Facile one-step preparation of laminated PDMS based flexible strain sensors with high conductivity and sensitivity via filler sedimentation
Introduction
Flexible force-sensing materials are valuable in that they detect stress or strain across wide ranges. These sensors have numerous applications, such as in human movement detection, health monitors and electrical skin (e-skin), thus attracting a lot of attention [[1], [2], [3]]. Polymer based conductive composites (PCC) are appealing for flexible sensors because of their easily tailorable flexibility, sensitvity, and detectability [[1], [2], [3]]. The morphology of the conductive network of PCC plays an important role in sensing performance. PCC with homogeneous and non-homogeneous networks have been studies.
A percolation network is a type of homogeneous conductive network wherein the filler is fully dispersed in the composite. Coleman et al. [4] embeded graphene into a lightly cross-linked polysilicone with an ultalow modulus, forming a percolation network. The composite sensors had a high gauge factor (GF) of about 500 when the graphene content was 6.8 vol%. In order to decrease the filler content, Liu [5] filled graphene into a binary blend rubber to form a segerated network in which graphene dispersed in the whole matrix, similar to a homogeneous networks. The percolation value decreased from 4.0 vol% for the non-segegrated network to 0.3 vol% for the segerated network composites. This low graphene content and double-interconnected network favored high flexiblity and sensitivity (GF ≈ 82.5) of the sensors.
Non-homogeneous networks with different morphologies have been used to further improve sensor flexibility and conductivity including sandwiched [[6], [7], [8]] and laminated stutures [9,10]. In these stuctures the rubber matrix and conductive phases perform their own functions; the former retains sensor flexiblity and the latter enables sensing function. The conductive network efficiencies of these sensors has been demonstrated to be higher than homogenous networks given lower filler contents are needed. Zhang et al. [6] diffused carbon nanotubes (CNTs) into polydimethylsiloxane (PDMS) to form a sandwiched structure with only 0.48 wt% CNTs in which the composite sensor could be stretched as pure PDMS with a workable strain of 352%.
Compared with a sandwiched structure, composite sensors with a laminated structure are easier to regulate gauge factors, stretchability, and linearity. Generally, laminated structure conductive phases are spay coated [9], bar coated [11], deposited [[12], [13], [14]], transfered [10], or wrapped under the assisstance of ultrasonication [15] on a prepared rubber. Fu et al. [9] formed PDMS/CNT composite sensors with a laminated stucture by spray coating a mixture of CNTs and 3-aminopropyltriethoxysilane (KH550) onto PDMS. Due to the layered structure and brittle CNTs conductive network, the GF, stretchability, and linearity (nonlinearity−linearity) were adjusted by changing the ratio of CNTs and KH550. The highest reported GF and working strian achieved were 1000 and 250%, respectively. Furthermore, the morphology of the conductive phase was also designed to tailor GF and stretchability. Niu et al. [14] sputtered a novel diffraction-induced Au film with a gradient thickness onto polyurethane (PU) and high sensitivity (GF ≈ 474.8) and stretchability (working strian ≈140%) were achieved. Another appoach is casting the rubber solution or melting onto conductive papers [16,17]. Pan et al. [16] casted PDMS onto prepared CNT bucky paper to obtain a composite sensor with GF as high as 25000 at a strain of 60%.
Despite the advantages of high conductivity, flexibility and sensitivity, preparing laminated and sandwich structured sensors is non-trivial. The reported methods usually containe two steps: preparation of the rubber matrix and then addition of the conductive phase, or vice versa. Thus, the procedures are sophisticated and the interface between the two layers needs to be regulated [10].
As a well-know phenomenon, the higher density phase of a composite of two materials will settle to the bottom of the matrix due to gravity. This occurrs often in preparing composites with metal or carbon fillers and great effort is needed to avoid filler sedimentation [18]. However, the higher density filler on the bottom of a composite could form a laminated structure composite [19]. Thus, this would be a facile method to prepare laminated composites in one step, allowing complicated procedures to be avoided. To the best of our knowledge, few literature reported flexible sensor fabrication using this method.
Herein, we repored a facile one-step method to prepare laminated structure PDMS based flexible sensors via filler sedimentation. The laminated structure composites obtained presented a two dimensional conductive network observed via SEM. Mechanical, electrical, and electrochemical properties were measusred. Experimental results were analyzed and fit to theoretical models of resistance.
Section snippets
Materials
PDMS (Sylgard 184) was supplied by Dow Corning (USA). Nickel-coated graphite (NCG) pellets with a mean size of 80 μm and density of 4 g/cm3 were purchased from Beijing Entrepreneur Science & Trading Co., Ltd, China. Fig. S1 shows their SEM images and size distribution in the Supplementary Information (SI). Hexane (C6H14) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals were used as received without further purification.
Sample preparation
The PDMS/NCG layered sample preparation
Results and discussion
The settlement of filler was determined by standing time, density of filler, specific surface area, viscosity of polymer. For the given NCG and PDMS, the sediment was regulated by standing time. Different standing times over the range of 0.5–2 h were tried and the SEM images of obtained composites were shown in Fig. S2 in SI. At 0.5 h, the settlement of NCG almost finished, but many bubbles appeared during the stirring to obtain a homogenous dispersion of NCG in PDMS. The vacuum was used to
Conclusion
In this invesitgation, PDMS/NCG conductive composite sensors with laminated structure were fabricated by a facile method via NCG sedimentation. This one step process had high effieciency and the resulting composites had a conductivity of 148 S/m, and high flxibility. The Younge's modulus of composites increased from 0.88 MPa of pure PDMS to 1.98 MPa for 11.11 vol% NCG composites. The resistance of composite sensors increased from 8 to 20 Ω to 4 × 108 Ω when loading stress and presented a gauge
Declaration of competing interest
We declare that we have no conflicts of interest to this work.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (NSFC) programs [grant number 51503061], the Natural Science Foundation of Hubei Province [grant numbers 2018AAA008, 2015CFB322], and the Hubei Chenguang Talented Youth Development Foundation. We thank Science Docs Inc. for language editing.
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These authors contributed equally to this work.