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Reinforcement of Polymeric Submicrometersized Fibers by Microfibrillated Cellulose Giuseppino Fortunato,* Tanja Zimmermann, Jo ¨rn Lu ¨bben, Nico Bordeanu, Rudolf Hufenus
To improve their mechanical properties, biodegradable network-forming microfibrillated cellulose (MFC) is introduced into PEO fibers by an electrospinning (e-spinning) procedure. The influence of MFC is investigated with respect to e-spinning process parameters as well as morphological and mechanical properties of the fibers. The determination of the Young’s modulus is established using both macro-tensile testing procedures of aligned fiber patches and AFM on single fibers. Highly filled fiber systems are obtained, showing an enhancement of Young’s modulus by a factor of up to ten compared to pure PEO fibers. The morphological investigations reveal fibers with circular cross-sections incorporating homogeneously dispersed MFC within the polymer matrix. The introduction of MFC has no relevant effect on processing and the appearance of the fiber.
1. Introduction E-spinning produces highly porous non-wovens consisting of ultrafine fibers. A wide variety of materials such as conventional petroleum-based polymers, bio-based polymers, and inorganic materials, but also composite strucDr. G. Fortunato, Dr. J. Lu ¨bben, Dr. R. Hufenus Laboratory for Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstr. 5, 9014 St. Gallen, Switzerland E-mail:
[email protected] Dr. G. Fortunato Laboratory for Protection and Physiology, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstr. 5, 9014 St. Gallen, Switzerland Dr. T. Zimmermann, Dr. N. Bordeanu Applied Wood Materials Laboratory, Empa, Swiss Federal ¨ berlandstr. Laboratories for Materials Science and Technology, U 129, 8600 Du ¨bendorf, Switzerland
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tures have been spun to form filaments with diameters from mm- to nm-scale. Due to the high surface to volume ratio of the products the envisaged applications lie mostly within filter systems and membranes,[1,2] catalysts,[3–5] drug delivery systems and tissue engineering.[6–14] In tissue engineering for instance, one of the basic requirements for a successful use of electrospun fibrous scaffolds are their adequate mechanical properties. Indeed, strength and Young’s moduli (YM) of nanofibers have been demonstrated to influence in vitro cell migration, proliferation, differentiation, along with cell morphology.[15,16] The characteristics of e-spun fibers are highly dependent upon the process parameters including the solution properties (electrical conductivity, viscosity, and surface tension),[17–24] the molecular characteristics of the polymer (molecular weight, polydispersity, degree of branching, degree of polymer chain entanglement),[25] and the instrumental parameters such as the applied electric potential, the distance between tip and counter-electrode,
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DOI: 10.1002/mame.201100408
Reinforcement of Polymeric Submicrometer-sized Fibers by Microfibrillated . . . www.mme-journal.de
the feeding rate and environmental conditions such as air humidity. Electrospinning of a polymer solution can produce a variety of structures including beads, beaded, flattened and regular fibers, and their formation is highly dependent on the characteristics of the starting solution. The determination of the mechanical properties of single sub-micrometer fibers is a challenging task. Whole patch properties can be determined using conventional mechanical testing,[26–29] and the results relate both to single fiber properties as well as to the patch architecture. Research has recently focused on the determination of mechanical properties of single sub-micrometer fibers by means of atomic force microscopy (AFM). Tan et al.[30] performed tensile tests on aligned electrospun poly(ethylene oxide) (PEO) fibers using a piezoresistive AFM cantilever tip and a custom-made setup. They found YM values similar to rollcast PEO films. Bellan et al.[31] deposited oriented PEO fibers over trenches etched in silicon and measured YMs using the AFM technique. They explained the observed increase in the YM compared to those of PEO bulk and films by molecular orientation in the fibers. Iwamoto et al.[32] determined the elastic moduli of cellulose microfibrils from tunicate using an AFM cantilever and a three-point bending set-up. The elastic moduli of single microfibrils prepared by TEMPO-oxidation and acid hydrolysis were 145.2 31.3 and 150.7 28.8 GPa, respectively. Yang et al.