Facile fabrication of photoinduced superhydrophobic–superhydrophilic surfaces on cellulose substrate without strength loss

Abstract

A novel strategy was reported on the design and fabrication of functional photosensitive hybrid sols (FPHSs) by non-alcoholic emulsification in the presence of a TiO₂ nanoparticle and photoinitiator via a sol-gel process using tetraethylorthosilicate, γ-methacryloxypropyltrimethoxysilane (MPS) and hydrophobic silane coupling agents as precursors. Smart cellulose substrates with alterable superhydrophobic–superhydrophilic conversion were fabricated using FPHS via the ultraviolet (UV) curing process. The liquid FPHS was photocured into solid gel during UV irradiation for 40 s with MPSs in FPHS, which was verified via Fourier transform infrared spectra. The cellulose substrates were modified with FPHSs, and the water contact angles of the modified cellulose substrates were more than 150°. The superhydrophobicity was improved by the gathering of hydrophobic chains and particle deposition of hybrid gel on the fiber surface. Nevertheless, the water contact angles of the modified cellulose substrates were receded with UV irradiation from 158° to 0° in 200 min, due to TiO₂ photoinduction. The irradiated cellulose substrates were placed in the dark, and the water contact angles were recovered to about 130°, gradually. What is more, the reversible process can be repeated more than eight times. The modified cellulose substrate presented excellent washing fastness, even suffering 10 times washing processing. The mechanical properties, including breaking strength and elongation rate, were improved after the coating and UV curing process, which considerably remedied the defects of the heating curing process on the mechanical properties.

Keywords

superhydrophobic–superhydrophilic ultraviolet curing hybrid sol cellulose substrate

Material surfaces with extreme wettability, such as superhydrophobic surfaces with a water contact angle higher than 150° and superhydrophilic surfaces with a water contact angle near to 0°, have been the subject of significant interest due to their importance in both scientific research and material applications.^1,2 Water droplets on superhydrophobic surfaces will remove contaminants from the surface via the droplets rolling off.^3,4 Conversely, surfaces with a 0° water contact angles can absorb all water droplets. Furthermore, a peculiar surface with alterable superhydrophobic and superhydrophilic characteristics on the same region has extended their applications to the fields of smart clothes, micro-fluidics, biomedical devices and so on.⁵ The peculiar wettability of a material surface is closely relevant to its geometric structures and chemical compositions^2,6 via light irradiation, thermal treatments, electrical potential and pH control.⁷ Among the induced methods, the ultraviolet (UV) photoswitch has attracted high attention because it can be quickly controlled in a convenient and non-contact process.^8,9

TiO₂ nanoparticles have been used as photoinduced material to control the surface wettability via UV photoinduction,^1,10 which has attracted strong interest from many areas.¹¹ A material surface containing TiO₂ can present superhydrophobicity via depositing the surface with low surface energy materials, such as long chain silane coupling agents, or building a surface with special morphologies.^7,12 Such surfaces are treated under different UV and dark conditions to drive superhydrophobic–superhydrophilic conversions.^13–15 Chagas and Weibel¹⁶ prepared smart superhydrophobic/superhydrophilic surfaces of polypropylene (PP) using a trimethoxypropyl silane-functionalized TiO₂ NPs nanocoating. Using UV light as a clean and external stimulus combined with a soft thermal treatment, the surface wettability can be switched from superhydrophobic to superhydrophilic. Zhu et al.¹⁷ produced TiO₂-based superhydrophobic–superhydrophilic patterns by UV or solar irradiation without a photomask. The fabricated superhydrophobic–superhydrophilic patterns with excellent mechanical properties have long lifetimes in outdoor applications or other UV environments because of a self-supply of low surface tension material. Lim et al.¹⁸ presented a facile method for the fabrication of a wetting surface that is photoswitchable from superhydrophobicity to superhydrophilicity, which combines layer-by-layer assembly and the introduction of photoresponsive moieties onto the top surface.

