High-Speed Actuation and Mechanical Properties of Graphene

Apr 28, 2014 - In the shape memory test, f-GO or r-GO-incorporated PU nanofibers showed actuation speed that was much faster than that of pure PU nano...
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High-Speed Actuation and Mechanical Properties of GrapheneIncorporated Shape Memory Polyurethane Nanofibers Hye Jin Yoo, Sibdas Singha Mahapatra, and Jae Whan Cho* Department of Organic and Nano System Engineering, Konkuk University, Seoul 143-701, Republic of Korea S Supporting Information *

ABSTRACT: We prepared poly(ε-caprolactone) (PCL)based shape memory polyurethane (PU) nanofibers incorporating three kinds of graphene, that is, graphene oxide (GO), PCL-functionalized graphene with PCL (f-GO), and reduced graphene (r-GO) to investigate their mechanical and shape memory properties. Incorporation of graphene into the PU nanofibers increased the modulus and breaking stress compared to that of pure PU nanofibers. In particular, the fGO nanofibers showed the largest enhancement in mechanical properties because of increased interaction between graphene and the polymer matrix. In the shape memory test, f-GO or rGO-incorporated PU nanofibers showed actuation speed that was much faster than that of pure PU nanofibers. The shape recovery time of 1 wt % f-GO or r-GO nanofibers was 8 s, whereas that of the PU nanofibers and GO-incorporated nanofibers were 27 and 13 s, respectively. This study demonstrates that incorporation of f-GO into shape memory PU nanofibers can be used effectively to achieve both high-speed shape recovery and high mechanical strength.

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to wrinkling and twisting of graphene sheet in the fibers as well as graphene dispersion, which may affect the fiber properties. Moreover, in the case of nanofibers, difficulty in controlling the curvature and alignment of 2-dimensional graphene sheets in the limited nanoscale diameter may result in lowering the mechanical properties of the resulting nanofibers. Polyurethane can show the shape memory effect by employing crystalline or amorphous segments in the soft segments.18,19 Shape memory polyurethane has many advantages such as flexibility, high deformation, low density, and easy processing relative to shape memory alloys, but has disadvantages in shape recovery force and shape recovery rate. To overcome these problems, polyurethane composites can be made by including nanomaterials such as carbon nanotubes, graphene, nanoclay, and metal particles.20−25 Large enhancements in the actuating properties of shape memory polymer composites with carbon nanotubes (CNT) and graphene nanocarbon have already been reported in many studies.16,23−25 Koerner et al.24 reported enhancement of actuation in the CNT-incorporated polymer composites when electrical, thermal, and optical stimuli were applied. Recently, Liang et al.16 reported infrared-triggered actuation of polymer composites with reduced and sulfonated graphene. Incorporation of sulfonated graphene showed good mechanical

INTRODUCTION Graphene has attracted much attention because of its excellent mechanical, thermal, and electrical properties, which can be used for a variety of applications such as electronic devices,1,2 energy storage devices,3,4 sensors and actuators,5−7 and polymer composites.8−11 Defect-free graphene can possess a Young’s modulus as high as 1.0 TPa, breaking stress of 130 GPa, thermal conductivity of 5000 W/(m K), and electrical conductivity of 7200 S/cm.12−14 Thus, graphene is considered to be an ideal reinforcing filler in polymer composites. Enhanced mechanical properties of graphene-reinforced composites have been reported for many polymers, including polystyrene, poly(methyl methacrylate), polypropylene, polyester, polyurethane, poly(vinylidene fluoride), and polycarbonate.15−17 Graphene oxide (GO), made from chemical exfoliation of graphite, is also preferable in fabrication of polymer composites because of the possibility of effective functionalization of graphene through covalent bonding between graphene oxide and the polymer in a variety of methods. The mechanical properties of graphene-reinforced polymer composites are dependent on dispersion of graphene in the polymer matrix, the interfacial strength between graphene and the polymer, and alignment of graphene in the polymer matrix. Although much research has been devoted to graphene-based polymer composites, relatively less study has been given to graphene−polymer composite fibers and enhancement of properties in this composite fibers has not yet been adequately realized.8 Difficulty in achieving superior properties may be due © 2014 American Chemical Society

