- Email: [email protected]
clay nanocomposites via “click” chemistry
European Polymer Journal 47 (2011) 937–941
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnolgy
Poly(epsilon-caprolactone)/clay nanocomposites via ‘‘click’’ chemistry Mehmet Atilla Tasdelen ⇑ Yalova University, Faculty of Engineering, Department of Polymer Engineering, 77100 Yalova, Turkey
i n f o
Article history: Received 14 September 2010 Received in revised form 17 December 2010 Accepted 3 January 2011 Available online 21 January 2011 Keywords: Nanocomposite ‘‘Click’’ chemistry Poly(epsilon-caprolactone) Ring-opening polymerization Montmorillonite
a b s t r a c t Poly(epsilon-caprolactone)/clay nanocomposites were prepared by copper(I) catalyzed azide/alkyne cycloaddition (CuAAC) ‘‘click’’ reaction. In this method, ring-opening polymerization of epsilon-caprolactone using propargyl alcohol as the initiator has been performed to produce alkyne-functionalized PCL and the obtained polymers were subsequently attached to azide-modified clay layers by a CuAAC ‘‘click’’ reaction. The exfoliated polymer/clay nanocomposites were characterized by X-ray diffraction spectroscopy, thermogravimetric analysis and transmission electron microscopy. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Polymer/clay nanocomposite materials, in which nanosized silicate plates of clay are uniformly dispersed in the polymer matrix, exhibit excellent physical properties such as high dimensional stability, gas barrier performance, flame retardancy, and mechanical strength when compared to the pure polymer or conventional composites (micro- and macrocomposites) [1–3]. Polymer/clay nanocomposites can be formed through four main processes namely, solution exfoliation, melt intercalation, in situ polymerization, and template synthesis [4]. The solution exfoliation process requires that the polymer species and inorganic clay materials are suspended in compatible solvents. This method is mostly used with water-soluble polymers, because of the need of large amounts of solvent to ensure a good clay dispersion [5]. The melt intercalation is a solvent-free method which enables mixing of the layered silicate with the polymer matrix in the molten state. This process is only viable for thermoplastic polymers, since the polymer must be in a molten state [6]. In the in situ polymerization technique [7], the monomer, together with the initiator and/or catalyst, is swelled
⇑ Tel.: +90 226 8112658. E-mail address: [email protected] 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.01.004
within the silicate layers and the polymerization is initiated by external stimulation such as thermal [8,9], photochemical [9–12] or chemical activation [13]. The chain growth in the clay galleries leads the clay exfoliation and hence the nanocomposite formation. In the template synthesis clay layers are synthesized within the polymer matrix, using an aqueous solution (or gel) containing the polymer and the silicate building blocks [4]. However, this method has serious disadvantages including decomposition of the polymers at high temperatures required for the synthesis of clay minerals. Recently, Tasdelen et al. have introduced a conceptually different approach, namely copper(I) catalyzed azide/ alkyne cycloaddition (CuAAC) ‘‘click’’ reaction [14–17], in which exfoliation is rooted in the functional groups of the intercalant that readily react with the antagonist groups of the preformed polymers [11,13]. This way, first, alkyne-functionalized polymers were synthesized by controlled polymerization methods and the obtained polymers were subsequently anchored to azide-modified clay layers by CuAAC ‘‘click’’ reaction. Later on, this approach was expanded by several groups using different compounds, particularly low-molecular weight molecules, and resulted in nanocomposite formation. The advantage of this technique is that many kinds of polymer chains with quantitative efficiency are easily attached on the surface of silicate layers [18–20].
