Preparation of fully hydrolyzed polyvinyl alcohol electrospun nanofibers with diameters of sub-200 nm by viscosity control

Abstract

Fully hydrolyzed polyvinyl alcohol (FH-PVA) electrospun fibers with uniform diameters of less than 200 nm were fabricated by reducing the viscosity of FH-PVA aqueous solutions. A novel viscosity-modifier (hydrazine monochloride [HMC]) gradually reduced the viscosity of FH-PVA aqueous solution over a period of several days. This phenomenon is counter to the effect of the usual salt addition. After being stored for several days, the viscosity decreased by up to 60% compared with that of an equivalent pure FH-PVA solution. From small angle X-ray scattering (SAXS) and proton nuclear magnetic resonance (¹H NMR) spectra observations it is evident that this effect results from the reconfiguration of hydrogen bonding. The viscosity control of FH-PVA solutions with HMC were used to electrospin highly uniform ultrafine fibers (diameter <200 nm).

Keywords

polyvinyl alcohol (PVA)electrospinning ultrafine fibers hydrogen bonding viscosity-modifier

Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer produced industrially by the hydrolysis of polyvinyl acetate.^1,2 Many different grades of PVA are commercially available, falling into two types, depending on the degree of hydrolysis (DH), fully hydrolyzed PVA (FH-PVA) and partially hydrolyzed PVA (PH-PVA).^3–5 The higher chemical stability, water resistance, and excellent physical and mechanical properties of FH-PVA have led to its wide use, especially in the textile industry.¹

Electrospinning is a relatively simple and inexpensive method to produce fibers with diameters in the nanometer range.^6–10 PVA is one of the most popular polymers used for ultrafine electrospun fiber production. Nano-scale PVA fibers have several interesting characteristics, such as high surface area to mass ratio, significant possibilities for surface functionalization, and high mechanical performance due to an improvement in the molecular organization of the spun fiber. These properties make electrospun PVA fibers excellent candidates for many applications, such as filtration, reinforcing materials, wound dressings, tissue scaffolding, and drug-releasing carriers.^11–16 FH-PVA is frequently described as a good material for producing nano-scale electrospun fibers;^17,18 however, it is difficult to fabricate electrospun FH-PVA fibers with diameters below about 200 nm due to the extremely high surface tension and viscosity of the solution.¹⁹ It has been suggested that reducing the high surface tension of FH-PVA in solution will address this problem.^20,21 However, although the surface tension can be demonstrably modified, controlled production of FH-PVA electrospun fibers with uniform nano-size diameters remains a challenge. The alternative approach to controlling fiber diameter, namely by decreasing solution viscosity, has not yet been reported.

This paper describes the fabrication of uniform electrospun FH-PVA fibers with sub-200 nm diameters by controlling solution viscosity. Hydrazine monochloride (HMC) is widely used in organic synthesis, but until now has not been used for reducing the viscosity of FH-PVA.²² The effect of HMC on the viscosity characteristics of the solutions was investigated and a reason for the change is presented. Furthermore, parameters such as concentration and storage time were systematically examined and their effects on the morphology and diameter of electrospun FH-PVA fibers are described. Based on this method, the spinnability of FH-PVA aqueous solutions can be greatly improved and it is now possible to produce ultrafine FH-PVA electrospun fibers with diameters of less than 200 nm.

Experimental details

Materials

PVA with a DH >99% and degree of polymerization (DP) = 3500 and PVA with DH = 86–90% and DP = 3500 were kindly provided by Kuraray Co. Ltd, Tokyo, Japan. In this paper the abbreviation form PVA-X-Y, is used to describe polymer molecular weight and DH. For example, PVA-35-99 represents a PVA with a DP of 3500 and a DH of 99%. HMC (chemical formula of N₂H₄·HCl), sodium chloride (NaCl), sodium sulfate (Na₂SO₄), potassium chloride (KCl), ammonium sulfocyanate (NH₄SCN) and isopropanol (IPA) were obtained from Tokyo Chemical Industry Co., Ltd. Deuterium oxide (99.9%) as a nuclear magnetic resonance (NMR) solvent was obtained from Wako Pure Chemical Industries, Ltd. All chemicals were of analytical grade and were used without further purification. Distilled water was used as the solvent.