[33] presented a study on the mechanical properties of single electrospun collagen fibers. AFM-based micromechanical bending tests were performed on native and glutaraldehyde cross-linked single electrospun fibers. Both bending and shear moduli of the electrospun collagen fibers were determined. The shear modulus was two orders of magnitude lower compared to the bending modulus. Cross-linking increased the shear modulus. Few investigations are available for reinforcement of e-spun fibers. Mainly, rod like structures such as single- and multi-walled carbon nanotubes (SWCNTs and MWCNTs, respectively) were used as their unique morphological and mechanical properties make them promising candidates for reinforcement.[34–36] The tensile strengths and moduli of the SWCNT/MWCNT containing polymer fibers were enhanced by 15–200% as compared to the pure polymer fibers. Interest in producing composite materials with renewable and biodegradable reinforcing components has grown strongly in recent years. Cellulose in particular, as the most abundant, natural polymer on earth with annual production of 1011 t[37] has attracted attention. In cell walls of wood tracheids or fibers that have diameters in the micrometer and lengths in the millimeter range, cellulose acts as a structural element with a content of 45%. The high tensile strength cellulose is organized as aggregates of fibrils embedded in the matrix substance lignin. One single cellulose fibril is 3–4 nm thick and several tens of
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micrometers long and consists of crystalline parts linked to amorphous domains.[38] Microfibrillated cellulose (MFC) with diameters in the nanometer range (10–100 nm) and aspect ratios (fiber length divided by diameter) between 50 and 100[39] is a form of expanded high-volume cellulose, obtained by a mechanical process of homogenization of wood pulp or other cellulosic raw materials. MFC is hydrophilic and builds up network structures by strong hydrogen bonds between the hydroxyl groups present on the surface of the MFC. Own measurements on pure MFC films show promising mechanical properties: modulus of elasticity 5–7 GPa; tensile strength 40–60 MPa as determined by tensile testing. Due to the hydrophilic character of the MFC, good compounding with hydrophilic, especially water-soluble polymers, can be achieved by preservation of the network structure and without re-agglomeration. In earlier studies, MFC-reinforced poly(vinyl alcohol) (PVOH) and hydroxypropylcellulose (HPC) showed – compared with the pure polymer films – an up to threefold higher stiffness and a fivefold higher tensile strength.[40] As an alternative, cellulose nanowhiskers (e.g., from cotton linter) were used to reinforce polymers as shown by Eichhorn et al.[41] PVOH films gave notably higher values for the elastic moduli with increasing filler content. This study investigates the potential of creating PEO fibers with MFC as reinforcing component using the e-spinning technique. The challenge is in both the homogeneous compounding of the two components and the transfer of the three-dimensional MFC network into the fiber during the spinning process. The research is focused on the interaction of both components within the dispersion and its influence with respect to the morphology and mechanical properties of fibers. Mechanical properties were both determined on aligned fiber patches as well as on single fibers.
2. Experimental Section 2.1. Materials PEO with an average molecular weight of 106 g mol1 was obtained from Sigma-Aldrich. For the dissolution of the PEO high purity water (conductivity 18 mS cm1) was used. For the isolation of the MFC refined bleached oat straw pulp (Avena sativa) was purchased from JELU GmbH & Co KG. The pulp contained 96.5% cellulose with a fiber diameter of 32 mm.
2.2. Preparation of the MFC Suspension 2.2.1. Pre-Treatment In a 10 L reactor, 200 g of dry oat straw pulp was added to 8 L of water (2.5 wt%). The resulting mixture was treated with an inline disperser (Megatron MT 3000 from Kinematica AG) at 20 000 rpm in
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and 1.12 wt% were obtained at a constant PEO concentration of 4.2 wt%. These values correspond to the following portions of MFC within the solid PEO fibers: 0, 3.3, 6.5, 11.7, and 21 wt%, respectively (Table 1).
2.3.2. Electrospinning Procedures
Figure 1. SEM micrograph of MFC mechanically isolated from oat straw pulp.
order to divide the pulp fibers into smaller portions (cellulose fibril bundles, CFB). After a processing time of 60 min, a homogeneous suspension of CFB in water was obtained.
