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Nanohybrid film consisted of hydrophobized imogolite and various aliphatic polyesters
Accepted Manuscript Nanohybrid Film Consisted of Hydrophobized Imogolite and Various Aliphatic Polyesters Kazuhiro Shikinaka, Ayaki Abe, Kiyotaka Shigehara PII:
S0032-3861(15)30002-1
DOI:
10.1016/j.polymer.2015.05.039
Reference:
JPOL 17877
To appear in:
Polymer
Received Date: 24 April 2015 Revised Date:
20 May 2015
Accepted Date: 23 May 2015
Please cite this article as: Shikinaka K, Abe A, Shigehara K, Nanohybrid Film Consisted of Hydrophobized Imogolite and Various Aliphatic Polyesters, Polymer (2015), doi: 10.1016/ j.polymer.2015.05.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Nanohybrid Film Consisted of Hydrophobized
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Imogolite and Various Aliphatic Polyesters
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Kazuhiro Shikinaka*, Ayaki Abe, Kiyotaka Shigehara
Graduate School of Engineering, Tokyo University of Agriculture and Technology,
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Koganei 184-8588, Japan
*Author to whom correspondence should be addressed: E-mail: [email protected]
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(K.S.) Tel.: +81-42-388-7406, Fax: +81-42-381-7058
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Abstract In this paper, we created a nanohybrids consisted of imogolite nanotube and various
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aliphatic polyesters through a hydrophobization of imogolite. The hydrophobization of imogolite via covalent bonding between aluminol group on imogolite and aldehyde group in benzaldehyde was firstly achieved. The hydrophobized imogolite is well miscible with aliphatic polyester and their mixture forms the transparent films via
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casting of their chloroform dispersion. An addition of hydrophobized imogolites to aliphatic polyester such as poly(L-lactic acid), poly(ε-caprolactone), and poly(butylene succinate) brought about the plasticization of polymer films that should be induced by the miniaturization of polymer crystals via the nucleation of polymer chains on
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Keywords imogolite, nanohybrid, plasticization
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hydrophobized IGs. Especially, the nanohybrids of poly(L-lactic acid) and hydrophobized imogolite showed an oriented crystallization under stretching.
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Introduction Nanohybrids consisted of inorganic nanofiller and organic molecules have been studied with the expectation of their usage for industrial applications [1-7]. Especially, an addition of inorganic nanofillers with anisotropic shape (rod, platelet etc.) gives the excellent improvement of mechanical strength to organic polymers. Furthermore, these nanohybrids exhibit some special properties such as frame-resistance [3] and anisotropic mechanical/optical nature [7]. Recently, imogolite (later denoted IG), a rigid hollow cylindrical inorganic clay with the composition of (HO)3Al2O3SiOH [8-12], have
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received widespread attention because of their anisotropic molecular shapes with rigidity that further derived various functions and potential applications of its composite materials. IG is single-walled alumino-silicate nanotube with extremely high aspect ratios [9-15]: the external and internal diameters are approximately 2 - 3 and 1 nm,
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respectively. In contrast, their length ranges from several tens of nanometers to several micrometers. Since IG is a perfectly rigid polyelectrolyte [16,17], it has been used as the constituent of inorganic-organic nanohybrids [18-21]. The outer and inner surfaces of IG are covered with the Al(OH)2 and Si(OH) groups, respectively, where protonation–deprotonation equilibria such as [outer surface]
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Al-(OH)2 + H+ ⇌ Al(OH)O+H2 and [inner surface] Si–OH ⇌ Si–O- + H+ occur. Because these groups act as the proton-capturing and -releasing functions, IGs disperse as thin bundles or monofilaments in acidic - neutral and relatively low ionic strength aqueous media (pH ≈ 2 ~ 7) to yield opaque to transparent solutions [21]. However, IG
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is non-miscible with hydrophobic components such as aliphatic polyesters and general organic solvents due to hydroxyl groups on its surface that limit the composition of IGs with various organic molecules. In one barely instance of the composition of IGs with hydrophobic organic molecule, there is the nanohybrids consisted of IG nanotubes
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grafted poly(methyl methacrylate) [22,23] or octadecylphosphonic acid [24,25], . In this paper, we firstly tried the hydrophobization of IG nanotubes via formation of covalent bonding between aluminol groups of IG and aldehyde groups of organic molecule. Here, the lower mobility of modified molecule (benzene ring) relative to previous experimental system (alkyl chain) [22] should induce both a hydrophobization of IG surface and a promotion of nucleation of polymer chains on IG surface for crystallization that allow effective plasticizing of polymer film. Further studies on the composition of hydrophobized IG with aliphatic polyester was carried out. The physical properties and structure of obtained nanohybrids were estimated by means of the tensile strain testing, polarized optical microscopy (POM), and differential scanning calorimetry (DSC). 3
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Experimental section Chemicals. Deionized water was further purified by a Milli-Q® Advantage A10® system (MilliporeTM, Eschborn, Germany) and used throughout our experiments. Benzaldehyde (BA; TCI, Japan), AlCl3·6H2O (Kanto Chem. Co. Inc., Japan), Na4SiO4 (Junsei Chem. Co. Ltd., Japan) and other reagent grade chemicals were used as received.
