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polystyrene nanocomposites
Applied Clay Science 75–76 (2013) 74–81
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Research paper
Effect of doubly organo-modified vermiculite on the properties of vermiculite/polystyrene nanocomposites Lan Wang a,⁎, Xu Wang b, Zhaoyang Chen c, Pengcheng Ma a a b c
Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi 830011, China Laboratory of Green Chemistry and Organic Synthesis, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi 830011, China Xinjiang Key Laboratory of Electronic Information Materials and Devices, Urumqi 830011, China
a r t i c l e
i n f o
Article history: Received 2 August 2011 Received in revised form 28 February 2013 Accepted 7 March 2013 Available online 9 April 2013 Keywords: Vermiculite Polystyrene Clay mineral polymer nanocomposite (CPN) Thermal stability Dynamic mechanical properties
a b s t r a c t Vermiculite (Verm)/polystyrene (PS) nanocomposites were prepared by dispersing a doubly organomodified Verm (DOVerm) in PS via in situ polymerization (DOVerm/PS 1/99, 3/97, 5/95, and 7/93 mass/ mass ratios). The morphology of Verm/PS nanocomposites evolved three stages as the content of DOVerm decreased in the nanocomposites: intercalation at high filler content, intermediate state of intercalation to exfoliation, and exfoliation of Verm in PS matrix with a low filler content. The morphological changes of Verm/PS nanocomposites were confirmed by the X-ray diffraction (XRD) patterns and the transmission electron microscopy (TEM) images. Compared with the pure PS, the nanocomposites filled with Verm showed significant enhancements on thermal stability and dynamic mechanical properties. Interestingly, the nanocomposites filled with 1 and 7 mass% of DOVerm exhibited more pronounced effects of Verm on the properties. It was proved that the double organo-modification clearly enhanced the ultimate properties of the Verm/PS nanocomposites. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Clay mineral/polymer nanocomposites (CPN) have attracted much interest for various applications in recent years owing to the excellent properties of the materials, as well as the low cost and easy availability of starting clay minerals (Alexandre and Dubois, 2000; Annabi-Bergaya, 2008; Bergaya and Lagaly, 2007; Gilman, 1999; Pavlidou and Papaspyrides, 2008; Ruiz-Hitzky and Van Meerbeek, 2006). Depending on the nature of components used and the method for preparation, intercalated, exfoliated or partially-intercalated-partially-exfoliated CPN can be fabricated (Alexandre and Dubois, 2000; Pavlidou and Papaspyrides, 2008). Usually, three main approaches are used to prepare CPN: solution blending, melt intercalation and in situ polymerization (Alexandre and Dubois, 2000). Among them, in situ polymerization is one of the most promising methods for preparing CPN with controlled morphology, especially when polymer matrix with low polarity was employed (Zeng and Lee, 2001; Zeng et al., 2002). Since clay minerals are naturally hydrophilic and inherently incompatible with most organic polymers, the compatibility between them is vital to produce CPN with superior properties (Owusu-Adom and Guymon, 2009; Ruiz-Hitzky and Van Meerbeek, 2006; Zanetti et al., 2000). In order to increase the interactions between clay minerals and polymer matrix, clay minerals were organically modified either by intercalating of alkyl chains (Lagaly and Beneke, 1991; Lagaly et al., ⁎ Corresponding author. Tel.: +86 991 3835879; fax: +86 991 3838957. E-mail address: [email protected] (L. Wang). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.03.005
2006) or introducing different functional groups (Pinnavaia, 1983; Ruiz-Hitzky and Van Meerbeek, 2006; Williams-Daryn et al., 2002). The surface treatment facilitates the intercalation and exfoliation of clay mineral layers inside the polymer matrices and thus offering reinforcing effects for the polymers. The most used clay minerals as nanoscaled filler are based on the smectite group (Ray and Okamoto, 2003), of which montmorillonite (Mt) is a typical representative (Panwar et al., 2011) in CPN. Vermiculite (Verm) is another 2:1 phyllosilicate (Brigatti et al., 2006), which is abundant and much cheaper than Mt (Tjong and Meng, 2003). It is generally used in packaging for antishocking purposes. Most of the Verm/polymer nanocomposites were developed by using the exfoliated Verm nanolayers as raw materials in polymers with low polarity or the intercalated organo-Verm (OVerm) as fillers in polymers with high polarity (Tjong et al., 2002a,b). However, the employment of low polarity polymer and intercalated OVerm to fabricate Verm/polymer nanocomposites with desired properties is rarely reported, especially for polystyrene (PS) which is widely used as an engineering plastic. Herein, we studied the effects of organo-modification on the compatibility between PS and Verm. In this work, the doubly organo-modified Verm (DOVerm) was prepared successively by the intercalating of cetyltrimethylammonium cations (CTA+) and grafting of 3-(trimethoxysilyl)propyl methacrylate (TMSPMA) on Verm. The obtained DOVerm was dispersed in styrene monomer to create intercalated and/or exfoliated Verm/PS nanocomposites via in situ polymerization. Structure and morphology of the Verm/PS nanocomposites were studied by X-ray diffraction (XRD)
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and transmission electron microscopy (TEM). Thermal stability and dynamic mechanical properties of the Verm/PS nanocomposites were also evaluated with a special emphasis on the effect arising from the content of DOVerm.
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2. Experimental section
prepared were labeled as X%DOVermPS, where X stands for the mass percentage of DOVerm in the CPN. For comparison purpose, a similar procedure was employed to prepare the pure PS and X%OVermPS. The unbound polymer in Verm/PS nanocomposites was removed with toluene using a Soxhlet extraction apparatus (Kim et al., 2002), and the residual Verm-PS powders were dried at 80 °C for 24 h.
2.1. Materials
2.5. Characterization
Verm was provided by Xinlong Verm Co. Ltd., Xinjiang, China. The raw Verm with size ranging from 2.8 to 8 mm was washed several times with deionized water and dried at 70 °C for 48 h, followed by crushing in a pulverizer for 2 min, and was then passed through a 325-mesh sieve and the fraction with size less than 45 μm was utilized for experiments. The cation exchange capacity (CEC) (70 mEq/100 g) of the pretreated Verm was determined by the ammonium acetate method (Bache, 1976; Chapman, 1965). 3-(Trimethoxysilyl)propyl methacrylate (TMSPMA, Tech) was purchased from Nanjing Yudeheng Fine Chemical Co. Ltd., China and used as received. Styrene (98%), 2,2′azobisisobutyronitrile (AIBN, CP), cetyltrimethylammonium bromide (CTA+B −, 99%) and other analytical grade reagents were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Styrene was washed with NaOH solution and deionized water successively, and dried over anhydrous sodium sulfate and distilled under reduced pressure, and then stored at −20 °C for further use.
X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer (40 kV, 40 mA) with nickel-filtered Cu Kα radiation (λ = 1.54 Å) and scintillation detector over the range of 1 to 15° (2θ) at a scanning rate of 1.5°/min by a step of 0.005 2θ. All samples were dried at 70 °C for 12–24 h before analysis. The specimen for XRD measurement was prepared by dropping powder into the cavity of a plastic sample holder and simply pressing it with a glass slide by applying a normal force to make the surface of the powder mount smooth and flat. Transmission electron microscopy (TEM) images of the Verm/PS nanocomposites were obtained using a JEOL JEM-2200 transmission electron microscope operating at an accelerating voltage of 150 kV. The samples of Verm/PS nanocomposites for TEM observation were ultramicrotomed to obtain thin sections with thickness of 70–100 nm using Leica Ultracut UCT ultramicrotome. The thin sections were deposited on carbon coated Cu grids for TEM observation. Fourier-transform infrared spectroscopy (FTIR) measurements were conducted on a BIO-RAD FTS 165 instrument from 400 to 4000 cm−1 with 16 scans at a resolution of 4 cm−1 using KBr method. All samples were dried at 100 °C for 12 h to remove adsorbed water prior to the analysis. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TGA, NETZSCH STA 449C) under a nitrogen flow from 50 to 800 °C with a heating rate of 20 °C/min. The TGA data were processed into differential thermogravimetry (DTG) curves to identify the bonding interactions between organically modified Verm and polymer matrix. Dynamic mechanical analysis (DMA) was carried out on a dynamic mechanical analyzer (DMAQ800, TA Instrument) at a fixed frequency of 1 Hz with an oscillation amplitude of 25 μm and in a temperature range of 25 to 150 °C with a heating rate of 3 °C/min.
