Quantification of residual liquid on repellent cotton fabrics after liquid roll off

Abstract

A method to detect and quantify any residual liquid left on coated cotton fabrics after liquid roll off was developed. Repellent cotton fabrics were prepared by grafting the diblock copolymer poly[3-(triisopropyloxysilyl)propyl methacrylate]-block-poly[2-(perfluorooctyl)ethyl methacrylate] (PIPSMA-b-PFOEMA) under acidic conditions. The PIPSMA block undergoes a sol–gel grafting reaction with the cotton, while the PFOEMA block imparts repellent properties to the cotton's surface. Liquid droplets loaded with quantum dots were placed on top of tilted coated cotton fabrics and allowed to roll off the surface. Fluorescence microscopy was then used to observe the path of the liquid droplets. The amount of quantum dots detected along this path was measured. The effects of varying the amount of polymer grafted to the cotton and the surface tension of the probe liquid on the residual liquid along the path of the rolling droplet was investigated.

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I. Introduction

In recent decades, the number of reports on water and oil repellent coatings has increased significantly due to their usefulness.^1–6 Highly repellent materials have contact angles greater than 150° and droplets easily roll off these surfaces.⁷ These properties can be achieved by incorporating hierarchical roughness and low surface energy into a material. Cotton textiles already possess micro-scale roughness due to their microfiber structure and can be turned to be repellent using a low surface energy substance. Repellent cotton textiles are particularly desirable for personal protection, for example in the healthcare and military sectors. Soldiers may come into contact with a variety of substances such as dust, oils, chemical and biological warfare agents, which can have a detrimental effect on their health.⁸ Thus, it is important to reduce exposure to these hazards.

Two main strategies are widely employed to produce repellent textiles. In the first approach, additional roughness is introduced to the cotton fibers which can be followed by the application of a low surface energy reagent.^9–12 The second approach involves directly modifying the cotton fibers with a hydrophobic substance.^13–15 Our group has applied this second strategy to create robust repellent fabrics using diblock copolymers.^16–19 The diblock copolymers were made from an anchoring block such as poly[3-(triisopropyloxysilyl)propyl methacrylate] (PIPSMA) which grafts to the cotton and crosslinks via a sol–gel reaction, and a low surface energy block such as poly[2-(perfluorooctyl)ethyl methacrylate] (PFOEMA) or poly(dimethylsiloxane) which imparts the repellent properties. This strategy provides the potential of an increase in grafting due to multiple anchoring units per chain, an increase in stability and resistance to etchants due to crosslinking and thickness, and facile thickness adjustment by changing the polymers' molecular weight.

Post-fabrication, there are a number of characterization techniques employed to measure repellent surface properties.^1,6,20 The most common measurement is the apparent contact angle, typically measured where a liquid droplet touches a solid coating. This is used to ascertain the degree of surface wetting for a given droplet type. Furthermore, another measurement is the rolling angle, the angle at which a surface must be inclined before a liquid droplet can roll off. Connected to this angle is the contact angle hysteresis, which is the difference between the advancing angle – the angle at the front side of the rolling droplet, and the receding angle – the angle at the back side of the droplet. This is used to understand the liquid adhesion to the surface. The shedding angle is the angle at which a droplet dispensed from a distance above the material's surface rolls off the surface. This allows for determining the ability of a surface to repel drops upon impact. An impressive characterization toolbox was developed over the years; however, these techniques do not study the actual amount of reagent transferred to the fabric.

In this study, we report a new method to identify and quantify the extent of reagent transfer to repellent cotton textiles using fluorescence microscopy. Liquid droplets were loaded with quantum dots, placed on tilted, coated cotton surfaces, and allowed to roll off. The path of the liquid droplets was then imaged and the amount of liquid transferred to the fabric was quantified by measuring the amount of quantum dots left behind. A number of liquids with decreasing surface tension were used to test this method on fabrics bearing different amounts of grafted copolymer. The diblock copolymer PIPSMA-b-PFOEMA, as shown in Scheme 1, was used to render liquid repellency to our test cotton fabrics.

Scheme 1 Chemical structure of PIPSMA-b-PFOEMA.

II. Experimental section

Materials

Tetrahydrofuran (THF, Aldrich, ≥99.9%), 2-propanol (Aldrich, 99.5%), and Trilite CdSSe/ZnS hydroxyl functionalized quantum dots with an emission wavelength of 540 nm (QD, Cytodiagnostics) were used as received. Hydrochloric acid (HCl, Aldrich, 37%) was diluted with THF to 0.13 M before use.

