- Email: [email protected]
Novel poly(vinyl alcohol) nanofibers prepared by heterogeneous saponification of electrospun poly(vinyl acetate)
Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Novel poly(vinyl alcohol) nanofibers prepared by heterogeneous saponification of electrospun poly(vinyl acetate) Seong Baek Yang a , Hyun Ji Lee b , Yeasmin Sabina b , Jong Won Kim c,∗∗ , Jeong Hyun Yeum a,b,∗ a
Department of Advanced Organic Materials Science & Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea Department of Bio-fibers and Materials Science, Kyungpook National University, Daegu 702-701, Republic of Korea c Regional Research Institute for Fiber & Fashion Materials, Yeungnam University, Kyeongsan 712-749, Republic of Korea b
h i g h l i g h t s
g r a p h i c a l
• Novel poly(vinyl alcohol) nanofibers
Generally, PVA nanofibers were prepared by the electrospinning of a PVA solution. In this work, we demonstrated a novel and facile technique to prepare PVA nanofibers from PVAc nanofibers, which are considered to be one of the most common precursors of PVA. This is a novel method for preparing PVA nanofibers through the heterogeneous saponification of PVAc electrospun nanofibers.
were prepared through the heterogeneous saponification of poly(vinyl acetate) nanofibers for the first time. • It was found that the degree of saponification depended on the temperature and alkali solution concentration. • The fully saponified PVA nanofibers had the appearance of wrinkled and wound fibers.
a r t i c l e
i n f o
Article history: Received 8 January 2016 Received in revised form 24 February 2016 Accepted 8 March 2016 Available online 10 March 2016 Keywords: Heterogeneous saponification Nanofiber Poly(vinyl alcohol) Poly(vinyl acetate)
a b s t r a c t
a b s t r a c t Novel poly(vinyl alcohol) (PVA) nanofibers were prepared through the heterogeneous saponification of poly(vinyl acetate) (PVAc) nanofibers for the first time. To prepare the saponified PVA nanofibers, the effects of the alkali solution concentration and temperature were studied. It was found that the degree of saponification depended on the temperature and alkali solution concentration. Field emission scanning electron microscopy was utilized to characterize the morphology and properties of mats of the saponified PVA nanofibers, and unusual wrinkled and wound fibers were found. The conversion of PVAc nanofibers to PVA nanofibers was measured using proton nuclear magnetic resonance spectrometry, X-ray diffraction measurements, and Fourier transform infrared spectroscopy. © 2016 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author at: Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea. ∗∗ Corresponding author. E-mail addresses: kjwfi[email protected] (J.W. Kim), [email protected] (J.H. Yeum). http://dx.doi.org/10.1016/j.colsurfa.2016.03.017 0927-7757/© 2016 Elsevier B.V. All rights reserved.
Nanofibers have been the subject of vigorous experimentation because they exhibit distinctive characteristics such as a large surface-area-to-volume ratio, flexible surface functionalities, and superior mechanical properties (e.g., stiffness and tensile strength), which give them a fascinating appeal in numerous disciplines. Electrospun nanofibrous mats have been implemented in wound
266
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270
Fig. 1. Schematic illustration of preparation of PVA nanofibers by heterogeneous saponification.
healing, sensor, tissue engineering, drug delivery, and antibacterial applications [1,2]. Poly(vinyl alcohol) (PVA) has generated significant interest, particularly for various pharmaceutical and biomedical applications, because of its many appealing features such as low protein adsorption properties, high water solubility, biocompatibility, and chemical resistance. Soft contact lenses, eye drops, tissue adhesion barriers, embolization particles, and artificial cartilage and menisci are some of the most common medical uses of PVA [3]. Ultrafine PVA fibers, which have high compressive and tensile strengths, and a high tensile modulus, may hold the potentialfor various applications because of their excellent functional properties [4,5]. Therefore, many attempts have been made to develop PVA-based nanofibers and microspheres. Lee et al. reported that in an aqueous alkali solution heterogeneous saponification obstructed the reaction on the surface of poly(vinyl acetate) (PVAc) and preservation of the spherical structure of the PVAc was possible. The PVA skin that adhered to the PVAc core was a hydrogel that was reversely swellable in water, and they used sodium sulfate to prevent the dissolution of this PVA skin during the heterogeneous saponification [6]. In another report, PVAc microspheres were saponified in an aqueous alkali solution containing sodium hydroxide (NaOH), sodium sulfate (Na2 SO4 ), and methanol (MeOH), which caused PVA to form at the surface of PVA/PVAc skin/coretype microspheres [7,8]. Our research group reported the novel preparation of PVA/clay nanocomposite microspheres via suspension polymerization and saponification [9]. Our group also studied PVAc/PVA/montmorillonite nanocomposite microspheres prepared by suspension polymerization and saponification [10]. All of the above reports were about microspheres prepared by suspension polymerization and saponification. Yang et al. prepared gelatin/PVA nanofibers and found their potential application in the controlled release of drugs [11]. In another study, the fabrication of PVA/chitosan blend nanofibers was studied [12]. Zeng et al. experimented with PVA nanofibers as a protein delivery system [13].
Shalumon et al. prepared carboxymethyl chitin/PVA electrospun nanofibrous scaffolds for tissue engineering applications [14]. The effect of deacetylation on the wicking behavior of a co-electrospun cellulose acetate/PVA nanofiber blend was studied by Khatri et al [15]. Ulvan-based uniform nanofibers were fabricated by blending with PVA [16]. All of these studies were about PVA nanofibers prepared by the electrospinning of a PVA solution. Our research group also fabricated many PVA nanocomposite nanofibers and examined their applications. However, the literature contains no reports on the saponification of PVAc nanofibers to form PVA nanofibers, which were fabricated for the first time in our lab. In this work, we demonstrated a novel and facile technique to prepare PVA nanofibers from PVAc nanofibers, which are considered to be one of the most common precursors of PVA [17]. This is a novel method for preparing PVA nanofibers through the heterogeneous saponification of PVAc electrospun nanofibers. The prepared nanofibers presented extraordinary wrinkled and wound fiber characteristics, as well as a large surface area compared to ordinary PVA nanofibers. The effects of the temperature and concentration of the alkali solution on the saponification rate were evaluated.
