- Email: [email protected]
Viscoelastic behavior of core-shell structured nanofibers of PLA and PVA produced by coaxial electrospinning
Accepted Manuscript Viscoelastic behavior of core-shell structured nanofibers of PLA and PVA produced by coaxial electrospinning Hamad F. Alharbi, Monis Luqman, H. Fouad, Khalil Abdelrazek Khalil, Nabeel H. Alharthi PII:
S0142-9418(18)30098-9
DOI:
10.1016/j.polymertesting.2018.02.026
Reference:
POTE 5350
To appear in:
Polymer Testing
Received Date: 16 January 2018 Accepted Date: 21 February 2018
Please cite this article as: H.F. Alharbi, M. Luqman, H. Fouad, K.A. Khalil, N.H. Alharthi, Viscoelastic behavior of core-shell structured nanofibers of PLA and PVA produced by coaxial electrospinning, Polymer Testing (2018), doi: 10.1016/j.polymertesting.2018.02.026. 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 Material Properties
Viscoelastic behavior of core-shell structured nanofibers of PLA and PVA produced by coaxial electrospinning Hamad F. Alharbi1,*, Monis Luqman1, H. Fouad2,3, Khalil Abdelrazek Khalil4, Nabeel H. Alharthi1 1
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Mechanical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia Department of Applied Medical Science, Riyadh Community College, King Saud University, Riyadh, 11437, Saudi Arabia 3 Faculty of Engineering, Helwan University, Department of Biomedical Engineering, P.O. Box 11792, Egypt 4 Mechanical Engineering Department, College of Engineering, University of Sharjah, Sharjah, 27272, United Arab Emirates
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2
Abstract
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In recent work, we demonstrated the successful fabrication of core-shell nanofiber composites with enhanced tensile, surface wetting and cytocompatibility properties via coaxial electrospinning using PLA and PVA. It has been shown that core/shell-structured PLA/PVA nanofiber mat has the hydrophilic benefits of PVA and the attractive biological properties of PLA. In this paper, we present detailed mechanical evaluation of the electrospun coaxial nanofiber mats under static, dynamic and creep loading. The core/shell-PLA/PVA
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nanofibers showed nearly 233% and 150% increase in tensile strength and ductility, respectively, compared to pristine PLA. Dynamic loading tests were employed to study the viscoelasticity of the coaxial core-shell composite nanofibers at various temperatures and frequencies. The results of the storage and loss moduli of the coaxial nanofibers suggested
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strong physical interaction between the PLA and PVA layers, that contribute to the observed good mechanical behaviour. The better creep resistance of the coaxial PLA/PVA nanofibers
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under axial loading compared to the pristine materials provides further evidence for the physical interaction between the two constituent materials. A simple linear viscoelastic model is used to quantify the evolution of creep strain in pristine and coaxial materials. This research may help to understand the physical relation between the observed viscoelastic behaviour and the internal structure of coaxial core/shell nanofibers of PLA and PVA.
Keywords: Poly lactic acid; Poly vinyl alcohol; coaxial electrospinning; viscoelastic behaviour *
Corresponding author. E-mail address: [email protected] (Hamad F. Alharbi)
ACCEPTED MANUSCRIPT 1.
Introduction Poly Lactic Acid (PLA) nanofiber, a synthetic thermoplastic biopolymer, has attracted
considerable attention in biomedical applications owing to its excellent biodegradability and non-toxicity [1]. However, the brittleness, low strength and hydrophobicity of PLA hinder its practical applications in important applications like tissue engineering. Many efforts have
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been devoted to improve the mechanical and surface properties of PLA by incorporating other materials [2-4]. Different types of polymers have been used to form PLA-based nanofiber composites with better properties, including poly(vinyl pyrrolidine) (PVP) [5], poly (glycerol sebacate) (PGS) [6], polydopamine (PDA) – cellulose nanofibrils (CNF) [7],
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poly(ethylene glycol) [8] and poly(vinyl alcohol) (PVA) [9].
Polymer-based nanoscale composites can be produced by different fabrication
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methods. Electrospinning is one of the most prominently used techniques for producing micro- to nano-metre fibers because it is a simple, cheap and versatile process [10]. A modified version of this technique, called coaxial electrospinning, has been recently used to prepare core-shell composite nanofibers with dissimilar materials in the core and shell [11, 12]. Coaxial electrospinning has shown tremendous promise to meet the next generation of polymers by producing unique nanoscale morphologies while maintaining the separate
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identities of the two constituent materials. In particular, this approach has gained great attention in drug delivery and tissue engineering by employing, for example, materials with different degradability rates [5, 13], biocompatible shell material with less biocompatible core material [14], and strong core material for reinforcing the structures [15].
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In many biomedical areas, the successful application of the synthetic nanostructure biopolymers depends largely on the mechanical behaviour. Important mechanical properties
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include high strength, good ductility, stable dynamic response under various temperature and strain rate, and good creep resistance under fixed applied load. The balance between these different attractive properties, along with deep understanding of the underlying mechanisms responsible for the improved properties, are a great challenge in most composite materials. In our last study, we demonstrated the successful fabrication of core-shell nanofiber composites with enhanced mechanical, surface wetting, and cytocompatibility properties via coaxial electrospinning of PLA and PVA [16]. In this paper, the aim is to expound the mechanical properties of this promising coaxial nanofiber composites under static, dynamic and creep deformation. More specifically, the strength and ductility under quasi-static tensile deformation, the storage and loss moduli under dynamic loading over a wide range of
ACCEPTED MANUSCRIPT temperature and frequency, and the evolution of creep strains under constant load are reported and discussed in detail. This investigation has also shed light on possible deformation mechanisms, and supports the notion of physical interaction between the constituent PLA and PVA materials. Such physical interaction is believed to strongly affect the segmental mobility of polymer chains and contribute to the excellent mechanical
2.
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behaviour observed.
Experimental procedures
2.1. Fabrication of core-shell structure of PVA and PLA composite nanofibers using
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coaxial electrospinning
Composite nanofibers with core-shell configurations of PVA/PLA and PLA/PVA were
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fabricated using coaxial electrospinning. To better examine the mechanical performance of the synthetic coaxial electrospun nanofiber mats compared with the constituent materials, pristine PLA and PVA nanofibers via single-nozzle electrospinning were also fabricated. A detailed description of the fabrication process of coaxial and pristine electrospun PLA and PVA nanofibers is described by Alharbi et al. [16]. For completeness, the fabrication process is briefly summarized in the following paragraph.
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A solution of PVA was first prepared by dissolving poly vinyl alcohol in distilled water and ethanol with a ratio of 9:1. The mixture of 10 wt.% was heated at 75 ºC for about 1 hour. For PLA solution, another viscous solution of 8 wt.% poly lactic acid was prepared by adding chloroform and dimethylformamide (DMF) with a ratio of 8:2. The mixture was then stirred
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at room temperature for about 5 hours. The general setup of the apparatus is shown in Figure 1. In this custom-made system, the spinneret was modified by inserting a concentric smaller
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(inner) needle inside the bigger (outer) needle to produce co-axial configuration. The diameters of the inner and outer needles used in this study were 0.8 mm and 1.2 mm, respectively. The outer needle was attached to a syringe pump containing the shell solution (either PLA or PVA) and the inner was connected to a pump having the core solution (either PVA or PLA). The core and shell solutions were delivered at feeding rates of 0.1 ml/h and 0.2 ml/h, respectively. The applied voltage was 19 KV and the rotating speed of the cylindrical drum that was used to collect the coaxial electrospun nanofibers was 80 rpm. The electrospinning process was run for at least 8 hours with a distance of 14 ±1 cm between the needle tip and the collector until a non-woven nanofiber membrane was produced. To compare the mechanical performance of the core-shell composite nanofibers with the original
ACCEPTED MANUSCRIPT pure material, pristine PLA and PVA nanofibers using the same PVA and PLA solutions were also fabricated. Here, a typical electrospinning apparatus with single-nozzle was used to produce the pure structure of PVA and PLA electrospun nanofiber.
