Carbon Nanotube

Research Article www.acsami.org

High Performance Shape Memory Epoxy/Carbon Nanotube Nanocomposites Yayun Liu,†,‡ Jun Zhao,*,‡ Lingyu Zhao,‡,§,∥ Weiwei Li,‡,∥ Hui Zhang,‡ Xiang Yu,*,⊥ and Zhong Zhang*,‡ †

School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing 100190, China § Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China ∥ University of Chinese Academy of Science, Beijing 100049, China ⊥ School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China ‡

ABSTRACT: A series of shape memory nanocomposites based on diglycidyl ether of bisphenol A (DGEBA) E51/methylhexahydrophthalic anhydride (MHHPA)/multiwalled carbon nanotube (MWCNT) with various stoichiometric ratios (rs) of DGEBA/ MHHPA from 0.5 to 1.2 and filler contents of 0.25 and 0.75 wt % are fabricated. Their morphology, curing kinetics, phase transition, mechanical properties, thermal conduction, and shape memory behaviors are systematically investigated. The prepared materials show a wide range of glass transition temperatures (Tg) of ca. 65−140 °C, high flexural modulus (E) at room temperature up to ca. 3.0 GPa, high maximum stress (σm) up to ca. 30 MPa, high strain at break (εb) above 10%, and a fast recovery of 32 s. The results indicate that a small amount of MWCNT fillers (0.75 wt %) can significantly increase all three key mechanical properties (E, σm, and εb) at temperatures close to Tg, the recovery rate, and the repetition stability of the shape memory cycles. All of these remarkable advantages make the materials good candidates for the applications in aerospace and other important fields. KEYWORDS: shape memory polymer, epoxy, carbon nanotube, nanocomposite, curing

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INTRODUCTION Shape memory materials are smart materials that can keep a temporary shape for a long time and recover to their permanent shape upon applying a stimulus such as heat,1,2 light,3,4 moisture,5 pH,6 or electrical7,8 or magnetic field.9,10 These features can meet the requirements of a variety of applications including deployable space structure,11 biomedical devices,12 textile materials,13 and actuators.14 Although shape memory alloys (SMAs) and shape memory ceramics (SMCs) are the most widely used shape memory materials, they have some obvious disadvantages such as high mass density, poor corrosion resistance, poor processability, high cost, and low strain.15 Compared with these materials, shape memory polymers (SMPs) can make up for these shortcomings, and they have, therefore, attracted increasing interest from both academia and industry.16−20 SMPs are generally classified into two groups depending on their chemical structures: thermoset and thermoplastic SMPs. The switch temperature (Tsw) of thermoset SMPs is usually the glass transition temperature (Tg), whereas that of thermoplastic SMPs can be either Tg or the crystal melting temperature (Tm). In the past few decades, research has mainly focused on the thermoplastic SMPs, such as polyurethane (PU),21,22 crosslinked polyolefins,23−25 and polystyrene (PS).26,27 Compared with thermoplastic SMPs extensively used in civilian © XXXX American Chemical Society

commodities, thermoset shape memory epoxies (SMEPs) are more suitable for the applications in industry and aerospace fields because of their unique thermal and mechanical properties and excellent shape memory performance.28 These materials offer high Tsw, high shape fixity ratio and shape recovery ratio, rapid response, superior environmental durability, excellent dimensional stability, and easy processing.19,29,30 There have been some reports on the SMEPs in the literature. Nair et al. reviewed the progress in the field of SMEPs and pointed out that the partial crystallization and verification were the basis of their shape memory effect (SME).31 By changing the chemistry and nature of the shape memory segment, the shape memory properties and phase transition temperature can be tuned in a wide range. Rousseau et al. discussed the relationship between the chemical composition and failure strains of SMEPs.32 Zhu et al. prepared a shape memory system by using hydro-epoxy, maleic anhydride, and poly(propylene glycol) diglycidyl ether (PPGDGE).33 The results indicated that the Tg linearly decreased from 124 to 60 °C as the PPGDGE content Received: September 16, 2015 Accepted: December 7, 2015

