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organically modified montmorillonite composite fibers
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JAAP-3961; No. of Pages 9
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Polylactide/organically modified montmorillonite composite fibers Esra Ozdemir a , Jale Hacaloglu a,b,∗ a b
Middle East Technical University, Department of Polymer Science and Technology, TR-06800 Ankara, Turkey Middle East Technical University, Department of Chemistry, TR-06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 5 December 2016 Received in revised form 27 January 2017 Accepted 1 February 2017 Available online xxx Keywords: Poly(lactide) Fibres Nanocomposites Electrospinning Pyrolysis mass spectrometry
a b s t r a c t Direct pyrolysis mass spectrometry technique was applied to investigate the characteristics of polylactide, (PLA) nanofibers containing organically modified montmorillonites, Cloisite 15A, (C15A) Cloisite 20A, (C20A) and Cloisite 30B, (C30B) prepared by electrospinning. As the amount of Cloisite present in the composites was increased, the fiber diameters became slightly narrower compared to neat PLA fiber due to the presence of quaternary ammonium salt as organic modifier increasing electrical conductivity. The diameters of the fibers were smallest for PLA/C15A composite and largest for PLA/C30B composite. This behavior was directly related to the amount of organic modifier present in the nanoclays. Significant increase in the thermal stability of PLA composite nanofibers was detected compared to not only neat PLA and PLA fiber but also compared to PLA/organoclay composites and associated with the increase in extent of delamination evidenced by pyrolysis mass spectrometry results. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Polymer nanocomposites with enhanced mechanical properties, increased thermal stabilities, improved gas barrier properties and reduced flammabilities are one of the most important research areas in materials chemistry. Several studies focused on incorporation of nano-diamonds, carbon nanotubes, halloysite nanotubes (HNT) and nanoclays into polylactide, PLA, matrix to improve the characteristics of electrospun fibers [1–13]. Increase in mechanical properties such as tensile strength, Young’s modulus, and elongation at break was detected upon incorporation of nano-diamonds into PLA nanofibers [1]. Electrospinning of PLA composites containing carbon nanotubes, CNTs, was studied in terms of solution concentrations and solvents effects and CNT loadings [2,3]. It has been determined that the fiber morphology is influenced with the loading levels of nanotubes and dispersion as entangled bundles was observed at high loading levels [3]. Uniformly dispersed electrospun mats of PLA were developed upon incorporation of multi-walled carbon nanotubes, MWCNT, with hydroxyapatite [4]. Conductive fibers of PLA involving MWCNT were produced by electrospinning [5]. In a recent study, the influence of MWCNTs on thermal and structural properties of electrospun fibers of PLA has
∗ Corresponding author at: Middle East Technical University, Department of Chemistry, TR-06800 Ankara, Turkey. E-mail addresses: [email protected] (E. Ozdemir), [email protected] (J. Hacaloglu).
been studied and reduction in metabolic activity of fibroblasts was detected [6]. Electrospun poly(lactide)/halloysite nanotube composite fibers were also prepared and characterized [7–12]. The influence of HNT content and modification has been investigated comprehensively [12]. The role of nanoclays on the characteristics of electrospun composite fibers has also been investigated [13–21]. Marras et al. determined that the electrospinning process altered the structure of the initially prepared nanocomposite material by increasing the basal spacing of the organomodified montmorillonite [13]. The exfoliated montmorillonite and PLLA solution was electrospun to provide montmorillonite reinforced PLLA nanofibers and the developed nanocomposite systems maintained the structural integrity during biodegradation reactions demonstrating their enhanced characteristics in engineered scaffold applications [16]. Dogan et al. found out that the organoclay has adjuvant effect both on tensile strength and dye-ability properties of PLA fiber [17]. It has been shown that the incorporation of clays and modified clays during the electrospinning of fibers was in particular showed to be a way to increase the thermal stability of the fibers [18–24]. In this paper, we successfully prepared PLA/montmorillonite composite fibers by electrospinning and investigated the morphological and the thermal characteristics of these fibers. The effect of type and amount of organoclay on thermal behavior were evaluated by pyrolysis mass spectrometry in addition to thermogravimetry analyses.
http://dx.doi.org/10.1016/j.jaap.2017.02.006 0165-2370/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: E. Ozdemir, J. Hacaloglu, Polylactide/organically modified montmorillonite composite fibers, J. Anal. Appl. Pyrol. (2017), http://dx.doi.org/10.1016/j.jaap.2017.02.006
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2.2. Preparation of fibers
Fig. 1. Electrospinning system for generation of nanofibers.
2. Experimental
2.1. Materials Polylactide, (Mn ∼ 190,000) was provided by Cargill Dow. Solvents chloroform, CHCl3 (99%) and dimethylformamide, DMF, (99%) were obtained from Sigma Aldrich. Organically modified montmorillonites, dimethyl ditallow ammonium modified montmorillonites, Cloisite 15 A and 20A (C15A and C20A) and methyl tallow bis-2-hydroxyethyl ammonium modified montmorillonite Cloisite 30 B (C30B), were provided by Southern Clay Products Inc. PLA and clays were dried at 60 ◦ C overnight under reduced pressure prior to mixing processes. PLA/clay composites (1, 3 or 5 wt% inorganic content) were melt-compounded using a DSM Xplore twin screw micro compounder at 190 ◦ C, with a screw speed of 100 rpm for 8 min or the solution was mixed by magnetic (1000 rph).
The electrospinning system was composed of a syringe pump (New Era Pump Systems, Inc NE 300), a high voltage power supply, a 1 mL syringe with metallic needle of 0.6 mm inner diameter, and a collector. The syringe was positioned horizontally on the syringe pump and the positive electrode of the high voltage power supply was clamped to the metal needle (Fig. 1). The electrospinning parameters were optimized as 0.5 mL H−1 for the flow rate of the polymer solution, 12.5 kV for the applied voltage and 10 cm for the tip-to-collector distance. Electrospun nanofibers were collected on a grounded stationary cylindrical metal collector covered by a piece of aluminum foil in an enclosed chamber at around 25 ◦ C and 20% relative humidity. The nanofibers obtained were dried over night at 25 ◦ C under vacuum in order to remove any residual solvent. Recent experiments done by changing the concentration of PLA from 3% to 20% (w/v) in CHCl3 /DMF solvent systems having varying compositions (v/v 100:0, 50:50, 60:40, 70:30, 80:20 and 90:10) indicated longer average distance between the beads on the fibers with the increase in the volume ratio of DMF in the solvent system, and reduction in beads and beaded fibers with the increase in concentration of PLA [18]. However, as the DMF and PLA concentrations were increased thicker fiber diameters were generated. The optimum concentration of PLA in CHCl3 /DMF solvent system was determined as 15% (w/v) PLA in 90:10 v/v CHCl3 /DMF and these concentration values were selected for generation of PLA composite fibers involving 1, 3 or 5 wt% organically modified Cloisite 15A, 20A, or C30 B by electrospinning. 2.3. Characterization The 2 XRD patterns of the composites, at 25 ◦ C in the 1◦ −10◦ range were recorded on a Rigaku X-ray diffractometer (Model, Miniflex) with CuK␣ (30 kV, 15 mA, = 1.54051 Å). Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai G2 Spirit BioTwin CTEM on the specimens cut from the middle portion of injection molded PLA composites used for generation of
Fig. 2. a. XRD diffractograms and b. TEM images of PLA fibers involving 3 wt% C15A, C20A and C30B.