Preparation of PVA solutions

FH-PVA in aqueous solution was prepared using the method described by Briscoe et al.²³ In brief, PVA powder was dispersed in distilled water at 25°C for 1 h and then this suspension was rapidly heated to 95°C and kept at this temperature with high-speed stirring for about 2 h. The solution was then cooled to room temperature. PVA powders with a range of DHs (86–90%) were directly dissolved in hot water (95°C) and stirred for 2 h. Various amounts of HMC were added to 6 wt% PVA solution to produce solutions with HMC concentrations of 0.05 mol/l, 0.1 mol/l and 0.3 mol/l. PVA solutions with NaCl, Na₂SO₄ and KCl (0.1 mol/l) were also prepared. These solutions were stirred for 2 h to ensure homogenization and then put into a water bath at 40°C for several days. Samples were taken every 24 h, cooled to 25°C. Viscosity, conductivity and surface tension were measured.

Electrospinning

The electrospinning experiments were performed at 25°C. The polymer solution was placed in a 12 ml syringe with a 0.8 mm ID blunt needle, and the needle was connected to the high-voltage power supply. A rotating cylinder (diameter: 12 cm; speed: 25 rpm) wrapped with aluminum foil was used as a grounded collector. The applied voltage and the tip-to-collector distance (TCD) were fixed at 10 kV and 15 cm and the syringe pump flow rate was set to 0.4 ml/h. Immediately before electrospinning, isopropanol (IPA) was added at 5 wt% to the PVA solutions to slightly decrease the surface tension.²⁰

Measurement

The PVA solutions were stored in a recycling-water bath at constant temperature and tested every 24 h. Rheological properties were measured using a Rheologia-A300 controlled stress rheometer (Elquest) at 25°C. A co-axial cylinder arrangement with a gap of 2.1 mm was used to measure the apparent viscosities using the Steady Flow Viscosity Testing mode. Apparent viscosities of different PVA solutions measured at a shear rate of 100 s⁻¹ were selected as reference viscosities. Conductivity was measured using a conductivity meter (Model SC72 personal SC meter, Yokogawa Electric Corporation) and the surface tension was measured using a Wilhelm plate method tension meter (CBVP-Z, Kyowa Interface Science Co. Ltd, Saitama, Japan). small angle X-ray scattering (SAXS) was also used to measure molecular weights. A SAXS camera (SAXSess mc², Anton Paar, Austria) attached to a sealed-tube anode X-ray generator (GE Inspection Technologies, Germany) was operated at 40 kV and 50 mA. A Göbel mirror and a block collimator provided a focused monochromatic X-ray beam of Cu Kα radiation (l = 0.1542 nm) with a well-defined line-shape. The morphology of the electrospun fibers was observed using scanning electron microscopy (SEM Hitachi S-3000 N). The SEM samples were sputtered with palladium-platinum. ¹H NMR spectra of FH-PVA/D₂O solutions were recorded on a Bruker DRX-400 spectrometer, operating at 400 MHz and 40°C.

Results and discussion

Effect of HMC on the viscosity of PVA solutions

PVA35-99 is nearly completely hydrolyzed, and so a preparation temperature above 95°C is required for the complete dissolution of the solid in an acceptable time. It is believed that inter- and intra-molecule hydrogen bonding in aqueous PVA solutions is disrupted by thermal energy during the solution preparation, but upon cooling the hydrogen bonds can re-form resulting in an increase in viscosity.^24–26

At concentrations of ≤10 wt% (a typical industrial strength), aqueous solutions of high-DH PVA undergo a viscosity increase with time and depending on temperature, concentration and additives, and may ultimately gel. It is reported that several additives, such as inorganic salts and organic compounds, can affect the viscosity of PVA in solution.^27–29 We have investigated the effect of Na₂SO₄, NaCl, KCl, NH₄SCN and IPA on the viscosity of FH-PVA solutions. As seen in Figure 1(a), most of the inorganic salts have a negative effect on viscosity stability, accelerating the time-dependent increase in the viscosity. However, NH₄SCN and IPA can moderate the process. HMC has an unexpected effect on the viscosity of aqueous FH-PVA, as can be seen in Figure 1(b) the apparent viscosity of PVA in solution decreased gradually with storage time in the presence of HMC with final viscosities reduced by up to 60% depending on the amount of HMC. The addition of HMC may disrupt PVA chain inter- and intra-chain hydrogen bonding resulting in a decrease in the viscosity.