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Imogolite (IG) Synthesis. The literature procedure [7] was performed in order to obtain homogeneously dispersive IGs. Aqueous solutions of AlCl3·6H2O (9.96 g in 369 mL) and Na4SiO4 (6.90 g in 362 mL) were mixed to prepare a solution containing 12.5 and 2.5 mol/L of Al and Si, respectively. The pH of the mixture was adjusted to 6.0 by rapidly adding about 26 mL of 1.0 mol/L NaOH aq. (localized high pH levels should be
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avoided by vigorous agitation), and the resulting solution was stirred for 1 h to obtain precipitates in the solution. The white precipitates were collected by centrifugation (5000 rpm for 30 min) and were then redispersed in 400 mL of water with stirring. After
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the addition of another 2400 mL of water, the solution was acidified (pH = 4.5) by adding 1.0 mol/L hydrochloric acid (7 - 8 mL), and then stirred at 100 °C continuously (should not be intermittent) for 4 days by SynFlexTM system (EYELA, Japan). After cooling to room temperature, a fine powder of sodium chloride (16.4 g, final concentration: 0.1 mol/L) was added to the solution with rapid agitation, and the resulting gel was collected by centrifugation (5000 rpm for 30 min) and then washed
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portionwise with 500 mL water on a 100 nm Millipore filter with suction. The wet products (caution, never allow to dry) were added to 1800 mL of tetrahydrofuran (THF; without stabilizer) with stirring for reprecipitation of IG, and the resulting fluffy precipitates were collected by filtration and dried in vacuo, resulting in a yield of 42%.
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Natrium and chloride in the resulting products could not be detected under the flame coloration test. The obtained IG white powder is miscible on ultra pure water. Surface modification of IG. An outline of procedure is shown in Scheme 1. A calculated amount of IG in pure water was sonicated for 4 h at 100 W at room temperature (FU-21H, SD-ULTRA. Ltd, KOREA), which was maintained by the occasional addition of ice into the sonicator bath. By this procedure, slightly opaque dispersions could be prepared, and the average length of IG in the solution was shortened to 131 nm, which was confirmed by transmission electron microscopic (TEM) observation [7]. The pH of IG dispersion (1.0 w/v%, 10 mL) was adjusted to 2.0 by adding approximately 1.0 mol/L of HCl aq. (2.0 mL). The resulting solution was mixed with 14 mol/L of BA (0.50 mL) and stirred for 72 h at 50 ºC. The white 4
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precipitates were collected by centrifugation (8000 rpm for 30 min) and then washed with 100 mL of water on a 200 nm Millipore filter with suction. The products were rinsed by acetone and dried in vacuo for 6 h at r.t. to give BA modified IG (later denoted as BAIG) as white powders typically in 20 - 30% yield.
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Cast film preparation of hybrid. A calculated amount of BA-IG in chloroform (CHCl3) was sonicated for 12 h at 100 W at room temperature (FU-21H, SD-ULTRA. Ltd, KOREA), which was maintained by the occasional addition of ice into the sonicator bath. By this procedure, slightly opaque dispersions up could be prepared. The
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BAIG dispersion was mixed with CHCl3 solution of poly(L-lactic acid) (PLA; UNITIKA, Ltd., Mw = 140,000, D ratio = 1.4%) or poly(ε-caprolactone) (PCL; Wako Pure Chem. Ind., Ltd., Mw = 70,000 - 100,000) or poly(butylene succinate) (PBS; Bionolle® 3001, Showa Denko K.K., melt flow rate = 1 - 3), respectively. The 4.0 mL
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of resulting mixture (0.2 w/v% of BAIG) was dropped, allowed to be spread on a glass surface and dried for about 2 days in air to give the hybrid films consisted of BAIG and organic polymer (later denoted as BAIG-PLA/-PCL/-PBS; thickness = approximately 100 µm). These films were further dried for 6 h at 70 ºC in vacuo before examining physical properties. The final concentration of IG in the hybrid films was adjusted to 5.0 w/v%.
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Fourier transform infrared (FT-IR) spectra. FT-IR spectra were recorded on a JASCO FT/IR-4100 spectrometer for KBr pellet. Thermogravimetric analysis (TGA). TGA was carried out on a Rigaku Thermo Plus TG8129, under nitrogen flow, at 10 ºC min-1.
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Transmission electron microscopy (TEM). TEM observations were performed using a JEM-2100 (JEOL, Tokyo, Japan) at an acceleration voltage of 200 kV. 5 µL of the sample solution was dropped on to carbon-coated grids (Oken Shouji Co., Tokyo),
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whose surface had previously been turned hydrophilic by glow discharge under reduced pressure. After 3 min, the sample on the grids was blotted by a filter paper, and then the grid was dried in ethanol vapor atmosphere according to the sample preparation method described in a previous study [26]. The digital TEM data were obtained using a slow-scan charge-coupled device (CCD) camera (Gatan USC1000, Gatan Inc.) and converted into images with a frame size of 1024 × 1024 pixels. A cold finger and a cold trap cooled with liquid nitrogen were used to prevent sample contamination by the electron beams. The length of the IG nanotubes was calculated from the TEM images using a DigitalMicrograph® (Gatan Inc., USA). Tensile measurement of films. For tensile testing, the films were cut into strips (length 20 mm, width 2 mm, thickness approximately 0.1 mm). Five millimeters from each end 5
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of the sample was clamped with polyimide tape to prevent the sample from slipping out
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of the machine. The tensile stress-strain measurements were performed on a tensile-compressive tester (Tensilon RTC-1310A, Orientec Co.) at a constant stretch rate of 100 mm/min. All measurements were carried out at ambient temperature (22 ± 2 °C) under a relative humidity (RH) = about 60%. Birefringence estimation. The center points of the sample strips described above were observed at room temperature by a polarized optical microscope (POM; BX51, Olympus, Japan) under crossed nicols. The images were obtained using a CCD camera
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(Olympus, Japan). All measurements were carried out at ambient temperature (22 ± 2 °C) under RH = about 60%.
