Date of Award

Spring 2021

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

First Advisor

John JT Tsavalas

Second Advisor

Wei WG Gao

Third Advisor

Arthur AG Greenberg

Abstract

Given the rising of environmental concern, the market of waterborne coating and adhesives has been constantly growing. Polymer latex particles produced by emulsion polymerization are one type of important components in their formulations. Thus, to tune mechanical properties of waterborne coatings and adhesives, one of the common ways is to utilize crosslinkers to form a macromolecular network structure, as known as “gel”, inside of the polymer latex. A major challenge to obtain desired properties of the material is how to control macromolecular architecture, such as sol-gel ratio and crosslinking density. Hence, this dissertation focuses on the study of predicting and controlling macromolecular network architectures in both bulk polymerization and emulsion polymerization. Bulk polymerizations share some similar mechanisms with emulsion polymerization but is simplified by being a homogeneous single phase reaction environment. Emulsion polymerization is a heterogeneous reaction environment with reactions in two phases. Due to the complexity of the emulsion polymerization environment, bulk polymerization is often used as a first test environment to simplify and study new reaction systems. Based on our previous work, a reduced reactivity parameter, Ψ, was introduced and applied to precisely describe the reactivity of crosslinking sites (pendent vinyl groups from crosslinker monomers). Most of our prior work used the bulk polymerization environment and here we now also extend to the emulsion polymerization environment. In addition to crosslinking reaction kinetics, we also explored macromolecular structure development as predicted by a hybrid Monte Carlo Model. Chapter 3 and Chapter 4 significantly extended our previous work. Chapter 3 explored a wide matrix of monomer-crosslinker pairs of varied molecular size and revealed a stronger understanding of the relationship between resulting Ψ values and the structures of each monomer-crosslinker pair. In Chapter 4, another mechanistic factor, reactivity ratio between the crosslinker and the main monomer, was also introduced. This chapter studied how both contributions, Ψ and reactivity ratio, influence the eventual crosslinking reaction. All polymerizations were conducted via bulk polymerization in Chapter 3 and Chapter 4. Emulsion polymerization is a heterophase polymerization and their polymers are known as “products by process”. Here, we fed the monomer mixture to the reaction in a semi-batch mode, and therefore, Chapter 5 mainly studied the influence of the monomer feeding profile on gel formation during emulsion polymerization. A relationship was observed that higher feeding rate can cause higher gel content. This was found to be explained by the dominance of a “micro loop” formation, an intramolecular first order reaction between the chain end radical and its own pendent vinyl group, when the unreacted free monomer concentration was very low (i.e. with slow monomer feed rate to the reactor). These microloops consumed pendent vinyls without allowing a crosslinking event to occur. At higher monomer feed rate, the mechanisms would start to favor the Ψ and reactivity ratio effects again. This was an important learning on the balance between favored mechanisms as a function of the polymerization process and environment. Chapter 6 is a side but still relevant topic. In this chapter, four different bio-based reactive surfactants were successfully synthesized. They could be used as stabilizers for emulsion polymerization. Two of four of reactive surfactants were two different types of bi-functional surfactants (divinyls, thus also considered crosslinkers). They were proved to not only be able to stabilize the particles, but also be able to crosslink the particles under certain conditions. Chapter 6 is a proof-of-concept study and should not be considered completed work. Nonetheless, it shows great promise for continuation by another student.

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