- Email: [email protected]
montmorillonite nanocomposites
Polymer Degradation and Stability 95 (2010) 651e655
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Structural design of imidazolium and its application in PP/montmorillonite nanocomposites Aihua He a, *, Limei Wang b, Wei Yao a, *, Baochen Huang a, Dujin Wang c, Charles C. Han d a
Key Laboratory of Rubber-Plastics (Ministry of Education), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China Department of Chemistry, Dezhou University, Shandong 253023, China c CAS Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, No. 2, 1st North Street, Zhongguancun, Beijing 100080, China d State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, No. 2, 1st North Street, Zhongguancun, Beijing 100080, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 November 2009 Accepted 5 December 2009 Available online 21 December 2009
Four kinds of imidazolium surfactants with high thermal stability were designed and synthesized accordingly. The structures of these surfactants were characterized by 1H NMR spectra. The TGA results indicated that the thermal stabilities of these surfactants with saturated alkyl groups were relatively high and the initial decomposition temperatures at 5% weight loss (T0.05) were higher than 250 C. Imidazolium(O) modified montmorillonite (MMT) was prepared by cation exchange. TGA results showed that the OMMT showed obviously higher thermal stability than the surfactants themselves and the T0.05 values of OMMT were higher than 330 C. The dihexadecane imidazolium (DHI) with two long tails has the ability to enlarge the interlayer spacing to a bigger degree compared with other imidazolium surfactants with only one long tail. Polypropylene(PP)/OMMT nanocomposites were prepared by solution blending and the effects of these surfactants with different structures on the silicate layer dispersion in PP matrix were measured. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Surfactants Montmorillonite Polypropylene Nanocomposites
1. Introduction Since the Nylon-6/clay nanocomposites with some excellent properties, such as enhanced mechanical properties, increased heat distortion temperature and decreased gas/vapor permeability [1e8] were successfully prepared by Toyota researchers [1,2] in the late 1980s, many research interests from both academic and industrial labs have been focused on the polymer/clay nanocomposites [9e22]. Recently, more efforts focus on the creating of high-performance poly(propylene) (PP)/clay nanocomposites for potential applications in the field of packaging and the automotive industry as a substitute of high-performance engineering plastics. PP, as one of the commodity plastics, is expected to have improved toughness, enhanced modulus and barrier properties, and could be used in automotive fields. PP nanocomposites filled with well dispersed silicate layers is thought to be an effective way to modify PP. However, it is difficult to prepare well dispersed PP/clay nanocomposites because of the incompatibility of hydrophobic PP and hydrophilic clay, and the strong self-agglomeration * Corresponding authors. Tel.: þ86 532 84022951; fax: þ86 532 84022951. E-mail addresses: [email protected] (A. He), [email protected] (W. Yao). 0141-3910/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.12.003
of silicate layers especially during PP processing process at high temperature. In order to improve the compatibility of clay and PP, alkyl-ammonium surfactants have been used to modify montmorillonite (AMMT) [10e17,23,24]. However, it was found that the alkyl-ammonium surfactant began to degrade at 104 C [20] and the thermal degradation of alkyl-ammonium surfactants at the PP processing temperature (200 10 C) not only accelerated the aging and decomposition of PP, but also led to the re-stacking of the silicate layers. This behavior has been reported in PP/clay composites via both melt- blending method and in-situ polymerization method [20,25]. In our previous work, monoalkylimidazolium (Im) modified montmorillonite (IMMT) were used to prepare novel ZieglereNatta /IMMT compound catalysts, and subsequently, the exfoliated PP/montmorillonite(MMT) nanocomposites with good thermal stability were successfully synthesized via intercalative polymerization method [20,21]. It has been accepted that the surfactants used in clay modification will play an important role on the processing stability of PP/clay nanocomposites. Therefore the design of surfactants with higher thermal stability, better compatibility for clay and PP, and enhanced clay exfoliation ability are becoming more and more crucial.
652
A. He et al. / Polymer Degradation and Stability 95 (2010) 651e655
In this paper, four kinds of imidazolium compounds with different chemical structures and relatively high thermal stability have been designed and synthesized as clay modification surfactants. The 1H NMR spectra were used to characterize those synthesized surfactants. Basic physical properties including thermal stability of those surfactants were investigated in detail. Organic montmorillonite (OMMT) modified with those surfactants were prepared by ionic-exchange and PP/OMMT composites were prepared by solution blending accordingly. The effects of surfactants with different chemical structures on the physical properties of OMMT and PP/OMMT nanocomposites were studied. 2. Experimental 2.1. Materials and instruments Naþ-MMT was supplied by Qinghe Chemical factory with about 80-100 mequiv. /100 g cationic exchange capacity (CEC). Pure polypropylene granule (PP) with melting index 44 was supplied kindly by the Institute of Chemistry, Chinese Academy of Sciences. 