Electrospun crosslinked polyvinyl alcohol membrane

Electrospun crosslinked polyvinyl alcohol membrane

Materials Letters 62 (2008) 3555–3557 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i...

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Materials Letters 62 (2008) 3555–3557

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t

Electrospun crosslinked polyvinyl alcohol membrane Enlong Yang ⁎, Xiaohong Qin, Shanyuan Wang College of Textile, Donghua University, Shanghai, 201620, China

A R T I C L E

I N F O

Article history: Received 21 January 2008 Accepted 20 March 2008 Available online 28 March 2008 Keywords: Nanomaterials Polymers Electrospinning Polyvinyl alcohol Maleic anhydride Crosslink

A B S T R A C T The electrospinning of polyvinyl alcohol (PVA) was performed with maleic anhydride (MA) as a cross linker to fabricate slightly soluble nanofiber membrane. The solubility, morphology and thermal behavior of electrospun PVA and PVA/MA membranes were characterized by water durability test, scanning electron microscope (SEM) and differential scanning calorimeter (DSC), respectively. Water durability test demonstrated that 8% PVA/MA (20/1, mole/mole) membrane had the least average mass loss and standard deviation. SEM images showed that fibers in PVA/MA membrane had a larger average diameter compared to those in PVA membrane. DSC investigated that crystal structure was formed in PVA/MA membrane. The results show that rapid evaporation of water and high electric field during electrospinning process may promote crosslinking of PVA and MA. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Electrospinning is quite a simple method for fabricating submicron fibers. In general, fibers with diameter less than 1000 nm are called nanofiber in electrospinning [1,2]. The electrospinning process is that a polymer solution or melt is placed into a syringe with a millimetersize nozzle and is subjected to high electric field. Under the applied electrostatic force, the polymer is ejected from the nozzle, whose diameter is reduced significantly as it is transported to and deposited on a collector, which also serves as the ground for the electrical charges. Due to unique properties such as high surface area to volume ratio, small pore sizes, high porosity and so on, the ultrafine fiber membranes prepared by electrospinning process have been extensively studied and widely used for its potential applications in filter media, composite materials, biomedical applications, etc [3–5]. PVA is a polyhydroxy polymer that has been studied intensively because of its good film forming and physical properties, high hydrophilicity, processability, biocompatibility, and good chemical resistance [6–8]. When the electrospun PVA nanofiber membrane was immersed in water, it would dissolve because PVA is a water soluble polymer. Therefore, it is necessary to crosslink the PVA polymer and stabilize the electrospun nanofiber membranes in wet condition [9]. All multifunctional compounds capable of reacting with the hydroxyl group may be used as a cross linker of PVA. Thus, dialdehydes, dicarboxylic acids, dianhydrides, etc. Dicarboxylic acid crosslinked PVA deserves more attention. Gohil et al. crosslinked PVA/MA cast membrane by heat treatment [10]. Ding et al. [11]

⁎ Corresponding author. Tel.: +86 21 67792700; fax: +86 21 67792627. E-mail address: [email protected] (E. Yang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.03.049

prepared electrospun PVA/glyoxal membranes then crosslinked them by heat treatment. Choi [12] prepared electrospun soluble polymer/crosslinking agent membranes then crosslinked them by heat or ultra violet (UV) lights treatment. In the present work, we

Fig. 1. Schematic diagram of electrospinning apparatus.

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Table 1 Average mass loss and standard deviation of electrospun 8% PVA/MA membranes PVA/MA (mole/mole)

30/1

21/1

20/1

19/1

10/1

Average mass loss (%) Standard deviation (%)

Soluble

5.13 0.92

3.28 0.68

4.95 1.03

8.37 1.18

found that PVA and MA crosslinked efficiently during electrospinning process instead of heat treatment or UV radiation. The prepared PVA/MA membranes were slightly soluble in boiling water. The results show that slightly soluble PVA/MA nanofiber membrane can be fabricated by electrospinning directly. 2. Experimental PVA powder (Mw = 88,000 g mol− 1, 88% hydrolyzed) was purchased from J & K Chemical ®. Maleic anhydride and sulfuric acid (98%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Distilled water was purchased from Pinjiang Chemical Co., Ltd. 5%, 6%, 7%, 8%, 9% and 10% PVA solutions were prepared from PVA powder and distilled water. PVA powder was added into distilled water slowly at room temperature. The mixture was stirred at 80 °C for 2 h to get homogenous solutions. After the solution was cooling down, proper amount of sulfuric acid was added into it to control the pH value within the range of 2–3 [11]. Then MA (PVA/MA = 30/1, 21/1, 20/ 1, 19/1, and 10/1, mole/mole) was added into the prepared PVA solutions respectively with vigorous stirring to get homogenous PVA/ MA solutions. PVA/MA solutions were electrospun immediately after being prepared. The electrospinning apparatus is schematically shown in Fig. 1. The electrospinning apparatus used in this study consist of a hypodermic syringe, syringe needle, aluminum collecting plate, and variable high voltage power supply. A syringe pump connected to the hypodermic syringe controlled the flow rate. Most of water evaporates rapidly in jet region and instability region during electrospinning. As they reach aluminum plate, the collected fibers are nearly dry. The polymer solutions were electrospun at a positive voltage of 20 kV, a tip-tocollector distance of 20 cm. The morphology of PVA and PVA/MA membranes was observed by JSM-5600LV scanning electron microscopy (SEM). A small piece of membrane was placed on the SEM sample holder and sputter-coated with gold. Accelerating voltage of 10 kV was employed to take the SEM photographs. The thermal behavior of PVA and PVA/MA membranes was investigated by differential scanning calorimeter (PYRIST DSE) over the temperature of 30–300 °C and at a heating rate of 10 °C min− 1. 3. Results and discussion To evaluate solubility, the prepared electrospun PVA and PVA/MA membranes were boiled in water for 1 h. The results show that mass loss of electrospun PVA membrane was 100%. In fact, electrospun PVA membrane can dissolve in water

