Here, we report a method to prepare cryogel particles with sponge-like mechanical properties, including high porosity and high elasticity. The preparation process of the cryogel particles of poly(2-hydroxythyl methacrylate) can be summarized in the two following steps: preparation of frozen droplets using the inverse Leidenfrost effect, followed by cryo-gelation by frozen polymerization. First, a polymer precursor was dropwise added into bulk liquid nitrogen (−196 °C). Then, frozen droplets were created by the inverse Leidenfrost effect, which were subsequently polymerized in liquid paraffine (−15 °C). After thawing and drying, the cryogel particles were obtained. The monolithic super-macroporous structure was observed by scanning electron microscopy (SEM). The mechanical properties of the cryogel particles were studied via compression−swelling tests. At maximum compression, the particles achieved 94.3% degree of deformation; remarkably, they returned to their original shape under the swelling state. The strategy proposed herein, which combines the inverse Leidenfrost effect with a cryopolymerization technique, could be applied to prepare various polymer particles without employing surfactants.

We have investigated the versatility of a two-step preparation method, without a detergent, that combines both the inverse Leidenfrost effect and the cryogelation technique by using the macroporous particles of different kinds of monomers (four vinyl monomers) or a natural polymer (agarose). First, the precursor of polymers was dropped into liquid nitrogen to prepare the spherical frozen droplet by the inverse Leidenfrost effect. Second, the frozen droplets were cryo-polymerized at the frozen temperature; then, cryogel particles were prepared after thawing. Subsequently, the basic characteristics of the macroporous polymer particles obtained above were compared, focusing on the appearances, porous morphologies, and mechanical properties. It was found that the similar polymer particles could be obtained by the two-step preparation method, while there was a slight difference in their characteristics, depending on the type of monomers. Especially for the mechanical properties, the cryogel particles of the hydrophilic polymer exhibited a shape memory function with sponge-like elasticity, whereas the hydrophobic polymer particles were observed to be cracked after compression (i.e., no shape memory function). This work provides a versatile method of adopting various kinds of monomers and natural polymers for the preparation of macroporous particles. Hence, the method possibly has a potential to prepare and design "tailor-made" macroporous polymer particles for the application purpose.

Here, we prepared hydrophobic cryogel particles with monolithic supermacropores based on poly-trimethylolpropane trimethacrylate (pTrim) by combining the inverse Leidenfrost effect and cryo-polymerization technique. The hydrophobic cryogel particles prepared by adopting this method demonstrated the separation of the stabilized O/W emulsion with surfactant. The prepared cryogel particles were characterized in terms of macroscopic shape and porous structure. It was found that the cryogel particles had a narrow size distribution and a monolithic supermacroporous structure. The hydrophobicity of the cryogel particles was confirmed by placing aqueous and organic droplets on the particles. Where the organic droplet was immediately adsorbed into the particles, the aqueous droplet remained on the surface of the particle due to repelling force. In addition, after it adsorbed the organic droplet the particle was observed, and the organic solvent was diffused into the entire particle. It was indicated that monolithic pores were distributed from the surface to the interior. Regarding the application of the hydrophobic cryogel particles, we demonstrated the separation of a stabilized oil-in-water emulsion, resulting in the successful removal of the organic solvent from the emulsion.

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