Water-based, heat-assisted preparation of water-repellent cotton fabrics using graft copolymers

Abstract

An inexpensive, facile, environmentally-friendly, water-based method to prepare water repellent cotton fabrics using non-fluorinated graft copolymers is reported. A series of graft copolymers, poly[(oligo(ethylene glycol) methacrylate)-co-(2-hydroxyethyl methacrylate)-co-(n-butyl methacrylate)-co-(methyl methacrylate)]-graft-poly(dimethylsiloxane), were synthesized. The oligo(ethylene glycol) methacrylate component was incorporated to impart water dispersibility to the polymer, the 2-hydroxyethyl methacrylate component facilitated polymer crosslinking around the cotton, while the grafted poly(dimethylsiloxane) provided water repellency. Cotton swatches were dipped into aqueous polymer micellar solutions, removed from the solutions, and then thermally annealed to produce robust water-repellent coatings. The water repellency improved with an increase in the polymer concentration of the coating dispersion, coating annealing temperature and time. The laundering durability of the coating improved as the graft copolymer backbone length and the PDMS content increased.

---

I. Introduction

Liquid repellent fabrics are highly desirable due to their usefulness in a wide variety of applications, ranging from everyday consumer products such as kitchen aprons or shirts to personal protective wear in the military and medical sectors and to water–oil separation materials.^1–5 Highly repellent surfaces usually possess hierarchical roughness, as well as low surface tension.^6–10 Cotton fabrics are already rough due to their micro-size fibers; thus, it is facile to turn them repellent using low surface tension compounds.

A common approach to fabricate highly liquid repellent fabrics involves incorporating additional surface roughness into the fabrics, which is sometimes followed by functionalization with low surface tension reagents such as fluorinated compounds.^11–19 This was achieved by growing inorganic nanoparticles in situ,^11,12 or depositing premade nanoparticles to the fibers via dip-coating^13–16 or layer-by-layer assembly,¹⁷ or vapor phase deposition.¹⁸ However, these methods have a number of drawbacks including poor particle bonding, fabric property alteration due to particle attachment, and tedious multi-step reactions.¹

A simpler approach is to directly modify the fabric with polymers or small molecules.^20–29 This was achieved by radiation induced graft polymerization,^20,21 or mist copolymerization,^22,23 or dip-coating into polymer solutions.^24,25 In previous work, our group also used the dip-coating approach to render cotton fabrics repellent.^25–30 Our strategy was to use diblock copolymers in which one of the blocks, such as poly[2-(perfluorooctyl)ethyl methacrylate] (PFOEMA), rendered the repellent properties to the cotton while the other block, such as poly[3-(triisopropyloxysilyl)propyl methacrylate] (PIPSMA), acted as the anchoring block and grafted onto the cotton fibers.²⁶ This strategy could yield a robust coating due to covalent bonding to the cotton from multiple anchoring units per chain and crosslinking of the polymer. In another strategy, we replaced the PIPSMA block with a photo-crosslinkable block, poly[2-(cinnamoyloxy)ethyl acrylate], which wrapped around the fabric in the dip-coating step and crosslinked when exposed to light afterwards.²⁸ In this case, we also replaced the fluorinated PFOEMA block with poly(dimethylsiloxane) (PDMS). PDMS is also a low surface tension compound with a value of 20 mN m⁻¹, and it is inexpensive, non-toxic and does not bio-accumulate as opposed to fluorinated reagents.^31–33 However, organic solvents were used in the dip coating process.

In this article, we report a facile, inexpensive, and water-based process to produce robust fluorine-free superhydrophobic cotton fabrics. Inspired by our recent use of graft copolymers in anti-smudge coatings on glass substrates,^34,35 we decided to utilize water-dispersible graft copolymers with a polyol backbone (P1 or P2) and PDMS side chains (Scheme 1) for fabric coating as well. Graft rather than block copolymers were used because the former were easier to synthesize. We used a random-copolymer polyol backbone consisting of oligo(ethylene glycol) methacrylate (OEGMA), 2-hydroxyethyl methacrylate (HEMA), n-butyl methacrylate (BMA), and methyl methacrylate (MMA) because the HEMA units crosslinked better in a copolymer system via a heat-assisted condensation reaction to eliminate methanol, ethylene glycol and butanol.^36–39 We further note that OEGMA was used to render water dispersion to the graft copolymer. To graft PDMS, the polyol was reacted with a limiting amount of PDMS-COCl, PDMS bearing a terminal acid chloride unit. PDMS was used to render superhydrophobicity to the cotton fabrics. We further discuss in this paper the factors affecting the coating's performance, the coating's water repellent properties and laundering stability.

Scheme 1 Chemical structures of (a) polyol P1, (b) polyol P2, and (c) PDMS-COCl.

We note that there have been several other reports on water-based methods to coat fabrics.^24,40–44 Li et al. prepared a binary graft copolymer bearing oligo(ethylene glycol) and fluorinated side chains and the copolymer was then used to coat cotton fabrics.²⁴ Aside from complex copolymer synthesis protocol, the coatings should not be durable because they were not covalently attached or crosslinked. In another case, Xu et al. coated cotton fabrics using silica hydrosols prepared via a water-based sol–gel reaction.⁴⁰ However, silica nanoparticles formed no stable bonds with cotton. Our group also reported water-based formulations to coat cotton fabrics.^25,41 In one case, PEG-S₂-PFOEMA-b-PCEMA was used, where PEG stands for poly(ethylene glycol), S₂ is a disulfide junction, and PCEMA is poly(2-cinnamoyloxyethyl methacrylate).⁴¹ The polymer synthesis was also complex. The system presented herein is a simpler and practical strategy to prepare stable coatings on cotton fibers using easily-synthesized polymers.

