Showing papers by "Alan J. Lesser published in 2023"
TL;DR: In this article , a fast and energy efficient method to produce anisotropic high performance polymeric foams via rapid radically induced cationic frontal polymerization coupled with chemical blowing agents is presented.
Abstract: Anisotropy in naturally occurring or synthetic microcellular structures is an important feature for the development of materials with high specific stiffness and strength, in addition to creating materials with unique physical properties. Polymeric foams constitute a broad class of materials that are widely used for their advantages of low density, high specific mechanical properties and high insulative properties. Traditional synthetic routes are slow, energy demanding processes that employ the use of high temperature ovens, freezers, or high-pressure equipment. Herein we present a convenient and energy efficient method to produce anisotropic high performance polymeric foams via rapid radically induced cationic frontal polymerization coupled with chemical blowing agents. The degree of pore orientation and degree of anisotropy are a result of the propagating front working in concert with the foam volume expansion. This paper presents results into FP foam formation to illustrate how changes in boundary conditions and front initiation position affect both the microcellular structure and their resulting physical and mechanical properties. Additionally, results are presented to show how changes in resin formulation, such as the addition of nanoparticles affect both properties as well as the microcellular structure and anisotropy.
TL;DR: In this article , the authors used silica particles of two different sizes (14 nm and 200-300 nm) in an epoxy-based frontal polymerization foam as a representative filler to probe the constraints imposed by non-reactive additives.
Abstract: Frontal polymerization (FP) is a solvent-free, energy-efficient process where a self-propagating polymerization reaction with a characteristic sharp temperature gradient at the front head propagates through the resin to provide the curing conditions. It relies on the enthalpic balance, which spreads the reaction to unreacted resin in the neighborhood. Therefore, the FP is sensitive to the presence of non-reactive volumes, such as boundaries, fillers, or other additives, that retain heat from the front but produce no enthalpy in return. On the other hand, the front's high temperature could be used to initiate other processes, such as foaming, incorporating them into a simple single-step fabrication procedure. This study used silica particles of two different sizes (14 nm and 200-300 nm) in an epoxy-based FP foam as a representative filler to probe the constraints imposed by non-reactive additives. The presence of particles visibly hindered the front propagation, increased the foam density and even corrupted the frontal regime in some cases. We show that preheating or chemical composition changes are viable approaches to address the fillers' adverse effects. Furthermore, we present evidence that the reduced reaction enthalpy caused by silica nanoparticles, was balanced by the lower heat capacity of our model system. At the same time, the front hindrance was attributed to changes in reaction kinetics and the heat distribution around the front. These results set up essential narratives for the design and practical applications of frontally polymerized foams with non-reactive fillers.