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Intelligent textiles with comfort regulation and inhibition of bacterial adhesion realized by cross-linking poly(n-isopropyl acrylamide-co-ethylene glycol methacrylate) to cotton fabrics Jiping Wang, Yangyi Chen, Jie An, Ke Xu, Tao Chen, Peter Muller-Buschbaum, and Qi Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01922 • Publication Date (Web): 30 Mar 2017 Downloaded from http://pubs.acs.org on March 31, 2017

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ACS Applied Materials & Interfaces

Intelligent textiles with comfort regulation and inhibition of bacterial adhesion realized by crosslinking poly(n-isopropyl acrylamide-co-ethylene glycol methacrylate) to cotton fabrics Jiping Wang, † Yangyi Chen,† Jie An,† Ke Xu,† Tao Chen,† Peter Müller-Buschaum ‡and Qi Zhong*,† †Key Laboratory of Advanced Textile Materials & Manufacturing Technology, Ministry of Education; Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education; National Base for International Science and Technology Cooperation in Textiles and Consumer-Goods Chemistry, Zhejiang Sci-Tech University, 310018 Hangzhou, China. ‡Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, James-Franck-Str. 1, D-85748 Garching, Germany KEYWORDS ： Comfort regulation, bacterial anti-adhesion, cross-linking, cotton fabrics, thermo-responsive copolymers, poly(methacrylate)

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ABSTRACT：：Comfort regulation and inhibition of bacteria adhesion to textiles is realized by cross-linking thermo-responsive random copolymer to the cotton fabrics. By introducing ethylene glycol methacrylate (EGMA) monomers into n-isopropyl acrylamide (NIPAM) with a molar ratio of 2:18, the obtained random copolymer poly(n-isopropyl acrylamide-co-ethylene glycol methacrylate), abbreviated as P(NIPAM-co-EGMA), presents a transition temperature (TT) of 40 oC in an aqueous solution with a concentration of 1 mg/mL. Because of the additional EGMA in the copolymer, the obtained P(NIPAM-co-EGMA) shows a glass transition temperature (Tg) of 0 oC, which is much lower than that of pure PNIPAM (Tg=140 oC). Therefore, the introduction of P(NIPAM-co-EGMA) into the cotton fabrics will have little influence on the softness of the fabrics. Due to the cross-linked P(NIPAM-co-EGMA) layer on the cotton fabrics, the porosity of the polymer layer can be adjusted by varying the external temperature below or above TT, inducing the regulation of the air and moisture permeability as well as the body comfort are feasible to the cotton fabrics cross-linked with P(NIPAM-coEGMA). In addition, the cross-linked P(NIPAM-co-EGMA) layer is capable of absorbing moisture in the ambient atmosphere and form a hydrated layer on top, which can inhibit bacterial adhesion to the textiles.

INTRODUCTION In the last decades, numerous pathogenic microorganisms have increased rapidly, causing diseases and threating people’s health.1,2 Among the microorganisms, pathogenic bacteria that cause serious infections has been attracted more and more attentions. For any bacterial infection the first and important step is the contact with the human body. Hence, bacterial adhesion3-10 to biomaterial surfaces is concerned due to its significance in


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biomedicine.1, 11-15 Because cloths play a role as the first line of body defense, extensive investigations are performed to pursue novel biomaterial textiles for universally preventing bacterial infection. It is well known that bacterial adhesion can be accomplished with two steps.16,17 Initially, the outer proteins of the bacteria combine with the surface, accompanied with the establishment of binding sites. Afterwards, abundant bacteria adhere to the surface and form a so-called biofilm. Therefore, a number of investigations about surface modifications have been performed to reduce or limit the bacterial adhesion and colonization. There are three effective strategies to prepare anti-bacterial surfaces.18-20 The first approach is to introduce antibiotic functional groups to kill the bacteria adhered on the material surface, which is named as contact killing, such as quaternary ammonium compounds (QACs),21 guanidine polymers22 and chitosan.23 The second approach to resist bacteria mainly focused on biocide leaching. The mechanism is that by depositing antibacterial agent24,25 (e.g., silver iron, Nitric oxide) or microcapsule on material surface, the cytotoxic compounds are released to kill the bacteria surrounding. However, the material treated with the two approaches have suffered from mechanically weakness, are lacking long-term stability and some biocide may be even noxious to the human body, which limits the application of the approaches. The third approach has been conducted by reducing the bacterial adhesion,26,27 such as the formation of a hydrated layer to avoid the contact of bacteria. In particular, poly(ethylene glycol) (PEG) and its derivatives are broadly used to prevent or retard the bacteria adhesion.28-34 PEG methacrylate polymers (PEGMA) have a side PEG chain of ethylene glycol units which are hydrophilic. At the presence of water, PEG chains change conformation to have a compact hydration shell,