II. Experimental section

Materials

All chemicals were purchased from Aldrich. 2-Hydroxyethyl methacrylate (HEMA), n-butyl methacrylate (BMA), and methyl methacrylate (MMA) were purified by reduced pressure distillation. Oligo(ethylene glycol) methacrylate (OEGMA, M_n = 500 g mol⁻¹) was purified by passing through an activated basic alumina column. Hexamethyldisilazane, trimethylsilyl chloride, copper(I) bromide (CuBr), 2,2′-bipyridine (bpy), ethyl-2-bromoisobutyrate (EBIB), methanol, anhydrous toluene, anhydrous tetrahydorfuran (THF), THF (≥99.9%), acetone (≥99.5%), oxalyl chloride ((COCl)₂, 98%), and monohydroxy-terminated poly(dimethylsiloxane) (PDMS-OH, M_n ∼ 4670 g mol⁻¹) were used as received.

Cotton fabrics were purchased from a local fabric store. The fabrics were washed thrice with soap and distilled water and dried in an oven at 120 °C for 2.0 h. The cotton swatches were subsequently dried in an oven at 120 °C for 2.0 h before each use.

Polyol synthesis

Two random copolymers, polyols P1 and P2, were synthesized by atom transfer radical polymerization (ATRP). First, 2-[(trimethylsilyl)oxy]ethyl methacrylate (HEMA-TMS) was synthesized following a literature method.⁴⁵ To synthesize P1, bpy (0.224 g, 1.44 mmol) and EBIB (0.142 g, 0.72 mmol) were placed in a flask and purged with N₂. HEMA-TMS (5.440 g, 25.2 mmol), BMA (2.162 g, 21.6 mmol), MMA (3.072 g, 21.6 mmol), OEGMA (1.296 g, 3.6 mmol) and anhydrous toluene were added to the flask under a N₂ flow. This was followed by degassing using three freeze–pump–thaw cycles before CuBr (0.103 g, 0.106 mmol) was added and another three freeze–pump–thaw cycles were executed. The final N₂-filled flask was immersed in an 80 °C oil bath for 20 h. The polymerization was terminated by putting the reaction mixture into the refrigerator and letting air into the flask. The reaction mixture was diluted with THF (15.0 mL) and passed through an alumina column to remove the copper salt. The solution was concentrated by rotary evaporation before adding 10.0 mL of THF and 10.0 mL of methanol to deprotect the TMS group. The final solution was added into excess hexane to precipitate the polymer. After re-dissolution in THF, the polymer solution was added into hexane again to precipitate the polymer. After drying under vacuum overnight, the polymer was obtained at a yield of 82%.

P2 was analogously prepared except only half the monomer amounts were used and the initiator to monomer molar ratios were changed. Specifically: HEMA-TMS 2.720 g (12.6 mmol), BMA 1.081 g (10.8 mmol), MMA 1.536 g (10.8 mmol), and OEGMA 0.648 g (1.8 mmol).

¹H NMR (in CDCl₃, at 300 MHz and 298 K): 4.20–4.10 (br, –CO₂CH₂ of OEGMA, OH of HEMA), 3.90–3.70 (br, –CO₂CH_2, of HEMA, BMA), 3.80 (br, –CH₂OH of HEMA), 3.65–3.60 (–CH₂CH₂O of OEGMA, –CO₂CH₃ of MMA), 2.5–1.35 (br, –CH₂ of main chain BMA, HEMA, MMA, and OEGMA, and 2 CH₂ of BMA), 1.3–0.5 (br, –CH₃ of polymer backbone BMA, MMA, HEMA, and OEGMA, and CH₃ of BMA) ppm.

Graft copolymer synthesis

Three graft copolymers, P1-g-PDMS_27%, P1-g-PDMS_66%, and P2-g-PDMS_41%, where the % refers to the weight fraction of PDMS in the graft copolymer, were synthesized in two steps according to a modified previously reported procedure.³⁵ In step 1, PDMS-COCl was prepared from PDMS-OH. To prepare PDMS-COCl, PDMS-OH in anhydrous THF (5.0 mL, 1.0 × 10⁻³ mol) was added dropwise into a N₂-filled flask containing (COCl)₂ (2.0 mL, 2.6 × 10⁻² mol) and the mixture was stirred for 24 h at room temperature. The solvent and unreacted (COCl)₂ were then removed under vacuum to yield PDMS-COCl as a clear liquid.

To prepare P1-g-PDMS_27%, P1 (1.37 g) was dissolved in anhydrous THF (3.0 mL). PDMS-COCl (0.47 g) in anhydrous THF (3.0 mL) was then added dropwise. After 48 h at room temperature, the solvent was removed by rotary evaporation to yield a white paste.

P1-g-PDMS_66% and P2-g-PDMS_41% were prepared analogously. The recipes consisted of 1.37 g P1, 1.41 g PDMS-COCl, and 6.0 mL THF in the first case and of 0.68 g P2, 0.47 g PDMS-COCl, and 6.0 mL THF in the latter case.