PIPSMA-b-PFOEMA (P1) was synthesized via sequential atom transfer radical polymerization (ATRP) and the detailed procedure is described in our recent report.²¹ The number-average molecular weight (M_n) was 58 kg mol⁻¹ and the polydispersity index (M_w/M_n) was 1.13 by size exclusion chromatography (SEC) based on polystyrene standards. The number of repeat units for the PIPSMA block was 15 and for the PFOEMA block was 31 as determined by ¹H NMR spectroscopy. The SEC trace and ¹H NMR spectrum can be seen in Fig. S1 in ESI.†

Cotton fabrics were purchased from a local fabric store. The fabrics were then 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.

Cotton fabric coating

P1 was dissolved in 2.0 mL THF at concentrations ranging from 0.30 to 10.00 mg mL⁻¹ in a vial. Then, 50.0 μL of 0.13 M HCl in THF solution was added to the vial and the mixture was stirred for 1.0 h. Two pieces of dried 2.0 × 2.0 cm² cotton fabric swatches were immersed into the polymer solution and the vial was placed in a pre-heated 50 °C oil bath for 30 min. The cotton fabrics were removed from the solution and annealed in an oven at 120 °C for 2.0 h.

Sol–gelled P1 samples were prepared by following the above protocol without the cotton pieces. The reaction was allowed to proceed overnight. The solution was centrifuged at 17 000g for 10 min to settle the product before oven drying.

Fluorescence liquid transfer measurements

First, the coated cotton QD uptake with time was measured by immersing three 1.0 × 1.0 cm² cotton pieces coated at 5.00 mg mL⁻¹ P1 in 300 μL of a 60 mass% 2-propanol in water mixture (f₆₀) containing 4.0 × 10⁻² mg QD for 20 min, 2.0 h, and 24.0 h. The fabrics were dried at room temperature for 1.0 h before attaching them to glass slides with double-sided adhesive tape. Ten random fluorescence images were then collected.

An intensity calibration curve was obtained by immersing 1.0 × 1.0 cm² coated cotton fabrics at 5.00 mg mL⁻¹ P1 into 100 μL of f₆₀ containing 1.0 × 10⁻² to 5.0 × 10⁻² mg QD for 20 min. The fabrics absorbed ∼45 μL of the solution. The fabrics were dried at room temperature for 1.0 h before attaching them to glass slides with double-sided adhesive tape. Ten random fluorescence images were then collected.

Liquid roll off experiments were performed at a 60° inclination from the horizontal position. 1.0 × 1.0 cm² coated cotton fabrics were attached to glass slides using double-sided adhesive tape. 10.0 μL liquid droplets of 2-propanol in water mixtures containing 4.0 × 10⁻² mg of QD were placed on the tilted samples and rolled off. Mixtures of 0 (f₀), 5.0 (f₅), and 10.0 (f₁₀) mass% of 2-propanol in water were used. The fabrics were dried at room temperature for 1.0 h. Fluorescence images were collected along the path of the liquid trace. 10–50 images were collected depending on the sample.

All of the experiments were repeated three times.

Characterization

The surface morphology of cotton fabrics before and after coating was obtained using scanning electron microscopy (SEM). A Philips XL-30 ESEM FEG instrument was operated at 2.0 kV. A Bomem MB100 Fourier Transform Infrared spectrometer (FTIR) was used to characterize P1 before and after sol–gel reaction using KBr as the sample matrix. Thermogravimetric analysis (TGA) was performed using a TA Q500 instrument. Samples were heated under an air flow from room temperature to 650 °C at a rate of 10 °C min⁻¹ with the temperature at 150 °C held for 15 min. The residual weight was normalized to the weight at 150 °C and represented the average of three measurements. Static and receding 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 droplets of f₀, f₅, and f₁₀. The surface tension of each liquid was measured using the pendant drop method.

Fluorescence microcopy was performed using a Nikon Eclipse TE2000-U microscope equipped with a 41001 HQFITC filter cube and an ND4 filter for the lamp. 0.63 × 0.47 mm² grayscale images were recorded as TIF files. The images were analyzed using the microscope software Simple PCI and ImageJ to obtain the mean gray pixel intensity in each QD area. The intensity in each area was multiplied by the area. The background obtained from an area with no QD was subtracted from this measurement.

