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Ultrasound response of aqueous poly(ionic liquid) solution
Ultrasonics Sonochemistry 30 (2016) 52–60
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Ultrasound response of aqueous poly(ionic liquid) solution Kai Li, Takaomi Kobayashi ⇑ Department of Materials Science and Technology, Nagaoka University of Technology, Kamitomioka 1603-1, Nagaoka 940-2188, Japan
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Article history: Received 31 August 2015 Received in revised form 30 October 2015 Accepted 30 October 2015 Available online 11 November 2015 Keywords: Ultrasound Poly(ionic liquid) Hydrogen bond Response behavior
a b s t r a c t Ultrasound (US) effects on aqueous poly(ionic liquid) (PIL) solution were investigated using viscosity and FT-IR spectroscopy after exposure to US of 23, 43, and 96 kHz frequencies at 50 W. The viscosity of the poly(1-vinyl-3-butyl-imidazolium chloride) (PIL) aqueous solution decreased during exposure to US. It then increased gradually within about 10 min as US stopped. The aqueous PIL behavior was then observed using FT-IR spectroscopy. The US exposure enhanced the FT-IR band intensity of the OH stretching. The band intensity returned to its original value after the US stopped. These results responded cyclically to the US on/off. Analysis of the FT-IR spectra revealed that US influenced the breakage and reformation of hydrogen bonds in the PIL and water. Two-dimensional correlation and deconvolution were used to analyze the change of components in the region of 3000–3700 cm1 for US exposure. Results of these analyses suggest that US exposure might break hydrogen bonds between PIL segments and water. In the absence of US, hydrogen bonds reformation was also observed between the PIL and water. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Poly(ionic liquid) (PIL), or polymerized ionic liquid, a polymer polymerized from ionic liquid (IL) monomer, has unique properties of low vapor pressure, a large chemical window, high thermal stability and tunable properties [1,2]. In fact, PILs possess some features of ionic conductivity, thermal stability, and tunable solution properties in addition to the common polymer properties of processability, durability and mechanical stability. Consequently, PILs behaves as polyelectrolytes and polymer solvents [3]. Recently, an attractive and promising research field related to PIL is the study of their stimulus-responsive behavior. Yan and Texter [4] first reported the reversibility of a PIL copolymer gel in response to solvent variation. Yuan et al. investigated the block copolymers of poly(N-isopropylacrylamide) and PILs, which were dual stimulusresponsive copolymers to changes in temperature and ionic strength in aqueous solution [3]. Mori et al. [5] demonstrated well-defined thermoresponsive ionic liquid block copolymers in which phase-separation behavior induced by temperature was observed. Most studies of the PIL have emphasized a study of the copolymer consisted with PIL segments. The stimulusresponsiveness of homopolymer is slight, especially to ultrasound (US).
⇑ Corresponding author. E-mail address: [email protected] (T. Kobayashi). http://dx.doi.org/10.1016/j.ultsonch.2015.10.021 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.
Actually, US is known to be related with sound having frequencies in the range of 20 kHz–100 MHz. It has been applied in several fields [6]. During US treatment, cavitation is generated in the aqueous media and greatly influencing the chemical and physical properties [7,8]. Therefore, US has been used in organic synthesis [6], material science [9], cleaning process [10], and food industries [11]. Utilization of the physical power of US has also been investigated. For example, acoustic streaming of the US is useful for rapid mixing between different compounds in separation processes [12]. Based on our studies for US responding to aqueous polymer solutions [13–15], it was reported that US can effectively influence the viscosity of aqueous polymer solution [13,14] or inorganic powder-aqueous polymer slurry [15]. As described in those reports, water soluble polymers such as PVA, PEG, PAA, and carrageenans showed viscosity decrease in US stimulus-response when US exposure-induced breakage of polymer–polymer and polymer–water hydrogen bonds led to a considerable decrease in viscosity [13,14]. The US effect was also applied recently to ionic liquids (ILs) having different counter anions for influencing IL with water molecule [16]. In the imidazole-based ILs, the IL–water interaction strongly influenced the Cl counter anion. Therefore, US broke the hydrogen bonds. However, little is known about the effect of US on the PIL solution. Moreover, PIL response to US has not been reported yet. This report describes the responsive behavior of a PIL, poly(1-vinyl-3-butylimidazolium chloride), to US exposure. Results show that the aqueous PIL viscosity changed periodically according to US exposure. The FT-IR spectra of aque-
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ous PIL solution were used to investigate US effects on the aqueous PIL. 2. Materials and methods 2.1. Materials All reagents were used directly after purchase with no further purification. For synthesis of the quaternized imidazole moiety, 1-vinylimidazole (98%) and 1-chlorobutane (98%) were purchased from TCI Co. Ltd. (Japan). In addition, 2,20 -azobis (2methylpropionitrile) (AIBN, 98%) was purchased from Nacalai Tesque Inc. (Japan). Poly(1-vinyl-3-butylimidazolium chloride) used for this study were synthesized according to methods described in earlier reports [17,18]. Fig. 1 shows the synthesis and polymerization as follows. Under N2 atmosphere, 3.0 g 1-vinyl-3-butylimidazolium chloride [19] and 0.06 g AIBN were dissolved in 30 mL CHCl3 with stirring, then the temperature was increased the temperature to 70 °C. After 18 h, the solution was cooled to room temperature and was precipitated in ethyl ether. Then, the white precipitate was dissolved into water and dialyzed (cutoff, 14 000, EIDIA Co. Ltd, Japan) in water for 2 days. The product was obtained through vaporization of water and drink in vacuum at 80 °C for 24 h. (Yield, 85%. 1 H NMR (400 MHz, D2O), d ppm, 7.63, 7.52, 7.38–7.02, 4.83, 4.95–3.57, 2.52, 1.78, 1.33, 0.94). Three aqueous solutions of PIL with concentrations of 40 wt%, 50 wt%, and 66 wt% were prepared for the experiments. 2.2. Shear viscosity measurement of aqueous PIL solution and the US exposure The experimental setup shown in Scheme 1 was for shear viscosity measurement in ultrasound water bath (8.5 cm 13.5 cm 13 cm) using a Brookfield rotating viscometer (Tokyo Keiki Inc., Japan). The shear viscosities of the aqueous solution were measured before and after US exposure with rotor No. 1 within a cylindrical glass vessel (40 mm diameter, 120 mm height). The water bath temperature was maintained at 25 °C by flowing water through whole experiments with thermostat (Ecoline E100, Lauda-Brinkmann, LP). During the experiments, the temperature of the PIL solution containing 26 wt% aqueous solution exposed to the US was monitored. No marked temperature change was detected in the solution. Sample solutions were exposed to US environment for 10 min with US power of 50 W for 23, 43, and 96 kHz, respectively. After US was stopped, viscosity measurements were conducted every minute until the shear viscosity returned to the original value. Similar experiments were conducted twice using the same solution. Before and after US, 1H NMR was used to ascertain whether the structure had been destroyed during the US exposure. Gel permeation chromatography (GPC) was con-
Fig. 1. The synthesis route of PIL.
