Macromolecular Rapid Communications


Self-Storage: A Novel Family of StimuliResponsive Polymer Materials for Optical and Electrochemical Switching Yixiao Dong, Chaocan Zhang,* Lili Wu, Yanjun Chen, Yuanyuan Hu For most stimuli-responsive polymer materials (SRPMs), such as polymer gels, micelles, and brushes, the responsive mechanism is based on the solubility or compatibility with liquid media. That basis always results in distorting or collapsing the material’s appearance and relies on external liquids. Here, a novel kind of SRPMs is proposed. Unlike most SRPMs, liquid is stored within special domains rather than expelled, so it is deforming-free and relying on no external liquid, which is referred to as self-storage SRPMs (SS-SRPMs). The facile and universal route to fabricate SS-SRPMs allows for another novel family of SRPMs. Furthermore, it is validated that SS-SRPMs can drastically respond to outside temperature like switchers, especially for optical and electrochemical responses. Those features hold prospects for applications in functional devices, such as smart optical lenses or anti-self-discharge electrolytes for energy devices.

1. Introduction Highly focused “smart” or stimuli-responsive polymer materials (SRPMs) represent a growing field of materials that supports a variety of applications.[1] Booming development of polymer science allows these materials to be designed with abundant types and also versatile functions.[2] There are quite a few families in this field, for instance, gels,[3] micelles,[4] colloids,[5] polymer brushes,[6] and the like. Some applications, such as delivering drugs,[7] tissue engineering,[8] diagnosis,[9] sensing,[10] and self-healing,[11] always regard SRPMs as ideal candidates. There are numerous studies about responsive polymer gels[11,12] and micelles,[7,8,13] which have been highlighted in recent reports. However, in most cases,

Y. Dong, Prof. C. Zhang, Dr. L. Wu, Prof. Y. Chen, Dr. Y. Hu School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China E-mail: [email protected] Macromol. Rapid Commun. 2014, 35, 1943−1948 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

these materials have several typical properties, such as serious deformation,[8,14] low mechanical strength, and the reliance of external substance (usually liquids for maintaining responsive properties), which may not be suitable for further applications in solid devices. Therefore, working out SRPMs that are deforming-free, independent to external substances with relatively high mechanical strength will probably extend its application in functional devices, like Li-ion batteries, supercapacitors, and smart windows. It is necessary to figure out the essential of SRPMs, so that we can fully recognize and solve the problems raised above. In the majority of cases, the responsive mechanism of SRPMs is based on the stretch and aggregation of polymer chains during a phase change. However, the process of either stretch or aggregation of polymer materials at large scales will lead to serious deformation and expulsion of substance (therefore, traditional SRPMs cannot recover themselves without external supplement). So trying to avoid phase changes at large scale instead of small, partial but intensive transformation is the key thought we applied to design

DOI: 10.1002/marc.201400356


Macromolecular Rapid Communications

Y. Dong et al.

a new type of SRPMs. But how is that small, partial, and intensive transformation achieved? Answers can be obtained from the Flory-Huggins theory[15] (or the lattice sites model). According to the Flory-Huggins theory, entropy always favors mixing. Therefore, if the entropic effect is dominant enough, the entropic penalty may restraint the extent of unfavorable phase separation, and small, partial but intensive phase transition will possibly be built. In this case, the Flory-Huggins theory was simplified by the following equation (details see Supporting Information), ∂2 Δ Fmix 1 = −2 χ kT + kT ∂φ 2 1 −φ


where φ stands for the volume fraction of polymer in this case, ΔFmix is the free energy of mixing per lattice site, T is the temperature (K), χ is the Flory interaction parameter, and k is the Boltzmann constant. The entropic part in this equation should be kT 1 . It points out the chance of 1 −φ making a predominant entropic effect, which is rising up the volume fraction φ of the polymers. When the volume fraction rises, the entropic effect becomes gradually dominant, hence it restrains the mixed components from large-scale aggregate after the external stimulus becomes effective. In this work, a novel family of SRPMs has been built, which is referred to as self-storage stimuli-responsive polymer materials (SS-SRPMs). Highlighted features of SS-SRPMs are the following: First, its responsive properties depend on a nearly non-shape-distorted way. Moreover, breaking through the reliance of external substances but a self-storage way can free its practical condition while using. Finally, it obeys the basic theory of polymer mixture, hence the approach of constructing this kind of material can be either universal or facile. The raw materials of SRPMs would be no more restrained by special functional monomers, such as N-isopropylacryl amide, acrylic acid, and so forth. Our work has further presented two kinds of responsive forms in SS-SRPMs, namely optical response and electrochemical response, which may become very potential for smart devices.

