(715e) Structure, Properties and Applications of a Model Thermoreversible Covalent Adaptable Network

Structure, Properties and
Applications of a Model Thermoreversible Covalent Adaptable Network

Richard J
Sheridan and Christopher N Bowman

Department of Chemical and Biological Engineering,
University of Colorado at Boulder, 3415 Colorado Avenue, Boulder, CO 80309

Abstract
–

Although
the gel point conversion of a thermoreversible
network is certainly a key parameter determining the properties of the material
under given conditions, it is not a liquid-solid transition as in common,
irreversible networks. Rather, the material's viscosity is finite at the gel
point and beyond, as bond breakage and diffusion work together to relax
stresses imposed on the forming transient network of the material. In this
work, we build upon our previous reports^{1, 2}
to demonstrate this process using a model Diels-Alder network, in which the
retro-Diels-Alder reaction rate, crosslink effective functionality, Diels-Alder
moiety conversion as a function of temperature, and gel point conversion are
all separately adjustable parameters, which can be manipulated simply by
adjusting the ratio of ingredients in a composition.

These
different knobs are unified by a simple relationship we derived from the work
of Semenov and Rubinstein³ for
associative transient networks. This relationship provides a toolkit for the
prediction of the important engineering and rheological properties of the
material in the immediate post-gel regime, such as viscosity, plateau modulus,
and relaxation time, based upon the straightforward estimation of one
material-dependent parameter.

Given
the simplicity of tailoring this model material, we revisited our earlier work⁴ on externally triggered healing, in
which the thermal reactivity of the material could be activated by self-limited
RF hysteresis heating. This composite of chromium(IV)
dioxide particles suspended in the model material is used to demonstrate the
ability of this, as well as other covalent adaptable networks, to be readily
tailored to applications that are constrained by factors not related directly
to the polymer, such as the Curie temperature of CrO₂.

Lastly,
to bring some of the flexibility of optical curing techniques to thermally
activated systems, we developed a robust custom core/shell/shell
gold/silica/organic nanoparticle to serve as a
durable light absorber. The particle was designed to efficiently convert laser
light into heat while maintaining surface plasmon resonance and colloidal
behavior even in the high temperature, poor solubility environment of a
Diels-Alder network. We probed the effectiveness of this technique in
applications such as fracture healing and scratch repair, and consider its
implications for the design of robust nanocomposite
polymer network materials.

References  ­–

(1)  Sheridan, R. J.; Adzima, B. J.; Bowman, C. N., Australian Journal of Chemistry. 2011, 64 (8), 1094-1099.

(2)  Adzima, B. J.; Aguirre, H. A.; Kloxin, C. J.; Scott, T. F.;
Bowman, C. N., Macromolecules. 2008, 41 (23), 9112-9117.

(3)  Rubinstein, M.; Semenov, A. N., Macromolecules. 1998, 31 (4), 1386-1397.

(4)  Adzima, B. J.; Kloxin, C. J.; Bowman, C. N., Advanced Materials. 2010, 22 (25), 2784-2787.