III. Results and discussion

Cotton fabric coating

Plain-weave cotton fabrics were purchased for this project from a local fabric store. The fabrics were then washed thrice with soap and water and dried in an oven at 120 °C for 2.0 h. The fabric morphology was investigated using SEM as can be seen in Fig. 1a and b. The bundle diameter was measured to be 310 ± 80 μm, while the spacing between the bundles was 130 ± 50 μm. At the single fiber scale, the fiber diameter was measured as 20 ± 6 μm, while the spacing between the fibers was 20 ± 11 μm. The SEM images also reveal rough fiber morphology.

Fig. 1 SEM images of (a) cotton fabric, and cotton fibers (b) before coating, and (c) after coating with P1.

Cotton fabrics were coated with P1 using dilute HCl as the catalyst. First, P1 was dissolved in 2.0 mL THF at concentrations ranging from 0.30 to 10.00 mg mL⁻¹ in a vial. Since THF is a good solvent for the PIPSMA block but a poor solvent for the PFOEMA block, micelles with a PFOEMA core and a PIPSMA corona would form. Then, 50.0 μL of 0.13 M HCl in THF solution was added to this vial and the mixture was stirred for 1.0 h. During this time the PIPSMA block hydrolyzes forming silanol groups as determined by FTIR (ESI, Fig. S2†). A new peak was observed in the FTIR spectrum at 3500 cm⁻¹. A dilute HCl solution was used as the catalyst to minimize damage to the cotton fabrics. When we used 12.2 M HCl, the reaction was faster and the resulting cotton had high repellence; however, the fabric was easily ripped by hand. After the PIPSMA block had hydrolyzed to some extent, two pieces of dried 2.0 × 2.0 cm² cotton fabric swatches were immersed into the polymer solution and the vial was placed in a pre-heated 50 °C oil bath for 30 min. Our previous studies on an analogous system showed that the higher temperature during the immersion step produces stable uniform coated cotton.¹⁷ The silanol groups of the PIPSMA block condense with the hydroxyl groups of the cotton fabrics as well as with themselves to produce a grafted crosslinked layer. The PFOEMA block would then protrude outward. Finally, the cotton fabrics were removed from the solution and annealed in an oven at 120 °C for 2.0 h to further facilitate the reaction and also dry the coated cotton.

The amount of P1 grafted onto the cotton was determined by TGA. Samples were heated in air from room temperature to 650 °C. The data were 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 at a concentration of 5.00 mg mL⁻¹, and sol–gelled P1 are shown in Fig. 2a (DTGA traces in ESI, Fig. S3†). The uncoated cotton was pyrolyzed by 600 °C, while the sol–gelled P1 shows a weight residue possible due to silicon oxide from the sol–gel reaction. The average weight residues at 600 °C for uncoated cotton, R_C, was (0.33 ± 0.02)%; the weight residue for sol–gelled P1, R_P, was (4.4 ± 0.5)%; and the weight residue for coated cotton at a P1 concentration of 5.00 mg mL⁻¹, R_PC, was (0.43 ± 0.04)%. These residues were used to calculate the grafted polymer mass fraction, x, from¹⁶

R_C(1 − x) + R_Px = R_PC (1)

Fig. 2 (a) TGA traces of cotton, sol–gelled P1, and coated cotton at a P1 concentration of 5.00 mg mL⁻¹, and (b) amount of grafted P1 at different P1 in THF concentrations.

The calculated grafted polymer amount is plotted in Fig. 2b as a function of P1 concentration in THF. As the concentration of P1 in the reaction vial increased, the amount of grafted polymer to the cotton also increased. At 5.00 and 10.00 mg mL⁻¹ P1, the deposited polymer appears saturated at a grafting amount of ∼2 wt%.

The fabric morphology after coating was investigated using SEM as can be seen in Fig. 1c. There was no noticeable morphological difference between the uncoated and coated fibers (Fig. 1b and c). A fully stretched P1 chain has a thickness of 12 nm.²¹ It will be difficult to observe such a thin layer. Our previous studies showed similar SEM results due to the thinness of the nanometer size coatings.¹⁶ It further showed that even AFM had trouble detecting this layer.

Wetting properties of coated cotton fabrics

P1 coated cotton fabrics showed liquid repellency due to the outward protruding, low surface tension fluorinated block. The coated cotton at a P1 concentration of 5.00 mg mL⁻¹ had a water contact angle of (156 ± 2)° using a 5.0 μL droplet. However, these angles were under-estimated due to the rough nature the cotton fabric and our inability to delineate the droplet baseline with precision. On the other hand, when water droplets were placed on uncoated cotton, the droplets spread and were absorbed by the cotton. This difference between the cotton swatches can be seen in ESI, Fig. S4.† The coated cotton also stayed afloat when it was placed in a vial with water as shown in ESI, Fig. S5.† When the cotton was forced into the water, a plastron air layer, trapped between the cotton and the water, was visible. The uncoated cotton sank in the vial and no plastron layer was observed.

