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Intercalation behavior of poly(ethylene glycol) in organically modified montmorillonite
Applied Surface Science 276 (2013) 502–511
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Intercalation behavior of poly(ethylene glycol) in organically modified montmorillonite Shipeng Zhu a,b , Hongmei Peng a , Jinyao Chen a , Huilin Li a,∗ , Ya Cao a,∗ , Yunhua Yang b , Zhihai Feng b a
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, PR China Science and Technology on Advanced Functional Composites Laboratory, Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, PR China b
a r t i c l e
i n f o
Article history: Received 19 July 2012 Received in revised form 16 March 2013 Accepted 19 March 2013 Available online 25 March 2013 Keywords: Intercalation Poly(ethylene glycol) Surfactant Montmorillonite
a b s t r a c t In this paper, two kinds of organically modified montmorillonite (OMMT) were prepared using alkylammonium surfactants with different alkyl chain numbers. XRD results showed the interlayer spacing of OMMT increased with low concentration surfactants. With further increasing the surfactants concentration, the interlayer spacing of OMMT was unchanged. Meanwhile, FTIR was used to characterize the local environments of surfactants in the interlayer space of OMMT. The results suggested that the double chain surfactant D-18 preferred to adopt highly ordered conformation compared with single chain surfactant S-18 in interlayer space of OMMT. It indicated that the surface property of the OMMT is affected by the concentration and configuration of the intercalated surfactants. Moreover, the effect of the OMMT type, or more particularly the chemical nature of the organic modifier in the interlayer spacing and the poly(ethylene glycol) (PEG) concentration onintercalation behavior of PEG chains in OMMT were investigated with XRD and DSC.The results indicated that PEG chains could not intercalate into Na-MMT when the surfactants were saturated in interlayer space of Na-MMT. PEG chains could intercalate into the interlayer space of SM when the S-18 concentration was lower than 2.00CEC, implying that the low surfactant concentration modified SM provided a better environment (presumably through the balanced hydrophobic and hydrophilic surfaces) for the PEG intercalation as well. However, PEG did not intercalate into the interlayer space of DM when the D-18 concentration was higher than 1.00CEC. It could be attributed to the hydrophobic double alkyl chains of DM increased with D-18. The increased hydrophobic properties in the interlayer space of 1.50DM hybrids can prevent the intercalation of hydrophilic PEG. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, surface modifications of clays have become increasingly important because it can be used to prepare polymer/clay nanocomposites and applied in some new applications such as adsorbents of organic pollutants in soil, water and air, rheological control agents, paints, medicine [1–5]. Natural clays are mostly hydrophilic, and their interactions are favorable only with polar polymers. In the case of hydrophobic polymers, intercalation or exfoliation can be achieved only with organophilized clays. Hence, proper organophilization procedure is a key step for successful intercalation and exfoliation of montmorillonite (MMT) particles in the polymer [6]. Generally, this progress can be done by ion-exchange reactions with cationic surfactants including primary, secondary, tertiary, and quaternary alkylammonium.
∗ Corresponding authors. Tel.: +86 28 85467166; fax: +86 28 85402465. E-mail addresses: [email protected] (H. Li), [email protected] (Y. Cao). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.123
Alkylammonium cations in the organically modified montmorillonite (OMMT) lower the surface energy of the inorganic host and improve the wetting characteristics of the polymer matrix, and result in a larger interlayer spacing [7]. Additionally, the alkylammonium cations can provide functional groups that can react with the polymer matrix, or in some cases initiate the polymerization of monomers to improve the strength of the interface between the inorganic and the polymer matrix [8–11]. The addition of OMMT into polymer improves mechanical, physical and chemical properties of the matrices and reduces cost in some cases [12–15]. Poly(ethylene oxide) (PEO) is a nonionic and water-soluble polymer with many applications due to its flocculent, thickening, sustained-release, lubrication, dispersing, and water-retention properties [16–18]. Its hydrophilicity, biocompatibility and versatility make it attractive as a biomaterial as well [19]. Additionally, PEO is a favorable candidate for the development of solid polymer electrolytes with high ionic conductivity because of its ability to dissolve large amounts of salt and its structure, which supports ion transport [20]. Recently, PEO/MMT nanocomposites are promising
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materials and showing great potential for various applications. Research interests on PEO/MMT nanocomposites have mainly originated from the area of polymer electrolytes [21–24]. PEO and its low molecular weight PEG can intercalate into the interlayer of MMT giving rise to a limited increase of the basal spacing (about 0.8 nm) [25,26]. Many studies have further more focused on the structure and conformation of the PEO chains in the confined space of MMT [24,25,26,27,28]. The findings concerning the effects of the Na-MMT on crystallization and polymer chain conformation are still controversial. In one case, a helical conformation for the intercalated PEO in MMT galleries was proposed based on FTIR and solid-state NMR [16]. In another case, it was suggested that the PEO chains within the galleries are not helical but they resemble single or double adsorbed polymer layers onto the clay surfaces [24,25]. Actually, it was suggested that the PEO chains within the galleries are less ordered than the most disordered bulk PEO systems. It was attributed to both the strong spatial confinement and to the strong coordination of the ether oxygens with the alkali cations present in the galleries [29–31]. The present paper concerned with local environments in the interlayer space of OMMT modified with different alkylammonium cations and intercalation behavior of PEG chains with these OMMT. A series of hybrids with different PEG content were prepared and characterized by X-ray diffraction and differential scanning calorimetry. The effects of the OMMT types, or more particularly the chemical nature of the organic modifier in the interlayer spacing and the PEG concentration were discussed. 2. Experimental 2.1. Materials Na+ -montmorillonite (Na-MMT), obtained from Fenghong Clay Co. (China), was denoted as Na-MMT. The cation exchange capacity (CEC) is 90 m mol/100 g. The surfactants used in this study are octadecyltrimethyl ammonium bromide (S-18) and dioctadecyl dimethylammonium bromide (D-18). They were provided by XiamenPioneer Technology Co. (China). PEG with weight-average molecular weight 6000, was obtained from Aoke Chemical Co. (China). 2.2. Preparation of the OMMT The syntheses of alkylammonium surfactant modified MMT were performed as the following procedure: a desired amount of surfactant was first dispersed in distilled water and stirred at 80 ◦ C for 0.5 h to form homogeneous liquor, into which then 3 g Na-MMT was slowly added. The concentrations of surfactants were 0.25, 0.50, 1.00, 1.50, 2.00 and 3.00 times CEC of MMT, respectively. The mass ratio of water/MMT was 100:1. The reaction mixtures were stirred for 3 h at 80 ◦ C in a water bath. All products were washed for 3 times by distilled water, dried at 60 ◦ C, and ground in an agate mortar. The OMMT prepared by using Na-MMT and S-18 (D-18) at a concentration of 0.50CEC was marked as 0.50SM (0.50DM) and the others were marked in a similar way. The hybrids of PEG intercalated OMMT were formed as the following procedure: a desired amount of PEG was first dispersed in ethanol and stirred at 60 ◦ C for 0.5 h to form homogeneous liquor, into which then 1.0 g 0.50SM was slowly added. The amounts of PEG were 0.1, 0.2, 0.3, 0.5 and 1.0 times mass of 0.50SM, respectively. And these samples were denoted as 0.1–0.50SM, 0.2–0.50SM, 0.3–0.50SM, 0.5–0.50SM and 1.0–0.50SM, respectively. The mass ratio of ethanol/MMT was 100:1. The reaction mixtures were stirred for 3 h at 60 ◦ C in a water bath. The PEG/MMT hybrids were air-dried in room temperature for several days and then ground in an agate
