Physical Structure Induced Hydrophobicity Analyzed from Electrospinning and Coating Polyvinyl Butyral Films

1 Industrial Research Institute of Nonwovens and Technical Textiles, College of Textiles and Clothing, Qingdao University, Qingdao 266071, China 2State Key Laboratory of Bio-Fibers and Eco-Textiles, Qingdao University, Qingdao 266071, China 3Collaborative Innovation Center for Eco-Textiles of Shandong Province, Qingdao University, Qingdao 266071, China 4Collaborative Innovation Center for Nanomaterials and Optoelectronic Devices, College of Physics, Qingdao University, Qingdao 266071, China


Introduction
The wetting behavior of solid surfaces by a liquid as a very important aspect of surface properties has been a focused interest for a long time [1][2][3][4] and has shown a wide variety of practical applications in various fields, such as protecting clothes [5], medical care [6][7][8], and filtration [9].Consequently, great efforts have been made to find the way to control the surface wettability [10][11][12][13].
Among various materials, polyvinyl butyral (PVB), which is characterized by high adhesion to glass, excellent mechanical strength, excellent biocompatibility, nontoxicity, and good solubility in alcohol, has been applied to many fields such as an adhesive interlayer in safety glass, wound dressing, and coating film [5-7, 10, 11].Moreover, PVB shows both hydrophobic and hydrophilic properties due to its chemical structure with both vinyl butyral group and vinyl hydroxyl group [10,25].And previous study has suggested that electrospun PVB nanofibrous structures and patterns showed larger water contact angle (WCA) than that of the solution casting film [7,10,11].However, these works most focus on the electrospun PVB meshes and rarely discuss the comparison of the structures between the different methods.
In this paper, we focused on the comparison of PVB films made by several methods, spin-coating, bar-coating, and electrospinning.The WCAs on these films were measured and the morphologies and surface roughness of them were also examined.By analyzing these results, we want to find a way to control the surface wettability.
. .Preparation of the Films.The PVB/alcohol solutions were used to produce films by spin-coating, bar-coating, and electrospinning, respectively.During spin-coating process, PVB/alcohol solutions were dripped onto cover slips (20 mm×20 mm) with 5 l and then set on the spin-coater (KW-4A, Institute of Microelectronics of the Chinese Academy of Sciences).The spinning velocities were selected at 250 r/s, 300 r/s, and 350 r/s, and spinning time was set at 30 seconds.In bar-coating process, 10 l PVB/alcohol solutions were dripped onto slide glasses (76 mm×26 mm), and the bars were selected with 6 m, 8 m, and 10 m film thickness.For electrospun meshes, both positive electrospinning and negative electrospinning were processed.PVB/alcohol solutions were firstly loaded into syringes (5 ml) and then set on the electrospinning apparatus (DW-P303-1ACF5 (positive), DW-N303-1ACDF0 (negative), Dongwen, China), the electrospinning voltage was set at 15 kV and -15 kV, and the distances were selected at 15 cm, 20 cm, and 25 cm.
. .Characterization.The static water contact angles (SWCA) of the prepared films were examined by an optical tensiometer (Attension Theta, Biolin Scientific, Germany) with a 2 L water droplet for three sites at room temperature and analysis was performed by fitting drops with a Young-Laplace formula using the Theta software.The surface roughness was also measured by optical tensiometer with 3D surface roughness module.The morphologies of the films were examined by a scanning electron microscope (SEM, Phenom ProX, Phenom Scientific Instruments Co., Ltd., China) at 10 kV, and all samples were coated with gold for 30 s before analysis.The fiber diameters were analyzed by Nano Measurer software.The Fourier transform infrared spectroscopy (FTIR) spectrums were measured by a Thermo Scientific Nicolet iN10 spectrometer.

Results and Discussion
To ensure the wettability of the prepared PVB films, SWCA were measured and shown in Table 1.One can find that the spin-coating and bar-coating PVB films showed hydrophilicity with SWCA <90 ∘ , while the electrospun films showed hydrophobicity with SWCA>90 ∘ , which agreed with the previous study [7,10,11].Moreover, the bar-coating PVB films had larger SWCA than spin-coating ones generally.
The SWCA of electrospun PVB films with positive electrospinning process is mostly larger than that of negative electrospinning.Nevertheless, as seen from Figure 1, the prepared PVB films showed similar absorption peaks, which indicated that these films had same chemical structures.It is interesting that the same material showed different wettability for its different film-forming methods.
To understand the interesting PVB film wettability, we firstly examined the morphologies of selected PVB films (10% concentration) with spin-coating 250 r/s, bar-coating 6m, and positive and negative electrospinning with distance of 15 cm.As shown in Figure 2(a), the spin-coating PVB film showed a macroscopically relative smooth surface with SWCA 78.8 ∘ ; the bar-coating film also had a smooth surface with SWCA 83.3 ∘ (Figure 2(b)).Both smooth films showed hydrophilicity.Meanwhile, both the positive and negative electrospun PVB films showed porous rough surfaces, as can be found in Figures 2(c) and 2(d), and the SWCA were 130.8 ∘ and 135.7 ∘ , respectively.Both the porous rough electrospun films showed hydrophobicity.According to these results, it seems that the surface roughness determined the wettability of the films.
For further investigation, we examined the roughness of the selected films, as shown in Figure 3.It is strange that the macroscopical smooth surfaces shown in Figure 2 have higher roughness than the electrospun films.In our opinion, this case may result from the different surface structures of the films that coating films showed relative dense surface structure, while electrospun films had porous surface structure, as can be seen in Figure 2.For the hydrophobilic coating PVB films [seen in Figures 3(a) and 3(b)], barcoating ones showed higher roughness than the spin-coating ones with r=3.715>2.351,where r is the factor of the surface roughness.According to the Wenzel model [1,22], where   < 90 ∘ , the larger the factor of roughness r is, the smaller   will get, and the increasing roughness will make the film more hydrophilic.The case in coating PVB films followed the Wenzel model; consequently the bar-coating films with more roughness showed smaller SWCA than spin-coating ones.Meanwhile, for the electrospun films, the porous and micro-nano surface structures made the Wenzel model not work.As suggested in Figures 3(c that liquids are assumed to only contact the solid through the top of the asperities and air pockets are assumed to be trapped underneath the liquid [1,22], may be considered; the function can be expressed as where   are the area fractions of the solid on the surface.From (2), one can find that the smaller   is, the more hydrophobicity will be obtained.Specifically to the electrospun PVB films, the smaller the fiber diameter is, the smaller   will be achieved, and then higher hydrophobicity will be shown [5,7,10,11].From lower inset images in Figure 2, we can obviously find that negative electrospun PVB fibers had smaller diameters and less uniform and consequently exhibited higher SWCA.

Conclusions
In conclusion, we have prepared several kinds of PVB films through spin-coating, bar-coating, and electrospinning.The FTIR spectra suggested that these films have the same chemical structures.Meanwhile, these films showed different wettability that coating films were hydrophilicity and electrospun films were hydrophobicity.SEM images indicated that the different films showed different physical surface structures, which may introduce the different wettability.Further investigation on the surface roughness confirmed that the wettability of coating films obeyed the Wenzel model, while the electrospun films agreed with the Cassie model.These results may help to understand the mechanism of the PVB films and then guide for its practical application.

Figure 1 :Figure 2 :
Figure 1: FTIR spectra of PVB films made by different methods.

Table 1 :
The SWCA of the prepared PVB films by different methods.