2. Materials and methods 2.1. Materials Vinyl acetate (VAc) (Sigma Aldrich) was washed with an aqueous solution of NaHSO4 and water; it was then dried with CaCl2 (anhydrous) and subsequently distilled in a nitrogen atmosphere under reduced pressure. PVA (Aldrich) was used as a suspending agent and the number-average molecular weight and the degree of saponification (DS) of PVA are 127,000 g/mol and 88% respectively. 2,2 -azobis(2,4-dimethylvaleronitrile) (ADMVN) (WakoCo.) was recrystallized twice in methanol and used as an initiator. We used NaOH (Duksan), Na2 SO4 (Duksan), and MeOH (Duksan) to pre-
Fig. 2. FE-SEM images of electrospun pure PVAc nanofibers prepared using various solution concentrations: (a) 10 wt.%, (b) 15 wt.%, and (c) 20 wt.% (applied voltage = 15 kV, TCD = 15 cm).
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270
267
Fig. 3. FE-SEM images of pure PVAc nanofibers (A), heterogeneous saponified PVA nanofibers (35 ◦ C, 24 h) (concentration of alkali solution = NaOH- Na2 SO4 - MeOH: 10- 1010) (B), and general PVA nanofibers prepared by electrospinning of PVA solution (C).
Fig. 4. 1 H NMR results for PVA nanofibers after saponification (25 ◦ C and 35 ◦ C, 24 h) (PVAc concentration = 15 wt%, concentration of alkali solution (NaOH- Na2 SO4 - MeOH) = A: 5- 5- 5 (25 ◦ C), B: 10- 10- 10 (25 ◦ C), C: 5- 5- 5 (35 ◦ C), D: 10- 10- 10 (35 ◦ C)).
pare an aqueous alkali solution for heterogeneous saponification. To conduct all the experiments deionized water was used.
2.2. Preparation of PVAc by suspension polymerization of VAc The suspension polymerization of VAc was conducted to prepare a PVAc resin. Under a nitrogen atmosphere, a suspending agent was dispersed in 120 mL of water with constant stirring in a 250 mL
reactor equipped with a condenser. The VAc monomer and ADMVN were degassed and added respectively at a polymerization temperature of 15 ◦ C, which was subsequently increased to 60 ◦ C. After a predetermined time, the reaction mixture was kept for 1 day to separate and allow the spherical PVAc particles to sink. The PVAc particles were washed with warm water and methanol to remove any residual VAc and suspending agent and dried at 40 ◦ C for 48 h in a vacuum oven.
268
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270
Fig. 5. XRD patterns of (a) saponified PVA nanofibers at 35 ◦ C for 24 h and (b) pure PVAc nanofibers.
Fig. 6. FT-IR patterns of (a) pure PVAc nanofibers and (b) saponified PVA nanofibers at 35 ◦ C for 24 h.
2.3. Electrospinning of PVAc nanofiber
Japan). The DS of the PVA nanofibers was determined by the ratio of the methyl and methylene proton peaks found using a proton nuclear magnetic resonance (1 H NMR) spectrometer (AVANCE III 500, Bruker, Germany). X-ray diffractometer (XRD) (D/Max-2500, Rigaku, Japan) and Fourier transform infrared (FT-IR) spectrometer (Frontier, Perkin Elmer, USA) were used to measure the conversion of PVAc nanofibers to PVA nanofibers.
To prepare the electrospinning solution, PVAc was dissolved in methanol at room temperature under magnetic stirring for 2 h. The PVAc concentrations used were 10, 15, and 20 wt.% based on the weight of the solution. In the electrospinning process, high-voltage electricity (model CPS- 60 K02 VIT < Chungpa EMT CO., Ltd., Seoul, Korea) was applied through an alligator clip fixed to a syringe needle to the PVAc solution in the syringe (30 mL). The applied voltage was fixed at 15 kV. To control the solution flow rate, it was delivered to the blunt needle tip via a syringe pump. An electrically grounded piece of aluminum foil was placed 15 cm below the needle tip, and the fibers were collected on it. In our lab, we found that these spinning conditions were the best for PVAc nanofiber mats. After air drying the fibers were peeled off from the aluminum foil manually using a knife and stored in a polyethylene bag for subsequent experiment and characterization. 2.4. Heterogeneous saponification of PVAc nanofibers A reflux condenser, thermocouple, stirring device and dropping funnel were fitted with a flask to conduct heterogeneous saponification of electrospun PVAc nanofiber to prepare PVA nanofibers. For the saponification 5–10 g of NaOH, 5–10 g of Na2 SO4 , 5–10 g of MeOH, and 100 g of water were used to prepare alkali solution. The electrospun PVAc nanofibers were slowly added to the alkali solution at two different temperatures (25 ◦ C and 35 ◦ C) with gentle stirring. At the appropriate time the saponification was discontinued, and the PVA nanofibers were formed on the surface of the PVAc nanofibers. The mixture was discharged into cold water after the requisite reaction time and kept the PVA nanofibers for 1 min to let the precipitation. Finally, the saponified nanofibers were washed several times with water and dried in a vacuum at room temperature for 24 h. 2.5. Characterization The molecular weights of the saponified PVA nanowebs were determined using gel permeation chromatography (GPC) [18]. A Waters GPC model 2 equipped with a model 590 programmable solvent delivery module, differential refractometer detector, and Styragel HT column was used for the GPC studies. The surface morphology of the saponified PVA nanofibers was examined using field emission scanning electron microscopy (FE-SEM) (SU8220, Hitachi,
