Letter www.acsami.org
Visible-Light Photolabile, Charge-Convertible Poly(ionic liquid) for Light-degradable Films and Carbon-Based Electronics Tongtong Zhou,†,‡ Yuan Lei,†,‡ Hanzhi Zhang,‡ Ping Zhang,‡ Casey Yan,§ Zijian Zheng,§ Yongming Chen,⊥ and You Yu*,‡ ‡
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069 China § Nanotechnology Center, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong, China ⊥ School of Chemistry and Chemical Engineering, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *
ABSTRACT: We report for the first time an innovative visible-light photolabile poly(ionic liquid) (VP-PIL). The as-prepared VP-PIL features low Tg (47 °C), good thermal stability (Td ≈ 284 °C) and solubility in ranges of polar solvents. Upon blue light irradiation (∼452 nm), C−O bonds of picolinuim units are photocleaved, and the charges of PILs are simultaneously converted from positive to negative. Taking full advantages of these excellent properties of VP-PIL, a visible light degradable film for the first time is fabricated. Moreover, to demonstrate its applications in electronics, we prepared high-quality VP-PIL-containing conductive ink for flexible interconnects and graphene electrodes for supercapacitors.
KEYWORDS: responsive poly(ionic liquid)s, visible-light-based photochemistry, degradable layer-by-layer films, picolinium ester, flexible electronics
P
and related areas. The major advantage of using light energy is that it can provide more specific and controllable responsive behavior to the polymers. Importantly, light at long wavelengths (>400 nm) is seemed as a safer and “greener” stimulus when comparing to those undesirable high-energy light sources (UV light or γ ray) which can pose potential hazards to human bodies. However, as to PILs, although it has been used in these areas as good biocompatible materials for antibacterial, antifouling layers, and substance delivery applications,17,22,23 there are still only rare reports on visible-light responsive PILs because of the difficulty in designing corresponding functional groups. To date, how to synthesize such functional PIL and explore applications remain a significant challenge. Herein, we report for the first time an innovative visible-light photolabile poly(ionic liquid) (VP-PIL). This picolinium-based PIL is low-cost and easily synthesized from commercial 4pyridylmethanol with a high yield. As-prepared VP-PIL features low Tg (47 °C), good thermal stability (Td ≈ 284 °C) and solubility in water and ranges of polar organic solvents. Upon light irradiation in visible range (∼452 nm), C−O bonds of picolinium units in VP-PIL chains are efficiently photocleaved
oly(ionic liquid)s (PILs), a distinctive subclass of polyelectrolytes with cationic and/or anionic species as repeated units, have garnered substantial attention because of the superior functions and properties that can be enabled only by the combination of ionic liquids and polymers, such as idiosyncratic charge delocalization, unique tunability, chemical and thermal stability, and low glass transition temperature (Tg).1−4 Fascinated by these unique properties, PILs are not only adopted as idea alternatives to traditional liquid electrolytes, but also extensively applied in fields of polymer science, energy storage and conversion, catalysis chemistry, and so on.5−11 With recent rise of responsive materials to enable “smart” systems, many research endeavors have also been devoted to the fabrication of responsive PILs. Generally, such PILs are synthesized by different strategies12−15 such as designing functional ionic monomers, introducing foreign functional monomers or polymeric segments into polymer chains, postmodifying neutral or ionic polymers, or mixing them with environmental sensitive materials to give responsive behaviors to the changing environments such as temperature,16,17 electricity,18 redox reaction,19 magnetism,20 and voltage.21 Light as a typical noncontact stimulus has been extensively used to trigger the responsive behaviors of light-sensitive polymer, offering various potential applications in biological © XXXX American Chemical Society
Received: July 22, 2016 Accepted: August 31, 2016
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DOI: 10.1021/acsami.6b09048 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. Scheme illustration of the synthesis and photocleavage of picolinium-based VP-PIL.