1-Methylimidazole (Analysis Purity, AP) and 1-iodohexadecane (AP) were purchased from Aldrich Co. Imidazole (AP) was purchased from Tianjin Fuchen chemical reagent Co. of China. 1-bromohexadecane (CP) was purchased from Beijing reagent Co. of China. 1-hexadecaneimidazolium was supplied kindly by our group member (the Institute of Chemistry, Chinese Academy of Sciences). 2-bromoethanol (CP) was purchased from Alfa Aesar Co. Allyl chloride (CP) was purchased from Shanghai Chemical reagent Co. of China. Other solvents such as acetonitrile, hexane, ether, ethanol, etc, were used after dried with 4A molecular sieves overnight. 1 H NMR spectra were recorded at 300 MHz on a Brucker DMX300 NMR Spectrometer with CDCl3 as the solvent and TMS as the internal reference. Thermogravimetric analysis was performed with Perkin-Elmer TGA at a heating rate of 20 C/min under nitrogen atmosphere. Differential Scanning Calorimetry (DSC) was conducted using a PerkineElmer DSC-7 thermal analyzer under nitrogen atmosphere with a heating rate of 10 C/min in a temperature range of 40e200 C for dynamic scanning, and Tm was determined in the second scan. Wide-angle X-ray diffraction (XRD) analysis was performed on a Japan Rigaku D/max-2500 diffractometer with Cu Ka radiation (l ¼ 0.1504 nm) at a generator voltage of 40 kV and generator current of 100 mA. Scaning was performed in a step of 0.02 at a speed of 2 /min. The interlayer spacing (d001) of MMT was calculated in accordance with Bragg equation: 2d sin q ¼ l. 2.2. Synthesis of the surfactants 2.2.1. Synthesis of the 1-hexadecane-3-methylimidazolium bromine (HMI) Equimolar quantities of 1-methylimidazole and 1-bromohexadecane were placed into a 500 mL flask with reflux condenser under the argon atmosphere. Then the reactor was degassed and sealed under vacuum. The reactants were left stirring for approximately 7 days at 120 C. The resulted solid was recrystallized from dry acetonitrile and washed by hexane for 3 times and then dried under vacuum overnight. The yield was 84 wt%. 2.2.2. Synthesis of the 1,3-dihexadecane imidazolium iodine (DHI) A precise amount of imidazole and 1-iodohexadecane were placed into a 500 mL flask with reflux condenser under the argon atmosphere (the molar ratio of imidazole to 1-iodohexadecane was 1:2.2). Then the reactor was degassed and sealed under vacuum. The reactants were left stirring for approximately 6 days at 110 C. The resulted solid was recrystallized from dry acetonitrile and
washed by hexane for 3 times and then dried under vacuum overnight. The yield was 41 wt%. 2.2.3. Synthesis of the 1-hydroxyethyl-3-hexadecane imidazolium bromine (HHI) 1-hexadecaneimidazolium and 2-bromoethanol with the molar ratio of 1:1.2 were placed into a reactor with reflux condenser under the argon atmosphere. The reactants were left stirring for approximately 2 days at 80 C. The resulted solid was washed by ether and then dried under vacuum overnight. The yield was 84 wt%. 2.2.4. Synthesis of the 1-allyl-3-hexadecane imidazolium chloride (AHI) 1-Hexadecaneimidazolium and 1-allyl chloride with the molar ratio of 1:1.25 were placed into a reactor with reflux condenser under the argon atmosphere. The reactants were left stirring for approximately 3 days at 60 C. The resulted solid was washed by ether and then dried under vacuum overnight. The yield was 63 wt%. 2.3. Preparation of organically modified montmorillonite (OMMT) The surfactant (twice the CEC of Naþ-MMT) was dissolved in ethanol at 50 C and the surfactant solution was added to a 10 wt% aqueous suspension of montmorillonite under vigorous stirring. The mixture was stirred for 8 h at 60 C, then the products were collected by filtration, washed with hot ethanol, and paper filtered until no halide anions were measured. OMMT was dried at 80 C under vacuum for 24 h. 2.4. Preparation of PP/OMMT nanocomposites The dried OMMT was dispersed in xylene, the pure PP was dissolved in xylene at 130 C. Then the above mixtures were blended at 130 C for 6 h at nitrogen atmosphere. The product was collected by precipitating in ethanol and dried at 80 C under vacuum for 2 days. 3. Results and discussion 3.1.
1
H NMR characterization of the synthesized surfactants
Imidazolium compounds have been generally studied as the ionic liquids or anti-bacterial agents [26,27]. However, as compatibilizers for clay and PP, imidazolium were little studied. The advantageous features of the imidazolium compounds include the imidazole ring which will offer the compound high thermal stability compared to that of alkyl-ammonium compounds, and the various alkyl substitute groups which will improve the hydrophobility of clay and benefit the enlargement of the gallery of silicate layers. Therefore, in order to obtain the desire surfactant for PP and clay composites, four kinds of imidazolium compounds were designed and synthesized accordingly. The structures of the synthesized surfactants were characterized by the 1H NMR measurements. The 1H NMR spectra of those surfactants were shown in Fig. 1. The chemical structure of each surfactant was given and the assignment of each peak in Fig. 1(aed) was also assigned according to the H sequence number of the formula, as shown in Fig. 1. The results of the 1H NMR spectra showed that 1-hexadecane-3methylimidazolium bromine (HMI), 1,3-dihexadecane imidazolium iodine (DHI), 1-hydroxyethyl-3-hexadecane imidazolium bromine (HHI) and 1-allyl-3-hexadecane imidazolium chloride (AHI) could be synthesized according to the above mentioned synthesized routes.
A. He et al. / Polymer Degradation and Stability 95 (2010) 651e655
653
Fig. 1. 1H NMR spectra of the surfactants. (a) HMI, (b) DHI, (c) HHI, (d) AHI.
