Transparent PMMA-based nanocomposite using electrospun graphene-incorporated PA-6 nanofibers as the reinforcement

Composites Science and Technology 89 (2013) 134–141

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Transparent PMMA-based nanocomposite using electrospun graphene-incorporated PA-6 nanofibers as the reinforcement Biyun Li, Huihua Yuan, Yanzhong Zhang ⇑ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China

a r t i c l e

i n f o

Article history: Received 1 September 2013 Received in revised form 26 September 2013 Accepted 30 September 2013 Available online 10 October 2013 Keywords: A. Nanocomposites A. Functional composites B. Mechanical properties E. Electro-spinning Graphene

a b s t r a c t This paper deals with development of a novel poly(methyl methacrylate) (PMMA) based transparent nanocomposite made from using electrospun graphene-incorporated-Nylon 6 (Gr/PA-6) nanofibers as the reinforcement, in which both the mechanical and optical properties of the developed Gr/PA-6/PMMA nanocomposite are paid particular attention. By introducing the concept of electrospun PA-6 nanofibers as the dispersing carrier for graphene nanosheets and by employing a facile self-blending co-electrospinning approach for homogeneously hybridizing nanocomposite nanofibers of Gr/PA-6 with PMMA fibers, aggregation issue of the involved nanofillers (i.e., the Gr nanosheets and the Gr-incorporated PA-6 nanofibers) within the PMMA matrix could be effectively addressed. Visible light transmittance and tensile mechanical properties of the hot-pressed Gr/PA-6/PMMA nanocomposite were examined in relation to the loading fractions of the Gr nanosheets in the nanocomposite. It was demonstrated that a significant enhancement in tensile mechanical properties of the Gr/PA-6/PMMA nanocomposite was accomplished at a Gr loading of merely 0.01 wt%; that is, a nearly 56%, 113% respective improvement of tensile strength, Young’s modulus, and noticeably above 250% increase of fracture toughness were achieved, while the transmittance of the nanocomposite was maintained above 70% (in other words, less than 10% loss in transparency in comparison with neat PMMA) in the visible wavelength range of 400–800 nm. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction It is well known that polymer composites reinforced with traditional micro-sized fibers offer the highest mechanical performance compared with other kinds of reinforcements. However, for transparent composites, as the name suggests, there is additional concern on transparency. In order to improve the mechanical properties with no sacrifice in transparency, one promising solution is to use nano-scaled reinforcements (e.g., nanofibers or nanoparticles) with dimension significantly less than visible light wavelength (400–800 nm) [1–3]. The available large surface area of nano-scaled reinforcements also offers the possibility of realizing efficient transfer of applied load from the matrix to the reinforcing element. Electrospinning is currently the most powerful technique for fabricating continuous nanofibers with typical diameters in a few hundreds of nanometers. Previous studies have demonstrated the feasibility of using electrospun nanofibrous membranes to prepare transparent nanocomposites with enhanced mechanical properties ⇑ Corresponding author. Address: College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Road, Shanghai 201620, China. Tel./fax: +86 21 6779 2374. E-mail address: [email protected] (Y. Zhang). 0266-3538/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2013.09.022

and minimal loss in transparency [1,3,4]. However, there is still much room for improvement by developing high performance nanofibers as reinforcement, conducting structural design via utilizing oriented nanofibers instead of non-wovens [5,6], and most of all having nanofibers homogeneously dispersed and distributed within the polymer matrices. A variety of proposed solutions such as coaxial electrospinning [4], suction filtration [7], and solution casting [8], have evidently demonstrated the necessity and critical role of nanofiber dispersion in achieving high performance transparent nanocomposites. Apart from the one-dimensional nanofibers, various inorganic nanoparticles such as clay, silica, metallic oxide and carbon nanotube, are also considered as favorable nano-reinforcements for improving mechanical, electrical, thermal, optical or other properties of the polymers of interest [6,9,10]. Particularly, graphene with a high optical transparency (97.4% at 550 nm for monolayer Gr film and is usually reduced by approximately 2.2–2.3% for an additional monolayer) [11–13] and Young’s modulus of 1 TPa and an ultimate strength of 130 GPa, is one of the strongest materials ever measured [14]. This makes graphene an outstanding reinforcement for polymers with optical transparency if the formation of aggregates or agglomerates is avoided. Considering the confinement effect with the electrospun nanofibers, electrospinning could be one of the most convenient dispersing methods to use [15].

