Well-ordered polymer nano-fibers with self-cleaning property by disturbing crystallization process
© Yang et al.; licensee Springer. 2014
Received: 30 April 2014
Accepted: 26 June 2014
Published: 15 July 2014
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© Yang et al.; licensee Springer. 2014
Received: 30 April 2014
Accepted: 26 June 2014
Published: 15 July 2014
Bionic self-cleaning surfaces with well-ordered polymer nano-fibers are firstly fabricated by disturbing crystallization during one-step coating-curing process. Orderly thin (100 nm) and long (5–10 μm) polymer nano-fibers with a certain direction are fabricated by external macroscopic force (Fblow) interference introduced by H2 gas flow, leading to superior superhydrophobicity with a water contact angle (WCA) of 170° and a water sliding angle (WSA) of 0-1°. In contrast, nano-wires and nano-bridges (1–8 μm in length/10-80 nm in width) are generated by “spinning/stretching” under internal microscopic force (FT) interference due to significant temperature difference in the non-uniform cooling medium. The findings provide a novel theoretical basis for controllable polymer “bionic lotus” surface and will further promote practical application in many engineering fields such as drag-reduction and anti-icing.
Bionic superhydrophobic (self-cleaning) surfaces with micrometer-nanometer-scale binary structure (MNBS) have aroused great interest of science and engineering fields[1–3], which can be attributed to their potential application prospects such as drag reduction on ship hulls, anti-biofouling in maritime industry, and anti-icing for power transmission. Their superhydrophobicity (a water contact angle (WCA) larger than 150° and a water sliding angle (WSA) less than 10°) strongly depends on MNBS structure[7, 8]. In the past few decades, many conventional attempts have been done to fabricate superhydrophobic surfaces with MNBS structure, such as creating a rough and well-ordered metallic or inorganic surface covered with low surface energy molecules, which is called two-step methods[9–14]. However, these methods usually are applied to small-scale substrates at severe conditions, and the surfaces did not exhibit long-term stability in the acid/alkali environment, thus greatly limiting their applications in practical engineering fields. On the other hand, a very simple one-step method involving solvent evaporation to fabricate a polymer superhydrophobic surface with disordered microstructure has been reported[15–17]; however, it is easily scraped off due to the weak cohesion between the coating and substrate and the low resistance to high and low temperature alternation, in addition no long-term stability over a wide pH range (such as acid rain) was achieved. In our previous work, we firstly demonstrated that bionic superhydrophobic poly-(tetrafluoroethylene)/poly(phenylene sulfide) (PTFE/PPS) coating surfaces with long-term stability, high cohesive strength, and anti-temperature change can be prepared by a simple, inexpensive, and conventional coating-curing process[18–20]. However, the nanometer-scale structure on these superhydrophobic PTFE/PPS coating was basically cross-linking and disorderly, leading to great obstacles for further exploration on its anti-icing mechanism. Recently, Wang and coworkers have reported that robust self-cleaning coatings with well-ordered arrays were specially prepared by grafting cross-linked polymers on the silicon wafer surfaces to investigate their anti-icing mechanisms[21, 22]. According to the above researches, up to now, the mechanism for self-cleaning surfaces with well-ordered polymer nano-fibers on various large-scale substrates has not been completely understood, and systematic study on it will significantly help explore new methods for polymer superhydrophobic surfaces in practical severe engineering fields.
