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Synthesis of poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) with low polydispersity using ultrasonic irradiation
Accepted Manuscript Synthesis of poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) with low polydispersity using ultrasonic irradiation Masaki Kubo, Takuya Sone, Masahiro Ohata, Takao Tsukada PII: DOI: Reference:
S1350-4177(18)30450-4 https://doi.org/10.1016/j.ultsonch.2018.08.022 ULTSON 4286
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
24 March 2018 10 July 2018 20 August 2018
Please cite this article as: M. Kubo, T. Sone, M. Ohata, T. Tsukada, Synthesis of poly(N-isopropylacrylamideco-2-hydroxyethyl methacrylate) with low polydispersity using ultrasonic irradiation, Ultrasonics Sonochemistry (2018), doi: https://doi.org/10.1016/j.ultsonch.2018.08.022
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Synthesis of poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) with low polydispersity using ultrasonic irradiation
Masaki Kubo*, Takuya Sone, Masahiro Ohata, Takao Tsukada
Department of Chemical Engineering, Tohoku University, 6-6-07 Aramaki, Aoba-ku, Sendai 980-8579, Japan
* Corresponding Author Masaki Kubo Department of Chemical Engineering, Tohoku University, 6-6-07 Aramaki, Aoba-ku, Sendai 980-8579, Japan Phone: +81-22-795-7261 Fax: +81-22-795-7261 E-mail : [email protected]
1
Abstract Poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) having low polydispersity was synthesized in mixed solvent of ethanol and water using ultrasonic irradiation without any chemical polymerization initiator. The effects of the volume fraction of ethanol in the solvent, the molar ratio of two monomers, the monomer concentration and the ultrasonic power intensity on the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity of synthesized polymer were investigated in order to determine the optimal conditions to synthesize the copolymers with a narrow molecular weight distribution (i.e. low polydispersity). The optimum volume fraction of ethanol in the solvent was 60 vol% to synthesize the copolymers with a low polydispersity. A higher ultrasonic power intensity resulted in a faster polymerization rate and a lower number average molecular weight. The polydispersity was less than 1.5 for all ultrasonic power intensities up to 450 W/dm3 applied in this work. A higher monomer concentration gave a faster polymerization rate and a higher number average molecular weight. The polydispersity was less than 1.5 when the monomer concentration was lower than 0.4 mol/dm3. A higher molar ratio of N-isopropylacrylamide resulted in a higher polymerization rate and a lower number average molecular weight. The copolymers with polydispersity less than 1.5 can be obtained regardless of the molar ratio of N-isopropylacrylamide. The copolymers synthesized by the ultrasonic polymerization method had a high temperature responsibility.
Keywords ultrasonic
polymerization;
copolymer;
N-isopropylacrylamide;
temperature responsivity
2
low
polydispersity;
1. Introduction Poly(N-isopropylacrylamide) (PNIPAM) is a temperature-responsive polymer with a lower critical solution temperature (LCST) [1-3]. Poly(2-hydroxyethyl methacrylate) (PHEMA) has a high mechanical strength [4], so that a copolymer of Poly(NIPAM-co-HEMA) is expected to be applied as advanced functional polymeric materials in various fields, such as sensors [5], actuators [6], and drug delivery system [7]. Since the properties of the copolymers are dependent on the molecular weight and polydispersity, it is necessary to control both molecular weight and polydispersity for their application. Ultrasonic irradiation to liquid results in cavitation phenomena, i.e., the formation and collapse of micro scale bubbles [8]. Collapsing bubbles generate a local high temperature and high pressure fields [9, 10], and extremely high shear flow in liquid [11]. When ultrasonic irradiation is applied to a solution containing vinyl monomers, the radical species are generated due to thermal decomposition of the monomers [12, 13]. These radical species can act as a polymerization initiator, so that radical polymerization takes place without any chemical polymerization initiator under ultrasonic irradiation. The synthesized polymer decomposes due to the high shear stress [14], simultaneously. Ultrasonic irradiation to a monomer solution has been applied to produce a wide variety of polymers such as polystyrene [15], poly(methyl methacrylate) [16, 17], poly(styrene-co-methyl methacrylate), and poly(styrene-co–n-butyl methacrylate) [18]. Since polymerization and polymer degradation proceed simultaneously in the polymer synthesis under ultrasonic irradiation [16], both the molecular weight and polydispersity of the polymer are expected to be controlled by regulating the conditions of ultrasonic irradiation. However, most of the researches focused on molecular weight only. We have demonstrated that the homopolymer, PHEMA, with a narrow molecular weight distribution (i.e. low polydispersity) can be obtained in the solution polymerization by controlling the condition of 3
ultrasonic irradiation [19]. In this study, copolymer poly(NIPAM-co-HEMA) having low polydispersity was synthesized in mixed solvent of ethanol and water using ultrasonic irradiation without any chemical polymerization initiator. The effects of the volume fraction of ethanol in the solvent, the monomer molar ratio of NIPAM, the monomer concentration and the ultrasonic power intensity on the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity of synthesized polymers were investigated in order to determine the optimal conditions for the synthesis of the copolymers with low polydispersity.
