Influence of Molecular Coherence on Surface Viscosity

Choi, Siyoung Q.,Kim, Kyuhan,Fellows, Colin M.,Cao, Kathleen D.,Lin, Binhua,Lee, Ka Yee C.,Squires, Todd M.,Zasadzinski, Joseph A. American Chemical Society 2014 Langmuir Vol.30 No.29

<P/><P>Adding small fractions of cholesterol decreases the interfacial viscosity of dipalmitoylphosphatidylcholine (DPPC) monolayers by an order of magnitude per wt %. Grazing incidence X-ray diffraction shows that cholesterol at these small fractions does not mix ideally with DPPC but rather induces nanophase separated structures of an ordered, primarily DPPC phase bordered by a line-active, disordered, mixed DPPC-cholesterol phase. We propose that the free area in the classic Cohen and Turnbull model of viscosity is inversely proportional to the number of molecules in the coherence area, or product of the two coherence lengths. Cholesterol significantly reduces the coherence area of the crystals as well as the interfacial viscosity. Using this free area collapses the surface viscosity data for all surface pressures and cholesterol fractions to a universal logarithmic relation. The extent of molecular coherence appears to be a fundamental factor in determining surface viscosity in ordered monolayers.</P>

Partially Oxidized Asphaltene as a Bitumen Viscosity Reducer

Choi, Seonung,Choi, Siyoung Q.,Kim, Jong-Duk,Nho, Nam-sun American Chemical Society 2017 ENERGY AND FUELS Vol.31 No.9

<P>Heavy oil/bitumen with asphaltene requires high energy consumption for transportation, because of its high viscosity. This often causes serious pipeline problems; therefore, it is necessary to reduce the viscosity to resolve these issues. In this paper, we propose a new simple method to reduce the viscosity of diluted bitumen using partially oxidized asphaltene without removing asphaltene. This partially oxidized asphaltene, modified by exposing asphaltene to ozone, has a strong affinity to nonoxidized asphaltene via pi-pi stacking, hydrogen bonding, and acid-base interaction. The added oxidized asphaltene thus facilitated the agglomeration of asphaltene, so that the hydrophilic part can be hidden, and a small amount (1250 ppm) decreased the viscosity of diluted bitumen by up to 45%. We investigated the change in rheological properties of the diluted bitumen for various oxidized fractions of asphaltene and for various fractions of oxidized asphaltene to asphaltene. We showed that the decreased shear viscosity is likely to originate with the increased particle size of the asphaltene, that is, not the change in interactions between the asphaltene particles, but rather from their nonuniform distribution in size.</P>

Importance of Critical Molecular Weight of Semicrystalline n-Type Polymers for Mechanically Robust, Efficient Electroactive Thin Films

Choi, Joonhyeong,Kim, Wansun,Kim, Donguk,Kim, Seonha,Chae, Junsu,Choi, Siyoung Q.,Kim, Felix Sunjoo,Kim, Taek-Soo,Kim, Bumjoon J. American Chemical Society 2019 Chemistry of materials Vol.31 No.9

<P>Mechanical properties of conducting polymers are an essential consideration in the design of flexible and stretchable electronics, but the guidelines for the material design having both high mechanical and electrical properties remain limited. Here we provide an important guideline for the design of mechanically robust, electroactive polymer thin films in terms of the molecular weight of the polymers. These studies based on a highly efficient, representative n-type conjugated polymer (P(NDI2OD-T2)) revealed a marked enhancement in mechanical properties across a narrow molecular weight range, highlighting the existence of a critical molecular weight that can be exploited to engineer films that balance processability and mechanical and electronic properties. We found the thin films formed from high molecular weight polymers (i.e., number-average molecular weight (<I>M</I><SUB>n</SUB>) ∼ 163 kg mol<SUP>-1</SUP>) to exhibit superior mechanical compliance and robustness, with a 114-fold enhanced strain at fracture and a 2820-fold enhanced toughness, as compared to those of low molecular weight polymer films (<I>M</I><SUB>n</SUB> = 15 kg mol<SUP>-1</SUP>). In particular, we observed a jump in the mechanical properties between the <I>M</I><SUB>n</SUB> = 48 and 103 kg mol<SUP><B>-</B>1</SUP>, yielding a 26-fold enhanced strain at fracture and a 160-fold enhanced toughness. The significant improvement of tensile properties indicates the presence of a critical molecular weight at which entangled polymer networks start to form, as supported by the analysis of the thermal and crystalline properties, specific viscosity, and microstructure. Our work provides useful guidelines for the design of conjugated polymers with recommendations for the best combinations of mechanical robustness and electrical performance for flexible and stretchable electronics.</P> [FIG OMISSION]</BR>