Through the past 5 years' research, it is firstly found that bionic self-cleaning surfaces with well-ordered polymer nano-wires/fibers can be fabricated by disturbing polymer crystallization during one-step coating-curing process. Both the external macroscopic force and internal microscopic force interferences on polymer aggregates can significantly affect the nucleation and crystal growth of polymer chains under various cooling conditions. Orderly polymer nano-fibers (5 to 10 μm in length/100 nm in width) with a certain direction are obtained due to an external macroscopic force ‘Fblow,’ which is on the same direction as the H2 gas flow. This orderly MNBS structure results in the coating with superior superhydrophobicity (WCA of 170° and WSA 0° to 1°), very similar with ‘lotus effect.’ More particularly, well-ordered nano-wires and nano-bridges (1 to 8 μm in length/10 to 80 nm in width) are generated at the non-continuous zone due to an internal microscopic tensile force (FT) by severe uneven shrinkage of adjacent continuous phases in the non-uniform medium (quenched in dry ice). The novel method for well-ordered polymer nano-fiber will provide a theoretical basis for other polymer self-cleaning surfaces with MNBS texture on various metal substrates and largely promote their practical application in many fields such as drag-reduction and anti-icing.
Bionic lotus polymer surfaces were fabricated through engineering materials, such as stainless steel or other metal substrates (Al/Cu), by using a certain volume of water-soluble PTFE emulsion and polyphenylene sulfide dispersion in mixed solvent (distilled water/ethanol/isobutyl alcohol in a volume fraction of 2:5:1), non-ionic surfactant (octylphenol polyoxyethylene ether: (C8H17-Ph-O(C2H4O)nH, n ~ 10), and industrial raw material ammonium carbonate ((NH4)2CO3)[18, 20]. The steel/alumina/copper block was polished with 500# and 900# sand papers in turn, and then cleaned with acetone in an ultrasonic bath for 5 min. The wet coatings on stainless steel or various metal substrate blocks were prepared by spraying the coating precursors with 0.2 MPa nitrogen gas and curing at temperature 150°C for 1 h and 390°C for 1.5 h.
In order to investigate the impact of external macroscopic force interference on polymer nano-fibers, pure PTFE coating (P1 coating) sample was naturally cooled to 20°C in the sintering furnace after curing at 390°C for 1.5 h. In contrast to P1 coating, H2 gas flow was passed into the sintering furnace during the same curing and cooling process as P1 coating for PTFE/PPS superhydrophobic coating (P2 coating) sample.
Various cooling conditions for superhydrophobic polymer coatings after curing
Crystallization interference methods
Thermal conductivity of the mediums
Quenched in the air at 20°C
K ≈ 0.026 [W/(m K)]
Quenched in the mixture of dry ice and ethanol at -60°C
K ≈ 0.24 [W/(m K)]
Quenched in the pure dry ice at -78.5°C
K ≈ 0.099 [W/(m K)]
Microstructures of the bionic lotus polymer coating surfaces were observed by a scanning electron microscopy (JSM-5600LV and field emission scanning electron microscopy (FE-SEM), JEOL, Akishima, Japan). Compositions of the surface of pure PTFE and PTFE/PPS coatings were analyzed by an X-ray photoelectron spectroscopy (XPS) on a VG Escalab 210 (VG Scientific, East Grinstead, UK) spectrometer with a Mg Ka X-ray source (1253.6 eV). The water static contact angle (WCA) and water sliding angle (WSA) of distilled water droplets of 5 μL on the superhydrophobic coating samples were tested by a contact angle apparatus (DSA-100, KRÜSS GmbH, Hamburg, Germany). Morphologies of the water droplets of 5 μL on the coatings were recorded with a digital camera.
In our previous work, we have demonstrated a simple and conventional coating-curing process to create PTFE/PPS superhydrophobic coatings with both MNBS roughness and the lowest surface energy hydrophobic groups (-CF3) on engineering materials such as stainless steel and other metals[18, 20]. However, the willow-leaf-like nanofibers are mostly cross-linking and disorderly, and the formation of these nanofibers is proposed to occur by means of a liquid-crystal ‘templating’ mechanism[24–26]. The method and mechanism for controllable fabrication of well-ordered nanofibers on the PTFE/PPS superhydrophobic coatings have always been a mystery and huge challenge for their engineering applications. In this work, we firstly found that external macroscopic force interference will help in the formation of well-ordered nanofibers.
