The purpose of 4D printing is to embed a product design into a deformable smart material using a traditional 3D printer. The 3D printed object can be assembled or transformed into intended designs by applying certain conditions or forms of stimulation such as temperature, pressure, humidity, pH, wind, or light. Simply put, 4D printing is a continuum of 3D printing technology that is now able to print objects which change over time. In previous studies, many smart materials were shown to have 4D printing characteristics. In this paper, we specifically review the current application, respective activation methods, characteristics, and future prospects of various polymeric materials in 4D printing, which are expected to contribute to the development of 4D printing polymeric materials and technology.
In this work, polymeric microgels with swift response to CO2 are synthesized by polymerization of tertiary-amine containing methacrylate monomers (N,N-diethylaminoethyl methacrylate, DEAEMA) and polyethylene glycol monomethyl ether acrylate (PEGMA) as stabilizers. The obtained microgels are stable but very sensitive to CO2, which can rapidly swell and further collapse within 5 s upon bubbling of CO2, or within minutes in an atmosphere of gaseous CO2. The protonation of the tertiary amine groups in the presence of CO2 induces sensitive swelling and further irreversible collapse of the microgels due to the internal charge repulsion and relatively low cross-linking density in the core area of microgels. This rapid response to CO2 may find further applications in the fields of sensitive detection or responsive loading and release upon CO2 stimulus.
Complex emulsions including double emulsions and high-internal-phase emulsions (HIPEs) are wonderful templates for producing porous polymeric materials. Yet, surfactants and multiple emulsifications are generally needed. In this work, surfactant-free complex emulsions are successfully prepared using a CO-responsive block copolymer through one-step emulsification. Phase inversion from HIPEs to double emulsions happens in one system upon the change in polymer amphiphilicity as a result of CO triggering. The one-step emulsification method offers great convenience for converting the block copolymer into porous 3D scaffolds and particles. Moreover, CO triggering is erasable so that the polymer can be repeatedly used for controllable complex emulsions as well as porous materials.
Alloy catalysts can achieve superior performance to single metal while reducing the cost by fine‐tuning the composition and morphology. Bimetallic Ru‐Fe nanoparticles were synthesized via liquid‐phase reduction method followed by impregnation with multiwall carbon nanotubes (CNTs) to prepare Ru‐Fe/CNTs catalysts. The Ru3Fe/CNTs catalyst yields a superior catalytic stability for ammonia decomposition compared to the Ru/CNTs catalyst. Hence, the ammonia synthesis rate of the Ru3Fe/CNTs catalyst was significantly higher than that of Ru/CNTs catalyst. The potential of bimetallic catalysts with reasonable composition and proportion will expand the research of efficient catalysts for ammonia decomposition and synthesis.
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