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M.A. Tasdelen / European Polymer Journal 47 (2011) 937–941
In this work, we report the synthesis of poly(epsiloncaprolactone) (PCL)/clay nanocomposites by combination of ring opening polymerization (ROP) and ‘‘click’’ chemistry. As the first step of this strategy, alkyne-functionalized PCL was synthesized by ROP of epsilon-caprolactone with tin(II) 2-ethyl-hexanoate as a catalyst and propargyl alcohol as an initiator. Subsequently, azido-functional montmorillonite were prepared by the conversion of hydroxyl groups of commercial clays into azides. Finally, the alkyne-PCL is attached onto the surface and into the interlayer of the montmorillonite, using the CuAAC reaction between azides and alkynes. 2. Experimental
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2.1. Materials Azide-functionalized montmorillonite clay (MMTAN3) [11,13] was synthesized as described previously. Organically modified clay, Cloisite 30B, was purchased from Southern Clay products, Gonzales, TX, USA. The organic content of the organo-modified montmorillonite, determined by thermal gravimetric analysis (TGA), was 21 wt.%. Before use, the clay was dried under vacuum at 110 °C for 1 h. Sodium azide (NaN3, Acros 99%), copper(II)sulfate.5H2O (Cu(II)SO4, Acros 99%), L-ascorbic acid sodium salt (NaAsc, Acros 99%), methanesulfonyl chloride (Acros 99.5%) and tin(II) 2-ethyl-hexanoate (Sn(Oct)2, Aldrich, 95%) were used as received. Epsilon-caprolactone (CL, Aldrich, 97%) was vacuum distilled over calcium hydride. Other solvents and chemicals were purified by conventional drying and distillation procedures. 2.2. Synthesis of alkyne-functionalized PCL Alkyne-functionalized poly(epsilon-caprolactone) was obtained according to the methods previously described in the literature [21] by ROP of CL using Sn(Oct)2 as a catalyst and propargyl alcohol as an initiator. Typically, propargyl alcohol (0.11 g, 1.88 mmol) and CL (8.55 g, 75 mmol) were charged in a 50-mL Schlenk flask with a magnetic stirring bar, and a solution of Sn(Oct)2 (7.6 mg, 0.019 mmol) in 0.5 mL of toluene was added using a syringe. The reactive mixture was degassed via three pump-freeze–thaw cycles and then immersed in a thermostated oil bath at 110 °C for 6 h. The obtained solid was dissolved in THF, and the solution was dropped into an excessive amount of methanol. The product was dried under vacuum overnight with a yield of 75%. 1 H NMR (CDCl3), d (TMS, ppm): 4.66 (s, 2H, CH2AC„CH), 4.00 (m, CH2O on PCL), 3.65 (t, 2H, CH2OH), 2.50 (s, 1H, CH2AC„CH), 2.35–2.27 (m, CH2C@O on PCL), 1.67–1.57 (m, CH2 on PCL), 1.40–1.38 (m, CH2 on PCL). GPC: Mn = 6500, PDI = 1.15. 2.3. Preparation of the PCL/MMT nanocomposites by ‘‘click’’ chemistry Azide-montmorillonite clay (0.25 g, 3 mmol), alkyne functionalized PCL (0.5 g, 1.5 mmol) and DMSO (20 mL)
were added in a round-bottomed flask and stirred. A solution of Cu(II)SO4 (0.02 g, 0.12 mmol) in 1 mL of water was added to the mixture, followed by addition of a solution of sodium ascorbate (0.09 g, 0.45 mmol) in 1 mL of water. The mixture was heated overnight in an oil bath at 70 °C. The particles were recovered by the same procedure as described above. 2.4. Characterization TGA was performed on Perkin–Elmer Diamond TA/TGA with a heating rate of 10 °C min under nitrogen flow. The powder X-ray diffraction (XRD) measurements were performed on a Siemens D5000 X-ray diffractometer equipped with graphite-monochromatized Cu Ka radiation (k = 1.5405 Å). Transmission electronic microscopy (TEM) micrographs were obtained with a Philips CM100 apparatus using an acceleration voltage of 100 kV. Ultrathin sections (ca. 80 nm thick) were cut at 100 °C from 3 mm thick hot-pressed plates by using a Reichert-Jung Ultracut FC4E microtome equipped with a diamond knife. Because of the large difference in electron density between silicate and polymer matrix, no selective staining was required. 3. Results and discussion Poly(epsilon-caprolactone) (PCL) is linear polyester synthesized by ROP of epsilon-caprolactone (CL) catalyzed with stannous octanoate (Sn(Oct)2) in the presence of an alcohol (initiator) [22]. The main drawback of PCL is its poor processing characteristics like low melt-strength, and for certain applications, inadequate thermo-mechanical properties like tear strength. There have been a lot of attempts to overcome these drawbacks including blending with other polymers, radiation cross-linking processes and clay nanocomposite formation [23–25]. Generally, PCL/clay nanocomposites can be obtained by both in situ polymerization [26–30] and melt intercalation [6,31] methods. For the preparation of PCL/clay nanocomposites by the CuAAC ‘‘click’’ reaction, the click components, namely, alkyne-functionalized PCL and azide-functionalized montmorillonite were prepared independently by the ROP and nucleophilic substitution reactions. Alkyne-functionalized PCL precursor was prepared by ROP of CL in bulk using Sn(Oct)2 as a catalyst and propargyl alcohol as an initiator and the molar ratio of [CL]:[propargyl alcohol]:[Sn(Oct)2] = 4000:100:1. The 1H NMR spectrum of alkyne-PCL showed the resonance signals of protons ACH2AC„CH at 4.66 and 2.50 ppm, and protons of repeating unit of PCL at 2.35–2.27, 1.67–1.57, 1.40–1.38, and 4.00 ppm, whereas the signals of protons ACH2AOH at 3.65 ppm were still observed, which indicated that ROP has completed. The GPC measurement revealed that the molecular weight (Mn,GPC) was 6500 with monomodal GPC trace and low molecular weight distribution (Mw/ Mn = 1.15). To introduce the azide group in the clay layers, the hydroxyl functions of the commercial Cloisite 30B clay were converted to methane sulfonate and subsequently transformed in the click reagent with sodium azide. It should be pointed out that the modification of the
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Scheme 1. Preparation of the PCL/MMT nanocomposites by CuAAC ‘‘click’’ reaction.