Cotton, which consists of biodegradable cellulose fibers, is one of the most practical fabrics for clothes.¹⁹ Cotton has moisture and water absorbing abilities owing to its microscale porous structure, which makes it suitable as an absorbent material for various applications.^20,21 However, cotton fabrics can be easily stained or contaminated by complex liquids, and suffer from bacterial growth because of the imbibition and natural superhydrophilic properties.⁶ In addition, if the mechanical property of the cotton product is not too high then the cotton suffers complicated chemical processes. An alterable superhydrophobic–superhydrophilic cotton fabric in different stimuli conditions with a higher mechanical property is the most appropriate.

Some efforts regarding alterable hydrophobic–hydrophilic or superhydrophobic–superhydrophilic cellulose substrates have been attempted and reported.^22–24 Sobczyk-Guzenda et al.²⁵ modified the cellulose substrate with a TiO₂ coating, applied with the assistance of the radio frequency plasma enhanced chemical vapor deposition technique. A hydrophilic cellulose substrate became strongly hydrophobic via deposition with a TiO₂ film. When illuminated with UV light, the water wettability of the TiO₂ coated material returned to highly hydrophilic. In this process, the tensile strength of the cellulose substrate was reduced to lower than 32.5 N from the original 89.5 N, and the elongation at maximum load was also reduced from 15.0% to lower than 12.9%. The lower tensile strength limits the application scope of cellulose substrates.

In the previous study, we synthesized different silica or TiO₂ hybrid sols, and they were used to deposit on different fabrics to present excellent UV-switchable wettability via electrochemical deposition¹¹ and other methods.²⁶ Although the wettability sensitivity were enhanced by doped F ions²⁷ or other attempts, nearly all the mechanical properties of modified cellulose samples were decreased. The main reason was that the cellulose fiber was damaged by the acidic sols at a high baking temperature (higher than 140℃). Based on this problem, we attempt to use a UV curing technology to replace the baking process during cellulose modification. Via the UV initiation of double-bond γ-methacryloxypropyltrimethoxysilane (MPS) in the functional hybrid sol, a mild film was deposited on the cellulose fibers. The UV curing performances were explored, and the contact angles of the modified cellulose substrates were analyzed by using methyltriethoxysilane (MTES), N-octyltriethoxysilane (OTES), dodecyltriethoxysilane (DTES) and hexadecyltrimethoxysilane (HDTMS). The damping and recovery of water contact angles under UV light or dark environments were investigated. Significantly, the mechanical properties and the mechanic-reinforced mechanism were investigated in detail.

Experimental details

Chemicals and materials

The tetraethylorthosilicate (TEOS), sodium dodecyl benzene sulfonate (SDBS), hydrochloric acid (HCl) and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd (China) and were analytical grade. HDTMS, OTES, MTES, MPS and DTES were supplied by Qufu Chenguang Chemical Co., Ltd (China). Photoinitiator 2-hydroxy-2-methyl phenyl-propane-1-one was offered by Guangzhou Guanchuan Trade Development Co., Ltd (China). The TiO₂ nanoparticle (P25) was prepared by Shanghai Degussa GmbH (China). The HDTMS, OTES, MTES, MPS, DTES, 2-hydroxy-2-methyl phenyl-propane-1-one and TiO₂ were technical grade. Cotton cellulose fabric weighing 141.0 g/m² was provided by Jiangsu Hongdou Industrial Co., Ltd (China).

Preparation of functional photosensitive hybrid sol

The silica sol was prepared via TEOS hydrolysis in SDBS emulsion.²⁸ Briefly, TEOS (12.0 g) and SDBS (0.8 g/L) were added to 200.0 g deionized water under magnetic stirring at room temperature for 1 h until an emulsion formed. A total of 6 mL of HCl (0.1 mol/L) was dropwise added into the emulsion. The acid silica sol was obtained after stirred at 30℃ for 3 h and aging for 24 h, and then 2% double-bond MPS was dropwise added into the prepared acid silica sol in a flask with a magnetic stirring apparatus at room temperature. After stirring for 1 h, the hydrophobic silane coupling agent and TiO₂ nanoparticle were added into the mixture, respectively. The mixture was dispersed with an ultrasonic dispersion device for 10 min. Finally, the functional photosensitive hybrid sol (FPHS) was stirred at 30℃ for 4 h and aged for 120 h.