2.2.2. Disintegration and Homogenization The aqueous CFB suspension (2.5 wt%) was subjected to high-shear homogenization by using a Microfluidizer type M-110Y (Microfluidics Corporation, USA) in order to isolate MFC from the CFB suspension. A stable MFC suspension was obtained within 10 passes at 1000 bar through the high shear generating interaction chambers of the Microfluidizer (chamber diameters, 200, 75 mm). The final concentration of the MFC suspension was 2.38 wt%. The typical appearance of MFC is presented in Figure 1.
2.3. Fabrication of Electrospun PEO Fibers 2.3.1. Preparation of PEO/MFC Dispersions The dispersions were prepared by weighing the constituents (high purity water, PEO powder and 2.38 wt% MFC suspension). First, PEO was dissolved in water by shaking for 24 h. Then, the MFC aqueous suspension was added followed by shaking the dispersion for another 24 h. In that way, MFC concentrations of 0, 0.14, 0.29, 0.56,
The electrospinning equipment consisted of an infusion pump to which a syringe with a stainless steel needle (internal diameter 0.8 mm) was attached. The positive electric potential (þ10 kV) was applied to the needle tip, a negative potential (10 kV) to the counter-electrode and the surrounding Faraday cage was attached to ground. In that way, the jet was targeted efficiently toward the counter electrode and a continuous and safe spinning procedure was accomplished. The fibers were collected using two types of counter electrodes: first, single fibers were spun onto embossed nickel foils fixed on the counter-electrode for AFM measurements. The average spacing between the parallel grooves, measured by AFM and used for the three-point bending tests, was 1.25 0.05 mm. Secondly, patches consisting of aligned fibers were spun onto a rotating drum with a radius of 10 cm and a width of 1 cm. In that way ribbons with a length of about 60 cm and a width of 1 cm were obtained. Solution feeding rate was 5 mL min1. The tip-to-counter-electrode distance was set to 20 cm. Table 1 lists other instrumental parameters for the electrostatic fiber spinning procedure.
2.4. Analysis 2.4.1. Electrical Conductivity and Viscosity Measurements The electrical conductivity of the solutions and dispersions used for electrostatic spinning was measured on a Metrohm 660 conductometer (Metrohm AG). The calibration of the conductivity measurement was performed using 0.001, 0.01, and 0.02 M potassium chloride (KCl) solutions. The KCl (Fluka, puriss. p.a. grade) was dried at 110 8C for 12 h before weighing to prepare the standards. 20 mL of each PEO dispersion with MFC contents of 0–1.12 wt% was used for the viscosity measurements (Table 1). A rheometer (Paar Physica MCR 300) equipped with a cylindrical system was used in controlled shear rate mode for determination of shear viscosity. A flow curve with shear rates varying from 10 to 100 s1 was recorded.
Table 1. PEO-MFC electrospinning conditions.
Sample
578
PEO conc.a) [wt%]
MFC conc. within fiber [wt%]
Conductivity [mS m1]
Viscosity at 0.1 s1 [Pa s]
PEOMFC0
0
4.2
0
85
10.5
PEOMFC1
0.14
4.2
3.3
69
11.4
PEOMFC2
0.29
4.2
6.5
107
13.5
PEOMFC3
0.56
4.2
11.7
173
19.0
PEOMFC4 a)
MFC conc.a) [wt%]
1.12
4.2
21.0
b)
–
39.8
Concentration of aqueous solution and dispersions used for e-spinning; b)Could not be determined due to high viscosity.
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2.4.2. Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscopy (STEM) The scanning electron micrographs of the e-spun fibers were recorded on a Hitachi S-4800 using an accelerating voltage of 2 kV. The pure cellulose structures and dried MFC/PEO dispersions were investigated on a JEOL 6300F (accelerating voltage of 5 kV). Prior to the measurements, a plasma gold or platinum coating of about 20 nm (thickness measured on a silicon wafer) was applied to the samples. Thirty values were measured for evaluation of the fiber diameters and their distributions. For STEM measurements, fibers were directly spun onto copper grids (Plano GmbH, 200 mesh). The images were recorded using 20 kV accelerating voltage.