3. Result and discussion A schematic illustration of the method for preparing PVA nanofibers from PVAc nanofibers using heterogeneous saponification is presented in Fig. 1. The PVAc nanofibers used as the precursor for the PVA nanofibers were prepared through the heterogeneous saponification of electrospun PVAc nanofibers. Changing the polymer concentration could dramatically change the morphology and fiber diameter, as shown in Fig. 2. We used 10, 15, and 20 wt.% PVAc solution concentrations with a fixed applied voltage (15 kV) and tip-to-collector distance (15 cm) and found that 15 wt.% was ideal to obtain thinner and uniform PVAc nanofibers. From our experiment we found that thinner PVAc fibers were needed to get nanoscale saponified PVA nanofibers. Thinner polymer fibers have several wonderful characteristics such as very large surface area to volume ratio, flexible surface functionalities exceptional mechanical performance (e.g. stiffness and tensile strength) [1], and uniform nanofibers are also needed for various applications. The saponification of PVAc nanofibers was conducted using a heterogeneous saponification system composed of sodium hydroxide, sodium sulfate, methanol, and water under different conditions. The morphology of the nanofibers is shown in Fig. 3, where (a), (b), and (c) show the FE-SEM images of pure PVAc, heterogeneous saponified PVA, and general PVA nanofibers prepared by the electrospinning of the PVA solution, respectively. Interestingly, unlike the general PVA nanofibers prepared by the electrospinning of the PVA solution, the alkali-treated saponified PVA nanofibers had the appearance of wrinkled and wound fibers as well as a large surface area compared to ordinary PVA nanofibers (saponification conditions:10, 10, 10, and 100 g of NaOH, Na2 SO4 , MeOH, and H2 O, respectively, at 35 ◦ C for 24 h). The unusual product shape might have been due to the (i) nanofibers forming through saponification increasing in density as a result of the density of the PVA skin being greater than that of the PVAc skin, as well as (ii) the dissolution of the PVA skins in the aqueous saponification solution [13].
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270 Table 1 Molecular weight of PVA nanofibers with DS of 99.9%. Sample
Mw (g/mol)
Mn (g/mol)
Polydispersity index (Mw /Mn )
PVA nanofibers
84,216
9868
8.53
269
ther studies are required to determine the effects of other factors such as Na2 SO4 , MeOH and H2 O. 4. Conclusion It is commonly known that PVA nanofibers can be prepared by electrospinning a PVA solution. However, in this study, fully saponified PVA nanofibers were successfully fabricated through the heterogeneous saponification of PVAc nanofibers. FE-SEM study showed that wrinkled and wound fibers were formed. 1 H NMR study determined that fully saponified nanofibers were clearly obtained. The experimental results showed that the DS of the PVAc nanofibers increased surprisingly with increases in both the temperature and alkali solution concentration. The detailed structure of the PVA was established using XRD, and FT-IR measurement further determined the conversion of the PVAc nanofibers to PVA nanofibers. Acknowledgments
Fig. 7. Effect of temperature on degree of saponification (concentration of alkali solution (NaOH- Na2 SO4 - MeOH) = (circle (䊉): 5- 5- 5, triangle (): 10- 10- 10)).
The 1 H NMR spectra of the PVA nanofibers after saponification for 24 h at different temperatures (25 ◦ C, 35 ◦ C) and alkali solution concentrations are shown in Fig. 4. The DS of the PVA was calculated using the ratio of the methyl and methylene proton peaks (shown as 1.74 and 1.4 ppm) in the 1 H NMR spectra. Fig. 4(D) shows the non-existence of methyl peaks. This might have been due to the complete conversion of the methyl group of the PVAc to the methylene group of the PVA. Therefore, it was found that fully saponified PVA nanofibers were obtained at saponification conditions of 10, 10, 10 and 100 g of NaOH, Na2 SO4 , MeOH and H2 O, respectively, at 35 ◦ C for 24 h. The molecular weights of the fully saponified PVA nanofibers were obtained by GPC and listed in Table 1. The structure of the saponified PVA nanofibers was established using an XRD analysis, as shown in Fig. 5. Here, the XRD pattern of the PVAc shows diffraction peaks at 2 values of 13.5◦ and 22.5◦ . The broadened background scattering areas of pure PVAc suggest the presence of an amorphous nature. PVAc is known to be an amorphous polymer and shows two broad peaks at the 2 values of 13.5◦ and 22.5◦ [19], which are similar to our experimental values. On the other hand, the diffraction pattern of the saponified PVA nanofibers shows a peak at the 2 value of 19.8◦ [20], which is similar to the diffraction pattern peak of the general PVA nanofibers. This suggests that PVA nanofibers were successfully fabricated from the PVAc nanofibers. The FT-IR spectra of the pure PVAc and saponified PVA nanofibers were inspected (Fig. 6). The vibrational band at 2923 and 2865 cm−1 are attributed to CH3 asymmetric stretching and symmetric stretching of PVAc respectively [21]. After saponification process the broad band observed in the region of 3200–3600 cm−1 are ascribed to the O H stretching from the intermolecular and intramolecular hydrogen bonds and the vibration band in the range of 3000–2800 cm−1 refers to the C H stretching from alkyl groups [22] further determined the formation of PVA nanofibers from PVAc nanofibers. The temperature and alkali solution concentration played important roles in the DS of the PVAc nanofibers, as shown in Fig. 7. The DS increased remarkably with increase in temperature. It was also found that DS increased as the entire alkali solution concentration increased this may result in an increase in the NaOH concentration [23], and fur-