A

DOI: 10.1021/acsami.5b08766 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

was purchased from Sinopharm Chemical Reagent, Co. Ltd., China. The chemical structures of these three reactants are shown in Scheme 1. MWCNTs with a purity greater than 95%, outer diameter of 20−30

increased. The PPGDGE content also had an effect on the tensile strength, the room-temperature bending strength, and the room-temperature elongation at break. Williams et al. realized that achieving large tensile elongations and large recovered stresses simultaneously was a difficult task if only based on chemical cross-linking.17 Therefore, they constructed some materials with chemical and physical cross-linking to acquire excellent mechanical properties, including relatively high tensile strains of ca. 75% and recovery stresses of ca. 3 MPa. One of the drawbacks of SMPs including SMEP compared to SMAs and SMCs is still their intrinsic low material stiffness and low recovery stress (σr). Adding a certain amount of fillers into the polymer matrix to produce SMP composites has become a typical strategy to reinforce the SMP materials and realize their multiple stimuli simultaneously.19,34 For example, the SMEP composites with TiNi macrowires as fillers showed much higher stiffness at elevated temperature than the pure SMEPs.35 The addition of 1 vol % TiNi wires was observed to increase the maximum (σm) from ca. 1.36 MPa to ca. 4.04 MPa. Another work by Leng et al. constructed an SMEP composite with sodium dodecyl sulfate (SDS) as the filler.5 The threedimensional (3D) microvoid was constructed on the shape memory composites surface by chemical interaction to show a conspicuous water-induced SME. It was found that the shape recovery rate of the water-induced composite was accelerated by raising the temperature or decreasing the specimen thickness because of the combined effect of the chemical interaction and physical swelling. Although some progress has been made in the research of SMEPs, it is still needed to further improve their performances for applications in the field of industry, aerospace, and other fields, especially the high switch temperature and strength, large recovery stress, and faster response. To prepare SMEPs as nanocomposites should be the most effective and reliable method. Carbon nanotubes are among the best choices of fillers due to their excellent comprehensive properties. In addition, the amine-cured epoxy networks are commonly used as SMEPs36,37 because of their relatively higher deformation capability than the anhydride-cured epoxy materials which have been widely used in the fields of electronics and electrics.38,39 However, some alicyclic anhydrides, such as hexahydrophthalic anhydride (HHPA) and methylhexahydrophthalic anhydride (MHHPA), have some outstanding advantages when they are mixed with a stoichiometric amount of epoxy resins.40 The resultant EP/anhydride mixtures generally exhibit a low initial viscosity and a long storage lifetime, which is beneficial to the subsequent processing. In this study, the Tsw (Tg), σr, and shape recovery rate of the anhydride-cured SMEP materials are supposed to be controlled by tuning the stoichiometric ratio of curing agent to epoxy resin and using multiwalled carbon nanotubes (MWCNTs) as the filler. MWCNTs are used because of their excellent specific strength and superb thermal and electrical conductivity, both of which will be beneficial to the improvement of σr and fast response.41,42

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Scheme 1. Schematic Presentation of the Chemical Structures of E51, MHHPA, and BDMA