2.4.3. Transmission Electron Microscopy (TEM) Samples of the non-woven material were embedded into epoxy resin (Spurr medium[42]). Ultrathin sections (70–100 nm) were cut with a diamond knife by an ultramicrotome (Ultracut E, Reichert-Jung AG). The samples were transferred to Formvar carbon-coated grids (Plano GmbH) and then studied under a Philips CM 30 TEM. As the non-woven material partially dissolves during the embedding procedure a second preparation pathway was used: PEO/MFC fibers were directly spun onto TEM grids and subsequently stained using concentrated uranyl acetate (dissolved in acetone) to enhance the contrast and to make the cellulose visible. Uranyl acetate attacks the hydroxyl groups of the MFC and forms stable bonds.
2.4.4. Mechanical Properties 2.4.4.1. AFM Topographical imaging and mechanical characterization were conducted using a commercial AFM (easyScan 2, Nanosurf AG). A cantilever (Nanoworld CONTR-10) with a nominal spring constant ktip ¼ 0.2 N m1 and a tip radius rtip < 10 nm was used both for contact and force spectroscopy mode. All measurements were carried out under constant ambient conditions (T ¼ 295 K, relative humidity RH < 40%). For deposition of pure MFC one droplet of 0.1 wt% MFC dispersion was applied to the embossed Ni substrates. The prepared samples were dried for 12 h under ambient conditions. The YMs of the sub-micrometer fibers were obtained by performing a nanoscale three-point bending test on a single fiber suspended perpendicularly over a groove of the grating embossed in the Ni foil. An AFM tip was used to apply a small deflection at the mid-span of the fiber along its suspended length. The YM was then calculated from beam bending theory for a beam with two fixed ends[43] YM ¼
l3 F 192I d
Figure 2. Typical force/distance curves of a PEO fiber incorporating 21 wt% MFC and of the Ni substrate reference.
of the cantilever and d being the deflection. The loading force F did not exceed 20 nN. Commercial software (Origin Professional) was used to analyze the force-displacement curves consisting of 256 data points (Figure 2). From the force-distance curves the displacement of the fiber d in the z-direction during bending was calculated using the relation d ¼ z d, in which z is the piezo movement in the z-direction. Maximum applied fiber displacements were in the range of 100 nm. The combined uncertainties of the YM were calculated by using error propagation law. The main uncertainties such as the fiber diameter, the suspended length, the loading force, the perpendicularity of the fiber with respect to the grating direction and the displacement of the fiber were included in the calculation of the combined measurement uncertainty.
2.4.4.2. Stress/Strain Measurements The YM of aligned fiber patches was assessed by stress/strain measurements on an Instron 4500 apparatus (Norwood, USA), equipped with a 100 N load cell and a displacement rate of 25 mm min1. Substrates of 1 4 cm2 dimension were conditioned at 23 8C and 50% relative humidity for 24 h. Stress was normalized to the cross-sectional areas of the fibrous patches, which were determined by use of a high precision caliper gauge (Futuro, Bru ¨ tsch-Ru ¨ egger). Typical measured patch thicknesses were in the range of 50 mm. Measurements of five individual patches per e-spinning condition were assessed. The YM was calculated using the slope of the curves within the linear range (typically within 0.5–10% of strain).
(1)
3. Results and Discussion where l is the suspended length of the PEO fiber spanning the distance between the groove maxima of the Ni grating. The fiber is considered to be a rod with a circular cross-section and with radius r, I is the moment of inertia and equals I ¼ p=4r4 , and d is the total displacement of the fiber. The applied force F is directly obtained from Hooke’s law F ¼ kd with k being the nominal spring constant
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3.1. Influence of the Dispersion Properties on the Fiber Morphology The introduction of MFC within the PEO starting solution has a distinct effect on viscosity and electrical conductivity.
