Hyungju Ahn

Fluorination on both D and A units in D–A type conjugated copolymers based on difluorobithiophene and benzothiadiazole for highly efficient polymer solar cells

Fluorination of conjugated polymers is one of the effective strategies to tune the frontier energy levels for achieving high efficiency polymer solar cells. In this study, three fluorinated D–A polymers, consisting of 3,3′-difluoro-2,2′-bithiophene and 2,1,3-benzothiadiazole (BT) with different numbers of fluorine substitution, were synthesized in order to investigate the effect of fluorination on their photovoltaic properties. The polymers with fluorinated BT show lower frontier energy levels, improved polymer ordering, and a narrower fibril size in the blend with PC71BM. The polymer with mono-fluorinated BT exhibits a superior PCE of 9.14% due to a high SCLC hole mobility, mixed orientation of polymer crystals in the active layer, and low bimolecular recombination. This result demonstrates that the fluorine content in conjugated polymers should be controlled for optimizing optoelectrical and photovoltaic properties of fluorinated conjugated polymers.

Directed Assembly of High Molecular Weight Block Copolymers: Highly Ordered Line Patterns of Perpendicularly Oriented Lamellae with Large Periods

The directed assembly of block copolymer nanostructures with large periods exceeding 100 nm remains challenging because the translational ordering of long-chained block copolymer is hindered by its very low chain mobility. Using a solvent-vapor annealing process with a neutral solvent, which was sequentially combined with a thermal annealing process, we demonstrate the rapid evolution of a perpendicularly oriented lamellar morphology in high molecular weight block copolymer films on neutral substrate. The synergy with the topographically patterned substrate facilitated unidirectionally structural development of ultrahigh molecular weight block copolymer thin films-even for the structures with a large period of 200 nm-leading to perfectly guided, parallel, and highly ordered line-arrays of perpendicularly oriented lamellae in the trenched confinement. This breakthrough strategy, which is applicable to nanolithographic pattern transfer to target substrates, can be a simple and efficient route to satisfy the demand for block copolymer assemblies with larger feature sizes on hundreds of nanometers scale.

Directed self-assembly of block copolymers in the extreme: guiding microdomains from the small to the large

The self-assembly of block copolymers (BCPs) is emerging as a promising route for numerous applications to generate templates and scaffolds for the fabrication of nanostructured materials. Here, we present an overview of recent progress in the directed self-assembly of BCPs with a focus on guiding the assemblies of extreme (small and large) features over large areas. We introduce inorganic-containing hybrid BCPs that enable access to ultra-small feature sizes. To achieve the desired orientation and lateral ordering of the BCP microdomains, we discuss routes that combine top-down (lithographic) processes with the bottom-up self-assembly of BCPs. On the extreme large size scale, high-molecular-weight BCPs and brush-type copolymers are discussed, covering the other extreme of this promising strategy.

Efficiency improvement of dye-sensitized solar cells using graft copolymer-templated mesoporous TiO2 films as an interfacial layer

Organized mesoporous TiO2 films with high porosity and good connectivity were synthesized via sol–gel by templating an amphiphilic graft copolymer consisting of poly(vinyl chloride) backbone and poly(oxyethylene methacrylate) side chains, i.e., PVC-g-POEM. The randomly microphase-separated graft copolymer was self-reorganized to exhibit a well-ordered micellar morphology upon controlling polymer–solvent interactions, as confirmed by atomic force microscope (AFM) and glazing incidence small-angle X-ray scattering (GISAXS). These organized mesoporous TiO2 films, 550 nm in thickness, were used an an interfacial layer between a nanocrystalline TiO2 thick layer and a conducting glass in dye-sensitized solar cells (DSSC). Introduction of the organized mesoporous TiO2 layer resulted in the increased transmittance of visible light, decreased interfacial resistance and enhanced electron lifetime. As a result, an energy conversion efficiency of DSSC employing polymer electrolyte was significantly improved from 3.5% to 5.0% at 100 mW cm−2.

Nanoporous block copolymer membranes for ultrafiltration: a simple approach to size tunability.

Nanoporous structures were obtained by the self-assembly of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymers (BCP) where, in thick films, cylindrical microdomains were oriented normal to the substrate and air interfaces, and in the interior of the films, the microdomains were randomly oriented. Continuous nanopores that penetrated through the film were readily produced by a simple preferential swelling of the PMMA microdomains. The confined swelling and rapid contraction of PMMA microdomains generated well-defined uniform pores with diameters to 17.5 nm. The size selectivity and rejection of Au nanoparticles (NPs) for these ultrafiltration (UF) membranes were demonstrated, suggesting an efficient route to tunable, noncomponent-degradative UF membranes.

Mesoporous monoliths of inverse bicontinuous cubic phases of block copolymer bilayers

Solution self-assembly of block copolymers into inverse bicontinuous cubic mesophases is a promising new approach for creating porous polymer films and monoliths with highly organized bicontinuous mesoporous networks. Here we report the direct self-assembly of block copolymers with branched hydrophilic blocks into large monoliths consisting of the inverse bicontinuous cubic structures of the block copolymer bilayer. We suggest a facile and scalable method of solution self-assembly by diffusion of water to the block copolymer solution, which results in the unperturbed formation of mesoporous monoliths with large-pore (>25 nm diameter) networks weaved in crystalline lattices. The surface functional groups of the internal large-pore networks are freely accessible for large guest molecules such as protein complexes of which the molecular weight exceeded 100 kDa. The internal double-diamond (Pn3m) networks of large pores within the mesoporous monoliths could be replicated to self-supporting three-dimensional skeletal structures of crystalline titania and mesoporous silica.

