Session P45: Surfaces, Interfaces, and Polymeric Thin Films - Surface Instabilities

Buckling in a particle film

When a thin rigid plate is adhered to a soft substrate and compressed, the plate will buckle out of plane to accommodate the applied stress. The out of plane bending is resisted by the substrate and the result is a sinusoidal topography (wrinkles). When the plate is replaced by a collection of closely packed particles similar phenomena results -- the positions of the particles move out of plane and follow a roughly sinusoidal curve. Due to the similarity of the end state of each system, the same continuum theory is often applied to model the behaviour. Here, we use a carefully constructed experimental system consisting of micron-scale polymer and silica spheres on a PDMS elastomer substrate to demonstrate the physical differences between a continuum plate and a discrete set of particles. In particular, because we can easily track the position of each particle in three dimensions with confocal microscopy, we have access to all of the particle motion. We note that the wrinkling is independent of particle modulus, and highly dependent on particle packing. This leads us to suggest that the underlying physics is granular (and not continuum) in nature. This result may have implications in biology, where elastic continua are often made of discrete building blocks (e.g. cells).   

Defect induced shape instabilities in textured Membranes

We study the dynamics of defect annihilation and quasi-equilibrium configurations in flexible textured membranes suffering a symmetry breaking phase transition. The phase separation process and relaxational properties are described through a Brazovskii-Helfrich-Canham Hamiltonian. Topological defects favor the development of local curvature to geometrically screen out the intrinsic stress field generated by the perturbations to the low symmetry phase. While in hexagonal systems the unbinding of dislocations and Carraro-Nelson interactions between disclinations slow down the dynamics, in smectic systems we found a wrinkled state where the bending forces are balanced by the out-of plane hoop stretching generated by positive disclinations. The wrinkled configuration found in the smectic systems show features that resemble those found in flexible thin films under small external loads.   

Dewetting Dynamics from Polymer Interfaces

The dewetting dynamics from metastable polymer/polymer interfaces is studied using X-ray Photon Correlation Spectroscopy (XPCS). In addition to more ``usual'' situations where XPCS correlation functions are used to measure out-of-plane height fluctuations (e.g. capillary waves), we show how this novel method can be used to measure in-situ in-plane motion of the dewetting film. The experimental correlation functions associated with this motion are fitted remarkably well by a simple model considering the growth of dewetting rims in the beam footprint.   

Nanoscale Confinement Induced Control of Polymer Thin Film Instabilities

Control of stability and wettability of polymer thin films is invaluable to a range of functional coatings with applications from electronics to biomedical coatings, yet simple non-chemical modification strategies to accomplish this are generally lacking. We demonstrate a novel route to effectively control instabilities in model polystyrene films on partially wetting and non-wetting solid substrates that would otherwise lead to film dewetting. The method involves top down confining capillary force lithography at various length scales. Systematic experimental studies on silicon and silicon oxide substrates supported by analytical theory shows that for confining pattern wavelengths less than $\sim $ 10 times film thickness, stabilizing surface tension forces dominate the overall energy balance of the system giving rise to stable films under confinement. Interestingly, thermal annealing at elevated temperatures after removal of confinement does not revert to growth of longer instability modes and stability of PS films is retained. These results pave the way for important new technological applications of otherwise unstable polymer films.   

The influence of the solid/liquid interface on the dewetting of ultra thin polymer films

In recent years, many studies showed that a thin liquid film on a solid surface in air bears more complexity than expected from a simple three-layer-system: e.g. a highly mobile surface layer in case the liquid is an unentangled polystyrene (PS) melt (Yang et al.,\textit{ Science} 2010; Seemann et al., \textit{J. of Polym. Sci. }2006) or the PS melt can slip over the solid substrate (Baeumchen et al., \textit{PRL} 2009). Our study focuses on such phenomena and explores their influence on dewetting (speed, morphology, etc.). We use hydrophilic and -phobic Si wafer (either covered by a highly ordered silane layer or by a thin layer of an amorphous fluoropolymer, AF 1600). On each of the substrates, one expects for a certain set of parameters spinodal dewetting for the PS melt. Yet experimentally, a much higher hole density is observed for both types of hydrophobic wafers than is theoretically expected. Moreover, the two hydrophobic coatings induce different dewetting speeds: the PS melt dewets faster on the silane covered Si wafer. The difference is attributed to slip (silane) or to no slip (AF 1600) conditions at the PS/substrate interface, which is also observable in the type of liquid front profile, which in turn changes the dewetting morphology.   

Viscoelastic properties of ultrathin polystyrene films by dewetting from liquid glycerol

There is considerable interest in studying the behaviors of polymers at the nanoscale. A liquid dewetting device originally proposed by Bodiguel and Fretigny[1] was built in our lab to study the size effect on the viscoelastic behaviors of ultrathin polystyrene (PS) films. PS with molecular weight of 278 kg/mol and 984 kg/mol and various thermal treatments were examined. The glass transition temperature (T$_{g})$ reduction and film stiffening were observed in films less than 20 nm in thickness and the properties of ultrathin PS films are different from the bulk PS. The value of the plateau compliance changes linearly with film thickness. No molecular weight effect was found on the liquid dewetting behaviors of these PS films. Interestingly, even though the dewetting occurs on liquid glycerol, the apparent T$_{g}$ reductions are less than observed in SiO$_{2}$ substrate supported films. \\[4pt] [1] H. Bodiguel and C. Fretigny, ``Viscoelastic dewetting of a polymer film on a liquid substrate,'' \textit{Eur.Phys. J. E}., 19, 185-193 (2006).   

