Expanded polystyrene via stabilized water droplet by in-situ modified starch nanocrystals

Colloids and Surfaces A 582 (2019) 123863

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Expanded polystyrene via stabilized water droplet by in-situ modified starch nanocrystals

T

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Zahra Ajeloua, Nasser Nikfarjama, , Yulin Dengb, Nader Taheri-Qazvinic,d a

Polymer Division, Department of Chemistry, Institute for Advanced Studies in Basic Sciences, Zanjan 45137-66731, Iran School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0620, United States c Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina, 29208, United States d Biomedical Engineering Program, University of South Carolina, Columbia, South Carolina, 29208, United States b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Starch nanocrystal w/o/w Inverse Pickering emulsion polymerization pH Water expanded Polystyrene

Synthesis of polystyrene beads containing well-dispersed water microdroplets viasurfactant free Pickering emulsion polymerization in water-in-oil-in-water system. The water microdroplets with a diameter of 8–15 μm were stabilized using in situ modified starch nanocrystals (SNCs). The morphological investigation revealed that the water droplets were surrounded by a densely layer of the modified SNCs. Since the acid hydrolyzed SNCs were sensitive to pH, the basic feature of polystyrene beads was changed with pH, so that the incorporated water content, encapsulation efficiency and droplet number density were increased with pH, but the volume-weighted mean diameter (d4,3) was decreased with pH. The thermogravimetric analysis confirmed two type of water inside the beads; free water and bound water with release temperatures of 103 and 120 °C, respectively. The bound water was responsible for higher water preservation ability and therefore long shelf-life of the beads, which over 40% of the initial entrapped water was kept 7 months after synthesis. The beads can be expanded at an optimum temperature of 135 °C due to water vapor pressure developed inside the beads. The best max expansion ratio of around 16 was found for the beads included 1.8 wt.% of water. During the expansion process, the SNCs were located in the cell wall to reinforce it from rupturing. Eventually, we report a new type of water expanded polystyrene beads synthesized by the use of SNCs, named SNCWEPS, with good expandability and extended shelf-life.

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Corresponding author. E-mail addresses: [email protected] (Z. Ajelou), [email protected] (N. Nikfarjam), [email protected] (Y. Deng), [email protected] (N. Taheri-Qazvini). https://doi.org/10.1016/j.colsurfa.2019.123863 Received 2 July 2019; Received in revised form 22 August 2019; Accepted 25 August 2019 Available online 26 August 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

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.

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 [2–4]. 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 [11–13], 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 [17–19]. 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 [19,21–23]. 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 [17–19]. 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-inwater (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 waterpartially 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.

2. Experimental 2.1. 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. 2.2. Preparation of starch nanocrystals (SNCs) The SNCs were prepared through acidic hydrolysis of starch granules [36]. Briefly, 30 g of potato starch granules were dispersed in 250 ml of H2SO4 solution (3.16 M) at 40 °C and stirred at 100 rpm for 5 days. The resultant suspension was diluted with deionized water and then centrifuged 8000 rpm for 15 min. The process of dispersing of resulted precipitated in deionized water followed by centrifuging was repeated for several times until pH around 6 was obtained. Finally, the SNC powder was obtained by lyophilizing the neutralized suspension. The lyophilized SNCs were redispersed in water with the assistance of ultrasonic treatment (Qsonica, model Q-700. USA, 5 min with an intensity of 50). The aquatic dispersion of SNCs was stored at 3–8 °C after adding 3 drops of chloroform to prevent microbial growth. 2

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2.4. Polystyrene beads fractionation

Table 1 Suspension polymerization recipe. Content (g)

Component

Styrene SNC# Polystyrene BPO# Maleic anhydride Water (emulsified) Water (suspension) HEC#

20-K K (18/100) ×(20-K) (0.5/100)×(20-K) 1 (0.5/100) ×(20-K) 60 0.06

To confirm the in situ surface-modification of nanoparticles during the polymerization-emulsification process, the grafted SNC was separated from the polymer matrix using Soxhlet extractor. For this purpose, 8 g of the dried polystyrene beads were transferred into a cellulose thimble and then placed in the extraction chamber of Soxhlet. To avoid transfer of nanoparticles into the distillation flask, the thimble was plugged and sealed with Whatman paper No.4. The extraction process was performed with toluene for 72 h with cycle of 20 min. The obtained surface-grafted SNCs was dried in vacuum oven at 50 °C for 24 h and then characterized by FT-IR and contact angle analysis.