Figure 2. (a, b) 1H NMR spectra of EPME (CDCl3) and VP-PIL (CD3OD), respectively. Stars show the chemical shifts of solvents. (c, d) DSC and TGA curves of PEPME and VP-PIL, respectively. (e) 1H NMR spectra of VP-PIL after visible light irradiation (D2O). (f) Transparence spectra of different samples at the range of 300−800 nm. Solid curves: aqueous solutions of VP-PIL before and after visible light irradiation. Dash curves and inserted digital images: the corresponding solutions after adding poly(acrylic acid ammonium salt). (g) CV curves of PME, EPME, and PEPME.
Next, the as-synthesized PME was undergone an ethylation reaction with low-cost bromoethane at a mild condition to obtain N-ethyl 4-picolyl methylacrylic ester (EPME), in which EPME subsequently acted as a monomer to polymerize via conventional free radical polymerization in DMSO to obtain poly(N-ethyl 4-picolyl methylacrylic ester) (PEPME) with a high yield of 92%. Finally, an anion-exchange process was conducted to result in the picolinium-based VP-PIL. It was worthy of note that there was no difference between the VPPILs that were prepared by the anion exchange before and after polymerization of EPME, respectively. In this study, the latter process was performed by replacing the Br− on PEPME to trifluoromethanesulfonate (OTf−). As shown in Figure 2a, b and Figure S2−S4, the characterization results of NMR and FT-IR indicate that the EPME monomer and picolinium-based PIL was successfully synthesized. The melting point of EPME is about 125 °C. This VP-PIL possesses good solubility in water and ranges of polar organic solvents (Table 1). The molecular weight was 58 000 g mol−1 that was indirectly calculated from the gel permeation chromatography (GPC) result of photocleaved polymer (Figure S5). By a comparison of PEPME and PIL, it was
via a mediate-electron-transfer (MET) process, in which the charges of PILs are converted from positive to negative. Taking full advantages of these excellent properties of VP-PIL, a visible light photodegradable film for the first time is fabricated. To show its versatile applications in electronics, we successfully fabricated high-quality VP-PIL-containing conductive ink for flexible interconnects and graphene electrodes for supercapacitors. Figure 1 illustrates the synthesis of VP-PIL which includes four sequential steps of (i) esterification of 4-pyridylmethanol, (ii) N-ethylation of picoly methylacrylic ester (PME), (iii) polymerization of N-ethyl picolinium methylacrylic ester (EPME) and (iv) anion-exchange of poly(N-ethyl picolinium methylacrylic ester) (PEPME) with trifluoromethanesulfonate anions (OTf−). In our previous work, the polymerizable picoly ester was synthesized with acyl chloride but the yield was very low (85%) by the esterification of 4-pyridylmethanol using methylacrylic acid and active ester chemistry (DCC/DMAP). B
DOI: 10.1021/acsami.6b09048 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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solution of Ru(II) and ASC. Based on these discussions, the photocleavage mechanism of VP-PIL is briefly described as follows (Figure S6): upon light irradiation, the excited Ru(II) is first reduced to Ru (I) by ASC, and then returns to its origin state (Ru(II)) by subsequent oxidation of picolinium in VPPIL. Simultaneously, C−O bonds in picolinium are cleaved and PIL is photodegraded. With the inspiration from the charge inversion of VP-PIL before and after visible light irradiation, a VP-PIL-based layerby-layer (LbL) film (Figure 3a) was fabricated for the first time
Table 1. Solubility of EPME, PEPME, and VP-PIL in Different Solventsa
a
solvents
H2O
CH3OH
CHCl3
PC
DMSO
EtOAc
CH3CN
EPME PEPME VP-PIL
+ + +
+ + +
+ − −