The static contact angles for water/2-propanol droplets were determined for P1 coated cotton swatches at different P1 coating solution concentrations, as plotted in Fig. 3. 5.0 μL f₀, f₅, and f₁₀ droplets were used. At these fractions of 2-propanol, the surface tension of the liquids at 20 °C is 72.75, 50.32 and 41.21 mN m⁻¹, respectively.²² Our measurements were carried out at a slightly warmer room temperature, 21–22 °C. The measured surface tension for the three liquids were close to the literature values at (72 ± 1), (51 ± 2), and (42 ± 1) mN m⁻¹, respectively. When QD were added to these liquids, the surface tension did not change significantly; it remained at (72 ± 1), (50 ± 1), and (43 ± 1) mN m⁻¹, respectively. As can be seen from Fig. 3, the contact angle decreased as the surface tension of the probe liquid decreased. This is expected as the energy required for liquid spreading diminishes. The contact angles also decreased as the P1 solution concentration decreased due to a lower amount of polymer grafted to the cotton as determined by TGA. Since the surface tension of the probe liquids were not affected by QD addition, the contact angle values measured with QD loaded droplets (ESI, Fig. S6†) were also similar to the values obtained without them (Fig. 3).

Fig. 3 Changes in static contact angles for 5.0 μL droplets of f₀, f₅, and f₁₀ on coated cotton fabrics at different P1 concentrations.

When a high surface tension liquid is placed on a repellent fabric in the dewetting regime, the liquid beads up and easily rolls off the surface. We attached 2.0 × 2.0 cm² coated cotton fabrics to glass slides using double-sided adhesive tape and tilted them at a 60° inclination from the horizontal position. The cotton swatches were coated at a P1 concentration of 0.30, 0.55, 5.00, and 10.00 mg mL⁻¹ 10.0 μL liquid droplets of f₀, f₅, and f₁₀ containing Rhodamine B (for ease of observation) were placed on the tilted cotton surfaces. The droplets then rolled off the surface. At f₀ (only water), the coating at 0.30 mg mL⁻¹ P1 showed a faint pink liquid trace, while the other coatings showed no visible liquid trace. When f₅ was used, the same trend was observed. In this case, the 0.30 mg mL⁻¹ P1 sample showed a larger liquid trace. When f₁₀ was used, the 0.30 mg mL⁻¹ P1 sample showed a larger liquid trace than before. The 0.55 mg mL⁻¹ P1 sample showed a slight liquid trace, while the 5.00 and 10.00 mg mL⁻¹ samples show no liquid trace once again. The four cotton surfaces after liquid roll off at f₁₀ can be seen in Fig. 4.

Fig. 4 Photographs of residuals left on cotton fabric pieces slanted at 60° after the dispensing and running off of a 10.0 μL f₁₀ droplet containing Rhodamine B. The fabrics were coated with P1 at a concentration of (a) 0.30, (b) 0.55, (c) 5.00, and (d) 10.00 mg mL⁻¹, respectively.

Fluorescence characterization of residual liquid after droplet roll off

In the previous section, we did not see any residual liquid on most of the samples tested. However, one would assume that some amount of liquid was left behind along the path of the rolling droplet. Therefore, we developed a method using QD loaded droplets which allows us to detect and quantify any residual trace using the QD fluorescence properties. Water soluble CdSSe/ZnS hydroxyl functionalized QD with an emission wavelength of 540 nm were used in this experiment. QD are very bright, resistant to photobleaching, have narrow emission bands and broad excitation bands.²³ In addition, the structure of the QD, a core–shell-polymer structure, where the polymer layer should minimize aggregation and concentration induced quenching.

First, we measured the QD uptake of the coated cotton with time. Three 1.0 × 1.0 cm² coated cotton pieces at 5.00 mg mL⁻¹ P1 were immersed in 300 μL of f₆₀ containing 4.0 × 10⁻² mg QD for 20 min, 2.0 h, and 24.0 h. We used f₆₀ (24.05 mN m⁻¹ surface tension)²² because we observed that it wetted the coated cotton. The intensity of the QD did not increase with time (ESI, Fig. S7†). The QD do not significantly adsorb onto the cotton, and the detected QD were possibly due to the QD initially entrenched in the cotton between the fibers and the threads. Therefore, we used a 20 min immersion time for further calibration experiments.

