Relative Humidity Effect on the Preparation of Porous ...

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Journal of Nanoscience and Nanotechnology Vol. 10, 3473–3477, 2010

Relative Humidity Effect on the Preparation of Porous Electrospun Polystyrene Fibers Ju-Young Park and In-Hwa Lee∗ Department of Environmental Engineering, BK21 Team for Biohydrogen Production, Chosun University, Gwangju, 501-759, Korea

RESEARCH ARTICLE

Porous polystyrene (PS) fibers were prepared by relative humidity control during electrospinning process. The relative humidity and solvent mixing ratio strongly affect the surface morphology and average diameter of electrospun PS fibers. In the circumstance of 30% relative humidity at MC/EtOH (90/10, v/v), pores did not form on the surface of polystyrene fibers. However, as the relative humidity increased to 60%, pores appeared on the fiber the same composition of solvent. In Delivered by surface Ingentaat to: comparison, solvent ratio of MC/EtOH (80/20, v/v) gave rather smooth surface of PS fibers. When Dental Library Seoul Natl Univ the MC/EtOH ratio are 90/10 (v/v) and 80/20 (v/v), electrospun PS fibers with minimum average IP : 147.46.182.248 diameter of 5,211 nm (SD = 1,986) and 5,315 nm (SD = 1,039) were prepared. Surface area and Sun, 26 Sep 2010 14:26:25 average pore size are found to be 30.7 m2 /g and 8.7 nm, respectively, with the relative humidity of 40%.

Keywords: Electrospinning, Polystyrene, Relative Humidity, Porous Fibers.

1. INTRODUCTION The surface morphology of electrospun fibers were affected by many parameters related to polymer concentration, applied voltage, spinning distance, air friction, gravity, temperature and ambient parameter.1 2 Porous membranes have been prepared from phase separation and electrospinning system.3 4 Porous fibers of a variety of polymers were prepared through electrospinning process using a bath of liquid nitrogen, which induced a phase separation between the polymer and the solvent.5 Porous polymer fibers can be obtained through thermally induced phase separation (TIPS) between the solvent-rich and solvent-poor regions in the fiber during electrospinning, followed by removal of solvent.5 Meanwhile, some researches have reported that porous fibers or fibers with nanoscaled structures were obtained when a highly volatile solvent is used in the electrospinning process. When a solution of poly(L-lactide) (PLLA), polycarbonate (PC), polyvinylcarbazole, poly(methyl methacrylate) (PMMA), poly(ethyleneoxide) (PEO), and polystyrene in highly volatile solvents was electrospun, many pores appear on the surface of electrospun fibers.3 5 Interestingly, some approaches considered the electrospinning of mixtures composed of two immiscible polymers and a common solvent. A phase-separated structure ∗

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J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 5

can be formed within the electrospun fibers after the evaporation of the solvent. In order to enhance the roughness of the fibers or yield overall porosity, selective remove one component is necessary. Selective dissolution can be employed to remove one component.6 7 Bognitzki group reported the preparation of nanostructured PLA and PVP fibers by selective removal of one of the polymer phases after electrospinning from a ternary solution.8 In a particular humidity environment, electrospinning can also generate porous fibers, and the pore size could be adjusted by altering the humidity values.3 9 The relative humidity of the electrospinning environment may have an influence in the polymer solution during electrospinning. At high relative humidity, it is likely that water condenses on the surface of the fibers when electrospinning carried out under normal atmosphere.3 8 9 The relative humidity of the environment will also determine the rate of evaporation of the solvent in the solution. At a very low relative humidity, a volatile solvent may dries very rapidly. The evaporation of the solvent may be faster than the removal of the solvent from the tip of the needle. As a result, the electrospinning process may only be carried out for a few minutes before the needle tip is clogged. This may also happen during electrospinning as water vapor may condense on the surface of the jet as a cooling of the surface of the jet as a result of rapid and solvent eventually evaporate.9

