Creating Honeycomb Structures in Porous Polymers by Osmotic Transport.

Understanding why honeycombs are shaped the way they are has moved biologists, physicists, chemists, and mathematicians alike. It was only recently that the honeycombs' shape "at birth" was included in the ongoing discussions: at birth, the cells are spherical but then transform into the well-known hexagons. It was proposed that a flow of wax-driven by surface tension effects-is the reason for this transformation. Our recent work on synthetic polymer foams with honeycomb-like structures points towards a very different mechanism. Just like in honeycomb cells, we observe that a spherical "initial state" transforms into a hexagon-shaped "final state" during polymerization. We have experimental evidence that a concentration gradient arises during polymerization, which transports monomers such that the spherical template becomes a honeycomb structure with walls of homogeneous thickness. The knowledge about this mechanism suggests promising strategies for the development of lightweight materials with optimized mechanical properties.

Diving into the Finestructure of Macroporous Polymer Foams Synthesized via Emulsion Templating: A Phase Diagram Study.

During our studies on emulsion-templated monodisperse polymer foams we found significant differences in the finestructure if the locus of initiation is changed. This motivated us to study the phase behavior of the liquid template. Our studies indicate that the template consists of droplets of three different length scales: The water droplets generated via microfluidics (∼70 µm) are surrounded by a continuous phase in which a w/o emulsion (≤100 nm) coexists with a w/o microemulsion (∼5 nm). We speculate that the w/o-emulsion droplets act as seeds for the porous finestructure observed in AIBN-initiated polymer foams. We have experimental evidence that the w/o emulsion inverts to an o/w emulsion with progressing polymerization. This explains the granular texture observed in KPS-initiated polymer foams. The control of the finestructure is important in the preparation of tailor-made polymer foams because it directly impacts the material's density and thus, in turn, its mechanical stability.

Liquid foam templating - A route to tailor-made polymer foams.

Solid foams with pore sizes between a few micrometres and a few millimetres are heavily exploited in a wide range of established and emerging applications. While the optimisation of foam applications requires a fine control over their structural properties (pore size distribution, pore opening, foam density, …), the great complexity of most foaming processes still defies a sound scientific understanding and therefore explicit control and prediction of these parameters. We therefore need to improve our understanding of existing processes and also develop new fabrication routes which we understand and which we can exploit to tailor-make new porous materials. One of these new routes is liquid templating in general and liquid foam templating in particular, to which this review article is dedicated. While all solid foams are generated from an initially liquid(-like) state, the particular notion of liquid foam templating implies the specific condition that the liquid foam has time to find its "equilibrium structure" before it is solidified. In other words, the characteristic time scales of the liquid foam's stability and its solidification are well separated, allowing to build on the vast know-how on liquid foams established over the last 20 years. The dispersed phase of the liquid foam determines the final pore size and pore size distribution, while the continuous phase contains the precursors of the desired porous scaffold. We review here the three key challenges which need to be addressed by this approach: (1) the control of the structure of the liquid template, (2) the matching of the time scales between the stability of the liquid template and solidification, and (3) the preservation of the structure of the template throughout the process. Focusing on the field of polymer foams, this review gives an overview of recent research on the properties of liquid foam templates and summarises a key set of studies in the emerging field of liquid foam templating. It finishes with an outlook on future developments. Occasional references to non-polymeric foams are given if the analogy provides specific insight into a physical phenomenon.