Received: January 21, 2014 Revised: March 27, 2014 Published: April 28, 2014 10408

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Figure 1. Synthesis scheme of PCL-functionalized graphene oxide.

temperature was gradually increased to 70 °C and the reaction was continued under nitrogen for 3 h. In the second step, the prepolymer was cooled to 0−5 °C and the calculated amount of BD with 10 mL of DMF and dibutyltindilaurate as a catalyst were added to it. The reaction temperature was increased gradually to 60 °C and maintained for 3 h. The final product was poured into a Petri dish and dried in a vacuum oven at 50 °C for 48 h. The PU and PU/graphene nanofibers were obtained by electrospinning. The PU was dissolved in an equal weight mixture of DMF and THF at 22 wt % concentration. The PU/ graphene solution was prepared with GO, f-GO, and r-GO at different graphene content of 0, 0.1, 0.5, and 1.0 wt %. The calculated amount of graphene was dispersed in DMF with sonication, and then the solution was added to the PU solution in THF and mixed by stirring for 24 h at room temperature. The electrospinning of the graphene/PU solution was carried out at room temperature using an electrospinning setup (NNCESR100, NanoNC, Korea). The applied voltage during electrospinning was fixed to 15 kV, and a tip-to-collector distance of 20 cm and volumetric flow rate of 1.2 mL/h were used. Characterization. Fourier transform infrared spectra (FTIR, FT-IR 300E, Jasco, Japan) were recorded using the KBr method. The IR spectra were scanned at a resolution of 4 cm−1 with 100 scans for each measurement. Raman spectra (LabRam Aramis, Horiba Jobin Yvon, France) were recorded with 785 nm laser excitation. X-ray photoelectron spectroscopy (XPS, ESCA 2000, U.K.) was used to investigate the surface compositions of the GO, f-GO, and r-GO. Al Kα radiation was used as X-ray source for the XPS measurements. The surface morphologies of GO, f-GO, and r-GO were observed by field emission scanning electron microscopy (FE-SEM, S4300SE, Hitachi, Japan) and transmission electron microscopy (TEM, JEM 2100F, JEOL, Japan). The X-ray diffraction measurements were carried out using an X-ray diffractometer (XRD, Bruker-AXS, Germany) in New D8-Advance with Cu Kα radiation at 40 kV and 40 mA. Thermogravimetric analysis (TGA, TA Q50, TA Instruments, U.S.) was performed under nitrogen flow and scanned up to 800 °C at a heating rate of 10 °C/min. The mechanical properties of the nanofiber webs were measured at room temperature using a tensile tester machine (Instron 4468, U.S.). The gauge length and cross head speed were 20 mm and 10 mm/min, respectively. Five samples were used for each tensile test. For measurements of shape memory properties, the specimen was stretched to 100% elongation (2L0) at 50 °C and then cooled to 0 °C and maintained for 5 min to fix the temporary elongation. After removal of load, the deformed length (L1) was measured. Finally, the specimen was

properties and excellent light-triggered actuation due to good solubility and a largely restored aromatic network. In this study, we investigated the electrospun shape memory polyurethane nanofibers reinforced with small amounts of three kinds of graphene nanomaterials: graphene oxide (GO), poly(ε-caprolactone)-functionalized graphene oxide (f-GO), and reduced graphene oxide (r-GO). The mechanical and shape memory properties of the graphene-incorporated nanofibers were analyzed along with morphological observation.