Cellulose substrate modification

To induce UV initiation on the cellulose substrate, 2% 2-hydroxy-2-methyl phenyl-propane-1-one was added into the FPHS before being used on cellulose substrates. The cellulose substrates were pre-cleaned in deionized water and then dried at 50℃ for 2 h before other treatments. The pre-cleaned cellulose substrates were impregnated into the FPHS for 5 min and padded two times with a P-130 padder offered by Xiamen Rapid Co., Ltd (China). The wet pick-up was 70–90%. Then the cellulose substrates were dried at 60℃ for 20 min.²⁷ The dried cellulose substrates were uniformly irradiated under UV light with the Intelli-ray 600 UV lightbox produced by USA Uvitron Company (light intensity 50.0 mW/cm², wavelength 253.7 nm) for 0–45 s to cure on the cellulose substrate (Figure 1).

Figure 1.

The mechanisms of functional photosensitive hybrid sol synthesis and modification for the cellulose substrate. TEOS: tetraethylorthosilicate; MPS: γ-methacryloxypropyltrimethoxysilane; UV: ultraviolet.

Photoinduced UV irradiation and dark storage

To convert into superhydrophilicity, the cellulose substrates modified with FPHS were irradiated in UV light (light intensity 21.0 mW/cm², main wavelength 253.7 nm) at about 25℃. When the water contact angle was decreased to 0°, the cellulose substrates were placed in a dark box under ambient environment (relative humidity of 30–40%), and after some time the recovered water contact angle was measured.¹¹

Instruments and characterization

The water contact angle values were recorded after 3 s while the water drop (8 µL) was on the cellulose substrates using the Kruüss DSA100 Drop Shape Analysis System (Germany) under ambient conditions at temperature of 25℃ and relative humidity of 40%. A Thermo Nicolet Nexus Fourier transform infrared (FTIR) spectrophotometer (Thermo Electron Corporation, MA, USA) equipped with an OMNI Sampler and a Ge-on-KBr beamsplitter was used for the FTIR measurements of changes in the characteristic groups on the sol film and cellulose substrate. The scanning electron microscope (SEM) micrographs of cellulose substrates, which were dried and sputter coated with gold, were tested by a JSM-5610 SEM (JEOL Ltd, Tokyo, Japan) under 1000, 4000 and 5000 magnifications, respectively. Thermo-gravimetric analysis (TGA) was measured by a Mettler-Toledo SDTA851e thermo-gravimetric analyzer (Mettler-Toledo, LLC, USA) to present the weight loss of the cellulose substrates with a scanned area from 25℃ to 600℃ at a constant heating rate of 10℃/min in a nitrogen atmosphere.

The washing fastness of the cellulose substrate was tested according to the standard of ISO 105-C10:2007 using 3 g/L soap at 40℃ for 30.0 min with the 12-A washing fastness tester supplied by Wenzhou Darong Textile Instrument Co., Ltd (China). The mechanical properties of the cellulose substrate were performed according to the ISO 13934-1:2013 standard using a YG (B) tester (Darong Textile Instrument Co., Ltd, Wenzhou, China) at the condition of 40% relative humidity. The handle and softness of cellulose substrates were evaluated using a KESFB2 Kawabata Evaluation System-Fabric (KES Kato Tech, Japan), and the changes of the substrate handle were described by the bending rigidity and bending hysteresis moment.

