Article pubs.acs.org/IECR
Production of Flexible and Electrically Conductive Polyethylene− Carbon Nanotube Shish-Kebab Structures and Their Assembly into Thin Films Tao Tao,† Ling Zhang,† Jan Ma,*,‡ and Chunzhong Li*,† †
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore ABSTRACT: Polyethylene−multiwalled carbon nanotube shish-kebab (PE/MWCNT SK) structures with a large aspect ratio were synthesized by solvothermal growth followed by recrystallization. The effects of different crystallization temperatures (95, 105, and 115 °C) on the polymer were investigated, and the diameter of the PE crystal lamellae on the nanotubes was found to decrease with increasing crystallization temperature. Moreover, isothermal recrystallization of the SK structure at 115 °C produced crystal lamellae that were uniform in diameter, with dimensions that could be controlled by the recrystallization time. We studied also the stretchable electrical properties of uniform PE/MWCNT SK structures films on elastomer substrates under different strains. The electrical resistance of an SK/elastomer film was found to increase more slowly, compared to that of a film without the SK configuration, where an abrupt transition at a critical point was observed.
1. INTRODUCTION Flexible materials with good electrical conductivity have attracted wide attention because of their potential applications in the areas of flexible displays, stretchable circuits, and largearea electronics.1−5 Stretchability is one of the major concerns in flexible electronics. Various approaches have been proposed to produce stretchable conductive materials, such as metal films in wavy layouts, conductive polymer films, and graphene or carbon nanotube composites.6−9 Wavy metal films are known to exhibit excellent conductivity and stretchability, but their complicated manufacturing process is a significant drawback for commercialization. Carbon nanotubes (CNTs), on the other hand, are considered as an ideal candidate because of their large aspect ratio and high electrical conductivity. A substantial amount of research has been reported on the construction of elastic conductors comprising millimeter-long single-walled carbon nanotubes (SWCNTs) and fluorinated copolymer composites.4,5 Noting that the key failure mechanism of conductive materials under stretching is the disconnection of the conductive pathways caused by the formation of cracks in the materials, the use of long CNTs is preferred, as their large aspect ratio can assist in the bridging of cracks under strain. In the present work, we present a new strategy to further enhance the stretchability of CNT conductive film materials that exploits not only the high aspect ratio but also the unique geometry of the polyethylene−multiwalled carbon nanotube shish-kebab (PE/MWCNT SK) structure. In the proposed SK structures, skewerlike polymer crystal lamellae are located periodically along the CNTs. These polymer crystal lamellae are considered to enhance the connection of the SK structures during stretching, so that the material can achieve better stretchable conductive properties than pristine CNTs. There have been reports on the synthesis of SK structures, such as polymer solution crystallization and supercritical carbon dioxide antisolvent process.10−15 In an earlier study, PE/ © 2012 American Chemical Society
MWCNT SK structures were also prepared using nonisothermal crystallization.16 The present work, on the other hand, employs a solvothermal approach, where PE/MWCNT SK structures with more uniform crystal lamellae size can be fabricated through subsequent recrystallization. The morphology of the present solvothermal product was found to be superior to that obtained using the previously reported methods because of the elevated temperature and pressure applied. The uniform PE/MWCNT SK structure was dispersed in cyclohexane and filtered to form a PE/MWCNT SK network film, which was then transferred onto an elastomer substrate to fabricate stretchable conductive film. The evaluation of the stretchable conductive properties was carried out under different strains. We also examined the correlation between the PE crystal lamella size and the stretchable conductive properties of the SK structured films.
2. EXPERIMENTAL SECTION Pristine MWCNTs (95% purity; average diameter range, 10− 20 nm; length range, 5−20 μm; Alfa Aesar) were used without any additional treatment to preserve the integrity of MWCNT sidewall structure. PE (7020, melt flow rate = 7.5 g/10 min) was obtained from Xinjiang Dushanzi International Petroleum & Chemical Ltd., and cyclohexane and p-xylene were obtained from Shanghai Lingfeng Chemical Reagents Co. Ltd. In the solvothermal process, 2 mg of MWCNTs and 2 mg of PE were dispersed in 18 mL of cyclohexane, and the mixture was ultrasonicated for 3 h. The as-prepared mixture was placed in a 20 mL solvothermal growth vessel and held at 130 °C for Received: Revised: Accepted: Published: 5456
April 17, 2011 March 21, 2012 March 23, 2012 March 23, 2012 dx.doi.org/10.1021/ie2008218 | Ind. Eng. Chem. Res. 2012, 51, 5456−5460
Industrial & Engineering Chemistry Research
Article
of the lamellae is due to the formation process of the SK structures. In our previous work, it was found that the growth of the lamellae is accompanied by the formation of new lamellae throughout the crystallization process.16 It is known that the dynamics of the transition of polymer chains from a random configuration to a folded chain configuration (the PE chain configuration in the lamellae) is strongly dependent on the molecular size and the initial configuration of the chains.17 As a result, larger chains usually crystallize earlier than smaller chains, and chains of the same size will also crystallize at different times because of their different initial configurations. Consequently, lamellae formed at different stages should have different diameters, as shown in these experiments. Figure 2 presents the distributions of the diameters of the PE crystal lamellae obtained at different crystallization temper-
30 min. The mixture was then quenched to different crystallization temperatures and further held for 3 h. After being allowed to cool to room temperature, samples were filtered and washed with hot p-xylene to remove the nonabsorbed PE and amorphous PE adhered to the surface of the SK structures. To improve the uniformity of the SK structures, samples were subsequently recystallized at 115 °C for 10 h. Before the recrystallization process, all SK structures were dispersed in cyclohexane and redissolved at 130 °C for 30 min. The morphology of the PE/MWCNT SK structure was examined by transmission electron microscopy (TEM; JEOL2100) at an accelerating voltage of 200 kV. For the stretchable conductive property tests, 1 mg of PE/MWCNT SK structures was dispersed in cyclohexane and ultrasonicated for 30 min. The mixture was filtered to form a PE/MWCNT SK network film, then placed on a 1 × 5 mm2 area elastomer substrate, and pressed at 100 °C for 1 min to improve the adhesion between the SK network and the elastomer substrate. Finally, the electrical resistance of the SK/elastomer film under different strains was measured using a four-point probe.
