Characterization of a Thermoresponsive Chitosan Derivative as a

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Characterization of a Thermoresponsive Chitosan Derivative as a Potential Draw Solute for Forward Osmosis Rumwald Leo G. Lecaros, Zih-Chi Syu, Yu-Hsuan Chiao, S. Ranil Wickramasinghe, Yan Li Ji, Quanfu An, Wei-Song Hung, Chien-Chieh Hu, Kueir-Rarn Lee, and Juin-Yih Lai Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02102 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016

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Environmental Science & Technology

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Characterization of a Thermoresponsive Chitosan

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Derivative as a Potential Draw Solute for Forward

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Osmosis

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Rumwald Leo G. Lecarosa, Zih-Chi Syua, Yu-Hsuan Chiaob, S Ranil Wickramasingheb, Yan-Li

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Jia, c, Quan-Fu Anc, Wei-Song Hunga*, Chien-Chieh Hua, Kueir-Rarn Leea, and Juin-Yih Laia

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a

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Yuan University, Chung Li 32023, Taiwan

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b

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United States

R&D Center for Membrane Technology and Department of Chemical Engineering, Chung

Department of Chemical Engineering, University of Arkansas, Fayetteville, Arkansas 72701,

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c

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Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of

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KEYWORDS: Thermoresponsive polymer, Draw solute, Forward osmosis

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ACS Paragon Plus Environment


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Abstract

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A thermoresponsive chitosan derivative was synthesized by reacting chitosan (CS) with butyl

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glycidyl ether (BGE) to break the inter- and intramolecular hydrogen bonds of the polymer. An

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aqueous solution of the thermoresponsive CS derivative exhibits a lower critical solution

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temperature (LCST) than CS, and it undergoes a phase transition separation when the

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temperature changes. Successful incorporation of BGE into the CS was confirmed by FTIR and

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XPS analyses. Varying the BGE content and the concentration of the aqueous solution produced

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different LCST ranges, as shown by transmittance vs. temperature curves. The particle size was

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observed by scanning electron microscopy, which revealed that the particles were smaller and

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well dispersed at 15 °C, whereas the particles became larger and tended to aggregate at 60 °C. A

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similar trend was observed with the mean particle size measured using dynamic light scattering.

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Positron annihilation lifetime spectroscopy data also revealed the reversibility of the particle

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properties as a function of temperature. Microstructure analysis showed that the particles had

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larger free-volume sizes at 15 °C than at 60 °C. The particles were also found to be non-toxic

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with 92% cell survival. A simple forward osmosis (FO) test for dye dehydration revealed the

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potential use of the thermoresponsive chitosan derivative as a draw solute with a flux of 8.6

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L/m2h and rejection of 99.8 %.

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Introduction

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Thermoresponsive polymers dissolve in aqueous solution at temperatures below their

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lower critical solution temperature (LCST) and are phase separated at temperatures above their

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LCST. Such stimuli-responsive polymers are important and have attracted much attention due to

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their diverse applications in biomedicine1 and separation processes.2-7 LCST behavior is

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observed when the inter- and intramolecular interactions vary with increasing temperature.8, 9

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Thus, grafting hydrophobic groups onto the polymer chain can tune the thermoresponsive

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behavior by disrupting the inter- and intramolecular hydrogen bonds of the polymers.1

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Thermoresponsive materials based on natural polymers have drawn great interest because of

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their attractive properties, which include biocompatibility, biodegradability and non-toxicity.