# SNC: starch nanoparticle, BPO: benzoyl peroxide and HEC: hydroxyethyl cellulose.

2.3. Synthesis of polystyrene beads in w/o/w system

2.5. Expansion of the WEPS beads

In the first step to synthesize polystyrene beads containing SNCstabilized water droplets, polystyrene along with benzoyl peroxide and maleic anhydride were dissolved in inhibitor-free styrene according to Table 1. After complete dissolution, the mixture was heated at 90 °C under N2 atmosphere for 75 min to attain monomer conversion of around 25%, determined by the gravimetric method. Afterward, an aqueous dispersion of SNC was added to the partially polymerized styrene mixture under stirring at 250 rpm and temperature of 90 °C for 2 min to create particulate stabilized inverse emulsion. To break the large droplets into the small ones and also obtain a homogenous dispersion of water droplets, the mixture was sonicated for 80 s at 500 W. Then, the obtained inverse emulsion was transferred quickly to a 250 ml three-neck glass reactor containing water and HEC to form a water/oil/water system (Table 1). The reactor was equipped with a mechanical stirrer, condenser and baffle to reduce the turbulence of fluid flow. The suspension was heated for 20 h at 90 °C under N2 atmosphere while stirring at 350 rpm to complete the polymerization. Finally, the suspension was cooled to room temperature, and the shiny white spherical beads were obtained and washed with water several times (Scheme 1). The yield of polystyrene beads was around 80% for all samples.

For all samples, beads with diameter of 4–5 mm were sieved and used for expansion. The beads were expanded by exposure to the hot oil bath (at an optimum temperature of 135 °C) followed by quenching by cold water at a given time. The expansion ratio (ε) was calculated by the following equation;

ε=

Ve d = e V0 d0

where, V0 , Ve , d 0 and de are the volume of a compact bead, the volume of an expanded bead, the diameter of a compact bead and the diameter of an expanded bead, respectively. The diameter of beads before and after the expansion was measured by micrometer three times to determine the average expansion ratio.

2.6. Characterization 2.6.1. Atomic force microscopy (AFM) The morphology and size of SNCs were studied using AFM (Ara Research Co., Iran) with a silicon tip operated in the tapping mode. For sample preparation, a drop of a highly diluted aqueous dispersion of SNCs was set on a clean mica surface and then dried in a vacuum oven at 60 °C for 24 h. The size and shape were evaluated using SPIP 6.7.4 software.

Scheme 1. Preparation of in situ modified SNC stabilized w/o emulsion (A) followed by polymerization of styrene in w/o/w system (B). 3

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powder (Fig. S1). The XRD peaks for SNCs were more prominent than starch peaks confirming elimination of amorphous regions of starch through acid hydrolysis (Fig. S1). The calculated crystallinity (using the Segal method) from the peak area of XRD patterns for starch powder and SNCs was around 50% and 67%, respectively [38].

2.6.2. Thermogravimetric analysis (TGA) The incorporated water content stabilized by SNC particles inside the as-synthesized WEPS beads was determined using a NETZSCH STA 409 PC/PG under N2 flow of 20 ml/min with the heating rate of 10 °C/ min from 30 to 600 °C. Also, the TGA measurements were performed 4.5 and 7 months after synthesis to study of water maintenance ability of the final PS beads.

3.2. Pickering emulsion polymerization of styrene in w/o/w system using SNCs

2.6.3. Fourier transform infrared spectroscopy (FT-IR) The native SNCs and the surface-modified SNCs extracted from PS beads were characterized by FT-IR spectroscopy using FT-IR spectrophotometer (BRUKER, Vector 22, Germany) by preparing their KBr pellets from 450 to 4000 cm−1. The samples were dried at 50 °C for 24 h before characterization.

