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Excellent Electroactive Shape Memory Performance of EVA/ PCL/CNT Blend Composites with Selectively Localized CNTs Zhi-xing Zhang, Wen-yan Wang, Jing-Hui Yang, Nan Zhang, Ting Huang, and Yong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06345 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016

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The Journal of Physical Chemistry

Excellent Electroactive Shape Memory Performance of EVA/PCL/CNT Blend Composites with Selectively Localized CNTs Zhi-xing Zhang, Wen-yan Wang, Jing-hui Yang, Nan Zhang, Ting Huang, Yong Wang* School of Materials Science & Engineering, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu, 610031, China Abstract:

In

this

work,

the

binary

blend

of

poly(ethylene

vinyl

acetate)

(EVA)/poly(ε-caprolactone) (PCL) with different compositions were prepared and the shape memory behaviors were evaluated firstly. Then, carbon nanotubes (CNTs) were introduced into the composition that exhibited relatively better shape memory effect (SME) to prepare the ternary blend composites with electrically actuated SME. The morphologies of the blend and/or blend composites, the crystallization of the component, and the selective location states of CNTs in the blend composites were comparatively investigated. The results demonstrated that the binary EVA/PCL blends exhibit a typical sea-island structure at low PCL content, a quasi-cocontinuous structure at mediate PCL content and then the sea-island structure at high PCL content. Among these binary blends, the EVA/PCL (60/40) sample exhibits relatively better SME with not only high shape fixing ratio but also shape recovery ratio. The presence of the CNTs further induces the changes of morphology and crystallization behavior in the blend composites. Due to the largely reduced electrical resistivity promoting more Joule heat generation at applied voltage, the blend composites containing relatively high content of CNTs exhibited excellent electrically actuated SME. This work demonstrated that introducing CNTs into the immiscible blends and controlling the selective location of CNTs in one component, the electrically actuated SME could be achieved at relatively lower voltage and the shape recovery speed could also be greatly increased.

1. Introduction Shape memory polymers (SMPs) were first introduced in the mid-1980s, and they attract much attention of researchers.1,

2

SMPs are a kind of smart materials which exhibit the following

features. They exhibit a temporary shape at an external force, and at an appropriate external stimulus, the temporary shape disappears and the materials recovered to their original shape again. The external stimulus is various, including thermal, mechanical, chemical, light, and electrical *

Corresponding authors: Tel: +86 28 87603042; E-mail: [email protected] 1

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stimulus,2-4 etc. To date, thermally responsive SMPs are the most researched type of SMPs. The common mechanism of thermally responsive SMPs is related to the existence of reversible switching structure and elastic network, and these SMPs can recover to their original shape when they are heated to the temperature above the transition temperature ( Ttrans ),5 which is a temperature range coinciding with reversible switching structure, such as the melting temperature ( Tm ) or glass transition temperature ( Tg ).6-8 However, the realization of the shape memory behaviors through direct heating is not convenient, especially in the area that requires the long-range controlling. Therefore, other strategies have been developed to prepare the SMPs with the shape memory effect (SME), including light-actuated,9 magnetic-actuated,10 and electrical-actuated SMPs,11 etc. Among these SMPs, the electrical-actuated SMPs attract much more attention of researchers because of the convenient manipulation, remote controllability and so on.2 However, it is well known to all that most polymers are electrically insulating and they cannot be driven by electrical means. The strategy to improve the electrical conductivity of these polymers is incorporating conductive fillers into them.12-14 In the electrical field, Joule heating can be induced when the electrical current passes through the material, which promotes the temperature enhancement of the sample and consequently, the SME of the SMPs can be achieved.15 To obtain the enough electrical current in the sample, the electrical conductive path must be constructed. Among the conductive fillers that have been widely used to prepare the electrically conductive polymer composites, carbon nanotubes (CNTs) are the most extensively investigated type due to their extremely high intrinsic conductivity, high aspect ratio and excellent self-entanglement ability,16,17 which facilitates the formation of the conductive path in the polymer composites. So far, many strategies have been developed to prepare the electroactive SMPs with CNTs.18,19 One of the most important aspects that must be considered when preparing the electroactive SMPs with CNTs is reducing the electrical resistivity of polymer composites at low CNT load. This is because that although increasing CNT content facilitates the conductive path formation, the CNT network structure with high density also restricts the relaxation of polymer chain segments, which is unfavorable to the shape fixity and shape recovery of the SMPs to a certain extent.20,21 Introducing CNTs into an immiscible polymer blends and controlling the selective localization of CNTs at the blend interface and/or in one component has been demonstrated a highly efficient 2


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strategy to reduce the percolation threshold of CNTs.22,23 This is so called “double percolation” conductive mechanism.24 Many factors, such as mixing sequence, mixing duration, the affinity of CNTs to each component, interfacial tension, and viscosity ratio of the two components,22,25,26 influence the selective location of CNTs in the polymer blend composites. To date, researches about the SME of the blend composites containing CNTs are rather few. Qi et al.27 prepared poly(propylene carbonate) (PPC)/poly(lactic acid) (PLA)/CNT composites in which CNTs selectively locate in the PPC component, and the composites with 3 wt% CNTs showed excellent SME, i.e. with 30 sec and 30 V, the shape recovery ratio can be 97%. In the present work, we attempt to introduce CNTs into a poly(ethylene vinyl acetate) (EVA)/poly(ε-caprolactone) (PCL) blend to prepare the blend composites with excellent SME. EVA exhibits excellent comprehensive performances, such as good melt processability, low-temperature flexibility, outstanding oil resistance, and good mechanical properties as well as good affinity with fillers.28,29 PCL has a low melting temperature of about 60 oC, and it is biocompatible and fully biodegradable with slower degradation rate.30 Therefore, the blends of EVA/PCL exhibit wide potential application in many fields. Specifically, once the material is endowed with electroactive SME, it can be widely used in the fields of biomedical device, sensor, and automatic control system, etc. Here, the SME of the binary EVA/PCL blends with varied compositions is firstly investigated, and then the blend composites with CNTs are prepared using the blend which exhibits relatively better SME. The microstructure-performance relationship of the blend composites is then systematically investigated. Interestingly, it is observed that the EVA/PCL/CNT blend composites show excellent electroactive shape memory behaviors.