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Chitosan (CS), the partially acetylated cationic (1,4)-2-amino-2-deoxy-β-D-glucan, possesses

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these distinct properties, which makes it a good candidate for various applications. CS is only

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soluble in dilute aqueous acid solution. It has two reactive groups (amino and hydroxyl groups)

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where a monomer can be added through different chemical modification techniques, potentially

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changing or enhancing its properties.10, 11

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One of the modern day concerns is having a sufficient supply of potable water for the

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future. The forward osmosis (FO) process has currently gained attention as an alternative method

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of desalination12

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less than reverse osmosis.14-18 An ideal FO membrane is capable of providing high water

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permeability, high rejection of solutes, substantially reducing internal concentration polarization

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(ICP), and has high chemical stability and mechanical strength. Likewise, a good draw solution

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must have the following characteristics: high water flux, minimal reverse draw solute flux, no

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toxicity, reasonably low cost, and easy recovery. Meanwhile, it is required to be compatible with

13

because it does not require high pressures, it is easy to operate, and it costs



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the FO membrane. Commonly available compounds as draw solutes can be classified into four

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types according to their physicochemical properties: volatile compounds, saccharides, inorganic

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salts, and organic salts and polymers.19-21 Butyl glycidyl ether (BGE) covalently bound to starch

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molecules has been previously demonstrated to undergo phase separation above its LCST.1 Other

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research has shown that a thermoresponsive cellulose ether composed of BGE was grafted onto

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hydroxyethyl cellulose and exhibited both LCST behavior and a critical flocculation temperature

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(CFT). This thermoresponsive cellulose ether was able to encapsulate dyes at temperatures above

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its LCST and to separate them from water at temperatures above the CFT.2 In another case, an

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ionic liquid at temperatures below its upper critical solution temperature (UCST) has been used

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in FO as a draw solute in treating high-salinity water;4 hydrophilic magnetic nanoparticles have

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also been used as draw solutes in FO.22, 23

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Herein, we developed a non-toxic draw solute using CS that has been modified using the

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hydrophobic monomer BGE, resulting in LCST behavior. The two reactive groups of CS, amine

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and hydroxyl groups, act as nucleophiles that attack the epoxide ring of BGE. In the case of the

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hydroxyl group of CS, it must first be attacked by a strong base such as NaOH, followed by the

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opening of the epoxide ring. The resulting draw solute was subsequently used in forward

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osmosis with Congo Red dye as the feed solution.


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Experimental Section

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Scheme 1. Synthesis of the thermoresponsive chitosan derivative and a schematic of its LCST

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behavior.

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Materials

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CS (90% deacetylated) was acquired from Shimakyu’s Pure Chemicals. BGE (95%) was

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purchased from Sigma-Aldrich. Glacial acetic acid (analytical grade, 99%) was acquired from

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Scharlau. All other reagents and solvents were commercially acquired and were used without

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further purification. The deionized water used in all experiment had a resistivity of 18 MΩ cm.

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Synthesis of the thermoresponsive CS derivative

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The synthesis reaction of CS with BGE is shown in Scheme 1. CS (2 g) was dissolved

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using 10 ml of 2% (v/v) acetic acid. It was stirred in a three-necked round-bottom flask at room



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temperature until all CS flakes were completely dissolved. The solution was heated under reflux

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to 75 °C in an oil bath, followed by the addition of 1.25 M NaOH and stirred for 1 h. Four

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formulations were prepared using various volumes of BGE: CS-BGE-1 (1 ml BGE); CS-BGE-2

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(3 ml BGE); CS-BGE-3 (6 ml BGE); and CS-BGE-4 (9 ml BGE). The reaction was conducted

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under continuous stirring at 75 °C. After 5 h, the mixture was cooled in an ice bath and

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neutralized to pH 7.0 using 1 M HCl. The product was washed three times with 90% (v/v)

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acetone. The product was placed in a dialysis bag and dialyzed in deionized water for 24 hours to

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purify the sample. The precipitate was filtered and then dried overnight at 50 °C.

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Characterization of the thermoresponsive chitosan derivative

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The compounds were analyzed by total reflectance Fourier transform infrared

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spectroscopy (ATR-FTIR) on a Perkin-Elmer Spectrum One over the wavenumber region from

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4000 to 900 cm-1. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo

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Scientific K-Alpha to characterize the elemental composition of the samples. LCSTs of 1 wt%

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CS-BGE aqueous solutions were measured through turbidity measurements using a UV-vis

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spectrophotometer (Perkin-Elmer Lambda 25) at 500 nm with a heating rate of 1 °C/min.

