- Email: [email protected]
polyurethane nanocomposites by introducing cationic groups into the polyurethane main chain
EUROPEAN POLYMER JOURNAL
European Polymer Journal 43 (2007) 2286–2291
www.elsevier.com/locate/europolj
Macromolecular Nanotechnology – Short communication
MACROMOLECULAR NANOTECHNOLOGY
Effective preparation of montmorillonite/polyurethane nanocomposites by introducing cationic groups into the polyurethane main chain Eun Hwan Jeong a, Jie Yang a, Ji Hye Hong a, Tae Gon Kim a, Jung Hyun Kim b, Ji Ho Youk a,* a
b
Department of Advanced Fiber Engineering, Division of Nano-Systems, Inha University, Incheon 402-751, Republic of Korea Department of Chemical Engineering, Yonsei University, 134 Shinchon-Dong, Sudaemoon-Ku, Seoul 120-749, Republic of Korea Received 11 January 2007; accepted 8 March 2007 Available online 14 March 2007
Abstract In this study, the effect of introducing a small amount of cationic groups into the polymer main chain on the exfoliation of montmorillonite (MMT) and the physical properties of the subsequent MMT/polymer nanocomposites were investigated. As a matrix polymer, a polyurethane cationomer (PUC) containing 3 mol% of quaternary ammonium groups was synthesized and MMT/PUC nanocomposites containing various amounts of MMT were prepared by the solution intercalation method. From the WAXS and TEM analyses, it was found that the MMT layers were completely exfoliated and dispersed in the PUC matrix. The Young’s modulus of the MMT/PUC nanocomposites significantly increased with increasing MMT content, but their elongation at break and maximum stress were maintained at a level close to that of the PU only at an MMT content of 1 wt% and decreased as the content of MMT increased above this level. The phase separation of the MMT/PUC nanocomposites was retarded with increasing content of MMT, due to the strong interactions between the PUC chains and the exfoliated MMT layers. It was found that the presence of small amounts of cationic groups in the main chain of the matrix polymer was very effective in facilitating the preparation of the MMT/polymer nanocomposites. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Polyurethane cationomer (PUC); Montmorillonite (MMT); Nanocomposites; Quaternary ammonium groups; Exfoliation
1. Introduction Natural montmorillonite (MMT) consists of layers made up of two silicate tetrahedrons fused * Corresponding author. Tel.: +82 32 860 7498; fax: +82 32 873 0181. E-mail address: [email protected] (J.H. Youk).
to an edge-shared octahedral sheet of either aluminum or magnesium hydroxide. The thickness of the layers is around 1 nm, and their lateral dimensions vary from 30 nm to several microns or even larger [1]. The high aspect ratio and available surface area of the MMT layers enhance the mechanical properties, thermal stability, flame retardancy, and gas-barrier properties of the matrix polymers.
0014-3057/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.03.015
The key to MMT/polymer nanocomposite technology is the exfoliation of the individual MMT layers [2]. Since MMT is hydrophilic and lacks affinity with hydrophobic organic polymers, it is necessary to modify it by introducing organic cationic molecules. Usually, organically modified MMT (O-MMT) is synthesized by introducing cationic organic modifiers such as alkylammonium or alkylphosphonium between the MMT layers by a cationic-exchange reaction. These cationic organic modifiers improve the exfoliation of the MMT layers, thereby improving the thermal and mechanical properties of the MMT/polymer nanocomposites. Recently, it has been reported that polymers having cationic groups in the polymer main chain can be effectively used for the preparation of MMT/polymer nanocomposites. Zhang et al. [3] first synthesized MMT/polystyrene-b-quaternized poly(4-vinylpyridine) (PS-b-QP4VP) nanocomposites by the cationic-exchange of PS-b-QP4VP with the cations in MMT and then blended it with commercial PS. The resulting MMT/PS-b-QP4VP/ PS nanocomposites contained 3 wt% and 5 wt% MMT and the MMT was homogeneously dispersed in the PS matrix. Sen et al. [4] also used quaternized poly(4-vinylpyridine) (P4VP) and PS-b-QP4VP for the modification of MMT. It is expected that the exfoliation of the individual MMT layers can be improved by introducing small amounts of cationic groups into the main chain of the matrix polymer. Furthermore, the strong interactions between the exfoliated MMT layer and the matrix polymer having cationic groups can enhance the properties of MMT/polymer nanocomposites. However, in practice, it is difficult to introduce cationic groups into the main chain of most commercial polymers. Polyurethanes (PUs) are segmented polymers with a microphase-separated morphology. They have very desirable properties, such as high abrasion resistance, flexibility, and elasticity. However, they have some disadvantages, such as their poor thermal stability and poor gas-barrier properties [5]. These disadvantages can be at least partially overcome by controlling their microstructure or by introducing inorganic fillers [6]. Recently, MMT/ PU nanocomposites have been extensively studied in order to enhance the properties of PUs and consequently extend their fields of application [5–15]. In practice, PU is a particularly useful polymer, since cationic groups can be readily incorporated into its main chain. Usually, cationic groups have been introduced into the main chain of PU by the poly-