Scheme 1. Shear viscosity measurement setup.
ducted for PIL samples before and after US exposure using a GPC system equipped with a differential refractive index detector (RI8012; Tosoh Corp.), a CCPS HPLC pump (Tosoh Corp.), packed columns for HPLC (SB-806MHQ, Shodex) and a Chino recorder EB22005. NaCl (50 mM) solution was used as eluent at a flow rate of 1 mL/min at 30 °C. 1H NMR was recorded using an NMR spectrometer (AL-400 NMR; JEOL Ltd., Japan), using D2O as solvent. In addition, the US profiles were tested by an US probe (Olympus Corp) connected with an oscilloscope (TDS3012; Tektronix Inc.) to compare US absorption of PIL solution under different frequencies. 2.3. Determination of US effect with FT-IR spectra FT-IR spectra were measured using a spectrometer (FT-IR/4100; Jasco Corp.) before and after US exposure. In the experiment, two CaF2 plates with 30 mm diameter and 2 mm thickness (Pier Optics Co. Ltd., Japan) were used for the PIL solution samples. Scheme 2 presents an illustration of the US experiments. Here, a small amount of the aqueous PIL solution was dropped on one CaF2 plate and was covered with another CaF2 plate. The PIL solution concentrations were, respectively, 40 wt%, 50 wt%, and 66 wt%. Then, the CaF2 plates with a sample were sealed with Teflon tape (0.1 mm 13 mm, Sanyo, Japan) to prevent water permeation into the sample. The sample-CaF2 window was exposed to 50 W US with the frequencies of 23, 43, and 96 kHz for 1, 3, 5 and 10 min. Then, FT-IR spectra were recorded with 4 cm1 spectral resolution. For peak deconvolution, FT-IR spectra were fitted with a Gaussian function using Origin software. In these cases, the chi-square value for each fitting curve was greater than 0.99. 2.4. Two-dimensional (2D) correlation spectroscopy Generalized two-dimensional correlation was applied to FT-IR spectra to analysis the interaction between different substrates in present work [20–22]. We conducted 2D correlation according to the method described by Noda et al. [23,24] to obtain the 2D synchronous and 2D asynchronous spectra. The correlation intensity (U (v1, v2)) in the 2D synchronous mapping reflected the inphase response. The correlation intensity (W (v1, v2)) in the asynchronous spectrum can be used to analyze the dissimilarity of the spectral intensity variations. From investigation of the crosspeaks in synchronous and asynchronous maps, the order of the spectral intensity changes under a specific environmental perturbation was obtained [23–25]. In the 2D synchronous map, autopeaks, which appeared along the diagonal and were always positive, were observed. Autopeaks were regarded as probes that
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Scheme 2. Processes of making samples (left) and experimental set (right) for US experiment.
changed the greatly under the environmental perturbations. Peaks, which appeared off the diagonal, were designated as peaks (U (v1, v2)). They represent the coincidental changes of spectra intensity variations measured at m1 and m2. The 2D asynchronous map had only off-diagonal cross peaks. They were positive or negative. According to Noda’s rule [23,24], when U (v1, v2) > 0, if W (v1, v2) was positive, then band m1 would change before band m2; while, if W (v1, v2) was negative, then band v1 would vary after v2. Moreover, this rule was reversed for U (v1, v2) < 0. FT-IR spectra with different US exposure times were used for generalized 2D correlation analysis using the software ‘‘2D shige” (Shigeaki Morita, Kwansei-Gakuin University, Japan). The FT-IR spectra for 2D correlation were smoothed and baseline corrected using the software (Spectra Manager 2.0, Jasco Corp.) in the FT-IR equipment. In the 2D correlation maps, which were drawn with origin software, the shaded areas were defined as the negative correlation intensities. Unshaded areas represented positive correlation intensities.
3. Results and discussion 3.1. Ultrasound effects on the shear viscosity of aqueous PIL solution The US effects on the shear viscosity of the aqueous PIL solution were measured immediately after US exposure for 10 min. Within 1 min intervals, the values were recorded until the measured value became equal to the original value. Fig. 2 shows the change in the shear viscosity with and without US exposure. Here, the US output
Fig. 2. Shear viscosity with and without US exposure at 50 W for PIL solution.
power was 50 W for each US frequency. Results show that the aqueous PIL solution viscosity was 1550 Cp. It then decreased to 1360 Cp under US exposure. It gradually returned to the original values, when the US was removed. After the shear viscosity returned to the original value, the aqueous solution was exposed to US a second time. The second and third times showed similar viscosity change. The three cycles indicated that the US could change the aqueous PIL solution shear viscosity periodically. Apparently, the viscosity change was significantly greater for 43 kHz US exposure than for either 23 kHz or 96 kHz. Moreover, the recovery was later for 43 kHz US because of the greater decline in viscosity. As reported, the shear viscosity change can reflect the change of hydrogen bonds in several aqueous polymers [13,14] and IL [16]. Results show that US exposure influenced hydrogen bonds in the aqueous PIL solution, especially for 43 kHz. In order to support above results, US intensity was monitored before and after the US transmitted through the sample solution, (Fig. 3 inserted). The US absorption at different frequencies was compared at different PIL concentrations. The calculation of the US absorption were based on our previous reports [14,26]. The results depicted that the value of US absorption increased with increasing PIL concentration. Moreover, the US absorption showed a tendency of 43 kHz > 96 kHz > 23 kHz, suggesting that the PIL solution strongly absorbed US energy at 43 kHz. This might be due to that hydrogen bond between Cl and water well absorbed the 43 kHz US wave and the absorbed energy might be used to
Fig. 3. US absorption of PIL solution with different concentrations under 23, 43 and 96 kHz US exposure. Inserted: scheme for measuring the US absorption.
