Photochemical Control of Covalent Bonds and Noncovalent Interactions in Polymer Materials - Gels, Thin Films and Colloids
Abstract: Understanding of structure-property relationships and advancements
in synthetic strategies of stimuli-responsive polymers allow the development of novel
functional materials for a variety of applications. Of particular interest are
photoresponsive polymers incorporating photolabile groups, as light can offer real-time
spatiotemporal control of material properties. In this thesis, we ... read moredescribe the use of
stimuli-responsive groups, especially photocleavable groups, to control covalent bonds and
non-covalent intermolecular interactions in polymer materials ranging from soft materials
(hydrogels and organogels) to solid-state polymer films (polyelectrolyte multilayers and
conjugated polymer thin films) to polyelectrolyte colloids. Incorporation of
photoresponsive moieties into polymer structures enables the photomodulation of material
properties (e.g., electrostatic properties, mechanical properties, and solubility), and
provides new approaches including triggered release and photolithographic patterning.
Chapter 2 describes a series of photodegradable ROMP gels covalently linked by structurally
different o-nitrobenzyl (ONB) groups of different photosensitivity. Inclusion of
photocleavable ONB groups in the crosslinkers enables photochemical control of the covalent
crosslinking density in the gels, and therefore over swelling, viscoelastic properties, and
solubility of these gels. Chapter 3 describes multi-responsive polyelectrolyte multilayers
(PEMs) prepared via layer-by-layer (LBL) assembly. A series of rationally designed and
synthesized stimuli-responsive polycations can "charge-shift" from positively charged
polymers to polymers with less positive net charge upon photodegradation or increasing pH.
Such charge-shifting can disrupt ion-pairing non-covalent intermolecular interactions in
PEMs comprising these polycations, and therefore render PEMs soluble in buffered solutions.
We also developed an approach to bond guest molecules to PEMs covalently via a redox
sensitive disulfide bond; cleavage of disulfide bond triggered by chemical reduction leads
to the release of guests from the PEMs. These two general approaches to tunable physical or
chemical bonds, combined with the capability for spatial segregation of chemical
functionality in LBL films, enable a versatile method (Chapter 3.2) to prepare
free-standing PEM films that can release covalently bound and physically trapped cargo upon
application of different stimuli (light, pH or chemical reduction). This general strategy
also allows for light-controlled selective disruption, multilevel patterning, and
sequential release with PEMs incorporating four photocleavable chromophores fabricated in
this manner (Chapter 3.3). Photo-disruption and triggered release with polyelectrolyte
colloidal nanoparticles is described in Chapter 4. Irradiation of polyelectrolyte complex
nanoparticles incorporating a photolabile polycation, which can "charge-shift" into a
polyanion upon photolysis, causes disruption of the attractive ion-pairing interactions
between the oppositely charged polymers and leads to the dissolution of the polyelectrolyte
colloid nanoparticles. Finally, chapter 5 describes photochemical control of conjugated
polymer solubility via a sidechain engineering approach. The use of ONB linkage between
solubilizing alkyl sidechains and a polythiophene mainchain enables the photocleavage of
the solubilizing alkyl sidechains and, therefore, yields a photolyzed polymer insoluble in
nonpolar solvents. Moreover, photolysis of the ONB junctions reveals carboxylic acid
pendant groups on the polymer backbone and, therefore, increases the solubility of the
photolyzed polymer in aqueous solutions. This conjugated polymer can be used as a positive-
negative dual-tone photoresist.
Thesis (Ph.D.)--Tufts University, 2017.
Submitted to the Dept. of Chemistry.
Advisor: Samuel Thomas.
Committee: Krishna Kumar, Katherine Mirica, and Ayse Asatekin.
Keywords: Chemistry, Polymer chemistry, and Organic chemistry.read less