2. Experimental Procedure Figure 1 shows an illustration of the experimental apparatus used for copolymer synthesis under ultrasonic irradiation. A horn type ultrasonic generator (VC 750, Sonics & Materials Inc., USA) operating at a frequency of 20 kHz was used. The diameter of the probe was 13 mm. The probe was immersed 30 mm from the surface of the reaction solution. The reactor was made of glass, and its inner diameter and outer diameter were 55 mm and 60 mm, respectively, and its height was 100 mm. There were an inlet for nitrogen gas, a syringe for sampling, and a condenser at the lid of the reactor. The reactor was placed in a water bath whose temperature was regulated using a water circulator (BF200, Yamato Science Co., Ltd., Tokyo, Japan). N-isopropylacrylamide (NIPAM) and 2-hydroxyethyl methacrylate (HEMA) were used as monomers. The chemical structures of the monomers are shown in Figure 2. The solvent was the mixture of ethanol and water. 100 cm3 of the solution containing the monomers was poured into the reactor. The height of the solution in the reactor was 42 mm. The probe was immersed 25 mm from the surface of the solution, so that the distance between the probe tip and the bottom of the reactor was 17 mm. The reaction solution was deoxygenated by 4
bubbling with nitrogen. The polymer synthesis was started by irradiating ultrasound to the reaction solution. The ultrasonic power intensity to the reaction solution was evaluated by the calorimetry method [20]. In this method, the ultrasonic power intensity was evaluated by the heat capacity of solution, the mass of solution, and the temperature rise per second after starting the ultrasonic irradiation. The temperature of the reaction solution was 303 K. The effects of the volume fraction of ethanol in the solvent, the molar ratio of NIPAM to HEMA, the ultrasonic power intensity, and the monomer concentration in reaction solution was investigated, as shown in Table 1. The standard experimental conditions were as follows: the volume fraction of ethanol in the solvent was 60 vol%, the molar ratio of NIPAM to HEMA was 5:5, the monomer concentration was 0.4 mol/dm3, and the ultrasonic power intensity was 300 W/dm3. During the reaction, 1 cm3 of the reaction solution was collected at specific time intervals. The conversion to the polymer and the molecular weight distribution of the polymer was measured using a gel permeation chromatography (GPC) (W2695, Waters Co., USA) equipped with a GPC column (Shodex LF-804, Showa Denko K.K., Japan) and a refractive index detector (W2414, Waters Co., USA). N,N-dimethylformamide containing 0.01 mol/dm3 LiBr was used as a mobile phase, and flow rate of the mobile phase was 0.5 cm3/min. The temperature of the column oven and detector was 313 K. The number average molecular weight and polydispersity of the polymer were estimated from the molecular weight distribution. Polystyrene standards were used to estimate the molecular weight. In order to evaluate the temperature responsivity of the polymer, the synthesized polymer was dried and dissolved in pure water with the concentration of 0.5 wt% and then the transmittance of the aqueous solution containing the polymer was measured by temperature-controllable ultraviolet-visible spectrophotometer (V-570, JASCO Co., Japan).
5
3. Results and discussion 3.1 Synthesis of homopolymers In the previous study on the ultrasonic synthesis of PHEMA homopolymer, the volume fraction of ethanol in solvent was fixed at 50 vol% [19]. However, the volume fraction of ethanol in solvent affects not only the polymerization rate but also the solubility of the polymer in solvent. Therefore, prior to the copolymer synthesis, PNIPAM and PHEMA homopolymers were synthesized, respectively, and the effect of the volume fraction of ethanol on polymerization was investigated. Figure 3 shows the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity at various volume fractions of ethanol for PNIPAM synthesis. The monomer concentration is 0.4 mol/dm3 and the ultrasonic power intensity is 300 W/dm3. The volume fractions of ethanol in the solvent are 60, 70 and 80 vol%. The higher the volume fraction of ethanol, the lower the conversion to the polymer, i.e. the slower the polymerization rate. Ethanol acts as a radical scavenger for radical polymerization, so that the polymerization rate is low at higher volume fraction of ethanol. When the volume fraction of ethanol was 70 and 80 vol%, the number average molecular weight increased in the early stage and then decreased. This result suggested that the polymerization and polymer degradation occurred simultaneously under the ultrasonic irradiation. In the early stage, the amount of the synthesized polymer was not so large that the polymerization was dominant compared to the polymer degradation. As the polymerization proceeded, the amount of the synthesized polymer became large so that the effect of the polymer degradation was dominant. When the volume fraction of ethanol was 80 vol%, the polymerization rate was so slow that the number average molecular weight was considered to reach the maximum at 30 min. When the volume fraction of ethanol was 60 vol%, the polymerization rate was so fast that the number average molecular weight was considered to reach the maximum in the very early 6
stage. The molecular weight in the early stage was higher than that in the case of 80 vol% of the volume fraction of ethanol. The polydispersity decreased with time. This result suggested that the degradation rate of a polymer with a higher molecular weight was faster than that of a polymer with lower molecular weight. The theoretical minimum of polydispersity for radical polymerization was 1.5 for recombination and 2.0 for disproportionation [21], respectively. Under ultrasonic irradiation, PNIPAM with polydispersity around 1.5 was obtained. Thus, the polymer with low polydispersity can be obtained by the ultrasonic polymerization method. Note that PNIPAM polymer with low polydispersity of 1.5 could not be obtained at volume fraction of ethanol below 50 vol%, because PNIPAM was precipitated at lower volume fraction of ethanol and inhibited the propagation of ultrasound to the reaction solution. Figure 4 shows the time courses of the conversion to the polymer, the number average molecular weight and the polydispersity at various volume fractions of ethanol in the solvent for PHEMA synthesis. The monomer concentration is 0.4 mol/dm3 and the ultrasonic power intensity is 300 W/dm3. The higher the volume fraction of ethanol, the slower the polymerization rate, similarly to the case of PNIPAM. The behavior of the number average molecular weight of PHEMA was almost similar to that of PNIPAM. When the volume fraction of ethanol was 70 vol%, the number average molecular weight in the early stage was highest and then decreased. The polymerization rate was so slow that the number average molecular weight was considered to reach the maximum at 120 min. When the volume fraction of ethanol was 60 and 50 vol%, the polymerization rate was so fast that the number average molecular weight was considered to reach the maximum in the very early stage. The molecular weight in the early stage was higher than that in the case of 70 vol% of the volume fraction of ethanol. Note that the conversion to copolymer prior to 60 min was too low to analyze the molecular weight. However, PHEMA with low polydispersity less than 1.5 was obtained even if the volume fraction of ethanol was relatively low, e.g. 50 vol%. Since 7
PHEMA dissolves well in both water and ethanol, the degradation of PHMEA occurs at any volume fraction of ethanol in the solvent. Note that the scale of the horizontal axis in Figure 4 is twice that in of the horizontal axis Figure 3, namely, the polymerization rate of PHEMA was slower than that of PNIPAM at the same volume fraction of ethanol.