Table 1 Thermal properties of pristine PCL and PCL/MMT nanocomposites. Clay (wt.%)
Conv.a (%)
PCL-Alkyne PCL-NC 1 PCL-NC 3 PCL-NC 5
0 1 2 5
– 91 93 96
Weight loss temperatureb (°C) 10 wt.%
50 wt.%
321 334 342 345
400 402 410 425
Char yieldb (%)
<1 3.3 7.4 10.1
Determined by gravimetrically. Determined by TGA analysis under a nitrogen flow at a heating rate of 10 °C/min.
hydroxyl groups could not be performed completely, giving unaltered reaction sites which are not accessible to the click reaction. However, this would not affect the ultimate target as the existing azide units would provide sufficient click sites for successful exfoliations (vide infra). Finally, the alkyne-PCL is attached onto the surface and into the interlayer of the silicate, using the copper catalyzed (I) Huisgen 1,3 dipolar cycloaddition reaction between azides and alkynes in the presence of Cu(II)SO4 and sodium ascorbate in a mixture of water and DMSO (Scheme 1). A series of PCL/MMT nanocomposites was prepared by CuAAC ‘‘click’’ reaction using different clay loadings and the results were collected in Table 1. XRD analyses of the PCL/MMT nanocomposites that are obtained by using respectively 1, 3 and 5 wt.% of organomodified nanofiller, are consistent with an exfoliated structure (Fig. 1). After the click reaction, the d0 0 1 spacing for organo-modified MMT initially present at 4.94°, which corresponds to a basal space of 1.79 nm, disappears completely in all nanocomposite samples. This indicates that the layers are likely to be exfoliated in the matrix. On the other hand, the characteristic diffraction peaks of crystalline PCL are detected in all samples at 21.41° and 23.76°. Dubois et al. have also obtained the similar results for the PCL/MMT nanocomposites prepared by in situ polymerization in the literature [28]. Thermal degradation behavior of nanocomposites was studied by thermogravimetric analysis (TGA) under a nitrogen flow at a heating rate of 10 °C/min. Typical TGA curves for pristine PCL and its nanocomposites are shown in Fig. 2 and the overall results are collected in Table 1. From the TGA data, it is clear that the decomposition onset and mid-point degradation temperature of all nanocomposites moved significantly toward a higher temperature compared to those of the pristine polymer. For the
Fig. 1. X-ray diffraction curves of MMTAN3 and PCL/MMT nanocomposites with different clay loaded (PCL-NC 1, PCL-NC 3 and PCL-NC 5).
comparison between the nanocomposites prepared by ‘‘click’’ reaction and in situ polymerization [28], the nanocomposites showed slightly improved thermal stability. This fact confirms the quantitative anchoring of the PCL in the latter case and the advantage of this ‘‘click’’ approach compared to the in situ polymerization. The final char yield of all PCL/MMT nanocomposites is improved by increasing the clay contents. The increase in char yield indicates the reduction of the polymer’s flammability and implies good thermal stability [26]. In order to obtain more information on the nanocomposite morphology, TEM analysis of PCL/MMT nanocomposites
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a b
Samples
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M.A. Tasdelen / European Polymer Journal 47 (2011) 937–941
MACROMOLECULAR NANOTECHNOLOGY
Fig. 2. TGA thermograms of PCL/MMT nanocomposites with different clay loaded (PCL-NC 1, PCL-NC 3 and PCL-NC 5) and PCL-Alkyne under nitrogen atmosphere.