B. Li et al. / Composites Science and Technology 89 (2013) 134–141

Nevertheless, well-dispersed Gr using nanofibers as a dispersing carrier has rarely been explored [16,17]. Poly(methyl methacrylate) (PMMA), a well-known transparent thermoplastic, has previously been used as a model polymer for making nanofiller-reinforced (e.g., PA-6 nanofibers) transparent nanocomposites [4,18,19]. In the present study, first of all Gr was proposed to enhance the mechanical properties of the high strength thermoplastic PA-6 by incorporating Gr nanosheets into PA-6 through electrospinning to form nanocomposite nanofibers of Gr/PA-6. Next, a novel self-blending co-electrospinning (SBCE) strategy [20] was employed to produce hybrid fibrous mats consisting of uniaxially aligned Gr/PA-6 nanofibers (as the reinforcement) and PMMA microfibers (as the matrix). Such hybrid fibers were then made into transparent nanocomposite via hot press molding, in which the PMMA fibers are in situ melted to become the matrix, whereas the Gr/PA-6 nanofibers with higher melting point maintained the original nanofiber morphology within the PMMA matrix. Thereafter, optical transmittance and mechanical properties of the Gr/PA-6 nanofibers reinforced PMMA nanocomposite were sequentially evaluated. 2. Experimental 2.1. Materials PMMA (Mw = 3.0  107 g/mol) and PA-6 (Mw = 52,800 g/mol) were purchased from the Aladdin Chemistry Co., Ltd. and the Shanghai Elite Plastic, respectively. Gr in powder form was kindly supplied by the Ningbo Institute of Material Engineering and Technology, Chinese Academy of Sciences. Hexafluoroisopropanol (HFIP, purity P98%) was obtained from the Shanghai Darui Fine Chemical Co., Ltd. These materials and chemicals were used as received without further purification.

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sets of solutions were charged in opposite-polarity voltages so that the generated jets/fibers could attract each other and then were wounded together onto a rotating drum for the sake of better blending. Whereas reciprocating movement of the spinneret for electrospinning Gr/PA-6 solutions allows for preferably achieving uniform thickness of the blended fibrous membranes of Gr/PA-6/ PMMA. By altering the composition of reinforcement systems (i.e., PA-6 or Gr/PA-6 solutions) while keeping the PMMA one constant during the electrospinning, the mass ratio of PA-6 to PMMA was predetermined to maintain a percentage of ca. 1 wt%, which means the Gr fraction can be accordingly varied at 0, 0.005, and 0.01 wt% to the PMMA matrix fibers. 2.3. Fabrication of Gr/PA-6/PMMA nanocomposite To fabricate Gr/PA-6/PMMA nanocomposite in the form of transparent sheets with different contents of Gr, the as-electrospun fibrous mats of Gr/PA-6 and PMMA hybrid collected via SBCE method in Section 2.2 were firstly cut into pieces along the fiber alignment direction to a dimension of 4  5 cm2, followed by stacking the fibrous mats layer-by-layer before transferring the fibrous assembly into a compression mold. Then, the mat assembly along with two pieces of polyimide films (as release agent) was placed in a thermocompressor (0.25MN XLB400X400-D/S, Shanghai Rubber Machinery, China) at 185 °C under 0.5 MPa for 3 min pre-pressing treatment. Then, compressing force was elevated to 1.5 MPa and maintained at this pressure level for 5 min to allow for complete melting of the PMMA fibers into the continuous matrix phase, within which the nanofibrous morphology of the Gr/PA6 remained as the reinforcement phase. Finally, the compressive force was removed and rectangular nanocomposite sheet was obtained (Fig. 1B). For the purpose of comparison, transparent nanocomposite sheets of Gr/PA-6/PMMA with a series of thickness at 0.3, 0.4, and 0.5 mm were accordingly prepared as well.