3.2 Synthesis of copolymer Figure 5 shows the time courses of the conversion to the polymer, the number average molecular weight and the polydispersity at various volume fractions of ethanol in the solvent for poly(NIPAM-co-HEMA) synthesis. The molar ratio of NIPAM to HEMA is 5:5, the monomer concentration is 0.4 mol/dm3, and the ultrasonic power intensity is 300 W/dm3. The higher the volume fraction of ethanol, the lower the polymerization rate. This tendency was the same as those of the homopolymers shown in Figures 3 and 4. When the volume fraction of ethanol was 70 vol%, the polymerization rate was slower than any other volume fraction. The number average molecular weight increased in the early stage and then decreased. As discussed for PNIPAM in Figure 3, the polymerization is dominant in the early stage because of the small amount of the synthesized polymer, and then the polymer degradation becomes dominant as the polymerization proceeds. When the volume fraction of ethanol was 60 and 50 vol%, the polymerization rate was so fast that the number average molecular weight was considered to reach the maximum in the very early stage. The molecular weight in the early stage was higher than that in the case of 70 vol% of the volume fraction of ethanol. The polydispersity was less than 1.5, similarly to the homopolymers. Thus, a copolymers with low polydispersity can be obtained by the ultrasonic polymerization method. When the volume fraction of ethanol was 50 vol%, the polydispersity increased in the later stage of the reaction because a part of copolymer was precipitated. Thus, the volume fraction of ethanol was set to 60 vol% in subsequent experiments. 8
Figure 6 shows the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity at various ultrasonic power intensities for copolymer synthesis. The molar ratio of NIPAM to HEMA is 5:5, and the monomer concentration is 0.4 mol/dm3. The higher the ultrasonic power intensity, the faster the polymerization rate. This was because the larger number of radical was generated at higher ultrasonic power intensity. The number average molecular weight was lower at higher ultrasonic power intensity, because the copolymer was exposed to higher shear stress due to the collapse of cavitation bubbles. The polydispersity was less than 1.5 for all the cases. Thus, the molecular weight of the copolymer can be controlled by ultrasonic power intensity while maintaining low polydispersity. Figure 7 shows the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity at various monomer concentrations for copolymer synthesis. The molar ratio of NIPAM to HEMA is 5:5, and the ultrasonic power intensity is 300 W/dm3. The higher the monomer concentration, the higher the conversion to the polymer and molecular weight of synthesized polymer. Those tendencies are consistent with general radical polymerization. However, the polymerization rate at monomer concentration of 0.8 mol/dm3 was almost the same as that at 0.4 mol/dm3. This is because higher monomer concentration results in higher solution viscosity. When the solution viscosity is high, the cavitation should be more difficult to generate [22]. The molecular weights at 0.4 and 0.8 mol/dm3 decreased with time due to the degradation of the generated polymer. In the case of the monomer concentration of 0.2 mol/dm3, the molecular weight increased in the early stage of the reaction, and then gradually decreased. This increase of the molecular weight in the early stage was due to the slow polymerization rate at lower monomer concentration and the polymerization was dominant due to the low conversion to polymer. In the case of monomer concentration of 0.1 mol/dm3, the molecular weight was almost constant. This was because 9
the molecular weight was too small to occur the degradation of generated polymer. The polydispersity was less than 1.5 when the monomer concentration was lower than 0.4 mol/dm3. In the case of 0.8 mol/dm3, however, the polydispersity was eventually high. In this case, the viscosity of the solution was relatively high in the late stage when the polymerization proceeded. Higher viscosity results in less generation of ultrasonic cavitation bubble, so that the rate of polymer degradation was low. Figure 8 shows the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity at various molar ratios of NIPAM and HEMA. The monomer concentration is 0.4 mol/dm3, and the ultrasonic power intensity is 300 W/dm3. The higher the molar ratio of NIPAM, the higher polymerization rate and the lower the molecular weight. This is because the NIPAM-terminated radical is secondary one so that its reaction activity is much higher than that of the HEMA-terminated radical, tertiary one. In all the molar ratios of NIPAM, the polymer with low polydispersity can be obtained by the ultrasonic polymerization method. In order to demonstrate the temperature responsivity of the polymers synthesized by ultrasonic polymerization method, the transmittance of the aqueous solutions containing the polymers was measured. Figure 9 shows the temperature dependence of the transmittance of the polymer aqueous solutions. The transmittance of unity denotes that the polymer is dissolved in water. As shown in Figure 9, the transmittance decreased sharply at a certain temperature depending on the molar ratio of NIPAM. This suggests that the polymers synthesized by the ultrasonic polymerization method have a sharp temperature responsivity. From the results in Figure 9, a lower critical solution temperature (LCST) was estimated for the polymers synthesized at various molar fractions of NIPAM. The LCST is defined as the temperature at the transmittance of 0.5. Figure 10 shows the effect of the molar fraction of NIPAM on LCST. The LCST of poly(NIPAM-co-HEMA) increased linearly with the molar 10
fraction of NIPAM. The LCST of the copolymer synthesized by general radical polymerization using chemical initiator [23] was also shown in Figure 10. The LCST in this study was almost the same as that by general radical polymerization. The decrease of LCST with decreasing NIPAM is attributed to the increment of hydrophobicity due to the hydrogen-bonding between the hydroxyl groups in HEMA and the amide groups in NIPAM [24, 25]. The sharpness of the transition, defined as the difference in temperature between high and low transmittances in Figure 9, was less than 3 K for all the condition. The temperature sharpness of the transition in this study was almost the same as that by general radical polymerization.
4. Conclusions In this study, poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) having low polydispersity was synthesized in the mixed solvent of ethanol and water using the ultrasonic polymerization method without any chemical polymerization initiator. Considering the polymerization rate and polydispersity of molecular weight, the optimum volume fraction of ethanol was 60 vol% to synthesize the copolymers with low polydispersity. The higher ultrasonic power intensity resulted in the faster polymerization rate and lower number average molecular weight due to the higher number of radical. The polydispersity was less than 1.5 for all the ultrasonic power intensity, so that the molecular weight can be controlled by ultrasonic power intensity while maintaining low polydispersity. The higher monomer concentration gave the higher conversion to polymer and higher number average molecular weight. The polydispersity was less than 1.5 when the monomer concentration was lower than 0.4 mol/dm3. The higher molar ratio of NIPAM resulted in higher polymerization rate and lower number average molecular weight. In all the monomer fraction, the polymer with low polydispersity can be obtained. 11
The polymers synthesized by the ultrasonic polymerization method have a temperature responsivity. The values of a lower critical solution temperature (LCST) in this study was almost the same as those by general radical polymerization.