1533-4880/2010/10/3473/005

doi:10.1166/jnn.2010.2349

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Relative Humidity Effect on the Preparation of Porous Electrospun Polystyrene Fibers

RESEARCH ARTICLE

For the preparation of porous electrospun fibers using electrospinning process, several researchers have been studied that by using a highly volatile solvent, selective removal of one component of electrospun polymer blend fibers and electrospinning environment (liquid nitrogen and relative humidity). There was no research about porous PS fibers by relative humidity control of electrosping process and different volatility of binary solvent. We report the effect of changing relative humidity and solvent ratios on the surface morphology of electrospun polystyrene fibers. Porous polystyrene fibers were characterized by field emission scanning electron microscopy (FE-SEM), atomic force microscope (AFM) and BrunaueEmmet-Teller (BET).

Park and Lee

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2. EXPERIMENTAL DETAILS 2.1. Materials

Delivered fromby Ingenta to: Polystyrene (PS) (Mn 280,000 Dalton) was purchased Dental Library Aldrich, and methylene chloride (GR grade) and ethanol Seoul Natl Univ IPwithout : 147.46.182.248 (EP grade) were purchased from Junsei, and used Sun, 26 Sep 2010 14:26:25 further purification. 2.2. Preparation and Characterization The morphology of electrospun PS fibers was observed by FE-SEM (S-4800, Hitachi Ltd., Japan), the average diameter was measured by Image J (National Institutes of Health, USA). Nitrogen physisorption isotherms were measureed on a nanoporosity (miraesi co, Ltd, korea) at 77 K. The specific surface areas were evaluated using the BrunaueEmmet-Teller (BET) method. The pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method based on the adsorption branch of the isotherms, and the average pore size was reported from the peak position of the distribution curve. The pore volume was estimated at a relative pressure range of 1.00.

3. RESULTS AND DISCUSSION In general, a broad range of porous polymer fibers with large surface areas have been produced by electrospinning from several highly volatile solvents.3 This experiment is carried out in binary solvent system by the relatively higher volatile methylene chloride and relatively lower volatile ethanol. The solvent mixing ratios cause significant changes in average diameter of fibers, surface of fibers and overall morphology due to the evaporation of the solvent and the variation of the viscosity.10 Figure 1 shows the FE-SEM images of electrospun PS fibers at various ratios of MC/EtOH binary solvent system. The influence by the vapor pressure of MC/EtOH binary solvent could be observed from the formed nanopores on the fibers surface. As MC/EtOH ratio increased, the volatility of the binary solvent decreased, and nanopores size 3474

Fig. 1. FE-SEM images of electrospun polystyrene fibers at different solvent ratio; (a, b) MC/EtOH = 95/5 (v/v), (c, d) MC/EtOH = 90/10 (v/v), (e, f) MC/EtOH = 80/20 (v/v). (concentration; 15 wt%, applied voltage; 15 kV, flow rate; 100 l/min, TCD; 10 cm, temperature; 25 C, humidity; 52%).

on fiber surface changed. In particular, as the MC/EtOH ratio increased, the nanopores size and distribution of PS fibers surface decreased. For the case of poly(L-lactic acid) (PLLA) porous nanofibers by Bognizke et al., the replacement of methylene chloride by chloroform with lower vapor pressure effected on the formation of pores. The average diameter of electrospun PS fiber were 6,696 nm (SD = 2,293), 5,852 nm (SD = 1,696) 6,940 nm (SD = 1,246), respectively when the MC/EtOH/(v/v) ratio were 95/5, 90/10 and 80/20. A solution of 15 wt% PS in MC/EtOH (90/10) was electrospun under different relative humidity range (30 to 70%) to prepare porous fibers. As the humidity changed, the surface morphology of electrpsun PS fibers was varied. When the relative humidity is 30%, electrospun fibers of twist form with smooth surface were observed. At the relative humidity 40%, PS fibers of some porous surface were prepared. Increasing the relative humidity up to 50∼60% caused a visible difference in the surface morphology of the fibers (Figs. 2(e∼h)). Figures 2(g, h) shows that PS fibers of much porous surface at the relative humidity 60%. Also, when the relative humidity is 60%, surface of the fibers change on uniform in shape and are slightly larger J. Nanosci. Nanotechnol. 10, 3473–3477, 2010

Park and Lee


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Fig. 3. AFM image of PS fiber (Fig. 2(h)).

