Electrospinning PVA Solution-Rheology and ... - Springer Link

Fibers and Polymers 2012, Vol.13, No.1, 44-50

DOI 10.1007/s12221-012-0044-9

Electrospinning PVA Solution-Rheology and Morphology Analyses Syang-Peng Rwei* and Cheng-Chiang Huang Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei, Taiwan, R.O.C. (Received March 29, 2011; Revised August 20, 2011; Accepted August 28, 2011) Abstract: This study investigates the electrospinning (ES) of poly(vinyl alcohol) (PVA). All of the electrospinning process or property parameters, including the concentration effect, the molecular weight effect, the pH effect, the salt effect, electrode voltage, surface tension, shear viscosity and extensional viscosity were examined. The pH variation had an insignificant effect on the formation of fibers. An increase in electrode voltage and salt concentration negatively affects the ES process. The salt concentration that yields an acceptable ES membrane without droplets was below 0.001 N. Also, the decrease in elongation viscosity rather than the variation in electric conductivity or surface tension was the main cause of the negative effect on the fiber formation when the salt was added to a PVA solution. The salt negative effects follow the order CaCl2 < NaCl < NaI < KBr < KI. Experimental results show that the ES processability of PVA solution depends mainly on the concentration and secondly on the molecular weight of the dissolved polymer. The PVA solution prepared with a larger molecular weight had a lower concentration window in the ES process. The concentration window of PVA solution with an MW of 88,000 in the ES process ranged between 6 and 14 wt%. Additionally, experimental results demonstrated that the upper limitation on PVA concentration depends strongly on the extension viscosity of the spun solution. Whenever the power law index n determined by extension test exceeds one, spinning is unfeasible, regardless of whether the index of the power law for shearing is within the normal range. Briefly, this work indicates that the extension viscosity can be adopted as a good indicator for predicting ES processability. Keywords: Electrospinning (ES), Poly(vinyl alcohol), Power law equation, Extension viscosity

the first related patent, until the early 1990s, when several research groups demonstrated that some organic polymers could be electrospun into nanofibers [10]. Investigations of the ES process then became very popular. Since the ES process shares characteristics of both electrospraying and conventional wet-spinning processes, several parameters might affect the fiber formation, including concentration of the solution [11], pH [12], molecular weight [13], electrode voltage [14], salts [15], surface tension [16] and solution rheology. Deitzel et al. discovered that an ES process must carry out in a certain range of concentrations. A low concentration (PEO solution < 4 %) will generate a mixture of fibers and droplets. However, a high concentration (PEO solution > 15 %) makes spinning impossible because capillary will block up. Son et al. found that the diameter of fibers is relative small in the ES process at high or low pH, because of the conductiveity is higher than neutral. Dhirendra et al. found that a voltage threshold must be exceeded for the formation of the Taylor cone but the fiber diameter decreases as the voltage increases. The process parameters associated with various materials have been sporadically investigated, as described above, and an extensive and systematic investigation of PVA solution in the ES process is still lacking. Koski et al. [13] concluded that the diameter of PVA fiber increased with the molecular weight (Mw), and the suitable range for ES is between 9000 and 85000 g/mole. At high Mw, 85000 to 180000 g/mole, a broad distribution of fiber diameter was observed. Zhang et al. [17] suggested that the DH (degree of hydrolysis) value of PVA should be no less than 88 % to achieve a good ES process. Li et al. [18] has

Introduction Polyvinyl alcohol (PVA) is a water-soluble polymer [1,2] with many hydroxyl groups pendant in the side chains [3]. It has been studied intensively because it has high hydrophilicity, processability, biocompatibility [4], good physical and mechanical properties, complete biodegradability [3], excellent chemical resistance, and a favorable capacity to form a film [3,4]. These properties have led to its broad industrial use in, for example, medical wrapping membranes, drug delivery, adhesive and thickener materials, filtration applications and gas barrier applications. In the fiber industry, PVA was spun to form monofilaments for concrete reinforcement. Recently, ultrafine PVA fibers [2], made by electrostatic spinning, have been investigated extensively for some biomedical applications. Electrospinning (ES) uses an electrical charge to draw very fine fibers from a liquid. In the electrospinning process, a high voltage is applied to create an electrically charged jet of polymer solution, which dries to leave a polymer fiber [5]. When the electric field strength is about to overcome the surface tension of the fluid, the free surface of the suspended drop changes to a cone, which is commonly referred to as the Taylor cone [6,7]. The mutually repulsive forces of the electric charges of the jets cause the polymer solution to be emitted from the Taylor cone [8]. The jet within the electric field is directed toward the grounded target, while the solvent evaporates and fibers are formed [9]. Little attention was paid to the ES process from 1902, when Cooley filed *Corresponding author: [email protected] 44

