J Polym Environ (2012) 20:967–975 DOI 10.1007/s10924-012-0486-6
ORIGINAL PAPER
Micromechanical Tensile Testing of Cellulose-Reinforced Electrospun Fibers Using a Template Transfer Method (TTM) Richard L. Andersson • Michaela Salajkova Peter E. Mallon • Lars A. Berglund • Mikael S. Hedenqvist • Richard T. Olsson
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Published online: 4 July 2012 Ó Springer Science+Business Media, LLC 2012
Abstract A template transfer method (TTM) and a fiber fixation technique were established for fiber handling and micro tensile stage mounting of aligned and non-aligned electrospun fiber mats. The custom-made template had been precut to be mounted on a variety of collectors, including a rapidly rotating collector used to align the fibers. The method eliminated need for direct physical interaction with the fiber mats before or during the tensile testing since the fiber mats were never directly clamped or removed from the original substrate. By using the TTM it was possible to measure the tensile properties of aligned poly(methyl methacrylate) (PMMA) fiber mats, which showed a 250 % increase in strength and 450 % increase in modulus as compared to a non-aligned system. The method was further evaluated for aligned PMMA fibers reinforced with cellulose (4 wt%) prepared as enzymatically derived nanofibrillated cellulose (NFC). These fibers showed an additional increase of 30 % in both tensile strength and modulus, resulting in a toughness increase of 25 %. The fracture interfaces of the PMMA–NFC fibers showed a low amount of NFC pull-outs, indicating favorable phase compatibility. The presented fiber handling technique is
R. L. Andersson M. S. Hedenqvist R. T. Olsson (&) Department of Fibre and Polymer Technology, Royal Institute of Technology, 100 44 Stockholm, Sweden e-mail:
[email protected] M. Salajkova L. A. Berglund R. T. Olsson Wallenberg Wood Science Center, Royal Institute of Technology, 100 44 Stockholm, Sweden P. E. Mallon Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Stellenbosch 7602, South Africa
universal and may be applied where conservative estimates of mechanical properties need to be assessed for very thin fibers. Keywords Electrospinning Template transfer method (TTM) Micro mechanical tensile testing PMMA Cellulose crystals (NFC)
Introduction Polymer and composite fibers with cross-sectional dimensions \1 lm can be prepared by the electrospinning method [1, 2]. These ultrathin fibers have an emerging use in applications ranging from filters and membranes [3] with confined functions such as reactive or catalytic properties [4] to wound dressings with antimicrobial activity [5]. The technique is attracting attention because the processing parameters can be varied to facilitate tuning of the fiber characteristics much more than is possible in traditional fiber processing. Examples range from solid to porous fibers [6], including hollow fibers with channels [7] and fibers with internal reinforcement [2, 8–12]. Their crosssectional dimensions stretch over a range from \20 nm to several lm [13]. A prerequisite for the use of electrospun fibers in many applications is that the fibers possess adequate mechanical strength and toughness. Methods of performing reliable mechanical testing on these materials are therefore becoming increasingly important, as the technique is currently being developed for large-scale industrial fiber fabrication [14]. Testing methods to evaluate the mechanical properties of individual fibers include three-point bending combined with atomic force microscopy (AFM) or mechanical tensile testing [15–26]. However, the handling and manipulation of
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these delicate fibers before and during testing have proven to be difficult and present significant challenges since the accuracy and reproducibility of the measurements decrease with decreasing cross-sectional dimensions of the fibers [22]. Rigidly defined testing procedures are therefore needed to enable more accurate assessment of fiber properties. In this paper, we present a convenient and reliable method to evaluate the mechanical properties of very thin randomly oriented as well as aligned electrospun fibers. The approach utilizes a template transfer methodology (TTM) that enables electrospun fiber mats to be very carefully handled to facilitate their micromechanical tensile testing. We present the method in detail for fiber mats of poly (methyl methacrylate) (PMMA) as well as for PMMA fibers containing flexible nanofibrillated cellulose (NFC) as reinforcement. The NFC was selected due to its high modulus (greater than 130 GPa for the cellulose crystal) and tensile strength (7–10 GPa) [27–34]. Lately, the NFC has received attention as a renewable structural building component material with ability to provide reinforcement in composites [35–37]. Our results show that the fibers can be accurately tested to give reproducible mechanical data using the TTM technique on a commercial micro-tensile-testing stage. Fibers were collected as aligned anisotropic mats directly on the templates (firmly attached to a rotating drum) or in isotropic arrangements on a static metal collector. The mats were immobilized by precision gluing onto the templates before being transferred to the micro-tensile-tester, allowing testing without disrupting the sensitive fibers. The procedure allowed the preparation and testing of PMMA– hybrid fibers with 25 % greater toughness than pristine PMMA fibers.