Effect of sodium lauryl sulfate on recurrent aphthous stomatitis: a randomized controlled clinical trial.

OBJECTIVE To compare the effects of sodium lauryl sulfate (SLS)-free and SLS-containing dentifrice in patient with recurrent aphthous stomatitis (RAS). MATERIALS AND METHODS The design of this study was a double-blind crossover trial. The 90 subjects were divided into three groups: group I used SLS-free (a commercially available SLS-free dentifrice) and SLS-A (SLS-free + 1.5% SLS), group II used SLS-A and SLS-B (a commercially available 1.5% SLS-containing dentifrice), and group III used SLS-free and SLS-B. The subjects used one of the two assigned dentifrices for 8 weeks and then the other for the following 8 weeks. The order of the dentifrices used was selected at random, and there was a 2-week washout period between the two phases. The clinical parameters (number of ulcers, number of episodes, duration of ulcers, mean pain score) were compared between the two phases for each group. RESULTS The number of ulcers and episodes did not differ significantly between SLS-A, SLS-B, and SLS-free. Only duration of ulcers and mean pain score was significantly decreased during the period using SLS-free. CONCLUSION Although SLS-free did not reduce the number of ulcers and episodes, it affected the ulcer-healing process and reduces pain in daily lives in patients with RAS.

Objective and subjective assessment of masticatory function for patients with temporomandibular disorder in Korea.

This study examined the differences in the masticatory function of patients with temporomandibular disorder (TMD) in Korea. The experimental groups were as follows: 23 patients with painful arthralgia classified as pain group according to the research diagnostic criteria for temporomandibular disorder (RDC/TMC) and 28 patients with pain-free disc displacement and reduction classified as clicking group. The subjects were obtained from those who had visited Yonsei University Dental Hospital from 2007 to 2008. Twenty dental students without TMD symptoms were enroled as the normal control group. The Mixing Ability Index (MAI) was used as the objective index, and the Food Intake Ability (FIA) Index, Visual Analogue Scale (VAS) and oral health impact profile (OHIP) were used as the subjective indices. The MAI, FIA and VAS were significantly lower in the pain group than in the normal and clicking groups (P<0·05). The pain group showed a MAI, FIA and VAS of 16%, 81% and 67%, respectively, compared to that of the normal group. However, there were no significant differences in the MAI, FIA and VAS between the clicking and normal groups. The pain and clicking groups showed a 1·7 and 1·4 times higher OHIP value than the normal group (P<0·05). The MAI and subjective indices, such as the FIA (r=0·40) and VAS (r=0·48), showed a moderate correlation (P<0·01). In conclusion, pain is the main factor for the reduced masticatory function in patients with TMD in Korea, and the joint sound, not the masticatory function, affects the declining OHIP.

Investigation of Thermally Induced Degradation in CH 3 NH 3 PbI 3 Perovskite Solar Cells using In-situ Synchrotron Radiation Analysis

In this study, we employ a combination of various in-situ surface analysis techniques to investigate the thermally induced degradation processes in MAPbI3 perovskite solar cells (PeSCs) as a function of temperature under air-free conditions (no moisture and oxygen). Through a comprehensive approach that combines in-situ grazing-incidence wide-angle X-ray diffraction (GIWAXD) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) measurements, we confirm that the surface structure of MAPbI3 perovskite film changes to an intermediate phase and decomposes to CH3I, NH3, and PbI2 after both a short (20 min) exposure to heat stress at 100 °C and a long exposure (>1 hour) at 80 °C. Moreover, we observe clearly the changes in the orientation of CH3NH3+ organic cations with respect to the substrate in the intermediate phase, which might be linked directly to the thermal degradation processes in MAPbI3 perovskites. These results provide important progress towards improved understanding of the thermal degradation mechanisms in perovskite materials and will facilitate improvements in the design and fabrication of perovskite solar cells with better thermal stability.

Lamellar microdomain orientation and phase transition of polystyrene-b-poly(methyl methacrylate) films by controlled interfacial interactions

The orientation of lamellar microdomains and phase transition for the thin films of a symmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) on random copolymer-grafted substrates were investigated by transmission electron microscopy (TEM) and grazing-incidence small-angle X-ray scattering (GISAXS). The styrene mole fraction (XS) in random copolymers of P(S-r-MMA) was controlled to tune the interfacial interactions at the substrates from PS-selective to PMMA-selective. Except in the case of a neutral substrate with XS = 0.55, all the films showed the parallel orientations of lamellar microdomains, which were validated by the TEM images. However, the GISAXS analysis of PS-b-PMMA films indicated that the intensity ratio of the out-of-plane scattering to the in-plane scattering can be a sensitive indicator for evaluating the degree of orientation of lamellar microdomains or surface neutrality at the substrates. The orientation of lamellar microdomains on a neutral substrate was influenced by the film thickness and molecular weight of PS-b-PMMA, which results from predominantly the entropic contribution to the free energy in competition between the chain-end effect and nematic term. Intriguingly, the order-to-disorder temperature (ODT) of PS-b-PMMA films on a series of the substrates shows a minimum at XS = 0.55. The ODT measurements, a new approach to evaluating the interfacial interactions at substrates, confirm that a neutral substrate induces surface compatibility between the PS and PMMA blocks at the substrate.