Acknowledgement This research was supported in part by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 16K14467.
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[19] M. Kubo, T. Kondo, H. Matsui, N. Shibasaki-Kitakawa, T. Yonemoto, Control of molecular weight distribution in synthesis of poly(2-hydroxyethyl methacrylate) using ultrasonic irradiation, Ultrason. Sonochem., 40 (2018) 736-741. [20] S. Koda, T. Kimura, T. Kondo, H. Mitome, A standard method to calibrate sonochemical efficiency of an individual reaction system, Ultrason. Sonochem. 10 (2003) 149. [21] F. W. Billmeyer, Textbook of polymer science, 2nd Ed, Wiley, New York, pp.280-310 (1971). [22] T.J. Mason and D. Peters, Practical Sonochemistry, Horwood Publishing, Chichester, UK, (2002). [23] Z. Shen, K. Terao, Y. Maki, T. Dobashi, G. Ma, T. Yamamoto, Synthesis and phase behavior
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Figure captions
Figure 1 Experimental apparatus for ultrasonic polymerization. Figure 2 Chemical structures of monomers. (a) N-isopropylacrylamide, (b) 2-hydroxyethyl methacrylate. Figure 3 Time course of PNIPAM polymerization at various volume fractions of ethanol. Monomer concentration is 0.4 mol/dm3, ultrasonic power intensity is 300 W/dm3. Figure 4 Time course of PHEMA polymerization at various volume fractions of ethanol. Monomer concentration is 0.4 mol/dm3, ultrasonic power intensity is 300 W/dm3. Figure 5 Time course of copolymerization at various volume fractions of ethanol. Monomer fraction of NIPAM to HEMA is 5:5, monomer concentration is 0.4 mol/dm3, and ultrasonic power intensity is 300 W/dm3. Figure 6 Time course of copolymerization at various ultrasonic power intensities. Volume fraction of ethanol is 60 vol%, molar ratio of NIPAM to HEMA is 5:5, and monomer concentration is 0.4 mol/dm3. Figure 7 Time course of copolymerization at various monomer concentrations. Volume fraction of ethanol is 60 vol%, molar ratio of NIPAM to HEMA is 5:5, and ultrasonic power intensity w is 300 W/dm3. Figure 8 Time course of copolymerization at various monomer fractions. Volume fraction of ethanol is 60 vol%, monomer concentration is 0.4 mol/dm3 and ultrasonic power intensity is 300 W/dm3. Figure 9 Temperature dependence of transmittance of polymer aqueous solution at various monomer fractions. Figure 10 Effect of molar ratio of NIPAM on LCST.
15
Figure 1 Experimental apparatus for ultrasonic polymerization.
16
Figure 2 Chemical structures of monomers. (a) N-isopropylacrylamide, (b) 2-hydroxyethyl methacrylate.
17
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6
80 vol% 70 vol% 60 vol%
0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
60
120
180
240
Time [min]
Figure 3 Time course of PNIPAM polymerization at various volume fractions of ethanol. Monomer concentration is 0.4 mol/dm3, ultrasonic power intensity is 300 W/dm3.
18
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6
70 vol% 60 vol% 50 vol%
0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 4 Time course of PHEMA polymerization at various volume fractions of ethanol. Monomer concentration is 0.4 mol/dm3, ultrasonic power intensity is 300 W/dm3.
19
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6 0.4
70 vol% 60 vol% 50 vol%
0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 5 Time course of copolymerization at various volume fractions of ethanol. Monomer fraction of NIPAM to HEMA is 5:5, monomer concentration is 0.4 mol/dm3, and ultrasonic power intensity is 300 W/dm3.
20
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6
450 W/dm3 300 W/dm3 150 W/dm3
0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 6 Time course of copolymerization at various ultrasonic power intensities. Volume fraction of ethanol is 60 vol%, molar ratio of NIPAM to HEMA is 5:5, and monomer concentration is 0.4 mol/dm3.
21
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.8 M 0.4 M 0.2 M 0.1 M
0.6 0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 7 Time course of copolymerization at various monomer concentrations. Volume fraction of ethanol is 60 vol%, molar ratio of NIPAM to HEMA is 5:5, and ultrasonic power intensity w is 300 W/dm3.
22
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6
NIPAM 9:1 7:3 5:5 HEMA
0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 8 Time course of copolymerization at various monomer fractions. Volume fraction of ethanol is 60 vol%, monomer concentration is 0.4 mol/dm3 and ultrasonic power intensity is 300 W/dm3.
23
Trancemittance [-]
1.0
NIPAM:HEMA
10:0 9:1 7:3 5:5
0.8 0.6 0.4 0.2 0.0
0
10
20
30
40
50
Temperature [℃]
Figure 9 Temperature dependence of transmittance of polymer aqueous solution at various monomer fractions.
24
40 Shen et al. This work
LCST [℃]
30 20 10 0 0.0
0.2 0.4 0.6 0.8 molar fraction of NIPAM [-]
1.0
Figure 10 Effect of molar ratio of NIPAM on LCST.
25
Table 1 range of operational factors.
operational factor
range
volume fraction of ethanol
50 – 80 vol%
molar ratio of NIPAM to HEMA
0:10 – 10:0
monomer concentration
0.1 – 0.8 mol/dm3
ultrasonic power intensity
150 – 450 W/dm3
26
Highlights
Copolymer based on N-isopropylacrylamide was synthesized using ultrasound.
Copolymer with low polydispersity can be obtained by ultrasonic polymerization.
Optimum solvent composition was determined by homopolymer synthesis.
Optimum conditions of ultrasonic irradiation was investigated.
Copolymer synthesized by ultrasonic method had a high temperature responsivity.