This research was supported by the Technology Commercialization Support Program (113042-3), Ministry of Agriculture, Food, and Rural Affairs. In addition, this work was partially supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and the National Research Foundation of Korea (2015H1C1A1035576). References [1] Z.M. Huang, Y.-Z. Zhang, M. Kotaki, S. Ramakrishna, A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Compos. Sci. Technol. 63 (2003) 2223–2253. [2] W. Li, X. Li, Y. Chen, X. Li, H. Deng, T. Wang, R. Huang, G. Fan, Poly(vinyl alcohol) /sodium alginate /layered silicate- based nanofibrous mats for bacterial inhibition, Carbohydr. Polym. 92 (2013) 2232–2238. [3] M.I. Baker, S.P. Walsh, Z. Schwartz, B.D. Boyan, A review of poly(vinyl alcohol) and its uses in cartilage and orthopedic applications, J. Biomed. Mater. Res. Part B 100 (2012) 1451–1457. [4] W.S. Lyoo, H.D. Ghim, J.H. Kim, S.K. Noh, J.H. Yeum, B.C. Ji, H.T. Jung, J. Blackwell, Critical molecular weight required for in situ fibrillation of syndiotactic poly(vinyl alcohol) during saponification, Macromolecules 36 (2003) 5428–5431. [5] W.S. Lyoo, J. Blackwell, H.D. Ghim, Structure of poly(vinyl alcohol) microfibrils produced by saponification of copoly(vinyl pivalate/vinyl acetate), Macromolecules 31 (1998) 4253–4259. [6] S.G. Lee, J.P. Kim, I.C. Kwon, K.H. Park, S.K. Noh, S.S. Han, W.S. Lyoo, Heterogeneous surface saponification of suspension-polymerized monodisperse poly(vinyl acetate) microspheres using various ions, J. Polym. Sci. Polym. Chem. 44 (2006) 3567–3576. [7] C.J. Kim, P.I. Lee, Synthesis and characterization of suspension-polymerized poly(vinyl alcohol) beads with core–shell structure, J. Appl. Polym. Sci. 46 (1992) 2147–2157. [8] S.G. Lee, J.P. Kim, W.S. Lyoo, J.W. Kwak, S.K. Noh, C.S. Park, J.H. Kim, Preparation of novel syndiotactic poly(vinyl alcohol) microspheres through the low-temperature suspension copolymerization of vinyl pivalate and vinyl acetate and heterogeneous saponification, J. Appl. Polym. Sci. 95 (2005) 1539–1548. [9] J.H. Yeum, Novel poly(vinyl alcohol)/clay nanocomposite microspheres via suspension polymerization and saponification, Polym. Plast. Technol. Eng. 50 (2011) 1149–1154. [10] H.M. Jung, E.M. Lee, B.C. Ji, Y. Deng, J.D. Yun, J.H. Yeum, Poly(vinyl acetate)/poly(vinyl alcohol)/montmorillonite nanocomposite microspheres prepared by suspension polymerization and saponification, Colloid Polym. Sci. 285 (2007) 705–710. [11] D. Yang, Y. Li, J. Nie, Preparation of gelatin/PVA nanofibers and their potential application in controlled release of drugs, Carbohydr. Polym. 69 (2007) 538–543. [12] Y.T. Jia, J. Gong, X.H. Gu, H.Y. Kim, J. Dong, X.Y. Shen, Fabrication and characterization of poly(vinyl alcohol)/chitosan blend nanofibers produced by electro spinning method, Carbohydr. Polym. 67 (2007) 403–409. [13] J. Zeng, A. Aigner, F. Czubayko, T. Kissel, J.H. Wendorff, A. Greiner, Poly(vinyl alcohol) nanofibers by electrospinning as a protein delivery system and the retardation of enzyme release by additional polymer coatings, Biomacromolecules 6 (2005) 1484–1488. [14] K.T. Shalumon, N.S. Binulal, N. Selvamurugan, S.V. Nair, D. Menon, T. Furuike, H. Tamura, R. Jayakumar, Electrospinning of carboxymethyl chitin /poly(vinyl
270
[15]
[16]
[17]
[18]
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270 alcohol) nanofibrous scaffolds for tissue engineering applications, Carbohydr. Polym. 77 (2009) 863–869. Z. Khatri, K. Wei, B.S. Kim, I.S. Kim, Effect of deacetylation on wicking behavior of co- electrospun cellulose acetate /polyvinyl alcohol nanofibers blend, Carbohydr. Polym. 87 (2012) 2183–2188. G. Toskas, R.D. Hund, E. Laourine, C. Cherif, V. Smyrniotopoulos, V. Roussis, Nanofibers based on polysaccharides from the green seaweed ulvarigida, Carbohydr. Polym. 84 (2011) 1093–1102. S.G. Lee, J.P. Kim, W.S. Lyoo, J.W. Kwak, S.K. Noh, C.S. Park, J.H. Kim, Preparation of novel syndiotactic poly(vinyl alcohol) microspheres through the low-temperature suspension copolymerization of vinyl pivalate and vinyl acetate and heterogeneous saponification, J. Appl. Polym. Sci. 95 (2005) 1539–1548. Y.H. Yu, C.Y. Lin, J.M. Yeh, W.H. Lin, Preparation and properties of poly(vinyl alcohol)-clay nanocomposite materials, Polymer 44 (2003) 3553–3560.
[19] Md.S. Islam, J.H. Yeum, A.K. Das, Synthesis of poly(vinyl acetate–methyl methacrylate) copolymer microspheres using suspension polymerization, J. Colloid Interface Sci. 368 (2012) 400–405. [20] J. Lu, T. Wang, L.T. Drzal, Preparation and properties of microfibrillated cellulose polyvinyl alcohol composite materials, Compos. Pt. A-Appl. Sci. Manuf. 39 (2008) 738–746. [21] R. Baskaran, S. Selvasekarapandian, G. Hirankumar, M.S. Bhuvaneswari, Dielectric and conductivity relaxations in PVAc based polymer electrolytes, Ionics 10 (2004) 129–134. [22] J. Wu, N. Wang, L. Wang, H. Dong, Y. Zhao, L. Jiang, Unidirectional water-penetration composite fibrous film via electrospinning, Soft Matter 8 (2012) 5996–5999. [23] W.S. Lyoo, H.W. Lee, Synthesis of high-molecular-weight poly(vinyl alcohol) with high yield by novel one-batch suspension polymerization of vinyl acetate and saponification, Colloid Polym. Sci. 280 (2002) 835–840.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Novel poly(vinyl alcohol) nanofibers prepared by heterogeneous saponification of electrospun poly(vinyl acetate) Seong Baek Yang a , Hyun Ji Lee b , Yeasmin Sabina b , Jong Won Kim c,∗∗ , Jeong Hyun Yeum a,b,∗ a
Department of Advanced Organic Materials Science & Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea Department of Bio-fibers and Materials Science, Kyungpook National University, Daegu 702-701, Republic of Korea c Regional Research Institute for Fiber & Fashion Materials, Yeungnam University, Kyeongsan 712-749, Republic of Korea b
h i g h l i g h t s
g r a p h i c a l
• Novel poly(vinyl alcohol) nanofibers
Generally, PVA nanofibers were prepared by the electrospinning of a PVA solution. In this work, we demonstrated a novel and facile technique to prepare PVA nanofibers from PVAc nanofibers, which are considered to be one of the most common precursors of PVA. This is a novel method for preparing PVA nanofibers through the heterogeneous saponification of PVAc electrospun nanofibers.