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merization of the base PU with chain extenders having a tertiary amino group and the subsequent quaternization of the tertiary nitrogen atoms with various acids or alkyl halides [15] The resulting PU cationomer (PUC) has been utilized in a variety of applications, such as in water treatment, coatings on medical devices, food packaging, and sanitary and health care related materials [16–18]. In this study, in order to improve the exfoliation of MMT in the PU matrix, PU containing 3 mol% of quaternary ammonium groups (the molar ratio of [N+]/[No] = 3%) was synthesized and used as the matrix polymer of MMT nanocomposites. The effect of these small amounts of cationic groups in the PU main chain on the exfoliation of MMT and the subsequently variation of the mechanical properties were investigated. 2. Experimental 2.1. Materials Poly(tetramethylene oxide) (PTMO, Mn 1800), 4,4 0 -diphenylmethylene diisocyanate (MDI), 1,4butanediol (BD), N-methyldiethanolamine (MDEA), dibutyltin dilaurate, iodomethane and N,N-dimethyl formamide (DMF), tetrahydrofuran (THF) were purchased from Aldrich Co and used as received. Cloisite-30B, as an organically modified MMT (O-MMT), was purchased from Southern Clay. The PTMO and Cloisite-30B were dried overnight at 90 °C in order to remove the moisture before use. 2.2. Synthesis of the PUC For the synthesis of the PUC, a two-step procedure was used. In the first step, isocyanate terminated prepolymers were synthesized by reacting PTMO with an excess of MDI at 50 °C for 1.5 h under a nitrogen atmosphere. In the second step, dibutyltin dilaurate was added to the prepolymers as a catalyst. A chain extension reaction of the prepolymers was carried out by dropping BD and MDEA dissolved in DMF onto them. The reaction temperature was slowly raised to 80 °C for 2 h with stirring. The subsequent molar ratio of MDI/BD/ MDEA/PTMO was 4.0/2.76/0.24/1.0. For the synthesis of the PUC, the additional quaternization of the PU backbones was carried out by introducing 1.0 equiv. of iodomethane for 1.0 equiv. of MDEA. The cationic content in the PUC was 3 mol% (the molar ratio of [N+]/[No] = 3%).
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E.H. Jeong et al. / European Polymer Journal 43 (2007) 2286–2291
E.H. Jeong et al. / European Polymer Journal 43 (2007) 2286–2291
MACROMOLECULAR NANOTECHNOLOGY
2.3. Preparation of the MMT/PU and MMT/PUC nanocomposites The MMT/PU and MMT/PUC nanocomposites were prepared via solution blending. For the preparation of the MMT/PUC nanocomposites, the PUC was dissolved in DMF at a concentration of 0.1 g/mL. A predetermined amount of Cloisite-30B was dispersed in 50 mL of DMF and ultrasonicated for 1.5 h. This dispersion was added to the PUC solution and stirred at 50 °C for 2 h. The resulting MMT/PUC solution was poured into a glass dish and dried slowly at 45 °C. The thickness of the resulting PU nanocomposite films ranged from 0.6 to 0.8 mm. The contents of O-MMT were 1, 3, 5, and 7 wt%. 2.4. Characterization The molecular weight of the base PU was determined by gel permeation chromatography (Young Lin SP930D solvent delivery pump, RI750F refractive index detector). THF was used as an eluent at a flow rate of 1.0 mL/min at 40 °C. Three columns of highly crosslinked polystyrene-divinylbenzene microspheres (PLGel; Polymer Laboratories, Inc.) ˚ were used with pore sizes of 103, 104 and 105 A for separation. Polystyrene standards were used for calibration. A wide angle X-ray scattering (WAXS) analysis was carried out using a Rigaku DMAX-2500 diffractometer to measure the d-spacing of O-MMT. Each sample was scanned from 2h = 1–10° at a scan rate of 0.01°/min. The wavelength of the X-ray beam was 0.154 nm (Cu Ka radiation). The transmission electron microscope (TEM) images of the PU nanocomposites were obtained using a transmission electron microscopy (Philips CM 200) with an operating voltage of 120 kV. The TEM specimens were prepared using a MTX ultra-microtome with a cryogenic system. Tensile tests were preformed using an Instron (Hounsfield, H10K-S) UTM according to ASTM D-638. All of the samples were punched out using an ASTM D-638 type-IV die. The crosshead speed was 25 mm/min. The small angle X-ray scattering (SAXS) measurements were performed using a Rigaku DMAX-2500 diffractometer. The experiments were carried out at room temperature using CuKa radiation (k = 0.154 nm, wavelength), operating at 40 kV and 200 mA. Each scan was recorded in the range of 2h = 0.08–2° in step-by-step mode with increments of 0.01°. The absolute intensity data
are presented as a function of the magnitude of the scattering vector, q, where q = 4p sin h/k, and 2h is the scattering angle. 3. Results and discussion In this study, PUC containing 3 mol% of quaternary ammonium groups was synthesized and used as the matrix polymer of MMT nanocomposites. The number average molecular weight and polydispersity index of the base PU were 52,000 g/mol and 2.56, respectively. In our previous study [19], the quaternization of the base PU was confirmed by XPS analysis. The MMT/PU and MMT/PUC nanocomposites were prepared using a solution intercalation method. The weight percentages of O-MMT were 1, 3, 5, and 7 wt%. Figs. 1 and 2 show the WAXS patterns of O-MMT and the MMT/PU nanocomposites, and the MMT/PUC nanocomposites, respectively. The d-spacing of the pristine O-MMT was 1.85 nm (2h = 4.8°). Generally, the d-spacing of organoclay in polymer nanocomposites depends on the method of preparing the nanocomposites and the interaction between the polymer matrix and organoclay [17,18]. This diffraction peak was slightly shifted to lower angles for all of the MMT/PU nanocomposites, indicating that most of the MMT was not successfully exfoliated. However, for all of the MMT/PUC nanocomposites, this
(a) (b) (c) (d) (e)
1 wt% MMT/PU 3 wt% MMT/PU 5 wt% MMT/PU 7 wt% MMT/PU O-MMT
Intensity
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(e) (d) (c) (b) (a)
2
4
6
8
10
2θ (degree) Fig. 1. WAXS patterns of O-MMT and the MMT/PU nanocomposites.
