Observing phase transition of a temperature-responsive polymer using

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Observing phase transition of a temperature-responsive polymer using electrochemical collisions on an ultramicroelectrode Nhung T. T. Hoang, Jinhee Lee, Byungyong Lee, Hae-Young Kim, Jungeun Lee, Truc Ly Nguyen, Myungeun Seo, Sang Youl Kim, and Byung-Kwon Kim Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00437 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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Analytical Chemistry

Observing phase transition of a temperature-responsive polymer using electrochemical collisions on an ultramicroelectrode Nhung T. T. Hoang,1,‡ Jinhee Lee,2,3,‡ Byungyong Lee,3,‡ Hae-Young Kim,1,‡ Jungeun Lee,1 Truc Ly Nguyen,1 Myungeun Seo,2 Sang Youl Kim,3,* Byung-Kwon Kim1,* 1

Department of Chemistry, Sookmyung Women’s University, Seoul, 04310, South Korea Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701, South Korea 3 Department of Chemistry, KAIST, Daejeon 305-701, South Korea 2

ABSTRACT: Herein, a study on a new lower critical solution temperature (LCST) polymer in an organic solvent by an electrochemical technique has been reported. The phase transition behavior of poly(arylene ether sulfone) (PAES) was examined on 1,2dimethoxyethane (DME). At a temperature above the LCST point, polymer molecules aggregate to create polymer droplets. These droplets subsequently collide with an ultramicroelectrode (UME), resulting in a new form of staircase current decrease. The experimental collision frequency and collision signal were analyzed in relation to the concentration of the polymer. In addition, the degree of polymer aggregation associated with temperature change was also observed.

Smart polymers with the ability to respond to slight variations in the local environment have been widely investigated for the development of new materials and applications.1,2 Due to the advantages of lower cost and easy tailoring, they have attracted more attention than other materials such as ceramics and metals. These stimuli-responsive polymers are capable of responding to small shifts in parameters of the surrounding environment, such as temperature, pH, magnetic field, ionic strength, and photo light.1 In particular, polymers that respond to temperature changes, known as temperature-responsive polymers, have been extensively developed for application in a wide range of fields, such as the biomedical field, architecture, and water-recovery strategies.1–5 There are two main types of temperature-responsive polymers: lower critical solution temperature (LCST) and upper critical solution temperature (UCST) polymers.1 While UCST polymers are infrequently discussed, significant research has been conducted on the synthesis of LCST polymers.5–11 However, while most of these studies have focused on the synthesis of LCST polymers, their behaviors in solvents have rarely been investigated.12,13 The phase behavior of LCST polymers in solutions is an important property with regard to the development and design of temperature-responsive polymers. To address this point, the study of cloud point temperature (Tcp), an important parameter for phase transition of LCST polymers in solution, is necessary.14–17 The single-phase region of a completely miscible polymer and solvent exists at a temperature below the Tcp. However, at a temperature above the LCST point, the clear and homogeneous solution is replaced by a cloudy solution of a two-phase region, in which the polymer molecules aggregate to form polymer droplets. Generally, turbidimetry, dynamic light scattering (DLS), calorimetry, and nuclear magnetic resonance (NMR) spectroscopy are commonly used in studying LCST phase behavior.14,18 Among them, the main methods

