ORIGINAL PAPER

The preparation of self-ordered porous polystyrene filmusing auxiliary of ultrasonic fogging device

Yu-Hsun Nien & Yan-Sheng Tsai & Chun-Heng Chen &

Pi-Chun Juan & Jia-Fu Dai & Tao-Ke Ou

Received: 7 July 2011 /Accepted: 26 February 2012 /Published online: 15 March 2012# Springer Science+Business Media B.V. 2012

Abstract One of the methods to fabricate orderedmicro-structured film is the usage of water dropletsfrom humid-air flow as template. However, humid-airflow on the surface of polymer solution during cast ofporous polymeric film may lead to uneven surface ofthe porous polymeric film. Moreover, the influence ofatmosphere humidity on the average pore size of porouspolymeric film is significant. In this study, we devel-oped a novel method to fabricate even surface of porouspolymeric film and reduce the influence of atmospherehumidity on the pore size. To achieve this purpose, anultrasonic fogging device was employed to producewater droplets instead of humid air. The water dropletsproduced by an ultrasonic fogging device may containnegative ions to prevent the water droplets from aggre-gation. Therefore, the pore size of water droplets is ableto be stabilized. In addition, the water droplets producedby an ultrasonic fogging device are able to fully fill incontainer in advance, which may avoid large humid-air

flow to agitate the surface of polymer solution. Thus theeven surface of porous polymeric film can be acquired.

Keywords Polymers . Porous materials . Polystyrene

Introduction

Porous polymeric film possesses versatile potential applica-tions such as antireflection coatings [1–3], tissue engineer-ing [4–9], sensors [10], electrolytes [11], templates as highresolution lithographic masks [12], and fuel cells [13]. Sev-eral methods to fabricate porous polymeric film arereported. Zhang et al. fabricated poly(styrene-co-metha-crylic acid) colloids endued with styrene rich in core andmethacrylic acid rich in hydrated shell and used solventextraction to drive the polystyrene units in the core rear-ranged leading to the pore formation [14]. Zhao et al. usedwater droplets as template to fabricate honeycomb orderedpolycarbonate films [15]. Sun et al. reported that surfactant-encapsulated polyoxometalate complex was able to func-tionalize the breath figure holes of polystyrene film [16].Shimomura et al. prepared honeycomb-like microporouspolymer films on substrate such as glass by casting polymersolutions under humid conditions [17–23].

Among the mentioned methods, the usage of waterdroplets or moist air as template to fabricate orderedmicro-structured films is convenient. However, humid-air flow on the surface of polymer solution during castof porous polymeric film may lead to uneven surface ofthe porous polymeric film. The uneven surface of po-rous polymeric film may restrict its application to pre-cise process. Moreover, the influence of atmospherehumidity on the average pore size of porous polymericfilm is significant [15]. Many key factors for the

Y.-H. Nien (*) :Y.-S. TsaiDepartment of Chemical and Materials Engineering,National Yunlin University of Science and Technology,Douliou, Yunlin 64002, Taiwane-mail: nienyh@yuntech.edu.tw

C.-H. ChenDepartment of Electrical Engineering and Institute of ElectronicEngineering, National Tsing-Hua University,Hsinchu 300, Taiwan

P.-C. Juan : J.-F. Dai : T.-K. OuDepartment of Materials Engineering and Center for Thin FilmTechnologies and Applications,Ming Chi University of Technology,Taipei 243, Taiwan

J Polym Res (2012) 19:9848DOI 10.1007/s10965-012-9848-y

determination of pore size were discussed by Shimomuraet al. and they have concluded that higher humiditygives a larger pore size and lower regularity of micro-pore size. In order to stabilize water droplets, thesurface-active amphiphilic polymers were used [19]. Inthis study, we developed a novel method to fabricateeven surface of porous polymeric film and reduce theinfluence of atmosphere humidity on the pore size. Toachieve this purpose, an ultrasonic fogging device wasemployed to produce water droplets instead of humidair. The water droplets produced by an ultrasonic fog-ging device may contain negative ions to prevent the

water droplets from aggregation. Therefore, the poresize of water droplets is able to be stabilized. In addi-tion, the water droplets produced by an ultrasonic fog-ging device are able to fully fill in container inadvance, which may avoid large humid-air flow to ag-itate the surface of polymer solution. Thus the evensurface of porous polymeric film can be acquired. Tocompare the conventional method of making waterdroplets by humid-air flow with the method by ultra-sonic fogging device, we chose polystyrene as the ma-terial of polymer film and there was no surfactant usedin this study. The results indicated that the usage ofultrasonic fogging device was able to fabricate evensurface of porous polymeric film and reduce the influ-ence of atmosphere humidity on the pore size.