E.H. Jeong et al. / European Polymer Journal 43 (2007) 2286–2291
1 wt% MMT/PUC 3 wt% MMT/PUC 5 wt% MMT/PUC 7 wt% MMT/PUC
Intensity
(a) (b) (c) (d)
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(d) (c) (b)
2
4
6 2θ (degree)
8
10
Fig. 2. WAXD patterns of the MMT/PUC nanocomposites.
diffraction peak completely disappeared, implying that most of the MMT was successfully exfoliated and well dispersed in the PUC matrix. Pattanayak and Jana [14] prepared Cloisite-30B/PU nanocomposites and characterized their various properties in terms of the degree of MMT exfoliation. Cloisite-30B contains the quaternary ammonium ion, N+(CH2CH2OH)2(CH3)T, as a cationic organic modifier, where T represents an alkyl group with approximately 65% C18H37, 30% C16H33, and 5% C14H29. The –CH2CH2OH groups reacted with the –NCO groups and produced MMT-tethered PU chains. When the content of Cloisite-30B was 1 and 3 wt%, a certain amount of intercalated MMT structures was observed by the WAXD and TEM analyses. However, interestingly, when the content of Cloisite-30B was 5 wt%, an exfoliated MMT structure was observed and it was assumed that this resulted from an increase in the number of MMTPU tethering reactions. These results imply that the complete exfoliation of the Cloisite-30B in the PU matrix is difficult to achieve and the MMT-tethered PU chains can successfully improve the exfoliation and dispersion of MMT, which is in good agreement with the results obtained in this study. Practically, the PUC has small amounts of cationic groups and that they can be tethered to MMT, resulting in the complete exfoliation of Cloisite30B. Fig. 3 shows the TEM micrographs of the 5 wt% and 7 wt% MMT/PUC nanocomposites. The exfoliated MMT layers were well distributed within both the 5 wt% and 7 wt% MMT/PUC nano-
Fig. 3. TEM micrographs of (a) 5 wt% MMT/PUC and (b) 7 wt% MMT/PUC nanocomposites.
composites. It was found that the dispersion of the MMT layers in the PUC matrix was also affected by the cationic groups in the PUC main chain. Fig. 4 shows the stress–strain curves of the PU, PUC, and MMT/PUC nanocomposites and their mechanical properties are summarized in Table 1. All of the samples were annealed at 110 °C for 24 h prior to the measurements. The PU and PUC showed very similar mechanical properties. The Young’s modulus of the MMT/PUC nanocomposites increased with increasing MMT content. The enhancement of the Young’s modulus is directly attributed to the reinforcement effects provided by the dispersed MMT layers. The Young’s modulus of the MMT/polymer nanocomposites is dependent
MACROMOLECULAR NANOTECHNOLOGY
(a)
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E.H. Jeong et al. / European Polymer Journal 43 (2007) 2286–2291
6 (a) (b) (c) (d) (e) (f)
(c) (b) (a)
5
4
(e)
Intensity
2
Stress (kgf/mm )
(f)
3 (a) (b) (c) (d) (e) (f)
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1
PU PUC 1 wt% MMT/PUC 3 wt% MMT/PUC 5 wt% MMT/PUC 7 wt% MMT/PUC
200
400
600
800
1000
Fig. 4. Tensile properties of the PU, PUC, and MMT/PUC nanocomposites.
of
the
PU,
PUC,
0.00
0.02
0.04
0.06
0.08
0.10
0.12
-1
q (A )
Elongation (%)
and
MMT/PUC
Samples
Maximum stress (kgf/mm2)
Elongation at break (%)
Young’s modulus (kgf/mm2)
PU PUC 1 wt% 3 wt% 5 wt% 7 wt%
4.88 4.90 4.93 3.86 3.66 3.45
806 824 760 568 495 487
4.11 4.15 5.60 6.26 7.20 9.43
MMT/PUC MMT/PUC MMT/PUC MMT/PUC
(c)
(a)
0
Table 1 Tensile properties nanocomposites
(d)
(b)
(f) (e) (d)
2
0
PU PUC 1 wt% MMT/PUC 3 wt% MMT/PUC 5 wt% MMT/PUC 7 wt% MMT/PUC
on the interfacial interaction between the MMT layers and polymer matrix [20]. However, the elongation at break and maximum stress of the MMT/ PUC nanocomposites were maintained at a level close to that of the PU only at an MMT content of 1 wt%, and decreased as the content of MMT was further increased. Tien and Wei [8] prepared both MMT-tethered PU and MMT-untethered PU nanocomposites and observed that the enhancement in the mechanical properties of the PU was much larger in the former nanocomposite. They suggested that the dramatic increase in the elongation at break and maximum stress was attributed to the reinforced branched structure formed by the tetheredMMT in the PU. However, in this study, although the MMT/PU nanocomposites have a branched
Fig. 5. SAXS patterns of the PU, PUC, and MMT/PUC nanocomposites.