used for observing the phase change of the LCST polymer with temperature are turbidimetry and DLS. Turbidity can be measured simply with turbidimetry. However, it is not sufficiently sensitive. The other method, DLS, is disadvantageous as it has large errors and low reproducibility in organic solvents. Moreover, DLS measures the average size of the various particles present in the solution. Thus, when the polymer is present in two forms (soluble and insoluble form) in solution near the Tcp, the method cannot sensitively measure the phase change because the average values of the two forms of polymers are measured. Therefore, it is necessary to develop a new method to observe the phase change with temperature change sensitively. Herein, for the first time, we use an electrochemical method, to observe the phase transition of LCST polymers with changing temperature. At temperatures lower than Tcp, the LCST polymer is fully dissolved in the solvent. However, when the temperature is higher than Tcp, the polymers form insoluble droplets. The study of droplets19 and other soft particles (e.g., vesicles,20–23 virus,24 bacteria25) using electrochemical techniques has been widely investigated because significant properties of the particles are well-characterized. In this experiment, electrochemical blocking technique was utilized to demonstrate the collision of polymer droplets.19 At a temperature above the Tcp point, the droplets collide with the electrode and block the flux of the redox species towards the electrode surface, resulting in the observed staircase current decrease (Figure 1). For this purpose, we synthesized a new LCST polymer in the solvent structured from poly(arylene ether sulfone) (PAES). PAES is a high-temperature polymer, possessing good chemical stability and mechanical properties, thus having various industrial and commercial applications.26 Moreover, PAES can be synthesized by chain-growth condensation polymerization (CGCP). Since PAES has a well-defined mo-

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lecular weight and low polydispersity, it has a great potential for studies on self-assembly.27

Figure 1. Schematic description of detection of an LCST polymer using electrochemical collision method.

EXPERIMENTAL SECTION Reagent. 2-Mercaptoethanol, 2,2-dimethoxy-2phenylacetophenone (DMPA), anthracene (99%), trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) amide (IL-PA), anhydrous dimethylsulfoxide (DMSO), anhydrous N,N-dimethylformamide (DMF), anhydrous benzene, and 1,2dimethoxyethane (DME) were purchased from Sigma-Aldrich (St. Louis, MO, USA). 18-crown-6-ether and potassium carbonate (K2CO3) were purchased from TCI (Chuo-ku, Tokyo, Japan). Carbon (99.99%) wire (10 µm diameter) was supplied by Goodfellow (Devon, PA, USA). Other commercially available reagent-grade chemicals were used without further purification. 4-Fluoro-4´-hydroxydiphenyl sulfone and 4-fluoro-4´hydroxydiphenyl sulfone potassium salt were synthesized according to the procedures in the literature.28–30 Preparation of ultramicroelectrode (UME). The C-UME was prepared following the general procedure.31 A cleaned 10 µm carbon fiber was placed in a capillary and sealed. The electrode was subsequently polished with a diamond pad (0.1 µm, Allied High Tech Products, Rancho Dominguez, CA, USA), followed by alumina powder (0.05 µm, Allied High Tech Products, Rancho Dominguez, CA, USA) to obtain a mirror surface. The electrode surface was checked with the standard redox electrochemistry of ferrocenemethanol solution. The electrode should be cleaned with acetone and ethanol, polished with double distilled water on the 0.1 µm diamond pad, and dried before every experiment. Instrumentation. All electrochemical experiments were performed on a CHI model 617B potentiostat (CH Instrument, Austin, TX), including three electrodes located in a Faraday cage. A 0.5 mm Ag wire was utilized as the reference electrode in cyclic voltammetry experiments. A 0.5 mm Pt wire auxiliary was used. Every experiment related to temperature was performed using the digital hotplate stirrer MaxtirTM 500 (Daihan, Seoul, Korea). All the experiments were set up in a 5 mL beaker-type cell. The cell was placed in a water bath on a stirrer/hot plate to maintain a certain temperature. The NMR spectra of the synthesized compounds were recorded on a Bruker Fourier Transform Avance 400 spectrometer. The NMR chemical shift was reported in parts per million (ppm) with reference to the peak of residual DMAO-d6 (2.49 ppm for 1 H). Gel permeation chromatography (GPC) diagrams were obtained with the Agilent 1260 Infinity system (Santa Clara, CA), equipped with a refractive index detector and a packing column (PLgel 10 µm MIXED-B) using chloroform as an eluent at 35 °C. The molecular weights of the polymers were calculated relative to linear polystyrene standards. DLS measurements were performed on an ELSZ-2000 (Otsuka Electronics Co.) system.

