A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Although gels are mostly liquid, they behave like solids due to a three-dimensional network formed through chemical or physical crosslinking of polymer chains. It is capable of absorbing liquid tens or hundreds of times the weight of the dry state. Various materials can be contained inside the porous network, and hydrogels, organo gels, ion gels, and aerogels can be made depending on the type. In addition, chemical changes such as hydrophilic/hydrophobic change and ionization of the polymer chain may occur due to external stimuli, and as a result, the volume or shape of the gel changes. By utilizing the functionality of these materials, they are being used as biomimetic soft robots, energy devices, and drug delivery systems.
1. Design of Sequence Controlled Multi-Block Polyelectrolytes by Modulating Morphology
Hydrogels, which are a three-dimensional (3D) network of cross-linked hydrophilic polymer chains with high water content (up to 90 wt%), are highly elastic and soft materials. In addition, they can be classified into chemical and physical hydrogels depending on the type of bond consisting the internal crosslinking point. Due to bio-friendly and flexible properties, hydrogels can be used in various fields including medical materials, however its mechanical properties and durability are not enough for practical use. Hydrogels composed of physical bonds, such as hydrophobic interactions and electrostatic interactions, can recover even if the network is damaged by force, because they are made up of reversible bonds, and can exhibit unique changes such as sol-gel transitions. In our lab, we are synthesizing a sequence-controlled multi-block copolymer through a one-pot synthesis process using reversible addition fragmentation chain transfer (RAFT) polymerization. In addition, the polymer type, sequence, and length of each block constituting the multi-block copolymer can be precisely controlled. Using this, various self-assembled structures including micelles can be made. Self-assembled structures can also create a hydrogel by forming a hierarchical three-dimensional network due to physical interactions. The internal structure of the hydrogel thus formed is confirmed through various image analysis methods. In addition, we analyze the mechanical properties, viscoelasticity, material diffusion, and viscosity of the hydrogel to determine the relationship between the polymer structure and the properties of the hydrogel. This research creates new polymer base building blocks that can lead the new material era, which can be used in a variety of new material designs.
2. Development of stimulus-responsive polymers
Stimulus-responsive polymer materials change the structure of the polymer, solubility in solvents, and mass transfer of ions or molecules by detecting changes in the surrounding environment such as temperature, light, electric field, magnetic field, and humidity. As a result, they can convert a variety of physical signals into chemical or biochemical signals. A typical temperature-sensitive polymer is poly(N-isopropylacrylamide), which dissolves in water below 32 °C and precipitates at higher temperatures.
In our lab, we synthesize poly(2-isopropyl-2-oxazoline) in addition to poly(N-isopropylacrylamide), and try to analyze and understand the temperature response mechanism of polymers. Furthermore, we are synthesizing polymers and hydrogels in a controlled topology to improve the response speed, response sensitivity, and volume change of the gel. It is expected to improve practicality and utilization as a material for soft actuators.
3. Development of gel actuators
With the increasing demand and interest in soft robots, research on biomimetic actuators is receiving great interest. Actuators are materials that convert externally applied physical, chemical and thermal energy into controllable mechanical motion. In particular, in recent years, research on soft actuators that can reproduce smooth movements is actively being conducted. Among them, hydrogel actuators have similar elasticity and strength to human muscles, so they are highly applicable as biomimetic actuators. And they are able to reproduce the smooth and flexible movements of living organisms. To develop more efficient and intelligent actuators, it is important to understand the underlying driving mechanisms of motion that can be found in natural and biological systems. In our laboratory, we are developing gel actuators that can induce large volume changes and rapid deformation in response to external stimuli, based on the rational design of stimulus-responsive polymer gels.
4. Development of gel-based energy materials
To realize wearable electronics, electrical components should be stable against mechanical deformation. Accordingly, research is being actively conducted to develop a material that exhibits excellent conductivity while the material itself has elasticity. In our lab, we are developing composite gels using functional polymers and inorganic materials. In addition, deformable energy materials with desired mechanical properties and electrochemical performance are produced by controlling the interaction between polymers and inorganic materials.