• PDF / 1,588,595 Bytes
• 5 Pages / 612 x 792 pts (letter) Page_size
• 21 Downloads / 149 Views

DOWNLOAD

REPORT

---

0922-U06-02

Tailoring of Carbon Nanotube Microstructure Using Poly(acrylic acid) and Poly(allylamine hydrochloride) Lei Liu, and Jaime C. Grunlan Mechanical Engineering, Polymer Technology Center, Texas A&M University, 3123 TAMU, College Station, Texas, 77843-3123 ABSTRACT Poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) were used to tailor the microstructure of single-walled carbon nanotubes (SWNTs) in both aqueous solutions and dry composite films. Microstructural changes were achieved as the pH of aqueous suspensions were changed, as evidenced by elevated suspension viscosity, scanning electron microscopy and electrical conductivity measurements. The ability to tailor microstructure is useful for aqueous SWNT processing and solid polymer nanocomposites. INTRODUCTION Single-walled carbon nanotubes (SWNTs) have the potential to significantly enhance polymer electrical conductivity [1], thermal conductivity [2] and mechanical properties [3]. However, the insoluble nature of SWNTs hinders their exfoliation and utilization, especially in water [4]. For this reason, both chemical [5-7] and physical modifications [8-10] are widely used to stabilize SWNTs. With stabilization, SWNT suspensions can be used as biocompatible transporters [11] or drug and gene delivery systems [12]. While numerous methods of nanotube stabilization have been examined, see [6-11] as examples, little attention has been paid to the interaction between stimuli-responsive polymers and SWNTs. The present study takes advantage of the relationship between pH, degree of ionization and conformational changes of PAA and PAH [13] that allow for controlled SWNT exfoliation and stabilization. EXPERIMENTAL METHODS Raw single-walled carbon nanotubes produced by the HiPco process were provided by Carbon Nanotechnologies (Houston, TX) and contain 27wt% residual impurity. To stabilize SWNTs in water, 1wt% aqueous solution of PAA (Mw 100,000; Aldrich) and PAH (Mw 70,000; Aldrich) were made and stored for two days before using. Next, an appropriate amount of SWNTs were introduced and the suspension was sonicated with a VirTis Virsonic 100 ultrasonic cell disrupter (SP industries, Warminster, PA) for 10 minutes at 50W. The pH of the resultant suspension was adjusted with 10M NaOH or 10M HCl followed by another 10 minute sonication period. Viscosities were recorded at room temperature with a Brookfield cone/plate LVDV-III Viscometer (Brookfield, Middleboro, MA). Solid composite films were made by drying the SWNT suspensions in plastic molds under ambient conditions. Sheet resistance of the composite films was measured with a home-built four-point probe apparatus with an Agilent E3644A DC power supply (Agilent Technologies, Inc., Englewood, CO), Keithley Model 2000 digital multimeter (Keithley Instruments, Inc., Cleveland, OH), and an SP4-62180TRS four-point probe head (Lucas Labs, Gilroy, CA). Scanning electron microscopy was performed with a Zeiss 1530 VP FE-SEM (Carl Zeiss, Inc., Thornwood, NY).

RESULTS AND DISCUSSION The viscosity-pH relations