Results and discussion

UV curing performances

The original FPHS was liquid before curing (Figure 2(a)), and the FPHS in the tube flowed distinctly when inclining the tubes of the FPHS. Within the UV curing for 5–20 s, the viscosity of the hybrid sol was increased and the fluidity of FPHS was decreased, leading to some solid gel fastening to the tube bottom (Figures 2(b)–(d)). When the curing time was more than 40 s, there was no liquid hybrid sol in the tube and a cylindrical solid gel was blocked at the tube bottom (Figures 2(e) and (f)). In hybrid sol preparation, the TEOS and silane coupling agents, including double-bond MPS and hydrophobic silane coupling agents, were condensated and crosslinked.

Figure 2.

Ultraviolet curing performances of functional photosensitive hybrid sol containing 6% dodecyltriethoxysilane at different curing times: (a) 0 s (without curing); (b) 5 s; (c) 10 s; (d) 20 s; (e) 40 s; and (f) 60 s.

During UV curing, the double bond in the FPHS was opened and crosslinked into net structure^29,30 (Figure 3) via free radical polymerization assisted with photoinitiator 2-hydroxy-2-methyl phenyl-propane-1-one. With the increased polymerization, the particle size of a liquid sol was improved (Figure 4). The polymerization degree of the net structure was increased, resulting in a phase transformation from liquid to solid.

Figure 3.

Condensation polymerization mechanism of functional photosensitive hybrid sol in acid catalysis under ultraviolet curing.

Figure 4.

Particle size of functional photosensitive hybrid sol containing 6% dodecyltriethoxysilane: (a) ultraviolet (UV) irradiation 0 s; and (b) UV irradiation 10 s.

The compared FPHS sample (a) was heated at 60℃ for 2 h in the vacuum oven and baked at 150℃ for 3 min, and the FPHS sample (b) was irradiated in UV light for 40 s. The heat curing and UV curing gel powders were grinded and tableted, respectively. The characteristic peak at 3437 cm^–1 represented the existence of –OH, which was mainly offered by Si-OH (Figure 5). The characteristic peak around 1636 cm^–1 was due to the stretching vibration of C=C from the double-bond MPS. The characteristic peak of the baked gel sample (a) at 1636 cm^–1 was distinct, whereas the characteristic peak nearly disappeared in the UV cured gel sample (b). This indicated that the UV curing process opened the C=C in double-bond MPS, but there was no effect on the C=C via the baking process for lower excitation energy.

Figure 5.

Four transform infrared spectra of functional photosensitive hybrid sol after baking and ultraviolet (UV) curing.

Water contact angles

The substrates were modified with FPHS containing MTES, OTES, DTES and HDTMS, respectively. All the substrates presented excellent hydrophobic or superhydrophobic (Figure 6(a)). The water contact angles of the cellulose substrates were higher than 150°, and were superhydrophobic. The roll contact angles were between 4.1° and 4.8° after the substrates were modified with FPHS containing MTES, OTES, DTES and HDTMS, respectively. The contact angle properties was similar to the data of the bibliographies, which were higher than 150°.^11,13 Although the cellulose substrates modified with FPHSs containing MTES and HDTMS were hydrophobic, the water contact angles were lower than 150°. With the increase of the hydrophobic functional chain length in the FPHS, the water contact angles of the modified cellulose substrates were increased from 129.7° to 159.5°. Although HDTMS has a cetyl group, the water contact angle was decreased to 143.5°. Usually, the surface tension of a silane coupling agent with a long hydrophobic functional chain is low, so the water contact angle should be increased when the surface tension of the cellulose substrate was decreased. The stability of HDTMS in sol-gel reaction was poor, and its hydrophobic functional chain was easily aggregated, so the water contact angle was lower.

Figure 6.

Water contact angles of cellulose substrates modified with functional photosensitive hybrid sols: (a) containing different silane coupling agents; (b) with different concentrations; (c) with different ultraviolet (UV) curing times; and (d) containing different TiO₂ contents. MTES: methyltriethoxysilane; OTES: N-octyltriethoxysilane; DTES: dodecyltriethoxysilane; HDTMS: hexadecyltrimethoxysilane.