3.2. Basic physical properties of the synthesized surfactants The melting behaviors of the synthesized surfactants were studied by the DSC measurements. It was found from Table 1 that all the surfactants are crystallized compounds at room temperature and melting points of these surfactants were higher than 44 C. In addition, both the melting point and crystallization temperature of DHI are the highest among all the four surfactants. The solubility of these surfactants was also studied. It showed that all the surfactants with long alkyl tails could not dissolve in water. Ethanol is the good solvent for all the surfactants. Therefore ethanol was used to dissolve the surfactants in this study. The thermal stability of the surfactants themselves was measured and TGA was employed to determine the initial decomposition temperature (temperature at 5% weight loss) (T0.05) and the onset decomposition temperature (Tonset), as summarized in Table 2. It was found that the T0.05 of all the surfactants was higher than 220 C, the T0.05 values of the surfactants with saturated alkyl groups were higher than 250 C. The T0.05 values of DHI and HHI were as high as 280 C and these results indicated that both DHI and HHI show higher thermal stability when compared with HMI and AHI. 3.2.1. Basic physical properties of the organic clay modified with the imidazolium surfactants The synthesized imidazoliums with relatively high thermal stability were used as surfactants for the organically modification of clay. Therefore, MHI, DHI, HHI and AHI modified montmorillonite (MHI-MMT, DHI-MMT, HHI-MMT and AHI-MMT) was prepared via ion-exchange procedures and characterized by XRD, respectively. Table 1 DSC results of the synthesized surfactants. Sample
Tm ( C)
DH (J/g)
Tc ( C)
DH (J/g)
HMI DHI HHI AHI
66.0 71.9 44.6 50.5
146.0 108.4 91.6 113.6
39.7 60.3 40.1 38.7
71.4 100.6 58.3 67.2
Fig. 2 shows that the (001) diffraction peaks of MHI-MMT, HHIMMT and AHI-MMT shifted to 2q ¼ 4.2-4.1 from 2q ¼ 9.05 of dried Naþ-MMT [22], which indicated that the d-spacing of these organically modified MMT increased to 2.1e2.2 nm according to the Brag equation. However, the (001) diffraction peaks of DHIMMT appears at 2q ¼ 3.0 , indicating that the d-spacing of DHI organically modified MMT increased to 2.9 nm. Therefore, it can be concluded that the imidazolium surfactants could intercalate into the galleries of the clay and enlarge the (001) d-spacing of the clay. The dihexadecane imidazolium with two long tails has the ability to enlarge the interlayer spacing to a bigger degree, while the other imidazolium surfactants with only one long tail have the similar ability to enlarge the interlayer spacing to some extent degree. The thermal stability of the organically modified clay was evaluated by TGA measurement. It was found that the T0.05 values of the imidazolium modified clays were all higher than 330 C, and nearly 50e100 C higher than that of the surfactants themselves, as shown in Table 2. This indicated that once the surfactants are intercalated into the interlayers of MMT, OMMT exhibit higher thermal stability compared to its organic salt, which is probably due to the barrier properties of silicate layers. The TGA results indicate that the imidazolium modified MMT can bear higher temperature when compared with the ammonium modified MMT [20,22], which will match the PP processing temperature and lead to the higher thermal stability of the nanocomposites for practical use. Table 2 TGA results of the surfactants. Sample
Temperature at 5 wt% weight loss ( C)
Temperature at onset ( C)
Surfactant mole content per gram clay (mmol/g)a
HMI DHI HHI AHI HMI-MMT DHI-MMT HHI-MMT AHI-MMT
249 278 277 226 336 331 344 349
272 288 301 256 305 316 310 321
e e e e 0.63 0.64 0.58 0.67
a
Weight loss at 400 C was calculated as the surfactant content.
654
A. He et al. / Polymer Degradation and Stability 95 (2010) 651e655
24000
Intensity (cps)
18000
12000
M H I-M M T 6000
H H I-M M T A H I-M M T D H I-M M T
0 5
10
15
20
25
30
35
40
2 T h e ta (d e g ) Fig. 2. XRD patterns of the organically modified clay.
The molar content of the surfactants intercalated into the gallery of clay was calculated according to the weight loss of the organics at 400 C in TGA curves and the results were shown in Table 2. It was found from Table 2 that the molar contents of the surfactants intercalated into the galleries of clay were nearly similar. It can be understood that the molar content of the surfactants were confined by CEC of the MMT. 3.3. Preparation and characterizations of the PP/OMMT nanocomposites via solution blending method PP/OMMT nanocomposites were prepared via solution blending method. The MMT was modified with HMI, DHI, HHI and AHI, respectively. The clay content in the PP/OMMT nanocomposites was constant at 2.5 wt%. The effect of surfactant structure on the clay dispersion in the PP matrix was studied in detail. The XRD patterns for PP/OMMT composites are shown in Fig. 3. It was found that the surfactant structure played an important role on the clay dispersion in the PP matrix. The XRD curve of the PP/ DHI-MMT composite displayed no (001) diffraction peak from the MMT, indicating that the average interlayer spacing of the MMT in 8000 7000
Intensity (cps)
6000 5000 4000 3000
PP/DHI-MMT
2000
PP/HHI-MMT PP/AHI-MMT
1000
PP/HMI-MMT 5
10
15
20
25
30
2 Theta (deg) Fig. 3. XRD patterns of the PP/OMMT nanocomposites.
35
PP/DHI-MMT nanocomposites was larger than 5.8 nm according to the Bragg equation. While for PP/HHI-MMT, PP/AHI-MMT and PP/ HMI-MMT, the XRD curves showed dispersed (001) diffraction peaks at about 2q ¼ 4.86 from MMT corresponding to the average d-spacing of 1.82 nm. However, for PP/HMI-MMT sample, the (001) diffraction peak was more obvious. These results indicate that surfactant containing two long tails plays a more powerful role on the compatibility of PP and MMT, and as a result, exfoliated PP/ MMT nanocomposites could be prepared. The surfactants containing only one long tail could lead to intercalated or partly exfoliated PP/MMT nanocomposites via solution blending method. However, among these surfactants containing only one long tail, HHI with polar groups is much more effective than other surfactants without polar groups. As shown in Table 3, the melting point of pure PP granule was about 160.6 C and the ones of PP in nanocomposites were similar to that of pure PP, which indicated that the crystal structures of PP in nanocomposites were changed little. However, the cold crystallization temperature (Tc) for PP nanocomposites was higher than that of pure PP. This can be explained by the effect that MMT layers act as nucleating agent for PP and enhance the crystallization rate. The thermal decomposition temperature at onset of these PP/ OMMT materials (Tonset) was investigated by using TGA. It was found that the Tonset values of the nanocomposites were approached to that of the neat PP except the PP/AHI-MMT nanocomposites. Generally speaking, the thermal stability of the PP/clay nanocomposites should be improved compared with that of pure PP. Anyway, in this case, the pure PP used was the lab-made degradated PP containing 0.1% antioxidant. However, in our nanocomposites, most antioxidant was removed during the solution blending process. The PP/AHI-MMT nanocomposite showed a relatively low Tonset value, which might be attributed to the instability of the double bonds in AHI. Table 3 Physical properties of the PP/OMMT nanocomposites.