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Scanning electron microscopy (SEM, Hitachi S4800) was performed to investigate the formed

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particles. The samples were dried on a Si sample stage at 15 °C and 60 °C. The mean particle

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size distribution at 15 °C and 60 °C was measured by dynamic light scattering (DLS) using a

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Beckman Coulter DelsaNano S. The samples were measured for 90 s at each temperature.

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Positron annihilation lifetime spectroscopy

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The free volume in the 1 wt% aqueous solution of the thermoresponsive chitosan

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derivative was measured using positron annihilation lifetime spectroscopy. A conventional fast-


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fast coincidence spectrometer with a time resolution of 250 ps was used. A radioactive source of

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22

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annihilation lifetimes were recorded using a fast-fast coincidence timing system. A time-to-

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amplitude converter was used to convert the lifetimes and to store the timing signals in a multi-

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channel analyzer (Ortec System). Two million counts were collected, and all positron

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annihilation lifetime spectra were analyzed by a finite-term lifetime analysis method using the

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PATFIT program. 24, 25

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Forward osmosis and microfiltration test

Na (0.74 MBq) sealed between 12-µm-thick Kapton films was placed in the solution. Positron

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The permeation flux was measured using small-scale U-shaped FO tubes from Narika

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Corp. (OP-25U F35-2111). A thin-film composite membrane from Hydration Technologies, Inc.

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(HTI) was placed and sealed between the two glass tubes. One side was filled with 0.025 wt%

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Congo Red dye as the feed solution. The other side was filled with 1 wt% aqueous solution of

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CS-BGE-3 as the draw solution. The temperature of the solutions was maintained at room

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temperature during the FO tests. The water permeation flux (L/m2h) was calculated as the

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volume change of the draw solution (L) over time (h) and effective surface area (m2) of the

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membrane. The percent rejection (%) was calculated as the difference of concentration in feed

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and permeate divided by the feed concentration times 100. The osmolality of the

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thermoresponsive chitosan derivatives and Congo Red dye was measured using an osmometer

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(model 3320 micro-osmometer, Advanced Instruments, Inc.). The draw solution was heated to

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60 °C to separate the water from the draw solute through microfiltration at operating pressure of

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1 bar (Gore® Microfiltration membrane, Product number: GMM-404, pore size: 0.45 µm).

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Results & Discussion

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Modification of Chitosan

134 135

Figure 1. FTIR spectra of BGE, CS, and CS derivatives: CS-BGE-1, CS-BGE-2, CS-BGE-3 and

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CS-BGE-4.

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The FT-IR spectra of BGE, CS, CS-BGE-1, CS-BGE-2, CS-BGE-3 and CS-BGE-4 in

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transmission mode are presented in Figure 1. The spectrum of BGE exhibited peaks at 2965 cm-1,

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2940 cm-1 and 2867 cm-1, which are all attributed to alkyl –CH stretching vibrations. The peak at

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1110 cm-1 is attributed to the C–O–C stretching of the epoxide ring. The characteristic peaks of

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the CS were observed at 3347 cm-1 and 3310 cm-1; these bands are assigned to the broad O–H

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and N–H stretching, respectively. The CS spectrum also showed a –CH stretching vibration 2935

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cm-1, a strong –NH2 bending vibration at 1650 cm-1, an N–H bending vibration at 1550 cm-1, a

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C–O–C asymmetric stretching of the glycosidic linkage at 1070 cm-1, and a C–O stretching

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vibration at 1022 cm-1.