+ − +
+ + +
− − −
− − +
+, soluble; −, insoluble
found that after anion-exchange, Tg was decreased from 69 to 47 °C, and the thermal stability was significantly improved with the degradation temperature increased by 96 °C due to the introduction of fluoride compound (Figure 2c, d). To identify the photolabile property of the as-prepared VP-PIL, a solution of VP-PIL, Ru(bpy)32+ complex (Ru(II)), and ascorbic acid (ASC) was exposed to blue light (∼452 nm). Photocleavage reaction of VP-PIL was evidenced by the disappearance of the peaks belonging to picolinium moieties in the 1NMR spectrum (Figure 2a, b, and e). This result indicates that picolinium moieties are photocleaved from polymer chains, and the VPPIL is photolabile as predicted. To further confirm the photolabile property of as-prepared VP-PIL, a visual experiment was performed. Figure 2f showed that the transmittance of VP-PIL were both about 90% at 800 nm, showing good solubility in methanol before and after visible light irradiation. However, when adding negatively charged poly(acrylic acid ammonium salt) (PAA, ∼450 000 g mol−1) into these two samples, there was a sharp decrease of transmittance observed in the sample without irradiation but not in the irradiated one. The change in transmittance was also observed by naked eyes as shown in the inset in Figure 2f. The reason for that can be ascribed to the charge inversion of PIL before and after irradiation: before irradiation, the picoliniumbased VP-PIL was positively charged, and a nonwater-soluble complex structure with PAA was formed leading to the decrease of transmittance. Nevertheless, after irradiation, the PIL was converted to a negatively charged polymer as shown in Figure 1, generating strongly electrostatic repulsion to PAA. As a consequence, both polymers were well dispersed and the transmittance still remained at a high level (∼90% @800 nm). The visible difference in transmittance strongly supported that the VP-PIL was photolabile and charge-convertible. To understand the photocleavage mechanism of the picolinium-contained compounds, we adopted cyclic voltammetry (CV) to compare the reduction potentials of PME, EPME and PEPME (Figure 2g).25,26 The reduction potentials were found to be −0.97 eV and −1.07 eV for EPME and PEPME, indicating the reduction properties of esters were improved after N-ethylation reaction. The reason for such an improvement can be ascribed to the fact that introduction of ethyl moiety can increase the electron density and mobility in pyridine rings. This explanation is supported by the increase of chemical shifts of hydrogen atoms in Figure S2. Importantly, when comparing the reduction potentials of EPME and PEPME, no obvious difference was observed which indicated that the polymerization process had negligible impact on the photolabile property of VP-PIL. Furthermore, based on the CV measurement, the driving force of photocleavage reaction was calculated according to the literature,25,26 and the Gibbs free energy (ΔG) for possible direct electron transfer (DET) and mediated electron transfer (MET) were 5.75 and −5.75 kcal mol−1, respectively. These results suggest that only the MET pathway is favorable for the photocleavage of PILs in the
Figure 3. (a) Schematic illustration of the fabrication and degradation of photolabile (PSS/VP-PIL)n LbL films. (b) UV−vis spectra of (PSS/ VP-PIL)n films with different deposition times. (c) UV−vis spectra of as-fabricated (PSS/VP-PIL)n films with increasing irradiation time. (d, e) AFM images of (PSS/VP-PIL)10 films with the top layer of PSS and VP-PIL, respectively. (f, g) AFM images of (PSS/VP-PIL)10 films with different irradiation time (f: 20 min, g: 60 min). The scale bars in d−g are 1 μm.