When PPS resin was added to PTFE coating (P2 coating), micrometer scale structure of porous gel network with micropapillae and isolated islands were generated. Micropores (approximately 60 μm in diameter) and micropapillae (20 to 30 μm in diameter) were scattered on the surface of porous gel network, which were similar with cauliflower pattern (Figure 1d). This porous structure could be attributed to phase separation of PPS phase[18, 20, 24]. Furthermore, thin and long PTFE nano-fibers with dimensions of 5 to 10 μm in length and 100 nm in width exhibited a needle-like morphology. They were distributed layer by layer on the surface of P2 coating (Figure 1e,f). The fluorine (F) was enriched at the top surface of P1 and P2 coating, as shown by the peak at 691.1 eV in the XPS survey spectra (Figure 2a). In addition, the C1s peak for P2 coating observed at 293.5 eV binding energy (C-F3) is similar to the peak at 292.1 eV (C-F2) for P1 coating (Figure 2b)[27, 28]. The above data indicates the composition of the nano-fibers on P2 coating surface is mainly PTFE.
In our previous work, disorderly willow-like PTFE nano-fibers (20 to 30 μm in width) formed on the PTFE/PPS coating during the cooling process in the furnace that was exposed to air[18, 20]. In our current work, these PTFE nano-fibers of P2 coating distinctly extended at a certain direction under continuous H2 gas flow; therefore, nano-wires and ‘nano-bridges’ formed with good directional consistency as well as uniform nano-pores (approximately 100 to 500 nm in width). In conclusion, the P2 coating surface shows superior superhydrophobicity as verified by WCA (170°) and WSA (0° to 1°) values.
Compared with P1 coating with only nano-scale fiber structure, nano-wires and nano-bridges with good directional consistency covered the microscale papillae and the interface between them on P2 coating surface, leading to formation of uniform nano-scale pores (100 to 500 nm in width). As large amount of air was captured by the nano-scale pores, the actual contact area between the water droplet and the coating surface greatly decreased[29, 30]; therefore, the WCA of P2 coating increased. Moreover, the adhesion of water droplets on the orderly thin and long nano-fibers was weakened resulting in the decrease of contact angle hysteresis; therefore, water droplets on P2 coating rapidly rolled down. Furthermore, the P2 coating shows better superhydrophobicity than the PTFE/PPS coating (WCA of 165° and WSA of 5°) by the same composition and curing process. It is mainly because external macroscopic force interference (H2 gas flow) can help to form MNBS structure with well-ordered nano-bridges and uniform nano-pores (approximately 100 to 500 nm in width) (Figure 1f). Therefore, external macroscopic force interference by H2 gas flow during the curing and cooling processes can be a good new method for controllable fabrication of well-ordered polymer MNBS structure with lotus effect.
Thus, a new stretching force Fblowy was added to the polymer chains. Therefore, polymer nano-fibers were stretched at a greater extent compared with P1 coating along the direction of Fblowy, leading to much thinner and longer ‘nano-needles’ and nano-bridges (100 nm in width/5 to 10 μm in length).
In our previous work, we have found that a higher curing temperature and longer cooling time resulted in longer crystallizing process during coating cooling process, which is beneficial to create the willow-leaf-like or wheat-haulm-leaf-like micro/nano-fiber on the atop surface of PTFE/PPS superhydrophobic coatings. Moreover, the PTFE/PPS coating was hardened in H2O after curing at 380°C to demonstrate the mechanism of the creation of micro-nano-scale binary structures (i.e., liquid-crystal ‘templating’ mechanism). The atop surface of the PTFE/PPS coating by hardening in H2O was covered with micro/nano-fluorocarbon papillae textures of 200 to 800 nm in diameter compared with that produced by natural cooling in air[18, 20]. However, the effect of internal microscopic force during the quenching process (crystallization process) on the nano-scale structure of the PTFE/PPS coating has still not been understood and systematically investigated.