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which will form repulsive hydration forces.35 Therefore, it prevents the adhesion of outer proteins of the bacteria to the surface. Combining the fact that PEG is nontoxic and will not harm active proteins or cells, PEG based surface have been extensively studied in the context of microbial adhesion. It was shown that the most commonly used method to suppress the bacterial adhesion is to use modified or coated surfaces with PEG based polymers. Examples are POEGMA brushes on gold surface,1 PEG-coated polystyrene36 and oligo (ethylene glycol) (OEG) self-assembled monolayers.37 In the applications mentioned above, the modification of surface with PEG based polymers can hinder the adhesion of bacteria. Simultaneously, the additional polymer layer on the surface will also result in a poor air/moisture permeability. The comfort of wearing should be considered not only for the textiles in daily lives, but also for textiles used in biomedicine. A good air/moisture permeability of textiles is required especially at high temperatures. For this reason, a simple PEG coating on the textiles might not be the optimal solution due to its limited air/moisture permeability. Previously we demonstrated that the adjustment of air/moisture permeability according to the external temperature can be realized by introducing thermo-responsive polymers onto cotton fabrics.38 Among thermo-responsive polymers, poly(n-isopropylacrylamide) (PNIPAM), is the most thoroughly investigated one, possessing a lower critical solution temperature (LCST) of 32 oC.39-43 As the LCST of PNIPAM is close to the human body temperature, it has been extensively investigated in the fields of biology, such as drug delivery,44-46 gene delivery,47 tissue engineering,48 protein absorption49 and cell attachment.50,51 However, PNIPAM possessing a glass transition temperature (Tg) around 140 oC. For this reason, simply introducing PNIPAM into cotton fabrics will make the obtained textile to feel


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hard. To overcome this drawback, ethylene glycol methacrylate (EGMA) is copolymerized with NIPAM. The obtained random copolymer P(NIPAM-co-EGMA) not only shows a much lower Tg than PNIPAM, but also provides hydroxyl groups to the copolymer. Therefore, P(NIPAM-co-EGMA) can be immobilized on cotton fabrics by the cross-linker citric acid. Therefore the P(NIPAM-co-EGMA) layer can simultaneously realize inhibition of bacteria adhesion and adjustment of air/moisture permeability after cross-linking to the cotton fabrics. To demonstrate such behavior, the thermo-responsive copolymer P(NIPAM-co-EGMA) is synthesized via atom transfer radical polymerization (ATRP). Afterwards it is crosslinked with the cotton fabric by the cross-linker citric acid. The air/moisture permeability of cotton fabric cross-linked with P(NIPAM-co-EGMA) is measured to characterize the breathability of the textiles. The attachment of S. aureus and E. coli on cotton fabrics cross-linked with P(NIPAM-co-EGMA) is used to investigate the inhibition of bacteria adhesion. It shows that the cotton fabrics with cross-linked P(NIPAM-co-EGMA) can effectively prevent the bacteria adhesion due to the hydrated P(NIPAM-co-EGMA) layer. While the porous structure of the cross-linked P(NIPAM-co-EGMA) layer on the cotton fabrics can be used to enhance the air and perspiration permeability at high temperatures.

EXPERIMENTAL SECTION Materials N-isopropyl acrylamide, abbreviated as NIPAM, (purity 97%) was purchased from Sigma Aldrich, USA and recrystallized twice from acetone/hexane (1:1 v/v) before use. Ethylene glycol methacrylate, abbreviated as EGMA, (Mn=360, purity 95%) was bought from Sigma Aldrich,


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USA. Tris (2-dimethylaminoethyl)amine (Me6TREN, purity >98%), cuprous bromide (CuBr, 99%), methyl 2-chloropropionate (MCP, purity 98%), isopropyl alcohol (i-PrOH, purity ≥99.5%) as well as sodium hypophosphite (SHP, purity 99%) were obtained from Aladdin, China and used as received. Citric acid (CA, ≥ 99.5%) was from Gaojing fine chemical, China. Tetrahydrofuran (THF, ≥ 99%) and n-hexane were received from Yongda Chemicals, China. The fabrics used in this investigation were 40×40 cotton poplin weighing 180 g/m2 and provided by Procter & Gamble Company. This type of cotton fabrics is extensively applied in the clothing.

Synthesis of P(NIPAM-co-EGMA) The scheme of synthesis process is presented in Figure 1. Monomer NIPAM (2.036 g, 18 mmol) were first dissolve in i-PrOH (3 mL) in a reaction tube. Afterwards monomer EGMA (570 µL, 2 mmol), ligand Me6TREN (26 µL, 0.1 mmol), catalyzer CuBr (14.3 mg, 0.1 mmol) were added into the previous solution. Then the reaction agent MCP (11.4 µL, 0.1 mmol) was added into the mixture. All the experiment mentioned above were performed in a glove box (Labstar, MBraun, Germany). The reaction tube was sealed and reacted at room temperature for 4 h. The reaction molar ratio was [monomers]: [MCP]: [Me6TREN]: [CuBr]=200:1:1:1. After the reaction, the mixture was dissolved in THF and went through a short neutral alumina column to remove the catalyst. The obtained polymer was purified by precipitation thrice in n-hexane, and then dried in a vacuum oven at 40 oC. The yield of the product is 75-80%.

Preparation of cotton fabrics cross-linked with thermo-responsive polymers The cotton fabrics with cross-linked P(NIPAM-co-EGMA) and PEGMA were prepared with a padder. The pre-weighted cotton fabrics were rolled with two dips and two nips with a finishing


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bath including 8%wt copolymer, 2%wt CA, 3%wt SHP and water. The wet pickup of padded fabric ranged from 70%-80%. Then the padded fabrics were dried at 80 oC for 8 min and cured at 130 oC for 3min. The cured fabrics were rinsed with distilled water twice to remove the unreacted polymers, then dried in an oven at a temperature of 60 oC for 12 h before the subsequent investigations.