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for 2 h and then washed. The enzyme was deactivated by incubation at 90 °C for 30 min. Subsequently, pre-treated fibers were disintegrated by a homogenization process (7 cycles) using a microfluidizer M-110EH (Microfluidics Inc., USA), resulting in an aqueous suspension of approximately 2 wt% of NFC (60–70 % crystallinity). The aqueous NFC suspension was solvent-exchanged into dimethylformamide (DMF) by repeated centrifugation (4754 RFC) and decantation, followed by ultra-homogenization (Ultra Turrax T18 basic, IKA Werke GmbH & Co. KG, Germany), and a gentle ultra-sonication (Sonics Materials Vibracell VCX750, 5 s at 21 % power) after each solvent exchange. The procedure was carried out until the water content in the solution was \1 % (4 exchanges with a ca. 80 % supernatant removal in each cycle). The concentration of NFC in the DMF was equal to 0.47 wt%. Preparation of Materials for Electrospinning The prepared solutions for electrospinning were selected from a prescreening of solutions with different solids contents (PMMA/NFC) in order to establish conditions for the preparation of uniform fibers with similar diameters to avoid uncertainties in the determination and comparison of mechanical strength between different fiber mats. The compared fiber mats included pure PMMA fibers (aligned and randomly oriented) and aligned PMMA fibers with a 4 wt% content of NFC. The PMMA with a Mw of 350,000 (Alfa Aesar) was dissolved in DMF (Assay 99.8 %) by mixing 10 wt% PMMA into 90 wt% DMF under stirring at a temperature of 50 °C for a minimum of 12 h. The PMMA–NFC–DMF solution was prepared in a similar manner by mixing 90 wt% of the NFC–DMF suspension with 10 wt% PMMA, resulting in 4.0 wt% of NFC to PMMA.
Experimental Electrospinning and Collection of Fibers Extraction of NFC The NFC was prepared from softwood sulphite pulp (Nordic Paper Seffle AB, Sweden); the degree of polymerization of the cellulose was 1200 and the cellulose content was approximately 86 % according to the enzymatic procedure developed by Henriksson et al. [38]. The pulp was first subjected to a pre-treatment step involving enzymatic degradation and mechanical beating. The enzyme used to degrade disordered regions of cellulose was an endoglucanase, Novozym 476 (Novozymes A/S, Denmark). During the enzymatic treatment, a buffer solution (prepared from 9 mM K2HPO4 and 11 mM KH2PO4) was used to keep the pH at 7. The amount of enzyme was 5 ll per 1 g of pulp and the fibers were incubated at 50 °C
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The fiber solutions were electrospun from a flat tip 18 gauge needle (internal diameter 0.84 mm), connected to a solvent-resistant syringe, pumped at a rate of 40 lL/min. The needle tip was positioned 20 cm vertically above the collector, and the electric field from the needle to the substrate was maintained at 45 kV/m. Two different collectors were used to collect non-aligned or aligned fiber mats; a flat metallic surface for the collection of nonaligned fiber mats, and a rapidly rotating cylindrical aluminum drum (diameter 50 mm, length 200 mm) for the collection of aligned and stretched fiber mats (Figs. 1a, 2). The surface of the cylindrical drum was smooth in order to reduce the aerodynamically induced airflow around the collector during high-speed rotation. The rotational speed
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was set to 2000 rpm based on an earlier evaluation of speeds from 200 to 6000 rpm, in order to minimize stretching of the fibers. The relative error and drift from the speed was \0.1 %. The fiber mats were collected over a time of ca. 30 min in all cases.
