THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING
GEORGIA INSTITUTE OF TECHNOLOGY
Under the provisions of the regulations for the degree
DOCTOR OF PHILOSOPHY
on Thursday, December 5, 2019
in MRDC 4404
will be held the
"Multiscale Modeling of Multicompartment Micelles: Influence of Triblock and Tetrablock Polymer Architecture on Morphological Variation"
Prof. Seung Soon Jang, Advisor, MSE
Prof. Zhiqun Lin, MSE
Prof. Paul Russo, MSE
Prof. Donggang Yao, MSE
Prof. Christopher Jones, ChBE
Efficient reaction design forms an important foundation of many processes in modern chemistry. Reaction optimization has far-reaching effects that greatly improve many other facets of polymer manufacturing, pharmaceutical production, and related industries. In particular, a field of growing interest during the past century is that of immobilized molecular catalysis. This topic holds great potential due to its combination of the best strengths of both homogeneous and heterogeneous catalysis. By allowing for high selectivity and reaction rates traditionally achieved by homogeneous catalysis while still yielding the excellent separability offered by heterogeneous catalysis, this field presents an opportunity to leverage the advantages of both techniques. Despite the strengths of immobilized molecular catalysis, however, systems containing multiple tandem non-orthogonal reactions (i.e., reactions which have the potential for mutual interference) still encounter difficulties. In extreme cases, a particular step of the multistep reaction may even be incompatible with another species present in the system; in such a case, the catalyzing agent could suffer drastically reduced efficacy or cease to function altogether.
A potential solution to these obstacles arises in the field of multicompartment micelles. These systems offer separate molecular “chambers” in which each of the non-orthogonal reactions can take place, allowing for one-pot synthesis and tandem catalysis. The multicompartment micelle (MCM) is simply an extension of the traditional micelle. It is well known that a typical micelle is generally composed of amphiphilic polymers; MCMs, then, are composed of polymers of three or more distinct portions; a common example results from triblock copolymers containing hydrophilic, lipophilic, and fluorophilic (HLF) blocks. For a proper choice of solvent, solutions of these polymers thus self-assemble into micellar structures containing three or more regions of microphase separation. By introducing catalysts into MCM-containing systems, it is possible to create a micelle nanoreactor with distinct catalytic regions within the structure that support simultaneous non-orthogonal reactions in the same chamber while still achieving high reaction rates and easy separability. The micelle nanoreactor (MNR) thus presents an elegant solution to many of the challenges facing immobilized molecular catalysis science.
It is natural to expect that the particular morphology of the micelles formed by a given polymer will in turn affect their utility in MNR applications. By extension, the particular architecture of the polymers selected for the formation of MCMs will have a marked effect on the performance of the resultant MNR system. For example, even if the species which define the different regions of solvophilicity of the polymer are unchanged, variations in the sequence, lengths, and length ratios of the respective blocks can lead to significant morphological changes in the resultant MCMs. Such changes can then lead to diminished catalyst effectiveness (e.g., due to decreased extent of compartmentalization) or less desirable reactant and product transport (leading to reduced reaction rates). Therefore, proper design of MCM systems for use in MNR applications requires complete understanding of how to control the polymer architecture and, consequently, the micelle structure. A systematic study of the effects of the relevant variables is, however, made difficult in experiment due to the time-consuming preparation and reactions involved. Computational techniques offer a more economical avenue for the study of large systems such as these, as they allow for direct analysis of the MCM structure without the need for structural synthesis.
Considering all of these factors, this thesis aims to study the effects of polymer architecture and composition on the structure of the resulting self-assembled MCMs. The foundation for the principal focus of this study lie in the development of a robust methodology for determining the miscibility of polymer species. From here, mesoscale simulation studies uncover the relationships between polymer architecture in triblock and tetrablock copolymer systems of various block sequences and the corresponding micelle morphology in aqueous conditions. Simulations of micelle self-assembly conducted via dissipative particle dynamics are used to examine these relationships and establish trends of behavior.