Session D18: Polymers at Surfaces

Surface Segregation in Blends of Chains with Two Architectures

Blends of chains of two architectures, e.g. linear chains with pom-pom branched polymers, present opportunities for tailoring bulk rheology and surface character of the blends. Pom-pom chains contain a central linear portion between two junction points from which multiple arms extend. We have studied the effect of varying the relative length of the central linear portion of the pom-pom chain on the size of a single chain and on the bulk and surface thermodynamics of blends of the pom-pom chains with linear analogs. The strength of segregation to the surface and substrate interface of a blend film is seen with neutron reflectivity and surface enhanced Raman spectroscopy to depend on the relative length of the central linear portion of the pom-pom. Comparison will be made to a linear response theory that considers the number and type of chain ends and junction points.   

Single chain mobility at an interface of a liquid polymer

Interfacial diffusion of single chains of polystyrene-b-polyisoprene (PS-b-PI) at the interface between polyisoprene and its non-solvent, DMF, was studied by fluorescence correlation spectroscopy. The diffusion coefficient of PS-b-PI probe was found to be two orders of magnitude high than that in the bulk PI, indicating a lower interfacial viscosity. The experimental data also exhibit a very weak dependence of the interfacial diffusion coefficient on the molecular weight of the liquid polymer. The possible mechanism was discussed.   

Molecular origin of oil resistance of polyacrylonitrile: CN interactions at the surface

Nitrile rubber (NR) is a random copolymer of acrylonitrile and butadiene and is one of the best oil resistance polymers. The superior oil resistance property of nitrile rubber is thought to be directly related to the amount of acrylonitrile used in NR. Here, we report for the first in-situ sum frequency generation spectroscopy characterization of polyacrylonitrile/oil interactions. We demonstrate that CN-CN interaction is the chief reason for superior oil resistance property of NR. At room temperature, the interaction between the polymer chains is much stronger than the interaction between the polymer and solvent molecules including water and heptane. However, at high temperatures, the interaction between the nitrile groups of the polymer weakens making the interaction between the nitrile groups and the surface hydroxyls of the substrate and water possible.   

Effect of Hydrogen Bonding on Colloidal Nanocrystal Growth: The Case for PbS

The adsorption of methylamine (CH$_{3}$NH$_{2})$ on rocksalt PbS(111)-S surface is studied by first-principles total energy calculation. It was found that nitrogen lone pairs on the CH$_{3}$NH$_{2 }$ can form dative bonds with surface sulfur atoms. Such an interaction is unique in two ways: first it is non-local. Charge transfer takes place from CH$_{3}$NH$_{2}$ not only to the closest S but also to surface S further away. Second, the interaction is strongly affected by hydrogen bonds formed between CH$_{3}$NH$_{2}$ and solution molecules such as H$_{2}$O. Only by the latter effect can the PbS nanocrystals assume the (111) facets in a colloidal growth in agreement with experiment. In the past, studies of nanostructures have taken the assumption that, in the presence of a solution, the relative stability among the various facets will not change, at least not dramatically. Our study shows that such an assumption is not necessarily true. The significance of the study therefore goes beyond just the PbS or PbTe systems. Its basic principles should apply to most colloidal growth of solid-state nanostructures with broad implications.   

Conformational behavior of polymers adsorbed on nanotubes

We study the interaction of a dilute solution of semiflexible polymers with a weakly attractive infinitely long nanotube using Monte Carlo simulation. Apart for bending stiffness of the polymer chains, the only interactions considered in our model are weakly attractive short-ranged Lennard-Jones interactions between the monomers and with the surface. These nonspecific interactions are found to result in stable helical and multi-helical adsorbed conformations for semiflexible chains. Adsorption of these chains is found to occur in a sequential manner through tight wrapping of the polymer around the nanotube. Adsorption occurs quickly and is characterized by a sharp peak in the heat capacity. A second transition follows whereby opening and reorganization of the adsorbed chains into nearly perfect helices and multiple helices. Extension of the model to block and triblock copolymers reveals rich conformational behavior. These results are discussed on physical grounds and implications towards polymer-carbon nanotubes composites are offered.   

Origin of glass transition temperature behavior in polymer nanocomposites

Local composition variations inherent in multi-component materials, even when the material constituents are miscible, generally lead to heterogeneous behavior in the properties of mixtures relative to their single component counterparts. In contrast, experiments have suggested that some polymer nanocomposite materials exhibit changes in their bulk T$_{g}$ without displaying excess heterogeneity in their dynamics, as measured mechanically, relative to the neat polymer. Incoherent neutron scattering measurements on materials that fit this description, C$_{60}$-PMMA mixtures, suggest that modifications of the polymer melt dynamics are limited to the vicinity of the particles. A model by which the localized modifications of polymer dynamics can account for the apparent homogeneous change in T$_{g}$ is proposed to explain the experimental findings. Computations based on percolation theory support the model.   