for demonstrating its potential applications in visible-light tunable ultrathin films and surfaces. First, poly(styrenesulfonate sodium) (PSS) and as-synthesized VP-PIL were employed as building blocks to fabricate (PSS/VP-PIL)n films. The whole assembly process driven by electrostatic interaction was monitored by UV−vis spectroscopy. As shown in Figure 3b, two characteristic peaks belonging to PSS (225 nm) and VPPIL (225 and 257 nm, Figure S7a) were observed, and the absorbance peaks increased with increasing times of LbL deposition. The as-fabricated film was immersed into an aqueous solution of Ru(II) and ASC, and irradiated with blue C
DOI: 10.1021/acsami.6b09048 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. (a) Flexible MWCNT-based conductor at twisting state (r = 2 mm). Inset shows the cotton fabric before and after immersing into the ink, and the change of resistance with different coating times. (b) Conductive characters drawn by a pen containing VP-PIL-stabilized MWCNT ink. (c) CV curves of the as-fabricated supercapacitor at scan rates ranging from 5 to 2000 mV s−1. Insets show the RGO powder and VP-PIL-dispersed RGO paste. (d) Galvanostatic charge/discharge (GCD) curves of as-fabricated supercapacitors at different current densities. (e) Ragone plots of asfabricated supercapacitors and other published data (RGO on Paper, MnO2/RGO composite and CNT fiber). Areal energy density: mWh cm−2; Areal power density (mW cm−2). (f, g) Eectrochemical impedance plots.
in dark (Figure S7b). Three control experiments were carried out to evaluate the reaction conditions, and no decrease of absorbance of assembled films was detected (Figure S8). These results revealed that a combination of visible light irradiation, Ru(II) and ASC was necessary for photodegrading of this film, which was consistent with the described photocleavage mechanism of VP-PIL in Figure 2. More importantly, the PILs composed of a picolinium moiety in each repeated unit behave as a good dispersant to disperse multiwall carbon nanotubes (MWCNTs) by strong specific cation-π and π−π interactions with aromatic rings. Figure S9 showed that MWCNTs were homogeneously dispersed in aqueous solution of VP-PIL and featured a longtime stability (>30 days), whereas MWCNTs without PILs rapidly aggregated and precipitated. The VP-PIL-dispersed MWCNTs were conductive and can be used as low-cost ink for fabricating flexible interconnects. As proof-of-concept, a conductive fabric was first prepared by a dip-coating method, and the resistance decreased with increasing coating times.
light (∼452 nm) for a predetermined time. As irradiation time went by, decrease of the UV absorbance was observed, indicating the film was photodegraded simultaneously (Figure 3c). Moreover, the fabrication and degradation processes of LbL films can also be traced by AFM characterization. As shown in Figure 3d, e, both VP-PIL and PSS were uniformly deposited to the substrates. When alternatively depositing the positive VP-PIL layer onto the negative PSS layer, the surface roughness was increased from 4 to 8 nm, indicating that the LbL films were successfully fabricated. Nevertheless, when exposing to light for the determined time (20 min), the partially degraded films were observed in Figure 3f, resulting in higher roughness (23 nm) comparing with that of the as-prepared films. If further increasing the irradiation time to 60 min, the films were fully degraded and the roughness of surface was sharply decreased to 2 nm that was very close to the roughness of bare quartz substrates (Figure 3g). Importantly, such a photodegrading process of VP-PIL was easily controllable: with irradiating, the film was gradually degraded, while it was stable D
DOI: 10.1021/acsami.6b09048 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces After drying, the interconnects were readily used in flexible and foldable circuits which integrate light-emitting diodes (LEDs), as shown in Figure 4a and Figure S10. The I−V characteristics of the circuits show no obvious variation observed upon twisting and folding of the interconnect (Figure S10). Additionally, this ink can also be adopted to prepare conductive patterns with a pen on a paper substrate, and the as-drawn characters were applied as patterned interconnects of a circuit (Figure 4b). As one of the essential components in electrodes, the binder affects the performance of devices. VP-PIL would be favorable as the binder for preparing electronic devices due to their good conductivity, stability and compatibility to carbon materials. Furthermore, because of its excellent thermal stability (Td ≈ 284 °C) (shown in Figure 2), VP-PIL-dispersed graphene oxide can be reduced to RGO by heating