Hydrochannel-Containing Hydrophobic Polymers by Inverse Emulsion Polymerization for Moisture-Driven Actuators

Two physical properties in polymers, hydrophobic and water-absorptive, are known to be incompatible. However, human skin in nature has a hydrophobic surface and yet absorbs water throughout hydrophilic amino acid sequences in filaggrin, one of the abundant proteins in our outermost skin layer. Although present in nature, a hydrophilic path network in a hydrophobic polymer is difficult to synthesize because of poor wettability and immiscibility between the two types of materials. Herein, we introduce a novel method for the creation of a hydrophobic absorptive polymer (HPHG), which overcomes the inherent incompatibility by increasing hydrophobicity of reaction sites in hydrophilic monomers. The methacrylate structure in hydrophilic monomers successfully contributes to stabilize reverse emulsions, which consist of polyethyleneglycol (PEG) methacrylate and polyethyleneglycol (PEG) dimethacrylate, in a hydrophobic matrix of polymethylhydrosiloxane and divinylpolydimethylsiloxane (PDMS material sources). The HPHG film (with a 11 weight percent water content) shows water repellency having over 100° contact angle with a water droplet and yet is capable of absorbing water by 19.1 weight percent while maintaining decent hydrophobicity on the surface (78° water contact angle). We have successfully demonstrated a moisture-driven actuator by constructing a bilayer of HPHG and PDMS (or a textile), which is delamination-free and transforms into a curvature geometry by a preferential expansion of the HPHG layer. HPHG is applicable for soft robotics and smart actuators where a hydrophobic artificial skin is needed to protect the surface against hydrophilic invasions of undesirable substances such as metal ions, bacteria, or viruses but absorptive for desirable evaporation and mobility by water migration in the polymer matrix.