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calorimeter. Samples weighing between 5 and 10 mg in aluminum sample holders were heated from -40 to 200 °C, at a scanning rate of 10 °C/min under a nitrogen atmosphere (10 mL/min).
Template Transfer Method (TTM) Development Characterization of Fibers The NFC was characterized by atomic force microscopy using a Veeco Instruments—Multimode IIIa AFM, equipped with a 8 nm tip radius RTESP silica cantilever. The electrospun fiber mats were characterized in a Hitachi S-4800 field emission scanning electron microscope (FESEM) at 0.8–1 kV accelerating voltage, before and after tensile testing. A thin coating of gold–palladium was sputtered onto the surface of the samples prior to observation in the SEM. The sputtering (20 s) was performed on a Cressington 208HR high-resolution sputter set to 80 mA current. The area density (mass per unit area) of the electrospun fiber mats was determined from the weight and area of fiber mats collected on the fiber collectors. Two samples were collected from each spinning occasion to ensure that an accurate area density value was determined for each fiber mat. The samples were taken from the same locations along the rotational axis (when using the rotating collector) as the samples taken for tensile measurements. For the fiber mats collected on the static collector, the area density was determined from an average value of two cut pieces from each side of the sample used for tensile measurement. The tensile tester was a Deben Microtest tensile tester, with a 50 N load cell. The micro-tensile-tester was modified to immediately interface with raw data from the analogue to digital converter, which resulted a greater resolution (*2 orders of magnitude) of the load cell and extensometer. All the samples were tested within 24 h after the electrospinning at a strain rate of 0.5 mm/min (temperature of 22 °C, relative humidity of ca. 20 %). A minimum of three samples was tested for each fiber composition. The size of each sample was 5 mm in length and 10 mm in width. The XRD (X-ray diffraction) measurements was conducted on a PANalytical X’Pert Pro diffractometer using Cu Ka radiation (45 kV, 35 mA) and a step size of 0.02° with 75 s per step. The samples were prepared by compressing the electrospun fibers into round (10 mm wide) 3 mm discs, using a Specac press. The crystalline phase and the peak parameters were determined by deconvolution of the experimental diffractogram according to Olsson et al. [2]. The thermal characteristics of the electrospun mats were assessed by a Mettler Toledo DSC 1 differential scanning
Figure 1 shows the arrangements in the TTM that served best to obtain reliable and reproducible data: Three layers build up the template-collecting surface; the first layer was a thin poly(tetrafluoro ethylene) (PTFE) film with adhesive on the rear side (thickness 250 lm), which was attached directly to the collector surface. A rigid (30 lm thick) custom-made template of aluminum foil (25 9 16 mm2) with a precut window (10 9 5 mm2) was then attached on top of the PTFE film. Finally, a 66 lm thick copper tape (20 9 5 mm2) was used to secure the template on top of the PTFE film to the collecting drum, Fig. 1b #1–3. The diameter of the drum gave space for fixing 3 templates along the circumference of the drum, perpendicular to its rotational axis. After the three templates had been properly fastened to the collecting drum, the set-up was placed in the electrical field and the electrospinning was started with the collector drum rotating at a speed of 2000 rpm. After 30 min of fiber collection from the electrospinning; a visible mat of aligned fibers was present over the pre-cut windows in the templates (Figs. 1c, d, 2). The aluminum templates were not removed from the drum until the fibers had been properly fixed along the upper and lower edge of the windows in the templates, using a low-vaporizing alkoxy-ethyl-cyanoacrylate adhesive (Loctite 460, Henkel AG & Co. KGaA, Germany), Fig. 1d, e and Fig. 2 inset. The adhesive was dispensed from a 1 mL syringe fitted with a flat tip 20-gauge needle, which had been modified to have a 0.32 mm thick metallic wire inside. The wire was inserted into the needle tube from the inside of the syringe and protruded a few mm from the tip. The adhesive was in this way allowed to spread evenly along the inserted wire section and could more easily be set in contact with the fibers (ca. 1 mm from the template windows), allowing a precise and uniform adhesive distribution along the window edge. More commonly encountered cyanoacrylates (methyl- or ethyl-cyanoacrylates) did not perform well in the gluing procedure due to vapors/fumes emitted from their formulations, which perturbed the fibers in a clearly visible manner (not shown). Figure 3 shows the resulting fiber-adhesive interface/border, where the fibers show no signs of deformation. Non-perturbed fibers were a prerequisite for the fiber breakage to occur between the separated template parts and not within the fiber fixation region (the micrograph was taken after tensile testing).