2.2. Electrospinning 2.4. Characterization PMMA (20 wt%) and PA-6 (6 wt%) solutions were prepared by dissolving weighed PMMA and PA-6 granules in HFIP, respectively. To prepare Gr-doped PA-6 solutions (i.e., Gr/PA-6 at the Gr to PA-6 mass ratios of 0, 0.5, and 1.0 wt%), different amounts of pristine PA6 granules were dissolved into Gr/HFIP dispersions, which were prior prepared and sonicated for 1 h to homogeneously disperse the Gr nanosheets in HFIP. Conductivity of the PA-6 solutions containing or free of Gr nanosheets was measured by using a conductivity meter (E-201-C, Shanghai INESA instrument, China). Prior prepared PMMA solution and Gr/PA-6 suspensions with varied loadings of Gr were simultaneously electrospun into hybrid fibers via self-blending co-electrospinning as depicted in Fig. 1A. The two

Raman spectroscopy was performed at room temperature by a Raman spectrometer (Renishaw, inVia-Reflex), which uses a laser excitation wavelength 514 nm and laser spot size 0.5 mm. A scanning electron microscope (SEM, JEOL JSM-5600LV, Japan) operated at an acceleration voltage of 8–10 kV was used to observe the morphology of the electrospun nanofibrous mats and the fractured surfaces of different Gr/PA-6/PMMA nanocomposite sheets. Transmission electron microscopy (TEM) observations of Gr nanosheets and Gr-doped PA-6 nanofibers were performed under an acceleration voltage of 200 kV with a Hitachi H-800 transmission electron microscope. Crystalline structure of the Gr-reinforced

Fig. 1. (A) Preparation of Gr/PA-6 nanofibers mingled with PMMA microfibers via SBCE method. (B) Fabrication of transparent PMMA-based nanocomposite reinforced with aligned electrospun Gr/PA-6 nanofibers through hot press molding.

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PA-6 nanofibers was determined by differential scanning calorimetry (DSC) (204 F1, NetZSch, Germany) from 30 to 250 °C at a heating rate of 10 °C/min in N2. X-ray diffraction (XRD) spectroscopy was carried out on a Rigaku D/max 2550 PC with Cu Ka radiation. The operating voltage and current were kept at 40 kV and 300 mA, respectively. The electrospun Gr/PA-6 nanofibrous samples were examined between 5° and 60° (2h) at a scanning rate of 5° (2h) per minute. Optical transmittance of the nanocomposite sheets with size of 40 mm  30 mm  (0.3–0.5) mm was inspected on a UV–Vis spectrometer (TU-1810, Beijing Purkinje General Instrument) in visible light wavelength range of 400–800 nm. Tensile properties of the electrospun nanofibrous membranes of Gr/PA-6 and the transparent nanocomposites of Gr/PA-6/PMMA were determined using a tabletop testing machine (H5K-S, Hounsfield, United Kingdom) equipped with a 1000 N load cell. Rectangular-shaped specimens with a dimension of 50 mm (length)  10 mm (wide)  0.4 mm (thickness) were stretched at a constant cross-head speed of 10 mm/min at room temperature. At least five specimens were tested for each type of samples.

3. Results and discussion 3.1. Electrospinning of aligned Gr/PA-6 nanofibers Aligned electrospun nanofibers can be generally achieved by using a rotating drum as the collector at high speed. In our case, by adjusting the rotating speed at 1000 rpm, electrospun PA-6, 0.5 and 1.0 wt% Gr/PA-6 nanofibers oriented generally along the rotational direction were prepared (Fig. 2A–C). The average diameters of aligned electrospun PA-6 based fibers were found to decrease from 500 ± 60, 479 ± 103, to 338 ± 76 nm with increasing the loading fractions of Gr in the PA-6 dominant polymer solutions from 0, 0.5, to 1.0 wt%, respectively (Fig. 2D). The decreasing fiber fineness is attributed to the presence of Gr in the PA-6 solution, which gave rise to increased electrical conductivity (Fig. 2D), in favor of attenuating the fiber thickness [21]. Fig. 3A shows TEM images of the Gr in surfactant-free HFIP dispersion. The typical wrinkled structures and diaphanous folding image of nanosheets are observed. The SAED pattern (inset of Fig. 3A) shows strong diffraction spots with 6-fold symmetry, which confirm the formation of a hexagonal crystalline Gr structure [22]. Fig. 3B and C shows the TEM micrographs of the electrospun PA-6 nanofibers containing 1 wt% Gr. Gr nanosheets were well dispersed in the electrospun PA-6 nanofibers at such a low Gr loading fraction (Fig. 3B). Also, the finely dispersed Gr nanosheets were aligned along the nanofiber axis, which is mainly ascribed to the large electrostatic field associated elongation forces