Thermal analysis The thermal properties of P(NIPAM-co-EGMA) with monomer ratio of 19:1, 18:2 17:3 and 0:20 were determined by DSC (Q2000, TA Instruments). For the heating process, the samples were initially cooled from room temperature to -80 oC with a cooling rate of 5 oC/min. After thermal-stated for 0.5 min, the temperature was raised to 150 oC with a heating rate of 1 oC/min. Then the cooling and heating cycle was repeated and simultaneously the DSC data was recorded. The data of the second cycle was used for analysis. Specially, the heating range of PNIPAM, denoted as P(NIPAM-co-EGMA) with a monomer ratio of 20:0, was set from -80 oC to 180 oC. The thermal stability of polymers was characterized by thermogravimetric analysis (TGA) (PYRIS 1, PerkinElmer). Approximately, 10 mg polymer was heated at a constant rate of 10 °C/min in a thermal range from 0 to 800 °C. The atmosphere applied was nitrogen gas.

Spectroscopies The changes of transmittance of P(NIPAM-co-EGMA) in aqueous solution was assessed by UV-Vis spectroscopy (UV/VIS Lambda 35 spectrometer, Perkin Elmer). The wave number was fixed at 500 nm during the measurements and the temperature was increased from 18 oC to 56 oC with a rate of 0.4 oC/min.


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The functional groups on the cotton fabrics cross-linked with P(NIPAM-co-EGMA) were characterized by ATR-FTIR spectroscopy (Vertex 70 spectrometer, Bruker). The scanned wavelength ranges from 600 cm-1 to 4000 cm-1 with a resolution of 4 cm-1. For comparison, the spectrum of the original cotton fabrics was measured as well.

Field emission scanning electron microscopy (FESEM) The surface morphology of original cotton fabrics and cotton fabrics with crossed-linked P(NIPAM-co-EGMA) or PEGMA were determined by FESEM (vltra55, Carl Zeiss) operating at 2 kV with the field emission gun. A thin layer of gold was sputtered on the sample surface prior to the measurements.

Contact angle measurements A drop shape analyzer (DSA20, Krüss) was used to probe the hydrophilicity of the cotton fabrics with cross-linked P(NIPAM-co-EGMA). 2 µL deionized water was dropped onto the cotton fabrics. Simultaneously the evolution of the drop shape was recorded by the drop shape analyzer. For each cotton fabric, the measurements were repeated 3 times to minimize possible errors.

Equilibrium swelling ratio measurements The equilibrium swelling ratio measurements were conducted by soaking original cotton fabrics and the cotton fabrics with cross-linked P(NIPAM-co-EGMA) in a temperaturecontrolled water bath. The temperatures of the water bath were periodically regulated to 30 oC and 40 oC. A swollen equilibrium state can be reached after 24 h. Afterwards the fabrics were


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moved out from the water bath. The residual water on the fabrics was removed by filter paper. Afterwards the fabrics were weighed again. The equilibrium swelling ratio (ESR) was calculated by the following equation: ‫= ܴܵܧ‬

ܹ − ܹ଴ × 100% ܹ଴

W and W0 denoted the weights of cotton fabric at swollen equilibrium state and dry cotton fabric respectively.

Washing fastness measurements The cotton fabrics cross-linked with P(NIPAM-co-EGMA) and PEGMA were firstly weighted in the dry state. Afterwards, the fabrics were immersed in the solution containing Tide detergent (1 g/L) and rotated for 30 min. Afterwards the fabrics were moved out and rinsed with water to remove the residual detergent. The fabrics were weighted again after drying in the oven. In order to investigate the washing fastness, the washing processes were repeated for five cycles. The weight loss ratio (WLR) was calculated by the following formula: ܹ‫= ܴܮ‬

݉ − ݉′ × 100% ݉ − ݉଴

Where m0 is the weight of original cotton fabric, m and m′ are the cotton fabrics cross-linked with polymer before and after washing, respectively.

Air/Moisture permeability measurements An air permeability tester (YG461E, Wenzhou Fangyuan) was used to determine the air permeability of the original cotton fabrics and the cotton fabrics cross-linked with P(NIPAM-coEGMA) and PEGMA at 30 oC (below TT) and 40 oC (above TT). To ensure no leakage during


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the measurements, the samples with an effective measured area of 50 cm2 were clamped on the worktable, and the pressure drop between two sides of fabrics was set to 100 Pa. Each fabric was measured three times to minimize experimental errors. A moisture permeability tester (YGB216-II, Wenzhou Darong) equipped with aluminum dishes was utilized to measure the moisture permeability. The dish was filled with 34 mL distilled water preheated to test temperature and sealed by the original cotton fabric and cotton fabric cross-linked with P(NIPAM-co-EGMA) and PEGMA. Afterwards, the dish was stored in the moisture permeability tester for 1 h, followed with weighed. Next, it was stored in the tester for another 1 h, then weighed again. The relative humidity and velocity of wind applied in the measurement were 50±2% and 0.5 m/s, respectively. The moisture permeability (water-vapor transmission rate, WVT) was calculated from the differences between the two weights. The temperature were set to 30 oC (below TT) and 40 oC (above TT) to compare the moisture permeability of original cotton fabric and cotton fabric cross-linked with P(NIPAM-co-EGMA) or PEGMA.