As the nano-scale pores between dense nano-papules and nano-spheres stacked on the micro-scale papillae of Q1, Q2 and Q3 coating were much smaller than the pores between orderly thin and long nano-fibers on P2 coating, leading to reduction of the amount of air captured by the pores; thus, the contact area between the water droplet and the coating surfaces increased[29, 30], and as a result, the WCA of Q1, Q2, and Q3 coating was smaller than P2 coating by more than 10°. In addition, the adhesion of water droplets on Q1, Q2, and Q3 coating was greater than that of P2 coating, due to poor directional consistency of nano-papules on Q1, Q2, and Q3 coating. Thus, the contact angle hysteresis of water droplets increased, and water droplets can be placed upside down on Q1, Q2, and Q3 coating. In conclusion, polymer surfaces with nano-fiber MNBS texture generated by external macroscopic force interference possessed superior non-wettability and superhydrophobicity compared with polymer surfaces with ‘nano-papules MNBS texture’ obtained by internal microscopic force interference.
Compared to Q1 coating, similar crystallization process took place in Q2 coating. The temperature of Q2 coating was dramatically reduced to about -60°C within just a few seconds (Table 1). It is believed that the cooling rate of the coating samples is closely related with the thermal conductivity of the cooling mediums. The nucleation and crystal growth processes of the PTFE aggregates were inhibited at a greater extent due to higher thermal conductivity compared to Q1 coating (Table 1), as the thermal motion of PTFE aggregates were greatly suppressed, and therefore, there was not enough time for the PTFE aggregates to crystallize and grow to form nano-fibers (Figure 4d,e)[31, 32]. On the other hand, there were large amount of protruding defects with high energy on the rough discontinuous interface between the gel network in Q2 coating (Figure 4d,f), which promote the nucleation and crystal growth of the PTFE aggregates. Thus, polymer nano-spheres/papules coexisted with smaller nano-fiber segments at the end of the cooling process.
In comparison to Q1 and Q2 coating, the Q3 coating was quenched at -78.5°C in the non-uniform medium (pure dry ice) after the same curing process. The smallest polymer nano-papules (20 to 100 nm in diameter) were scattered most uniformly and densely on the continuous zone due to the highest cooling rate (Table 1). In addition, cracks/gaps were generated at the discontinuous interface (discontinuous zone) (Figure 5a,d), which can be attributed to shrinkage tension from adjacent continuous phase (continuous zone) during the abrupt intense cooling process. Thus, PTFE macromolecular chains covered on the discontinuous zone crystallized similar with Q1 and Q2 coating, and they were rapidly ‘spinned/stretched’ to form more slender polymer nano-wires and nano-bridges (10 to 80 nm in diameter), as shown in Figure 5e,f,g,h.
Where E is Young's modulus, al is coefficient of linear expansion, and T0 and T1 are the initial and final temperatures, respectively. The force FT was derived from the intense shrinkage of surrounding macromolecular chains on the cooling process. As the temperature decreased at the same rate for the continuous zones during the whole quenching (crystallization) processes, F s and FT were at the equilibrium state, respectively (ΣF s ≈ 0, ΣFT ≈ 0); therefore, the crystallization of polymer chains at continuous zone of Q1, Q2, and Q3 coating was in an unconstrained environment similar with P1 coating. However, the crystal growth of polymer chains was different because crystallization time of Q1, Q2, and Q3 coating was much shorter than P1 coating (Table 1). Therefore, only nano-spheres/papules formed in the continuous zone for Q1, Q2, and Q3 coating. Moreover, increasing the cooling rate gradually from Q1 to Q3 coating (Table 1) resulted in a size reduction of polymer nano-spheres with a higher degree of overlap.
