Thermoresponsive Cellulose AcetateâPoly(N-isopropylacrylamide

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Thermoresponsive Cellulose Acetate-Poly(N-isopropylacrylamide) Core-Shell Fibers for Controlled Capture and Release of Moisture Neha Thakur, Anupama Sargur Ranganath, Kostiantyn V Sopiha, and Avinash Baji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07559 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

Thermoresponsive Cellulose Acetate-Poly(N-isopropylacrylamide) Core-Shell Fibers for Controlled Capture and Release of Moisture Neha Thakur, Anupama Sargur Ranganath, Kostiantyn Sopiha, Avinash Baji* Division of Engineering Product Development, Singapore University of Technology and Design, (SUTD), 8 Somapah Rd, Singapore – 487372 Abstract In this study, we used core-shell electrospinning to fabricated cellulose acetate-poly(Nisopropylacrylamide) (CA-PNIPAM) fibrous membrane and demonstrated the ability of these fibers to capture water from a high humid atmosphere and release it when thermally stimulated. The wettability of the fibers was controlled by using a thermoresponsive PNIPAM as the shell layer. Scanning electron microscope and fluorescence microscopes are used to investigate the microstructure of the fibers and confirm the presence of the core and shell phases within the fibers. The moisture capturing and releasing ability of these core-shell CA-PNIPAM fibers was compared with neat CA and neat PNIPAM fibers at room temperature as well as at elevated temperature. At room temperature, the CA-PNIPAM core-shell fibers are shown to have the maximum moisture uptake capacity among the three samples. The external temperature variations which triggers the moisture-response behaviour of these CA-PNIPAM fibers falls within the range of typical day and night cycles of deserts demonstrating the potential use of these fibers for water harvesting applications.

Electrospinning: Cellulose Acetate; Poly(N-isopropylacrylamide); thermoresponsive; stimuli responsive; Water Harvesting ____________________________________________ * Corresponding author: Tel: +65 6499 4502; Fax: +65 6779 5161 Email: [email protected] Keywords:

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1.

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Introduction Development of smart or stimuli responsive polymeric materials that possess the ability

to change their functional properties in response to external stimuli have gathered huge amount of interest among the researchers.1,2 Some of the external stimuli which are responsible for inducing a change in the physical or chemical properties of these smart polymers include pH, temperature, light, humidity, electric/magnetic fields, mechanical force and presence of small biomolecules.3-5 Among all stimuli responsive polymer systems, temperature responsive polymer systems have found widespread interest as the temperature can be easily applied, externally monitored and controlled. 6-9 Temperature-responsive polymers also known as thermoresponsive polymers are often characterised by a critical solution temperature at which the polymer system undergoes a change in its solubility behaviour. A typical temperature sensitive polymer solution possesses an upper critical solution temperature (UCST) below which a clear phase separation can be noticed and above which only one particular phase of polymer exists.10,11 However, certain polymer solutions have a tendency to remain monophasic below a particular temperature and phase separate above this temperature. These polymer systems are generally considered to possess lower critical solution temperature (LCST). Depending upon the chemistry of the groups present in the polymer system as well the mechanism of interactions between the groups and the adjacent solvent molecules, different thermoresponsive polymers have been reported such as poly(Nvinylcaprolactam)

(PNVC),

poly(N-isopropylacrylamide)

(PNIPAM),

poly

(N,N-

diethylaminoethyl methacrylate) (PDEAEMA) and copolymers such as poly(l-lactic acid)poly(ethyleneglycol)-poly(l-lactic

acid)

(PLLA-PEG-PLLA).12-15

Majority

of

these

thermoresponsive polymers have been used for biomedical applications such as tissue 2 ACS Paragon Plus Environment

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engineering, drug and gene delivery.