Bacterial adhesion assay S. aureus (ATCC 29213) and E. coli (ATCC 8739) were selected to probe the bacterial adhesion on original cotton fabrics and cotton fabrics cross-linked with P(NIPAM-co-EGMA) or PEGMA. The cotton fabrics were sterilized under ultraviolet disinfection lamp for 15 min and cleaned with 0.85 wt% saline. S. aureus (2×107 cfu/mL) and E. coli (2×108 cfu/mL) were stained with LIVE/DEAD® BacLightTM Bacterial Viability Kit (Invitrogen, USA) of an equal volume for 15 min and stored in an ambient atmosphere without light. The culture solution applied in this investigation was 1‰ dilute fluid Luria-Bertani (LB) medium, which provides essential nutrients


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for the reproduction of bacteria and simultaneously avoids the enrichment of bacteria. Then 0.2 g scrap cotton fabric was mixed with 100 µL labelled bacterial suspension and 5 mL dilute culture solution in a culture tube. The bacteria in the sealed tube were cultivated after a gently shaking. The cotton fabrics were moved out and washed with sterile saline to remove the bacteria without adhesion after 24 h. Then the cotton fabrics were sonicated for 2 min at 40 kHz in fluid LB medium to obtain the adhered bacteria. The residual bacterial suspension and the bacteria without adhesion were observed with confocal microscopy (Nikon C2). The bacterial suspension after sonication was diluted to gradient volume concentrations (10-2, 10-3, 10-4, 10-5 and 10-6) and subsequently cultured for 24 h to determine the bacteria number by plate count. In order to compare the inhibition of bacterial adhesion of PEGMA with that of P(NIPAM-co-EGMA), the cotton fabrics cross-linked with PEGMA was investigated as well.

RESULTS AND DISCUSSTION Characterization of P(NIPAM-co-EGMA) The molecular weights of copolymers with different monomer ratios are measured by gel permeation chromatography (GPC) and presented in Table 1. A range from 28 to 43k is covered. The GPC values of the polymers obtained are between 1.26 and 1.58, indicating the narrow polydispersity of the polymers. The details for the characteristic 1H NMR spectrum of P(NIPAM-co-EGMA) (18:2) in DMSO-d6 can be found in the supporting information.

Table 1. Molar mass (Mn), polymer dispersity index (PDI), glass transition temperature (Tg) and TT of P(NIPAM-co-EGMA) with different molar ratio of NIPAM to EGMA


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P(NIPAM-co-EGMA) monomer ratios

Mn (g/mol)

PDI

Tg (oC)

TT (oC)

20:0

28600

1.26

138

32

19:1

31800

1.42

60

34

18:2

29800

1.35

0

40

17:3

32600

1.58

-36

46

0:20

43200

1.32

-a

＞90

a

The Tg cannot be measured by DSC due to over range.

The influence of the EGMA ratio on the glass transition temperature (Tg) of the copolymers P(NIPAM-co-EGMA) is measured with DSC (Figure 2). The measured Tg of PNIPAM is 138 o

C, which agrees well with literature values.30 The Tg values of P(NIPAM-co-EGMA) with

monomer ratios (NIPAM:EGMA) of 19:1, 18:2 and 17:3 are changing from 60 oC, 0 oC to -36 o

C, respectively. Thus, Tg is significantly influenced by the amount of the EGMA monomer,

which has a relative low Tg value. Hence, by increasing the molar ratio of NIPAM:EGMA above 18:2, the obtained copolymer P(NIPAM-co-EGMA) possesses a Tg below 0 oC, indicating that it is in a gel state at room temperature. This property of the copolymer will be beneficial to the soft handle of a textile coated with the copolymer. The thermal stability of the polymers is also considered and tested by TGA (the details can be found in the supporting information). CH 3

H

+ n H2C

m H2C C C

O

NH

C

H i-PrOH, RT, 4h

C

CH3

CuBr/Me6TREN, MCP

CH 2 C

m

C

O

O

NH

O

CH 3

CH 2

H 3C

CH3

4-5

EGMA

O

O

CH2 OH

OH

NIPAM

n

C

CH2

CH 2 H3C

CH2 C

4-5

P(NIPAM-co-EGMA)

Figure 1. Scheme for the synthesis of P(NIPAM-co-EGMA)


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Figure 3a presents the transmittance of P(NIPAM-co-EGMA) with different monomer ratios of NIPAM to EGMA in aqueous solution as a function of temperature. The aqueous solution concentration is fixed at 1 mg/mL. The TT value is determined by plotting the first derivative of the transmittance with respect to the temperature (Figure 3b). The obtained TT values of P(NIPAM-co-EGMA) with monomer ratios of NIPAM to EGMA of 20:0, 19:1, 18:2 and 17:3 are 32 oC, 34 oC, 40 oC and 46 oC, respectively. Thus, the TT shifts towards higher temperatures with increasing the monomer ratio of EGMA due to the hydrophilicity of EGMA. The EGMA applied in the study contains 4 to 5 EG in the side chain, which makes it more hydrophilic than PNIPAM. Moreover, the hydroxyl group in the end of the side chain will form hydrogen bonds with neighboring water molecules as well. Therefore, the phase transition of P(NIPAM-coEGMA) will occur at a higher temperature than that of the pure PNIPAM. Further increasing the monomer ratio of EGMA, the TT will even shift towards higher temperatures.