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break the hydrogen bonds. However, the reason is still not very clear at this time for frequency dependence. To ascertain whether US irradiation destroyed the polymer chains or not, 1H NMR data were compared before and after the US exposure in D2O (Fig. 4). Results showed that no new peak for the segmental residuals broken from the PIL appeared in the 1H NMR data after US exposure. Moreover, as presented in the Fig. 4B, GPC results showed that the retention time of the PIL samples before and after US exposure were both 11.25 min. There is no additional peak after US exposure in the chromatography. This indicated that the PIL molecular weight had no change during US exposure. These results strongly suggested that US did not destroy the PIL structure.
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exposure. Therefore, the hydrogen bond change is reflected in the FT-IR spectra as well as in ILs. To confirm the results, FT-IR spectra of the aqueous PIL solution were recorded after different durations of US exposure. Fig. 5 presents FT-IR results of three PIL solutions with different concentrations of 40 wt% (a), 50 wt% (b), and 66 wt% (c). In each figure, different frequencies of US exposure was applied for 10 min. Fig. 5a shows that the broad peak intensity in the 3000– 3700 cm1 region was enhanced by US exposure. For the three frequencies of 23, 43, and 96 kHz, the sample with 40 wt% concentration presented a somewhat broader band in the region. Moreover,
3.2. Effect of US in changing the FT-IR spectra In our previous work with low molecular weight IL [16], hydrogen bonds between water and the ionic liquid responded to US
Fig. 4. (A) 1H NMR (400 MHz, D2O) results of PIL before and after US exposure. (B) GPC results before (B) and after (C) US exposure.
Fig. 5. FT-IR spectra of different concentration PIL solution before and after 23, 43 and 96 kHz US exposure for 10 min. ((a) 40 wt%, (b) 50 wt%, (c) 66 wt%).
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among them, the intensity was intensified mostly by 43 kHz US exposure. Regarding the US effect on the aqueous IL system [16], FT-IR spectra of the imidazole IL with Cl as the counter anion increased significantly during US exposure. Similar results were obtained when the solution concentrations were changed to 50 wt% (Fig. 5b) and 66 wt% (Fig. 5c). Because the 43 kHz US influenced the results to the greatest degree, 43 kHz US was exposed to targets for 1, 3, 5, and 10 min. The resultant FT-IR spectra are shown in Fig. 6. It is apparent from Fig. 6a that US exposure gradually enhanced the intensities of the
3000–3700 cm1 region. The intensities increased with increasing of exposure time of 1 to 10 min. As reported earlier in the literature [27,28], this region is related to the O–H stretching of water and C– H stretching in imidazole rings. Both bands are highly overlapped because of the water solvation. Enhancement of this region strongly indicates breakage of the hydrogen bonds in the system. When concentrations of the PIL in the solution were changed, a similar tendency was observed, indicating that the US could break the hydrogen bonds in the solution. Therefore, the results of the OH band enhancement corresponded well to the shear viscosity change, as presented in Fig. 2. To confirm the results by which the shear viscosities returned to the original value, a recovery experiment of the FT-IR spectra peak was conducted. Fig. 7 shows the relative ratio (It/I0) of the peaks center of the OH region at 3409 nm1. Here, the relative intensity of the FT-IR band was designated as It/I0, where I0 and It denote the intensities of the OH stretching region before and after US exposure for t time, respectively. The results demonstrated that the value of It/I0 was increased by US exposure for different PIL concentrations. After the US exposure, the relative intensities returned to the original value gradually, which indicated the reforming of hydrogen bonds in the aqueous PIL solution after removal of the US. In the present PIL solution, water molecules were hydrogen-bonded with the PIL cation and anion. During US exposure, the hydrogen bonds between water and PIL were broken. Then free water came from PIL polymer networks. After removing the US exposure, water molecules might interact into the PIL polymer networks by forming hydrogen bonds. Therefore, the It/I0 value decreased gradually and returned to the initial value. The FT-IR spectral change suggested that in 40 wt% concentration, the water molecules might be easier to move into the PIL polymer networks after US exposure. However, in higher water contents of 66 wt% concentration, it might be difficult to move into the PIL polymer network after US exposure since the recovery of the It/I0 value took a longer time. In 40 wt%, the molar ratio of Cl and water was 12/Mn; while in 66 wt%, the molar ratio was 36/Mn, which was three times compared with 40 wt%. Where, Mn represented the molecular weight of PIL, Mn > 14000. Therefore, the results of Fig. 7 might be suggested that water molecules hardly interacted with the polymer chain after removing the US exposure. 3.3. Analysis with 2D correlation analysis and peak deconvolution for FT-IR spectra Fig. 8 presents 2D correlation results for the region of 3000– 3700 cm1. Here, the synchronous spectra are shown in a, c, and e, asynchronous ones are in b, d, and f. In the synchronous spectrum of Fig. 8a for 40 wt% concentration, one autopeak is apparent at 3418 cnm1. Two positive cross-peaks are found at (3418, 3136) and (3418, 3093). In the corresponding asynchronous spectrum
Fig. 6. FT-IR spectra of different concentration PIL solution before and after 43 kHz US exposure from 1 to 10 min. ((a) 40 wt%, (b) 50 wt%, (c) 66 wt%).
Fig. 7. Intensity ratio of the center of OH band for different PIL solution after 43 kHz US exposure for 10 min.
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Fig. 8. Synchronous (a, c and e) and asynchronous (b, d and f) 2D correlation IR spectra of different concentration PIL solution in the 3000–3700 cm1 region. (a and b) 40 wt%, (c and d) 50 wt%, and (e and f): 66 wt%.
presented in Fig. 8b, five negative cross-peaks are found as following: (3093, 3418), (3136, 3418), (3226, 3418), (3524, 3418), (3588, 3418) and (3648, 3418). Therefore, seven components might exist in the region of 3000–3700 cm1: those for 3093, 3136, 3226, 3418, 3524, 3588, and 3648 cm1. Their bands are centered at 3093 and 3136 cm1 belong to the C–H stretching in the imidazole ring, five belong to different O–H stretching in the OH region of the aqueous PIL solution. Similar results were obtained as shown in Fig. 8c and d, respectively, for 50 wt% and 66 wt% concentration. The bands involved in the region of 300–3700 cm1 were similarly observed as well as that of the 40 wt% concentration. Consequently, it might be inferred that five bands exist in the O–H stretching band with peak centers at about 3093, 3136, 3226, 3418, 3524, 3588, and 3648 cm1.