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EXPERIMENTAL SECTION

Materials. GO was purchased from NanoInnova Technologies (Madrid, Spain). 1,3-Dicyclohexyl carbodiimide (DCC), 4-(dimethylamino)pyridine (DMAP), anhydrous dimethyl sulfoxide (DMSO), acetic acid, hydriodic acid (HI), 4,4-methylene bis(phenylisocyanate) (MDI), 1,4-butanediol (BD), and dibutyltin dilaurate were purchased from SigmaAldrich Co. (Korea). Poly(ε-caprolactone)diol (PCL; Mw, 3000 g/mol) was obtained from Solvay (U.K.). Dimethylformamide (DMF) and acetone were purchased from Samchun Chemicals (Korea). Functionalization and Reduction of Graphene Oxide. The PCL-functionalized GO was obtained by covalent bonding of GO with PCL through an esterification reaction. A mixture of 0.5 g of GO and 30 g of PCL was suspended in anhydrous DMSO (100 mL) at 70 °C for 3 days under nitrogen. After being cooled to room temperature, a solution of DCC (18.5 g) and DMAP (1.35 g) in anhydrous DMSO (200 mL) was added to the GO/PCL solution. The mixed solution was stirred for reaction at room temperature for 3 days. After complete reaction (Figure 1), the reacted solution was filtered using PTFE membrane (pore size of 1 μm), washed with excess DMSO and acetone, and dried at 50 °C under vacuum to produce PCL-functionalized GO.26,27 The r-GO was prepared from the GO using acetic acid and HI as a reducing agent. GO (0.5 g) was dispersed in 200 mL of acetic acid with sonication for 30 min. HI (10 mL) was added to the mixture and stirred at 40 °C for 40 h.28 After completion of the reaction, the mixture was filtered; washed with saturated sodium bicarbonate, distilled water, and acetone to remove the HI; and dried at room temperature to obtain the r-GO. Synthesis of PU and Preparation of Nanofibers by Electrospinning. The PU with 22 wt % hard segment content was synthesized by a two-step method. In the first step, the prepolymer was synthesized in a four-neck cylindrical vessel equipped with a mechanical stirrer. PCL (2.67 mmol) was dissolved in 25 mL of dry DMF, and then 2 g of MDI (8.0 mmol) in 10 mL of DMF solution was added slowly to the vessel at room temperature in nitrogen atmosphere. Then 10409

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heated to 50 °C to allow shape recovery with the resulting elongation returned (L2) under free-standing state. Dimensions of the samples used for shape memory testing were 40 × 3 × 0.15 mm3. The shape retention and shape recovery were calculated using eqs 1 and 2, respectively. Shape retention (%) =

L1 − L0 × 100 L0

(1)

Shape recovery (%) =

2L0 − L 2 × 100 L0

(2)

The shape recovery time was measured for the shape memory nanofiber webs as follows. In the same method as the shape retention test, the specimen was first stretched to 100% elongation at 50 °C and then cooled to 0 °C and maintained for 5 min to fix the temporary elongation. In the next step, after removal of the load, the specimen was loaded by applying a weight of 6.5 gf to the end of the nanofiber webs and then heated to 50 °C in a constant temperature bath. The shape recovery behavior of the loaded specimen was recorded with a digital camera, and the shape recovery time up to 80% shape recovery was counted for each specimen.

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RESULTS AND DISCUSSION Characterization of f-GO and r-GO. The PCL-functionalized GO was obtained by an esterification reaction between OH groups of PCL and carboxylic acid groups of GO, and GO was reduced (r-GO) by using HI/acetic acid as a nonnitrogenous reducing agent. The functionalization and reduction of GO was confirmed by FT-IR, Raman, and X-ray diffraction spectra as shown in Figure 2. The IR spectrum of GO in Figure 2a show the presence of acid, epoxy, alcohol, and alkoxy groups in graphene oxide, and the 1727 and 3400 cm−1 peaks correspond to carbonyl and hydroxyl groups, respectively. The IR peak at 1643 cm−1 is due to adsorbed water, while O−H deformation of carboxylic groups and C−O of carboxyl appear at 1417 and 1388 cm−1, respectively. The peaks at 1226 and 1060 cm−1 correspond to C−OH stretching and C−O bending, respectively.26 The f-GO sample shows the IR peak shift in carbonyl group from 1724 to 1720 cm−1 due to a change of the carboxylic acid to ester group, and the peak at 2938 cm−1 became prominent because of asymmetric CH stretching. This confirms grafting of long-chain aliphatic C−C groups containing PCL to GO. It is also observed that after reduction of GO, the IR peaks become less dominant because of removal of oxygen functional groups during reduction. According to the Raman spectra in Figure 2b, GO shows two strong peaks at 1561 and 1363 cm−1 corresponding to sp2 and sp3 hybridizations of carbon, respectively. The functionalization of graphene can be evaluated using the intensity ratio of D and G bands. The calculated intensity ratio ID/IG for GO was 1.0. For the f-GO, the same intensity ratio was obtained and further increase of D band was not observed, whereas for r-GO, ID/IG was 1.44. This indicates that the PCL molecules reacted well with most sites of carboxylic acid groups of GO, but the reduction of GO still left many D band defects in graphene.29,30 X-ray diffraction curves of GO, f-GO, and r-GO are shown in Figure 2c. The X-ray peak of GO appeared at 2θ = 11.1° corresponding to a lattice spacing of 0.79 nm, while f-GO showed the X-ray peak at 2θ = 9.8° corresponding to a lattice spacing of 0.90 nm. The increased lattice spacing in f-GO compared to that of GO indicates the successful functionaliza-