Fig. 3. TEM images of PCL/MMT nanocomposite (PCL-NC 3); intercalated (A) and exfoliated (B) structures.
was performed. As displayed in Fig. 3, exfoliated silicate sheets (as highlighted by white circles) are observed together with small stacks of intercalated montmorillonite. The same morphology and distribution were also observed for the PCL/MMT nanocomposites prepared by either melt intercalation [6,31] or in situ polymerization [28,31] techniques. These structures might result from a limited diffusion of the macromolecules within the clay layers or an incomplete activation of hydroxyl group during the azide modification. 4. Conclusions In conclusion, PCL/clay nanocomposites have been prepared by combination of ROP and CuAAC ‘‘click’’ reactions. The random dispersion of clay layers in the polymer matrix was confirmed by XRD and TEM measurements. TGA traces showed that the nanocomposites have higher thermal stabilities relative to that of the pristine PCL. The CuAAC ‘‘click’’ method is simple and distinctly different from existing techniques since it is quantitative and no byproduct is formed. Apart from these advantages, the ‘‘click’’
chemistry approach can be used to incorporate the different type of polymers onto silicate layers. Acknowledgment The author thanks to Prof. Yusuf Yagci and Prof. Filip Du Prez for their support of using equipments. References [1] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater Sci Eng R 2000;28(1–2):1–63. [2] Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater 1996;8(1):29–35. [3] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28(11):1539–641. [4] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Prog Polym Sci 2008;33(12):1119–98. [5] Ma J, Xu H, Ren JH, Yu ZZ, Mai YW. A new approach to polymer/ montmorillonite nanocomposites. Polymer 2003;44(16):4619–24. [6] Lepoittevin B, Devalckenaere M, Pantoustier N, Alexandre M, Kubies D, Calberg C, et al. Poly(epsilon-caprolactone)/clay nanocomposites prepared by melt intercalation: mechanical, thermal and rheological properties. Polymer 2002;43(14):4017–23.
[7] Tasdelen MA, Kreutzer J, Yagci Y. In situ synthesis of polymer/clay nanocomposites by living and controlled/living polymerization. Macromol Chem Phys 2010;211(3):279–85. [8] Yenice Z, Tasdelen MA, Oral A, Guler C, Yagci Y. Poly(styrene-btetrahydrofuran)/clay nanocomposites by mechanistic transformation. J Polym Sci Part A: Polym Chem 2009;47(8):2190–7. [9] Akat H, Tasdelen MA, Du Prez F, Yagci Y. Synthesis and characterization of polymer/clay nanocomposites by intercalated chain transfer agent. Eur Polym J 2008;44(7):1949–54. [10] Oral A, Tasdelen MA, Demirel AL, Yagci Y. Poly(cyclohexene oxide)/ clay nanocomposites by photoinitiated cationic polymerization via activated monomer mechanism. J Polym Sci Part A: Polym Chem 2009;47(20):5328–35. [11] Oral A, Tasdelen MA, Demirel AL, Yagci Y. Poly(methyl methacrylate)/clay nanocomposites by photoinitiated free radical polymerization using intercalated monomer. Polymer 2009;50(16):3905–10. [12] Nese A, Sen S, Tasdelen MA, Nugay N, Yagci Y. Clay-PMMA nanocomposites by photoinitiated radical polymerization using intercalated phenacyl pyridinium salt initiators. Macromol Chem Phys 2006;207(9):820–6. [13] Tasdelen MA, Van Camp W, Goethals E, Dubois P, Du Prez F, Yagci Y. Polytetrahydrofuran/clay nanocomposites by in situ polymerization and ‘‘click’’ chemistry processes. Macromolecules 2008;41(16): 6035–40. [14] Fournier D, Hoogenboom R, Schubert US. Clicking polymers: a straightforward approach to novel macromolecular architectures. Chem Soc Rev 2007;36(8):1369–80. [15] Binder WH, Sachsenhofer R. ‘Click’ chemistry in polymer and materials science. Macromol Rapid Commun 2007;28(1): 15–54. [16] Lutz JF. 1,3-Dipolar cyclo additions of azides and alkynes: a universal ligation tool in polymer and materials science. Angew Chem Int Ed 2007;46(7):1018–25. [17] Kolb HC, Finn MG, Sharpless KB. Click chemistry: diverse chemical function from a few good reactions. Angew Chem Int Ed 2001;40(11):2004–21. [18] Ye YS, Yen YC, Cheng CC, Syu YJ, Huang YJ, Chang FC. Polytriazole/ clay nanocomposites synthesized using in situ polymerization and click chemistry. Polymer 2010;51(2):430–6. [19] Chen JC, Wang HD, Luo WQ, Xiang JM, Zhang LH, Sun BB. Preparation of poly(epsilon-caprolactone)@attapulgite nanocomposites via a