2.2. Morphological observation
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Morphological analysis of the pristine and coaxial composite nanofibers was performed using a Field-Emission Scanning Electron Microscope (FE-SEM JSM-7600) supplied by JOEL Ltd., Japan. The nanofiber samples were platinum coated by electron sputtering before the microscopic examination. The diameter of many different nanofibers were measured. A
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transmission electron microscopy (TEM) supplied by JOEL Ltd., Japan (TEM JSM-2100) was used to authenticate the core/shell structure of the fibers. The TEM sample was prepared
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by collecting fibers directly onto the copper grid.
2.3. Thermal analysis
Differential scanning calorimetry (DSC) was performed on pristine and coaxial nanofiber samples using a SDT Q600 machine, TA-USA. Samples of 5-8 grams were placed in ceramic pans. The temperature was increased from room temperature to about 300° C at a
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rate of 2 ºC/min.
2.4. Quasi-static mechanical properties
The tensile mechanical properties of the electrospun pristine PLA and PVA as well as
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the electrospun PVA/PLA and PLA/PVA core-shell mats were measured using a uniaxial tensile test. A dumbbell test sample of each material was cut following ISO 527-1:2012. The
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gauge length and thickness of each sample were approximately 10 mm and 0.2 mm, respectively. A micro-tensile test machine (MTD-500 PLUS) was used to run the tensile test at room temperature with 0.5 mm/min crosshead speed. Five nanofiber membrane samples from each type were tested in order to check the repeatability of the data.
2.5. Dynamic mechanical analysis Dynamic mechanical analysis machine (AR-G2 DMA (TA Instruments, USA)) was used to evaluate the viscoelastic behaviour of the four fabricated electrospun pristine and composite nanofiber sheets (PLA, PVA, core/shell-PLA/PVA, core/shell-PVA/PLA). The specimens were cut from the nanofibrous mats in rectangular shape with initial dimensions of 20 mm (length) x 10 mm (width) x 0.3 mm (thickness). The storage
and loss
ACCEPTED MANUSCRIPT moduli were determined in tension mode over (1) different temperature range with fixed frequency, and (2) different frequency range at room temperature. For the first run, the tests were carried out over a temperature range from 10 °C to 150 °C at a heating rate of 3.0 °C /min and frequency of 1 Hz. The second run was conducted over a frequency range from 0.01 Hz to 100 Hz at room temperature (37 °C). All runs were carried out under 0.05 % strain and
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0.5 N axial force. A minimum of two samples of each material were tested, and the data was found to have low variability.
2.6. Creep – recovery testing
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A creep-recovery testing was conducted to examine the response of the electrospun nanofibrous mats to constant load and its behaviour after removal of that load. The same DMA machine described above was used for the creep – recovery testing with similar sample
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dimensions of about 20 mm (length) x 10 mm (width) x 0.3 mm (thickness). The creep test was conducted under a constant load of about 4 N for about 4.0 hours at room temperature. The materials were then recovered for another 4.0 hours at the same temperature. The strain value was recorded every 2 seconds.
Results and discussion
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3.
3.1. Morphological aspects
The SEM images of pristine PLA, PVA and core/shell-structures of PVA/PLA and PLA/PVA nanofiber mats are depicted in Error! Reference source not found.. The figure
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illustrates the successful fabrication of uniform nanofibers with average diameters of 170 ± 15 nm and 200 ± 20 nm, respectively, for the pristine and coaxial electrospun nanofibers.
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During the SEM analysis of the coaxial composite nanofibers with PVA in the core and PLA in the shell, few fibers were observed to have broken shells due to the brittle nature of PLA. The core structure was clearly revealed, providing evidence for the successful fabrication of a core/shell structure using coaxial electrospinning (see Figure 3). The core/shell structure of the coaxial nanofiber was also confirmed using TEM analysis, as shown in Figure 3.
3.2. Thermal properties The DSC analysis of the nanofiber mats is shown in Figure 4. The DSC result of PVA indicates that the peaks related to glass transition and melting point occurred at about 59 °C
ACCEPTED MANUSCRIPT and 220 °C, respectively. For the PLA nanofiber sheet, the peaks of the glass transition and melting point observed at about 65 °C and 155 °C, respectively. The results of the coaxial PLA/PVA and PVA/PLA nanofibers were similar. The glass transition and melting temperature can be observed at about 57 oC and 146 oC, respectively. It is interesting to note that the coaxial composite nanofibers mats have lower glass transition temperature and
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melting temperature than the pristine nanofiber mats. These lower temperatures will influence the thermo-mechanical response of the composite nanofiber mats, as will be demonstrated in the following sections.
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3.3. Mechanical properties
The mechanical properties of the electrospun nanofibers are best described by tests on
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both the single-electrospun fiber and the fiber mat. However, the characterization of the mechanical properties of single-electrospun fiber is challenging [17-21]. For this reason, most of the reported mechanical properties of electrospun nanofibers are solely based on the measurements of the nanofibrous mats using a universal testing machine [22, 23]. In this context, one should be aware that the mechanical properties of the nanofiber mats can be influenced by other factors such as the distribution of nanofiber diameters, length, alignment
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and entanglements of the fibers inside the mats [24, 25]. Furthermore, for composite coreshell electrospun nanofibers, the type of polymers in the core and shell as well as the chemical and/or physical interactions between the layers have additional contributions to the
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observed mechanical response of the composite mats. The experimental quantification of all these features (nanofiber diameter, length, alignment, entanglement, defects, core/shell material and interaction) in a statistical representative framework and linking them to the
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measured mechanical response demand major investments in time and effort. The reported mechanical results can give useful trends about the expected mechanical performance of electrospun nanofibrous materials, but more experimental work might be needed for critical applications.
In this study, the mechanical performance of the fabricated electrospun core/shellstructured PLA/PVA and core/shell-structured PVA/PLA was examined using three different techniques including 1) quasi-static tensile mechanical response, 2) dynamic mechanical analysis, and 3) creep-recovery analysis. The mechanical response of the coaxial composite materials were also compared with the basic constituent materials, pristine PLA and PVA.
ACCEPTED MANUSCRIPT 3.3.1. Quasi-static mechanical properties Typical tensile stress-strain curves of the pristine PVA, pristine PLA, core/shellstructured PLA/PVA and core/shell-structured PVA/PLA are shown in Figure 5. The values of the ultimate tensile strength and strain at failure are listed in Table 1. The result of the pristine PLA sheet showed the expected brittle and weak mechanical response with a tensile
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strength of about 4.2 ± 0.15 MPa and a final strain of about 0.4. The pristine PVA sheet exhibited better mechanical properties compared to pristine PLA, with increase in strength and ductility of about 143% and 90%, respectively. The average values of the tensile strength and strain at failure for pristine PVA were estimated to be about 10.2 ± 1.2 MPa and 0.76.