Please cite this article in press as: E. Ozdemir, J. Hacaloglu, Polylactide/organically modified montmorillonite composite fibers, J. Anal. Appl. Pyrol. (2017), http://dx.doi.org/10.1016/j.jaap.2017.02.006
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Fig. 3. SEM images of PLA fibers involving 3 and 5% Cloisite 15 A, electrospun 15% (w/v) PLA/nanoclay in 90:10 v/v CHCl3 /DMF solvent systems.
fibers. Scanning electron microscope (SEM, Quanta 400 FEG, FEI) was used to investigate the morphology of nanofibers. Samples were coated with 5 nm Au/Pd (PECS-682) prior to the SEM imaging. Thermogravimetry analyses (TGA) were performed on a Perkin Elmer Instrument STA6000 having temperature accuracy and reproducibility ±0.5 ◦ C under nitrogen atmosphere at a flow rate of 20 mL/min and a heating rate of 10◦ C/min. Direct pyrolysis mass spectrometry (DP-MS) analyses were performed on a 5973 HP quadrupole mass spectrometry system, with a mass range 1.6–800 Da and resolution of unit mass, coupled to a JHP SIS direct insertion probe pyrolysis system, 70 eV EI mass spectra, at a rate of 2 scan s−1 , were recorded. About 0.10 mg of PLA composite fibers in flared glass sample vials, were heated to 450 ◦ C at a rate of 10 ◦ C/min. All experiments were repeated at least twice to assess reproducibility.
3. Results and discussions 3.1. Morphology of PLA/montmorillonite composites The diffraction pattern of Cloisite 15A, three peaks at around 2 = 3.34, 5.18 and 7.72◦ corresponding to inter-reticular distances of d001 = 2.64, 1.70 and 1.16 nm, almost totally disappeared and a weak peak located at around 1.8◦ corresponding to interreticular distance of d001 = 2.6 nm and small budges located around 4◦ were detected (Fig. 2a). Cloisite 20A shows two broad peaks at around 2 = 3.9 and 7.4◦ corresponding to inter-reticular distances of d001 = 2.21 and 1.19 nm in its diffractogram. A weak peak located at around 3.0◦ was observed in the XRD of PLA/C20A. The peak at around 2 = 5.0◦ corresponding to an inter-reticular distance of d001 = 1.8 nm present in the diffractogram of Cloisite 30 B was almost totally disappeared for all the samples of PLA/C30 B nanocomposite. The XRD diffractograms of PLA/15A and PLA/20A fibers show very weak peaks around 3.3◦ .
The TEM images taken from representative regions of the PLA nanocomposites showed mainly regions of tactoids. Yet, especially for PLA/C15A and PLA/20A single unit layers can be observed indicating presence of both intercalated and exfoliated structures. 3.2. Generation of PLA composite fibers The SEM images of PLA fibers involving C15A, C20A and C30 B depicted in Figs. 3–5 respectively revealed generations of beadfree fibers having diameters around 400–200 nm indicating slightly narrower fibers compared to neat PLA fibers in the presence of organically modified montmorillonites. As the amount of clay present in the composite was increased, the fiber diameter became slightly narrower. A similar behavior was observed by Shia and coworkers for electrospun fibers of PLA incorporated with cellulose nanocrystals [23]. It has been proposed that the quaternary ammonium salt present as an organic modifier in the nanoclays increases the electrical conductivity, thus the diameter of fibers formed become substantially smaller as the amount of nanoclay is increased [23,24]. However, high magnification of the SEM images indicated non-homogenous surface structure especially for the PLA fibers involving 5 wt% nanoclays, most probably due to aggregation of clay at high loading levels. In Fig. 6, SEM images of neat PLA fiber and PLA composite fibers involving 3 wt% C15A, C20A and C30 B are shown together for comparison. It can be noted from this figure that the diameters of the fibers were smallest for PLA/C15A composite and largest for PLA/C30 B composite. This behavior may directly be related to amount of organic modifier present in the nanoclay. 3.3. DSC analyses In Fig. 7, DSC images of PLA and PLA fibers involving 3% C15A, C20A and C30 B prepared by melt blending are shown. The endothermic melting peak of PLA at around 172 ◦ C, was shifted
Please cite this article in press as: E. Ozdemir, J. Hacaloglu, Polylactide/organically modified montmorillonite composite fibers, J. Anal. Appl. Pyrol. (2017), http://dx.doi.org/10.1016/j.jaap.2017.02.006
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Fig. 4. SEM images of PLA fibers involving 1, 3 and 5% Cloisite 20 A electrospun 15% (w/v) PLA/nanoclay in 90:10 v/v CHCl3 /DMF solvent systems.
to 167.5, 169.2 and 168.8 for the corresponding fibers involving 3 wt% C15A, C20A or C30 B respectively, indicating a smaller decrease in the melting temperature upon fiber formation [22]. The cold crystallization peaks observed for all the PLA composite fibers were associated with the nanofiller-induced nucleating effect. As a consequence, the rate of crystallinity was accelerated generating crystalline defects and yielding less ordered crystals melting at lower temperatures. Lowest melting and cold crystallization temperatures were detected for PLA involving C15A, whereas, highest values were recorded for the composite involving C30B.
Table 1 TGA dataa for PLA composite fibers involving 3 wt% C15A, C20A and C30B. T5%
T10%
Tmax
% char
PLA
324.4
332.2
355.1
0.37
PLA-C15A PLA-C15A fiber
318.2 314.1
331.3 327.3
360.9 361.2
1.16 3.22
PLA-C20A PLA-C20A fiber
323.4 322.5
334.5 333.2
362.0 361.8
1.71 2.43
PLA-C30B PLA-C30 B fiber
328.3 324.9
337.6 334.9
363.3 363.7
2.05 1.91
a
Temperature accuracy and reproducibility is ±0.5 ◦ C.
