed concentrations, which is based on the result shown in panel a. The weight percentages of TiN NP in crosslinked PCLs were calculated from the weight losses of TiN−PCL hybrids by thermogravimetric analysis. The dashed line shows 1:1 (ideal) relationship between content and feed. Inset photographs show cross-linked PCL with different TiN NP compositions.

replacing nitrogen with oxygen.46 Subsequently, the final weights at 600 °C were plotted against the original concentration of the TiN NPs in the PCL films in Figure 2b. Although the actual concentration of the 20 wt % PCL film is slightly smaller (17.6 wt %), the plot confirms that TiN NPs were indeed mixed in cross-linked PCL hybrid films. This linear relationship of TiN NPs content between the feed and crosslinked hybrid surely indicates the homogeneous dispersion of TiN NPs in the cross-linked PCL matrix. This high dispersibility in PCL matrix would be advantageous for achieving efficient light absorption property. Figure 3 show the raw data and retrieved data from differential scanning calorimetry (DSC) measurements for the

monotonically deceases in accordance to the increasing concentration of TiN NP. On the other hand, the ΔH of matrix forming PCL component was also estimated from DSC curve combined with TGA data, suggesting the ΔH values of PCL domain were almost constant and independent of TiN NP contents as shown in Figure 3b. This implies that the TiN NPs dispersed in the matrix has little effect on structural and thermal properties of cross-linked PCL, suggesting TiN−PCL hybrid should retain the high ability of shape memory performance. The effect of hybridizing TiN NPs into PCL becomes obvious when the optical properties of the TiN−PCL hybrids are discussed. Figure 4 shows the transmittance of the TiN−

Figure 4. Transmittance of the TiN−PCL hybrid films. The inset shows a magnified view. The photographs below show the CCD images of the TiN−PCL hybrid films where a white light LED illuminated the films from the bottom.

Figure 3. Thermal properties of the TiN−PCL hybrids characterized by differential scanning calorimetry (DSC). (a) Thermal property (melting temperature, Tm and endothermic enthalpy change, ΔH) of the TiN−PCL hybrids as a function of TiN NP content. Tm and ΔH were calculated from peak top and peak area of DSC curves in Figure S4, respectively. (b) Endothermic enthalpy change of the PCL component in the hybrids.

PCL hybrids. The extinction of TiN NPs themselves can be found in ref 5. Just adding 1 wt % of TiN NPs into PCL makes the TiN−PCL hybrid completely opaque in visible and NIR ranges. This result is consistent with Figure 1 for the homogeneous dispersion of light absorbing TiN NPs in cross-linked PCL matrix even in the lower TiN NP content. Such property is beneficial to absorb broadband light source such as sunlight. The performances of the TiN−PCL hybrids as shape memory materials were examined as follows. To program

TiN−PCL hybrids (Figure S4). From Figure 3a, the melting temperature (estimated from the peak top of DSC curve) increased from ∼45.5 to ∼46.6 °C by adding 1 wt % of TiN NPs into PCL, but further addition of TiN NPs had little effect on the melting temperatures. In contrast, the ΔH (estimated from the peak area of DSC curve) also plotted in Figure 3a has different dependence to the TiN NP concentration as it C

DOI: 10.1021/acsami.5b12658 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Photos of the TiN−PCL hybrids before and after the irradiation by a solar simulator for 5 min. First column, initial shape; second column, temporary shape after fixing; third column, irradiance at 100 mW/cm2; fourth column, irradiance at 130 mW/cm2; fifth column, irradiance at 160 mW/cm2.

as shown in the fifth column of Figure 5. Hence, the threshold irradiance lies between 130 and 160 mW/cm2. In other words, perfect shape recovery of TiN−PCL hybrids was induced by light irradiation greater than threshold irradiance. Because the transmittances of the TiN NPs containing PCL are on the same order, it is expected that the threshold irradiances were nearly identical for all the hybrids. Because the pure PCL film had little change, even at the 160 mW/cm2 irradiance, our results provide experimental proof that the irradiation by a solar simulator heated the TiN NPs, which subsequently heated the PCL. The temperature increase of the TiN−PCL hybrid was also confirmed by numerical simulations based on finite element method (see Figure S5). These studies show that TiN NPs can effectively work as sunlight-to-heat transducer even in a cross-linked polymer matrix. As mentioned earlier, 160 mW/cm2 is a little higher than the standard solar irradiance. Nevertheless, solar irradiance of 160 mW/cm2 can be easily achieved by focusing sunlight by a Fresnel lens or parabolic mirror. It is also worth mentioning that others have used much higher irradiance for the photoresponsive shape memory polymers31,32,41,47 and the threshold irradiance of ours is the lowest to the best of our knowledge (see Table S1 for the comparison of the threshold irradiances of photoresponsive polymers). The light-to-heat transducer function of TiN NP should be useful to induce not only the bulk shape memory effect as discussed above, but also the surface shape memory effect because of the lower light transmittance of the hybrid allow more efficient generation of heat at the surface region. The