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The mechanical response of pure PLA and PVA have been reported in many studies (see for example, Refs. [5, 26, 27]). The overall mechanical properties of the core/shell-structured nanofiber mats are expected to depend on the contribution of both the core and shell
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nanofibers. The quasi-static tensile behaviour of the coaxial electrospun core/shell composite nanofiber sheets produced in this study have different response compared to pristine PLA and PVA. There was no major improvement observed in the mechanical response of the core/shell-structured PVA/PLA nanofiber mats. The measured tensile strength and strain at failure for this material were estimated to be 5.9 ± 0.85 MPa and 0.45, respectively (see
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Figure 5). These values are close to those of pristine PLA (4.2 MPa and 0.4) and much less than those of pristine PVA (10.2 MPa and 0.76). However, the core/shell-structured PLA/PVA nanofiber mat shows significant improvements in both strength and ductility compared to pristine PLA and PVA. This coaxial PLA/PVA mat exhibits a tensile strength of
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about 14.0 ± 1.25 MPa and a good ductility with a strain at failure of about 1.0. The tensile strength of this material was nearly 233 % and 37 % increase compared to pristine PLA and
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PVA, respectively. The ductility was also improved by about 150 % and 32 % compared to pristine PLA and PVA, respectively. Thus, the coaxial electrospun core-shell nanofiber mats with PLA in the core and PVA in the shell exhibited significant improvements in both tensile strength and ductility compared to the electrospun PLA or PVA alone. As discussed above, the mechanical properties of composite core/shell nanofiber scaffolds can be strongly influenced by different factors, including the type of polymers in the core and shell, chemical and/or physical interactions between the two layers, nanofibers diameter and length distributions, entanglement, molecular weight, processing history and defects. Because of these many different factors and the challenge of statistically quantifying their separate effects experimentally, there is large variability in the reported mechanical properties of electrospun composite mats and the exact underlying mechanisms are usually
ACCEPTED MANUSCRIPT difficult to elucidate with high certainty. For example, Sun et al. [5] has investigated the mechanical properties of core/shell structure of poly(vinyl pyrrolidine) (PVP) and PLA ultrafine fibers produced by coaxial electrospinning and found that the tensile modulus and tensile strength of the core/shell-structured PVP/PLA membrane were lower than those of the electrospun pure PLA membrane. On the other hand, Merkle et al. [26, 28] has reported an
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increase in the values of Young’s modulus and ultimate strength in the core/shell structure of PVA/gelatin composite scaffolds produced by coaxial electrospinning when compared with the PVA or gelatin scaffolds. Such improvements were related to the possible enhancement of the molecular alignment of core PVA by the gelatin shell, which was assumed to transform
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the PVA from weak plastic PVA into a strong elastic PVA. The mechanical properties of poly (glycerol sebacate) (PGS)-PLA core-shell membranes produced via coaxial electrospinning were investigated in detail using uniaxial tensile testing in Ref. [6]. This
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study examined the influence of the shell and core solutions and the addition of poly (ethylene oxide) (PEO) to the shell on the overall mechanical properties of the fabricated membranes, but not compared with the individual core or shell nanofibrous mats. In another study by HUANG et al. [15], it was shown that the ultimate strength and ultimate strain of coaxially electrospun nanofibers with gelatin in the core and poly(ε-caprolactone) (PCL) in
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the shell were at least 68% and 244% higher, respectively, than those of pure gelatin and PCL nanofibers. These improved mechanical properties were achieved using 7.5% w/v gelatin as the core and 10% w/v PCL as the shell solutions, which produced the finest nanofibrous membrane resulting in the largest contact area and highest bond strength in between the
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nanofibers. The exact underlying mechanisms for the significant improvement in the mechanical properties of the coaxial gelatin/PCL composite nanofibers compared to the pure
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gelatin and PCL nanofibers were not discussed in detail. In this study, the tensile strength and ductility of the fabricated core-shell PLA/PVA composite nanofiber scaffolds were significantly enhanced compared to the electrospun PLA or PVA alone. The improvement in these mechanical properties can be related to some sort of interactions between the PVA and PLA layers. The DMA analysis, explained in the following section, supports the notion of possible interactions between the constituent PLA and PVA materials. Furthermore, since the process history and the degree of dense packing of the electrospun mats can influence the mechanical properties, the weight of all electrospun nanofibrous mats with similar dimensions (~ 10 mm x 10 mm x 0.2 mm) were measured. The weight of PLA, PVA, PLA/PVA, and PVA/PLA samples were found to be 3.4 mg, 3.37 mg, 3.94 mg and 3.67 mg, respectively. The weight of the coaxial core-shell PLA/PVA and
ACCEPTED MANUSCRIPT PVA/PLA mats were higher than the pristine materials by about 16 % and 8 %, respectively. This increase in weight may also contribute to the increase of strength for the coaxial PLA/PVA and PVA/PLA composite nanofiber mats. Additionally, the lower glass transition of the coaxial nanofiber mats compared to the pristine materials (see Figure 4) can influence the ductility of the material. This is related to the fact that deceasing the thermal stability of
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polymers generally make the polymer more soft and ductile.
Table 1: The values of the tensile strength and strain at failure for pristine and coaxial nanofiber sheets tested in this study
4.2 ± 0.15
10.2 ± 1.2
0.4
0.76
3.3.2. Dynamic mechanical analysis
Core/shellPLA/PVA
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PVA
Core/shellPVA/PLA
14.0 ± 1.25
5.9 ± 0.85
1.0
0.44
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Tensile strength [MPa] Strain at failure
PLA
The dynamical mechanical analysis was carried out to obtain the viscoelastic behaviour of the coaxial nanofibrous mats compared to the constituent pure materials under different temperature and frequency ranges. Figure 6 shows the storage and loss moduli versus
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temperature for the pristine PLA, PVA and core/shell-structures of PVA/PLA and PLA/PVA nanofiber mats under a fixed frequency of 1 Hz. It is obvious that all four materials display three different regions with increasing temperatures, the glassy region, glass transition region
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and rubbery region [29]. For all four materials, the storage modulus is high in the glassy region due to the hard chain mobility. With increasing temperature, the vibrational and thermal energy increase causing high mobility of the polymer chains in the transition region,
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which lead to sharp drop in the storage modulus [29]. After the glass transition temperature, the polymer tends to act more like a viscous material than the elastic (solid) response. Subsequently, the storage modulus reached a plateau in the rubbery region due to strong local interactions between neighbouring chains. It can be seen that the storage modulus of PLA increased slightly above 100 °C, which is usually related to the effect of cold crystallization process [30, 31]. The values of the storage modulus
in the glassy region are about 210 MPa, 150 MPa,
and 20 MPa for the pristine PVA, core/shell composite and pristine PLA nanofibrous mats, respectively (see Figure 6 (a)). Although, the storage modulus of the pristine PVA is higher in the glassy region, the values of the storage modulus for the coaxial composite nanofibrous
ACCEPTED MANUSCRIPT mats in the rubbery regions are comparable to that of PVA. It is also interesting to note that the rate of decrease of the storage modulus in the transition region is higher in the pristine PVA compared to the core-shell PLA/PVA composite nanofiber mats. To make this clear, we show in Figure 7 the derivative storage modulus as a function of temperature. This figure reflects the variation rate of the material structure with increasing temperature. It is clear that
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the PLA/PVA core-shell exhibits better heat resistance since the rate of decrease is lower and occurs at a higher temperature compared to all other materials investigated in this study. This figure also shows that the shape of the derivative storage modulus for the coaxial PLA/PVA
sharp drops in the curve. Figure 6 (b) shows the loss modulus
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composite combines the characteristics of both pristine PLA and PVA with two separate
as a function of temperature for pristine and
coaxial nanofiber mats. The loss modulus increased and reached a maximum in the glass
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transition region and then decreased in the rubbery region. The peak values are usually reported to occur at or very close to the glass transition temperature. In this figure, the corresponding temperature for the peak values are about 62 °C, 57 °C, 58.3 °C and 63 °C for the pristine PLA, pristine PVA, PVA/PLA and PLA/PVA nanofiber mats, respectively. These values are slightly higher than the glass transition temperature obtained by using DSC (see
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Figure 4). It was reported in other studies that DMA experiments usually give higher values for the glass transition temperature compared to DSC [32]. It is obvious from Figure 6 (b) that the intensity of the loss modulus peak decreased for the coaxial core/shell nanofiber composites compared to neat PVA. It is also interesting to note that the peak of the loss
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modulus curve for the PLA/PVA composite is shifted to the right (to higher temperature) compared to the pristine PLA and PVA. Additionally, the loss modulus curve of the coaxial
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PLA/PVA composite is more shallow (broader) in the transition region compared to the other materials.
