Methacrylate-based polymer foams with controllable connectivity, pore shape, pore size and polydispersity.

Polymer foams are becoming increasingly important in industry, especially biodegradable polymer foams are in demand. Depending on the application, polymer foams need to have characteristic properties, which include connectivity and polydispersity. We show how polymer foams with tailor-made structures can be synthesized from water-in-monomer emulsions which were generated via microfluidics. As monomer we used 1,4-butanediol dimethacrylate (1,4-BDDMA). Firstly, we synthesised monodisperse open- and closed-cell poly(1,4-BDDMA) foams with either spherical or hexagonal pore shapes by varying the locus of initiation. Secondly, we were able to control the pore diameters and obtained polymer foams of both connectivities and pore shapes with pore sizes from ∼70 µm up to ∼120 µm by means of one microfluidic chip. Finally, we synthesized poly(1,4-BDDMA) foams with controllable polydispersity. Here, the mean droplet diameter was the same as that of the monodisperse counterparts in order to be able to compare the properties of the resulting polymer foams.

Formulation and polymerization of foamed 1,4-BDDMA-in-water emulsions.

Emulsion and foam templating allow the synthesis of tailor-made polymer foams. A complementary templating route is foamed emulsion templating. The concept is based on the generation of a monomer-in-water emulsion which is subsequently foamed. After polymerization of the foamed emulsion, one obtains open-cell polymer foams with porous pore walls. In the paper at hand, we generated foamed emulsions and synthesized polymer foams which are based on the monomer 1,4-butanediol dimethacrylate (1,4-BDDMA). The main challenge was to find the optimal composition of the emulsion by varying the components systematically. We will discuss that the composition of the monomer-in-water emulsion is key for the stability of the foamed emulsion and thus for the structure of the resulting polymer foam. The final composition of the continuous phase was found to be 65 vol% 1,4-BDDMA, 30 vol% water and 5 vol% glycerol. We foamed and polymerized this emulsion to check the foamed emulsion's suitability as a template for solid polymer foams. We generated a foamed emulsion with a mean bubble diameter of 151 µm ± 90 µm and obtained a highly porous poly(1,4-BDDMA) foam with a pore mean diameter of 366 µm ± 91 µm. Furthermore, the polymer foam has a "sub-porosity" within the pore walls.

Monodisperse liquid foams via membrane foaming.

HYPOTHESIS: It is possible to generate fairly monodisperse liquid foams by a dispersion cell, which was originally designed for the generation of fairly monodisperse emulsions. If this is the case, scaling-up the production of monodisperse liquid and solid foams will be no longer a problem. EXPERIMENTS: We used the dispersion cell - a batch process - and examined the influence of stirrer speed, membrane pore diameter and injection rate on the structure of the resulting liquid foams. We used an aqueous surfactant solution as scouting system. Once the experimental conditions were known we generated gelatin-based liquid foams and methacrylate-based foamed emulsions. FINDINGS: We found that (a) the bubble size of the generated liquid foams can be adjusted by varying the membrane pore diameter, (b) no stirrer should be used to obtain monodisperse foams, and (c) the bubble size is not influenced by the air injection rate. Since (i) the output for all investigated systems is up to two orders of magnitude larger compared to microfluidics and (ii) the membrane technology can very easily be scaled-up if run in a continuous process, the use of membrane foaming is expected to be heavily used for the generation of monodisperse liquid and solid foams, respectively.