nm, and length of 10−30 μm were purchased from Chengdu Organic Chemical Co. Ltd., Chinese Academy of Sciences, China. All materials were used as received without further purification. Reaction Mechanism. Scheme 2 shows the proposed curing procedures of the SMEP materials synthesized from E51, MHHPA, and BDMA, based on the literature.36 First, the accelerator BDMA reacts with the anhydride MHHPA and generates a carboxylic acid salt anion. Then, the produced carboxylate anion attacks the epoxy group and the epoxy group is opened to form an oxygen anion. Afterward, the oxygen anion reacts with another anhydride group and generates a carboxylate anion. In this way, the reaction proceeds in alternation between anhydride groups and epoxy groups. Finally, cross-linked networks are formed in the system. Sample Preparation. For the preparation of SMEP materials, 100−200 g of E51 epoxy was added into a glass beaker and heated up to ca. 60 °C in an oven. A certain stoichiometric amount of curing agent, MHHPA, with 1 wt % accelerator, BDMA, was then added into the epoxy, and the beaker was moved to an oil bath at 60 °C. The mixture was mechanically stirred for 20 min before it was degassed in a vacuum oven at 60 °C for 45 min to remove the trapped air bubbles. Afterward, the degassed mixture was poured into a preheated steel mold and cured following the stepwise schedule: 90 °C for 30 min, 120 °C for 60 min, 140 °C for 30 min, and 160 °C for 120 min. The pure SMEP samples with the stoichiometric ratio of MHHPA to E51 varying from 0.5 to 1.2 are denoted as aEP, where the “a” is the ratio value. For the preparation of SMEP nanocomposite materials, a certain amount of MWCNTs was mechanically mixed with epoxy resin in a high-speed mixer (Dispermat AE, Germany) for ca. 1 h and then processed in a three-roll mill (Exakt 80E, Germany) to get a homogeneous dispersion of fillers in the polymer matrix. Afterward, the curing of the SMEP nanocomposite materials followed exactly the same procedure as that of the SMEP materials, but with the prepared epoxy/MWCNT master batch instead of the pure epoxy. SMEP nanocomposite materials with various MWCNT contents (0−0.75 wt % relative to the whole materials) are denoted as aEP-b, where “a” is the stoichiometric ratio value of MHHPA to E51 and “b” is the mass fraction (in wt %) of MWCNTs in the materials. Scanning Electron Microscopy (SEM). The SMEP and SMEP nanocomposite materials were cryofractured in liquid nitrogen, and the fracture surfaces were observed on an SEM (Jeol SM-J7500F, Japan) running at an accelerating voltage of 15 kV. The fracture surfaces of the samples after the three-point-bending tests were also observed by the SEM. All the surfaces were sputtered with a thin gold film before observation. Fourier Transform Infrared (FTIR) Spectrometry. The cured SMEP samples with various stoichiometric ratios (rs) were tested on an FTIR spectrometer (Spectrum One, PerkinElmer, USA) in

EXPERIMENTAL SECTION

Materials. Epoxy resin, DGEBA E51 (WSR 618), with an epoxy equivalent weight of 185 g (equiv)−1, was purchased from Nantong Xingchen Synthetic Materials Co. Ltd., China. Curing agent, MHHPA, was purchased from Puyang Huicheng Electronic Material Co. Ltd., China. Accelerator, N,N-benzyl dimethyl amine (BDMA) (CP purity), B


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Scheme 2. Schematic Presentation of the Curing Reaction Procedures of the SMEPs Synthesized from E51, MHHPA, and BDMA

reflective mode. The wavenumber (ν) range was 4000−450 cm−1 with a resolution of 4 cm−1 and 40 scans. Modulated Temperature Differential Scanning Calorimetry (MTDSC). The curing kinetics and phase transitions of SMEP and SMEP nanocomposite materials were measured on a DSC (TA Instruments Q2000, USA) in temperature modulation mode under a nitrogen atmosphere (50 mL min−1). The calibration of temperature and enthalpy was performed with indium. The 5−10 mg samples were sealed in nonhermetic (for solid samples) or hermetic (for liquid samples) aluminum crucibles. A standard modulation condition with an amplitude of 0.5 K and period of 60 s was used. The scan rate for nonisothermal measurements was 3.0 K min−1. Three samples were tested for each composition, and the most representative one was plotted. Thermogravimetric Analysis (TGA). The thermal stability of SMEP and SMEP nanocomposite materials was checked on a TGA (TA Instruments Q500, USA) under a nitrogen atmosphere (60 mL min−1) with the temperature range from a room temperature of ca. 25.0 °C to 800.0 °C at a heating rate of 10.0 K min−1. Three samples were tested for each composition, and the most representative curve was plotted. Three-Point-Bending Tests. Three-point-bending tests of the rectangular SMEP and SMEP nanocomposite materials with dimensions of 80 × 10 × 4 mm3 were carried out on a testing machine (Zwick/Roell BT2-FR010TE.A50, Germany) at both a room temperature of ca. 25 °C and a temperature of ca. 50 °C below their Tg’s measured by MTDSC as above. The span of the testing machine was 64 mm, and the running speed of the crosshead was 10 mm min−1.