Electrospinning PVA Solution

Fibers and Polymers 2012, Vol.13, No.1

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Table 1. A summary of the literature survey of PVA electrospinning No 1

Ref. Koski et al. (2004) [13]

2

Zhang et al. (2005) [17]

Mw (DH%) of PVA 9000-10,000 (98-99 %) 13,000-23,000 (98 %) 31,000-50,000 (98-99 %) 50,000-85,000 (97 %) 89,000-98,000 (98-99 %) 124,000-186,000 (99 %) 72,600-77,000 (80-99 %)

Effect of parameters Molecular weight, Concentration (wt%)

Concentration (wt%), Voltage (kV), Collector distance (cm), Flow rate (ml/h), Shear viscosity (p), Surface tension (mN/m), Conductivity (mS/cm), Salt (NaCl), co-solvent (ethanol) 3 Li et al. (2008) [18] 88,000 (unknown) Concentration (wt%), Salt (LiCl), 4 Phachamud1et al. (2011) [19] Unknown (97.5-99.5 %) Concentration (wt%), Shear viscosity (cp), Conductivity (µS), Collector distance (cm), Voltage (kV) 5 Rwei et al. (this work) 16,000 (98 %) Concentration (wt%), Voltage (kV), Shear viscosity (p), Surface 88,000 (98 %) tension (mN/m), pH, Conductivity (mS/cm), Extensional viscosity (p), Salt (NaCl, NaI, KBr, KI, CaCl2) Note: The terms in bold and slant character are the unique properties studied in this paper only.

studied the effect of LiCl on the ES process of PVA and a higher bond of LiCl concentration, 0.8 wt% was concluded. Phachamud et al. [19] pointed out that the concentration is the main factor to affect the morphology of PVA fiber; the concentration to achieve an acceptable ES process was from 8 to 12 wt%. Unfortunately, Phachamud did not report the molecular weight and gave further explanation for the optimum concentration range from a physical point of view. This study examines the factors in the ES process of PVA solution beyond the studied issues or ranges mentioned above - focusing especially on the rheological and morphological aspects. A summary of the above surveys along with the special properties of this study are tabulated in Table 1. The final objective of this study is hopefully to identify the factor that dominates the ES process.

Experimental The ES setup in this work consists of a hypodermic syringe needle, connected to a high-voltage (10 to 30 kV) power supply and a grounded collector plate. A DC power supply (ES30P-5w/DAM, Gamma High Voltage Research Co., USA), a copper board (22×22 cm), a capillary needle (No.20, Hung Duen Co., Taiwan) and a syringe (5 ml, Hung Duen Co., Taiwan) were chosen herein to conduct the ES process. The PVA (M.W. 88,000, 16,000 denoted as PVA1 and PVA2, respective, DH: 98 %, Scientific Polymer Products, USA) aqueous solution with various concentrations is loaded into the syringe and is extruded from the tip of the needle at a constant rate using a syringe pump, yielding a droplet at the tip. As the voltage was increased to a threshold of around 10 kV, when the repulsive electrostatic force overcomes the surface tension, a charged PVA stream began to be ejected from the tip of the Taylor Cone. The PVA

Figure 1. Illustration of CaBER operation principle.

solutions were prepared by adding a given amount of PVA1 or PVA2 powder to the water at 85 oC with stirring. The concentrations of PVA1 and PVA2 examined herein were 6 to 14 wt% and 14 to 24 wt%, respectively. The rheological measurements in shear mode were made using a Rheometrics SR-5 rheometer (SR5, Rheometric Scientific Co., USA). A parallel plate geometry comprising a titanium upper plate and an aluminum-coated lower plate both of diameter 2.5 cm were used. The rheological measurements in extension mode were made using a Capillary Breakup Extentional Rheometer (abbreviate CaBER, HAKKE Caber1, Thermo Co., USA), which included a laser beam as a detector to measure the diameter of the extended drop (Figure 1). The main data obtained from the CaBER refer to the evolution of the midpoint diameter of fluid samples with time. This evolution is driven by the surface tension of the stretched drop and resisted by the extensional stress in the fluid. The analysis proceeds using equation (1). σ ηapp ( ε ) = ------------dDmid ------------dt

(1)

Where η is the apparent extensional viscosity; σ is the

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surface tension, and dDmid/dt is the rate of shrinkage of the diameter of the stream midpoint. Other equipment used to characterize the PVA solution in this work includes a surface tensiometer (CBVP-Z, Face Co., USA), a pH-meter (6010, JENCO, Co., China), and a scanning electron microscope (S-4300, HITACHI Co., Japan).

