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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>
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>