Water-Expandable Polystyrene Using Cross-Linked Starch

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Water Expandable Polystyrene Using Crosslinked Starch Nanoparticle as Water Stabilizing Agent Nasser Nikfarjam, Mohammad Sabzi, Yulin Deng, and Nader Taheri Qazvini Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00835 • Publication Date (Web): 16 Jun 2015 Downloaded from http://pubs.acs.org on June 23, 2015

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Industrial & Engineering Chemistry Research

Water Expandable Polystyrene Using Crosslinked Starch Nanoparticle as Water Stabilizing Agent †‡

§

‡*

Nasser Nikfarjam , Mohammad Sabzi , Yulin Deng , Nader Taheri Qazvini

£†*

†

Polymer Division, School of Chemistry, College of Science, University of Tehran, Tehran, Iran

‡

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta,

Georgia 30318, USA §

£

Iran Polymer and Petrochemical Institute, Tehran, Iran

Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA

KEYWORDS: water expandable polystyrene; crosslinked starch nanoparticle; Pickering emulsion; w/o/w system; bound water.

ABSTRACT

Despite several attempts to synthesize water expandable polystyrene (WEPS) beads, the product is still far from commercialization. This is mainly due to the inability of the PS beads to preserve the blowing agent (water) during storage which leads to the loss of expandability. Here we report a new generation of WEPS beads with extended shelf-life and good expandability. The beads were synthesized through Pickering emulsion polymerization in which crosslinked starch

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nanoparticles (CSTNs) were used as water stabilizing agent. The synthesized beads with different CSTN content could encapsulate 5.0-12 wt.% of the water droplets of 3-4 µm in diameter. The thermogravimetric analysis showed that the entrapped water is mainly bound water with a strong adsorption to the starch nanoparticles. The bound water can be preserved over 88 % of the initial content 3 months after synthesis and therefore prolong shelf-life of the CSTNWEPS beads. The expansion behavior of CSTNWEPS beads were investigated in hot oil and at an optimized temperature of 135 °C. The best results (expansion ratio of ~7 for assynthesized beads and ~3 after 3 months) were obtained for the sample containing 1 wt.% of CSTN and 5.6 wt.% of water in the form of uniformly distributed microdroplets. Finally, the morphological investigation of the expanded beads revealed that the CSTNs not only stabilize the water microdroplets in the beads, but also reinforce the foam during expansion and inhibit the cell wall rupture. INTRODUCTION Expanded polystyrene (EPS) is an important industrial product with a wide spectrum of applications, especially in the fields of construction and packaging. EPS is generally prepared via a modified route for styrene suspension polymerization 1. The route starts with the reaction of styrene monomer dispersed in the water medium containing suitable suspension agents and an organic blowing agent such as pentane. To avoid the use of a flammable blowing agents and its harmful environmental effects

2, 3

, the concept of water expandable polystyrene (WEPS) was

proposed by Crevecoeur et al. 4 in which the organic blowing agent is replaced by water. Due to complete immiscibility of water and polystyrene (PS), however, to encapsulate enough water in polystyrene beads is still a challenge. So far, two alternative methods have been reported.


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In the first method, developed by Crevecoeur et al. 4, water was emulsified in a pre-polymerized styrene/PS mixture in the presence of emulsifiers. Subsequently, the inverse emulsion was suspended in a water medium containing suspension agents and polymerization was continued until a complete conversion. The final product was spherical PS beads with entrapped micrometer-scaled water droplets. Instead of using emulsifiers, Pallay et al.

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used a water

swellable polymer, e.g. starch to stabilize water inside the PS beads. In their method, prepolymerization of the styrene/starch mixture was carried out to a conversion of approximately 30%. The viscous reaction phase was subsequently transferred to a water medium containing suitable suspension agents and polymerization was completed. During polymerization, water was directly absorbed into the starch inclusions. As a blowing agent for PS, however, water has some significant disadvantages. First, the conventional expansion process using saturated steam cannot be used for WEPS because of identical pressures inside and outside the beads. Second, during storage and expansion, a substantial fraction of the blowing agent permeate out of the beads leading to very low expandability of WEPS. The water loss during the expansion occurs by water diffusion out of PS beads through polymer matrix and also via scape of water through ruptured cell walls. The scape through ruptured cell walls can possibly be reduced by modification of the rheological properties of the polymer matrix during expansion, for instance by blending the PS matrix with a high Tg polymer e.g. poly(2,6-dimethyl-1,4-phenylene ether) (PPE) 6. The diffusional water loss can also be reduced by taking advantages of barrier effect of nanoclay platelets. Shen et al.