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The spectra of the CS derivatives showed a broad stretching of O–H absorption at 3347

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cm-1, which is attributed to the CS backbone. Compared with spectrum of the pristine CS, the


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spectra of the CS derivatives showed an increased band intensity for the alkyl groups (–CH

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stretching) at 2965 cm-1, 2940 cm-1, and 2867 cm-1; this increase in intensity was due to the

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addition of BGE. The band intensity also increased with increasing BGE content. The spectra of

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the CS derivatives also showed weaker bands representing N–H bending absorptions at 1650 cm-

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1

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BGE bonded to the free amine groups of CS. The C–O–C band of the epoxide ring from BGE

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was not observed in the spectra of the CS derivatives; however, the spectra of the CS derivatives

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showed a shift of the C–O stretching band at 1100 cm-1, indicating that the epoxide ring was

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opened during the reaction and bound to either the free –OH or the free –NH2 groups of the CS,

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thereby forming an ether linkage. These results indicate that BGE was successfully grafted onto

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the molecular chains of CS.

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Table 1. XPS elemental analyses of CS and its derivatives.

and 1550 cm-1 as the BGE content was increased; the emergence of these bands indicates that

CS CS-BGE-1 CS-BGE-2 CS-BGE-3 CS-BGE-4

C (%) 61.64 65.56 68.29 67.67 68.87

O (%) 31.81 30.37 28.74 30.16 28.34

N (%) 6.55 4.07 2.97 2.17 1.79

N/C 0.106 0.062 0.043 0.032 0.026

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XPS was performed to confirm changes in the elemental composition of the CS

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derivatives prepared by the addition of BGE. Table 1 lists the elemental compositions and N/C

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ratios of CS and its thermoresponsive derivatives. Compared to the carbon composition of the

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unmodified CS, that of the CS derivatives increased when BGE was added. As a result, a

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decrease in the nitrogen content was observed. These results show that upon the addition of more

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alkyl groups, the N/C ratio decreased. This noticeable trend indicates that changes occurred in



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the molecular structure due to covalent bonding of BGE to the CS backbone. Both the FTIR and

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XPS analyses demonstrate the presence of the hydrophobic moiety on the CS derivatives.

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Thermoresponsive solubility

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The LCST can be identified as the initial break point,26, 29

27

as the temperature

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corresponding to 90% transmittance28,

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transmittance1, 9, 27 in transmittance vs. temperature curves obtained by UV-Vis spectroscopy. In

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this study, we chose to define the LCST as the temperature corresponding to 90% transmittance.

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The initial break point does not apply to our curves because the curve drops too rapidly. On the

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other hand, some curves did not reach 50% transmittance.

or as the temperature corresponding to 50%

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80

Transmittance(%)

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Transmittance(%)

100

CS-BGE-1 0.5wt% 1wt% 1.5wt%

60

40

20


60

40

20

0

0 0

10

20

30

40

50 o

60

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0

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Temperature( C)

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Transmittance(%)

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Transmittance(%)

100

60


40

50 o

60

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0

0 0

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40

20

20

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Temperature( C)

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20

30

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50 o

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0

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Temperature( C)

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40

50 o

Temperature( C)

Figure 2. Transmittance vs. temperature curves of CS derivatives at 500 nm.


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Zhang et al. previously used thermoresponsive 2-hydroxy-3-butoxypropyl starch (i.e.,

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starch molecules grafted with BGE) to demonstrate that the LCST behavior of the starch changed

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with changes in molar substitution.1, 9 Figure 2 shows the transmittance vs. temperature curves

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of the modified CS derivatives with various BGE contents and different aqueous concentrations.

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LCST behavior is observed from the temperature where the solution is clear showing high

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transmittance up to the temperature where the solutions turns into a turbid solution exhibiting

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low transmittance. Neither CS-BGE-1 nor CS-BGE-2 exhibited temperature-dependent light

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transmittance. These results are attributed to the low extent of BGE addition to the amine groups

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of the CS, as confirmed by the FTIR spectra. However, CS-BGE-3 and CS-BGE-4 exhibited

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temperature-dependent light transmittance, even at different concentrations of the aqueous

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solutions. These results demonstrate that sufficient amounts of hydrophobic reagents must be

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added to the mixture to observe the LCST behavior. We also observed that lowering the polymer

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concentration increases the LCST. The 0.5 wt% solutions of CS-BGE-3 and CS-BGE-4 did not

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reach transmittances lower than ~80% and ~90%, respectively. This is due to the few particles

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formed by the presence of low solute. The particles will take time to aggregate and show a very

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slow LCST behavior. Thus a less turbid solution is observed with high transmittance. CS-BGE-3

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exhibited the best observable thermoresponsive behavior among the investigated formulations, as

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demonstrated by its sharp phase transition near ambient temperature. With an increase in the

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BGE content, the number of hydrophobic groups on the CS derivatives increased, which

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disrupted the inter- and intramolecular hydrogen bonds of the CS backbone. The excessive

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disruption of hydrogen bonds resulted in a reduced thermoresponsive effect.



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(b)

(a)

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Figure 3. (a) Transmittance vs. temperature curves of CS-BGE-3 at different concentrations. (b)

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The effect of concentration on LCST and ∆T1/2.

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CS-BGE-3 was further characterized for its phase-transition behavior. Figure 3a displays

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the effect of polymer concentration on the phase-transition behavior. Polymer solutions with

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concentrations from 0.5 wt% to 2 wt% exhibited different LCST ranges. Solutions with lower

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polymer concentrations exhibited higher LCSTs and broader phase transitions. With decreasing

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polymer concentration, the light transmittance of the solutions decreased. This result is attributed

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to the presence of less solute and insufficient polymer to aggregate and form a completely white

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turbid solution, unlike the solutions with higher polymer concentrations. A sharp phase transition

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is important for thermoresponsive smart materials. The microcalorimetric endotherm (∆T1/2),

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which is the half-value of the temperature difference between the final and initial break points in

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the transmittance vs. temperature curves, was used to better evaluate the sharpness of the phase

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transitions.27 Figure 3b shows that the phase transition of the 0.5 wt% solution was broad and

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that its ∆T1/2 was 19.5 °C. When the polymer concentration was increased to 2 wt%, the phase

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transition became sharper, with a ∆T1/2 of 5.1 °C. This result can be explained by the effect of the


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polymer concentration on the time required for aggregation.9 That is, when the polymer

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concentration is low, additional time is needed to form aggregates.

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Thermoresponsive behavior of CS derivatives (a) (b)

217 218

Figure 4. (a) Digital photographs of 1 wt% CS-BGE-3 aqueous solution at 15 °C and 60 °C,

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along with corresponding SEM images of the particles. (b) Reversible changes in the mean

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particle size of CS-BGE-3 aqueous solution at 15 °C and 60 °C.

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We observed that the 1 wt% CS-BGE-3 solution turned turbid when heated beyond its

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LCST and was able to return to its original state when cooled below its LCST (Figure 4a). This

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observation reveals that the aqueous solution exhibits reversible thermoresponsiveness. The

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SEM images reveal that particles were formed in the solution. The size of particles obtained by

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drying the solution at 15 °C was approximately 10 µm, whereas that of particles obtained from

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drying the solution at 60 °C was approximately 30 µm. The SEM images further show that the

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particles were uniformly dispersed and that aggregates formed at higher temperatures. A similar

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trend was reported on the basis of TEM images of a thermoresponsive cellulose ether.2



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To thoroughly investigate the size of the particle and aggregation with respect to temperature

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changes, dynamic light scattering (DLS) measurements were performed at 15 °C and 60 °C

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(Figure 4b). The sample was analyzed at 15 °C for 90 s. The temperature of the sample was then

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increased to 60 °C and analyzed for another 90 s. This test was repeated for three temperature

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cycles. Interestingly, the mean particle size at 15 °C ranged from approximately 1100 nm to

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1300 nm, whereas that at 60 °C initially ranged from 3000 nm and then abruptly increased with

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time to approximately 4250–4750 nm. The size observed in the SEM images correlates with the

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DLS results. Notably, the particle size at 60 °C continually increased with time, demonstrating

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that the particles continued to aggregate. A possible reason for this behavior is the continued

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disruption of the inter- and intramolecular hydrogen bonds in the polymer. The CS derivative has

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an amphiphilic nature; thus, the hydrophobic groups from the hydrophobic reagent bonded onto

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the CS chain provide a means for the molecule to form particles in solution.

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Microstructure analysis of the thermoresponsive property by positron annihilation lifetime

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spectroscopy

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(a)

(b)


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Figure 5. Positron annihilation lifetime spectroscopy analysis for 1 wt% CS-BGE-3 aqueous

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solution at 15 °C and 60 °C. Reversible thermoresponsive behavior effect on (a) o-Ps lifetime

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and free-volume size and (b) o-Ps intensity.

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Positron annihilation lifetime spectroscopy (PALS) is a highly sensitive technique for

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monitoring conformational, structural and microenvironmental transformations arising from

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subtle geometric changes in the molecular geometry of polymers.30-34 Quantitative analysis of the

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free-volume size and the free-volume size distribution for the thermoresponsive polymeric

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aqueous solutions were determined by the PALS technique for the first time. Figure 5 shows the

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o-Ps lifetime and o-Ps intensity of a 1 wt% CS-BGE-3 solution. The o-Ps lifetime is related to

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the free-volume that is available between polymer chains. A longer o-Ps lifetime corresponds

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with free-volumes with larger radii. As evident in Figure 5a, the free-volume size of the solution

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was larger at lower temperatures than when heated to higher temperatures. At 15 °C, the o-Ps

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lifetime was high at approximately 3.8–4.0 ns, corresponding to radii between 4.02 and 4.24 Å.

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When the solution temperature was increased to 60 °C, the o-Ps lifetime decreased substantially

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to 2.1–2.4 ns, corresponding to radii between 2.86 and 3.20 Å. The temperature was then cycled.

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These experiments show that the particles at 15 °C exhibit a larger free-volume than those at 60

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°C, suggesting a reversible thermoresponsiveness. Next, we monitored the o-Ps intensities,

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shown in Figure 5b, to assess the available free-volume. Large o-Ps intensity values indicate

262

large amounts of free-volume holes. A similar trend was observed at 15 °C, where the intensity

263

is higher, indicating approximately 21–22% free-volume. However, when the temperature was

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increased to 60 °C, the o-Ps intensity decreased to approximately 10–12%. The data also show

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the reversibility of the free-volume. These results suggest that at low temperatures, the

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hydrophobic part of the CS derivative is hidden within the well-dispersed particles. By contrast,



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at high temperatures, the polymer chains tend to rearrange to expose the hydrophobic moiety and

268

aggregate, which leads to smaller free-volume holes. These events explain the dramatic changes

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in the o-Ps lifetime and o-Ps intensity during the phase transition. A better representation of this

270

idea is shown in Scheme 1. At temperatures below the LCST, the polymer chains are linear and

271

form hydrogen bonds with water molecules, resulting in larger free-volume spaces. During the

272

transition at temperatures greater than the LCST, the polymer chains tend to fold and rearrange.

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In this phase, the hydrogen bonds decrease, and the free-volume spaces shrink.

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Thermoresponsive chitosan derivative as a draw solute 800

Osmolality (mOsm)

700 600 500 400 300

CS-BGE-1 CS-BGE-2 CS-BGE-3 CS-BGE-4

200 100 0

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congo 0.0 red dye

0.5

1.0

1.5

2.0

Concentration (wt%)

276

Figure 6. Osmolality of Congo Red dye and different concentrations of CS derivatives aqueous

277

solution.

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Current challenges in FO involve the development of good draw solutes. In this study, we

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used a simple FO setup to initially study the potential of the thermoresponsive CS-BGE-3

280

derivative as a draw solute. The osmolality of the Congo Red dye and different concentrations of

281

CS derivatives aqueous solution was evaluated; the results are shown in Figure 6. The

282

osmolality of the dye was lower than that of the thermoresponsive CS derivative at different


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concentrations in aqueous solutions. The lower BGE content resulted to a higher osmolality.