Hydrophobization of particles derived from biopolymers is necessary to act as an emulsion stabilizer for Pickering emulsions. However, in-situ modification of particles during the emulsification and polymerization brings several advantageous including; (I) reduction of processing steps, (II) shorter formulation cycle, (III) reduction or excluding of surfactants as well as organic solvents, IV) well-dispersion of particles in polymer matrix, V) capability of upgrading to a continuous polymerization process to make it easier to be industrialized. In this study, the SNCs were utilized as solid particulate emulsifiers to stabilize water droplets inside partially polymerized styrene in the first stage, and to entrap the water droplets inside PS beads after complete polymerization (Scheme 1). During the emulsification step (Scheme 1A), an optimized viscosity of styrene/PS phase was required to fixate the water droplets in styrene/PS phase and also break-down the bigger SNC aggregates into the individual and fresh particles to be efficiently acted as stabilizer. This required viscosity can also minimize the washing-out of SNCs from styrene/PS phase to the suspension media. The optimized viscosity was attained by adding 18 wt.% of PS (Mn = 100,000 g.mol−1), relative to styrene content, and pre-polymerization of styrene up to 25% of monomer conversion. To give suitable wettability to the SNCs acting as an efficient emulsifier, an optimized content of maleic anhydride, 0.5 wt.% related to styrene content, was initially added to the styrene phase before polymerization. In the pre-polymerization step, both PS and styrene-maleic anhydride (SMA) copolymer were obtained. PS and SMA chains modify the SNC's surface, and thus the hydrophilicity of SNC was changed to partial hydrophobicity which is suitable for stabilizing w/o emulsions (modification mechanisms will be discussed in the next section). This surface modification also led to better compatibility of SNCs with PS matrix which in turn prevented washing-out of SNCs from PS bead to the suspension media during the final polymerization step. To characterize the internal morphology, the pearle-like WEPS beads were cut by a razor (Fig. 2A). The cross-section FE-SEM micrographs of the PS beads showed that the water droplets were uniformly dispersed throughout the bead (Fig. 2B–C). Fig. 2E–F show the morphology of the internal wall of water droplets in more detail. The protruding spots on the wall are related to the modified SNCs which form a wall around the droplets and finally stabilize the water droplets. It seems that the particles can be arranged around the water droplets and form a dense layer (Fig. 2G-F). This close-packed form of particles around the droplets can be attributed to the particle-particle interactions. Although SMA modified the SNCs during the emulsification and polymerization, it is expected that there are still some hydroxyl functional groups on the surface of SNCs. It is worth to note that WEPS beads were prepared at different content of maleic anhydride (0, 0.25, 0.5, 0.75, 1 and 2 wt.% related to the styrene content) and in agreement with previous findings [27,28,33,39], the best results were obtained at 0.5 wt.% of maleic anhydride. In both higher and lower contents of maleic anhydride, nonexpandable PS beads were collected. Low maleic anhydride content results in insufficient surface modification of SNCs with SMA and weak anchoring strength of particles at the oil-water surface to stabilize water droplets. On the other hand, at a high content of maleic anhydride, because of the aggregation of SMA chains in the PS matrix, a phase separation occurs which leads to the unstable emulsion and rough morphology of the final PS beads.

2.6.4. X-ray diffraction (XRD) The crystallinity of native starch and SNCs were studied using XRD (X-PERT PRO model, PANalytical Company) through Cu radiation with λ = 1.54 nm at 40 KV and 40 mA. The XRD patterns were obtained at angel range of 2θ = 5–40°. 2.6.5. Dynamic light scattering (DLS) & zeta potential The size and size distribution of SNC, as well as its zeta potential at different pH values, were measured using DLS (Zetasizer Nano ZS™3600, UK). The measurements were performed at λ = 532 nm at 25 °C. 2.6.6. Contact angle The extracted grafted-SNCs were pressed for 5 min at 74,000 bar using stainless steel die-set hydraulic press into compact pellets 7 mm in diameter. The contact angle was determined using deionized water (5 μl), imaging by Canon SX230HS camera and calculating by Meazure software. For more accuracy, three readings were taken. The pellets were placed in a vacuum oven at 50 °C for 12 h before measurements. 2.6.7. Field emission scanning electron microscopy (FE-SEM) The WEPS compact beads and the related expanded samples were cut by a razor blade and then coated with a gold layer. Digital micrographs from the cross-section of the samples were acquired using a Hitachi S4160 field emission scanning electron microscope operating at 20 kV. The obtained micrographs were used to evaluate the morphology and size of the water droplets in the unexpanded beads and the foam cells in the expanded beads. The volume-weighted mean diameter (d4,3) as well as droplet number density were calculated by d4.3 = ∑ ni di4 / ∑ ni di3 and NO = (n/ A)3/2 , respectively. Where, ni is the number of droplets with a diameter of di and n is the number of droplets in the defied area A, derived from the FE-SEM images at a magnification of X300. At least 200 droplets were manually counted for each samples and their diameters were measured by the use of JMicrovision 1.2.7 software. The measured diameters for the micrographs were multiplied by a factor of 2/√3 to have a better estimation of the real void (droplet) diameter [37]. This statistical correction was applied in order to compensate for the underestimation of values obtained by direct measurements from the micrographs. 3. Result and discussion 3.1. 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 4