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mortar. The other PEG intercalated OMMT hybrids were prepared and marked in the same ways. 2.3. Characterization X-ray diffraction patterns were collected on X’Pert Pro X-ray diffractometer (Philps, Netherlands) using Ni-filtered Cu Kr radiation from 1.5◦ to 30◦ , the scanning speed being 2◦ /min with a step of 0.03◦ . Powder samples were packed in horizontally held trays. The changes in the XRD peak positions reflect the intercalation of the polymer into layered silicates. The Bragg equation was applied to calculate the basal spacing of MMT platelets. The gallery sizes in intercalated hybrids were deduced from XRD peak positions of (0 0 1) of the hybrids. FTIR spectra of samples were collected on a Nicolet-560 spectrometer (Thermal Nicolet Co., USA) from 400 to 4000 cm−1 with a nominal resolution of 4 cm−1 . For each spectrum 64 runs were collected and averaged. The MMT specimens were prepared by adding approximately 1% of the sample powder to dry KBr powder. Crystallization and melting behaviors were recorded on a DSC1 (Mettler Toledo Co., Switzerland). The DSC cell was purged with dry nitrogen flowing at a rate of 50 mL/min. Sample weight was maintained at low level (10 mg) for all measurements. All samples were first heated from 0 ◦ C to 100 ◦ C with a rate 10 ◦ C/min, held at 100 ◦ C for 5 min, and then cooled with a rate 10 ◦ C/min–0 ◦ C. 3. Results and discussion 3.1. Intercalation of surfactants into MMT With the ion-exchange of the sodium ion for the cationic surfactant, expansion of the Na-MMT layers occurred. This expansion was readily measured by XRD. Fig. 1a showed the XRD patterns recorded for the surfactant S-18 modified Na-MMT at room temperature from 2 = 0◦ to 10◦ . The basal spacing of the (0 0 1) plane (d0 0 1 ) for MMT estimated by Bragg’s formula n = 2d sin 2. Pure Na-MMT exhibited a main diffraction peak at 2 = 7.3◦ , which was corresponding to a basal spacing of 1.21 nm. Upon addition of 0.25CEC S-18, this peak shifted to 2 = 6.6◦ , corresponding to d0 0 1 = 1.34 nm. The increased basal spacing indicated that surfactant S-18 had intercalated between the inorganic layers. As S-18 concentration increased up to 1.50CEC, the d0 0 1 of 1.50SM was 2.78 nm. With further increasing the S-18 concentration to 2.00CEC and 3.00CEC, the d0 0 1 of SM was up to 4.01 nm. And the corresponding peaks were sharp and strong. It indicated the surfactant S-18 had formed some order structures between the MMT layers. Meanwhile, the result demonstrated that there was a saturation concentration when S-18 intercalated Na-MMT. When surfactant S-18 concentration increased to 2.00CEC, the cationic chains of S-18 did not intercalate into the interlayer space of Na-MMT. A similar situation was observed in Fig. 1b. Upon addition of 0.25CEC D-18, the d0 0 1 increased to 1.31 nm. The increased basal spacing indicated that surfactant D-18 had intercalated into the MMT layers. As the D-18 concentration increased up to 1.50CEC, the d0 0 1 of 1.50DM was 3.68 nm. With further increasing the D-18 concentration, the basal spacing of DM hybrids remained unchanged. It could also be attributed to saturation of D-18 concentration in interlayer space of Na-MMT. 3.2. Characteristic vibrations of surfactant intercalated into the MMT interlayer Vibrational spectroscopy has been extensively used to probe the local environments of surfactants in the interlayer space of MMT [32–35]. FTIR studies have led to detailed correlation of the spectra with structural features. The position, splitting and intensities of
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Fig. 1. XRD patterns of (a) SM series and (b) DM series.
methylene stretching modes in the FTIR spectra were extensively used in the study of conformation of alkyl chain assemblies [36–39]. Figs. 2 and 3 showed the CH2 vibrational spectra of the OMMT modified with S-18 and D-18. The FTIR absorption bands at 2951 and 2819 cm−1 were attributed to the symmetric CH2 stretching (s (CH2 )) and antisymmetric (as (CH2 )) modes. It was well established that the frequencies of the CH2 stretching bands of hydrocarbon chains were extremely sensitive to the gauche/trans conformer ratio of the hydrocarbon chains [33,40]. Only when the chains were highly ordered (all-trans), the narrow absorption bands appeared around 2851 and 2919 cm−1 in the infrared spectrum. If conformational disorder was included in the chains, their frequencies shifted upward, depending upon the average content of gauche conformers [34]. As shown in Figs. 2 and 3, the positions of these bands for SM and DM hybrids shifted to lower frequencies with the surfactant concentration increased. The band shifted from higher to lower frequency means that the number of gauche conformers of alkyl chain decreased whereas the number of all-trans conformers increased. This reflected that, with the increase of surfactant packing density, the liquid-likemolecular environment of the intercalated surfactants changed to a solid-like environment [34,36,37]. For example, at 0.25CEC of surfactant concentration, the bands were located at 2925 and 2853 cm−1 for 0.25SM, indicating that at this low surfactant concentration the alkyl chains adopted a more gauche-like monolayer configuration. With the increase of surfactant concentration, s (CH2 ) shifted slightly from 2853 to 2851 cm−1 and as (CH2 ) shifted from 2925 to 2919 cm−1 for the specimens from 0.25SM to 3.00SM. These methylene stretching mode frequencies indicated that a majority of the methylene units of the intercalated surfactants adopteda highly ordered conformation. And for the DM hybrids, the shift of antisymmetric stretching mode was 3 cm−1 . In the high surfactant concentration range, the frequency of both as (CH2 ) and s (CH2 ) for the confined alkyl chains kept relatively constant. They were very close to the frequency of the pure surfactant D-18. This suggested that, in this relatively high surfactant concentration range, the confined D-18 chains adopted an essentially all-trans conformation. However, in the relatively low S-18 concentration range, the frequency shifted significantly to high wave number, indicating that a large number of the gauche conformation was introduced into the alkyl chains. Meanwhile, both as (CH2 ) and s (CH2 ) for DM hybrids with a slight shifts suggested that the double chain surfactant D-18 preferred to adopt highly ordered conformation when compared with single chain surfactant S-18.