11,16-18

More recently, some of these thermoresponsive

polymers have been explored for water harvesting applications. 19-21 It has been reported that 13,000 trillion litres of water is present in form of fog or dew in the atmosphere.22 This air-laden moisture often acts as a direct source of water and can be harvested to meet the ever growing demand for fresh water resources. It is known that in dry coastal areas of Namib Desert, the humid air currents of the oceans often give rise to earlymorning fog which acts as a rich source of water for Namib desert beetle and other plant species.23,24 Inspired by the water capturing mechanism of these natural species, researchers have fabricated several synthetic smart materials with tuned wettability to harvest water from fog or dew.25-30 Grafting stimuli-responsive polymers on the surface of cotton (99% cellulose) is one approach that is used for harvesting water from fog.31 For example, thermoresponsive polymer PNIPAM is grafted on the surface of cellulose using atom trap radical polymerisation (ATRP) for extracting water from humid air.18,32-34 However, surface modification via various polymerisation methods often involves complicated synthetic procedures that limits its use for large scale applications. Herein, we report the fabrication of thermoresponsive fibers using core-shell electrospinning technique and demonstrate its use for moisture harvesting applications. Cellulose acetate (CA) is used as the core and thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM) is used as the shell component in fabricate thermoresponsive core-shell fibers. In comparison to the two step grafting approach, we have employed a single step method to tailor the wettability of CA for its use for water harvesting applications. Cellulose acetate (acetate ester of cellulose) is chosen as it has good thermal stability, low adsorption characteristic and its surface can be easily modified.35 Obtaining CA in nanometer sized fibers is advantageous for 3 ACS Paragon Plus Environment


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water harvesting applications as it presents high-surface-to-volume ratio.36 PNIPAM is chosen as the shell phase due to its temperature responsive switchable wettability behaviour. The switchable wettability behaviour of these core-shell fibers in response to the thermal stimulus is investigated. Our results demonstrate that at ambient temperature, these core-shell samples absorb water and release it at elevated temperature. The temperature variations are chosen such that they are within the range of a typical day and night temperature cycle of deserts.

2.

Experimental

2.1

Materials Cellulose acetate (CA, Mw = 30,000, 39.8 wt% acetyl content) and poly(N-

isopropylacrylamide) (PNIPAM, Mw = 300,000) was purchased from Sigma Aldrich and Scientific Polymer Products Inc. respectively. N,N-dimethyl formamide (DMF; 99%), tetrahydrofuran (THF; 99%) and 2-ethyl-4-methylimidazole (EMI) were procured from SigmaAldrich, Singapore whereas octaglycidyl polyhedral oligomeric silsesquioxane (OG-POSS, (C6H11O2)n(SiO1.5)n; n=8,10,12) was obtained from Hybrid Plastics, Inc. United States. 1,1’dioctadecyl-3,3,3’,3-tetramethylindocarocyanine perchlorate was procured from ThermoFisher Scientific, Singapore. Dimethyl acetamide (DMAc; 99%) and acetone were obtained from Sigma-Aldrich and were used as received. 2.2

Sol-gel preparation The solution for electrospinning of neat CA and neat PNIPAM fibrous membranes was

prepared as follows. Known amount of CA was dissolved in a binary solvent mixture of DMAc: Acetone (1:2; w/w) to obtain 12 wt% CA solution. Similarly, 15wt% solution of PNIPAM was


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prepared by dissolving known amount of PNIPAM in THF and DMF (1:1 w/w) solution mixture.37 Appropriate amount of cross-linker, OG-POSS and initiator, EMI was also added to the PNIPAM solution. Homogeneous solutions were obtained after mechanically stirring all the solutions for 8 h and then subjected to electrospinning. Nanospinner-24 (Inovenso, Turkey) was used to obtain neat CA and neat PNIPAM fibers. The as prepared solutions were loaded into a 5 mL syringe and positioned vertically on the syringe pump. The flow rate of the pump and the voltage used during electrospinning was 0.5 mL h-1 and 12 kV respectively. The distance between the needle (21G) and the flat collector was set at 15 cm. The electrospun fibers were deposited on the grounded stationary metal collector covered by a piece of aluminum foil. The obtained fibers were heat treated at 60 °C for 6 h to evaporate any residual solvent. The core-shell electrospinning setup was used to obtain CA-PNIPAM core-shell fibers. For this purpose, the CA solution was fed to the inner capillary and PNIPAM solution was fed to the outer shell capillary of the setup. Two separate syringe pumps were used to control the flow rates of the shell and core solutions. A coaxial spinneret with 1 mm inner capillary diameter and 3 mm outer capillary diameter was used. The flow rate of core solution was fixed at 0.6 mLh-1 whereas the flow rate of shell solution was varied from 0.6 mLh-1 to 1.8 mLh-1. The applied voltage used was 9 kV and the distance between the needle and the stationary collector was maintained at 15 cm. Uniform core-shell fibers were obtained when the electrospinning was performed using a vertical setup instead of a horizontal setup. In the horizontal setup, the gravitational force was found to distort the pendant droplet that lead to the formation of nonuniform core-shell fibrous structures. Temperature and relative humidity (RH) during the electrospinning process was maintained at ~25˚C and ~50%, respectively. The obtained CAPNIPAM and neat PNIPAM fibers were heat treated at 150 ˚C for 8 h to induce cross-linking.