temporary shape, the PCL and TiN−PCL hybrid films were elongated to a strain of 100% above melting temperature and fixed by cooling below crystallization temperature while still elongated. The temporary fixed shape was successfully obtained after releasing of the applied strain as shown in the second column of Figure 5. Similar with PCL film (0 wt %), high shape fixing ratio, which is given by the ratio of the strain in stress-free (initial) state and elongated state after cooling, was exhibited for all TiN−PCL hybrids, and those were almost independent of TiN concentration. Thermal heating of those elongated samples confirmed that all the hybrids showed thermally induced shape memory effects (not shown). Photoresponsive shape memory effects of the TiN−PCL hybrids as bulk are summarized in Figure 5. The samples irradiated at 100 mW/ cm2, which is the standard irradiance of sunlight, are shown in the third column of Figure 5. Although the hybrid films showed measurable changes, the irradiance was not strong enough to return to their original shape. The changes seen for the 1 wt % sample was a little larger than the others. We anticipate that adding TiN NPs increased the rigidity of the PCL hybrid, thus only the 1 wt % sample might be more flexible than that of higher contents and showed the largest shape change (see the photographs in Figure 4). The detailed study with this regard will be carried out elsewhere. By increasing the irradiance to 130 mW/cm2, all the TiN NP containing samples shrank almost to their original lengths; however, the irradiance at 130 mW/cm2 was not enough to fully recover the shapes (see the fourth column of Figure 5). When the irradiance was increased to 160 mW/cm2, all the samples expect the pure PCL recovered to their original shapes D

DOI: 10.1021/acsami.5b12658 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (a) Photographs of the TiN−PCL hybrid, before surface patterning, after the surface patterning and after the NIR irradiation. (b) 3D AFM surface (top) and topographic images (bottom) of PUA mold with dimension 800 nm 1:1 (=groove: ridge) and TiN−PCL hybrid with 20 wt % TiN NPs before and after near-infrared light irradiation observed at room temperature (∼20 °C). All images were obtained in the 20 μm × 20 μm scan area.

performance of the 20 wt % TiN−PCL hybrid as a surface shape memory was further examined and is summarized in Figure 6. The mold pattern transferred on the TiN−PCL hybrid was erased by the NIR irradiation (Figure 6a), suggesting the cross-linked PCL matrix remains the surface shape memory in addition to bulk shape memory ability after the hybridization with TiN NPs. In addition, the local surface morphological transition was observed in microscopic level. In fact, we clearly observed the surface transition from the nanogrooves to flat structure in response to the NIR irradiation by the surface and cross-sectional images of AFM images as shown in Figure 6b. As the absorption in NIR is large as shown in Figure 4, the observed surface shape memory effect was also induced by the light absorption of the TiN NPs in the hybrid. Because the shape memory transition is spatially limited to the NIR-light irradiated region, changes in surface nanopatterns can be driven remotely and locally. As we have demonstrated so far, the hybridization of TiN NPs into PCL can be done by a facile method and TiN−PCL hybrids functioned as shape memory polymer as well as surface shape memory by shining weak intensity light. Although we only worked with a single type of PCL, the shape memory property as well as melting temperature of PCL can be easily tuned by modifying the chemical structure of PCL.38 By doing so, the sensitivity of TiN−PCL hybrid to light intensity can be adjusted for particular purposes. It is important to recall that PCL itself is biodegradable and TiN is chemically stable. Together with light responsive property TiN−PCL hybrid, TiN−PCL hybrid is suitable for biological and clinical applications. Sunlight responsive property is attractive for the sake of saving energy to activate shape memory polymers. With such results, we believe this platform can allow for simple and rational design of multifunctional shape memory polymers in response to broadband light that may not be achievable by other established shape memory composite systems.

simulator and the TiN−PCL hybrid with the concentration ranging from 1 to 20 wt % returned to their original shape at the irradiance of 160 mW/cm2. The surface shape memory analysis was carried out by irradiating the TiN−PCL film by a NIR source and the surface went back to its original geometry after the irradiation. In both cases, the TiN NPs functioned as an efficient light absorber to generate heat and the threshold irradiance was much lower than other shape memory polymers containing NPs. The photoresponsive function added to a shape memory polymer is quite beneficial for remote operation and noninvasive assessment. The photoresponsive function in NIR range makes TiN−PCL particularly attractive for biological applications such as cell manipulations. Because TiN NPs are nontoxic and inexpensive, we believe that the use of TiN NPs as a light absorber can be extended to other smart polymers in addition to shape memory polymers.

CONCLUSION To summarize, we synthesized hybrid PCL and TiN NPs, which absorb light in broad spectrum range, to add photoresponsive functionality to the original PCL. The photoinduced shape memory performance as bulk were studied by a solar

ACKNOWLEDGMENTS This work is partially supported by the JSPS KAKENHI Grant Number 15K17447. KU appreciates the contributions by the Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowships for Research Abroad.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12658. Scheme of surface modification of TiN NPs, TEM image and XRD pattern of TiN NPs, EDX mapping of the TiN NPs in TiN−PCL hybrid, threshold irradiances of photoresponsive shape memory polymers, heat transfer simulation (PDF).

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AUTHOR INFORMATION

Corresponding Author

*S. Ishii. E-mail: [email protected]. Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsami.5b12658 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.5b12658 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b12658 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX