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Enhanced Stain Removal and Comfort Control Achieved by Cross-linking Light- and Thermo- Dual Responsive Copolymer onto Cotton Fabrics Qi Zhong, Min Lu, Sophie Nieuwenhuis, Bi-Sheng Wu, GuangPeng Wu, Zhi-Kang Xu, Peter Muller-Buschbaum, and Ji-Ping Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19908 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Applied Materials & Interfaces

Enhanced Stain Removal and Comfort Control Achieved by Cross-linking Light- and Thermo- Dual Responsive Copolymer onto Cotton Fabrics Qi Zhong1,2,*, Min Lu1, Sophie Nieuwenhuis1, Bi-Sheng Wu1, Guang-Peng Wu3, Zhi-Kang Xu3, Peter Müller-Buschbaum2 and Ji-Ping Wang1,* 1Key

Laboratory of Advanced Textile Materials & Manufacturing Technology, Ministry of

Education; National Base for International Science and Technology Cooperation in Textiles and Consumer-Goods Chemistry, Zhejiang Sci-Tech University, 310018 Hangzhou, China

2Technische

Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748 Garching, Germany

3MOE

Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

*Corresponding author. [email protected] Phone +86 571 86843436 fax +86 571 86843436 [email protected] Phone +86 571 86843665 fax +86 571 86843436 Key words: smart textiles, light-responsive, thermo-responsive, copolymer, crosslinking, cotton fabrics 1

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Abstract Enhanced capabilities of stain removal and comfort control are simultaneously achieved by the light- and thermo- dual responsive copolymer poly(triethylene glycol methyl ether methacrylate-co-ethylene glycol methacrylate-co-acrylamide azobenzene) (P(MEO3MA-co-EGMA-co-AAAB)) cross-linked on cotton fabrics. P(MEO3MA-coEGMA-co-AAAB) is synthesized by sequential atom transfer radical polymerization (ATRP) with a molar ratio of 8 (MEO3MA): 1 (EGMA): 1 (AAAB). The MEO3MA units induce a thermo-responsive behavior to the copolymer. The hydrophilicity of the copolymer films can be further improved by the light-induced trans-cis isomerization of the AAAB units with UV radiation. The copolymer is facilely immobilized onto cotton fabrics with 1,2,3,4-butane tetracarboxylic acid (BTCA) as cross-linker. Due to the immobilization of P(MEO3MA-co-EGMA-co-AAAB), the hydrophilicity of the fabrics surface is increased under UV radiation. Therefore, by simply installing a UV light source in the washing machine, the better capability of stain removal is realized for the cross-linked cotton fabrics. It can prominently reduce the consumption of energy, water and surfactants in laundry. In addition, the trans- AAAB units of the copolymer cause the cross-linked P(MEO3MA-co-EGMA-co-AAAB) layer to be more hydrophobic under ambient conditions. Hence, the copolymer can more easily collapse and form a porous structure on the fabrics. Thereby, the air permeability of cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) is enhanced by 13% at human body temperature as compared to P(MEO3MA-co-EGMA), giving an improved comfort control during daily wear. 2


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Introduction In recent decades, stimuli-responsive polymers have attracted great interests because of the capability of having a spontaneous response to an external stimulus (temperature, 1-5

light,6-10 pH11-20 and magnetic field21-23). Among them, thermo-responsive polymers

are intensively investigated because of the easy realization of temperature control which renders it compatible with many applications.2 When the temperature is below the transition temperature (TT) of the polymer, intermolecular hydrogen bonds can be formed between the polymer chains and the water molecules.2 Therefore, the thermoresponsive polymers are hydrophilic and can dissolve in water. Increasing the temperature above its TT, the polymer chains tend to turn hydrophobic and collapse. This transition behavior is reversible, meaning that the formerly collapsed polymers can switch back to a hydrophilic and swollen state when the temperature is decreased below the TT again. Based on this unique switching capability, thermo-responsive polymers possess a broad range of applications, such as use in drug delivery,24,25 cell adhesion26,27 and smart textiles.28-31 In particular smart textiles offer new possibilities with multiple functions when adhering thermo-responsive polymer to the fabrics. Recently, for example the thermo-responsive polymer (poly(2-(2-methoxyethoxy) ethoxyethyl methacrylate-co-ethylene glycol methacrylate), denoted as P(MEO2MAco-EGMA)), was cross-linked onto cotton fabrics to improve cleaning28 and air/moisture permeability29 of the fabrics. Due to heating from the human body, the cross-linked P(MEO2MA-co-EGMA) turned hydrophobic and porous. Thereby, the air/moisture permeability and comfort of fabrics were controlled in daily wear. During 3



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laundry, P(MEO2MA-co-EGMA) turned hydrophilic because of the low water temperature. Hence, the cleaning performance of the cotton fabrics was improved as well.28 Although both functions were successfully demonstrated, the realized TT of 38 oC

was quite high with respect to applications.28 The high value was caused by the

additional EGMA in P(MEO2MA-co-EGMA). To realize the cross-linking of the copolymer to cotton fabrics, monomer EGMA containing -OH groups in the end of the side chain were introduced into the copolymer. However, EGMA also increased the hydrophilicity of the copolymer and shifted the TT to more moderate values which were above the human skin temperature (32-33 oC). As a consequence, a higher temperature was required to switch the polymer to its hydrophobic state and form a porous polymer structure. However, an expanded polymer layer with a significantly lower porosity showed less air permeability with correspondingly lower comfort. Therefore, the comfort of the fabrics turned poor, if the hydrophilicity was too good. To overcome these challenges, one possible approach is to introduce acrylate based analogues with a lower TT into the copolymer, such as poly(2-methoxyethoxy ethoxyethyl methacrylate), abbreviated as PMEOMA (TT=15 oC).32 However, due to less ethoxy groups in the side chain, PMEOMA possesses a glass transition temperature (Tg) much higher than room temperature. For this reason, it is in the glassy state under ambient conditions. When cross-linking onto the cotton fabrics, PMEOMA significantly influences its softness. Hence, this approach is not reasonable. Another option is to add hydrophobic components, such as polystyrene, into the copolymer. The hydrophobicity of the copolymer is increased and the TT shifts towards lower values. Although lower TT 4


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improves the comfort control in daily wear, it also causes the copolymer to maintain hydrophobic in a broader temperature region. Thus, the cleaning performance is reduced during laundry, which is again disadvantageous. Based on the above discussion, it seems difficult to realize better stain removal and comfort control simultaneously. Besides thermo-responsive polymers, light-responsive polymers containing azobenzene were also broadly investigated in the last decades.33-35 Under UV radiation, the conformation of azobenzene switches from the trans- to the cis- state. Thereby its polarity as well as hydrophilicity increases. When UV radiation is off, azobenzene immediately turns back to the trans- and hydrophobic state.36 Based on this unique and reversible property, the hydrophobicity of thermo-responsive polymers can be further tuned by adding azobenzene units. The obtained copolymer turns more hydrophobic under ambient conditions.37-39 When such copolymer is coated to the fabrics and in contact with the human body, the copolymer layer has a more porous structure. Therefore the air permeability as well as the comfort of the fabrics is improved. Under UV radiation, the cis- state of azobenzene increases the layer hydrophilicity, favoring for the stain removal during laundry. Therefore, the desire for better cleaning performance and comfort control can be simultaneously realized by introducing a lightand thermo- dual responsive copolymer onto the cotton fabrics. In order to achieve the function described above, light- and thermo- dual responsive copolymers are immobilized onto cotton fabrics. General approaches for the immobilization include grafting30,40,41 and cross-linking.28,29,42 As compared to grafting, cross-linking is more effective and suitable for a potential mass production of the 5



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fabrics. For this reason, we decide for cross-linking with 1,2,3,4-butane tetracarboxylic acid (BTCA). Due to the hydrophobicity of the azobenzene component (acrylamide azobenzene, AAAB) under ambient conditions, the introduction of AAAB shifts the TT towards lower values. To maintain the TT of the copolymer close to the ambient temperature, triethylene glycol methyl ether methacrylate (MEO3MA), possessing a higher TT than 2-(2-methoxyethoxy) ethoxyethyl methacrylate (MEO2MA), is selected as the thermo-responsive monomer. Additionally, ethylene glycol methacrylate (EGMA) containing -OH groups at the end of side chain is also introduced into the copolymer to realize the cross-linking. The structure of this article is as follows: The introduction is followed by an experimental section describing the synthesis of light- and thermo- dual responsive copolymer

P(MEO3MA-co-EGMA-co-AAAB)

as

well

as

thermo-responsive

copolymer P(MEO3MA-co-EGMA), preparation of cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA), the measurements applied for testing the hydrophilicity under different thermal and light stimuli. The third section compares the structure, swelling capability, the cleaning performance and air permeability between cotton fabrics cross-linked with P(MEO3MA-co-EGMA-coAAAB) and P(MEO3MA-co-EGMA). In the end, a conclusion is presented about the enhanced capabilities of stain removal and comfort control achieved by cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB).

Experimental section 6


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Materials The monomers triethylene glycol methyl ether methacrylate (MEO3MA, purity 95%) and ethylene glycol methcrylate (EGMA, purity 95%) were purchased from J & K Chemical. P-aminoazobenzene (purity 98%), acryloyl chloride (purity 96%), 1,2,3,4butane tetracarboxylic acid (BTCA, purity 99%), sodium hypophosphite (purity 99%), tris(2-dimethylaminoethy)amine (Me6TREN, purity 98%), methyl 2-chloropropionate (purity 98%) and anisole (purity 99%) were received from Aladdin. N-Hexane (purity 98%), tetrahydrofuran (purity 99%) and triethylamine (purity 99%) were obtained from Gaojing fine chemical. CuBr (purity 98.5%) was bought from Qiangshun Chemicals. Cotton fabrics used in the investigation were 40 × 40 cotton poplin weighing 180 g m2,

provided by Procter & Gamble Company. They were extensively applied in clothing.

Synthesis of monomer acrylamido azobenzene (AAAB) Light-responsive monomer acrylamido azobenzene (AAAB) was synthesized as follows: 1.0 g p-aminoazobenzene and 0.8 mL triethylamine were dissolved in 20 mL dichloromethane. Afterwards the mixed solution was transferred into a 100 mL round bottom flask. Then 0.3 mL acryloyl chloride was slowly added into the flask in 20 min. After that, the flask was immersed into an oil bath with a condensing tube. After heating at 60 oC for 3 h, the flask was moved out from the oil bath and quenched in ice bath. The rotary evaporator was used to remove the solvent in the obtained solution. The residual product was washed with deionized water once and recrystallized in ethanol thrice. The obtained monomer AAAB was yellow powder. 7



Synthesis

of

random

copolymer


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and

P(MEO3MA-co-EGMA) The light- and thermo- dual responsive random copolymer poly(triethylene glycol methyl ether methacrylate-co-ethylene glycol methcrylate-co-acrylamido azobenzene), donated as P(MEO3MA-co-EGMA-co-AAAB), was synthesized by MEO3MA, EGMA and AAAB with a molar ratio of 8:1:1. During the synthesis, MEO3MA (3.63 mL; 16 mmol), EGMA (0.68 mL; 2 mmol), AAAB (502 mg; 2 mmol), Me6TREN (56 mL; 0.21 mmol), catalyzer CuBr (20 mg; 0.14 mmol), 20 mL anisole, initiator MCP (21 mL; 0.14 mmol) were sequently added into a tube in the glove box. Afterwards, the tube was sealed and immersed into an oil bath thermo-stated at 70 oC. After reaction for 3 h, it was quenched in ice bath. The mixture was first diluted by THF, and then eluted in the aluminum oxide column by THF to remove residual CuBr. Finally, it was precipitated thrice in hexane. The obtained random copolymer P(MEO3MA-co-EGMA-co-AAAB) was a yellow and viscous gel. The synthesis of P(MEO3MA-co-EGMA) was similar as P(MEO3MA-co-EGMA-coAAAB). The molar ratio of MEO3MA to EGMA was set to 8:1. Thus, the added amount of MEO3MA and EGMA were 2.01 mL (8.88 mmol) and 0.38 mL (1.11 mmol), respectively.

Contact angle measurements for deposited and cross-linked films Non-cross-linked, deposited films were prepared by directly dropping the P(MEO3MAco-EGMA-co-AAAB) 0.4 mL solution out of 1,4-dioxane onto blank Si substrates. The 8


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concentration was varied from 2 mg/mL to 5 mg/mL and 10 mg/mL. Thereby different film thickness were obtained (1.1 µm, 2.7 µm and 5.4 µm). In case of cross-linked P(MEO3MA-co-EGMA-co-AAAB) films, 0.2 mL solution containing 2% (wt%) P(MEO3MA-co-EGMA-co-AAAB), 2.66% (wt%) BTCA, 1.33% (wt%) sodium hypophosphite, 88% (wt%) water was dropped onto blank Si substrates. After deposition, the Si substrates were placed in an oven thermo-stated at 60 oC for 8 min. Then, the temperature was raised to 130 oC and thermo-stated for 5 min. The obtained thickness was 12.5 µm. By increasing the concentration to 5% and 10%, the thickness increased to 31.3 µm and 62.5 µm, respectively. The contact angles of the non-cross-linked, deposited and cross-linked P(MEO3MAco-EGMA-co-AAAB) films were measured at different scenarios (below/above TT, before/after UV radiation) with the drop shape analyzer (Krüss DSA20). The UV irradiation was performed in a chamber with UV light (wavelength of 365 nm) for 20 min. To avoid the absorption of water by the P(MEO3MA-co-EGMA-co-AAAB) films, paraffin oil was applied. During the measurements, 3 µL paraffin oil was dropped onto the film. The contact angles of the oil droplets approaching the film surface were monitored. For each film, the measurements were repeated three times to minimize the experimental errors.

Preparation of cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) The cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) were prepared 9



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as follows: The cotton fabrics were first immersed in a solution containing 8% (wt%) P(MEO3MA-co-EGMA-co-AAAB), 2.66% (wt%) BTCA, 1.33% (wt%) sodium hypophosphite, 88% (wt%) water for one day. Afterwards, the fabrics were moved out from the solution and rolled. To ensure a sufficient amount of copolymer was attached on the cotton fabric, the immersion and rolling processes were repeated twice. Afterwards, the fabrics were placed in an oven thermo-stated at 60 oC for 8 min. Then, the temperature was raised to 130 oC and thermo-stated for 5 min. When the thermal treatment was finished, the fabrics were flushed with water to remove all unreacted copolymer and cross-linker. Finally, the cotton fabrics cross-linked with P(MEO3MAco-EGMA-co-AAAB) were mounted in the oven at 60 oC for 10 min to remove the residual water. To address the influence of AAAB to the stain removal and comfort control, thermo-responsive copolymer P(MEO3MA-co-EGMA) was cross-linked onto the cotton fabrics as well. The preparation protocol was same as that for P(MEO3MAco-EGMA-co-AAAB).

UV-Vis spectroscopy By UV-Vis spectroscopy (Perkin Elmer UV/Vis lambda 35 spectrometer), both lightand thermo- responsive behaviors of P(MEO3MA-co-EGMA-co-AAAB) in aqueous solutions were investigated. The concentration was fixed as 3 mg mL-1. The wavelength applied was 700 nm. During the measurements, the copolymer was heated from 14 oC to 21 oC with a heating rate of 1 oC min-1. To address the effect of trans-cis isomerization to the transition behavior, the copolymer was first exposed to UV 10


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radiation (wavelength of 365 nm) for 10 min. Afterwards, the radiated copolymer solution was heated from 14 oC to 22 oC with same heating rate. To focus on the influence of AAAB to the transition behavior, P(MEO3MA-co-EGMA) was also measured with the same concentration and measurement protocol.

ATR-FTIR spectroscopy The functional groups in P(MEO3MA-co-EGMA-co-AAAB) as well as the intermolecular hydrogen bonds formed between copolymer and water were investigated by ATR-FTIR spectroscopy (Bruker Vertex 70 spectrometer). The scanned wavelength covered a range from 600 cm-1 to 4000 cm-1 with a resolution of 4 cm-1. Before measurements, P(MEO3MA-co-EGMA-co-AAAB) was dissolved in 1,4dioxane. The solution concentration was 50 mg mL-1. By repeatedly dropping the solution onto PET foils, the films with a thickness of 100 m were obtained. During the measurements, the as-prepared P(MEO3MA-co-EGMA-co-AAAB) film was first probed. Another as-prepared film was placed in a sealed plastic cell thermostated at 15 oC (below TT under ambient condition, TTam). 2 mL distilled water was injected into the cell to install a saturated water vapor atmosphere. Thereby the absorption of water molecules set in. After waiting for 0.5 h, the swollen film was probed by ATR-FTIR. To investigate the breaking of the hydrogen bonds above TTam, the third as-prepared film was exposed to saturated water vapor at 30 oC (above TTam) for 0.5 h and measured by ATR-FTIR as well. To further study the effect of trans-cis isomerization to the hydrogen bonds in P(MEO3MA-co-EGMA-co-AAAB) films, UV 11



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radiation (wavelength of 365 nm) and exposure to water vapor at 15 oC (below TT under UV radiation, TTUV) or 30 oC (above TTUV) were simultaneously applied on the fourth and fifth as-prepared films for 0.5 h. Then the equilibrated films were measured by ATR-FTIR. To emphasize the increase of hydrogen bonds by the trans-cis isomerization from AAAB, P(MEO3MA-co-EGMA) films were also exposed to saturated water vapor at 30 oC (below TT) for 0.5 h and measured by ATR-FTIR.

Scanning electron microscopy (SEM) The surface morphology of the original cotton fabrics, cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) was probe by scanning electron microscopy (SEM, JEOL, JSM-5610LV). The selected magnification was 2000. The working distance and the voltage applied in the measurements were 6 mm and 30 kV, respectively.

X-ray photoelectron spectroscopy (XPS) The surface components on the original cotton fabrics, cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) were analyzed by Xray photoelectron spectrometer (XPS, Axis Ultra from Kratos Analytical, UK). A monochromatized Al-Kα X-ray with an energy of 1486.7 eV was selected as the excitation source. The survey scans were obtained with a pass energy and step of 100 eV and 1 eV, respectively.

12


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Dynamic light scatting (DLS) measurements The temperature evolution of the average particle size in P(MEO3MA-co-EGMA-coAAAB) aqueous solutions was measured by Dynamic light scattering (Zetasizer Nano S, Malvern, UK). The solution concentration was fixed as 3 mg mL-1. The temperature range was between 15 oC to 24 oC and the temperature was increase in steps of 1 oC. The measurements were performed 120 s after each change of temperature to ensure that the copolymer particle size reached an equilibrium state. In order to study the influence of UV radiation on the particle size, the aqueous solution was also exposed to UV radiation (365 nm) for 20 min before each DLS measurement.

Equilibrium swelling ratio (ESR) measurements The equilibrium swelling ratio measurements were conducted by soaking original cotton fabrics and cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) in a temperature and UV light-controlled water bath. The initial water temperature was set to 10 oC. After swelling in the ambient condition for 24 h, the equilibrated fabrics were moved out from the water bath. The residual water on the fabrics was absorbed by placing two pieces of KIMTECH filter paper below and above the fabrics. In addition, a piece of glass (200 g) with similar area as the cotton fabrics was mounted on top of the filter paper to ensure that the force on the fabrics was constant during the absorption. Then, the fabrics were weighed again. The equilibrium swelling ratio (ESR) was calculated by the following equation: ESR =

𝑊 - 𝑊0 𝑊0

× 100% 13



In which

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W0 and W were the weights of cotton fabrics in the initial and at equilibrium

state, respectively. To study the correlation between the temperature and absorption capability of fabrics, the water temperature was increased from 10 oC to 35 oC, with a step of 5 oC. In addition, UV radiation (wavelength of 365 nm) was applied on the fabrics at different temperatures to probe the influence of chain conformation to the absorption capability. To further address the change of absorption capability by the introduction of AAAB, the ESR of cotton fabrics cross-linked with P(MEO3MA-co-EGMA) was measured at different temperatures as well.

Washing fastness measurements The stability of the cross-linked P(MEO3MA-co-EGMA-co-AAAB) on cotton fabrics was investigated by washing fastness measurements. Before washing, the cross-linked cotton fabric (1 g) was placed in a beaker containing 50 mL detergent aqueous solution (1 mg mL-1). Thereby, the weight ratio of fabric to solution was fixed as 1:50. Then a magnetic rotator was placed in the beaker. During the washing, the rotation speed and washing time were set to 200 rpm and 20 min, respectively. Afterwards, the fabrics were moved out and flushed with distilled water to remove the residual detergent. It was weighed again after drying in the oven for 12 hours. The washing process was repeated 10 times to test the stability of the cross-linked P(MEO3MA-co-EGMA-coAAAB) on the cotton fabric.

14


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Differential scanning calorimetry (DSC) measurements The glass transition temperatures (Tg) of P(MEO3MA-co-EGMA-co-AAAB) and P(MEO3MA-co-EGMA) were measured by differential scanning calorimetry (DSC, TA Instruments Q2000). The measurements were performed in a nitrogen atmosphere. Before the measurements, the copolymers were first cooled down from room temperature to -80 oC with a cooling rate of 10 oC min-1. After thermo-stated for 30 s, the temperature was increased to 200 oC with a heating rate of 10 oC min-1. The cooling and heating were repeated once with same protocol. Simultaneously DSC data was recorded.

Air permeability measurements To study the increase of air permeability by the additional AAAB component, the air permeability of original cotton fabrics, cotton fabrics cross-linked with P(MEO3MAco-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) was measured by air permeability tester (YG461E, Wenzhou Fangyuan) at room temperature (25 oC). The fabric area measured was 50 cm2. The decline of air pressure was set to 100 Pa. To mimic the air permeability of fabrics during wearing, the fabrics were pre-heated to 40 oC and measured again. During the measurements, the temperature of fabrics was around 33 oC

(slowly reduced from 35 oC to 30 oC). As the temperature was close to the skin

temperature (33 oC), it was able to evaluate the air permeability during daily wear.

Stain removal measurements 15



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To study the enhanced stain removal on cotton fabrics cross-linked with P(MEO3MAco-EGMA-co-AAAB), the confocal microscopy (CM) from Nikon C2 was used. The stain on the fabrics was mimicked by the cooking oil, while Nile red was used as the florescence dye. Before the measurements, the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) were immersed into the cooking oil and Nile red in sequence. Each immersion lasted for 60 s. Afterwards, the fabrics were moved into an oven and dried at 30 oC for 24 hours. The fiber from the dyed fabrics was measured by CM under ambient condition. The second dyed fiber was rinsed by distilled water thermo-stated at 15 oC (below TTam) for 10 times and measured by CM. To investigate the influence of hydrophilicity-hydrophobicity switching to the cleaning, the third dyed fiber was rinsed by distilled water thermo-stated at 30 oC (above TTam) for 10 times and measured by CM as well. The fourth and fifth dyed fibers were rinsed by distilled water thermo-stated at 15 oC (below TTUV) or 30 oC (above TTUV) under UV radiation for 10 times to address the trans-cis isomerization to the stain removal. In addition, the cotton fabrics cross-linked with P(MEO3MA-co-EGMA) were measured as as-prepared, after rinsing by distilled water thermo-stated at 30 oC (below TT) or 50 oC (above TT). To further address the cleaning capability, the rinsing was prolonged to 40 times for the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) at 15 oC under UV irradiation and P(MEO3MA-co-EGMA) at 30 oC. Thus, the effect of light-responsive AAAB component to the stain removal can be obtained. The average fluorescence intensity was calculated from the total fluorescence intensity divided by the total fluorescence area. The fluorescence area and the fluorescence intensity for each area 16


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on the fiber can be obtained directly from the CM images by the software Image Pro 10.1.

Results and discussion Characterization of P(MEO3MA-co-EGMA-co-AAAB) and P(MEO3MA-coEGMA) The schematic presentation for the synthesis of P(MEO3MA-co-EGMA-co-AAAB) and P(MEO3MA-co-EGMA) is shown in Figure 1. The ratio of MEO3MA, EGMA to AAAB is selected as 8:1:1 in P(MEO3MA-co-EGMA-co-AAAB). While the ratio of MEO3MA to EGMA is 8:1 in P(MEO3MA-co-EGMA).

Figure 1: Schematic presentation for the synthesis of (a) P(MEO3MA-co-EGMA-co-AAAB) and (b) P(MEO3MA-co-EGMA).

17



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The characteristic 1H-NMR spectrum of P(MEO3MA-co-EGMA-co-AAAB) is presented in Figure S1a. Signal e is from hydrogen belonging to the methoxy group (O-CH3) in MEO3MA. While signal g is from hydrogen in hydroxyl group (-CH2-OH) in EGMA. In addition, signals a, b and c are from hydrogen in AAAB. Hence, the presence of MEO3MA, EGMA and AAAB is confirmed in the copolymer. In the 1H-NMR spectrum of P(MEO3MA-co-EGMA), signal e and g are also observed, whereas signals a, b and c are not found in the spectrum (Figure S1b). Thereby it can be concluded that both copolymers are successfully synthesized. The size exclusion chromatography (SEC) is performed in THF. The number average molecular weight (Mn) and polydispersity (PDI) of P(MEO3MA-co-EGMA-co-AAAB) are 15,600 g mol-1 and 1.28, respectively. To P(MEO3MA-co-EGMA), Mn and PDI are 15,700 g mol-1 and 1.27, respectively. Tg is crucial for the polymers used in fabrics. Higher Tg causes the polymer to stay in the glassy state under ambient conditions. Hence, the fabrics turn stiff after introduction of these polymers with high Tg. Figure S2a shows the DSC curve of P(MEO3MA-co-EGMA-co-AAAB). Its Tg is around -23 oC. Comparing to Tg of P(MEO3MA-co-EGMA) (-45 oC, Figure S2b), the additional AAAB in P(MEO3MA-co-EGMA-co-AAAB) shifts the Tg towards higher values. However, both values are still well below 0 oC, indicating the copolymers stay in the rubbery state under ambient conditions and will not influence the hand feel of the fabrics. Figure 2a shows the transmittance of the P(MEO3MA-co-EGMA-co-AAAB) 18


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aqueous solutions with (blue curve) and without (red curve) UV radiation as well as P(MEO3MA-co-EGMA) aqueous solutions (green curve) as a function of temperature probed by UV-Vis spectroscopy. The solution concentration is 3 mg mL-1.

Figure 2. (a) Transmittance of the P(MEO3MA-co-EGMA-co-AAAB) aqueous solution with (blue curve) and without (red curve) UV radiation as well as P(MEO3MA-co-EGMA) aqueous solution (green curve) as a function of temperature probed by UV-Vis spectroscopy. (b) The first derivative of transmittance with respect to temperature as a function of the temperature. The solution concentration is 3 mg mL-1.

For the P(MEO3MA-co-EGMA-co-AAAB) aqueous solution, the transmittance starts dropping at 16.0 oC and reaches its minimum at 21 oC under ambient conditions. Obviously, P(MEO3MA-co-EGMA-co-AAAB) is thermo-responsive. It should be noted that the aqueous solution presents a relatively broad transition region, which is 19



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related to the acrylate based thermo-responsive polymer applied.2,43,44 By plotting the first derivative of transmittance with respect to temperature as a function of the temperature (red curve in Figure 2b), the TTam of P(MEO3MA-co-EGMA-co-AAAB) is determined as the temperature showing the minimum in the first derivative, which is 19 oC. After UV radiation (wavelength of 365 nm) for 10 min, radiated P(MEO3MA-coEGMA-co-AAAB) aqueous solution starts to turn turbid at 15.0 oC and reaches its minimum in transmission at 20 oC. Surprisingly, both temperatures are even lower than those without UV radiation. In general, the expectation is that the increased hydrophilicity of AAAB will shift the TT towards higher values after the trans-cis isomerization.34-36 However, the observed unexpected reduction of the TT after UV radiation has been reported before only for light-responsive, amphiphilic poly(NIPAM) derivatives.45 Following this idea, it can be related to the different self-assembly behavior based on the AAAB molar fraction in P(MEO3MA-co-EGMA-co-AAAB). When the fraction is small, AAAB is randomly distributed in the copolymer. Thus, it can only contribute to the change of hydrophilicity before and after UV radiation. However, a large fraction of AAAB induces the formation of tiny AAAB blocks in copolymer chains. These blocks from different chains can aggregate together and selfassemble the micelle-like structure (core: hydrophobic AAAB, shell: hydrophilic MEO3MA and EGMA). Upon heating, the outer shell will turn hydrophobic and aggregate together to form large particles. When the micelles are exposed to UV radiation, the switching of AAAB from hydrophobic to hydrophilic state does not 20


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disassemble the micelles but only enlarge their volumes. The expanded AAAB cores favor the formation of large particles upon heating and shift TT towards lower value. To further confirm the unusual observation of a lower TT under UV radiation, the temperature evolution of the average particle size in P(MEO3MA-co-EGMA-co-AAAB) aqueous solution (3 mg mL-1) is measured by dynamic light scattering (DLS) with and without UV radiation. As presented in Figure S3, the particle size is (27 ± 5) nm at 15 oC without UV radiation (red dots). Between 15 oC and 17 oC, it stays unchanged. When

the solution is heated to 18 oC, the size is slightly increased to (60 ± 5) nm. As the size is smaller than 100 nm (marked by black dashed line), the solution remains clear. Further heating to 19 oC (marked by red arrow in Figure S3), the size suddenly enlarges to (136 ± 10) nm. Because this size is larger than 100 nm, the solution turns turbid. This temperature fits the TTam obtained from UV-Vis spectroscopy. The size of particles continues increasing when the solution is further heated. It is caused by the aggregation of the hydrophobic particles at temperatures above the TTam. After exposure to UV radiation, the particle size of P(MEO3MA-co-EGMA-co-AAAB) aqueous solution rises to (74 ± 5) nm at 15 oC. This value is almost three times to that without UV radiation. It confirms our assumption that the UV radiation enhances the hydrophilicity of P(MEO3MA-co-EGMA-co-AAAB). Thereby it expands the volume at low temperature. This conclusion also agrees with the observation from Liu et al.45 When the solution is heated to 17 oC (marked by blue arrow in Figure S1), the size turns to (95 ± 5) nm. As this value is very close to 100 nm, the solution starts turning turbid. Further increasing the temperature to 18 oC, the size continues enlarging to (169 ± 10) 21



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nm and the solution is completely turbid. This observation agrees with the data obtained from UV-Vis spectroscopy: TTUV is even lower than TTam. Thereby, it can be concluded that P(MEO3MA-co-EGMA-co-AAAB) is light- and thermo- dual responsive. The transmittance of P(MEO3MA-co-EGMA) aqueous solution also reduces with temperature (green curve in Figure 2a). From the first derivative of transmittance with respect to temperature (green curve in Figure 2b), the TT of P(MEO3MA-co-EGMA) is determined as 48 oC, which is remarkably higher than P(MEO3MA-co-EGMA-co-AAAB). As a consequence, the hydrophobicity of the AAAB component significantly influences the TT, although the AAAB molar fraction is only 10%. To compare the transition behavior between aqueous solutions and films, ATR-FTIR measurements are applied to the P(MEO3MA-co-EGMA-co-AAAB) and P(MEO3MAco-EGMA) films coated on PET foils. Figure 3a shows the spectrum of the as-prepared P(MEO3MA-co-EGMA-co-AAAB) film (black curve) in the wavenumber range between 750 cm-1 and 3750 cm-1. The characteristic peaks related to the C-O and C=O are visible at 1104 cm-1 and 1720 cm-1, respectively. The O-H and N-H bands in the range of 3300-3600 cm-1 and 3000-3300 cm-1 are observed as well. The absorption band in the range of 2800-3000 cm-1 is assigned to C-H stretching.44,46

22


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Figure 3. a) ATR-FTIR spectra of P(MEO3MA-co-EGMA-co-AAAB) films measured in different conditions: as-prepared film (black curve), exposed in water vapor atmosphere at 15 oC

(below TTam) for 0.5 h (red curve), exposed in water vapor atmosphere at 30 oC (above TTam)

for 0.5 h (blue curve), exposed in water vapor atmosphere at 15 oC (below TTUV) for 0.5 h under UV radiation (cyan curve), exposed in water vapor atmosphere at 30 oC (above TTUV) for 0.5 h under UV radiation (magenta curve). P(MEO3MA-co-EGMA) film exposed in water vapor atmosphere at 30 oC (below TT) for 0.5 h (green curve). The wavenumber range applied in the measurements is 750-3750 cm-1. b) Zoom in of ATR-FTIR spectra in the range of 750-2000 cm-1. The hydrogen bond formation is marked by the red box in the spectra.

After exposure to water vapor atmosphere at 15 oC (below TTam) for 0.5 h, the amplitude of O-H band in swollen P(MEO3MA-co-EGMA-co-AAAB) film is prominently enhanced (red curve). The increase is attributed to the absorbed water molecules. Moreover, a characteristic peak at 1640 cm-1 is emerging (marked by red 23



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box in Figure 3b).44,46 It is related to the intermolecular hydrogen bonds formed between C=O and water molecules. Further increasing the temperature to 30 oC (above TTam), both O-H band and hydrogen bond peak shrink (blue curve). It means the disruption of hydrogen bonds and repellence of absorbed water occur when the temperature is above the TTam. When the P(MEO3MA-co-EGMA-co-AAAB) film is simultaneously exposed to water vapor and UV radiation at 15 oC (below TTUV) for 0.5 h, the amplitude of O-H band as well as hydrogen bonds peak (cyan curve) are further enhanced comparing to the one only exposed to water vapor (red curve). The more pronounced O-H band (related to the absorbed water) indicates that the hydrophilicity of P(MEO3MA-co-EGMA-co-AAAB) is even larger than that of P(MEO3MA-coEGMA) (green curve) under UV radiation. This unreported behavior reveals that UV radiation triggered trans-cis isomerization favors for the water absorption and formation of intermolecular hydrogen bonds. Therefore, the hydrophilicity as well as water absorption capability are improved. Similar as the collapse of P(MEO3MA-coEGMA-co-AAAB) under ambient conditions, the collapse is also observed when the temperature is above the TTUV (magenta curve). However, the amplitude of the O-H band and hydrogen bond peak under UV radiation is still more prominent than that under ambient condition. Besides the ATR-FTIR measurements, contact angle measurements are performed to investigate the surface properties under different scenarios. In case of the non-crosslinked, deposited P(MEO3MA-co-EGMA-co-AAAB) film with a thickness of 1.1 m, the contact angle is (95 ± 3)o when the external temperature is 15 oC (white column with left sparse pattern in Figure 4a). As the droplet used for the measurement is 24


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paraffin oil, the contact angle above 90o indicates the surface is hydrophilic when the temperature is below TTam (TTUV). After UV radiation, the contact angle (white column with right sparse pattern in Figure 4a) increases to (107 ± 3)o, which is 12.6% higher than the value without UV radiation at the same temperature. This increase is caused by the trans-cis isomerization of the AAAB component. The increased polarity of cis AAAB induces the enhanced hydrophilicity. When the temperature is increased to 30 oC,

the contact angle (white column with left dense pattern in Figure 4a) significantly

deceases to (71 ± 3)o. The deposited film surface turns hydrophobic when the temperature is above its TTam. After UV radiation, the contact angle only slightly increases to (75 ± 3)o, which is only 5.6% higher. The significantly lower increase of the hydrophilicity might be explained by the already hydrophobic polymer chains. The influence of hydrophilicity introduced by an increased polarity will not be as pronounced at low temperatures. When the film thickness is increased to 2.7 m and 5.4 m, the observed allover behavior is the same. Thus, no thickness dependence is observed in case of the P(MEO3MA-co-EGMA-co-AAAB) films. In case of the cross-linked P(MEO3MA-co-EGMA-co-AAAB) film with a thickness of 12.5 m, the contact angle is (92 ± 3)o at 15 oC (white column with left sparse pattern in Figure 4b). This value is very close to the deposited and non-cross-linked one. It means that the hydrophilicity is not influenced by the cross-linking process and film thickness. However, the contact angle only rises to (101 ± 3)o after UV irradiation (white column with right sparse pattern in Figure 4b), which is only 9.8% higher. This smaller increase in the contact angle might be related to the cross-linking process. After the cross-linking, the rearrangement of the chain conformation is prominently hindered. Therefore, the increases of hydrophilicity by trans-cis isomerization is less pronounced. When the temperature is 30 oC, the contact angle decreases to (74 ± 3)o and (78 ± 3)o 25



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before (white column with left dense pattern in Figure 4b) and after (white column with left dense pattern in Figure 4b) UV radiation, respectively. These values are also close to the ones obtained in case of deposited and non-cross-linked films. Interestingly, the absolute value of the increase (4 o) does not depend on cross-linking. Thus, the effect of cross-linking is not visible when the temperature is above the TT. Increasing the film thickness to 31.3 m and 62.5 m does not change behavior and the same tendency is observed. Again, no thickness dependence is found in the cross-linked P(MEO3MAco-EGMA-co-AAAB) films.

Figure 4. Contact angles of (a) non-crosslinked, deposited P(MEO3MA-co-EGMA-co-AAAB) films with different film thicknesses (white, 1.1 m; red, 2.7 m and blue, 5.4 m) and (b) cross-linked P(MEO3MA-co-EGMA-co-AAAB) films with different film thickness (white, 12.5 m; red, 31.3 m and blue, 62.5 m) under different scenarios: 15 oC (column with left sparse pattern), 15 oC after UV radiation (column with right sparse pattern), 30 oC (column with left dense pattern) and 30 oC under UV radiation (column with right dense pattern). 26


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Characterization of cotton fabrics cross-linked with P(MEO3MA-co-EGMA-coAAAB) After confirming the structure as well as light- and thermo- dual responsive property of P(MEO3MA-co-EGMA-co-AAAB), it is cross-linked onto the cotton fabrics by the cross-linker BTCA. Figure 5 shows SEM images of the original cotton fabrics and the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MAco-EGMA).

Figure 5. SEM images of (a) original cotton fabrics, (b) cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB), (c) cotton fabrics cross-linked with P(MEO3MA-coEGMA).

27



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From Figure 5a, it is clear that the original cotton fabrics possess a smooth surface. No dust or polymer is visible on and between the fibers. After cross-linking with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA), a thin layer is observed not only on the fiber surface, but also between the two neighboring fibers (Figure 5b and 5c). It presents the successful immobilization of copolymer onto cotton fabrics. Further comparing these two SEM images, no prominent differences are observed although AAAB is absent in Figure 5c. As the solution concentrations used for preparation are the same (8%), the amounts of P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) cross-linked onto the cotton fabrics are similar (weight gain ratio of 6%). Due to the fact that the AAAB cannot be observed from SEM images, the surface morphology does not show remarkable differences in these two cross-linked cotton fabrics. The coating thickness is measured by a thickness gauge (Exploit 033004, Suzhou Qile, China). The original cotton fabrics and the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) were measured 20 times and averaged to minimize the experimental errors. The thickness of the original cotton fabrics is 261.6 m. After cross-linking with P(MEO3MA-co-EGMA-co-AAAB), the thickness increases to 269.2 m. Because the cross-linked polymer layer is on both sides of the fabrics, the coating thickness of cross-linked P(MEO3MA-co-EGMA-co-AAAB) is 3.8 m. To ensure the observed copolymer layers are cross-linked P(MEO3MA-co-EGMAco-AAAB) and P(MEO3MA-co-EGMA), XPS is used to analyze the surface component before and after cross-linking. The spectrum obtained for the original cotton 28


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fabrics and cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) are shown in Figure 6 with black, red and blue curves, respectively. Comparing between black and red curves, it is obvious that the peak related to O decreases while that related to C increases after the cross-linking (Figure 6a). Simultaneously, the peak corresponding to N is emerging in the spectrum (Figure 6b). By integrating the peak area, the fractions of C, O and N in the original cotton fabrics are determined to be 65.4%, 34.6% and 0%, respectively. In case of the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB), these values change to 73.1% (C), 24.7% (O) and 2.2% (N). As there is no N in the original cotton fabrics, the presence of the N peak indicates the successful cross-linking of the polymer P(MEO3MA-co-EGMA-co-AAAB) onto the cotton fabrics. In case of cotton fabrics cross-linked with P(MEO3MA-co-EGMA), the values of C and O are 71.42% and 28.58%, respectively. No N is observed (blue curve in Figure 6b). The reason for this change is that the thermo-responsive polymer P(MEO3MA-co-EGMA) contains a larger amount of C than the original cotton fabrics. Thus it also confirms that P(MEO3MA-co-EGMA) is successfully cross-linked onto the cotton fabrics.

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Figure 6. XPS spectrum of (a) original cotton fabrics (black curve), cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) (red curve) or P(MEO3MA-co-EGMA) (blue curve), (b) the zoom in of spectrum in the range of 390-415 eV to emphasize the peak corresponding to N.

Due to the limited amount of cross-linked P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) on the cotton fabrics, the amplitudes of O-H band and hydrogen bonds peak from cross-linked P(MEO3MA-co-EGMA-co-AAAB) are not so pronounced as those from copolymer film in ATR-FTIR spectra. Their changes under thermo- and lightstimuli are barely detectable. Therefore, instead of ATR-FTIR measurements, the equilibrium swelling ratio (ESR) is used to study the hydrophilicity of the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or 30


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P(MEO3MA-co-EGMA) under different thermo- and lightstimuli.

Figure 7. The ESRs of original cotton fabrics (black dots), cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) under ambient condition (red dots) and UV radiation (blue dots) as a function of temperature. To further address on the influence of AAAB to the ESR, ESR of cotton fabrics cross-linked with P(MEO3MA-co-EGMA) (green dots) is also measured. The solid lines in the graph are guides to the eye.

As presented in Figure 7, the ESR of original cotton fabrics fluctuates between 69.5% and 72% upon heating from 10 oC to 50 oC (black dots), showing that the original cotton fabrics is not thermo-responsive. The cotton fabrics cross-linked with P(MEO3MA-coEGMA-co-AAAB) show an ESR value of 97% at 10 oC (red dots in Figure 8), which is 39% higher than that of the original cotton fabrics. Considering the limited amount of

cross-linked


on

the

fabrics

surface,

P(MEO3MA-co-EGMA-co-AAAB) is more hydrophilic than the cotton fabrics, even though the AAAB is still hydrophobic under ambient conditions. This property is beneficial for the stain removal during laundry. When the temperature increases to 15 oC,

which is close to the TTam, a minor reduction of the ESR from 97% to 95% is 31



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observed. Further increasing the temperature to 20 oC (above TTam), the ESR significantly drops to 80%. When the temperature is higher than TTam, the ESR almost stays unchanged. Therefore, the ESR evolution as function of temperature confirms that cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) is thermoresponsive. After exposure to UV radiation, the ESR profoundly rises to 111%, which is 14% higher than that without UV radiation. Considering the limited molar fraction of AAAB (10%), the trans-cis isomerization significantly varies the chain conformation and provides more space for the formation of hydrogen bonds. Hence, the amount of absorbed water is prominently increased after UV radiation. When the temperature increases, the collapse of P(MEO3MA-co-EGMA-co-AAAB) and repellence of water are visible as well. Interestingly, the pronounced increase of the ESR occurs between 20 oC and 25 oC, which is 5 oC higher than the one under ambient conditions. Thus, the special behavior found in solutions is not transferred to fabrics. Its TT increases under UV radiation. This changed behavior might be related to the restricted mobility of the P(MEO3MA-co-EGMA-co-AAAB) chains in the cross-linked films. It should be noted that even the temperature does not reach its TTUV, the collapse has already set in. It can be attributed to the broad transition of acrylate based thermoresponsive polymers.43,44 Hence, the collapse occurs when the temperature approaches to TTUV. Surprisingly, the UV radiation induces the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) to present a larger ESR value even in the collapsed state. This property is crucial as it indicates that even the external temperature is much higher than its TTUV, the hydrophilicity under UV radiation is still better than that of 32


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original cotton fabrics or cotton fabrics cross-linked with thermo-responsive polymers. Thereby, the stain removal from the cotton fabrics cross-linked with dual responsive copolymer is much easier. Since the realized light- and thermo- dual responsive fabrics have the potential applications for clothes, a high stability of the cross-linked P(MEO3MA-co-EGMA-coAAAB) on cotton fabrics is required. For this reason, the washing fastness is measured on the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) (Figure S4). After washing for the first time, the cross-linked cotton fabrics show a weight loss of (8.7 ± 0.5) %. This reduction is related to the removal of only physically loosely attached P(MEO3MA-co-EGMA-co-AAAB) chains from the cotton fabrics. Increasing the washing time to 2 and 3 yields a significantly less pronounced weight loss, since only a small amount of physically loosely attached copolymer remained on the cotton fabrics after the initial washing. Further increasing the washing time to 5 and 10, the weight loss is (14.5 ± 0.5) % and (15.1 ± 0.5) %, respectively. It is clear that the weight almost stays unchanged after washing for 5 times, indicating almost all physically attached copolymer has been washed away by the previous washing steps. It should be noted that there is still 85% copolymer left on the surface after washing for 10 times. Hence, it can be concluded that the cross-linked P(MEO3MA-co-EGMA-coAAAB) is stable on the cotton fabrics and unable to be washed away easily.

Enhanced stain removal in cotton fabrics cross-linked with P(MEO3MA-coEGMA-co-AAAB) 33



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To investigate the enhancement of stain removal by applying dual thermo- and lightstimuli in cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB), the confocal microscopy (CM) measurements are performed. In our investigation, the stain on the clothes is mimicked by cooking oil. Because of the good affinity between cooking oil and Nile red, the stain removal capability can be easily evaluated by the fluorescent intensity of the Nile red on the fabrics before and after laundry. The cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-coEGMA) are first measured in three different scenarios: as-prepared, rinsing by water thermo-stated below and above TT (TTam). Figure 8a and 8g show the CM images of fibers from the as-prepared cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA). The fluorescent intensities on the fibers can be obtained by integration with the software Image Pro Plus. As shown in Figure 9, the intensities of the fibers with cross-linked P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) before rinsing are 80 ± 3 (red column) and 83 ± 3 (blue column), respectively. Considering the measurement error, the fluorescent intensities show no difference between the fibers, illustrating the amount of Nile red is identical on these two fibers.

Figure 8. CM images of fibers from cotton fabrics cross-linked with P(MEO3MA-co-EGMA34


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co-AAAB) (as-prepared (a), rinsing by distilled water thermo-stated below TTam (15 oC) for 10 times (b), above TTam (30 oC) for 10 times (c), below TTUV under UV radiation (15 oC) for 10 times (d), below TTUV under UV radiation (15 oC) for 40 times (e), above TTUV under UV radiation (30 oC) for 10 times (f)) and P(MEO3MA-co-EGMA) (as-prepared (g), below TT (30 oC)

for 10 times (h), below TT (30 oC) for 40 times (i), above TT (50 oC) for 10 times (j)).

Figure 8b presents the CM images of the fiber cross-linked with P(MEO3MA-coEGMA-co-AAAB) after rinsing by distilled water thermo-stated at 15 oC for 10 times. As the temperature is below the TTam, the cross-linked P(MEO3MA-co-EGMA-coAAAB) is hydrophilic and the affinity between the copolymer and oil is weak. For this reason, the fluorescent intensity of cotton fiber cross-linked with P(MEO3MA-coEGMA-co-AAAB) (63 ± 3, red column with left sparse pattern in Figure 9) is lower than that of the as-prepared fiber. However, the additional AAAB in P(MEO3MA-coEGMA-co-AAAB) causes the less hydrophilicity of P(MEO3MA-co-EGMA-coAAAB) than P(MEO3MA-co-EGMA) in ambient condition. Consequently, the fluorescent intensity of the fiber cross-linked with P(MEO3MA-co-EGMA) is only 55 ± 3 after rinsing by distilled water thermo-stated at 30 oC (below its TT) for 10 times (blue column with left sparse pattern in Figure 9). This value is 13% lower than that of the fiber cross-linked with P(MEO3MA-co-EGMA-co-AAAB) in the same scenario. When the cross-linked fibers are rinsed by water thermo-stated above the TTam or TT, the copolymers switch from a hydrophilic state to a hydrophobic state. The affinity between the copolymer and the cooking oil is strengthened and thereby the removal of cooking oil is even more difficult. Thus, the probed fluorescent intensities rise to 73 ± 3 (red column with right sparse pattern in Figure 9) and 75 ± 3 (blue column with right 35



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sparse pattern in Figure 9). To address the enhancement of stain removal under thermo- and light- dual stimuli, the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) are also rinsed by water thermo-stated below and above the TTUV under UV radiation. Interestingly, when dual stimuli (below TTUV together with UV radiation) are applied, the intensity dramatically drops to 40 ± 3 (red column with left medium pattern in Figure 9). This value is 36% lower than that without UV radiation. Considering the limited amount of AAAB in P(MEO3MA-co-EGMA-co-AAAB), the UV radiation triggered trans-cis isomerization indeed enhances the hydrophilicity and remarkably improves the stain removal capability. It should be noted that this fluorescent intensity is even 28% lower than that cross-linked with P(MEO3MA-co-EGMA) below its TT. Surprisingly, the fluorescent intensity of cross-linked fiber rinsed by water thermo-stated above TTUV under UV radiation is only 44 ± 3 (red column with right medium pattern in Figure 9). This value is the second minimum obtained in our measurements when the rinsing time is 10. Hence, it can be concluded that the trans-cis conformation transition from AAAB indeed enhances the hydrophilicity even the temperature is well above TTUV. For this reason, the laundry temperature is no longer required a low value under UV radiation. To further address the cleaning capability of the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA), the rinsing is prolonged to 40 times. Figure 8i present the CM images of the cotton fiber cross-linked with P(MEO3MA-co-EGMA) after rinsing for 40 times at 30 oC. The florescence intensity is 14.6 ± 3 (blue column with left dense pattern in Figure 9), which is much 36


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less than the value after rinsing for 10 times. In case of the fiber cross-linked with P(MEO3MA-co-EGMA-co-AAAB), the florescence is barely seen after rinsing for 40 times at 15 oC (Figure 8e). The residual intensity is only 7.1 ± 3 (red column with left dense pattern in Figure 9). As this value is very close to 0, it means the residual amount of Nile red on the fiber is extremely small. In addition, this value is only half of the fluorescence intensity from the fiber with cross-linked P(MEO3MA-co-EGMA) after rinsing for 40 times. Thus, the extra AAAB component can improve the stain removal capability. It should be noted that only distilled water is applied to test cleaning capability. There is no detergent used in our measurements. Moreover, rinsing 40 times only takes 240 s, which is significantly shorter than the usual washing time in our daily lives. For this reason, we believe the cotton fabrics cross-linked with P(MEO3MA-coEGMA-co-AAAB) present an enhanced capability for stain removal. In addition, the UV radiation used for switching the azobenzene units during the laundry has a second advantage. UV light is harmful to bacteria on the clothes and thereby reduce them in number. Concerning the practical use in daily life, the UV light source in the washing machine is not harmful for humans, because it is installed inside the machine. It will be only switched on during the washing and the cover of the washing machine will avoid the direct exposure of UV light to human.

