Session U34: Thin Films, Surfaces and Interfaces I

Comparison of experimental and computational estimation of non-freezing interfacial molecules

Recently, we have estimated that about 2.14 +/- 0.14 nm of interfacial cyclohexane molecules do not participate in phase transition. This estimation was determined from calorimetric measurements of physically confined cyclohexane in silica nanopores. In agreement with previous work, melting and freezing temperatures of the confined cyclohexane were lower than that of the bulk cyclohexane, and the apparent heat of fusion changed with silica pore size. Correcting for the layers of molecules at the interface that do not participate in the phase transition keeps the heat of fusion independent of the confined size scale. In the current study, we used molecular dynamics simulations to investigate the behavior of the cyclohexane molecules at the interface and compared their behavior to those in the bulk (away from the interface).   

A molecular view of latex-water interfaces

Latex paints and coatings are colloidal suspensions, in which amorphous polymer particles are dispersed in an aqueous phase. The polymer-water interface plays a key role in the stability and rheology of the suspension. To obtain a molecular level view of this interface, atomistic simulations were performed for a slab of poly(methyl methacrylate)-poly(butyl acrylate) random copolymer in water, focusing on polymer and water density profiles, the hydrogen bonding of water with polymer carbonyl groups, and surface tension. The carbonyl groups at the interface were found to orient significantly towards water. We also calculated the temperature dependence of the surface tension between the polymer/water and polymer/ vacuum interfaces, including tail corrections for cut-off dispersion interactions, and we predict the contact angle of a water droplet at room temperature.   

Pathways and Time Scales for Water Movement to a Metal/Polymer Interface

The movement of water underneath polymer films is important in the process of corrosion of metals protected by polymer coatings. Here the ingress of water to such an interface is studied with Neutron Reflectometry (NR), which allows the measurements to be done in situ. We have shown that water incursion along the interface between an epoxy hybrid coating and aluminum is fast compared to incursion through the face of the coating. With a more highly crosslinked coating, after a small amount of water has entered along the interface, water incursion slows dramatically. When the sample is once again in a dry environment, swelling of the coating caused by water is not fully reversible, but is reduced slowly over a period of a month.   

Desorption Kinetics of Water from Poly (methyl methacrylate) Films and other Polymer Films

We present herein the water desorption from the dipole oriented poly (methyl methacrylate) PMMA. Water desorption from PMMA presents the ``ice species'' at 150 K and a bulk peak at about 280 K. We note that the desorption peak temperature does not vary greatly with increasing coverage. The energy of desorption is obtained by employing the Arrhenius and Polany-Wigner equations. The comparison with previous thermal desorption spectra of water from two ferroelectric polymers is also discussed. [1] Dowben, P.A., Rosa, Luis G., Ilie, C.C., Zeitschrift f\"{u}r Physikalische Chemie 222 (2008) 755-778. [2] Ilie, C.C., Rosa, L.G., Poulsen, M., Takacs, J., Integrated Ferroelectrics (2011) 125:1, 98-103.   

A Molecular Dynamics Simulation Study on the Wetting Behavior of Water on Oxidized and Non-Oxidized atactic Polystyrene Surface

All-Atomistic Molecular dynamics simulations have been carried out to study the wetting of oxidized and non-oxidized atactic polystyrene (aPS) thin films by water droplets. The dependence of the contact angle on droplet size has been studied using spherical and hemispherical water droplets of varying sizes. The effect of oxidation of the aPS surface on the contact angle has been studied as a function of oxygen concentration. Oxidation of the aPs has been achieved by randomly replacing the ortho and/or meta hydrogen on the phenyl rings within 1 nm of the aPS surface by oxygen until the desired concentration of oxygen on the surface is reached. The simulated contact angle is found to decrease monotonically with oxygen concentration consistent with recent experimental results. We will present results on the variation of water contact angle with oxygen concentration on the aPS surface. In addition, the effect of oxidization on the roughness of the polystyrene surface, the ordering of the phenyl rings and the water molecules and the number of hydrogen bonding between water molecules and the polystrene at the interface have been investigated and will be presented.   