The shift of the peak value and the broadening in the loss modulus curve are usually related in other composite polymer materials to some sort of local physical interactions. For example, Jonoobi et al. [31] observed a shift in the tan δ position (tan δ curve is obtained by dividing the loss modulus by the storage modulus) when adding cellulose nanofiber reinforcements to neat PLA. The shift in the curve was related to the physical interaction between the polymer and reinforcements which restricts the segmental mobility of the polymer chains in the vicinity of the reinforcements. Spinella et al. [33] have also related the shift in the tan delta peak maximum and broadening of the transition for PLA nanocomposites with cellulose nanocrystals to the interactions between PLA and the
ACCEPTED MANUSCRIPT cellulose nanocrystals. Thus, the shift in the loss modulus peak of the core/shell-structured PLA/PVA nanofiber mat and the broadening of the transition zone can be attributed to physical interaction between the two layers that restricts the segmental mobility of the polymer chains. The effect of loading frequency on the storage and loss moduli of the pristine and coaxial
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electrospun nanofiber mats at room temperature is shown in Figure 8. All four materials tested in this study exhibit the typical behavior of most solid polymers where increasing frequency leads to an increase in the storage modulus and decrease in the loss modulus. This can be visualized as moving from viscous-like behavior (at low frequency where more time is
storage modulus
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available for viscos flow) to solid-like behavior (at higher frequency) [29] . The values of the of the coaxial nanofibers are very close to the pristine PVA and much
higher than those of pristine PLA. The present results show that the storage and loss moduli
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of the coaxial core-shell nanofiber mats are insignificantly influenced by the loading frequency compared to the pristine PVA and PLA. This can be clearly seen in the plot of the loss modulus as a function of frequency where
for pristine nanofibers decreased by about
96% compared to only 9% for the coaxial nanofibers. Thus, the relative trends of the curves in the frequency scan indicate that the viscoelastic behavior of the coaxial nanofiber mats are
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slightly less dependent on strain rate compared to the pristine materials. It should be remembered that the current frequency loading was carried out at room temperature and different observations could be obtained at different temperatures.
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3.3.3. Creep – recovery results
The mechanical behaviour of nanoscale structures is usually influenced by the time and
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strain rate [34, 35]. The time-dependent strains of polymeric nanostructured films can be attributed to multiple relaxation mechanisms, such as chain decomposition, molecular separation and side group rotation [36]. In this study, the creep response of the fabricated coaxial electrospun PLA/PVA nanofiber mats subjected to nominally elastic strains was investigated and compared against the corresponding pristine materials of PVA and PLA. The experimental creep and recovery curves of PLA, PVA, core/shell-PLA/PVA and core/shell-PVA/PLA at room temperature are shown in Figure 9. It is noteworthy that the coaxial electrospun composite nanofiber mats have remarkable reduction in the creep strains compared to pure PLA and PVA. It can also be seen that the steady-state or relaxed strains are obtained at a short time interval in the coaxial nanofibers compared to the pristine materials. The high creep resistance of the coaxial nanofiber mats observed in our experiment
ACCEPTED MANUSCRIPT can be attributed to the interaction between the constituent layers of PVA and PLA that may hinder the segmental mobility of polymer chains in the vicinity of interface [37]. To get a better quantitative description of the creep behaviour, the strain time-dependent response of the pristine and coaxial nanofibers was modelled using a linear viscoelastic model. This phenomenological standard linear model, also called Zener model, employs a
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combination of springs and dashpot to describe the creep behaviour, as shown in Figure 10 (a) [38]. The linear spring controls the mechanical elastic response of the fibers and the dashpot accounts for the rate-dependent flow. In this model, the strain as a function of time for a given constant stress σ can be written as
where
and
+
−
1 − exp
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=
denote the initial (elastic) and relaxed strains, respectively, and refers to
the relaxation time. Further, the creep compliance
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= where
and
can be calculated as
σ
+
1 − exp
are the elastic moduli of the springs describing the instantaneous and
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retarded elastic response of the material, respectively. The calculated creep strains from this model are compared against the corresponding experimental results in Figure 10 (b). It was seen that the predicted curves are in good agreement with the experimental data with an rsquare value of at least 94%. The parameters of the model for each creep curve are shown in
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Table 2. The values of the parameters clearly show that the coaxial core-shell composite nanofibers have better creep resistance compared to the pristine materials. For example, the elastic modulus of PLA/PVA are approximately 232% and 46% higher than
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retardant
those of PLA and PVA, respectively. The
values for PVA/PLA are also higher than those
of PLA and PVA by about 319% and 84%, respectively.
Table 2: Parameters of the creep standard linear model used in this study
!" !" #
%$PLA
PVA
45 77 6.5
63 175 5
Core/shellPLA/PVA 68 256 4
Core/shellPVA/PLA 116 323 4
ACCEPTED MANUSCRIPT 4.
Conclusions The mechanical properties of the recently synthesized core-shell nanofiber composites of
PLA and PVA fabricated by coaxial electrospinning were thoroughly investigated. Various types of mechanical testing were applied (1) uniaxial tension, (2) dynamic loading over a
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wide range of temperature and frequency, and (3) creep deformation. The mechanical response of core-shell nanofiber mats were always compared against the constituent pristine materials of PVA and PLA. The main conclusions of this study are summarized below based on the experimental observations: core/shell-structured
PLA/PVA
nanofiber
mat
showed
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(a) The
significant
improvements in both strength and ductility compared to pristine PLA, with
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increase in tensile strength and strain at failure of about 233% and 150%, respectively.
(b) The decrease of the storage modulus
with increasing temperature (at fixed
frequency) was found to be lower for the core-shell PLA/PVA nanofiber mats and occurs at a higher temperature compared to the pristine PLA and PVA. This indicates better heat resistance for this material with less variation of the material
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structure with increasing temperature. (c) The curve of the loss modulus
as a function of temperature (at fixed
frequency) showed a shift in the peak to higher temperature, and broadening at the transition zone for the core-shell PLA/PVA composite compared to the pristine
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PLA and PVA. This was assumed to be related to physical interaction between the two constituent layers that restricts the segmental mobility of the polymer chains.
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(d) The storage and loss moduli of the coaxial core-shell nanofiber mats were found to be slightly less influenced by the loading frequency at room temperature compared to the pristine PLA and PVA.
(e) The creep test showed that the coaxial electrospun PLA/PVA composite nanofiber mats exhibit better creep resistance compared to the pristine constituent materials. This was attributed to the interaction between the constituent layers of PVA and PLA that is assumed to hinder the segmental mobility of polymer chains in the vicinity of interface.
Acknowledgements
ACCEPTED MANUSCRIPT The authors extend their appreciation to the Deanship of Scientific Research at King
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Saud University for funding this work through Research Group no. RGP-1438-035.
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Figure 1. A schematic of the co-axial electrospinning apparatus
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Figure 2: SEM results of (A) pristine PLA, (B) pristine PVA, (C) core/shell-structured PLA/PVA, and (D) core/shell-structured PVA/PLA.
Figure 3: SEM (left) and TEM (right) images showing the core-shell structures of the coaxial PVA/PLA nanofibers
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Figure 4: DSC results of pristine and coaxial nanofiber mats
Figure 5: The measured tensile stress-strain responses of pristine and coaxial nanofiber mats tested in this study
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Figure 6: Temperature dependence of (a) storage modulus and (b) loss modulus , for pristine and coaxial nanofiber mats tested at a heating rate of 3.0 °C/min and frequency of 1 Hz.
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Figure 7: The derivative storage modulus as a function of temperature for the electrospun pristine and coaxial core/shell nanofibrous mats tested in this study.
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Figure 8: Frequency dependence of (a) storage modulus and (b) loss modulus for pristine and coaxial nanofiber mats tested at room temperature.
(a)
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Figure 9: Experimental creep-recovery curves of pristine (PLA and PVA) and core/shell-structured (PLA/PVA and PVA/PLA) nanofiber mats at room temperature
(b)
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: Moduli of the springs
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: Viscosity of the dashpot
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Figure 10: (a) Standard linear model with springs and dashpot used in this study to describe the creep response of pristine and coaxial electrospun nanofiber mats, (b) Comparison of the predicted creep strains against the corresponding experimental results
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Highlights •
Coaxial electrospinning of composite core-shell nanofibers made of PLA and
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Detailed mechanical response of the electrospun coaxial nanofiber mats under static, dynamic, and creep loading.