2.2. Preparation of CTA +-modified Verm (OVerm) The procedure to prepare Na+-Verm was described in the previous paper (Wang et al., 2011). Its CEC was 86.5 mEq/100 g. The OVerm was prepared by ion-exchange reaction between the interlayer cations of Na +-Verm and the CTA+ cations under the optimized conditions using the hot solution method. The OVerm was recovered by filtering the solution, followed by repeated washings of the filtered cake with deionized water and ethanol to remove excess ions. 2.3. Preparation of functionalized Verm (DOVerm) The OVerm was chemically modified using a two-step procedure to obtain the doubly organo-modified vermiculite (DOVerm). In a typical experiment, 30 mL of TMSPMA was dispersed in 600 mL anhydrous ethanol with stirring. The pH of mixture was adjusted to 3.5–5.5 with acetic acid and the hydrolysis reaction of TMSPMA was maintained at room temperature under stirring for 1 h. Subsequently, 12.23 g OVerm was added into the mixture and the dispersion was further stirred at 50 °C for silanization for 6 h. The product was collected by centrifugation followed by repeated washing with ethanol for four times and subsequently with deionized water for one time, and dried in a vacuum oven at 70 °C for 24 h. 2.4. Preparation of Verm/PS nanocomposites Verm/PS nanocomposite samples with DOVerm contents of 1, 3, 5 and 7 mass% (abbreviated as %) were prepared via in situ polymerization of styrene in the presence of DOVerm. A typical synthesis procedure has been previously described (Uthirakumar et al., 2004; Zhong et al., 2005). Briefly, an appropriate amount of DOVerm was dispersed into the styrene monomer, and the mixture was ultrasonicated for 15 min and then mechanically stirred for 12 h at room temperature to allow the dispersion and swelling of the DOVerm in the styrene. Subsequently, the mixture was added into a 250-mL three-neck round-bottomed flask equipped with a mechanical stirrer, a reflux condenser and a nitrogen gas inlet. Then, AIBN initiator (1.0% based on styrene) was added, followed by inlet nitrogen gas for 15 min. The resulting mixture was slowly heated to 80 °C under stirring, and polymerized at this temperature for 48 h to obtain Verm/PS nanocomposites. The samples
3. Results and discussion The aim of double organo-modification for Verm is to design silicate nanolayers which are compatible with styrene monomer and more convenient to be intercalated or exfoliated by PS matrix. The overall scheme for Verm treatment and polymerization is shown in Fig. 1. The introduction of long alkyl chain and silane coupling agent on Verm offers two major functions: (1) to increase the interlayer space of Verm layers for the insertion of styrene molecules and to convert the surface of Verm layers from hydrophilic to hydrophobic (Lee and Lin, 2006; Osman, 2006); (2) The silane treatment of Verm introduces C_C groups on Verm surface, which can participate in the polymerization process, further improves the compatibility of silicate with PS. Finally, depending on the different contents of DOVerm added into the polymer matrix, three types of Verm/PS nanocomposites (intercalated, exfoliated, and intermediate state of intercalation to exfoliation) were prepared via in situ polymerization. 3.1. Organically modified Verm The XRD patterns of the Verm, Na +-Verm, OVerm and DOVerm are shown in Fig. 2 (Lower panel). In the case of Verm, the treatment of sodium chloride and hydrochloric acid was applied with a consideration that Na + ions can be easily exchanged by alkylammonium cations, and the diluted hydrochloric acid can remove some insoluble salts on the surface of Verm and further purify the material (Carrado et al., 2006). Compared with the reflections of the Verm, which agree
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Fig. 1. Schematic representation of the processes of preparing the DOVerm and the Verm/PS nanocomposites based on the DOVerm.
with observations of Marcos et al. (2009), the XRD pattern of Na +-Verm presented the loss of the diffuse basal reflection, the decreased intensity of the reflection (d = 1.01 nm) of phlogopite
Fig. 2. (Lower panel) X-ray diffraction (XRD) patterns of (a) Verm, (b) Na+-Verm, (c) OVerm and (d) DOVerm; (Upper panel) XRD patterns of (a) 1%DOVermPS, (b) 3%DOVermPS, (c) 5%DOVermPS, (d) 7%DOVermPS and (e) 3%OVermPS, and all samples were extracted with THF in Soxhlet extraction apparatus and the residual Verm-PS were ground into powder prior to the analysis.
and the remaining reflection (d = 1.12 nm) of the Na +-Verm. The results indicated that the treatment effectively removed the insoluble salts and enhanced the purity and homogeneity of Verm while retaining its layer structure (Tjong and Meng, 2003). The XRD peak of phlogopite decreased significantly due to the replacement of Na + ions for some K + ions in the interlayer space to form Na +-saturated phlogopite (Stout and Komarneni, 2002). Combining the XRD results and the increased CEC, it was confirmed that the process produced homoionic Na +-Verm. Furthermore, the improved structure of Verm was beneficial for the further organic modification of Verm. As shown in Fig. 2, the shift of the 001 peak of Na+-Verm toward lower 2θ angle after ion-exchange treatment confirmed the intercalation of the CTA+ cations into the interlayer space. The high-intensity and sharp peak at 2θ = 1.95° (d = 4.53 nm) in the XRD pattern of OVerm clearly showed a regular distribution of the alkylammonium cations in the interlayer space. The interlayer distance of the OVerm is estimated to be 3.57 nm by subtracting the Verm platelet thickness (about 0.96 nm) from the d spacing, suggesting that the CTA+ cations (chain length is 2.36 nm) built a tilting paraffin-type bilayer arrangement in the interlayer space (Ibrahim et al., 1999). When the OVerm was further modified by TMSPMA, the initial 001 peak of the OVerm sample was shifted toward a higher 2θ angle of 2.46° for the DOVerm, corresponding to a d spacing of 3.59 nm. The observed decrease in interlayer distance following silane surface treatment may be attributed to the fact that the introduction of silane molecules destroyed the ordered paraffin-type bilayer arrangement of CTA + in the interlayer space, which was partly confirmed by the relatively larger peak width of the XRD pattern of the DOVerm. Qualitative evidences of organo-modifications were provided by FTIR. The representative FTIR spectra of the Na +-Verm, OVerm and DOVerm are shown in Fig. 3. Compared with the unmodified Na+-Verm in Fig. 3a, the OVerm in Fig. 3b showed the new vibration bands at 2921 cm−1 and 2852 cm−1 originated from the CH2 stretching vibration, with a bending vibration at the band of 1473 cm−1. This is direct evidence for the successful intercalation of CTA+ cations into the interlayer space of Verm (Hsu and Jehng, 2009), which is consistent with the XRD results. Furthermore, peak intensity weakening of the stretching and bending vibration of H-O groups at 3431 cm−1 and
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1644 cm−1 respectively attested that the amount of hydrated inorganic cations in the OVerm decreased after ion-exchange compared with that of the Na+-Verm, an indication of enhanced hydrophobic properties of the Na+-Verm. The spectrum of the DOVerm in Fig. 3c exhibited the presence of C\O bond at around 1720 cm−1, indicating that TMSPMA was grafted on the edges and surface of the silicate layers (Herrera et al., 2004; Yao and Zhou, 2009). These FTIR observations well attested that the Verm was doubly organo-modified by alkyl chain and silane coupling agent. To gain more insight into the organo-modification process, we investigated the thermal stability of Na +-Verm and OVerm. Fig. 4 shows the TGA curves of samples before and after the organomodification. The Na +-Verm was very stable in the temperature range of 100–700 °C, which is consistent with previous reports (El Mouzdahir et al., 2009; Tjong et al., 2002b). In the whole temperature range, the mass loss was less than 2%, which can be ascribed to the loss of free water, interlayer water and a part of structure water. However, the OVerm exhibited a greatly different behavior under heating: in the temperature region between 180 and 520 °C, approximately 16% mass loss appeared due to the decomposition of CTA + in the modified Verm. Compared with the OVerm, as expected, the TGA curve of the DOVerm exhibited another decomposing platform beginning at about 250 °C, which corresponds to the onset decomposition temperature of silane attached on the layered silicate (Herrera et al., 2004). Furthermore, the decomposition process of the DOVerm was slower than that of the OVerm in the range from 300 to 700 °C, and a higher mass loss found in DOVerm, which can be attributed to the grafting of silane molecules with higher thermal stability based on the OVerm. Results on the thermal stability of materials provided additional evidence that the double organo-modification for the Verm layers was successful. 3.2. Structure of Verm/PS nanocomposites The XRD patterns of the Verm/PS nanocomposites with different contents of DOVerm are shown in Fig. 2 (Upper panel). In the case of the Verms compounded with PS, the d spacings were larger than that of the DOVerm. Significant differences can be observed in the shape, intensity and position of the silicate reflections depending on the different contents of DOVerm. Fig. 2d and c (Upper panel) shows that the 001 reflections for 7%DOVermPS and 5%DOVermPS shifted consistently from 2.46° of 2θ to 1.79° even with different contents of DOVerm, corresponding to an increase of d spacing from 3.59 to 4.93 nm. The increase of interlayer distance and the ordered reflection indicated the formation of an intercalated structure between
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Fig. 4. TGA curves of Na+-Verm, OVerm and DOVerm.
