Expanded polystyrene via stabilized water droplet by in-situ modified starch nanocrystals
Graphical abstract
Introduction
Solid particulate stabilized emulsions, commonly known as Pickering emulsions, has gained more attention due to the superior long-lasting stability against coalescence and the elimination of side effects of surfactants in conventional emulsions [1]. In Pickering emulsions, the solid particulate emulsifiers with intermediate wettability are attached at the interfaces of two immiscible liquids to provide a steric hindrance against droplet coalescence phenomenon. The main force of the stabilization process arising from the Gibbs free energy penalty caused by detaching of solid particles from the interface [[2], [3], [4]]. Various particles with different shape and dimension ranging from nanometer to micrometer have been used as particle emulsifier in Pickering emulsions [1]. Among them, particles derived from natural biopolymers such as protein [5], chitin [6], chitosan [7], casein [8], egg yolk granules [9], soy glycinin [10], cellulose [[11], [12], [13]], zein [14,15] and starch [16] has recently drawn considerable attention due to the low-cost raw materials, safe application, broad range of chemical applications, biodegradability, non-toxic character, wide availability, renewability and sustainability.
Moreover, the nanoparticles from biopolymers can combine the advantageous properties of biopolymers with the features of nanomaterials, which in turn has broadened the spectrum of potential applications. Starch, as the second abundant polysaccharide, is an excellent candidate to prepare Pickering emulsions used in food technology, cosmetic formulations, pharmaceutical products, and composite industry. However, the emulsions resulted from the micro-sized starch granules such as quinoa starch, rice starch, waxy maize starch, wheat starch showed poor stability because of the instability of large granules against the gravity force and tending to sediment [[17], [18], [19]]. Also, the native starch granules are highly hydrophilic and therefore are generally not absorbing at the water-oil interface to stabilize emulsion droplets. The hydrophobicity of starch granules can be improved through surface modification with alkyl bearing molecules like octenyl succinic anhydride (OSA) [9,20]. For example, OSA-surface modified starch granules with 1–5 μm in size have been utilized as a particulate emulsifier to stabilize o/w emulsion droplets with a size range of 10–100 μm [19,[21], [22], [23]]. It was found that the size of particles had more effect than shape on emulsion stability, so that stabilized emulsion droplets by the smaller particles showed better stability against Ostwald ripening and coalescence and due to their higher packing efficiency in forming a dense, uniform and homogenous layer around the droplets [[17], [18], [19]]. Therefore, in recent years, several research groups have reported the preparation of Pickering emulsion using hydrophobized starch based nanoparticles such as OSA modified starch nanospheres [24,25], acid hydrolyzed starch nanocrystals [13], enzymolysis prepared starch nanoparticles [26].
In our previous works, we have reported the synthesis of polystyrene (PS) beads by the polymerization of styrene in water-in-oil-in-water (w/o/w) system, in which water microdroplets are stabilized by the crosslinked starch nanoparticles [27], and the cellulose nanofibrils (CNF) [28]. Both of these nanoparticles were in situ modified by the during the synthesis of styrene-maleic anhydride (SMA) copolymer during the emulsification and polymerization. It was found that this strategy leads to a better attachment of SMA onto the nanoparticle’s surface and modifies thier wettability for absorbing at the the water-partially polymerized styrene interphase. The formation of chemical and physical linkages occurs through forming of esteric and hydrogen bonds between hydroxyl groups on the surface of CNFs and CSTNs and maleic anhydride groups present in the SMA chains [27,28]. The obtained PS beads containing water microdroplets can be expanded over 7–14 folds by immersing in a hot oil bath at 130 °C [29,30]. In this case, water acts as a blowing agent, to form water expanded polystyrene (WEPS) materials. The WEPS materials are new type of expanded polystyrene (EPS) foam in which water is used as a physical blowing agent instead of using volatile organic compounds (VOCs) like pentane. This can be a promising and eco-friendly product because, in its production process, the flammable blowing agents and therefore their harmful environmental effects are avoided. Due to the complete immiscibility of water and polystyrene, some specific methods have been proposed to the synthesis of WEPS materials such as entrapping water droplets through inversion emulsion followed by suspension polymerization [31,32] or introducing water using water absorbent materials during suspension polymerization of styrene [33]. One of the biggest obstacles to the industrialization of WEPS materials is their low expandability. During storage and expansion process, a substantial fraction of water permeates out of the beads through polystyrene matrix leading to a shortage of water as a blowing agent to expansion. In spite of some attempts to put barriers against water diffusion-out [32,34,35], the water loss and therefore low expandability are still the biggest challenge for WEPS materials. Recently, our research group has proposed Pickering emulsion polymerization technique to introduce and stabilize water droplets inside the polystyrene beads using water absorbents solid particles such as CSTNs and CNFs [29,30]. The prepared WEPS materials in these two work were termed CSTNWEPS and CNFWEPS, respectively. The higher ability of these nanoparticles to stabilize water microdroplets along with their ability to preserve water inside the polystyrene beads led to the expansion ratio of around 7 for CSTNWEPS and around 14 for CNFWEPS which have not been reported before.
Despite this, the water-loss during expansion process and storage period is still primary challenge to the commercialization of WEPS. Therefore, with the aim of obtaining a higher expansion ratio, a more efficient processing than can effectively avoid water permeation out was developed in this study. The platelet shape along with the packed and crystalline structure of SNCs encouraged us to utilize them in the synthesis of new WEPS materials through Pickering emulsion polymerization. SNCs in this system act simultaneously as water absorbing agent and particulate emulsifier to stabilizer water droplets inside the polystyrene beads. As well, locating SNC particles inside the polymer matrix is expected to result in enhanced mechanical strength of polymer matrix and act as an efficient barrier against water diffusing out of the PS beads.
Section snippets
Materials
The polystyrene with Mn = 100,000 g.mol−1 was supplied by Alfa Aesar. Potato starch, styrene, benzoyl peroxide with a half-life of 145 min at 90 °C, maleic anhydride, toluene, sulfuric acid, and hydrochloric acid were obtained from Merck. Hydroxyethyl cellulose (HEC, with average molecular weight of 250,000 g.mol−1) as suspension stabilizer was purchased from Sigma-Aldrich and used as received. All reagents were used without further purification. Distilled water was used for all experiments.
Preparation of starch nanocrystals (SNCs)
The
Characterization of SNCs
The AFM micrographs revealed a flake-like morphology for SNCs with a length of 300–500 nm and a thickness of 30 nm (Fig. 1). This result was in agreement with DLS measurements (Fig. 7A). But, DLS results showed some aggregation in a larger z-average at lower pH values (Fig. 7A); this will be discussed in more detail later. In the XRD pattern of starch granules, the distinct peaks at 2θ of 34.5, 23.0 and 17.5° confirmed the crystalline structure of B-type for potato starch powder (Fig. S1). The
Conclusion
Polystyrene beads included monodispersed water microdroplets were successfully synthesized through Pickering emulsion polymerization in the water-in-oil-in-water system using potato starch nanocrystals (SNCs). The SNCs were in situ surface modified by poly(styrene-co-maleic anhydride) (SMA) during emulsification and polymerization process to be compatibilized with polystyrene matrix. The modified SNCs have suitable wettability to stabilize water droplets inside the styrene/polystyrene phase.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The Institute for Advanced Studies in Basic Sciences (IASBS) are gratefully appreciated for financial support (with Grant number of G2015IASBS32632).
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