structure formed by the tethered-MMT, their elongation at break and maximum stress were not increased. In order to understand this result, the microphase separation of the PU, PUC, and MMT/PUC nanocomposites was investigated by SAXS. Fig. 5 shows the SAXS patterns of the PU, PUC, and MMT/PUC nanocomposites. The peak at q 0.032 A1 corresponds to the scattering resulting from the periodicity in the PU microdomain structure. The intensity of the PU peak is related to the degree of microphase separation. The peak maxima of the PU and PUC were very similar, indicating that the microphase separation of the PUC was not affected by the presence of small amounts of quaternary ammonium groups in the main chain. However, the higher the content of MMT in the MMT/PUC nanocomposites, the less phase-separation occurred in the matrix PUC. This result indicates that the microphase separation was retarded by the completely exfoliated MMT layers. Finnigan et al. [21] synthesized PTMO-based PU nanocomposites by incorporating 3 and 7 wt% of Cloisite-15A via melt compounding and solvent casting. Their WAXS patterns revealed a small increase in the interlayer spacing and large amounts of tactoids were observed by TEM. At a loading of 7 wt%, the decrease in the elongation at break and maximum stress was explained by the reduction of the mobility of the PU chains and the presence of large amounts of tactoids. These tactoids did not
E.H. Jeong et al. / European Polymer Journal 43 (2007) 2286–2291
4. Conclusion PUC containing 3 mol% of quaternary ammonium groups was synthesized and used as the matrix polymer of MMT nanocomposites. The number average molecular weight of the base PU was 52,000 g/mol. The MMT/CPU nanocomposites were successfully prepared by the solution intercalation method. The diffraction peak ascribed to the d-spacing of O-MMT in the WAXS patterns completely disappeared for all contents of O-MMT, implying that most of the MMT was completely exfoliated and well dispersed in the PUC matrix. Further evidence of the dispersion of the MMT layers was provided by the TEM micrographs. The Young’s modulus of the MMT/PUC nanocomposites were significantly increased with increasing content of MMT, however their elongation at break and maximum stress were maintained at a level close to that of the PU only at an MMT content of 1 wt% and decreased as the content of MMT increased above this level. The higher the content of MMT in the MMT/PUC nanocomposites, the less phase-separated was the PUC, indicating that the microphase separation was retarded by the exfoliated MMT layers.
Acknowledgements The authors of this study would like to thank the Korea Science and Engineering Foundation (KOSEF) for sponsoring this research through the SRC/ERC Program of MOST/KOSEF (R11-2005065) and Ministry of Commerce, Industry and Energy (MOCIE) through the project of NGNT (No. 10024135-2005-11). References [1] Ray SS, Okamoto M. Prog Polym Sci 2003;28:1539–641. [2] Berta M, Lindsay C, Pans G, Camino G. Polym Degrad Stabil 2006;91:1179–91. [3] Zhang BQ, Chen GD, Pan CY, Luan B, Hong CY. J Appl Polym Sci 2006;102:1950–8. [4] Sen S, Nugay N, Nugay T. Polym Int 2006;55:552–7. [5] Yao KJ, Song M, Hourston DJ, Luo DZ. Polymer 2002;43:1017–20. [6] Zilg C, Thomann R, Mu¨lhaupt R, Finter J. Adv Mater 1999;11:49–52. [7] Chen TK, Tien YI, Wei KH. Polymer 2000;41:1345–53. [8] Tien YI, Wei KH. Macromolecules 2001;34:9045–52. [9] Tien YI, Wei KH. J Appl Polym Sci 2002;86:1741–8. [10] Tortora M, Gorrasi G, Vittoria V, Galli G, Ritrovati S, Chiellini E. Polymer 2002;43:6147–57. [11] Zhang X, Xu R, Wu Z, Zhou C. Polym Int 2003;52:790–4. [12] Xiong J, Liu Y, Yang X, Wang X. Polym Degrad Stabil 2004;86:549–55. [13] Song L, Hu Y, Tang Y, Zhang R, Chen Z, Fan W. Polym Degrad Stabil 2005;87:111–6. [14] Pattanayak A, Jana SC. Polymer 2005;46:3275–88. [15] Buruiana EC, Buruiana T. J Photochem Photobiol A: Chem 2002;151:237–52. [16] Kenawy ER, Abdel-Hay FI, El-Shanshoury AERR, ElNewehy MH. J Polym Sci Polym Chem 2002;40:2384–93. [17] Oh ST, Ha CS, Cho WJ. J Appl Polym Sci 1994;54:859–66. [18] Grapski JA, Cooper SL. Biomaterials 2001;22:2239–46. [19] Jeong EH, Yang J, Youk JH. Mater Lett, accepted for publication. [20] Choi WJ, Kim SH, Kim YJ, Kim SC. Polymer 2004; 45:6045–57. [21] Finnigan B, Martin D, Halley P, Truss R, Campbell K. J Appl Polym Sci 2005;97:300–9. [22] Zheng J, Ozisik R, Siegel RW. Polymer 2006;47:7786–94.