RESEARCH ARTICLE

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Delivered by Ingenta to: Dental Library Seoul Natl Univ IP : 147.46.182.248 Sun, 26 Sep 2010 14:26:25 (g)

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Fig. 2. FE-SEM images of electrospun polystyrene fibers at different relative humidity; (a, b) 30%, (c, d) 40%, (e, f) 50%, (g, h) 60%, (i, j) 70% (concentration; 15 wt%, binary solvent; MC/EtOH = 90/10 (v/v), applied voltage; 15 kV, flow rate; 100 l/min, TCD; 10 cm, temperature; 19 ± 1 C).

than those obtained from 50%. The results of these phenomenon is similar to PS/THF experimental by Cheryl et al.9 When the solvent evaporates from the fibers surface to the air, the vaporization for the solvent takes a latent heat of the fiber surface and the surrounding air. When the temperature of both the surface and the atmosphere decreases to the dew point, the relative humidity increased to 100% and the vapor is saturated. Therefore, fast solvent evaporation at higher relative humidity has more chance to condense the moisture on the surface of electropun fibers. However, low relative humidity cannot reach to the dew J. Nanosci. Nanotechnol. 10, 3473–3477, 2010

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Fig. 4. FE-SEM images of electrospun polystyrene fibers at different relative humidity; (a, b) 30%, (c, d) 40%, (e, f) 50%, (g, h) 60%, (i, j) 70% (concentration; 15 wt%, binary solvent; MC/EtOH = 80/20 (v/v), applied voltage; 15 kV, flow rate; 100 l/min, TCD; 10 cm, temperature; 19 ± 1 C).

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Relative Humidity Effect on the Preparation of Porous Electrospun Polystyrene Fibers (a)

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Table I. BET surface area, BJH mesorpore Area, BJH mesopore volume and BJH mesorpore diameter of 15 wt% PS under MC/EtOH (90/10) solvent ratio with different humidity. Humidity a

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BET surface area (m /g) Mesorpore area (m2 /g) c Mesopore volume (cm3 /g) d Mean mesorpore diameter (nm) b

30%

40%

50%

60%

718 2196 017 1329

307 3447 014 87

158 3390 024 1236

249 3262 012 723

SBET: Specific surface area was calculated by BET method. b Smeso: Mesopore (1.7∼5000 nm) surface area calculated with Barret, Joyner and Halenda (BJH) metod on the Kelvin equation. c Vmeso: Mesopore (1.7∼5000 nm) volume calculated with Barret, Joyner and Halenda (BJH) metod on the Kelvin equation. d Wmeso: Average mesopore width calculated with BJH method.