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Differential scanning calorimetry (DSC). DSC measurement was performed on a Rigaku Thermo Plus DSC 8230 under nitrogen flow at 10 °C min-1. Results and discussion In this study, we performed the dehydration condensation between the aluminol
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groups on the surface of IG and the aldehyde groups of BA via covalent bonding as shown in Scheme 1. The reaction progress of IG and BA was confirmed by the presence of peaks originated from vibration of benzene ring and C-O-Al bonds [27] in FT-IR spectra of BAIG (Figure 1). TGA curves (Figure 2) indicate that the 47% of BAIG is
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organic component (equal to BA molecules). From these results, it was revealed that the amount of BA units is equal to 1.15 times for that of aluminum atoms on the surface of IGs, i.e., the approximately quantitative reaction of BA and IG was occurred in the presented experiment system. The dispersibility of BAIG to hydrophobic components such as CHCl3 was estimated by visual contact and TEM observation (Figure 3). As shown in Fig. 3a, the
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BAIG disperses in CHCl3 after sonication of mixture for 12 h. Because the pristine IG never disperses in CHCl3 under the same condition (Figure 3 (b)), it seems that the modification of BA gives the hydrophobic nature to IG. The BAIG CHCl3 dispersion is miscible with the PLA/PCL/PBS CHCl3 solution. The casting and drying of BAIG-polymer mixture in CHCl3 gives the films without some aggregation under visual contact (Figure S1). The DSC curves of BAIG-PLA (Figure 4) clearly indicate the uniform dispersion of BAIG in PLA matrix because of no change in the peak temperature and no emergence of additional peaks by BAIG mixing to PLA that are sensitive to the microstructure of the presented hybrid system as shown in other studies [22]. The sharp peaks in the DSC curves of BAIG-PLA and pristine PLA correspond to the melting of PLA crystals [28]. The peak of BAIG-PLA has larger 6
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area, i.e., higher enthalpy of melting (∆H) of PLA crystals, than pristine PLA that
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indicates higher crystallinity of BAIG-PLA than pristine PLA. The crystallization of PLA nucleated on the hydrophobized IG results in the increase in their crystallinity. To estimate the mechanical properties of films, the tensile stress-strain measurements were performed as shown in Figure 5. The BAIG-PLA [+BAIG] exhibited the fracture stress and strain of 44 MPa and 0.24 mm/mm, respectively, corresponding to 92% and 171% of the pristine PLA [-BAIG]. Furthermore, the yielding stress of BAIG-PLA (45 MPa) is lower than that of PLA (56 MPa). Thus, an
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addition of BAIG brings about a plasticization to PLA that is on-target result as described in Introduction part. The BAIG-PLA showed birefringence only after stretching (Figure 6 (a)). Under crossed nicols, the complete extinction was observed at the angle of
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analyzer-to-stretching direction = 0°, while maximum brightness was observed at 45°, indicating the ordering of IG nanotubes along with the stretching direction. On the other hand, such birefringence change could not be recognized in the pristine PLA. These
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POM results indicate that an introduction of BAIG prompts the oriented crystallization of PLA film because of an alignment of IG nanotubes parallel to tensile direction as shown in the composite gels consisted of organic polymer network and IGs [21]. This oriented crystallization in accordance with stretch of BAIG-PLA should cause the plasticization of films as shown in Figure 5. The BAIG-PCL and BAIG-PBS also give the films without some additional aggregation that is estimated from DSC curves (Figure S2 and S3). Furthermore, BAIG-PCL and BAIG-PBS also exhibited the higher fracture strain than pristine PCL
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and PBS (Figure 7). As shown in POM images (Figure 8), spherulites in the polymer films were miniaturized by an addition of BAIG despite of no change of ∆H (Figure S2 and S3). The nucleation of polymer chains on the surface of BAIG would prevent coarsening of polymer crystals (spherulites) in the films that diminishes the stress concentration of films as shown in other polymer-silica nanocomposite materials [29]. This structural change should induce the plasticization of the films. The plasticization of polymer films by composition of IG nanotube is firstly achieved in this study, i.e., pristine IG and alkyl chain-modified IG act only as hardening agents in previous reports [18,22]. Since an addition of nano-sized silicates (silica [29] and Laponite [30]) usually brings about hardening of polymer films, it seems that the BAIG is unique silicate modifier as plasticizing agent. The low dimensionally of IG nanotube and the high hydrophobicity of modified aromatic rings
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on IG induce both the orient crystallization of hybrid materials and the miniaturization of polymer crystals that allow plasticization of polymer film.
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Conclusion In the presented study, we achieved the hydrophobization of IG nanotubes and the effective plasticization of aliphatic polyester films by composition of hydrophobized IGs. The hydrophobization via covalent bonding between aluminol groups on IG and aldehyde groups in BA was firstly performed. An addition of hydrophobized IG should
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promote the nucleation of aliphtic polymers on IG that induces the miniaturization of spherulites, i.e., the plasticization of polymer films. Especially, the hybrid films of PLA and hydrophobized IG showed the oriented crystallization. As described above, the nanohybrid formation of aliphatic polymers and hydrophobized IGs will enable us to
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use the IG nanotubes to various applications such as biomass-based nanofiller reinforced plastics and/or the anisotropic materials. There is also the usage of the presented hybrids for various electronic industry materials (e.g., insulated cable). The
Acknowledgements
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detailed electrochemical/thermal properties of the hybrids (volume resistivity, thermal expansion rate etc.) will be studied in near future to estimate their material performance.
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This work was supported by the JGC-S Scholarship Foundation (No. 1335) and by the JSPS KAKENHI Grant Number 26870179.
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Notes and References [1] Leroy CM, Achard M-F, Babot O, Steunou N, Massé P, Livage J, Binet L, Brun N, Backov R. Chem Mater 2007; 19: 3988-99. [2] Frydrych M, Wan C, Stengler R, O’Kelly KU, Chen B. J Mater Chem 2011; 21: 9103-11. [3] Walther A, Bjurhager I, Malho J-M, Ruokolainen J, Berglund L, Ikkala O. Angew Chem Int Ed 2010; 49: 6448-53. [4] Haraguchi K, Takehisa T. Adv Mater 2002; 14: 1120-24. [5] Tang Z, Kotov NA, Magonov S, Ozturk B. Nature Mater 2003; 2: 413-8.