•
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PVA.
Strong physical interaction between the PLA and PVA layers contribute to the
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good mechanical behavior of core-shell PLA/PVA nanofibers.
Coaxial PLA/PVA mats show better creep resistance compared to pristine PLA
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and PVA.
S0142-9418(18)30098-9
DOI:
10.1016/j.polymertesting.2018.02.026
Reference:
POTE 5350
To appear in:
Polymer Testing
Received Date: 16 January 2018 Accepted Date: 21 February 2018
Please cite this article as: H.F. Alharbi, M. Luqman, H. Fouad, K.A. Khalil, N.H. Alharthi, Viscoelastic behavior of core-shell structured nanofibers of PLA and PVA produced by coaxial electrospinning, Polymer Testing (2018), doi: 10.1016/j.polymertesting.2018.02.026. 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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Viscoelastic behavior of core-shell structured nanofibers of PLA and PVA produced by coaxial electrospinning Hamad F. Alharbi1,*, Monis Luqman1, H. Fouad2,3, Khalil Abdelrazek Khalil4, Nabeel H. Alharthi1 1
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Mechanical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia Department of Applied Medical Science, Riyadh Community College, King Saud University, Riyadh, 11437, Saudi Arabia 3 Faculty of Engineering, Helwan University, Department of Biomedical Engineering, P.O. Box 11792, Egypt 4 Mechanical Engineering Department, College of Engineering, University of Sharjah, Sharjah, 27272, United Arab Emirates
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Abstract
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In recent work, we demonstrated the successful fabrication of core-shell nanofiber composites with enhanced tensile, surface wetting and cytocompatibility properties via coaxial electrospinning using PLA and PVA. It has been shown that core/shell-structured PLA/PVA nanofiber mat has the hydrophilic benefits of PVA and the attractive biological properties of PLA. In this paper, we present detailed mechanical evaluation of the electrospun coaxial nanofiber mats under static, dynamic and creep loading. The core/shell-PLA/PVA
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nanofibers showed nearly 233% and 150% increase in tensile strength and ductility, respectively, compared to pristine PLA. Dynamic loading tests were employed to study the viscoelasticity of the coaxial core-shell composite nanofibers at various temperatures and frequencies. The results of the storage and loss moduli of the coaxial nanofibers suggested
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strong physical interaction between the PLA and PVA layers, that contribute to the observed good mechanical behaviour. The better creep resistance of the coaxial PLA/PVA nanofibers
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under axial loading compared to the pristine materials provides further evidence for the physical interaction between the two constituent materials. A simple linear viscoelastic model is used to quantify the evolution of creep strain in pristine and coaxial materials. This research may help to understand the physical relation between the observed viscoelastic behaviour and the internal structure of coaxial core/shell nanofibers of PLA and PVA.
Keywords: Poly lactic acid; Poly vinyl alcohol; coaxial electrospinning; viscoelastic behaviour *
Corresponding author. E-mail address: [email protected] (Hamad F. Alharbi)
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Introduction Poly Lactic Acid (PLA) nanofiber, a synthetic thermoplastic biopolymer, has attracted
considerable attention in biomedical applications owing to its excellent biodegradability and non-toxicity [1]. However, the brittleness, low strength and hydrophobicity of PLA hinder its practical applications in important applications like tissue engineering. Many efforts have
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been devoted to improve the mechanical and surface properties of PLA by incorporating other materials [2-4]. Different types of polymers have been used to form PLA-based nanofiber composites with better properties, including poly(vinyl pyrrolidine) (PVP) [5], poly (glycerol sebacate) (PGS) [6], polydopamine (PDA) – cellulose nanofibrils (CNF) [7],
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poly(ethylene glycol) [8] and poly(vinyl alcohol) (PVA) [9].
Polymer-based nanoscale composites can be produced by different fabrication
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methods. Electrospinning is one of the most prominently used techniques for producing micro- to nano-metre fibers because it is a simple, cheap and versatile process [10]. A modified version of this technique, called coaxial electrospinning, has been recently used to prepare core-shell composite nanofibers with dissimilar materials in the core and shell [11, 12]. Coaxial electrospinning has shown tremendous promise to meet the next generation of polymers by producing unique nanoscale morphologies while maintaining the separate
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identities of the two constituent materials. In particular, this approach has gained great attention in drug delivery and tissue engineering by employing, for example, materials with different degradability rates [5, 13], biocompatible shell material with less biocompatible core material [14], and strong core material for reinforcing the structures [15].
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In many biomedical areas, the successful application of the synthetic nanostructure biopolymers depends largely on the mechanical behaviour. Important mechanical properties
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include high strength, good ductility, stable dynamic response under various temperature and strain rate, and good creep resistance under fixed applied load. The balance between these different attractive properties, along with deep understanding of the underlying mechanisms responsible for the improved properties, are a great challenge in most composite materials. In our last study, we demonstrated the successful fabrication of core-shell nanofiber composites with enhanced mechanical, surface wetting, and cytocompatibility properties via coaxial electrospinning of PLA and PVA [16]. In this paper, the aim is to expound the mechanical properties of this promising coaxial nanofiber composites under static, dynamic and creep deformation. More specifically, the strength and ductility under quasi-static tensile deformation, the storage and loss moduli under dynamic loading over a wide range of
ACCEPTED MANUSCRIPT temperature and frequency, and the evolution of creep strains under constant load are reported and discussed in detail. This investigation has also shed light on possible deformation mechanisms, and supports the notion of physical interaction between the constituent PLA and PVA materials. Such physical interaction is believed to strongly affect the segmental mobility of polymer chains and contribute to the excellent mechanical
2.
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behaviour observed.
Experimental procedures
2.1. Fabrication of core-shell structure of PVA and PLA composite nanofibers using
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coaxial electrospinning
Composite nanofibers with core-shell configurations of PVA/PLA and PLA/PVA were
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fabricated using coaxial electrospinning. To better examine the mechanical performance of the synthetic coaxial electrospun nanofiber mats compared with the constituent materials, pristine PLA and PVA nanofibers via single-nozzle electrospinning were also fabricated. A detailed description of the fabrication process of coaxial and pristine electrospun PLA and PVA nanofibers is described by Alharbi et al. [16]. For completeness, the fabrication process is briefly summarized in the following paragraph.
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A solution of PVA was first prepared by dissolving poly vinyl alcohol in distilled water and ethanol with a ratio of 9:1. The mixture of 10 wt.% was heated at 75 ºC for about 1 hour. For PLA solution, another viscous solution of 8 wt.% poly lactic acid was prepared by adding chloroform and dimethylformamide (DMF) with a ratio of 8:2. The mixture was then stirred
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at room temperature for about 5 hours. The general setup of the apparatus is shown in Figure 1. In this custom-made system, the spinneret was modified by inserting a concentric smaller
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(inner) needle inside the bigger (outer) needle to produce co-axial configuration. The diameters of the inner and outer needles used in this study were 0.8 mm and 1.2 mm, respectively. The outer needle was attached to a syringe pump containing the shell solution (either PLA or PVA) and the inner was connected to a pump having the core solution (either PVA or PLA). The core and shell solutions were delivered at feeding rates of 0.1 ml/h and 0.2 ml/h, respectively. The applied voltage was 19 KV and the rotating speed of the cylindrical drum that was used to collect the coaxial electrospun nanofibers was 80 rpm. The electrospinning process was run for at least 8 hours with a distance of 14 ±1 cm between the needle tip and the collector until a non-woven nanofiber membrane was produced. To compare the mechanical performance of the core-shell composite nanofibers with the original
ACCEPTED MANUSCRIPT pure material, pristine PLA and PVA nanofibers using the same PVA and PLA solutions were also fabricated. Here, a typical electrospinning apparatus with single-nozzle was used to produce the pure structure of PVA and PLA electrospun nanofiber.
