Anisotropic Foams Via Frontal Polymerization

Abstract

The properties of foams, an important class of cellular solids, are most sensitive to the volume fraction and openness of its elementary compartments; size, shape, orientation, and the interconnectedness of the cells are other important design attributes. Control of these morphological traits would allow the tailored fabrication of useful materials including highly porous solids, anisotropic heat conductors, tough composites, among others. While approaches like ice templating has produced foams with elongated cells, there is a need for rapid, versatile, and energy efficient methods that also control the local order and macroscopic alignment of cellular elements. Here we describe a fast and convenient method to obtain anisotropic structural foams using frontal polymerization. We fabricated foams by curing mixtures of dicyclopentadiene and a physical blowing agent via frontal ring opening metathesis polymerization (FROMP). The materials were characterized using micro-computed tomography and an image analysis protocol to quantify morphological characteristics including volume fraction and anisotropy. The cellular structure, porosity, and hardness of the foams changed with blowing agent, concentration, and resin viscosity. Moreover, we used a full factorial combination of variables to correlate each parameter with the structure of the obtained foams. We found a strong correlation between the resin viscosity and the foam’s cellular structure. Furthermore, a specific combination of input parameters controlled the transitions from (i) isotropic to anisotropic cellular structures, (ii) porous to non-porous, and (iii) soft to hard foams. Our results demonstrate the controlled production of foams with specific morphologies using the simple and efficient method of frontal polymerization. This work shows promise for creating foams with aligned cellular structures that allow anisotropic mass and energy transport properties in high performance structural solids.

Frontal Polymerizations: From Chemical Perspectives to Macroscopic Properties and Applications

“…Intriguingly, the oscillations in Tmax are reminiscent of patterns generated in the classical single-and multi-slit diffraction experiments; constructive and destructive interference of propagating waves provides a unique splitting pattern! Recently, Alzate-Sanchez and coworkers 152 demonstrated that well-defined pattern formation occurs in FROMP resins with high concentration of added solvent. In this system, FP is triggered at the base of the resin inside a test-tube; the heat of the reaction induces solvent boiling.…”

“…Apart from ATRP and polyurethanes, which have not been thoroughly investigated, only FROMP occurs catalytically with the use of the commercial Grubbs-type ruthenium alkylidene complexes (e.g., G1 and G2). 32,41,49,66,69,70,73,102,106,124,[145][146][147]151,152,160,162,163 Highly strained, cyclic olefins undergo living catalytic polymerization; frontally derived polymers of dicyclopentadiene (DCPD) exhibit desirable mechanical properties via extensive crosslinking during catalysis. Perhaps counterintuitively to most catalyst design principles, FROMP and polyurethane-based systems 68,72,[193][194][195] require worse catalysts to circumvent rapid background polymerization.…”