Figure 2. (a) FT-IR spectra of (i) PCL, (ii) GO, (iii) f-GO, and (iv) rGO; (b) Raman spectra and (c) X-ray diffraction curves of (i) GO, (ii) f-GO, and (iii) r-GO.

tion of PCL molecules on the graphene surface. Whereas the rGO showed the X-ray peak at 2θ = 24.5° corresponding to a lattice spacing of 0.36 nm, it reflects a recovery of carbon− carbon structure with fewer structural defects in the reduced graphene oxide. X-ray photoelectron spectroscopy results are shown in Figure 3. The C1s and O1s peaks were observed at 285 and 533 eV, respectively. The peak intensity ratio of O1s and C1s was decreased with functionalization and reduction (Figure 3a). The high-resolution C1s spectra are shown in Figure 3b−d. The XPS spectrum of GO shows the characteristic peaks at 10410

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Figure 3. (a) Wide scan X-ray photoelectron spectra of (i) GO, (ii) f-GO, and (iii) r-GO and high-resolution C1s spectrum of (b) GO, (c) f-GO, and (d) r-GO.

Figure 4. TEM and SEM images of GO (a, d), f-GO (b, e), and r-GO (c, f).

corresponding to oxygen functional groups of CO, COOH, and OCO−O at the higher binding energy. The increased intensity at 284.6 eV and decreased intensities at three weak peaks are due to removal of the oxygen functional groups, reflecting the reduction of GO. Measurements of 13C NMR spectra also strongly confirmed the functionalization of the PCL molecules on GO surface (Figure S1 of Supporting Information).

284.6, 286.5, and 288.4 eV corresponding to C−C, C−O, and COOH groups, respectively, and the f-GO also shows three XPS peaks at binding energy that is the same as that of GO with different intensities. However, the intensity of C−C peak for the PCL-functionalized GO increased compared to that of GO, which is due to the presence of the grafted aliphatic C−C groups in the PCL-functionalized GO. The r-GO spectrum shows a strong peak at 284.6 eV and three weak peaks 10411

