Nanostructured Elastomers: From Smectic Liquid Crystals to Noble Metal Nanocomposites

Open Access
Lentz, Daniel Michael
Graduate Program:
Materials Science and Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
May 06, 2010
Committee Members:
  • Evangelos Manias, Dissertation Advisor
  • Ronald C Hedden, Committee Chair
  • Evangelos Manias, Committee Chair
  • Ralph H Colby, Committee Member
  • Thomas E Mallouk, Committee Member
  • main chain liquid crystalline elastomers
  • noble metal nanoparticles
  • nanocomposites
Noble metal/polymer nanocomposites are a desirable and useful class of material due to their combination of the beneficial processibility and mechanical properties of polymers with the optical, electrical, barrier, and other engineering properties of metal nanoparticles. Potential applications of such materials include non-linear optical materials with gold nanoparticles or conductive polymer substrates with percolated silver nanoparticles. A processing approach has been developed whereby metal nanoparticles, especially silver and gold, can be infused into the surface of a thermoplastic elastomer following the melt processing operation. 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 (Bayer MaterialScience AURA® 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 of 10-500 μm. The metal salt is subsequently reduced to produce zero-valent metal nanoparticles by a second infusion of a reducing agent or a thermal or photochemical reduction process. The growth and agglomeration of the nanoparticles is arrested by the high viscosity of the polymer matrix, producing a stable nanocomposite. In order to examine how nanoparticle size distribution and concentration are affected by soak times in the salt and reducing agent solutions, combinatorial, high-throughput screening methods have been applied. Particle size distributions are characterized rapidly by small-angle x ray scattering (SAXS) using a "dual gradient" nanoinfusion matrix. In addition, an improved method of the nanoinfusion process has been demonstrated that displays significant enhancements to nanoparticle concentration (volume fraction) in thermoplastic polyurethane elastomers (TPUs), as well as decreased average particle size. This latter method involves the creation of an interpenetrating layer of a functionalized monomer via an infusion-polymerization approach. Said functional group is subsequently used to reduce the metal precursor as it is infused using the same processing step as the above method. TEM images show significantly higher volume fraction of nanoparticles using this method, providing the potential for more drastic improvements in optical and other properties. Another material of interest for synthesis of nanocomposites are liquid crystalline elastomers. Liquid crystalline elastomers have received attention for their unique mechanical properties, which underlie their potential for use in applications such as artificial muscles (due to the electroclinic effect) and constrained vibration damping applications (due to their broad peak in tan δ versus either temperature or frequency). An interesting feature observed in the liquid crystalline elastomers produced in our groups is the formation of a neck in the sample under uniaxial tension. The mechanical response of these smectic main-chain liquid crystalline elastomers (MCLCE) has been characterized at a variety of strain rates and temperatures in order to understand the cause of the observed neck formation. A well-defined yield stress is observed at a critical strain that is essentially independent of strain rate, followed by necking and cold-drawing. Cold-drawing is rarely observed in liquid crystalline elastomers, but we believe that it is observed in MCLCE due to the unfolding of hairpin chains at the start of the polydomain to monodomain transition. A neck forms as the hairpins straighten out, resulting in a decreased cross-sectional area that promotes yielding. Infusions of both metal nanoparticles and dichroic dyes were attempted.