An intensity calibration curve was obtained by immersing 1.0 × 1.0 cm² coated cotton fabrics at 5.00 mg mL⁻¹ P1 into 100 μL of f₆₀ containing 1.0 × 10⁻² to 5.0 × 10⁻² mg QD for 20 min. The cotton fabrics absorbed ∼45 μL of this solution, thus, 0.4 × 10⁻² to 2.2 × 10⁻² mg QD per swatch. Grayscale fluorescence images of these samples were acquired for 10 random areas of 0.63 × 0.47 mm² size in three separate experiments. The images were analyzed to obtain the mean gray pixel intensity per area. The results are plotted in Fig. 5. The y-axis is the measured area multiplied by the mean pixel intensity, while the x-axis is the amount of QD in that area. Background images were obtained by measuring a coated cotton sample that was not immersed into a QD solution and the signal from these images was subtracted from our measurements. As shown in the plot, the QD fluorescence intensity increased linearly with the QD amount deposited onto the coated cotton.

Fig. 5 Calibration curve of fluorescence intensity as a function of QD concentration.

Liquid roll off experiments were performed again at a 60° inclination from the horizontal position using QD loaded droplets. A 60° inclination was used to minimize contact time between the droplet and the fabric, which mitigates adsorption, and because no visible droplet trace was observed for most samples in the previous experiment. 1.0 × 1.0 cm² coated cotton fabrics at P1 solution coating concentrations of 0.30, 0.55, 5.00, and 10.00 mg mL⁻¹ were attached to glass slides using double-sided adhesive tape and tilted. 10.0 μL f₀, f₅, or f₁₀ droplets containing 4.0 × 10⁻² mg of QD were placed on the tilted samples. The liquid rolled off the samples. After drying, fluorescence images were collected along the path of the liquid trace. The mean gray pixel intensity in each QD area was obtained. The calibration curve from Fig. 5 was then used to calculate the QD amount in each area of the liquid trace path. These values were added to obtain the total QD amount deposited along each liquid trace path. These experiments were repeated 3 times.

Typical fluorescence images can be seen in Fig. 6 and in ESI Fig. S8 and S9.† The total QD amount deposited along the liquid trace path is summarized in ESI Table S1,† and in ESI Table S2† and in Fig. 7 as a percent of the initial amount in the droplet. Two major trends can be identified. As the surface tension of the liquid decreased by increasing the amount of 2-propanol in the mixture, more of the QD were detected on the cotton. This can be again explained by a decrease in the energy required to spread the liquid on the surface, causing more of it to be left behind. The second trend is more interesting; in general as the amount of P1 attached to the cotton increased, the amount of QD left behind on the cotton decreased. This was expected from our experiments and the literature since a higher amount of low surface tension polymer on the cotton improves its repellent properties.^16,17 The static contact angle values of the coatings increased as the amount of P1 increased as seen from Fig. 3.

Fig. 6 Fluorescence images of fabrics slanted at 60° after roll off of QD-containing water (f₀). The fabrics were coated at a P1 concentration of (a) 0.30, (b) 0.55, (c) 5.00, and (d) 10.00 mg mL⁻¹. Images were cropped to 0.19 × 0.19 mm². Images (b), (c), and (d) were post-processed to be brighter for this display only. The arrow in part (a) shows the direction of the droplet rolling for all the images.

Fig. 7 Percent of residual liquid left behind on fabrics slanted at 60° after roll off of QD-containing f₀, f₅, and f₁₀ droplets.

The exciting result was that we were able to detect and quantify residual liquid on repellent samples that did not show a trace in Fig. 4 or show visually discernible traces. The quantity of these residual liquids was very small as seen from Fig. 7. In general, coatings with a static contact angle higher than ∼145° (Fig. 3) showed this behavior. Rioboo et al. showed that for liquid repellency, the receding contact angle should also be taken into consideration.²⁴ In particular, a receding contact angle higher than 135° was necessary to reach superhydrophobicity. This receding contact angle threshold also correlates well with the amount of residual liquid observed on our surfaces. Coatings with a receding contact angle higher that ∼133° (ESI, Fig. S10†) showed low quantities of residual liquid.

