Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N28: Polymer Adsorption and Surface Modification |
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Sponsoring Units: DPOLY Chair: Eric Lin, National Institute of Standards and Technology Room: Baltimore Convention Center 325 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N28.00001: PEG Surface Modification by Thermoreversible Ligand Cleavage in Nanoparticle Composites. Rick Beyer, Philip Costanzo The control of surface chemistry is an increasingly important area of research in the polymer science community; a simple example of a need for controlled surface properties can be found in the need for surfaces that are resistant to bacterial growth for medical applications. In this study, we have successfully modified the surface properties of solvent cast poly(ethylene glycol) (PEG) films, triggered by exposure to an elevated environmental temperature. PEG matrices of varying molecular weights have modified with gold nanoparticles functionalized with thiol terminated, poly(styrene)-PEG block copolymer (P(S-$b$-PEG)) ligands. Gold nanoparticles approximately 15 nm in diameter were first synthesized via reduction of HAuCl$_{4}$ with oleyl amine. Diels-Alder chemistry was then used to create P(S-$b$-EG) ligands that, with increasing temperature, dissociate into simple thiol-terminated PS ligands and PEG oligomers. The ligand-modified gold particles were characterized via small-angle X-ray scattering and TEM. After dissociation occurs, around 90 \r{ }C, the gold particles are suddenly functionalized with only a PS ligand and thus immiscible in the surrounding PEG matrix. The gold nanoparticles are then driven to the surface of the films, measurably denoted by a change in contact angle. After cooling below 60 \r{ }C, the Diels-Alders linkages reform, stabilizing the film surfaces with the new morphology trapped both chemically and kinetically. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N28.00002: Smart Polymeric Surfaces: Responsiveness and Reconstruction Julie Crowe, Jan Genzer The ultimate responsive surface is one that instantaneously responds to its environment with a measurable property change. In our research we utilize model poly(vinylmethyl)siloxane elastomer (SE) networks modified with thiol alkanes to provide hydrophobic or hydrophilic surface properties. The cooperative effects of polymer mobility, arising from the high flexibility of the siloxane backbone, and the enthalpic interactions between the outside medium and the SE functionalized surface control the degree of responsiveness as measured by dynamic and static contact angle. The initial parameters screened were alkane chain length, medium temperature, and end-group functionality. Real-time wetting force measurements have been obtained with dynamic contact angle, where the surface reconstruction is measured continuously providing a means to determine the kinetics of reconstruction and reversibility. Our examples show that not only are SE networks excellent stimuli-responsive substrates, but that the magnitude of change and repeated reversibility are unparallel to most polymeric surfaces. [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N28.00003: Conformations of Amphiphilic Comb Copolymer Chains Confined to Two Dimensions Through Self-Organization at the Polymer/Water Interface William Kuhlman, Elsa A. Olivetti, Linda G. Griffith, Anne M. Mayes Amphiphilic comb copolymers composed of a hydrophobic poly(methyl methacrylate) (PMMA) backbone and short, hydrophilic PEO side chains (PMMA$-g-$PEO) are known to self-organize at the polymer/water interface, resulting in the effective confinement of the backbone to two dimensions for chains at the surface of a PMMA-$g$-PEO film. Conformations of polymers thus confined were studied through selective nanoparticle labeling of PEO side chains of polymer molecules at the film surface. Transmission electron microscopy was used trace the backbone trajectory of nanoparticle labeled chains. The distribution of observed chain lengths is found in good agreement with the distribution determined by gel permeation chromatography. The 2D radius of gyration ($R_{g})$ calculated from the observed conformations was found to scale with number of backbone segments ($N)$ as $R_{g}$\textit{$\sim $N}$^{0.69\pm 0.02}$. This value agrees with Monte Carlo simulations for a system of similar polydispersity, which yield a scaling exponent between that for 2D isolated chains and monodisperse polymer melts ($R_{g}$\textit{$\sim $N}$^{0.64\pm 0.03})$. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N28.00004: Temperature-responsive polymers and brushes with tunable onset of response Theresa Foley, Kiril Efimenko, Jan Genzer, Evangelos Manias Temperature-responsive polymers are of high interest in the scientific field of stimuli responsive materials, in particular water soluble polymers with a response at $\sim$36.5$^{o}$C. However, difficulties in tailoring this T-response, as illustrated for example from studies of PNIPAM in numerous functionalized and copolymer forms, has hampered their proliferation. Here we present a systematic series of temperature-responsive polymers, which were designed, synthesized, and studied, and we show that we can tailor with high sensitivity their onset of T-response via the design of their monomer. Specifically, we demonstrate lower critical solution temperature (LCST) in water finely tuned between 5 and 70$^{o}$C, by controlling the hydrophilic/hydrophobic balance in the monomer (closely following predictions of phase behavior theories). In addition, we will also show that these polymers maintain their T-responsive characteristics when end-tethered to solid surfaces, over a wide range of grafting densities in combinatorial brushes. This approach allows for controlling contact angle, adhesion and tackiness as a function of temperature. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N28.00005: Single Molecule Experiments with Adsorbed Polyelectrolytes Sergiy Minko, Yuri Roiter We report on the AFM study of single polyelectrolyte (positively charged) molecules (PE) adsorbed on mica surfaces at different conditions (we vary pH and salt concentration). The study was carried out under aqueous solutions in a liquid cell. We observed behavior of PE in real time. The AFM experiments were experiments when possible effects of the AFM tip on PE conformations were minimized. A series of experiments were carried out when PE was adsorbed between two electrodes at applied electrical potential. The AFM images were processed to extract contour length, end-to-end distance, and radii of center of mass. The experiments revealed several interesting facts about adsorption of PE. The chain statistics was found to be consistent with the 2D random walk model. A decrease of charge density resulted in the coil-to-globule transition. The globules appear as a strongly deformed swollen polymer globule. In saline solutions the globules resemble necklace-like globules. PE chains were mobile if an electrical field was applied. The motion of PE chains can be describes as a caterpillar-like motion. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N28.00006: Quantifying How Polymer Interfacial Diffusion Differs from Bulk Liang Hong, Steve Granick Whereas polymer adsorption-desorption kinetics are reasonably well explored, in-plane diffusion is not. This talk will describe the molecular weight (M) and surface coverage dependence of two polymers, polystyrene (PS) and polydimethylsiloxane (PDMS) adsorbed to quartz from organic solvent. The M scaling of surface diffusion is quantified, and a surprising dependence on surface coverage is described. Time permitting, additional studies will be described in which, for the first time, polymer self-diffusion has been studied within a surface forces apparatus designed for fluorescence spectroscopy. Using FRAP (fluorescence recovery after photobleaching) to study slow diffusion and FCS (fluorescence correlation spectroscopy) to study rapid diffusion, we quantify how the self-diffusion coefficient of PDMS oligomer melts slows with diminishing surface separation, when PDMS is confined between mica surfaces. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N28.00007: Balancing size exclusion and adsorption of polymers in nanopores Won Kim, Chang Y. Ryu The liquid chromatography at critical condition (LCCC) presents the condition, at which the size exclusion and adsorption of polymer chains are balanced upon interactions with nanoporous substrates. In this study, we investigate how the polymer interactions with nanopores are affected by the solvent quality and nanopore size. Specifically, we measure the retention times of monodisperse polystyrenes in C18-bonded nanoporous silica column as a function of molecular weight, when a mixed solvent of methylene chloride and acetonitrile are used as elutent. C18-bonded silica particles with 70, 100, and 250 A pore size are used as a stationary phase to study how the transition from SEC-like to IC-like retention behavior depends on the condition of temperature and solvent composition. To locate the LCCC at various nanopore sizes, the temperature and solvent composition have been varied from 0 to 60 C and from 51 to 62 v/v{\%} of methylene chloride, respectively. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N28.00008: Polymer brushes dynamics by evanescent wave dynamic light scattering Benoit Loppinet, Vassiliki Michailidou, George Fytas, Juergen Ruehe Dynamics of swollen brush is experimentally measured by dynamic light scattering in the total internal reflection geometry. Dense thick polymer polystyrene brushes with varying grafting densities are obtained through grafting from synthesis. When highly swollen in good solvent, concentration fluctuations are found to decay through a fast diffusive mechanism, attributed to the expected cooperative diffusion, akin to semi-dilute polymer solutions. Its hydrodynamic size is found to be comparable to the estimated distance between grafting chains. De-swelling of the brush by lowering solvent quality (using cyclohexane at different temperatures) leads to qualitative different dynamics. An extra slower relaxation with a broad distribution of relaxation times is observed, which strongly depends on solvent quality, with an extensive slow down of it characteristic time and an increase of scattered intensity upon cooling. This complex dynamics is discussed in relation to the dynamics in entangled semi-dilute solution in theta solvent. Finally, the brownian dynamics of colloidal particles (radius from 17nm to 100nm) in contact with the brushes are reported. The particles are found to marginally penetrate the brush, but to nonetheless exhibit dynamics reflecting their interactions with the outer part of the brush. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N28.00009: Electromechanical Recognition of Molecules Adsorbed on Microcantilevers Sangmin Jeon, Dongkyu Lee, Thomas Thundat An alternating current was applied to gold-coated silicon microcantilevers in sodium chloride solution. The cantilever is coated on one side with a thin layer of gold. Since the applied electric field through the cantilever attracts oppositely charged ions onto the gold layer, the variation of surface charges induces the oscillation of the cantilever. The larger the applied voltage is, the more the cantilever oscillates. When the experiment was repeated with self-assembled monolayer coated cantilever, the amplitude of the oscillation is decreased because the monolayer hinders the ions from approaching to the cantilever. In-situ measurements of the adsorption of mercaptohexanol molecules under the electric field clearly showed that the decrease of the oscillation resulted from the formation of the layer. When a square electric field was applied to 1-mercaptoethanol, 1-mercaptopropanol, and 1-mercaptohexanol coated cantilever, the bending profile of the cantilever depended on the kinds of the monolayer. This method can be used to study the diffusion of small ions through thin films as well as the structure of self-assembled monolayer. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N28.00010: Dependence of surface diffusivity on the molecular conformation of single hydrophobic polyelectrolytes molecules Jiang Zhao, Shengqin Wang Hydrophobic polyelectrolytes are found to have their conformation change from an extended chain to globule via neck- lace structures. In this work, surface diffusion of single poly (2-vinylpryridine) (P2VP) molecule was studied under different chemical environment (pH value and ionic strength). Via hydrophobic interaction, P2VP molecules adsorbed to a hydrophobic surface. By fluorescence correlation spectroscopy, fluorescence labeled P2VP molecules were found to raise their surface diffusivity moderately but monotonously when the pH value was tuned from 2.0 to 6.5. The physical mechanism of the diffusivity dependence on molecular conformation is discussed. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N28.00011: Adsorption of polymers onto selective mixed brushes. A.I Chervanyov, G. Heinrich Reversible adsorption of polymers onto selective mixed brushes is studied theoretically. Mixed brushes are recently developed self-adoptive materials that reversibly change their morphology in response to altering external factors (e.g. quality of the solvent). The above changes in the morphology result in the formation of different patterns on the outer surface of the brush. It is shown that thus achieved patterning of the adsorbing surface of the mixed brush drastically enhances the adsorption of polymers, as compared to the adsorption onto the homogeneous brush surface. The density profiles and absorbances of the selected homo- and co-polymers are calculated for the three different morphologies (`ripple', `dimple' and random) of the binary brush. The interplay between conformational entropy and binding energy of the adsorbed polymer leads to the reach adsorption-desorption behavior that is described by the developed theory. The calculated isotherms are compared with the experimental data and Monte-Carlo simulation results. In addition, the developed theory is applied to the study of the polymer adsorption onto the non-uniform binary brush in the presence of the gradient of chemical composition. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N28.00012: Nano-meter structured three-phase contact line and its surface-guided alignment effect Gang Liu, Jiang Zhao We report our studies on the air-liquid-solid three-phase contact line on the periodically patterned surface made of polystyrene-b-polymethylmethacrylate (PS-b-PMMA). The difference of the contact angle of the liquid (water and polymer solution) on PS and PMMA generates the contact line with periodic structures of 40 nm length scale. Such a structured contact line was found to produce surface guided alignment effect for single DNA molecules by the molecular combing process and to generate surface-guided morphology of the polymer films through a combination of de-pinning and de- wetting process. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N28.00013: Surface diffusion of adsorbed polymers studied by molecular dynamics simulation Wonki Roh, Erik Luijten We study the diffusion of adsorbed polymers near a flat surface by means of molecular dynamics simulations, as a function of chain length~$N$, adsorption energy, and surface coverage~$\phi$. We find that the two-dimensional diffusion coefficient scales as $D\sim N^{-1.017\pm 0.011}$, in agreement with other experimental and simulation results. The relation between lateral diffusion coefficient and surface coverage shows an exponential decay. We also investigate the conformation of the adsorbed chains. The number of ``trains,'' ``loops,'' and ``tails'' per chain, as well as the number of monomers in tails and loops increase as surface density increases and adsorption energy decreases, whereas the number of monomers in trains decreases. The parallel radius of gyration increases as a power law of the chain length, $\langle R_{g\parallel}^2 \rangle \sim N^{2\nu}$, with a power that is in good agreement with the Flory exponent $\nu = 3/4$ for two-dimensional chains. $\langle R_{g\parallel}^2 \rangle$ decreases with increasing surface density and decreasing adsorption energy, whereas $\langle R_{g\perp}^2 \rangle$ remains almost constant with increasing surface density and increases with decreasing surface energy. [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N28.00014: Solvent and salt effects on the adsorption of polymers to charged surfaces Govardhan Reddy, Arun Yethiraj The effect of solvent quality and salt concentration on the adsorption of charged polymers to a planar uniformly charged surface is studied using molecular dynamics simulation. The polyion chains are modeled as chains of charged spheres, the counter ions to the polyions and the surface are modeled as charged spheres, and the solvent molecules are modeled as uncharged spheres. The polyion adsorption is studied as a function of monomer and salt concentration, solvent quality and surface charge density of the surface. The amount of polyion adsorbed on the surface increases with the decrease in solvent quality as the system approaches a bulk phase transition. There are some surprising and counter-intuitive results in this regime. For example, incoprporating a short-ranged attraction between the polymers and the surfaces decreases the number of adsorbed polymers, and the amount of polyion adsorbed decreases as the salt concentration increases. [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N28.00015: Mean-field theory of planar absorption of RNA molecules Toan Nguyen, Robijn Bruinsma, William Gelbart Interaction between the viral RNA and the protective protein capsid plays a very important role in the cell infection and self-assembly process of a virus. To better understand this interaction, we study a similar problem of absorption of RNA on an attractive wall. It is known that the secondary structure of a folded RNA molecules without pseudo-knots has the same topology as that of a branched polymer. We use a mean-field theory for branched polymers to analytically calculate the RNA concentration profile. The results are compared to known exact scaling calculations and computer simulations. [Preview Abstract] |
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