Polyvinyl alcohol (PVA) is a highly hydrophilic, biocompatible, and biodegradable polymer [1
] with good chemical stability and mass transfer properties [4
]. PVA nanofibers can be used as wound dressings, drug carriers, biomedical materials, and matrices for tissue regeneration [7
]. The drawback of PVA nanofiber films is with respect to their low mechanical properties. The addition of nanofillers can improve mechanical, electrical, thermal, and optical properties. For example, graphene (GR) is one commonly used nanofiller [10
] capable of increasing the mechanical properties considerably and retaining the intrinsic biocompatibility, which massively strengthens the polymer matrix composites [12
]. Graphene also features a high specific surface area, surface conductivity, and transmission capacity, and even accelerates the transmission of drugs and target cells [14
Nanofibers have a tremendously high specific surface area and mass ratio, both of which are inversely proportional to the diameter, and can achieve a greater diameter ratio and porosity [20
]. Hence, nanofibers are commonly seen in biomedical, environmental, and optoelectronic applications [23
]. In addition to the micro/nano processing methods including photolithography, electron beam exposure, and ion beam cutting, nanofibers can also be produced through vapor deposition methods such as the template method, self-assembly solution growth method, nanoimprinting, and electrospun [24
]. By contrast, the electrospun technique is a newer and more efficient, low-cost, non-polluting method that has been proven to be the most effective and direct technique. Electrospun nanofibers, which have been widely used due to their efficient properties [26
], can be produced by needleless electrospun and needle electrospun.
Needleless electrospun avoids clogged needles and magnificently increases the spinning efficiency and the yields of nanofibers [27
]. The process of needleless electrospun has undergone development. In addition to the magnetic fluid auxiliary electrospun [28
] and bubble electrospun [29
], other needleless electrospun methods use different spinnerets such as cylinders [30
], conical coils [31
], pyramids [32
], disk nozzles, and spirals [33
]. The drawback is that the jet flow is directly drawn from the free surface of the electrospun liquid to form nanofibers, and the unpredictable process difficult to manage [35
]. Changing the spinning electrode is one measure to secure the spinning process to a certain extent. For example, Niu et al. invented a spinning electrode in a spiral line and obtained a more powerful and more even electric field surrounding the jet than that of cylinder and disk nozzles electrodes. The nanofibers were of better quality and could be produced in greater quantity [33
]. The advanced study by Huang et al. produced GR-PVA nanofibers using the electrospun technique, and the microscopic structure of graphene nanosheets (GNS)/PVA nanofibers was observed. The average diameter of the nanofibers was 371 nm [7
]. Golafshan et al. investigated the graphene/poly (vinyl alcohol)/sodium alginate (Gr-AP) fibrous scaffolds for engineering neural constructs and found that the scaffolds that were composed of 1 wt % Gr-AP had an average nanofiber diameter of 296 ± 40 nm [36
Nevertheless, there are relatively fewer studies incorporating needleless electrospun with the preparation of PVA/GR nanofiber films. In this study, the custom-made copper wires are used as the spinning electrode for the electrospun of the PVA/GR nanofiber films with a finer diameter, and the nanofiber films are then evaluated in terms of the potential of mass production. The influences of viscosity and conductivity of the PVA/GR mixtures on the morphology and diameter as well as the influence of the content of graphene on the wettability, thermal stability, electric conduction, and electromagnetic shielding performance of the PVA/GR nanofiber films are evaluated.
Polyvinyl alcohol (PVA, Changchun Chemical, Jiangsu, China) was purchased with a molecular weight of 84,000–89,000 g/mol. Sodium dodecyl sulfate (SDS) was purchased from Shanghai Macklin Biochemical Co. Ltd, Shanghai, China. Graphene (GR, P-ML20) was purchased from Enerage Inc., Yilan, Taiwan.