Figure 2. DSC curves of P(NIPAM-co-EGMA), shifted vertically for clarity. The monomers ratios of NIPAM to EGMA are 20:0 (black), 19:1 (red), 18:2 (blue) and 17:3 (pink). Tg values are indicated by vertical dashed lines.


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Figure 3. a) Transmittance of P(NIPAM-co-EGMA) aqueous solutions (1 mg/mL) as a function of temperature investigated by UV-Vis spectroscopy. b) The first derivatives of the transmittances with respect to the temperature plotted as a function of temperature. The monomers ratios of NIPAM to EGMA are 20:0 (black), 19:1 (red), 18:2 (blue) and 17:3 (pink).

Characterization of cotton fabrics cross-linked with P(NIPAM-co-EGMA) The schematic of crossing-linking reaction during the preparation of cotton fabric cross-linked with P(NIPAM-co-EGMA) is presented in Figure 4. Immobilization of P(NIPAM-co-EGMA) on cotton fabrics by cross-linking is confirmed by ATR-FTIR measurements. As shown in Figure 5a, the black curve presents a typical ATR-FTIR spectrum of the original cotton fabrics, including the main absorption peaks, such as O-H (3600-3300 cm-1), C-H (3000-2800 cm-1) and C-O (1020 cm-1) bands. After cross-linking P(NIPAM-co-EGMA) to the cotton fabrics, the C=O


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peak emerges at 1730 cm-1 (guide line 1 in Figure 5b), which can be attributed to the presence of PEGMA. The PNIPAM component is identified by the emergence of the characteristic peak of the C=O at 1670 cm-1 (guide line 2 in Figure 5b) and the band resulting from N−H stretching at 1570 cm−1 (guide line 3 in Figure 5b). The characteristic peak of C-O-C in PEGMA superimposes with that of the carbon oxygen ring in cellulose, inducing the peaks around 1100 cm-1 to show no prominent change after the cross-linking.

Figure 4. Schematic of crossing-linking reaction during the preparation of cotton fabric crosslinked with P(NIPAM-co-EGMA).


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Figure 5. a) ATR-FTIR spectra of original cotton fabrics (black) and the cotton fabrics crosslinked with P(NIPAM-co-EGMA) (red) ranging from 4000 cm-1 to 600 cm-1. b) Zoom-in the ATR-FTIR spectra ranging from 1900 cm-1 to 800 cm-1. The spectra were shifted vertically for clarity.

Figure 6 presents the morphology of the original cotton fabrics and cotton fabrics cross-linked with P(NIPAM-co-EGMA) obtained from FESEM. As seen in Figure 6a, the surface of the original cotton fiber is smooth and clean. After cross-linking with P(NIPAM-co-EGMA) or PEGMA, a thin layer is observed on the surface (Figure 6b and 6c). Therefore, it can be concluded that P(NIPAM-co-EGMA) and PEGMA were successfully cross-linked to the cotton fabrics.


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Figure 6. FESEM images of a) original cotton fabrics, b) cotton fabrics cross-linked with P(NIPAM-co-EGMA) and c) cotton fabrics cross-linked with PEGMA.

It is well-known that the hydrophilicity of the surface significantly influences the bacteria adhesion. Generally, the more hydrophilic is the surface, the more effectively it can resist to the bacteria adhesion. For this reason, the hydrophilicity of original cotton fabrics, cotton fabrics cross-linked with PEGMA or P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2 and 19:1) are investigated by measuring the variation of the water contact angle with temperature (Figure 7). The original cotton fabrics exhibits an initial contact angle of 56o. With increasing the temperature to 45 oC, the contact angle decreases to 48o. This slight change of the water contact angle can be attributed to the enhanced water evaporation at higher temperatures. Similarly, the surface of cotton fabrics with cross-linked PEGMA shows no prominent changes of the contact angle in the investigated temperature range. However, the contact angle of cotton fabrics crosslinked with P(NIPAM-co-EGMA) exhibits a prominent increase when the temperature crosses 36 oC. When the temperature is 36 oC, the contact angle is only 53o (hydrophilic). Further increasing the temperature to 39 oC, the contact angle rises to 96o (hydrophobic). This abrupt change of the contact angle indicates that the cotton fabrics cross-linked with P(NIPAM-coEGMA) (NIPAM:EGMA=18:2) possess thermo-responsive properties. TT measured for cotton fabrics cross-linked with P(NIPAM-co-EGMA) is slightly lower than that of P(NIPAM-co-


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EGMA) (NIPAM:EGMA=18:2) in aqueous solution, due to the influence of the cotton fabrics and cross-linking. Furthermore, it should be noted that the contact angles of the cotton fabrics cross-linked with PEGMA and with P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2) are lower than that of the original cotton fabrics at 33 oC. This decrease is caused by the introduction of the hydrophilic polymers to the cotton fabrics, resulting in a relatively more hydrophilic surface than the original cotton fabric surface. Hence, below a temperature of 36 oC (normal human skin temperature), cotton fabrics cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2) remains relatively hydrophilic, which can be used to suppress the bacteria adhesion. The cotton fabrics cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=19:1) has the similar transition behavior as compared with P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2). Only the TT shifts towards lower value because the molar ratio of EGMA is decreased. When increasing the skin temperature, e.g. doing sport or during work, will cause the cross-linked P(NIPAM-co-EGMA) layer to switch to a more hydrophobic state. The collapsed copolymer layer will form a porous structure, which can be used for a desired comfort regulation.