Results and Discussion Figures 2(a) to 2(g) depict the SEM pictures of PVA fibers made by the ES process with various concentrations of PVA1. Figure 2(a) indicates that no fiber can be formed when a dilute PVA1 solution is used with concentration less than 6 wt%, because the PVA polymer is too dilute to enable the molecules to be entangled for forming a fiber. In contrast, Figure 2(g) demonstrates that fiber formation is infeasible when the concentration of PVA1 solution is over 14 wt%. PVA2 solutions with a low molecular weight yielded similar results but with a higher concentration window of 14

Figure 2. The SEM images of PVA1 with different concentrations; (a) 6, (b) 8, (c) 10, (d) 12, (e) 13, (f) 13.5, and (g) 14 wt% (5000×).

Syang-Peng Rwei and Cheng-Chiang Huang

to 24 wt%. To form a fiber via ES process, the concentration of a solution of a polymer with a lower molecular weight, PVA2, must be increased to maintain the entanglement points of PVA1. The major factor that determines the upper concentration limit, 14 wt% for the PVA1, or 24 wt% for the PVA2, may be the surface tension, the ES field strength, the shear viscosity or the elongation viscosity. This topic is particularly interesting and demands further study. Notably, for quickly identifying the dominant factors, only PVA1 was used for all the subsequent examinations except the rheological test. Briefly, Figures 2 and 3 demonstrate that the ES processability of the PVA solution depends primarily on the concentration and secondly on the molecular weight of the dissolved polymer. A PVA solution with higher molecular weight exhibited a lower concentration window in the ES process. Figure 4 indicate that regular ES fiber can be formed at low pH, medium pH, or high pH: in Figures 4(a), 4(b), and 4(c) pH equals 2, 7, and 12, respectively. Figure 5 reveals that the diameter of the fiber over a broad pH range was constant, except for a slight decrease at a pH of 2 because of the decrease in viscosity in an acidic environment. Figures 4 and 5 reveal that the variation of pH negligibly affects the formation of PVA fiber via the ES process. Figures 6(a) to (d) exhibit the salt effect on the ES process. Continuously increasing the salt concentration to certain

Figure 3. The SEM images of PVA2 with different concentrations; (a) 14, (b) 16, (c) 18, (d) 20, (e) 22, and (f) 24 wt% (5000×).


Figure 4. The SEM images of PVA1 with different pH values; (a) 2, (b) 7, and (c) 12 (5000×).


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Figure 6. The SEM images of 8 wt% PVA1 with different concentrations of calcium chloride in an ES process under 10 kV (a) 0.0001 N, (b) 0.0005 N, (c) 0.001 N, and (d) 0.002 N (5000×).

Figure 5. The dependence of fiber diameter on the pH of PVA solution in an ES process.

level can cause the nonwoven surface to be filled with droplets. The salt concentration that yields an acceptable ES membrane without any droplet was below 0.001 N. Increasing the salt concentration from 0.0001 to 0.002 N increases the electrical conductivity of the PVA solution by a factor of four and reduces the elongation viscosity by a factor of five. Figure 7 provide advanced information to elucidate the main cause, between the electric conductivity and the viscosity, of the salt effect. Various salts with a fixed concentration (0.001 N) were added to the 8 wt% PVA solution to study the ES spinnability. Figures 7(a) to (e) display the SEM images. Figure 7 reveal great variation of fiber formation with the species of salt added to the PVA solution. For example, the SEM images reveal a perfectly fibrous shape without any droplets when calcium chloride was added; in

Figure 7. The SEM images of 8 wt% PVA1 with different kind salts (0.001 N) in an ES process under 10 kV; (a) calcium chloride, (b) sodium chloride, (c) sodium iodide, (d) potassium bromide, and (e) potassium iodide (5000×).

contrast, the worst fibers, with non-uniform diameters and many droplets, were obtained when the sodium iodide or potassium iodide was added. Interestingly, the electrical

