Session D6: Dynamics of Polymers on Multi-Length Scales: Interfaces

Polymer Loop Formation on a Functionalized Hard Surface: Quantitative Insight by Comparison of Experimental and Monte Carlo Simulation Results

The grafting of telechelic polystyrene terminated with carboxylic acid end groups from the melt onto an epoxidized silicon wafer is examined experimentally to monitor the kinetics of the grafting and loop formation processes. These ellipsometry and fluorimetry results are quantitatively correlated with bond fluctuation Monte Carlo (BFMC) simulations, which provides unique insight into the process of polymer loop formation. Specifically, this correlation provides a calibration of the fluorescence intensity to the amount of singly bound chains present on the surface, revealing that about 80{\%} of the bound chains form loops on the surface at the longest reaction time studied, and provides the time evolution of singly and doubly bound chains during the reaction. Moreover, this correlation is broadly applicable and can be used to readily monitor the impact of a broad range of reaction conditions (e.g., temperature, telechelic concentration, surface density of functional groups) on the loop formation process. This correlation, therefore, provides a method to access fundamental information that is not available by experiment alone, and yet is required to tailor surface properties through adjusting the coverage and fraction of loops in the grafted layer and to correlate surface sensitive properties to specific grafted layer structure.   

Surface Dynamics of ``Dry'' Homopolymer Brushes

The dynamics of the surface height fluctuations of polymer melts can be profoundly altered by tethering of the chains. Surface height fluctuations on layers of covalently tethered, nearly monodisperse polymer chains synthesized by atom transfer radical polymerization were studied using x-ray photon correlation spectroscopy. The data reveal that both polystyrene and poly($n$-butyl acrylate) ``brushes'' have structure at the surface with length scales in the region of 620 nm to 3100 nm, but the surface features show no relaxation in a time window of 0.1 seconds to 1000 seconds, even at temperatures more than 130 \r{ }C above the glass transition temperature of corresponding untethered chains. This remarkable alteration of the dynamics is compared with the suppression of fluctuations on this length scale anticipated by thermodynamic theories. The alteration of surface dynamics by tethering has implications for wetting, friction, and adhesion. \\[4pt] Collaborators: Bulent Akgun and G\"{o}k\c{c}e Ugur, University of Akron; Zhang Jiang, X-ray Science Division, Argonne National Laboratory and University of California, San Diego; Suresh Narayanan, X-ray Science Division, Argonne National Laboratory; Heeju Lee and Sanghoon Song, Department of Physics, Sogang University, Seoul, South Korea; William J. Brittain, University of Akron; Hyunjung Kim, Department of Physics, Sogang University, Seoul, South Korea; and Sunil K. Sinha, University of California, San Diego.   

Development of Biomimetic and Functionally Responsive Surfaces

Controlling the surface morphology of solids and manufacturing of functional surfaces with special responsive properties has been the subject of intense research. We report a methodology for creating multifunctionally responsive surfaces by irradiating silicon wafers with femtosecond laser pulses and subsequently coating them with different types of functional conformal coatings. Such surfaces exhibit controlled dual-scale roughness at the micro- and the nano-scale, which mimics the hierarchical morphology of water repellent natural surfaces. When a simple alkylsilane coating is utilized, highly water repellent surfaces are produced that quantitatively compare to those of the Lotus leaf. When a polymer brush is ``grafted from" these surfaces based on a pH-sensitive polymer, the surfaces can alter their behavior from super-hydrophilic (after immersion in a low pH buffer) to super-hydrophobic and water-repellent (following immersion to a high pH buffer). We quantify the water repellency of such responsive systems by drop elasticity measurements whereas we demonstrate that the water repellent state of such surface requires appropriate hydrophobicity of the functionalizing polymer. When a photo-responsive azobenzene-type polymer is deposited, a dynamic optical control of the wetting properties is obtained and the surface can be switched from super-hydrophilic (following UV irradiation) to hydrophobic (following green irradiation). In all the above cases we show that the principal effect of roughness is to cause amplification of the response to the different external stimuli.   

