Silver-Polyimide Nanocomposite Films Yielding Highly Reflective Surfaces
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Silver-Polyimide Nanocomposite Films Yielding Highly Reflective Surfaces Robin E. Southward,1 C. J. Dean,2 J. L. Scott,2 S. T. Broadwater,2 and D. W. Thompson2 1 Structure and Materials Competency, NASA, Langley Research Center, Hampton, VA 23681 2 Department of Chemistry, College of William and Mary, Williamsburg, VA 23187 ABSTRACT Highly reflective surface-metallized flexible polyimide films have been prepared by the incorporation of the soluble silver ion complex (1,1,1-trifluoroacetylacetonato)silver(I) into dimethylacetamide solutions of the poly(amic acid) prepared from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF). Thermal curing of solution cast silver(I)-poly(amic acid) films leads to cycloimidization of the amic acid with concommitant silver(I) reduction and formation of a reflective surface-silvered film at 8 and 13 weight percent silver. The metallized films are thermally stable and flexible with mechanical properties similar to those of the parent polyimide. TEM reveals that the bulk (interior) of the polyimide composite films have 5-20 nanometer-sized silver particles with a surface layer of silver metal ca. 80 nm thick. Neither the bulk nor the surface of the films is electrically conductive. Adhesion of the surface metal to polyimide is excellent. INTRODUCTION The fabrication of specularly reflective and electrically conductive surface metallized polyimide films is of enormous interest as reviewed by Matienzo and Unertl [1]. Applications are numerous including: anti-infective coatings, contacts and circuit lines in microelectronics, enhancement of thermal conductivity, flexible surface conductive tapes, patternable conductive surfaces on dielectric bases, the terrestrial concentration of solar radiation for power generation, and gas permeability barriers. Of particular interest to us are space applications of metallized polyimides. These include highly reflective thin film reflectors and concentrators in space environments for solar thermal propulsion [2] and solar dynamic power generation [3,4], reflectors for flat panel solar power arrays for satellites [5], large scale radiofrequency antennas for the management of EM signals [6], solar sails [7,8], and sunshields to control device temperatures for projects such as the Next Generation Space Telescope. Polymeric supports offer advantages in weight, flexibility, elasticity, fragility, and deployability relative to inorganic supports such as glass and ceramics. Southward et al. [9] and Taylor et al. [10] have have been successful in preparing surfacemetallized, in particular silver-metallized, polyimide films by a novel single-stage, internal metallization technique which leads to flexible films with excellent specular reflectivity and/or electrical conductivity. “Single-stage” denotes the fabrication in one step of metallized films from a homogeneous solution of a positive valent metal precursor and a poly(amic acid). In contrast to traditional metallized film preparat
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