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        Analysis of the static yield stress for giant electrorheological fluids

        Youngwook P. Seo,최형진,서용석 한국유변학회 2017 Korea-Australia rheology journal Vol.29 No.3

        Cheng et al. (2010)'s experimental results for the static yield stress of giant electrorheological (GER) fluids over the full range of electric field strengths were reanalyzed by applying Seo’s scaling function which could include both the polarization and the conductivity models. The Seo’s scaling function could correctly fit the yield stress behavior of GER suspensions behavior after if a proper normalization of the yield stress data was taken which collapse them onto a single curve. The model predictions were also contrasted with recently proposed Choi et al.’s scaling function to rouse the attention for a proper consideration of the GER fluid mechanisms.

      • Dispersion and reaggregation of nanoparticles in the polypropylene copolymer foamed by supercritical carbon dioxide

        Oh, Kyunghwan,Seo, Youngwook P.,Hong, Soon Man,Takahara, Atsushi,Lee, Kyoung Hwan,Seo, Yongsok The Royal Society of Chemistry 2013 Physical chemistry chemical physics Vol.15 No.26

        <P>For the preparation of nanocomposites, we conducted environmentally benign foaming processing on polypropylene (PP) copolymer/clay nanocomposites <I>via</I> a batch process in an autoclave. We investigated the dispersion and the exfoliation of the nanoclay particles. Full exfoliation was achieved by the foamability of the matrix PP copolymer using supercritical carbon dioxide (sc CO<SUB>2</SUB>) and subcritical carbon dioxide (sub CO<SUB>2</SUB>). More and smaller cells were observed when the clay was blended as heterogeneous nuclei and sc CO<SUB>2</SUB> was used. Small angle X-ray scattering showed that highly dispersed states (exfoliation) of the clay particles were obtained by the foaming process. Since the clay particles provided more nucleating sites for the foaming of the polymer, a well dispersed (or fully exfoliated) nanocomposite exhibited a higher cell density and a smaller cell size at the same clay particle concentration. Expansion of the adsorbed CO<SUB>2</SUB> facilitated the exfoliation of the clay platelets; thus, sc CO<SUB>2</SUB> at lower temperature was more efficient for uniform foaming-cell production. Fully dispersed clay platelets were, however, re-aggregated when subjected to a further melting processing. The reprocessed nanocomposites still had some exfoliated platelets as well as some aggregated intercalates. The dual role of the nanoclay particles as foaming nucleus and a crystallization nucleus was confirmed by cell growth observation and nonisothermal crystallization kinetics analysis. A low foaming temperature and a high saturation pressure were more favorable for obtaining a uniform foam. The PP copolymer was found to be foamed more easily than polypropylene. A small amount of other olefin moieties in the backbone of the polymer facilitated better foamability than the neat polypropylene.</P> <P>Graphic Abstract</P><P>Polypropylene copolymer nanocomposite was more easily foamed than polypropylene by supercritical CO<SUB>2</SUB>, the expansion of which helped the exfoliation of clay platelets. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3cp51068a'> </P>

      • A simplified model for analyzing the flow behavior of electrorheological fluids containing silica nanoparticle-decorated polyaniline nanofibers

        Seo, Youngwook P.,Choi, Hyoung Jin,Seo, Yongsok The Royal Society of Chemistry 2012 SOFT MATTER Vol.8 No.17

        <P>A new rheological model was applied to the analysis of the electrorheological behavior of a fluid containing silica nanoparticle-decorated polyaniline nanofibers. The model's predictions were compared with the experimental data, revealing that the proposed model correctly predicted the shear stress behavior both quantitatively and qualitatively. The shear stress data of the electrorheological fluid showing aligned fibers' structural reformation as a function of the shear rate agreed well with the new model which required fewer parameters than the CCJ (Cho–Choi–Jhon) model. The static yield stress was found to be quadratically dependent on the field strength, in agreement with the predictions of the polarization model. A scaling function was used to model the yield stress behavior of the electrorheological fluid over a range of electric fields, and it correctly predicted the static yield stress behavior both quantitatively and qualitatively.</P> <P>Graphic Abstract</P><P>A new rheological model was applied to the analysis of the electrorheological behavior of a fluid containing silica nanoparticle-decorated polyaniline nanofibers. The model's predictions were compared with the experimental data, revealing that the proposed model correctly predicted the shear stress behavior both quantitatively and qualitatively. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2sm07275k'> </P>


        High-Performance Magnetorheological Suspensions of Pickering-Emulsion-Polymerized Polystyrene/Fe<sub>3</sub>O<sub>4</sub> Particles with Enhanced Stability

        Han, Sangsuk,Choi, Junsok,Seo, Youngwook P.,Park, In Jun,Choi, Hyoung Jin,Seo, Yongsok American Chemical Society 2018 Langmuir Vol.34 No.8

        <P>The magnetorheological (MR) performance of suspensions based on core–shell-structured foamed polystyrene (PSF)/Fe<SUB>3</SUB>O<SUB>4</SUB> particles was investigated by using a vibrating sample magnetometer and a rotational rheometer. Core–shell-structured polystyrene (PS)/Fe<SUB>3</SUB>O<SUB>4</SUB> was synthesized by using the Pickering-emulsion polymerization method in which Fe<SUB>3</SUB>O<SUB>4</SUB> nanoparticles were added as a solid surfactant. Foaming the PS core in PS/Fe<SUB>3</SUB>O<SUB>4</SUB> particles was carried out by using a supercritical carbon dioxide (scCO<SUB>2</SUB>) fluid. The density was measured by a pycnometer. The densities of PS/Fe<SUB>3</SUB>O<SUB>4</SUB> and PSF/Fe<SUB>3</SUB>O<SUB>4</SUB> particles were significantly lowered from that of the pure Fe<SUB>3</SUB>O<SUB>4</SUB> particle after Pickering-emulsion polymerization and foaming treatment. All tested suspensions displayed similar MR behaviors but different yield strengths. The important parameter that determined the MR performance was not the particle density but rather the surface density of Fe<SUB>3</SUB>O<SUB>4</SUB> on the PS core surface. The morphology was observed by scanning electron microscopy and transmission electron microscopy. Most Fe<SUB>3</SUB>O<SUB>4</SUB> particles stayed on the surface of PS/Fe<SUB>3</SUB>O<SUB>4</SUB> particles, making the surface topology bumpy and rough, which decreased the particle sedimentation velocity. Finally, Turbiscan apparatus was used to examine the sedimentation properties of different particle suspensions. The suspensions of PS/Fe<SUB>3</SUB>O<SUB>4</SUB> and PSF/Fe<SUB>3</SUB>O<SUB>4</SUB> showed remarkably improved stability against sedimentation, much better than the bare Fe<SUB>3</SUB>O<SUB>4</SUB> particle suspension because of the reduced density mismatch between the nanoparticles and the carrier medium as well as the surface topology change.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/langd5/2018/langd5.2018.34.issue-8/acs.langmuir.7b04043/production/images/medium/la-2017-040436_0010.gif'></P>

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