Fig. 3. DSC thermograms of a) electrospun 8% PVA membrane, b) electrospun 8% PVA/ MA (20/1, mole/mole) membrane.

completely at room temperature in a short period of time. Average mass loss and standard deviation of electrospun 8% PVA/MA (30/1, 21/1, 20/1, 19/1 and 10/1, mole/ mole) membranes are shown in Table 1. As demonstrated in Table 1, electrospun 8% PVA/MA (30/1, mole/mole) membrane was soluble because the crosslinking was not efficient. Mass loss of 8% PVA/MA (10/1, mole/mole) membrane was larger because unreacted MA dissolved in boiling water. Compared to 8% PVA/MA (21/1 and 19/1, mole/mole) membranes, 8% PVA/MA (20/1, mole/mole) membrane had the least average mass loss and standard deviation; crosslinking of it may be the most efficient. Electrospun 8% PVA/MA (20/1, mole/mole) membrane is slightly soluble after boiling in water for 1 h; this is mainly due to the efficient crosslinking of PVA and MA during electrospinning. The crosslinking chemical agent forms three dimensional structural molecules which can prevent the polymer from dissolving in water [12]. Another possible reason is attributed to the decrease of hydrophilic hydroxyl groups. SEM images of electrospun 8% PVA and 8% PVA/MA (20/1, mole/mole) membranes are shown in Fig. 2. Average diameter of twenty fibers in Fig. 2a and b was separately calculated. Average diameter of fibers in PVA and PVA/MA membranes is about 193 nm and 238 nm, respectively. Obviously the later average diameter is larger than the former. The crosslinking of PVA and MA during electrospinning enhances solution viscosity, so average diameter of fibers in PVA/MA membrane is larger. DSC thermograms of electrospun 8% PVA and 8% PVA/MA (20/1, mole/mole) membranes are shown in Fig. 3. It is generally believed that the melting endothermic peak (Tm) of PVA appears around 257 °C [13]. It can be seen from thermogram of PVA/MA membrane that clear melting endothermic peak appears around 250 °C and the enthalpy is 248.5 J g− 1, but there is no clear melting endothermic peak in thermogram of PVA membrane. Crystal structure was destroyed when PVA powder dissolved in water. The rapid solidification process of stretched chains under high elongation rate during the later stages of electrospinning may hinder the development of crystal as the chains do not have time to form crystalline registration [14]. The crystallinity and molecular orientation of electrospun 8% PVA were in a low degree, so no clear cold crystallization and melting endothermic peak was shown in thermogram of PVA membrane. But three dimensional structural molecules and crystal structure were formed in electrospun 8% PVA/MA (20/1, mole/mole) membrane, so clear melting endothermic peak was shown in thermogram of PVA/ MA membrane.

Fig. 2. SEM images of membranes of a) electrospun 8% PVA membrane, b) electrospun 8% PVA/MA (20/1, mole/mole) membrane.

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Fig. 4. Schematic representation of the reaction between PVA and MA.

The esterification reaction of PVA and cross-linker is a slow process. Gohil, Ding and Choi promoted esterification reaction of PVA and cross-linker by heat treatment or UV radiation [10–12]. The present work discovered that PVA and MA crosslinked efficiently during electrospinning process instead of heat treatment or UV radiation. PVA and MA react to form either mono-esters or bis-esters during electrospinning as shown in Fig. 4 [10,15]. Sulfuric acid is a catalyst to increase the rate of crosslinking. High electric field can enhance chemical activity of molecules. The prompt evaporation of produced water may also increase equilibrium constant, so esterification reaction of PVA and MA is efficient during electrospinning.

4. Conclusion Cosslinked PVA/MA membranes were fabricated by electrospinning. Water durability test found that average mass loss and standard deviation of electrospun 8% PVA/MA (20/1, mole/mole) membrane was the least after boiling in water for 1 h. SEM observed that average diameter of fibers in PVA/MA membrane was larger than that in PVA membrane. DSC investigated that crosslinked PVA/MA membrane had crystal structure. The results indicate that rapid evaporation of water and high electric field during electrospinning process can promote esterification reaction. Acknowledgement The work is supported by Grant 10602014 from National Natural Science Foundation of China.

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