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Fig. 1. AFM micrographs of the prepared SNCs. Size evaluation revealed the thickness of approximately 30 nm and the length of 300–500 nm for SNCs.

besides typical bands for SNC, additional bands at 1157, 1450, 1492 1582 and 1601 cm−1 are assigned to the aromatic ring of styrene in SMA and bands at 1865 and 1781 cm−1 are assigned to the cyclic anhydride group in SMA [41]. As well, the band at 1735 cm−1 is assigned to C=O stretching of ester groups confirming the grafting of SMA through the formation of ester group between anhydride groups in SMA and hydroxyl groups in SNC surface. The modified SNC was not soluble in water and therefore dispersed in toluene after stirring at room temperature for a day. The affinity of the native and modified SNC to water were evalute by measuring their contact angles (θ) (Fig. S2). The measured values for the native and modified SNC were 33 ± 1° and 92 ± 2°, respectively. One can be concluded that the modified SNC has less affinity to water than the native SNC and also it has suitable wettability to stabilize w/o emulsions. Furthermore, it has been proved that the energy needed to detach a particle from the interface is the highest when θ is around 90° [42]. The high detaching energy is expected to give a robust and dense particle network around the water droplets, resulting in high stability of droplets against coalescence phenomenon which in turn lead to a decrease in the average droplet size and narrow size distribution [43].

3.3. In-situ modification of SNCs Due to the hydrophilicity nature of SNC, it is immiscible with most of the synthetic polymers. This immiscibility can lead to the phase separation and agglomeration of SNCs in the PS matrix and therefore the poor performance of the final products. Moreover, in the case of this study, the hydrophobicity of SNCs can be a good reason to washing-out from PS matrix to the media during the suspension polymerization. One of the ways to cope with this problem is the introduction of polar functional groups (carboxylic acid, anhydride, amine or epoxy) in the PS chain. This partial manipulation of PS structure will lead to effective interaction and hence good compatibility between the two components. Among functional groups, the cyclic anhydride is expected to react more quickly than the other ones. Consequently, in this work an optimized content of maleic anhydride (0.5 wt.% relative to the styrene content) was added to the initial formulation. It is postulated that maleic anhydride can compatibilize SNCs and polystyrene chains in two ways; (I) Grafting onto; styrene can copolymerize with maleic anhydride to produce styrene-maleic anhydride (SMA) copolymers in different molecular weight and maleic anhydride content [39]. SMA can be grafted on the SNC surfaces during the polymerization process through the interaction (hydrogen bonding) and reaction (ester bond formation) of the maleic anhydride groups in the copolymer with the hydroxyl groups of the starch chains in SNCs. (II) Grafting from; absorbed maleic anhydride on the SNC surface through hydrogen bonding can be copolymerize with styrene to produce SMA copolymers. In both of these grafting mechanisms, formed radical sites by the initiator or radical transfer from radical monomers on the SNC surfaces can directly initiate co/homopolymerization of styrene from the SNC surfaces. Both grafting mechanisms are illustrated in more details in the supplementary data (Schemes S1 and S2). To verify the in situ surface modification, the modified SNCs were separated from the PS beads Soxhlet setup and investigated carefully. AFM micrographs of the modified SNC showed a flake-like morphology for the modified SNC (Fig. 3) similarly with the native SNCs (Fig. 1). The surface evaluation revealed a rough surface for the modified SNC, while the native SNC showed a smooth surface. Also, the size evaluation disclosed an increase in thickness for the modified SNC in comparison with the native SNC (Fig. 3). The other evidence for surface modification of SNCs can be obtained by FTIR. The FT-IR spectra of neat polystyrene, maleic anhydride, SNC and modified SNC are compared in Fig. 4. The native SNC shows some characteristic bands for starch; CH2 bending at 1452 cm−1, C–O–C stretching in glycoside ring at 1153, 1069 and 1080 cm−1, O–H bending of adsorbed water at 1635 cm−1, hydrogen bonded O–H stretching at 2900-3800 cm−1, C–H stretching at 2850-2980 cm−1, S=O and S–O stretching of sulfate groups (existing sulfate at the surface of SNC) at 1109 and 842 cm−1, respectively (Fig. 4) [40]. While, the spectrum of SMA grafted SNC shows that