Figure 1.

The variation of apparent viscosities of fully hydrolyzed polyvinyl alcohol (FH-PVA) aqueous solutions with storage time: (a) FH-PVA solutions with additives; (b) FH-PVA solutions with varying amounts of hydrazine monochloride (HMC). (Shear rate: 100 s⁻¹; temperature: 25°C).

Figure 2 shows the relationship between shear rate and apparent viscosity for different PVA solutions after 120 h. Apparent viscosities of the PVA-35-99 solutions are much higher than those of the PVA-35-86 solution, and pronounced shear thinning behavior is observed in the pure PVA-35-99 solution. This is due to the higher DH causing more inter- and intra-chain hydrogen bonding in the solution and hence more interaction and entanglements between the PVA chains.²³ The obvious shear thinning behavior also indicates the presence of stronger polymer chain associations. In addition, after the viscosities had decreased, the properties of the PVA-35-99 solutions with varying amounts of HMC show similar behavior to the PVA-35-86 solutions with no shear thinning. It is probable that the addition of HMC reduces inter- and intra-chain hydrogen bonding and reduces the degree of interaction and entanglement between the molecular chains in PVA-35-99.

Figure 2.

Relationship between apparent viscosity and shear rate for polyvinyl alcohol (PVA) aqueous solutions with varying concentrations of hydrazine monochloride (HMC) at 25°C. (Storage time: 120 h).

SAXS of different FH-PVA solutions

SAXS was used to investigate the structural morphology of the solutions. The sample solutions were tested at original, twice-diluted and thrice-diluted concentrations of the original solution. The scattered intensities of the aqueous solutions at 25°C are shown in Figure 3 and all I(q) data were corrected for the background scattering from the capillary and the solvents, and the absolute scale calibration was made using water as a secondary standard. According to the Ornstein–Zernike theory, the scattering intensity near q = 0 is represented by the following equation:

I (q) = \frac{I (0)}{1 + ξ^{2} q^{2}}

(1)

Figure 3.

Small-angle X-ray scattering (SAXS) intensities, I(q), for fully hydrolyzed polyvinyl alcohol (FH-PVA) aqueous solutions at 25°C. (A: pure 6 wt% FH-PVA solutions; A_1/2 twice-diluted A solution; A_1/3: thrice-diluted A solution; B: PVA solution with hydrazine monochloride [HMC]; B_1/2: twice-diluted B solution; B_1/3: thrice-diluted B solution.) (Storage time: 72 h).

where ξ is the Ornstein–Zernike correlation length which is believed to be related to the inter-molecule coil distance.^30–33 In our experiment, the correlation lengths (ξ) were calculated from the lower q-range by using the Ornstein–Zernike model to fit our original data (the solid curves in Figure 3), which are shown in Table 1. The concentration of HMC in the FH-PVA solutions is proportional to the correlation length (ξ). This indicates that the physical junction points formed by hydrogen bonding between molecules were broken, resulting that the interaction between FH-PVA molecules was weakened, which is in agreement with our inference drawn from the decrease in viscosity.

Table 1.

Correlation length (ξ) calculated by using the Ornstein–Zernike equation

	A	A _1/2	A _1/3	B	B _1/2	B _1/3
ξ/nm	12.1	14.0	14.2	13.0	16.5	16.2

A: pure 6 wt% PVA solutions; A_1/2: twice-diluted A solution; A_1/3: thrice-diluted A solution; B: PVA solution with HMC; B_1/2: twice-diluted B solution; B_1/3: thrice-diluted B solution). (Storage time: 72 h.)

