With the rapid development of industry, volatile organic gases (VOCs) due to improper emissions in the environment have posed a serious hazard to human health and safety, the environment and socially sustainable development. It has been well recognized that most VOCs are strongly carcinogenic, such as benzene, xylene, ammonia, formaldehyde, polycyclic aromatic hydrocarbons, aldehydes and nitrosamines in the atmosphere have carcinogenic effects on the organism [1, 2]. Long-term exposure to VOCs can bring about chronic intoxication and neurological disorder syndrome. Since these VOCs are poisonous and nonreversible to people’s bodies, it is essential to find and develop effective methods to detect or remove these VOCs. Gas-sensitive sensors with excellent selectivity, good sensitive responsiveness, and low energy expending and room-temperature availability are thus imperative [3, 4].
Traditional gas-sensitive sensors were mainly focused on metal or oxide semiconductors, such as WO3 [5], ZnO [6] and SnO2 [7], etc., which have shown excellent responsiveness to various VOCs. However, they are also suffering from some deficiencies, such as high operating temperatures (higher than 200oC), poor repeatability and high costs. In contrast, polymer-based gas-sensitive nanocomposites exhibited many advantages including a simple preparation process, low cost, good gas-sensitive selectivity and low operating temperature, which currently have become the research hotspots in gas-sensitive nanocomposites and much progress has been achieved [8–10].
Polymer conductive nanocomposites can be fabricated by filling them with conductive nanoparticles (CNTs, CB, etc.) [11]. The conductive nanoparticles uniformly dispersed in the polymer matrix led to the formation of conductive networks and thus contributed to the excellent electrical conductivity of nanocomposites [12, 13]. When these nanocomposites are exposed to VOCs, the original conductive networks will be fleetly destroyed and as a result, it causes changes in the resistance due to the interactions between the nanocomposites and the target VOCs. Thus, the gas-sensitive responsiveness is reflected. When the gas-sensitive response is completed, the conductive networks can be quickly recovered, and the gas-sensitive recovery and stability are thus reflected [14–17]. Although there have been a lot of reports on the gas-sensitive of polymer conductive nanocomposites, there are still many bottleneck problems that need to be solved: (1) the gas-sensitive properties are based on the changes in electrical properties. Preparation of perfect, but easily destroyed and self-healing conductive networks are still the focus of this system; (2) although the responsiveness of filled conductive polymer gas-sensitive nanocomposites is high, the sensitivity and recovery performance is still inferior to those of semiconductor gas-sensitive counterparts, and the investigation on the gas-sensitive responsiveness mechanism of nanocomposite is not enough [18].
Interestingly, previous studies have shown that double percolation could result in a lower conductive percolation value. Fillers, such as CNTs [19], and CB [20, 21] have been introduced into the two-phase continuous incompatible system to form good conductive networks in order to further reduce the conductive percolation value. Chen et al. [22] realized the selective distribution of CB in incompatible LDPE blends and obtained LDPE/CB composites. An easy way for the formation of conductive polycarbonate (PC)/styrene-acrylonitrile (SAN)/multi-walled MWCNTs nanocomposites with very little MWCNT content by melt blending of PC and SAN-MWCNT has also been reported [23]. An immiscible blend of PC/SAN was formed without MWCNT. After mixing PC with SAN-MWCNT mixtures during melt blending, the MWCNT in the SAN phase was well dispersed and formed a consecutively conductive interconnected network structure extending all over the matrix. Das et al. [24] produced conductive composites with low conductive permeability values by filling CB into EVA, EPDM, and EVA/EPDM (50/50) blends, respectively. The permeability threshold was found to be related to viscosity, polarity, compatibility and polymer blend type. Gong et al. [25] prepared a neoteric continuous separation structure in PP/CB composites the percolation threshold obtained was low to 0.37 vol%, which was remarkable lower than the percolation threshold of melt-laminated PP/CB composites (2.75 vol%) and PP/ PS/CB composites (3.23 vol%). Dong et al. [26] prepared PMMA/CB conductive composites by in situ polymerization with the presence of CB. Similarly, CB/PS composites [27] and CB/polyurethane composites [28] were also fabricated. The results found that the permeability threshold largely decreased compared to the two-phase continuously compatible system.
Thermoplastic polyurethane (TPU) showed relatively high mechanical strength, good processing properties and excellent biocompatibility. The internal micro-phase separation structure from polyester and polyether segments endows the possibility of to use of TPU as gas-sensitive material. Polycaprolactone (PCL) is a semi-crystalline aliphatic polyester [29]. Previous studies have found that two-phase continuous structure conductive composites with TPU/PCL as matrix could be prepared, which not only formed conductive nanocomposites with double percolation structure; but also reduced conductive percolation value [30, 31]. By integrating ethylene dimethyl EMA and EOC with a conductor (CB), the results show the selective distribution of the filler particles in the EMA phase favors the double percolation phenomenon, which implies better electrical conductivity [32]. Toolbox polymers dispersed by carbon nanotubes to get conductive nanocomposites were prepared by Grossiord et al. [33].
In order to obtain polymer-based gas-sensitive conductive nanocomposites for use at room temperature, this study is intended to prepare TPU/PCL/MWCNTs conductive nanocomposites and investigate their gas-sensitive properties to obtain materials with low conductive percolation value and excellent gas-sensitive properties. Since the conductivity of the nanocomposites containing high MWCNTs loadings was mainly attributed to the conductivity of MWCNTs, which did not reflect the conductivity of the polymer matrix. In addition, according to the existing research, the polymer matrix with a double percolation structure could generally undergo percolation behavior when it is filled with low conductive filler and showed excellent conductivity. Therefore, the polymer nanocomposites filled with low MWCNTs content (1%-5%) were selected for research in this work. This work can be prospected to offer a novel strategy for designing and manufacturing conductive polymer nanocomposites-based gas-sensitive sensors for rapid and selective response VOC applications.