Figure 2. (a) Chemical structure and (b) 1H NMR in DMSO-d6 (400 MHz) of PAES-OH.

LCST polymer structure. The polycondensation of 4-fluoro4´-hydroxydiphenyl sulfone potassium salt, which undergoes chain growth polycondensation, with 1-(allyloxy)-4-((4fluorophenyl)sulfonyl) benzene (Figure S-1) forms allyl-PAES (Figure S-2), eventually yielding PAES-OH through the thiolene click reaction (Figure S-3). The structure of the LCST polymer PAES-OH was finally determined using NMR before its thermo-responsive properties were examined using DME (Figure 2).

RESULTS AND DISCUSSION Anthracene, which is a common redox substance in organic media, was selected as the redox species, while IL-PA was used as the electrolyte. A series of cyclic voltammetry (CV) analyses of a mixture of 1 mM anthracene and 100 mM IL-PA in DME were measured at room temperature (RT) and 50 °C on a UME. In the absence of a polymer, anthracene was oxidized under the diffusion-controlled process and reversible steady-state current (iss), measured at both RT and 50 °C (Figure 3(a)). The iss value increases with an increase in temperature.

Figure 3. CVs of 1 mM anthracene and 100 mM IL-PA in DME solution with (a) no PAES-OH present, and (b) 4 mg PAES-OH present at RT (black line) and 50 °C (red line) at a scan rate of 50 mV/s using C-UME.

The iss of anthracene oxidation was calculated using the following equation:32


iss = 4 nFD An Cr

(1)

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Analytical Chemistry where n is the number of electrons transferred in the reaction, F is the Faraday constant (96485 C/mol), DAn is the diffusion coefficient of anthracene (cm2/s), C is the initial concentration of anthracene (M), and r is the radius of the C-UME (5 µm). The iss value was increased from 7.9 nA at RT to 16.7 nA at 50 °C due to the increase in temperature. The same phenomenon was observed in CVs of the mixture containing 4 mg of polymer (5.5 nA at RT to 6.2 nA at 50 °C). The presence of the polymer has a negative effect on the anodic current of anthracene. As seen from Eq. (1), the DAn is only affected by temperature, leading to different values of iss. The DAn can be obtained by the Stokes-Einstein equation:32

DAn =

kBT 6πηrAn

homogeneous. However, at temperatures above the LCST, a slightly milky color was observed, which became stronger as the temperature increased (Figure S-5). Amperometric timecurrent (i-t) curves were measured to monitor the collision of polymer particles on the UME over the temperature range of RT to 55 °C at a concentration of 4 mg/mL PAES-OH (Figure 4). As seen in Figure 4, certain collision signals caused by the aggregation of the polymer initially appeared at 35 °C. Several collision signals were measured with the current fluctuation at this temperature because polymer aggregation begins at this temperature, creating various sizes of polymer particles. However, at 45 °C, polymer aggregations are nearly complete and clear collision signals can be observed. The same phenomenon was observed at 50 °C and 55 °C.

(2)

where kB is the Boltzmann's constant (1.38 × 10−23 J/K), T is the temperature (K), η is the absolute viscosity of the solvent medium (Pa·s), and rAn is the radius of the diffusing species (0.2 nm).33 The diffusion coefficient and viscosity of the DME calculated from CVs are reported in Table 1. Table 1. Diffusion coefficient (D) of anthracene and the viscosity (η) of the DME of the mixture with and without polymer at room temperature (RT) and 50 °C. iss (nA)

D (cm2/s)

η (Pa·s)