40
Sample
Tm ( C)
DHm (J/g)
Tc ( C)
Degradation temperature at onset ( C)
PP granule PP/HMI-MMT PP/DHI-MMT PP/HHI-MMT PP/AHI-MMT
160.6 160.5 160.6 160.0 161.3
96.9 97.4 99.4 101.4 104.8
112.2 119.1 118.7 118.5 119.5
367.7 351.3 360.5 368.9 330.2
A. He et al. / Polymer Degradation and Stability 95 (2010) 651e655
4. Conclusion Four kinds of imidazolium surfactants with high thermal stability were designed and synthesized. The structures of these surfactants were characterized and then affirmed by 1H NMR spectra. The TGA results indicated that the thermal stabilities of these surfactants with saturated alkyl groups were relatively high and the T0.05 values were higher than 250 C. Once the imidazolium surfactants were intercalated into the galleries of silicate layers, the OMMT showed obviously higher thermal stability than the surfactants themselves and the T0.05 values of OMMT were higher than 330 C. The dihexadecane imidazolium (DHI) with two long tails has the ability to enlarge the interlayer spacing to a bigger degree when compared with other imidazolium surfactants with only one long tail. It was found that exfoliated PP/OMMT nanocomposites could be prepared with DHI modified MMT by solution blending. Acknowledgements This work was financially supported by the National Nature Science Foundation of China through funds No. 20774098, No. 50973123, No. 50503023. National ‘‘973'’ Project (G2003 CB615600) is also acknowledged for its support. This work was also supported by the Scientific Research Startup Foundation of Qingdao University of Science and Technology for talents. References [1] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. Mechanical properties of nylon6eclay hybrid. J Mater Res 1993;8:1185e99. [2] Okada A, Kawasumi M. Synthesis and characterization of a nylon 6-clay hybrid. Polym Prepr Jpn 1987;28(2):447e8. [3] Hotta S, Paul DR. Nanocomposites formed from linear low density polyethylene and organoclays. Polymer 2004;45(22):7639e54. [4] Galgali G, Agarwal S, Lele A. Effect of clay orientation on the tensile modulus of polypropyleneenanoclay composites. Polymer 2004;45(17):6059e69. [5] Zanetti M, Costa L. Preparation and combustion behaviour of polymer/layer silicate nanocomposites based upon PE and EVA. Polymer 2004;45(13):4367e73. [6] Gopakumar TG, Lee JA, Kontopoulou M, Parent JS. Influence of clay exfoliation on the physical properties of montmorillonite/polyethylene composites. Polymer 2002;43(20):5483e91. [7] Qin HL, Su QS, Zhang SM, Zhao B, Yang MS. Thermal stability and flammability of polyamide 66/montmorillonite nanocomposites. Polymer 2003;44(24): 7533e8. [8] Gilman JW. Flammability and thermal stability studies of polymer layeredsilicate (clay) nanocomposites. Appl Clay Sci 1999;15(1):31e49.
655
[9] Kurokawa Y, Yasuda H, Oya A. Preparation of a nanocomposite of polypropylene and smectite. J Mater Sci Lett 1996;15(17):1481e3. [10] Sun T, Garcés MJ. High-performance polypropylene-clay nanocomposites by in situ polymerization with metallocene/clay catalysts. Adv Mater 2002;14(2): 128e30. [11] JP 09118792. N Tetsuo Chem Abstr 1997;127:51586. [12] Tudor J, Willington L, O'Hare D, Royan B. Intercalation of catalytically active metal complexes in phyllosilicates and their application as propene polymerization catalysts. Chem Commun 1996;17:2031e2. [13] Tudor J, O'Hare D. Stereospecific propene polymerization catalysis using an organometallic modified mesoporous silicate. Chem Commun 1997:603e4. [14] Bergman JS, Chen Hua, Giannelis EP, Thomas MG, Coates GW. Synthesis and characterization of polyolefin-silicate nanocomposites: a catalyst intercalation and in situ polymerization approach. Chem Commun 1999;21:2179e80. [15] Kawasumi M, Hasegawa N, Kato M, Usuki A, Okada A. Preparation and mechanical properties of polypropylene-clay hybrids. Macromolecules 1997;30(20):6333e8. [16] Hasegawa N, Kawasumi M, Kato M. Preparation and mechanical properties of polypropylene-clay hybrids using a maleic anhydride-modified polypropylene oligomer. J Appl Polym Sci 1998;67(1):87e92. [17] Nam PH, Maiti P, Okamoto M. A hierarchical structure and properties of intercalated polypropylene/clay nanocopmposites. Polymer 2001;42(23): 9633e40. [18] Reichert P, Nitz H, Klinke S, Brandsch R, Thomann R, Muelhaupt R. Poly(propylene)/organoclay nanocomposite formation: influence of compatibilizer functionality and organoclay modification. Macromol Mater Eng 2000;275(1): 8e17. [19] Zhang Q, Fu Q, Jiang L, Lei Y. Preparation and properties of poly-propylene/ montmorillonite layered nano-composites. Polym Int 2000;49(12):1561e4. [20] He AH, Hu HQ, Huang YJ, Dong JY, Han Charles C. Isotactic poly(propylene)/ monoalkylimidazolium-modified montmorillonite nanocomposites: preparation by intercalative polymerization and thermal stability study. Macromol Rapid Commun 2004;25:2008e13. [21] Du K, He AH, Liu X, Han Charles C. High-performance exfoliated poly (propylene)/clay nanocomposites by in situ polymerization with a novel Z-N/ clay compound catalyst. Macromol Rapid Commun 2007;28(24):2294e9. [22] He AH, Wang LM, Li JX, Dong JY, Han Charles C. Preparation of exfoliated isotactic polypropylene/alkyl-triphenylphosphonium-modified montmorillonite nanocomposites via in-situ intercalative polymerization. Polymer 2006; 47(6):1767e71. [23] Kurokawa Y, Yasuda H, Kashiwagi M, Oya A. Structure and properties of a montmorillonite/polypropylene nanocomposite. J Mater Sci Lett 1997;16 (20):1670e2. [24] Oya A, Kurokawa Y, Yasuda H. Factors controlling mechanical properties of clay mineral/polypropylene nanocomposites. J Mater Sci 2000;35(5): 1045e50. [25] Zanetti M, Camino G, Reichert P, Mülhaupt R. Thermal behavior of poly (propylene) layered silicate nanocomposites. Macromol Rapid Commun 2001;22:176e80. [26] Sergei VD, Richard AB. New room-temperature ionic liquids with C2symmetrical imidazolium cations. Chem Commun 2001:1466e7. [27] Khabnadideh S, Rezaei Z, Khalafi-Nezhad A, Bahrinajafi R, Mohamadi R, Farrokhroz AA. Synthesis of N-Alkylated derivatives of imidazole as antibacterial agents. Bioorg Med Chem Lett 2003;13:2863e5.