2.2. Preparation of PVA/GR Nanofiber Films
Graphene (0, 0.01, 0.1, 0.25, 0.5, 1, and 2 wt %) was added to 1 wt % SDS with ultrasonic treatment for 3 h, after which PVA powders were added with magnetic stirring at 90 °C for 2 h and ultrasonic treatment for 3 h, forming different PVA/GR mixtures. Based on our previous study on the spinnability of PVA using a linear electrode, the PVA solution had a specified concentration of 7.5 wt % [37
]. The electric conductivity and viscosity of the PVA/GR mixture were measured using a portable multiple parameter tester (ST3100MZH/F, OHAUS, Pine Brook, NJ, USA) and a digital viscosity meter (Bangxi Instrument Technology, Shanghai, China). Afterwards, the PVA/GR mixtures underwent needleless electrospun into nanofiber films at 25 °C with a humidity of 23% using a linear electrospun device which included a linear spinning head, a high-voltage power supply, and a grounded mesh collector (see Figure 1
). The linear nozzle had a length of 15 cm and a diameter of 0.8 mm. The rate of the linear spinning head was 72 r/h, with a spinning voltage of 70 kV and a spinning distance of 30 cm [37
]. The pure PVA nanofiber films represented the control group and the PVA/GR nanofiber films the experimental group. Both the control group and the experimental group were adhered with aluminum foil for electromagnetic interference shielding effectiveness (EMSE) measurement only.
2.3. Morphology and Characterizations of PVA/GR Nanofiber Film
Scanning electron microscopy (SEM, TM3030, HITACHI, Tokyo, Japan) was used to observe the morphology of the nanofibers. A bundle of 100 nanofibers was used to compute the average diameter. A surface contact angle instrument (JC2000DM, Shanghai Zhongchen Digital Technic Apparatus, Shanghai, China) and deionized water were used to measure the surface contact angle at 25 °C every 10 seconds, thereby examining the wettability of the PVA/GR nanofiber films. The thermogravimetric (TG) measurement was conducted using a thermogravimetric analyzer (TG 209F3, NETZSCH, Bavaria, Germany) with nitrogen gas at a flow rate of 60 mL/min. The relative mass loss of the samples was recorded from 25 °C to 700 °C with a heating rate of 10 °C/ min, thereby characterizing the thermal stability of the PVA/GR nanofiber films. A surface resistance instrument (RT-1000, OHM-STAT, Static Solutions Inc., Hudson, NY, USA) was used to measure the surface resistivity of the PVA/GR nanofiber films as specified in JIS L1094. The instrument equipped a 5-pound weight ensured that the two parallel electrodes were in good contact with the surface of the sample. Twenty samples for each specification were taken for the mean. The EMSE of PVA/GR nanofiber films shielding electromagnetic waves at frequencies between 0.1 MHz and 1.5 GHz was measured using an EMSE tester (EM-2107A, TS RF Instrument, Taoyuan, Taiwan) as specified in ASTM D4935. The cylinder samples had a diameter of 80 mm.
A custom-made linear electrode-electrospun technique is used to accomplish the controllable large-scale preparation of electrospun nanofiber membranes. The viscosity and conductivity of the mixture as well as the diameter and properties of the PVA/GR nanofiber films are evaluated in order to examine the influence of the content of graphene. The viscosity of the PVA/GR mixtures first decreases and then increases when the content of graphene increases, but the opposite is the case for the conductivity of the PVA/GR mixtures. The average diameter of the nanofibers is dependent on the conductivity of the mixture, and a small amount of graphene improves the fineness of the PVA/GR nanofibers. By contrast, an excessive amount of graphene causes bead-shaped and merged nanofibers. Specifically, 0.1 wt % graphene generates the optimal PVA/GR nanofiber films with 120-nm-thick nanofibers. Moreover, the addition of graphene has a negative influence on the surface contact angle. The PVA/GR nanofiber films containing 0.1 wt % graphene have a surface contact angle of 31°, which is 12° lower than that of the pure PVA nanofiber films. It is also equivalent to a greater wettability as a full saturation takes 190 s. The presence of graphene strengthens the thermal stability of PVA/GR nanofiber films at second and third stages, and the decomposition temperature reaches 255.2 °C. However, excessive graphene adversely affects the decomposition temperature. Compared to the pure PVA nanofiber films with a surface resistivity of 6.26 × 1010 (ohms/sq), the PVA/GR nanofiber films have a surface resistivity of 4.99 × 104 (ohms/sq) due to the presence of graphene. Specifically, PVA/GR nanofiber films containing 0.1 wt % graphene have a surface resistivity of 2.60 × 109 (ohms/sq), a maximum yield of 2.24 g/h, and an EMSE of 114 dB which is 21 dB greater than that of the control group. The test results serve a useful reference for the mass production of PVA/GR nanofiber films in the future that can be applied to the medical hygiene field.