at 180 °C for several hours without adding any reduction agent. Figure S9 showed the homogeneous, ultrastable PIL-dispersed reduced graphene oxide (RGO) both in water and organic solvent (propylene carbonate, PC). The concentrated RGO/VP-PIL composite was highly viscous and can be used as high-quality solid electrodes for fabricating devices. To further demonstrate versatile applications of the VP-PIL, we fabricated a highperformance solid-state supercapacitor, where RGO/VP-PIL composite, ionic liquid, filter paper, and nickel sponge were respectively used as electrode, electrolyte, separator, and current collector. Cyclic voltammetry (CV) curves of asfabricated supercapacitor in Figure 4c exhibited slightly slopped rectangular-like shapes within a potential window from 0 to 1.0 V. The shape at high scan rates (up to 2000 mV s−1) were similar to those at low scan rates without obvious distortions, which indicated a high rate performance and efficient ionic and electronic transports within RGO/VP-PIL composites. The galvanostatic charge/discharge (GCD) curves at different current exhibited triangular shapes with a columbic efficiency of ∼95%, indicating excellent reversibility of the device and good charge propagation between the electrodes (Figure 4d). Maximum areal capacitance was 15 mF cm−2 at scan rate of 5 mV s−1 and the highest areal energy density was 2 μWh cm−2 at the areal power density of 0.036 mW cm−2, showing that the RGO/VP-PIL-based supercapacitor was among the best value compared with other published data (Figure 4e and Figure S11).27−29 Electrochemical impedance spectroscopies (EIS) measurement was conducted to further characterize the device performance. The device showed a small semicircle at highfrequency regions, which was due to small charge-transfer resistance, and linear behavior in the low-frequency region with 75°, also revealing an ideal capacitive behavior of the supercapacitor (Figure 4f, g). And we also configured an equivalent circuit comprising a set of resistors and capacitors in series and parallel to fit the EIS plots (Figure 4f, g and Figure S12). The as-prepared supercapacitor not only had a low equivalent series resistance (Rs = 1.513 Ω) but also showed much short Warburg region portion, which indicated better ion diffusion efficiency due to smaller contact resistance among the material and ion (Figure S12). The reason for this can be attributed to the highly conductive VP-PIL used and its good dispersion efficiency to RGO sheets. In conclusion, we have reported, for the first time, a picolinium-based photolabile poly(ionic liquid). With blue light irradiation (∼452 nm), picolinium moieties are rapidly photocleaved from main chains. Such novel VP-PIL shows several unique advantages as follows: (i) by optimizing the
synthetic procedure, this low-cost VP-PIL is simply synthesized from cheap, commercial chemicals with a high yield and unfavorable side reactions are prevented. (ii) Because of excellent thermal and chemical stability, this picolinium-based VP-PIL is for dispersing 2D or carbon materials by strong specific cation-π and π−π interactions. (iii) The VP-PIL is altered by a nontoxic “green” visible light in a spatial fashion that is beneficial for its applications in biological systems, compared with the strategies based on high-energy light sources (UV light or γ ray). (iv) With light irradiation, the polymer charge is converted from positive to negative, which can be applied as matrix to load and release other useful guest molecules, such as DNA, drugs and enzymes. (v) The VP-PIL is conductive and photocleavable, thereby providing a photodegradable and solid polymer electrolyte, which can be used for fabricating flexible and degradable electronic devices. Taking full advantages of unique physical and chemical properties, it is anticipated that such picolinium-based VP-PIL shows greatly potential applications in fields of material science, electronics, biology, etc.
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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/acsami.6b09048. Description of the experimental procedures and supporting figures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
T.Z. and Y.L. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the Natural Science Foundation of Shaanxi Province (2016JQ5035), NFFTBS (J1210057) and the NSFC (21604069) for the financial support of this work. The authors thank Dr. Jian Kang for the DSC measurement, and Prof. Shuxun Cui for the AFM measurement.
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REFERENCES
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DOI: 10.1021/acsami.6b09048 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