Because the OH region was highly overlapped, it was difficult to tell the change of each component. Deconvolution of the 3000– 3700 cm1 region was conducted with a Gaussian function to elucidate the change of hydrogen bonds in the system. Peak fitting was conducted based on the second derivatives. Results were combined with analysis by 2D correlation. The peak fitting results are presented in Table 1. The region of the 3000–3700 cm1 band could be well fitted with seven components. All chi-squared were greater than 0.99. Fig. 9A presents FT-IR results of aqueous PILs. Here, for the drying condition, the aqueous PIL solution was dried in vacuum at 80 °C for 24 h. The wet condition was in 66 wt% water concentration. It is apparent that the O–H region became somewhat broader than with the drying condition because of the solvation of PIL with water. However, O–H stretching was observed in the drying condi-
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Table 1 Peak centers of the fitting curves. Peak index
Dry film
Wet film
40 wt % No US
40 wt % US 10 min
50 wt % No US
50 wt % US 10 min
60 wt % No US
60 wt % US 10 min
a b c d e f g
3093 3132 3335 3386 3502
3093 3138 3259 3418 3526 3585 3646
3092 3137 3258 3418 3522 3585 3646
3092 3137 3250 3418 3522 3585 3646
3092 3136 3256 3418 3524 3589 3648
3091 3136 3257 3420 3525 3589 3648
3093 3139 3242 3402 3521 3585 3639
3092 3136 3243 3409 3521 3585 3633
tion in the PIL because small amounts of water in the polymer chain was difficult to remove completely. As shown in the deconvolution components in the drying condition, several Gaussian components were found for c, e, and d (Fig. 9B) as presented in the chemical structure of Fig. 9D. Five peaks for the dry film and seven peaks for the wet film were obtained. Among them in the D spectrum, the peaks of a and b were assigned to the C–H stretching on the imidazole rings. The other three were assigned to O–H stretching. When the water con-
tent in the drying condition was extremely low, all the water molecules might be hydrogen-bonded with the IL [29]. For the drying condition for PIL, the water molecule might also be hydrogenbonded with the polymer as shown in Fig. 9D. Therefore, the fitting peaks of c, d, and e were assigned respectively to the hydrogen bonds in the structure of H2O Cl, H2O C2–H, and H2O C4–H. In contrast, in the wetting condition, the spectrum was divided into five components for the OH stretching region and two for the CH stretching. These peaks were assigned as shown in Fig. 9E as follows: peaks of c, Cl H2O Cl; peaks of d, H2O; peak of e, H2O Cl; peak of f, H2O C2–H and peak of g, H2O C4–H as described in Fig. 9E. Fig. 10 presents deconvolution results for FT-IR in the region of 3000–3700 cm1 before and after US exposure for different PIL aqueous solutions. The results of the 40 wt% samples (Fig. 10a and b) showed peaks of c and d enhanced after US exposure for 10 min, whereas peak g and f were decreased, which indicated that breakage of the hydrogen bonds occurred in the PIL solution. Here, the peak of c was assigned to Cl H2O Cl. The concentration of free water, which possesses the peak of d, increased after US exposure, strongly suggesting that free water was released from the polymer networks during US exposure.
Fig. 9. (A) FT-IR spectra of the dry and wet PIL. Deconvolution results of the dry (B) and wet (C) PIL for the range of 3000–3700 cm1. Interaction mode in drying condition (D) and wet condition (E).
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Fig. 10. Deconvolution results for PIL solution before (a, c and e) and after (b, d and f) US exposure.
Scheme 3. Illustration of hydrogen bond change during US exposure.
Fig. 10c and d present results found for the 50 wt% samples. The figures show that the intensities of peak c, d, and f were higher after US exposure. The peak e intensity decreased slightly, meaning
that the hydrogen bond of Cl H2O Cl and H2O C2–H were easily broke easily by US exposure. In addition, the intensity of free water was enhanced as well as in the case of 40 wt% concentration.
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Almost identical trends were observed in Fig. 10e and f for the 66 wt% concentration. After the US exposure, the intensities of peaks c, d, e, and f were intensified, although peak g intensity decreased. Therefore, these changes in the intensities after the US exposure were followed by the breakage of hydrogen bonding in the cases of water and the PIL, as shown in the Scheme 3. 4. Conclusion This study investigated the US effect on the PIL solution using viscosity measurements and FT-IR spectroscopy. Results showed that the PIL solution viscosity changed periodically in the presence and absence of US exposure. Moreover, the FT-IR intensities were enhanced after US exposure and were able to return to the original value after moving the US setup. These results demonstrated that the aqueous PIL solution can respond to US exposure through breakage and reformation of hydrogen bonds. Results show that US exposure can produce effective responses such as the breakage of hydrogen bonds in an aqueous PIL solution, causing marked decline in viscosity. References [1] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071–2084. [2] J.P. Hallett, T. Welton, Room-temperature ionic liquids: solvents for synthesis and catalysis. 2, Chem. Rev. 111 (2011) 3508–3576. [3] J. Yuan, H. Schlaad, C. Giordano, M. Antonietti, Double hydrophilic diblock copolymers containing a poly(ionic liquid) segment: controlled synthesis, solution property, and application as carbon precursor, Eur. Polym. J. 47 (2011) 772–781. [4] F. Yan, J. Texter, Solvent-reversible poration in ionic liquid copolymers, Angew. Chem. Int. Ed. 119 (2007) 2492–2495. [5] H. Mori, M. Yahagi, T. Endo, Raft polymerization of N-vinylimidazolium salts and synthesis of thermoresponsive ionic liquid block copolymers, Macromolecules 42 (2009) 8082–8092. [6] T.J. Mason, Ultrasound in synthetic organic chemistry, Chem. Soc. Rev. 26 (1997) 443–451. [7] K.S. Suslick, The chemical effects of ultrasound, Sci. Am. 260 (1989) 80–86. [8] E.B. Flint, K.S. Suslick, The temperature of cavitation, Science 253 (1991) 1397– 1399. [9] K.S. Suslick, G.J. Price, Applications of ultrasound to materials chemistry, Annu. Rev. Mater. Res. 29 (1999) 295–326. [10] T. Kobayashi, T. Kobayashi, Y. Hosaka, N. Fujii, Ultrasound-enhanced membrane-cleaning processes applied water treatments: influence of sonic frequency on filtration treatments, Ultrasonics 41 (2003) 185–190.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Ultrasound response of aqueous poly(ionic liquid) solution Kai Li, Takaomi Kobayashi ⇑ Department of Materials Science and Technology, Nagaoka University of Technology, Kamitomioka 1603-1, Nagaoka 940-2188, Japan
a r t i c l e
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Article history: Received 31 August 2015 Received in revised form 30 October 2015 Accepted 30 October 2015 Available online 11 November 2015 Keywords: Ultrasound Poly(ionic liquid) Hydrogen bond Response behavior