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[6] Bonderer LJ, Studart AR, Gauckler LJ. Science 2008; 319: 1069-73. [7] Shikinaka, K, Koizumi, Y, Shigehara, K. J Appl Polym Sci 2015; 132: 41691 (6 page).
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[8] Karube J. Clays Clay Miner 1998; 46(5): 583-5. [9] Cradwick PDG, Farmer V C, Russell JD, Masson CR, Wada K, Yoshinaga N. Nature Phys Sci 1972; 240: 187-9.
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[10] Bursill LA, Peng JL, Bourgeois LN. Philos Mag A 2000; 80: 105-17. [11] Mukherjee S, Bartlow VM, Nair S. Chem Mater 2005; 17: 4900-9.
[12] Levard C, Masion A, Rose J, Doelsch E, Borschneck D, Dominici C, Ziarelli F,
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Bottero J-Y. J Am Chem Soc 2009; 131: 17080-1.
[13] Farmer VC, Adams MJ, Fraser AR, Palmieri F. Clay Miner 1983; 18: 459-72. [14] Farmer VC, Fraser AR, Tait JM. J Chem Soc Chem Comm 1977; 13: 462-3.
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[15] Yucelen GI, Kang D-Y, Guerrero-Ferreira RC, Wright ER, Beckham HW, Nair S. Nano Lett 2012; 12: 827-32. [16] Donkai N, Inagaki H, Kajiwara K, Urakawa H, Schmidt M. Makromol Chem 1985; 186: 2623-38.
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[17] Harsh JB, Traina SJ, Boyle J, Yang Y. Clays Clay Miner 1992; 40(6): 700-6.
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[18] Yamamoto K, Otsuka H, Wada S, Sohn D, Takahara A. Soft Matter 2005; 1: 372-7. [19] Yang H, Chen Y, Su Z. Chem Mater 2007; 19: 3087-9. [20] Kang D-Y, Tong HM, Zang J, Choudhury RP, Sholl DS, Beckham HW, Jones CW, Nair, ACS Appl Mater Interfaces 2012; 4(2): 965-76. [21] Shikinaka K, Koizumi Y, Kaneda K, Osada Y, Masunaga H, Shigehara K. Polymer 2013; 54: 2489-92. [22] Ma W, Otsuka H, Takahara A. Polymer 2011; 52: 5543-50. [23] Yamamoto K, Otsuka H, Wada S-I, Sohn D, Takahara A. Polymer 2005; 46: 12386-92. 9
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[24] Yamamoto K, Otsuka H, Wada S-I, Takahara A. Chem Lett 2001; 30: 1162-3. [25] Yamamoto K, Otsuka H, Takahara A, Wada S-I. J Adhesion 2002; 78: 591-602. [26] Yang H, Su Z. Chinese Sci Bull 2007; 52(16): 2301-3.
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[27] Woodward S, Dagorne S. Modern Organoaluminum Reagents. Springer: Berlin; 2013. [28] Nampoothiri KM, Nair NR, John R.P. Bioresource Technol 2010; 101: 8493-501.
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[29] Pustak A, Leskovac M, Denac M, Švab I, Pohleven J, Makarovič M. J Reinf Plast Compos 2014; 33(9): 851-61.
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[30] Perotti GF, Barud HS, Messaddeq Y, Ribeiro SJL, Constantino VRL. Polymer 2011; 52: 157-63.
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Figure Captions
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Scheme 1 Schematic illustration of reaction route of IG and BA Figure 1 FT-IR spectrum of (a) BAIG, (b) BA, and (c) IG. Figure 2 TGA curves of (a) BAIG and (b) IG. Figure 3 TEM images of (a) BAIG and (b) IG dispersed in CHCl3 at 1.0 w/v%. Inserted photographs are the visual contact images of dispersions. The scale bars are 20 nm. Figure 4 DSC curve of (a) BAIG-PLA and (b) pristine PLA. The peak temperature (ºC) and enthalpy of melting (J/g) are shown at the side of peaks.
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Figure 5 Typical tensile stress-strain curves of BAIG-PLA [+BAIG] and pristine PLA [-BAIG].
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Figure 6 POM images of (a) BAIG-PLA and (b) pristine PLA before stretching and after fracturing. The arrows of A and P represent the analyzer and polarizer setting direction. Figure 7 Typical tensile stress-strain curves (a) of BAIG-PCL [+BAIG] and pristine PCL [-BAIG] (b) of BAIG-PBS [+BAIG] and pristine PBS [-BAIG].
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Figure 8 POM images of (a) BAIG-PCL and (b) pristine PCL before stretching and after fracturing.
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Scheme 1 Shikinaka et al.
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Figure 1 Shikinaka et al.
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Graphical Abstract
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Highlight · Nanohybrid consisted of hydrophobized imogolite and aliphatic polyester was created.
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· Hydrophobization of imogolite by benzaldehyde via covalent bonding was achieved.
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· Nanohybrid of poly(L-lactic acid) and imogolite showed an oriented crystallization.
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Supplementary Information for Publication
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of Paper in Polymer Nanohybrid Film Consisted of Hydrophobized
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Imogolite and Various Aliphatic Polyesters
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Kazuhiro Shikinaka*, Ayaki Abe, Kiyotaka Shigehara
Graduate School of Engineering, Tokyo University of Agriculture and Technology,
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Koganei 184-8588, Japan
*Author to whom correspondence should be addressed: E-mail: [email protected]
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(K.S.) Tel.: +81-42-388-7406, Fax: +81-42-381-7058
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Fig. S1 Typical photographs of BAIG-poly(L-lactic acid).
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Fig. S2 DSC curve of (a) BAIG-poly(ε-caprolactone) (PCL) and (b) PCL. The peak temperature and enthalpy of melting are shown at the side of peaks.