¹H NMR (in CDCl₃, at 300 MHz and 298 K): 4.6–4.3 (br, CH₂O₂C₂O₂-PDMS), 4.20–4.10 (br, –CO₂CH₂ of OEGMA, OH of HEMA), 3.90–3.70 (br, –CO₂CH_2, of HEMA, BMA), 3.80 (br, –CH₂OH of HEMA), 3.65–3.60 (–CH₂CH₂O of OEGMA, –CO₂CH₃ of MMA), 2.5–1.35 (br, –CH₂ of main chain BMA, HEMA, MMA, and OEGMA, and 2 CH₂ of BMA), 1.3–0.5 (br, –CH₃ of polymer backbone BMA, MMA, HEMA, and OEGMA, and CH₃ of BMA), 0–0.5 (br, 6H, CH₃ of PDMS) ppm.

Polymer characterization

¹H NMR measurements were performed using a Bruker Avance-300 instrument using chloroform-d (CDCl₃) as the solvent. Size exclusion chromatography (SEC) was performed on a Wyatt Optilab rEX refractive index detector using chloroform as the eluent at a 1.00 mL min⁻¹ flow rate. The columns were packed by American Polymer Standards Corporation with 5 μm crosslinked gel at the nominal pore sizes of 1000 Å and 10 000 Å. The system was calibrated with monodisperse polystyrene (PS) standards.

Cotton fabric coating

The first step involved preparing the coating dispersions. In one case, 5.52 mg of graft copolymer was dissolved in 0.60 mL acetone. Under vigorous stirring, 1.00 mL of distilled water was pumped into the polymer solution at a 0.10 mL min⁻¹ flow rate resulting in a light white emulsion. The acetone was then removed from this solution using rotary evaporation at 45 °C for 5 min, yielding a 0.92 mL of polymer solution at a concentration of 6.0 mg mL⁻¹. Similar protocols were used to prepare polymer dispersions with concentrations from 0.3 mg mL⁻¹ to 10.7 mg mL⁻¹.

To coat cotton fabric, clean 2.0 × 2.0 cm² cotton pieces were then dipped in a polymer dispersion solution for 20 min. This was followed by vacuum oven annealing at 180 °C for 3.0 h. The effect of varying experimental parameters on the coatings was investigated by changing the annealing temperature from 150 to 190 °C and time from 0.5 h to 4.0 h. Finally, all samples were extracted in THF at 60 °C for 1.0 h.

In one control experiment, vials containing only 6.0 mg P1-g-PDMS_27% were annealed in a vacuum oven at 180 °C for 0.5 h to 3.0 h. Another control experiment involved dipping a cotton swatch into a 6.0 mg mL⁻¹ PDMS-OH solution in THF and annealing resultant fabric in a vacuum oven at 180 °C for 3.0 h.

Characterization

Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed using a Philips XL-30 ESEM FEG instrument operated at 15 kV. The samples were coated with a thin Au layer prior to measurements. Attenuated total reflectance infrared (ATR-IR) analysis was performed using an ALPHA-Bruker instrument. Thermogravimetric analysis (TGA) was performed using a TA Q500 instrument. Samples were heated under an air flow from room temperature to 750 °C at a rate of 5 °C min⁻¹. The residual weight was normalized to the weight at 150 °C and represented the average of two measurements. Contact angles were measured at room temperature using a Dataphysics OCA 15Pro optical contact angle measuring system. Contact angles were measured at 5 different positions on a coating using 5.0 μL water droplets for static contact angles and 10.0 μL water droplets for the shedding angles. The shedding angle was measured according to procedures previously reported.^28,46

Laundering durability

Laundry durability was evaluated using a method similar to that reported previously.^20,25 In brief, the three types of coated cotton swatches were laundered together in a 250 mL flask containing 150 mL of a 0.16 wt% aqueous detergent solution (Sunlight brand) at 50 °C. The cotton swatches were stirred at 300 rpm for 15 min. They were rinsed with distilled water and then stirred in 100 mL of distilled water for 15 min. Afterwards, they were stirred in 50 mL THF for 15 min. The swatches were dried in air at room temperature for 10 min, which was followed by drying in the oven at 120 °C for 0.5 h. This process was considered one laundering cycle. Before measuring the water static contact angles and shedding angles, the cotton swatches were dried in the oven at 140 °C for 0.5 h, except for cycles 1 and 5.

III. Results and discussion

Polymers

To prepare P1-g-PDMS_27%, P1-g-PDMS_66%, and P2-g-PDMS_41%, the polyols P1 and P2 were first synthesized via ATRP. Fig. 1 shows the ¹H NMR spectra of the polyols. Using the peaks at 3.63 ppm for OEGMA, 3.55 ppm for HEMA, 3.90 ppm for BMA, and 3.84 ppm for MMA, we obtained for P1 the molar fractions of 5%, 40%, 17%, and 38% for OEGMA, HEMA, BMA, and MMA, respectively. These numbers changed for P2 to 5%, 54%, 20%, and 21%, respectively. To determine the numbers of repeat units, we measured the monomer conversions at the end of polymerization. The conversions and the monomer to initiator molar feed ratios were then used to calculate the repeat unit number of 88 for P1 and 30 for P2. Further, the number-average molecular weight (M_n) were calculated and were listed in Table 1 along with the SEC polydispersity values (PDI). The PDI of the polyols were high particularly for P1 probably due to the challenges of making copolymers using ATRP of so many components including OEGMA. The SEC trace for P1 can be seen in Fig. 3a, while the SEC trace of P2 can be seen in Fig. 3c.

Fig. 1 The ¹H NMR spectra of (a) P1, and (b) P2.

Table 1 ¹H NMR and SEC characterization

Polyols	^1H NMR Mn (g mol^−1)	SEC Mw/Mn
P1	12.2 × 10^3	2.06
P2	4.3 × 10^3	1.56

---

Fig. 2 The ¹H NMR spectra of (a) P1-g-PDMS_27%, (b) P1-g-PDMS_66%, and (c) P2-g-PDMS_41%.