a b s t r a c t Ultrasound (US) effects on aqueous poly(ionic liquid) (PIL) solution were investigated using viscosity and FT-IR spectroscopy after exposure to US of 23, 43, and 96 kHz frequencies at 50 W. The viscosity of the poly(1-vinyl-3-butyl-imidazolium chloride) (PIL) aqueous solution decreased during exposure to US. It then increased gradually within about 10 min as US stopped. The aqueous PIL behavior was then observed using FT-IR spectroscopy. The US exposure enhanced the FT-IR band intensity of the OH stretching. The band intensity returned to its original value after the US stopped. These results responded cyclically to the US on/off. Analysis of the FT-IR spectra revealed that US influenced the breakage and reformation of hydrogen bonds in the PIL and water. Two-dimensional correlation and deconvolution were used to analyze the change of components in the region of 3000–3700 cm1 for US exposure. Results of these analyses suggest that US exposure might break hydrogen bonds between PIL segments and water. In the absence of US, hydrogen bonds reformation was also observed between the PIL and water. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Poly(ionic liquid) (PIL), or polymerized ionic liquid, a polymer polymerized from ionic liquid (IL) monomer, has unique properties of low vapor pressure, a large chemical window, high thermal stability and tunable properties [1,2]. In fact, PILs possess some features of ionic conductivity, thermal stability, and tunable solution properties in addition to the common polymer properties of processability, durability and mechanical stability. Consequently, PILs behaves as polyelectrolytes and polymer solvents [3]. Recently, an attractive and promising research field related to PIL is the study of their stimulus-responsive behavior. Yan and Texter [4] first reported the reversibility of a PIL copolymer gel in response to solvent variation. Yuan et al. investigated the block copolymers of poly(N-isopropylacrylamide) and PILs, which were dual stimulusresponsive copolymers to changes in temperature and ionic strength in aqueous solution [3]. Mori et al. [5] demonstrated well-defined thermoresponsive ionic liquid block copolymers in which phase-separation behavior induced by temperature was observed. Most studies of the PIL have emphasized a study of the copolymer consisted with PIL segments. The stimulusresponsiveness of homopolymer is slight, especially to ultrasound (US).
⇑ Corresponding author. E-mail address: [email protected] (T. Kobayashi). http://dx.doi.org/10.1016/j.ultsonch.2015.10.021 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.
Actually, US is known to be related with sound having frequencies in the range of 20 kHz–100 MHz. It has been applied in several fields [6]. During US treatment, cavitation is generated in the aqueous media and greatly influencing the chemical and physical properties [7,8]. Therefore, US has been used in organic synthesis [6], material science [9], cleaning process [10], and food industries [11]. Utilization of the physical power of US has also been investigated. For example, acoustic streaming of the US is useful for rapid mixing between different compounds in separation processes [12]. Based on our studies for US responding to aqueous polymer solutions [13–15], it was reported that US can effectively influence the viscosity of aqueous polymer solution [13,14] or inorganic powder-aqueous polymer slurry [15]. As described in those reports, water soluble polymers such as PVA, PEG, PAA, and carrageenans showed viscosity decrease in US stimulus-response when US exposure-induced breakage of polymer–polymer and polymer–water hydrogen bonds led to a considerable decrease in viscosity [13,14]. The US effect was also applied recently to ionic liquids (ILs) having different counter anions for influencing IL with water molecule [16]. In the imidazole-based ILs, the IL–water interaction strongly influenced the Cl counter anion. Therefore, US broke the hydrogen bonds. However, little is known about the effect of US on the PIL solution. Moreover, PIL response to US has not been reported yet. This report describes the responsive behavior of a PIL, poly(1-vinyl-3-butylimidazolium chloride), to US exposure. Results show that the aqueous PIL viscosity changed periodically according to US exposure. The FT-IR spectra of aque-
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ous PIL solution were used to investigate US effects on the aqueous PIL. 2. Materials and methods 2.1. Materials All reagents were used directly after purchase with no further purification. For synthesis of the quaternized imidazole moiety, 1-vinylimidazole (98%) and 1-chlorobutane (98%) were purchased from TCI Co. Ltd. (Japan). In addition, 2,20 -azobis (2methylpropionitrile) (AIBN, 98%) was purchased from Nacalai Tesque Inc. (Japan). Poly(1-vinyl-3-butylimidazolium chloride) used for this study were synthesized according to methods described in earlier reports [17,18]. Fig. 1 shows the synthesis and polymerization as follows. Under N2 atmosphere, 3.0 g 1-vinyl-3-butylimidazolium chloride [19] and 0.06 g AIBN were dissolved in 30 mL CHCl3 with stirring, then the temperature was increased the temperature to 70 °C. After 18 h, the solution was cooled to room temperature and was precipitated in ethyl ether. Then, the white precipitate was dissolved into water and dialyzed (cutoff, 14 000, EIDIA Co. Ltd, Japan) in water for 2 days. The product was obtained through vaporization of water and drink in vacuum at 80 °C for 24 h. (Yield, 85%. 1 H NMR (400 MHz, D2O), d ppm, 7.63, 7.52, 7.38–7.02, 4.83, 4.95–3.57, 2.52, 1.78, 1.33, 0.94). Three aqueous solutions of PIL with concentrations of 40 wt%, 50 wt%, and 66 wt% were prepared for the experiments. 2.2. Shear viscosity measurement of aqueous PIL solution and the US exposure The experimental setup shown in Scheme 1 was for shear viscosity measurement in ultrasound water bath (8.5 cm 13.5 cm 13 cm) using a Brookfield rotating viscometer (Tokyo Keiki Inc., Japan). The shear viscosities of the aqueous solution were measured before and after US exposure with rotor No. 1 within a cylindrical glass vessel (40 mm diameter, 120 mm height). The water bath temperature was maintained at 25 °C by flowing water through whole experiments with thermostat (Ecoline E100, Lauda-Brinkmann, LP). During the experiments, the temperature of the PIL solution containing 26 wt% aqueous solution exposed to the US was monitored. No marked temperature change was detected in the solution. Sample solutions were exposed to US environment for 10 min with US power of 50 W for 23, 43, and 96 kHz, respectively. After US was stopped, viscosity measurements were conducted every minute until the shear viscosity returned to the original value. Similar experiments were conducted twice using the same solution. Before and after US, 1H NMR was used to ascertain whether the structure had been destroyed during the US exposure. Gel permeation chromatography (GPC) was con-
Fig. 1. The synthesis route of PIL.