NMR of different FH-PVA solutions

Our work indicates that the viscosity of FH-PVA aqueous solutions decreases relative to the proportion of broken hydrogen inter- and intra-molecular bonds. To further confirm this, NMR spectroscopy, as the most direct way of investigating hydrogen bonding, was used to observe hydroxyl protons in FH-PVA solutions.^34–38 In the NMR samples, there is an extremely small residual quantity of water due to the bound water existing in FH-PVA and HMC, which is impossible to remove completely. It is known that in FH-PVA aqueous solutions, hydrogen bonding exists between the FH-PVA molecules and water, as well as the inter- and intra-molecular FH-PVA bonding, which will lead to hydroxyl protons with different hydrogen bond configurations.²⁷ As a result, the peak of hydroxyl protons is much wider than that of water, which has homogeneous hydrogen bonding as shown in Figure 4(a) and (b). However, in the presence of HMC, the situation became more complex, due to the strong electron supplying capacity of the imido group, which promotes the action of HMC in the formation of hydrogen bonds with protons.

Figure 4.

Proton nuclear magnetic resonance (¹H NMR) spectra of hydroxyl protons: (a) fully hydrolyzed polyvinyl alcohol (FH-PVA)/D₂O solution; (b) H₂O/D₂O solution; (c) FH-PVA/hydrazine monochloride (HMC)/D₂O immediately after solution preparation; (d) FH-PVA/HMC/D₂O solution after 72 h. (Measurement temperature: 40°C).

As seen in Figure 4(c), the peak from water protons appears at 4.595 ppm. Furthermore, an up-field shift was observed for hydroxyl protons from FH-PVA (from 4.582 to 4.591 ppm) due to the weakening of inter- and intra-molecule FH-PVA hydrogen bonding and hydrogen bonds between FH-PVA and solvent molecules. After storage at 40°C for 3 days, the peak from water protons at 4.595 ppm disappeared and the main peak of hydroxyl proton became sharp, indicating the formation of a configuration with a narrower distribution of hydrogen bonding between FH-PVA, HMC, water and solvent, which lead to a net de-shielding effect on hydroxyl protons (Figure 4(d)).

Electrical conductivity and surface tension

To investigate the spinnability of FH-PVA in detail, we examined the solution properties of electrical conductivity and surface tensions, which are important factors in electrospinning.²⁰ As seen in Figure 5, the surface tension was slightly reduced by adding HMC while the conductivities increased sharply with the concentration of HMC. However, both the surface tension and conductivity changed very little during the period of storage although during the same period the viscosity decreased greatly.

Figure 5.

Surface tension (a) and conductivity (b) of polyvinyl alcohol (PVA) in aqueous solution with different concentrations of hydrazine monochloride (HMC) and varying storage time at 25°C.

Electrospinnability of FH-PVA

In the electrospinning process, as a consequence of the applied electric force, the pendant drop of solution on the tip of the nozzle forms a conical protrusion known as the Taylor cone.^39–41 When the applied electric field strength overcomes the surface tension; a straight jet is ejected from the apex of Taylor cone. This jet remains straight for some distance and then it bends and follows a looping and spiraling path: the so-called instability. Solvent is evaporated from the strand surface as the jet flies to the collector. Finally, the jet solidifies and deposits on the grounded collector forming a non-woven fiber mat. These fundamental processes in electrospinning are related to solution viscosity and surface tension; these properties also significantly affect the morphology and diameter of the fibers.^42–46

The surface tension and viscosity of FH-PVA in aqueous solution is much higher than PH-PVA,¹⁹ this is the root cause of the difficulty in fabricating electrospun FH-PVA fibers with diameters below 200 nm. To make ultrafine FH-PVA fibers, previous studies concentrated in reducing the solution surface tension.^19–21 This paper describes the effects of reducing the viscosity of FH-PVA and examines the relationship between solution viscosity and the morphologies and diameters of the electrospun fiber product. Electrospun FH-PVA fibers fabricated from solutions with different viscosities, controlled by HMC addition and aging are shown in Figure 6. For the pure PVA solution (Figure 6A), long threads with irregular fiber structures like tree roots were formed from the high surface tension and viscosity precursor solutions. It is difficult to say what an ideal Taylor cone geometry formed during the spinning process.⁴⁷ However, in the presence of HMC, the diameters of electrospun fibers are significantly decreased (when the concentration of HMC is above 0.1 M and after 72 h maturation) and fibers with diameter of 140–260 nm could be fabricated. This is not only because of the decrease of surface tension and the increase in conductivity, but also because of the decrease in viscosity. It is thought to be impossible to get regular morphology ultrafine fibers without modifying these three factors. However, SEM observations show a heavily beaded morphology (Figure 6C and 6D) when the concentration of HMC was 0.3 M, which is enough to reduce the viscosity.