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affect the microphase morphology of the PU nanocomposites. However, in this study, no tactoids were observed in the MMT/PUC nanocomposites. Zheng et al. [22] recently reported that the phase separation in PU could be disrupted by the addition of a small amount of nano-size fillers. When 5 wt% of Cloisite-20A was added to the PU, its Young’s modulus and maximum stress decreased by more than 80%. The disruption of the phase separation of the PU resulted in a decrease of the modulus and maximum stress. Therefore, it can be concluded that the decrease in the elongation at break and maximum stress of the MMT/PUC nanocomposites was due to the less phase-separated morphology of the PUC caused by the strong interactions between the PUC and the exfoliated MMT layers.
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European Polymer Journal 43 (2007) 2286–2291
www.elsevier.com/locate/europolj
Macromolecular Nanotechnology – Short communication
MACROMOLECULAR NANOTECHNOLOGY
Effective preparation of montmorillonite/polyurethane nanocomposites by introducing cationic groups into the polyurethane main chain Eun Hwan Jeong a, Jie Yang a, Ji Hye Hong a, Tae Gon Kim a, Jung Hyun Kim b, Ji Ho Youk a,* a
b
Department of Advanced Fiber Engineering, Division of Nano-Systems, Inha University, Incheon 402-751, Republic of Korea Department of Chemical Engineering, Yonsei University, 134 Shinchon-Dong, Sudaemoon-Ku, Seoul 120-749, Republic of Korea Received 11 January 2007; accepted 8 March 2007 Available online 14 March 2007
Abstract In this study, the effect of introducing a small amount of cationic groups into the polymer main chain on the exfoliation of montmorillonite (MMT) and the physical properties of the subsequent MMT/polymer nanocomposites were investigated. As a matrix polymer, a polyurethane cationomer (PUC) containing 3 mol% of quaternary ammonium groups was synthesized and MMT/PUC nanocomposites containing various amounts of MMT were prepared by the solution intercalation method. From the WAXS and TEM analyses, it was found that the MMT layers were completely exfoliated and dispersed in the PUC matrix. The Young’s modulus of the MMT/PUC nanocomposites significantly increased with increasing MMT content, but their elongation at break and maximum stress were maintained at a level close to that of the PU only at an MMT content of 1 wt% and decreased as the content of MMT increased above this level. The phase separation of the MMT/PUC nanocomposites was retarded with increasing content of MMT, due to the strong interactions between the PUC chains and the exfoliated MMT layers. It was found that the presence of small amounts of cationic groups in the main chain of the matrix polymer was very effective in facilitating the preparation of the MMT/polymer nanocomposites. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Polyurethane cationomer (PUC); Montmorillonite (MMT); Nanocomposites; Quaternary ammonium groups; Exfoliation
1. Introduction Natural montmorillonite (MMT) consists of layers made up of two silicate tetrahedrons fused * Corresponding author. Tel.: +82 32 860 7498; fax: +82 32 873 0181. E-mail address: [email protected] (J.H. Youk).
to an edge-shared octahedral sheet of either aluminum or magnesium hydroxide. The thickness of the layers is around 1 nm, and their lateral dimensions vary from 30 nm to several microns or even larger [1]. The high aspect ratio and available surface area of the MMT layers enhance the mechanical properties, thermal stability, flame retardancy, and gas-barrier properties of the matrix polymers.
0014-3057/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.03.015
The key to MMT/polymer nanocomposite technology is the exfoliation of the individual MMT layers [2]. Since MMT is hydrophilic and lacks affinity with hydrophobic organic polymers, it is necessary to modify it by introducing organic cationic molecules. Usually, organically modified MMT (O-MMT) is synthesized by introducing cationic organic modifiers such as alkylammonium or alkylphosphonium between the MMT layers by a cationic-exchange reaction. These cationic organic modifiers improve the exfoliation of the MMT layers, thereby improving the thermal and mechanical properties of the MMT/polymer nanocomposites. Recently, it has been reported that polymers having cationic groups in the polymer main chain can be effectively used for the preparation of MMT/polymer nanocomposites. Zhang et al. [3] first synthesized MMT/polystyrene-b-quaternized poly(4-vinylpyridine) (PS-b-QP4VP) nanocomposites by the cationic-exchange of PS-b-QP4VP with the cations in MMT and then blended it with commercial PS. The resulting MMT/PS-b-QP4VP/ PS nanocomposites contained 3 wt% and 5 wt% MMT and the MMT was homogeneously dispersed in the PS matrix. Sen et al. [4] also used quaternized poly(4-vinylpyridine) (P4VP) and PS-b-QP4VP for the modification of MMT. It is expected that the exfoliation of the individual MMT layers can be improved by introducing small amounts of cationic groups into the main chain of the matrix polymer. Furthermore, the strong interactions between the exfoliated MMT layer and the matrix polymer having cationic groups can enhance the properties of MMT/polymer nanocomposites. However, in practice, it is difficult to introduce cationic groups into the main chain of most commercial polymers. Polyurethanes (PUs) are segmented polymers with a microphase-separated morphology. They have very desirable properties, such as high abrasion resistance, flexibility, and elasticity. However, they have some disadvantages, such as their poor thermal stability and poor gas-barrier properties [5]. These disadvantages can be at least partially overcome by controlling their microstructure or by introducing inorganic fillers [6]. Recently, MMT/ PU nanocomposites have been extensively studied in order to enhance the properties of PUs and consequently extend their fields of application [5–15]. In practice, PU is a particularly useful polymer, since cationic groups can be readily incorporated into its main chain. Usually, cationic groups have been introduced into the main chain of PU by the poly-