Studies on model repellent micropillar surfaces have shown that liquid droplets slide off these surfaces with a combination of rolling and slipping motions.^25–28 As the droplet slides off the surface, the advancing contact line continuously reforms. However, at the receding contact line, microcapillary bridges can form and they can rupture leaving behind microdroplets on top of the posts. This receding line pinning could be responsible for some of the QD depositions on our surface. Thus, brighter areas should be visible towards the dewetting direction. This is not always the case as shown in Fig. 6a where the top of the fibers are brighter as pointed by the arrow. Additionally, these QD deposits could also be due to defects in the coating. The coating fabricated at a P1 solution concentration of 10.00 mg mL⁻¹ showed higher QD deposits than expected. The high polymer concentration in the reaction coating solution would result in a P1 decrease in solubility in THF due to the long PFOEMA block. P1 would form more compact micelles, which will not deform as readily as before. The PFOEMA block could then be trapped in the core of the micelle, as the PIPSMA block cross-links around it. This would result in the PIPSMA block at the surface of the coating, and non-uniformity in the coating. If these defects are small/spread apart, the change in the coating's properties might not be noticeable with high surface tension liquids, such as water. These defects would start to become apparent as the surface tension of the liquid decreases.

Our method showed that the best performing coating was the one fabricated at a P1 solution concentration of 5.00 mg mL⁻¹. Even in this case, some QD were observed on the surface. This minute amount of deposit (e.g. (2.0 ± 0.4) ng using f₀) is more than sensitive enough to detect even the most toxic substances; for example, sulphur mustard causes blisters starting at about 10 μg.²⁹ Thus, additional methods such as our facile fluorescence approach should be used to fully test a new coating's repellent properties in order to better understand its safety potential and limitations.

IV. Conclusions

The diblock copolymer PIPSMA-b-PFOEMA, P1, was used to prepare repellent cotton fabrics. The PIPSMA block in the presence of HCl catalyst underwent a sol–gel reaction with the cotton fabric as well as with itself to yield cotton-grafted crosslinked polymer. The fluorinated block, PFOEMA, imparted liquid repellent properties to the cotton's surface due to its low surface energy. A series of coated cotton fabrics were prepared with increasing amounts of grafted polymer by increasing the amount of P1 in the coating solution from 0.30 to 10.00 mg mL⁻¹. As the grafted polymer amount decreased, water or water/2-propanol contact angles on the fabrics decreased.

The resulting coated cotton was used to develop and test a method to assess the repellence of this material. A number of test liquids with surface tensions from 72.75 to 41.21 mN m⁻¹ were prepared by mixing 2-propanol with water and impregnating them with QDs. The roll off behavior of these liquid droplets on the coated cotton swatches mounted on glass slides and tilted at 60° was investigated. The amount of liquid transferred to the fabric after droplet roll off was detected and quantified using the fluorescence intensity of quantum dots that were deposited. The amount of residual liquid left behind on the coated cotton increased as the surface tension of the test liquid decreased, and as the amount of polymer grafted to the cotton decreased. In addition, we were also able to detect the transfer of a trace amount of liquid, e.g. 0.005% of the original droplet size of 10.0 μL containing 4.0 × 10⁻² mg of QD, onto coated fabrics in the traditionally-called dewetting regime. This facile and practical method provides further insights into repellency of textiles and probably other related materials.

Acknowledgements

We thank the Canadian Department of National Defence for sponsoring this research and Drs Eva F. G. Dickson and E. J. Scott Duncan for their helpful discussions. G. L. also thanks the Canada Research Chair program for a Tier I Canada Research Chair Position in Materials Science.

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Footnote

† Electronic supplementary information (ESI) available: SEC trace and ¹H NMR spectrum of the PIPSMA-b-PFOEMA copolymer P1. FTIR spectra of P1 before and after 1 h of sol–gel reaction. DTGA traces for cotton fabric, sol–gelled P1, and coated cotton at a P1 concentration of 5.00 mg mL⁻¹. Photographs of uncoated cotton and coated cotton at a P1 concentration of 5.00 mg mL⁻¹ with 5.0 μL water droplets on top and in a vial with water. Static contact angles measured with QD loaded droplets. QD fluorescence intensity as a function of time for coated cotton fabrics at a P1 concentration of 5.00 mg mL⁻¹. Fluorescence images of fabrics slanted at 60° after roll off of QD-containing f₅ and f₁₀ using cotton fabrics coated at a P1 concentration of 0.30, 0.55, 5.00, and 10.00 mg mL⁻¹. Tables of the amount and the percent of residual liquid left behind on fabrics at slanted at 60° after roll off of QD-containing 10.0 μL droplets. Receding contact angles. See DOI: 10.1039/c5ra20782g

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