Figure 6.

Scanning electron microscopy (SEM) images of electrospun fully hydrolyzed polyvinyl alcohol (FH-PVA) fibers produced from different FH-PVA solutions: (A) pure FH-PVA solution; (B) FH-PVA solution with 0.05 M hydrazine monochloride (HMC); (C) FH-PVA solution with 0.1 M HMC; (D) FH-PVA solution with 0.3 M HMC. (Storage time: 72 h).

To reveal the dependence on viscosity of the fiber diameter and morphology, FH-PVA solution with 0.1 M HMC was selected as a model solution since its surface tension and conductivity remained virtually constant (shown in Figure 5), even as the viscosity gradually decreased during the storage period: electrospun fibers from this solution are shown in Figure 7. With the original solution, both the conductivity and viscosity are high and it is difficult to form a stable jet and only some spotty deposition was observed on the aluminum foil target (Figure 7A). After several days’ storage, the solution viscosity decreased to 0.25–0.35 Pa·s, and uniform electrospun fibers could be fabricated. However, when the viscosity became too low, a beaded morphology resulted. All of the results indicate that viscosity is a predominant factor affecting fiber diameter. Electrospun fibers with diameter down to about 180 nm can be fabricated by this method of controlling viscosity.

Figure 7.

Scanning electron microscopy (SEM) images of fully hydrolyzed polyvinyl alcohol (FH-PVA) nanofibers produced from solutions containing 0.1 M hydrazine monochloride (HMC) at various storage times: (A) immediately after preparation; (B) 24 h storage; (C) 72 h storage; (D) 120 h storage; (E) 168 h storage.

Conclusion

We present a novel use of HMC for reducing the viscosity of FH-PVA. The viscosity of FH-PVA in aqueous solution decreased gradually with storage time in proportion to the amount of HMC present. This is opposite to the effect of ionic salts. In addition, after 7 days of storage, the viscosity of the FH-PVA test solutions decreased by 60% (related to the concentration of HMC) compared with those of the original solutions. These phenomena are related to the reconfiguration of hydrogen bonding in the solutions, confirmed by SAXS and ¹H NMR spectra observations. The morphology of FH-PVA electrospun fibers produced from solutions with different viscosities were investigated by SEM, and it was observed that the electrospun fibers became more uniform and the diameters became smaller as the concentration of HMC or the storage time increased. A beaded morphology resulted if the viscosity became too low. Therefore, the spinnability of FH-PVA aqueous solutions can be significantly improved using this method, and it is possible to readily produce ultrafine FH-PVA electrospun fibers with diameters below 200 nm.

Footnotes

Funding

This work was mainly supported by the Grant-in-Aid for Global COE Program by the Ministry of Education, Culture, Sports, Science, and Technology, Japan and partially supported by Program for Fostering Regional Innovation in Nagano, granted by Ministry of Education, Culture, Sports, Science, and Technology, Japan (the SAXS studies).

References

Shao

Kim

Gong

Ding

Lee

Park

. Fiber mats of poly(vinyl alcohol)/silica composite via electrospinning. Mater Lett 2003; 57: 1579–1584.

Krumova

López

Benavente

Mijangos

Peren

. Effect of crosslinking on the mechanical and thermal properties of poly(vinyl alcohol). Polymer 2000; 41: 9265–9272.

Okaya

. General properties of polyvinyl alcohol in relation to its applications. In: Finch

(ed.) Polyvinyl alcohol: Development. Chichester: John Wiley & Sons Ltd, 1992, pp. 1–29.