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Fig. 2 Photograph of the alignment drum, with (top) magnified image of the aligned fiber mats over the window in the template and (bottom) the adhesive application procedure
supporting sides of the template window were cut open, Figs. 1d, e. This procedure prevented the unintentional application of stresses to the fragile fiber mats before testing, since the template hindered any external forces from reaching the fiber mat. This technique is different from previously reported methods where the fiber mats were manually transferred to a template before fixation of the fibers [26]. The motivation for avoiding removal of the fiber mats from the template substrate can be found in the brittle fiber characteristics (see ‘‘Mechanical Properties of TTM-tested Neat PMMA Fibers as Related to Their Orientation’’ Section), which made it practically impossible to manually transfer the fibers without the support. In the case of collecting non-aligned fiber mats, the same methodology was used, with the only difference that the templates were attached to a flat static collector.
Fig. 1 The apparatus for collecting aligned fibers on a a rotating collector cylinder/drum, and b the tensile template attachment on the collector surface. c An example of aligned fibers deposited on the rotating collector on top of the tensile test transfer template, followed by d the fiber mat secured to the template with a low vaporizing alkoxy-ethyl-cyanoacrylate. e Template cut along edges prior to tensile test
After the adhesive was fully cured (30 min), the templates were cut out (released from the drum) with a scalpel and transferred to the micromechanical stage. After the ends of a template had been clamped in the tester, the
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Fig. 3 Scanning electron micrograph of the fiber-adhesive interface/ border after tensile testing, the apparent disorientation of the originally aligned mat was a result of the rupture
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Fig. 4 Atomic force micrograph (phase imaging) of the NFC used in the composite PMMA-NFC fibers. Inset photograph show PMMA– DMF (left) and PMMA–NFC–DMF (right) electrospinning solutions
Results Characterization of NFC and Electrospinning Solution The enzymatically obtained NFC fibrils used to serve as reinforcement phase inside the PMMA matrix are displayed in Fig. 4. The NFC showed a consistent and large aspect ratio. The lengths of the fibrils were mainly between 0.5 and 2 lm with a diameter between 15 and 30 nm, although a significant fraction (ca. 50 % of the total fibril number) of shorter (\200 nm) and thinner fibrils (5–10 nm) was also present after the enzymatic pre-treatment and homogenization. The NFC was solvent exchanged into DMF, which was subsequently used to prepare the electrospinning solution by addition of PMMA. The electrospinning solution containing NFC and PMMA in DMF showed no visible phase separation or flocculation of the fibrils. The inset photograph in Fig. 4 shows the electrospinning solutions two weeks after homogenization and sonication. Characteristics of Electrospun Fibers for Mechanical Testing The morphologies of the non-aligned and aligned fiber mats are shown in Fig. 5. The individual fibers showed a uniform shape without any beads or other distortion or damage that would reduce their effective load-bearing
Fig. 5 Scanning electron micrograph of a a non-aligned fiber mat (static collector) and b a mat of aligned fibers in the horizontal direction (rotating cylindrical collector), and magnified parts in each case
capacity. The non-aligned fiber mats showed both a random fiber orientation and sometimes coils of fibers, Fig. 5a. In the aligned fiber system, the standard deviation of the fiber axis (from perfectly aligned fibers) was 14.3° for the PMMA and 14.7° for the PMMA–NFC fiber systems (as determined from 300 manual measurements in SEM micrographs). This corresponded to optical orientation factors of 0.922 and 0.921 respectively (a value of 1 corresponds to a perfect alignment of the fibers), using the formula derived by Hermans and Platzek [39]. The diameter of the aligned PMMA fibers was 1.74 ± 0.35 lm, whereas that of the randomly oriented fibers was 2.02 ± 0.34 lm (Fig. 6), indicating that the aligned fibers were slightly stretched by the collection/deposition on the drum surface at a rotational speed of 2000 rpm. The PMMA–NFC fibers showed a larger diameter, 2.70 ± 0.58 lm (Fig. 6c), even though they were collected at the same speed as the pristine PMMA fibers. This was suggested as related to the increased viscosity of the electrospinning solution as a result of the inclusion of the NFC.