3.4. Thermal analyses Thermal decompositions of PLA composites involving C15A and C20A and the corresponding fibers started at slightly lower temperatures compared to neat PLA. However, for all the composites and their corresponding fibers noticeably higher Tmax values compared to neat PLA were recorded (Fig. 8). Although the degradation of fibers started at slightly lower temperatures than the corresponding composites, their Tmax values were almost identical and the decomposition of PLA nanocomposite fibers occurred over a
broader temperature region [22]. In Table 1, TGA data for PLA composite fibers involving 3 wt% C15A, C20A and C30 B are collected. The DP-MS analyses of PLA and its electrospun fiber indicated decrease in thermal stability of PLA upon fiber formation [22]. On the other hand, noticeable increase in thermal stability was detected upon incorporation of organically modified nanoclays, Cloisite 15A, 20A and 30 B into PLA matrix [25]. Although identical products were detected during the pyrolysis of PLA, its fiber and PLA composites, the relative intensities of the products gen-
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Fig. 5. SEM images of PLA fibers involving 1, 3 and 5% Cloisite 30 B electrospun 15% (w/v) PLA/nanoclay in 90:10 v/v CHCl3 /DMF solvent systems.
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Please cite this article in press as: E. Ozdemir, J. Hacaloglu, Polylactide/organically modified montmorillonite composite fibers, J. Anal. Appl. Pyrol. (2017), http://dx.doi.org/10.1016/j.jaap.2017.02.006
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Fig. 6. SEM images of PLA fiber and PLA fibers involving 3% Cloisite 15A, 20A or 30 B electrospun 15% (w/v) PLA/nanoclay in 90:10 v/v CHCl3 /DMF solvent systems.
erated by trans-esterification and cis-elimination reactions were changed noticeably. The elimination of these products at lower temperatures during the pyrolysis of PLA fiber indicated that the inter-molecular interactions in the aligned fiber structure are more effective, and take place at lower temperatures. In contrast, increase in the relative yields of the products due to cis-elimination reactions were detected during the pyrolysis of PLA/nanoclay composites [25]. The increase was more pronounced for the products generated
by cis-elimination reactions, especially for the composite involving C30B. In Fig. 9, the TIC, (the variation of total ion yield as a function of temperature) curves and the mass spectra recorded at peak maxima of PLA fibers involving 3% organically modified montmorillonites C15A, C20A or C30 B electrospun from 15% (w/v) CHCl3 /DMF solution system (v/v = 90:10) are depicted. The maximum product yields are detected at 387, 398 and 410 ◦ C in the presence of C15A,
Please cite this article in press as: E. Ozdemir, J. Hacaloglu, Polylactide/organically modified montmorillonite composite fibers, J. Anal. Appl. Pyrol. (2017), http://dx.doi.org/10.1016/j.jaap.2017.02.006
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Fig. 7. DSC images of PLA and PLA fibers involving 3% C15A, C20A and C30 B prepared by melt blending.
C20A and C30 B respectively indicating noticeable enhancement in thermal stability not only compared to PLA fibers with lowest thermal stability but also compared to PLA and corresponding PLA composites. The noticeable increase in the thermal stability of PLA composite nanofibers compared to the corresponding PLA/organoclay composites may be associated with the increase in extent of delamination and better insertion of aligned PLA chains in fiber structure into clay layers. Single ion evolution profiles of some of the representative fragments of PLA, such as C2 H4 CO+ (56 Da) generated by homolysis reactions and dissociative ionization of high mass products, (C2 H4 CO2 )x H+ (217, 361 and 577 Da for x = 3, 5 and 8 respectively) due to cis-elimination reactions and (C2 H4 CO2 )x C2 H4 CO+ (200, 344 and 704 Da for x = 2, 4 and 9 respectively) generated by trans-esterification reactions are depicted in Fig. 10. The evolution profiles of the most abundant product of the organic modifiers, 296 Da fragment ion recorded during the pyrolysis of PLA composites are also included in Fig. 10. Contrary to the trends observed for PLA, its fiber and PLA nanocomposites, the evolution profiles of the products generated by cis-elimination and trans-esterification reactions during the pyrolysis of PLA composite fibers show similar behavior. The relative yields of the high mass products generated by trans-esterification and cis-elimination reactions were significantly diminished. For PLA fiber, the increase in the relative yields of products generated by trans-esterification reactions was associated with the increase in intermolecular interactions in the aligned structure [22]. It may be thought that the presence of tactoids and intercalated structures restricts the intermolecular interactions of PLA chains at least to a certain extent in the fiber structure. The loss of organic modifiers was detected over a broad temperature range. For a better understanding of the effects of electrospinning process and fiber formation on organoclays, the evolution profiles the characteristic degradation products of the organic modifiers, C15A, C20A and C30 B recorded during the pyrolysis of organo-nanoclays, PLA/organoclays and their fibers are compared. Pyrolysis mass spectra of the organically modified montmorillonites, C15A, C20A and C30 B were dominated with diagnostic peaks for amines and were almost similar to the free organic modifiers showing the base peak at 296 Da and intense peaks at 268 and 56 Da. These peaks were attributed to C18 H37 (CH3 )NHCH2 + (296 Da), C16 H33 (CH3 )NCH2 + , (268 Da) and C3 H6 N+ , (56 Da) fragments for C15A and C20A, whereas for C30B, were associated with C16 H31 N(CH2 )C2 H4 OH+
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Fig. 8. TGA data for PLA composite fibers involving 3 wt% C15A, C20A and C30B.