Fig. 3 The SEC traces of (a) P1-g-PDMS_27%, (b) P1-g-PDMS_66%, and (c) P2-g-PDMS_41% and their precursors.

In the next step, commercial PDMS-OH (M_n ∼ 4670 g mol⁻¹) was modified to PDMS-COCl, before grafting it onto the polyol backbone to obtain the final polymers. The PDMS-COCl reacted with only some of the hydroxyl groups of the HEMA component of the polyol. P1 was used as the backbone for two of the graft copolymers, 27 or 66 wt% PDMS. The ¹H NMR spectrum of P1-g-PDMS_27% is shown in Fig. 2a. The strong peak at 0.1 ppm corresponds to the methyl groups of PDMS, while the peak at 4.5 ppm corresponds to the methylene group of the ester linkage indicative of the successful grafting of PDMS onto the polyol. Further, the PDMS peak at 0.1 ppm with the BMA peak of the polyol at 3.9 ppm were used to calculate the PDMS weight fraction of the copolymer and its total molecular weight (Table 2). The SEC traces in Fig. 3a of P1, PDMS-OH, and P1-g-PDMS_27% also show evidence of successful grafting. The PDMS-OH peak was negative due to its lower refractive index than chloroform. Compared to the P1 and PDMS-OH peaks, the graft copolymer peak was shifted to the higher molecular weight side. The final graft copolymer peak was still positive due to a stronger contribution of the polyol to the peak than PDMS. On the other hand, the P1-g-PDMS_66% SEC trace peak in Fig. 3b is negative due to the larger contribution of the PDMS side chains. The ¹H NMR spectrum in Fig. 2b also shows a very strong PDMS peak at 0.1 ppm. Finally, the P2-g-PDMS_41% ¹H NMR spectrum and SEC trace can be seen in Fig. 2c and 3c, respectively. The SEC trace was again positive due to a lower amount of PDMS in the copolymer.

Table 2 ¹H NMR characterization

Polymers	Mn (g mol^−1)
P1-g-PDMS27%	15.5 × 10^3
P1-g-PDMS66%	20.3 × 10^3
P2-g-PDMS41%	6.1 × 10^3

---

Cotton fabric coating

Fig. 4a shows an SEM image of the plain-weave cotton fabric used. The fabric has a thread diameter of (310 ± 80) μm and an inter-thread spacing of (130 ± 50) μm. Furthermore, a fiber diameter of (14 ± 4) μm and an inter-fiber spacing of (20 ± 11) μm were measured. Fig. 4b shows a close-up image of the fabric and the fibers appear to have rough surfaces.

Fig. 4 SEM images of (a) uncoated cotton fabric, (b) uncoated cotton fibers, and (c) cotton coated with P1-g-PDMS_27%.

To coat the fabric, a dispersion of a graft copolymer in water was first prepared (Scheme 2, A → B). This involved dissolving the graft copolymer in acetone, pumping in water at 0.10 mL min⁻¹ under vigorous stirring, and rota-evaporation of acetone. While the polymer solution was clear in acetone, the dispersion formed after water addition was whitish (ESI, Fig. S1a and b†) and the whish appearance remained after acetone evaporation (ESI, Fig. S1c†). When left still for a week, the dispersion concentrated onto the bottom of the vial. However, a simple vial shaking by hand resulted in a uniform dispersion again. We suspect that a micellar solution was prepared at this stage. While the PDMS side chains of the graft copolymer formed the micellar cores, the water-soluble OEGMA-bearing polyol backbone formed the corona of the micelle and provided the dispersibility.

Scheme 2 Schematic cross-sectional view of the preparation of the cotton coating after cotton soaking in a concentrated spherical micellar solution of a graft copolymer. The light blue parts in the scheme represent crosslinked polyol. While the deposition of spherical micelles is depicted above, cylindrical and vesicular micelles as well as micelles or nano- or micro-aggregates of any shape may deposit and fuse analogously to yield a rough polymer coating on fiber surfaces.

Then, clean cotton fabric pieces were dipped in the polymer dispersion for 20 min (B → C). The micelles would infiltrate the cotton. Upon fabric removal and water evaporation, the micelles would deposit on the fibers. The subsequent annealing of the fabric in a vacuum oven at 180 °C for 3.0 h (C → D) should cause the micelles to fuse, smoothen the coating somewhat, and cause the low-surface-tension PDMS chains to stratify to the surface. More important, it should cause the polyol backbone to crosslink. We suspect that the hydroxyl groups of the HEMA component would attack ester groups to eliminate alcohols such as methanol, butanol, or ethylene glycol and result in formation of ethylene glycol dimethacrylate linkages or crosslinks. Previous studies have shown that under thermal treatment, PHEMA degrades via depolymerisation, but also crosslinks due to ethylene glycol elimination.^36–39 The crosslinking reaction becomes more dominant when HEMA is present in a copolymer.³⁹ We confirmed a crosslinking reaction by subjecting P1-g-PDMS_27% to thermal annealing at 180 °C for 3.0 h without the cotton swatch. The polymer that was not heated or heated for only 0.5 h was soluble in acetone. It became partially soluble after heating at 180 °C for 1.0 or 2.0 h. No solubility was noticed after the polymer was heated for 3.0 h. Finally, the fabrics were extracted in THF at 60 °C for 1.0 h to remove non-attached polymer (D → E).