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Fig. 7 X-ray diffractograms of the two electrospun fiber mats (offset for clarity), with an inset showing the corresponding NFC diffractogram
Fig. 6 Size distribution of a non-aligned, b aligned PMMA fibers and c aligned PMMA–NFC fibers
The presence of NFC in the PMMA–NFC hybrid fibers was confirmed by the presence of the peak from the 200 lattice plane (22.9°) in the cellulose I structure [40] (Fig. 7). The crystalline content inside the hybrid fibers was calculated from the ratio between the crystalline peak area and that of the total integrated area as explained by Olsson et.al. [2]. This resulted in a 2.5 % crystallinity, which confirmed the 4 wt% loading of the NFC having a ca. 60 % crystalline structure. The Tg of the PMMA in the PMMA–NFC fibers increased from 124.4 to 125.2 °C in the presence of 4 wt% NFC. An increase in the glass transition temperature (Tg) has previously been reported as an indication of a good dispersion of the cellulose inside the fibers due to hydrogen bond interactions between hydroxyl groups present on the cellulose and ester functional groups present on the PMMA, which in turn would restrict the molecular motions of the polymer segments [19]. Mechanical Properties of TTM-Tested Neat PMMA Fibers as Related to Their Orientation The stress applied during the tensile testing was calculated as the load divided by the effective thickness and width of the fiber mats. The effective thickness was determined by dividing the measured area density of the fiber mat
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(typically 9–10 g/m2) by the known density of the material in the fibers (1.17 g/cm3 for PMMA [41], 1.50 g/cm3 for NFC [42]). The nominal stress was thus estimated as the global force divided by the nominal accumulated crosssectional area for all fibers. This was found to be a more reliable method than using a digital micrometer due to the fragile nature of the fibers. In addition, the thicknesses obtained using a digital micrometer gave measurement deviations of up to 50 % depending on the applied torque. The maximum tensile stresses for the respective mats were 5.0 ± 0.7 MPa for the non-aligned PMMA fibers and 16.8 ± 3.7 MPa for the aligned PMMA fibers (Fig. 7), i.e. the non-aligned mats showed only 1/3 of the tensile strength recorded for the aligned fibers. The Young’s modulus was 0.27 ± 0.05 GPa for the randomly oriented fiber mats and 1.49 ± 0.29 GPa for the aligned PMMA fiber mats. The stiffness ratio of the non-aligned and aligned fibers was 0.18. This corresponds to a ratio of 1.6:8, which is lower than the theoretical prediction of 3:8 derived for a random in-plane fiber orientation [43, 44]. This was a result of the looping of fibers visible in the nonaligned fiber mat (shown in Fig. 5a), which contributed to a less uniform stress transfer because a large fraction of the fibers did not carry load initially. Accordingly, a considerable number of fibers were progressively aligned during the testing in the random system, whereas the fibers that had been aligned before the testing were instantaneously stretched (without any significant fiber entanglements) as the measurements started. This resulted in a higher initial modulus for the aligned mats. It also explained the larger strain to failure for the non-aligned mats, as well as the gradual decrease in strength for the tested non-aligned fiber mats. Similar trends for non-aligned fiber mats have been reported for poly(caprolactone), poly(ethylene oxide) and PMMA [18, 45, 46] as well as for composites of
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impregnated electrospun fibers [10]. Interestingly, the aligned fiber system showed mechanical properties approaching those of individual fibers. The strength of ca. 17 MPa recorded for the aligned fiber mats also agreed well with the reported values measured for single fibers of PMMA by Liu et al. (15–25 MPa) [47].