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Samples for contact angle measurements were collected on microscopic glass slides which were attached to the flat collector. 2.3

Characterisation The samples were coated with a thin layer of gold-palladium using a sputter coater for 30

seconds to increase their conductivity before their morphology was examined using a scanning electron microscope (SEM, JEOL, JSM-6700F) operated at 5 kV. The instrument was also used to obtain energy-dispersive X-ray spectroscopy (EDS) measurements to confirm the presence of both the core and shell components in the fabricated CA-PNIPAM fibers. To visualize the core and shell phases of the CA-PNIPAM fibers, 1 µgmL-1 1,1’dioctadecyl-3,3,3’,3-tetramethylindocarocyanine perchlorate; DilC18(3) was added to the CA solution prior to the core-shell electrospinning process. The fibers were collected on the glass slides and directly imaged using an inverted research microscope (Axio Observer D1, Carl Zeiss) equipped with a digital camera. Filters were chosen with regard to the emission wavelength (λex = 545 nm and λem= 565 nm) of the incorporated dye. Chroma Filter Set 49005 (DsRed/Cy3) was used to capture the fluorescence image of the stained fibers whereas optical image was captured using bright field imaging mode of the microscope. The infrared spectra of the fibrous membranes was obtained using a Fourier transform infrared spectrometer (FTIR) (Bruker Alpha spectrometer). Attenuated total reflectance (ATR) mode was used to obtain the spectra of different electrospun membranes and 64 scans were recorded between 4000 and 400 cm−1 at a resolution of 4 cm-1. Differential scanning calorimetry (DSC) was used to determine the LCST of the samples. The samples were heated from 20 oC to 60 oC at a heating rate of 5 oC min-1. 3-5mg of fibrous 6 ACS Paragon Plus Environment

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samples was placed in T0-Hermetic pans (TA instruments). 30 µL of distilled water was added to the samples to pre-wet them. The samples were then sealed in the respective pans and were further subjected to two consecutive heating and cooling cycles. Static water contact angle on the fibrous membranes was measured using a contact angle measurement setup (KINO-Optical contact angle and interface tension meter) in sessile drop technique to evaluate the wetting properties of the samples. Furthermore, the thermoresponsive behaviour of the samples was investigated by measuring the contact angle at room temperature (25˚C) and at an elevated temperature (45˚C). The elevated temperature chosen is above the LCST of PNIPAM. A temperature controlled sample chamber associated with the goniometer was used to measure the contact angle at elevated temperature. A water droplet of 3 µL was used and the static contact angle was obtained from the average of at-least seven measurements taken on different regions of the samples. The moisture sorption behaviour of the samples was investigated using the setup as explained in our previous work.38 Briefly the setup consisted of two flowmeters and a humidity sensor (Sensirion (SHT1x) that was connected to the thermogravimetric analysis setup (TGA; TAQ90, TA Instruments). The flowmeters were used to control the proportion of dry and humid air that was supplied to the TGA. This enabled us to control the relative humidity (RH) within the TGA chamber. The dynamic weight change of the samples as a function of supplied humidity as well as the temperature around the sample was measured using the TGA. Moisture collection efficiency of the samples as a function of temperature was also investigated using a controlled environmental chamber (BINDER, USA). To investigate the sample’s ability to collect and release the moisture, the samples were placed in vials and then weighed. These vials were then placed in the environmental chamber for 24 h. The relative 7 ACS Paragon Plus Environment


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humidity within the chamber was maintained at 95%. To determine the amount of moisture absorbed by the sample, the weight of the samples before and after conditioning in the environmental chamber was measured. An average of 3 measurements was used to determine the moisture collection ability of the samples. In order to optically observe the moisture collection behaviour, few strands of CA-PNIPAM fibers were collected and exposed to the high humidity generated by a conventional humidifier. The formation of water droplets on the fibers was recorded using a digital camera. The velocity of the humidifier was kept to a minimum to mimic the actual fog conditions.

3.