the Verm and PS matrix (Korley et al., 2006). However, the reflection intensity of 5%DOVermPS was lower than that of 7%DOVermPS, which can be ascribed mainly to the fact that a fraction of outside layers of the DOVerm in 5%DOVermPS has an expanded interlayer space with d spacing out of the detection range of the diffractometer (exfoliation-like structure). In the case of 3%DOVermPS, the XRD pattern shows a lower-intensity peak at 2θ = 1.56°, corresponding to d spacing = 5.65 nm. This means that the intercalation of the polymer chains further increased the interlayer distance of the Verm, which shifted the XRD peak toward a lower angle. The lower intensity of the peak can be attributed to the partial disruption of parallel stacking of the DOVerm layers (Chen et al., 2005). But with further decrease in the content of DOVerm to 1%, the Verm/PS nanocomposites exhibited no peak in the XRD pattern, which can be explained by the total exfoliation of Verm. Obviously, these differences could be interpreted by the difference content of DOVerm (Chen and Evans, 2006). When the content of DOVerm is lower, it appears that the relative high amount of styrene molecules is kinetically feasible to diffuse into the interlayer space of DOVerm. Accordingly, there are sufficient styrene molecules with availability and mobility to polymerize in the interlayer space and develop a greater interlayer distance. However, in the mixture containing styrene and higher content of DOVerm, the styrene monomer could not be able to diffuse into the silicate interlayer effectively, leading to an insufficient swelling of DOVerm with styrene molecules before the polymerization. To further examine the effect of TMSPMA on the final structure of the Verm/PS nanocomposites, the XRD pattern of 3%OVermPS was obtained, as shown in Fig. 2e (Upper panel). The sample exhibited reflections at 2θ = 1.90° and 2θ = 7.65°, corresponding to d spacings of 4.64 and 1.16 nm, respectively, which were almost similar to that of the OVerm. This can be attributed to the fact that the styrene molecules rarely penetrate into the interlayer space of the OVerm to obtain chain growth triggering the expansion of OVerm layers (Kim et al., 2002).
3.3. Morphology of Verm/PS nanocomposites
Fig. 3. FTIR spectra of (a) Na+-Verm, (b) OVerm and (c) DOVerm.
In order to confirm the deductions from XRD results, we performed TEM measurement on the Verm/PS nanocomposites. The morphology evolution was described in order to understand clearly the process of the breaking up of the multilayered silicate into the individual layers. Fig. 5 shows the typical TEM images of CPN. The dark areas at the edge of the images were attributed to the nonuniform thickness of the samples. The TEM images of 7%DOVermPS (Fig. 5d) show the large lateral size of the silicate layers and the tightly stacked silicate region,
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revealing a typical intercalated structure with narrow interlayer space, which is consistent with the corresponding XRD results (Fig. 2d, Upper panel). The TEM images of 5%DOVermPS (Fig. 5c) clearly show the similar intercalated morphology as 7%DOVermPS, but with occasional exfoliated layers, suggesting the stacking of silicate layers arising from the single and double layers. However, the TEM images of 3%DOVermPS (Fig. 5b) show a mixture of both intercalated and exfoliated structures, which is the result of partial disruption of parallel stacking of the DOVerm layers, suggesting an intermediate state from intercalation to exfoliation. It also explains the presence of a broad shoulder of reflection in the XRD pattern (Fig. 2b, Upper panel). When the DOVerm content decreased to 1%, each single (or, at most, double) silicate layer in the matrix kept free from adjacent silicate layers, and the individual silicate layers were well dispersed in the polymer matrix (Fig. 5a). This is consistent with the absence of reflection in the XRD pattern (Fig. 2a, Upper panel). According to these results, it is concluded that that an intercalated CPN is more feasible with higher silicate contents, while an exfoliated system is observed for the CPN with a lower silicate content (about 1%). 3.4. Properties of Verm/PS nanocomposites 3.4.1. Thermal stability The thermal stability of the pure PS and Verm/PS nanocomposites with different DOVerm contents was studied and the results are shown in Fig. 6. The derivative thermogravimetry (DTG) curves that further transformed from the TGA are shown in Fig. 7. The onset decomposition temperatures (T−5%) (5% mass loss temperature), the mid-point decomposition temperatures (T−50%) (50% mass loss temperature) and the maximum mass loss temperatures (Tmax, obtained from DTG) are listed in Table 1. From the TGA and DTG curves, it is clear that the thermal
stability of Verm/PS nanocomposites was significantly improved compared with pure PS. This enhancement is higher than that of the other report (Tang et al., 2008). More specifically, the onset decomposition temperature (T−5%) was about 40 °C higher for the 1%DOVermPS than that of pure PS, which can be attributed to the exfoliated twodimensional nanoscale Verm layers with large surface having very strong interactions with PS chains, preventing out-diffusion of the volatile decomposition products (Kotsilkova et al., 2001). A decrease of 54 °C was noticed in 3%DOVermPS, which can be explained by two reasons: i) the higher content of organic modifiers in 3%DOVermPS can be decomposed easily in the early stage of thermal degradation, resulting in the lower onset decomposition temperature; and ii) for the intergradation structure, the layers with partial disruption and low consistency of orientation in the matrix cannot exhibit a great barrier effect that stops the spreading of volatile molecules generated during the thermal degradation process, as confirmed by previous reports (Alexandre et al., 2001; Uthirakumar et al., 2005). As the further increase of the DOVerm content, an increase of 23 °C and 36 °C of the onset decomposition temperature was observed in 5%DOVermPS and 7%DOVermPS, respectively. Therefore, the highest onset decomposition temperature was obtained in 1%DOVermPS (exfoliated), and the lowest one was found in 3%DOVermPS (partially-intercalated-partiallyexfoliated), followed by a gradual shift towards higher temperatures in 5%DOVermPS and 7%DOVermPS (intercalated). At 50% mass loss, the T−50% was approximately 30 °C higher for the Verm/PS nanocomposites than that observed for the pure PS. Also, in the DTG curves, the Tmax of the Verm/PS nanocomposites tended to increase obviously with the incorporation of DOVerm and shifted toward higher temperature as the amount of DOVerm increases. These indicated that the DOVerm exhibited a beneficial effect on the enhancement of thermal stability of PS and more layers
Fig. 5. TEM images of (a) 1%DOVermPS, (b) 3%DOVermPS, (c) 5%DOVermPS and (d) 7%DOVermPS at different magnifications.
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Table 1 TGA data of pure PS and Verm/PS nanocomposites. Sample
T−5%a (°C)
T−50%b (°C)
Tmaxc (°C)
1%DOVermPS 3%DOVermPS 5%DOVermPS 7%DOVermPS 3%OVermPS PS
393 300 377 390 333 354
440 443 444 445 434 412
444 453 454 455 439 421
a
The 5% mass loss temperature, onset decomposition temperature under nitrogen. The 50% mass loss temperature, mid-point decomposition temperature of the degradation process. c The maximum mass loss temperature obtained from DTG. b
Fig. 6. TGA curves of pure PS and Verm/PS nanocomposites.
were better than fewer layers. Additionally, it is clear that there is no significant difference among the temperatures of the Verm/PS nanocomposites with 3–7% DOVerm, suggesting the saturation of enhancement on thermal stability above a critical content of filler. An explanation for this enhancement may be that the amount of intercalated layers is high enough to promote the thermal stability of PS through char formation, which acts as a physical barrier preventing the heat to spread quickly and limiting further degradation (Burnside and Giannelis, 1995; Doh and Cho, 1998; Fu and Qutubuddin, 2001; Huang and Brittain, 2001). For further comparison of effect of TMSPMA on the thermal stability of Verm/PS nanocomposites, the TGA and DTG curves of 3%OVermPS are represented in Figs. 6 and 7, respectively, and the T−5%, T−50% and Tmax are summarized in Table 1. It is interesting to note that the CTA +-modified Verm-containing PS nanocomposite displayed lower thermal stability than that of the double-modified one. The result revealed that the presence of TMSPMA with C_C groups reacting with styrene increased interfacial interactions between the Verm and PS chains, leading to the improved interface and marked enhancement on the thermal stability. From the results presented so far, it can be concluded that the thermal stability of Verm/PS nanocomposites depends not only on the silicate content, but also on the extent to which the silicate layers interact with the polymer chains, which can be enhanced by the organic modification to the fillers.