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applied to the solution jet during electrospinning [6,15]. At high magnification (Fig. 3C), fine features of Gr nanosheets in the form of three-layer stacking are observed in the PA-6 nanofibers. The calculated thickness of single layer of Gr nanosheet, namely, nearly 0.34 nm, is consistent with the value reported previously [23]. To further confirm the existence of Gr nanosheets in PA-6 nanofibers, the Gr/PA-6 nanofibrous mat samples were monitored by a Raman spectrometer. It is known that the Raman spectrum of Gr in many cases is typically characterized by two main features, the G band from the first-order scattering of the E2g phonon of sp2 C atoms (usually observed at 1575 cm 1) and the D band from a breathing mode of j-point photons of A1g symmetry (1350 cm 1) [24]. As can be seen from Fig. 3D, both the Gr powder and Gr/PA-6 nanofibrous mat showed two typical peaks at 1300–1400 cm 1 and 1500–1600 cm 1, which were assigned as D band and G band, respectively. The intensity ratios of D band and G band (i.e., ID/IG) are 1.34 and 1.31 for Gr and Gr/PA-6 samples, respectively. Previous studies have shown that the crystallization behavior of polymers is affected when interacting with nanofillers [25,26]. Likewise, in our study the investigation of crystallographic structure through XRD and DSC has clarified the existence of interaction between Gr nanosheets and the PA-6 nanofiber matrix. As shown in Fig. 4A, the Gr diffraction peak located at 24.5° becomes broad and exhibits the location of the (0 0 2) plane of the graphite. This peak corresponds to a 0.36–0.37 nm layer thickness [22,27], which is coincident with the above TEM results of Gr nanosheets. PA-6 has been reported to exhibit two main crystalline forms, namely a and c [28]. Raw PA-6 granules exhibit a1 and a2 phases at 20.0° and 23.8°, respectively. Whereas the electrospun PA-6 and Gr/PA-6 nanofibers only show c phase that appears at 21.4°. This may be attributed to the fact that electrospinning is, as with high-speed spinning, to predominantly give rise to PA-6 the c-form crystalline structure. Furthermore, the increased amount of Gr incorporation into the PA-6 nanofibers leads to increased crystallinity of the c phase crystal. This may be attributed to the promotional role of Gr in increasing the heterogeneous nucleation of the c-form and accelerating the crystallization of c phase in electrospinning of PA-6 nanofibers [29]. On the other hand, absence of the characteristic peak of Gr in hybrid nanofibers would suggest the well dispersion of Gr in the PA-6 nanofibers. Fig. 4B is the DSC thermograms of raw PA-6 granules, electrospun PA-6 nanofibers and Gr/PA-6 nanofibers. The a and c forms of PA-6 crystalline are usually associated with two different melting temperatures at ca. 220 and 214 °C, respectively [25,28]. An obvious enhancement of c form crystal peak can be clearly seen with increasing the content of Gr in PA-6 nanofibers, which is consistent with the aforementioned XRD results. Tensile tests were conducted to prove that introduction of Gr enables to achieve a reinforcement effect to the PA-6 nanofibers.