The water contact angles of substrates modified with FPHSs containing different OTES and DTES were above 140.0° (Figure 6(b)). Via the film-forming of FPHS on the cellulose substrate, the hydrophobicity was improved. Assisted with hydrophobic silane coupling agents, the water contact angles of the substrates were increased gradually with the increase of silane coupling agents, and then almost remained constant when the silane coupling agent concentration was higher than 6%. The replacement of –OH groups by hydrophobic groups in silane coupling agents on the silica film was almost completed and no further dehydration or condensation occurred as the silane coupling agent was higher than 6%.²⁸

The cellulose substrate was modified with the FPHSs containing OTES and DTES, and then cured in UV light for different time. The water contact angle of the cured cellulose substrate was increased with the UV curing time in the first 35 s (Figure 6(c)). During the UV curing process, the double bond in the FPHS was opened and crosslinked into the net structure via free radical polymerization. The polymerization reaction increased the irregularity of the network structure, and the surface roughness was increased. According to the Wenzel model, higher roughness of a hydrophobic surface leads to an improved water contact angle,¹¹ so the water contact angle after UV curing was increased. Meanwhile, there was some heating effect during UV curing, and some H₂O in the functional photosensitive hybrid gel would further volatilize. The water droplet presented low wettability to a dry and hydrophobic film surface.

The TiO₂ nanoparticle was added into the FPHS. Although it would not affect the hydrophobic property of the functional photosensitive hybrid gel, it would increase the roughness of the cellulose substrate surface, so the water contact angle property was improved (Figure 6(d)). When the TiO₂ content was higher than 0.3 wt.%, the water contact angle was not increased and the water contact angle remained between 155° and 157°. During the UV curing process, the UV irradiation time was short (less than 50 s), and it mainly affect the C=C in MPS, and the TiO₂ nanoparticle was not photoinduced in the short UV irradiation.

Properties of modified cellulose substrates

The SEM image indicated that the surface of the cellulose fiber was rough, and there were abundant groove constructions and natural distortions (Figure 7(a)).²⁷ The roughness was increased after deposition, and the surface roughness of the modified fiber with DTES (Figure 7(b)) was lower than that of the fiber surface containing OTES (Figure 7(c)). The roughness distribution of the modified cellulose fiber was related to the water contact angle, and superhydrophobicity of the modified cellulose substrates containing OTES and DTES were remarkable.¹¹

Figure 7.

The surfaces of the cellulose substrates modified with functional photosensitive hybrid sols: (a) original cellulose substrate; (b) containing dodecyltriethoxysilane; and (c) containing N-octyltriethoxysilane.

In Figure 8, the absorption peak at 2866 cm⁻¹ in the spectra was attributed to the stretching vibration of the C–H in –CH₂–. The absorption peak intensity of C–H at 2866 cm⁻¹ was weakened, which indicated that the surface –OH of the cellulose fiber was covered with functional photosensitive hybrid gel film. The absorption peaks at 1126 and 899 cm⁻¹ represented the dissymmetry and symmetry stretching vibration absorptions of Si–O–Si.^31,32 All the absorption peaks demonstrated that the functional photosensitive hybrid gel film was deposited on the fiber surface.

Figure 8.

Fourier transform infrared spectra of cellulose substrates: (a) original cellulose substrate; and (b) modified with functional photosensitive hybrid sol containing dodecyltriethoxysilane.

From the TGA curve of the original cellulose substrate (Figure 9), the weight remaining of the original cellulose substrate sintered at 500℃ temperature was 2.67%, which indicated that the cellulose fiber was nearly degraded. Within the modification by FPHS containing DTES, the weight remaining at 500℃ temperature was 10.01%. The SiO₂ and TiO₂ were resistant to heat, and they were stable at 500℃.

Figure 9.

