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Functional nanocomposites and dispersions: Synthesis, characterization and performance evaluation

Tiwari, MK; (2009) Functional nanocomposites and dispersions: Synthesis, characterization and performance evaluation. Doctoral thesis (Ph.D), UNSPECIFIED.

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Abstract

Nanocomposites and nanoparticle dispersions offer opportunities to develop advanced materials with tunable properties that are unattainable through any single material. Identifying strategies to synthesize nanoparticle dispersions with desired rheological properties as well as their use to fabricate nanocomposites with well-controlled functionalities such as liquid repellency, mechanical durability, electrical conductivity is of great importance in a variety of existing and potential applications. With this broad outlook, the following topics have been investigated in this thesis. Synthesis of flexible coatings with extremely high liquid repellency (e.g. superhydrophobicity when the liquid is water) with high adhesion to substrate remains a major technological challenge. In addition, imparting additional functionality such as electrical conductivity and thermal stability, as well as being able to apply (fabricate) such coatings on substrates using techniques that can be potentially scaled up to large area applications, are critical. The current thesis investigates wet-processed polymer nanocomposites (consisting of polymer(s) and filler particles) to obtain coatings with these qualities. A new approach to such nanocomposites is presented wherein polymer blends are used as dispersant cum binder for particle fillers. Judicious choice of polymer blends as binder for nanocomposites helps achieve the ambitious goal of good particle dispersion, high adhesion, and low surface energy which when coupled with texture control through incorporation of filler particles, lead to highly liquid repellent coatings with versatile functionalities. Blends of poly(vinylidene fluoride) (PVDF) with acrylic polymers are prepared as binder for particle dispersions used to produce the liquid repellent coatings. To this end, a novel blend of PVDF with ethyl 2-cyanoacrylate (ECA) is introduced through control and inhibition of rapid polymerization of the ECA monomer. The PVDF and polymerized ECA (PECA) blends are analyzed chemically and thermally. The solution blends of PVDF/PECA used to prepare nanocomposite coatings using potentially scalable spray atomization process. The wettability of the liquid repellent coatings is tested with water and water/alcohol mixtures. The effect of filler particle surface energy on liquid repellency is tested and interpreted in the framework of existing wetting models from the literature. In addition, conductive superhydrophobic coatings with applications to electromagnetic interference (EMI) shielding are also demonstrated using a combination of conductive carbon nanofibers (CNFs) and poly(tetrafluoroethylene) (PTFE) filler particles dispersed with PVDF and poly(methyl-methacrylate) (PMMA) blends. Another important technological challenge addressed by the current thesis is to demonstrate proof-of-concept of polymer nanocomposites as lightweight, compliant (flexible) and permeable strain sensors that can be easily integrated into load bearing systems for preventing failure. The adopted approach involves preparing fibrous nanocomposites using electrospinning. It is hypothesized that the electrical resistance a conductive fibrous nanocomposite mat should increase when the mat is subjected to a mechanical deformation (strain) due to joint change geometry of the mat as well as percolation network structure of filler particles inside the mat. The validity of this hypothesis is demonstrated using electrospun fibrous nanocomposites consisting of poly e-caprolactone (PCL) polymer and electrically conductive carbon black (CB) filler nanoparticles. The results provide proof-of-concept for fibrous nanocomposites as strain sensor through measurements of strain/resistance variation in uniaxial stretching. A novel layered percolation model accounting for both changes in sensor geometry as well as percolation structure under mechanical strain is formulated to interpret the strain/resistance relationships in conductive nanocomposites. As an additional advantage, the percolation threshold for conductivity of electrospun nanocomposites is experimentally determined to be well below the value predicted from random percolation models. A theoretical framework for the application of such strain sensors to report local-clogging in large filter assemblies is also presented. Proper determination of rheological (fluid) properties of suspensions with large aspect ratio nanoparticles such as carbon nanotubes (CNT) is vital to their applications such as for designing field-responsive fluids, in printed electronics, in preparation of any nanocomposites, etc. Flows in such applications are invariably predominantly elongational in nature, where the rheological properties can be significantly different from simple or oscillatory shear mode measurements used thus far in the literature for such fluids. In the current work, a capillary self-thinning filament rheometer is used to determine the elongational properties of such suspensions. A quasi-1D model is developed to determine the elongational properties of suspension from high-speed video recorded self-thinning of a suspension filament. The effects of CNT concentration, aspect ratio, filament diameter and suspending liquid viscosity are carefully analyzed. The elongational rheological parameters are compared with those obtained from shear mode measurements as well. Both elongational and shear mode measurements indicate the rheology of CNT suspensions to be well represented by the Herschel-Bulkley model of liquids (i.e. power law fluids with yield stress). Our measurements also indicate flow dependent yield-stress for such suspensions, alluding to care that must be adopted in using the properties of such fluids. The present thesis demonstrates the potential and promise of nanocomposites and nanoparticle dispersions for advancing many practical technologies requiring tunable material properties.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Functional nanocomposites and dispersions: Synthesis, characterization and performance evaluation
Event: University of Illinois at Chicago
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering
URI: https://discovery.ucl.ac.uk/id/eprint/1418896
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