This paper aims to exploit shape-memory polymers as self-healable materials. The underlying mechanism involved the thermal transitions as well as the enrichment of the healing reagents and the closure of the crack surfaces due to shape recovery. The multi-stimuli-triggered shape memory composite was capable of self-healing under not only direct thermal but also electrical stimulations.
The shape memory epoxy polymer composites comprising the AgNWs and poly (ε-caprolactone) were fabricated by dry transfer process. The morphologies of the composites were investigated by the optical microscope and scanning electron microscopy (SEM). The electrical conduction and the Joule heating effect were measured. Furthermore, the healing efficiency under the different stimuli was calculated, whose dependence on the compositions was also discussed.
The AgNWs network maintained most of the pathways for the electrons transportation after the dry transfer process, leading to a superior conduction and flexibility. Consequently, the composites could trigger the healing within several minutes, as applied with relatively low voltages. It was found that the composites having more the AgNWs content had better electrically triggered performance, while 50 per cent poly (ε-caprolactone) content endowed the materials with max healing efficiency under thermal or electrical stimuli.
The findings may greatly benefit the application of the intelligent polymers in the fields of the multifunctional flexible electronics.
Most studies have by far emphasized on the direct thermal triggered cases. Herein, a novel, flexible and conductive shape memory-based composite, which was capable of self-healing under the thermal or electrical stimulations, has been proposed.
Luo, H., Zhou, X., Xu, Y., Wang, H., Yao, Y., Yi, G. and Hao, Z. (2018), "Multi-stimuli triggered self-healing of the conductive shape memory polymer composites", Pigment & Resin Technology, Vol. 47 No. 1, pp. 1-6. https://doi.org/10.1108/PRT-03-2017-0032Download as .RIS
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Self-healing polymers, as imitating living organisms for self-curing and self-repairing wounds processes, can spontaneously mend micro-/macro-structural after original breakage to recover on-demand properties such as mechanical tensile strength and thermal/electric conductivity (Thakur and Kessler, 2015; Someya et al., 2016; Wang et al., 2013). This is an alternative view to give a tradeoff from ensuring reliability of fundamental physical properties to prolong the whole lifespan of materials, while conventional methods have to be added other reinforced phases to prevent the occurence of cracks and fractures with inevitably increasing cost and weight. Thus, this new kind of polymers exhibits huge potential applications in next-generation organic-material-based electronics and portable electrodes, especially for further sophisticated systems of smart electronic skins (E-skins) (Benight et al., 2013; Hammock et al., 2013; Huynh and Haick, 2016; Mei et al., 2016) due to its interesting self-healing functionality. Commonly, such polymers can be passive self-healing processes with encapsulated healing agents or active self-healing with external energy, being categorized two kinds of self-healing approaches (Thakur and Kessler, 2015; Benight et al., 2013). In recent flexible polymer-based electric conductor application, for example, great efforts have been addressed to improve electric conductivity and compatibility of electric medium and polymer matrix by discussing homogeneity and optimizing structure of conductor electrodes (Cheng et al., 2015; Han et al., 2014). The cracks are usually found from conductor matrix due to frequent usage or accidents. In addition, efficient self-healing polymer with high electric conductivity is still a limitation.
Owning to the shape-memory effect (SME), the shape-memory polymers (SMPs) are another multifunctional polymers (Lu et al., 2014; Lu et al., 2015; Lu et al., 2016). Being akin to the second type of self-healing polymers, the SMPs can also be triggered by external stimuli to experience a large recoverable deformation, being observed with macro-scale observation (Liu et al., 2009). Poly (ε-caprolactone) (PCL), as one of important SMPs, is an eco-friendly (Peponi et al., 2013) polymer and has low glass transition and melting temperature (Gurarslan et al., 2015; Salvekar et al., 2015), being found in many dual and triple responses SMP applications. It is noted that Rodriguez et al. (Rodriguez et al., 2011) developed a shape-memory-assisted self-healing (SMASH) system, where the shape-memory properties originated from the covalently cross-linked network by end-linking end-functionalized PCL, and the healing mechanism was due to the diffusion of linear PCL. Wei et al. (2015a, 2015b) constructed epoxy-based shape memory polymer (ESMP) and PCL composites based on the phase separation phenomenon between these two polymers. Self-healing process of ESMP/PCL composites is controlled by thermal stimuli via tuning–melting transition and glass transition temperatures. EMSP/PCL composites also show excellent SMP effects with full recovery process within 80 s. Recently, the same group (Yao et al., 2016) fabricated an SMP composite with self-healing properties based on PCL microfibers and epoxy resin. In this work, PCL was electrospun into a laminated network layer of epoxy matrix to optimize interaction between them and enhance the SME geometrically and structurally. Additionally, Wang et al. (Wei et al., 2015a, 2015b) reported triple-shape effect of upytelechelic poly(tetremethylene ether) glycol (PTMEG) and PCL composites with self-healing features, displaying a good tensile strength recovery of 87 per cent. However, the abovementioned literatures mainly concentrate on the optimizing SME effects of dual polymer-based composites, while their healing functionality is solely actuated by thermal energy. To fulfill complex self-healing conditions, it is necessary to develop multi-triggered self-healing polymers to prompted self-healing efficiency and degree to retain origin properties. Designing novel multi-self-healing polymer is still a big challenge toward building smart portable devices like E-skins.
