Effect of the surfactant coverage on the preparation of polystyrene–clay nanocomposites prepared by melt intercalation

doi:10.1016/j.matlet.2005.03.006

Materials Letters

Volume 59, Issue 18, August 2005, Pages 2292-2295

https://doi.org/10.1016/j.matlet.2005.03.006 Get rights and content

Abstract

The melt intercalation of polystyrene (PS) into organoclay with variation in the degree of surfactant coverage by melt intercalation was investigated. The surface coverage of the organoclay was found to play a major role in controlling the type of the final composites formation. Two major types of composites were found depending upon the surface treatment of the organoclay. These are conventional composites and intercalated nanocomposites. At the high degree of the clay's surface coverage, the conventional composite was obtained. It is characterized by the micron size aggregate of the organoclay particle. An intercalated nanocomposite was observed in the organoclay with a lower surface coverage. The organoclay is dispersed into a smaller stack along with the intercalation of polystyrene into the organoclay interlayer. This is originated from the affinity between the organoclay surface and a molten polystyrene.

Introduction

Polymer–clay nanocomposites have emerged as one of the most attractive engineering materials due to their unique properties. The basic properties of the nanocomposites such as mechanical and thermal are improved over a pristine polymer [1], [2], [3], [4]. The properties of polymer–clay nanocomposites are depending upon the formation of composite as reported by Kojima in 1993, the strength of nylon-6 clay composite is increased more than 50% and the heat distortion temperature improved from 65 to 145 °C [1], [2], [5], [6]. The formation of polymer–clay nanocomposites has been classified into three major types depending upon the dispersion of the clay in the polymer matrix. The first type is a conventional composite where the clay layers form an aggregate as a microparticle or tactoid. Second is the intercalated nanocomposite. The interlayer spacing of the clay layer is expanded by polymer insertion while the order of the clay layer is still maintained. The last one is exfoliated nanocomposites where the clay layer is delaminated as an individual layer and distributed randomly in the polymer matrix [7], [8], [9]. The goal of this study is to investigate the effect of the clay surface modification on its dispersion in polystyrene via melt intercalation [7].

Section snippets

Materials

Na-bentonite was obtained from Thai Nippon Chemical Co. Ltd. The surfactant used in this study is a mixture of dioctadecyl dimethyl ammonium chloride salts with alkyl chain lengths of 14 (4%), 16 (32%), and 18 (58%) and often referred as tallow alkyl ammonium. It will be referred as D18 throughout this study. The surfactant was obtained from Thai Specialty Chemical Co., Ltd. Polystyrene (GP 110) was obtained from Thai Petrochemical Industry Public Co., Ltd. Thailand. All the materials were used

Result and discussion

To understand the role of the clay surface coverage on the polystyrene–clay nanocomposites formation, two organoclay with different degrees of surface coverage were synthesized. The controlling of the surface coverage was done by treating the clay with the same surfactant but using a different surfactant loading. The surfactant is a mixed surfactant of the D18 surfactant. The organoclay was prepared by cation exchange reaction. Two loading levels at 0.5 and 2.0 mmol were used. They will be

Conclusion

The effect of clay surface coverage on the preparation of PS–clay nanocomposites by melt intercalation was investigated. Polystyrene shows a good affinity toward the organoclay with a lower surface coverage D1805 where the intercalated nanocomposites was observed. The clay with the high surface coverage hinders the formation of the nanocomposites due to poor wettability of the polystyrene on the clay surface. This leads to a phase separation between the polymer and the organoclay which results

Acknowledgement

Financial support from the Commission on Higher Education (Ministry of Education of Thailand) is gratefully acknowledged.