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Fig. 2. SEM images of as-spun (A) PA-6 nanofibers, (B) 0.5%Gr/PA-6 nanofibers, and (C) 1.0%Gr/PA-6 nanofibers. (D) Plot on the correlations of fiber diameters (black hollow diamond) and electrical conductivity (blue solid circle) versus PA-6 solutions containing varied amount of Gr nanosheets. Parameters used are: concentration 6 wt%, applied voltage 18 kV, feeding rate 0.05 mL/h, collecting distance 15 cm, and rotating speed 1000 rpm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 4. (A) XRD patterns of Gr nanosheets, electrospun aligned nanofibers of PA-6 and Gr/PA-6, and raw PA-6 granules (Inset plot shows degree of crystallinity versus Gr loading in PA-6 nanofibers); (B) DSC thermograms of raw PA-6 granules, electrospun aligned nanofibers of PA-6 and Gr/PA-6; (C) typical tensile stress–strain curves of electrospun aligned nanofibers of PA-6 and Gr/PA-6 with different Gr loadings.

Fig. 4C gives typical stress-strain curves of the electrospun aligned Gr/PA-6 nanofiber mats relative to the mass content of Gr in PA-6 nanofibers. Of note, the tensile strength of 0.5%Gr/PA-6 (38.6 ± 2.5 MPa) and 1.0%Gr/PA-6 fibrous membranes (52.1 ± 4.6 MPa) were about 1.5 and 2 times as high as that of pure PA-6 nanofibrous mats (25.1 ± 4.8 MPa), respectively, revealing a positive correlation of Gr loading versus strength enhancement. Furthermore, the Young’s modulus of Gr/PA-6 nanofiber mats follows the same trend, namely, nearly 178% increase obtained by 1.0% Gr/PA-6 fibrous mats (216.8 ± 32.0 MPa) compared with neat PA-6 fibrous mats (78.0 ± 19.4 MPa). The significant enhancement of mechanical properties of Gr/PA-6 nanofibers is likely due to its capacity of Gr in promoting higher crystalline degree of PA-6 as discussed in the above XRD and DSC results. 3.2. PMMA nanocomposites reinforced by the Gr/PA-6 nanofibers With respect to the matrix PMMA, 1 wt% of PA-6 nanofibers was chosen as the primary reinforcing phase in our study because the optical transmittance of the PA-6/PMMA nanocomposite could drop minimally when the content of PA-6 nanofibers was lower than 1.5 wt% [4,30]. Such a judicious choice of loading fraction of the PA-6 nanofibers exactly allows us to examine the reinforcing effect upon introducing Gr nanosheets. As expected, the incorporation of Gr nanosheets resulted in decreased fineness of electrospun PA-6 nanofibers with improved mechanical properties as shown in Figs. 2 and 4C. Thus, the obtained results in Section 3.1 provide solid evidences for us to further explore the feasibility of using the electrospun Gr/PA-6 nanocomposite nanofibers as a novel type of nanofiller for reinforcing the PMMA matrix. In order to have the Gr/PA nanofibers homogeneously dispersed in the PMMA matrix, we proposed a novel strategy by using the

SBCE method [20] to first of all prepare fibrous membranes composed of Gr/PA-6 nanofibers mingled with PMMA microfiber matrix. As can be seen in Fig. 5, Gr/PA-6 nanofibers (the thinner fibers labeled with green1 arrows) were homogeneously mingled with the PMMA microfibers (the thicker ones indicated by orange arrows). Then, after hot pressing as illustrated in Fig. 1B, the resultant Gr/PA-6/PMMA nanocomposite sheets (thickness: 0.4 mm, Fig. 6) were found transparent, still maintaining a high degree of transparency when the Gr loading fraction is lower than the current upper limit of 0.01 wt% relative to the PMMA matrix. Fig. 7A shows quantitatively measured profiles of light transmittance versus the wavelength of visible light for the fabricated pure PMMA and nanocomposite sheets of 1%PA-6/PMMA, 0.005%Gr/1%PA-6/PMMA and 0.01%Gr/1%PA-6/PMMA examined by a UV–Vis spectrometer. The neat PMMA was an optically transparent sheet with light transmittance above 85% in the visible light wavelength range (400–800 nm), while the transmittance of PMMA nanocomposites reinforced with pure PA-6 and 0.5%Gr/ PA-6 nanofibers maintained about 80% in the full range of visible light wavelength, indicative of excellent transparency (Fig. 6B and C). Whereas, further improving the content of Gr in PA-6 nanofibers up to 1.0%Gr/PA-6, generated slight decrease of its transparency but still remained above 70% (throughout 400– 800 nm), suggesting that incorporation of small quantities of Gr nanosheets (i.e., no more than 0.01 wt%) will not remarkably influence the transparence (When Gr loading fraction was above 0.01 wt% in PMMA matrix, the transparency of the nanocomposites had a serious decline. Data not shown). Furthermore, in order to investigate the influence of sheet thickness on transparency, the 1 Please note that Fig. 5 will appear in B/W in print and color in the web version. Based on this, please approve the footnote 1 which explains this.

