Dynamic wettability and contact angles of …

1 1. IntroductionPVDF has been intensively studied due to its excel-lent bulk properties, such as high electric resistanceas well as good thermostability, light weight andgood processability [1 3]. These properties havemade PVDF increasingly used in various fieldssuch as filtration, air cleaning, and rechargeablebatteries. These applications require materials withwell-defined properties and functionalities. Nano-fibers produced by electrospinning have attracted agreat deal of attentions in these applications due toits remarkable properties, such as small fiber diam-eters, porous structures as well as high surface area[4, 5].

2 However, their applications are hampered inmany cases because of its poor wettability andadhesion property with other materials [6].In recent years, various techniques have been triedto improve the wettability of PVDF materials, suchas plasma treatment, iron irradiation and sputtercoating [6, 7]. In these modifications, plasma-induced graft polymerization of vinyl monomershas been found to be an extremely attractive tech-nique for chemically modifying the surfaces ofpolymeric materials [8]. Although both surfaceenergy and surface roughness are the dominant fac-tors for wettability of materials, surface roughnessis the key factor once the components of materialshave been this study, electrospun PVDF nanofiber mem-branes were modified by plasma-induced graftingof acrylic acid to improve their wettablity.

3 ThePVDF membranes with different structures were551*Corresponding author, e-mail: BME-PTDynamic wettability and contact angles of poly(vinylidenefluoride) nanofiber membranes grafted with acrylic acidF. L. Huang1, Q. Q. Wang1, Q. F. Wei1*, W. D. Gao1, H. Y. Shou2, S. D. Jiang11 Key Laboratory of Eco-Textiles(Jiangnan University), Ministry of Education, 214122 Wuxi, China2 Zhejiang Province New Textile Research & Development Emphasised Laboratory, Zhejiang Textile & Garment Science& Technology Co., Ltd., 310009 Hangzhou, ChinaReceived 18 March 2010; accepted in revised form 20 May has been recognized as one of the most important properties of fibrous materials for both fundamen-tal and practical applications.

4 In this study, the plasma induced grafting of acrylic acid (AAc) was applied to improve thewettability of the electrospun poly(vinylidene fluoride) (PVDF) nanofiber membranes. The diameter and chemical struc-ture of the modified PVDF nanofibers were characterized by scanning electron microscopy (SEM) and Fourier transforminfrared (FTIR). Nitrogen adsorption based on BET (Brunauer, Emmett and Teller) principle was employed to measure thespecific surface areas and porosities of the modified nanofiber membrances. The contact angles of the modified membranewere evaluated by drop shape analysis (DSA) and the modified Washburn method. The dependence of contact angles onspecific surface area and porosity was also discussed in this paper.

5 Water adsorptions were used to evaluate the dynamicwetting behavior of the grafted membranes by a Dynamic adsorption apparatus (CDCA100-F). The experimental resultsrevealed that the wettablity of the modified PVDF membrane was significantly affected by both surface and porous :polymer membranes, electrospinning, contact angles , poly (vinylidene fluoride)eXPRESS Polymer Letters , (2010) 551 558 Available online at : by electrospinning for the investigationinto the relationship between Dynamic wettablityand fibrous wettability of a material can be characterizedby contact angles . Two types of measurements,drop shape analysis and Washburn method [9],have been widely used to characterize the contactangles.

6 However, due to the complicated surfacestructures, few literatures have focused on the sur-face contact angle of nanofiber membrane. Andalso, the complicated internal geometry in thenanofiber membrane has also been the obstructivefactor for the analysis of Washburn contact this work, the Washburn equation was modifiedto evaluate the porous contact angles of the graftedPVDF nanofiber membrane, and the effect ofporous structures on contact angles was also Materials PVDF with average molecular weight (Mn) 105g mol 1was purchased from Shanghai 3 FNew Materials Co., Ltd (Shanghai, China).

7 N,N-Dimethyl formamide (DMF), acetone and acrylicacid (AAc) were supplied by Sinopharm ChemicalReagent Co., Ltd (Shanghai, China) and used with-out further Preparation of PVDF nanofibersPVDF nanofiber membranes with four differentfiber diameters were prepared in this study. PVDFwas dissolved in a mixture of DMF and acetone(20/80, w/w) to make 15% (wt) solutions for apparatus for electrospinning included a plasticsyringe, an 18 gauge stainless steel needle, amicroinfusion pump (Medical Instrument Co., Zhe-jiang, China), a high-voltage power supply (Dong-wen Co., Tianjing, China), and an aluminum foil asthe fiber collector.

8 The ejection rate of the solutionwas set at ml/h, and the distance between theneedle tip and the fiber collector was was performed at the electrostaticvoltages of 8, 12, 16 and 20 kV respectively withthe adjustable-voltage power supply. The electro-spinning was performed in a fume chamber at thetemperature of 20 C and humidity of 45%. Afterthree hours deposition, all the membranes were col-lected on the aluminium foil substrate. The mem-branes were dried in vacuum to remove the membranes were then cut at the weight Plasma-induced grafting of PVDF nanofibersThe process of plasma pretreatment followed bysurface grafting with acrylic acid (AAc) on themembrane is described in Figure 1.

9 The nanofibermembrane was treated with an argon plasma whichwas performed in a HD-1A vertical plasma treat-ment machine (Changzhou Shitai Co, China). Thepretreatment was performed at a pressure of 15 Paand power of 75 W for 120 s. After being taken outfrom the reactor, the sample was allowed to stay inair atmosphere for 30 min and then immersed in asolution of acrylic acid (30%, wt). The graftingreaction was carried out by placing ampoules at60 C temperature in a water bath for 2 h. The sam-ples were, then, washed extensively in deionizedwater at 60 C in a water bath to remove the poly(acrylic acid) which was not covalently bound tothe PVDF surface.

10 The membranes were then driedin an oven at 70 SEM and FTIRThe fibrous structures of the nanofibers were exam-ined using a scanning electron microscopy (SEM,JEOL JSM-5610LV, Japan). Before SEM imaging,552 Huang et al. eXPRESS Polymer Letters , (2010) 551 558 Figure diagram of pretreatment and graftingon PVDF nanfiber membranethe samples were sputtered with a thin layer ofgold. The diameters of nanofibers were also meas-ured using the software Photoshop surface chemistry of the modified PVDF nanofiber membranes was examined by FourierTransform Infrared (FTIR) (Thermo Electron Cor-poration). Spectra were recorded in air by use of aFTIR Nexus spectrometer.