Figure 7. Contact angle of the original cotton fabrics (black), cotton fabrics cross-linked with PEGMA (red) and P(NIPAM-co-EGMA) with a molar ratio of NIPAM:EGMA=18:2 (blue) and 19:1 (pink).


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The contact angle of the cotton fabrics cross-linked with the thermo-responsive polymers is also related with the relative humidity (RH). An increase of RH will enhance the hydration of the thermo-responsive polymer layers and induce the contact angle to be lower (details can be found in the supporting information). Besides contact angle measurements, moisture sorption experiments of the textiles were conducted as well and the results are shown in Figure 8. Original cotton fabrics reach to the equilibrium state with a moisture regain of 8.2% after 70 minutes. After cross-linking with the thermo-responsive polymers, the ability of moisture sorption is improved and the equilibrium moisture regain raises with the increase of EGMA molar ratio. The reason is that the content of the thermo-responsive polymer on the cotton fabric is determined by the EGMA molar ratio. As mentioned in our manuscript, with increasing the EGMA molar ratio, the amount of polymer cross-linked on the cotton fabrics is increased as well. Hence, moisture regain is also increased. In comparison, when increasing the temperature above the TT, the moisture regain of the cotton fabrics cross-linked with P(NIPAM-co-EGMA) decreases, which can be attributed to the hydrophobic surface above the TT.


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Figure 8. Moisture regain of fabrics at a) 30 oC (below TT) and b) 40 oC (above TT). The measured fabrics are original cotton fabrics (black), the cotton fabrics cross-linked with P(NIPAM-co-EGMA) with a molar ratio of NIPAM-co-EGMA=19:1 (red) and 18:2 (blue) as well as PEGMA (pink).

The resistance of cross-linked thermo-responsive polymer is also concerned in our investigation. For this reason, washing fastness measurements are performed with original cotton fabrics and cotton fabrics cross-linked with thermo-responsive polymers. Figure 9a illustrates the weight loss as a function of the number of washing cycles. The original cotton fabrics (black) show a slight decrease of weight, which can be attributed to the loss of fiber during washing. The cotton fabrics cross-linked with P(NIPAM-co-EGMA) and PEGMA present an obvious weight loss after the first washing cycle, which could be due to a small amount of non cross-linked


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polymer residues. They only entangle with each other or physically adhere on the surface. Hence, they will be easily removed after washing. Further increasing the washing cycle number, the weight loss is getting less prominent. After four washing cycles, all cotton fabrics with crosslinked polymers show no further reduction of weight. It should be noted that the final weight loss of cotton fabrics cross-linked with PEGMA and P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2 and 19:1) are 5.5%, 6.3% and 8.3%, respectively. In other words, still 94.5%, 93.7% and 91.7% polymers are remaining on the cotton fabrics. Thus, it can be concluded that the resistance of cross-linked PEGMA or P(NIPAM-co-EGMA) on cotton fabrics is good. Because the hydroxyl groups perform as the reactive groups during the cross-linking, higher amount of EGMA in the polymer will enhance the cross-linking between polymers and fabrics. For this reason, the cotton fabrics cross-linked with PEGMA (red) and P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2, blue) present a better washing fastness than the one cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=19:1, pink).


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Figure 9. a) Weight loss of original cotton fabrics (black), the cotton fabrics cross-linked with PEGMA (red), with P(NIPAM-co-EGMA) with a molar ratio of NIPAM:EGMA=18:2 (blue) and 19:1 (pink) presented as a function of the number of washing cycles. b) Swelling ratio of the original cotton fabrics (black), cotton fabrics cross-linked with PEGMA (red), P(NIPAM-coEGMA) with a molar ratio of NIPAM:EGMA=18:2 (blue) and 19:1 (pink) at temperature below and above TT. The temperatures are 30 oC (open square) and 40 oC (solid square).

In addition, the swelling ratios of the cotton fabrics cross-linked with or without thermoresponsive polymers are also measured below or above TT to confirm that the thermo-responsive property still exists after washing (Figure 9b). Since the swelling ratios of the original cotton fabrics stay unchanged when varying the temperature, no thermo-responsive properties are


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present. A similar behavior is observed for the cotton fabrics cross-linked with PEGMA due to the fact that the TT of EGMA is well above 40 oC. In contrast, the cotton fabrics cross-linked with P(NIPAM-co-EGMA) with a molar ratio of NIPAM:EGMA=18:2 (blue) and 19:1 (pink) both present a prominent change of the swelling ratio during the switching of the temperature between 30 oC (below TT) and 40 oC (above TT). This switching capability of the swelling ratio stays unchanged when the washing cycle number is increased. The wettability of the as-prepared cotton fabrics cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2) and the ones stored in ambient condition for 6 months as a function of temperature were measured as well to investigate the reproduction of wettability as a function of time (details can be found in the supporting information).

It can be confirmed that the

wettability and thermo-responsive property still remained even after storage for 6 month. Besides comparing the wettability of the as-prepared ones with the ones stored for 6 months, we also measured the wettability as a function of moisture/temperature (details can be also found in the supporting information). The wettability of the cotton fabrics cross-linked P(NIPAM-coEGMA) (NIPAM:EGMA=18:2) depends on the RH and temperature. In addition, it shows a good reproducibility by repeatedly switching the moisture and temperature.