Results and Discussions It is known that the viscosity of the shell component plays an important role in the core-

shell fiber formation as the electrically charged shell solution draws out the core solution by shear force to obtained compound fibers.41 In the first step, the electrospinning processing variables are controlled. To obtain CA-PNIPAM core-shell fibers, 12 wt% CA solution is used as the core solution and 15 wt% PNIPAM solution is used as the outer shell solution. The use of 0.6 mLh-1 and 1.8 mLh-1 flow rate for core and shell capillaries respectively led to the formation of thin uniform CA-PNIPAM core-shell fibers. 9 kV electrostatic voltage is used during the process to obtain consistent fibers as the voltage below and above this led to the formation of nonuniform core-shell fibers. Figure 1 shows the SEM images of neat CA, neat PNIPAM and CAPNIPAM core-shell fibers. Inset image in Figure 1(a) shows that the CA fibers have a rough surface morphology. This can be attributed to the use of highly volatile acetone as the solvent solution. The average fiber diameter for the neat CA fibers is determined to be 0.19 ± 0.082 µm. In contrast, Figures 1(b) and 1(c) shows that the PNIPAM and CA-PNIPAM fibers have smooth 8 ACS Paragon Plus Environment

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morphology. The average diameter for PNIPAM and CA-PNIPAM is determined to be 0.87 ± 0.29 µm and 0.98 ± 0.32 µm respectively. These fibers are used to investigate their ability to capture water. Typically, the membranes are required to be mechanically robust as they are required to support the weight of the water. Gopiraman et al.39 investigated the mechanical properties of electrospun CA membranes and demonstrated that the modulus of the CA membranes can be ~250 MPa. They show that the mechanical property can be improved by 3 times when graphene is used to reinforce the fibers. Similarly, our group investigated the mechanical properties of electrospun membranes and demonstrate that the mechanical strength and stiffness of the membranes can be improved when there is fiber to fiber bonding and interaction.40

In this study, the CA-PNIPAM membranes are first fabricated and then

crosslinked, which should improve the strength of the fibers. Additionally, we observe that the fibers have sufficient strength and can withstand the handling as well as operational conditions. Similar membranes have been used in our lab for oil-water separation and the membranes are shown to display good mechanical integrity.9 To verify the presence of both CA and PNIPAM in the core-shell fibers, energy dispersive X-Ray measurements (EDX) are obtained. Figures 2(a) and 2(b) show the corresponding EDX atomic mapping as well as the X-ray spectrum of the CA-PNIPAM fibers. EDX atomic mapping confirms the presence of both CA and PNIPAM phases in core-shell fibers. It is evident that the atomic composition of the fibers show all the X-ray peaks corresponding to the presence of various elements in the core as well as the shell component. The X-ray peak associated with silicon is attributed to its presence in the OG-POSS, a crosslinker that is added to the PNIPAM phase to improve its stability when exposed to moisture. The presence of nitrogen in PNIPAM is also reflected in the spectrum. The peaks associated with



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carbon and oxygen in the spectrum corresponds to the presence of CA and PNIPAM. Peaks associated with palladium and gold is due to the coating of the polymer sample with gold and palladium. Following this, the core-shell structure formation is visualized by incorporating a fluorescent dye, DilC18(3) into the core solution prior to electrospinning. The bright field optical image of as-spun core-shell fibers is shown in Figure 3(a) and the corresponding fluorescence image obtained using a DsRed/Cy3 filter is shown in Figure 3(b). Following this, a compound image of the fibers is obtained by overlaying these images one on top of the other using FIJI ImageJ. The compound image is shown in Figure 3(c). The image clearly shows the presence of dye stained core component surrounded by a thin layer of shell component (inset marked with arrows). This confirms the presence of a distinct core and shell phases. Additionally, the core is also selectively stained with bismuth nitrate. Briefly, a known concentration of bismuth nitrate is added to the core CA solution prior to the electrospinning. The obtained fibers are then cryofractured and their cross-sectional morphology is investigated using a SEM. It is evident in the Figure S1 (Supporting Information) that the fibers display a heterogeneous structure having a dark core surrounded by a lighter shell. The presence of high density bismuth in the core component results in its darker appearance compared to the shell component. The appearance of an inner ring surrounded by an outer thin ring provides clear evidence that the obtained fibers have core-shell morphology. In the next step, FTIR is used to investigate the presence of various functional groups in neat as well as in core-shell fibers. Figure 4 shows the FTIR spectra of the samples. FTIR spectra of pure CA fibers displays its characteristic ester band at 1735 cm-1. The medium intensity band from 3499 cm-1 to 3260 cm-1 with a peak centred at 3430 cm-1 corresponds to the 10 ACS Paragon Plus Environment

