Robust porous organosilica monoliths via a surfactant-free high internal phase emulsion process for efficient oil-water separation

Highlights

• Robust porous PDMS were prepared via a HIPE process for the first time.

• Polyethoxysiloxane could in-situ grow into silica particles and strengthen the POMs.

• The POMs exhibit outstanding oil-water separation performance.

Abstract

Elastic porous organosilica monoliths (POMs) are successfully prepared by water in oil (w/o) high internal phase emulsions (HIPEs) with a novel silica precursor named polyethoxysiloxane (PEOS) as sole stabilizer. The stability of the HIPEs and the mechanical strength of the POMs are investigated as functions of PEOS and crosslinker contents in the oil phase. FESEM reveals that PEOS molecules play a significant role of in-situ growth into silica particles to strengthen the POMs, which enables the successful preparation of elastic POMs. Furthermore, oil-water separation behavior of the POMs is studied. Both the absorbent capacity and speed are considerably superior to the previously reported PDMS sponges.

Introduction

Although water covers 70% of earth surface area, only less than 2 percent can be used for human needs [1]. Water pollutions are threatening the public health and marine ecosystem, which are mainly caused by the increasing amount of industrial toxic wastewater [2], the leakage of organic solvents such as toluene, hexane, dichloromethane, especially, the crude oil spilling [3], [4]. For example, the oil spilling in Gulf of Mexico has caused severe damage to marine life [3], [5]. To work out this issue, various advanced materials and technologies for cleanup oil from water have been developed in recent years [4], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], especially, porous oil sorbent materials exhibit good performances for their oil collection and reuse properties [4], [6]. For porous oil sorbent materials, higher porosity can produce larger absorption capacity [16]. Accordingly, various highly porous oil sorbents such as kinds of aerogels [17], [18], porous carbon materials [19], [20], metal organic frameworks [21], [22], porous organosilicons [23], [24], [25] have been developed. These porous oil sorbents can be devided into two categories: the rigid and the elastic sorbents. The elastic sorbents are much more preferred because they can be easily compressed to collect absorbed oils, while the rigid sorbents could only through some complicated desorption methods, such as washing [26], [27], centrifuging [28], heat treatment [29], or burning [30], etc.

As an alternative, the emerging porous polydimethylsiloxane (PDMS) monolith is considered to be one of the most promising oil sorbents for its excellent recyclability and selectivity, high oil capacity, low fabrication cost and anticorrosion properties [4], [31], [32], [33]. Its outstanding oil absorbing performances mainly attribute to the elastic and swellable property of PDMS. However, the porous PDMS monolith is usually prepared using hard templates like sugar [32], salt particles [33], and porous polycaprolactone (PCL) [31], which need to be removed by subsequent etching or calcination that seriously limits the large-scale fabrication of the PDMS sponges.

High internal phase emulsion (HIPE) that features an internal phase volume fraction of more than 74% is usually used as template to prepare well-defined porous materials [34], [35], [36]. However, this method has rarely been successfully applied to prepare elastic porous PDMS monoliths due to the great capillary force and the flexibility of PDMS molecular chains [37], [38].

Here, we have successfully prepared the superelastic porous PDMS monolith from HIPE templating for the first time. Instead of the conventional HIPEs stabilized by surfactants which are toxic and environmental unfriendly, we utilized a silica precursor polymer, hyperbranched polyethoxysiloxane (PEOS) as stabilizer. PEOS is highly hydrophobic and becomes amphiphilic upon hydrolysis at the oil/water interface, and can further hydrolyze and condense into silica frameworks at the oil/water interfaces and in-situ grow into silica particles in PDMS matrix to strengthen the porous PDMS monoliths. The obtained superelastic porous organosilica monoliths (POMs) are hydrophobic and oleophylic and possess high porosity and robust mechanical strength. They can completely recover to their initial state after compressing for 100 times with strain of 60%. Furthermore, the POMs exhibit outstanding oil-water separation performance and they were applied to set up an oil collection apparatus which could quickly, continually and massively collecting pure oil from the water in oil emulsion.