methacrylate (MMA) purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Methyl methacrylate and styrene were distilled under reduced pressure and followed by three freeze–pump– thaw cycles to remove gas. Materials were used as received except noted.

2.2. IPA/PMMA SS-SRPMs Fabrication The mass ratio between IPA and poly(methyl methacrylate) (PMMA) may be varied as the experiment required. But typically, a four-neck flask was equipped with a magnetic stir bar, a reflux condenser, a nitrogen adapter, and a thermometer. MMA (50 g) and BPO (0.25 g) were added in the flask which was filled with N2 and heated in an oil bath at 80 °C. The mixture was stirred for 20 min, and then added IPA (25 g). After 5 min, the prepolymer syrup was cooled to about 50 °C and filled in required moulds. The curing process of prepolymer syrup was placed in a blowing air oven at 60 °C for 3 h at first, followed by 50 °C for 14 h. Aftermost, the bulk polymer was annealed at 60 °C for four times. Notably, each annealing process continues heating for 30 min and cooled to room temperature (25 °C).

2.3. NBA/PMMA SS-SRPMs Fabrication The PMMA/NBA (mass ratio of PMMA to NBA is 2:1) SS-SRPMs were prepared in similar way like PMMA/IPA SS-SRPMs but annealed at 70 °C for four times as last step.

2.4. DMSO/PS SS-SRPMs Fabrication Styrene (50 g) was prepolymerized in same equipment of PMMA/ IPA system in an oil bath at 90 °C with BPO (0.20 g) for 1.5 h. Then, DMSO (16.70 g) and BPO (0.05 g) were mixed in and kept on heating for 5 min. The prepolymer syrup was cooled to 65 °C and filled in required moulds. The curing process was conducted in a blowing air oven at 65 °C for 48 h. Finally, the polymer bulk was annealed at 80 °C for four times.

2.5. 1-Butyl-3-methylimidazolium Hexafluorophosphate (BMIMPF6)-Modified IPA/PMMA SS-SRPMs Fabrication 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), namely ionic liquid (IL), modified IPA/PMMA SS-SRPMs were prepared similar to PMMA/IPA SS-SRPMs but alter the IPA into different ratio of IPA/IL mixture in 1:4, 2:3, 3:2, 4:1, respectively.

2.6. Preparation of SS-SRPMs Samples for Analysis

2. Experimental Section 2.1. Materials Isopropyl alcohol (IPA), n-butyl alcohol (NBA), benzoyl peroxide (BPO), styrene (St), and dimethyl sulfoxide (DMSO) were purchased from Sinopharm Chemical Reagent Co., Ltd. 1-butyl3-methylimidazolium hexafluorophosphate (BMIMPF6) was purchased from Shanghai Chengjie Chemistry Co., Ltd. Methyl


Samples for SEM and EDS measurement were made by a fastfrozen and cracking procedure. Typically, the sample was frozen by liquid nitrogen in a very short time, then cracking it with a hammer to make fracture surfaces. Samples for UV–vis spectra were acquired by solid the polymer syrup in cuvettes. Samples for EIS test were SS-SRPMs plates, which are about 13 mm in diameter and 3 mm in thickness. The round surfaces of the plates were coated with silver as electrodes.

Macromol. Rapid Commun. 2014, 35, 1943−1948 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Self-Storage: A Novel Family of Stimuli-Responsive Polymer Materials for Optical and Electrochemical Switching

Macromolecular Rapid Communications

Figure 1. a) Schematics of SS-SRPMs. b) Scanning electron microscopy (SEM) image of a fractured surface in ambient temperature. c) A SSSRPMs plate was heated in 60 °C water leading to a gradual transition from opaque to transparent until half of the hided picture “Tai chi” clearly emerged. d,e) UV–vis test for SS-SRPMs.