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Structural design of imidazolium and its application in PP/montmorillonite nanocomposites Aihua He a, *, Limei Wang b, Wei Yao a, *, Baochen Huang a, Dujin Wang c, Charles C. Han d a
Key Laboratory of Rubber-Plastics (Ministry of Education), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China Department of Chemistry, Dezhou University, Shandong 253023, China c CAS Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, No. 2, 1st North Street, Zhongguancun, Beijing 100080, China d State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, No. 2, 1st North Street, Zhongguancun, Beijing 100080, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 November 2009 Accepted 5 December 2009 Available online 21 December 2009
Four kinds of imidazolium surfactants with high thermal stability were designed and synthesized accordingly. The structures of these surfactants were characterized by 1H NMR spectra. The TGA results indicated that the thermal stabilities of these surfactants with saturated alkyl groups were relatively high and the initial decomposition temperatures at 5% weight loss (T0.05) were higher than 250 C. Imidazolium(O) modified montmorillonite (MMT) was prepared by cation exchange. TGA results showed that the OMMT showed obviously higher thermal stability than the surfactants themselves and the T0.05 values of OMMT were higher than 330 C. The dihexadecane imidazolium (DHI) with two long tails has the ability to enlarge the interlayer spacing to a bigger degree compared with other imidazolium surfactants with only one long tail. Polypropylene(PP)/OMMT nanocomposites were prepared by solution blending and the effects of these surfactants with different structures on the silicate layer dispersion in PP matrix were measured. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Surfactants Montmorillonite Polypropylene Nanocomposites
1. Introduction Since the Nylon-6/clay nanocomposites with some excellent properties, such as enhanced mechanical properties, increased heat distortion temperature and decreased gas/vapor permeability [1e8] were successfully prepared by Toyota researchers [1,2] in the late 1980s, many research interests from both academic and industrial labs have been focused on the polymer/clay nanocomposites [9e22]. Recently, more efforts focus on the creating of high-performance poly(propylene) (PP)/clay nanocomposites for potential applications in the field of packaging and the automotive industry as a substitute of high-performance engineering plastics. PP, as one of the commodity plastics, is expected to have improved toughness, enhanced modulus and barrier properties, and could be used in automotive fields. PP nanocomposites filled with well dispersed silicate layers is thought to be an effective way to modify PP. However, it is difficult to prepare well dispersed PP/clay nanocomposites because of the incompatibility of hydrophobic PP and hydrophilic clay, and the strong self-agglomeration * Corresponding authors. Tel.: þ86 532 84022951; fax: þ86 532 84022951. E-mail addresses: [email protected] (A. He), [email protected] (W. Yao). 0141-3910/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.12.003
of silicate layers especially during PP processing process at high temperature. In order to improve the compatibility of clay and PP, alkyl-ammonium surfactants have been used to modify montmorillonite (AMMT) [10e17,23,24]. However, it was found that the alkyl-ammonium surfactant began to degrade at 104 C [20] and the thermal degradation of alkyl-ammonium surfactants at the PP processing temperature (200 10 C) not only accelerated the aging and decomposition of PP, but also led to the re-stacking of the silicate layers. This behavior has been reported in PP/clay composites via both melt- blending method and in-situ polymerization method [20,25]. In our previous work, monoalkylimidazolium (Im) modified montmorillonite (IMMT) were used to prepare novel ZieglereNatta /IMMT compound catalysts, and subsequently, the exfoliated PP/montmorillonite(MMT) nanocomposites with good thermal stability were successfully synthesized via intercalative polymerization method [20,21]. It has been accepted that the surfactants used in clay modification will play an important role on the processing stability of PP/clay nanocomposites. Therefore the design of surfactants with higher thermal stability, better compatibility for clay and PP, and enhanced clay exfoliation ability are becoming more and more crucial.
652
A. He et al. / Polymer Degradation and Stability 95 (2010) 651e655
In this paper, four kinds of imidazolium compounds with different chemical structures and relatively high thermal stability have been designed and synthesized as clay modification surfactants. The 1H NMR spectra were used to characterize those synthesized surfactants. Basic physical properties including thermal stability of those surfactants were investigated in detail. Organic montmorillonite (OMMT) modified with those surfactants were prepared by ionic-exchange and PP/OMMT composites were prepared by solution blending accordingly. The effects of surfactants with different chemical structures on the physical properties of OMMT and PP/OMMT nanocomposites were studied. 2. Experimental 2.1. Materials and instruments Naþ-MMT was supplied by Qinghe Chemical factory with about 80-100 mequiv. /100 g cationic exchange capacity (CEC). Pure polypropylene granule (PP) with melting index 44 was supplied kindly by the Institute of Chemistry, Chinese Academy of Sciences. 