ABSTRACT
Micro to Macro: Translating the behavior of small molecules to functional materials
through the use of external stimuli
by
Kyle Darrin Clark
As the scientific community continues to develop a vast array of functional
materials, the demand for improved control over a broad range of properties becomes all
the more prevalent. Stimuli such as light and temperature have been employed
consistently throughout history as easily accessible tools for chemists, and in recent years
they have shown great promise as a means to control polymer property on-demand. The
discovery of novel photo-responsive small molecules and temperature triggered reactions
and applications of the materials are presented.
Self-immolative polymers (SIPs) are a sub-class of stimuli-responsive materials
that, when exposed to an appropriate stimulus, degrade end-to-end into their composite
monomers and release a given cargo. One challenge with current SIPs is that once
initiated, unraveling of the polymer typically occurs in an uncontrolled fashion. To
address this challenge, we designed a SIP system capable of dual-stimulus activation,
allowing on/off control of depolymerization. Specifically, furan-endcapped polyurethanes
were prepared and the controlled depolymerization was studied with both acid and heat
triggers. From these SIPs, a new class of temporally controlled degradable materials
could be accessed which hold potential for on/off drug release.
Photo-responsive polymers represent another class of stimuli responsive material
with myriad of applications that range from controlled micelle rupture and surface
patterning to mechanical actuators. Advances in this area has been fueled by
photoswitching molecules – compounds that change molecular conformation and
numerous properties in response to light. Donor–acceptor Stenhouse adducts (DASAs), a
novel class of photochromic developed in our group, exhibit large changes in color,
polarity and molecular volume upon visible light irradiation. The first generation of these
photochromic molecules were limited to alkyl amine donors, absorbing green light. To
expand their capabilities, we developed second and third generations DASA derivatives,
vastly expanding the scope of both structure and properties. Second generation DASAs
were synthesized and using aniline derivatives, yielding compounds with red-shifted
absorption windows. Although advantageous, we discovered that these new derivatives
lost control over the thermodynamic equilibrium between the open and closed state in the
dark. As such, a third generation DASA derivative was developed. By modifying the
acceptor unit, we demonstrated both tunable equilibria between the open and closed form
and an expanded absorption window (550 to 750 nm).
With optimized DASA derived photochromic material in hand, we focused on
demonstrating the ability convert light into mechanical work. To accomplish this, we
developed a new approach to liquid crystal elastomers (LCEs) based on mild, additive
free Diels–Alder step-growth polymerization reaction. This approach expands the
functional-group tolerance of current click chemistry platforms, enabling Michael-
acceptor-bearing compounds such as DASAs to be incorporated into LCE networks. In
addition, this new method provides a platform to study the performance of a multitude of
photoswitches under a unified polymer system. This holds the potential to enable for the
first time structure-property relationships between the photochromic material and LCE
architecture that are needed to advance this field.