(296 Da), C14 H27 N(CH2 )C2 H4 OH+ (268 Da) and C3 H6 N+ , (56 Da) fragments for C30 B [25]. In Fig. 11, the single ion evolution profiles of the most abundant characteristic degradation product, with m/z value 296 Da due to C18 H37 N(CH3 )2 + (296 Da) for C15A and C20A and C16 H31 N(CH2 )C2 H4 OH+ fragments for C30 B recorded during the pyrolysis of organonanoclays, PLA/organoclays and their fibers are shown in Fig. 11 as an example. It is clear that the loss of organic modifier shifted to low temperature regions upon incorporation of organically modified montmorillonites, C15A and C20A into PLA matrix. This behavior may be associated with separation of clay layers. In a recent study, it has been determined that as the spacing between the layers of clay increases the loss of organic modifier takes place at lower temperatures [25]. However, during the pyrolysis of PLA/C15A and C20A fibers, significant amounts of the organic modifier of C15A and C20A were lost immediately, upon insertion of pyrolysis probe inside the mass spectrometer. The loss of organic modifier even at room temperature may be associated not only with the separation of clay layers in accordance with the literature findings but also with the degradation of the organic modifier to a certain extent during the electrospinning process. The elimination of organic modifier of C30 B was also observed at initial stages of pyrolysis confirming the decomposition of the organic modifier, but, was noticeably low. Contrary to what was observed for C15A and C20A involving composites and fibers, the single ion evolution profiles of characteristic fragments of the organic modifier of C30 B were shifted to high temperature regions during the pyrolysis of PLA/C30 B composite and its fiber. It may be thought that, as also proposed in our previous work, due to the trans-esterification reactions between the ester groups of PLA and the hydroxide groups of organic modifier, a crosslinked structure that decomposes at high temperatures eliminating the organic modifier in the temperature region where PLA decomposition occurred, was generated [25]. The highest thermal stability of PLA/C30 B fiber can then be associated with also the crosslinking effect of the organic modifier of C30B. 4. Conclusions PLA composite nanofibers involving variable amounts of Cloisite 15A, Cloisite 20A, and Cloisite 30B were prepared by electrospinning successfully. The fiber diameter became slightly narrower compared to neat PLA fibers as the amount of Cloisite present in the composite was increased. The diameters of the fibers were smallest for PLA/C15A composite and largest for PLA/C30 B composite. This behavior was directly be related to the increase in electrical
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Fig. 9. a) The TIC curves and b) the pyrolysis mass spectra recorded at peak maxima detected during the pyrolysis PLA fibers involving I. C15A, II. C20A and III. C30B.
Fig. 10. Single ion evolution profiles of representative products of detected during the pyrolysis PLA, PLA-fiber and PLA fibers involving C15A, C20A and C30B.
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Fig. 11. Single ion evolution profiles of most abundant fragment of organic modifiers recorded during the pyrolysis of organoclays, PLA/organoclay composites, and PLA/organoclay fibers.
conductivity of the solution with the increase in the amount of the quaternary ammonium salt present as an organic modifier in the nanoclays. The loss of organic modifiers of Cloisites especially those of C15A and C20A indicated increase in extent of exfoliation and degradation of the modifiers at least to a certain extent during the electrospinning process. The noticeable increase in the thermal stability of PLA composite nanofibers compared to the corresponding PLA/organoclay composites may then be associated with the increase in extent of delamination and better insertion of aligned PLA chains in fiber structure into clay layers. In addition, decrease in the relative yields of high mass pyrolysis products generated by trans-esterification reactions was detected compared to PLA fiber indicating reduction in intermolecular interactions in the presence of nanoclays. Acknowledgment This work is partially supported by TUBITAK Research Fund112T493. References [1] N. Cai, Q. Dai, Z. Wang, X. Luo, Y. Xue, F. Yu, Preparation and properties of nanodiamond/poly(lactic acid) composite nanofiber scaffolds, Fiber Polym. 15 (2014) 2544–2552. [2] Y.X. Kong, J. Yuan, Z.M. Wang, J. Qui, Study on the preparation and properties of aligned carbon nanotubes/polylactide composite fibers, Polym. Compos. 33 (2012) 1613–1619. [3] T. Yang, D. Wu, L. Lu, W. Zhou, M. Zhang, Electrospinning of polylactide and its composites with carbon nanotubes, Polym. Compos. 32 (2011) 1280–1288. [4] F. Mei, J.S. Zhong, X.P. Yang, Improved biological characteristics of Poly(l-Lactic Acid) electrospun membrane by incorporation of multiwalled carbon nanotubes/hydroxyapatite nanoparticles, Biomacromolecules 8 (2007) 3729–3735. [5] S.J. Shao, S.B. Zhou, L. Li, J.R. Li, C. Luo, J. Wang, X.H. Li, J. Weng, Osteoblast function on electrically conductive electrospun PLA/MWCNTs nanofibers, Biomaterials 32 (2011) 2821–2833. [6] Y. Zhu, C. Li, P. Cebe, Poly(lactides co-electrospun with carbon nanotubes: thermal and cell culture properties, Eur. Polym. J. 75 (2016) 565–576. [7] Y. Dong, T. Bickford, H.J. Haroosh, K.T. Lau, H. Takagi, Multi-response analysis in the material characterization of electrospun poly(lactic acid)/halloysite nanotube composite fibres based on Taguchi design of experiments: fibre diameter, non-intercalation and nucleation effects, Appl. Phys. A-Mater. Sci. Process 112 (2013) 747–757. [8] H.J. Haroosh, Y. Dong, D.S. Chaudhary, G.D. Ingram, S. Yusa, Appl. Phys. A-Mater. Sci. Process 110 (2) (2013) 433–442.