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stretching vibrations of residual aliphatic hydroxyl group (-OH) present in CA. The presence of two strong well defined peaks at 1230 cm-1 and 1046 cm-1 are attributed to the -C-O stretching modes of the ester group. In the case of neat PNIPAM, secondary amide as well as -CH stretching region between 3000 cm-1 and 2800 cm-1 dominates the spectra. The sharp peaks in the fingerprint region at 1635 cm-1 and 1541cm-1 is attributed to the amide-I and amide-II stretching whereas the peaks at approximately 1460 cm-1 and 1369 cm-1 corresponds to the asymmetric and symmetric deformation of isopropyl groups of acrylamide in PNIPAM chains. Also, the C-H stretching region has three prominent peaks. Peak at ~2970 cm−1 is associated with isopropyl CH3 asymmetric stretch, peak at 2919 cm−1 is associated with -CH2 asymmetric stretch and the peak at 2850 cm−1 corresponds to isopropyl -CH3 symmetric stretch.42 The spectra recorded for the CA-PNIPAM fibers shows all the characteristic peaks associated with the CA and PNIPAM phases. This is an evidence that both CA and PNIPAM phases are present in the core-shell fibers. LCST temperature of neat PNIPAM and CA-PNIPAM fibers is determined using differential scanning calorimeter (DSC). For this purpose, the pre-wet samples are subjected to consecutive heating and cooling cycles as shown in Figure 5. The LCST transition is usually confirmed by the presence of an endothermic peak during the heating cycle. It is evident from Figure 5 that the endothermic peak corresponding to the LCST is seen at ~34 °C for neat PNIPAM as well as for CA-PNIPAM fibers. This transition can be attributed to the presence of water in the pre-wet samples. LCST is caused by a critical balance between the hydrophobic/hydrophilic groups in a molecule. As the temperature is increased above LCST, the polymer-polymer interactions between the hydrophobic isopropyl-methyl groups of PNIPAM are energetically favoured compared to the interactions between the amide groups of PNIPAM and water molecules. This causes the existing H-bonds to break thereby leading to an endothermic



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peak at 34 °C.43 Therefore the presence of an endothermic peak in CA-PNIPAM fibrous samples similar to neat PNIPAM confirms the LCST behaviour of electrospun fibers which is responsible for temperature induced wettability change. However the narrow endothermic peak in neat PNIPAM is due to its sharp deswelling behaviour at the LCST whereas the broad peak in the CA-PNIPAM can be attributed to the presence of CA along with PNIPAM which leads to a gradual deswelling behaviour. The wettability behaviour of the samples is investigated by measuring the contact angle made by a water drop on the surface of the samples. For this purpose, a 3 µL droplet of deionised water is placed on the surface of the fabricated membranes and its image at room temperature and at an elevated temperature is captured at a speed of 2 frames per second and 0.5 frames per second respectively. The angle between the tangential lines of the liquid interface and the solid surface measured using drop-cast plugin in FIJI ImageJ software gave the static contact angle (θs). Figure 6(a) shows the contact angle (θs) values of the samples measured at ambient temperature. The water contact angle on neat PNIPAM is measured to be 8 ± 1.3°. The hydrophilic nature of PNIPAM ensures that the water droplet is immediately absorbed by the PNIPAM fibers. It is interesting to note that the contact angle measured on CA-PNIPAM coreshell fibers is 19.8 ± 1.2°. This indicates that the fibers are hydrophilic at room temperature which is attributed to the presence of hydrophilic PNIPAM in the shell region of these core-shell fibers. On the contrary, the contact angle on electrospun CA is measured to be 131 ± 1.1°. This demonstrates that the electrospun CA fibers are hydrophobic. This is surprising as bulk CA is known to be hydrophilic. We also measured the contact angle on bulk solvent casted CA film. Figure S2 (Supporting Information) shows that the contact angle measured on CA film is 62 ± 1.1°. This shows that the bulk CA is hydrophilic. The apparent difference in the wettability