Molecular Dynamics Simulations of Responsive Semi-Fluorinated Interfaces

Responsive polymeric thin films with controlled surface energies, dielectric constants and structure are critical for a variety of emerging nano and micro-scale technologies including fluidics, electro-optical devices and biotechnology. Introducing nanometer sized fluorinated segments offers a means to tune the polymer properties while significantly enhancing chemical and thermal stability. The interfacial structure and dynamics of multiblock semi fluorinated copolymers at their liquid/vapor interface and at interfaces with water and protonated alkanes has been studied using explicit atom molecular dynamic simulations. For semifluorinated diblocks H$_{3}$C(CH$_{2})_{n-1}$(CF$_{2})_{m-1}$CF$_{3}$ of varying fluorine content, fluorinated groups proliferate and reside longer at the liquid/vapor interface as expected for the lower surface tension components. Aqueous interfaces of these diblocks are sharp and well defined with an enhanced density of protonated groups owing to their reduced hydrophobicity in comparison to fluorinated groups. The enhancement increases with temperature. Protonated alkanes are found to be mutually miscible with the semifluorinated diblock copolymers. Similar surface behavior is observed in semifluorinated multiblock copolymers of the form H-[(CH$_{2})_{n }$(CF$_{2})_{n}$]$_{m}$-F where m varies from 3 to 48 with nxm=48. The fluorine enhancement at the liquid-vapor interface depends on both the temperature and block length, with the longest blocks showing the greatest enhancement. Due to mutual phobicity of protonated and fluorinated groups, nm-scale fluorine and hydrogen rich regions occur at the surfaces of these materials, with sizes that also depend on block length and temperature. \newline \newline Work in collaboration with Dvora Perahia and Gary S. Grest.   

Physical Properties and Responsive Behavior of Semi-fluorinated Polymer Interfaces

The macromolecular platform for this research is a polyoxetane, which has a {\-}C{\-}C-C-O- main chain, specifically P[({\-}CH$_{2}$CMe(\textbf{A)}CH$_{2}$O{\-})(CH$_{2}$CMe(\textbf{B)}CH$_{2}$O-)]. If side chain \textbf{A} = \textbf{B}, the polyoxetane is semicrystalline; If \textbf{A} $\ne $ \textbf{B}, the copolyoxetane is an amorphous low $T_{g}$ telechelic used as a soft block. Dynamic interfacial behavior is described for (\textbf{\textit{1}}) \textbf{A} = \textbf{B} = CF$_{3}$CH$_{2}$OCH$_{2}$-, P(B-3FOx), and (\textbf{\textit{2}}) a soft block where \textbf{A} = 3FOx and \textbf{B} = C12, a CH$_{3}$(CH$_{2})_{11}$N$^{+}$(CH$_{3})_{2}$-(CH$_{2})_{4}$O- side chain used for introducing surface quaternary charge as a \textit{polymer surface modifier} (PSM). For P(B-3FOx) (21kDa), differences in cooling rates from the melt have substantial effects on crystal phase, percent crystallinity, surface topography, and wetting behavior. DSC and WAXD reveal that slow cooling from the melt ($\le $ 5 \r{ }C /min) gives $\alpha $-P(B-3FOx) while quenching from the melt results in $\beta $-P(B-3FOx), which forms an ordered mesophase. TM-AFM and SEM for $\alpha $-P(B-3FOx) shows cold crystallization (25 \r{ }C) brings about sharp asperities and lath-shaped crystals. A 30\r{ } increase in water contact angle is associated with the change from a relatively smooth surface (Wenzel) to an asperity-rich surface yielding a discontinuous three-phase contact line (composite of Wenzel and Cassie-Baxter). Prior research established P[\textbf{AB}]-copolyoxetane polyurethanes having soft blocks \textbf{\textit{2}} with\textbf{ A} = 3FOx and \textbf{B} = dodecylammonium-butoxy (C12) are effective \textit{contact} antimicrobial PSMs, but accessible quaternary charge density was unknown. Streaming potential (SP) measurements in microfluidic capillaries have been employed for estimating surface accessible charge. Inner capillary walls were coated with a base polyurethane modified by 1 wt{\%} \textbf{\textit{2}}\textbf{-PU}, that is, [HMDI{\-}BD(30){\-}P[(3FOx)(C12){\-}87:13-(5100)]. The neat PSM has a constant SP, but SPs for 1 wt{\%} PSM coatings decreased with time. TM-AFM showed that the dynamic behavior of modified surfaces was correlated with PSM phase separation. The results are important in providing a facile method for screening candidate coatings prior to time consuming antimicrobial testing.