Kari Dalnoki-Veress

The glass transition in thin polymer films

Abstract In this article, we present a detailed account of important recent developments in the rapidly evolving area of glass transitions in thin polymer films. We review the case of polymer films supported by substrates, and show that a definite experimental consensus exists. We consider recent results from experimental studies of free-standing films of polystyrene. These studies have provided a thorough quantification of the behavior of glass transition anomalies in free-standing polymer films, and have served to motivate recent attempts at theoretical descriptions. We introduce and examine models, which have been proposed to explain the experimental observations and discuss the significance of these models.

A Direct Quantitative Measure of Surface Mobility in a Glassy Polymer

Polymer Film Behavior An ongoing debate in the understanding of the behavior of thin-film glassy polymers is whether there is nanoconfinement of large molecules or enhanced mobility near a free surface. Chai et al. (p. 994; see the Perspective by Chen et al.) prepared polymer films with a sharp step in the profile by depositing broken film fragments onto a uniform underlay. Atomic force microscopy revealed changes to the overall film profile with time at various temperatures. A transition was observed from localized motions to relaxation of the entire film at a temperature close to that of the bulk glass transition temperature. A step geometry is used to probe the behavior of a glassy polymer above and below its glass transition temperature. [Also see Perspective by Chen et al.] Thin polymer films have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature Tg. Above Tg the entire film flows, whereas below Tg only the near-surface region responds to the excess interfacial energy. An analytical thin-film model for flow limited to the free surface region shows excellent agreement with sub-Tg data. The system transitions from whole-film flow to surface localized flow over a narrow temperature region near the bulk Tg. The experiments and model provide a measure of surface mobility in a simple geometry where confinement and substrate effects are negligible. This fine control of the glassy rheology is of key interest to nanolithography among numerous other applications.

PHASE SEPARATION MORPHOLOGY OF SPIN-COATED POLYMER BLEND THIN FILMS

We present the results of a study of the morphology of phase separation in thin films of two different polymer blend systems: polystyrene/polyisoprene and polystyrene/poly(methyl methacrylate). For each blend system, the two polymer components are dissolved in a common solvent. Spin coating of the ternary solutions (polymer blend/solvent) is used to confine the blends to a thin film geometry and to produce phase separation because of rapid evaporation of the solvent (solvent quench). As a quantitative measure of the phase separation morphology the average domain area of the minority component is measured as a function of the polystyrene mass fraction. For both blend systems we identify a small range of composition corresponding to a large increase in the average domain area. We show that the strong dependence of the average domain area on spin speed allows control over the quench time of the polymer blend thin films.

Effect of atmosphere on reductions in the glass transition of thin polystyrene films

We have used nulling ellipsometry to measure the glass transition temperature, Tg , of thin films of polystyrene in ambient, dry nitrogen, and vacuum environments. For all environments, the measured Tg values decrease with decreasing film thickness in a way that is quantitatively similar to previously reported studies in ambient conditions. These results provide strong reinforcement of previous conclusions that such reduced Tg values are an intrinsic property of the confined material. Furthermore, the results are in contrast to recent reports which suggest that the Tg reductions measured by many researchers are the results of artifacts (i.e. degradation of the polymer due to annealing in ambient conditions, or moisture content).

Phase separation morphology of thin films of polystyrene/polyisoprene blends

We present the results of a study of the morphology of phase separation in a thin film blend of polystyrene (PS) and polyisoprene (PI) in a common solvent of toluene. The blend is quenched by rapid solvent evaporation using a spincoating technique rather than a temperature quench. The mass fraction of polystyrene is varied to determine the effect of the substrate on thin film phase separation morphology. We compare the phase separation morphology for very thin films of the PS/PI blend cast onto three different substrates: Si(001) with a native oxide layer (Si (SINGLEBOND) SiOx), Si(001) etched in hydrofluoric acid (Si-H), and a Au/Pd alloy sputtered onto Si(001). We observe large differences between the morphologies of 1000 A thick blend films on the Si(SINGLEBOND) SiOx and Si-H substrates as the mass fraction is varied due to the difference in the wetting properties of PS on the two substrates. Smaller differences are observed between the films on the Si(SINGLEBOND) SiOx and Au/Pd substrates only for film thicknesses h < 600 A.