PAESOH

RT

50 °C

RT

50 °C

RT

50 °C

0 mg/mL

7.9

16.7

2.04 × 10−5

4.33 × 10−5

5.36 × 10−4

2.73 × 10−4

4 mg/mL

5.5

6.2

1.43 × 10−5

1.61 × 10−5

7.66 × 10−4

7.37 × 10−4

In both experimental cases, as the temperature increased, the diffusion of anthracene increased 2.13 times in the absence of the polymer (2.04 × 10−5 cm2/s (RT), 4.33 × 10−5 cm2/s (50 °C)) and 1.13 times in the presence of the polymer (1.43 × 10−5 cm2/s (RT), 1.61 × 10−5 cm2/s (50 °C)). In contrast, the viscosity of the solvent decreased by the same values (no polymers; 5.36 × 10−4 Pa·s (RT), and 2.73 × 10−4 Pa·s (50 °C), with the polymer; 7.66 × 10−4 Pa·s (RT), and 7.37 × 10−4 Pa·s (50 °C)) because of the increase in temperature. It is worth noting that in the presence of the polymer, the electrode surface was affected by polymer particle. The adsorption of these polymer particles increases the resistance of the electrode surface. To verify this, we carried out further experiments at a lower scan rate (10 mV/s; Figure S-4). The measurement time is 44 s when the scan rate is 50 mV/s, while at 10 mV/s, the measurement time is 220 s. Due to the longer measurement time, the adsorption of the polymer particles to the electrode also increases. The adsorption of these polymer particles increases the resistance of the electrode surface. Therefore, the backward scans are not similar to the forward scans (Figures 3(b) and S-4). However, according to the CV results, 1.4 V is a sufficiently high potential to observe collision events of aggregated polymer particles at 50 °C during the blocking experiment. The phase transition of the mixture in the presence of a polymer was visually observed (Figure S-5). At temperatures below the LCST of polymers, the polymer solution was clear and

Figure 4. i-t curves of 4 mg/mL PAES-OH polymer in DME solution containing 1 mM anthracene and 100 mM IL-PA at different temperatures. The UME was biased at +1.4 V (vs. Ag wire).

This thermal induced phase separation behavior can be easily varied by the degree of polymerization, polydispersity, and concentration of the polymer solution. Here, this electrochemical collision experiment shows that the Tcp of 4 mg/mL polymer is ca. 35 °C. We compared this result with the UV/VIS transmittance result, which is the commonly used method to obtain the Tcp point of polymers. There are several references for determining the Tcp point as a result of reduced transmittance. Researchers have often reported 10%, 20%, or 50% reduced points in transmittance as the LCST point. We set the LCST to the point where the transmittance is half of the initial value (50% reduced).37,38 Thus, a polymer solution with 4 mg/mL concentration has the Tcp of 33.2 °C from the UV/VIS data. This indicates that the Tcp value of 4 mg/mL PAES-OH performed using our technique was in good agreement with



the value of 33.2 °C recorded by the UV/VIS spectrometer (Figure S-6).

Figure 6. Collision of 4 mg/mL PAES-OH in DME solution containing 1 mM anthracene and 100 mM IL-PA at 50 °C on C-UME. The UME was biased at +1.4 V (vs. Ag wire).

Figure 5. Comparative schematic description of detection of (a) emulsion or latex bead, and (b) polymer using electrochemical collision method.

From previous research on single particle (emulsions, latex beads, bacteria, and cells) detection using the blocking technique, a stable current measurement was expected after each observed staircase current reduction (Figure 5(a)).19,34,35 However, in this study, the measured current increased again, immediately after a decrease in the staircase current (Figure 5(b)). We believe this is a newly reported phenomenon caused by the structure of aggregated polymer particles. During the aggregation process, the polymer particles still have several pores on the structures, through which the flux of redox species can move towards the electrode surface. As a result, a gradually increasing current was measured (Figures 4 and 6). Here, the time required to recover the decreasing current value is referred to as relaxation time. At a concentration of 4 mg/mL of PAES-OH, collision processes were complete in 0.4 ± 0.2 s time scale, while relaxation processes required a significantly longer time (usually more than 3.0 ± 0.7 s; Figure 6). As the temperature increased above the LCST point, the polymer molecules shrunk and aggregated together to form larger polymer particles in the collision experiment. Therefore, the relaxation times of the polymer chains increased.36 Explanations of these collision and relaxation processes of polymers are not clear at the current stage. Therefore, we are suggesting this as a hypothesis and will attempt to verify it later through further experiments.