Five samples were tested for each composition, and the most representative curve was plotted. Dynamic Mechanical Thermal Analysis (DMTA). The dynamic mechanical properties of SMEP and SMEP nanocomposite materials were measured on a DMTA (TA Instruments Q800, USA) in the single-cantilever mode from a room temperature of ca. 25.0 °C to 200.0 °C at a heating rate of 3.0 K min−1 and under a frequency of 1 Hz. The dimensions of the rectangular samples were ca. 40.0 × ca. 12.9 × ca. 2.0 mm3. Three samples were tested for each composition, and the most representative curve was plotted. Shape Memory Effect (SME) Analysis. The SME behavior of the SMEP and SMEP nanocomposite materials was evaluated in a singlecantilever and force-controlling mode on the DMTA. The process was divided into four steps. In the first step, a certain amount of stress (σ) was loaded at ca. 10 °C above the sample’s Tg to get an ε of ca. 10%. In the second step, the sample was cooled at 5.0 K min−1 to 20.0 °C under the load. In the third step, the σ was unloaded at a uniform rate in 5 min. In the last step, the sample was heated at 3.0 K min−1 to ca. 20 °C above their Tg for the complete recovery of shape deformation. Strain fixity ratio (Rf) and strain recovery ratio (Rr) for the SME were calculated as follows1

R f = εfix /εload × 100%

(1)

R r = (εfix − εrec)/εfix × 100%

(2)

where εload, εfix, and εrec were the maximum ε under load, the fixed ε after the cooling and load removal, and the ε after recovery, respectively. C


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ACS Applied Materials & Interfaces To investigate the shape recovery speed and cycling effect of SMEP and SMEP nanocomposite materials, a fold-deploy shape memory test was performed as illustrated in Figure 1. Two rectangular samples with

same MWCNT content of 0.75 wt %. It can be seen that, for both samples, the fillers have a uniform distribution in the polymer matrix, although some content of agglomeration can be seen somewhere. Some holes can be seen on the fracture surface of 1.0EP-0.75%, probably due to the debonding between the fillers and the matrix and the pull-out of the fillers from the matrix or the residual bubble. The chemical structures of SMEP samples with various stoichiometric ratios (rs) from 0.5 to 1.2 were confirmed by their FTIR spectra, as shown in Figure 3. The characteristic

Figure 1. Photographs showing the fold-deploy shape memory test for an SMEP sample (yellow) and an SMEP nanocomposite sample (black): original shape (left), oil bath at 150 °C on an oven (middle), and temporary shape (right). the dimensions of ca. 80 × ca. 10 × ca. 1 mm3 were heated in an oil bath from a room temperature of ca. 25.0 °C to ca. 10 °C above their Tg for the deformation. The samples were then bent along the inner wall of a circular glass dish with a diameter of ca. 65 mm. After the deformation, the samples in the glass dish were cooled down in air to a room temperature of ca. 25.0 °C. For the shape recovery, the bent samples were put back to the oil bath at ca. 10 °C above their Tg (the same as the deformation temperature) and the recovery process was recorded by a camera. Bending and compression mode shape recovery tests were also performed to test and exhibit the shape memory performance. Three rectangular samples with the dimensions of ca. 85 × ca. 5 × ca. 2 mm3 were heated in an oil bath from a room temperature of ca. 25.0 °C to ca. 150 °C for the deformation. Then, the samples were bent into “S”, “M”, and “P” shapes by tweezers. Three rectangular samples with the dimensions of ca. 10 × ca. 10 × ca. 4 mm3 and two rectangular samples with the dimensions of ca. 10 × ca. 20 × ca. 4 mm3 were heated from a room temperature of ca. 25.0 °C to ca. 150 °C in a thermocompressor (Wuhan Qien Science & Technology Development Co., Ltd., R-3221, China) with a pressure of ca. 0.1 MPa. For the shape recovery, these bent and compressed samples were put into the oil bath at ca. 150 °C and the recovery process was recorded by a camera. Thermal Conductivity (λ) Measurements. The λ in the direction of thickness of rectangular SMEP and SMEP nanocomposite materials with dimensions of ca. 5 × ca. 5 × ca. 1 mm3 was measured on a comprehensive physical property measurement system (PPMS-9, American Quantum Design, USA). The test temperatures were set at 20, 90, and 120 °C, respectively. Three samples were tested for each composition.