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combination of controlled ring-opening polymerization and click chemistry. Colloid Polym Sci 2010;288(2):173–9. Chen JC, Xiang JM, Cai ZW, Yong H, Wang HD, Zhang LH, et al. Synthesis of hydrophobic polymer brushes on silica nanoparticles via the combination of surface-initiated ATRP, ROP and click chemistry. J Macromol Sci Pure Appl Chem 2010;47(7):655–62. Hoogenboom R, Moore BC, Schubert US. Synthesis of star-shaped poly(epsilon-caprolactone) via ‘click’ chemistry and ‘supramolecular click’ chemistry. Chem Commun 2006;38:4010–2. Woodruff MA, Hutmacher DW. The return of a forgotten polymer–polycaprolactone in the 21st century. Prog Polym Sci 2010;35(10):1217–56. Ray SS, Bousmina M. Biodegradable polymers and their layered silicate nano composites: in greening the 21st century materials world. Prog Mater Sci 2005;50(8):962–1079. Kiersnowski A, Piglowski J. Polymer-layered silicate nanocomposites based on poly(epsilon-caprolactone). Eur Polym J 2004;40(6): 1199–207. Chen BQ, Evans JRG. Poly(epsilon-caprolactone)-clay nanocomposites: structure and mechanical properties. Macromolecules 2006;39(2):747–54. Viville P, Lazzaroni R, Pollet E, Alexandre M, Dubois P. Controlled polymer grafting on single clay nanoplatelets. J Am Chem Soc 2004;126(29):9007–12. Kubies D, Pantoustier N, Dubois P, Rulmont A, Jerome R. Controlled ring-opening polymerization of epsilon-caprolactone in the presence of layered silicates and formation of nanocomposites. Macromolecules 2002;35(9):3318–20. Lepoittevin B, Pantoustier N, Devalckenaere M, Alexandre M, Kubies D, Calberg C, et al. Poly(epsilon-caprolactone)/clay nanocomposites by in-situ intercalative polymerization catalyzed by dibutyltin dimethoxide. Macromolecules 2002;35(22):8385–90. Messersmith PB, Giannelis EP. Polymer-layered silicate nanocomposites: in situ intercalative polymerization of epsiloncaprolactone in layered silicates. Chem Mater 1993;5(8):1064–6. Messersmith PB, Giannelis EP. Synthesis and barrier properties of poly(epsilon-caprolactone)-layered silicate nanocomposites. J Polym Sci Part A: Polym Chem 1995;33(7):1047–57. Lepoittevin B, Pantoustier N, Devalckenaere M, Alexandre M, Calberg C, Jerome R, et al. Polymer/layered silicate nanocomposites by combined intercalative polymerization and melt intercalation: a masterbatch process. Polymer 2003;44(7):2033–40.
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M.A. Tasdelen / European Polymer Journal 47 (2011) 937–941
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnolgy
Poly(epsilon-caprolactone)/clay nanocomposites via ‘‘click’’ chemistry Mehmet Atilla Tasdelen ⇑ Yalova University, Faculty of Engineering, Department of Polymer Engineering, 77100 Yalova, Turkey
i n f o
Article history: Received 14 September 2010 Received in revised form 17 December 2010 Accepted 3 January 2011 Available online 21 January 2011 Keywords: Nanocomposite ‘‘Click’’ chemistry Poly(epsilon-caprolactone) Ring-opening polymerization Montmorillonite
a b s t r a c t Poly(epsilon-caprolactone)/clay nanocomposites were prepared by copper(I) catalyzed azide/alkyne cycloaddition (CuAAC) ‘‘click’’ reaction. In this method, ring-opening polymerization of epsilon-caprolactone using propargyl alcohol as the initiator has been performed to produce alkyne-functionalized PCL and the obtained polymers were subsequently attached to azide-modified clay layers by a CuAAC ‘‘click’’ reaction. The exfoliated polymer/clay nanocomposites were characterized by X-ray diffraction spectroscopy, thermogravimetric analysis and transmission electron microscopy. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Polymer/clay nanocomposite materials, in which nanosized silicate plates of clay are uniformly dispersed in the polymer matrix, exhibit excellent physical properties such as high dimensional stability, gas barrier performance, flame retardancy, and mechanical strength when compared to the pure polymer or conventional composites (micro- and macrocomposites) [1–3]. Polymer/clay nanocomposites can be formed through four main processes namely, solution exfoliation, melt intercalation, in situ polymerization, and template synthesis [4]. The solution exfoliation process requires that the polymer species and inorganic clay materials are suspended in compatible solvents. This method is mostly used with water-soluble polymers, because of the need of large amounts of solvent to ensure a good clay dispersion [5]. The melt intercalation is a solvent-free method which enables mixing of the layered silicate with the polymer matrix in the molten state. This process is only viable for thermoplastic polymers, since the polymer must be in a molten state [6]. In the in situ polymerization technique [7], the monomer, together with the initiator and/or catalyst, is swelled