Fig. 7. DTG curves of pure PS and Verm/PS nanocomposites.
3.4.2. Dynamic mechanical properties The dynamic storage modulus (E′) and tan δ of the pure PS and Verm/PS nanocomposites are plotted as a function of temperature, as shown in Figs. 8 and 9, respectively. The representative values of the storage modulus at 25, 70 and 90 °C and the glass transition temperature (Tg) obtained from the tan δ are listed in Table 2. The Verm/PS nanocomposites showed substantial enhancement on E′ in the presence of DOVerm compared with pure PS. This was more pronounced when the temperature was near Tg. In order to clarify the effect of the DOVerm on the E′ of PS, the relative storage modulus of the Verm/PS nanocomposites to those of PS (E′-CPN/E′-PS ratio) was calculated and listed in Table 2. Below 70 °C, the enhancement of E′ was clear in the Verm/PS nanocomposites that was either exfoliated or intercalated. The relative modulus was relatively small and was about 1.2–1.4 to those of PS, which is comparable to the reported values (Hasegawa et al., 1999). Above 70 °C, the four Verm/PS nanocomposites exhibited much higher enhancement of E′ as compared with pure PS. Their relative modulus increased drastically up to 11.73–75.57 at 93 °C and then decreased to melt gradually, which can be attributed to the restricted segmental motions at the organic-inorganic interface because of the confinement of PS chains by the Verm layers at the nanoscale (Hasegawa et al., 1999; Park et al., 2004). Interestingly, the E′ of the exfoliated Verm/PS nanocomposites (1%DOVermPS) was higher than those of the corresponding intercalated Verm/PS nanocomposites (3%DOVermPS and 5%DOVermPS) above 69 and 73 °C. This is probably due to the creation of a three-dimensional network of interconnected long silicate layers in the exfoliated CPN, the improved dispersion and interfacial interactions of filler with polymer matrix. The synergistic effects strengthened the material through mechanical percolation (Alexandre and Dubois, 2000). This improvement is higher than that of other researchers' report (Uthirakumar et al., 2005). However, in the case of 3%DOVermPS, it exhibited relatively weak
Fig. 8. Temperature dependence of storage modulus for pure PS and Verm/PS nanocomposites.
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For 3%OVermPS, it was found that the incorporation of OVerm did not substantially enhance the E′ and Tg of PS. This can be primarily attributed to its incompatibility with PS matrix because PS chains were ineffectively intercalated into the interlayer space of Verm. In other words, grafting reaction between TMSPMA and Verm is responsible for the improved interfacial interactions between the Verm layers and PS matrix. In this case, the grafted silane coupling agent acts as the “bridge” connecting the layers and polymer chains, and the DOVerm layer acts as a highly polyfunctional crosslinking agent, leading to an increase in the dynamic mechanical properties of Verm/PS nanocomposites. 4. Conclusions
Fig. 9. Temperature dependence of tan δ for pure PS and Verm/PS nanocomposites.
enhancements in E' as compared with those of other samples. It seems that a partly exfoliated structure existed in 3%DOVermPS also contributed to the enhancement of E′, however, the large number of disrupted parallel DOVerm layers with intercalated structure improved the properties less than the exfoliated one (Shi et al., 2007). On the other hand, the enhancement monotonically increased with increasing DOVerm content from 3 to 7% in the investigated temperature range, implying that increasing the content of DOVerm with intercalated morphology in the matrix favors the mechanical properties of corresponding Verm/PS nanocomposites. As seen from the TEM images, the 7%DOVermPS had relatively whole layers and more layers as compared with those of 5%DOVermPS and 3%DOVermPS. This also can be responsible for the enhancement trend of mechanical properties in the Verm/PS nanocomposites (Ray et al., 2003). As shown in Fig. 9, the tan δ curves of Verm/PS nanocomposites shifted significantly to higher temperature with the addition of DOVerm. Correspondingly, the Tg obtained from the tan δ is listed in Table 2. The significant shifts mean that the molecular mobility of PS chains was restricted by the presence of DOVerm layers. Interestingly, 1%DOVermPS with exfoliated layers exhibited more increase in Tg than 3%DOVermPS and 5%DOVermPS. It can be concluded that the improvement was greater when the silicate layers were well exfoliated in the polymer matrix than those with intercalated fillers. The reasons for this are two-folds: i) The improved dispersion and compatibility of exfoliated silicate layers with polymers restricted the mobility of matrix; ii) PS chains anchored on the silicate due to the polymerization of C_C bonds of silane, which is covalently bonded on silicate surface. On the other hand, the incorporation of 3–7% DOVerm into the PS matrix produced a progressive increase of Tg. This is possibly due to the fact that the restricting effect was enhanced with increasing silicate content in the intercalated CPN.
Table 2 Dynamic mechanical properties of pure PS and Verm/PS nanocomposites. Sample
1%DOVermPS 3%DOVermPS 5%DOVermPS 7%DOVermPS 3%OVermPS PS
Storage modulusa (GPa) 70 °C
90 °C
2.20 2.26 2.52 2.91 2.45 2.20
1.96 (1.20) 1.94 (1.19) 1.98 (1.21) 2.31 (1.42) 1.58 (0.97) 1.63
1.12 (43.8) 0.34 (13.3) 0.61 (23.8) 1.31 (51.2) 0.0355 (1.39) 0.0256
(1.00) (1.03) (1.15) (1.32) (1.11)
Acknowledgments The authors acknowledged the financial support from the Western Light Program of Chinese Academy of Sciences (CAS, XBBS201211) and the High Technology Research and Development Program in Xinjiang Uygur Autonomous Region (200816121). PC Ma was supported by the One Hundred Talent Program of CAS. Particular acknowledgment was made for the use of characterization facilities in the CAS Key Laboratory of Engineering Plastics. References
Tgb (°C)
25 °C
Double organo-modification of Verm improved its compatibility with PS, allowing the formation of exfoliated and/or intercalated structures of Verm in PS-based nanocomposites. At 1% DOVerm content, the clay mineral layers were delaminated as thin platelets in the PS matrix, as evidenced by XRD and TEM results. When the contents were increased to 5–7%, the CPN with intercalated DOVerm was obtained through the intermediate state of exfoliation to intercalation with 3% filler. The introduction of DOVerm significantly enhanced the thermal stability and dynamic mechanical properties of PS, and this became more pronounced in the Verm/PS nanocomposites filled with 1% exfoliated DOVerm and 7% intercalated one. 1%DOVermPS displayed an about 40 °C increase in onset decomposition temperature compared to that of pure PS, which was higher than that of other samples. T−50% and Tmax of the Verm/PS nanocomposites were increased with increasing the DOVerm content, and the highest increases were obtained in 7%DOVermPS. For the dynamic mechanical properties of the Verm/PS nanocomposites, the enhancement effect increased monotonously with increasing the DOVerm content (except that when 1% of DOVerm was present). Interestingly, the E′ of 1%DOVermPS was significantly enhanced around the Tg of PS, and the Tg value of 1%DOVermPS was as high as that of 7%DOVermPS. Thus, the content and surface functionality of Verm had pronounced effects on the morphology and properties of Verm/PS nanocomposites. The approach of double organo-modification of Verm and in situ polymerization of styrene was successful in producing exfoliated and/or intercalated Verm/PS nanocomposites with enhanced thermal stability and mechanical properties, making the Verm a high-value-added multifunctional filler for polymers.
105 98 102 105 92 92
a The values in parentheses are the relative dynamic storage modulus (E′-CPN/E′-PS ratio) of the CPN to those of PS. b Obtained from the temperature that corresponds to the tan δ peak.