were prepared through the heterogeneous saponification of poly(vinyl acetate) nanofibers for the first time. • It was found that the degree of saponification depended on the temperature and alkali solution concentration. • The fully saponified PVA nanofibers had the appearance of wrinkled and wound fibers.
a r t i c l e
i n f o
Article history: Received 8 January 2016 Received in revised form 24 February 2016 Accepted 8 March 2016 Available online 10 March 2016 Keywords: Heterogeneous saponification Nanofiber Poly(vinyl alcohol) Poly(vinyl acetate)
a b s t r a c t
a b s t r a c t Novel poly(vinyl alcohol) (PVA) nanofibers were prepared through the heterogeneous saponification of poly(vinyl acetate) (PVAc) nanofibers for the first time. To prepare the saponified PVA nanofibers, the effects of the alkali solution concentration and temperature were studied. It was found that the degree of saponification depended on the temperature and alkali solution concentration. Field emission scanning electron microscopy was utilized to characterize the morphology and properties of mats of the saponified PVA nanofibers, and unusual wrinkled and wound fibers were found. The conversion of PVAc nanofibers to PVA nanofibers was measured using proton nuclear magnetic resonance spectrometry, X-ray diffraction measurements, and Fourier transform infrared spectroscopy. © 2016 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author at: Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea. ∗∗ Corresponding author. E-mail addresses: kjwfi[email protected] (J.W. Kim), [email protected] (J.H. Yeum). http://dx.doi.org/10.1016/j.colsurfa.2016.03.017 0927-7757/© 2016 Elsevier B.V. All rights reserved.
Nanofibers have been the subject of vigorous experimentation because they exhibit distinctive characteristics such as a large surface-area-to-volume ratio, flexible surface functionalities, and superior mechanical properties (e.g., stiffness and tensile strength), which give them a fascinating appeal in numerous disciplines. Electrospun nanofibrous mats have been implemented in wound
266
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270
Fig. 1. Schematic illustration of preparation of PVA nanofibers by heterogeneous saponification.
healing, sensor, tissue engineering, drug delivery, and antibacterial applications [1,2]. Poly(vinyl alcohol) (PVA) has generated significant interest, particularly for various pharmaceutical and biomedical applications, because of its many appealing features such as low protein adsorption properties, high water solubility, biocompatibility, and chemical resistance. Soft contact lenses, eye drops, tissue adhesion barriers, embolization particles, and artificial cartilage and menisci are some of the most common medical uses of PVA [3]. Ultrafine PVA fibers, which have high compressive and tensile strengths, and a high tensile modulus, may hold the potentialfor various applications because of their excellent functional properties [4,5]. Therefore, many attempts have been made to develop PVA-based nanofibers and microspheres. Lee et al. reported that in an aqueous alkali solution heterogeneous saponification obstructed the reaction on the surface of poly(vinyl acetate) (PVAc) and preservation of the spherical structure of the PVAc was possible. The PVA skin that adhered to the PVAc core was a hydrogel that was reversely swellable in water, and they used sodium sulfate to prevent the dissolution of this PVA skin during the heterogeneous saponification [6]. In another report, PVAc microspheres were saponified in an aqueous alkali solution containing sodium hydroxide (NaOH), sodium sulfate (Na2 SO4 ), and methanol (MeOH), which caused PVA to form at the surface of PVA/PVAc skin/coretype microspheres [7,8]. Our research group reported the novel preparation of PVA/clay nanocomposite microspheres via suspension polymerization and saponification [9]. Our group also studied PVAc/PVA/montmorillonite nanocomposite microspheres prepared by suspension polymerization and saponification [10]. All of the above reports were about microspheres prepared by suspension polymerization and saponification. Yang et al. prepared gelatin/PVA nanofibers and found their potential application in the controlled release of drugs [11]. In another study, the fabrication of PVA/chitosan blend nanofibers was studied [12]. Zeng et al. experimented with PVA nanofibers as a protein delivery system [13].
Shalumon et al. prepared carboxymethyl chitin/PVA electrospun nanofibrous scaffolds for tissue engineering applications [14]. The effect of deacetylation on the wicking behavior of a co-electrospun cellulose acetate/PVA nanofiber blend was studied by Khatri et al [15]. Ulvan-based uniform nanofibers were fabricated by blending with PVA [16]. All of these studies were about PVA nanofibers prepared by the electrospinning of a PVA solution. Our research group also fabricated many PVA nanocomposite nanofibers and examined their applications. However, the literature contains no reports on the saponification of PVAc nanofibers to form PVA nanofibers, which were fabricated for the first time in our lab. In this work, we demonstrated a novel and facile technique to prepare PVA nanofibers from PVAc nanofibers, which are considered to be one of the most common precursors of PVA [17]. This is a novel method for preparing PVA nanofibers through the heterogeneous saponification of PVAc electrospun nanofibers. The prepared nanofibers presented extraordinary wrinkled and wound fiber characteristics, as well as a large surface area compared to ordinary PVA nanofibers. The effects of the temperature and concentration of the alkali solution on the saponification rate were evaluated.
