Tailoring Properties of Carbon Nanotube Dispersions and Nanocomposites Using Temperature-Responsive Copolymers of Pyrene-Modified Poly(N-cyclopropylacrylamide)

Abstract

Despite their immense potential, the ability to control the dispersion and microstructure of carbon nanotubes remains a hurdle for their widespread use. Stimuli-responsive polymers show conformational changes with an applied external stimulus (pH, temperature, light, etc.). The dispersion of carbon nanotubes by thermoresponsive polymers is shown to enable the macroscopic properties of aqueous suspensions to be tailored as a function of temperature. This work presents the synthesis, characterization, and use of temperature-responsive poly(N-cyclopropylacrylamide) (PNCPA) polymers containing 1, 3, and 5 mol % pyrene-bearing repeat units to tailor the dispersion state of single-walled carbon nanotubes (SWNT) in water. Turbidity measurements show that the lower critical solution temperature (LCST) of PNCPA decreases with increasing pyrene content. Viscosity measurements on aqueous SWNT suspensions stabilized with pyrene-functionalized PNCPA show highly entangled and well-dispersed nanotube microstructure above and below the LCST of the polymer, respectively. UV−vis spectra also confirm that nanotube stabilization by these polymers is dependent upon the pyrene content. Drying of these suspensions produces composites whose microstructure and electrical conductivity vary with drying temperature and pyrene content of the stabilizing polymer. This temperature-dependent dispersion behavior has significant implications for the processing of carbon nanotubes and tailoring of composite properties. Such stimuli-controlled dispersion of carbon nanotubes could have a variety of applications in nanoelectronics, sensing, and drug and gene delivery systems.