Chiellini

Corti

D’Antone

Solaro

. Biodegradation of poly(vinyl alcohol) based materials. Prog Polym Sci 2003; 28: 963–1014.

DeMerlis

Schoneker

. Review of the oral toxicity of polyvinyl alcohol (PVA). Food Chem Toxicol 2003; 41: 319–326.

Tan

Inai

Kotaki

Ramakrishna

. Systematic parameter study for ultra-fine fiber fabrication via electrospinning process. Polymer 2005; 46: 6128–6134.

Lee

Ohsawa

Lee

Park

Kim

. A study on the PVAc/FeSO4 composite nanofiber via electrospinning. Seni Gakkaishi 2008; 64: 306–311.

Park

Kim

Yoo

Khil

Kim

. Enhanced mechanical properties of multilayer nano-coated electrospun nylon 6 fibers via a layer-by-layer self-assembly. J Appl Polym Sci 2007; 107: 2211–2216.

Doshi

Reneker

. Electrospinning process and applications of electrospun fibers. J Electrostat 1995; 35: 151–160.

10.

Wei

Ohta

Kim

Lee

Khil

Kim

. Development of electrospun metallic hybrid nanofibers via metallization. Polym Adv Technol 2010; 21: 746–751.

11.

Deitzel

Kleinmeyer

Harris

Beck Tan

. The effect of processing variables on the morphology of electrospun nanofibers and textiles. Polymer 2001; 42: 261–272.

12.

Chen

Zhu

Bao

Yang

. Physically controlled cross-linking in gelated crystalline colloidal array photonic crystals. Appl Mater Interfaces 2010; 2: 1499–1504.

13.

Jensen

BEB

Smith

AAA

Fejerskov

Postma

Senn

Reimhult

. Poly(vinyl alcohol) physical hydrogels: noncryogenic stabilization allows nano- and microscale materials design. Langmuir 2011; 27: 10216–10223.

14.

Ossipov

Hilborn

. Poly(vinyl alcohol)-based hydrogels formed by “click chemistry”. Macromolecules 2006; 39: 1709–1718.

15.

Ossipov

Piskounova

Hilborn

. Poly(vinyl alcohol) cross-linkers for in vivo injectable hydrogels. Macromolecules 2008; 41: 3971–3982.

16.

Cavalieri

Chiessi

Villa

Vigano

Zaffaroni

Telling

. Novel PVA-based hydrogel microparticles for doxorubicin delivery. Biomacromolecules 2008; 9: 1967–1973.

17.

Song

Kim

. Characteristic rheological features of PVA solutions in water-containing solvents with different hydration states. Polymer 2004; 45: 2381–2386.

18.

Koski

Yim

Shivkumar

. Effect of molecular weight on fibrous PVA produced by electrospinning. Mater Lett 2004; 58: 493–497.

19.

Park

Ito

Kim

Khil

. Electrospun poly(vinyl alcohol) nanofibers: effects of degree of hydrolysis and enhanced water stability. Polym J 2010; 42: 273–276.

20.

Yao

Haas

Guiseppi-Elie

Bowlin

Simpson

Wnek

. Electrospinning and stabilization of fully hydrolyzed poly(Vinyl Alcohol) fibers. Chem Mater 2003; 15: 1860–1864.

21.

Zhang

Yuan

Han

Sheng

. Study on morphology of electrospun poly(vinyl alcohol) mats. Euro Polym J 2005; 41: 423–432.

22.

Wang

Zhu

Ruan

. Microwave-assisted synthesis and magnetic property of magnetite and hematite nanoparticles. J Nanopart Res 2007; 9: 419–426.

23.

Briscoe

Luckham

Zhu

. The effects of hydrogen bonding upon the viscosity of aqueous poly(vinyl alcohol) solutions. Polymer 2000; 41: 3851–3860.

24.

Hara

Matsuo

. Phase separation in aqueous poly(vinyl alcohol) solution. Polymer 1995; 36: 603–609.

25.

Komatsu

Inoue

Miyasaka

. Light-scattering studies on sol-gel transition in aqueous Solutions of Poly(vinyl alcohol). J Polym Sci Phys Ed 1986; 24: 303–311.