Directed Self-Assembly of Gradient Concentric Carbon Nanotube Rings

Hundreds of gradient concentric rings of linear conjugated polymer, (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene], i.e., MEH-PPV) with remarkable regularity over large areas were produced by controlled, repetitive ``stick- slip'' motions of the contact line in a confined geometry consisting of a sphere on a flat substrate (i.e., sphere-on-flat geometry). Subsequently, MEH-PPV rings exploited as template to direct the formation of gradient concentric rings of multiwalled carbon nanotubes (MWNTs) with controlled density. This method is simple, cost effective, and robust, combining two consecutive self-assembly processes, namely, evaporation-induced self- assembly of polymers in a sphere-on-flat geometry, followed by subsequent directed self-assembly of MWNTs on the polymer- templated surfaces.   

Use of Fluorescence Correlation Spectroscopy for Studying Polyelectrolyte-Nanoparticle Interaction in Aqueous Solution

Formation of polyelectrolyte complexes is very facile via layer-by-layer sequential adsorption of oppositely charged species method. This method has been used to fabricate versatile materials with tailored properties. However, the fundamental assembly mechanisms of these advanced engineering materials and the adsorption kinetics of these systems is not completely understood. We use fluorescence correlation spectroscopy (FCS), which analyzes fluctuations in the fluorescence emission of molecular ensembles and provides the concentration, mobility, and dynamics of fluorescently labeled molecules, to study the interaction of polyelectrolyte-nanoparticle assembled structures to establish what's driving the adsorption of these systems, the dependence of molecular weight and the effects of variations in the solution environment with pH and ionic strength. Layer-by-layer assembly was performed on fluorescent dye-labeled spherical nanoparticles and amine and carboxyl functionalized polyelectrolytes in aqueous solution. Changes in the dynamics of the polyelectrolyte-nanoparticle system in response to various salt and pH conditions will be presented.   

Directed Nanoparticle Assembly onto Random Copolymer Templates: Kinetics and Surface Considerations

Recent efforts have focused on the development of nanoparticle arrays with controlled spacing. In this study, poly(styrene-ran-acrylic acid) films were prepared by spin-casting poly(styrene-ran-t-butyl acrylate), followed by thermal deprotection. Silica nanoparticles (10-15 nm in diameter) coated with self-assembled monolayers (SAMs) of (3-aminopropyl)triethoxysilane were covalently attached to the PS-ran-PAA films with an EDC/NHS coupling reaction. To measure the kinetics of nanoparticle attachment, films of either 25 or 50 weight percent acrylic acid were reacted with nanoparticle suspensions from 0.005 to 0.1 weight percent for varying lengths of time. SEM imaging of the nanoparticle surfaces showed that the particles were well dispersed, and that particle coverage increased with increasing AA and nanoparticle concentration, and time. SAMs containing an acrylic acid moiety were used as a non-swelling control surface, and particle attachment to these surfaces follow different kinetics than those observed for the polymeric substrates. The swelling of the polymeric substrates under the reaction conditions was found to influence the observed coverage kinetics, so film swelling was monitored with environmental AFM.   

Studies of the Dynamics of Alkane Nanoparticles

Our AFM and synchrotron x-ray scattering measurements on dotriacontane (C$_{32}$H$_{66}$ or C32) deposited on SiO$_{2}$-coated Si(100) substrates reveal mesa-shaped nanoparticles that have an orthorhombic structure in which the C32 molecules are aligned perpendicular to the SiO$_{2}$ surface.$^{2}$ To investigate their dynamical properties, we have used both the backscattering and disk chopper spectrometers at NIST covering a wide range of time scales (1 ps -- 4 ns). Elastic scans obtained on both spectrometers show step-like changes in intensity as a function of temperature indicating the existence of phase transitions below the bulk C32 melting point. One of these steps occurs at the crystalline-to-rotator phase transition of bulk C32, but there is a second step at still lower temperature of unknown origin. The similarity of the elastic scans on the two spectrometers suggests that the crystalline-to-rotator phase transition involves diffusive motion spanning a wide range of time scales. $^{2}$M. Bai\textit{ et al.}, Europhys. Lett. \textbf{79}, 26003 (2007).   