2. Materials and methods 2.1. Materials Vinyl acetate (VAc) (Sigma Aldrich) was washed with an aqueous solution of NaHSO4 and water; it was then dried with CaCl2 (anhydrous) and subsequently distilled in a nitrogen atmosphere under reduced pressure. PVA (Aldrich) was used as a suspending agent and the number-average molecular weight and the degree of saponification (DS) of PVA are 127,000 g/mol and 88% respectively. 2,2 -azobis(2,4-dimethylvaleronitrile) (ADMVN) (WakoCo.) was recrystallized twice in methanol and used as an initiator. We used NaOH (Duksan), Na2 SO4 (Duksan), and MeOH (Duksan) to pre-
Fig. 2. FE-SEM images of electrospun pure PVAc nanofibers prepared using various solution concentrations: (a) 10 wt.%, (b) 15 wt.%, and (c) 20 wt.% (applied voltage = 15 kV, TCD = 15 cm).
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270
267
Fig. 3. FE-SEM images of pure PVAc nanofibers (A), heterogeneous saponified PVA nanofibers (35 ◦ C, 24 h) (concentration of alkali solution = NaOH- Na2 SO4 - MeOH: 10- 1010) (B), and general PVA nanofibers prepared by electrospinning of PVA solution (C).
Fig. 4. 1 H NMR results for PVA nanofibers after saponification (25 ◦ C and 35 ◦ C, 24 h) (PVAc concentration = 15 wt%, concentration of alkali solution (NaOH- Na2 SO4 - MeOH) = A: 5- 5- 5 (25 ◦ C), B: 10- 10- 10 (25 ◦ C), C: 5- 5- 5 (35 ◦ C), D: 10- 10- 10 (35 ◦ C)).
pare an aqueous alkali solution for heterogeneous saponification. To conduct all the experiments deionized water was used.
2.2. Preparation of PVAc by suspension polymerization of VAc The suspension polymerization of VAc was conducted to prepare a PVAc resin. Under a nitrogen atmosphere, a suspending agent was dispersed in 120 mL of water with constant stirring in a 250 mL
reactor equipped with a condenser. The VAc monomer and ADMVN were degassed and added respectively at a polymerization temperature of 15 ◦ C, which was subsequently increased to 60 ◦ C. After a predetermined time, the reaction mixture was kept for 1 day to separate and allow the spherical PVAc particles to sink. The PVAc particles were washed with warm water and methanol to remove any residual VAc and suspending agent and dried at 40 ◦ C for 48 h in a vacuum oven.
268
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270
Fig. 5. XRD patterns of (a) saponified PVA nanofibers at 35 ◦ C for 24 h and (b) pure PVAc nanofibers.
Fig. 6. FT-IR patterns of (a) pure PVAc nanofibers and (b) saponified PVA nanofibers at 35 ◦ C for 24 h.
2.3. Electrospinning of PVAc nanofiber
Japan). The DS of the PVA nanofibers was determined by the ratio of the methyl and methylene proton peaks found using a proton nuclear magnetic resonance (1 H NMR) spectrometer (AVANCE III 500, Bruker, Germany). X-ray diffractometer (XRD) (D/Max-2500, Rigaku, Japan) and Fourier transform infrared (FT-IR) spectrometer (Frontier, Perkin Elmer, USA) were used to measure the conversion of PVAc nanofibers to PVA nanofibers.
To prepare the electrospinning solution, PVAc was dissolved in methanol at room temperature under magnetic stirring for 2 h. The PVAc concentrations used were 10, 15, and 20 wt.% based on the weight of the solution. In the electrospinning process, high-voltage electricity (model CPS- 60 K02 VIT < Chungpa EMT CO., Ltd., Seoul, Korea) was applied through an alligator clip fixed to a syringe needle to the PVAc solution in the syringe (30 mL). The applied voltage was fixed at 15 kV. To control the solution flow rate, it was delivered to the blunt needle tip via a syringe pump. An electrically grounded piece of aluminum foil was placed 15 cm below the needle tip, and the fibers were collected on it. In our lab, we found that these spinning conditions were the best for PVAc nanofiber mats. After air drying the fibers were peeled off from the aluminum foil manually using a knife and stored in a polyethylene bag for subsequent experiment and characterization. 2.4. Heterogeneous saponification of PVAc nanofibers A reflux condenser, thermocouple, stirring device and dropping funnel were fitted with a flask to conduct heterogeneous saponification of electrospun PVAc nanofiber to prepare PVA nanofibers. For the saponification 5–10 g of NaOH, 5–10 g of Na2 SO4 , 5–10 g of MeOH, and 100 g of water were used to prepare alkali solution. The electrospun PVAc nanofibers were slowly added to the alkali solution at two different temperatures (25 ◦ C and 35 ◦ C) with gentle stirring. At the appropriate time the saponification was discontinued, and the PVA nanofibers were formed on the surface of the PVAc nanofibers. The mixture was discharged into cold water after the requisite reaction time and kept the PVA nanofibers for 1 min to let the precipitation. Finally, the saponified nanofibers were washed several times with water and dried in a vacuum at room temperature for 24 h. 2.5. Characterization The molecular weights of the saponified PVA nanowebs were determined using gel permeation chromatography (GPC) [18]. A Waters GPC model 2 equipped with a model 590 programmable solvent delivery module, differential refractometer detector, and Styragel HT column was used for the GPC studies. The surface morphology of the saponified PVA nanofibers was examined using field emission scanning electron microscopy (FE-SEM) (SU8220, Hitachi,
3. Result and discussion A schematic illustration of the method for preparing PVA nanofibers from PVAc nanofibers using heterogeneous saponification is presented in Fig. 1. The PVAc nanofibers used as the precursor for the PVA nanofibers were prepared through the heterogeneous saponification of electrospun PVAc nanofibers. Changing the polymer concentration could dramatically change the morphology and fiber diameter, as shown in Fig. 2. We used 10, 15, and 20 wt.% PVAc solution concentrations with a fixed applied voltage (15 kV) and tip-to-collector distance (15 cm) and found that 15 wt.% was ideal to obtain thinner and uniform PVAc nanofibers. From our experiment we found that thinner PVAc fibers were needed to get nanoscale saponified PVA nanofibers. Thinner polymer fibers have several wonderful characteristics such as very large surface area to volume ratio, flexible surface functionalities exceptional mechanical performance (e.g. stiffness and tensile strength) [1], and uniform nanofibers are also needed for various applications. The saponification of PVAc nanofibers was conducted using a heterogeneous saponification system composed of sodium hydroxide, sodium sulfate, methanol, and water under different conditions. The morphology of the nanofibers is shown in Fig. 3, where (a), (b), and (c) show the FE-SEM images of pure PVAc, heterogeneous saponified PVA, and general PVA nanofibers prepared by the electrospinning of the PVA solution, respectively. Interestingly, unlike the general PVA nanofibers prepared by the electrospinning of the PVA solution, the alkali-treated saponified PVA nanofibers had the appearance of wrinkled and wound fibers as well as a large surface area compared to ordinary PVA nanofibers (saponification conditions:10, 10, 10, and 100 g of NaOH, Na2 SO4 , MeOH, and H2 O, respectively, at 35 ◦ C for 24 h). The unusual product shape might have been due to the (i) nanofibers forming through saponification increasing in density as a result of the density of the PVA skin being greater than that of the PVAc skin, as well as (ii) the dissolution of the PVA skins in the aqueous saponification solution [13].
