Researches

1. Theory

1-1. Photo Polymerization

A photopolymerizable resin is a material that is involved in polymerization reaction by absorbing the light energy of ultraviolet or visible light region of the electromagnetic spectrum. It is composed of monomer, oligomer, cross-linker, photoinitiator. The chemical reaction of the photopolymerizable resin proceeds through a free radical mechanism, largely divided into initiation, propagation and termination steps.

Here in, R• is a radical generated at the initiation and M is a monomer. Photo-initiators form free radicals from light energy absorption, and active monomers are produced through reaction with monomers. In the propagation step, the reaction takes place at the double bond of the chain radical and the monomer or oligomer, as the result, chain growth proceeds. The termination step generally proceeds through a disproportionation reaction in which two chain radicals are bonded together or atoms are being transferred from one radical chain to another to form two polymer chains. Curing materials are widely used in medical, printing and photoresist technologies because they can form polymers in a short period of time through radical chain growth.

1-2. Electrochemical Polymerization

Electrochemistry is defined as a discipline that deals with the transfer of electrons and the chemical reactions that accompany them. Simply, electrochemistry is a method to analyze the chemical response of a system to a given stimulus when an electrical stimulus is given to the system. The movement of the electrons accompanying the electric stimulation brings about much information about the concentration of the active species, the reaction mechanism, the electron transfer reaction occurring on the electrode surface, and the adsorption. Previously, electrochemistry was mainly used as an analytical method for the oxidation and reduction properties of materials, but recently, it has been used in many fields such as sensor development, functional thin film production, battery, and capacitor material development.

In the polymer thin film material laboratory, a conducting polymer film is formed on the surface of the working electrode by cyclic voltammetry performing electropolymerization. The cyclic voltammetric is a method that can directly analyze the reaction occurring on the electrode surface. Electropolymerization requires a potentiostat, an electrode, and an electrolytic solution containing a polymeric monomer. To measure the potential of the electrode in the solution, the potential difference between the two electrodes should be measured using two electrodes. An electrode to be measured in each cell is called a working electrode, and another electrode, reference electrode, is connected to the electrode to measure the potential difference. In the case of a two-electrode cell, when a large current flows through the reference electrode, the concentration of the electrochemically active species at the electrode/electrolyte interface, which determines the potential of the reference electrode is changed. And then, the potential of the reference electrode deviates from the equilibrium value. A large current is required for the electrochemical polymerization. Therefore, when a large current flows, a three-electrode cell is used. In this case, the electrode cell is composed of a working electrode, a reference electrode, and a counter electrode. In the three-electrode cell, a current flows between the working electrode and the counter electrode, and the potential of the working electrode is adjusted by the potential controller with respect to the reference electrode. At this time, the potential difference between the working electrode and the reference electrode can be accurately measured regardless of the current value flowing by the electrode reaction.

Fig. 1. (a) a potential scanning type in the cyclic voltammetric method, (b) the cyclic voltammetric curve obtained for the potential scanning

The cyclic voltammetric is a method of measuring the current by circulating the potential of the working electrode at a constant rate as shown in Fig. 1. In the experiment, the initial potential (Ei) is set to the potential at which the Faraday current does not flow, the scanning starts from Ei, the scanning is performed at a constant speed, the scanning direction is reversed at the reverse potential, and the potential is scanned at the same scanning speed as the forward direction. The method of performing such circulation is a single scanning method, and the method of repeatedly scanning the same type of potential scanning is a multiple scanning method. In the experiment, the initial potential and the reverse potential should be appropriately set in order to observe the redox signal of the electrode. At this time, the obtained cyclic voltammetric curve can be understood by examining the change in the concentration of O and R in the electrode surface or near the electrode. When the electrode reaction is reversible, the concentration ratios of O and R on the electrode surface are expressed by the following Nernst equation.

Where Eo ‘is the formal potential of the electrode, and Red and Ox are the concentrations of reducing species and oxidizing species on the electrode surface, respectively. When the potential of the working electrode is positive (+) compared to Eo, O is predominantly present. When the potential of the electrode is injected in the negative direction, a reduction reaction occurs to generate a reduction product R. In addition, when the potential direction is reversed and re-injected in the (+) direction, the oxidation reaction occurs to produce the oxidation product O and the concentration of R decreases. This method is frequently used when a specific substance is synthesized on the surface of an electrode mainly by an electrochemical method.