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Fig. S3 DSC curve of (a) BAIG-poly(butylene succinate) (PBS) and (b) PBS. The peak temperature and enthalpy of melting are shown at the side of peaks.
S0032-3861(15)30002-1
DOI:
10.1016/j.polymer.2015.05.039
Reference:
JPOL 17877
To appear in:
Polymer
Received Date: 24 April 2015 Revised Date:
20 May 2015
Accepted Date: 23 May 2015
Please cite this article as: Shikinaka K, Abe A, Shigehara K, Nanohybrid Film Consisted of Hydrophobized Imogolite and Various Aliphatic Polyesters, Polymer (2015), doi: 10.1016/ j.polymer.2015.05.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Nanohybrid Film Consisted of Hydrophobized
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Imogolite and Various Aliphatic Polyesters
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Kazuhiro Shikinaka*, Ayaki Abe, Kiyotaka Shigehara
Graduate School of Engineering, Tokyo University of Agriculture and Technology,
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Koganei 184-8588, Japan
*Author to whom correspondence should be addressed: E-mail: [email protected]
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(K.S.) Tel.: +81-42-388-7406, Fax: +81-42-381-7058
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Abstract In this paper, we created a nanohybrids consisted of imogolite nanotube and various
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aliphatic polyesters through a hydrophobization of imogolite. The hydrophobization of imogolite via covalent bonding between aluminol group on imogolite and aldehyde group in benzaldehyde was firstly achieved. The hydrophobized imogolite is well miscible with aliphatic polyester and their mixture forms the transparent films via
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casting of their chloroform dispersion. An addition of hydrophobized imogolites to aliphatic polyester such as poly(L-lactic acid), poly(ε-caprolactone), and poly(butylene succinate) brought about the plasticization of polymer films that should be induced by the miniaturization of polymer crystals via the nucleation of polymer chains on
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Keywords imogolite, nanohybrid, plasticization
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hydrophobized IGs. Especially, the nanohybrids of poly(L-lactic acid) and hydrophobized imogolite showed an oriented crystallization under stretching.
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Introduction Nanohybrids consisted of inorganic nanofiller and organic molecules have been studied with the expectation of their usage for industrial applications [1-7]. Especially, an addition of inorganic nanofillers with anisotropic shape (rod, platelet etc.) gives the excellent improvement of mechanical strength to organic polymers. Furthermore, these nanohybrids exhibit some special properties such as frame-resistance [3] and anisotropic mechanical/optical nature [7]. Recently, imogolite (later denoted IG), a rigid hollow cylindrical inorganic clay with the composition of (HO)3Al2O3SiOH [8-12], have
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received widespread attention because of their anisotropic molecular shapes with rigidity that further derived various functions and potential applications of its composite materials. IG is single-walled alumino-silicate nanotube with extremely high aspect ratios [9-15]: the external and internal diameters are approximately 2 - 3 and 1 nm,
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respectively. In contrast, their length ranges from several tens of nanometers to several micrometers. Since IG is a perfectly rigid polyelectrolyte [16,17], it has been used as the constituent of inorganic-organic nanohybrids [18-21]. The outer and inner surfaces of IG are covered with the Al(OH)2 and Si(OH) groups, respectively, where protonation–deprotonation equilibria such as [outer surface]
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Al-(OH)2 + H+ ⇌ Al(OH)O+H2 and [inner surface] Si–OH ⇌ Si–O- + H+ occur. Because these groups act as the proton-capturing and -releasing functions, IGs disperse as thin bundles or monofilaments in acidic - neutral and relatively low ionic strength aqueous media (pH ≈ 2 ~ 7) to yield opaque to transparent solutions [21]. However, IG
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is non-miscible with hydrophobic components such as aliphatic polyesters and general organic solvents due to hydroxyl groups on its surface that limit the composition of IGs with various organic molecules. In one barely instance of the composition of IGs with hydrophobic organic molecule, there is the nanohybrids consisted of IG nanotubes
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grafted poly(methyl methacrylate) [22,23] or octadecylphosphonic acid [24,25], . In this paper, we firstly tried the hydrophobization of IG nanotubes via formation of covalent bonding between aluminol groups of IG and aldehyde groups of organic molecule. Here, the lower mobility of modified molecule (benzene ring) relative to previous experimental system (alkyl chain) [22] should induce both a hydrophobization of IG surface and a promotion of nucleation of polymer chains on IG surface for crystallization that allow effective plasticizing of polymer film. Further studies on the composition of hydrophobized IG with aliphatic polyester was carried out. The physical properties and structure of obtained nanohybrids were estimated by means of the tensile strain testing, polarized optical microscopy (POM), and differential scanning calorimetry (DSC). 3
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Experimental section Chemicals. Deionized water was further purified by a Milli-Q® Advantage A10® system (MilliporeTM, Eschborn, Germany) and used throughout our experiments. Benzaldehyde (BA; TCI, Japan), AlCl3·6H2O (Kanto Chem. Co. Inc., Japan), Na4SiO4 (Junsei Chem. Co. Ltd., Japan) and other reagent grade chemicals were used as received.
