Simple Preparation of Nanoporous Honeycomb Film by Low Volatile Solvent

ZANG Bing

(Qingdao University, Qingdao Shandong 266071, China)

【Abstract】In this paper, the traditional water-templates technology has been improved. We introduce a simple effective method to prepare honeycomb materials with nanopores by low volatile solvent.

【Key words】Honeycomb; Nanoporous; Low volatility; Copolymer

0 Introduction

The Water-templates method is a convenient and effective technology to prepare the porous film, which has broad prospects for development in the field of electronic skin［1］, membranes［2］, cytoskeletal［3］, superhydrophobic materials［4］, ect. The technique utilizes a solvent volatilization and condensation of water vapor atmosphere prepared honeycomb structures by forming droplets templates. In so far for a long time, researchers generally believed that rapid evaporation of solvent and condensation of water vapor in a short time are two necessary conditions to form a porous structure of the desired material effectively. The solvent with high volatility are often applied such as carbon disulfide, chloroform, tetrahydrofuran, etc. Some even facilitate the evaporation and the condensation by raising the temperature, humidity or vacuum degrees［5］.However, the pore size of resultant material is mostly micron-scale. It has even been asserted that the ordered nanoporous honeycomb material can’t be prepared by the water-templates technology.

Here, we improved the traditional water-templates technology, introduce a simple effective method to prepare honeycomb materials with nanopores by low volatile solvent. The strategy applied conventional block copolymer to create the nanoporous structure in the moist atmosphere.

1 Experimental Section

1.1 Materials

Commercially available polystyrene-b-polyethylene oxide (PS-PEO) was bought from Polymer Source Inc. Cyclohexanone was obtained from Aladdin Industrial Inc. Concentrated sulfuric acid, hydrogen peroxide, ethyl alcohol and acetone was purchased from Shanghai Chemical Reagent Plant.

1.2 Pretreatment of the glass Substrate

The Piranha solution was prepared by slowly adding hydrogen peroxide (30wt%) into the concentrated sulfuric acid (98wt%).Their volume ratio is about 1:3. After the lotion was cool down, the glass substrates (1×1cm2) were soaked in the Piranha solution for 10 min. Then the substrates were washed by distilled water two times and ultrasonic cleaned by acetone, ethanol and water three times in turn.

1.3　Preparation of the nanoporous Film

The honeycomb polymer film was prepared in a glass sealed chamber filled with saturated vapor. The vapor was obtained beforehand by adding 3 ml of water on the bottom of the chamber. First, the temperature was controlled at 28℃ in a water bath. A piece of glass substrates pretreated as mentioned above was placed carefully on the support one centimeter away from the level of the pre-added water. Then the glass chamber was sealed until system was stabilized with saturated vapor. The PS-PEO was dissolved in cyclohexanone and the concentration was 30mg/ml. When the system stabilized 30μl of the polymer solutions were placed on the glass substrate with a micropipettor in the chamber. After complete evaporation of the solvent, the polymer was removed from the chamber.

2 Results and Discussion

Fig. 1　Representative SEM image of the honeycomb film with nanopores

2.1 Selection of solutions and polymers

The volatility of solvent is a key factor to form the pores at the nanoscale. Cyclohexane, cyclohexanone and n-butanol, which boiling points are higher than 80℃, are suitable for the preparation of the nanoporous films. The low volatility and sealed chamber greatly extended the time of the volatilization and inhibited the growth of the water droplets templates. At the same time, hydrophilic segments of the copolymer promote the enrichment and nucleation of water vapor. In this paper, we choose cyclohexanone as the solvent of PS-PEO copolymer.

2.2 Preparation of the nanoporous Film

In the preparation of the nanoporous film(figure 1), the volatile process of block copolymer solution lasts more than 30 min, which can be minutes in traditional water-templates method. Then the temperature difference between the surface of solution and the water atmosphere is small. However, water droplets still nucleate, condensate and grow. They gather to the hydrophilic segments of the copolymer forming droplets templates with radius of less than one hundred nanometers. After the complete volatilization of solvent and templates, the honeycomb material with templates nanostructures(Figure 2) is left.

Fig.2 The AFM 3D image of the PS-PEO honeycomb film with nanopores

3 Conclusion

In summary, we improved the water-templates method to fabricate a nanoporous honeycomb by low volatile solvent and segmented copolymer overcoming the technical prejudice that the water droplets should condensate rapidly to forming the porous structure.

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