2287
merization of the base PU with chain extenders having a tertiary amino group and the subsequent quaternization of the tertiary nitrogen atoms with various acids or alkyl halides [15] The resulting PU cationomer (PUC) has been utilized in a variety of applications, such as in water treatment, coatings on medical devices, food packaging, and sanitary and health care related materials [16–18]. In this study, in order to improve the exfoliation of MMT in the PU matrix, PU containing 3 mol% of quaternary ammonium groups (the molar ratio of [N+]/[No] = 3%) was synthesized and used as the matrix polymer of MMT nanocomposites. The effect of these small amounts of cationic groups in the PU main chain on the exfoliation of MMT and the subsequently variation of the mechanical properties were investigated. 2. Experimental 2.1. Materials Poly(tetramethylene oxide) (PTMO, Mn 1800), 4,4 0 -diphenylmethylene diisocyanate (MDI), 1,4butanediol (BD), N-methyldiethanolamine (MDEA), dibutyltin dilaurate, iodomethane and N,N-dimethyl formamide (DMF), tetrahydrofuran (THF) were purchased from Aldrich Co and used as received. Cloisite-30B, as an organically modified MMT (O-MMT), was purchased from Southern Clay. The PTMO and Cloisite-30B were dried overnight at 90 °C in order to remove the moisture before use. 2.2. Synthesis of the PUC For the synthesis of the PUC, a two-step procedure was used. In the first step, isocyanate terminated prepolymers were synthesized by reacting PTMO with an excess of MDI at 50 °C for 1.5 h under a nitrogen atmosphere. In the second step, dibutyltin dilaurate was added to the prepolymers as a catalyst. A chain extension reaction of the prepolymers was carried out by dropping BD and MDEA dissolved in DMF onto them. The reaction temperature was slowly raised to 80 °C for 2 h with stirring. The subsequent molar ratio of MDI/BD/ MDEA/PTMO was 4.0/2.76/0.24/1.0. For the synthesis of the PUC, the additional quaternization of the PU backbones was carried out by introducing 1.0 equiv. of iodomethane for 1.0 equiv. of MDEA. The cationic content in the PUC was 3 mol% (the molar ratio of [N+]/[No] = 3%).
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E.H. Jeong et al. / European Polymer Journal 43 (2007) 2286–2291
E.H. Jeong et al. / European Polymer Journal 43 (2007) 2286–2291
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2.3. Preparation of the MMT/PU and MMT/PUC nanocomposites The MMT/PU and MMT/PUC nanocomposites were prepared via solution blending. For the preparation of the MMT/PUC nanocomposites, the PUC was dissolved in DMF at a concentration of 0.1 g/mL. A predetermined amount of Cloisite-30B was dispersed in 50 mL of DMF and ultrasonicated for 1.5 h. This dispersion was added to the PUC solution and stirred at 50 °C for 2 h. The resulting MMT/PUC solution was poured into a glass dish and dried slowly at 45 °C. The thickness of the resulting PU nanocomposite films ranged from 0.6 to 0.8 mm. The contents of O-MMT were 1, 3, 5, and 7 wt%. 2.4. Characterization The molecular weight of the base PU was determined by gel permeation chromatography (Young Lin SP930D solvent delivery pump, RI750F refractive index detector). THF was used as an eluent at a flow rate of 1.0 mL/min at 40 °C. Three columns of highly crosslinked polystyrene-divinylbenzene microspheres (PLGel; Polymer Laboratories, Inc.) ˚ were used with pore sizes of 103, 104 and 105 A for separation. Polystyrene standards were used for calibration. A wide angle X-ray scattering (WAXS) analysis was carried out using a Rigaku DMAX-2500 diffractometer to measure the d-spacing of O-MMT. Each sample was scanned from 2h = 1–10° at a scan rate of 0.01°/min. The wavelength of the X-ray beam was 0.154 nm (Cu Ka radiation). The transmission electron microscope (TEM) images of the PU nanocomposites were obtained using a transmission electron microscopy (Philips CM 200) with an operating voltage of 120 kV. The TEM specimens were prepared using a MTX ultra-microtome with a cryogenic system. Tensile tests were preformed using an Instron (Hounsfield, H10K-S) UTM according to ASTM D-638. All of the samples were punched out using an ASTM D-638 type-IV die. The crosshead speed was 25 mm/min. The small angle X-ray scattering (SAXS) measurements were performed using a Rigaku DMAX-2500 diffractometer. The experiments were carried out at room temperature using CuKa radiation (k = 0.154 nm, wavelength), operating at 40 kV and 200 mA. Each scan was recorded in the range of 2h = 0.08–2° in step-by-step mode with increments of 0.01°. The absolute intensity data
are presented as a function of the magnitude of the scattering vector, q, where q = 4p sin h/k, and 2h is the scattering angle. 3. Results and discussion In this study, PUC containing 3 mol% of quaternary ammonium groups was synthesized and used as the matrix polymer of MMT nanocomposites. The number average molecular weight and polydispersity index of the base PU were 52,000 g/mol and 2.56, respectively. In our previous study [19], the quaternization of the base PU was confirmed by XPS analysis. The MMT/PU and MMT/PUC nanocomposites were prepared using a solution intercalation method. The weight percentages of O-MMT were 1, 3, 5, and 7 wt%. Figs. 1 and 2 show the WAXS patterns of O-MMT and the MMT/PU nanocomposites, and the MMT/PUC nanocomposites, respectively. The d-spacing of the pristine O-MMT was 1.85 nm (2h = 4.8°). Generally, the d-spacing of organoclay in polymer nanocomposites depends on the method of preparing the nanocomposites and the interaction between the polymer matrix and organoclay [17,18]. This diffraction peak was slightly shifted to lower angles for all of the MMT/PU nanocomposites, indicating that most of the MMT was not successfully exfoliated. However, for all of the MMT/PUC nanocomposites, this
(a) (b) (c) (d) (e)
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4
6
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2θ (degree) Fig. 1. WAXS patterns of O-MMT and the MMT/PU nanocomposites.