SEM characterization of the polymer coated cotton swatch can be seen in Fig. 4c for P1-g-PDMS_27% (ESI, Fig. S2a and S2b† for P1-g-PDMS_66%, and P2-g-PDMS_41%, respectively). The fibers appear to be covered by a thin polymer layer. The center of the fibers were smoother without the small striations visible in the uncoated cotton, while closer to the edges, mesh-like rough structures were visible (ESI, Fig. S3† additional P1-g-PDMS_27% SEM).

Wetting properties of coated cotton fabrics

As hypothesized in the coating protocol, the PDMS chains in the coating should be present at the air interface and impart water repellency due to their low surface tension. The coating's properties should depend on the polymer solution concentration, the temperature, and the time of oven annealing. The wetting properties of the coatings were investigated by measuring the water static contact angle and shedding angle.

The oven annealing or coating curing temperature and time was first arbitrarily fixed at 180 °C and 2.0 h, while the polymer solution concentration was varied. Fig. 5a shows the change in wetting properties for these coatings for each of the three graft copolymers. As the P1-g-PDMS_27% concentration increased, the water static contact angle increased to a maximum of (151 ± 2)°, while the shedding angle decreased to (32 ± 2)°. Similar wetting property trends were observed for the other two graft copolymers.

Fig. 5 Change in static water contact angle (closed symbols) and shedding angle (open symbols) for cotton coated by P1-g-PDMS_27% (), P1-g-PDMS_66% (), or P2-g-PDMS_41% () as a function of (a) polymer dispersion concentration, and for P1-g-PDMS_27% () as a function of coating annealing (b) temperature and (c) time. In the shaded areas dispensed water droplets on the coated fabrics would eventually get adsorbed.

In contrast, water droplets were absorbed in seconds by an uncoated piece of cotton (ESI,† photographs of water on uncoated and coated cotton swatches in Fig. S4†). While the coated cotton also floated when placed in a beaker with water and showed a reflective plastron layer when forced into water, the uncoated cotton swatch quickly sunk when placed in a beaker of water and showed no plastron layer (ESI, Fig. S5†). Additionally, water droplets were also fast absorbed by a control sample that was a cotton fabric coated with hydroxyl-terminated PDMS alone. To prepare this coating, the cotton fabric was dipped into a 6.0 mg mL⁻¹ PDMS-OH in THF solution, since PDMS-OH did not dissolve in water. Then the coating was annealed under the standard conditions of 180 °C for 3.0 h. Therefore, the P1 backbone of the graft copolymer was essential for providing the multiple attachment sites to the cotton as well as the water dispersion.

Furthermore, the cotton coatings from 0.3 mg mL⁻¹ to 1.2 mg mL⁻¹ were unstable for all three polymers as can be seen in the shaded area of Fig. 5a. The water droplets penetrated into the cotton after they stayed on the cotton for a certain amount of time such as 1.0 min for the 0.3 mg mL⁻¹ P1-g-PDMS_27% coating (ESI, Fig. S6a†). The time at which the droplet started to absorb into the cotton increased as the polymer concentration increased, indicating an increase in coating stability. Also, the droplet stability varied with the droplet position. At these lower concentrations, there was not enough copolymer deposited onto the cotton to form a layer dense enough to stop the water absorption. This water absorption is not unusual, Deng et al. also observed unstable coating at low polymer grafting degrees.²⁰ As the concentration of the polymer increased, the wetting properties levelled off. If some of the polymer which deposited onto the cotton during the dipping step did not crosslink, it would have been removed by the THF extraction step.

Then, the annealing temperature was varied, while the P1-g-PDMS_27% solution concentration was fixed at 6.0 mg mL⁻¹ and the annealing time at 2.0 h. As the annealing temperature increased, the wetting properties improved as seen in Fig. 5b. The cotton coatings were not stable at temperatures between 150 to 170 °C (shaded area) and the dispensed water droplets got absorbed into the cotton (ESI, Fig. S6b†). However, when the coating was annealed at the lowest temperature, 150 °C, the coating became stable when the annealing time was increased to 18.0 h. Higher temperatures facilitated the coating reaction, but time is also an important factor.

The annealing time was varied, while the P1-g-PDMS_27% solution concentration was fixed to 6.0 mg mL⁻¹ and the annealing temperature at 180 °C. As the annealing time increased, the wetting properties were seen to improve, Fig. 5c. At 1.0 h and lower annealing times, the cotton coatings were not stable with the water droplets absorbing into the cotton (ESI, Fig. S6c†).

Therefore, the standard coating conditions for the cotton were chosen as 6.0 mg mL⁻¹ polymer dispersion concentration, annealing temperature of 180 °C, and annealing time of 3.0 h. Under these conditions, the P1-g-PDMS_27% coating had a water static angle of (151 ± 2)° with a shedding angle of (32 ± 2)°, while the P1-g-PDMS_66% coating had values of (152 ± 3)° and (32 ± 2)°, and P2-g-PDMS_41% coating had values of (150 ± 2)° with a shedding angle of (33 ± 2)°. These wetting properties were not as high as fluorine-based systems due to PDMS's higher surface tension, 20 mN m⁻¹ (ref. 31) as opposed to ∼7 mN m⁻¹ (ref. 47) for fluorinated polymers. Also, we have a water soluble component in our polymer, which could potentially decrease the water repellency.

The cotton weave-type, thread and fibre spacing also played a critical role. We tested this effect with a swatch of cotton that had smaller pores and thread diameter. As determined by optical microscopy, this new cotton swatch had a thread diameter ∼40% smaller than the typical cotton used above, while the area between the bundles, the pores, was ∼10% smaller (ESI, Fig. S7†). This P1-g-PDMS_27% coating had similar water static angle of (152 ± 2)°, but the shedding angle was much lower at (15 ± 2)°.

