The development and characterization of stimuli-responsive systems for performance materials
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
In nature, living organisms adjust to their surroundings by responding to
environmental cues, such as light, temperature or force. Stimuli-triggered processes,
such as the contraction of eyes in response to bright light or wound healing in skin after
a cut, motivate the design of “smart” materials which are designed to respond to
environmental stimuli. Responsive materials are used as self-healing materials, shape
memory polymers and responsive coatings; moreover, responsive materials may also be
employed as model systems, which enhance understanding of complex behavior. ☐ The overall goal of this work is to design a material that offers self-healing
functionality, which will allow for self-repair following material fatigue or failure, and
increased strength in response to ballistic or puncture threats through the incorporation
of colloidal particles. The target application for this material is as a protective barrier
in extreme environments, such as outer space. Towards this end, the dissertation is
focused on the development and characterization of each component of the protective
material by (1) designing and testing novel light- and force-sensitive polymers for selfhealing
applications and (2) examining and characterizing long-time behavior (i.e.,
aging) in model thermoreversible colloidal gels and glasses. ☐ Towards the development of novel stimuli-responsive materials, a photoresponsive
polymer network is developed in which a dynamic bond is incorporated into
the network architecture to enable a light-triggered, secondary polymerization, which
increases the modulus by two orders of magnitude while strengthening the network by
over 100%. Unlike traditional two-stage polymerization systems, in which the
secondary polymerization is triggered by a leachable photoinitiator, the dynamic nature
is imparted by the material itself via the dissociation of its own crosslinks to become
stronger in response to light. Several attributes of the photo-responsive network are
shown including: (1) photo-induced healing and strengthening of a specimen after it has
been severed, (2) photopatterning for effecting spatially confined property changes on
demand, and (3) locking in the film’s 3D geometry using light after reshaping. The
utility of the photo-responsive dynamic bond is enhanced by demonstrating that it is
also responsive to mechanical force. Force-responsive materials are activated by the
energy from the damage event itself, thereby enabling healing without human
intervention. Specifically, selective cleavage of a polymer containing a dynamic
trithiocarbonate group initiates a force-driven radical polymerization, thus enabling the
material to constructively respond to force via gelation on an experimentally relevant
timescale. ☐ To enhance the stress response of the self-healing materials described above, a
protective material composed of colloidal particles is proposed. Toward this goal, the
second half of this dissertation investigates the microstructural basis of rheological
aging in colloidal gels and glasses using a model thermoreversible colloidal dispersion.
In this work, rheological aging is quantitatively related to microstructural aging in
glasses and gels by simultaneously measuring the bulk properties and sample
microstructure using rheometry and small angle neutron scattering (Rheo-SANS),
respectively. A one-to-one correspondence between the evolution in storage modulus
and microstructure as the sample ages is observed, which is investigated as a function
of thermal and shear history. The microstructural measurements are consistent with the
hypothesis of aging as a trajectory in a free energy landscape, which combined with
analysis with mode coupling theory, support local particle rearrangements as the
mechanism of aging. Moreover, by using a system that is fully rejuvenated by thermal
cycling, the effectiveness of shear as a rejuvenation method is investigated by directly
comparing microstructure and bulk properties following thermal and mechanical
rejuvenation. The conclusions of this study may be industrially relevant to products that
age on commercial timescales, such as pharmaceuticals, applicable to other dynamically
arrested systems, such as metallic glasses, and provide pathways to advanced composite
materials such as those envisioned in this work.