Quantitative analysis for lipophilic drug transport through a model lipid membrane with membrane retention

Lee, Yohan,Choi, Siyoung Q. Elsevier 2019 European journal of pharmaceutical sciences Vol.134 No.-

<P><B>Abstract</B></P> <P>Drug absorption after oral administration mostly occurs in the intestine, and is controlled mainly by the aqueous solubility and the intestinal permeability of the drug. In modern drug discovery, the proportion of lipophilic drug candidates with poor water solubility has increased, and these lipophilic drugs are known to undergo membrane retention during transport across the cell membrane, depending on the hydrophobic interaction between the drug and lipid molecules. Hence, a precise and effective permeability assay for lipophilic drug compounds, which can also quantify membrane retention, is required. In this study, we developed a permeability assay for lipophilic drugs with poor water solubility using a freestanding lipid bilayer. The lipid bilayer was created within a UV cuvette, and the number of transported molecules through the bilayer was estimated by measuring the UV absorbance over time. We then measured the permeability of six tested compounds, and there was a significant difference in permeability between the Biopharmaceutics Classification System (BCS) class 2 and class 4 compounds. In addition, the fraction of molecules trapped in the lipid bilayer were estimated for each compound as well. As a result, the drugs with higher lipophilicity were found to undergo more membrane retention, which agrees with the previous reports, supporting that our model lipid membrane system could also be applied to investigate the drug-lipid interaction.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Nonlinear chiral rheology of phospholipid monolayers

Kim, KyuHan,Choi, Siyoung Q.,Zasadzinski, Joseph A.,Squires, Todd M. The Royal Society of Chemistry 2018 SOFT MATTER Vol.14 No.13

<P>Microbutton rheometry reveals that the chiral morphology of dipalmitoylphosphatidylcholine (DPPC) monolayers imparts a chiral nonlinear rheological response. The nonlinear elastic modulus and yield stress of DPPC monolayers are greater when sheared clockwise (C), against the natural winding direction of DPPC domains, than counter-clockwise (CC). Under strong CC shear strains, domains deform plastically; by contrast, domains appear to fracture under strong C shearing. After CC shearing, extended LC domains develop regular patterns of new invaginations as they recoil, which we hypothesize reflect the nucleation and growth of new defect lines across which the tilt direction undergoes a step change in orientation. The regular spacing of these twist-gradient defects is likely set by a competition between the molecular chirality and the correlation length of the DPPC lattice. The macroscopic mechanical consequences of DPPC's underlying molecular chirality are remarkable, given the single-component, non-cross-linked nature of the monolayers they form.</P>

Blending Mechanism of PS-<i>b</i>-PEO and PS Homopolymer at the Air/Water Interface and Their Morphological Control

Kim, Baekmin Q.,Jung, Yunji,Seo, Myungeun,Choi, Siyoung Q. American Chemical Society 2018 Langmuir Vol.34 No.35

<P>We report a blending mechanism of polystyrene-<I>b</I>-poly(ethylene oxide) (PS-<I>b</I>-PEO) and PS homopolymer (homoPS) at the air/water interface. Our blending mechanism is completely different from the well-known “wet-dry brush theory” for bulk blends; regardless of the size of homoPS, the domain size increased and the morphology changed without macrophase separation, whereas the homoPS of small molecular weight (MW) leads to a transition after blending into the block copolymer domains, and the large MW homoPS is phase-separated in bulk. The difference in blending mechanism at the interface is attributed to adsorption kinetics at a water/spreading solvent interface. Upon spreading, PS-<I>b</I>-PEO is rapidly adsorbed to the water/spreading solvent interface and forms domain first, and then homoPS accumulates on them as the solvent completely evaporates. On the basis of our proposed mechanism, we demonstrate that rapid PS-<I>b</I>-PEO adsorption is crucial to determine the final morphology of the blends. We additionally found that spreading preformed self-assemblies of the blends slowed down the adsorption, causing them to behave similar to bulk blends, following the “wet-dry brush theory”. This new mechanism provides useful information for various block copolymer-homopolymer blending systems with large fluid/fluid interfaces such as emulsions and foams.</P> [FIG OMISSION]</BR>