Tuteja and coworkers showed that surface texture has a large impact on wettability, and a robust highly repellent surface can be designed by taking into consideration of two design parameters D*, the feature spacing ratio, and A*, the robustness factor.^7,8,48 In the case of cylindrical textures, such as meshes⁴⁹ or fabrics,⁴ increasing the spacing between the features and/or decreasing the radius of the cylinders leads to an increase in D*, and consequently an increase in apparent contact angle. In one of their experiments, they increased the spacing between features by stretching a fabric sample, which lead to an initial slightly higher apparent contact angle.⁴ However, as they continued to stretch the fabric, the contact angle decreased and the liquid wetted the surface. This was due to a decrease in the robustness factor A* caused by an increase in the spacing between the features. As A* decreased, the sagging of the liquid–air interface become more severe leading to the liquid touching the next level of solid and transitioning to a wetted state. This sagging can also be affected by the application of external pressure or impact from a droplet released from a height as in the case of shedding angle measurements.^48,50 As in the case of our two cotton fabrics, the contact angle was not highly affected by the change in feature dimensions possible because A* was still above the threshold to sustain a non-wetted state, but the shedding angle was higher for the cotton with the larger dimensions due to an increase in the sagging of the interface and a decrease in A*.

Grafted polymer amount

The amount of polymer grafted onto the cotton fabrics was determined by TGA. Samples were heated in air from room temperature to 750 °C. The data were then normalized to the weight of the sample at 150 °C to eliminate the contribution from adsorbed water. The TGA traces of uncoated cotton, cotton coated by P1-g-PDMS_27% at a concentration of 6.0 mg mL⁻¹, and P1-g-PDMS_27% are shown in Fig. 6 (DTGA traces in ESI, Fig. S8†). The uncoated cotton was pyrolyzed by 600 °C, while the P1-g-PDMS_27% sample shows a weight residue even at 700 °C possible due to silicon oxide. The average weight residues at 700 °C for uncoated cotton, R_C, was (0.27 ± 0.05)%; the weight residue for P1-g-PDMS_27%, R_P, was (10.8 ± 0.1)%; and the weight residue for coated cotton at a P1-g-PDMS_27% concentration of 6.0 mg mL⁻¹, R_PC, was (0.58 ± 0.02)%. These residues were used to calculate the grafted polymer mass fraction, x, from:²⁶

R_C(1 − x) + R_Px = R_PC (1)

Fig. 6 TGA traces of uncoated cotton, cotton coated with P1-g-PDMS_27% at 6.0 mg mL⁻¹ solution concentration, and P1-g-PDMS_27%.

For this coating, the amount of grafted polymer to the cotton was (3.0 ± 0.5) wt%. This low polymer grafting amount is not unusual in the literature for polymer coated cotton swatches.^26,28

Using the average diameter determined by SEM for the cotton fibers and assuming the same density for the fiber and cellulose and the smoothness of the fibers, we were able to calculate the total length of the fibers making up each gram of cotton.²⁵ This length was then used in combination with the fiber diameter to calculate the specific surface area of the fibers. The further use of the determined grafting density of (3.0 ± 0.5) wt% allowed us to estimate the thickness of 140 nm for the grafted polymer layer (see ESI† for detailed calculations). This suggests a layer should be sufficiently thick to be visible by SEM, in agreement with our experimental observation.

Chemical characterization

The uncoated cotton fabrics, the graft copolymers, and the coated cotton fabrics were analyzed by ATR-IR to determine their chemical composition. Fig. 7 shows the spectra of P1-g-PDMS_27%, uncoated cotton, and cotton coated with P1-g-PDMS_27%. In the P1-g-PDMS_27% spectrum, the peak at 1720 cm⁻¹ corresponds to the carbonyl group stretching vibration from the polyol backbone, while peaks from the PDMS side chains can be seen at 800 cm⁻¹ and 1015 cm⁻¹, which correspond to the Si–CH₃ stretching and the Si–O–Si stretching, respectively.^31,51 Similar absorption peaks were also seen in the P1-g-PDMS_66% and P2-g-PDMS_41% spectra (ESI, Fig. S9†); however, the intensity of the PDMS peaks was stronger in the latter sample due to its higher PDMS content. The uncoated cotton spectrum shows peaks in the 1060–1100 cm⁻¹ region corresponding to C–O–C stretching vibrations, and strong peaks in the 3200–3600 cm⁻¹ region corresponding to the H-bonded OH stretching.⁵² These peaks were also present in the spectrum of the cotton coated with P1-g-PDMS_27%. In addition, a new peak appeared at 1720 cm⁻¹, which was due to the carbonyl group stretching from the polyol. Similar absorption peaks were also observed in the spectra of cotton coated with P1-g-PDMS_66% and P2-g-PDMS_41% (ESI, Fig. S10†). However, the peak at 1720 cm⁻¹ was not seen in the spectrum of cotton coated with P1-g-PDMS_27% after 0.5 h of oven annealing (ESI, Fig. S11†). This was expected since the coating annealed at 0.5 h was not stable and water spread on the cotton.

Fig. 7 ATR-IR spectra of (a) P1-g-PDMS_27%, (b) uncoated cotton, and (c) cotton coated with P1-g-PDMS_27%. Left side is a zoom in of the 1500–1900 cm⁻¹ range for the (b) and (c) spectra.