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volume fraction of NFC. g1 can be estimated from the shear lag model obtained by Cox [49]: g1 ¼ 1
tanhðbL2 Þ
ð2Þ
bL 2
where
Mechanical and Thermal Data for Aligned PMMA Fibers with 4 wt% Dispersed NFC
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u G bL L u m ¼2 t D 2 Ef ln j
Figure 8 includes the mechanical properties of PMMA fibers with 4 wt% NFC. The brittle nature of the fibers is clearly displayed for the PMMA samples as well as the PMMA fibers containing cellulose. However, the trend in the testing series revealed stronger and stiffer fibers as expected from the properties of the cellulose. The tensile strength of the cellulose-containing samples was 21.9 ± 1.7 MPa and the average Young’s modulus was 1.93 ± 0.34 GPa, which is equivalent to a ca. 30 % increase in strength and modulus for the reinforced PMMA–NFC hybrid fibers compared with fibers without NFC. The toughness of the aligned fibers (fracture energy calculated from the area under the stress-strain curve) increased by ca. 25 % from 126 ± 38 kJ/m3 to 157 ± 18 kJ/m3 with the addition of NFC. To evaluate the measured modulus for fibers with the presence of NFC to the theoretically expected modulus based on the material data for the components of the fibers, the modified rules of mixture was applied [48]: Ec ¼ g o g 1 / f Ef þ 1 / f Em ð1Þ
L and D are the length and diameter of the NFC and Gm is the shear modulus of the PMMA matrix. j takes the value of 0.907 if a hexagonal packing of the fibers is assumed. Using the full range of the L and D data described in ‘‘Characterization of NFC and Electrospinning Solution’’Section, it is possible to estimate a maximum and a minimum L/D ratio equal to 133 and 17. The volume fraction of NFC was obtained from the weight fraction and the densities of the NFC and the PMMA. Gm was estimated from: Gm ¼ Em =ð2ð1 þ tÞÞ where Em is the measured 1.49 GPa, and t is the Poisson’s ratio for PMMA (0.4 [50]). Ef was assumed to be 60 % of the 130 GPa modulus of 100 % crystalline cellulose (corresponding to a sample with 60 % crystallinity). If the NFC orientation factor is 1 (completely uniaxial oriented NFC) the Ec values are 2.42 and 3.69 GPa, depending on the L/D ratio used. These values are higher than the measured 1.93 GPa and the explanation to this lower measured value as compared to the theoretical estimate may stem from several factors:
where go is the NFC orientation factor and g1 is the fiber length efficiency. Ec, Ef and Em are the modulus of the composite, NFC and PMMA matrix, respectively. hf is the
1.
2.
3.
4.
5.
Fig. 8 Stress-strain data for the aligned PMMA and cellulosereinforced PMMA fiber mats, and for the non-aligned (random) PMMA fibers
ð3Þ
/f
The NFC orientation factor inside the electrospun fibers may have been lower than 1, i.e. smaller NFC may occasionally be oriented in a random manner inside the core of the electrospun fibers, i.e. in a direction deviating from the main fiber direction. The adhesion between the NFC and the matrix PMMA may occasionally be lacking. This may result in a less than perfect stress-transfer between the fibrils and the matrix. The true modulus of the NFC may deviate from the assumed value for weaker fibrils due to an uneven enzymatic degradation of the amorphous phase during the extraction of the crystals. The electrospun fibers with NFC have some nonuniformity along the fibers present between the micro tensile clamps. This can be seen in the size distribution shown in Fig. 6c. The rule of mixtures also assumes that both the NFC and the main fibers are stretched in the tensile direction, which was not a perfectly satisfied condition since the electrospun fibers themselves were not perfectly aligned, Herman orientation factor 0.92.
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Fig. 9 Scanning electron micrograph of fracture surfaces of a electrospun PMMA and b PMMA–NFC composite fibers after tensile testing, with a magnified image illustrating NFC pull-out
Of the factors mentioned above, the compatibility between the PMMA and the NFC appeared favorable from the micrographs, and the hybrid fibers showed fracture surfaces where the NFC (visible as small dots) was broken at the same interface as the PMMA. Only very short NFC pull-out lengths were observed, Fig. 9 inset. The features referred to as ‘‘dots’’ in the fracture surface were present on all the fracture surfaces of the broken PMMA–NFC fibers, shown in Fig. 9b. The size of the dots corresponded well with the diameters of the thinner fraction of the cellulose NFC fibrils presented in Fig. 4. It was difficult to find fracture surface regions with apparent NFC fibril pull-out. This was in contrast to the hybrid fibers reported in a previous study where more uniform acidhydrolyzed cellulose nanofibers extracted from bacterial cellulose (BC) was used and numerous BC fibrils protruded from the fracture surfaces [2].
