Superhydrophobic and superelastic thermoplastic polyurethane/multiwalled carbon nanotubes porous monolith for durable oil/water separation

Highlights

• Superflexible TPU/cMWNTs monoliths with hierarchical porous structure were fabricated.

• Superhydrophobicity of TPU/cMWNTs monoliths was ascribed to its peculiar microstructure.

• The monoliths exhibited excellent oil/water separation selectivity and durability.

• The monoliths are capable of collecting a large area of floating oil by the pumping technique.

Abstract

Absorbing materials with high hydrophobicity, porosity and mechanical resilience are urgently needed for oil/water separation. In this work, three dimensional (3D) porous thermoplastic polyurethane-based (TPU) composite monoliths with hierarchical porous structure were prepared by a feasible thermally induced phase separation (TIPS) method. The morphology analysis revealed the hierarchical porous structure was effectively perfected by adding 2 wt% carboxyl multiwalled carbon nanotubes (cMWNTs), which showed a uniform dispersion and compatibility in TPU skeleton. As a result, the hydrophobicity (151°), saturated adsorption capacity (6.9–42.3 g g⁻¹) and mechanical compression and resilience property of TPU/cMWNTs monolith were improved significantly when comparing to pure monolith. Moreover, the composite monolith could maintain stable hydrophobicity and absorption capacity in corrosive environment. Besides, arising from the excellent mechanical elasticity, the monolith can be easily reused by absorbing-squeezing cycles. The recycled monolith can maintain high separation efficiency of 98.4% even reusing 40 cycles. As a kind of highly efficient absorbent, porous TPU/cMWNTs monolith shows great prospect in the application of large-scale oil/water separation.

Introduction

Oil spill and chemical leakage events are causing catastrophic damages to our environment nowadays [[1], [2], [3], [4]]. The leaked toxic substances also threaten various species through the food chain, from low-grade algae to high-level animals and even humans. During past decades, a great deal of attempts including oil skimmers [5], direct burning [6], physical diffusion [7], absorbent [8], and biodegradation [9], have been used to overcome this challenge. Among them, absorption is one of the most popular approaches in view of its advantages of no secondary pollution, simplicity of operator and low cost [10,11]. A desired absorbent material should be highly porous with super-hydrophobicity and super-oleophilicity in order to selectively absorb oils and hydrophobic organic solvents from water to the maximum extent [12,13]. High reliability and reusability are also hoped in practical application [14]. Recently, various kinds of materials including carbon-based aerogels, inorganic or organic silicon sponges have been proposed for oil/water separation [[15], [16], [17]]. However, although the outstanding absorption capacity, these aerogels or sponges are usually limited by low mechanical properties and high-cost of raw materials and complex synthesis [18].

Hydrophobic porous polymer-based monoliths are believed to be one of the most promising candidates used in oil/water separation because of their ultralow material/preparation costs and satisfying mechanical strength [19]. The adjustable pore structure and thermal/chemical resistance endow them high selectivity and efficiency for oil absorption, showing good application prospect in oil/water separation [20]. Benefiting from the versatile processing of polymers, electrospinning, sol-gel process, chemical/physical foaming, phase separation, self-assembly process, template methods etc., have been developed to prepare porous polymer monoliths [[21], [22], [23]]. Among them, the simple solution phase separation process without complex equipment requirement is believed to be an ideal mass-produce way. On the other hand, porous polymer monoliths with high mechanical resilience are desired when taking the high-efficient reusability into consideration [24]. The mechanical resilience allows the absorbed oils to be removed by the convenient and high-efficient squeezing process rather than the time-consuming centrifugation or evaporation treatment. Up to now, it is still a challenge to develop the high-performance porous monoliths produced by commercial polymers with desired absorption capacity and mechanical resilience, simultaneously.

Generally, the mechanical characters of polymer monoliths are decided by their raw materials. Commercial thermoplastic elastomers are thus thought to be the promising alternatives to prepare high-elastic monoliths [25]. Thermoplastic polyurethane (TPU) comprising of adjusting soft and hard segments shows a controllable mechanical elasticity and resilience [26,27]. Our group has recently reported a porous TPU monolith with well oil/water separation ability showing an excellent mechanical elasticity [28]. However, the oversize pore structure with loose skeleton results in the relatively low mechanical strength which is not conducive to the reusability performance of TPU monoliths during the long-term reuse. Besides, the hydrophobicity of pure TPU monolith is also not ideal (<150°) due to the undesired pore structure and surface morphology. To solve these questions, a feasible approach of introducing carbon-based nanomaterials into polymer monolith has been proposed based on our previous reports [[29], [30], [31]]. On the one hand, the strength and rigidity of monolith's skeleton can be effectively improved by combing the rigid nanoparticles [32]. On the other hand, the introducing nanoparticles can significantly affect the phase separation process by changing the viscosity and compatibility, further adjust the pore structure of polymer monolith [33].

For this reason, carboxyl multiwalled carbon nanotubes (cMWNTs), with high specific surface area, excellent oil affinity and outstanding mechanical property were introduced to tailor pore structure of TPU monolith in this work. Typically, TPU/cMWNTs monolith with hierarchical porous structure was first produced by thermally induced phase separation (TIPS) method assisted by non-solvent. Comparing to pure TPU monolith, the pore structure of composite monolith was significantly improved in terms of the pore size and skeleton. As a result, TPU/cMWNTs monolith with super-hydrophobicity (151°) and super-oleophilicity (0°) exhibited high separation efficiency for oils/organic solvents in different thermal/corrosive environments. It is worth noting that the mechanical elasticity and resilience of monolith were obviously increased, which ensured its durability and reusability. Moreover, the high-performance TPU/cMWNTs monolith showed ability to achieve continuous absorption and removal of a large amount of oil on water surface through the pump-assisted system, which meant the high potential in practical application.