Wetting of star-shaped macromolecules

We~show that the equilibrium contact angles and line tensions of macroscopic droplets of star-shaped polystyrene (PS) macromolecules of functionality, f, and degree of polymerization per arm, Narm, on oxidized silicon substrates, may be as much as one and two orders of magnitude, respectively, smaller than their linear analogs, depending on f and~Narm. The dewetting characteristics of the linear and star polymers also differ. Thin film of LPS and SPS dewet SiOx~substrates due to destabilizing long-range intermolecular forces.~~However, while macroscopic droplets surrounded by droplets of nanoscale dimensions characterize the late-stage dewetting morphology of the LPS system, the macroscopic droplets of the SPS molecules reside on a stable layer of molecules adsorbed to the substrate. The thickness of the adsorbed layer depends on both f and Narm. We provide evidence that the wetting/dewetting characteristics of the SPS macromolecules are largely determined by the competition between interfacially attractive conformational entropic effects and steric repulsion effects, for molecules of sufficiently large f and small Narm.   

The Role of Acid-Base Interactions in Controlling Interfacial Segregation in Polymer Blends

We have studied segregation of polymethylmethacrylate (PMMA)/polystyrene (PS) blends next to solid surfaces using interface sensitive infrared-visible sum frequency generation (SFG) spectroscopy. We have monitored the SFG spectra as a function of blend compositions and used the shift in the surface hydroxyl peak, due to acid-base interactions, to determine the concentration of PMMA groups next to the sapphire substrate. A quantitative connection between the extent of interfacial segregation and the strength of the acid-base interactions will be discussed.   

Measurement of monolayer viscosity using non-contact microrehology

Microrheological studies of phospholipid monolayers, bilayers, and other surfactant monolayer systems present a particularly useful avenue for studying the flow properties of fragile, complex fluid systems. Unfortunately, in some cases microscopic particle tracking methods disagree with macroscopic flow methods by several orders of magnitude. This ``missing modulus'' problem has been speculated to originate in the heterogeneity of the monolayer under study, as well as the unknown boundary conditions and uncertainty in particle position intrinsically associated with coupling the tracer bead to the monolayer. In this talk we discuss an alternative method for performing microrheology experiments, where the tracer bead is submerged a known depth beneath the monolayer. Using both theory and experiment, we demonstrate that despite the weaker coupling between the tracer and the monolayer, the well-characterized hydrodynamics between the bulk sub-phase and the surface allows for the calculation of particle response functions and recovery of the ``missing modulus'' for several model monolayer systems.   

Evidence of Phase Separation during Vapor Deposition Polymerization

Initiated chemical vapor deposition (iCVD) is a solventless, free radical technique predominately used to deposit homogeneous films of linear and crosslinked polymers directly from gas phase feeds. We are developing multicomponent iCVD techniques to induce phase separation during film growth. Small molecule porogens and crosslinkers are introduced into the iCVD process during film growth of poly(glycidyl methacrylate). Analogous to well established polymerization induced phase separation (PIPS) processes, porogens, such as dimethyl phthalate, are well mixed at the growing gas-film interface but are immiscible with high molecular weight polymer. Polymerization, crosslinking and PIPS are intended to occur simultaneously on the substrate, resulting in a vitrified microstructure. A series of films were grown by varying deposition rate, porogen type, and reagent flowrates. Deposited films were studied by electron microscopy and spectroscopic techniques. Experiments are compared to Cahn-Hilliard theory predictions that relate the length and time scale of the phase separation to the polymer-porogen interaction energy, the rate of polymerization and the species mobility.   

Surface phase separation between polythylene oxide of different molecular weight

In-plane phase separation of polyelethylene oxide of different molecular weight has been observed. A systematic investigation on a broad range of Mw show that the process is originated from the conformatinal entropy for a polymer confined on a surface (thin film).   

Understanding diblock copolymer colloidal particle anisotropy

Colloidal particles are formed by emulsifying a mixture of PS-$b$-P2VP, nanoparticles and chloroform in water with surfactant and then evaporating the chloroform. With the addition of a sufficient number of nanoparticles, the colloids form prolate ellipsoids with lamellae oriented along the major axis. These colloidal particles are of interest for potential applications such as photonic materials and drug delivery. In order to explain the colloidal particle anisotropy and its dependence on colloidal particle size, a theoretical model that balances internal and external surface tension was developed. Agreement between the model and experimental results suggests that thermodynamic factors control the particle anisotropy.   