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The conductivity increases by a factor of two from 85 mS cm1 for pure PEO solution to 173 mS cm1 for 0.56 wt% MFC (Table 1). The apparent viscosity values (at 0.1 s1) rise from 10.5 Pa s for 0 wt% MFC content to 39.8 Pa s for 1.12 wt% MFC content. The viscosity versus shear rate plot reveals increasing values with increasing MFC concentrations accompanied by pseudo-plastic Figure 4. (a) and (b) FE-SEM micrographs showing dried suspensions with MFC loadings behavior for all of the dispersions of (a) 6.5 and (b) 21.0 wt% within the nanocomposite, respectively. Homogeneous (Figure 3). These increased viscosities compounding with visible network structures of MFC. are mainly due to MFC network forming structures within the dispersions, while the shear thinning behavior is caused by an alignment of these fibrils with increasing shear rate. concentration is kept constant, and the MFC concentration Similar shear thinning behavior is found for carbon is increased up to 1.12 wt% (Table 1). SEM measurements of nanofibers (CNFs) suspended in epoxy resins, where pure dried MFC/PEO dispersions reveal a homogeneous fibrous epoxy resins showed Newtonian behavior at low shear composition (Figure 4a and b) and an intact MFC network within dissolved PEO. The importance of percolating rates and shear thinning at higher shear rates.[44] With network formation for a high reinforcement efficiency increasing concentration of CNFs (>1%) a more pronounced even with small amounts of cellulose is a main topic in shear thinning behavior is found, in which the viscosity many studies carried out with fibrils obtained from decreases linearly with an increase of the shear rate. different organic fibers.[49–51] Therefore, a homogeneous Typically, the conductivity and viscosity of e-spinning solutions or dispersions have a strong effect on both dispersion of MFC networks without agglomeration as morphology and the diameter of the fibers. Increased found in our study is one of the key points for the fabrication conductivity values allow the fiber diameter to be of any type of reinforced composites.[52] During electrospinning, this MFC network is embedded minimized as more charges are carried by the jet and thus a higher drawing ratio is obtained.[45,46] Furthermore, low within the PEO fibers. In that way, circular fiber morphologies are obtained with diameters varying between 400 conductivity of the solution results in insufficient elongaand 500 nm for single fibers collected on Ni foils, which is a tion of a jet by electrical force, so that beads or spindle-like typical size distribution when using the e-spinning fibers are obtained.[47] An increase in viscosity typically technique (Figure 5). In contrast, fibers collected on a leads to fibers of increased diameter[22,48] and is accomrotating drum show smaller fiber diameters with a linear plished by higher polymer concentrations in the solution or decrease from pure PEO to 21 wt% MFC/PEO fibers from 300 by the use of larger molecular polymer weight. Regarding to 150 nm. These slightly smaller values are due to a the dispersions used within this study, the polymer
Figure 3. Shear viscosity vs. shear rate. Plots for PEO/MFC suspensions.
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Figure 5. SEM images showing the fiber morphology for pure PEO and PEO/MFC fibers. (a) 0% MFC (single fiber), (b) 11.7% MFC (single fiber), (c) 11.7% MFC aligned fiber patch.
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Figure 6. (a) PEO particles linked by MFC (6 wt% MFC loading). 1: PEO particle, 2: MFC fibril, 3: epoxy particle. (b) PEO/MFC fiber showing a network structure (11 wt% MFC loading). (c) Pure PEO fiber.
