Electrospun multi-scale hybrid nanofiber/net with enhanced water swelling ability in rubber composites
Introduction
Water-swellable rubber (WSR) with high elasticity, resilience, high toughness, and also high water swelling ability is widely used as sealing material in caulking applications, preventing water leakage from pipes and blocking connections in constructions such as subways and subsea tunnels, sealing of gaps with waterproof sealing material, and preservation of airtightness in machinery and apparatus [1], [2]. Both chemical grafting and physical mixing methods can be used to prepare WSR. Synthesis of amphiphilic polymer is an important chemical modification method for the grafting of hydrophilic chains onto hydrophobic rubber, such as grafting co-polymerization of N, N-dimethylacrylamide onto natural rubber [3], grafting of poly(ethylene oxide) onto chlorosulphonated polyethylene and poly(ethylene oxide) onto butyl rubber [4], [5], but the chemical method is more complicated and more environmentally unfriendly than the physical method. In the physical method, common hydrophobic rubbers such as chloroprene rubber [6], chlorohydrin rubber [7], and natural rubber [8] have been mixed with various water-absorbent materials such as crosslinked forms of polyacrylate [9], poly(vinyl alcohol) [10], starch-acrylate copolymer [11] and some other fillers [2]. Water-absorbent materials known as superabsorbent polymers (SAPs), which are kinds of hydrogel with three-dimensional (3D) network structures, possess a high degree of absorption and absorbing rate, as well as stability. However, hydrophilic super water-absorbent resin does not disperse well in hydrophobic rubber, having very poor interfaces between them, so that the hydrophilic part can easily break off from rubber networks [12]. Thus swelling ability is ultimately lost, strength is remarkably decreased, and water is polluted as well. To overcome these problems, a silica coupling agent was used as a compatibilizer in our previous work [13] to increase the interactions between SAP and rubber. Improved swelling and mechanical properties were achieved. However, isolated SAPs were trapped within the rubber and water could not efficiently transfer between SAP particles in the rubber.
Previous work [14], [15] has shown that by using even low nanofiber loadings in rubber composites, mechanical properties such as Young's modulus and tensile strength increased greatly. Because of the hydrophilic nature of the fibers they could act as internal water channels in hydrophobic materials for superabsorbent nanocomposites and help to transfer water from the surface of the rubber matrix to hydrophilic SAP particles as well as between encapsulated SAP particles in the rubber matrix, enhancing the water swelling ability of WSR composites.
Electrospinning of ultrafine fibers ranging from submicron to nanometer size, with smaller pores and higher surface area than regular fibers, can be used in an enormous variety of applications [14], [16], [17]. In 2006, a two-dimensional (2D) spiderweb-like nanofiber/net (NFN) with ultrafine fiber diameter less than 20 nm was prepared by electrospinning/netting (ESN) [18], [19]. It exhibited extremely large specific surface area, high porosity, and large stacking density, providing optimal candidates for applications as sensor materials [20], biomedical wound dressing [21], and other significant applications [22], [23]. Ding et al. [18] were the first to find a procedure for generating novel 2D nanowebs in 3D fibrous mats by optimization of various processing parameters during electrospinning, but multi-scaled hybrid PAA NFN with enhanced properties has not yet been reported.
The present study is the first to report improved water absorption property and stability of WSR resulting from the use of electrospun multi-scaled hybrid NFN in 3D fibrous mats of crosslinked PAA functioning as water channels. Electrospinning of various superabsorbent SAP (PAA with hyper branched polymer and/or GO) 3D fibrous mats was performed. The effects and mechanisms of those fibers in enhancing the water swelling properties of WSR are discussed. This novel water channel system could be an effective method for enhancing the absorption property of WSR as well as for developing the feasibility of using the electrospun fine fibers in sealing material.
Section snippets
Materials
Poly(acrylic acid) (PAA, average Mw ~ 450 000 g · mol− 1), poly(acrylic acid) solution (average Mw ~ 250 000 g · mol− 1, 35 wt.% in H2O), ethylene glycol (EG, anhydrous, 99.8%), hyperbranched bis-MPA polyester-64-hydroxyl (HB), (3-Aminopropyl) triethoxysilane, sulfuric acid and ethanol were purchased from Sigma Aldrich Australia. Banana skin silicone rubber was provided by Barnes Products Pty Ltd, Australia. Graphene oxide (GO) was prepared by oxidizing graphite powder following a modified Hummers method
Morphology of electrospun NFN mats
The desire for ever-higher specific surface area and porosity has encouraged researchers to improve electrospinning techniques [28]. In this work, the PAA and PAA hybrid NFN mats were prepared for high water absorption. The SEM images of PAA-EG, PAA-EG + HB, PAA-EG + GO and PAA-EG + HB + GO NFN mat are shown in Fig. 3. The image of the PAA-EG NFN mat (Fig. 3(a)) shows an interlinked 1D ultrafine fibrous mat formed from 700–800 nm diameter fibers with a small number of thinner fibers (< 100 nm) generated
Conclusion
Superabsorbent SAP fiber mats with hyperbranched polymer and/or graphene oxide having multi-scaled structure fibers were electrospun. The mats with fibers of nano- and submicro-diameter had enhanced water swelling abilities (swelling rate and swelling ratio). When hyperbranched polymer and graphene oxide were added, the hybrid mats showed synergistic effects on water swelling ability, due to the formation of spiderweb-like multi-scale structures and increased specific surface areas. The
Acknowledgements
Y Tang is grateful for the support of the Australian Research Council with a Discovery Early Career Research Award Grant (DE120102784) for the research work.
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