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Research paper
Effect of doubly organo-modified vermiculite on the properties of vermiculite/polystyrene nanocomposites Lan Wang a,⁎, Xu Wang b, Zhaoyang Chen c, Pengcheng Ma a a b c
Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi 830011, China Laboratory of Green Chemistry and Organic Synthesis, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi 830011, China Xinjiang Key Laboratory of Electronic Information Materials and Devices, Urumqi 830011, China
a r t i c l e
i n f o
Article history: Received 2 August 2011 Received in revised form 28 February 2013 Accepted 7 March 2013 Available online 9 April 2013 Keywords: Vermiculite Polystyrene Clay mineral polymer nanocomposite (CPN) Thermal stability Dynamic mechanical properties
a b s t r a c t Vermiculite (Verm)/polystyrene (PS) nanocomposites were prepared by dispersing a doubly organomodified Verm (DOVerm) in PS via in situ polymerization (DOVerm/PS 1/99, 3/97, 5/95, and 7/93 mass/ mass ratios). The morphology of Verm/PS nanocomposites evolved three stages as the content of DOVerm decreased in the nanocomposites: intercalation at high filler content, intermediate state of intercalation to exfoliation, and exfoliation of Verm in PS matrix with a low filler content. The morphological changes of Verm/PS nanocomposites were confirmed by the X-ray diffraction (XRD) patterns and the transmission electron microscopy (TEM) images. Compared with the pure PS, the nanocomposites filled with Verm showed significant enhancements on thermal stability and dynamic mechanical properties. Interestingly, the nanocomposites filled with 1 and 7 mass% of DOVerm exhibited more pronounced effects of Verm on the properties. It was proved that the double organo-modification clearly enhanced the ultimate properties of the Verm/PS nanocomposites. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Clay mineral/polymer nanocomposites (CPN) have attracted much interest for various applications in recent years owing to the excellent properties of the materials, as well as the low cost and easy availability of starting clay minerals (Alexandre and Dubois, 2000; Annabi-Bergaya, 2008; Bergaya and Lagaly, 2007; Gilman, 1999; Pavlidou and Papaspyrides, 2008; Ruiz-Hitzky and Van Meerbeek, 2006). Depending on the nature of components used and the method for preparation, intercalated, exfoliated or partially-intercalated-partially-exfoliated CPN can be fabricated (Alexandre and Dubois, 2000; Pavlidou and Papaspyrides, 2008). Usually, three main approaches are used to prepare CPN: solution blending, melt intercalation and in situ polymerization (Alexandre and Dubois, 2000). Among them, in situ polymerization is one of the most promising methods for preparing CPN with controlled morphology, especially when polymer matrix with low polarity was employed (Zeng and Lee, 2001; Zeng et al., 2002). Since clay minerals are naturally hydrophilic and inherently incompatible with most organic polymers, the compatibility between them is vital to produce CPN with superior properties (Owusu-Adom and Guymon, 2009; Ruiz-Hitzky and Van Meerbeek, 2006; Zanetti et al., 2000). In order to increase the interactions between clay minerals and polymer matrix, clay minerals were organically modified either by intercalating of alkyl chains (Lagaly and Beneke, 1991; Lagaly et al., ⁎ Corresponding author. Tel.: +86 991 3835879; fax: +86 991 3838957. E-mail address: [email protected] (L. Wang). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.03.005
2006) or introducing different functional groups (Pinnavaia, 1983; Ruiz-Hitzky and Van Meerbeek, 2006; Williams-Daryn et al., 2002). The surface treatment facilitates the intercalation and exfoliation of clay mineral layers inside the polymer matrices and thus offering reinforcing effects for the polymers. The most used clay minerals as nanoscaled filler are based on the smectite group (Ray and Okamoto, 2003), of which montmorillonite (Mt) is a typical representative (Panwar et al., 2011) in CPN. Vermiculite (Verm) is another 2:1 phyllosilicate (Brigatti et al., 2006), which is abundant and much cheaper than Mt (Tjong and Meng, 2003). It is generally used in packaging for antishocking purposes. Most of the Verm/polymer nanocomposites were developed by using the exfoliated Verm nanolayers as raw materials in polymers with low polarity or the intercalated organo-Verm (OVerm) as fillers in polymers with high polarity (Tjong et al., 2002a,b). However, the employment of low polarity polymer and intercalated OVerm to fabricate Verm/polymer nanocomposites with desired properties is rarely reported, especially for polystyrene (PS) which is widely used as an engineering plastic. Herein, we studied the effects of organo-modification on the compatibility between PS and Verm. In this work, the doubly organo-modified Verm (DOVerm) was prepared successively by the intercalating of cetyltrimethylammonium cations (CTA+) and grafting of 3-(trimethoxysilyl)propyl methacrylate (TMSPMA) on Verm. The obtained DOVerm was dispersed in styrene monomer to create intercalated and/or exfoliated Verm/PS nanocomposites via in situ polymerization. Structure and morphology of the Verm/PS nanocomposites were studied by X-ray diffraction (XRD)
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and transmission electron microscopy (TEM). Thermal stability and dynamic mechanical properties of the Verm/PS nanocomposites were also evaluated with a special emphasis on the effect arising from the content of DOVerm.
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2. Experimental section
prepared were labeled as X%DOVermPS, where X stands for the mass percentage of DOVerm in the CPN. For comparison purpose, a similar procedure was employed to prepare the pure PS and X%OVermPS. The unbound polymer in Verm/PS nanocomposites was removed with toluene using a Soxhlet extraction apparatus (Kim et al., 2002), and the residual Verm-PS powders were dried at 80 °C for 24 h.
2.1. Materials
2.5. Characterization
Verm was provided by Xinlong Verm Co. Ltd., Xinjiang, China. The raw Verm with size ranging from 2.8 to 8 mm was washed several times with deionized water and dried at 70 °C for 48 h, followed by crushing in a pulverizer for 2 min, and was then passed through a 325-mesh sieve and the fraction with size less than 45 μm was utilized for experiments. The cation exchange capacity (CEC) (70 mEq/100 g) of the pretreated Verm was determined by the ammonium acetate method (Bache, 1976; Chapman, 1965). 3-(Trimethoxysilyl)propyl methacrylate (TMSPMA, Tech) was purchased from Nanjing Yudeheng Fine Chemical Co. Ltd., China and used as received. Styrene (98%), 2,2′azobisisobutyronitrile (AIBN, CP), cetyltrimethylammonium bromide (CTA+B −, 99%) and other analytical grade reagents were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Styrene was washed with NaOH solution and deionized water successively, and dried over anhydrous sodium sulfate and distilled under reduced pressure, and then stored at −20 °C for further use.
X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer (40 kV, 40 mA) with nickel-filtered Cu Kα radiation (λ = 1.54 Å) and scintillation detector over the range of 1 to 15° (2θ) at a scanning rate of 1.5°/min by a step of 0.005 2θ. All samples were dried at 70 °C for 12–24 h before analysis. The specimen for XRD measurement was prepared by dropping powder into the cavity of a plastic sample holder and simply pressing it with a glass slide by applying a normal force to make the surface of the powder mount smooth and flat. Transmission electron microscopy (TEM) images of the Verm/PS nanocomposites were obtained using a JEOL JEM-2200 transmission electron microscope operating at an accelerating voltage of 150 kV. The samples of Verm/PS nanocomposites for TEM observation were ultramicrotomed to obtain thin sections with thickness of 70–100 nm using Leica Ultracut UCT ultramicrotome. The thin sections were deposited on carbon coated Cu grids for TEM observation. Fourier-transform infrared spectroscopy (FTIR) measurements were conducted on a BIO-RAD FTS 165 instrument from 400 to 4000 cm−1 with 16 scans at a resolution of 4 cm−1 using KBr method. All samples were dried at 100 °C for 12 h to remove adsorbed water prior to the analysis. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TGA, NETZSCH STA 449C) under a nitrogen flow from 50 to 800 °C with a heating rate of 20 °C/min. The TGA data were processed into differential thermogravimetry (DTG) curves to identify the bonding interactions between organically modified Verm and polymer matrix. Dynamic mechanical analysis (DMA) was carried out on a dynamic mechanical analyzer (DMAQ800, TA Instrument) at a fixed frequency of 1 Hz with an oscillation amplitude of 25 μm and in a temperature range of 25 to 150 °C with a heating rate of 3 °C/min.