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have shown that sodium montmorillonite (Na+-MMT) platelets can reside around the droplet wall to avoid premature water diffusion. Furthermore, Amiri et al.

8

used different organically

modified clays to reduce the water diffusion rate via the barrier effect of nanoclay’s sheets.


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Despite advancements in these attempts, the expansion ratio is still far from the theoretical values. In our previous report 9, we showed that free surfactant Pickering emulsion polymerization of styrene in w/o/w system can be exploited to obtain expandable polystyrene beads containing well-dispersed water microdroplets. We used spherical crosslinked starch nanoparticles (CSTN), with average size of 250 nm, as the stabilizing agent for the emulsified water 9. This is a new generation of WEPS (CSTNWEPS), in which solid nanoparticles are responsible for stabilization of the water microdroplets inside the PS beads. In this report, we focus on the expansion characteristics of the beads and describe the effect of incorporated water content on the expansion behavior and foam morphology. Furthermore, the ability of the beads to preserve the entrapped water during storage is investigated by thermogravimetric analysis (TGA) measurements. EXPERIMENTAL SECTION CSTNWEPS beads The CSTNWEPS samples that were used for expansion experiments are listed in Table 1 with the most important characteristics, such as incorporated water content, water droplet number density and volume-weighted average water droplet diameter (d4,3). These samples were prepared and characterized according to the procedure described in 9.

Table 1. Composition and characteristics of prepared CSTNWEPS grades; CSTN (crosslinked starch nanoparticle) 9.


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CSTN* (wt.%)

Water droplet number density (#/mm3)

d4,3† of water droplets (µm)

Incorporated water ±0.1 (wt.%)

0

-

-

-

0.5

3.9×104

4.0±0.2

5.0

CSTNWEPS-1

1

8.6×104

3.0±0.1

5.6

CSTNWEPS-2

2

1.8×105

3.1±0.1

10.1

CSTNWEPS-3

3

2.4×105

3.0±0.1

11.9

Sample CSTNWEPS-0 CSTNWEPS-0.5

*

based on initial styrene content

†

d4,3 can be calculated by the following formula; , = ∑

∑

, where ni is the number of water

droplets with diameter of di.

Expansion of CSTNWEPS beads in hot oil A sieve fraction of 3-4 mm CSTNWEPS beads was used for expansion experiments and the beads were expanded in an oil bath. The heating media was set at an optimum temperature of 135 °C. After exposure to the hot medium for a given time, the expanded beads were quenched by cold air, liquid Nitrogen or ice-water mixture. The diameter of the compact bead (d˳) and the expanded bead (de) were measured with an accuracy of ±0.01 mm. Finally, the expansion ratio (є) was calculated by є=Ve/V˳=(de/d˳)3. In which, Vo and Ve are the volume of a bead before and after expansion, respectively. The average expansion ratio was determined from four measurements at each exposure time. Characterization To determine the emulsified water content in the CSTNWEPS beads, TGA measurements were performed using a Perkin Elmer STA6000 under an N2 flow of 20 ml.min-1 with the heating rate of 20 °C.min-1 from 30 °C to 600 °C. Differential thermogravimetric (DTG) thermograms were


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also obtained for more precise characterizations. The final CSTNWEPS beads before and after expansion were cut by a razor blade and subsequently were coated with a gold layer. Digital images of the cross sections were acquired with a Hitachi S4160 field emission scanning electron microscope (FE-SEM) operating at 20 kV. The obtained FE-SEM micrographs were used to measure the size of water droplets in unexpanded beads (Table 1) or the size of the foam cells in expanded beads. JMicroVision 1.2.7 software was used for analysis of cell structure. For each sample, as least 400 cells were analyzed. RESULT AND DISCUSSION Characteristics and morphology of CSTNWEPS compact beads The morphology of the compact beads (before expansion) was studied by FE-SEM (Figure 1A). The water microdroplets with an average droplet size of 3-4 µm were well-dispersed in PS matrix (Table 1 and Figure 1A). Obviously, the incorporated water content and water droplet number density increases with the CSTN content (Table 1) 9. The CSTNs were naturally hydrophilic, however after in situ grafting of poly(styrene-co-maleic anhydride) (SMA) on their surface, the hydrophobicity of the CSTNs was enhanced 9. Formation of ester bonds and/or hydrogen bond interaction between cyclic maleic anhydride on SMA and hydroxyl groups on starch were proposed as the main mechanism 9. Due to this surface modification, the CSTNs preferentially reside at the water-styrene/PS interphase and stabilize the water microdroplets, though some particles can also remain in the water phase (Figure 1B).


