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Date of Award

2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering–Engineering Mechanics (PhD)

College, School or Department Name

Department of Mechanical Engineering-Engineering Mechanics

First Advisor

Dennis Desheng Meng

Co-Advisor

Qingli Dai

Abstract

Encapsulation is a key enabling technology of self-healing materials for which incorporation of reactive materials into a composite, without loss of functionality, is required for damage repair. The functionalized particles resulting from such processes must be readily incorporable into a composite and have minimal detrimental impact on its undamaged properties. At the same time, their morphology must preferentially promote the release of their content during a damage event. However, there is still a need for new techniques capable of fine tuning particle properties for the controlled design of composite performance. To introduce superior processing control, two microfluidics based encapsulation processes have been developed, one each for the individual components of a two-part chemical healing system, namely dicyclopentadiene and Grubb's catalyst. These processes have enabled significantly enhanced performance of self-healing epoxy composites by introducing unprecedented control over particle morphology.

The microfluidics based encapsulation platform is first demonstrated by emulsification, using droplet microfluidics, and subsequent encapsulation of dicyclopentadiene. The reported approach allows for facile control of mean microcapsule diameter thru variation of fluid flow rates. The microcapsules exhibit coefficients of variation (CV) of diameter in the range 1-3 (i.e. monodisperse is typically defined as CV smaller than 5), an order of magnitude reduction when compared with conventional batch emulsification methods whose typical CV is 20-40. This control over microcapsule uniformity has led to significant improvement in self-healing composite performance as exemplified by ~25% higher undamaged fracture toughness. A microfluidic solution spinning process is then developed to encapsulate Grubb's catalyst, the most expensive component of this particular material system, in a novel fibrous morphology. The continuous, on-chip fiber production allows for controllable fabrication of uniform diameters with CV in the range 6-10 and the solution processing allows for control of catalyst content. The microfibers exhibit several advantageous characteristics such as uniform catalyst loading, sub-micron catalyst particle size, and amorphous catalyst structure. As a result, the fibers enable faster gelation times during polymerization of dicyclopentadiene. The microfluidic solution spinning process enables dramatically improved catalyst utilization in self-healing epoxy composites. As compared with their traditionally spherical analogue, the microfibers speed the rate of healing by 25% and increase the overall healed strength by 75%.

The impacts of particle size distribution on healing performance and the use of fibrous particles are then investigated theoretically through development of two separate analytical models. Both models relate known composite design parameters such as the weight fraction and dimensions of the particles to the respective healing agent delivery modes for solid and liquid phase healing agent. These are the number of particles exposed on a crack surface and the mass of healing agent released into the damage volume. These models provide a theoretical frame work for the controlled design and optimization of composite healing performance through development of novel encapsulation processes.

Throughout this work, evaluation of composite properties and healing performance was experimentally performed using a tapered double cantilever beam specimen. Quantification of such composite performance metrics requires careful calibration of this fracture specimen to determine geometry dependent parameters used for calculations. A method is developed here, for this purpose, using finite element analysis of 3D specimen models and through experimental calibration. The development of this method helps to elucidate the influence of side grooving on the parameters and to align the method of evaluating self-healing material performance with previously developed theory.

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