2.2. Preparation of CTA +-modified Verm (OVerm) The procedure to prepare Na+-Verm was described in the previous paper (Wang et al., 2011). Its CEC was 86.5 mEq/100 g. The OVerm was prepared by ion-exchange reaction between the interlayer cations of Na +-Verm and the CTA+ cations under the optimized conditions using the hot solution method. The OVerm was recovered by filtering the solution, followed by repeated washings of the filtered cake with deionized water and ethanol to remove excess ions. 2.3. Preparation of functionalized Verm (DOVerm) The OVerm was chemically modified using a two-step procedure to obtain the doubly organo-modified vermiculite (DOVerm). In a typical experiment, 30 mL of TMSPMA was dispersed in 600 mL anhydrous ethanol with stirring. The pH of mixture was adjusted to 3.5–5.5 with acetic acid and the hydrolysis reaction of TMSPMA was maintained at room temperature under stirring for 1 h. Subsequently, 12.23 g OVerm was added into the mixture and the dispersion was further stirred at 50 °C for silanization for 6 h. The product was collected by centrifugation followed by repeated washing with ethanol for four times and subsequently with deionized water for one time, and dried in a vacuum oven at 70 °C for 24 h. 2.4. Preparation of Verm/PS nanocomposites Verm/PS nanocomposite samples with DOVerm contents of 1, 3, 5 and 7 mass% (abbreviated as %) were prepared via in situ polymerization of styrene in the presence of DOVerm. A typical synthesis procedure has been previously described (Uthirakumar et al., 2004; Zhong et al., 2005). Briefly, an appropriate amount of DOVerm was dispersed into the styrene monomer, and the mixture was ultrasonicated for 15 min and then mechanically stirred for 12 h at room temperature to allow the dispersion and swelling of the DOVerm in the styrene. Subsequently, the mixture was added into a 250-mL three-neck round-bottomed flask equipped with a mechanical stirrer, a reflux condenser and a nitrogen gas inlet. Then, AIBN initiator (1.0% based on styrene) was added, followed by inlet nitrogen gas for 15 min. The resulting mixture was slowly heated to 80 °C under stirring, and polymerized at this temperature for 48 h to obtain Verm/PS nanocomposites. The samples
3. Results and discussion The aim of double organo-modification for Verm is to design silicate nanolayers which are compatible with styrene monomer and more convenient to be intercalated or exfoliated by PS matrix. The overall scheme for Verm treatment and polymerization is shown in Fig. 1. The introduction of long alkyl chain and silane coupling agent on Verm offers two major functions: (1) to increase the interlayer space of Verm layers for the insertion of styrene molecules and to convert the surface of Verm layers from hydrophilic to hydrophobic (Lee and Lin, 2006; Osman, 2006); (2) The silane treatment of Verm introduces C_C groups on Verm surface, which can participate in the polymerization process, further improves the compatibility of silicate with PS. Finally, depending on the different contents of DOVerm added into the polymer matrix, three types of Verm/PS nanocomposites (intercalated, exfoliated, and intermediate state of intercalation to exfoliation) were prepared via in situ polymerization. 3.1. Organically modified Verm The XRD patterns of the Verm, Na +-Verm, OVerm and DOVerm are shown in Fig. 2 (Lower panel). In the case of Verm, the treatment of sodium chloride and hydrochloric acid was applied with a consideration that Na + ions can be easily exchanged by alkylammonium cations, and the diluted hydrochloric acid can remove some insoluble salts on the surface of Verm and further purify the material (Carrado et al., 2006). Compared with the reflections of the Verm, which agree
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Fig. 1. Schematic representation of the processes of preparing the DOVerm and the Verm/PS nanocomposites based on the DOVerm.
with observations of Marcos et al. (2009), the XRD pattern of Na +-Verm presented the loss of the diffuse basal reflection, the decreased intensity of the reflection (d = 1.01 nm) of phlogopite
Fig. 2. (Lower panel) X-ray diffraction (XRD) patterns of (a) Verm, (b) Na+-Verm, (c) OVerm and (d) DOVerm; (Upper panel) XRD patterns of (a) 1%DOVermPS, (b) 3%DOVermPS, (c) 5%DOVermPS, (d) 7%DOVermPS and (e) 3%OVermPS, and all samples were extracted with THF in Soxhlet extraction apparatus and the residual Verm-PS were ground into powder prior to the analysis.
and the remaining reflection (d = 1.12 nm) of the Na +-Verm. The results indicated that the treatment effectively removed the insoluble salts and enhanced the purity and homogeneity of Verm while retaining its layer structure (Tjong and Meng, 2003). The XRD peak of phlogopite decreased significantly due to the replacement of Na + ions for some K + ions in the interlayer space to form Na +-saturated phlogopite (Stout and Komarneni, 2002). Combining the XRD results and the increased CEC, it was confirmed that the process produced homoionic Na +-Verm. Furthermore, the improved structure of Verm was beneficial for the further organic modification of Verm. As shown in Fig. 2, the shift of the 001 peak of Na+-Verm toward lower 2θ angle after ion-exchange treatment confirmed the intercalation of the CTA+ cations into the interlayer space. The high-intensity and sharp peak at 2θ = 1.95° (d = 4.53 nm) in the XRD pattern of OVerm clearly showed a regular distribution of the alkylammonium cations in the interlayer space. The interlayer distance of the OVerm is estimated to be 3.57 nm by subtracting the Verm platelet thickness (about 0.96 nm) from the d spacing, suggesting that the CTA+ cations (chain length is 2.36 nm) built a tilting paraffin-type bilayer arrangement in the interlayer space (Ibrahim et al., 1999). When the OVerm was further modified by TMSPMA, the initial 001 peak of the OVerm sample was shifted toward a higher 2θ angle of 2.46° for the DOVerm, corresponding to a d spacing of 3.59 nm. The observed decrease in interlayer distance following silane surface treatment may be attributed to the fact that the introduction of silane molecules destroyed the ordered paraffin-type bilayer arrangement of CTA + in the interlayer space, which was partly confirmed by the relatively larger peak width of the XRD pattern of the DOVerm. Qualitative evidences of organo-modifications were provided by FTIR. The representative FTIR spectra of the Na +-Verm, OVerm and DOVerm are shown in Fig. 3. Compared with the unmodified Na+-Verm in Fig. 3a, the OVerm in Fig. 3b showed the new vibration bands at 2921 cm−1 and 2852 cm−1 originated from the CH2 stretching vibration, with a bending vibration at the band of 1473 cm−1. This is direct evidence for the successful intercalation of CTA+ cations into the interlayer space of Verm (Hsu and Jehng, 2009), which is consistent with the XRD results. Furthermore, peak intensity weakening of the stretching and bending vibration of H-O groups at 3431 cm−1 and
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1644 cm−1 respectively attested that the amount of hydrated inorganic cations in the OVerm decreased after ion-exchange compared with that of the Na+-Verm, an indication of enhanced hydrophobic properties of the Na+-Verm. The spectrum of the DOVerm in Fig. 3c exhibited the presence of C\O bond at around 1720 cm−1, indicating that TMSPMA was grafted on the edges and surface of the silicate layers (Herrera et al., 2004; Yao and Zhou, 2009). These FTIR observations well attested that the Verm was doubly organo-modified by alkyl chain and silane coupling agent. To gain more insight into the organo-modification process, we investigated the thermal stability of Na +-Verm and OVerm. Fig. 4 shows the TGA curves of samples before and after the organomodification. The Na +-Verm was very stable in the temperature range of 100–700 °C, which is consistent with previous reports (El Mouzdahir et al., 2009; Tjong et al., 2002b). In the whole temperature range, the mass loss was less than 2%, which can be ascribed to the loss of free water, interlayer water and a part of structure water. However, the OVerm exhibited a greatly different behavior under heating: in the temperature region between 180 and 520 °C, approximately 16% mass loss appeared due to the decomposition of CTA + in the modified Verm. Compared with the OVerm, as expected, the TGA curve of the DOVerm exhibited another decomposing platform beginning at about 250 °C, which corresponds to the onset decomposition temperature of silane attached on the layered silicate (Herrera et al., 2004). Furthermore, the decomposition process of the DOVerm was slower than that of the OVerm in the range from 300 to 700 °C, and a higher mass loss found in DOVerm, which can be attributed to the grafting of silane molecules with higher thermal stability based on the OVerm. Results on the thermal stability of materials provided additional evidence that the double organo-modification for the Verm layers was successful. 3.2. Structure of Verm/PS nanocomposites The XRD patterns of the Verm/PS nanocomposites with different contents of DOVerm are shown in Fig. 2 (Upper panel). In the case of the Verms compounded with PS, the d spacings were larger than that of the DOVerm. Significant differences can be observed in the shape, intensity and position of the silicate reflections depending on the different contents of DOVerm. Fig. 2d and c (Upper panel) shows that the 001 reflections for 7%DOVermPS and 5%DOVermPS shifted consistently from 2.46° of 2θ to 1.79° even with different contents of DOVerm, corresponding to an increase of d spacing from 3.59 to 4.93 nm. The increase of interlayer distance and the ordered reflection indicated the formation of an intercalated structure between
77
Fig. 4. TGA curves of Na+-Verm, OVerm and DOVerm.