1-Methylimidazole (Analysis Purity, AP) and 1-iodohexadecane (AP) were purchased from Aldrich Co. Imidazole (AP) was purchased from Tianjin Fuchen chemical reagent Co. of China. 1-bromohexadecane (CP) was purchased from Beijing reagent Co. of China. 1-hexadecaneimidazolium was supplied kindly by our group member (the Institute of Chemistry, Chinese Academy of Sciences). 2-bromoethanol (CP) was purchased from Alfa Aesar Co. Allyl chloride (CP) was purchased from Shanghai Chemical reagent Co. of China. Other solvents such as acetonitrile, hexane, ether, ethanol, etc, were used after dried with 4A molecular sieves overnight. 1 H NMR spectra were recorded at 300 MHz on a Brucker DMX300 NMR Spectrometer with CDCl3 as the solvent and TMS as the internal reference. Thermogravimetric analysis was performed with Perkin-Elmer TGA at a heating rate of 20 C/min under nitrogen atmosphere. Differential Scanning Calorimetry (DSC) was conducted using a PerkineElmer DSC-7 thermal analyzer under nitrogen atmosphere with a heating rate of 10 C/min in a temperature range of 40e200 C for dynamic scanning, and Tm was determined in the second scan. Wide-angle X-ray diffraction (XRD) analysis was performed on a Japan Rigaku D/max-2500 diffractometer with Cu Ka radiation (l ¼ 0.1504 nm) at a generator voltage of 40 kV and generator current of 100 mA. Scaning was performed in a step of 0.02 at a speed of 2 /min. The interlayer spacing (d001) of MMT was calculated in accordance with Bragg equation: 2d sin q ¼ l. 2.2. Synthesis of the surfactants 2.2.1. Synthesis of the 1-hexadecane-3-methylimidazolium bromine (HMI) Equimolar quantities of 1-methylimidazole and 1-bromohexadecane were placed into a 500 mL flask with reflux condenser under the argon atmosphere. Then the reactor was degassed and sealed under vacuum. The reactants were left stirring for approximately 7 days at 120 C. The resulted solid was recrystallized from dry acetonitrile and washed by hexane for 3 times and then dried under vacuum overnight. The yield was 84 wt%. 2.2.2. Synthesis of the 1,3-dihexadecane imidazolium iodine (DHI) A precise amount of imidazole and 1-iodohexadecane were placed into a 500 mL flask with reflux condenser under the argon atmosphere (the molar ratio of imidazole to 1-iodohexadecane was 1:2.2). Then the reactor was degassed and sealed under vacuum. The reactants were left stirring for approximately 6 days at 110 C. The resulted solid was recrystallized from dry acetonitrile and
washed by hexane for 3 times and then dried under vacuum overnight. The yield was 41 wt%. 2.2.3. Synthesis of the 1-hydroxyethyl-3-hexadecane imidazolium bromine (HHI) 1-hexadecaneimidazolium and 2-bromoethanol with the molar ratio of 1:1.2 were placed into a reactor with reflux condenser under the argon atmosphere. The reactants were left stirring for approximately 2 days at 80 C. The resulted solid was washed by ether and then dried under vacuum overnight. The yield was 84 wt%. 2.2.4. Synthesis of the 1-allyl-3-hexadecane imidazolium chloride (AHI) 1-Hexadecaneimidazolium and 1-allyl chloride with the molar ratio of 1:1.25 were placed into a reactor with reflux condenser under the argon atmosphere. The reactants were left stirring for approximately 3 days at 60 C. The resulted solid was washed by ether and then dried under vacuum overnight. The yield was 63 wt%. 2.3. Preparation of organically modified montmorillonite (OMMT) The surfactant (twice the CEC of Naþ-MMT) was dissolved in ethanol at 50 C and the surfactant solution was added to a 10 wt% aqueous suspension of montmorillonite under vigorous stirring. The mixture was stirred for 8 h at 60 C, then the products were collected by filtration, washed with hot ethanol, and paper filtered until no halide anions were measured. OMMT was dried at 80 C under vacuum for 24 h. 2.4. Preparation of PP/OMMT nanocomposites The dried OMMT was dispersed in xylene, the pure PP was dissolved in xylene at 130 C. Then the above mixtures were blended at 130 C for 6 h at nitrogen atmosphere. The product was collected by precipitating in ethanol and dried at 80 C under vacuum for 2 days. 3. Results and discussion 3.1.
1
H NMR characterization of the synthesized surfactants
Imidazolium compounds have been generally studied as the ionic liquids or anti-bacterial agents [26,27]. However, as compatibilizers for clay and PP, imidazolium were little studied. The advantageous features of the imidazolium compounds include the imidazole ring which will offer the compound high thermal stability compared to that of alkyl-ammonium compounds, and the various alkyl substitute groups which will improve the hydrophobility of clay and benefit the enlargement of the gallery of silicate layers. Therefore, in order to obtain the desire surfactant for PP and clay composites, four kinds of imidazolium compounds were designed and synthesized accordingly. The structures of the synthesized surfactants were characterized by the 1H NMR measurements. The 1H NMR spectra of those surfactants were shown in Fig. 1. The chemical structure of each surfactant was given and the assignment of each peak in Fig. 1(aed) was also assigned according to the H sequence number of the formula, as shown in Fig. 1. The results of the 1H NMR spectra showed that 1-hexadecane-3methylimidazolium bromine (HMI), 1,3-dihexadecane imidazolium iodine (DHI), 1-hydroxyethyl-3-hexadecane imidazolium bromine (HHI) and 1-allyl-3-hexadecane imidazolium chloride (AHI) could be synthesized according to the above mentioned synthesized routes.
A. He et al. / Polymer Degradation and Stability 95 (2010) 651e655
653
Fig. 1. 1H NMR spectra of the surfactants. (a) HMI, (b) DHI, (c) HHI, (d) AHI.
