Title

CONTROLLING THE MORPHOLOGY OF POLYMER BLENDS USING THE NONLINEAR OPTICAL PROPERTIES OF LIGHT

Date of Award

May 2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Ian D. Hosein

Keywords

Morphology, Nonlinear Optics, Phase Separation, Polymer Blends

Subject Categories

Engineering

Abstract

Nonlinear optics and polymer systems are distinct fields that have been studied for decades. These two fields intersect with the observation of nonlinear wave propagation in photoreactive polymer systems. Combining nonlinear pattern formation mechanisms to polymer systems is attractive for directing materials synthesis. This has led to studies on the nonlinear dynamics of transmitted light in polymer media, particularly for optical self-trapping and optical modulation instability. The irreversibility of polymerization leads to permanent capture of nonlinear optical patterns in the polymer structure, which is a new synthetic route to complex structured soft materials. This dissertation discusses the work to date on nonlinear optical pattern formation processes in polymers. A brief overview of the nonlinear optical phenomenon is provided to set the stage for understanding their effects. We review the accomplishments of the field on studying nonlinear waveform propagation in photopolymerizable systems, then discuss our most recent progress in coupling nonlinear optical pattern formation to polymer blends and phase separation.

We demonstrate how morphology evolution and phase separation in polymer blends can be controlled through irradiation with arrays of self-trapping of optical beams. We studied the effects of different weight fractions and exposure intensities on morphology evolution during photopolymerization. An in situ microscopy experiments were conducted to elucidate the nature of the phase separation behavior and morphology evolution in a blended system. In situ confocal Raman measurements of polymer conversion were done to obtain better understating of the kinetics and formation dynamics associated with phase separation induced by self-trapped light. Control over morphology depends strongly on the competitive processes of phase separation and photo-crosslinking. Our findings demonstrate a fundamentally new approach to the patterning of polymer blends, which is important for controlling their critical physical phenomenon and establishing advanced structure-property relationships.

The fabrication of a new type of solar cell encapsulation architecture comprising a periodic array of step-index waveguides is reported. The materials are fabricated through patterning with light in a photoreactive binary blend of crosslinking acrylate and urethane, wherein phase separation induces the spontaneous, directed formation of broadband, cylindrical waveguides. This microstructured material efficiently collects and transmits optical energy over a wide range of entry angles. Silicon solar cells comprising this encapsulation architecture show greater total external quantum efficiencies and enhanced wide-angle light capture and conversion. This is a rapid, straightforward, and scalable approach to process light-collecting structures, whereby significant increases in cell performance may be achieved.

We present a new approach to synthesize microporous surfaces through the combination of photopolymerization-induced phase separation and light pattern formation in photopolymer-solvent mixtures. The mixtures are irradiated with a wide-area light pattern consisting of high and low-intensity regions. This light pattern undergoes self-focusing and filamentation, thereby preserving its spatial profile through the mixture. Over the course of irradiation, the mixture undergoes phase separation, with the polymer and solvent located in the bright and dark regions of the light profile, respectively, to produce a binary phase morphology with a congruent arrangement as the optical pattern. A congruently-arranged microporous structure is attained upon solvent removal. The microporous surface structure can be varied by changing the irradiating light profile via photomask design. The porous architecture can be further tuned through the relative weight fractions of photopolymer and solvent in the mixture, resulting in porosities ranging from those with discrete and uniform pore sizes to hierarchical pore distributions. All surfaces become superhydrophobic (water contact angles > 150) when spray-coated with a thin layer of polytetrafluoroethylene nanoparticles. The water contact angles can be enhanced by changing the surface porosity via the processing conditions. This is a scalable and tunable approach to precisely control microporous surface structure in thin-films to create functional surfaces and anti-wetting coatings.

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