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Figure 1. Cross-section FE-SEM micrographs of CSTNWEPS-0.5 unexpanded bead (A), the CSTN spherical particles reside at the interface and physically stabilize the emulsion and some of them are encapsulated inside the water droplets (B). Figure 1B reprinted with permission from reference 9. Copyright 2014 Springer. Thermogravimetric analysis of the CSTNWEPS beads Water content in starch granules has been evaluated by nuclear magnetic spectroscopy (NMR) 1013

and thermogravimetric analysis 14-17. The term of unbound water is corresponded to the freely

available water that behaves physically as pure water and diffuses from porous structure of amylose gel located between partially swollen starch granules. On the other hand, bound water molecules are associated chemically with starch chains surfaces and their motion is severely limited. For example, Fessas et al. investigated water state in wheat flour by TGA and found two main water release peaks at around 80 °C and 125-130 °C, relating to unbound and bound water, respectively 14, 15. The water content of the beads was measured directly after filtration and drying the beads. TGA was used to determine the water content. The incorporated water content increases with CSTN content which is in consistence with the other studies in solid stabilized emulsions (Table 1) 18.


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Interestingly, two water release profiles at around 105 °C and 128 °C were found (Figure 2A, B). Obviously, the former is related to free water (also known as unbound water), and the latter can be attributed to the bound water that forms strong hydrogen bonding with starch molecules. The bound water is not released at boiling temperature of free water and needs higher temperature to overcome this interaction

14, 15

. By increasing CSTN content more free water and bound water

entrapped in the beads, however the ratio of free water to bound water increased slightly with CSTN content (Figure 2C).

Figure 2. Thermogravimetric analysis data for CSTNWEPS samples; (A) TGA, (B) DTG and (C) free water and bound water content variations with the CSTN content.


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In order to understand the effect of long time storage on the water content, CSTNWEPS-2 sample containing 2 wt.% of CSTN was evaluated over months by TGA. The peak corresponding to free water was disappeared after one month, while bound water existed even after 3 months (Figures 3A and 3B). As well, the bound water release rate is less than free water release rate. The residual water content after 3 months for bound and free waters are 92.0 and 61.3 % of the initial water content, respectively (Figure 3C). Comparing to the previous water expandable polystyrenes reported in the literature e.g., WEPS by Crevecoeur 4 and WEPSCN by Shen 7, our CSTNWEPS samples show good water preservation ability over months (Figure 3D). This help us to overcome rapid water diffusion, which is known as the main reason of low expandability in WEPS samples, and move toward a higher expansion ratio even after 3 months. The preserved water content after 3 months for WEPS 4, WEPSCN 7 and CSTNWEPS were 2.6, 27.1 and 88.33 % of the initial water content in the beads, respectively. These results are promising and suggest a new route to prepare water expandable polystyrene materials with an extended shelf-life along with good expandability.


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Figure 3. Thermogravimetric analysis data for CSTNWEPS-2 containing 2 wt.% of CSTN particle over months; (A) TGA, (B) DTG, (C) free and bound water content variation over months, (D) overall water content over months for WEPS, WEPSCN and CSTNWEPS-2 samples, (WEPS and WEPSCN data are taken from Refs. 4 and 7, respectively). Expansion behavior Aside from its eco-friendliness, compared to pentane as the most common blowing agent, water has lower molar mass, 18 versus 72 g.mol-1. This leads to a water vapor pressure four times higher than that of created by pentane at the same amount of blowing agent at the same


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temperature. Therefore, theoretically, a maximum expansion ratio which is four times higher than the EPS expanded by pentane