Actually, some reports had figured out the relative shifts of CH2 vibrational energies could be used to assess the relative hydrophobic properties of the loaded surfactant, with lower energies reflecting a more structured and hydrophobic environment [39–41]. The results mentioned above reflected that, the degree of the hydrophobic properties might be increased with the increment of surfactant concentration and chain number. The infrared absorption bands between 1480 and 1440 cm−1 , due to the CH2 scissoring modes, were quite similar to the CH2 rocking modes between 750 and 700 cm−1 in the position and shape of the bands (Fig. 4). In the spectra of pure S-18, doublets at 1474 and 1463 cm−1 corresponded to the scissoring modes and those at 730 and 719 cm−1 to the rocking modes, respectively. The splitting with 11 cm−1 of the CH2 scissoring and rocking bands was attributed to the intermolecular interaction between the two adjacent hydrocarbon chains in a perpendicular orthorhombic sub cell, and further required an all-trans conformation for its detection [42,43]. When the alkyl chains of surfactant S-18 intercalated into Na-MMT, doublets at 1474 and 1463 cm−1 disappeared, which were substituted by a broad single band at 1470 cm−1 (Fig. 4a). With the increase of surfactant concentration, the band split again, but the intensity was weak. For the CH2 rocking modes, when the surfactant concentration was lower than 1.00CEC, the FTIR spectra only displayed a broad single band at 721 cm−1 , similar to CH2 scissoring modes. These broad single absorption bands were related to either a liquid-like structure or relatively disordered hexagonal subcell packing, where the hydrocarbon chain freely rotated around its long axis. However, with the further increase of surfactant loading, two well resolved vibration bands at 730 and 721 cm−1 were observed. There were only single bands at 1471 and 719 cm−1 in the vibration spectra of pure D-18 (Fig. 4b). For the DM hybrids modified by D-18, there were no doublets observed for both CH2 scissoring and rocking modes, instead of only single band occurring at 1469 and 721 cm−1 . The slight shifts suggested that the double chain surfactant D-18 adopted highly ordered conformation when the surfactant loading was low. The above results suggested that splitting of the CH2 scissoring and rocking modes not only related with surfactant loading, but also related with the configuration of the used surfactants. 3.3. Intercalation behaviorof PEG into OMMT Fig. 5 showed the XRD patterns recorded for PEG intercalated OMMT(SM series) at room temperature from 2 = 0◦ to 30◦ . PEG
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Fig. 2. The vibration spectra of CH2 in OMMT prepared from S-18 (a) and D-18 (b).
Fig. 3. Change of frequency of as (CH2 ) (a) and s (CH2 ) (b) of OMMT prepared from S-18 and D-18 as a function of surfactant concentration.
showed two crystalline reflections at 2 = 19.2◦ and 23.3◦ . Due to the nature of this system, the state of intercalation of PEG into OMMT as judged by XRD can thus be seen both by any d spacing changes of OMMT and any peaks presented due to PEG crystallites. The latter aspect was particularly useful in that the absence of any PEG crystal peaks had been shown to indicate the lack of any residual PEG outside of the galleries (this interpretation is based on the fact that it has been reported that semi crystalline polymers such
as PEO and PEG which are confined inside the galleries are unable to crystallize) [44–47]. In Fig. 5a, the d0 0 1 of 0.25SM was 1.34 nm. When PEG was incorporated with 0.25SM, the d0 0 1 spacings of 0.1–0.25SM and 0.2–0.25SM increased to 1.58 nm and 1.72 nm, respectively. These increases indicated that PEG chains had intercalated into 0.25SM. Equally important, in these composition ranges, no peaks were observed at higher angles that could be assigned to the crystalline
Fig. 4. The vibration spectra of CH2 in OMMT prepared from S-18 (a) and D-18 (b).
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Fig. 5. XRD patterns of PEG intercalated SM series.
structure of PEG. It indicated that the PEG chains were fully intercalated into the interlayer space. However, when the ratio of PEG to SM was 0.3 or higher, the d0 0 1 of SM hybrids increased to a constant about 1.77 nm and crystalline PEG peaks were observed. In Fig. 5b, the d0 0 1 of 0.50SM increased to 1.77 nm and kept constantly. PEG intercalated 1.00SM and 1.50SM both had a similar results compared to 0.25SM.The d0 0 1 spacings of 1.00SM and 1.50SM increased with adding PEG, up to 3.20 nm and 3.24 nm, respectively. These results indicated that there was a saturation ratio when PEG intercalated OMMT. In our cases, as discussed above, the saturation ratio of PEG to SM was between 0.2 and
0.3 when the surfactant concentration was lower than 2.00CEC. However, quite different PEG intercalation behavior was observed in Fig. 5e and f. The interlayer spaces of PEG intercalated 2.00SM and 3.00SM were all the same compared with 2.00SM and 3.00SM, corresponding to 4.01 nm. And the crystalline PEG peaks were always observed in the presence of PEG. This could be attributed to the PEG chains did not intercalate into the interlayer space of 2.00SM and 3.00SM. It was because with addition of surfactant S-18, the hydrophobic alkyl chains of SM increased. When the surfactant concentration increased to 2.00CEC, the increased hydrophobic properties in the interlayer space of 2.00SM could
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Fig. 6. XRD patterns of PEG intercalated DM series.
prevent the intercalation of hydrophilic PEG. It was noted that the saturation concentration of S-18 intercalated Na-MMT was also 2.00CEC. It indicated that PEG chains couldn’t intercalate into the interlayer space of Na-MMT when the S-18 was saturated. Similarly, PEG intercalation behaviors in OMMT modified by D-18 were showed in Fig. 6. In Fig. 6a, the d0 0 1 of 0.25DM was 1.31 nm. When low concentration PEG intercalated into 0.25DM, the d0 0 1 spacings of 0.1-0.25DM and 0.2-0.25DM increased to 1.63 nm and 1.73 nm, respectively. These increased d0 0 1 spacings indicated that PEG chains intercalated into 0.25DM. Moreover, no PEG crystalline peaks were observed at 2 = 19.2◦ and 23.3◦ in these composition ranges. It indicated that the PEG chains were fully
intercalated into the interlayer space of 0.25DM. However, when the mass ratio of PEG/0.25DM was 0.3 or higher, the d0 0 1 of 0.25DM increased to a constant about 1.77 nm and crystalline PEG peaks were observed. Fig. 6b showed that the d0 0 1 of 0.50DM increased to 1.83 nm and kept constantly. There was a similar result in PEG intercalated 1.00DM when compared to 0.25DM. The d0 0 1 spacing of 1.00DM increased up to 3.68 nm with adding PEG. These results indicated that there was also a saturation ratio when PEG intercalated OMMT modified by D-18. In our cases, as discussed above, the saturation ratio of PEG/DM was between 0.2 and 0.3 when the surfactant concentration was lower than 1.50CEC. However, quite different behavior was observed in Fig. 6d. The interlayer spaces of
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Fig. 7. DSC curves of SM hybrids with varying polymer content.