Hierarchically Ordered Plasmonic Mask for Photo-lithography

A new high density nanolithography method for the fabrication of a hierarchically ordered plasmonic mask employs silver (Ag) nano-particles (NPs) attached to the surface of an amine- functionalized two-dimensional (2D) pattern fabricated by laser interference lithography (IL). The bi-functional sol-gel hybrid material (BFHM) is a negative-tone resist and can be directly patterned by IL. Since the BFHM has both an amine-function and a methacryl function on each molecule, photopolymerization provides network formation and a set of binding sites for the Ag NPs. The Ag NPs were then attached onto the BFHM pillars by immersing the patterned sample in a solution. Hierarchically ordered arrays of Ag NPs could be made by a block copolymer comprised of 40nm diameter spherical P2VP domains having a spacing of 88nm, forming a hexagonal pattern covering the hexagonally arrayed BFHM pillars. Lithography experiments using 430 nm wavelength light demonstrate transfer of both a 350 nm periodic pattern and a 88nm patten to a positive-tone photoresist via plasmonic field enhancement arising from the collective and individual excitation of the closely spaced interacting Ag NPs on the hierarchically patterned BFHM.   

Dynamics of an Adsorbed Polymer Chain

Because of the current precision in fluorescent labeling, it is possible to label single polymers such as DNA or PEG and track their dynamical and equilibrium properties in the bulk as well as near attractive surfaces[Maier et. al., Macro 2000][Sukhishvili et. al., Macro 2002]. Recent evidence from these experiments and related simulations has shown that the dynamics of a single polymer near an attractive surface appear diffusive, however further evidence coming from the 'diffusion coefficient' implies a different process other than diffusion is at work such as reptation, `hovercrafting', or `hopping' [Sukhishvili et. al., Macro 2002]. In general, these possible dynamical behaviors are determined by the length of the polymer itself as well as the microscopic details of the attractive surface which include the density, strength, and distribution of attractive surface sites[Desai et. al., PRL 2007][Qian et. al., PRL 2007]. In this presentation, we investigate the effects of microscopic surface effects on single polymer dynamics through Monte-Carlo and molecular dynamics simulations.   

Chasing drops: Following escaper and pursuer drop couple systems

We study the Marangoni flow induced by two chemically different drops resting on a solid surface in air. We show that in all the systems studied, the Marangoni flow is induced at the solid-liquid interface as opposed to the air-liquid interface. This is true even for the case of water drop and alcohol drop on a glass surface (corresponding to the classical ``tears of wine'' case). Thus we explain the drop motion as a result of a surface tension gradient which takes place primarily at the air-surface region (and less at the drop-substrate or drop-air interfaces). The discontinuous motion of the drops, characterized by stops and jumps as in a ``stick slip'' mechanism is explained by an increase in the Laplace pressure that creates a higher anchoring pinning effect at the front edge of the moving drop. We discuss this in connection to the ``tears of wine'' case.   

The Measurement of Surface Rheological and Surface Adhesive Properties using Nanosphere Embedment

In previous work, we determined the actual rheological behavior at the surface of a polystyrene film with nanometer scale resolution by applying a viscoelastic contact mechanics model to experimental data in the literature. The goal of our current research is to build upon this analysis and use nanosphere embedment experiments to probe the nanorheological behavior of polymer surfaces near the glass transition, in the melt state and in the solid rubbery state. An atomic force microscope (AFM) is used to measure the embedment depth as nanoparticles are pulled into the surface by the thermodynamic work of adhesion. The results show that, with properly designed experiments, both the surface adhesion properties and the surface rheological properties can be extracted from nanosphere embedment rates. We include work on a phase separated copolymer and a commercially available polydimethylsiloxane (PDMS) rubber.   

Slip behavior of the confined polymer melt near periodically roughened surface: comparison between molecular dynamics and continuum simulations

Molecular dynamics (MD) simulations are used to investigate the behavior of the slip length in the Couette flow of a polymer melt. For atomically smooth surfaces and weak wall-fluid interactions, the shear rate dependence of the slip length is a non-monotonic function, with a distinct local minimum. For corrugation wavelengths larger than the radius of gyration of polymers, the decay of the slip length with corrugation amplitude obtained from MD simulations agrees well with the continuum predictions for the following cases: (1) Stokes solution with constant local slip length, (2) Stokes solution with local shear-rate-dependent slip length, and (3) Navier- Stokes solution with local rate-dependent slip length. If the corrugation wavelength is less than or on the order of the radius of gyration, the continuum predictions (the Stokes solution) overestimate the values of the slip length extracted from MD simulations. The analysis of the conformational properties of the polymer melt indicates that polymer chains tend to stretch in the direction of shear at the peaks of the sinusoidal wave and align themselves along the bottom of the grooves.