[9] H.J. Haroosh, D.S. Chaudhary, Y. Dong, Electrospun PLA: PCL composites embedded with unmodified and 3-aminopropyltriethoxysilane (ASP) modified halloysite nanotubes (HNT), J. Appl. Polym. Sci. 124 (5) (2012) 3930–3939. [10] Y. Dong, D.S. Chaudhary, H.J. Haroosh, T. Bickford, Development and characterization of novel electrospun polylactic acid/tubular clay nanocomposites, Mater. Sci. 46 (2011) 6148–6153. [11] A. Touny, J. Lawrence, A. Jones, S. Bhaduri, Effect of electrospinning parameters on the characterization of PLA/HNT nanocomposite fibers, J. Mater. Res. 25 (2010) 857–865. [12] Y. Dong, J. Marshall, H.J. Haroosh, S. Mohammadzadehmoghadam, D. Liu, X. Qi, K.T. Lau, Polylactic acid (PLA)/halloysite nanotube (HNT) composite mats: influence of HNT content and modification, Compos. Part A Appl. Sci. Manuf. 76 (2015) 28–36. [13] S.I. Marras, K.P. Kladi, I. Tsivintzelis, I. Zuburtikudis, C. Panayiotou, Biodegradable polymer nanocomposites: the role of nanoclays on the thermomechanical characteristics and the electrospun fibrous structure, Acta Biomater. 4 (2008) 756–765. [14] T.D. Hapuarachchi, T. Peijs, Multiwalled carbon nanotubes and sepiolite nanoclays as flame retardants for polylactide and its natural fibre reinforced composites, Compos. Part A Appl. Sci. Manuf. 42 (8) (2010) 954–963. [15] K. Piekarska, P. Sowinski, E. Piorkowska, M.M. U.l. Haque, M. Pracella, Structure and properties of hybrid PLA nanocomposites with inorganic nanofillers and cellulose fibers, Compos. Part A-Appl. Sci. Manuf. 82 (2016) 34–41. [16] Y.H. Lee, J.H. Lee, I.G. An, C. Kim, D.S. Lee, Y.K. Lee, J.D. Nam, Electrospun dual-porosity structure and biodegradation morphology of Montmorillonite reinforced PLLA nanocomposite scaffolds, Biomaterials 26 (2005) 3165–3172. [17] U. Tayfun, M. Dogan, Improving the dyeability of poly(lactic acid) fiber using organoclay during melt spinning, Poym. BulI. 3 (6) (2016) 1581–1593. [18] K. Saeed, S.Y. Park, Effect of nanoclay on the thermal, mechanical, and crystallization behavior of nanofiber webs of nylon-6, Polym. Compos. 33 (2) (2012) 192–195. [19] J.H. Hong, E.H. Jeong, H.S. Lee, D.H. Baik, S.W. Seo, J.H. Youk, Electrospinning of polyurethane/organically modified montmorillonite nanocomposites, J. Polym. Sci. B: Polym. Phys. 43 (22) (2005) 3171–3177. [20] Q. Li, D. Gao, Q. Wei, M. Ge, W. Liu, L. Wang, K. Hu, Thermal stability and crystalline of electrospun polyamide 6/organo-montmorillonite nanofibers, J. Appl. Polym. Sci. 117 (3) (2010) 1572–1577. [21] H.W. Lee, M.R. Karim, H.M. Ji, J.H. Choi, H.D. Ghim, S.M. Park, W. Oh, J.H. Yeum, Electrospinning fabrication and characterization of poly(vinyl alcohol)/montmorillonite nanofiber mats, J. Appl. Polym. Sci. 113 (3) (2009) 1860–1867. [22] E. Ozdemir, J. Hacaloglu, Thermal degradation of polylactide and its electrospun fiber, Fiber Polym. 17 (1) (2016) 66–73. [23] Q. Shia, C. Zhou, Y. Yue, W. Guo, Y. Wu, Q. Wu, Mechanical properties and in vitro degradation of electrospun bio-nanocomposite mats from PLA and cellulose nanocrystals, Carbohydr. Polym. 90 (2012) 301–308. [24] A. Greiner, J.H. Wendorff, Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angew. Chem-Int. Edit. 46 (30) (2007) 5670–5703. [25] E. Ozdemir, T.O. Lekesiz, J. Hacaloglu, Polylactide/organically modified montmorillonite composites; effects of organic modifier on thermal characteristics, Polym. Deg. Stabil. 134 (2016) 87–96.
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Polylactide/organically modified montmorillonite composite fibers Esra Ozdemir a , Jale Hacaloglu a,b,∗ a b
Middle East Technical University, Department of Polymer Science and Technology, TR-06800 Ankara, Turkey Middle East Technical University, Department of Chemistry, TR-06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 5 December 2016 Received in revised form 27 January 2017 Accepted 1 February 2017 Available online xxx Keywords: Poly(lactide) Fibres Nanocomposites Electrospinning Pyrolysis mass spectrometry
a b s t r a c t Direct pyrolysis mass spectrometry technique was applied to investigate the characteristics of polylactide, (PLA) nanofibers containing organically modified montmorillonites, Cloisite 15A, (C15A) Cloisite 20A, (C20A) and Cloisite 30B, (C30B) prepared by electrospinning. As the amount of Cloisite present in the composites was increased, the fiber diameters became slightly narrower compared to neat PLA fiber due to the presence of quaternary ammonium salt as organic modifier increasing electrical conductivity. The diameters of the fibers were smallest for PLA/C15A composite and largest for PLA/C30B composite. This behavior was directly related to the amount of organic modifier present in the nanoclays. Significant increase in the thermal stability of PLA composite nanofibers was detected compared to not only neat PLA and PLA fiber but also compared to PLA/organoclay composites and associated with the increase in extent of delamination evidenced by pyrolysis mass spectrometry results. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Polymer nanocomposites with enhanced mechanical properties, increased thermal stabilities, improved gas barrier properties and reduced flammabilities are one of the most important research areas in materials chemistry. Several studies focused on incorporation of nano-diamonds, carbon nanotubes, halloysite nanotubes (HNT) and nanoclays into polylactide, PLA, matrix to improve the characteristics of electrospun fibers [1–13]. Increase in mechanical properties such as tensile strength, Young’s modulus, and elongation at break was detected upon incorporation of nano-diamonds into PLA nanofibers [1]. Electrospinning of PLA composites containing carbon nanotubes, CNTs, was studied in terms of solution concentrations and solvents effects and CNT loadings [2,3]. It has been determined that the fiber morphology is influenced with the loading levels of nanotubes and dispersion as entangled bundles was observed at high loading levels [3]. Uniformly dispersed electrospun mats of PLA were developed upon incorporation of multi-walled carbon nanotubes, MWCNT, with hydroxyapatite [4]. Conductive fibers of PLA involving MWCNT were produced by electrospinning [5]. In a recent study, the influence of MWCNTs on thermal and structural properties of electrospun fibers of PLA has
∗ Corresponding author at: Middle East Technical University, Department of Chemistry, TR-06800 Ankara, Turkey. E-mail addresses: [email protected] (E. Ozdemir), [email protected] (J. Hacaloglu).
been studied and reduction in metabolic activity of fibroblasts was detected [6]. Electrospun poly(lactide)/halloysite nanotube composite fibers were also prepared and characterized [7–12]. The influence of HNT content and modification has been investigated comprehensively [12]. The role of nanoclays on the characteristics of electrospun composite fibers has also been investigated [13–21]. Marras et al. determined that the electrospinning process altered the structure of the initially prepared nanocomposite material by increasing the basal spacing of the organomodified montmorillonite [13]. The exfoliated montmorillonite and PLLA solution was electrospun to provide montmorillonite reinforced PLLA nanofibers and the developed nanocomposite systems maintained the structural integrity during biodegradation reactions demonstrating their enhanced characteristics in engineered scaffold applications [16]. Dogan et al. found out that the organoclay has adjuvant effect both on tensile strength and dye-ability properties of PLA fiber [17]. It has been shown that the incorporation of clays and modified clays during the electrospinning of fibers was in particular showed to be a way to increase the thermal stability of the fibers [18–24]. In this paper, we successfully prepared PLA/montmorillonite composite fibers by electrospinning and investigated the morphological and the thermal characteristics of these fibers. The effect of type and amount of organoclay on thermal behavior were evaluated by pyrolysis mass spectrometry in addition to thermogravimetry analyses.
http://dx.doi.org/10.1016/j.jaap.2017.02.006 0165-2370/© 2017 Elsevier B.V. All rights reserved.