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270 Table 1 Molecular weight of PVA nanofibers with DS of 99.9%. Sample
Mw (g/mol)
Mn (g/mol)
Polydispersity index (Mw /Mn )
PVA nanofibers
84,216
9868
8.53
269
ther studies are required to determine the effects of other factors such as Na2 SO4 , MeOH and H2 O. 4. Conclusion It is commonly known that PVA nanofibers can be prepared by electrospinning a PVA solution. However, in this study, fully saponified PVA nanofibers were successfully fabricated through the heterogeneous saponification of PVAc nanofibers. FE-SEM study showed that wrinkled and wound fibers were formed. 1 H NMR study determined that fully saponified nanofibers were clearly obtained. The experimental results showed that the DS of the PVAc nanofibers increased surprisingly with increases in both the temperature and alkali solution concentration. The detailed structure of the PVA was established using XRD, and FT-IR measurement further determined the conversion of the PVAc nanofibers to PVA nanofibers. Acknowledgments
Fig. 7. Effect of temperature on degree of saponification (concentration of alkali solution (NaOH- Na2 SO4 - MeOH) = (circle (䊉): 5- 5- 5, triangle (): 10- 10- 10)).
The 1 H NMR spectra of the PVA nanofibers after saponification for 24 h at different temperatures (25 ◦ C, 35 ◦ C) and alkali solution concentrations are shown in Fig. 4. The DS of the PVA was calculated using the ratio of the methyl and methylene proton peaks (shown as 1.74 and 1.4 ppm) in the 1 H NMR spectra. Fig. 4(D) shows the non-existence of methyl peaks. This might have been due to the complete conversion of the methyl group of the PVAc to the methylene group of the PVA. Therefore, it was found that fully saponified PVA nanofibers were obtained at saponification conditions of 10, 10, 10 and 100 g of NaOH, Na2 SO4 , MeOH and H2 O, respectively, at 35 ◦ C for 24 h. The molecular weights of the fully saponified PVA nanofibers were obtained by GPC and listed in Table 1. The structure of the saponified PVA nanofibers was established using an XRD analysis, as shown in Fig. 5. Here, the XRD pattern of the PVAc shows diffraction peaks at 2 values of 13.5◦ and 22.5◦ . The broadened background scattering areas of pure PVAc suggest the presence of an amorphous nature. PVAc is known to be an amorphous polymer and shows two broad peaks at the 2 values of 13.5◦ and 22.5◦ [19], which are similar to our experimental values. On the other hand, the diffraction pattern of the saponified PVA nanofibers shows a peak at the 2 value of 19.8◦ [20], which is similar to the diffraction pattern peak of the general PVA nanofibers. This suggests that PVA nanofibers were successfully fabricated from the PVAc nanofibers. The FT-IR spectra of the pure PVAc and saponified PVA nanofibers were inspected (Fig. 6). The vibrational band at 2923 and 2865 cm−1 are attributed to CH3 asymmetric stretching and symmetric stretching of PVAc respectively [21]. After saponification process the broad band observed in the region of 3200–3600 cm−1 are ascribed to the O H stretching from the intermolecular and intramolecular hydrogen bonds and the vibration band in the range of 3000–2800 cm−1 refers to the C H stretching from alkyl groups [22] further determined the formation of PVA nanofibers from PVAc nanofibers. The temperature and alkali solution concentration played important roles in the DS of the PVAc nanofibers, as shown in Fig. 7. The DS increased remarkably with increase in temperature. It was also found that DS increased as the entire alkali solution concentration increased this may result in an increase in the NaOH concentration [23], and fur-
This research was supported by the Technology Commercialization Support Program (113042-3), Ministry of Agriculture, Food, and Rural Affairs. In addition, this work was partially supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and the National Research Foundation of Korea (2015H1C1A1035576). References [1] Z.M. Huang, Y.-Z. Zhang, M. Kotaki, S. Ramakrishna, A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Compos. Sci. Technol. 63 (2003) 2223–2253. [2] W. Li, X. Li, Y. Chen, X. Li, H. Deng, T. Wang, R. Huang, G. Fan, Poly(vinyl alcohol) /sodium alginate /layered silicate- based nanofibrous mats for bacterial inhibition, Carbohydr. Polym. 