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The article reports upon the preparation and characterization of organo-functionalized NiAl layered double hydroxide (LDH)-polystyrene (PS) nanocomposites. Initially, pristine NiAl LDH was synthesized via the co-precipitation technique and was subsequently treated using sodium dodecyl sulfate to obtain organo-functionalized NiAl LDH (ONiAl LDH). PS nanocomposites were fabricated by melt intercalation using a twin screw extruder in presence of ONiAl LDH nanofiller (1, 3, 5, and 7 wt.%). The PS nanocomposites were characterized for their structural, thermal and mechanical properties. The dispersion and morphology of the obtained PS nanocomposites were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Mechanical and thermal properties of the PS nanocomposites as a function of LDH content were examined by tensile tests, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The XRD and TEM results revealed the formation of an exfoliated structure of the PS nanocomposite with 1 wt.% ONiAl LDH loading. The maximum improvements of the mechanical and thermal properties of the nanocomposites with ONiAl LDH loading over pristine PS included tensile strength = 34.5% (1 wt.%), thermal decomposition temperatures (T_15%) = 27.4 °C (7 wt.%), and glass transition temperature (T_g) = 4.3 °C (7 wt.%). The PS nanocomposites possessed higher mechanical strength and thermal degradation resistance compared to the pristine PS. The activation energy (E_a) and reaction mechanism with respect to thermal degradation of the pristine PS and its nanocomposites were evaluated by the Coats-Redfern and Criado model, respectively.
Effect of CEC coverage of hexadecyltributylphosphonium modified montmorillonite on polymer compatibility
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According to Morgan and Harris (2003) the excess of surfactant is necessary to allow the PP chains to intercalate between the clay layers. Some contradictory results have also been reported in the literature, where cationic surfactant modified Mt at low loading showed better dispersability than Mt modified with excess surfactant loading (Fornes et al., 2002; Limpanart et al., 2005; Panek et al., 2006). The Mt modified with surfactant below CEC coverage yielded better exfoliation characteristics than Mt modified with excess of surfactant in Nylon 6 matrix (Fornes et al., 2002).
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Processing and characterization of polystyrene nanocomposites based on Co–Al layered double hydroxide
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% LDH exhibited improved thermal stability (25 °C) over pristine PMMA. Limpanart et al. [15] followed melt compounding technique to synthesis PS/clay nanocomposites. They observed that two major types of composites namely, conventional and intercalated nanocomposites obtained depending on the modification of the organoclay.
The present work deals with the development of polystyrene (PS) nanocomposites through solvent blending technique with diverse contents of modified CoAl layered double hydroxide (LDH). The prepared PS as well as PS/CoAl LDH (1–7 wt.%) nanocomposites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), rheological analysis, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The XRD results suggested the formation of exfoliated structure, while TEM images clearly indicated the intercalated morphology of PS nanocomposites at higher loading. The presence of various functional groups in the CoAl LDH and PS/CoAl LDH nanocomposites was verified by FTIR analysis. TGA data confirmed that the thermal stability of PS composites was enhanced significantly as compared to pristine PS. While considering 15% weight loss as a reference point, it was found that the thermal degradation (T_d) temperature increased up to 28.5 °C for PS nanocomposites prepared with 7 wt.% CoAl LDH loading over pristine PS. All the nanocomposite samples displayed superior glass transition temperature (T_g), in which PS nanocomposites containing 7 wt.% LDH showed about 5.5 °C higher T_g over pristine PS. In addition, the kinetics for thermal degradation of the composites was studied using Coats-Redfern method. The Criado method was ultimately used to evaluate the decomposition reaction mechanism of the nanocomposites. The complex viscosity and rheological muduli of nanocomposites were found to be higher than that of pristine PS when the frequency increased from 0.01 to 100 s⁻¹.
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Clays have been used for the last 30 years as nanometric fillers to reinforce polymeric matrices (Fornes et al., 2001; Hedley et al., 2007) with the aim of improving specific properties, mainly thermal, barrier, impact and mechanical, for an important number of applications (Limpanart et al., 2005; Roelofs and Berben, 2006; Xie et al., 2002).
In this work, a study of exchange of soy lecithin, a natural product, in bentonite was performed in order to synthesize bio-organoclays. The effects of initial amount of modifier and reaction time were studied at a fixed reaction temperature. Organoclays thus obtained were characterized by means of X-ray diffraction (XRD) analysis, X-ray fluorescence spectrometry (XRF), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and water absorption tests. An effective intercalation of soy lecithin between the clay layers was obtained. The ionic exchange reaction was completed at short times whereas variations in the initial amount of modifier produced organoclays with different final properties. At low ratios of soy lecithin to bentonite, a slight increment in basal spacing of organoclays was observed due to intercalation of the organic modifier between the clay layers and a significant diminution on water absorption was achieved. When the organic content increased, the interlayer spacing increased but thermal stability of organoclays decreased compared to the samples with low organic content, whereas the water absorption was not affected. The obtained bio-organoclays are potential environmental-friendly fillers for the development of clay/biopolymer nanocomposites.
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The aim of this study is to analyze different strategies of modification of bentonite (Bent) combining reactions of cationic exchange, silylation and acid activation.
Six samples of modified bentonite were prepared: silylated bentonite (S-Bent), acid-activated bentonite (A-Bent), exchanged clay (E-Bent), silylated and acid-activated clay (S–A-Bent), exchanged and acid-activated clay (E–A-Bent), and exchanged, silylated and acid-activated bentonite (E–S–A-Bent). To study the effectiveness of the different treatments, X-ray diffraction analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy and water absorption tests were performed.
The silylated bentonite presented a significant reduction in the equilibrium water uptake percentage with respect to the unmodified clay, even when small amounts of silane were incorporated to the clay. On the other hand, no significant changes in the organic content were obtained when the silylation reaction was performed on previously acid-activated bentonite. When a sample of silylated and activated bentonite was further modified by cationic exchange reaction, it was found that the previous treatments allowed increasing the uptaken amount of surfactant cations during the exchange reaction. In fact, the combination of the three treatments (acid-activation, silylation and cationic exchange) gave place to the modified bentonite with the biggest basal spacing and the lower equilibrium water uptake percentage.
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