Deformation–morphology correlations in electrically conductive carbon nanotube—thermoplastic polyurethane nanocomposites

Abstract

Addition of small amounts (0.5–10 vol%) of multiwall carbon nanotubes (CNT) to thermoplastic elastomer Morthane produced polymer nanocomposites with high electrical conductivity (σ∼1–10 S/cm), low electrical percolation (ϕ∼0.005) and enhancement of mechanical properties including increased modulus and yield stress without loss of the ability to stretch the elastomer above 1000% before rupture. In situ X-ray scattering during deformation indicated that these mechanical enhancements arise not only from the CNTs, but also from their impact on soft-segment crystallization. The deformation behavior after yielding of the nanocomposites, irrespective of CNT concentration, is similar to the unfilled elastomer, implying that the mechanistics of large deformation is mainly governed by the matrix. The relative enhancement of the Young's modulus of the nanocomposites is comparable to other elastomeric nanocomposites, implying that to the first order specific chemical details of the elastomeric system is unimportant.

Introduction

Elastomers with high electrical conductivity are critical for applications ranging from seals between pipes used for transferring flammable gases, electrostatic automotive painting and electromagnetic shielding for mobile electronics. Traditionally, conductive fillers, such as carbon black, chopped carbon fiber or metallic flakes are used. Conductivity is established by percolative network formation of the fillers and limited by carrier transport (hopping or tunneling) between filler particles. Thus the extent of filler dispersion, aspect ratio of the filler, and wettability of the filler by the elastomeric medium are key morphological characteristics in determining the conductivity of the system. The relatively large volume fractions (>20 vol%) necessary though, negatively impact deformability, processibility, surface finish, and limit the ability to maintain desired conductivity at extreme deformations (>100%). Furthermore, the use of metallic fillers results in galvanic corrosion issues in many service environments.

The broad availability of nanoscale multiwall carbon nanotubes with large aspect ratios (>100) and high electrical conductivity (σ∼18,000 S/cm along tube axis) have lead to a resurgence of applied and fundamental investigations of filled polymers, driven by the potential to address limitations of classic conductive fillers. Whether tubes (e.g. single and multiwall carbon nanotubes) or plates (e.g. exfoliated graphite), the nanoscopic dimensions and high aspect ratios inherent in these polymer nanocomposites result in six interrelated characteristics distinguishing them from classic fillers: (1) low percolation threshold (∼0.1–2 vol%); (2) particle-particle correlation (orientation and position) arising at low volume fractions (ϕ_C<10⁻³); (3) large number density of particles per particle volume (10⁶–10⁸ particles/μm³); (4) extensive interfacial area per volume of particles (10³–10⁴ m²/ml); (5) short interparticle spacing (10–50 nm at ϕ∼1–8 vol%); and (6) comparable size scales between the rigid nanoparticle inclusion, distance between particles, and the relaxation volume of the polymer matrix. These characteristics provide materials with properties of traditional filled systems, but at lower loadings and thus enhanced processibility. Additionally, novel properties absent in traditional filled systems are reported. Recent reviews of materials fabrication and fundamental structure-property correlations can be found in the literature [1]. Much of the current effort in polymer nanocomposites though is plagued by uncertainties as to what is possible, necessitating substantially more detailed structure-processing-property investigations.

Carbon nanotube filled plastics are currently under intensive investigation for enhancement of structural [2], [3], stress-recovery [4], electrical [5], [6] and thermal performance [7], [8], while maintaining the inherent processibility and deformability of the matrix resin. Optimization of electrical characteristics presents an interesting dilemma. Maximum dispersion of the nanotubes is desired to provide the lowest percolation threshold; however, this necessitates favorable polymer–tube interactions, which will result in a strongly bound polymer interfacial layer. Conversely, this will establish a minimum tube–tube distance, which ultimately determines electrical conductivity by limiting carrier transport and creating capacitive contacts between tubes. In addition, for conductive elastomers maximization of deformability implies minimizing the strength of tube–polymer interactions. One possibility to address these conflicting requirements is to utilize a nanotube with a heterogeneous surface, which spatially segregates strong tube–tube interactions favoring aggregation from maximum tube–polymer interactions favoring dispersion.

Following these suppositions, aspects of the deformation-morphology correlation of an electrically conductive, elastomeric nanocomposite are presented. Uniaxial elongation in combination with in situ X-ray studies indicate that nanotube orientation and polymer deformation are coupled, altering the strain induced soft-segment crystallinity and the mechanical response of the polyurethane at increasing strain. The complex interrelation implies that the extensive characterization discussed herein is necessary, but still not sufficient to effectively establish deformation-morphology relationships for nanotube containing elastomers.