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is attainable. However, some essential differences in

expansion behavior can be expected for WEPS with respect to the conventional EPS. Due to the absolute immiscibility of water and PS, the blowing agent in WEPS is located in discrete pools, water droplets, entrapped by the PS matrix (Figure 1A). The size distribution and number of these water droplets can be assumed to determine the dimensions of the final foam cells. The use of a nucleating agent is redundant for WEPS since the droplets themselves initiate cell growth 19. Although, these differences are advantageous of employing water as a blowing agent to expand polystyrene, the water loss during storage and expansion decreases the practical expansion ratio of the WEPS beads. CSTNWEPS samples containing water as blowing agent and possessing comparable droplet sizes were expanded in hot oil bath. During the expansion, a substantial fraction of water would diffuse out of the beads rather than participating in expansion. If the surrounding medium is hot air, this diffusion would be expedited

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. However, if the surrounding medium is hot oil, more

water can be trapped within the beads and is utilized as the blowing agent. Therefore, a higher expansion ratio can be obtained 7. The expansion behavior of CSTNWEPS-1 included 1 wt.% of CSTN and 5.6 wt.% of water were investigated in hot oil at an optimum temperature of 135 °C (Figure 4A) followed by quenching in cold water. In general, the expansion of water expandable polystyrene (WEPS) beads follows three pronounced stages: (I) induction, (II) processing window, including expansion up to a maximum and initiation of collapse and (III) collapse

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.

During the induction period, the material is heated, but the temperature is too low to make sufficient vapor pressure and make the PS matrix enough softer for expansion. However, during this induction period, the content of blowing agent is reduced because of the high diffusion rate


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of water through PS 20. After the induction period, the water vapor pressure exceeds the elastic retractive forces of the softened PS matrix and the beads expand quickly. Once the maximum expansion stage reached, the expanded beads start to collapse from the edge where the blowing agent escapes 19. All these stages were obviously recognizable for the CSTNWEPS beads (Figure 4A). In order to quantify the expansion behavior, expansion ratio of the samples with different content of CSTN and therefore different water content is illustrated in Figure 4B as a function of exposure time in hot oil. The maximum expansion ratio of around 7 was found for CSTNWEPS1. Interestingly, in spite of increasing in water content and droplet number density with CSTN content (Table 1), the maximum expansion ratio goes down with CSTN content. From these results, it can be concluded that a higher content of water does not necessarily result in a higher maximum expansion and other factors seems to be responsible for this behavior. First, during preparation of the emulsion and then polymerization process, water droplets are surrounded by CSTNs and stabilized, but some of the nanoparticles can reside in the PS matrix (Figure 5A) and some others are capsulated inside the water droplets (Figure 5B). Consequently, the number of CSTNs in PS matrix and inside droplets increases with CSTN content. This leads to a poor morphology of the beads with a wide range of water droplet size (Figures 5C and 5D). As well, it has been shown that the non-uniformity in water droplet size leads to a low expansion ratio due to heterogeneous and asynchronous growth of cells during the expansion process

19

.

Moreover, the melt strength of matrix rises more likely with CSTN content, which is caused by the strong connections of the modified CSTNs and PS matrix forming of a 3D network aggregation by CSTNs (Figure 5A)

23

21, 22

, and probably due to the

. Thus, the matrix is almost not


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softened during the expansion process, causing to a decrease in maximum expansion (Figure 4B). It is worth pointing out that after primary experiments, an optimum temperature of 135 °C was found for the expansion process. Also, three different media including liquid nitrogen, cold water and ambient air were utilized to investigate the effect of quenching media on the expansion ratio. After immersing the beads in oil bath at 135 °C for 30 s, the expanded beads were quickly transferred to the quenching media, i.e. liquid nitrogen, cold water or ambient air. For CSTNWEPS-1 sample, for instance, the final expansion ratios were 7.4±0.3, 6.7±0.1 and 4.8±0.1, in liquid nitrogen, cold water and ambient air, respectively. Due to the ease of use and the comparable results to the liquid nitrogen, cold water was selected as the quenching media for all expansion processes in this study. To know the effect of time on expandability of synthesized WEPS beads, the expansion of CSTNWEPS-1 beads was performed after 3 months using the same expansion procedure. The expansion ratio of ~ 3 was found for these beads. Regarding losing 38.7 % of initial free water content and 7.8 % of initial bound water after 3 months, we found that free water has more effectiveness on expansion than bound water.


