Tannic Acid-Terminated Poly(Propylene Oxide) as Efficient Dispersant for Multi-Walled Carbon Nanotube to Prepare Highly Conductive Composites
Abstract
In this study, multi-functional tannic acid (TA) was incorporated with soft poly(propylene oxide) (PPO) chain to result in a TA-terminated JTA polymer, in which the TA terminals impart reversible aromatic and hydrogen bond (H bond) interactions to JTA polymer. Moreover, the aromatic TA terminals of JTA can preferably bind to the surface carbon rings of multi-walled carbon nanotubes (MWCNTs) to result in JCTx composites containing large amounts (up to 96 wt%) of MWCNTs. The sorption isotherm study indicated a two-step sorption mechanism of the sorbed JTA on MWCNT. Dependent on the MWCNT content, JCTx composites can be healable elastomers with high fracture strain (up to 700%) or plastics with outstanding conductivity (up to 1.21×106 m-1). Experimental mechanical modulus and conductivity of JCTx also were approached by the theoretical Kolarik model and scaling law, respectively. The result was explained in term of morphological arrangement of the MWCNT, evaluated from scanning and transmission) electron microscopies in JTA matrix. This study provided theoretical evaluation of highly MWCNT-loaded composites.
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Introduction
Molecular engineering using delicate design and testing of molecular structures and interactions enables the fabrication of advanced materials with different application fields. Given the extraordinary mechanical, electrical, and thermal properties, carbon nanotubes (CNTs) provided versatile routes for the preparations of advanced materials, among them, polymer/CNT nanocomposites1,2were studied frequently despite the existing difficulty in fabricating homogenous composite products. Primarily, dispersing CNTs without destroying the superior mechanical and electrical properties, with some notable exceptions (<10%)3-5 , were hampered by the lack of effective method in dispersing large amounts of CMTs. Primarily, it is difficult to disperse nanotubes in water due to the repulsive interactions6-8 between nanotubes and water. Moreover, the large attractive interparticle potential between the nanoscale tubes9,10 caused the easy formation of aligned aggregated tubes. Several methods have been developed to disperse CNTs11-13 previously, nevertheless, the amounts of nanotubes incorporated in the polymer composites are limited (< 10 wt%) due to the preferable aggregation tendency of nanotubes in the polymer solutions.
Introducing conductive materials into insulating hosts14- 16always is an attracting topic because of the scientific and practical importance. The electrical conductivity of nanotubes can reache 106 S m-1 , but polymer matrices usually are poor in conductivity with values between 10-16 and 10-12 S m-1 . Adding small amounts of nanotubes to polymer matrices significantly increased the conductivity17-24 (to 10-6 S m-1 , approximately), which stimulated the practical applications of polymer/carbon nanotube (PCNT) composites in electronics, sensors, and actuators, as well as many others25,26 in the electronics industry. The critical concentration27,28, known as percolation threshold, at which conductive networks start to form is inversely related to the aspect ratio (length per diameter) of the dispersed nanomaterials. Regarding the high aspect ratio of nanotubes, PCNT nanocomposites generally exhibit extremely low (< 0.5 wt%) percolation thresholds29-33. Experimental and theoretical approaches for the percolation thresholds indicated that conductivity od PCNTs was limited by the potential barriers between nanotubes and polymer because the surfaces of the nanotube bundles always were covered by polymer coatings.
Conclusion
Through deliberate molecular engineering, we synthesized a TA-terminated JTA polymer with inherent aromatic phenolic rings and hydroxyl OH groups to promote the desired elasticity. The aromatic phenolic rings of JTA also bind to the surface carbon rings of MWCNTs, thus generating homogeneous JCTx composites with low (e.g. 10 wt%) and high (96 wt%) MWCNT contents.
Tree branches, constituted by inner MWCNT bundles and outer JTA walls, are interconnected with each other to form a 3 D crosslinked network of JCTx composites. With concentration well above percolation threshold, soft JCTx (x from 1 to 40 wt%) exhibited the elastic modulus correlated with theoretical Kolarix model based on All JCTx (x from 1 to 96) composites are conductive materials and the increase of MWCNT content largely promoted the composite conductivity to an ultimate excellent value of 1.2 x 106 S m-1 for the JCT96. The conductivity of the JCTx composites can be appropriately fitted by the scaling law of ∝1.92 according to a 3D network model. The tree branches are responsible for the effective electron transfer and the ideal conductivity behaviors of the JCTx samples.
Dependent on the MWCNT content, the JCTx composite sensors can be highly stretchable or highly conductive. The highly conductive JCT96 is a sensitive sensor with keen response toward minor pulse beating. With good stretchability, the soft JCT40 is a stable sensor for the steady detection of large elbow bending motion. With the lowest MWCNT content, JCT10 is capable of repeatedly detecting a high strain of 220%. Through manipulating composite composition, JCTx sensors with the desired mechanical and conductive properties can be conveniently prepared and characterized. The developed JCTx composites in this study can also be applied to other practical application fields such as supercapacitor or lithium ion battery.