In this paper, the AgNWs/SMPs composites for self-healing conductive electrode with dual-triggered stimuli (thermal and electrical responsiveness) were proposed. Precisely controlling of AgNWs film from dip-coating process and utilization of different melting transition temperature between PCL and epoxy were both in favor of healing efficiency under thermal and electric stimulus. The polymer systems of the PCL and epoxy were chosen in this study due to the well compatibility of the two contents as well as the self-healing and shape-memory efforts according to the previous literature. Due to excellent electrical and thermal inherent properties from AgNWs, the SMP composites achieved better self-healing efficiency through dual-triggered, self-healing processes. Such flexible self-healing SMP composites may pave the way toward advanced multifunctional conductive electrodes for smart electronics.
The materials, including PCL (Mw = 65000 g/mol), poly(diglycidyl ether of bisphenol A) (DGEBA), neopentyl glycol diglycidyl ether (NGDE) and poly(propylene glycol) bis(2-amino-propyl) ether (Jeffamine D230) were purchased from Sigma-Aldrich, and used as received. The shape-memory epoxy polymers were synthesized according to the previous literature (Luo and Mather, 2013). Briefly speaking, equimolar DGEBA (preheated at 70∼80°C to melt the crystals); NGDE and Jeffamine D230 were homogeneously mixed and solidified at 110°C for 3 h. The PCL content was 60 Wt.%. The AgNW-containing composites were fabricated via dry transfer process. The samples, containing the AgNWs whose densities were 6.4, 5.3 and 4.2 μg/mm2, were denoted as SHP-01, SHP-02 and SHP-03, respectively. For the self-healing experiments, the damage was introduced by a fresh razor blade. The surface morphologies and the conduction were investigated by SEM (Hitachi, S-4800) and an electrochemical station (CHI660D, ChenHua). The tensile tests were performed with the tensile machine (SUST, China). The thermal–electrical effects were monitored with a thermal infrared imager (MAG 11, Shanghai Magnify, China).
3. Results and discussion
3.1 Fabrication and morphologies of the composites
The self-healing composites were fabricated via the transfer process, which was similar to that in our previous report (Luo et al., 2014). The PCL-containing epoxy composites exhibited good compatibility. The pristine polymer films were visibly compatible without obvious phase separation. The conductive composites were further fabricated based on the pristine polymers. Figure 1(a) schematically illustrates the process, typically containing three steps, namely, dip-coating of the AgNW films, hot compression of the epoxy polymers and peeling off the composites. The AgNWs formed a percolating network possessing homogeneity and conduction whose thickness could be well controlled by the volume and the concentrations of the suspension during the dip-coating. On the other hand, the PCL, reported to be rather compatible with the epoxy matrix, played the role of the healing reagent (Luo and Mather, 2013). The hot compression was performed at around 85°C under the pressure of around 25 N/mm2 for 10 min. The temperature was above the melting transition temperature (Tm, once determined to be in the range of 45° and 50°C) of the PCL. The melting-crystallization transition of the PCL during the hot compressions was expected to benefit the complete transferring of the AgNWs network. The composites were flexible and electrically conductive, facial to be peeled off from the release substrate [shown in Figure 1(b)]. The AgNWs network embedded in the epoxy-surface layer could even be clearly observed by an optical microscope.
Furthermore, the conductive networks constructed by the AgNWs were observed using the SEM measurements [shown in Figure 2(a-d)], which was consistent with the optical results. Comparatively, the AgNWs densities of sample SHP-01 were significantly higher than that of sample SHP-03. Spherical PCL crystals were found to distribute in the nanowire networks. Although the conduction decreased in the magnitude of 1∼2, as transferred from the release substrate to the polymer matrix, the AgNWs network maintained most of the pathways for the electrons transportation, leading to superior conduction and flexibility after the dry transfer process.