Conclusions A template transfer method (TTM) was established that allows reliable deposition, transfer and micro-tensile
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testing of aligned as well as isotropic fiber mats prepared by electrospinning. The technique includes the fixation of sensitive fibers to the transfer substrate by a low volatile acrylate liquid adhesive (using a custom-made syringe unit), which proved to be less damaging to the fibers than any kind of tape application. The only physical contact with the fibers prior to the testing was the liquid adhesive itself. After fixation of the fibers (curing of the liquid adhesive), the template was able to carry the intact fibers to the testing stage where mechanical testing was performed. The TTM is a universal method and may apply other fibertesting procedures where mechanical data need to be conservatively assessed on intact and non-disrupted very thin and sensitive fibers. Three different types of electrospun fiber mats were mechanically evaluated using the TTM. Aligned fiber mats of PMMA were compared to mats of non-aligned fibers. The aligned fiber system showed a significantly higher modulus (*450 %) and tensile strength (*250 %), which was related to a larger proportion of load-carrying fibers. With addition of enzymatically pretreated NFC to PMMA, it was possible to successfully prepare uniform electrospun nanocomposite PMMA–NFC fibers, despite the large aspect ratio of NFC. This resulted in an additional increase of 30 % in modulus and tensile strength of aligned fiber mats compared to the neat PMMA fiber reference, corresponding to a 25 % increase of the toughness (work to fracture) of the fibers. Microscopy revealed a flat fracture surfaces with very few NFC pull-outs in the cellulose-PMMA hybrid fibers, suggesting good phase compatibility. This indicates the potential for NFC reinforced composite fibers with further improvements in mechanical performance. Acknowledgments The Swedish International Development Cooperation Agency is acknowledged for providing the financial support of this project. Dr. Andreas Fisher (Division of Inorganic Chemistry, Royal Institute of Technology) is thanked for performing the X-ray diffraction characterization.
References 1. Doshi J, Reneker DH (1995) J Electrostat 35(2–3):151–160 2. Olsson RT, Kraemer RH, Lopez-Rubio A, Torres-Giner S, Ocio MJ, Lagaron JM (2010) Macromolecules 43(9):4201–4209 3. Barhate RS, Ramakrishna S (2007) J Memb Sci 296(1–2):1–8 4. Zhou Y, Yang D, Chen X, Xu Q, Lu F, Nie J (2008) Biomacromolecules 9(1):349–354 5. Sheikh FA, Barakat NAM, Kanjwal MA, Chaudhari AA, Jung I-H, Lee JH, Kim HY (2009) Macromol Res 17(9):688–696 6. Zhang YZ, Feng Y, Huang ZM, Ramakrishna S, Lim CT (2006) Nanotechnology 17(3):901–908 7. Li D, Xia YN (2004) Nano Lett 4(5):933–938 8. Sen R, Zhao B, Perea D, Itkis ME, Hu H, Love J, Bekyarova E, Haddon RC (2004) Nano Lett 4(3):459–464 9. Swart M, Olsson RT, Hedenqvist MS, Mallon PE (2010) Polym Eng Sci 50(11):2143–2152