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2.2. Preparation of fibers
Fig. 1. Electrospinning system for generation of nanofibers.
2. Experimental
2.1. Materials Polylactide, (Mn ∼ 190,000) was provided by Cargill Dow. Solvents chloroform, CHCl3 (99%) and dimethylformamide, DMF, (99%) were obtained from Sigma Aldrich. Organically modified montmorillonites, dimethyl ditallow ammonium modified montmorillonites, Cloisite 15 A and 20A (C15A and C20A) and methyl tallow bis-2-hydroxyethyl ammonium modified montmorillonite Cloisite 30 B (C30B), were provided by Southern Clay Products Inc. PLA and clays were dried at 60 ◦ C overnight under reduced pressure prior to mixing processes. PLA/clay composites (1, 3 or 5 wt% inorganic content) were melt-compounded using a DSM Xplore twin screw micro compounder at 190 ◦ C, with a screw speed of 100 rpm for 8 min or the solution was mixed by magnetic (1000 rph).
The electrospinning system was composed of a syringe pump (New Era Pump Systems, Inc NE 300), a high voltage power supply, a 1 mL syringe with metallic needle of 0.6 mm inner diameter, and a collector. The syringe was positioned horizontally on the syringe pump and the positive electrode of the high voltage power supply was clamped to the metal needle (Fig. 1). The electrospinning parameters were optimized as 0.5 mL H−1 for the flow rate of the polymer solution, 12.5 kV for the applied voltage and 10 cm for the tip-to-collector distance. Electrospun nanofibers were collected on a grounded stationary cylindrical metal collector covered by a piece of aluminum foil in an enclosed chamber at around 25 ◦ C and 20% relative humidity. The nanofibers obtained were dried over night at 25 ◦ C under vacuum in order to remove any residual solvent. Recent experiments done by changing the concentration of PLA from 3% to 20% (w/v) in CHCl3 /DMF solvent systems having varying compositions (v/v 100:0, 50:50, 60:40, 70:30, 80:20 and 90:10) indicated longer average distance between the beads on the fibers with the increase in the volume ratio of DMF in the solvent system, and reduction in beads and beaded fibers with the increase in concentration of PLA [18]. However, as the DMF and PLA concentrations were increased thicker fiber diameters were generated. The optimum concentration of PLA in CHCl3 /DMF solvent system was determined as 15% (w/v) PLA in 90:10 v/v CHCl3 /DMF and these concentration values were selected for generation of PLA composite fibers involving 1, 3 or 5 wt% organically modified Cloisite 15A, 20A, or C30 B by electrospinning. 2.3. Characterization The 2 XRD patterns of the composites, at 25 ◦ C in the 1◦ −10◦ range were recorded on a Rigaku X-ray diffractometer (Model, Miniflex) with CuK␣ (30 kV, 15 mA, = 1.54051 Å). Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai G2 Spirit BioTwin CTEM on the specimens cut from the middle portion of injection molded PLA composites used for generation of
Fig. 2. a. XRD diffractograms and b. TEM images of PLA fibers involving 3 wt% C15A, C20A and C30B.
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Fig. 3. SEM images of PLA fibers involving 3 and 5% Cloisite 15 A, electrospun 15% (w/v) PLA/nanoclay in 90:10 v/v CHCl3 /DMF solvent systems.
fibers. Scanning electron microscope (SEM, Quanta 400 FEG, FEI) was used to investigate the morphology of nanofibers. Samples were coated with 5 nm Au/Pd (PECS-682) prior to the SEM imaging. Thermogravimetry analyses (TGA) were performed on a Perkin Elmer Instrument STA6000 having temperature accuracy and reproducibility ±0.5 ◦ C under nitrogen atmosphere at a flow rate of 20 mL/min and a heating rate of 10◦ C/min. Direct pyrolysis mass spectrometry (DP-MS) analyses were performed on a 5973 HP quadrupole mass spectrometry system, with a mass range 1.6–800 Da and resolution of unit mass, coupled to a JHP SIS direct insertion probe pyrolysis system, 70 eV EI mass spectra, at a rate of 2 scan s−1 , were recorded. About 0.10 mg of PLA composite fibers in flared glass sample vials, were heated to 450 ◦ C at a rate of 10 ◦ C/min. All experiments were repeated at least twice to assess reproducibility.
3. Results and discussions 3.1. Morphology of PLA/montmorillonite composites The diffraction pattern of Cloisite 15A, three peaks at around 2 = 3.34, 5.18 and 7.72◦ corresponding to inter-reticular distances of d001 = 2.64, 1.70 and 1.16 nm, almost totally disappeared and a weak peak located at around 1.8◦ corresponding to interreticular distance of d001 = 2.6 nm and small budges located around 4◦ were detected (Fig. 2a). Cloisite 20A shows two broad peaks at around 2 = 3.9 and 7.4◦ corresponding to inter-reticular distances of d001 = 2.21 and 1.19 nm in its diffractogram. A weak peak located at around 3.0◦ was observed in the XRD of PLA/C20A. The peak at around 2 = 5.0◦ corresponding to an inter-reticular distance of d001 = 1.8 nm present in the diffractogram of Cloisite 30 B was almost totally disappeared for all the samples of PLA/C30 B nanocomposite. The XRD diffractograms of PLA/15A and PLA/20A fibers show very weak peaks around 3.3◦ .