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Imogolite (IG) Synthesis. The literature procedure [7] was performed in order to obtain homogeneously dispersive IGs. Aqueous solutions of AlCl3·6H2O (9.96 g in 369 mL) and Na4SiO4 (6.90 g in 362 mL) were mixed to prepare a solution containing 12.5 and 2.5 mol/L of Al and Si, respectively. The pH of the mixture was adjusted to 6.0 by rapidly adding about 26 mL of 1.0 mol/L NaOH aq. (localized high pH levels should be
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avoided by vigorous agitation), and the resulting solution was stirred for 1 h to obtain precipitates in the solution. The white precipitates were collected by centrifugation (5000 rpm for 30 min) and were then redispersed in 400 mL of water with stirring. After
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the addition of another 2400 mL of water, the solution was acidified (pH = 4.5) by adding 1.0 mol/L hydrochloric acid (7 - 8 mL), and then stirred at 100 °C continuously (should not be intermittent) for 4 days by SynFlexTM system (EYELA, Japan). After cooling to room temperature, a fine powder of sodium chloride (16.4 g, final concentration: 0.1 mol/L) was added to the solution with rapid agitation, and the resulting gel was collected by centrifugation (5000 rpm for 30 min) and then washed
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portionwise with 500 mL water on a 100 nm Millipore filter with suction. The wet products (caution, never allow to dry) were added to 1800 mL of tetrahydrofuran (THF; without stabilizer) with stirring for reprecipitation of IG, and the resulting fluffy precipitates were collected by filtration and dried in vacuo, resulting in a yield of 42%.
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Natrium and chloride in the resulting products could not be detected under the flame coloration test. The obtained IG white powder is miscible on ultra pure water. Surface modification of IG. An outline of procedure is shown in Scheme 1. A calculated amount of IG in pure water was sonicated for 4 h at 100 W at room temperature (FU-21H, SD-ULTRA. Ltd, KOREA), which was maintained by the occasional addition of ice into the sonicator bath. By this procedure, slightly opaque dispersions could be prepared, and the average length of IG in the solution was shortened to 131 nm, which was confirmed by transmission electron microscopic (TEM) observation [7]. The pH of IG dispersion (1.0 w/v%, 10 mL) was adjusted to 2.0 by adding approximately 1.0 mol/L of HCl aq. (2.0 mL). The resulting solution was mixed with 14 mol/L of BA (0.50 mL) and stirred for 72 h at 50 ºC. The white 4
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precipitates were collected by centrifugation (8000 rpm for 30 min) and then washed with 100 mL of water on a 200 nm Millipore filter with suction. The products were rinsed by acetone and dried in vacuo for 6 h at r.t. to give BA modified IG (later denoted as BAIG) as white powders typically in 20 - 30% yield.
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Cast film preparation of hybrid. A calculated amount of BA-IG in chloroform (CHCl3) was sonicated for 12 h at 100 W at room temperature (FU-21H, SD-ULTRA. Ltd, KOREA), which was maintained by the occasional addition of ice into the sonicator bath. By this procedure, slightly opaque dispersions up could be prepared. The
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BAIG dispersion was mixed with CHCl3 solution of poly(L-lactic acid) (PLA; UNITIKA, Ltd., Mw = 140,000, D ratio = 1.4%) or poly(ε-caprolactone) (PCL; Wako Pure Chem. Ind., Ltd., Mw = 70,000 - 100,000) or poly(butylene succinate) (PBS; Bionolle® 3001, Showa Denko K.K., melt flow rate = 1 - 3), respectively. The 4.0 mL
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of resulting mixture (0.2 w/v% of BAIG) was dropped, allowed to be spread on a glass surface and dried for about 2 days in air to give the hybrid films consisted of BAIG and organic polymer (later denoted as BAIG-PLA/-PCL/-PBS; thickness = approximately 100 µm). These films were further dried for 6 h at 70 ºC in vacuo before examining physical properties. The final concentration of IG in the hybrid films was adjusted to 5.0 w/v%.
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Fourier transform infrared (FT-IR) spectra. FT-IR spectra were recorded on a JASCO FT/IR-4100 spectrometer for KBr pellet. Thermogravimetric analysis (TGA). TGA was carried out on a Rigaku Thermo Plus TG8129, under nitrogen flow, at 10 ºC min-1.
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Transmission electron microscopy (TEM). TEM observations were performed using a JEM-2100 (JEOL, Tokyo, Japan) at an acceleration voltage of 200 kV. 5 µL of the sample solution was dropped on to carbon-coated grids (Oken Shouji Co., Tokyo),
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whose surface had previously been turned hydrophilic by glow discharge under reduced pressure. After 3 min, the sample on the grids was blotted by a filter paper, and then the grid was dried in ethanol vapor atmosphere according to the sample preparation method described in a previous study [26]. The digital TEM data were obtained using a slow-scan charge-coupled device (CCD) camera (Gatan USC1000, Gatan Inc.) and converted into images with a frame size of 1024 × 1024 pixels. A cold finger and a cold trap cooled with liquid nitrogen were used to prevent sample contamination by the electron beams. The length of the IG nanotubes was calculated from the TEM images using a DigitalMicrograph® (Gatan Inc., USA). Tensile measurement of films. For tensile testing, the films were cut into strips (length 20 mm, width 2 mm, thickness approximately 0.1 mm). Five millimeters from each end 5
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of the sample was clamped with polyimide tape to prevent the sample from slipping out
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of the machine. The tensile stress-strain measurements were performed on a tensile-compressive tester (Tensilon RTC-1310A, Orientec Co.) at a constant stretch rate of 100 mm/min. All measurements were carried out at ambient temperature (22 ± 2 °C) under a relative humidity (RH) = about 60%. Birefringence estimation. The center points of the sample strips described above were observed at room temperature by a polarized optical microscope (POM; BX51, Olympus, Japan) under crossed nicols. The images were obtained using a CCD camera
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(Olympus, Japan). All measurements were carried out at ambient temperature (22 ± 2 °C) under RH = about 60%.
