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Haladhara NAIK,Guinyun KIM,Annareddy Venkat Ramann REDDY,Devesh RAJ,Duc Khue PHAM,In Soo KO,Kwangsoo KIM,Moo-Hyun CHO,Srinivasan GANESAN,남궁원,Young Do OH 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.3
The post-neutron-emission mass yield distribution and the photo-neutron cross-section σ (γ,3n)) of 209Bi with 65-MeV bremsstrahlung have been determined using an off-line gamma-ray spectrometric technique in the test electron linac at Pohang Accelerator Laboratory. The post-neutron mass yield distribution in 65-MeV bremsstrahlung induced fission of 209Bi is symmetric with average mass around 103 and a FWHM of 22 mass units, which is higher than the literature value of 19 mass units in the energy range of 28 -- 40 MeV. The increasing trend of the FWHM of the post-neutron mass yield distribution with increasing breamsstrahlung energy has been explained from the point of view of the increase in the multi-chance fission probabilities with increasing excitation energy. The (γ,3n) reaction cross-section is determined for the first time. It was also calculated using the TALYS code and was found to be in good agreement with the experimental value. The post-neutron-emission mass yield distribution and the photo-neutron cross-section σ (γ,3n)) of 209Bi with 65-MeV bremsstrahlung have been determined using an off-line gamma-ray spectrometric technique in the test electron linac at Pohang Accelerator Laboratory. The post-neutron mass yield distribution in 65-MeV bremsstrahlung induced fission of 209Bi is symmetric with average mass around 103 and a FWHM of 22 mass units, which is higher than the literature value of 19 mass units in the energy range of 28 -- 40 MeV. The increasing trend of the FWHM of the post-neutron mass yield distribution with increasing breamsstrahlung energy has been explained from the point of view of the increase in the multi-chance fission probabilities with increasing excitation energy. The (γ,3n) reaction cross-section is determined for the first time. It was also calculated using the TALYS code and was found to be in good agreement with the experimental value.
Lee, Wooseop,Park, Sungmin,Kim, Yeongsik,Sethuraman, Vaidyanathan,Rebello, Nathan,Ganesan, Venkat,Ryu, Du Yeol American Chemical Society 2017 Macromolecules Vol.50 No.15
<P>We modulated the grafting density (sigma) of a random copolymer brush of poly(styrene-r-methyl-methacrylate) on substrates to probe its effect on the formation of perpendicularly aligned lamellae of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA). Supported by coarse-grained simulation results, we hypothesized that an increase in sigma will allow us to systematically tune the block-copolymer interfacial interactions with substrates from being preferential to one 2 3 of the blocks to being neutral toward both blocks and will thereby facilitate enhanced regimes of perpendicularly aligned lamellae. We verified such a hypothesis by using a simple grafting-to approach to modify the substrates and characterized the thickness window for perpendicular lamellae as a function of brush thickness (or sigma) on the grafted substrates using scanning force microscopy (SFM) images and grazing incidence small-angle X-ray scattering (GISAXS) measurements. The experimental results validated our hypothesis and suggested that the sigma of random copolymer brushes can be used as an additional versatile parameter to modulate the interfacial interactions and the resulting alignment of block copolymer films.</P>
Pryamitsyn, Victor,Han, Sung Hyun,Kim, Jin Kon,Ganesan, Venkat American Chemical Society 2012 Macromolecules Vol.45 No.21
<P>We present a strong stretching theory model for microphase segregation of AB + AC block copolymer blends in which the B and C segments possess strongly attractive (hydrogen bonding) interactions. In microphase separated morphologies, we demonstrate that the attraction between the B and the C segments causes a bending force toward the A layers. Such bending forces may induce transitions from lamellar and A-majority cylindrical morphologies in the pure component systems to “inverted” cylindrical and spherical morphologies in blends in which the B and C segments constitute the matrix phase. Similar driving forces may also drive transitions from A-majority spherical phases in pure component systems to highly asymmetric lamellar morphologies in blends. The predictions of our model are in excellent agreement with the trends observed in recent experimental results.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/mamobx/2012/mamobx.2012.45.issue-21/ma301402n/production/images/medium/ma-2012-01402n_0012.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ma301402n'>ACS Electronic Supporting Info</A></P>
Nonmonotonic Glass Transition Temperature of Polymer Films Supported on Polymer Brushes
Lee, Hoyeon,Sethuraman, Vaidyanathan,Kim, Yeongsik,Lee, Wooseop,Ryu, Du Yeol,Ganesan, Venkat American Chemical Society 2018 Macromolecules Vol.51 No.12
<P>We modulated the grafting density (σ) and chain length of polystyrene (PS) brushes on substrates to probe their effect on the glass transition temperature (<I>T</I><SUB>g</SUB>) in overlaying PS thin films. The <I>T</I><SUB>g</SUB> of PS films was analyzed as a function of brush thickness (or σ<I>R</I><SUB>g</SUB><SUP>2</SUP>), where <I>R</I><SUB>g</SUB> is the radius of gyration of PS brushes. Our results indicate that PS films below 90 nm exhibit a nonmonotonic dependence of <I>T</I><SUB>g</SUB> on grafting density with the maximum <I>T</I><SUB>g</SUB> occurring near the regime of wetting-dewetting transition of the overlaying melt. The maximum <I>T</I><SUB>g</SUB> was found to be higher than the value of bulk PS system. Computer simulation results suggest that such trends arise as a consequence of the environment of enhanced friction presented by the brush layer and the overlap between the polymer film and the grafted layer.</P> [FIG OMISSION]</BR>