2. Experimental 2.1 Materials EVA (with a trade name of Elvax 3120) was obtained from DuPont Industrial Polymers (USA). The melt flow rate (MFR) and density of the material were 1.2 g/10 min (190°C /2.16 kg) and 0.93 g/cm3, respectively. PCL (with a trade name of CAPA*6806) was obtained from Solvay (Belgium). It exhibited the MFR of 7.3 g/10 min (160 °C/2.16 kg). CNTs (with a trade name of TNIM2) were purchased from Chengdu Institute of Organic Chemistry, Chinese Academy of Science (Chengdu, China). The length of a single CNT and the outer diameters of the CNTs were 3



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30–50 µm and 8–15 nm, respectively. 2.2 Sample preparation Before they were used, EVA and PCL were dried for about 8 h in an oven at 40 °C. The melt compounding was carried out at a melt temperature of 160 °C and a screw speed of 50 rpm, which was conducted on an internal mixer ZJL-300 (Changchun Zhineng Instrument, China). The mixing duration was 5 min. At first, the binary EVA/PCL blends with different PCL contents (0-100 wt%) were prepared. To prepare the EVA/PCL-based composites, 15 wt% CNTs were firstly incorporated into EVA to prepare the master batch. After that, EVA and PCL were melt-compounded with the master batch to prepare the blend composites with varied CNT contents from 0.5 to 5 wt%. For all the blend composites, the weight ratio of EVA and PCL was maintained at 60:40. The binary blend without CNTs was named as ExPy, where x represented the content of EVA component while y represented the content of PCL component. For example, E80P20 represented that the contents of EVA and PCL were 80 and 20 wt%, respectively. The ternary blend composites were named as E-CNTx. For example, E-CNT2 represented that 2 wt% CNTs were present in the EVA/PCL/CNT sample. At last, the plates with a thickness of 0.5 mm were obtained through compression-molding process which was carried out at a pressure of 5 MPa and a melt temperature of 170 °C. 2.3 Scanning electron microscopy (SEM) The phase morphologies of samples were characterized using a SEM Fei Inspect (FEI, the Netherlands) which was operated at 20 kV. Sample was firstly immersed into liquid nitrogen for 30 min and then it was cryogenically fractured. Before SEM characterization, a thin layer of gold was coated on the cryo-fractured surfaces. 2.4 Differential scanning calorimetry (DSC) A differential scanning calorimetry (DSC) (Perkin-Elmer pyris-1, DSC) was used to investigate the thermal properties of samples. The measurements were done in nitrogen atmosphere. During the measurements, the weight of sample was maintained at about 5 mg. First, the sample was heated from 0 °C to 150 °C with a heating rate of 10 °C/min, and then it was maintained at 150 °C for 5 min to erase any thermal history, finally the sample was cooled down to 0 °C with a cooling rate of 5 °C/min. 2.5 Contact angle measurements 4


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A contact angle measurement was carried out at room temperature to calculate the surface tensions of all the components. The measurements were conducted on a drop shape analysis system DSA100 (KRÜSS, Germany) using the compression-molded films of EVA and PCL. Each sample was measured for 5 times and the average value of contact angles was reported. The probe liquids were double distilled water (H2O) and methylene iodide (CH2I2). 2.6 Rheological measurements To investigate the rheological properties of samples, a stress-controlled rheometer DHR-1 (TA Instrument, USA) was used. The sample disk was prepared through a compression molding processing and it had a diameter of 20 mm and a thickness of 0.5 mm. The rheological measurements were carried out at a melt temperature of 160 °C and a frequency range of 0.01-100 Hz, and the sample was protected in nitrogen atmosphere. 2.7 Electrical resistivity measurements Two methods were adopted to obtain the electrical properties of samples. At extremely high electrical resistance, A Digital High Resistance Test Fixture PC68 (Shanghai Precision Instrument Manufacture, China) was used. These samples were prepared through compression molding processing and they had a diameter of 80.0 mm and a thickness of 1.0 mm. For the samples with relatively lower electrical resistance, a universal meter (DT9208, China) was used, and the rectangular cross-sectional bar had a length of 20 mm and a width of 5 mm. 2.8 Shape memory measurements Samples which were cut from the compression-molded plates were used for shape memory measurements. They had a length of 50 mm, a width of 5 mm and a thickness of 0.5 mm. A four-step measurement strategy was designed as follows: First, the sample was deformed as a “U” shape at water bath of 60 °C. Then, under a constant force, the deformed sample was cooled down to the room temperature (22 °C). After removing the external force, the angle between the two sides of the bent sample was measured ( θ 0

CNT

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