3. Results and Discussion 3.1. Optical Switching for SS-SRPMs To begin with, a typical kind of SS-SRPMs (with a mass ratio of 2:1 of PMMA/IPA couple) was selected for the optical switching test. It can transform from opaque to transparent when temperature rises without an obvious deformation (Figure 1c and Figure S3, Supporting Information). Scanning electron microscopy (SEM) images of the fracture surface (Figure 1b) in ambient temperature further indicate the self-storage model, since a porous structure has been found in PMMA bulk. Each pore stands for a domain, which is directed by microscale phase separation. The UV–vis spectra were mainly applied to characterize the transformation in details. Figure 1d clearly presents an opaque/transparent transition, as the transmittance of SS-SRPMs is close to 1 after heated into 55 °C. The microdomains, formed in room temperature, contribute to the opaque state that coincides with the Mie’s scattering theory.[16] During the heating process to transition temperature, IPA diffuses into PMMA matrix gradually and heals the domains inside as a responsive mechanism. Furthermore, the repeatability of the responsive feature had also been taken into consideration, so that 20 heating and cooling cycles have done for testing. Notably, in order to present the transition process more directly, we fixed 470 nm as a measure wavelength. The four curves which stand for 1st, 5th, 10th, and 20th measurement are really consistent that prove a well repeatability of this material (Figure 1e).

The construction of SS-SRPMs through PMMA and IPA should be no coincidence. Since it was applied the most basic theory of polymer mixture, other couples of liquid and polymer should also be reasonable. To prove that, first, isopropyl alcohol (IPA) was altered into n-butyl alcohol (NBA) that also acquired self-storage structure inside with responsive feature as the temperature fluctuate in 60–65 °C (Figure 2a,b). In addition, different homopolymer and solvent was attempted as polystyrene (PS)/dimethyl sulfoxide (DMSO) couple, which can responsive in 75–80 °C (Figure 2c,d).

Figure 2. Characterization of SS-SRPMs fabricated by other polymer and liquid couples. a,c) SEM images of the fracture surface of NBA/PMMA and DMSO/PS couples. b,d) UV–vis spectra for responsive feature of NBA/PMMA and DMSO/PS couples according to light absorbance at 470 nm.

Macromol. Rapid Commun. 2014, 35, 1943−1948 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


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Figure 3. Characterization of IL-modified SS-SRPMs. a–d) SEM images of fracture surfaces with different ratios of IL and IPA, respectively. e) Comparison-responsive ability based on light absorbance at 470 nm.

3.2. Electrochemical Switching for SS-SRPMs The SS-SRPMs can be more valuable, if we can take advantage of the temperature-guided special domains one step further. Electrochemical devices, such as Li-ion battery, super capacitor, fuel cell, and solar cell, are really promising fields in current researches. Electrolytes are always important to be a conductive medium for those devices. So if we can switch the conductivity on purpose, it may be able to reduce the energy loss by self-discharge. Since the SS-SRPMs discussed in this work are based on the interaction between polymer and liquid, there is a chance to build electrochemical-responsive materials if we introduced the conductive liquid into polymer matrix. Ionic liquid (IL) is a kind of molten salt in room temperature. Due to its thermal stability and high ionic conductivity, IL was always applied to build solid electrolyte


with polymer. So, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) was employed as a third component to PMMA/IPA couple in order to make a kind of stimuli-responsive solid electrolyte (SRSE). It appears that as more amounts of IPA replaced by IL, size of the microdomains has become smaller until totally disappeared (Figure 3a–d). So was the responsive ability presented by UV spectrum, which corresponds to size and density of the domains (Figure 3e). That is probably because IL did not contribute to build the microdomains but distributed uniformly in the PMMA matrix. Thus, the decreasing amount of IPA is responsible for the “malfunction” of SSSRPMs, which can be explained by too dominant entropic effect as Figure S1 (Supporting Information) indicates. Followed control groups (Figure S4, Supporting Information) and energy-dispersed X-ray spectroscopy (EDS) mapping of fluorine and phosphorous (Figure S5, Supporting