92 (2013) 2232–2238. [3] M.I. Baker, S.P. Walsh, Z. Schwartz, B.D. Boyan, A review of poly(vinyl alcohol) and its uses in cartilage and orthopedic applications, J. Biomed. Mater. Res. Part B 100 (2012) 1451–1457. [4] W.S. Lyoo, H.D. Ghim, J.H. Kim, S.K. Noh, J.H. Yeum, B.C. Ji, H.T. Jung, J. Blackwell, Critical molecular weight required for in situ fibrillation of syndiotactic poly(vinyl alcohol) during saponification, Macromolecules 36 (2003) 5428–5431. [5] W.S. Lyoo, J. Blackwell, H.D. Ghim, Structure of poly(vinyl alcohol) microfibrils produced by saponification of copoly(vinyl pivalate/vinyl acetate), Macromolecules 31 (1998) 4253–4259. [6] S.G. Lee, J.P. Kim, I.C. Kwon, K.H. Park, S.K. Noh, S.S. Han, W.S. Lyoo, Heterogeneous surface saponification of suspension-polymerized monodisperse poly(vinyl acetate) microspheres using various ions, J. Polym. Sci. Polym. Chem. 44 (2006) 3567–3576. [7] C.J. Kim, P.I. Lee, Synthesis and characterization of suspension-polymerized poly(vinyl alcohol) beads with core–shell structure, J. Appl. Polym. Sci. 46 (1992) 2147–2157. [8] S.G. Lee, J.P. Kim, W.S. Lyoo, J.W. Kwak, S.K. Noh, C.S. Park, J.H. Kim, Preparation of novel syndiotactic poly(vinyl alcohol) microspheres through the low-temperature suspension copolymerization of vinyl pivalate and vinyl acetate and heterogeneous saponification, J. Appl. Polym. Sci. 95 (2005) 1539–1548. [9] J.H. Yeum, Novel poly(vinyl alcohol)/clay nanocomposite microspheres via suspension polymerization and saponification, Polym. Plast. Technol. Eng. 50 (2011) 1149–1154. [10] H.M. Jung, E.M. Lee, B.C. Ji, Y. Deng, J.D. Yun, J.H. Yeum, Poly(vinyl acetate)/poly(vinyl alcohol)/montmorillonite nanocomposite microspheres prepared by suspension polymerization and saponification, Colloid Polym. Sci. 285 (2007) 705–710. [11] D. Yang, Y. Li, J. Nie, Preparation of gelatin/PVA nanofibers and their potential application in controlled release of drugs, Carbohydr. Polym. 69 (2007) 538–543. [12] Y.T. Jia, J. Gong, X.H. Gu, H.Y. Kim, J. Dong, X.Y. Shen, Fabrication and characterization of poly(vinyl alcohol)/chitosan blend nanofibers produced by electro spinning method, Carbohydr. Polym. 67 (2007) 403–409. [13] J. Zeng, A. Aigner, F. Czubayko, T. Kissel, J.H. Wendorff, A. Greiner, Poly(vinyl alcohol) nanofibers by electrospinning as a protein delivery system and the retardation of enzyme release by additional polymer coatings, Biomacromolecules 6 (2005) 1484–1488. [14] K.T. Shalumon, N.S. Binulal, N. Selvamurugan, S.V. Nair, D. Menon, T. Furuike, H. Tamura, R. Jayakumar, Electrospinning of carboxymethyl chitin /poly(vinyl
270
[15]
[16]
[17]
[18]
S.B. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 265–270 alcohol) nanofibrous scaffolds for tissue engineering applications, Carbohydr. Polym. 77 (2009) 863–869. Z. Khatri, K. Wei, B.S. Kim, I.S. Kim, Effect of deacetylation on wicking behavior of co- electrospun cellulose acetate /polyvinyl alcohol nanofibers blend, Carbohydr. Polym. 87 (2012) 2183–2188. G. Toskas, R.D. Hund, E. Laourine, C. Cherif, V. Smyrniotopoulos, V. Roussis, Nanofibers based on polysaccharides from the green seaweed ulvarigida, Carbohydr. Polym. 84 (2011) 1093–1102. S.G. Lee, J.P. Kim, W.S. Lyoo, J.W. Kwak, S.K. Noh, C.S. Park, J.H. Kim, Preparation of novel syndiotactic poly(vinyl alcohol) microspheres through the low-temperature suspension copolymerization of vinyl pivalate and vinyl acetate and heterogeneous saponification, J. Appl. Polym. Sci. 95 (2005) 1539–1548. Y.H. Yu, C.Y. Lin, J.M. Yeh, W.H. Lin, Preparation and properties of poly(vinyl alcohol)-clay nanocomposite materials, Polymer 44 (2003) 3553–3560.
[19] Md.S. Islam, J.H. Yeum, A.K. Das, Synthesis of poly(vinyl acetate–methyl methacrylate) copolymer microspheres using suspension polymerization, J. Colloid Interface Sci. 368 (2012) 400–405. [20] J. Lu, T. Wang, L.T. Drzal, Preparation and properties of microfibrillated cellulose polyvinyl alcohol composite materials, Compos. Pt. A-Appl. Sci. Manuf. 39 (2008) 738–746. [21] R. Baskaran, S. Selvasekarapandian, G. Hirankumar, M.S. Bhuvaneswari, Dielectric and conductivity relaxations in PVAc based polymer electrolytes, Ionics 10 (2004) 129–134. [22] J. Wu, N. Wang, L. Wang, H. Dong, Y. Zhao, L. Jiang, Unidirectional water-penetration composite fibrous film via electrospinning, Soft Matter 8 (2012) 5996–5999. [23] W.S. Lyoo, H.W. Lee, Synthesis of high-molecular-weight poly(vinyl alcohol) with high yield by novel one-batch suspension polymerization of vinyl acetate and saponification, Colloid Polym. Sci. 280 (2002) 835–840.