1-3. Atom Transfer Radical Polymerization

Atom Transfer Radical Polymerization is a method of preparing polymer through an oxidation-reduction reaction using a transition metal as a catalyst. This method, also called as atom transfer radical addition polymerization, was discovered in 1995 by Mitsuo Sawamoto and Krzyszt of Matyjaszewski.

The general mechanism of ATRP is as mentioned above. A polymer is generated through a reversible redox process in which halogen atoms (X) migrate from R-X (dormant species) through a transition metal complex (ligand). This process occurs repeatedly during the activation and deactivation phases. Here, the deactivation rate (Kdeact) is much larger than the activation rate (Kact). Therefore, it is possible to keep the concentration of the radical species constant in order to suppress the radical disappearance due to the stationary reaction. The transition between dormant and growing species occurs rapidly and all chains grow at the same rate, resulting in narrow molecular weight distribution. The polymer chain grows at a growth rate (kp) by adding an intermediate radical to the monomer in a manner similar to conventional radical polymerization. The termination reaction (kt) occurs primarily through radical coupling and disproportionation.

ATRP can modulate the radical polymerization by keeping the concentration of active sites low and keeping the dynamic equilibrium of the active species. In order for the polymer obtained by living polymerization to have a narrow molecular weight distribution and to control the molecular weight, the initiator should be utilized at the initial stage of polymerization to form a polymerization active site and satisfy the condition that the exchange reaction among various polymerization active sites occurs very rapidly. It is possible to form the living polymerization system by using the rapid equilibrium reaction between the dormant species and the active species without generating the stopping reaction or the chain transfer reaction while the radical activity concentration is kept almost constant during the polymerization reaction.

In ATRP, there are almost no termination and chain transfer reactions, so the active sites of the polymer chain are alive until the monomer is all utilized. The reaction resumes when the monomer is exhausted and then reintroduced. Homopolymer can be prepared by injecting the same kind of monomers. Block copolymers can be prepared by injecting different kinds of monomers, and It is possible to prepare a polymer having a desired reactive end group by selectively killed a living polymer chain using appropriate additives. Further, by using a multifunctional initiator, the polymer chains can be grown in various directions.

2. Technology

2-1. Molecularly Imprinting

Molecularly imprinting has been established by Professor Wulff of Germany in 1972 as a technique for forming a cavity that can be chemically bonded to a specific molecule in a polymer matrix. Generally, a molecularly imprinted polymer is prepared by polymerizing a mixture of a functional monomer capable of a covalent bond or noncovalent bond with a specific target substance called a template, and a crosslinking agent constituting the polymeric matrix. When the template is removed from the polymerized polymer, the pore is complementary to the chemical function of the template. Thus, molecularly imprinted polymers are capable of selective recognition of a specific template.

Figure 2. Mechanism of molecularly imprinted polymer

In the polymer thin film material laboratory, functional polymers are synthesized and capable of selectively detecting various organic compounds such as drugs, physiologically active substances, insecticides and herbicides such as proteins, alkaloids, nerve stabilizers, and antibiotics by introducing molecular imprinting technology. Besides, we are also conducting a response characteristics analysis and applied research.

2-2. Lithography

2-2-1. Soft Lithography

Soft lithography is the first technique introduced by the Whitesides group from Harvard University, in which a patterned master mold is replicated with an elastomeric stamp to form a pattern. The use of an elastomer such as PDMS as a replica mold is referred to as “soft” and is classified into five types according to a pattern forming method.

Micro-Contact Printing (μCP): A method widely used when a patterned self-assembly monolayer (SAM) is formed by placing a patterned elastomer on a substrate and transferring the patterned to a substance having a contact area.

Replica Molding (REM): A method of replicating an existing shape or structure on the surface of an elastomer stamp in order to form a polymer. A UV curable or free-solvent thermosetting pre-polymer has a shrinkage of less than about 3% upon curing. Therefore, an inverted structure can be created with respect to the surface shape of the stamp.