Thermo-gravimetric analysis of cellulose substrates: (a) original cellulose substrate; and (b) modified with functional photosensitive hybrid sol containing dodecyltriethoxysilane.

Switchable wettability

The wettability of the modified cellulose substrate was improved in UV irradiation. The water contact angles were decreased from 158° to 0° in UV light for 200 min (Figure 10(a)), and the cellulose substrate was changed to a superhydrophilic surface. The mechanism of dynamic wettability was that the excited energy of the TiO₂ nanoparticle in the UV irradiation via the generation of e⁻ pairs at the conduction and valence bands is higher than the band gap energy.^33,34 During UV irradiation, excess holes on the TiO₂ surface are trapped on oxygen sites of the lattice.³⁵ Consequently, the bonds between titanium and lattice oxygen become weak^36,37 and they are easily broken and reacted with water molecules,¹³ which will lead to new hydroxyl groups (Equations (1)–(4)). So, the cellulose surface presents remarkable surface hydrophilicity, or superhydrophilicity³⁸

{TiO}_{2} + h ν \to {TiO}_{2} + h^{+} + e^{-}

(1)

h^{+} + e^{-} \to h ν

(2)

H_{2} O + h^{+} \to \cdot OH + H^{+}

(3)

{OH}^{-} + h^{+} \to OH

(4)

Figure 10.

Photoinduced time of the modified cellulose substrates from superhydrophobicity to superhydrophilicity: (a) damping process of the water contact angle; (b) with different silane coupling agents; (c) with different concentrations of silane coupling agents; and (d) with different TiO₂ contents. UV: ultraviolet; MTES: methyltriethoxysilane; OTES: N-octyltriethoxysilane; DTES: dodecyltriethoxysilane; HDTMS: hexadecyltrimethoxysilane.

The cellulose substrates modified with FPHSs containing MTES, OTES, DTES and HDTMS were irradiated under UV exposure; the photoinduced time of the modified cellulose substrate from superhydrophobicity to superhydrophilicity was different. In Figure 10(b), the photoinduced times of the cellulose substrates modified with FPHSs containing OTES and DTES were 190.0 and 200.0 min, respectively, and the photoinduced times of the cellulose substrates modified with FPHSs containing MTES and HDTMS were 150.0 and 130.0 min, respectively. The hydrophobic chains of the silane coupling agents in the FPHSs were not the same, and the water contact angles of the modified cellulose substrates before UV irradiation were also different. The photoinduced time distribution was similar to the water contact angle distribution of the corresponding substrates.

In Figure 10(c), with the increases of the OTES and DTES concentrations, the photoinduced times of the modified cellulose substrates were increased. When the OTES and DTES concentrations were 8%, the photoinduced times were 190 and 200 min, respectively, and the photoinduced time would not be prolonged when the OTES and DTES concentrations were further improved. The increase of the hydrophobic chains would enhance the cellulose substrate hydrophobicity, and when the OTES and DTES concentrations were increased, the hydrophobicity of the modified cellulose substrate was improved and the photoinduced time from superhydrophobicity to superhydrophilicity was increased. When the hydrophobic chain on the cellulose fiber was saturated, a redundant hydrophobic chain would superimposed, and it would not improve the substrate hydrophobicity. So, the photoinduced time would not increase any more.

The TiO₂ content would affect the photoinduced effect. In Figure 10(d), with the increase of TiO₂ content from 0.1% to 0.9%, the photoinduced time would decrease from 280 to 180 min. When the TiO₂ content was higher than 0.5%, the photoinduced time would not decrease. The wettability change was determined by the TiO₂ valence state, and the titanium in the TiO₂ nanoparticle was changed to Ti³⁺ from Ti⁴⁺ via UV photoinduction. With the increase of TiO₂ content, more TiO₂ was photoinduced, which would promote the coating hydrophilicity. If using the superfluous TiO₂ nanoparticle, the redundant TiO₂ nanoparticle was inserted in the interior of the functional photosensitive hybrid gel film, and the internal TiO₂ nanoparticle would not affect the wettability obviously.