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and their surface was less uniform compared to that of pure PU nanofibers, especially with higher graphene content. Mechanical Properties of PU and PU/Graphene Nanofibers. The mechanical properties of pure PU and PU/ graphene nanofiber webs are shown in Table 1 and Figure S4 of Supporting Information. The average breaking stress of pure PU nanofibers was 7.8 MPa, whereas the GO, f-GO, and r-GO composite nanofibers showed increased breaking stress. For example, the breaking stress of the composite nanofiber webs at 0.1 wt % f-GO was 9.3 MPa, which is 1.2 times higher than that of the pure PU nanofibers. The modulus of the nanofiber webs also increased with an increase of graphene content. The f-GO nanofibers at 1 wt % graphene content showed a modulus of 41.4 MPa, whereas the modulus of pure PU nanofiber webs was 31.5 MPa. Moreover, the breaking stress and modulus of f-GO nanofibers were higher than those of GO and r-GO nanofibers at the same graphene loading. In particular, the f-GO nanofibers at 0.1 wt % f-GO loading showed a higher modulus than the GO and r-GO nanofibers at 1.0 wt % graphene loading. f-GO was well-dispersed in polymer matrix compared to GO and rGO, and plays a role in good load transfer. In addition, the elongation-at-break of the GO, f-GO, and r-GO nanofibers was higher than that of pure PU nanofibers and increased with increasing graphene content, which may be attributed to the elasto-plastic behavior of graphene.33−35 Shape Memory Properties of PU and PU/Graphene Nanofibers. The thermoresponsive shape memory properties were measured for the nanofiber webs for each of the three samples with five cycles. The shape retention of pure PU and PU/graphene nanofibers was higher than 95% in the first cycle, which was retained for all the samples even in the increased cycles (Table 2). The shape recovery of the pure PU nanofiber webs was 88.3%, while the PU/graphene nanofiber webs showed shape recovery higher than 90% in the first cycle (Figure 7). Despite this, the shape recovery decreased with increasing cycles, although their values remained higher than 80%. The graphene-incorporated nanofibers showed higher shape recovery because of the high thermal conductivity of graphene. In particular, the thermoresponsive shape recovery force and recovery rate of the pure PU, GO, f-GO, and r-GO nanofibers were compared and analyzed. Figure 8 shows the shape recovery behavior when the nanofiber samples were kept under a loading of 6.5 gf at 50 °C. The width and thickness of the samples were 4 mm and 150 μm, respectively. The PU/GO nanofiber webs showed a shape recovery of 50% in 8 s under 6.5 gf loading, while the pure PU nanofiber webs showed the same recovery in 25 s. It should be noted that the pure PU nanofiber webs and PU film have a shape recovery of 80% after 40 and 120 s, respectively, under free-standing state. This indicates that the shape recovery of nanofiber webs is very fast compared to that of films because of the high surface area of nanofibers.33,36,37 In particular, the shape recovery time decreased significantly when graphene was incorporated into the nanofibers (Figure 9), with the shape recovery time similar among the GO, f-GO, and r-GO nanofibers. The fast shape recovery rate of graphene-incorporated nanofibers is ascribed to the combined effects of high thermal conductivity of graphene and the high surface area of the nanofibers. Consequently, f-GO nanofibers could show good shape memory properties as well as enhanced mechanical properties due to good interactions between the graphene and polymer molecules in addition to excellent thermal conductivity and high surface area of graphene nanofibers. However, with an increase of graphene

Thermogravimetric measurements were performed for GO, f-GO, and r-GO (Figure S2 of Supporting Information). The GO showed significant weight loss near 100 °C, which is attributed to an elimination of moisture adsorbed in the GO layers. The weight loss at a higher temperature of 200 °C is due to loss of oxygen present in the GO platelets.31 f-GO began to degrade around 200 °C because of removal of oxygen groups, followed by weight loss due to degradation of PCL molecules in the temperature range of 200 to 400 °C, whereas r-GO showed thermal stability much better than that of GO. Figure 4 shows TEM and SEM images of GO, f-GO, and rGO. The dimensions of GO, f-GO, and r-GO were roughly estimated to be about 2 μm from the TEM measurements (Figure S3 of Supporting Information), and a significant dimension difference among GO, f-GO, and r-GO was not observed.32 GO shows a partially wrinkled surface due to the presence of oxygen functional groups, whereas r-GO shows relatively flat and neat surfaces of graphene sheets because of reduction of GO. In the case of PCL-functionalized GO, its surface is slightly blurred and appears similar to amorphous carbon at the edge. According to the SEM measurements, the PCL-functionalized GO shows a more bulky and peeled-off morphology compared to that of the GO. Figure 5 shows the

Figure 5. SEM images of (a) PU, (b) PU/GO05, (c) PU/f-GO05, (d) PU/r-GO05, (e) PU/f-GO01, and (f) PU/f-GO1 nanofibers, where 01, 05, and 1 indicate the graphene wt %.

morphology of GO, f-GO, and r-GO nanofibers obtained by SEM measurements. All the nanofiber webs showed randomly oriented nanofibers with diameter of 1 to 1.5 μm. There was almost no significant difference in the diameter of nanofibers due to the presence of GO, f-GO, or r-GO, nor due to their amount of loading. According to TEM measurements (Figure 6), the graphene nanofibers showed wrinkled or curved structure of graphene in the GO, f-GO, and r-GO nanofibers, 10412

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Figure 6. TEM images of (a) PU, (b) PU/f-GO05, (c) PU/r-GO05, (d) PU/GO01, (e) PU/GO05, and (f) PU/GO1 nanofibers.