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Differential scanning calorimetry (DSC). DSC measurement was performed on a Rigaku Thermo Plus DSC 8230 under nitrogen flow at 10 °C min-1. Results and discussion In this study, we performed the dehydration condensation between the aluminol
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groups on the surface of IG and the aldehyde groups of BA via covalent bonding as shown in Scheme 1. The reaction progress of IG and BA was confirmed by the presence of peaks originated from vibration of benzene ring and C-O-Al bonds [27] in FT-IR spectra of BAIG (Figure 1). TGA curves (Figure 2) indicate that the 47% of BAIG is
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organic component (equal to BA molecules). From these results, it was revealed that the amount of BA units is equal to 1.15 times for that of aluminum atoms on the surface of IGs, i.e., the approximately quantitative reaction of BA and IG was occurred in the presented experiment system. The dispersibility of BAIG to hydrophobic components such as CHCl3 was estimated by visual contact and TEM observation (Figure 3). As shown in Fig. 3a, the
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BAIG disperses in CHCl3 after sonication of mixture for 12 h. Because the pristine IG never disperses in CHCl3 under the same condition (Figure 3 (b)), it seems that the modification of BA gives the hydrophobic nature to IG. The BAIG CHCl3 dispersion is miscible with the PLA/PCL/PBS CHCl3 solution. The casting and drying of BAIG-polymer mixture in CHCl3 gives the films without some aggregation under visual contact (Figure S1). The DSC curves of BAIG-PLA (Figure 4) clearly indicate the uniform dispersion of BAIG in PLA matrix because of no change in the peak temperature and no emergence of additional peaks by BAIG mixing to PLA that are sensitive to the microstructure of the presented hybrid system as shown in other studies [22]. The sharp peaks in the DSC curves of BAIG-PLA and pristine PLA correspond to the melting of PLA crystals [28]. The peak of BAIG-PLA has larger 6
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area, i.e., higher enthalpy of melting (∆H) of PLA crystals, than pristine PLA that
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indicates higher crystallinity of BAIG-PLA than pristine PLA. The crystallization of PLA nucleated on the hydrophobized IG results in the increase in their crystallinity. To estimate the mechanical properties of films, the tensile stress-strain measurements were performed as shown in Figure 5. The BAIG-PLA [+BAIG] exhibited the fracture stress and strain of 44 MPa and 0.24 mm/mm, respectively, corresponding to 92% and 171% of the pristine PLA [-BAIG]. Furthermore, the yielding stress of BAIG-PLA (45 MPa) is lower than that of PLA (56 MPa). Thus, an
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addition of BAIG brings about a plasticization to PLA that is on-target result as described in Introduction part. The BAIG-PLA showed birefringence only after stretching (Figure 6 (a)). Under crossed nicols, the complete extinction was observed at the angle of
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analyzer-to-stretching direction = 0°, while maximum brightness was observed at 45°, indicating the ordering of IG nanotubes along with the stretching direction. On the other hand, such birefringence change could not be recognized in the pristine PLA. These
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POM results indicate that an introduction of BAIG prompts the oriented crystallization of PLA film because of an alignment of IG nanotubes parallel to tensile direction as shown in the composite gels consisted of organic polymer network and IGs [21]. This oriented crystallization in accordance with stretch of BAIG-PLA should cause the plasticization of films as shown in Figure 5. The BAIG-PCL and BAIG-PBS also give the films without some additional aggregation that is estimated from DSC curves (Figure S2 and S3). Furthermore, BAIG-PCL and BAIG-PBS also exhibited the higher fracture strain than pristine PCL
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and PBS (Figure 7). As shown in POM images (Figure 8), spherulites in the polymer films were miniaturized by an addition of BAIG despite of no change of ∆H (Figure S2 and S3). The nucleation of polymer chains on the surface of BAIG would prevent coarsening of polymer crystals (spherulites) in the films that diminishes the stress concentration of films as shown in other polymer-silica nanocomposite materials [29]. This structural change should induce the plasticization of the films. The plasticization of polymer films by composition of IG nanotube is firstly achieved in this study, i.e., pristine IG and alkyl chain-modified IG act only as hardening agents in previous reports [18,22]. Since an addition of nano-sized silicates (silica [29] and Laponite [30]) usually brings about hardening of polymer films, it seems that the BAIG is unique silicate modifier as plasticizing agent. The low dimensionally of IG nanotube and the high hydrophobicity of modified aromatic rings
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on IG induce both the orient crystallization of hybrid materials and the miniaturization of polymer crystals that allow plasticization of polymer film.
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Conclusion In the presented study, we achieved the hydrophobization of IG nanotubes and the effective plasticization of aliphatic polyester films by composition of hydrophobized IGs. The hydrophobization via covalent bonding between aluminol groups on IG and aldehyde groups in BA was firstly performed. An addition of hydrophobized IG should
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promote the nucleation of aliphtic polymers on IG that induces the miniaturization of spherulites, i.e., the plasticization of polymer films. Especially, the hybrid films of PLA and hydrophobized IG showed the oriented crystallization. As described above, the nanohybrid formation of aliphatic polymers and hydrophobized IGs will enable us to
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use the IG nanotubes to various applications such as biomass-based nanofiller reinforced plastics and/or the anisotropic materials. There is also the usage of the presented hybrids for various electronic industry materials (e.g., insulated cable). The
Acknowledgements
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detailed electrochemical/thermal properties of the hybrids (volume resistivity, thermal expansion rate etc.) will be studied in near future to estimate their material performance.
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This work was supported by the JGC-S Scholarship Foundation (No. 1335) and by the JSPS KAKENHI Grant Number 26870179.
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Notes and References [1] Leroy CM, Achard M-F, Babot O, Steunou N, Massé P, Livage J, Binet L, Brun N, Backov R. Chem Mater 2007; 19: 3988-99. [2] Frydrych M, Wan C, Stengler R, O’Kelly KU, Chen B. J Mater Chem 2011; 21: 9103-11. [3] Walther A, Bjurhager I, Malho J-M, Ruokolainen J, Berglund L, Ikkala O. Angew Chem Int Ed 2010; 49: 6448-53. [4] Haraguchi K, Takehisa T. Adv Mater 2002; 14: 1120-24. [5] Tang Z, Kotov NA, Magonov S, Ozturk B. Nature Mater 2003; 2: 413-8.
