(110c) Polymer Nanoinfusion Processing: Nanocomposites of Silver and Gold in Thermoplastic Polyurethanes

Colorants, UV stabilizers, antioxidants, and nano-fillers are examples of additives that are traditionally blended with thermoplastic polymer resins prior to melt-state shaping operations such as profile extrusion or injection molding. However, the processing of polymer-metal nanocomposites presents a challenge due to the tendency of metal nanoparticles to agglomerate during melt processing at typical shear rates. In addition, metal nanoparticles can exhibit surprisingly low melting points compared to their respective bulk metals, complicating the processing of nanoparticles at elevated temperatures. Although much effort has been directed at functionalized nanoparticle synthesis to approach these issues, custom-synthesized nanoparticles are not necessarily economically attractive, limiting commercial opportunities. Recently, we demonstrated a novel processing approach whereby metal nanoparticles, especially silver and gold, can be infused into the surface of a thermoplastic or thermoset article following the melt processing operation.¹ This reaction-diffusion approach (nanoinfusion) allows metal nanoparticles to be introduced at relatively low cost while avoiding the issues of thermal degradation, microphase separation, or agglomeration that can occur at elevated temperatures in the melt state.

The nanoinfusion process involves immersion of a molded, cast, or extruded plastic article in an aqueous plasticizer solution (AURA® Color Infusion Technology) containing a metal salt such as HAuCl₄ or AgNO₃. Infusion of the metal salt into the plastic surface is achieved well below the melt-processing temperature due to plasticization of a thin surface layer. The metal salt is generally able to penetrate up to 30 μm into the surface of the article within a few minutes. The metal salt is subsequently reduced to produce zero-valent metal nanoparticles, either by spontaneous reaction with functional groups in the plastic, or by a second infusion of a reducing agent. In either case, the growth and agglomeration of the nanoparticles is arrested by the high viscosity of the polymer matrix, producing a stable nanocomposite.

Earlier work demonstrated rapid infusion of Au and Ag nanoparticles into thermoplastic polyurethanes and a coated acrylic.¹ Combinatorial, high-throughput screening methods have subsequently been applied to examine how nanoparticle size distribution and concentration are affected by soak times in the salt and reducing agent solutions. Particle size distributions are characterized rapidly by small-angle X-ray scattering (SAXS) using a "dual gradient" nanoinfusion matrix. In addition, recent work has demonstrated significant enhancements in nanoparticle concentration (volume fraction) in thermoplastic polyurethanes that contain a reactive interpenetrating network (IPN) at the surface. Patterned nanoinfusions of both gold and silver nanoparticles are possible in thermoplastic polyurethanes by introducing a photopatterned IPN prior to infusion of the metal. Infusion of metal nanoparticles into elastomeric IPNs creates nanocomposites with high volume fractions of metal near the surface, potentially leading to highly flexible plastics with electrical conductivity above the percolation threshold.

References

1. "Surface Infusion of Gold Nanoparticles into Processed Thermoplastics." Lentz, D.M.; Pyles, R.A.; and Hedden, R.C. Polym. Eng. Sci. 2009 (In Press).