27
S1350-4177(18)30450-4 https://doi.org/10.1016/j.ultsonch.2018.08.022 ULTSON 4286
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
24 March 2018 10 July 2018 20 August 2018
Please cite this article as: M. Kubo, T. Sone, M. Ohata, T. Tsukada, Synthesis of poly(N-isopropylacrylamideco-2-hydroxyethyl methacrylate) with low polydispersity using ultrasonic irradiation, Ultrasonics Sonochemistry (2018), doi: https://doi.org/10.1016/j.ultsonch.2018.08.022
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Synthesis of poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) with low polydispersity using ultrasonic irradiation
Masaki Kubo*, Takuya Sone, Masahiro Ohata, Takao Tsukada
Department of Chemical Engineering, Tohoku University, 6-6-07 Aramaki, Aoba-ku, Sendai 980-8579, Japan
* Corresponding Author Masaki Kubo Department of Chemical Engineering, Tohoku University, 6-6-07 Aramaki, Aoba-ku, Sendai 980-8579, Japan Phone: +81-22-795-7261 Fax: +81-22-795-7261 E-mail : [email protected]
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Abstract Poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) having low polydispersity was synthesized in mixed solvent of ethanol and water using ultrasonic irradiation without any chemical polymerization initiator. The effects of the volume fraction of ethanol in the solvent, the molar ratio of two monomers, the monomer concentration and the ultrasonic power intensity on the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity of synthesized polymer were investigated in order to determine the optimal conditions to synthesize the copolymers with a narrow molecular weight distribution (i.e. low polydispersity). The optimum volume fraction of ethanol in the solvent was 60 vol% to synthesize the copolymers with a low polydispersity. A higher ultrasonic power intensity resulted in a faster polymerization rate and a lower number average molecular weight. The polydispersity was less than 1.5 for all ultrasonic power intensities up to 450 W/dm3 applied in this work. A higher monomer concentration gave a faster polymerization rate and a higher number average molecular weight. The polydispersity was less than 1.5 when the monomer concentration was lower than 0.4 mol/dm3. A higher molar ratio of N-isopropylacrylamide resulted in a higher polymerization rate and a lower number average molecular weight. The copolymers with polydispersity less than 1.5 can be obtained regardless of the molar ratio of N-isopropylacrylamide. The copolymers synthesized by the ultrasonic polymerization method had a high temperature responsibility.
Keywords ultrasonic
polymerization;
copolymer;
N-isopropylacrylamide;
temperature responsivity
2
low
polydispersity;
1. Introduction Poly(N-isopropylacrylamide) (PNIPAM) is a temperature-responsive polymer with a lower critical solution temperature (LCST) [1-3]. Poly(2-hydroxyethyl methacrylate) (PHEMA) has a high mechanical strength [4], so that a copolymer of Poly(NIPAM-co-HEMA) is expected to be applied as advanced functional polymeric materials in various fields, such as sensors [5], actuators [6], and drug delivery system [7]. Since the properties of the copolymers are dependent on the molecular weight and polydispersity, it is necessary to control both molecular weight and polydispersity for their application. Ultrasonic irradiation to liquid results in cavitation phenomena, i.e., the formation and collapse of micro scale bubbles [8]. Collapsing bubbles generate a local high temperature and high pressure fields [9, 10], and extremely high shear flow in liquid [11]. When ultrasonic irradiation is applied to a solution containing vinyl monomers, the radical species are generated due to thermal decomposition of the monomers [12, 13]. These radical species can act as a polymerization initiator, so that radical polymerization takes place without any chemical polymerization initiator under ultrasonic irradiation. The synthesized polymer decomposes due to the high shear stress [14], simultaneously. Ultrasonic irradiation to a monomer solution has been applied to produce a wide variety of polymers such as polystyrene [15], poly(methyl methacrylate) [16, 17], poly(styrene-co-methyl methacrylate), and poly(styrene-co–n-butyl methacrylate) [18]. Since polymerization and polymer degradation proceed simultaneously in the polymer synthesis under ultrasonic irradiation [16], both the molecular weight and polydispersity of the polymer are expected to be controlled by regulating the conditions of ultrasonic irradiation. However, most of the researches focused on molecular weight only. We have demonstrated that the homopolymer, PHEMA, with a narrow molecular weight distribution (i.e. low polydispersity) can be obtained in the solution polymerization by controlling the condition of 3
ultrasonic irradiation [19]. In this study, copolymer poly(NIPAM-co-HEMA) having low polydispersity was synthesized in mixed solvent of ethanol and water using ultrasonic irradiation without any chemical polymerization initiator. The effects of the volume fraction of ethanol in the solvent, the monomer molar ratio of NIPAM, the monomer concentration and the ultrasonic power intensity on the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity of synthesized polymers were investigated in order to determine the optimal conditions for the synthesis of the copolymers with low polydispersity.