Figure 7. i-t curves and first-order derivatives corresponding to i-t curves at varying amounts of PAES-OH polymer particle collisions on the C-UME in the DME solution containing 1 mM anthracene and 100 mM IL-PA at 50 °C. The UME was biased at +1.4 V (vs. Ag wire).

The collision signals are more easily observed by taking the first derivative of current by time corresponding to the current transient record at varying concentrations of polymers. As seen from Figures 7 and 8(a), the collision frequency is proportional to the concentrations of the polymer. At 1 mg/mL of polymer (at 50 °C), the mean diameter of the polymer particles is ca. 280 nm, showing a broad curve and a low intensity value with DLS measurement (Figure S-7). The diameter measured by DLS is the size of the original polymer in its natural state (unaggregated). At the same experimental concentration, no collision events were detected in the electrochemical experiment (Figure 7). This means that the polymer particles are not aggregated at this concentration. Collision events could be observed in the electrochemical experiments when the concentration of the polymer increased to 2 mg/mL, whereas the mean diameter of polymer particles in this case was not significantly different from that at 1 mg/mL. As the number of particles increased, they were able to aggregate to form larger particles, which were detectable in the electrochemical experiment. As the concentration of the polymer increased to 3 mg/mL, larger and more collision signals were observed. At 4, 5, and 6 mg/mL of polymer, the current magnitudes were slightly increasing, while the number of collisions continued to follow the trend.


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Analytical Chemistry ‡These authors contributed equally Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

Figure 8. Plots of (a) collision frequency from collision experiments and (b) diameter of aggregated polymer particles obtained by DLS measurements with changing PAES-OH concentration at 50 °C.

Similar results were observed in the DLS experiments. The DLS results revealed that the average size distribution increased dramatically with increasing concentrations of the polymer, where they increased slightly in diameter once the concentration reached 3 mg/mL (Figure S-7). From these results, we can observe some correlation between collision frequencies from the electrochemical method and size data from the DLS results. However, additional experiments are necessary to reveal the exact association.

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, and ICT (NRF2015R1C1A1A01055250, and NRF-2018R1C1B6008668).

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CONCLUSIONS

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We have reported a new LCST polymer that exhibits temperature reactivity in DME solvent. The phase transition behavior of PAES-OH polymers has been successfully demonstrated using the electrochemical blocking technique. The increase in polymer concentration correlated well with the electrochemical collision results. Moreover, the degree of aggregation of the polymer with temperature change was also determined through these experiments. Most importantly, the type of staircase current change observed at the time of the collision is a new one, which has not been reported so far. Hence, the technique used in this experiment can provide a new way of measuring the density of aggregated polymers. Further research is needed to confirm this. However, it is expected that the study of the behavior of various polymer particles in solution using this method will be actively pursued in the future.

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ASSOCIATED CONTENT Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website. Synthesis of 1-(allyloxy)-4-((4-fluorophenyl) sulfonyl) benzene (Figure S-1); synthesis of Allyl-PAES (Figure S-2); Thiol-ene Click reaction (Figure S-3); CVs data and visually observation of 4 mg/mL PAES-OH in DME solution (Figure S-4 and Figure S5); UV/VIS transmittance spectra and DLS data of 4 mg/mL of PAES-OH solution (Figure S-6 and Figure S-7).

AUTHOR INFORMATION Corresponding Author [email protected] (S.Y. Kim) phone: +82 42 350 2834. Fax: +82 42 350 8177 [email protected] (B.-K. Kim) phone: +82 2 2077 7808. Fax: +82 2 2077 7321 Author Contributions

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