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However, these CS derivatives with low BGE content do not exhibit LCST behavior. Notably,

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the osmolality of the best thermoresponsiveness at 1 wt% CS-BGE-3 (492 mOsm) was higher

286

than that of the dye (133 mOsm). Thus, a 0.025 wt% Congo Red dye solution was used as the

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feed solution and 1 wt% CS-BGE-3 as the draw solution. A commercial FO membrane was used.

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The experiment was performed at room temperature, which is below the LCST of the 1 wt% CS-

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BGE-3 solution.

290 291

Figure 7. Simple FO test for 1 wt% CS-BGE-3 as draw solute and microfiltration recovered CS

292

derivatives. Feed solution: 0.025 wt% Congo Red dye.

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Figure 7 shows the setup used for FO and microfiltration. Initially, an equal volume of

294

both the feed and draw solutions were placed on either side of a commercial FO membrane. The

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ability of the draw solute to draw water from the feed solution was initially investigated using

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deionized water as the feed solution. The flux was 9.1 L/m2h and CS derivative rejection 99.8 %.

297

In the case of dye dehydration, a 10 cm difference in height was observed between the feed and

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draw solutions after a period of time. The calculated flux was 8.6 L/m2h and CS derivative

299

rejection 99.8 %, indicating good permeation and negligible solute back diffusion. The small



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particles may have enhanced the drawability of the solution because they are more hydrophilic.

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The water from the draw solution was collected and heated to 60 °C, a temperature above its

302

LCST. The CS derivative aggregation can facilitate separation of the particles via microfiltration

303

(pore size 1.0 µm), 99.9 % was recovered enabling the particles to be reused (See Figure 7.

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right). We followed the ISO 10993-535 method for CSBGE-3 cytotoxicity test. The CSBGE-3

305

was extracted for cell culture. Results showed a cell survival of 92% and it has been determined

306

to be zero-grade (non-toxic). The synthesized thermoresponsive CS derivative showed potential

307

to be used as a draw solute in the FO process.

308

In conclusion, a thermoresponsive CS derivative, CS-BGE, was synthesized from

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hydrophilic CS reacted with hydrophobic BGE. The successful addition of BGE functional

310

groups was confirmed by FTIR spectroscopy and XPS elemental analysis. These experiments

311

showed that BGE was bound to both the amine and the hydroxyl reactive groups of the CS.

312

Varying the BGE content changed the LCST range, and the CS-BGE-3 formulation was

313

observed to exhibit the best response via a turbidity test by UV-vis spectroscopy. Increasing the

314

concentration of the thermoresponsive CS aqueous solution led to a decrease in the LCST. This

315

change was due to aggregation of more particles in solution with increasing temperature. SEM

316

images showed the aggregation of particles from 15 °C to 60 °C. A similar trend was observed

317

for the mean particle size determined by DLS; particles aggregated when the temperature was

318

increased. When the temperature was increased to 60 °C, the mean particle size continually

319

increased within 90 s and returned to a smaller size when the temperature was lowered back to

320

15 °C. Interestingly, the PALS data also showed the reversibility of the particle formation. The

321

free-volume sizes at lower temperatures was larger than those at higher temperatures. These

322

results demonstrate that changes in the microstructures occurred during the phase transition.


Page 19 of 24


323

Furthermore, the thermoresponsive CS derivative was found to be non-toxic with 92% cell

324

survival. The results of simple FO tests suggest that the synthesized thermoresponsive CS

325

derivative can potentially be used as a draw solute because it gives a flux of 8.6 L/m2h and

326

rejection of 99.8 % upon dye dehydration.

327

Corresponding Author

328

*E-mail: [email protected]

329

Acknowledgements

330

The authors wish to sincerely thank the Ministry of Science and Technology of Taiwan for

331

financially supporting this work.

332

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Table of Contents Graphic

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Reaction between chitosan and butyl glycidyl ether, producing a thermoresponsive chitosan

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derivative; the photographs show particles exhibiting lower critical solution temperature

433

behavior.


Characterization of a Thermoresponsive Chitosan Derivative as a

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