The water contact angle of the irradiated cellulose substrate modified with FPHSs containing DTES was increased gradually in the dark environment (Figure 11). The water contact angle was increased to the maximum recovery (about 130.0°) when the irradiated cellulose substrate was placed in the dark for 14 days. The surface of TiO₂ gradually transformed into a metastable state, and some −OH groups were adsorbed. The adsorbed −OH groups were replaced gradually by atmospheric oxygen when the sample was in the dark environment.¹¹ The surface film of the cellulose substrate would revert back to its original state, and the fiber surface wettability recovered to hydrophobic again. So, the water contact angle was increased gradually. Because the replacement of –OH was incomplete, some −OH groups were still on the surface of the reverted cellulose substrate in the dark. So, the hydrophobic property was inferior and the water contact angle was lower than that of the original modified cellulose substrate.

Figure 11.

Water contact angle recovery of cellulose substrate in a dark environment.

The reversible process can be repeated more than eight times, as shown in Figure 12. The contact angles were between 0° and around 130° during all the cycles when the cellulose substrate modified with FPHSs containing DTES was in UV and dark for some time. We can clearly see that there is no obvious degradation of the reversibility after eight cycles. A high stability of the film on the cellulose substrate modified with FPHSs leads to a relatively stable contact angle when the cellulose substrates were in UV light and dark during different cycles.

Figure 12.

Contact angle curves of the modified cellulose substrate in ultraviolet light and dark.

The cellulose substrate modified with FPHSs containing OTES and DTES revealed high surface roughness (Figure 13), and the water contact angle was higher than 150° (Figure 14). Within washing 10 times, the water contact angle of the cellulose substrate was about 120° (Figure 14) and the washed cellulose substrates were still hydrophobic (Figure 13). During the UV curing, the double bond in the FPHS was opened and crosslinked into the net structure, which improved the resistance to washing or friction force.^31,32

Figure 13.

The surface of the cellulose substrates: (a) containing dodecyltriethoxysilane (DTES) and without washing; (b) containing DTES and washing 10 times; (c) containing N-octyltriethoxysilane (OTES) and without washing; and (d) containing OTES and washing 10 times.

Figure 14.

Water contact angles of the modified cellulose substrate during the washing process. DTES: dodecyltriethoxysilane; OTES: N-octyltriethoxysilane.

Mechanical property

Table 1 shows that there was considerable influence of the modified process with FPHS containing DTES on breaking strengths and elongation rates. The breaking strengths of the modified cellulose substrates were increased to 781.3 and 421.0 N from 726.0 and 357.1 N, respectively, and the breaking strengths were enhanced by 7.6% and 17.9% in the warp and weft directions, respectively. During this process, the breaking elongations were significantly increased to 7.4% and 10.6% from 6.2% and 9.0% in the warp direction and weft direction, respectively, and the enhancement rates of breaking elongation were 19.4% and 17.8%, respectively.

Table 1.

Mechanical properties of cellulose substrates

Samples	Breaking strength (N)		Breaking elongation (%)
Samples	Warp	Weft	Warp	Weft
Original cellulose substrate	726.0 ± 15.9	357.1 ± 12.9	6.2 ± 0.5	9.0 ± 0.2
Modified cellulose substrate	781.3 ± 21.3	421.0 ± 16.2	7.4 ± 0.7	10.6 ± 0.3
Improvement	7.6%	17.9%	19.4%	17.8%

Most the mechanical properties of the cellulose substrate modified with functional sol materials declined in our and other previous experiments (Table 2). After summarizing the reasons for the decrease of the breaking strength in Table 2, the main functional sols used in the references were acidic, and the cellulose fiber would be partially damaged by the acidic component¹¹ during the baking process at 140–180℃ (Figure 15). Via the intermolecular forces, the fibers and the deposited films were combined. The mobility of the basic units was confined and the ability of burdening forces was not even,⁴¹ which led to the decrease of strength and deformability of the cellulose substrate.