E.H. Jeong et al. / European Polymer Journal 43 (2007) 2286–2291
1 wt% MMT/PUC 3 wt% MMT/PUC 5 wt% MMT/PUC 7 wt% MMT/PUC
Intensity
(a) (b) (c) (d)
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(d) (c) (b)
2
4
6 2θ (degree)
8
10
Fig. 2. WAXD patterns of the MMT/PUC nanocomposites.
diffraction peak completely disappeared, implying that most of the MMT was successfully exfoliated and well dispersed in the PUC matrix. Pattanayak and Jana [14] prepared Cloisite-30B/PU nanocomposites and characterized their various properties in terms of the degree of MMT exfoliation. Cloisite-30B contains the quaternary ammonium ion, N+(CH2CH2OH)2(CH3)T, as a cationic organic modifier, where T represents an alkyl group with approximately 65% C18H37, 30% C16H33, and 5% C14H29. The –CH2CH2OH groups reacted with the –NCO groups and produced MMT-tethered PU chains. When the content of Cloisite-30B was 1 and 3 wt%, a certain amount of intercalated MMT structures was observed by the WAXD and TEM analyses. However, interestingly, when the content of Cloisite-30B was 5 wt%, an exfoliated MMT structure was observed and it was assumed that this resulted from an increase in the number of MMTPU tethering reactions. These results imply that the complete exfoliation of the Cloisite-30B in the PU matrix is difficult to achieve and the MMT-tethered PU chains can successfully improve the exfoliation and dispersion of MMT, which is in good agreement with the results obtained in this study. Practically, the PUC has small amounts of cationic groups and that they can be tethered to MMT, resulting in the complete exfoliation of Cloisite30B. Fig. 3 shows the TEM micrographs of the 5 wt% and 7 wt% MMT/PUC nanocomposites. The exfoliated MMT layers were well distributed within both the 5 wt% and 7 wt% MMT/PUC nano-
Fig. 3. TEM micrographs of (a) 5 wt% MMT/PUC and (b) 7 wt% MMT/PUC nanocomposites.
composites. It was found that the dispersion of the MMT layers in the PUC matrix was also affected by the cationic groups in the PUC main chain. Fig. 4 shows the stress–strain curves of the PU, PUC, and MMT/PUC nanocomposites and their mechanical properties are summarized in Table 1. All of the samples were annealed at 110 °C for 24 h prior to the measurements. The PU and PUC showed very similar mechanical properties. The Young’s modulus of the MMT/PUC nanocomposites increased with increasing MMT content. The enhancement of the Young’s modulus is directly attributed to the reinforcement effects provided by the dispersed MMT layers. The Young’s modulus of the MMT/polymer nanocomposites is dependent
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6 (a) (b) (c) (d) (e) (f)
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1
PU PUC 1 wt% MMT/PUC 3 wt% MMT/PUC 5 wt% MMT/PUC 7 wt% MMT/PUC
200
400
600
800
1000
Fig. 4. Tensile properties of the PU, PUC, and MMT/PUC nanocomposites.
of
the
PU,
PUC,
0.00
0.02
0.04
0.06
0.08
0.10
0.12
-1
q (A )
Elongation (%)
and
MMT/PUC
Samples
Maximum stress (kgf/mm2)
Elongation at break (%)
Young’s modulus (kgf/mm2)
PU PUC 1 wt% 3 wt% 5 wt% 7 wt%
4.88 4.90 4.93 3.86 3.66 3.45
806 824 760 568 495 487
4.11 4.15 5.60 6.26 7.20 9.43
MMT/PUC MMT/PUC MMT/PUC MMT/PUC
(c)
(a)
0
Table 1 Tensile properties nanocomposites
(d)
(b)
(f) (e) (d)
2
0
PU PUC 1 wt% MMT/PUC 3 wt% MMT/PUC 5 wt% MMT/PUC 7 wt% MMT/PUC
on the interfacial interaction between the MMT layers and polymer matrix [20]. However, the elongation at break and maximum stress of the MMT/ PUC nanocomposites were maintained at a level close to that of the PU only at an MMT content of 1 wt%, and decreased as the content of MMT was further increased. Tien and Wei [8] prepared both MMT-tethered PU and MMT-untethered PU nanocomposites and observed that the enhancement in the mechanical properties of the PU was much larger in the former nanocomposite. They suggested that the dramatic increase in the elongation at break and maximum stress was attributed to the reinforced branched structure formed by the tetheredMMT in the PU. However, in this study, although the MMT/PU nanocomposites have a branched
Fig. 5. SAXS patterns of the PU, PUC, and MMT/PUC nanocomposites.