Scheme 1. Shear viscosity measurement setup.
ducted for PIL samples before and after US exposure using a GPC system equipped with a differential refractive index detector (RI8012; Tosoh Corp.), a CCPS HPLC pump (Tosoh Corp.), packed columns for HPLC (SB-806MHQ, Shodex) and a Chino recorder EB22005. NaCl (50 mM) solution was used as eluent at a flow rate of 1 mL/min at 30 °C. 1H NMR was recorded using an NMR spectrometer (AL-400 NMR; JEOL Ltd., Japan), using D2O as solvent. In addition, the US profiles were tested by an US probe (Olympus Corp) connected with an oscilloscope (TDS3012; Tektronix Inc.) to compare US absorption of PIL solution under different frequencies. 2.3. Determination of US effect with FT-IR spectra FT-IR spectra were measured using a spectrometer (FT-IR/4100; Jasco Corp.) before and after US exposure. In the experiment, two CaF2 plates with 30 mm diameter and 2 mm thickness (Pier Optics Co. Ltd., Japan) were used for the PIL solution samples. Scheme 2 presents an illustration of the US experiments. Here, a small amount of the aqueous PIL solution was dropped on one CaF2 plate and was covered with another CaF2 plate. The PIL solution concentrations were, respectively, 40 wt%, 50 wt%, and 66 wt%. Then, the CaF2 plates with a sample were sealed with Teflon tape (0.1 mm 13 mm, Sanyo, Japan) to prevent water permeation into the sample. The sample-CaF2 window was exposed to 50 W US with the frequencies of 23, 43, and 96 kHz for 1, 3, 5 and 10 min. Then, FT-IR spectra were recorded with 4 cm1 spectral resolution. For peak deconvolution, FT-IR spectra were fitted with a Gaussian function using Origin software. In these cases, the chi-square value for each fitting curve was greater than 0.99. 2.4. Two-dimensional (2D) correlation spectroscopy Generalized two-dimensional correlation was applied to FT-IR spectra to analysis the interaction between different substrates in present work [20–22]. We conducted 2D correlation according to the method described by Noda et al. [23,24] to obtain the 2D synchronous and 2D asynchronous spectra. The correlation intensity (U (v1, v2)) in the 2D synchronous mapping reflected the inphase response. The correlation intensity (W (v1, v2)) in the asynchronous spectrum can be used to analyze the dissimilarity of the spectral intensity variations. From investigation of the crosspeaks in synchronous and asynchronous maps, the order of the spectral intensity changes under a specific environmental perturbation was obtained [23–25]. In the 2D synchronous map, autopeaks, which appeared along the diagonal and were always positive, were observed. Autopeaks were regarded as probes that
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Scheme 2. Processes of making samples (left) and experimental set (right) for US experiment.
changed the greatly under the environmental perturbations. Peaks, which appeared off the diagonal, were designated as peaks (U (v1, v2)). They represent the coincidental changes of spectra intensity variations measured at m1 and m2. The 2D asynchronous map had only off-diagonal cross peaks. They were positive or negative. According to Noda’s rule [23,24], when U (v1, v2) > 0, if W (v1, v2) was positive, then band m1 would change before band m2; while, if W (v1, v2) was negative, then band v1 would vary after v2. Moreover, this rule was reversed for U (v1, v2) < 0. FT-IR spectra with different US exposure times were used for generalized 2D correlation analysis using the software ‘‘2D shige” (Shigeaki Morita, Kwansei-Gakuin University, Japan). The FT-IR spectra for 2D correlation were smoothed and baseline corrected using the software (Spectra Manager 2.0, Jasco Corp.) in the FT-IR equipment. In the 2D correlation maps, which were drawn with origin software, the shaded areas were defined as the negative correlation intensities. Unshaded areas represented positive correlation intensities.
3. Results and discussion 3.1. Ultrasound effects on the shear viscosity of aqueous PIL solution The US effects on the shear viscosity of the aqueous PIL solution were measured immediately after US exposure for 10 min. Within 1 min intervals, the values were recorded until the measured value became equal to the original value. Fig. 2 shows the change in the shear viscosity with and without US exposure. Here, the US output
Fig. 2. Shear viscosity with and without US exposure at 50 W for PIL solution.
power was 50 W for each US frequency. Results show that the aqueous PIL solution viscosity was 1550 Cp. It then decreased to 1360 Cp under US exposure. It gradually returned to the original values, when the US was removed. After the shear viscosity returned to the original value, the aqueous solution was exposed to US a second time. The second and third times showed similar viscosity change. The three cycles indicated that the US could change the aqueous PIL solution shear viscosity periodically. Apparently, the viscosity change was significantly greater for 43 kHz US exposure than for either 23 kHz or 96 kHz. Moreover, the recovery was later for 43 kHz US because of the greater decline in viscosity. As reported, the shear viscosity change can reflect the change of hydrogen bonds in several aqueous polymers [13,14] and IL [16]. Results show that US exposure influenced hydrogen bonds in the aqueous PIL solution, especially for 43 kHz. In order to support above results, US intensity was monitored before and after the US transmitted through the sample solution, (Fig. 3 inserted). The US absorption at different frequencies was compared at different PIL concentrations. The calculation of the US absorption were based on our previous reports [14,26]. The results depicted that the value of US absorption increased with increasing PIL concentration. Moreover, the US absorption showed a tendency of 43 kHz > 96 kHz > 23 kHz, suggesting that the PIL solution strongly absorbed US energy at 43 kHz. This might be due to that hydrogen bond between Cl and water well absorbed the 43 kHz US wave and the absorbed energy might be used to
Fig. 3. US absorption of PIL solution with different concentrations under 23, 43 and 96 kHz US exposure. Inserted: scheme for measuring the US absorption.