2.2. Morphological observation
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Morphological analysis of the pristine and coaxial composite nanofibers was performed using a Field-Emission Scanning Electron Microscope (FE-SEM JSM-7600) supplied by JOEL Ltd., Japan. The nanofiber samples were platinum coated by electron sputtering before the microscopic examination. The diameter of many different nanofibers were measured. A
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transmission electron microscopy (TEM) supplied by JOEL Ltd., Japan (TEM JSM-2100) was used to authenticate the core/shell structure of the fibers. The TEM sample was prepared
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by collecting fibers directly onto the copper grid.
2.3. Thermal analysis
Differential scanning calorimetry (DSC) was performed on pristine and coaxial nanofiber samples using a SDT Q600 machine, TA-USA. Samples of 5-8 grams were placed in ceramic pans. The temperature was increased from room temperature to about 300° C at a
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rate of 2 ºC/min.
2.4. Quasi-static mechanical properties
The tensile mechanical properties of the electrospun pristine PLA and PVA as well as
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the electrospun PVA/PLA and PLA/PVA core-shell mats were measured using a uniaxial tensile test. A dumbbell test sample of each material was cut following ISO 527-1:2012. The
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gauge length and thickness of each sample were approximately 10 mm and 0.2 mm, respectively. A micro-tensile test machine (MTD-500 PLUS) was used to run the tensile test at room temperature with 0.5 mm/min crosshead speed. Five nanofiber membrane samples from each type were tested in order to check the repeatability of the data.
2.5. Dynamic mechanical analysis Dynamic mechanical analysis machine (AR-G2 DMA (TA Instruments, USA)) was used to evaluate the viscoelastic behaviour of the four fabricated electrospun pristine and composite nanofiber sheets (PLA, PVA, core/shell-PLA/PVA, core/shell-PVA/PLA). The specimens were cut from the nanofibrous mats in rectangular shape with initial dimensions of 20 mm (length) x 10 mm (width) x 0.3 mm (thickness). The storage
and loss
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0.5 N axial force. A minimum of two samples of each material were tested, and the data was found to have low variability.
2.6. Creep – recovery testing
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A creep-recovery testing was conducted to examine the response of the electrospun nanofibrous mats to constant load and its behaviour after removal of that load. The same DMA machine described above was used for the creep – recovery testing with similar sample
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dimensions of about 20 mm (length) x 10 mm (width) x 0.3 mm (thickness). The creep test was conducted under a constant load of about 4 N for about 4.0 hours at room temperature. The materials were then recovered for another 4.0 hours at the same temperature. The strain value was recorded every 2 seconds.
Results and discussion
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3.
3.1. Morphological aspects
The SEM images of pristine PLA, PVA and core/shell-structures of PVA/PLA and PLA/PVA nanofiber mats are depicted in Error! Reference source not found.. The figure
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illustrates the successful fabrication of uniform nanofibers with average diameters of 170 ± 15 nm and 200 ± 20 nm, respectively, for the pristine and coaxial electrospun nanofibers.
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During the SEM analysis of the coaxial composite nanofibers with PVA in the core and PLA in the shell, few fibers were observed to have broken shells due to the brittle nature of PLA. The core structure was clearly revealed, providing evidence for the successful fabrication of a core/shell structure using coaxial electrospinning (see Figure 3). The core/shell structure of the coaxial nanofiber was also confirmed using TEM analysis, as shown in Figure 3.
3.2. Thermal properties The DSC analysis of the nanofiber mats is shown in Figure 4. The DSC result of PVA indicates that the peaks related to glass transition and melting point occurred at about 59 °C
ACCEPTED MANUSCRIPT and 220 °C, respectively. For the PLA nanofiber sheet, the peaks of the glass transition and melting point observed at about 65 °C and 155 °C, respectively. The results of the coaxial PLA/PVA and PVA/PLA nanofibers were similar. The glass transition and melting temperature can be observed at about 57 oC and 146 oC, respectively. It is interesting to note that the coaxial composite nanofibers mats have lower glass transition temperature and
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melting temperature than the pristine nanofiber mats. These lower temperatures will influence the thermo-mechanical response of the composite nanofiber mats, as will be demonstrated in the following sections.
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3.3. Mechanical properties
The mechanical properties of the electrospun nanofibers are best described by tests on
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both the single-electrospun fiber and the fiber mat. However, the characterization of the mechanical properties of single-electrospun fiber is challenging [17-21]. For this reason, most of the reported mechanical properties of electrospun nanofibers are solely based on the measurements of the nanofibrous mats using a universal testing machine [22, 23]. In this context, one should be aware that the mechanical properties of the nanofiber mats can be influenced by other factors such as the distribution of nanofiber diameters, length, alignment
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and entanglements of the fibers inside the mats [24, 25]. Furthermore, for composite coreshell electrospun nanofibers, the type of polymers in the core and shell as well as the chemical and/or physical interactions between the layers have additional contributions to the
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observed mechanical response of the composite mats. The experimental quantification of all these features (nanofiber diameter, length, alignment, entanglement, defects, core/shell material and interaction) in a statistical representative framework and linking them to the
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measured mechanical response demand major investments in time and effort. The reported mechanical results can give useful trends about the expected mechanical performance of electrospun nanofibrous materials, but more experimental work might be needed for critical applications.
In this study, the mechanical performance of the fabricated electrospun core/shellstructured PLA/PVA and core/shell-structured PVA/PLA was examined using three different techniques including 1) quasi-static tensile mechanical response, 2) dynamic mechanical analysis, and 3) creep-recovery analysis. The mechanical response of the coaxial composite materials were also compared with the basic constituent materials, pristine PLA and PVA.
ACCEPTED MANUSCRIPT 3.3.1. Quasi-static mechanical properties Typical tensile stress-strain curves of the pristine PVA, pristine PLA, core/shellstructured PLA/PVA and core/shell-structured PVA/PLA are shown in Figure 5. The values of the ultimate tensile strength and strain at failure are listed in Table 1. The result of the pristine PLA sheet showed the expected brittle and weak mechanical response with a tensile
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strength of about 4.2 ± 0.15 MPa and a final strain of about 0.4. The pristine PVA sheet exhibited better mechanical properties compared to pristine PLA, with increase in strength and ductility of about 143% and 90%, respectively. The average values of the tensile strength and strain at failure for pristine PVA were estimated to be about 10.2 ± 1.2 MPa and 0.76.