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[6] Bonderer LJ, Studart AR, Gauckler LJ. Science 2008; 319: 1069-73. [7] Shikinaka, K, Koizumi, Y, Shigehara, K. J Appl Polym Sci 2015; 132: 41691 (6 page).
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[8] Karube J. Clays Clay Miner 1998; 46(5): 583-5. [9] Cradwick PDG, Farmer V C, Russell JD, Masson CR, Wada K, Yoshinaga N. Nature Phys Sci 1972; 240: 187-9.
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[10] Bursill LA, Peng JL, Bourgeois LN. Philos Mag A 2000; 80: 105-17. [11] Mukherjee S, Bartlow VM, Nair S. Chem Mater 2005; 17: 4900-9.
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[15] Yucelen GI, Kang D-Y, Guerrero-Ferreira RC, Wright ER, Beckham HW, Nair S. Nano Lett 2012; 12: 827-32. [16] Donkai N, Inagaki H, Kajiwara K, Urakawa H, Schmidt M. Makromol Chem 1985; 186: 2623-38.
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[17] Harsh JB, Traina SJ, Boyle J, Yang Y. Clays Clay Miner 1992; 40(6): 700-6.
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[18] Yamamoto K, Otsuka H, Wada S, Sohn D, Takahara A. Soft Matter 2005; 1: 372-7. [19] Yang H, Chen Y, Su Z. Chem Mater 2007; 19: 3087-9. [20] Kang D-Y, Tong HM, Zang J, Choudhury RP, Sholl DS, Beckham HW, Jones CW, Nair, ACS Appl Mater Interfaces 2012; 4(2): 965-76. [21] Shikinaka K, Koizumi Y, Kaneda K, Osada Y, Masunaga H, Shigehara K. Polymer 2013; 54: 2489-92. [22] Ma W, Otsuka H, Takahara A. Polymer 2011; 52: 5543-50. [23] Yamamoto K, Otsuka H, Wada S-I, Sohn D, Takahara A. Polymer 2005; 46: 12386-92. 9
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[24] Yamamoto K, Otsuka H, Wada S-I, Takahara A. Chem Lett 2001; 30: 1162-3. [25] Yamamoto K, Otsuka H, Takahara A, Wada S-I. J Adhesion 2002; 78: 591-602. [26] Yang H, Su Z. Chinese Sci Bull 2007; 52(16): 2301-3.
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[27] Woodward S, Dagorne S. Modern Organoaluminum Reagents. Springer: Berlin; 2013. [28] Nampoothiri KM, Nair NR, John R.P. Bioresource Technol 2010; 101: 8493-501.
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[29] Pustak A, Leskovac M, Denac M, Švab I, Pohleven J, Makarovič M. J Reinf Plast Compos 2014; 33(9): 851-61.
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[30] Perotti GF, Barud HS, Messaddeq Y, Ribeiro SJL, Constantino VRL. Polymer 2011; 52: 157-63.
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Figure Captions
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Scheme 1 Schematic illustration of reaction route of IG and BA Figure 1 FT-IR spectrum of (a) BAIG, (b) BA, and (c) IG. Figure 2 TGA curves of (a) BAIG and (b) IG. Figure 3 TEM images of (a) BAIG and (b) IG dispersed in CHCl3 at 1.0 w/v%. Inserted photographs are the visual contact images of dispersions. The scale bars are 20 nm. Figure 4 DSC curve of (a) BAIG-PLA and (b) pristine PLA. The peak temperature (ºC) and enthalpy of melting (J/g) are shown at the side of peaks.
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Figure 5 Typical tensile stress-strain curves of BAIG-PLA [+BAIG] and pristine PLA [-BAIG].
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Figure 6 POM images of (a) BAIG-PLA and (b) pristine PLA before stretching and after fracturing. The arrows of A and P represent the analyzer and polarizer setting direction. Figure 7 Typical tensile stress-strain curves (a) of BAIG-PCL [+BAIG] and pristine PCL [-BAIG] (b) of BAIG-PBS [+BAIG] and pristine PBS [-BAIG].
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Figure 8 POM images of (a) BAIG-PCL and (b) pristine PCL before stretching and after fracturing.
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Scheme 1 Shikinaka et al.
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Figure 1 Shikinaka et al.
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Figure 2 Shikinaka et al.
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Figure 3 Shikinaka et al.
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Figure 4 Shikinaka et al.
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Figure 5 Shikinaka et al.
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Figure 6 Shikinaka et al.
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Figure 7 Shikinaka et al.
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Figure 8 Shikinaka et al.
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Graphical Abstract
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Highlight · Nanohybrid consisted of hydrophobized imogolite and aliphatic polyester was created.
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· Hydrophobization of imogolite by benzaldehyde via covalent bonding was achieved.
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· Nanohybrid of poly(L-lactic acid) and imogolite showed an oriented crystallization.
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Supplementary Information for Publication
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of Paper in Polymer Nanohybrid Film Consisted of Hydrophobized
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Imogolite and Various Aliphatic Polyesters
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Kazuhiro Shikinaka*, Ayaki Abe, Kiyotaka Shigehara
Graduate School of Engineering, Tokyo University of Agriculture and Technology,
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Koganei 184-8588, Japan
*Author to whom correspondence should be addressed: E-mail: [email protected]
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(K.S.) Tel.: +81-42-388-7406, Fax: +81-42-381-7058
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Fig. S1 Typical photographs of BAIG-poly(L-lactic acid).
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Fig. S2 DSC curve of (a) BAIG-poly(ε-caprolactone) (PCL) and (b) PCL. The peak temperature and enthalpy of melting are shown at the side of peaks.
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Fig. S3 DSC curve of (a) BAIG-poly(butylene succinate) (PBS) and (b) PBS. The peak temperature and enthalpy of melting are shown at the side of peaks.