2. Experimental Procedure Figure 1 shows an illustration of the experimental apparatus used for copolymer synthesis under ultrasonic irradiation. A horn type ultrasonic generator (VC 750, Sonics & Materials Inc., USA) operating at a frequency of 20 kHz was used. The diameter of the probe was 13 mm. The probe was immersed 30 mm from the surface of the reaction solution. The reactor was made of glass, and its inner diameter and outer diameter were 55 mm and 60 mm, respectively, and its height was 100 mm. There were an inlet for nitrogen gas, a syringe for sampling, and a condenser at the lid of the reactor. The reactor was placed in a water bath whose temperature was regulated using a water circulator (BF200, Yamato Science Co., Ltd., Tokyo, Japan). N-isopropylacrylamide (NIPAM) and 2-hydroxyethyl methacrylate (HEMA) were used as monomers. The chemical structures of the monomers are shown in Figure 2. The solvent was the mixture of ethanol and water. 100 cm3 of the solution containing the monomers was poured into the reactor. The height of the solution in the reactor was 42 mm. The probe was immersed 25 mm from the surface of the solution, so that the distance between the probe tip and the bottom of the reactor was 17 mm. The reaction solution was deoxygenated by 4
bubbling with nitrogen. The polymer synthesis was started by irradiating ultrasound to the reaction solution. The ultrasonic power intensity to the reaction solution was evaluated by the calorimetry method [20]. In this method, the ultrasonic power intensity was evaluated by the heat capacity of solution, the mass of solution, and the temperature rise per second after starting the ultrasonic irradiation. The temperature of the reaction solution was 303 K. The effects of the volume fraction of ethanol in the solvent, the molar ratio of NIPAM to HEMA, the ultrasonic power intensity, and the monomer concentration in reaction solution was investigated, as shown in Table 1. The standard experimental conditions were as follows: the volume fraction of ethanol in the solvent was 60 vol%, the molar ratio of NIPAM to HEMA was 5:5, the monomer concentration was 0.4 mol/dm3, and the ultrasonic power intensity was 300 W/dm3. During the reaction, 1 cm3 of the reaction solution was collected at specific time intervals. The conversion to the polymer and the molecular weight distribution of the polymer was measured using a gel permeation chromatography (GPC) (W2695, Waters Co., USA) equipped with a GPC column (Shodex LF-804, Showa Denko K.K., Japan) and a refractive index detector (W2414, Waters Co., USA). N,N-dimethylformamide containing 0.01 mol/dm3 LiBr was used as a mobile phase, and flow rate of the mobile phase was 0.5 cm3/min. The temperature of the column oven and detector was 313 K. The number average molecular weight and polydispersity of the polymer were estimated from the molecular weight distribution. Polystyrene standards were used to estimate the molecular weight. In order to evaluate the temperature responsivity of the polymer, the synthesized polymer was dried and dissolved in pure water with the concentration of 0.5 wt% and then the transmittance of the aqueous solution containing the polymer was measured by temperature-controllable ultraviolet-visible spectrophotometer (V-570, JASCO Co., Japan).
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3. Results and discussion 3.1 Synthesis of homopolymers In the previous study on the ultrasonic synthesis of PHEMA homopolymer, the volume fraction of ethanol in solvent was fixed at 50 vol% [19]. However, the volume fraction of ethanol in solvent affects not only the polymerization rate but also the solubility of the polymer in solvent. Therefore, prior to the copolymer synthesis, PNIPAM and PHEMA homopolymers were synthesized, respectively, and the effect of the volume fraction of ethanol on polymerization was investigated. Figure 3 shows the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity at various volume fractions of ethanol for PNIPAM synthesis. The monomer concentration is 0.4 mol/dm3 and the ultrasonic power intensity is 300 W/dm3. The volume fractions of ethanol in the solvent are 60, 70 and 80 vol%. The higher the volume fraction of ethanol, the lower the conversion to the polymer, i.e. the slower the polymerization rate. Ethanol acts as a radical scavenger for radical polymerization, so that the polymerization rate is low at higher volume fraction of ethanol. When the volume fraction of ethanol was 70 and 80 vol%, the number average molecular weight increased in the early stage and then decreased. This result suggested that the polymerization and polymer degradation occurred simultaneously under the ultrasonic irradiation. In the early stage, the amount of the synthesized polymer was not so large that the polymerization was dominant compared to the polymer degradation. As the polymerization proceeded, the amount of the synthesized polymer became large so that the effect of the polymer degradation was dominant. When the volume fraction of ethanol was 80 vol%, the polymerization rate was so slow that the number average molecular weight was considered to reach the maximum at 30 min. When the volume fraction of ethanol was 60 vol%, the polymerization rate was so fast that the number average molecular weight was considered to reach the maximum in the very early 6
stage. The molecular weight in the early stage was higher than that in the case of 80 vol% of the volume fraction of ethanol. The polydispersity decreased with time. This result suggested that the degradation rate of a polymer with a higher molecular weight was faster than that of a polymer with lower molecular weight. The theoretical minimum of polydispersity for radical polymerization was 1.5 for recombination and 2.0 for disproportionation [21], respectively. Under ultrasonic irradiation, PNIPAM with polydispersity around 1.5 was obtained. Thus, the polymer with low polydispersity can be obtained by the ultrasonic polymerization method. Note that PNIPAM polymer with low polydispersity of 1.5 could not be obtained at volume fraction of ethanol below 50 vol%, because PNIPAM was precipitated at lower volume fraction of ethanol and inhibited the propagation of ultrasound to the reaction solution. Figure 4 shows the time courses of the conversion to the polymer, the number average molecular weight and the polydispersity at various volume fractions of ethanol in the solvent for PHEMA synthesis. The monomer concentration is 0.4 mol/dm3 and the ultrasonic power intensity is 300 W/dm3. The higher the volume fraction of ethanol, the slower the polymerization rate, similarly to the case of PNIPAM. The behavior of the number average molecular weight of PHEMA was almost similar to that of PNIPAM. When the volume fraction of ethanol was 70 vol%, the number average molecular weight in the early stage was highest and then decreased. The polymerization rate was so slow that the number average molecular weight was considered to reach the maximum at 120 min. When the volume fraction of ethanol was 60 and 50 vol%, the polymerization rate was so fast that the number average molecular weight was considered to reach the maximum in the very early stage. The molecular weight in the early stage was higher than that in the case of 70 vol% of the volume fraction of ethanol. Note that the conversion to copolymer prior to 60 min was too low to analyze the molecular weight. However, PHEMA with low polydispersity less than 1.5 was obtained even if the volume fraction of ethanol was relatively low, e.g. 50 vol%. Since 7
PHEMA dissolves well in both water and ethanol, the degradation of PHMEA occurs at any volume fraction of ethanol in the solvent. Note that the scale of the horizontal axis in Figure 4 is twice that in of the horizontal axis Figure 3, namely, the polymerization rate of PHEMA was slower than that of PNIPAM at the same volume fraction of ethanol.