PEG intercalated 1.50DM were all the same when compared with 1.50DM, corresponding to 3.68 nm. And the crystalline PEG peaks werealways observed in their corresponding XRD patterns. The same results were found in 2.00DM and 3.00DM (Fig. 6e and f). It was because the PEG chains did not intercalate into the interlayer space of DM with high D-18 concentration. With addition of surfactant D-18, the hydrophobic alkyl chains of DM increased. When the surfactant D-18 concentration increased to 1.50CEC or higher, the increased hydrophobic properties in the interlayer space of DM could prevent the intercalation of hydrophilic PEG. Ha and Char figured out a bridge conformation of dodecyldiamine chains in the confined space of MMT can also prevent PEO chains form intercalation [34]. As discussed above, the saturation concentration of D-18 intercalated Na-MMT was 1.50CEC. It indicated that PEG chains could not intercalate into the interlayer space of Na-MMT when the D-18 was saturated. And it could be anticipated
the structures of PEG incorporated with high D-18 concentration of DM (1.50DM, 2.00DM and 3.00DM) were almost the same. Fig. 7 showed the DSC results of mixture samples with different PEG contents before annealing for SM hybrids. The melting peak of neat PEG was about 59.1 ◦ C. In Fig. 7a–d, the melting peak of crystalline PEG was not present in DSC curves of SM which did not show crystalline PEG peaks in their XRD patterns when the mass ratios of PEG/MMT were 0.1 and 0.2. When the ratio of PEG/MMT was 0.3 or higher, the melting peaks crystalline PEG were observed. And with PEG increasing, the melting peak of PEG decreased. The melting temperature was reduced by addition of PEG and it was ascribed to disruption of large scale crystallite formation by the presence of MMT. However, DSC curves showed the melting peaks of crystalline PEG when the mass ratios of PEG to 2.00SM and 3.00SM were lower than 0.3 (Fig. 7e and f). The excited crystalline PEG peaks indicated PEG did not intercalate into the interlayer space
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Fig. 8. DSC curves of DM hybrids with varying polymer content.
of 2.00SM and 3.00SM. It demonstrated that PEG chains could not intercalate into the interlayer space of Na-MMT when the S-18 was saturated. These results of DSC curves were well consistent with the XRD results discussed in Fig. 5. Fig. 8 showed the DSC results of mixture samples with different PEG contents before annealing for DM hybrids, respectively. In the case of DM hybrids, two melting peaks can be seen in most samples. The lower was due to endothermic disruption of the alkylammoniumions attached to the MMT when the higher temperature peaks related to the melting of the PEG crystallites (except for PEG-3.00DM). In some cases, for example, in 1.0–1.00DM and 1.0–1.50DM, the two peaks were merged. In Fig. 8a–c, the melting peak of PEG was not present in DSC curves of DM hybrids which did not show crystalline PEG peaks in their XRD patterns when the mass ratios of PEG/MMT were 0.1 and 0.2. Especially, in Fig. 8d–f, DSC curves showed the melting peaks of crystalline PEG about 60 ◦ C when the mass ratios of PEG/DM were 0.1 and 0.2. The excited
crystalline PEG peaks indicated PEG did not intercalate into the interlayer space of 1.50DM. As discussed in Fig. 6, PEG chains could not intercalate into the interlayer space of Na-MMT when the D-18 was saturated. These results of DSC curves were all well consistent with the XRD results discussed in Fig. 6. In addition, it was noted that the melting temperatures of DM increased with the D18 concentration. It indicated the structures of surfactant chains in high D-18 concentration of DM were more order than low D-18 concentration of DM. Based on the above results, an intercalation mechanism of PEG in OMMT was proposed (Fig. 9). When surfactants concentration in OMMT was unsaturated, the alkyl chains of surfactants adopted a more gauche-like configuration. PEG chains could intercalate into the interlayer space of OMMT and enlarged the d-spacing. With addition of surfactants, the alkyl chains in interlayer space of OMMT increased and adopted a highly ordered conformation. When the surfactant increased to the saturated concentration, the increased
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Fig. 9. Schematics of PEG intercalation mechanism in OMMT.
hydrophobic properties in the interlayer space of OMMT could prevent the intercalation of hydrophilic PEG. 4. Conclusions Two kinds of OMMT (SM and DM) were prepared with single alkyl chains and double alkyl chains ammonium salts. XRD results showed the interlayer spacing of OMMT increased with surfactants in low concentration. With further increasing the surfactants concentration, the interlayer spacings of SM and DM were unchanged. FTIR results suggested that the double chain surfactant D-18 preferred to adopt highly ordered conformation compared with single chain surfactant S-18 in interlayer space of OMMT. These results indicated the degree of the hydrophobic properties might increase with the increment of surfactant loading and chain number. The intercalation behaviors of PEG chains in OMMT were investigated with XRD and DSC. The results indicated that PEG chains could not intercalate into Na-MMT when the surfactants were saturated in interlayer space of Na-MMT. PEG chains intercalated into the interlayer space of SM when the S-18 concentration was lower than 2.00CEC, implying that the low surfactant concentration modified SM provided a better environment (presumably through the balanced hydrophobic and hydrophilic surfaces) for the PEG intercalation as well. Similarly, PEG did not intercalate into the interlayer space of DM when the D-18 concentration was higher than 1.00CEC. It could be attributed to the hydrophobic double alkyl chains of DM increased with D-18. The increased hydrophobic properties in the interlayer space of 1.50DM hybrids could prevent the intercalation of hydrophilic PEG. Acknowledgements This work was supported by the Program for New Century Excellent Talents in University (NCET-10-0562) and the National Basic Research Program of China (2005CB623800). References [1] S. Sinha Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Progress in Polymer Science 28 (2003) 1539–1641. [2] P.C. LeBaron, Z. Wang, T.J. Pinnavaia, Polymer-layered silicate nanocomposites: an overview, Applied Clay Science 15 (1999) 11–29. [3] S. Pavlidou, C. Papaspyrides, A review on polymer-layered silicate nanocomposites, Progress in Polymer Science 33 (2008) 1119–1198. [4] C. Aguzzi, P. Cerezo, C. Viseras, C. Caramella, Use of clays as drug delivery systems: possibilities and limitations, Applied Clay Science 36 (2007) 22–36.