Macromol. Rapid Commun. 2014, 35, 1943−1948 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Self-Storage: A Novel Family of Stimuli-Responsive Polymer Materials for Optical and Electrochemical Switching

Macromolecular Rapid Communications

Figure 4. Electrochemical results for IL-modified SS-SRPMs. a) Equivalent circuit used for calculating the resistance of SS-SRPMs. b–e) EIS data and their fitting results: IL:IPA:PMMA of 1:4:10 (b), IL:IPA:PMMA of 2:3:10 (c), IL:IPA:PMMA of 3:2:10 (d), and IL:IPA:PMMA of 4:1:10 (e). f) Calculated conductivity data from EIS results and change rate of conductivity for different IL ratio-modified SS-SRPMs, respectively.

Information) confirmed our assumption. Moreover, it was found that BMIMPF6 is not a good solvent to PMMA and there is no reported article suggests that BMIMPF6 would react with MMA while polymerization conducted. The uniform distribution of IL can probably be answered by the difficult migration of IL molecules in PMMA matrix, which suggests BMIMPF6 was actually “frozen.” Therefore, one way to switch the electrochemical properties of SRSE is by altering the “frozen” state of BMIMPF6. The release of IPA from the self-storage structure can lead to an unfrozen state of BMIMPF6. After IPA totally

diffuses into PMMA matrix as one phase, the chains of PMMA are plasticized hence building “channels” that allow BMIMPF6 migrating easily. Results from electrochemical impedance spectroscopy (EIS) correspond well with UV results. EIS Nyquist plots (Figure 4b,c) show that the resistance of SRSE may have a change in order of magnitude from 23 to 50 °C, if responsive ability performed well (according to UV plots). But if the responsive ability has totally gone as Figure 3e shows, the electrochemical switching will neither perform well (Figure 4d,e). Figure 4a presents an equivalent circuit we have built to

Macromol. Rapid Commun. 2014, 35, 1943−1948 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


Macromolecular Rapid Communications

Y. Dong et al.

simulate the resistance of solid electrolytes, in which R1 stands for the resistance of the silver painting which was applied as electrode to SRSE; R2 stands for the intrinsic resistance of SRSE; Zw stands for diffusion impedance represented by the Warburg element; and constant phase element (CPE) stands for nonideal capacitor. Variation of conductivity was calculated out from Equations S3 and S4 (Supporting Information) as Figure 4f shows. When the ratio of IL is as small as 1 to 15 (compare to the whole material), it does not result for good conductivity even in a relative high temperature at 50 °C. When the ratio of IL is as large as 3 to 15 or 4 to 15, switching ability drops down fast due to the absence of liquid domains. Thereby, in this case, the optimal ratio of IL/IPA/PMMA should be 2:3:10 as a balance between conductivity and switching ability. There is also an anomaly in Figure 4e, since the resistance became much bigger if the ratio of the BMIMPF6 becomes dominant. In fact, no matter the domains exist or not at room temperature, there is always part of IPA that combines with PMMA. So the larger resistance is probably because this part of combined IPA which can act as plasticizer in polymer matrix is replaced by BMIMPF6. So the immigration of IL becomes more difficult that cause the rising of electrical resistance.

4. Conclusions We have successfully developed a novel kind of SRPMs based on a self-storage mechanism. Dominated entropic effects force the phase separation to conduct in a small but intensive way when the material is subjected to a lower temperature. Compared with general SRPMs, the strategy of storing “dislikes” inside avoids either the shape deformation or the reliance of external substance. Its significance was further discussed for two responsive forms that hold perspectives for future applications, for example, a smart optical window which can control the light from getting inside, or a smart electrolyte which can reduce self-charging for long-term preservation. Most importantly, there is no need of special monomers to build SRPMs. Thus, finding out more potential couples of liquid and polymers may help to build a universal family of SRPMs in future work.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.