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Figure 9. Fluorescent intensity of fibers from the cotton fabrics cross-linked with P(MEO3MAco-EGMA-co-AAAB) in following conditions: as-prepared (red column without pattern), rinsing by distilled water thermo-stated below TTam (15 oC) for 10 times (red column with left sparse pattern), above TTam (30 oC) for 10 times (red column with right sparse pattern), below TTUV under UV radiation (15 oC) for 10 times (red column with left medium pattern) and 40 times (red column with left dense pattern), above TTUV under UV radiation (30 oC) for 10 times (red column with right medium pattern) as well as the cotton fabrics cross-linked with P(MEO3MA-co-EGMA) in the following conditions: as-prepared (blue column without pattern), rinsing by distilled water thermo-stated below TT (30 oC) for 10 times (blue column with left sparse pattern) and 40 times (blue column with left dense pattern), above TT (50 oC) for 10 times (blue column with right sparse pattern).

Improved comfort control in cotton fabrics cross-linked with P(MEO3MA-coEGMA-co-AAAB) To investigate the improvement of comfort control in cross-linked cotton fabrics, the air permeability of original cotton fabrics as well as cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) are probed. As presented in Figure 10, the air permeability of original cotton fabric is 126 mm/s at 25 38


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oC.

After cross-linking with P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-

EGMA), the air permeability drops to 115 mm/s and 105 mm/s, respectively. Both values are lower than that of the original cotton fabrics, which is attributed to the coverage of fabrics by the cross-linked copolymer. Because the cross-linking processes and the solution concentrations applied are the same, the weight gain ratios of both cross-linked cotton fabrics are around 6%. As a consequence, the amount of crosslinked P(MEO3MA-co-EGMA-co-AAAB) or P(MEO3MA-co-EGMA) is same on the cotton fabrics. As the measurements are performed at 25 oC and the TTam of P(MEO3MA-co-EGMA-co-AAAB) is around 20 oC, the collapse of cross-linked P(MEO3MA-co-EGMA-co-AAAB) has already set in. On the contrary, the TT of crosslinked P(MEO3MA-co-EGMA) is much higher (around 48 oC) and the cross-linked P(MEO3MA-co-EGMA) layer stays in the expanded state at 25 oC. Thereby, the reduction of air permeability from 115 mm/s to 105 mm/s is caused by the more porous structure in cross-linked P(MEO3MA-co-EGMA-co-AAAB) layer. To further emphasize the change of air permeability during daily wear, the temperature of fabrics is set to 33 oC in the measurements. This temperature region is close to the temperature when the fabrics are in contact with the human body. The air permeability of the original cotton fabrics and cotton fabrics cross-linked with P(MEO3MA-co-EGMA) almost stay unchanged (126 mm/s and 106 mm/s), whereas that cross-linked with P(MEO3MA-coEGMA-co-AAAB) rises to 120 mm/s. Thus, it is clear that introducing AAAB to the copolymer causes the air permeability to increase by 13%. It should be noted that the weight gain ratio of the cross-linked P(MEO3MA-co-EGMA-co-AAAB) on the cotton 39



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fabrics is only 8%, it means there is only a very thin coating layer on the surface. Additionally, the molar fraction of AAAB in P(MEO3MA-co-EGMA-co-AAAB) is only 10%. Thereby, the amount of AAAB in the coating layer is very limited. Based on this, the increase of air permeability by 13% is a prominent enhancement, which is attributed to the further collapse of the copolymer P(MEO3MA-co-EGMA-co-AAAB). Especially we do not need further change or modification of the cotton fabrics. As the TTam of P(MEO3MA-co-EGMA-co-AAAB) and TT of P(MEO3MA-co-EGMA) are 20 oC and 45 oC, the cross-linked P(MEO3MA-co-EGMA-co-AAAB) layer presents a more porous structure than in case of P(MEO3MA-co-EGMA) at the skin temperature (around 33 oC). For this reason, the improvement of comfort control in cross-linked cotton fabrics is also successfully realized.

Figure 10. The air permeability of original cotton fabrics (black), cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) (cyan) or P(MEO3MA-co-EGMA) (magenta) at 25 oC

(filled) and 33 oC (slashed).

Conclusion The simultaneous enhancement of stain removal and comfort control in cotton fabrics 40


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is realized by simply cross-linking light- and thermo- dual responsive P(MEO3MA-coEGMA-co-AAAB) onto cotton fabrics. The light- and thermo- dual responsive random copolymer P(MEO3MA-co-EGMA-co-AAAB) with a molar ratio of 8:1:1 and the single thermo-responsive random copolymer P(MEO3MA-co-EGMA) with a molar ratio of 8:1 are applied in our investigation. The copolymers are successfully immobilized onto cotton fabrics by cross-linking. Under UV radiation, the water absorption capability of the cotton fabrics cross-linked with P(MEO3MA-co-EGMAco-AAAB) is even better than that cross-linked with P(MEO3MA-co-EGMA). It indicates that the trans-cis isomerization increases the polarity of the AAAB units, which further enhances the hydrophilicity of the copolymer P(MEO3MA-co-EGMAco-AAAB). Confocal microscopy measurements confirm that the fluorescent intensity of fibers cross-linked with P(MEO3MA-co-EGMA-co-AAAB) under UV radiation is much lower than that of its analogue without AAAB. This reduction of intensity is realized by the increased hydrophilicity of cis- AAAB triggered by UV radiation, which weakens the affinity between copolymer and cooking oil. Only rinsing with distilled water for 240 s is sufficient to almost remove all of the oil. Thus, by simply installing a UV light source in the washing machine, the better stain removal can be realized during laundry. On the contrary, the trans- AAAB induces the cross-linked P(MEO3MA-co-EGMA-co-AAAB) layer to exhibit a more hydrophobic and porous state under ambient conditions. The air permeability of cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) is 13% higher. The improved permeability is favorable for the heat transfer from the human body to the external atmosphere. 41



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Therefore, the cotton fabrics cross-linked with P(MEO3MA-co-EGMA-co-AAAB) are promising for application in intelligent textiles with a capability to enhance stain removal and comfort control simultaneously. Its use can significantly reduce the consumption of energy, water and surfactants needed in laundry. It can also improve the fabrics comfort during daily wear, which is highly required by humans.

Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 51403186 and 51611130312) and National Key R&D Program of China (2017YFB0309600). PMB thanks for the support by DFG project MU 1487/23-1.

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