3.2 Synthesis of copolymer Figure 5 shows the time courses of the conversion to the polymer, the number average molecular weight and the polydispersity at various volume fractions of ethanol in the solvent for poly(NIPAM-co-HEMA) synthesis. The molar ratio of NIPAM to HEMA is 5:5, the monomer concentration is 0.4 mol/dm3, and the ultrasonic power intensity is 300 W/dm3. The higher the volume fraction of ethanol, the lower the polymerization rate. This tendency was the same as those of the homopolymers shown in Figures 3 and 4. When the volume fraction of ethanol was 70 vol%, the polymerization rate was slower than any other volume fraction. The number average molecular weight increased in the early stage and then decreased. As discussed for PNIPAM in Figure 3, the polymerization is dominant in the early stage because of the small amount of the synthesized polymer, and then the polymer degradation becomes dominant as the polymerization proceeds. When the volume fraction of ethanol was 60 and 50 vol%, the polymerization rate was so fast that the number average molecular weight was considered to reach the maximum in the very early stage. The molecular weight in the early stage was higher than that in the case of 70 vol% of the volume fraction of ethanol. The polydispersity was less than 1.5, similarly to the homopolymers. Thus, a copolymers with low polydispersity can be obtained by the ultrasonic polymerization method. When the volume fraction of ethanol was 50 vol%, the polydispersity increased in the later stage of the reaction because a part of copolymer was precipitated. Thus, the volume fraction of ethanol was set to 60 vol% in subsequent experiments. 8
Figure 6 shows the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity at various ultrasonic power intensities for copolymer synthesis. The molar ratio of NIPAM to HEMA is 5:5, and the monomer concentration is 0.4 mol/dm3. The higher the ultrasonic power intensity, the faster the polymerization rate. This was because the larger number of radical was generated at higher ultrasonic power intensity. The number average molecular weight was lower at higher ultrasonic power intensity, because the copolymer was exposed to higher shear stress due to the collapse of cavitation bubbles. The polydispersity was less than 1.5 for all the cases. Thus, the molecular weight of the copolymer can be controlled by ultrasonic power intensity while maintaining low polydispersity. Figure 7 shows the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity at various monomer concentrations for copolymer synthesis. The molar ratio of NIPAM to HEMA is 5:5, and the ultrasonic power intensity is 300 W/dm3. The higher the monomer concentration, the higher the conversion to the polymer and molecular weight of synthesized polymer. Those tendencies are consistent with general radical polymerization. However, the polymerization rate at monomer concentration of 0.8 mol/dm3 was almost the same as that at 0.4 mol/dm3. This is because higher monomer concentration results in higher solution viscosity. When the solution viscosity is high, the cavitation should be more difficult to generate [22]. The molecular weights at 0.4 and 0.8 mol/dm3 decreased with time due to the degradation of the generated polymer. In the case of the monomer concentration of 0.2 mol/dm3, the molecular weight increased in the early stage of the reaction, and then gradually decreased. This increase of the molecular weight in the early stage was due to the slow polymerization rate at lower monomer concentration and the polymerization was dominant due to the low conversion to polymer. In the case of monomer concentration of 0.1 mol/dm3, the molecular weight was almost constant. This was because 9
the molecular weight was too small to occur the degradation of generated polymer. The polydispersity was less than 1.5 when the monomer concentration was lower than 0.4 mol/dm3. In the case of 0.8 mol/dm3, however, the polydispersity was eventually high. In this case, the viscosity of the solution was relatively high in the late stage when the polymerization proceeded. Higher viscosity results in less generation of ultrasonic cavitation bubble, so that the rate of polymer degradation was low. Figure 8 shows the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity at various molar ratios of NIPAM and HEMA. The monomer concentration is 0.4 mol/dm3, and the ultrasonic power intensity is 300 W/dm3. The higher the molar ratio of NIPAM, the higher polymerization rate and the lower the molecular weight. This is because the NIPAM-terminated radical is secondary one so that its reaction activity is much higher than that of the HEMA-terminated radical, tertiary one. In all the molar ratios of NIPAM, the polymer with low polydispersity can be obtained by the ultrasonic polymerization method. In order to demonstrate the temperature responsivity of the polymers synthesized by ultrasonic polymerization method, the transmittance of the aqueous solutions containing the polymers was measured. Figure 9 shows the temperature dependence of the transmittance of the polymer aqueous solutions. The transmittance of unity denotes that the polymer is dissolved in water. As shown in Figure 9, the transmittance decreased sharply at a certain temperature depending on the molar ratio of NIPAM. This suggests that the polymers synthesized by the ultrasonic polymerization method have a sharp temperature responsivity. From the results in Figure 9, a lower critical solution temperature (LCST) was estimated for the polymers synthesized at various molar fractions of NIPAM. The LCST is defined as the temperature at the transmittance of 0.5. Figure 10 shows the effect of the molar fraction of NIPAM on LCST. The LCST of poly(NIPAM-co-HEMA) increased linearly with the molar 10
fraction of NIPAM. The LCST of the copolymer synthesized by general radical polymerization using chemical initiator [23] was also shown in Figure 10. The LCST in this study was almost the same as that by general radical polymerization. The decrease of LCST with decreasing NIPAM is attributed to the increment of hydrophobicity due to the hydrogen-bonding between the hydroxyl groups in HEMA and the amide groups in NIPAM [24, 25]. The sharpness of the transition, defined as the difference in temperature between high and low transmittances in Figure 9, was less than 3 K for all the condition. The temperature sharpness of the transition in this study was almost the same as that by general radical polymerization.
4. Conclusions In this study, poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) having low polydispersity was synthesized in the mixed solvent of ethanol and water using the ultrasonic polymerization method without any chemical polymerization initiator. Considering the polymerization rate and polydispersity of molecular weight, the optimum volume fraction of ethanol was 60 vol% to synthesize the copolymers with low polydispersity. The higher ultrasonic power intensity resulted in the faster polymerization rate and lower number average molecular weight due to the higher number of radical. The polydispersity was less than 1.5 for all the ultrasonic power intensity, so that the molecular weight can be controlled by ultrasonic power intensity while maintaining low polydispersity. The higher monomer concentration gave the higher conversion to polymer and higher number average molecular weight. The polydispersity was less than 1.5 when the monomer concentration was lower than 0.4 mol/dm3. The higher molar ratio of NIPAM resulted in higher polymerization rate and lower number average molecular weight. In all the monomer fraction, the polymer with low polydispersity can be obtained. 11
The polymers synthesized by the ultrasonic polymerization method have a temperature responsivity. The values of a lower critical solution temperature (LCST) in this study was almost the same as those by general radical polymerization.