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Intercalation behavior of poly(ethylene glycol) in organically modified montmorillonite Shipeng Zhu a,b , Hongmei Peng a , Jinyao Chen a , Huilin Li a,∗ , Ya Cao a,∗ , Yunhua Yang b , Zhihai Feng b a
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, PR China Science and Technology on Advanced Functional Composites Laboratory, Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, PR China b
a r t i c l e
i n f o
Article history: Received 19 July 2012 Received in revised form 16 March 2013 Accepted 19 March 2013 Available online 25 March 2013 Keywords: Intercalation Poly(ethylene glycol) Surfactant Montmorillonite
a b s t r a c t In this paper, two kinds of organically modified montmorillonite (OMMT) were prepared using alkylammonium surfactants with different alkyl chain numbers. XRD results showed the interlayer spacing of OMMT increased with low concentration surfactants. With further increasing the surfactants concentration, the interlayer spacing of OMMT was unchanged. Meanwhile, FTIR was used to characterize the local environments of surfactants in the interlayer space of OMMT. The results suggested that the double chain surfactant D-18 preferred to adopt highly ordered conformation compared with single chain surfactant S-18 in interlayer space of OMMT. It indicated that the surface property of the OMMT is affected by the concentration and configuration of the intercalated surfactants. Moreover, the effect of the OMMT type, or more particularly the chemical nature of the organic modifier in the interlayer spacing and the poly(ethylene glycol) (PEG) concentration onintercalation behavior of PEG chains in OMMT were investigated with XRD and DSC.The results indicated that PEG chains could not intercalate into Na-MMT when the surfactants were saturated in interlayer space of Na-MMT. PEG chains could intercalate into the interlayer space of SM when the S-18 concentration was lower than 2.00CEC, implying that the low surfactant concentration modified SM provided a better environment (presumably through the balanced hydrophobic and hydrophilic surfaces) for the PEG intercalation as well. However, PEG did not intercalate into the interlayer space of DM when the D-18 concentration was higher than 1.00CEC. It could be attributed to the hydrophobic double alkyl chains of DM increased with D-18. The increased hydrophobic properties in the interlayer space of 1.50DM hybrids can prevent the intercalation of hydrophilic PEG. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, surface modifications of clays have become increasingly important because it can be used to prepare polymer/clay nanocomposites and applied in some new applications such as adsorbents of organic pollutants in soil, water and air, rheological control agents, paints, medicine [1–5]. Natural clays are mostly hydrophilic, and their interactions are favorable only with polar polymers. In the case of hydrophobic polymers, intercalation or exfoliation can be achieved only with organophilized clays. Hence, proper organophilization procedure is a key step for successful intercalation and exfoliation of montmorillonite (MMT) particles in the polymer [6]. Generally, this progress can be done by ion-exchange reactions with cationic surfactants including primary, secondary, tertiary, and quaternary alkylammonium.
∗ Corresponding authors. Tel.: +86 28 85467166; fax: +86 28 85402465. E-mail addresses: [email protected] (H. Li), [email protected] (Y. Cao). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.123
Alkylammonium cations in the organically modified montmorillonite (OMMT) lower the surface energy of the inorganic host and improve the wetting characteristics of the polymer matrix, and result in a larger interlayer spacing [7]. Additionally, the alkylammonium cations can provide functional groups that can react with the polymer matrix, or in some cases initiate the polymerization of monomers to improve the strength of the interface between the inorganic and the polymer matrix [8–11]. The addition of OMMT into polymer improves mechanical, physical and chemical properties of the matrices and reduces cost in some cases [12–15]. Poly(ethylene oxide) (PEO) is a nonionic and water-soluble polymer with many applications due to its flocculent, thickening, sustained-release, lubrication, dispersing, and water-retention properties [16–18]. Its hydrophilicity, biocompatibility and versatility make it attractive as a biomaterial as well [19]. Additionally, PEO is a favorable candidate for the development of solid polymer electrolytes with high ionic conductivity because of its ability to dissolve large amounts of salt and its structure, which supports ion transport [20]. Recently, PEO/MMT nanocomposites are promising
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materials and showing great potential for various applications. Research interests on PEO/MMT nanocomposites have mainly originated from the area of polymer electrolytes [21–24]. PEO and its low molecular weight PEG can intercalate into the interlayer of MMT giving rise to a limited increase of the basal spacing (about 0.8 nm) [25,26]. Many studies have further more focused on the structure and conformation of the PEO chains in the confined space of MMT [24,25,26,27,28]. The findings concerning the effects of the Na-MMT on crystallization and polymer chain conformation are still controversial. In one case, a helical conformation for the intercalated PEO in MMT galleries was proposed based on FTIR and solid-state NMR [16]. In another case, it was suggested that the PEO chains within the galleries are not helical but they resemble single or double adsorbed polymer layers onto the clay surfaces [24,25]. Actually, it was suggested that the PEO chains within the galleries are less ordered than the most disordered bulk PEO systems. It was attributed to both the strong spatial confinement and to the strong coordination of the ether oxygens with the alkali cations present in the galleries [29–31]. The present paper concerned with local environments in the interlayer space of OMMT modified with different alkylammonium cations and intercalation behavior of PEG chains with these OMMT. A series of hybrids with different PEG content were prepared and characterized by X-ray diffraction and differential scanning calorimetry. The effects of the OMMT types, or more particularly the chemical nature of the organic modifier in the interlayer spacing and the PEG concentration were discussed. 2. Experimental 2.1. Materials Na+ -montmorillonite (Na-MMT), obtained from Fenghong Clay Co. (China), was denoted as Na-MMT. The cation exchange capacity (CEC) is 90 m mol/100 g. The surfactants used in this study are octadecyltrimethyl ammonium bromide (S-18) and dioctadecyl dimethylammonium bromide (D-18). They were provided by XiamenPioneer Technology Co. (China). PEG with weight-average molecular weight 6000, was obtained from Aoke Chemical Co. (China). 2.2. Preparation of the OMMT The syntheses of alkylammonium surfactant modified MMT were performed as the following procedure: a desired amount of surfactant was first dispersed in distilled water and stirred at 80 ◦ C for 0.5 h to form homogeneous liquor, into which then 3 g Na-MMT was slowly added. The concentrations of surfactants were 0.25, 0.50, 1.00, 1.50, 2.00 and 3.00 times CEC of MMT, respectively. The mass ratio of water/MMT was 100:1. The reaction mixtures were stirred for 3 h at 80 ◦ C in a water bath. All products were washed for 3 times by distilled water, dried at 60 ◦ C, and ground in an agate mortar. The OMMT prepared by using Na-MMT and S-18 (D-18) at a concentration of 0.50CEC was marked as 0.50SM (0.50DM) and the others were marked in a similar way. The hybrids of PEG intercalated OMMT were formed as the following procedure: a desired amount of PEG was first dispersed in ethanol and stirred at 60 ◦ C for 0.5 h to form homogeneous liquor, into which then 1.0 g 0.50SM was slowly added. The amounts of PEG were 0.1, 0.2, 0.3, 0.5 and 1.0 times mass of 0.50SM, respectively. And these samples were denoted as 0.1–0.50SM, 0.2–0.50SM, 0.3–0.50SM, 0.5–0.50SM and 1.0–0.50SM, respectively. The mass ratio of ethanol/MMT was 100:1. The reaction mixtures were stirred for 3 h at 60 ◦ C in a water bath. The PEG/MMT hybrids were air-dried in room temperature for several days and then ground in an agate