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Figure 4. (A) Expansion of CSTNWEPS-1 bead containing 1 wt.% of CSTN and 5.6 wt.% water in oil bath at temperature of 135 °C. (B) Expansion ratio versus time for CSTNWEPS samples with different CSTN content.


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Figure 5. Cross-section FE-SEM micrographs of CSTNWEPS beads before expansion: during the emulsion polymerization the CSTN particles mainly reside at the oil-water interface to stable emulsion. However, some of the particles remain in the PS matrix (A) or encapsulate inside the water droplets (B). With increasing the CSTN content the water droplets distribute nonuniformly inside the beads leading to poor expansion behavior, CSTNWEPS-2 (C) and CSTNWEPS-3 (D). Foam morphology The expansion of the beads happens after the induction period via rapid initiation of cell growth all through the bead (Figure. 6). Figs 6A, B, C and D show the foam morphology at the


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maximum expansion for CSTNWEPS-1 (expansion ratio ~ 7) at different magnifications. A homogeneous distribution of the cell size with more frequency at 70 µm is observed (Fig. 7). During the expansion process, CSTNs around the water droplet along with PS matrix are pushed back, therefore the CSTNs are resided within the cell walls (Figs 6E-6H). We believe that the presence of the surface-modified CSTNs at the cell wall with strong connections with PS chains, i.e. entanglements of PS and/or SMA chains on the modified CSTNs with PS chains in the matrix, can improve the strength of melted PS around the cell walls and therefore inhibit the cell wall rupture. This, consequently, decreases the probability of premature escape of water during induction period, reduces water diffusion during expansion and therefore leads to higher expansion ratios. More importantly, despite severe conditions of preparation, including polymerization and expansion steps, the CSTNs remained unchanged in size and shape (Figure 6H and Figure 3B in ref 9). This reveals high dimensional stability of CSTNs.


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Figure 6. FE-SEM micrographs of foam morphology at maximum expansion for CSTNWEPS-1 with 1 wt.% of CSTN and 5.6 wt.% of water at different magnifications (A, B, C, D), during the expansion process the PS matrix along with the CSTN particles are forced back to form polyhedral cells and thus the CSTN particles are located in cell wall (E, F, G, H).

Figure 7. The cell size distribution of the obtained foam from CSTNWEPS-1 with 1 wt.% of CSTN; Solid curve is Gaussian fit to the data. CONCLUSIONS A new generation of water expandable polystyrene was successfully synthesized using crosslinked starch nanoparticles (CSTN) in Pickering emulsion polymerization. The CSTNs not only have a key role in stabilization of water droplets inside polystyrene beads, but also can affect the expansion behavior of the beads. First, the CSTNs can tightly surround water inside the beads by formation of dense layers around the water microdroplets. Second, the presence of the CSTNs in the cell wall reinforce the wall against rupture and therefore prevent the premature escape of water from the beads during expansion. Third, the CSTNs enhance the melt strength of


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the PS matrix which in turn slow down the diffusion of blowing agent, i.e. water out of the beads. In addition, due to significant amount of bound water inside the CSTNs, the water preservation ability of the prepared CSTNWEPS beads was exceptionally improved. Specifically, 3 months after synthesis of the beads, the residual water was approximately 88% of the initial water, a high value that has not been reported before. As well, the maximum expansion ratio of around 7 was found for the as-prepared CSTNWEPS containing 1 wt.% of CSTN and 5.6 wt.% of water. After 3 months, the beads showed an expansion ratio of around 3. Exploiting Pickering emulsion polymerization to replace volatile organic compound with water as blowing agent gives us an opportunity to insert hydrophilic compounds into the expanded foam. For instance, as a natural flame retardant, montmorillonite nanocaly platelets can be dispersed in water phase and distributed uniformly in the beads. We are currently working to optimize the formulation and to develop hybrid WEPS with enhanced flame retardant properties. AUTHOR INFORMATION Corresponding Author * Yulin Deng, Email: [email protected] * Nader Taheri Qazvini, Email: [email protected] Present Addresses Author: Nasser Nikfarjam, Present Address: Institute for Advanced Studies in Basic Sciences, Zanjan, Iran. ACKNOWLEDGMENT


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The authors thank the Institute of Paper Science and Technology (IPST), Georgia Institute of Technology (Georgia Tech) for experimental facilities and infrastructure. Rui Zhao, Sudhier Sharma, and Wei Mu in IPST are greatly acknowledged for providing technical support.

