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.