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mortar. The other PEG intercalated OMMT hybrids were prepared and marked in the same ways. 2.3. Characterization X-ray diffraction patterns were collected on X’Pert Pro X-ray diffractometer (Philps, Netherlands) using Ni-filtered Cu Kr radiation from 1.5◦ to 30◦ , the scanning speed being 2◦ /min with a step of 0.03◦ . Powder samples were packed in horizontally held trays. The changes in the XRD peak positions reflect the intercalation of the polymer into layered silicates. The Bragg equation was applied to calculate the basal spacing of MMT platelets. The gallery sizes in intercalated hybrids were deduced from XRD peak positions of (0 0 1) of the hybrids. FTIR spectra of samples were collected on a Nicolet-560 spectrometer (Thermal Nicolet Co., USA) from 400 to 4000 cm−1 with a nominal resolution of 4 cm−1 . For each spectrum 64 runs were collected and averaged. The MMT specimens were prepared by adding approximately 1% of the sample powder to dry KBr powder. Crystallization and melting behaviors were recorded on a DSC1 (Mettler Toledo Co., Switzerland). The DSC cell was purged with dry nitrogen flowing at a rate of 50 mL/min. Sample weight was maintained at low level (10 mg) for all measurements. All samples were first heated from 0 ◦ C to 100 ◦ C with a rate 10 ◦ C/min, held at 100 ◦ C for 5 min, and then cooled with a rate 10 ◦ C/min–0 ◦ C. 3. Results and discussion 3.1. Intercalation of surfactants into MMT With the ion-exchange of the sodium ion for the cationic surfactant, expansion of the Na-MMT layers occurred. This expansion was readily measured by XRD. Fig. 1a showed the XRD patterns recorded for the surfactant S-18 modified Na-MMT at room temperature from 2 = 0◦ to 10◦ . The basal spacing of the (0 0 1) plane (d0 0 1 ) for MMT estimated by Bragg’s formula n = 2d sin 2. Pure Na-MMT exhibited a main diffraction peak at 2 = 7.3◦ , which was corresponding to a basal spacing of 1.21 nm. Upon addition of 0.25CEC S-18, this peak shifted to 2 = 6.6◦ , corresponding to d0 0 1 = 1.34 nm. The increased basal spacing indicated that surfactant S-18 had intercalated between the inorganic layers. As S-18 concentration increased up to 1.50CEC, the d0 0 1 of 1.50SM was 2.78 nm. With further increasing the S-18 concentration to 2.00CEC and 3.00CEC, the d0 0 1 of SM was up to 4.01 nm. And the corresponding peaks were sharp and strong. It indicated the surfactant S-18 had formed some order structures between the MMT layers. Meanwhile, the result demonstrated that there was a saturation concentration when S-18 intercalated Na-MMT. When surfactant S-18 concentration increased to 2.00CEC, the cationic chains of S-18 did not intercalate into the interlayer space of Na-MMT. A similar situation was observed in Fig. 1b. Upon addition of 0.25CEC D-18, the d0 0 1 increased to 1.31 nm. The increased basal spacing indicated that surfactant D-18 had intercalated into the MMT layers. As the D-18 concentration increased up to 1.50CEC, the d0 0 1 of 1.50DM was 3.68 nm. With further increasing the D-18 concentration, the basal spacing of DM hybrids remained unchanged. It could also be attributed to saturation of D-18 concentration in interlayer space of Na-MMT. 3.2. Characteristic vibrations of surfactant intercalated into the MMT interlayer Vibrational spectroscopy has been extensively used to probe the local environments of surfactants in the interlayer space of MMT [32–35]. FTIR studies have led to detailed correlation of the spectra with structural features. The position, splitting and intensities of
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Fig. 1. XRD patterns of (a) SM series and (b) DM series.
methylene stretching modes in the FTIR spectra were extensively used in the study of conformation of alkyl chain assemblies [36–39]. Figs. 2 and 3 showed the CH2 vibrational spectra of the OMMT modified with S-18 and D-18. The FTIR absorption bands at 2951 and 2819 cm−1 were attributed to the symmetric CH2 stretching (s (CH2 )) and antisymmetric (as (CH2 )) modes. It was well established that the frequencies of the CH2 stretching bands of hydrocarbon chains were extremely sensitive to the gauche/trans conformer ratio of the hydrocarbon chains [33,40]. Only when the chains were highly ordered (all-trans), the narrow absorption bands appeared around 2851 and 2919 cm−1 in the infrared spectrum. If conformational disorder was included in the chains, their frequencies shifted upward, depending upon the average content of gauche conformers [34]. As shown in Figs. 2 and 3, the positions of these bands for SM and DM hybrids shifted to lower frequencies with the surfactant concentration increased. The band shifted from higher to lower frequency means that the number of gauche conformers of alkyl chain decreased whereas the number of all-trans conformers increased. This reflected that, with the increase of surfactant packing density, the liquid-likemolecular environment of the intercalated surfactants changed to a solid-like environment [34,36,37]. For example, at 0.25CEC of surfactant concentration, the bands were located at 2925 and 2853 cm−1 for 0.25SM, indicating that at this low surfactant concentration the alkyl chains adopted a more gauche-like monolayer configuration. With the increase of surfactant concentration, s (CH2 ) shifted slightly from 2853 to 2851 cm−1 and as (CH2 ) shifted from 2925 to 2919 cm−1 for the specimens from 0.25SM to 3.00SM. These methylene stretching mode frequencies indicated that a majority of the methylene units of the intercalated surfactants adopteda highly ordered conformation. And for the DM hybrids, the shift of antisymmetric stretching mode was 3 cm−1 . In the high surfactant concentration range, the frequency of both as (CH2 ) and s (CH2 ) for the confined alkyl chains kept relatively constant. They were very close to the frequency of the pure surfactant D-18. This suggested that, in this relatively high surfactant concentration range, the confined D-18 chains adopted an essentially all-trans conformation. However, in the relatively low S-18 concentration range, the frequency shifted significantly to high wave number, indicating that a large number of the gauche conformation was introduced into the alkyl chains. Meanwhile, both as (CH2 ) and s (CH2 ) for DM hybrids with a slight shifts suggested that the double chain surfactant D-18 preferred to adopt highly ordered conformation when compared with single chain surfactant S-18.