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Table 2. Salt effect of PVA solution Concentration 10 % PVA1+CaCl2 10 % PVA1+NaCl 10 % PVA1+NaI 10 % PVA1+KBr 10 % PVA1+KI

Surface tension (mN/m) 54.3 56.8 56.8 53.7 56.3

Extensional viscosity (P) 18.0717×10-2 6.3445×10-2 3.7523×10-2 1.3077×10-2 0.1516×10-2

Figure 8. The SEM images of 8 wt% PVA1 with 0.0005 N sodium iodide in an ES process under different voltages (a) 10 kV and (b) 15 kV (5000×).

conductivity under all conditions was approximately 5.5, regardless of the salt species. However, the elongation viscosity, shown in Table 2, varied significantly, in a manner that was related to the feasibility of the formation of the fiber (Figure 7). Table 2 demonstrates the independence of the surface tension from the salt species. However, the decrease in elongation viscosity was the major cause of the negative effect of the salt in the PVA solution during the ES process. The decrease in viscosity indicates that the disentanglement of the polymer chains suppresses the spinning process. This process will be discussed in detail later. Finally, both Table 2 and Figure 7 reveal that the negative effect of the salt on the

ES process increases the order CaCl2 < NaCl < NaI < KBr < KI. Figure 8 reveal that increasing the output voltage above the crucial ES threshold negatively affects the fiber formation. Increasing the electrical power from 10 to 15 kV actually produces several droplets, as revealed by comparing Figures 8(b) to 8(a). This result is consistent with those of Deitzel et al. [11], who found that a high voltage might cause a spin-instability from a regular Taylor-cone shape to a random multi-jet situation. Surface tension is another factor related to the ES process that must be clarified. Table 3 indicates that surface tension decreases as the PVA concentration increases. The ES process depends on charge repulsion to overcome the surface tension. Increasing the concentration reduces the surface tension but equilibrium is reached at a crucial point, such as 10 wt% for PVA1, as shown in Table 3. The lower surface tension in the PVA solution of higher concentration does not reasonably explain the upper concentration limit in the ES process for two reasons. First, the drop in surface tension is too small to be correlated with the abrupt breakdown of the ES process above the concentration threshold. Moreover, the decrease in surface tension as the concentration increases actually promotes spinning because the shrinking force decreases as the fiber is extended. Other factors that are responsible for the upper limit of ES are investigated later. Nevertheless, the surface tension is still the main driver of the formation of droplets after a failed ES process, regardless of whether the failure is associated with excessively dilute or concentrated spinning conditions. In the ES process, polymer solution was extended by an attractive force between electrodes. This action can be duplicated and analyzed using an extensional rheology apparatus called a CaBER rheometer. The Experimental section above, presented details of the principles that underline the CaBER design. Figure 9 presents the raw data

Table 3. Concentration effect of PVA solution Concentration 6 % PVA1 (A) 8 % PVA1 (B) 10 % PVA1 (C) 12 % PVA1 (D) 13 % PVA1 (E) 13.5 % PVA1 (F) 14 % PVA1 (G) 16 % PVA2 (H) 18 % PVA2 (I) 20 % PVA2 (J) 22 % PVA2 (K) 24 % PVA2 (L)

Surface tension (mN/m) 43.05 42.57 41.35 39.27 39.01 38.89 38.46 43.10 42.90 41.20 40.86 40.55

n by extension 1.00 1.00 0.98 0.93 0.89 0.94 1.38 0.93 0.99 0.97 0.96 1.28

m by extension 0.82 0.99 28.89 46.64 73.16 106.44 540.92 12.75 20.70 24.50 40.94 320.15

n by shearing 1.00 1.00 0.99 0.97 0.99 0.99 0.94 0.99 0.99 0.99 0.98 0.98

m by shearing 0.96 3.83 10.86 32.06 41.79 48.28 82.17 4.36 7.90 15.08 32.65 49.32

Average fiber diameter (nm) 236±5 256±5 303±5 422±5 426±5 434±5 462±5 113±5 140±5 154±5 202±5 320±5


Figure 9. Illustration of the normalized filament diameter versus time obtained from CaBER test. Conditions (A)-(G) and conditions (H)-(L) represent different concentrations of PVA1 and PVA2, respectively. PVA1: (A) 6 %, (B) 8 %, (C) 10 %, (D) 12 %, (E) 13 %, (F) 13.5 %, (G) 14 %, PVA2: (H) 16 %, (I) 18 %, (J) 20 % (K) 22 %, (L) 24 %.