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behaviour of bulk and fibrous CA is attributed to the different orientation of the functional groups on the surface of both solvent casted film and electrospun fibers. CA behaves as an amphiphilic molecule as it contains both hydrophobic -CH3COO and hydrophilic -OH groups. Electrospinning of CA solution can result in the strong orientation of its methyl groups at the air/solid surface which makes these fibers hydrophobic.44-46 The exposure of the methyl groups on the surface minimizes the surface energy and makes it hydrophobic. This can also be used to explain the difference in wettability of CA-PNIPAM core-shell fibers and wettability of neat PNIPAM fibers. The presence of CA in the core-shell fibers slightly reduces the hydrophilicity of the fibers. The water droplet when placed on these core-shell fibers is restricted to penetrate into the inner regions of the fibers due to the presence of hydrophobic CA. The water droplet spreads only within the shell regions of the fibers. Furthermore, thermoresponsive behaviour of the core-shell fibers is investigated by measuring the contact angle at an elevated temperature (45° C). Figure 6(b) shows the plot of contact angle measured at ambient and at elevated temperature as a function of time. It is clear from this plot that the contact angle of fibers is higher at elevated temperature compared to ambient temperature. For example, after 30 s time interval, the contact angle measured at ambient temperature is 19.8 ± 1.2° and at elevated temperature is 102 ± 1.9°. This shows that the fibers are hydrophobic at elevated temperature and hydrophilic at room temperature, demonstrating their thermoresponsive behavior.

This switchable wettability behaviour is

attributed to the presence of PNIPAM in the core-shell fibers. Due to the complex polarity of the PNIPAM groups, the surface exhibits a reversible switching between hydrophobic and hydrophilic states. At temperature below the LCST, hydrogen bonds can form between the amide groups of PNIPAM and the surrounding water molecules. Whereas at elevated temperature, these



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hydrogen bonds break and the hydrophobic-inter and intramolecular interactions between the methyl groups of CA and PNIPAM dominates. The switchable behaviour of the membranes is also investigated by varying the temperature between 25 and 45 °C for three cycles (Figure 6(c)). The wettability of the membranes is experimentally seen to switch between the two extreme states of hydrophilic and hydrophobic on variation of temperature below and above LCST. This behavior of temperature triggered wettability change can be employed efficiently to collect and release water from a humid atmosphere. A TGA setup is then used to investigate the water capture and release behaviour of the samples. The relative humidity (RH) around the samples is controlled by varying the ratio of dry and humid air. A constant 40 sccm air flow is maintained within the TGA furnace during the experiment. In the first step, the fibers are heated to 110 °C in nitrogen atmosphere for few mins to remove any moisture from the samples and then cooled to 25 °C. Following this, the RH inside the chamber is increased from 7% to 90% in the steps of 150 min interval. The weight of samples is seen to increase with an increase in supplied RH. The weight gained by the fibers indicates the amount of water that is captured by the fibers when exposed to humid air. Plots of relative humidity vs time and relative weight vs time are shown in Figures 7 (a) and 7 (b) respectively. The sorption and desorption behaviour of the samples is also recorded and shown in Figure 8 (a) and 8 (b) respectively. It is evident from Fig 8 (a) that at the beginning of the humidity exposure cycle, all the samples show more or less similar weight gain. This is attributed to the physical adsorption of water on the samples. However, ~10 minutes later, the resultant weight gain of the PNIPAM and CA-PNIPAM fibers is higher than the pristine CA fibers. This is attributed to the chemisorption of water molecules on the surface of PNIPAM. In case of pristine CA fibers, most of the hydroxyl groups have been replaced by the comparatively


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inert acetyl (-CH3COO) groups. These acetyl groups do not attract water molecules as strongly as the amide groups of PNIPAM. Therefore, the weight gain is lower in case of CA fibers. This is surprising as the contact angle measured on the CA demonstrated that CA is hydrophobic. However, here, the cellulose acetate fibers show a net moisture sorption ability. This can be explained based on the amphiphilic nature of CA and ability of water molecules to diffuse inside the porous membranes. It is evident from the FTIR spectra that the electrospun CA also has -OH groups. The presence of these hydrophilic groups explains the sample’s ability to form intermolecular hydrogen bonds between the polymer molecules and the adjacent water molecules in the amorphous regions of cellulose acetate. This results in a net moisture sorption. The moisture sorption ability of neat CA reaches 11% maximum whereas maximum moisture ability of hydrophilic PNIPAM reaches to ~16%. However, CA-PNIPAM fibers show 21% sorption ability. This can be attributed to the presence of higher number of repeatable functional hydrophilic units in the core-shell fibers compared with the pristine PNIPAM fibers. The presence of hydrophilic groups in the shell component of PNIPAM and in core component of CA is responsible for attracting the adjacent water molecules. PNIPAM shell can also act as an interfacial bridge that promotes the flow of water molecules to the inner CA core. This explains why CA-PNIPAM has higher moisture sorption ability. Fig 8 (b) shows the response of the fibers when the RH is reduced. These curves represent the corresponding desorption behaviour of moisture loaded fibers. It is evident that the weight of the sample decreases as the RH is reduced. This indicates that the moisture absorbed by the samples is lost as the RH is reduced. These results confirm that the core-shell CA-PNIPAM fibers demonstrate the highest moisture sorption behaviour when exposed to humid air. These samples also have an ability to release the