On the other hand, for the discontinuous zone of Q1, Q2, and Q3 coating (Figures 4 and5) between the porous gel network and micropapillae, the nucleation and crystal growth of polymer chains were promoted because of high interfacial energy. At the same time, the cooling time in the discontinuous zone was longer than the continuous zone because of less exposure in the cooling medium. Although a tensile force (FT) was generated by the uneven shrinkage from adjacent continuous phase of the coatings under the quenching interference[35–37], FT was much smaller than the critical value (Fcr) for both Q1 and Q2 coating. Thus, the crystallization process of polymer chains was dominated by the crystallization driving force and crystallization time[32, 38]; therefore, nano-willow and nano-fiber segments were obtained in the discontinuous zone of Q1 coating, while nano-spheres/papules coexisted with smaller nano-fiber segments in the discontinuous zone of Q2 coating.
However, when Q3 coating was quenched in a non-uniform medium interference, the polymer chains at discontinuous zone suffered much larger tensile force FT than the discontinuous zone of Q1 and Q2 coating, due to the significant temperature difference between the continuous zone and discontinuous zone (Table 1). The tensile force FT was large enough (FT> > Fcr, and ΣFT> > 0) to pull the discontinuous zone off to form cracks and gaps, as shown by the discontinuous zone in Figures 5e,f,g,h and6b. Therefore, nano-wires and nano-bridges can be formed by spinning polymer aggregates (Figure 5e,f,g,h).
MNBS texture and surface behaviors of the coatings
Disordered nano-grass (500 nm in width)
Well-ordered nano-fibers (5 to 10 μm in length/100 nm in width)
Well-ordered nano-fibers (5 to 10 μm in length/100 nm in width)
0 to 1
Nano-scale spheres/papules (80 to 200 nm in diameter)
Willow-like nano-scale segments (approximately 1 μm in length/100 to 500 nm in width)
Nano-scale spheres/papules (60 to 150 nm in diameter)
Nano-scale fiber segments (100 to 500 nm in length/200 to 400 nm in width)
Nano-scale spheres/papules (20 to 100 nm in diameter)
Orderly nano-scale wires/bridges (1 to 8 μm in length/10 to 80 nm in width)
By disturbing crystallization during one-step coating-curing process, bionic lotus surfaces with controllable polymer nano-spheres/papules, nano-wires/fibers were firstly fabricated. It is demonstrated that both macroscopic force interference and internal microscopic force interference on polymer aggregates under different cooling conditions will significantly affect the crystallization of polymer chains. Polymer chains stretched and elongated freely to form a disordered micro-nano-scale grass/leaf-like morphologies in pure PTFE coating (P1 coating), while orderly polymer nano-fibers (100 nm in length/5 to 10 μm in width) with a certain direction are obtained by the force Fblow along the direction of H2 gas flow. During the quenching process in the uniform and non-uniform mediums, nano-papules/spheres (20 to 200 nm in diameter) formed in the continuous zone, while polymer aggregates are partially stretched to nano-fiber segments (1 μm in length/100 to 500 nm in width) during the crystallization process in the discontinuous zone.
However, by polymer crystallization interference in the non-uniform medium, the polymer chains at discontinuous zone of Q3 coating suffered much greater tensile force (FT) in comparison to Q1 and Q2 coating, which can be attributed to the temperature difference between the continuous zone and discontinuous zone. The tensile force was large enough (FT> > Fcr and ΣFT> > 0) to generate cracks and gaps in the discontinuous zone for Q3 coating. Therefore, nano-wires and nano-bridges (1 to 8 μm in length/10 to 80 nm in width) formed. We bring a novel perspective to controllable polymer nano-fibers; this study will provide a theoretical basis for polymer superhydrophobic surface with MNBS texture and promote development of polymer superhydrophobic surfaces in many engineering fields such as drag reduction and anti-icing.
water contact angle
water sliding angle
micrometer-nanometer-scale binary structure
scanning electron microscopy
X-ray photoelectron spectroscopy.
The authors thank Chongqing Key Scientific and Technological Program Project (No. cstc2011ggC0037) and the ‘Western Light’ Talent Key Projects of the Chinese Academy of Sciences (No. 3ZR12BH010) for the financial support and Dr. Zakaria A. Mirza and Dr. Wenjing Wang for the instructive and fruitful discussions.
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