Table 2.

Mechanical properties of cellulose substrates in previous references

Functional sol	Breaking strength related to control cellulose sample		Breaking elongation related to control cellulose sample		References
Functional sol	Warp	Weft	Warp	Weft	References
Hybrid electrochemical sol	–6.3%	–4.0%	–14.5%	–15.4%	¹¹
Alkaline silica sol, acidic TiO₂ sol and cationic sol	2.2%	–4.8%	–50.0%	–50.0%	³¹
P25 sol	–0.5%	–13.9%	–	–	³⁹
CC18-ZnO sol	–18%		–5%		⁴⁰
cationic gemini silica sol (200℃, 3 min)	–45.6		–8.9%		⁴¹
ZnO sol	–19.7%		–	–	⁴²
DMDAAC-ZnO sol	–29.1%		–	–	⁴²

Figure 15.

Hydrolysis mechanism of the cellulose substrate in acidic and high temperature conditions.

Nevertheless, in this experiment, although the FPHS was acidic, the pH was not too low (pH 5–5.5). We replaced the superacid TiO₂ sol with nearly neutral TiO₂ nanoparticle powder. What is more important is that we used the UV curing process in this experiment to replace the previous heating curing process at a high temperature, and the acid degradation effect was weak. Furthermore, during UV curing, the double bond in the FPHS was opened and crosslinked into the net structure on the cellulose substrate surface via free radical polymerization (Figure 2), and the organic–inorganic hybrid film would remedy the yarn flaw and offer additional tension before the fiber was broken during the stretching process. Meanwhile, the organic components in the hybrid film were combined within the UV curing process, and the toughness of the organic components improved the breaking elongation. Due to the same reason, the bending rigidity and bending hysteresis moment of the modified cellulose substrate were also improved (Table 3). The decreases of bending rigidity and bending hysteresis moment indicated that the modified substrate handle and softness were improved.

Table 3.

Bending rigidity and bending hysteresis moment of cellulose substrates

Samples	Bending rigidity (N·cm²/cm)		Bending hysteresis moment (N·cm/cm)
Samples	Warp	Weft	Warp	Weft
Original cellulose substrate	0.1342	0.0317	0.0764	0.0117
Modified cellulose substrate	0.1083	0.0274	0.0400	0.0161

Conclusions

In this work, a novel FPHS was synthesized via dehydration and condensation with TEOS, MPS and hydrophobic silane coupling agents as the precursors in an acidic system, and the FPHS was used to modify the cellulose substrate via UV curing. The modified cellulose substrate was superhydrophobic, and its water contact angle was higher than 150° when tried to different varieties and concentrations of silane coupling agents, different UV curing times and different TiO₂ contents. The modified fiber surface was rough, and the double bond was opened in the UV curing process. The water contact angle was decreased from 158° to 0° in 200 min when the samples were irradiated under UV light, and the water contact angle of the irradiated cellulose substrate was recovered gradually in a dark environment. The reversible process can be repeated more than eight times. The silane coupling agent variety and concentration, as well as the TiO₂ content, would affect the photoinduced time. The cellulose substrate modified with FPHSs containing OTES and DTES showed excellent washing resistance. The breaking strength and elongation rate of the modified cellulose substrates were higher than that of the original cellulose substrate. The bending rigidity and bending hysteresis moment data indicated that the cellulose substrate handle and softness were improved. The UV curing process sets the stage for fully exploiting the potential of smart cellulose materials to realize controllable superhydrophobic–superhydrophilic conversion performance with reinforced mechanical properties.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (51403083), the China Postdoctoral Science Foundation (2016M590409), the Fundamental Research Funds for the Central Universities (JUSRP51724B), the open project of Key Laboratory of Textile Science & Technology (Donghua University), the Ministry of Education (KLTST201603) and the International Joint Research Laboratory for Advanced Functional Textile Materials of Jiangnan University.

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