Figure 3. FTIR spectra of SMEP materials with various rs from 0.5 to 1.2. The peaks corresponding to the benzene ring and epoxy group are marked.

absorption peaks of the benzene ring and epoxy group are at 1510 cm −1 (C−C stretching) and 910 cm −1 (C−O stretching),43 respectively. It can be seen that the intensity of the benzene ring peak remains constant for all the samples, whereas that of the epoxy group peak decreases with increasing rs from 0.5 to 0.8. The epoxy peak almost disappears for the rs of 1.0 and 1.2, which means that all the epoxy groups in the system have been consumed by the curing reaction. Figure 4 shows the curing kinetics of E51/MHHPA mixtures with various rs by following the MTDSC heat flow (HF) signal during heating and quasi-isothermal treatment. As shown in Figure 4a, the peak temperature (Tp) of all samples is almost the same, ca. 141.0 °C. However, with increasing rs from 0.5 to 1.0, the enthalpy of curing reaction (ΔH c ) increases continuously, which indicates the increasing amount of cured epoxy. Besides, the value of ΔHc remains almost constant when the rs is further increased from 1.0 to 1.2. These results suggest that all the epoxy is consumed for the ratio of 1.0, which is consistent with the FTIR results, as shown in Figure 3. Because all the SMEP and SMEP nanocomposite materials were prepared by stepwise isothermal curing, the quasiisothermal DSC measurements in temperature modulation mode can provide some useful information about the curing kinetics.44,45 Figure 4b shows the DSC HF curves of the 1.0EP at different curing temperatures (Tc). With increasing Tc, the time needed to reach the exothermic peak point (tp) gradually decreases and the time required for the completion of the curing reaction is also shortened. For example, it takes ca. 4.3 min, ca. 2.0 min, and ca. 1.2 min to reach the exothermic peak point for 120, 140, and 160 °C, respectively. Clearly, the stepwise curing procedure employed in this work (90 °C for 30 min, 120 °C for 60 min, 140 °C for 30 min, and 160 °C for 120 min) can ensure the complete curing of the samples. The Tg of the SMEP material is usually used as its Tsw, so it is a key parameter defining the temperature range of SMEP

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RESULTS AND DISCUSSION Figure 2 shows the SEM images of cryo-fractured surfaces of the SMEP nanocomposite materials with different rs and the

Figure 2. SEM images of the cryo-fractured surfaces of 0.5EP-0.75% (a) and 1.0EP-0.75% (b). D


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Figure 4. MTDSC thermographs showing HF during heating of E51/MHHPA mixtures with various rs (a) and quasi-isothermal curing of 1.0EP (b). The inset of part a shows the dependence of ΔHc on the rs, and the inset of part b shows the dependence of tp on Tc.

materials’ shape fixity and recovery. The Tg values of the SMEP and SMEP nanocomposite materials with various rs are shown in Figure 5 by the MTDSC reversible heat capacity (cpr) curves,

Figure 6 shows the weight loss during the heating of SMEP and SMEP nanocomposite materials with various rs. It can be

Figure 6. TGA curves during heating of SMEP and SMEP nanocomposite materials with various rs.

cpr

Figure 5. MTDSC thermographs showing during heating of SMEP and SMEP nanocomposite materials with various rs. The dashed and dashed-dotted lines are the nanocomposites with 0.25 and 0.75 wt % MWCNTs, respectively.

