Elsevier

Polymer

Volume 167, 22 March 2019, Pages 85-92
Polymer

Effect of incorporation of sulfonate (single bondSO3-) on surface sealing of polystyrene (PS)-based bowl

https://doi.org/10.1016/j.polymer.2019.01.072Get rights and content

Highlights

  • Sealing behavior of polymer hollow particles with a surface opening was studied.

  • The sealing behavior was strongly dependent on the surface property of the particle.

  • The surface property affected the structure of the surface-sealed particles.

Abstract

We study thermal annealing-driven surface sealing of polymer hollow particles with a different surface property. Use of phase separation between polystyrene (PS) solid particles incorporated with sulfonate (single bondSO3-) group and decane allows their transformation into hollow particles with an opening on the surface. Control of the phase separation enables us to tune the structure (outer and inner diameters, and size of the opening) of the hollow particles. While thermally annealed at a temperature above Tg of the polymer, structural change of the hollow particles is experimentally observed and theoretically analyzed. The results demonstrate that the change in the surface property of the hollow particles due to the incorporation of the single bondSO3- group can have a direct impact on their sealing behavior and surface-sealed structure.

Introduction

Over the past several decades, extensive studies have been performed to design and fabricate polymer particles with a hollow structure, which are characterized as high surface area and large interior void, for a broad range of applications such as lightweight fillers, catalysis, selective separation, electronics, optics, chromatography, and so on [[1], [2], [3], [4], [5], [6], [7]]. Recently, research on these particles has been extended to development of hollow particles with a microporous shell, as a new class of functional materials, due to their ease in the encapsulation of targeted components into the hollow interior [8]. Guo and coworkers demonstrated a facile approach for fabrication of poly(o-methoxyaniline) hollow particles with an opening on their surface [9]. Droplets formed by o-methoxyaniline monomer acted as a template for the formation of the hollow particles, and the diffusion flux of the monomer during the polymerization generated the opening. Use of Self-assembling of Phase Separated Polymer (SaPSeP) method in the presence of sodium dodecyl sulfate (SDS) was also reported to produce polymer hollow particles [10]. The surfactant changed the interfacial tension among the components involved in the synthesis to form an opening on the surface of the polymer particles. Similarly, self-assembly of polystyrene (PS) particles at the interface of emulsion droplets was used to make poly(methyl methacrylate) (PMMA) and poly(vinyl acetate) (PVA) particles with a hollow interior, accompanied by a porous shell [11]. Recently, Tang and coworkers reported a one-step template-free synthesis of polymer hollow particles with a surface opening through microwave-assisted dispersion polymerization [12]. However, practical use of the hollow particles with a microporous shell is hindered as the opening and pores still remain after encapsulation, making stable loading of payloads challenging.

In order to address this issue, use of thermal and solvent annealing has been demonstrated for sealing of the opening on the surface of the hollow particles. Xia and coworkers fabricated PS, PMMA, and poly(ε-caprolactone) (PCL) hollow particles with a well-defined open hole on their surface through solvent swelling and freeze-drying processes, followed by successful sealing of the opening by thermally annealing the sample at a temperature slightly above the glass transition temperature (Tg) or melting point (Tm) of each polymer [13,14]. Consequently, the hollow particles could still have an interior void together with a closed shell. They also achieved surface sealing without thermal annealing by the additional introduction of toluene and 1,4-dioxane serving as a plasticizer into the suspension of the hollow particles with a surface opening [13,15]. A similar result was reported by Yates and coworkers working with compressed carbon dioxide and dichloromethane for completely sealing an open hole on the surface of PS and poly(d,l-lactic acid) (PLA) hollow particles [16,17]. They inferred that minimization of the interfacial free energy would be a driving force for the sealing [17].

Although these studies have expanded the applicability of hollow particles with a surface opening to diverse fields, particularly to biomedical applications including controlled release and delivery of drugs [[14], [15], [16]], the sealing behavior has not been fully understood. In a recent study, we systematically investigated the surface sealing of PS hollow particles with an open hole on their surface during thermal annealing [18]. The theoretical analysis revealed that the reduction of the interfacial free energy driven by the decrease in the surface area of the particles would be a driving force for their surface sealing while maintaining the interior void. Furthermore, it was found that the structure of the as-prepared hollow particles could determine the successfulness of their sealing. However, it still remains a pending question if a change in the surface property of the hollow particles has an impact on their sealing behavior.

In this study, we investigate the thermal annealing-driven surface sealing of polymer hollow particles with different surface property. To accomplish this, we incorporate sulfonate (single bondSO3-) group into PS solid particles by adding sodium styrene sulfonate as an extra monomer during their synthesis, and then induce phase separation in the particles for their transformation to hollow particles with a hole on the surface. Control in the phase separation enables the formation of single bondSO3--incorporated PS hollow particles with different structures (outer diameter, hollow interior diameter, and opening size). While thermally annealed at a temperature above Tg of the polymer, structural change in the hollow particles is experimentally observed and theoretically analyzed. The results demonstrate that the change in the surface property of the hollow particles by the incorporation of single bondSO3- group can affect their sealing behavior and surface-sealed structure.

Section snippets

Materials

Styrene (St), sodium 4-vinylbenzenesulfonate (NaSS), 2-ethylhexyl methacrylate (EHMA, 98%), decane, poly(vinyl pyrrolidone) (PVP, MW ≈ 55,000), benzoyl peroxide (BPO), and isoamyl alcohol were purchased from Sigma-Aldrich. Azobisisobutyronitrile (AIBN) was obtained from Junsei Chemical. Anhydrous ethanol and anhydrous methanol were purchased from Duksan Chemical. Deionized (DI) water (18.2 MΩ) produced by an ultra water purification system (ROMAX, Human Science) was used as a dispersion medium

Preparation of poly(St-NaSS) solid particles

Fig. 1 shows SEM images of the poly(St-NaSS) particles produced by dispersion polymerization, which demonstrate that the particles were spherical and monodisperse in size. The mean diameters of the PSN1, PSN2, and PSN3 particles, evaluated from the images, were 0.96 ± 0.02, 0.92 ± 0.02, and 0.96 ± 0.03 μm, respectively, which were in good agreement with the results obtained by DLS (Fig. S1, Supporting Information). The TEM images in the insets show that the particles had a solid structure. In

Conclusions

We investigated the thermal annealing-driven surface sealing behavior of the polymer hollow particles with different surface property. To accomplish this, we incorporated the different amount of the single bondSO3- group into PS solid particles, and subsequently transformed them to hollow particles with an opening on their surface by inducing phase separation between the polymer and decane. The control in the phase separation allowed the formation of single bondSO3- -incorporated PS hollow particles with different

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgement

D.C.H. thanks the support from Basic Science Research Program (NRF- 2018R1D1A1B07043878) funded by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF). J.K. acknowledges the support from the Energy Technology Development Project of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 1711026557).

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