3. RESULTS AND DISCUSSION 3.1. Effects of Crystallization Temperature. Three different crystallization temperatures were chosen, namely, 95, 105, and 115 °C, and the resultant SK structure morphologies are shown in parts a−c, respectively, of Figure 1. It can be seen
Figure 2. Distributions of lamella diameters of PE/MWCNT SK structures prepared by crystallization at 95, 105, and 115 °C.
atures, based on the TEM images of 350−400 lamellae. For samples crystallized at 105 and 115 °C, the diameters were mainly in the range of 20−70 and 20−60 nm, respectively. It is evident that the diameters became more uniform as the crystallization temperature increased, and the SK structures also became more regular. The narrower distribution of lamella diameters for the sample with the higher crystallization temperature resulted from the lower crystallization rate of the PE chains. A previous study showed that SK structures prepared by nonisothermal crystallization could contain lamellae with a wide range of diameters (30−150 nm).16 Hence, the present solvothermal approach is able to produce more uniform SK structures than those obtained from the previously reported nonisothermal crystallization process, especially at high crystallization temperatures. On the other hand, it can be seen that the periodicities of the SK structures prepared by the two methods were in the same range of 35−80 nm. This is consistent, because the crystallization process does not influence the periodicity of the SK structure. 3.2. Effects of Recrystallization. It has been shown that uniform SK structures can be achieved by a solvothermal approach at high crystallization temperatures; however, the lamella size of the SK structures was small because of the slow crystallization process. To synthesize uniform PE/MWCNT SK structures with larger lamellae, a recrystallization process was applied at 115 °C for 10 h. In addition, for comparison, a PE/ MWCNT SK structure was fabricated by the solvothermal process at 115 °C for 10 h. The resulting distributions of lamella diameters are shown in Figure 3. It can be seen that the mean lamella diameter of the recrystallized SK structures was
Figure 1. TEM images of PE/MWCNT SK structures prepared by crystallizing at (a) 95, (b) 105, and (c) 115 °C, along with the selected-area electron diffraction (SAED) pattern (inset in a).
that the MWCNTs were periodically decorated with PE crystal lamellae. The polycrystalline rings of the selected-area electron diffraction (SAED) pattern correspond to the (110) crystal faces of the PE lamellae (inset in Figure 1a). For the sample crystallized at 95 °C (Figure 1a), the diameters of the PE crystal lamellae are in the range of 30−90 nm. The diameter deviation 5457
dx.doi.org/10.1021/ie2008218 | Ind. Eng. Chem. Res. 2012, 51, 5456−5460
Industrial & Engineering Chemistry Research
Article
Figure 5. TEM image of the SK structures after dissolution in p-xylene at 130 °C.
Figure 3. Distributions of lamella diameters of PE/MWCNT SK structures crystallized at 115 °C for 3 h (blue) and 10 h (green) and recrystallized at 115 °C for 10 h (red).
process (Figure 4d). On the other hand, the distance between subglobules was too small for the formation of new subglobules because of geometric confinement, so very few subglobules formed in the later stages of the recrystallization process. As a result, a post-treatment of recrystallization provides a narrow lamella size distribution (Figure 3). 3.3. Stretchable Conductive Properties of PE/MWCNT SK/Elastomer Films. In light of the unique morphology of SK structures, we sought to demonstrate the electrical properties of SK/elastomer films under strain, which is an essential parameter for flexible electronics applications. PE/MWCNT SK structures with two different PE lamella sizes were used to prepare SK/elastomer films, as indicated in Table 1. An
around 70 nm, larger than that of the structures obtained upon crystallization for 3 h (