26.

Liu

Cheng

Qian

. Effect of solution concentration on the gelation of aqueous polyvinyl alcohol solution. J Polym Sci Part B: Polym Phys 1995; 33: 1731–1735.

27.

Fong

Chun

Reneker

. Beaded nanoﬁbers formed during electrospinning. Polymer 1999; 40: 4585–4592.

28.

Zong

Kim

Fang

Ran

Hsiao

Chu

. Structure and process relationship of electrospun bioabsorbable nanofiber membranes. Polymer 2002; 43: 4403–4412.

29.

Bianchi

Conio

. Intrinsic viscosity of polyvinyl alcohol in aqueous salt solutions. J Phys Chem 1967; 71: 4563–4564.

30.

Stanley

. Introduction to Phase Transitions and Critical Phenomena. London: Oxford University Press, 1971, pp. 121–121.

31.

Nishikawa

Kasahara

Ichioka

. Inhomogeneity of mixing in acetonitrile aqueous solution studied by small-angle x-ray scattering. J Phys Chem B 2002; 106: 693–700.

32.

Fukasawa

Sato

. Versatile application of indirect Fourier transformation to structure factor analysis: from x-ray diffraction of molecular liquids to small angle scattering of protein solutions. Phys Chem Phys 2011; 13: 3187–3196.

33.

Taylor

. Disintegration of water drops in an electric field. Proc Roy Soc A 1964; 280: 383–397.

34.

Adams

Lerner

. Observation of hydroxyl protons of sucrose in aqueous solution: no evidence for persistent intramolecular hydrogen bonds. J Am Chem Soc 1992; 114: 4821–4829.

35.

Poppe

Van Halbeek

. Nuclear magnetic resonance of hydroxyl and amido protons of oligosaccharides in aqueous solution: evidence for a strong intramolecular hydrogen bond in sialic acid residues. J Am Chem Soc 1991; 113: 361–363.

36.

Poppe

Stuike-Prill

Meyer

Van Halbeek

. The solution conformat ion of sialyl-α(2→6)-lactose studied by modern NMR techniques and Monte Carlo simulations. J Biomol NMR 1992; 2: 109–136.

37.

Harris

Rutherford

Milton

Homans

. Three-dimensional heteronuclear NMR techniques for assignment and conformational analysis using exchangeable protons in uniformly ¹³C-enriched oligosaccharides. J Biomol NMR 1997; 9: 47–54.

38.

Van Halbeek

Poppe

. Conformation and dynamics of glycoprotein oligosaccharides as studied by ¹H NMR spectroscopy. Magn Reson Chem 1992; 30: 74–86.

39.

Loscertales

Barrero

Guerrero

Cortijo

Marquez

Gañán-Calvo

. Micro/nano encapsulation via electrified coaxial liquid jets. Science 2002; 295: 1695–1698.

40.

Reneker

Yarin

Fong

Koombhongse

. Bending instability of electrically charged liquid jets of polymer solutions in electrospinning. J Appl Phys 2000; 87: 4531–4547.

41.

Yarin

Koombhongse

Reneker

. Bending instability in electrospinning of nanofibers. J Appl Phys 2001; 89: 3018–3026.

42.

Zeleny

. The electrical discharge from liquid points and a hydrostatic method of measuring the electric Intensity at their surfaces. Phys Rev 1914; 3: 69–91.

43.

Morton WJ. Method of dispersing fluids. US Patent 705691, 1902.

44.

Baumgarten

. Electrostatic spinning of acrylic microfibers. J Coll Interf Sci 1971; 36: 71–79.

45.

Larrondo

John Manley

. Electrostatic fibre spinning from polymer melts. I. experimental observations on fiber formation and properties. J Polym Sci: Polym Phys Ed 1981; 19: 909–920.

46.

Doshi

Reneker

. Electrospinning process and applications of electrospun fibers. J Electrost 1995; 35: 151–160.

47.

Srinivasan

Reneker

. Structure and morphology of small diameter electrospun aramid fibers. Polym Int 1995; 36: 195–201.