structure formed by the tethered-MMT, their elongation at break and maximum stress were not increased. In order to understand this result, the microphase separation of the PU, PUC, and MMT/PUC nanocomposites was investigated by SAXS. Fig. 5 shows the SAXS patterns of the PU, PUC, and MMT/PUC nanocomposites. The peak at q 0.032 A1 corresponds to the scattering resulting from the periodicity in the PU microdomain structure. The intensity of the PU peak is related to the degree of microphase separation. The peak maxima of the PU and PUC were very similar, indicating that the microphase separation of the PUC was not affected by the presence of small amounts of quaternary ammonium groups in the main chain. However, the higher the content of MMT in the MMT/PUC nanocomposites, the less phase-separation occurred in the matrix PUC. This result indicates that the microphase separation was retarded by the completely exfoliated MMT layers. Finnigan et al. [21] synthesized PTMO-based PU nanocomposites by incorporating 3 and 7 wt% of Cloisite-15A via melt compounding and solvent casting. Their WAXS patterns revealed a small increase in the interlayer spacing and large amounts of tactoids were observed by TEM. At a loading of 7 wt%, the decrease in the elongation at break and maximum stress was explained by the reduction of the mobility of the PU chains and the presence of large amounts of tactoids. These tactoids did not
E.H. Jeong et al. / European Polymer Journal 43 (2007) 2286–2291
4. Conclusion PUC containing 3 mol% of quaternary ammonium groups was synthesized and used as the matrix polymer of MMT nanocomposites. The number average molecular weight of the base PU was 52,000 g/mol. The MMT/CPU nanocomposites were successfully prepared by the solution intercalation method. The diffraction peak ascribed to the d-spacing of O-MMT in the WAXS patterns completely disappeared for all contents of O-MMT, implying that most of the MMT was completely exfoliated and well dispersed in the PUC matrix. Further evidence of the dispersion of the MMT layers was provided by the TEM micrographs. The Young’s modulus of the MMT/PUC nanocomposites were significantly increased with increasing content of MMT, however their elongation at break and maximum stress were maintained at a level close to that of the PU only at an MMT content of 1 wt% and decreased as the content of MMT increased above this level. The higher the content of MMT in the MMT/PUC nanocomposites, the less phase-separated was the PUC, indicating that the microphase separation was retarded by the exfoliated MMT layers.
Acknowledgements The authors of this study would like to thank the Korea Science and Engineering Foundation (KOSEF) for sponsoring this research through the SRC/ERC Program of MOST/KOSEF (R11-2005065) and Ministry of Commerce, Industry and Energy (MOCIE) through the project of NGNT (No. 10024135-2005-11). References [1] Ray SS, Okamoto M. Prog Polym Sci 2003;28:1539–641. [2] Berta M, Lindsay C, Pans G, Camino G. Polym Degrad Stabil 2006;91:1179–91. [3] Zhang BQ, Chen GD, Pan CY, Luan B, Hong CY. J Appl Polym Sci 2006;102:1950–8. [4] Sen S, Nugay N, Nugay T. Polym Int 2006;55:552–7. [5] Yao KJ, Song M, Hourston DJ, Luo DZ. Polymer 2002;43:1017–20. [6] Zilg C, Thomann R, Mu¨lhaupt R, Finter J. Adv Mater 1999;11:49–52. [7] Chen TK, Tien YI, Wei KH. Polymer 2000;41:1345–53. [8] Tien YI, Wei KH. Macromolecules 2001;34:9045–52. [9] Tien YI, Wei KH. J Appl Polym Sci 2002;86:1741–8. [10] Tortora M, Gorrasi G, Vittoria V, Galli G, Ritrovati S, Chiellini E. Polymer 2002;43:6147–57. [11] Zhang X, Xu R, Wu Z, Zhou C. Polym Int 2003;52:790–4. [12] Xiong J, Liu Y, Yang X, Wang X. Polym Degrad Stabil 2004;86:549–55. [13] Song L, Hu Y, Tang Y, Zhang R, Chen Z, Fan W. Polym Degrad Stabil 2005;87:111–6. [14] Pattanayak A, Jana SC. Polymer 2005;46:3275–88. [15] Buruiana EC, Buruiana T. J Photochem Photobiol A: Chem 2002;151:237–52. [16] Kenawy ER, Abdel-Hay FI, El-Shanshoury AERR, ElNewehy MH. J Polym Sci Polym Chem 2002;40:2384–93. [17] Oh ST, Ha CS, Cho WJ. J Appl Polym Sci 1994;54:859–66. [18] Grapski JA, Cooper SL. Biomaterials 2001;22:2239–46. [19] Jeong EH, Yang J, Youk JH. Mater Lett, accepted for publication. [20] Choi WJ, Kim SH, Kim YJ, Kim SC. Polymer 2004; 45:6045–57. [21] Finnigan B, Martin D, Halley P, Truss R, Campbell K. J Appl Polym Sci 2005;97:300–9. [22] Zheng J, Ozisik R, Siegel RW. Polymer 2006;47:7786–94.
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affect the microphase morphology of the PU nanocomposites. However, in this study, no tactoids were observed in the MMT/PUC nanocomposites. Zheng et al. [22] recently reported that the phase separation in PU could be disrupted by the addition of a small amount of nano-size fillers. When 5 wt% of Cloisite-20A was added to the PU, its Young’s modulus and maximum stress decreased by more than 80%. The disruption of the phase separation of the PU resulted in a decrease of the modulus and maximum stress. Therefore, it can be concluded that the decrease in the elongation at break and maximum stress of the MMT/PUC nanocomposites was due to the less phase-separated morphology of the PUC caused by the strong interactions between the PUC and the exfoliated MMT layers.
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