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Fig. 5. SEM images of (A) pure electrospun PMMA microfibers, hybrid fibers of (B) PMMA + 1%PA-6, (C) PMMA+0.5%Gr/PA-6, and (D) PMMA + 1.0%Gr/PA-6. Parameters used are: concentration 6 wt% (Gr)/PA-6 and 20 wt% PMMA; applied voltage 18 kV (Gr/PA-6) and 15 kV (PMMA); feeding rate 0.05 mL/h (Gr/PA-6) and 2 mL/h (PMMA); collecting distance 15 cm and rotating speed 1000 rpm.

Fig. 6. Optically clear transparent PMMA-based nanocomposites: (A) pure PMMA; (B) 1%PA-6/PMMA; (C) 0.005%Gr/1%PA-6/PMMA; (D) 0.01%Gr/1%PA-6/PMMA. Sample thickness used in these demonstrations is 0.4 mm. The photos were taken by putting these sheets one by one in front of a pomegranate flower to compare visually the transparence differences.

transmittance of 0.01%Gr/1%PA-6/PMMA nanocomposite sheets with various thicknesses of 0.3, 0.4, and 0.5 mm were similarly examined via a UV–Vis spectrometer. The results revealed that the loss in optical transmittance is negligible when the sheet thickness of Gr/PA-6/PMMA nanocomposite is less than 0.4 mm. However, a further increase of sheet thickness to 0.5 mm led to significant loss in transparency (Fig. 7B). Currently, there are quite some reports in literature pertaining to using electrospun nanofibers for reinforcing transparent polymers. While this approach is advantageous in giving rise to enhanced mechanical performance and minimal loss in optical transparency, some factors such as fraction of fibers incorporated, fineness of fibers, physical and chemical characteristics of the fiber material (e.g., synthetic poly-

mers versus natural biopolymers, refractive index), fiber dispersing ability in matrix, interface between electrospun fibers and matrix, and so on, do have direct and confounding effects on the transparency of the resultant composites [1,4,7,18,31]. In this sense, further decreasing the fineness of electrospun Gr/PA-6 nanofibers, improving dispersing ability of the Gr nanosheets (within PA-6, and preferably in the form of monolayer [12] and Gr/PA-6 nanofibers (within PMMA), and reducing internal defects from process would warrant to generate Gr/PA-6/PMMA nanocomposites with negligible influence of electrospun fibers on transparency. As shown in Fig. 8, the hot press compressed PMMA sheet exhibited a tensile strength of 32 MPa, Young’s modulus of around 0.59 GPa and fracture toughness of around 0.96 MJ/m3. The tensile

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Fig. 7. The light transmittance of (A) different PMMA-based nanocomposites (sample thickness: 0.4 mm); (B) influence of sheet thickness of the 0.01%Gr/1%PA-6/PMMA nanocomposite on transparency.

strength of the Gr/PA-6/PMMA nanocomposite containing 0.01 wt% Gr, for instance, is up to 50 MPa, showing a dramatic increase by 56%. Semblable trend also can be noted in modulus versus Gr loadings, that is, 113% enhancement in modulus of Gr/PA-6/ PMMA nanocomposites with the 0.01%Gr loading. The tensile toughness of the composite containing 0.01 wt% Gr is up to 3.36 MJ/m3, indicative of a significant improvement by 250% compared with neat PMMA sheet. The dramatic enhancement of toughness in Gr/PA-6/PMMA nanocomposites is directly ascribed to the utilization of high performance of Gr/PA-6 nanocomposite nanofibers, containing increased c form crystalline in the PA-6 nanofibers with the introduction of Gr nanosheets. As discussed in previous XRD and DSC results, there are two possible crystal structures, i.e, a (monoclinic) and c (pseudo hexagonal) crystals for PA-6; since the monoclinic structure is more difficult to slip than the pseudo-hexagonal structure, a crystal has higher strength and less breaking strain than c crystal [15], the variation of the crystalline ratio of a to c crystal in PA-6 as a consequence leads to improved