Further evidence supporting polymer grafting onto the cotton can be seen from EDS measurements. The EDS spectrum of uncoated cotton (ESI, Fig. S12a†) shows peaks corresponding to C and O as expected, while the EDS spectra of the three types of coatings also show an additional peak corresponding to Si, which comes from the PDMS side chains of the polymers (ESI, Fig. S12b–d†).

Laundering durability

The laundering durability of coated cotton fabrics is an important consideration if they are to be used for practical applications. Nevertheless, this property is not always reported in the literature. The resistance of the three types of coatings was evaluated using a simulated washing cycle by stirring the samples in hot water at 50 °C with laundry detergent. This was followed by rinsing with water and THF to remove the detergent, and finally drying in an oven. The THF rinsing step was a much harsher treatment than that in real applications. It was used to test our coatings' resistance to organic solvents.

Fig. 8 shows the change in the water static contact angles and shedding angles as a function of laundering cycles. The worst performing coating was the P2-g-PDMS_41%. After 10 cycles, water droplets penetrated into the cotton after a few minutes even after the coating was additionally heated to 180 °C for 0.5 h. The high PDMS content of this polymer provides the coating with good repellence, but the shorter polyol backbone probably decreased the mixing of different backbone chains and the degree of crosslinking. The P1-g-PDMS_27% coating showed a decrease in repellence, which could be due to a combination of lower PDMS content, polymer detachment, and surface reconstruction. However, this coating was more durable due to more facile crosslinking from a longer backbone. The best performing coating was the one based on P1-g-PDMS_66%. This polymer had both a high PDMS content and a long backbone resulting in good repellence and durability.

Fig. 8 Change in static water contact angle (closed symbols) and shedding angle (open symbols) as a function of laundering cycles for P1-g-PDMS_27% (), P1-g-PDMS_66% (), and P2-g-PDMS_41% () coated at a 6.0 mg mL⁻¹ solution concentration.

IV. Conclusions

Three graft copolymers, P1-g-PDMS_27%, P1-g-PDMS_66%, and P2-g-PDMS_41%, were synthesized and characterized. The P1/P2 backbone of the copolymer was synthesized using the monomers HEMA, BMA, MMA, and OEGMA. The OEGMA component imparted water dispersion to the copolymer, while the HEMA component should facilitate polymer crosslinking around the cotton fibers. The PDMS side chains imparted water repellency due to their low surface tension. Cotton swatches were coated with the copolymer by dipping them into aqueous polymer dispersions, removing them from the solutions, and then thermally annealing in an oven. The water repellency of the coatings improved with an increase in polymer concentration, coating curing temperature and time. Optimizing these parameters produced a robust coating that resisted THF extraction. Additionally, the laundering durability improved with an increase in both the anchoring sites of the graft copolymer backbone and PDMS content. A robust water repellent coating was fabricated using an easy, inexpensive, water-based approach with non-fluorinated, environmentally-friendly polymers.

Acknowledgements

NSERC of Canada is thanked for sponsoring this research. G. L. thanks the Canada Research Chair program for a Tier I Canada Research Chair Position in Materials Science.

