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 polymerization-triggered simultaneous ring-opening metathesis polymerization and cross metathesis affords anisotropic macroporous dicyclopentadiene cellulose nanocrystal foam

“…Recently, the introduction of various metal NPs and lipase enabled the realization of distinctive foam morphologies and functionalities 36 . Similar phenomena were observed from FROMP-mediated foams by embedding secondary solvent 24 and polymeric microparticles 21 in the monomer layer. While different combinations of monomer-secondary phase systems still remain widely unexplored in DCPD FROMP, we were inspired to examine how the intrinsic morphology of rigid rod-like CNCs would affect the construction of the pDCPD monolith microstructure in situ.…”

“…For example, M. Garg et al 23 recently reported a production of porous or vascularized DCPD monolith via a synchronized ring-opening metathesis polymerization (ROMP) and depolymerization of DCPD and a poly(propylene carbonate) sacrificial layer, respectively. Moreover, D. M. Alzate-Sanchez et al 24 studied the relationship between blowing agents, gelation time, and resin viscosity in the production of anisotropic channels within DCPD foams. Despite these cutting-edge results, a more homogeneous pore organization may be achieved by reducing the dependency of foam porosity to the number of sacrificial layers and the long delay time derived from precursor formulations.…”

Scalable Frontal Oligomerization: Insights from Advanced Mass Analysis

“…In FROMP, Grubbs-type Ru catalysts rapidly transform liquid or gel-state resins of dicylopentadiene ( DCPD ) into thermoset materials. After an initial thermal ,− or photo , trigger, the heat generated by ring-opening drives subsequent catalytic events in advance of the reaction zone. The resultant polymerization front traverses through the monomer resin with a well-defined monomer to polymer interface at a steady-state velocity (ν f ).…”

Switching Frontal Polymerization Mechanisms: FROMP and FRaP

“…Our group recently leveraged blowing agents (solvents) for the generation of foams from FROMP of DCPD. 22 Rather than the addition of blowing agents, we envisioned using the inherent generation of nitrogen from the radical initiator as well as the boiling of monomer to generate voids in the propagating polymer front. Using tandem FRaP and FROMP upward, in situ mixing of the two layers would form gradient foams through bubble formation.…”

“…As the gas bubbles form, they are trapped by the propagating front to create a foam. Our group recently leveraged blowing agents (solvents) for the generation of foams from FROMP of DCPD . Rather than the addition of blowing agents, we envisioned using the inherent generation of nitrogen from the radical initiator as well as the boiling of monomer to generate voids in the propagating polymer front.…”