3.2. Basic physical properties of the synthesized surfactants The melting behaviors of the synthesized surfactants were studied by the DSC measurements. It was found from Table 1 that all the surfactants are crystallized compounds at room temperature and melting points of these surfactants were higher than 44 C. In addition, both the melting point and crystallization temperature of DHI are the highest among all the four surfactants. The solubility of these surfactants was also studied. It showed that all the surfactants with long alkyl tails could not dissolve in water. Ethanol is the good solvent for all the surfactants. Therefore ethanol was used to dissolve the surfactants in this study. The thermal stability of the surfactants themselves was measured and TGA was employed to determine the initial decomposition temperature (temperature at 5% weight loss) (T0.05) and the onset decomposition temperature (Tonset), as summarized in Table 2. It was found that the T0.05 of all the surfactants was higher than 220 C, the T0.05 values of the surfactants with saturated alkyl groups were higher than 250 C. The T0.05 values of DHI and HHI were as high as 280 C and these results indicated that both DHI and HHI show higher thermal stability when compared with HMI and AHI. 3.2.1. Basic physical properties of the organic clay modified with the imidazolium surfactants The synthesized imidazoliums with relatively high thermal stability were used as surfactants for the organically modification of clay. Therefore, MHI, DHI, HHI and AHI modified montmorillonite (MHI-MMT, DHI-MMT, HHI-MMT and AHI-MMT) was prepared via ion-exchange procedures and characterized by XRD, respectively. Table 1 DSC results of the synthesized surfactants. Sample
Tm ( C)
DH (J/g)
Tc ( C)
DH (J/g)
HMI DHI HHI AHI
66.0 71.9 44.6 50.5
146.0 108.4 91.6 113.6
39.7 60.3 40.1 38.7
71.4 100.6 58.3 67.2
Fig. 2 shows that the (001) diffraction peaks of MHI-MMT, HHIMMT and AHI-MMT shifted to 2q ¼ 4.2-4.1 from 2q ¼ 9.05 of dried Naþ-MMT [22], which indicated that the d-spacing of these organically modified MMT increased to 2.1e2.2 nm according to the Brag equation. However, the (001) diffraction peaks of DHIMMT appears at 2q ¼ 3.0 , indicating that the d-spacing of DHI organically modified MMT increased to 2.9 nm. Therefore, it can be concluded that the imidazolium surfactants could intercalate into the galleries of the clay and enlarge the (001) d-spacing of the clay. The dihexadecane imidazolium with two long tails has the ability to enlarge the interlayer spacing to a bigger degree, while the other imidazolium surfactants with only one long tail have the similar ability to enlarge the interlayer spacing to some extent degree. The thermal stability of the organically modified clay was evaluated by TGA measurement. It was found that the T0.05 values of the imidazolium modified clays were all higher than 330 C, and nearly 50e100 C higher than that of the surfactants themselves, as shown in Table 2. This indicated that once the surfactants are intercalated into the interlayers of MMT, OMMT exhibit higher thermal stability compared to its organic salt, which is probably due to the barrier properties of silicate layers. The TGA results indicate that the imidazolium modified MMT can bear higher temperature when compared with the ammonium modified MMT [20,22], which will match the PP processing temperature and lead to the higher thermal stability of the nanocomposites for practical use. Table 2 TGA results of the surfactants. Sample
Temperature at 5 wt% weight loss ( C)
Temperature at onset ( C)
Surfactant mole content per gram clay (mmol/g)a
HMI DHI HHI AHI HMI-MMT DHI-MMT HHI-MMT AHI-MMT
249 278 277 226 336 331 344 349
272 288 301 256 305 316 310 321
e e e e 0.63 0.64 0.58 0.67
a
Weight loss at 400 C was calculated as the surfactant content.
654
A. He et al. / Polymer Degradation and Stability 95 (2010) 651e655
24000
Intensity (cps)
18000
12000
M H I-M M T 6000
H H I-M M T A H I-M M T D H I-M M T
0 5
10
15
20
25
30
35
40
2 T h e ta (d e g ) Fig. 2. XRD patterns of the organically modified clay.
The molar content of the surfactants intercalated into the gallery of clay was calculated according to the weight loss of the organics at 400 C in TGA curves and the results were shown in Table 2. It was found from Table 2 that the molar contents of the surfactants intercalated into the galleries of clay were nearly similar. It can be understood that the molar content of the surfactants were confined by CEC of the MMT. 3.3. Preparation and characterizations of the PP/OMMT nanocomposites via solution blending method PP/OMMT nanocomposites were prepared via solution blending method. The MMT was modified with HMI, DHI, HHI and AHI, respectively. The clay content in the PP/OMMT nanocomposites was constant at 2.5 wt%. The effect of surfactant structure on the clay dispersion in the PP matrix was studied in detail. The XRD patterns for PP/OMMT composites are shown in Fig. 3. It was found that the surfactant structure played an important role on the clay dispersion in the PP matrix. The XRD curve of the PP/ DHI-MMT composite displayed no (001) diffraction peak from the MMT, indicating that the average interlayer spacing of the MMT in 8000 7000
Intensity (cps)
6000 5000 4000 3000
PP/DHI-MMT
2000
PP/HHI-MMT PP/AHI-MMT
1000
PP/HMI-MMT 5
10
15
20
25
30
2 Theta (deg) Fig. 3. XRD patterns of the PP/OMMT nanocomposites.
35
PP/DHI-MMT nanocomposites was larger than 5.8 nm according to the Bragg equation. While for PP/HHI-MMT, PP/AHI-MMT and PP/ HMI-MMT, the XRD curves showed dispersed (001) diffraction peaks at about 2q ¼ 4.86 from MMT corresponding to the average d-spacing of 1.82 nm. However, for PP/HMI-MMT sample, the (001) diffraction peak was more obvious. These results indicate that surfactant containing two long tails plays a more powerful role on the compatibility of PP and MMT, and as a result, exfoliated PP/ MMT nanocomposites could be prepared. The surfactants containing only one long tail could lead to intercalated or partly exfoliated PP/MMT nanocomposites via solution blending method. However, among these surfactants containing only one long tail, HHI with polar groups is much more effective than other surfactants without polar groups. As shown in Table 3, the melting point of pure PP granule was about 160.6 C and the ones of PP in nanocomposites were similar to that of pure PP, which indicated that the crystal structures of PP in nanocomposites were changed little. However, the cold crystallization temperature (Tc) for PP nanocomposites was higher than that of pure PP. This can be explained by the effect that MMT layers act as nucleating agent for PP and enhance the crystallization rate. The thermal decomposition temperature at onset of these PP/ OMMT materials (Tonset) was investigated by using TGA. It was found that the Tonset values of the nanocomposites were approached to that of the neat PP except the PP/AHI-MMT nanocomposites. Generally speaking, the thermal stability of the PP/clay nanocomposites should be improved compared with that of pure PP. Anyway, in this case, the pure PP used was the lab-made degradated PP containing 0.1% antioxidant. However, in our nanocomposites, most antioxidant was removed during the solution blending process. The PP/AHI-MMT nanocomposite showed a relatively low Tonset value, which might be attributed to the instability of the double bonds in AHI. Table 3 Physical properties of the PP/OMMT nanocomposites.