ABBREVIATIONS EPS, Expanded Polystyrene; CSTN, Crosslinked Starch Nanoparticle; WEPS, Water Expandable Polystyrene; CSTNWEPS, Water Expandable Polystyrene which is prepared by CSTN; PPE, Poly(2,6-dimethyl-1,4-phenylene ether); Na+-MMT, Sodium montmorillonite; Nuclear Magnetic Spectroscopy, NMR; TGA, Thermogravimetric Analysis; DTG, Differential Thermogravimetric. REFERENCES (1) Mark, H. F.; Gaylord, N. G.; Bikales, N. M. Encyclopedia of polymer science and technology. Plastics, resins, rubbers, fibers. Step-reaction polymerization to thermoforming; Interscience Publishing: Michigan, 1970, Vol. 13, pp. 844. (2) Babley, C. J. Control of VOC emissions from polystyrene foam manufacturing. US Office of Air Quality Planning and Standards; Washington: 1990, EPA-450/3-90-020. (3) Wolff, G. T. Kirk-Othmer encyclopedia of chemical technology; Wiley: New York, 1991. (4) Crevecoeur, J. J.; Nelissen, L.; Lemstra, P. J., Water expandable polystyrene (WEPS): Part 1. Strategy and Procedures. Polymer 1999, 40, 3685. (5) Pallay, J.; Kelemen, P.; Berghmans, H.; Van Dommelen, D., Expansion of polystyrene using water as the blowing agent. Macromol. Mater. Eng. 2000, 275, 18. (6) Snijders, E. A.; Nelissen, L.; Lemstra, P. J., Water expandable polystyrene (WEPS): Part 4. Synthesis of the water expandable blend of polystyrene and poly(2,6-dimethyl-1,4-phenyl


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ether). e-Polym. 2006, 6, 994. (7) Shen, J.; Cao, X.; Lee, L. J., Synthesis and foaming of water expandable polystyrene-clay nanocomposites. Polymer 2006, 47, 6303. (8) Amiri, R. N.; Taheri Qazvini, N.; Sharifi Sanjani, N., Water Expandable PolystyeneOrganoclay Nanocomposites: Role of Clay and Its Dispersion State. J. Macromol. Sci., Part B: Phys. 2009, 48, 955. (9) Nikfarjam, N.; Taheri Qazvini, N.; Deng, Y., Cross-linked starch nanoparticles stabilized Pickering emulsion polymerization of styrene in w/o/w system. Colloid Polym. Sci. 2014, 292, 599. (10) Belton, P. S., NMR and the mobility of water in polysaccharide gels. Int. J. Bio. Macromol. 1997, 21, 81. (11) Le Botlon, D.; Rugraff, Y.; Martin, C.; Colonna, P., Quantitative determination of bound water in wheat starch by time domain NMR spectroscopy. Carbohydr. Res. 1998, 308, 29. (12) Charles, A. L.; Kao, H. M.; Huang, T. C., Physical investigations of surface membranewater relationship of intact and gelatinized wheat-starch systems. Carbohydr. Res. 2003, 338, 2403. (13) Choi, S. G.; Kerr, W. L., 1H NMR studies of molecular mobility in wheat starch. Food Res. Int. 2003, 36, 341. (14) Fessas, D.; Schiraldi, A., Water properties in wheat flour dough II: classical and Knudsen thermogravimetry approach. Food Chem. 2005, 90, 61.