The TEM images taken from representative regions of the PLA nanocomposites showed mainly regions of tactoids. Yet, especially for PLA/C15A and PLA/20A single unit layers can be observed indicating presence of both intercalated and exfoliated structures. 3.2. Generation of PLA composite fibers The SEM images of PLA fibers involving C15A, C20A and C30 B depicted in Figs. 3–5 respectively revealed generations of beadfree fibers having diameters around 400–200 nm indicating slightly narrower fibers compared to neat PLA fibers in the presence of organically modified montmorillonites. As the amount of clay present in the composite was increased, the fiber diameter became slightly narrower. A similar behavior was observed by Shia and coworkers for electrospun fibers of PLA incorporated with cellulose nanocrystals [23]. It has been proposed that the quaternary ammonium salt present as an organic modifier in the nanoclays increases the electrical conductivity, thus the diameter of fibers formed become substantially smaller as the amount of nanoclay is increased [23,24]. However, high magnification of the SEM images indicated non-homogenous surface structure especially for the PLA fibers involving 5 wt% nanoclays, most probably due to aggregation of clay at high loading levels. In Fig. 6, SEM images of neat PLA fiber and PLA composite fibers involving 3 wt% C15A, C20A and C30 B are shown together for comparison. It can be noted from this figure that the diameters of the fibers were smallest for PLA/C15A composite and largest for PLA/C30 B composite. This behavior may directly be related to amount of organic modifier present in the nanoclay. 3.3. DSC analyses In Fig. 7, DSC images of PLA and PLA fibers involving 3% C15A, C20A and C30 B prepared by melt blending are shown. The endothermic melting peak of PLA at around 172 ◦ C, was shifted
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Fig. 4. SEM images of PLA fibers involving 1, 3 and 5% Cloisite 20 A electrospun 15% (w/v) PLA/nanoclay in 90:10 v/v CHCl3 /DMF solvent systems.
to 167.5, 169.2 and 168.8 for the corresponding fibers involving 3 wt% C15A, C20A or C30 B respectively, indicating a smaller decrease in the melting temperature upon fiber formation [22]. The cold crystallization peaks observed for all the PLA composite fibers were associated with the nanofiller-induced nucleating effect. As a consequence, the rate of crystallinity was accelerated generating crystalline defects and yielding less ordered crystals melting at lower temperatures. Lowest melting and cold crystallization temperatures were detected for PLA involving C15A, whereas, highest values were recorded for the composite involving C30B.
Table 1 TGA dataa for PLA composite fibers involving 3 wt% C15A, C20A and C30B. T5%
T10%
Tmax
% char
PLA
324.4
332.2
355.1
0.37
PLA-C15A PLA-C15A fiber
318.2 314.1
331.3 327.3
360.9 361.2
1.16 3.22
PLA-C20A PLA-C20A fiber
323.4 322.5
334.5 333.2
362.0 361.8
1.71 2.43
PLA-C30B PLA-C30 B fiber
328.3 324.9
337.6 334.9
363.3 363.7
2.05 1.91
a
Temperature accuracy and reproducibility is ±0.5 ◦ C.
3.4. Thermal analyses Thermal decompositions of PLA composites involving C15A and C20A and the corresponding fibers started at slightly lower temperatures compared to neat PLA. However, for all the composites and their corresponding fibers noticeably higher Tmax values compared to neat PLA were recorded (Fig. 8). Although the degradation of fibers started at slightly lower temperatures than the corresponding composites, their Tmax values were almost identical and the decomposition of PLA nanocomposite fibers occurred over a
broader temperature region [22]. In Table 1, TGA data for PLA composite fibers involving 3 wt% C15A, C20A and C30 B are collected. The DP-MS analyses of PLA and its electrospun fiber indicated decrease in thermal stability of PLA upon fiber formation [22]. On the other hand, noticeable increase in thermal stability was detected upon incorporation of organically modified nanoclays, Cloisite 15A, 20A and 30 B into PLA matrix [25]. Although identical products were detected during the pyrolysis of PLA, its fiber and PLA composites, the relative intensities of the products gen-
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Fig. 5. SEM images of PLA fibers involving 1, 3 and 5% Cloisite 30 B electrospun 15% (w/v) PLA/nanoclay in 90:10 v/v CHCl3 /DMF solvent systems.
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Fig. 6. SEM images of PLA fiber and PLA fibers involving 3% Cloisite 15A, 20A or 30 B electrospun 15% (w/v) PLA/nanoclay in 90:10 v/v CHCl3 /DMF solvent systems.
erated by trans-esterification and cis-elimination reactions were changed noticeably. The elimination of these products at lower temperatures during the pyrolysis of PLA fiber indicated that the inter-molecular interactions in the aligned fiber structure are more effective, and take place at lower temperatures. In contrast, increase in the relative yields of the products due to cis-elimination reactions were detected during the pyrolysis of PLA/nanoclay composites [25]. The increase was more pronounced for the products generated
by cis-elimination reactions, especially for the composite involving C30B. In Fig. 9, the TIC, (the variation of total ion yield as a function of temperature) curves and the mass spectra recorded at peak maxima of PLA fibers involving 3% organically modified montmorillonites C15A, C20A or C30 B electrospun from 15% (w/v) CHCl3 /DMF solution system (v/v = 90:10) are depicted. The maximum product yields are detected at 387, 398 and 410 ◦ C in the presence of C15A,
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Fig. 7. DSC images of PLA and PLA fibers involving 3% C15A, C20A and C30 B prepared by melt blending.
C20A and C30 B respectively indicating noticeable enhancement in thermal stability not only compared to PLA fibers with lowest thermal stability but also compared to PLA and corresponding PLA composites. The noticeable increase in the thermal stability of PLA composite nanofibers compared to the corresponding PLA/organoclay composites may be associated with the increase in extent of delamination and better insertion of aligned PLA chains in fiber structure into clay layers. Single ion evolution profiles of some of the representative fragments of PLA, such as C2 H4 CO+ (56 Da) generated by homolysis reactions and dissociative ionization of high mass products, (C2 H4 CO2 )x H+ (217, 361 and 577 Da for x = 3, 5 and 8 respectively) due to cis-elimination reactions and (C2 H4 CO2 )x C2 H4 CO+ (200, 344 and 704 Da for x = 2, 4 and 9 respectively) generated by trans-esterification reactions are depicted in Fig. 10. The evolution profiles of the most abundant product of the organic modifiers, 296 Da fragment ion recorded during the pyrolysis of PLA composites are also included in Fig. 10. Contrary to the trends observed for PLA, its fiber and PLA nanocomposites, the evolution profiles of the products generated by cis-elimination and trans-esterification reactions during the pyrolysis of PLA composite fibers show similar behavior. The relative yields of the high mass products generated by trans-esterification and cis-elimination reactions were significantly diminished. For PLA fiber, the increase in the relative yields of products generated by trans-esterification reactions was associated with the increase in intermolecular interactions in the aligned structure [22]. It may be thought that the presence of tactoids and intercalated structures restricts the intermolecular interactions of PLA chains at least to a certain extent in the fiber structure. The loss of organic modifiers was detected over a broad temperature range. For a better understanding of the effects of electrospinning process and fiber formation on organoclays, the evolution profiles the characteristic degradation products of the organic modifiers, C15A, C20A and C30 B recorded during the pyrolysis of organo-nanoclays, PLA/organoclays and their fibers are compared. Pyrolysis mass spectra of the organically modified montmorillonites, C15A, C20A and C30 B were dominated with diagnostic peaks for amines and were almost similar to the free organic modifiers showing the base peak at 296 Da and intense peaks at 268 and 56 Da. These peaks were attributed to C18 H37 (CH3 )NHCH2 + (296 Da), C16 H33 (CH3 )NCH2 + , (268 Da) and C3 H6 N+ , (56 Da) fragments for C15A and C20A, whereas for C30B, were associated with C16 H31 N(CH2 )C2 H4 OH+
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Fig. 8. TGA data for PLA composite fibers involving 3 wt% C15A, C20A and C30B.