⇑ Tel.: +90 226 8112658. E-mail address: [email protected] 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.01.004
within the silicate layers and the polymerization is initiated by external stimulation such as thermal [8,9], photochemical [9–12] or chemical activation [13]. The chain growth in the clay galleries leads the clay exfoliation and hence the nanocomposite formation. In the template synthesis clay layers are synthesized within the polymer matrix, using an aqueous solution (or gel) containing the polymer and the silicate building blocks [4]. However, this method has serious disadvantages including decomposition of the polymers at high temperatures required for the synthesis of clay minerals. Recently, Tasdelen et al. have introduced a conceptually different approach, namely copper(I) catalyzed azide/ alkyne cycloaddition (CuAAC) ‘‘click’’ reaction [14–17], in which exfoliation is rooted in the functional groups of the intercalant that readily react with the antagonist groups of the preformed polymers [11,13]. This way, first, alkyne-functionalized polymers were synthesized by controlled polymerization methods and the obtained polymers were subsequently anchored to azide-modified clay layers by CuAAC ‘‘click’’ reaction. Later on, this approach was expanded by several groups using different compounds, particularly low-molecular weight molecules, and resulted in nanocomposite formation. The advantage of this technique is that many kinds of polymer chains with quantitative efficiency are easily attached on the surface of silicate layers [18–20].
MACROMOLECULAR NANOTECHNOLOGY
a r t i c l e
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M.A. Tasdelen / European Polymer Journal 47 (2011) 937–941
In this work, we report the synthesis of poly(epsiloncaprolactone) (PCL)/clay nanocomposites by combination of ring opening polymerization (ROP) and ‘‘click’’ chemistry. As the first step of this strategy, alkyne-functionalized PCL was synthesized by ROP of epsilon-caprolactone with tin(II) 2-ethyl-hexanoate as a catalyst and propargyl alcohol as an initiator. Subsequently, azido-functional montmorillonite were prepared by the conversion of hydroxyl groups of commercial clays into azides. Finally, the alkyne-PCL is attached onto the surface and into the interlayer of the montmorillonite, using the CuAAC reaction between azides and alkynes. 2. Experimental
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2.1. Materials Azide-functionalized montmorillonite clay (MMTAN3) [11,13] was synthesized as described previously. Organically modified clay, Cloisite 30B, was purchased from Southern Clay products, Gonzales, TX, USA. The organic content of the organo-modified montmorillonite, determined by thermal gravimetric analysis (TGA), was 21 wt.%. Before use, the clay was dried under vacuum at 110 °C for 1 h. Sodium azide (NaN3, Acros 99%), copper(II)sulfate.5H2O (Cu(II)SO4, Acros 99%), L-ascorbic acid sodium salt (NaAsc, Acros 99%), methanesulfonyl chloride (Acros 99.5%) and tin(II) 2-ethyl-hexanoate (Sn(Oct)2, Aldrich, 95%) were used as received. Epsilon-caprolactone (CL, Aldrich, 97%) was vacuum distilled over calcium hydride. Other solvents and chemicals were purified by conventional drying and distillation procedures. 2.2. Synthesis of alkyne-functionalized PCL Alkyne-functionalized poly(epsilon-caprolactone) was obtained according to the methods previously described in the literature [21] by ROP of CL using Sn(Oct)2 as a catalyst and propargyl alcohol as an initiator. Typically, propargyl alcohol (0.11 g, 1.88 mmol) and CL (8.55 g, 75 mmol) were charged in a 50-mL Schlenk flask with a magnetic stirring bar, and a solution of Sn(Oct)2 (7.6 mg, 0.019 mmol) in 0.5 mL of toluene was added using a syringe. The reactive mixture was degassed via three pump-freeze–thaw cycles and then immersed in a thermostated oil bath at 110 °C for 6 h. The obtained solid was dissolved in THF, and the solution was dropped into an excessive amount of methanol. The product was dried under vacuum overnight with a yield of 75%. 