Air/Moisture Permeability Breathability and perspiration permeability are two critical facts for applications in the field of textiles. The air and moisture permeability are measured to predict the breathability and perspiration permeability. In general, the air permeability of cotton fabrics is not influenced by the relative humidity (RH) unless the RH is extremely high. In this scenario, extremely high RH will induce the cotton fibers to swell. Therefore, the air permeability of the cotton fabrics will reduce. With respect to the moisture permeability, a higher RH will induce the transfer of the


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moisture to be more difficult. Hence, the moisture permeability of the cotton fabrics will decrease with increase of RH. In case of the cotton fabrics cross-linked with the thermoresponsive polymers, the rise of RH will cause the polymer to be more hydrated at a temperature below the TT. When the temperature is above the TT, a more pronounced collapse of the thermoresponsive polymer can be observed. Hence, the smart control of air/moisture permeability can be more prominent. The air permeability of cotton fabrics is presented in Figure 10. At 30 oC (below TT), the air moisture permeability of cotton fabrics cross-linked with PEGMA (182 mm/s) and P(NIPAMco-EGMA) with a molar ratio of NIPAM:EGMA=18:2 (181 mm/s) decrease as compared with that of the original cotton fabrics (198 mm/s). As the polymer was cross-linked on the cotton fabrics, a compact polymer layer formed on the fiber surface as confirmed by FESEM measurements. Polymer layer filling the gaps between the fibers prevents the circulation of air between the body and the surrounding atmosphere. As a result, the air permeability of cotton fabrics cross-linked with polymers decreases. When the temperature is raised to 40 oC (above TT), the air permeability of the original cotton fabrics shows only a tiny increase of 2 mm/s, which can be explained by the expanded gaps between the fibers in a hotter atmosphere. However, the air permeability of cotton fabrics cross-linked with P(NIPAM-co-EGMA) with a molar ratio of NIPAM:EGMA=18:2 is dramatically increase from 181 mm/s to 195 mm/s, a value which is close to that of the original cotton fabrics. The reason is the collapse of the former densely packed P(NIPAM-co-EGMA) layer. The collapsed layer will form a porous structure and enhance the air permeability enabling a recovery of values of the uncoated fabrics.38 Because PEGMA possesses no transition between 30 oC and 40 oC, there is no prominent change


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observed in the air permeability, meaning that the coated fabrics still show a poor air permeability. As moisture is the major content of perspiration, the perspiration permeability of cotton fabrics is simulated by detecting the change of moisture in a permeability cup. The results are combined in Table 2. The moisture permeability of the original cotton fabrics is higher as compared to cotton fabrics with cross-linked PEGMA or copolymer at 30 oC. It increases slightly for the original cotton fabrics and for the ones cross-linked with PEGMA at 40 oC. In contrast, the moisture permeability of cotton fabrics cross-linked with P(NIPAM-co-EGMA) increases significantly from 879 g/(m2·24h) at 30 oC (below TT) to1268 g/(m2·24h) when the temperature is increased to 40 oC (above TT). Again the value of the original cotton fabrics is nearly recovered although the fabrics are polymer coated, due to the porous structure formed on the cotton fabrics cross-linked with P(NIPAM-co-EGMA) above the transition temperature. In addition, the cotton fabrics cross-linked with P(NIPAM-co-EGMA) with a molar ratio of NIPAM:EGMA=19:1 also exhibits enhanced air and moisture permeability. Concerning air and moisture permeability, the copolymer P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2 and 19:1) outperforms simple PEGMA coatings.


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Figure 10. Air permeability of the original cotton fabrics, cotton fabrics cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2 and 19:1) and PEGMA at 30 oC (black) and 40 oC (red). Table 2. Moisture permeability of the original cotton fabrics, cotton fabrics cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2 and 19:1) and PEGMA at 30 oC and 40 oC.

Temperature (oC)

Original cotton g/(m2·24h)

NIPAM:EGMA=18:2 g/(m2·24h)

NIPAM:EGMA=19:1 g/(m2·24h)

PEGMA g/(m2·24h)

30

1126

879

884

862

40

1305

1268

1275

901

Bacterial adhesion The attachment of S. aureus and E. coli on original cotton fabrics and cotton fabrics crosslinked with P(NIPAM-co-EGMA) or PEGMA are characterized by a bacterial culture experiment. After 24 h, the accumulation of bacteria on cotton fabrics is quantitatively analyzed (Figure 11).

Figure 11. Number (N) of S. aureus and E. coli attachment on original cotton fabrics (black), cotton fabrics cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=19:1, red), cotton


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fabrics cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2, dark grey), cotton fabrics cross-linked with PEGMA (light grey).

In case of S. aureus on original cotton fabrics, the number of bacteria on the original cotton is 2×108 cfu/mL. In contrast, the bacteria on cotton fabrics cross-linked with PEGMA is only 3.5×104 cfu/mL, which is a reduction by 4 orders of magnitude. Hence it can be concluded that although the PEGMA polymers are cross-linked to the cotton fabrics, the cross-linked PEGMA still can form a hydrophilic layer on the surface, which reduces the adhesion of bacteria to the cotton

fabrics.