References

1. B. Cortese, D. Caschera, G. Padeletti, G. M. Ingo and G. Gigli, Surf. Innovations, 2013, 1, 140–156.
2. X. Yao, Y. Song and L. Jiang, Adv. Mater., 2011, 23, 719–734.
3. Q. Truong, N. Pomerantz, P. Yip, M. Sieber, J. M. Mabry and S. M. Ramirez, Surf. Innovations, 2014, 2, 79–93.
4. W. Choi, A. Tuteja, S. Chhatre, J. M. Mabry, R. E. Cohen and G. H. McKinley, Adv. Mater., 2009, 21, 2190–2195.
5. H. Teisala, M. Tuominen and J. Kuusipalo, Adv. Mater. Interfaces, 2014, 1, 1300026.
6. H. Bellanger, T. Darmanin, E. Taffin de Givenchy and F. Guittard, Chem. Rev., 2014, 114, 2694–2716.
7. A. Tuteja, W. Choi, G. H. McKinley, R. E. Cohen and M. F. Rubner, MRS Bull., 2008, 33, 752–758.
8. A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S. A. Mazzella, G. C. Rutledge, G. H. McKinley and R. E. Cohen, Science, 2007, 318, 1618–1622.
9. A. K. Kota, W. Choi and A. Tuteja, MRS Bull., 2013, 38, 383–390.
10. S. Wang and L. Jiang, Adv. Mater., 2007, 19, 3423–3424.
11. B. Leng, Z. Shao, G. de With and W. Ming, Langmuir, 2009, 25, 2456–2460.
12. X. Chao-Hua, J. Shun-Tian, C. Hong-Zheng and W. Mang, Sci. Technol. Adv. Mater., 2008, 9, 035001.
13. J. Zhang, B. Li, L. Wu and A. Wang, Chem. Commun., 2013, 49, 11509–11511.
14. L. Wu, J. Zhang, B. Li and A. Wang, J. Mater. Chem. B, 2013, 1, 4756–4763.
15. J. Lin, C. Zheng, W. Ye, H. Wang, D. Feng, Q. Li and B. Huan, J. Appl. Polym. Sci., 2015, 132, 41458.
16. Y. Kong, Y. Liu and J. H. Xin, J. Mater. Chem., 2011, 21, 17978–17987.
17. Y. Zhao, Z. Xu, X. Wang and T. Lin, Langmuir, 2012, 28, 6328–6335.
18. M. Ma, Y. Mao, M. Gupta, K. K. Gleason and G. C. Rutledge, Macromolecules, 2005, 38, 9742–9748.
19. J. Chen, X. Zhong, J. Lin, I. Wyman, G. Zhang, H. Yang, J. Wang, J. Wu and X. Wu, Compos. Sci. Technol., 2016, 122, 1–9.
20. B. Deng, R. Cai, Y. Yu, H. Jiang, C. Wang, J. Li, L. Li, M. Yu, J. Li, L. Xie, Q. Huang and C. Fan, Adv. Mater., 2010, 22, 5473–5477.
21. J. Wu, J. Li, Z. Wang, M. Yu, H. Jiang, L. Li and B. Zhang, RSC Adv., 2015, 5, 27752–27758.
22. L. Wang, G. H. Xi, S. J. Wan, C. H. Zhao and X. D. Liu, Cellulose, 2014, 21, 2983–2994.
23. G. Xi, W. Fan, L. Wang, X. Liu and T. Endo, J. Polym. Sci., Part A: Polym. Chem., 2015, 53, 1862–1871.
24. Y. Li, X. Zheng, H. Zhu, K. Wu and M. Lu, RSC Adv., 2015, 5, 46132–46145.
25. H. Zou, S. Lin, Y. Tu, G. Liu, J. Hu, F. Li, L. Miao, G. Zhang, H. Luo, F. Liu, C. Hou and M. Hu, J. Mater. Chem. A, 2013, 1, 11246–11260.
26. D. Xiong, G. Liu and E. J. S. Duncan, Langmuir, 2012, 28, 6911–6918.
27. Z. Shi, I. Wyman, G. Liu, H. Hu, H. Zou and J. Hu, Polymer, 2013, 54, 6406–6414.
28. Y. Wang, X. Li, H. Hu, G. Liu and M. Rabnawaz, J. Mater. Chem. A, 2014, 2, 8094–8102.
29. M. Rabnawaz, Z. Wang, Y. Wang, I. Wyman, H. Hu and G. Liu, RSC Adv., 2015, 5, 39505–39511.
30. C. M. Grozea, M. Rabnawaz and G. Liu, RSC Adv., 2015, 5, 103722–103728.
31. J. E. Mark, Polymer Data Handbook, Oxford University Press, New York, 1999.
32. S. Seethapathy and T. Górecki, Anal. Chim. Acta, 2012, 750, 48–62.
33. A. B. Lindstrom, M. J. Strynar and E. L. Libelo, Environ. Sci. Technol., 2011, 45, 7954–7961.
34. M. Rabnawaz and G. Liu, Angew. Chem., Int. Ed., 2015, 54, 6516–6520.
35. M. Rabnawaz, G. Liu and H. Hu, Angew. Chem., Int. Ed., 2015, 54, 12722–12727.
36. J. Rázga and J. Petránek, Eur. Polym. J., 1975, 11, 805–808.
37. K. Demirelli, M. Coşkun and E. Kaya, Polym. Degrad. Stab., 2001, 72, 75–80.
38. F. Bennet, P. J. Barker, T. P. Davis, A. H. Soeriyadi and C. Barner-Kowollik, Macromol. Chem. Phys., 2010, 211, 2034–2052.
39. M. S. Choudhary and K. Lederer, Eur. Polym. J., 1982, 18, 1021–1027.
40. L. Xu, W. Zhuang, B. Xu and Z. Cai, J. Text. Inst., 2011, 103, 311–319.
41. M. Rabnawaz and G. Liu, Macromolecules, 2014, 47, 5115–5123.
42. J. E. Mates, T. M. Schutzius, I. S. Bayer, J. Qin, D. E. Waldroup and C. M. Megaridis, Ind. Eng. Chem. Res., 2014, 53, 222–227.
43. A. Fuchs, W. Artner and B. Zwlklrsch, US Pat., 8969492 B2, Huntsman Textile Effects GmBH, 2015.
44. M. Rabnawaz and G. Liu, Macromolecules, 2012, 45, 5586–5595.
45. A. Hirao, H. Kato, K. Yamaguchi and S. Nakahama, Macromolecules, 1986, 19, 1294–1299.
46. J. Zimmermann, S. Seeger and F. A. Reifler, Text. Res. J., 2009, 79, 1565–1570.
47. A. Hirao, K. Sugiyama and H. Yokoyama, Prog. Polym. Sci., 2007, 32, 1393–1438.
48. A. Tuteja, W. Choi, J. M. Mabry, G. H. McKinley and R. E. Cohen, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 18200–18205.
49. S. S. Chhatre, W. Choi, A. Tuteja, K.-C. Park, J. M. Mabry, G. H. McKinley and R. E. Cohen, Langmuir, 2010, 26, 4027–4035.
50. D. Quéré and M. Reyssat, Philos. Trans. R. Soc., A, 2008, 366, 1539–1556.
51. B. H. Stuart, Infrared spectroscopy: fundamentals and applications, John Wiley & Sons, Chichester, UK, 2004.
52. C. Chung, M. Lee and E. K. Choe, Carbohydr. Polym., 2004, 58, 417–420.

---

Footnote

† Electronic supplementary information (ESI) available: Photographs of polymers in acetone, acetone/water mixture, and water. Typical photographs of 5.0 μL water droplets on uncoated and coated cotton. Additional SEM images of polymer coated cotton. Graphs of the time at which the first water droplet and the fifth (last) water droplet spread on the unstable cotton coatings prepared as a function of concentration, temperature, and time. Optical microscopy images of cotton swatches. Additional DTGA trace, ATR-IR spectra, and EDS spectra of uncoated cotton, polymer coated cotton, and polymers. See DOI: 10.1039/c5ra27056a

---