Actually, some reports had figured out the relative shifts of CH2 vibrational energies could be used to assess the relative hydrophobic properties of the loaded surfactant, with lower energies reflecting a more structured and hydrophobic environment [39–41]. The results mentioned above reflected that, the degree of the hydrophobic properties might be increased with the increment of surfactant concentration and chain number. The infrared absorption bands between 1480 and 1440 cm−1 , due to the CH2 scissoring modes, were quite similar to the CH2 rocking modes between 750 and 700 cm−1 in the position and shape of the bands (Fig. 4). In the spectra of pure S-18, doublets at 1474 and 1463 cm−1 corresponded to the scissoring modes and those at 730 and 719 cm−1 to the rocking modes, respectively. The splitting with 11 cm−1 of the CH2 scissoring and rocking bands was attributed to the intermolecular interaction between the two adjacent hydrocarbon chains in a perpendicular orthorhombic sub cell, and further required an all-trans conformation for its detection [42,43]. When the alkyl chains of surfactant S-18 intercalated into Na-MMT, doublets at 1474 and 1463 cm−1 disappeared, which were substituted by a broad single band at 1470 cm−1 (Fig. 4a). With the increase of surfactant concentration, the band split again, but the intensity was weak. For the CH2 rocking modes, when the surfactant concentration was lower than 1.00CEC, the FTIR spectra only displayed a broad single band at 721 cm−1 , similar to CH2 scissoring modes. These broad single absorption bands were related to either a liquid-like structure or relatively disordered hexagonal subcell packing, where the hydrocarbon chain freely rotated around its long axis. However, with the further increase of surfactant loading, two well resolved vibration bands at 730 and 721 cm−1 were observed. There were only single bands at 1471 and 719 cm−1 in the vibration spectra of pure D-18 (Fig. 4b). For the DM hybrids modified by D-18, there were no doublets observed for both CH2 scissoring and rocking modes, instead of only single band occurring at 1469 and 721 cm−1 . The slight shifts suggested that the double chain surfactant D-18 adopted highly ordered conformation when the surfactant loading was low. The above results suggested that splitting of the CH2 scissoring and rocking modes not only related with surfactant loading, but also related with the configuration of the used surfactants. 3.3. Intercalation behaviorof PEG into OMMT Fig. 5 showed the XRD patterns recorded for PEG intercalated OMMT(SM series) at room temperature from 2 = 0◦ to 30◦ . PEG
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Fig. 2. The vibration spectra of CH2 in OMMT prepared from S-18 (a) and D-18 (b).
Fig. 3. Change of frequency of as (CH2 ) (a) and s (CH2 ) (b) of OMMT prepared from S-18 and D-18 as a function of surfactant concentration.
showed two crystalline reflections at 2 = 19.2◦ and 23.3◦ . Due to the nature of this system, the state of intercalation of PEG into OMMT as judged by XRD can thus be seen both by any d spacing changes of OMMT and any peaks presented due to PEG crystallites. The latter aspect was particularly useful in that the absence of any PEG crystal peaks had been shown to indicate the lack of any residual PEG outside of the galleries (this interpretation is based on the fact that it has been reported that semi crystalline polymers such
as PEO and PEG which are confined inside the galleries are unable to crystallize) [44–47]. In Fig. 5a, the d0 0 1 of 0.25SM was 1.34 nm. When PEG was incorporated with 0.25SM, the d0 0 1 spacings of 0.1–0.25SM and 0.2–0.25SM increased to 1.58 nm and 1.72 nm, respectively. These increases indicated that PEG chains had intercalated into 0.25SM. Equally important, in these composition ranges, no peaks were observed at higher angles that could be assigned to the crystalline
Fig. 4. The vibration spectra of CH2 in OMMT prepared from S-18 (a) and D-18 (b).
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Fig. 5. XRD patterns of PEG intercalated SM series.
structure of PEG. It indicated that the PEG chains were fully intercalated into the interlayer space. However, when the ratio of PEG to SM was 0.3 or higher, the d0 0 1 of SM hybrids increased to a constant about 1.77 nm and crystalline PEG peaks were observed. In Fig. 5b, the d0 0 1 of 0.50SM increased to 1.77 nm and kept constantly. PEG intercalated 1.00SM and 1.50SM both had a similar results compared to 0.25SM.The d0 0 1 spacings of 1.00SM and 1.50SM increased with adding PEG, up to 3.20 nm and 3.24 nm, respectively. These results indicated that there was a saturation ratio when PEG intercalated OMMT. In our cases, as discussed above, the saturation ratio of PEG to SM was between 0.2 and
0.3 when the surfactant concentration was lower than 2.00CEC. However, quite different PEG intercalation behavior was observed in Fig. 5e and f. The interlayer spaces of PEG intercalated 2.00SM and 3.00SM were all the same compared with 2.00SM and 3.00SM, corresponding to 4.01 nm. And the crystalline PEG peaks were always observed in the presence of PEG. This could be attributed to the PEG chains did not intercalate into the interlayer space of 2.00SM and 3.00SM. It was because with addition of surfactant S-18, the hydrophobic alkyl chains of SM increased. When the surfactant concentration increased to 2.00CEC, the increased hydrophobic properties in the interlayer space of 2.00SM could
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Fig. 6. XRD patterns of PEG intercalated DM series.
prevent the intercalation of hydrophilic PEG. It was noted that the saturation concentration of S-18 intercalated Na-MMT was also 2.00CEC. It indicated that PEG chains couldn’t intercalate into the interlayer space of Na-MMT when the S-18 was saturated. Similarly, PEG intercalation behaviors in OMMT modified by D-18 were showed in Fig. 6. In Fig. 6a, the d0 0 1 of 0.25DM was 1.31 nm. When low concentration PEG intercalated into 0.25DM, the d0 0 1 spacings of 0.1-0.25DM and 0.2-0.25DM increased to 1.63 nm and 1.73 nm, respectively. These increased d0 0 1 spacings indicated that PEG chains intercalated into 0.25DM. Moreover, no PEG crystalline peaks were observed at 2 = 19.2◦ and 23.3◦ in these composition ranges. It indicated that the PEG chains were fully
intercalated into the interlayer space of 0.25DM. However, when the mass ratio of PEG/0.25DM was 0.3 or higher, the d0 0 1 of 0.25DM increased to a constant about 1.77 nm and crystalline PEG peaks were observed. Fig. 6b showed that the d0 0 1 of 0.50DM increased to 1.83 nm and kept constantly. There was a similar result in PEG intercalated 1.00DM when compared to 0.25DM. The d0 0 1 spacing of 1.00DM increased up to 3.68 nm with adding PEG. These results indicated that there was also a saturation ratio when PEG intercalated OMMT modified by D-18. In our cases, as discussed above, the saturation ratio of PEG/DM was between 0.2 and 0.3 when the surfactant concentration was lower than 1.50CEC. However, quite different behavior was observed in Fig. 6d. The interlayer spaces of
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Fig. 7. DSC curves of SM hybrids with varying polymer content.
