Elsevier

Applied Surface Science

Volume 396, 28 February 2017, Pages 616-624
Applied Surface Science

Full Length Article
Assessment of morphology, topography and chemical composition of water-repellent films based on polystyrene/titanium dioxide nanocomposites

https://doi.org/10.1016/j.apsusc.2016.10.205Get rights and content

Highlights

  • Self-cleaning behavior was attained for PS/TiO2 nanocomposite films.

  • A modified phase separation process resulted in a hierarchical morphology.

  • A proper level of uniformity in surface roughness is mandatory for superhydrophobicity.

  • The required amount of nanoparticles was highly reduced via the presented method.

Abstract

In this study, polystyrene (PS)/titanium dioxide (TiO2) films were fabricated through simple solution casting technique via a modified phase separation process. The presented approach resulted in a remarkable reduction in the required amount of nanoparticles for achieving superhydrophobicity. Scanning electron microscopy (SEM) and 3D confocal microscopy were utilized to characterize surface morphology and topography of samples, respectively. An attempt was made to give an in-depth analysis on the surface rough structure using 3D roughness profiles. It was found that high inclusions of non-solvent and nanoparticles resulted in a stable self-cleaning behavior due to the strong presence of hydrophobic TiO2 nanoparticles on the surface. Quite unexpectedly, low inclusions of nanoparticles and non-solvent also resulted in superhydrophobic property mainly due to the proper level of induced surface roughness. XPS analysis was also utilized to determine the chemical composition of the films’ surfaces. The results of falling drop experiments showed that the sample containing a higher level of nanoparticles had a much lower mechanical resistance against the induced harsh conditions. All in all, the presented method has shown promising potential in fabrication of superhydrophobic surfaces with self-cleaning behavior using the lowest content of nanoparticles.

Introduction

Tremendous potential of superhydrophobic surfaces and coatings have made this subject extremely intriguing for researchers from both industrial and scientific viewpoints, and as a result, numerous studies have been reported on this subject within the last 15 years [1], [2], [3], [4], [5], [6]. In order to achieve water contact angles (WCAs) above 150° and sliding angles (SAs) below 10°, the wettability of surface has to be designed by tuning surface roughness and chemical composition. It is well-known that enhancing the surface roughness and reduction of surface energy can dramatically improve the water-repellency of the surface [7]. The lotus flowers provide a great example in which superhydrophobicity is employed as the basis of a mechanism to control the surface morphology for the protection and self-cleaning of their surfaces [8]. Generally, in order to create a superhydrophobic surface, two strategies have been employed. One is induction of roughness to a low surface energy material and the other is to modify an intrinsically rough surface with low surface energy materials. Superhydrophobic surfaces with a hierarchical structure in both nano and micro-scales mimicking that of a lotus leaf have been abundantly reported in the literature [9], [10], [11]. A variety of methods were utilized for fabrication of such surfaces including chemical etching [12], surface embedding [13], phase separation [14], etc. Phase separation methods are simple and inexpensive yet effective, and have been utilized to fabricate self-cleaning surfaces since explored by Erbil et al. [15]. A modified version of phase separation method for fabrication of superhydrophobic surfaces has been recently presented by our group [16]. In this method, nanoparticles are used simultaneously with non-solvent and accelerate the phase separation process.

In this study, polystyrene (PS), which is a very popular thermoplastic and has numerous applications, was employed as the polymer matrix. For instance, to utilize PS in microfluidic devices, superhydrophobic property has to be developed [17]. In many researches, nanoparticles were used instead of non-solvent [18]; however, very high inclusions were necessary to induce superhydrophobic property which is commercially undesirable [19], [20], [21], [22]. Moreover, very high loadings of nanoparticles may also result in poor mechanical stability and durability of the coatings due to the poor adhesion to the substrates. For instance, Qing et al. [19] reported that the weight ratio of ZnO nanoparticles to PS should be 7:3 in order to achieve superhydrophobicity on cotton textiles. In another study, it was reported that the concentration of molybdenum disulfide nanoparticles should be 55 wt.% to fabricate superhydrophobic polyurethane surfaces [22]. In the current study, titanium dioxide (TiO2) nanoparticles are incorporated into the phase separation process and act synergistically in the presence of non-solvent, which results in a remarkable reduction in the required contents of nanoparticles to 5 and 10 wt.% for achieving superhydrophobicity. Therefore, the presented approach is commercially desirable, and also, highly efficient in fabrication of durable PS surfaces with self-cleaning property.

Section snippets

Materials

Polystyrene (Mn = 140,000 g/mol, Mw = 230,000 g/mol) was obtained from Sigma-Aldrich (St. Louis, MO, USA). The hydrophobic fumed titanium dioxide (titania) used in this study is a commercial product (AEROXIDE® TiO2 T 805) which was purchased from Evonic Industries (Essen, Germany) and used as received. AEROXIDE® R805 has a specific surface area of 45 ± 10 m2/g and primary particle size of 12 nm. It was produced by treating TiO2 with octylsilane (C8H17SiH3). Tetrahydrofuran (THF) and ethanol were supplied

Wettability

The water contact angle (WCA) and sliding angle (SA) values are illustrated for all the samples in Fig. 1. At first, the sole effect of nanoparticles was studied by measuring the WCA for Ti5 and Ti10 samples in which 5 and 10 wt.% of TiO2 nanoparticles (with respect to the solid content) were used, respectively. It can be observed that addition of 5 and 10 wt.% of TiO2 nanoparticles did not considerably change the WCA values. Based on the obtained results, the WCA was increased from 92° for the

Conclusions

In this paper, an investigation on the surface morphology, topography and composition of the water-repellent polystyrene films containing TiO2 nanoparticles was presented. A solution casting technique was employed to prepare the nanocomposite films through a modified phase separation process. In fact, the simultaneous use of nanoparticles and non-solvent caused the surface layer of the films to become more enriched by the hydrophobic TiO2 nanoparticles. Such process resulted in a hierarchical

Acknowledgment

Partial financial support from the Iranian Nanotechnology Initiative is gratefully appreciated.

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