Micro-transfer Molding (μTM): The pre-polymer solution is dropped onto the patterned elastomer stamp, and the channel is transferred by contacting the substrate, which is suitable for patterning of sub-micron and nano-units in soft lithography.

Micro-Molding in Capillaries (MIMIC): A method of placing the patterned stamp on the substrate, injecting the pre-polymer solution at one end, and finally, the solution is injected along the pattern by capillary phenomenon. After the curing process, the stamp is removed to produce a patterned polymer. This method requires good penetration of the solution along the channel and is an effective method for patterning in micro units.

Solvent-Assisted Micro-molding (SAMIM): The solvent is dropped on the patterned stamp, and the patterned stamp is placed on the substrate in which the polymer film is formed. The solvent must be capable of dissolving the polymer film. In the polymer thin film material laboratory, various patterning techniques using elastomer stamps are available, and applied research are currently being conducted.

Figure 3. Fabrication of striped patterned atrazine sensors using soft lithography

2-2-2. Colloidal Lithography

A colloid is a mixture in which particles of 1nm -1μm size are uniformly dispersed in a solvent. The colloidal particles can be uniformly aligned due to their self-assembly properties. There are several typical methods such as spin coating, dip coating, and solvent evaporation.

The spin coating is a method of placing colloidal particles on a substrate by using centrifugal force. It is necessary to control conditions such as spin coating speed, time, the concentration of a colloidal solution, substrate-particle interaction and so on. Although the particles can be aligned in a large area in a short time, there is a disadvantage in term of low uniformity. Consequently, the spin coating and floating method are widely utilized in order to complement the conventional spin coating method.

The dip coating is a method of arranging the colloid particles by immersing the substrate in the colloid solution and lifting the substrate at a specific speed. Various parameters such as substrate withdrawal speed, concentration in colloidal solution, substrate-particle interaction, solvent evaporation rate, immersion angle, and substrate hydrophilicity should be considered.

The solvent evaporation is the most popular method of immersing the substrate at a certain angle in the colloid solution and evaporating the solvent. This is an easy method to form opal structures under controlled conditions such as the evaporation rate of the solvent, the immersion angle and the concentration of the colloidal solution, but it has a drawback that it takes a long time to fabricate.

In the polymer thin film material laboratory, we are conducting various research based on colloidal arrangement methods that are capable to produce a various structure such as line, zigzag, and unaligned structure as well as a highly ordered structure of colloidal particles.

Figure 4. SEM images of a porous structured sensor for theophylline recognition using PS colloidal monolayer

Figure 5. SEM image of the hierarchically structured sensor for caffeine recognition

2-3. Electrospinning technology

Electrospinning is a technique in which a polymer solution having a sufficient viscosity which can produce a large number of nanofibers in a short time under an electrostatic force. The electrospinning is a combined term of spinning and electrostatic force. Basically, spinning has meaning to produce fibers and the electrostatic force is used to control the type of fiber. The basic equipment required for electrospinning consists of a syringe pump, a needle, a high voltage power supply, and a collector to pick up the spun fibers.

When the polymer solution is discharged through the needle, a positive or negative charge is accumulated in the polymer solution by the electric field under high voltage between 0 and 30 kV conditions. At this time, due to the mutual repulsive force of the same charge, the surface tension of the polymer solution is exceeded, and the polymer is drawn into a fiber form and collected by a collector. In electrospinning, the shape and thickness of the fiber are determined by the characteristics of the polymer, the polarity of the solvent, the material properties such as viscosity and surface tension, and the driving conditions such as voltage and scanning speed. The obtained fibers have a diameter of between 20 nm and 1 μm and can be fabricated into structures of bead or pillar depending on the type of needles. Electrospinning has extensively applied to various industrial fields (sensor technology, optoelectronics, catalysis, filtration, and medicine) due to the simple process and the advantages of a wide selection of materials. In the polymer thin film material laboratory, functional nanofibers are fabricated by electrospinning of polymer solutions containing functional molecules and are being applied to the field of sensors and filters.

Figure 6. Fabrication and SEM images of nanofibers for creatinine recognition

3. Applications

3-1. Chemical/Bio Sensor

3-2. Stimuli-Responsive Hydrogel

3-3. Filter