RESEARCH ARTICLE

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Casper group reported that the PS/THF applied to electrospinning processs as a function of different humidity, (b) where humidity does not affect the shape or diameter of the electrospun PS fibers. These results do show that humidity has a large effect on the surface morphology of Delivered by Ingenta to: the fibers.9 On the hand, when PS in THF/DMF binary Dental Library Seoul Natl Univ solvent system was applied to electrospinning as a funcIP : 147.46.182.248 tion of various humidity, the humidity do affect not only Sun, 26 Sep 2010 14:26:25 the surface morphology of the PS fiber but also the average diameter of PS fibers.2 Our experimental results are similar to Kim group’s results. The BET surface area, BJH mesorpore area, BJH mesorpore volume and BJH mesorpore diameter for porous electrospun PS fibers at various relative humidity are listed in Table I. The surface area determined by nitrogen adsorption/desorption isotherm data was found to be highest Fig. 5. Average diameters of the electorospun PS fibers as a function 30.7 m2 /g at relative humidity 40%, where the average of relative humidity; (a) MC/EtOH = 90/10, (b) MC/EtOH = 80/20. pore size calculated by BJH method was 8.7 nm. point of water so that there is no pore distribution on the surface. As the relative humidity increases, bigger water droplets can be formed.2 However, when the humidity was over 70%, pore of surface disappeared. Figure 3 shows the AFM image of PS fiber (Fig. 2(h)). Figure 4 shows result of electrospun PS fibesr as relative humidity at MC/EtOH ratio of 80/20 (v/v). On the whole, surface of electrospun PS fibers are not regular. As the relative humidity increased, the surface of PS fibers was observed to have micro hole. The change of the PS fibers surface according to relative humidity are different from MC/EtOH of 90/10 (v/v) ratio. When the EtOH was added to, the surface morphology of PS fibers was not affected to relative humidity due to the lower volatility of EtOH than MC. As the relative humidity increased, an average diameter decreased (Fig. 5). When the humidity is too high, the electrospun with higher diameter is noted. When the MC/EtOH ratio are 90/10 (v/v) and 80/20 (v/v), electrospun PS fibers with minimum average diameter of 5,211 nm (SD = 1,986) and 5,315 nm (SD = 1,039) was respectively obtained. 3476

4. CONCLUSION A solution of 15 wt% PS in MC/EtOH (90/10) was electrospun under different relative humidity range (30 to 70%) to prepare porous fiber. The fibers have smooth surfaces and having twisted form, when the humidity is less than 40%. The porous surface of PS fibers was found with increasing humidity from 40 to 60%. When the MC/EtOH ratio are 90/10 (v/v) and 80/20 (v/v), electrospun PS fibers with the minimum average diameter of 5,211 nm (SD = 1,986) and 5,315 nm (SD = 1,039) were prepared. When the relative humidity is 40%, surface area and average pore size are 30.7 m2 /g and 8.7 nm, respectively.

References and Notes 1. J. M. Deitzel, J. Kleinmeyer, K. Harris, and N. C. B. Tan, Polym. 42, 261 (2001). 2. G. T. Kim, J. S. Lee, J. Y. Sin, Y. C. Ahn, Y. J. Hwang, H. S. Sin, J. K. Lee, and C. M. Sung, Korean J. Chem. Eng. 22, 783 (2005). 3. S. Megelski, J. S. Stephens, D. B. Chase, and J. F. Rabolt, Macromolecules 35, 8456 (2002).

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4. P. Gibson, H. S. Gibson, and D. Rivin, Colloids and Surfaces A 187, 469 (2001). 5. J. T. McCann, M. Marquez, and Y. J. Xia, Am. Chem. Soc. 128, 146 (2006). 6. Y. You, J. H. Youk, S. W. Lee, B. M. Min, S. J. Lee, and W. H. Park, Mater. Lett. 60, 757 (2006). 7. W. S. Lyoo, J. H. Youk, S. W. Lee, and W. H. Park, Mater. Lett. 59, 3558 (2005).

8. M. Bognizki, W. Czado, T. Frese, A. Schaper, M. Hellweg, M. Steinhart, A. Greiner, and J. H. Wendorff, Adv. Mater. 13, 70 (2001). 9. C. L. Casper, J. S. Stephens, N. G. Tassi, D. B. Chase, and J. F. Rabolt, Macromolecules. 37, 573 (2004). 10. S. Ramakrishna, K. Fujihara, W. E. Teo, T. C. Lim, and M. Zuwei, Introduction Electrospinning and Nanofibers, World Scientific Publishing Company, Singapore (2005).

Received: 1 January 2009. Accepted: 1 July 2009.

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