Figure 10. Illustration of apparent extensional viscosity versus strain rate of PVA1 and PVA2 solution in an ES process. The experimental conditions were defined same as previous plot (Figure 9).

of filament diameter against extensional time obtained using the CaBER. Given the surface tension, the extensional viscosity against the elongation rate can be calculated using equation (1), yielding the results in Figure 10. Figure 10 shows an abrupt increase in the extension viscosity as the concentration increases above a critical value. Interestingly, the critical concentration in the CaBER test, which is 14 wt% PVA1 and 24 wt% PVA2, respectively, matches exactly the threshold for the non-feasibility on the ES process, as determined in previous concentration part. The extension actually causes each polymer chain to be oriented in the extensional direction. In a dilute solution, the


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Figure 11. The plot of shear viscosity against shear rate. The experimental conditions were defined same as previous plot (Figure 9).

orientation causes some disentanglement among the polymers that are in contact with each other, the extension deformation can therefore be increased. However, increasing the concentration promotes the entanglements of the chains. Serious entanglements finally form a 3-D network when the concentration exceeds a threshold, and the system then becomes a true gel, making the extension process infeasible thereafter. The extensional viscosity therefore increases abruptly, in what is extensional thickening. The ES process fails thereafter because elongation to form fibers becomes difficult. The following quantitative approach based on typical rheological analysis was implemented to investigate the ES process. Figure 11 plots the shear viscosity against the shear rate at various concentrations. The power law model was used to analyze quantitatively the relationship between shear viscosity and strain rate, as follows (equation (2)). η = mγ

n–1

(2)

Where η represents the shear viscosity; m represents the fluid consistency coefficient; γ is the shear strain rate, and n represents the power law index. The logarithm of both sides of equation (2) was taken, yielding equation (3), as follows. log( η ) = ( n – 1 )log( γ ) + log ( m )

(3)

Table 3 presents the fitting results of n and m for the shearing teat. All values of the power law index n determined by shearing are close to, but less than one. This result demonstrates that a concentrated PVA solution exhibits shear thinning, as exhibited by a normal polymer, because shearing orientates the chain. The power law index n decreases steadily as the concentration increases. The “steady decrease” does not provide any insight into the sudden breakdown termination of the ES process.

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As mentioned above, Figure 10 displays the relationship between extension viscosity and extensional rate. According to the same analysis as in equations (2) and (3), the power law index n determined by extension and the fluid consistency coefficient m can be calculated; they are also presented in Table 3. Table 3 demonstrates that when the power law index n determined by extension exceeds one, spinning is infeasible, regardless of whether the power law index determined by shearing is still in normal range. The extension thickening phenomenon, where n is greater than one, indicates that the extension produces hard entanglements among the polymer network, inhibiting further extension of the polymer chains. This process occurred as the solution changes to a gel. The loosened entanglements in the gel state can easily be tightened up under tension, causing extension thickening. Notably, this result indicates that the higher bound of concentration for ES spinnability has strong relationship with the extensional viscosity, which is probably the most important in this study. However, the lower bond of concentration for ES spinnability, which can actually be easily determined by the droplet formation in a SEM image, still has no correlation with the rheological properties.

Conclusion This study examined the electrospinning of PVA. All the major factors in the electrospinning process were examined. Varying the pH insignificantly affected the formation of fibers. Increasing the electrode voltage and the salt concentration negatively influenced the ES process. The salt concentration that yielded an acceptable ES membrane without droplets was below 0.001 N. Also, the decrease in elongation viscosity, instead of the increase in electrical conductivity, was the major cause of the negative effect of the addition of salt to a PVA solution. The negative effect of adding the salt increased in the order CaCl2 < NaCl < NaI < KBr < KI. Experimental results demonstrated that the ES processability of PVA solution depends primarily on the concentration and secondly on the molecular weight of the dissolved polymer. The PVA solution prepared with higher molecular weight exhibited a lower concentration window in an ES process. The concentration window of the PVA solution, with an MW of 88,000 in the ES process falls between 6 and 14 wt%. Moreover, experimental results demonstrated that the upper limit on the PVA concentration, 14 wt%, is strongly related to the extension viscosity of the spun solution. Whenever the power law index n associated with extension exceeded one, spinning was infeasible, regardless of whether the power law index n associate with shearing was still in a normal range. In short, this study


demonstrates that the extension viscosity is a good indicator for predicting ES processability.

Acknowledgements The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 95-2622E-027-033-CC3.

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