Plateau-Rayleigh instability in a torus: formation and breakup of a polymer ring

A liquid jet can break up into a stream of droplets as a result of the Plateau-Rayleigh instability. The droplet formation decreases the jets surface area and hence its free energy. Here we present the results of experiments in an unconventional geometry where this instability can be observed: a toroidal section. We discuss the formation of these polystyrene toroids with nanometer length scales. The constraints imposed by this geometry affect its observed instability in comparison to a simple linear jet. Specifically, we show that the additional curvature imposed by the torus can have a significant impact on the energy minimization route.

Self-similarity and energy dissipation in stepped polymer films.

The surface of a thin liquid film with a nonconstant curvature is unstable, as the Laplace pressure drives a flow mediated by viscosity. We present the results of experiments on one of the simplest variable curvature surfaces: a thin polymer film with a step. Height profiles are measured as a function of time for a variety of molecular weights. The evolution of the profiles is shown to be self-similar. This self-similarity offers a precise measurement of the capillary velocity by comparison with numerical solutions of the thin film equation. We also derive a master expression for the time dependence of the excess free energy as a function of the material properties and film geometry. The experiment and theory are in excellent agreement and indicate the effectiveness of stepped polymer films to elucidate nanoscale rheological properties.

Adhesion and membrane tension of single vesicles and living cells using a micropipette-based technique

The fundamental study of the adhesion of cells to each other or to a substrate is a key research topic in cellular biophysics because cell adhesion is important to many biological processes. We report on the adhesion of a model cell, a liposome, and a living HeLa cell to a substrate measured with a novel experimental technique. The cells are held at the end of a micropipette mounted on a micromanipulator and brought into contact with a surface. The adhesion energy and membrane tension are measured directly using the deflection of the micropipette when binding or unbinding the cell from the substrate. Since the force applied on the cells is known throughout the experiment, the technique presented enables the measurement of dynamics such as changes in the adhesion, elasticity, and membrane tension with time.

Viscoelastic properties of the nematode Caenorhabditis elegans, a self-similar, shear-thinning worm

Undulatory motion is common to many creatures across many scales, from sperm to snakes. These organisms must push off against their external environment, such as a viscous medium, grains of sand, or a high-friction surface; additionally they must work to bend their own body. A full understanding of undulatory motion, and locomotion in general, requires the characterization of the material properties of the animal itself. The material properties of the model organism Caenorhabditis elegans were studied with a micromechanical experiment used to carry out a three-point bending measurement of the worm. Worms at various developmental stages (including dauer) were measured and different positions along the worm were probed. From these experiments we calculated the viscoelastic properties of the worm, including the effective spring constant and damping coefficient of bending. C. elegans moves by propagating sinusoidal waves along its body. Whereas previous viscoelastic approaches to describe the undulatory motion have used a Kelvin–Voigt model, where the elastic and viscous components are connected in parallel, our measurements show that the Maxwell model, where the elastic and viscous components are in series, is more appropriate. The viscous component of the worm was shown to be consistent with a non-Newtonian, shear-thinning fluid. We find that as the worm matures it is well described as a self-similar elastic object with a shear-thinning damping term and a stiffness that becomes smaller as one approaches the tail.

Nanoparticle Flotation Collectors: Mechanisms Behind a New Technology

This is the first report describing a new technology where hydrophobic nanoparticles adsorb onto much larger, hydrophilic mineral particle surfaces to facilitate attachment to air bubbles in flotation. The adsorption of 46 nm cationic polystyrene nanoparticles onto 43 μm diameter glass beads, a mineral model, facilitates virtually complete removal of the beads by flotation. As little as 5% coverage of the bead surfaces with nanoparticles promotes high flotation efficiencies. The maximum force required to pull a glass bead from an air bubble interface into the aqueous phase was measured by micromechanics. The pull-off force was 1.9 μN for glass beads coated with nanoparticles, compared to 0.0086 μN for clean beads. The pull-off forces were modeled using Scheludkos classical expression. We propose that the bubble/bead contact area may not be dry (completely dewetted). Instead, for hydrophobic nanoparticles sitting on a hydrophilic surface, it is possible that only the nanoparticles penetrate the air/water interface to form a three-phase contact line. We present a new model for pull-off forces for such a wet contact patch between the bead and the air bubble. Contact angle measurements of both nanoparticle coated glass and smooth films from dissolved nanoparticles were performed to support the modeling.