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break the hydrogen bonds. However, the reason is still not very clear at this time for frequency dependence. To ascertain whether US irradiation destroyed the polymer chains or not, 1H NMR data were compared before and after the US exposure in D2O (Fig. 4). Results showed that no new peak for the segmental residuals broken from the PIL appeared in the 1H NMR data after US exposure. Moreover, as presented in the Fig. 4B, GPC results showed that the retention time of the PIL samples before and after US exposure were both 11.25 min. There is no additional peak after US exposure in the chromatography. This indicated that the PIL molecular weight had no change during US exposure. These results strongly suggested that US did not destroy the PIL structure.
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exposure. Therefore, the hydrogen bond change is reflected in the FT-IR spectra as well as in ILs. To confirm the results, FT-IR spectra of the aqueous PIL solution were recorded after different durations of US exposure. Fig. 5 presents FT-IR results of three PIL solutions with different concentrations of 40 wt% (a), 50 wt% (b), and 66 wt% (c). In each figure, different frequencies of US exposure was applied for 10 min. Fig. 5a shows that the broad peak intensity in the 3000– 3700 cm1 region was enhanced by US exposure. For the three frequencies of 23, 43, and 96 kHz, the sample with 40 wt% concentration presented a somewhat broader band in the region. Moreover,
3.2. Effect of US in changing the FT-IR spectra In our previous work with low molecular weight IL [16], hydrogen bonds between water and the ionic liquid responded to US
Fig. 4. (A) 1H NMR (400 MHz, D2O) results of PIL before and after US exposure. (B) GPC results before (B) and after (C) US exposure.
Fig. 5. FT-IR spectra of different concentration PIL solution before and after 23, 43 and 96 kHz US exposure for 10 min. ((a) 40 wt%, (b) 50 wt%, (c) 66 wt%).
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among them, the intensity was intensified mostly by 43 kHz US exposure. Regarding the US effect on the aqueous IL system [16], FT-IR spectra of the imidazole IL with Cl as the counter anion increased significantly during US exposure. Similar results were obtained when the solution concentrations were changed to 50 wt% (Fig. 5b) and 66 wt% (Fig. 5c). Because the 43 kHz US influenced the results to the greatest degree, 43 kHz US was exposed to targets for 1, 3, 5, and 10 min. The resultant FT-IR spectra are shown in Fig. 6. It is apparent from Fig. 6a that US exposure gradually enhanced the intensities of the
3000–3700 cm1 region. The intensities increased with increasing of exposure time of 1 to 10 min. As reported earlier in the literature [27,28], this region is related to the O–H stretching of water and C– H stretching in imidazole rings. Both bands are highly overlapped because of the water solvation. Enhancement of this region strongly indicates breakage of the hydrogen bonds in the system. When concentrations of the PIL in the solution were changed, a similar tendency was observed, indicating that the US could break the hydrogen bonds in the solution. Therefore, the results of the OH band enhancement corresponded well to the shear viscosity change, as presented in Fig. 2. To confirm the results by which the shear viscosities returned to the original value, a recovery experiment of the FT-IR spectra peak was conducted. Fig. 7 shows the relative ratio (It/I0) of the peaks center of the OH region at 3409 nm1. Here, the relative intensity of the FT-IR band was designated as It/I0, where I0 and It denote the intensities of the OH stretching region before and after US exposure for t time, respectively. The results demonstrated that the value of It/I0 was increased by US exposure for different PIL concentrations. After the US exposure, the relative intensities returned to the original value gradually, which indicated the reforming of hydrogen bonds in the aqueous PIL solution after removal of the US. In the present PIL solution, water molecules were hydrogen-bonded with the PIL cation and anion. During US exposure, the hydrogen bonds between water and PIL were broken. Then free water came from PIL polymer networks. After removing the US exposure, water molecules might interact into the PIL polymer networks by forming hydrogen bonds. Therefore, the It/I0 value decreased gradually and returned to the initial value. The FT-IR spectral change suggested that in 40 wt% concentration, the water molecules might be easier to move into the PIL polymer networks after US exposure. However, in higher water contents of 66 wt% concentration, it might be difficult to move into the PIL polymer network after US exposure since the recovery of the It/I0 value took a longer time. In 40 wt%, the molar ratio of Cl and water was 12/Mn; while in 66 wt%, the molar ratio was 36/Mn, which was three times compared with 40 wt%. Where, Mn represented the molecular weight of PIL, Mn > 14000. Therefore, the results of Fig. 7 might be suggested that water molecules hardly interacted with the polymer chain after removing the US exposure. 3.3. Analysis with 2D correlation analysis and peak deconvolution for FT-IR spectra Fig. 8 presents 2D correlation results for the region of 3000– 3700 cm1. Here, the synchronous spectra are shown in a, c, and e, asynchronous ones are in b, d, and f. In the synchronous spectrum of Fig. 8a for 40 wt% concentration, one autopeak is apparent at 3418 cnm1. Two positive cross-peaks are found at (3418, 3136) and (3418, 3093). In the corresponding asynchronous spectrum
Fig. 6. FT-IR spectra of different concentration PIL solution before and after 43 kHz US exposure from 1 to 10 min. ((a) 40 wt%, (b) 50 wt%, (c) 66 wt%).
Fig. 7. Intensity ratio of the center of OH band for different PIL solution after 43 kHz US exposure for 10 min.
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Fig. 8. Synchronous (a, c and e) and asynchronous (b, d and f) 2D correlation IR spectra of different concentration PIL solution in the 3000–3700 cm1 region. (a and b) 40 wt%, (c and d) 50 wt%, and (e and f): 66 wt%.
presented in Fig. 8b, five negative cross-peaks are found as following: (3093, 3418), (3136, 3418), (3226, 3418), (3524, 3418), (3588, 3418) and (3648, 3418). Therefore, seven components might exist in the region of 3000–3700 cm1: those for 3093, 3136, 3226, 3418, 3524, 3588, and 3648 cm1. Their bands are centered at 3093 and 3136 cm1 belong to the C–H stretching in the imidazole ring, five belong to different O–H stretching in the OH region of the aqueous PIL solution. Similar results were obtained as shown in Fig. 8c and d, respectively, for 50 wt% and 66 wt% concentration. The bands involved in the region of 300–3700 cm1 were similarly observed as well as that of the 40 wt% concentration. Consequently, it might be inferred that five bands exist in the O–H stretching band with peak centers at about 3093, 3136, 3226, 3418, 3524, 3588, and 3648 cm1.