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The mechanical response of pure PLA and PVA have been reported in many studies (see for example, Refs. [5, 26, 27]). The overall mechanical properties of the core/shell-structured nanofiber mats are expected to depend on the contribution of both the core and shell
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nanofibers. The quasi-static tensile behaviour of the coaxial electrospun core/shell composite nanofiber sheets produced in this study have different response compared to pristine PLA and PVA. There was no major improvement observed in the mechanical response of the core/shell-structured PVA/PLA nanofiber mats. The measured tensile strength and strain at failure for this material were estimated to be 5.9 ± 0.85 MPa and 0.45, respectively (see
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Figure 5). These values are close to those of pristine PLA (4.2 MPa and 0.4) and much less than those of pristine PVA (10.2 MPa and 0.76). However, the core/shell-structured PLA/PVA nanofiber mat shows significant improvements in both strength and ductility compared to pristine PLA and PVA. This coaxial PLA/PVA mat exhibits a tensile strength of
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about 14.0 ± 1.25 MPa and a good ductility with a strain at failure of about 1.0. The tensile strength of this material was nearly 233 % and 37 % increase compared to pristine PLA and
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PVA, respectively. The ductility was also improved by about 150 % and 32 % compared to pristine PLA and PVA, respectively. Thus, the coaxial electrospun core-shell nanofiber mats with PLA in the core and PVA in the shell exhibited significant improvements in both tensile strength and ductility compared to the electrospun PLA or PVA alone. As discussed above, the mechanical properties of composite core/shell nanofiber scaffolds can be strongly influenced by different factors, including the type of polymers in the core and shell, chemical and/or physical interactions between the two layers, nanofibers diameter and length distributions, entanglement, molecular weight, processing history and defects. Because of these many different factors and the challenge of statistically quantifying their separate effects experimentally, there is large variability in the reported mechanical properties of electrospun composite mats and the exact underlying mechanisms are usually
ACCEPTED MANUSCRIPT difficult to elucidate with high certainty. For example, Sun et al. [5] has investigated the mechanical properties of core/shell structure of poly(vinyl pyrrolidine) (PVP) and PLA ultrafine fibers produced by coaxial electrospinning and found that the tensile modulus and tensile strength of the core/shell-structured PVP/PLA membrane were lower than those of the electrospun pure PLA membrane. On the other hand, Merkle et al. [26, 28] has reported an
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increase in the values of Young’s modulus and ultimate strength in the core/shell structure of PVA/gelatin composite scaffolds produced by coaxial electrospinning when compared with the PVA or gelatin scaffolds. Such improvements were related to the possible enhancement of the molecular alignment of core PVA by the gelatin shell, which was assumed to transform
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the PVA from weak plastic PVA into a strong elastic PVA. The mechanical properties of poly (glycerol sebacate) (PGS)-PLA core-shell membranes produced via coaxial electrospinning were investigated in detail using uniaxial tensile testing in Ref. [6]. This
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study examined the influence of the shell and core solutions and the addition of poly (ethylene oxide) (PEO) to the shell on the overall mechanical properties of the fabricated membranes, but not compared with the individual core or shell nanofibrous mats. In another study by HUANG et al. [15], it was shown that the ultimate strength and ultimate strain of coaxially electrospun nanofibers with gelatin in the core and poly(ε-caprolactone) (PCL) in
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the shell were at least 68% and 244% higher, respectively, than those of pure gelatin and PCL nanofibers. These improved mechanical properties were achieved using 7.5% w/v gelatin as the core and 10% w/v PCL as the shell solutions, which produced the finest nanofibrous membrane resulting in the largest contact area and highest bond strength in between the
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nanofibers. The exact underlying mechanisms for the significant improvement in the mechanical properties of the coaxial gelatin/PCL composite nanofibers compared to the pure
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gelatin and PCL nanofibers were not discussed in detail. In this study, the tensile strength and ductility of the fabricated core-shell PLA/PVA composite nanofiber scaffolds were significantly enhanced compared to the electrospun PLA or PVA alone. The improvement in these mechanical properties can be related to some sort of interactions between the PVA and PLA layers. The DMA analysis, explained in the following section, supports the notion of possible interactions between the constituent PLA and PVA materials. Furthermore, since the process history and the degree of dense packing of the electrospun mats can influence the mechanical properties, the weight of all electrospun nanofibrous mats with similar dimensions (~ 10 mm x 10 mm x 0.2 mm) were measured. The weight of PLA, PVA, PLA/PVA, and PVA/PLA samples were found to be 3.4 mg, 3.37 mg, 3.94 mg and 3.67 mg, respectively. The weight of the coaxial core-shell PLA/PVA and
ACCEPTED MANUSCRIPT PVA/PLA mats were higher than the pristine materials by about 16 % and 8 %, respectively. This increase in weight may also contribute to the increase of strength for the coaxial PLA/PVA and PVA/PLA composite nanofiber mats. Additionally, the lower glass transition of the coaxial nanofiber mats compared to the pristine materials (see Figure 4) can influence the ductility of the material. This is related to the fact that deceasing the thermal stability of
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polymers generally make the polymer more soft and ductile.
Table 1: The values of the tensile strength and strain at failure for pristine and coaxial nanofiber sheets tested in this study
4.2 ± 0.15
10.2 ± 1.2
0.4
0.76
3.3.2. Dynamic mechanical analysis
Core/shellPLA/PVA
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PVA
Core/shellPVA/PLA
14.0 ± 1.25
5.9 ± 0.85
1.0
0.44
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Tensile strength [MPa] Strain at failure
PLA
The dynamical mechanical analysis was carried out to obtain the viscoelastic behaviour of the coaxial nanofibrous mats compared to the constituent pure materials under different temperature and frequency ranges. Figure 6 shows the storage and loss moduli versus
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temperature for the pristine PLA, PVA and core/shell-structures of PVA/PLA and PLA/PVA nanofiber mats under a fixed frequency of 1 Hz. It is obvious that all four materials display three different regions with increasing temperatures, the glassy region, glass transition region
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and rubbery region [29]. For all four materials, the storage modulus is high in the glassy region due to the hard chain mobility. With increasing temperature, the vibrational and thermal energy increase causing high mobility of the polymer chains in the transition region,
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which lead to sharp drop in the storage modulus [29]. After the glass transition temperature, the polymer tends to act more like a viscous material than the elastic (solid) response. Subsequently, the storage modulus reached a plateau in the rubbery region due to strong local interactions between neighbouring chains. It can be seen that the storage modulus of PLA increased slightly above 100 °C, which is usually related to the effect of cold crystallization process [30, 31]. The values of the storage modulus
in the glassy region are about 210 MPa, 150 MPa,
and 20 MPa for the pristine PVA, core/shell composite and pristine PLA nanofibrous mats, respectively (see Figure 6 (a)). Although, the storage modulus of the pristine PVA is higher in the glassy region, the values of the storage modulus for the coaxial composite nanofibrous
ACCEPTED MANUSCRIPT mats in the rubbery regions are comparable to that of PVA. It is also interesting to note that the rate of decrease of the storage modulus in the transition region is higher in the pristine PVA compared to the core-shell PLA/PVA composite nanofiber mats. To make this clear, we show in Figure 7 the derivative storage modulus as a function of temperature. This figure reflects the variation rate of the material structure with increasing temperature. It is clear that
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the PLA/PVA core-shell exhibits better heat resistance since the rate of decrease is lower and occurs at a higher temperature compared to all other materials investigated in this study. This figure also shows that the shape of the derivative storage modulus for the coaxial PLA/PVA
sharp drops in the curve. Figure 6 (b) shows the loss modulus
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composite combines the characteristics of both pristine PLA and PVA with two separate
as a function of temperature for pristine and
coaxial nanofiber mats. The loss modulus increased and reached a maximum in the glass
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transition region and then decreased in the rubbery region. The peak values are usually reported to occur at or very close to the glass transition temperature. In this figure, the corresponding temperature for the peak values are about 62 °C, 57 °C, 58.3 °C and 63 °C for the pristine PLA, pristine PVA, PVA/PLA and PLA/PVA nanofiber mats, respectively. These values are slightly higher than the glass transition temperature obtained by using DSC (see
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Figure 4). It was reported in other studies that DMA experiments usually give higher values for the glass transition temperature compared to DSC [32]. It is obvious from Figure 6 (b) that the intensity of the loss modulus peak decreased for the coaxial core/shell nanofiber composites compared to neat PVA. It is also interesting to note that the peak of the loss
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modulus curve for the PLA/PVA composite is shifted to the right (to higher temperature) compared to the pristine PLA and PVA. Additionally, the loss modulus curve of the coaxial
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PLA/PVA composite is more shallow (broader) in the transition region compared to the other materials.