When

cotton

fabrics

are

cross-linked

with

P(NIPAM-co-EGMA)

(NIPAM:EGMA=19:1 and 18:2), the numbers of the bacteria are 4.1×106 and 4.3×104 cfu/mL. If compared with the original cotton fabrics, it is obvious that both cotton fabrics cross-linked with P(NIPAM-co-EGMA) exhibit an anti-adhesion behavior for bacteria. For a molar ratio of NIPAM to EGMA of 19:1, the anti-bacteria adhesion ability of the cotton fabrics cross-linked with P(NIPAM-co-EGMA) does not reach that of the cotton fabrics cross-linked with PEGMA or with P(NIPAM-co-EGMA) with a molar ratio of 18:2. The reason might be the limited amount of EGMA giving rise to an inhomogeneous P(NIPAM-co-EGMA) layer on the cotton fabrics. Cross-linking of P(NIPAM-co-EGMA) on the cotton fabrics is realized by the reaction between EGMA and citric acid. When the molar ratio of NIPAM to EGMA is 19:1, there might be not sufficient EGMA in the copolymer to form a homogeneous layer on the surface of the cotton fabrics. Therefore, the anti-bacteria adhesion ability is limited. Thus, it needs an increased molar ratio of EGMA in the copolymer to have sufficient EGMA for the cross-linking and thereby getting anti-bacteria adhesion similar to pure PEGMA.


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As seen in Figure 11, the E. coli accumulation on cotton fabrics is similar as compared with the S. aureus accumulation, which reveals that the cotton fabrics cross-linked with P(NIPAM-coEGMA) has an identical good resistance against adhesion of S. aureus and E. coli. Therefore, a monomer ratio of NIPAM:EGMA of 18:2 is sufficient for the cross-linked cotton fabrics to effectively prevent the attachment of bacteria. Bacteria without adhesion are observed by confocal microscopy (Figure 12). In case of original cotton fabrics, the fluorescence intensity of S. aureus is relatively weak, meaning that there are rarely bacteria observed even after culturing for 24 h. As a consequence, most of the bacteria are adhered to the original cotton fabrics. On the contrary, Figure 12b and 12c present much higher fluorescence intensities, showing that there is much more S. aureus left in the suspensions in case of cotton fabrics cross-linked with polymers. In case of E. coil, the behavior is similar as compared to that of S. aureus. Hence, we can conclude that the two kinds of bacteria investigated prefer to stay in suspension instead of adhering to the cotton fabrics with a hydrophilic thermoresponsive polymer layer. The anti-adhesion of bacteria is realized in our investigation by the hydrated thermo-responsive polymer layer on top of the cotton fabrics. For this reason, an increase of the RH will enhance the hydration of the cross-linked thermo-responsive polymers and the adhesion of bacteria will be more difficult.


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Figure 12. Confocal microscopy graphs of the bacteria which are not adhered to the cotton fabrics. S. aureus treated with a) original cotton fabric, b) cotton fabric cross-linked with PEGMA and c) cotton fabric cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2). E. coli treated with d) original cotton fabric, e) cotton fabric cross-linked with PEGMA and f) cotton fabric cross-linked with P(NIPAM-co-EGMA) (NIPAM:EGMA=18:2).

CONCLUSIONS Thermo-responsive random copolymers P(NIPAM-co-EGMA) with different monomer ratios of NIPAM to EGMA are synthesized via ATRP. The TT of P(NIPAM-co-EGMA) rises while the Tg significantly drops with the increase of EGMA molar ratio, which can be attributed to the hydrophobicity and low Tg of the EGMA monomer. The obtained copolymers are cross-linked to cotton fabrics. The obtained cotton fabrics with cross-linked P(NIPAM-co-EGMA) present thermo-responsive properties as well. By increasing the temperature above itsTT, the crosslinked P(NIPAM-co-EGMA) layer forms a porous structure, inducing the air/moisture


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permeability of cotton fabric cross-linked with P(NIPAM-co-EGMA) to enhanced significantly above TT. The values of original cotton fabrics are nearly recovered although the fabrics are polymer coated. In addition, due to the hydrophilicity of the cross-linked P(NIPAM-co-EGMA) layer at room temperature, there will be a hydrated layer formed on the cotton fabrics crosslinked with P(NIPAM-co-EGMA). This hydrated layer can prevent bacteria adhesion. Therefore, it can be concluded that the cotton fabrics cross-linked with P(NIPAM-co-EGMA) has dual functions: It combines a comfort regulation at high temperatures by reaching air/moisture permeability similar to original cotton and much larger than PEGMA coatings with inhibition of bacterial adhesion at room temperature being significantly increased as compared to original cotton and reaching values of PEGMA coatings. Best conditions for comfort regulation and antibacteria adhesion are reached if the monomer molar ratio of NIPAM to EGMA is fixed to 18:2 in P(NIPAM-co-EGMA) copolymer.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant no. 51403186 and 51611130312), Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology of Zhejiang Sci-Tech University (CETT2015002) and The Young Researchers Foundation of Key Laboratory of Advanced Textile Materials and Manufacturing


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Technology, Ministry of Education, Zhejiang Sci-Tech University (2015QN01). PMB thanks for support by the DFG priority program “Intelligente Hydrogele” (MU 1487/23-1).

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