seen that the starting decomposition (ca. 2 wt % weight loss) of 0.5EP is at ca. 243 °C, whereas that of 1.0EP is at ca. 335 °C. The reason might be that there is still a large amount of uncured epoxy remaining in the former, which is much less thermally stable than the cured epoxy. For both 0.5EP and 1.0EP samples, the fastest weight loss happens at almost the same temperature of ca. 412 °C. Besides, the addition of 0.25 and 0.75 wt % fillers only increases the decomposition temperature very slightly by ca. 3 °C in terms of the fastest weight loss. To keep consistent with the following SME measurements, the mechanical properties of SMEP and SMEP nanocomposite materials were measured in three-point-bending mode. Figure 7a shows their typical σ−ε curves at both a room temperature of ca. 25 °C and 90 °C (ca. 50 °C below the Tg of 1.0EP sample). The data for modulus (E) are summarized in Figure 7b. At 25 °C, 0.5EP has lower E, lower maximum stress (σm), and higher strain at break (εb) than 1.0EP, which results in the former sample’s higher ductility and is consistent with the SEM observation of the bending-fractured surfaces, as shown in Figure 8a,b. Figure 8a shows cracks with sharp edges spread in one direction in the smooth surface, which indicates a typical brittle breakage. However, Figure 8b shows less smooth surfaces with cracks spread around, which indicates a less brittle breakage. According to Figure 7a, when the temperature increases from 25 to 90 °C, 1.0EP SMEP and SMEP nanocomposite materials become much more ductile, as evidenced by lower E, lower σm,

which are much more sensitive than the normal DSC heat capacity (cp) curves.44,45 When the rs increases from 0.5 to 1.0, the Tg increases from ca. 65 °C to ca. 140 °C due to the increasing degree of curing in the materials. When the rs further increases to 1.2, the Tg decreases slightly by ca. 6 °C because of the remaining curing agent, which is a small molecule and will be volatile at high temperature, probably causing the reduced degree of cross-linking. Besides, Figure 5 shows that the addition of up to 0.75 wt % MWCNTs can produce an increase of Tg of ca. 10 °C for 0.5EP but only ca. 3 °C for 1.0EP. The Tg of cured epoxy reflects the segmental mobility which is related to the degree of cross-linking in the system. Figures 3 and 4a show that a change in the rs can produce different degrees of curing and cross-linking. Therefore, our results show that the Tsw (Tg) of SMEP and SMEP nanocomposite materials can be well controlled by simply changing the added content of curing agent and fillers. The reason for different effects of fillers on the Tg of 0.5EP and 1.0EP might be that the former has a lower degree of cross-linking than the latter. Therefore, the addition of fillers produces a more pronounced confinement effect on the segmental mobility of the former than the latter. Compared with the reported Tsw almost below 90 °C in the literature,17,46 our work has achieved a higher Tsw which can be changed over a wide temperature range. E


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Figure 7. σ−ε curves (a) and filler content dependence of E (b) for SMEP and SMEP nanocomposite materials at both a room temperature of ca. 25 °C and 90 °C.

Figure 8. SEM images of 1.0EP (a) and 0.5EP (b) bending-fractured at 25 °C and 1.0EP (c) and 1.0EP-0.75% (d−f) bending-fractured at 90 °C. Part e is a zoom-in view of part d, and part f is a zoom-in view of part e.

Figure 9. DMTA curves showing the E′ change during heating of SMEP materials with various rs (a) and SMEP and SMEP nanocomposite materials with various rs and filler contents (b).

and higher εb. This observation is also consistent with the SEM observation as shown in Figure 8a,c for 1.0EP. As shown in Figure 7, the comparison between SMEP and SMEP nanocomposite materials shows that the enhancement effect by the fillers is strongly dependent on the temperature. At a low temperature of 25 °C, although the addition of fillers significantly increases both the E and the σm, it does decrease

the εb. At a high temperature of 90 °C, however, the addition of fillers improves all of these three properties. It is also interesting to notice that 1.0EP and its nanocomposite materials do not show any yielding at 25 °C, while 0.5EP and its nanocomposite materials show slight yielding at 25 °C. 1.0EP and its nanocomposite materials show pronounced yielding at 90 °C. Figure 8c−f shows the typical ductile fracture behaviors of F


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Figure 10. Shape memory cycle on DMTA of SMEP nanocomposite materials: 0.5EP-0.25% (a) and 1.0EP-0.25% (b).