3.4. The features of the WEPS beads In order to find the effect of the various parameters on the content of emulsified water inside the PS beads, 4 series of these beads were prepared by varying of the following parameters; the SNC content (Table S1), the initial emulsified water (Table S2), NaCl content (Table S3) and pH (Table 2). The amount of water content of the PS beads was obtained by TGA. The encapsulation efficiency (EE%) of the beads, defined as the amount of internal aqueous phase preserved after emulsification and polymerization, was calculated by dividing the total incorporated water in the final bead to the initial water (SNC aqueous dispersion) that used for emulsification. Based on initial experiments (Tables S1-S3), the sample E-0.5-1 with a water content of 1.80 wt.% and encapsulation efficiency of 34.6% with no electrolyte was selected for further study. The selection of this sample was based on the uniformity in size and morphology as well as the maximum expansion ratio (will be discussed later) of the beads. Since the acid hydrolysis of the starch results in the formation of carboxylate and sulfate groups on the surface of the SNCs [44], these particles are sensitive to the pH of media. So, we were curious to know how pH can affect the essential features of the PS beads. Therefore, a series of PS beads were prepared using the SNC aqueous dispersion with different pH value (Table 2). The TGA and DTG results showed two profiles for water release at around 103 and 120 °C for all beads (Fig. 5). The former one is attributed to free water or unbound water (behaves physically as pure water) which is entrapped inside the water droplets. And, the latter one is 5

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Fig. 2. The image (A) and cross-section FE-SEM micrographs (B–C) of prepared polystyrene beads (sample E-0.5-1-pH = 6.5). Cross-section FE-SEM micrographs of the water droplet and its interior wall at different magnifications (D–F); The modified SNCs reside at the interface and physically stabilize the water droplets.

water content is always higher than the free water (Fig. 5B). Moreover, the incorporated water content, as well as the encapsulation efficiency, increases with pH (Table 2 and Fig. 5). In addition to increasing the water content and the encapsulation efficiency with pH, the water droplet number density was also increased with pH (Figs. 6–7). But, the size distribution and the average diameter of droplets (d4,3) were narrowed and decreased with pH,

related to bound water which is associated chemically with the starch chain existed in the SNC surfaces. Because of severely limited motion of the bound water molecules, it is not released at the boiling point of free water and therefore more energy, i.e., higher temperature, needs to overcome these interactions [45,46]. These temperatures were also reported in our previous works [29,30]. Obviously, the peak area of both free water and bound water increases with pH and the bound 6

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Fig. 3. AFM micrographs of the isolated in situ modified SNC. Size evaluation revealed that the modified SNC has a rougher surface than the native SNC confirming surface modification by SMA and/or PS chains.

Fig. 4. FT-IR spectra of polystyrene, maleic anhydride, native SNC and SMA-modified SNC (A) and extended FT-IR spectra of SNC before and after modification by SMA and/or PS chains. Table 2 Characteristic of the prepared PS beads using the aqueous dispersion of SNC with different pH. To prepare the beads, 1 ml of the SNC dispersion with a concentration of 0.5 wt.% was used. Sample*

SNC/Styrene content ratio (wt.%)

pH

Water content (wt. %)

Encapsulation efficiency (%)

Bead size (± 0.1 mm)

Water droplet number density (No, #/mm3)

d4,3 (μm)

E-1-0.5-pH = 3.0 E-1-0.5-pH = 6.5 E-1-0.5-pH = 10.0 E-1-0.5-pH=12.0

0.025 0.025 0.025 0.025

3.0 6.5 10.0 12.0

1.49 1.80 2.85 3.01

29.8 34.6 57.0 60.2

4.0 5.4 4.6 3.7

0.0045 2014 2193 1968

15.9 11.4 9.11 8.8

* Sample Nomination: E-x-y, where x is the concentration of nanoparticle and y is the emulsified water.