toughness of PA-6 nanofibers. Collectively, our results in Figs. 6–8 evidently indicate that adopting electrospun nanofibers as dispersing carriers for Gr nanosheets and using SBCE method to homogeneously distribute high performance Gr/PA-6 nanofibers provides a novel, simple, and effective in situ processing solution to the development of transparent PMMA-based nanocomposites with remarkably improved mechanical characteristics and high optical transparency. In order to understand the dispersion of Gr/PA-6 nanofibers in PMMA matrix and interfacial compatibility between reinforcement nanofibers and PMMA matrix, the fractured surfaces of PMMA, PA6/PMMA and Gr/PA-6/PMMA sheets were examined via SEM (Fig. 9). As can be seen, the fractured surface of the PMMA is smooth and homogeneous (Fig. 9A). With respect to the nanocomposites, neat PA-6 nanofibers were found to be intimately surrounded by the PMMA matrix (Fig. 9B), which can be ascribed to some degree of compatibility between PA-6 nanofibers and PMMA matrix [32]. This explains why the mechanical strength of the

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Fig. 8. Mechanical properties of Gr/PA-6/PMMA nanocomposites with varying Gr loadings: (A) representative stress–strain curves, (B) tensile strength, (C) Young’s modulus, and (D) toughness.

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Fig. 9. Cross-sectional view of the transparent PMMA-based material systems after tensile testing: (A) pure PMMA; (B) 1%PA-6/PMMA; (C) 0.005%Gr/1%PA-6/PMMA; and (D) 0.01%Gr/1%PA-6/PMMA.

nanofibers reinforced PMMA nanocomposites was significantly improved compared with the pure PMMA. However, for the cases of 0.5 wt% Gr/PA-6 and 1.0 wt% Gr/PA-6 nanofibers reinforced PMMA nanocomposites (Fig. 9C and D), it can be occasionally seen that the electrospun Gr/PA-6 nanofibers are individually protruded out of the fractured surfaces, indicating mechanical enhancement of PA6 fibers. On the other hand, this is probably because the interfacial compatibility between Gr-laden PA-6 nanofibers and PMMA matrix is not as strong as that of the neat PA-6 nanofibers. Incorporation of carbon fillers is likely to give rise to poor interfacial compatibility between nanofibers and the polymer matrix [6] suggesting a necessity to perform surface functionalization in future. 4. Conclusion In this study, high-performance PMMA-based nanocomposites with good transparency and improved mechanical properties were achieved by addressing the dispersion issue of nanofillers and using aligned electrospun Gr/PA-6 nanocomposite nanofibers as a novel type of reinforcement. With the aligned electrospun Gr/PA6 nanofibers as the reinforcement, a significant enhancement of mechanical properties with the Gr/PA-6/PMMA nanocomposite was accomplished at a Gr loading of merely 0.01 wt%; that is, a nearly 56%, 113% respective improvement of tensile strength, modulus, and noticeably above 250% increase of toughness were achieved, while the transmittance of the nanocomposite maintained above 70% (less than 10% loss in transparency in comparison with neat PMMA) in the visible light wavelength range of 400– 800 nm. This demonstration could pave the way toward utilizing different nanofillers (like Gr nanosheets) with well-dispersed ability to achieve practically applicable high-performance nanocomposites that are transparent, ultrastrong and tough. Acknowledgments This work was supported by the Doctoral Student Innovation Fund (No. CUSF-DH-D-2013043) of Donghua University, and partially supported by the National Natural Science Foundation of China (51073032) and Scientific Research Foundation for Returned Scholars (ZX201106000004).

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