the Verm and PS matrix (Korley et al., 2006). However, the reflection intensity of 5%DOVermPS was lower than that of 7%DOVermPS, which can be ascribed mainly to the fact that a fraction of outside layers of the DOVerm in 5%DOVermPS has an expanded interlayer space with d spacing out of the detection range of the diffractometer (exfoliation-like structure). In the case of 3%DOVermPS, the XRD pattern shows a lower-intensity peak at 2θ = 1.56°, corresponding to d spacing = 5.65 nm. This means that the intercalation of the polymer chains further increased the interlayer distance of the Verm, which shifted the XRD peak toward a lower angle. The lower intensity of the peak can be attributed to the partial disruption of parallel stacking of the DOVerm layers (Chen et al., 2005). But with further decrease in the content of DOVerm to 1%, the Verm/PS nanocomposites exhibited no peak in the XRD pattern, which can be explained by the total exfoliation of Verm. Obviously, these differences could be interpreted by the difference content of DOVerm (Chen and Evans, 2006). When the content of DOVerm is lower, it appears that the relative high amount of styrene molecules is kinetically feasible to diffuse into the interlayer space of DOVerm. Accordingly, there are sufficient styrene molecules with availability and mobility to polymerize in the interlayer space and develop a greater interlayer distance. However, in the mixture containing styrene and higher content of DOVerm, the styrene monomer could not be able to diffuse into the silicate interlayer effectively, leading to an insufficient swelling of DOVerm with styrene molecules before the polymerization. To further examine the effect of TMSPMA on the final structure of the Verm/PS nanocomposites, the XRD pattern of 3%OVermPS was obtained, as shown in Fig. 2e (Upper panel). The sample exhibited reflections at 2θ = 1.90° and 2θ = 7.65°, corresponding to d spacings of 4.64 and 1.16 nm, respectively, which were almost similar to that of the OVerm. This can be attributed to the fact that the styrene molecules rarely penetrate into the interlayer space of the OVerm to obtain chain growth triggering the expansion of OVerm layers (Kim et al., 2002).
3.3. Morphology of Verm/PS nanocomposites
Fig. 3. FTIR spectra of (a) Na+-Verm, (b) OVerm and (c) DOVerm.
In order to confirm the deductions from XRD results, we performed TEM measurement on the Verm/PS nanocomposites. The morphology evolution was described in order to understand clearly the process of the breaking up of the multilayered silicate into the individual layers. Fig. 5 shows the typical TEM images of CPN. The dark areas at the edge of the images were attributed to the nonuniform thickness of the samples. The TEM images of 7%DOVermPS (Fig. 5d) show the large lateral size of the silicate layers and the tightly stacked silicate region,
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revealing a typical intercalated structure with narrow interlayer space, which is consistent with the corresponding XRD results (Fig. 2d, Upper panel). The TEM images of 5%DOVermPS (Fig. 5c) clearly show the similar intercalated morphology as 7%DOVermPS, but with occasional exfoliated layers, suggesting the stacking of silicate layers arising from the single and double layers. However, the TEM images of 3%DOVermPS (Fig. 5b) show a mixture of both intercalated and exfoliated structures, which is the result of partial disruption of parallel stacking of the DOVerm layers, suggesting an intermediate state from intercalation to exfoliation. It also explains the presence of a broad shoulder of reflection in the XRD pattern (Fig. 2b, Upper panel). When the DOVerm content decreased to 1%, each single (or, at most, double) silicate layer in the matrix kept free from adjacent silicate layers, and the individual silicate layers were well dispersed in the polymer matrix (Fig. 5a). This is consistent with the absence of reflection in the XRD pattern (Fig. 2a, Upper panel). According to these results, it is concluded that that an intercalated CPN is more feasible with higher silicate contents, while an exfoliated system is observed for the CPN with a lower silicate content (about 1%). 3.4. Properties of Verm/PS nanocomposites 3.4.1. Thermal stability The thermal stability of the pure PS and Verm/PS nanocomposites with different DOVerm contents was studied and the results are shown in Fig. 6. The derivative thermogravimetry (DTG) curves that further transformed from the TGA are shown in Fig. 7. The onset decomposition temperatures (T−5%) (5% mass loss temperature), the mid-point decomposition temperatures (T−50%) (50% mass loss temperature) and the maximum mass loss temperatures (Tmax, obtained from DTG) are listed in Table 1. From the TGA and DTG curves, it is clear that the thermal
stability of Verm/PS nanocomposites was significantly improved compared with pure PS. This enhancement is higher than that of the other report (Tang et al., 2008). More specifically, the onset decomposition temperature (T−5%) was about 40 °C higher for the 1%DOVermPS than that of pure PS, which can be attributed to the exfoliated twodimensional nanoscale Verm layers with large surface having very strong interactions with PS chains, preventing out-diffusion of the volatile decomposition products (Kotsilkova et al., 2001). A decrease of 54 °C was noticed in 3%DOVermPS, which can be explained by two reasons: i) the higher content of organic modifiers in 3%DOVermPS can be decomposed easily in the early stage of thermal degradation, resulting in the lower onset decomposition temperature; and ii) for the intergradation structure, the layers with partial disruption and low consistency of orientation in the matrix cannot exhibit a great barrier effect that stops the spreading of volatile molecules generated during the thermal degradation process, as confirmed by previous reports (Alexandre et al., 2001; Uthirakumar et al., 2005). As the further increase of the DOVerm content, an increase of 23 °C and 36 °C of the onset decomposition temperature was observed in 5%DOVermPS and 7%DOVermPS, respectively. Therefore, the highest onset decomposition temperature was obtained in 1%DOVermPS (exfoliated), and the lowest one was found in 3%DOVermPS (partially-intercalated-partiallyexfoliated), followed by a gradual shift towards higher temperatures in 5%DOVermPS and 7%DOVermPS (intercalated). At 50% mass loss, the T−50% was approximately 30 °C higher for the Verm/PS nanocomposites than that observed for the pure PS. Also, in the DTG curves, the Tmax of the Verm/PS nanocomposites tended to increase obviously with the incorporation of DOVerm and shifted toward higher temperature as the amount of DOVerm increases. These indicated that the DOVerm exhibited a beneficial effect on the enhancement of thermal stability of PS and more layers
Fig. 5. TEM images of (a) 1%DOVermPS, (b) 3%DOVermPS, (c) 5%DOVermPS and (d) 7%DOVermPS at different magnifications.
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Table 1 TGA data of pure PS and Verm/PS nanocomposites. Sample
T−5%a (°C)
T−50%b (°C)
Tmaxc (°C)
1%DOVermPS 3%DOVermPS 5%DOVermPS 7%DOVermPS 3%OVermPS PS
393 300 377 390 333 354
440 443 444 445 434 412
444 453 454 455 439 421
a
The 5% mass loss temperature, onset decomposition temperature under nitrogen. The 50% mass loss temperature, mid-point decomposition temperature of the degradation process. c The maximum mass loss temperature obtained from DTG. b
Fig. 6. TGA curves of pure PS and Verm/PS nanocomposites.
were better than fewer layers. Additionally, it is clear that there is no significant difference among the temperatures of the Verm/PS nanocomposites with 3–7% DOVerm, suggesting the saturation of enhancement on thermal stability above a critical content of filler. An explanation for this enhancement may be that the amount of intercalated layers is high enough to promote the thermal stability of PS through char formation, which acts as a physical barrier preventing the heat to spread quickly and limiting further degradation (Burnside and Giannelis, 1995; Doh and Cho, 1998; Fu and Qutubuddin, 2001; Huang and Brittain, 2001). For further comparison of effect of TMSPMA on the thermal stability of Verm/PS nanocomposites, the TGA and DTG curves of 3%OVermPS are represented in Figs. 6 and 7, respectively, and the T−5%, T−50% and Tmax are summarized in Table 1. It is interesting to note that the CTA +-modified Verm-containing PS nanocomposite displayed lower thermal stability than that of the double-modified one. The result revealed that the presence of TMSPMA with C_C groups reacting with styrene increased interfacial interactions between the Verm and PS chains, leading to the improved interface and marked enhancement on the thermal stability. From the results presented so far, it can be concluded that the thermal stability of Verm/PS nanocomposites depends not only on the silicate content, but also on the extent to which the silicate layers interact with the polymer chains, which can be enhanced by the organic modification to the fillers.