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moisture when the RH is reduced. Thus, the ability of core-shell fibers to capture and release moisture is well demonstrated. Due to the limitation of our current TGA setup in maintaining the RH at elevated temperatures, a controlled environmental chamber is used to investigate the moisture sorption behaviour of the electrospun fibers as a function of temperature. In the environmental chamber, the relative humidity as well as the temperature around the sample can be effectively controlled. Four different temperatures are chosen well above and below the LCST of PNIPAM and the relative humidity is maintained at maximum (95%). In the first step, all the samples are placed in a small open glass vials with no direct contact with water and subjected to 95% RH for 24 h. The weight of the fibers before (W0) and after (Wf) exposure to humidity is accurately measured. In the second step, percentage moisture uptake (Wm) is then determined as; =

∗ 100

(1)

All these set of experiments are repeated thrice and an average value of these three experiments is determined to investigate the moisture sorption behaviour of the fibers (Figure S3, Supporting Information). Table 1 shows the quantitative moisture collection efficiency of fibers at various temperatures. It is clear from Table 1 that the core-shell fibers show the maximum moisture uptake compared to the neat CA and neat PNIPAM fibers. It is also evident that as the temperature is increased, the water harvesting ability of the core-shell fibers is significantly decreased. These thermoresponsive measurements clearly support the temperature induced switchable wettability behaviour of fabricated core-shell CA-PNIPAM electrospun fibers. At the temperatures below the LCST, the fabricated CA-PNIPAM fibers have the maximum moisture uptake capacity (~208 ± 6%) among all three samples. These results are also seen to be


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consistent with the moisture sorption behaviour characterized using the TGA setup. However, at elevated temperatures (>34 °C), the results of moisture uptake are significantly reversed. It is to be noted that the moisture uptake tendency of neat CA fibers increases with an increase in temperature. This can be attributed to the fact that increase in temperature accelerates the diffusion of water molecules within the less crystalline areas of CA resulting in a higher moisture gain. Liu et al. has also reported an increase in the moisture regain for cotton fibers with an increase in the temperature. 32 The quantitative decrease in the moisture uptake of PNIPAM and CA- PNIPAM fibers at elevated temperature is attributed to the well reported coil to globule transition of the PNIPAM molecules above the LCST. This transition is accompanied by the alignment of methyl groups of PNIPAM in the outer regions of the fibers. The protruding methyl group shows negligible affinity to the adjacent water molecules thereby decreasing the moisture uptake. This effect is also known as “Umbrella effect” and has been observed with nanoparticles displaying superhydrophobic character.47 In contrast, at lower temperatures, the interfacial layer of water on PNIPAM surface further binds with the adjacent water molecules leading to a continuous water uptake. These results clearly demonstrate that the core-shell fibers are capable of absorbing and releasing a significant amount of water in response to the external temperature variations. The droplet condensation behaviour on CA-PNIPAM fibers at ambient temperature is also investigated. For this purpose, few strands of fibers are collected using a ungrounded parallel collector. This explains why the fibers appear to adhere to one another in Figure 9. The charges on the fibers are not dissipated due to the use of an ungrounded collector. These fibers are placed at a distance of 15 cm from the nozzle of humidifier and the velocity of humid air is kept at minimum. This setup is used to direct the artificial fog generated from the ultrasonic humidifier onto the fibers. The ultrasonic humidifier produces many micron sized droplets in the



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form of mist. When the fibers are exposed to this mist flowing at very low velocities, the fog droplets begin to condense on the surface of the fibers. Figure 9 shows the real time optical images of the fog droplets condensing on the fibers. The droplets started condensing as soon as the fibers were placed in high humidity environment. It is evident that after 5 s, visible smaller droplets appear on the surface of fibers which can be attributed to the presence of a hydrophilic layer of PNIPAM on CA fibers. Initial water condensation involves the growth of individual droplets without significant interaction with each other. As the water condensation continues, the size of the tiny droplets (marked with arrows in Figure 9.) begins to increase. The joints on the fiber surface also acts as a nuclei for the cohesive aggregation of the droplets without any external force as evident in Fig 9. Due to the self-propelled coalescence of these droplets, their inter-droplet distance decreases resulting in the formation of bigger droplets. Once these fully grown droplets exceed a threshold diameter, the force of gravity is expected to overcome the cohesive force between the fibers and droplets and the latter can eventually be collected. The droplet size is seen to increase to ~15 µm in diameter before it detaches from the fibers. This droplet growth behaviour on the surface of the CA-PNIPAM fibers clearly demonstrates its water collection ability, which shows its tremendous potential for use in water harvesting applications.