The shift of the peak value and the broadening in the loss modulus curve are usually related in other composite polymer materials to some sort of local physical interactions. For example, Jonoobi et al. [31] observed a shift in the tan δ position (tan δ curve is obtained by dividing the loss modulus by the storage modulus) when adding cellulose nanofiber reinforcements to neat PLA. The shift in the curve was related to the physical interaction between the polymer and reinforcements which restricts the segmental mobility of the polymer chains in the vicinity of the reinforcements. Spinella et al. [33] have also related the shift in the tan delta peak maximum and broadening of the transition for PLA nanocomposites with cellulose nanocrystals to the interactions between PLA and the
ACCEPTED MANUSCRIPT cellulose nanocrystals. Thus, the shift in the loss modulus peak of the core/shell-structured PLA/PVA nanofiber mat and the broadening of the transition zone can be attributed to physical interaction between the two layers that restricts the segmental mobility of the polymer chains. The effect of loading frequency on the storage and loss moduli of the pristine and coaxial
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electrospun nanofiber mats at room temperature is shown in Figure 8. All four materials tested in this study exhibit the typical behavior of most solid polymers where increasing frequency leads to an increase in the storage modulus and decrease in the loss modulus. This can be visualized as moving from viscous-like behavior (at low frequency where more time is
storage modulus
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available for viscos flow) to solid-like behavior (at higher frequency) [29] . The values of the of the coaxial nanofibers are very close to the pristine PVA and much
higher than those of pristine PLA. The present results show that the storage and loss moduli
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of the coaxial core-shell nanofiber mats are insignificantly influenced by the loading frequency compared to the pristine PVA and PLA. This can be clearly seen in the plot of the loss modulus as a function of frequency where
for pristine nanofibers decreased by about
96% compared to only 9% for the coaxial nanofibers. Thus, the relative trends of the curves in the frequency scan indicate that the viscoelastic behavior of the coaxial nanofiber mats are
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slightly less dependent on strain rate compared to the pristine materials. It should be remembered that the current frequency loading was carried out at room temperature and different observations could be obtained at different temperatures.
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3.3.3. Creep – recovery results
The mechanical behaviour of nanoscale structures is usually influenced by the time and
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strain rate [34, 35]. The time-dependent strains of polymeric nanostructured films can be attributed to multiple relaxation mechanisms, such as chain decomposition, molecular separation and side group rotation [36]. In this study, the creep response of the fabricated coaxial electrospun PLA/PVA nanofiber mats subjected to nominally elastic strains was investigated and compared against the corresponding pristine materials of PVA and PLA. The experimental creep and recovery curves of PLA, PVA, core/shell-PLA/PVA and core/shell-PVA/PLA at room temperature are shown in Figure 9. It is noteworthy that the coaxial electrospun composite nanofiber mats have remarkable reduction in the creep strains compared to pure PLA and PVA. It can also be seen that the steady-state or relaxed strains are obtained at a short time interval in the coaxial nanofibers compared to the pristine materials. The high creep resistance of the coaxial nanofiber mats observed in our experiment
ACCEPTED MANUSCRIPT can be attributed to the interaction between the constituent layers of PVA and PLA that may hinder the segmental mobility of polymer chains in the vicinity of interface [37]. To get a better quantitative description of the creep behaviour, the strain time-dependent response of the pristine and coaxial nanofibers was modelled using a linear viscoelastic model. This phenomenological standard linear model, also called Zener model, employs a
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combination of springs and dashpot to describe the creep behaviour, as shown in Figure 10 (a) [38]. The linear spring controls the mechanical elastic response of the fibers and the dashpot accounts for the rate-dependent flow. In this model, the strain as a function of time for a given constant stress σ can be written as
where
and
+
−
1 − exp
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=
denote the initial (elastic) and relaxed strains, respectively, and refers to
the relaxation time. Further, the creep compliance
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= where
and
can be calculated as
σ
+
1 − exp
are the elastic moduli of the springs describing the instantaneous and
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retarded elastic response of the material, respectively. The calculated creep strains from this model are compared against the corresponding experimental results in Figure 10 (b). It was seen that the predicted curves are in good agreement with the experimental data with an rsquare value of at least 94%. The parameters of the model for each creep curve are shown in
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Table 2. The values of the parameters clearly show that the coaxial core-shell composite nanofibers have better creep resistance compared to the pristine materials. For example, the elastic modulus of PLA/PVA are approximately 232% and 46% higher than
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retardant
those of PLA and PVA, respectively. The
values for PVA/PLA are also higher than those
of PLA and PVA by about 319% and 84%, respectively.
Table 2: Parameters of the creep standard linear model used in this study
!" !" #
%$PLA
PVA
45 77 6.5
63 175 5
Core/shellPLA/PVA 68 256 4
Core/shellPVA/PLA 116 323 4
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Conclusions The mechanical properties of the recently synthesized core-shell nanofiber composites of
PLA and PVA fabricated by coaxial electrospinning were thoroughly investigated. Various types of mechanical testing were applied (1) uniaxial tension, (2) dynamic loading over a
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wide range of temperature and frequency, and (3) creep deformation. The mechanical response of core-shell nanofiber mats were always compared against the constituent pristine materials of PVA and PLA. The main conclusions of this study are summarized below based on the experimental observations: core/shell-structured
PLA/PVA
nanofiber
mat
showed
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(a) The
significant
improvements in both strength and ductility compared to pristine PLA, with
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increase in tensile strength and strain at failure of about 233% and 150%, respectively.
(b) The decrease of the storage modulus
with increasing temperature (at fixed
frequency) was found to be lower for the core-shell PLA/PVA nanofiber mats and occurs at a higher temperature compared to the pristine PLA and PVA. This indicates better heat resistance for this material with less variation of the material
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structure with increasing temperature. (c) The curve of the loss modulus
as a function of temperature (at fixed
frequency) showed a shift in the peak to higher temperature, and broadening at the transition zone for the core-shell PLA/PVA composite compared to the pristine
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PLA and PVA. This was assumed to be related to physical interaction between the two constituent layers that restricts the segmental mobility of the polymer chains.
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(d) The storage and loss moduli of the coaxial core-shell nanofiber mats were found to be slightly less influenced by the loading frequency at room temperature compared to the pristine PLA and PVA.
(e) The creep test showed that the coaxial electrospun PLA/PVA composite nanofiber mats exhibit better creep resistance compared to the pristine constituent materials. This was attributed to the interaction between the constituent layers of PVA and PLA that is assumed to hinder the segmental mobility of polymer chains in the vicinity of interface.
Acknowledgements
ACCEPTED MANUSCRIPT The authors extend their appreciation to the Deanship of Scientific Research at King
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Saud University for funding this work through Research Group no. RGP-1438-035.
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Figure 1. A schematic of the co-axial electrospinning apparatus
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Figure 2: SEM results of (A) pristine PLA, (B) pristine PVA, (C) core/shell-structured PLA/PVA, and (D) core/shell-structured PVA/PLA.
Figure 3: SEM (left) and TEM (right) images showing the core-shell structures of the coaxial PVA/PLA nanofibers
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Figure 4: DSC results of pristine and coaxial nanofiber mats
Figure 5: The measured tensile stress-strain responses of pristine and coaxial nanofiber mats tested in this study
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Figure 6: Temperature dependence of (a) storage modulus and (b) loss modulus , for pristine and coaxial nanofiber mats tested at a heating rate of 3.0 °C/min and frequency of 1 Hz.
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Figure 7: The derivative storage modulus as a function of temperature for the electrospun pristine and coaxial core/shell nanofibrous mats tested in this study.
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Figure 8: Frequency dependence of (a) storage modulus and (b) loss modulus for pristine and coaxial nanofiber mats tested at room temperature.
(a)
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Figure 9: Experimental creep-recovery curves of pristine (PLA and PVA) and core/shell-structured (PLA/PVA and PVA/PLA) nanofiber mats at room temperature
(b)
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,
: Moduli of the springs
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: Viscosity of the dashpot
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Figure 10: (a) Standard linear model with springs and dashpot used in this study to describe the creep response of pristine and coaxial electrospun nanofiber mats, (b) Comparison of the predicted creep strains against the corresponding experimental results
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Highlights •
Coaxial electrospinning of composite core-shell nanofibers made of PLA and
•
Detailed mechanical response of the electrospun coaxial nanofiber mats under static, dynamic, and creep loading.
•
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PVA.
Strong physical interaction between the PLA and PVA layers contribute to the
•
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good mechanical behavior of core-shell PLA/PVA nanofibers.
Coaxial PLA/PVA mats show better creep resistance compared to pristine PLA
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and PVA.