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(15) Fessas, D.; Schiraldi, A., Water properties in wheat flour dough I: classical thermogravimetry approach. Food Chem. 2001, 72, 237. (16) Orlowska, M.; Utzig, E.; Randzio, S. L., Thermogravimetric study of water state in wheat starch gels obtained under high pressures. Ann. N. Y. Acad. Sci. 2010, 1189, 55. (17) Schiraldi, A.; Fessas, D., Classical and Knudsen thermogravimetry to check states and displacements of water in food systems. J. Therm. Anal. Calorim. 2003, 71, 225. (18) Midmore, B. R., Effect of Aqueous Phase Composition of the Properties of a SilicaStabilized W/O Emulsion. J. Colloid Interface Sci. 1999, 213, 352. (19)

Crevecoeur, J. J.; Coolegem, J. F.; Nelissen, L.; Lemstra, P. J., Water expandable

polystyrene (WEPS): Part 3. Expansion behaviour. Polymer 1999, 40, 3697. (20) Pogany, G. A., Anomalous diffusion of water in glassy polymers. Polymer 1976, 17, 690. (21) Vaidya, U. R.; Bhattacharya, M., Properties of blends of starch and synthetic polymers containing anhydride groups. J. Appl. Polym. Sci. 1994, 52, 617. (22) Yang, Z.; Bhattacharya, M.; Vaidyat, U. R., Properties of ternary blends of starch and maleated polymers of styrene and ethylene propylene rubber. Polymer 1996, 37, 2137. (23) Binks, P. B.; Philip, J.; Rodrigues, J. A., Inversion of Silica-Stabilized Emulsions Induce by Particle Concentration. Langmuir 2005, 21, 3296.

GRAPHIC ABSTRACT


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Water expandable polystyrene (WEPS) beads were synthesized through Pickering emulsion polymerization in the presence of crosslinked starch nanoparticles (CSTNs). Then the WEPS beads were expanded by exposing to hot oil at optimum temperature of 135 °C. The maximum expansion ratio of around 7 was obtained for WEPS beads containing 1 wt.% of CSTN and 5.6 wt.% of water. Interestingly, 88 wt.% of initial water were kept inside the beads after 3 months. This water preservation ability caused by formation of CSTN layers around the uniformly distributed water microdroplets as well as bound water inside the CSTNs.


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Figure 1. Cross-section FE-SEM micrographs of CSTNWEPS-0.5 unexpanded bead (A), the CSTN spherical particles reside at the interface and physically stabilize the emulsion and some of them are encapsulated inside the water droplets (B) 9 81x31mm (300 x 300 DPI)


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Thermogravimetric analysis data for CSTNWEPS samples; (A) TGA, (B) DTG and (C) free water and bound water content variations with the CSTN content. 130x108mm (300 x 300 DPI)



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Figure 3. Thermogravimetric analysis data for CSTNWEPS-2 containing 2 wt.% of CSTN particle over months; (A) TGA, (B) DTG, (C) free and bound water content variation over months, (D) overall water content over months for WEPS, WEPSCN and CSTNWEPS-2 samples, (WEPS and WEPSCN data are taken from Refs. 4 and 7, respectively). 254x217mm (300 x 300 DPI)


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Figure 4. (A) Expansion of CSTNWEPS-1 bead containing 1 wt.% of CSTN and 5.6 wt.% water in oil bath at temperature of 135 °C. (B) Expansion ratio versus time for CSTNWEPS samples with different CSTN content. 242x209mm (300 x 300 DPI)



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Figure 5. Cross-section FE-SEM micrographs of CSTNWEPS beads before expansion: during the emulsion polymerization the CSTN particles mainly reside at the oil-water interface to stable emulsion. However, some of the particles remain in the PS matrix (A) or encapsulate inside the water droplets (B). With increasing the CSTN content the water droplets distribute non-uniformly inside the beads leading to poor expansion behavior, CSTNWEPS-2 (C) and CSTNWEPS-3 (D). 140x105mm (300 x 300 DPI)


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Figure 6. FE-SEM micrographs of foam morphology at maximum expansion for CSTNWEPS-1 with 1 wt.% of CSTN and 5.6 wt.% of water at different magnifications (A, B, C, D), during the expansion process the PS matrix along with the CSTN particles are forced back to form polyhedral cells and thus the CSTN particles are located in cell wall (E, F, G, H). 105x157mm (300 x 300 DPI)



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Figure 7. The cell size distribution of the obtained foam from CSTNWEPS-1 with 1 wt.% of CSTN; Solid curve is Gaussian fit to the data. 150x104mm (300 x 300 DPI)


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Graphical abstract 643x360mm (150 x 150 DPI)


Water-Expandable Polystyrene Using Cross-Linked Starch

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