Fig. 5. TGA and DTG thermograms of the prepared PS beads in different pH value.

respectively (Table 1, Figs. 6–7). The reason for all these variations with pH is the changing in particle size of SNCs which in turn results from zeta potential variation with pH. It means that in low pH values (acidic

conditions), a broad size distribution with a large particle size average was obtained (Fig. 8A). Where the size distribution and the average particle size were narrowed and decreased with pH (Fig. 8A). It is 7

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Fig. 6. Cross-section FE-SEM micrographs of the prepared PS beads in different pH value. A) E-0.5-1-pH = 3.0, B) E-0.5-1-pH = 6.5, C) E-0.5-1-pH = 10.0 and D) E0.5-1-pH = 12.0.

Fig. 7. pH effect on the size distribution (A), the average diameter and number density of the water droplets (B). The curves of size distribution were obtained by the data fitting with Gaussian equation.

Fig. 8. Average particle size and size distribution of SNC in different pH values obtained by DLS (A) and the zeta potential variation of SNC with pH (B).

particles can produce large emulsion droplets with a broad size distribution (Fig. 7). These SNC aggregations can also be observed in the final PS beads prepared at low pH values, i.e., sample E-0.5-1-pH = 3.0, while these aggregations were not found in other samples prepared in higher pH values (Fig. 9). But, with an increase in pH values and

believed that the obtained large particle size in low pH values is caused by the protonation of the existing functional groups on the SNC surface, i.e., low zeta potential values. These low negative zeta potential values are not large enough to prevent particle-particle aggregations through mainly hydrogen bonding interaction. Therefore the obtained large 8

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Fig. 9. FE-SEM micrographs of sample E-0.5-1-pH = 3.0; water droplet inside at different magnifications.

Fig. 10. TGA and DTG thermograms of sample E-0.5-1-pH = 6.5 as-synthesized and over time.

calculated ES% for sample E-0.5-1-pH = 6.5 after 4.5 and 7 months after synthesis was 58 and 40.7% of the initial encapsulated water content, respectively. Which in comparison with its analogous, i.e.polystyrene beads containing water droplets stabilized by dioctyl sulfosuccinate sodium salt (AOT) with ES of 2.63% [31] and polystyrene bead included water droplets stabilized by AOT aid with sodium montmorillonite platelet with ES of 27.1% [32], the prepared sample in this work showed the best encapsulation stability

therefore the deprotonation of the functional groups, the zeta potential of the particles goes down to the significant negative values and consequently the aggregation between the particles was prevented (Fig. 8). As results, with increasing of pH, the number of effective available particles to stabilize emulsion droplets is so high resulting in small droplets with narrow in size distribution (Fig. 7). The plenty of particles to stabilize emulsion droplets can well prevent from coalescence phenomenon. This higher ability of particles to stabilize internal phase could lead to an increase in water content, encapsulation efficiency and droplet number density (Table 2). These results are consistent with the previous reports, in which more solid particulate emulsifiers led to the more emulsified internal phase and small droplets [27,47,48]. The water preservation ability during storage was determined by calculating the encapsulation stability (ES%). The ES was obtained by dividing the remained encapsulated water after storage into the original encapsulated water. Therefore, the water content of the sample E-0.5-1pH = 6.5 was determined by TGA after 4.5 and 7 months and was compared with the as-synthesized sample (Fig. 10). It seems that the release rate of the free water is higher than the bound water because the peak corresponding to the free water was totally disappeared after 4.5 months, while the bound water existed over 7 months (Fig. 10). The

3.5. Expansion behavior of the beads In the line of the studies carried out in our research group on the WEPS materials, i.e.CSTNWEPS and CNFWEPS [29,30], in the present work we continue trying to reach more expansion ratio by using starch nanocrystals (SNCs) with platelet-like morphology. The SNCs were undertaken with the following aims; (I) stabilizing water microdroplets inside PS matrix, (II) enhancing water maintainability of PS beads by water absorbing because of their nature of hydrophilicity and (III) placing individually in the PS matrix to cause tortuous pathway against water molecules (barrier effect). As mentioned before in the in situ modification section, during the emulsification and polymerization 9

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Fig. 11. FE-SEM micrographs of foam morphology at max expansion for E-0.5-1-pH = 6.5 (SNCWEPS) with water content of 1.8 wt.% at different magnifications (A, B, C, D). During the expansion process, the SNC particles along with the softened polystyrene matrix are forced back to create polyhedral cells; thus, the SNC particles are located within the cell wall (E and F).