Table 1. SME Parameters of SMEP and SMEP Nanocomposite Materials as Measured in Figure 10 samples Rf (%) Rr (%)

0.5EP

0.5EP-0.25%

0.5EP-0.75%

1.0EP

1.0EP-0.25%

1.0EP-0.75%

96.5 86.7

96.7 87.6

97.6 83.7

97.9 96.4

98.1 92.1

98.0 91.4

Figure 11. Stoichiometric ratio dependence of tr (a) and temperature dependence of λ (b) of SMEP and SMEP nanocomposite materials.

°C to ca. 142 °C with increasing rs from 0.5 to 1.0 and keeps almost constant when the rs is further increased to 1.2. This is consistent with the MTDSC results shown in Figure 5. In addition, the E′ at a temperature above the Tg increases from ca. 2 MPa to ca. 23 MPa with increasing rs from 0.5 to 1.0 and keeps almost constant for a further increase of rs to 1.2. Figure 9b shows the effect of filler content on the E′ curves. It can be seen that the addition of MWCNT fillers up to 0.75 wt % causes a larger shift of the step to higher temperature for the 0.5EP materials than the 1.0EP: ca. 15 °C and ca. 5 °C, respectively. Besides, the addition of 0.75 wt % fillers produces an 80% increase of E′ at high temperature above the Tg for 0.5EP materials but only 30% increase for 1.0EP due to the lower degree of cross-linking of the former one. Figure 10 presents the shape memory cycle data from DMTA for two different SMEP nanocomposite materials as examples. High shape fixity and recovery can be seen for both samples. The Rr and Rf are ca. 97% and ca. 88%, respectively, for 0.5EP-0.25% and ca. 98% and ca. 92%, respectively, for 1.0EP-0.25%. Besides, for the same ε of ca. 10%, 0.5EP-0.25% requires a lower σ than 1.0EP-0.25%: ca. 1.0 MPa and ca. 3.5 MPa, respectively. As shown in Table 1, the Rf of all SMEP and SMEP nanocomposite materials keeps at a high value above 96.5% and it increases slightly with increasing rs and filler

1.0EP and 1.0EP-0.75% at 90 °C. The pulling out of MWCNTs, as shown in Figure 8e,f, can partly explain the significantly increased toughness of the SMEP nanocomposite materials at high temperature. In addition, when the 1.0EP series samples are heated to 90 °C, the bending strain can reach more than 24% and the samples cannot be broken due to the machine’s limitation of measurements (results not shown), which indicates that these SMEP and SMEP nanocomposite materials have a large deformation capacity above their Tsw. It is interesting to compare the properties of our samples with those reported in the literature. In ref 29, the σm and εb above the Tg were ca. 18.9 MPa and ca. 12%.29 In ref 35, the σm and εb at room temperature were ca. 25 MPa and ca. 2.5%, and ca. 4.0 MPa and ca. 15% at the Tsw.35 In ref 47, the σm and εb at room temperature were ca. 37.7 MPa and ca. 3.5%.47 Clearly, the materials reported in this work have a larger σm and εb, especially at a temperature above the Tsw. Figure 9a shows the temperature effect of storage modulus (E′) of SMEP materials with various rs. A stepwise decrease of E′, indicating the glass transition, can be seen for all the samples. For all of these SMEP materials, the E′ at room temperature is ca. 2.0−3.0 GPa. For most of the amine-cured SMEP materials reported in the literature, the E is usually less than 2 GPa.37,47 It can be seen that the step shifts from ca. 74 G


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ACS Applied Materials & Interfaces content. The Rr of all SMEP and SMEP nanocomposite materials also keeps at a high value above 83.7%, and it increases significantly with increasing rs but decreases significantly with increasing filler content. These results can be explained by the increased degree of cross-linking with increasing rs and the decreased segmental mobility with increasing filler content. Figure 11a shows the dependence of the recovery time (tr) as tested in Figures 1 and 12 of SMEP and SMEP nanocomposite

Figure 13 shows the shape memory cycle of 1.0EP deformed in different modes. It can be seen that both bending and