3.2 Conductivity and the Joule-heating effect
Figure 3(a) shows the current–voltage curves of the composites under scanning. The electrical current nearly linearly increased along with the voltage increase in the range of 0-5 V. The resistance was calculated according to the slope of the curves, which were determined to be 4.14, 15.4 and 22.3 Ohm/mm2 for samples SHP-01, SHP-02 and SHP-03, respectively. The electro-triggered behaviors attributing to the Joule-heating effect have been extensively studied for the conductive shape-memory polymers in the previous literature (Luo et al., 2014). In our study, the AgNW-containing composites significantly increased the temperature within several seconds during the voltages scanning. The increase was monitored, as shown in Figure 3(b). All the composites enhanced the temperature above 60°C within 1 min. Particularly, the composites having more AgNWs content had much greater increase in temperature.
The thermal infrared technology was used to monitor the surface temperature changes, as applied with the voltages. Representatively, the thermal infrared images of the sample SHP-01 with different stimulation durations are shown in Figure 4(a) to (c). The sample was heated to 50°C for 5 s and even heated up to 105°C for 10 s under 3 V. According to the Joule law:
3.3 Multi-stimuli triggered self-healing of the composites
To study the healing behaviors under different stimuli, the composites were partially cut along with the direction perpendicular to that of the current flow in a control manner. Figure 5(a) to (c) representatively demonstrate the photographical thermal-triggered healing process even as the composites were cut down totally. Direct heating at 85°C for several minutes allowed the healing to occur. Meanwhile, the transparency of the composites visibly increased, indicating that the PCL content experienced the melting transitions [shown in Figure 5(d)]. The shape-memory-assisted self-healing was expected to occur for the cracked composites under the thermal stimulations. Speaking in details, cutting generated new interface in the composites. The crack size was in the range of several dozens of micrometers, e.g. 20-50 μm. On the other hand, we also studied the healing performance of the composites cut off totally. The heating triggered the shape recovery of the polymer matrix, which made the interface closer. The shape-memory composite matrix comprised the shape-fixing phase of the crystalline PCL and the elastic epoxy network. The entropic elasticity of the network driven the shape recovery occurs when the crystalline PCL was melted as exposed upon the heating. Meanwhile, the PCL content locally in the cracked area experienced melting and crystallization transitions during the heating and cooling programs. The combination of the interface closure with the phase transitions of the PCL content allowed the completion of healing.
Furthermore, the healing efficiency was calculated according to the ratio of the maximum stress in the pristine and the healed states. The calculation equations were expressed as below:
where ηt and ηi stood for the healing efficiency triggered by the direct thermal and electrical stimulations, respectively; σ and I were the maximum stress and the current of the samples in the electro-mechanic measurements respectively; and the subscripts hel and pri represented the healed and pristine states, respectively. The dependence of the healing efficiency against the compositions under the thermal and electrical stimulations is shown in Figure 6. Both the direct thermal and the Joule-heating stimuli were capable of triggering the self-healing, the efficiency of which was in the range of 40 to 70 per cent. The maximum healing efficiency was obtained as the PCL content increased up to 50 per cent, being indicative of the healing reagent role of the PCL content.
On the other hand, the AgNWs content had significant influence on the electrically triggered performance. The healing efficiency increased from 45 to 57 per cent for sample SHP-01, which had the maximum AgNWs density compared with SHP-03. It was reasonable considering the Joule heating derived from the conduction of the AgNWs. On the contrary, given that the PCL crystals’ melting transition was mainly involved, the AgNWs content had negligible influence on the thermal-triggered healing efficiency, which remained around 53 per cent. The healing process was expected to include two steps (Yao et al., 2016):
Thermal-/electrically triggered shape recovery made the cracks surface closer.
The healing reagent melted and flowed to the cracks for rebuilding the bulk phase.
The multi-stimuli responsiveness of the shape-memory polymers provided an avenue to achieve the self-healing behaviors in extended manners (Figure 7).
Shape-memory epoxy polymer composites comprising AgNWs and PCL were fabricated by dry transfer process. The composites exhibited flexibility, electrical conduction and multi-stimuli-triggered self-healing. As exposed upon the partial structural damages, the composites were capable of self-healing under direct thermal or electrical stimulations. The healing efficiency could be improved by optimizing the compositions for the different healing process. The findings may be of great benefit to the development of the multifunctional flexible electronics.
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The authors thank China Postdoctoral Science Foundation (No. 2015M580709), Pearl River (Guangzhou) of Nova in Science and Technology (2014J2200090), National Natural Science Foundation of China (No. 11772108), for providing financial support.