PEG intercalated 1.50DM were all the same when compared with 1.50DM, corresponding to 3.68 nm. And the crystalline PEG peaks werealways observed in their corresponding XRD patterns. The same results were found in 2.00DM and 3.00DM (Fig. 6e and f). It was because the PEG chains did not intercalate into the interlayer space of DM with high D-18 concentration. With addition of surfactant D-18, the hydrophobic alkyl chains of DM increased. When the surfactant D-18 concentration increased to 1.50CEC or higher, the increased hydrophobic properties in the interlayer space of DM could prevent the intercalation of hydrophilic PEG. Ha and Char figured out a bridge conformation of dodecyldiamine chains in the confined space of MMT can also prevent PEO chains form intercalation [34]. As discussed above, the saturation concentration of D-18 intercalated Na-MMT was 1.50CEC. It indicated that PEG chains could not intercalate into the interlayer space of Na-MMT when the D-18 was saturated. And it could be anticipated
the structures of PEG incorporated with high D-18 concentration of DM (1.50DM, 2.00DM and 3.00DM) were almost the same. Fig. 7 showed the DSC results of mixture samples with different PEG contents before annealing for SM hybrids. The melting peak of neat PEG was about 59.1 ◦ C. In Fig. 7a–d, the melting peak of crystalline PEG was not present in DSC curves of SM which did not show crystalline PEG peaks in their XRD patterns when the mass ratios of PEG/MMT were 0.1 and 0.2. When the ratio of PEG/MMT was 0.3 or higher, the melting peaks crystalline PEG were observed. And with PEG increasing, the melting peak of PEG decreased. The melting temperature was reduced by addition of PEG and it was ascribed to disruption of large scale crystallite formation by the presence of MMT. However, DSC curves showed the melting peaks of crystalline PEG when the mass ratios of PEG to 2.00SM and 3.00SM were lower than 0.3 (Fig. 7e and f). The excited crystalline PEG peaks indicated PEG did not intercalate into the interlayer space
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Fig. 8. DSC curves of DM hybrids with varying polymer content.
of 2.00SM and 3.00SM. It demonstrated that PEG chains could not intercalate into the interlayer space of Na-MMT when the S-18 was saturated. These results of DSC curves were well consistent with the XRD results discussed in Fig. 5. Fig. 8 showed the DSC results of mixture samples with different PEG contents before annealing for DM hybrids, respectively. In the case of DM hybrids, two melting peaks can be seen in most samples. The lower was due to endothermic disruption of the alkylammoniumions attached to the MMT when the higher temperature peaks related to the melting of the PEG crystallites (except for PEG-3.00DM). In some cases, for example, in 1.0–1.00DM and 1.0–1.50DM, the two peaks were merged. In Fig. 8a–c, the melting peak of PEG was not present in DSC curves of DM hybrids which did not show crystalline PEG peaks in their XRD patterns when the mass ratios of PEG/MMT were 0.1 and 0.2. Especially, in Fig. 8d–f, DSC curves showed the melting peaks of crystalline PEG about 60 ◦ C when the mass ratios of PEG/DM were 0.1 and 0.2. The excited
crystalline PEG peaks indicated PEG did not intercalate into the interlayer space of 1.50DM. As discussed in Fig. 6, PEG chains could not intercalate into the interlayer space of Na-MMT when the D-18 was saturated. These results of DSC curves were all well consistent with the XRD results discussed in Fig. 6. In addition, it was noted that the melting temperatures of DM increased with the D18 concentration. It indicated the structures of surfactant chains in high D-18 concentration of DM were more order than low D-18 concentration of DM. Based on the above results, an intercalation mechanism of PEG in OMMT was proposed (Fig. 9). When surfactants concentration in OMMT was unsaturated, the alkyl chains of surfactants adopted a more gauche-like configuration. PEG chains could intercalate into the interlayer space of OMMT and enlarged the d-spacing. With addition of surfactants, the alkyl chains in interlayer space of OMMT increased and adopted a highly ordered conformation. When the surfactant increased to the saturated concentration, the increased
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Fig. 9. Schematics of PEG intercalation mechanism in OMMT.
hydrophobic properties in the interlayer space of OMMT could prevent the intercalation of hydrophilic PEG. 4. Conclusions Two kinds of OMMT (SM and DM) were prepared with single alkyl chains and double alkyl chains ammonium salts. XRD results showed the interlayer spacing of OMMT increased with surfactants in low concentration. With further increasing the surfactants concentration, the interlayer spacings of SM and DM were unchanged. FTIR results suggested that the double chain surfactant D-18 preferred to adopt highly ordered conformation compared with single chain surfactant S-18 in interlayer space of OMMT. These results indicated the degree of the hydrophobic properties might increase with the increment of surfactant loading and chain number. The intercalation behaviors of PEG chains in OMMT were investigated with XRD and DSC. The results indicated that PEG chains could not intercalate into Na-MMT when the surfactants were saturated in interlayer space of Na-MMT. PEG chains intercalated into the interlayer space of SM when the S-18 concentration was lower than 2.00CEC, implying that the low surfactant concentration modified SM provided a better environment (presumably through the balanced hydrophobic and hydrophilic surfaces) for the PEG intercalation as well. Similarly, PEG did not intercalate into the interlayer space of DM when the D-18 concentration was higher than 1.00CEC. It could be attributed to the hydrophobic double alkyl chains of DM increased with D-18. The increased hydrophobic properties in the interlayer space of 1.50DM hybrids could prevent the intercalation of hydrophilic PEG. Acknowledgements This work was supported by the Program for New Century Excellent Talents in University (NCET-10-0562) and the National Basic Research Program of China (2005CB623800). References [1] S. Sinha Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Progress in Polymer Science 28 (2003) 1539–1641. [2] P.C. LeBaron, Z. Wang, T.J. Pinnavaia, Polymer-layered silicate nanocomposites: an overview, Applied Clay Science 15 (1999) 11–29. [3] S. Pavlidou, C. Papaspyrides, A review on polymer-layered silicate nanocomposites, Progress in Polymer Science 33 (2008) 1119–1198. [4] C. Aguzzi, P. Cerezo, C. Viseras, C. Caramella, Use of clays as drug delivery systems: possibilities and limitations, Applied Clay Science 36 (2007) 22–36.
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