Figure 13. Photographs showing the shape memory cycle of 1.0EP deformed in different modes. Figure 12. Photographs exhibiting the shape recovery process of the samples 1.0EP (yellow) and 1.0EP-0.75% (black) for various repetition times. t1 and t2 are the times needed for the complete recovery of 1.0EP-0.75% and 1.0EP, respectively. 1st, 3rd, 10th, and 15th are the shape memory cycle numbers.

compression modes can make good shape memory cycles. For the compression mode, the superficial area can increase by ca. 21−33% after deformation. Definitely, these properties are beneficial to the application of the SMEP and SMEP nanocomposite materials in various fields. The SMEP nanocomposite materials prepared in this work could be used in aerospace as the truss boom or hinge in deployable structures of space components because of their high strength, high deformability, and high thermal and electrical conductivities produced by the combination of epoxy matrix and MWCNT fillers.28

materials on the rs and filler content. It can be seen that, for SMEP materials, the tr decreases with increasing rs. The recovery of 1.0EP at 150 °C is as ca. 1.5 times fast as that of 0.5EP at 90 °C. Besides, for both 0.5EP and 1.0EP, the addition of 0.75 wt % fillers significantly increases the recovery rate. The reason might be the increase of λ with increasing rs and filler content, as shown in Figure 11b. Figure 11b clearly shows that, for all three different temperatures of 20, 90, and 120 °C, the value of λ increases with both increasing rs and increasing filler content. Compared with the increase of rs, the addition of a small amount of MWCNT fillers can increase the λ much more effectively. For example, at 120 °C, the λ of SMEP materials increases by less than 3% when the rs increases from 0.5 to 1.0, whereas it increases by ca. 40% after 0.75 wt % MWCNT fillers are added into 1.0EP materials. Figure 11b also shows that the λ of all the samples increases with increasing temperature, which is a general characteristic of polymer materials.48,49 Figure 12 shows the shape memory behaviors of 1.0EP and 1.0EP-0.75% after different cycling times. The shape recovery of 1.0EP-0.75% is always much faster than that of 1.0EP for each repetition at 150 °C due to the higher λ of the former sample, as shown in Figure 11b. Besides, with the proceeding of cycling, the shape recovery of 1.0EP-0.75% only slows down slightly, whereas that of 1.0EP slows down sharply, which means that the former has higher mechanical stability due to the contribution of the MWCNT fillers. For the 15th repetition, the recovery of 1.0EP-0.75% is almost twice as fast as that of 1.0EP.

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CONCLUSIONS

A series of shape memory nanocomposites of DGEBA E51MHHPA/MWCNT with various rs from 0.5 to 1.2 and filler contents up to 0.75 wt % have been fabricated, and their comprehensive properties have been systematically investigated. The prepared SMEP materials showed a wide range of Tsw of ca. 65−140 °C, high E′ at room temperature up to ca. 3.0 GPa, high deformation ability above 10% at a high temperature close to the Tg, high recovery stress up to ca. 30 MPa, and fast recovery within 32 s. It was also found that a small amount of MWCNT fillers (0.75 wt %) could significantly increase all three key mechanical properties (E, σm, and εb) at high temperature close to the Tg, recovery rate, and the cycling stability of mechanical properties. The results have demonstrated that the properties of SMEP nanocomposite materials could be further improved by better controlling the chemical composition and microstructure of the system. H


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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 10 82322048. Phone: +86 10 82322048. E-mail: [email protected] (X.Y.). *Fax: +86 10 82545586. Phone: +86 10 82545586. E-mail: [email protected] (Z.Z.). *Fax: +86 10 82545602. Phone: +86 10 82545602. E-mail: [email protected] (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Basic Research Program of China (Grant Nos. 2012CB937503 and 2013CB934203), the National High Technology Research and Development Program of China (Grant No. 2013AA031803), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA09030200), the National Natural Science Foundation of China (Grant Nos. 11225210 and 51571183), and the Fundamental Research Funds for the Central Universities. The authors also acknowledge Prof. Dr. Ronald C. Hedden of Texas Tech University, USA, for his assistance in the language revision of the manuscript.

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