Table 1. Tensile Properties of PU/Graphene Nanofiber Webs samples PU PU/ GO

PU/fGO

PU/rGO

graphene content (wt %)

tensile strength (MPa)

modulus (MPa)

elongation-atbreak (%)

0 0.1

7.8 7.4

31.5 32.9

406.7 299.5

0.5 1.0 0.1

8.3 7.9 9.3

33.7 35.7 35.9

412.9 422.8 429.2

0.5 1.0 0.1

8.5 8.9 8.6

36.9 41.4 31.3

515.4 515.6 350.6

0.5 1.0

9.1 8.0

33.1 32.4

553.6 450.6

Figure 7. Shape recovery of the pure PU and PU/graphene nanofiber webs with different cycles (red and solid symbols, first cycle; blue and open symbols, fifth cycles; ●, PU; ■, PU/GO; ▲, PU/f-GO; and ▼, PU/r-GO).

content, the shape recovery of f-GO decreased. This is because the increased graphene content can disturb the polymer flexibility significantly in the elongated nanofibers.38 Whereas, in the case of r-GO nanofibers, the shape recovery increased with an increase of graphene content. This is attributed to the increased thermal conductivity of r-GO with fewer structural defects because of the recovered C−C structure.

graphene and electrospinning. GO, f-GO, and r-GO nanofibers showed enhanced breaking stress and modulus compared to those of the pure PU nanofibers, and the f-GO nanofibers showed the highest results. The graphene-based nanofiber webs showed fast shape recovery actuation with more than 90% shape recovery. Functionalized graphene-based nanofibers can have excellent mechanical and shape memory properties by

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CONCLUSIONS Mechanically strong and high-speed shape memory nanofibers were obtained through incorporation of PCL-functionalized Table 2. Shape Retention (%) of PU/Graphene Nanofiber Webs PU/GO

PU/f-GO

PU/r-GO

cycle

PU

0.1

0.5

1.0

0.1

0.5

1.0

0.1

0.5

1.0

1st 2nd 3rd 4th 5th

98.1 99.9 97.8 97.3 98.9

99.8 99.2 98.5 98.3 99.3

97.8 96.8 99.6 98.9 97.0

96.2 96.7 97.2 97.9 97.6

95.1 96.1 95.1 98.2 96.9

97.1 97.0 98.1 97.5 97.1

99.4 98.0 99.9 98.7 98.7

94.9 95.1 97.7 96.9 95.9

96.8 96.8 96.2 95.9 96.6

95.0 97.5 96.7 96.9 97.3

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ACKNOWLEDGMENTS

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REFERENCES

Article

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2012R1A2A2A01015155).

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Figure 8. Shape recovery images of (a) PU and (b) PU/GO05 nanofibers, where the shape recovery test was carried out at constant temperature of 50 °C under loading of the weight of 6.5 gf with increasing time.

Figure 9. Shape recovery time of the pure PU and PU/graphene nanofiber webs with different graphene content.

controlling the interaction between graphene and the polymer matrix as well as the effect of large surface area of the nanofibers.

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ASSOCIATED CONTENT

S Supporting Information *

13

C NMR spectra (Figure S1); TGA thermograms (Figure S2); TEM images of GO, f-GO, r-GO, and PCL (Figure S3); and stress−strain curves of PU, PU/GO, PU/f-GO, and PU/r-GO nanofibers (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82 (2) 4503513. Fax: +82 (2) 4578895. Notes

The authors declare no competing financial interest. 10414

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp500709m | J. Phys. Chem. C 2014, 118, 10408−10415