Fig. 7. DTG curves of pure PS and Verm/PS nanocomposites.
3.4.2. Dynamic mechanical properties The dynamic storage modulus (E′) and tan δ of the pure PS and Verm/PS nanocomposites are plotted as a function of temperature, as shown in Figs. 8 and 9, respectively. The representative values of the storage modulus at 25, 70 and 90 °C and the glass transition temperature (Tg) obtained from the tan δ are listed in Table 2. The Verm/PS nanocomposites showed substantial enhancement on E′ in the presence of DOVerm compared with pure PS. This was more pronounced when the temperature was near Tg. In order to clarify the effect of the DOVerm on the E′ of PS, the relative storage modulus of the Verm/PS nanocomposites to those of PS (E′-CPN/E′-PS ratio) was calculated and listed in Table 2. Below 70 °C, the enhancement of E′ was clear in the Verm/PS nanocomposites that was either exfoliated or intercalated. The relative modulus was relatively small and was about 1.2–1.4 to those of PS, which is comparable to the reported values (Hasegawa et al., 1999). Above 70 °C, the four Verm/PS nanocomposites exhibited much higher enhancement of E′ as compared with pure PS. Their relative modulus increased drastically up to 11.73–75.57 at 93 °C and then decreased to melt gradually, which can be attributed to the restricted segmental motions at the organic-inorganic interface because of the confinement of PS chains by the Verm layers at the nanoscale (Hasegawa et al., 1999; Park et al., 2004). Interestingly, the E′ of the exfoliated Verm/PS nanocomposites (1%DOVermPS) was higher than those of the corresponding intercalated Verm/PS nanocomposites (3%DOVermPS and 5%DOVermPS) above 69 and 73 °C. This is probably due to the creation of a three-dimensional network of interconnected long silicate layers in the exfoliated CPN, the improved dispersion and interfacial interactions of filler with polymer matrix. The synergistic effects strengthened the material through mechanical percolation (Alexandre and Dubois, 2000). This improvement is higher than that of other researchers' report (Uthirakumar et al., 2005). However, in the case of 3%DOVermPS, it exhibited relatively weak
Fig. 8. Temperature dependence of storage modulus for pure PS and Verm/PS nanocomposites.
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For 3%OVermPS, it was found that the incorporation of OVerm did not substantially enhance the E′ and Tg of PS. This can be primarily attributed to its incompatibility with PS matrix because PS chains were ineffectively intercalated into the interlayer space of Verm. In other words, grafting reaction between TMSPMA and Verm is responsible for the improved interfacial interactions between the Verm layers and PS matrix. In this case, the grafted silane coupling agent acts as the “bridge” connecting the layers and polymer chains, and the DOVerm layer acts as a highly polyfunctional crosslinking agent, leading to an increase in the dynamic mechanical properties of Verm/PS nanocomposites. 4. Conclusions
Fig. 9. Temperature dependence of tan δ for pure PS and Verm/PS nanocomposites.
enhancements in E' as compared with those of other samples. It seems that a partly exfoliated structure existed in 3%DOVermPS also contributed to the enhancement of E′, however, the large number of disrupted parallel DOVerm layers with intercalated structure improved the properties less than the exfoliated one (Shi et al., 2007). On the other hand, the enhancement monotonically increased with increasing DOVerm content from 3 to 7% in the investigated temperature range, implying that increasing the content of DOVerm with intercalated morphology in the matrix favors the mechanical properties of corresponding Verm/PS nanocomposites. As seen from the TEM images, the 7%DOVermPS had relatively whole layers and more layers as compared with those of 5%DOVermPS and 3%DOVermPS. This also can be responsible for the enhancement trend of mechanical properties in the Verm/PS nanocomposites (Ray et al., 2003). As shown in Fig. 9, the tan δ curves of Verm/PS nanocomposites shifted significantly to higher temperature with the addition of DOVerm. Correspondingly, the Tg obtained from the tan δ is listed in Table 2. The significant shifts mean that the molecular mobility of PS chains was restricted by the presence of DOVerm layers. Interestingly, 1%DOVermPS with exfoliated layers exhibited more increase in Tg than 3%DOVermPS and 5%DOVermPS. It can be concluded that the improvement was greater when the silicate layers were well exfoliated in the polymer matrix than those with intercalated fillers. The reasons for this are two-folds: i) The improved dispersion and compatibility of exfoliated silicate layers with polymers restricted the mobility of matrix; ii) PS chains anchored on the silicate due to the polymerization of C_C bonds of silane, which is covalently bonded on silicate surface. On the other hand, the incorporation of 3–7% DOVerm into the PS matrix produced a progressive increase of Tg. This is possibly due to the fact that the restricting effect was enhanced with increasing silicate content in the intercalated CPN.
Table 2 Dynamic mechanical properties of pure PS and Verm/PS nanocomposites. Sample
1%DOVermPS 3%DOVermPS 5%DOVermPS 7%DOVermPS 3%OVermPS PS
Storage modulusa (GPa) 70 °C
90 °C
2.20 2.26 2.52 2.91 2.45 2.20
1.96 (1.20) 1.94 (1.19) 1.98 (1.21) 2.31 (1.42) 1.58 (0.97) 1.63
1.12 (43.8) 0.34 (13.3) 0.61 (23.8) 1.31 (51.2) 0.0355 (1.39) 0.0256
(1.00) (1.03) (1.15) (1.32) (1.11)
Acknowledgments The authors acknowledged the financial support from the Western Light Program of Chinese Academy of Sciences (CAS, XBBS201211) and the High Technology Research and Development Program in Xinjiang Uygur Autonomous Region (200816121). PC Ma was supported by the One Hundred Talent Program of CAS. Particular acknowledgment was made for the use of characterization facilities in the CAS Key Laboratory of Engineering Plastics. References
Tgb (°C)
25 °C
Double organo-modification of Verm improved its compatibility with PS, allowing the formation of exfoliated and/or intercalated structures of Verm in PS-based nanocomposites. At 1% DOVerm content, the clay mineral layers were delaminated as thin platelets in the PS matrix, as evidenced by XRD and TEM results. When the contents were increased to 5–7%, the CPN with intercalated DOVerm was obtained through the intermediate state of exfoliation to intercalation with 3% filler. The introduction of DOVerm significantly enhanced the thermal stability and dynamic mechanical properties of PS, and this became more pronounced in the Verm/PS nanocomposites filled with 1% exfoliated DOVerm and 7% intercalated one. 1%DOVermPS displayed an about 40 °C increase in onset decomposition temperature compared to that of pure PS, which was higher than that of other samples. T−50% and Tmax of the Verm/PS nanocomposites were increased with increasing the DOVerm content, and the highest increases were obtained in 7%DOVermPS. For the dynamic mechanical properties of the Verm/PS nanocomposites, the enhancement effect increased monotonously with increasing the DOVerm content (except that when 1% of DOVerm was present). Interestingly, the E′ of 1%DOVermPS was significantly enhanced around the Tg of PS, and the Tg value of 1%DOVermPS was as high as that of 7%DOVermPS. Thus, the content and surface functionality of Verm had pronounced effects on the morphology and properties of Verm/PS nanocomposites. The approach of double organo-modification of Verm and in situ polymerization of styrene was successful in producing exfoliated and/or intercalated Verm/PS nanocomposites with enhanced thermal stability and mechanical properties, making the Verm a high-value-added multifunctional filler for polymers.
105 98 102 105 92 92
a The values in parentheses are the relative dynamic storage modulus (E′-CPN/E′-PS ratio) of the CPN to those of PS. b Obtained from the temperature that corresponds to the tan δ peak.
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