(296 Da), C14 H27 N(CH2 )C2 H4 OH+ (268 Da) and C3 H6 N+ , (56 Da) fragments for C30 B [25]. In Fig. 11, the single ion evolution profiles of the most abundant characteristic degradation product, with m/z value 296 Da due to C18 H37 N(CH3 )2 + (296 Da) for C15A and C20A and C16 H31 N(CH2 )C2 H4 OH+ fragments for C30 B recorded during the pyrolysis of organonanoclays, PLA/organoclays and their fibers are shown in Fig. 11 as an example. It is clear that the loss of organic modifier shifted to low temperature regions upon incorporation of organically modified montmorillonites, C15A and C20A into PLA matrix. This behavior may be associated with separation of clay layers. In a recent study, it has been determined that as the spacing between the layers of clay increases the loss of organic modifier takes place at lower temperatures [25]. However, during the pyrolysis of PLA/C15A and C20A fibers, significant amounts of the organic modifier of C15A and C20A were lost immediately, upon insertion of pyrolysis probe inside the mass spectrometer. The loss of organic modifier even at room temperature may be associated not only with the separation of clay layers in accordance with the literature findings but also with the degradation of the organic modifier to a certain extent during the electrospinning process. The elimination of organic modifier of C30 B was also observed at initial stages of pyrolysis confirming the decomposition of the organic modifier, but, was noticeably low. Contrary to what was observed for C15A and C20A involving composites and fibers, the single ion evolution profiles of characteristic fragments of the organic modifier of C30 B were shifted to high temperature regions during the pyrolysis of PLA/C30 B composite and its fiber. It may be thought that, as also proposed in our previous work, due to the trans-esterification reactions between the ester groups of PLA and the hydroxide groups of organic modifier, a crosslinked structure that decomposes at high temperatures eliminating the organic modifier in the temperature region where PLA decomposition occurred, was generated [25]. The highest thermal stability of PLA/C30 B fiber can then be associated with also the crosslinking effect of the organic modifier of C30B. 4. Conclusions PLA composite nanofibers involving variable amounts of Cloisite 15A, Cloisite 20A, and Cloisite 30B were prepared by electrospinning successfully. The fiber diameter became slightly narrower compared to neat PLA fibers as the amount of Cloisite present in the composite was increased. The diameters of the fibers were smallest for PLA/C15A composite and largest for PLA/C30 B composite. This behavior was directly be related to the increase in electrical
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Fig. 9. a) The TIC curves and b) the pyrolysis mass spectra recorded at peak maxima detected during the pyrolysis PLA fibers involving I. C15A, II. C20A and III. C30B.
Fig. 10. Single ion evolution profiles of representative products of detected during the pyrolysis PLA, PLA-fiber and PLA fibers involving C15A, C20A and C30B.
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Fig. 11. Single ion evolution profiles of most abundant fragment of organic modifiers recorded during the pyrolysis of organoclays, PLA/organoclay composites, and PLA/organoclay fibers.
conductivity of the solution with the increase in the amount of the quaternary ammonium salt present as an organic modifier in the nanoclays. The loss of organic modifiers of Cloisites especially those of C15A and C20A indicated increase in extent of exfoliation and degradation of the modifiers at least to a certain extent during the electrospinning process. The noticeable increase in the thermal stability of PLA composite nanofibers compared to the corresponding PLA/organoclay composites may then be associated with the increase in extent of delamination and better insertion of aligned PLA chains in fiber structure into clay layers. In addition, decrease in the relative yields of high mass pyrolysis products generated by trans-esterification reactions was detected compared to PLA fiber indicating reduction in intermolecular interactions in the presence of nanoclays. Acknowledgment This work is partially supported by TUBITAK Research Fund112T493. References [1] N. Cai, Q. Dai, Z. Wang, X. Luo, Y. Xue, F. Yu, Preparation and properties of nanodiamond/poly(lactic acid) composite nanofiber scaffolds, Fiber Polym. 15 (2014) 2544–2552. [2] Y.X. Kong, J. Yuan, Z.M. Wang, J. Qui, Study on the preparation and properties of aligned carbon nanotubes/polylactide composite fibers, Polym. Compos. 33 (2012) 1613–1619. [3] T. Yang, D. Wu, L. Lu, W. Zhou, M. Zhang, Electrospinning of polylactide and its composites with carbon nanotubes, Polym. Compos. 32 (2011) 1280–1288. [4] F. Mei, J.S. Zhong, X.P. Yang, Improved biological characteristics of Poly(l-Lactic Acid) electrospun membrane by incorporation of multiwalled carbon nanotubes/hydroxyapatite nanoparticles, Biomacromolecules 8 (2007) 3729–3735. [5] S.J. Shao, S.B. Zhou, L. Li, J.R. Li, C. Luo, J. Wang, X.H. Li, J. Weng, Osteoblast function on electrically conductive electrospun PLA/MWCNTs nanofibers, Biomaterials 32 (2011) 2821–2833. [6] Y. Zhu, C. Li, P. Cebe, Poly(lactides co-electrospun with carbon nanotubes: thermal and cell culture properties, Eur. Polym. J. 75 (2016) 565–576. [7] Y. Dong, T. Bickford, H.J. Haroosh, K.T. Lau, H. Takagi, Multi-response analysis in the material characterization of electrospun poly(lactic acid)/halloysite nanotube composite fibres based on Taguchi design of experiments: fibre diameter, non-intercalation and nucleation effects, Appl. Phys. A-Mater. Sci. Process 112 (2013) 747–757. [8] H.J. Haroosh, Y. Dong, D.S. Chaudhary, G.D. Ingram, S. Yusa, Appl. Phys. A-Mater. Sci. Process 110 (2) (2013) 433–442.
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Please cite this article in press as: E. Ozdemir, J. Hacaloglu, Polylactide/organically modified montmorillonite composite fibers, J. Anal. Appl. Pyrol. (2017), http://dx.doi.org/10.1016/j.jaap.2017.02.006