1 H NMR (CDCl3), d (TMS, ppm): 4.66 (s, 2H, CH2AC„CH), 4.00 (m, CH2O on PCL), 3.65 (t, 2H, CH2OH), 2.50 (s, 1H, CH2AC„CH), 2.35–2.27 (m, CH2C@O on PCL), 1.67–1.57 (m, CH2 on PCL), 1.40–1.38 (m, CH2 on PCL). GPC: Mn = 6500, PDI = 1.15. 2.3. Preparation of the PCL/MMT nanocomposites by ‘‘click’’ chemistry Azide-montmorillonite clay (0.25 g, 3 mmol), alkyne functionalized PCL (0.5 g, 1.5 mmol) and DMSO (20 mL)
were added in a round-bottomed flask and stirred. A solution of Cu(II)SO4 (0.02 g, 0.12 mmol) in 1 mL of water was added to the mixture, followed by addition of a solution of sodium ascorbate (0.09 g, 0.45 mmol) in 1 mL of water. The mixture was heated overnight in an oil bath at 70 °C. The particles were recovered by the same procedure as described above. 2.4. Characterization TGA was performed on Perkin–Elmer Diamond TA/TGA with a heating rate of 10 °C min under nitrogen flow. The powder X-ray diffraction (XRD) measurements were performed on a Siemens D5000 X-ray diffractometer equipped with graphite-monochromatized Cu Ka radiation (k = 1.5405 Å). Transmission electronic microscopy (TEM) micrographs were obtained with a Philips CM100 apparatus using an acceleration voltage of 100 kV. Ultrathin sections (ca. 80 nm thick) were cut at 100 °C from 3 mm thick hot-pressed plates by using a Reichert-Jung Ultracut FC4E microtome equipped with a diamond knife. Because of the large difference in electron density between silicate and polymer matrix, no selective staining was required. 3. Results and discussion Poly(epsilon-caprolactone) (PCL) is linear polyester synthesized by ROP of epsilon-caprolactone (CL) catalyzed with stannous octanoate (Sn(Oct)2) in the presence of an alcohol (initiator) [22]. The main drawback of PCL is its poor processing characteristics like low melt-strength, and for certain applications, inadequate thermo-mechanical properties like tear strength. There have been a lot of attempts to overcome these drawbacks including blending with other polymers, radiation cross-linking processes and clay nanocomposite formation [23–25]. Generally, PCL/clay nanocomposites can be obtained by both in situ polymerization [26–30] and melt intercalation [6,31] methods. For the preparation of PCL/clay nanocomposites by the CuAAC ‘‘click’’ reaction, the click components, namely, alkyne-functionalized PCL and azide-functionalized montmorillonite were prepared independently by the ROP and nucleophilic substitution reactions. Alkyne-functionalized PCL precursor was prepared by ROP of CL in bulk using Sn(Oct)2 as a catalyst and propargyl alcohol as an initiator and the molar ratio of [CL]:[propargyl alcohol]:[Sn(Oct)2] = 4000:100:1. The 1H NMR spectrum of alkyne-PCL showed the resonance signals of protons ACH2AC„CH at 4.66 and 2.50 ppm, and protons of repeating unit of PCL at 2.35–2.27, 1.67–1.57, 1.40–1.38, and 4.00 ppm, whereas the signals of protons ACH2AOH at 3.65 ppm were still observed, which indicated that ROP has completed. The GPC measurement revealed that the molecular weight (Mn,GPC) was 6500 with monomodal GPC trace and low molecular weight distribution (Mw/ Mn = 1.15). To introduce the azide group in the clay layers, the hydroxyl functions of the commercial Cloisite 30B clay were converted to methane sulfonate and subsequently transformed in the click reagent with sodium azide. It should be pointed out that the modification of the
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Scheme 1. Preparation of the PCL/MMT nanocomposites by CuAAC ‘‘click’’ reaction.
Table 1 Thermal properties of pristine PCL and PCL/MMT nanocomposites. Clay (wt.%)
Conv.a (%)
PCL-Alkyne PCL-NC 1 PCL-NC 3 PCL-NC 5
0 1 2 5
– 91 93 96
Weight loss temperatureb (°C) 10 wt.%
50 wt.%
321 334 342 345
400 402 410 425
Char yieldb (%)
<1 3.3 7.4 10.1
Determined by gravimetrically. Determined by TGA analysis under a nitrogen flow at a heating rate of 10 °C/min.