Because the OH region was highly overlapped, it was difficult to tell the change of each component. Deconvolution of the 3000– 3700 cm1 region was conducted with a Gaussian function to elucidate the change of hydrogen bonds in the system. Peak fitting was conducted based on the second derivatives. Results were combined with analysis by 2D correlation. The peak fitting results are presented in Table 1. The region of the 3000–3700 cm1 band could be well fitted with seven components. All chi-squared were greater than 0.99. Fig. 9A presents FT-IR results of aqueous PILs. Here, for the drying condition, the aqueous PIL solution was dried in vacuum at 80 °C for 24 h. The wet condition was in 66 wt% water concentration. It is apparent that the O–H region became somewhat broader than with the drying condition because of the solvation of PIL with water. However, O–H stretching was observed in the drying condi-
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Table 1 Peak centers of the fitting curves. Peak index
Dry film
Wet film
40 wt % No US
40 wt % US 10 min
50 wt % No US
50 wt % US 10 min
60 wt % No US
60 wt % US 10 min
a b c d e f g
3093 3132 3335 3386 3502
3093 3138 3259 3418 3526 3585 3646
3092 3137 3258 3418 3522 3585 3646
3092 3137 3250 3418 3522 3585 3646
3092 3136 3256 3418 3524 3589 3648
3091 3136 3257 3420 3525 3589 3648
3093 3139 3242 3402 3521 3585 3639
3092 3136 3243 3409 3521 3585 3633
tion in the PIL because small amounts of water in the polymer chain was difficult to remove completely. As shown in the deconvolution components in the drying condition, several Gaussian components were found for c, e, and d (Fig. 9B) as presented in the chemical structure of Fig. 9D. Five peaks for the dry film and seven peaks for the wet film were obtained. Among them in the D spectrum, the peaks of a and b were assigned to the C–H stretching on the imidazole rings. The other three were assigned to O–H stretching. When the water con-
tent in the drying condition was extremely low, all the water molecules might be hydrogen-bonded with the IL [29]. For the drying condition for PIL, the water molecule might also be hydrogenbonded with the polymer as shown in Fig. 9D. Therefore, the fitting peaks of c, d, and e were assigned respectively to the hydrogen bonds in the structure of H2O Cl, H2O C2–H, and H2O C4–H. In contrast, in the wetting condition, the spectrum was divided into five components for the OH stretching region and two for the CH stretching. These peaks were assigned as shown in Fig. 9E as follows: peaks of c, Cl H2O Cl; peaks of d, H2O; peak of e, H2O Cl; peak of f, H2O C2–H and peak of g, H2O C4–H as described in Fig. 9E. Fig. 10 presents deconvolution results for FT-IR in the region of 3000–3700 cm1 before and after US exposure for different PIL aqueous solutions. The results of the 40 wt% samples (Fig. 10a and b) showed peaks of c and d enhanced after US exposure for 10 min, whereas peak g and f were decreased, which indicated that breakage of the hydrogen bonds occurred in the PIL solution. Here, the peak of c was assigned to Cl H2O Cl. The concentration of free water, which possesses the peak of d, increased after US exposure, strongly suggesting that free water was released from the polymer networks during US exposure.
Fig. 9. (A) FT-IR spectra of the dry and wet PIL. Deconvolution results of the dry (B) and wet (C) PIL for the range of 3000–3700 cm1. Interaction mode in drying condition (D) and wet condition (E).
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Fig. 10. Deconvolution results for PIL solution before (a, c and e) and after (b, d and f) US exposure.
Scheme 3. Illustration of hydrogen bond change during US exposure.
Fig. 10c and d present results found for the 50 wt% samples. The figures show that the intensities of peak c, d, and f were higher after US exposure. The peak e intensity decreased slightly, meaning
that the hydrogen bond of Cl H2O Cl and H2O C2–H were easily broke easily by US exposure. In addition, the intensity of free water was enhanced as well as in the case of 40 wt% concentration.
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Almost identical trends were observed in Fig. 10e and f for the 66 wt% concentration. After the US exposure, the intensities of peaks c, d, e, and f were intensified, although peak g intensity decreased. Therefore, these changes in the intensities after the US exposure were followed by the breakage of hydrogen bonding in the cases of water and the PIL, as shown in the Scheme 3. 4. Conclusion This study investigated the US effect on the PIL solution using viscosity measurements and FT-IR spectroscopy. Results showed that the PIL solution viscosity changed periodically in the presence and absence of US exposure. Moreover, the FT-IR intensities were enhanced after US exposure and were able to return to the original value after moving the US setup. These results demonstrated that the aqueous PIL solution can respond to US exposure through breakage and reformation of hydrogen bonds. Results show that US exposure can produce effective responses such as the breakage of hydrogen bonds in an aqueous PIL solution, causing marked decline in viscosity. References [1] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071–2084. [2] J.P. Hallett, T. Welton, Room-temperature ionic liquids: solvents for synthesis and catalysis. 2, Chem. Rev. 111 (2011) 3508–3576. [3] J. Yuan, H. Schlaad, C. Giordano, M. Antonietti, Double hydrophilic diblock copolymers containing a poly(ionic liquid) segment: controlled synthesis, solution property, and application as carbon precursor, Eur. Polym. J. 47 (2011) 772–781. [4] F. Yan, J. Texter, Solvent-reversible poration in ionic liquid copolymers, Angew. Chem. Int. Ed. 119 (2007) 2492–2495. [5] H. Mori, M. Yahagi, T. Endo, Raft polymerization of N-vinylimidazolium salts and synthesis of thermoresponsive ionic liquid block copolymers, Macromolecules 42 (2009) 8082–8092. [6] T.J. Mason, Ultrasound in synthetic organic chemistry, Chem. Soc. Rev. 26 (1997) 443–451. [7] K.S. Suslick, The chemical effects of ultrasound, Sci. Am. 260 (1989) 80–86. [8] E.B. Flint, K.S. Suslick, The temperature of cavitation, Science 253 (1991) 1397– 1399. [9] K.S. Suslick, G.J. Price, Applications of ultrasound to materials chemistry, Annu. Rev. Mater. Res. 29 (1999) 295–326. [10] T. Kobayashi, T. Kobayashi, Y. Hosaka, N. Fujii, Ultrasound-enhanced membrane-cleaning processes applied water treatments: influence of sonic frequency on filtration treatments, Ultrasonics 41 (2003) 185–190.
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