Multiblock copolymer adsorption on a hydrophobic surface: A Monte Carlo simulation study

Dilute solutions of long multiblock copolymers with alternating hydrophilic and hydrophobic segments in contact with a hydrophobic surface have been investigated by Monte Carlo simulation in order to characterize the structure of the adsorption layer. Its properties are determined as a function of the bulk hydrophobicity, the surface hydrophobicity and the monomer concentration. The influence of the copolymer length and its block structure is also investigated. Interesting features appear close to the bulk critical micelle concentration: surface micelles, a secondary surface layer of bulk micelles, depletion effects. Depending on the interaction strengths the surface layer consists of individually adsorbed hydrophobic segments or of surface micelles, at equilibrium with bulk micelles, as found in a previous study of the bulk properties [1]. At higher surface coverage the surface micelles form a regularly spaced layer of hydrophilically connected micellar cores. For sufficiently long copolymers a layer of bulk micelles is hydrophilically attached to the layer of surface micelles. \\[4pt] [1] Hugouvieux, V. et al., Soft Matter 7, 2580 (2011)   

Capillary Levelling of Stepped Polymer Films - A Nanofluidic Probe of the Slip Boundary Condition

For flows on small length scales, the hydrodynamic boundary condition of a liquid at a solid surface plays an enormous role. In recent years much has been learned about this slip boundary condition from flows that are driven by internal, capillary, forces such as dewetting of thin liquid films. For the case of dewetting, holes in the film grow, driven by exposing the underlying substrate. Here, we present the opposite approach: We show that the capillary levelling of initially curved surfaces, in our case stepped polymer films, is sensitive to the nano-rheological properties of the liquid and the dependence on the slip boundary condition at the buried liquid/substrate interface. A thin film model which includes the slip boundary condition enables us to quantify the boundary condition at the buried interface and the dependence of slip on the molecular weight of the polymers used.   

Relaxation of non-equilibrium entanglement networks in thin polymer films

It is well established that polymer films, prepared by spincoating, inherit non-equilibrium chain conformations which can affect macroscopic film properties. Here we present the results of crazing measurements that elucidate the non-equilibirum chain configurations in spin-cast films. Furthermore, we find that the entanglement network equilibrates on a time scale comparable to one reptation time. In a second set of experiments, we confine polymers to films with thickness comparable to the molecular size. By stacking two such films at room temperature, a glassy bilayer film with a buried entropic interface is created. According to Silberberg's reflection principle, such an interface has an entropic cost associated with the restricted configurations of molecules that cannot cross the mid-plane of the bilayer. In the melt, the interface heals as chains from the two layers mix and entangle with one another. Crazing measurements reveal that it takes less than one bulk reptation time for a bilayer to become indistinguishable from a single film.   

Ion Dispositions in Polyelectrolyte Multilayer Films

Polyelectrolyte multilayers (PEMs) fabricated from sodium chloride-containing solutions of poly(diallyldimethylammonium chloride) (PDDA) and poly(styrene sulfonate) (PSS) were examined by various techniques to determine the dispositions of polyelectrolytes and counterions across the PEM thickness. The key technique was dry film QCM, which quantified incremental mass depositions during PEM assembly. Counterion dispositions depended strongly on salt concentration, and three salt regimes were identified: zero to near zero salt ([NaCl] less than 0.1M), low salt ([NaCl] between 0.1M and 0.75M), and high salt ([NaCl] greater than 0.5M]). The first two are associated with linear PEM growth while the latter is associated with exponential PEM growth. At zero salt, no counterions are present in the PEM bulk (middle), while at low salt, an excess of PDDA charge across the bulk coincides with an excess of counteranions. Differently, at high salt, deposited PSS permeates the PEM bulk, conveying an excess of countercations. At all salt concentrations, the PEM surface charge alternates according to the capping polyelectrolyte's identity. Accumulations of small ions in the bulk can be ascribed to as yet poorly understood property asymmetries between the two deposited polyelectrolytes.