Thermomechanics of nano-filled elastomers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.Includes bibliographical references.The incorporation of nanoparticles into engineering thermoplastic elastomers affords engineers an opportunity to formulate flexible, tough and multifunctional polymer nanocomposites that potentially rival the most advanced materials in nature. Development of these materials is difficult since thermodynamic and kinetic barriers inhibit the dispersal of inorganic, hydrophilic nanoparticles into inherently hydrophobic polymer matrices. Thermoplastic polyurethanes (TPUs) are particularly attractive nanocomposite matrix materials due to their vast range of potential applications (e.g. in artificial organs, coatings, foams, and active wear), their mechanical versatility, and tunable block-polymeric structure. In this thesis we explore methods for systematically nanoreinforcing such materials by exploiting the microphase structure, differential polarities and multiple thermomechanical phase transitions of the macromolecular blocks that constitute the elastomeric matrix. Using a solvent exchange technique we show that it is possible to preferentially nanoreinforce the hard micro-domains of thermoplastic elastomers with smectic clay nanofillers that have characteristic dimensions similar to the hard segment. The adhesion between the clay and the hard micro-domains coupled with the formation of a percolative network not only stiffens and toughens, but increases the heat distortion temperature (HDT) of the material. The discotic clay platelets induce morphological ordering over a range of length scales that results in significant thermomechanical enhancement and expands high temperature applications. This thesis seeks to further enhance the understanding and utility of thermoplastic polyurethane nanocomposites by answering two questions: (1) what thermo-physical interactions between nano-clay and elastomeric thermoplastic polyurethane are taking place? and (2) how can these thermo-physical interactions be exploited?(cont.) To answer these questions the nano-reinforced-hard micro-domain morphology was monitored during deformation using in-situ wide angle x-ray scattering and combined with the results of extensive quasi-static mechanical testing which enabled the identification two characteristic relaxation times. A one-dimensional constitutive model to account for such morphological changes augmenting the previous model for unfilled polyurethanes developed by Qi and Boyce (2005) is discussed. Finally, the thermo-mechanical influence of nano-clay fillers on the shape memory effects exhibited by polyurethane nanocomposites is examined and multi-responsive shape memory polyurethane fibrous mats are developed via electrospinning. Quantifying and controlling the thermo-physical interactions between a block-copolymer with polar segments (e.g. thermoplastic polyurethane) and inorganic nanoparticles (e.g. nano-clay) is important for future nanocomposite processing strategies: the efficacy of nanoreinforcement hinges upon the close matching of characteristic length scale and the adhesion of the nanoparticles to the targeted polymer phase morphology. Exploiting the different polarity of the blocks in conjunction with solvent exchange approach developed in this thesis and solution processing techniques such as electro-spinning, offers an avenue toward the development of high performance, hierarchically-ordered materials that rival natural materials.by Shawna M. Liff.Ph.D

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