drawing process during collection of the fibers by high speed drum rotation. Similar differences for non-oriented and oriented fibers were found for polycaprolactone in our laboratory.[53] The introduction of MFC into the PEO solution has no distinct effect either on the surface morphology of the fibers nor on its average diameters. Additionally, the SEM micrographs reveal fully polymerwrapped cellulose structures. All suspensions were spun using constant PEO but increasing MFC concentrations. Nevertheless, the fiber diameters for single fiber deposition remained constant. This means that the MFC content has no influence on the drawing process during fiber formation. Presumably, the drawing process is only driven by the elastic PEO resulting in constant fiber diameters. Further investigations on the impact of MFC in water soluble polymers during the drawing process are necessary to clarify the detailed mechanisms. Two sample preparation methods were used for TEM investigations. For the first method, the non-wovens were embedded in an epoxy resin. During the embedding process the PEO fibers dissolved. Despite this mobilization the TEM micrograph shows finely dispersed MFC at the boundaries of PEO particles (in Figure 6a assigned to the round black particles). This means that no agglomeration of the MFC takes place during either the preparation of the dispersion or the spinning. Based on the chemical nature of the MFC and PEO, respectively, we assume that they are interacting among themselves through hydrogen bonds between the hydroxyl groups of MFC and of PEO situated at the boundaries of the particles. An interaction of this kind was found by De Rodriguez et al.[54] for a similar composite film system composed of sisal cellulose whiskers and polyvinyl acetate. To prove that the MFC is non-agglomerated within the PEO fibers, TEM measurements of stained samples reveal a three-dimensional homogeneously dispersed network of MFC (Figure 6b). The black parts of the fiber depict the stained MFC and the grayish areas the PEO. Domains of pure PEO are visible having dimensions of 10–20 nm, which are percolated extensively by the MFC structures. Interestingly, no evident alignment of the
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cellulose fibrils is found as compared to reinforcement by CNT-filled nanofibers.[34] No network is visible for pure PEO e-spun fibers (Figure 6c). 3.2. Mechanical Properties of the Fibers 3.2.1. Stress/Strain Measurements The stress/strain measurements on aligned fiber patches show a distinct influence of the MFC’s regarding, e.g., ultimate tensile strength, YMs, and plastic behavior (Figure 7, Table 2). MFC improved the modulus and tensile strength of the composite nanofiber patch. Concentrating on the values of the YMs, a linear dependency is found with increasing MFC concentration from 12.4 MPa for pure PEO to 170 MPa for highly filled fiber patches, which means an increase of a factor of about 14 (Figure 8). The strain at break became smaller with increasing MFC concentration. 3.2.2. AFM Measurements The determination of the YM by AFM requires the acquisition of force-distance curves in the center of the
Figure 7. Stress/strain curves for aligned PEO fibers with varying concentrations of MFC.
Macromol. Mater. Eng. 2012, 297, 576–584 ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 2. Mechanical properties of pure PEO and PEO-MFC e-spun fibers.
AFM measurement Modulus of elasticity [MPa]
MFC conc. within fiber [wt%]
Modulus of elasticity [MPa]
66 17
0
12.4 1.2
3.3
38 10
3.1
31.8 3.2
6.5
240 60
6.3
48.5 4.9
11.7
349 87
11.3
118.3 11.8
21.0
468 117
21.0
170.0 17.0
MFC conc. within fiber [wt%] 0
100 a)
Macrotensile testing
a)
6300 450
Pure and dried MFC.
fibers. Topographical AFM images were used to position the AFM tip on the midspan of the fibers spanning grooves of a grating (Figure 9a and b). The YM of the fibers was calculated using Equation 1 (see Section 2.4), valid for fibers that adhere strongly to the Ni substrate. In case of freely supported ends the theory predicts a four times larger YM.[43] In order to evaluate the adhesion of the fibers to the Ni substrate several AFM topographical images were acquired before and after the bending tests. The vertical forces applied to the fibers both for the imaging (Fsetpoint ¼ 20 nN) and for the three-point bending tests (Fload < 20 nN) were of the same order of magnitude. No displacement of the fibers was observed during imaging, which proves that good anchoring of the fibers to the Ni substrate took place, allowing an accurate determination of the YM. Moreover, it has been reported that the movement of the supporting or load points during the bending tests would result in a nonlinear force/distance relationship.[43] Nonlinearities have not been observed in our studies. The goodness of the linear fit of the cantilever
Figure 8. Young’s moduli (YMs) of the MFC-reinforced PEO nanofibers as a function of MFC concentration.