40
Sample
Tm ( C)
DHm (J/g)
Tc ( C)
Degradation temperature at onset ( C)
PP granule PP/HMI-MMT PP/DHI-MMT PP/HHI-MMT PP/AHI-MMT
160.6 160.5 160.6 160.0 161.3
96.9 97.4 99.4 101.4 104.8
112.2 119.1 118.7 118.5 119.5
367.7 351.3 360.5 368.9 330.2
A. He et al. / Polymer Degradation and Stability 95 (2010) 651e655
4. Conclusion Four kinds of imidazolium surfactants with high thermal stability were designed and synthesized. The structures of these surfactants were characterized and then affirmed by 1H NMR spectra. The TGA results indicated that the thermal stabilities of these surfactants with saturated alkyl groups were relatively high and the T0.05 values were higher than 250 C. Once the imidazolium surfactants were intercalated into the galleries of silicate layers, the OMMT showed obviously higher thermal stability than the surfactants themselves and the T0.05 values of OMMT were higher than 330 C. The dihexadecane imidazolium (DHI) with two long tails has the ability to enlarge the interlayer spacing to a bigger degree when compared with other imidazolium surfactants with only one long tail. It was found that exfoliated PP/OMMT nanocomposites could be prepared with DHI modified MMT by solution blending. Acknowledgements This work was financially supported by the National Nature Science Foundation of China through funds No. 20774098, No. 50973123, No. 50503023. National ‘‘973'’ Project (G2003 CB615600) is also acknowledged for its support. This work was also supported by the Scientific Research Startup Foundation of Qingdao University of Science and Technology for talents. References [1] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. Mechanical properties of nylon6eclay hybrid. J Mater Res 1993;8:1185e99. [2] Okada A, Kawasumi M. Synthesis and characterization of a nylon 6-clay hybrid. Polym Prepr Jpn 1987;28(2):447e8. [3] Hotta S, Paul DR. Nanocomposites formed from linear low density polyethylene and organoclays. Polymer 2004;45(22):7639e54. [4] Galgali G, Agarwal S, Lele A. Effect of clay orientation on the tensile modulus of polypropyleneenanoclay composites. Polymer 2004;45(17):6059e69. [5] Zanetti M, Costa L. Preparation and combustion behaviour of polymer/layer silicate nanocomposites based upon PE and EVA. Polymer 2004;45(13):4367e73. [6] Gopakumar TG, Lee JA, Kontopoulou M, Parent JS. Influence of clay exfoliation on the physical properties of montmorillonite/polyethylene composites. Polymer 2002;43(20):5483e91. [7] Qin HL, Su QS, Zhang SM, Zhao B, Yang MS. Thermal stability and flammability of polyamide 66/montmorillonite nanocomposites. Polymer 2003;44(24): 7533e8. [8] Gilman JW. Flammability and thermal stability studies of polymer layeredsilicate (clay) nanocomposites. Appl Clay Sci 1999;15(1):31e49.
655
[9] Kurokawa Y, Yasuda H, Oya A. Preparation of a nanocomposite of polypropylene and smectite. J Mater Sci Lett 1996;15(17):1481e3. [10] Sun T, Garcés MJ. High-performance polypropylene-clay nanocomposites by in situ polymerization with metallocene/clay catalysts. Adv Mater 2002;14(2): 128e30. [11] JP 09118792. N Tetsuo Chem Abstr 1997;127:51586. [12] Tudor J, Willington L, O'Hare D, Royan B. Intercalation of catalytically active metal complexes in phyllosilicates and their application as propene polymerization catalysts. Chem Commun 1996;17:2031e2. [13] Tudor J, O'Hare D. Stereospecific propene polymerization catalysis using an organometallic modified mesoporous silicate. Chem Commun 1997:603e4. [14] Bergman JS, Chen Hua, Giannelis EP, Thomas MG, Coates GW. Synthesis and characterization of polyolefin-silicate nanocomposites: a catalyst intercalation and in situ polymerization approach. Chem Commun 1999;21:2179e80. [15] Kawasumi M, Hasegawa N, Kato M, Usuki A, Okada A. Preparation and mechanical properties of polypropylene-clay hybrids. Macromolecules 1997;30(20):6333e8. [16] Hasegawa N, Kawasumi M, Kato M. Preparation and mechanical properties of polypropylene-clay hybrids using a maleic anhydride-modified polypropylene oligomer. J Appl Polym Sci 1998;67(1):87e92. [17] Nam PH, Maiti P, Okamoto M. A hierarchical structure and properties of intercalated polypropylene/clay nanocopmposites. Polymer 2001;42(23): 9633e40. [18] Reichert P, Nitz H, Klinke S, Brandsch R, Thomann R, Muelhaupt R. Poly(propylene)/organoclay nanocomposite formation: influence of compatibilizer functionality and organoclay modification. Macromol Mater Eng 2000;275(1): 8e17. [19] Zhang Q, Fu Q, Jiang L, Lei Y. Preparation and properties of poly-propylene/ montmorillonite layered nano-composites. Polym Int 2000;49(12):1561e4. [20] He AH, Hu HQ, Huang YJ, Dong JY, Han Charles C. Isotactic poly(propylene)/ monoalkylimidazolium-modified montmorillonite nanocomposites: preparation by intercalative polymerization and thermal stability study. Macromol Rapid Commun 2004;25:2008e13. [21] Du K, He AH, Liu X, Han Charles C. High-performance exfoliated poly (propylene)/clay nanocomposites by in situ polymerization with a novel Z-N/ clay compound catalyst. Macromol Rapid Commun 2007;28(24):2294e9. [22] He AH, Wang LM, Li JX, Dong JY, Han Charles C. Preparation of exfoliated isotactic polypropylene/alkyl-triphenylphosphonium-modified montmorillonite nanocomposites via in-situ intercalative polymerization. Polymer 2006; 47(6):1767e71. [23] Kurokawa Y, Yasuda H, Kashiwagi M, Oya A. Structure and properties of a montmorillonite/polypropylene nanocomposite. J Mater Sci Lett 1997;16 (20):1670e2. [24] Oya A, Kurokawa Y, Yasuda H. Factors controlling mechanical properties of clay mineral/polypropylene nanocomposites. J Mater Sci 2000;35(5): 1045e50. [25] Zanetti M, Camino G, Reichert P, Mülhaupt R. Thermal behavior of poly (propylene) layered silicate nanocomposites. Macromol Rapid Commun 2001;22:176e80. [26] Sergei VD, Richard AB. New room-temperature ionic liquids with C2symmetrical imidazolium cations. Chem Commun 2001:1466e7. [27] Khabnadideh S, Rezaei Z, Khalafi-Nezhad A, Bahrinajafi R, Mohamadi R, Farrokhroz AA. Synthesis of N-Alkylated derivatives of imidazole as antibacterial agents. Bioorg Med Chem Lett 2003;13:2863e5.