J Polym Environ (2012) 20:967–975 10. Bayley GM, Hedenqvist M, Mallon PE (2011) Polymer 52(18):4061–4072 11. Martinez-Sanz M, Olsson RT, Lopez-Rubio A, Lagaron JM (2012) J Appl Polym Sci 124(2):1398–1408 12. Martinez-Sanz M, Olsson RT, Lopez-Rubio A, Lagaron JM (2011) Cellulose 18(2):335–347 13. Jaeger R, Bergshoef MM, Batlle CMI, Schonherr H, Vancso GJ (1998) Macromol Symp 127:141–150 14. http://www.elmarco.com/products/ns-organic-production-lines1600/ 15. Sanders JE, Nicholson BS, Mitchell SB, Ledger RE (2003) J Biomed Mater Res, Part A 67A(4):1412–1416 16. Tan EPS, Lim CT (2004) Rev Sci Instrum 75(8):2581–2585 17. Tan EPS, Lim CT (2004) Appl Phys Lett 84(9):1603–1605 18. Croisier F, Duwez AS, Je´roˆme C, Le´onard AF, van der Werf KO, Dijkstra PJ, Bennink ML (2012) Acta Biomater 8(1):218–224 19. Dong H, Strawhecker KE, Snyder JF, Orlicki JA, Reiner RS, Rudie AW (2012) Carbohydr Polym 87(4):2488–2495 20. Inai R, Kotaki M, Ramakrishna S (2005) Nanotechnology 16(2): 208–213 21. Tan EPS, Ng SY, Lim CT (2005) Biomaterials 26(13):1453–1456 22. Tan EPS, Lim CT (2006) Compos Sci Technol 66(9):1102–1111 23. Burgert I, Fruhmann K, Keckes J, Fratzl P, Stanzl-Tschegg SE (2003) Holzforschung 57(6):661–664 24. Wong S-C, Baji A, Leng S (2008) Polymer 49(21):4713–4722 25. Baker SC, Atkin N, Gunning PA, Granville N, Wilson K, Wilson D, Southgate J (2006) Biomaterials 27(16):3136–3146 26. Huang ZM, Zhang YZ, Ramakrishna S, Lim CT (2004) Polymer 45(15):5361–5368 27. Hsieh YC, Yano H, Nogi M, Eichhorn SJ (2008) Cellulose 15(4): 507–513 28. Nishiyama Y (2009) J Wood Sci 55(4):241–249 29. Iwamoto S, Kai W, Isogai A, Iwata T (2009) Biomacromolecules 10(9):2571–2576 30. Hepworth DG, Bruce DM (2000) J Mater Sci 35(23):5861–5865 31. Mark RE (1967) Cell wall mechanics of tracheids. Yale University Press, New Haven
975 32. Kroonbatenburg LMJ, Kroon J, Northolt MG (1986) Polym Commun 27(10):290–292 33. Helbert W, Cavaille JY, Dufresne A (1996) Polym Compos 17(4):604–611 34. Nakagaito AN, Yano H (2005) Appl Phys A Mater Sci Process 80(1):155–159 35. Fernandes SCM, Freire CSR, Silvestre AJD, Neto CP, Gandini A, Berglund LA, Salmen L (2010) Carbohydr Polym 81(2):394–401 36. Sehaqui H, Salajkova M, Zhou Q, Berglund LA (2010) Soft Matter 6(8):1824–1832 37. Sehaqui H, Zhou Q, Berglund LA (2011) Soft Matter 7(16):7342–7350 38. Henriksson M, Henriksson G, Berglund LA, Lindstrom T (2007) Eur Polym J 43(8):3434–3441 39. Hermans PH, Platzek P (1939) Kolloid-Z 88:68–72 40. O’Connor RT (1972) Instrumental analysis of cotton cellulose and modified cotton cellulose. M. Dekker, New York 41. Bashford D (1997) Thermoplastics: directory and databook, 1st edn. Chapman & Hall, London 42. Henriksson M, Berglund LA, Isaksson P, Lindstrom T, Nishino T (2008) Biomacromolecules 9(6):1579–1585 43. McCrum NG, Buckley CP, Bucknall CB (1997) Principles of polymer engineering, 2nd edn. Oxford University Press, Oxford 44. van den Akker JA, Lathrop AL, Voelker MH, Dearth LR (1958) Tappi 41(8):416–425 45. Zhou C, Chu R, Wu R, Wu Q (2011) Biomacromolecules 12(7):2617–2625 46. Baek W-I, Pant HR, Nirmala R, Nam K-T, Oh H-J, Kim H-Y (2012) Polym Int 61(5):844–849 47. Liu L-Q, Tasis D, Prato M, Wagner HD (2007) Adv Mater 19(9):1228–1233 48. Madsen B, Joffe R, Peltola H, Nattinen K (2011) J Compos Mater 45(20):2119–2131 49. Cox HL (1952) Br J Appl Phys 3(3):72–79 50. van Krevelen DW (1990) Properties of polymers, 3rd edn. Amsterdam, New York, Elsevier
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