Shape-controlled percolation transition in 2D random packing of asymmetric dimers

Han, Youngkyu,Lee, Juncheol,Choi, Siyoung Q.,Choi, Myung Chul,Kim, Mahn Won IOP Publishing - Europhysics Letters 2015 Europhysics letters Vol.109 No.6

Controllable one-step double emulsion formation <i>via</i> phase inversion

Kim, Subeen,Kim, KyuHan,Choi, Siyoung Q. The Royal Society of Chemistry 2018 SOFT MATTER Vol.14 No.7

<P>Double emulsions, the simplest form of multiple emulsion, have been intensively utilized in various industries as well as in fundamental research. A variety of strategies to effectively form double emulsions have been developed, but no simple yet controlled and scalable technique has been achieved yet. Herein, we examine the mechanism of the entire process of double emulsion formation by phase inversion, and we propose a universal one-step strategy for the formation of an oil/water/oil double emulsion using oil soluble polymers and hydrophobic silica nanoparticles. We demonstrate that this new approach enables control of both the fraction and the number of inner small droplets; even high internal phase double emulsions could be achieved.</P>

지질막에서의 단백질 자기조립과 퇴행성 질환과의 관계

박수진(Sujin Park),최시영(Siyoung Q. Choi) 한국고분자학회 2019 고분자 과학과 기술 Vol.30 No.2

타닌산-전이 금속-고분자로 구성된 젤의 단일 단계 합성과 점착제로의 이용

이재홍,이경문,최시영,Lee, Jaehong,Lee, Kyoungmun,Choi, Siyoung Q. 한국화학공학회 2020 Korean Chemical Engineering Research(HWAHAK KONGHA Vol.58 No.2

이 연구에서는 작은 유기 분자 말단의 하이드록실기와 전이 금속 사이의 배위 결합을 통해 고분자와 유사하게 연결된 복합체를 제작하고, 점착 부여제를 추가하여 해당 물질의 점착제로의 사용 가능성을 확인하였다. 점착제 합성에 사용한 타닌산(tannic acid, TA)은 하이드록실기를 풍부하게 보유하고 있어 전이 금속과는 배위 결합이 가능하고 친수성 고분자와는 수소 결합이 가능하다. 위의 성질을 이용하여 타닌산과 전이 금속, 고분자 세 가지 성분을 한 번에 간단히 섞어 기판에 잘 펴지며 점착 능력을 보유한 특별한 유변 물성을 가지는 물질을 제작하였다. 합성에 사용한 전이 금속의 종류(Fe³⁺, Ti⁴⁺), 고분자의 종류, 처리 조건 등에 따른 유변 물성의 변화를 확인하는 과정을 통해 점착제로 사용하기에 가장 적합한 성분의 조합을 발견하였으며, 인체에 무해하며 높은 응집력과 접착력을 보유한 다목적 점착제로의 사용 가능성을 확인하였다. In this study, synthesis of a hydrogel consisted of a coordination bond network between small organic molecules and transition metals had been carried out. By adding a tackifying material to the gel, the potential of the gel to be used as an adhesive material had been also confirmed. Synthesis of the adhesive had been done with simple mixing of 3 components: tannic acid, transition metal, and polymer. The tannic acid molecule possesses multiple hydroxyl groups that can form coordination bonds with the transition metals and hydrogen bonds with the hydrophilic polymers. Due to the morphology of the metal-organic complex and polymer dispersed in water, the fabricated material exhibited high adhesiveness and cohesiveness. Optimizing the rheological property had been conducted for use in adhesive by the synthesis with varying the transition metal (Fe³⁺, Ti⁴⁺), polymer, and treatment conditions. Rheological measurement results demonstrate the promising potential of the material as a bio-compatible and versatile pressure-sensitive adhesive with both high adhesiveness and cohesiveness.