Acknowledgements: This work was supported by the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) No. 20120143110002. The authors especially thank Huiying Xiang and Yumei Li for the assistance at SEM, Dandan Wang and Huazhang Zhang for EIS tests, Wanyu Chen and Dong Xie for their critical advice, and Feihua Liu for the discussion of this work. Received: June 24, 2014; Revised: August 31, 2014; Published online: September 25, 2014; DOI: 10.1002/marc.201400356 Keywords: optical response; polymers; responsive electrolytes; self-storage; stimuli-responsive [1] P. Theato, B. S. Sumerlin, R. K. O’Reilly, T. H. Epps, III, Chem. Soc. Rev. 2013, 42, 7055. [2] D. Roy, J. N. Cambre, B. S. Sumerlin, Prog. Polym. Sci. 2010, 35, 278. [3] a) S. Hackelbusch, T. Rossow, H. Becker, S. Seiffert, Macromolecules 2014, 47, 4028; b) S.-k. Ahn, R. M. Kasi, S.-C. Kim, N. Sharma, Y. Zhou, Soft Matter 2008, 4, 1151. [4] H. Chen, L. H. Liu, L. S. Wang, C. B. Ching, H. W. Yu, Y. Y. Yang, Adv. Funct. Mater. 2008, 18, 95. [5] M. I. Gibson, R. K. O'Reilly, Chem. Soc. Rev. 2013, 42, 7204. [6] M. Motornov, R. Sheparovych, R. Lupitskyy, E. MacWilliams, O. Hoy, I. Luzinov, S. Minko, Adv. Funct. Mater. 2007, 17, 2307. [7] N. Rapoport, Prog. Polym. Sci. 2007, 32, 962. [8] M. A. C. Stuart, W. T. Huck, J. Genzer, M. Müller, C. Ober, M. Stamm, G. B. Sukhorukov, I. Szleifer, V. V. Tsukruk, M. Urban, Nat. Mater. 2010, 9, 101. [9] H. Koo, H. Lee, S. Lee, K. H. Min, M. S. Kim, D. S. Lee, Y. Choi, I. C. Kwon, K. Kim, S. Y. Jeong, Chem. Commun. 2010, 46, 5668. [10] D. Buenger, F. Topuz, J. Groll, Prog. Polym. Sci. 2012, 37, 1678. [11] L. He, D. E. Fullenkamp, J. G. Rivera, P. B. Messersmith, Chem. Commun. 2011, 47, 7497. [12] a) C. d. l. H. Alarcon, S. Pennadam, C. Alexander, Chem. Soc. Rev. 2005, 34, 276; b) Y. Ma, Y. Tang, N. C. Billingham, S. P. Armes, A. L. Lewis, Biomacromolecules 2003, 4, 864; c) D. D. Díaz, D. Kühbeck, R. J. Koopmans, Chem. Soc. Rev. 2011, 40, 427. [13] a) D. Han, X. Tong, Y. Zhao, Langmuir 2012, 28, 2327; b) Q. Zhang, N. Re Ko, J. Kwon Oh, Chem. Commun. 2012, 48, 7542. [14] a) L. Hu, Z. Chen, M. J. Serpe, Soft Matter 2012, 8, 10095; b) C. de las Heras Alarcón, S. Pennadam, C. Alexander, Chem. Soc. Rev. 2005, 34, 276; c) L. Dong, A. K. Agarwal, D. J. Beebe, H. Jiang, Nature 2006, 442, 551. [15] M. Rubinstein, R. H. Colby, Polymer Physics, Oxford University Press, Oxford 2003. [16] a) P. Marston, D. Langley, D. Kingsbury, Appl. Sci. Res. 1982, 38, 373; b) J. W. Gooch, Encyclopedic Dictionary of Polymers, 2nd ed., Springer, New York 2011.

Macromol. Rapid Commun. 2014, 35, 1943−1948 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Self-storage: a novel family of stimuli-responsive polymer materials for optical and electrochemical switching.

For most stimuli-responsive polymer materials (SRPMs), such as polymer gels, micelles, and brushes, the responsive mechanism is based on the solubilit...
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