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Figure 9. (a) AFM topography image of a PEO fiber on a grid containing 6.5 wt% MFC (P1-3 denotes bending positions). (b) AFM topography image of pure MFC (P1-3 denotes the bending positions).
deflection in the repulsive regime reveals a correlation coefficient R2 for all curves >0.98, thus allowing precise determination of the displacement d. The YMs values show an interesting dependence on MFC content: at very low MFC concentrations no change of the YM is found, then a strong increase with addition of up to 21 wt% is measured. The increase in the YMs between 6.5 and 21.0 wt% MFC within the fibers demonstrates transfer of the mechanical load from the PEO matrix to the MFC. We assume that the measured trend as a function of the MFC concentration can be explained as follows: at very low MFC concentration the fiber composite structure is discontinuous and partial fiber regions contain less or no amounts of MFC, thus no increase of YMs values can be measured. The different behavior with respect to the macro-tensile testing result might be explained by the low drawing process when spun on the Ni gratings. For fibers spun on the rotating drum the discontinuous MFC dispersion, and thus lower YM, might be superimposed by the higher molecular orientation of the PEO chains caused by the drawing process giving a higher value. Further investigations would be necessary to clarify these assumptions. At medium and high MFC concentrations strong bonding and a percolating homogeneous distribution between the two constituents is found, as shown by the TEM measurements of PEO/MFC composite fibers (Figure 6b). The comparison of the YMs of pure and dried MFC using the AFM technique presented here and conventional tensile testing on pure MFC films gives values in the same range of 5–7 GPa (Table 2). The ratio of the measured YMs between conventional tensile testing and AFM technique are in the range of a mean factor of 4 1.3, except for very low MFC addition (Table 2). The general difference can be explained by the non-fully aligned fibers within the patches and their high patch porosity. During mechanical exposure first an alignment of the fibers takes place followed by the elongation process. This lowers the slope of the linear
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range of the measured stress/strain curves resulting in smaller YMs. Our results are underpinned by the work of Croisier et al., where single polycaprolactone fibers and fully non-oriented patches were investigated by AFM and macro-tensile testing methods for mechanical characterization. Same trend was found for the fibers YM for both methods.[55] Similar behavior of mechanical properties as a function of reinforcing component content, as depicted in our study, has been reported by Hou et al. for polyacrylonitrile/ MWCNT e-spun fibers.[36] The dependence of the MWCNT concentration versus the YM reveals a similar increase with increasing concentration of the additive. However, the investigation of PVOH/MWCNT e-spun fiber patches by Jeong et al. shows a different pattern.[34] After a slow increase in the YM at low MWCNT concentrations, the modulus decreased markedly at higher concentrations, which was attributed to weaker bonding between the wrapped polymer matrix and the filler nanotube bundles. Compared to our values other studies on the mechanical properties of pure PEO e-spun fibers show a consistent figure: Tan et al. investigated the mechanical properties of single PEO (Mw ¼ 0.9 106 g mol1) e-spun fibers having a diameter of about 700 nm, also using an AFM-based technique.[30] They found a value of 45 MPa from tensile tests in the longitudinal direction. This YM value is in good agreement with our results using a similar polymer molecular weight. The same authors demonstrated on electrospun poly(lactic acid) fibers that the two methods, nano-scale tensile testing and three-point bending, provide comparable results.[56] The electrospun PEO fibers having an average Mw of 1 105 g mol1 investigated by Bellan et al. show much higher values for the modulus (7 GPa > YM > 100 MPa).[31] We assume that this higher value is influenced by the lower molecular weight and the relatively smaller fiber diameters, which correspond to a higher degree of drawing and, as a consequence, a higher YM.
4. Conclusion The focus of this study was the mechanical improvement of PEO e-spun fibers using MFC as reinforcing element. The espinning technique allows uniform fibers to be obtained with average fiber diameters in the sub-micrometer range. Although parameters such as electrical conductivity and viscosity of the dispersions changed markedly with increasing MFC concentration, no influence was revealed with respect to surface morphology or fiber diameter up to a MFC concentration of 21 wt%. The MFC is able to form finely dispersed network structures within the e-spun fibers and a high impact of the reinforcing component was manifested. An increase in the modulus of elasticity from about
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38–66 MPa for PEO fibers having no or little content (