hydroxyl groups could not be performed completely, giving unaltered reaction sites which are not accessible to the click reaction. However, this would not affect the ultimate target as the existing azide units would provide sufficient click sites for successful exfoliations (vide infra). Finally, the alkyne-PCL is attached onto the surface and into the interlayer of the silicate, using the copper catalyzed (I) Huisgen 1,3 dipolar cycloaddition reaction between azides and alkynes in the presence of Cu(II)SO4 and sodium ascorbate in a mixture of water and DMSO (Scheme 1). A series of PCL/MMT nanocomposites was prepared by CuAAC ‘‘click’’ reaction using different clay loadings and the results were collected in Table 1. XRD analyses of the PCL/MMT nanocomposites that are obtained by using respectively 1, 3 and 5 wt.% of organomodified nanofiller, are consistent with an exfoliated structure (Fig. 1). After the click reaction, the d0 0 1 spacing for organo-modified MMT initially present at 4.94°, which corresponds to a basal space of 1.79 nm, disappears completely in all nanocomposite samples. This indicates that the layers are likely to be exfoliated in the matrix. On the other hand, the characteristic diffraction peaks of crystalline PCL are detected in all samples at 21.41° and 23.76°. Dubois et al. have also obtained the similar results for the PCL/MMT nanocomposites prepared by in situ polymerization in the literature [28]. Thermal degradation behavior of nanocomposites was studied by thermogravimetric analysis (TGA) under a nitrogen flow at a heating rate of 10 °C/min. Typical TGA curves for pristine PCL and its nanocomposites are shown in Fig. 2 and the overall results are collected in Table 1. From the TGA data, it is clear that the decomposition onset and mid-point degradation temperature of all nanocomposites moved significantly toward a higher temperature compared to those of the pristine polymer. For the
Fig. 1. X-ray diffraction curves of MMTAN3 and PCL/MMT nanocomposites with different clay loaded (PCL-NC 1, PCL-NC 3 and PCL-NC 5).
comparison between the nanocomposites prepared by ‘‘click’’ reaction and in situ polymerization [28], the nanocomposites showed slightly improved thermal stability. This fact confirms the quantitative anchoring of the PCL in the latter case and the advantage of this ‘‘click’’ approach compared to the in situ polymerization. The final char yield of all PCL/MMT nanocomposites is improved by increasing the clay contents. The increase in char yield indicates the reduction of the polymer’s flammability and implies good thermal stability [26]. In order to obtain more information on the nanocomposite morphology, TEM analysis of PCL/MMT nanocomposites
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Fig. 2. TGA thermograms of PCL/MMT nanocomposites with different clay loaded (PCL-NC 1, PCL-NC 3 and PCL-NC 5) and PCL-Alkyne under nitrogen atmosphere.
Fig. 3. TEM images of PCL/MMT nanocomposite (PCL-NC 3); intercalated (A) and exfoliated (B) structures.
was performed. As displayed in Fig. 3, exfoliated silicate sheets (as highlighted by white circles) are observed together with small stacks of intercalated montmorillonite. The same morphology and distribution were also observed for the PCL/MMT nanocomposites prepared by either melt intercalation [6,31] or in situ polymerization [28,31] techniques. These structures might result from a limited diffusion of the macromolecules within the clay layers or an incomplete activation of hydroxyl group during the azide modification. 4. Conclusions In conclusion, PCL/clay nanocomposites have been prepared by combination of ROP and CuAAC ‘‘click’’ reactions. The random dispersion of clay layers in the polymer matrix was confirmed by XRD and TEM measurements. TGA traces showed that the nanocomposites have higher thermal stabilities relative to that of the pristine PCL. The CuAAC ‘‘click’’ method is simple and distinctly different from existing techniques since it is quantitative and no byproduct is formed. Apart from these advantages, the ‘‘click’’
chemistry approach can be used to incorporate the different type of polymers onto silicate layers. Acknowledgment The author thanks to Prof. Yusuf Yagci and Prof. Filip Du Prez for their support of using equipments. References [1] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater Sci Eng R 2000;28(1–2):1–63. [2] Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater 1996;8(1):29–35. [3] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28(11):1539–641. [4] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Prog Polym Sci 2008;33(12):1119–98. [5] Ma J, Xu H, Ren JH, Yu ZZ, Mai YW. A new approach to polymer/ montmorillonite nanocomposites. Polymer 2003;44(16):4619–24. [6] Lepoittevin B, Devalckenaere M, Pantoustier N, Alexandre M, Kubies D, Calberg C, et al. Poly(epsilon-caprolactone)/clay nanocomposites prepared by melt intercalation: mechanical, thermal and rheological properties. Polymer 2002;43(14):4017–23.
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