(∼135 °C), well above the glass transition temperature of the matrix, the rubbery softened polystyrene matrix was expanded owing to the developed water vapor pressure inside the beads. This results in a foamlike structure with rather polyhedral cells (Fig. 11A–D) with cell size ranged from 0 to 160 μm centered at 80 μm (Fig. 12A). The beads were exposed to hot oil and then quickly quenched in cold water bath. The

processes the SNC particles were surface-modified with SMA copolymer to be miscible with polystyrene matrix. Hence, it is not difficult to imagine that the modified SNCs are located at the water-in-oil interface and also in the PS matrix. When the E-0.5-1-pH = 6.5 sample beads included small and welldispersed water microdroplets are heated at an optimum temperature 10

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Fig. 12. Expansion behavior of the E-0.5-1-pH = 6.5 (SNCWEPS) beads. (A) Cell size distribution of the obtained foam from SNCWEPS. Solid curve is Gaussian fit to the data. (B) Three regimes in expanded polystyrene using water as blowing agent I; induction II; expansion III; collapse.

obtained expansion behavior for these beads is in agreement with the results of other reports [29,30]. Generally, the WEPS materials show three regimes during expansion; (I) induction, (II) expansion and (III) cell collapsing. In the induction period, the applied heating is too low to soften polystyrene matrix and make sufficient vapor pressure for expansion, however, during this period the content of water (blowing agent) is lost due to water diffusion through polystyrene matrix. In the expansion period, the sufficient water vapor pressure forces the well softened polystyrene matrix and the beads were expanded quickly. After the max expansion stage, the foam cells start to collapse from the edge due to the lack of blowing agent, i.e. water (Fig. 12B). During the expansion process, the polystyrene matrix along with the SNC particles around the water microdroplets are forced back, consequently the SNC particles located inside the cell walls (Fig. 11E–F). It is believed that the presence of the modified SNCs at the cell wall with secure connection with the polystyrene matrix through interaction and physical entanglement, can improve the strength of the melted polystyrene around the cell wall. This enhancing mechanism inhibits rupturing of the cell wall and consequently, decreases the chance of premature escape of water during the induction period and reduces water diffusion-out during the expansion process. All these effects lead to higher expansion ratios. The max expansion ratio of around 16 was obtained for E-0.5-1pH = 6.5 (Fig. 12B). Since these materials are a new generation of water expandable polystyrene (WEPS) materials, therefore these materials obtained in this work call SNCWEPS. As mentioned before, the TGA results proved that the prepared beads in this work have an excellent ability to preserve water inside the beads over months (Fig. 10). So, around 40% of initial water was retained 7 months after the synthesis of the beads. This ability aids us to overcome the problem of rapid water diffusion, which is the main reason for low expandability in WEPS materials, and approach a higher expansion ratio even after 4.5 and 7 months.

pH. Finally, the prepared PS beads containing SNC stabilized water droplets was introduced as a new type of water expandable polystyrene which is named as SNCWEPS materials. The SNCs not only have a crucial role in particulate stabilization of water droplets inside polystyrene beads but also can efficiently affect expansion behavior of the bead. Moreover, because of the significant amount of bound water in the SNCs, the water preservation ability of the prepared SNCWEPS beads was exceptionally enhanced. Specifically, 7 months after synthesis of the beads, the residual water was around 40% of the initial water. This water maintainability of these beads has positively effect on expansion process. So, a max expansion ratio of about 16 was found for the as-prepared SNCWEPS containing 1.8 wt.% of water. As a final conclusion, exploiting Pickering emulsion polymerization to replace pentane with water as blowing agent gives us an opportunity to insert renewable hydrophilic compound into the expanded polystyrene foam.

4. Conclusion

References

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). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.123863.

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