4.

Conclusion This study demonstrated that core-shell cellulose acetate-poly(N-isopropylacrylamide)

fibers can be used for temperature controlled capture and release of water from humid air. PNIPAM was chosen as a shell component because of its well-known thermoresponsive


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behaviour. The wettability behaviour of the fibers was investigated. The fibers are shown to be hydrophilic at room temperature and hydrophobic at elevated temperatures. These fibers are also shown to collect water from moisture. Their moisture capturing and releasing ability is investigated and compared with the neat CA and neat PNIPAM fibers. The core-shell fibers were shown to capture the highest amount of water when exposed to humid air at ambient temperature. The temperature responsive property can be employed for an efficient collection and release of water from high humid air/fog in arid regions where the external temperature stimuli can be provided by the continuous day and night temperature cycles of nature.

5. Associated Contents Supporting Information. Cross-sectional SEM image of bismuth stained CA-PNIPAM coreshell fibers, SEM image of solvent cast CA film and electrospun CA fibers, moisture sorption behaviour of CA, PNIPAM and CA-PNIPAM fibers as a function of temperature.

6. Acknowledgements The author AB would like to acknowledge the financial support from Ministry of Education (MOE - Tier 2), Singapore [Grant T2-MOE-1302].

7.

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Figure Captions 1. Representative SEM micrographs of (a) neat CA fibers; (b) neat PNIPAM fibers; and (c) CA-PNIPAM core-shell fibers. The insets show magnified SEM image of the fibers. 2. (a) SEM image of CA-PNIPAM core-shell fibers and their corresponding elemental mapping images of carbon, oxygen, nitrogen, and silicon; and (b) EDS spectra of CAPNIPAM core-shell fibers. The inset table shows the detected elements and their weight percentage. 3. (a) Bright field optical microscopy image of the CA-PNIPAM core-shell fibers; (b) fluorescent image of the CA-PNIPAM core-shell fibers obtained by incorporating DilC18(3) dye inside the CA matrix of the core-shell fibers; and (c) compound image obtained by overlaying the optical microscopy image on top of the fluorescent image. The shell regions of the fibers are shown with the help of the arrows. 4. Comparison of the ATR-FTIR spectra of CA, PNIPAM and CA-PNIPAM core-shell fibers. 5. DSC thermograms showing the LCST phase transition of PNIPAM and CA-PNIPAM electrospun fibers. The endothermic peak at ~34 ˚C recorded indicates the LCST transition in PNIPAM and CA-PNIPAM core-shell fibers. 6. (a) Plot shows static water contact angle (θs) values measured on CA, PNIPAM and CA-PNIPAM core-shell fibers at 25 °C. The insets show the images of the droplets on the samples; (b) plot shows contact angle as a function of time measured on CAPNIPAM membranes at room temperature and at elevated temperature. The inset shows digital image of the droplets on the CA-PNIPAM membrane at room temperature and at elevated temperature; and (c) reversible wettability of the CAPNIPAM core-shell fibers triggered by temperature. 7. (a) Representative plot of relative humidity as a function of time. This plot shows the duration of relative humidity maintained around the samples within the TGA setup; and (b) representative plots of relative weight gain by the samples as a function of time corresponding to the relative humidity within the TGA setup. 8. (a) Plot of relative weight gain by the samples as a function of relative humidity. These plots demonstrate the sorption behaviour of the samples. The samples gain weight as the relative humidity around them is increased; and (b) plot of relative weight loss by the samples as a function of relative humidity. The samples lose 23 ACS Paragon Plus Environment


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weight when the relative humidity around them is reduced. This plot shows the desorption behaviour of the samples. 9. Optical images demonstrating the condensation behavior of the water droplet on the CA-PNIPAM core-shell fibers. The arrows in the figure show the initiation of water condensation on the surface of the fibers. The water begins to condense as soon as they are placed in high humidity environment.


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Figure 2


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Figure 6


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Figure 8


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Table 1. Comparison of the moisture sorption efficiency of various electrospun membranes at different temperatures


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Thermoresponsive Cellulose AcetateâPoly(N-isopropylacrylamide

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