Preferential nanoreinforcement of thermoplastic polyurethane elastomers with dispersed nano-clay

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (p. 94-99).It is difficult for scientists to engineer elastomeric materials that are both strong and tough like spider dragline silk. Inspired by the morphology of spider dragline silk and motivated to develop strong, tough, elastomeric polyurethanes to be used in soldier applications I have prepared polyurethane/clay nanocomposites. Polymer/clay nanocomposites have exhibited great potential for providing enhanced and possibly-tunable thermomechanical behavior. However, the biggest challenge facing advances in polymer/clay nanocomposites is the complete dispersion of nano- clay within the polymer matrix due to thermodynamic and kinetic limitations. A novel solvent exchange method to fully exfoliate and disperse discotic smectic clay, Laponite (diameter = 25 nm, thickness = 1 nm), in three thermoplastic polyurethane elastomers (TPUs) -- Elasthane 80A, HDI-PTMO PU, and PU-1-33 -- has been developed. This clay was selected because the diameter of one platelet is similar to the lateral dimension of a single hard-domain in block-polymeric TPU. WAXD, TEM, and AFM phase imaging of cast films following solvent exchange show that the nano-clay is well dispersed in the TPUs. Uniaxial mechanical testing showed that as much as a 23-fold increase in elastic modulus, 100% increase in toughness, and 50% increase in strength can be achieved without a reduction in extensibility when Laponite is added to Elasthane. Furthermore, the heat distortion temperature of the Elasthane can be increased from 101⁰C to more than 200⁰C, as measured by DMA, when 20 wt% Laponite is added.(cont.) The HDI-PTMO PU/Laponite nanocomposites behave like the Elasthane/Laponite nanocomposites, exhibiting an increase in elastic modulus, strength, and toughness without a loss in extensibility. In contrast, a PU-1-33 thin film exhibits a significant decrease in extensibility, strength, and toughness with no significant change in elastic modulus when filled with Laponite. Characterization shows that the Laponite is preferentially embedded within the polar hard domains of the Elasthane and HDI-PTMO PU and embedded within the soft domain of PU-1-33. The Laponite is attracted to the polar, hydrophilic soft segment constituent, polyethylene oxide, in PU-1-33. Ultimately, Laponite can be used to strengthen and toughen TPUs and the location of Laponite reinforcement can be altered by adjusting the polarity and hydrophilicity of the soft segment.by Shawna M. Liff.S.M