Acknowledgement This research was supported in part by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 16K14467.
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[19] M. Kubo, T. Kondo, H. Matsui, N. Shibasaki-Kitakawa, T. Yonemoto, Control of molecular weight distribution in synthesis of poly(2-hydroxyethyl methacrylate) using ultrasonic irradiation, Ultrason. Sonochem., 40 (2018) 736-741. [20] S. Koda, T. Kimura, T. Kondo, H. Mitome, A standard method to calibrate sonochemical efficiency of an individual reaction system, Ultrason. Sonochem. 10 (2003) 149. [21] F. W. Billmeyer, Textbook of polymer science, 2nd Ed, Wiley, New York, pp.280-310 (1971). [22] T.J. Mason and D. Peters, Practical Sonochemistry, Horwood Publishing, Chichester, UK, (2002). [23] Z. Shen, K. Terao, Y. Maki, T. Dobashi, G. Ma, T. Yamamoto, Synthesis and phase behavior
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Figure captions
Figure 1 Experimental apparatus for ultrasonic polymerization. Figure 2 Chemical structures of monomers. (a) N-isopropylacrylamide, (b) 2-hydroxyethyl methacrylate. Figure 3 Time course of PNIPAM polymerization at various volume fractions of ethanol. Monomer concentration is 0.4 mol/dm3, ultrasonic power intensity is 300 W/dm3. Figure 4 Time course of PHEMA polymerization at various volume fractions of ethanol. Monomer concentration is 0.4 mol/dm3, ultrasonic power intensity is 300 W/dm3. Figure 5 Time course of copolymerization at various volume fractions of ethanol. Monomer fraction of NIPAM to HEMA is 5:5, monomer concentration is 0.4 mol/dm3, and ultrasonic power intensity is 300 W/dm3. Figure 6 Time course of copolymerization at various ultrasonic power intensities. Volume fraction of ethanol is 60 vol%, molar ratio of NIPAM to HEMA is 5:5, and monomer concentration is 0.4 mol/dm3. Figure 7 Time course of copolymerization at various monomer concentrations. Volume fraction of ethanol is 60 vol%, molar ratio of NIPAM to HEMA is 5:5, and ultrasonic power intensity w is 300 W/dm3. Figure 8 Time course of copolymerization at various monomer fractions. Volume fraction of ethanol is 60 vol%, monomer concentration is 0.4 mol/dm3 and ultrasonic power intensity is 300 W/dm3. Figure 9 Temperature dependence of transmittance of polymer aqueous solution at various monomer fractions. Figure 10 Effect of molar ratio of NIPAM on LCST.
15
Figure 1 Experimental apparatus for ultrasonic polymerization.
16
Figure 2 Chemical structures of monomers. (a) N-isopropylacrylamide, (b) 2-hydroxyethyl methacrylate.
17
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6
80 vol% 70 vol% 60 vol%
0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
60
120
180
240
Time [min]
Figure 3 Time course of PNIPAM polymerization at various volume fractions of ethanol. Monomer concentration is 0.4 mol/dm3, ultrasonic power intensity is 300 W/dm3.
18
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6
70 vol% 60 vol% 50 vol%
0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 4 Time course of PHEMA polymerization at various volume fractions of ethanol. Monomer concentration is 0.4 mol/dm3, ultrasonic power intensity is 300 W/dm3.
19
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6 0.4
70 vol% 60 vol% 50 vol%
0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 5 Time course of copolymerization at various volume fractions of ethanol. Monomer fraction of NIPAM to HEMA is 5:5, monomer concentration is 0.4 mol/dm3, and ultrasonic power intensity is 300 W/dm3.
20
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6
450 W/dm3 300 W/dm3 150 W/dm3
0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 6 Time course of copolymerization at various ultrasonic power intensities. Volume fraction of ethanol is 60 vol%, molar ratio of NIPAM to HEMA is 5:5, and monomer concentration is 0.4 mol/dm3.
21
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.8 M 0.4 M 0.2 M 0.1 M
0.6 0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 7 Time course of copolymerization at various monomer concentrations. Volume fraction of ethanol is 60 vol%, molar ratio of NIPAM to HEMA is 5:5, and ultrasonic power intensity w is 300 W/dm3.
22
Conversion [-]
0.0
Polydispersity [-]
0.8
Molecular weight [x104 g/mol]
1.0
0.6
NIPAM 9:1 7:3 5:5 HEMA
0.4 0.2
15 10 5 0 2.0
1.5
1.0
0
120
240
360
480
Time [min]
Figure 8 Time course of copolymerization at various monomer fractions. Volume fraction of ethanol is 60 vol%, monomer concentration is 0.4 mol/dm3 and ultrasonic power intensity is 300 W/dm3.
23
Trancemittance [-]
1.0
NIPAM:HEMA
10:0 9:1 7:3 5:5
0.8 0.6 0.4 0.2 0.0
0
10
20
30
40
50
Temperature [℃]
Figure 9 Temperature dependence of transmittance of polymer aqueous solution at various monomer fractions.
24
40 Shen et al. This work
LCST [℃]
30 20 10 0 0.0
0.2 0.4 0.6 0.8 molar fraction of NIPAM [-]
1.0
Figure 10 Effect of molar ratio of NIPAM on LCST.
25
Table 1 range of operational factors.
operational factor
range
volume fraction of ethanol
50 – 80 vol%
molar ratio of NIPAM to HEMA
0:10 – 10:0
monomer concentration
0.1 – 0.8 mol/dm3
ultrasonic power intensity
150 – 450 W/dm3
26
Highlights
Copolymer based on N-isopropylacrylamide was synthesized using ultrasound.
Copolymer with low polydispersity can be obtained by ultrasonic polymerization.
Optimum solvent composition was determined by homopolymer synthesis.
Optimum conditions of ultrasonic irradiation was investigated.
Copolymer synthesized by ultrasonic method had a high temperature responsivity.
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