Experimental

Polystyrene (Mw0192 K) and trichloromethane used assolvent were purchased from Aldrich and ShimakyuChemical Co., respectively. Several weight percentagesof polystyrene solution were prepared, which were0.5%, 1%, 2%, and 4% [13]. The schematic of thesetup with the auxiliary of ultrasonic fogging device isshown in Fig. 1. The water droplets (fog) producedfrom ultrasonic fogging device (labeled as No.1) wasintroduced into a container (labeled as No. 3) by a tube

Fig. 1 The schematic of the setup with the auxiliary of ultrasonicfogging device (1: ultrasonic fogging device, 2: tube, 3: container, 4:glass and 5: dish)

Fig. 2 An ordered porouspolystyrene film coated on theglass

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(labeled as No.2) at low flow rate. Distilled water wasused as the source of water droplets. A piece of glass

(labeled as No. 4) with dimension of 3 cm×3 cm wasfirst laid inside a dish (labeled as No. 5). The polystyrene

Fig. 3 The SEM figures of theordered porous polystyrene filmmade from polystyrene solution(a: 0.5 wt.%, b: 1 wt.%, c:2 wt.%, and d: 4 wt.%)observed from top view

Fig. 4 The SEM figures of theordered porous polystyrene filmmade from polystyrene solution(a: 0.5 wt.%, b: 1 wt.%, c:2 wt.%, and d: 4 wt.%)observed from cross section

J Polym Res (2012) 19:9848 Page 3 of 5

solution was poured into the dish until the glass was immersedcompletely. Then the whole dish was moved into the containerwhich was fully filled by fog inside. The temperature insidethe container was at room temperature (about 30 °C).The humidity in the container filled by fog was morethan 90%. After the trichloromethane was evaporated,an ordered porous polystyrene film was formed on theglass. The ordered porous polystyrene films were exam-ined using optical microscope and scanning electronmicroscope (JEOL JSM6701F). The ordered porouspolystyrene films were fixed on the sample holder bycarbon adhesive tape. Then the sample holder with theordered porous polystyrene films was coated with plat-inum. After sputter coating, the ordered porous polysty-rene films were observed by a scanning electronmicroscope.

Results and discussion

Figure 2 is ordered porous polystyrene film made frompolystyrene solution of 1% coated on the glass. Oncethe micro-porous film was formed, it appeared translu-cent. Based on our setup, it was able to produce largescale ordered porous polystyrene film. Figures 3 and 4are the SEM figures of the ordered porous polystyrenefilms made from polystyrene solution (a: 0.5%, b: 1%,c: 2%, and d: 4%) observed from top view and crosssection, respectively. The morphology of the porouspolystyrene films made from polystyrene solutions atthe weight percent of 0.5%, 1% and 2% retained anappearance of regularity. As the concentration of poly-styrene solution up to 4 wt.%, the porous polystyrenefilm became irregular due to the increase of the viscos-ity of the polystyrene solution. The average pore sizesof the ordered porous polystyrene film are summarizedin Table 1. The average pore sizes of the orderedporous polystyrene film ranged from 3.113±0.714 μmto 3.901±1.548 μm. The average pore sizes of theporous polystyrene films made from polystyrene solu-tion at the weight percent of 0.5%, 1% and 2% did notshow significant difference. The pore sizes were not

affected by the concentration of the polystyrenesolution.

Based on the study of Zhao in the fabrication ofhoneycomb ordered polycarbonate films, the averagepore size decreased when the related humidity becamelower and the average pore size at the relative humidityof 90% was about 5 μm. In addition, the average poresize decreased with the increase of the concentration ofpolymer solution [15]. Peng et al. also reported that thepore size was found to linearly increase with increasedhumidity [24]. In our study, with the auxiliary of ultra-sonic fogging device, the average pore sizes are notrelated to the concentration of the polymer solution atthe range from 0.5 wt.% to 4 wt.%. Even at the relativehumidity of 90%, the average pore sizes are still rangedfrom 3.113±0.714 μm to 3.901±1.548 μm rather than5 μm. The reason may be caused by the fog producedby ultrasonic fogging device. The fog is water dropletswith electricity phenomena [25]. These water dropletsmay contain negative ions to prevent the water dropletsfrom aggregation. The size of the water droplets do notchanged significantly. Therefore, the fog produced byultrasonic fogging device is able to use as stable waterdroplet to prepare ordered porous polystyrene film.

In Fig. 4, these SEM figures of the porous polystyrenefilms made from polystyrene solution (a: 0.5 wt.%, b:1 wt.%, c: 2 wt.%) observed from cross section show evensurface. That illustrates that our method of making waterdroplets can avoid agitating the surface of polymer solution.

Conclusions

The ordered porous polystyrene film was coated on theglass successfully with the auxiliary of ultrasonic fog-ging device. The morphology of the porous polystyrenefilm made from polystyrene solution at the weight per-cent of 0.5 wt.%, 1 wt.% and 2 wt.% retained anappearance of regularity. With the auxiliary of ultrasonicfogging device, the average pore sizes were not affectedby the concentration of the polymer solution at therange from 0.5 wt.% to 4 wt.%.

The water droplets produced by an ultrasonic foggingdevice may contain negative ions to prevent the water drop-lets from aggregation. Therefore, the pore size of waterdroplets is able to be stabilized. Therefore, the fog producedby ultrasonic fogging device is able to use as stable waterdroplets to prepare ordered porous polystyrene film. Thewater droplets produced by an ultrasonic fogging deviceare able to fully fill in container in advance, which mayavoid large humid-air flow to agitate the surface of polymersolution. Thus the even surface of porous polymeric filmcan be acquired.

Table 1 The average pore sizes of the ordered porous polystyrene filmmade from polystyrene solutions at the weight percent of 0.5%, 1%,2% and 4%

0.5 wt% 1 wt% 2 wt% 4 wt%

Pore size(μm)

3.113±0.714 3.901±1.548 3.509±0.496 3.809±1.043

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