Evolution of non-equilibrium entanglement networks in spincast thin polymer films

Measuring the rheology of non-equilibrium thin polymer films has received significant attention recently. Experiments are typically performed on thin polymer films that inherit their structure from spin coating. While the results of several rheological experiments paint a clear picture, details of molecular configurations in spincast polymer films are still unknown. Here we present the results of crazing measurements which demonstrate that the effective entanglement density of thin polymer films changes as a function of annealing toward a stable equilibrium value. The effective entanglement density plateaus with a time scale on the same order as the bulk reptation time.   

Evolution of Chitosan Film Thickness Profiles During Spincoating

Many hygroscopic biopolymers can be processed using aqueous solutions. For example, biopolymer films can be prepared by spincoating from dilute biopolymer solutions. We have spincoated ultrathin films of chitosan onto silicon substrates under controlled relative humidity (\textit{RH}) using acetic acid solutions. Since the solvent is much less volatile than organic solvents that are typically used to spincoat thin films of synthetic polymers, the dynamics associated with the spincoating process can be several orders of magnitude slower. Because of the slow evaporation of the solvent, it is possible to control the thickness by controlling the \textit{RH} value in the spincoating chamber. To gain insight into the spincoating process, we have collected images of the film during spincoating using a high-speed camera. This has allowed us to determine the evolution of the radial profile of the chitosan film thickness, which can be correlated with the final film thickness values measured using ellipsometry. We compare the measured film thickness profiles with those predicted by theoretical models of spincoating.   

Spreading of polymer droplets on thin polymer films

We present experimental results of small (r$\sim $10 $\mu $m) polystyrene droplets spreading on thin polystyrene films. Previous experimental work has extensively explored droplets spreading on various types of solid substrates. However, to our knowledge, micron-sized liquid droplets spreading on the same liquid substrate have not been previously studied due to the difficulty of preparing such a geometry. Initially a glassy droplet is placed atop a glassy thin film and a distinct interface exists, upon heating the interface heals quickly. During spreading we must thus consider not only the flow of the polymer droplet but also that of the supporting film. This droplet-on-liquid geometry is fundamentally different from previous studies and allows us to probe the nanorheology of thin polymer films. We observe a characteristic power law spreading that is dependent on the size of the droplet as well as the height of the substrate film. The preparation and study of such samples provides many opportunities because the droplet and supporting film may be of the same polymer, the same polymer but differing molecular weight, or two different polymers.   

Experimental Study of Particle Behavior on a Curved Polymer Interface

A spherical particle bound to an anisotropically curved liquid interface, such as a cylinder or catenoid, cannot maintain a constant contact angle without deforming the interface. Theory predicts that because of this deformation, individual particles will experience a capillary force toward lower negative Gaussian curvature. To test this prediction, particles are deposited from suspension onto interfaces of non-uniform shape. Melted polystyrene (PS) confined on chemically patterned surfaces creates semi-cylinders a few hundred microns in diameter. Microscopic catenoids are created by placing a melted PS film in an electric field. After cooling, PS vitrifies and the particles are frozen in place. The location of particles is observed by optical, scanning electron, and scanning force microscopy (SFM). Particles are observed to migrate to the rims of the catenoids while particles on semi-cylinders cluster, but show no preference for location. At these size scales and particle concentrations, the predicted single-particle behavior is not observed. SFM is used to determine the validity of the assumed constant contact angle boundary condition at the particle's surface. The implication of these results on curvature induced particle assembly will be discussed.   

Tension amplification in branched macromolecules

A molecule's topology can greatly affect the distribution of tension within its bonds. Pom-pom molecules consist of a short linear spacer linking two star polymers, each containing z long arms. The striking ability of this molecular architecture to magnify spacer tension in solution by several orders of magnitude, from the pN to the nN level is due to the steric repulsion between densely packed side chains. In fact, the tension can increase to values that significantly alter the molecule's chemical properties, or even initiate the scission of a carbon-carbon covalent bond in the spacer chain. We study the tension distribution in the spacer and in side branches using molecular dynamics simulations and scaling theory. The dependences of observables such as spacer tension and root-mean-square spacer length on the number of side chains, chemical spacer length, and length of side branches have been quantified. Scaling models are used to explain the interrelated phenomena of tension amplification and spacer elongation and to interpret the results of molecular dynamics simulations.   

Viscous memory effects on the generation of hierarchical morphologies at an emulsified oil/water interface

A defining feature of biological materials is their fractal morphology. Cancellous bone, pulmonary alveoli, small intestine villi, neural networks, and bladder epithelium are just a few examples of biological structures with hierarchically organized topographies spanning multiple length scales. Herein we present a self-assembly method that faithfully reproduces the topographic features of these biomaterials. The system consists of a photocurable monomer and water. To this quasi-two-component system we add surfactants that sculpt the interface into the desired shape. The resulting structures are then solidi?ed by crosslinking with UV light. Drawing from the rich phase behavior of oil/water/surfactant systems, we demonstrate complex fractal morphologies over many length scales ranging from several mm down to 100 nm. Quantitative image analysis reveals fractal morphologies with at least four distinct levels of hierarchy. Increasing viscosity, in particular, shows a strong correlation with the number of hierarchical levels.   

Effect of Molecular Architecture on the Wetting Properties of Polymers

We show that the wetting properties of star-shaped polystyrene (PS) macromolecules possessing sufficiently high functionality, f higher than 4, differ significantly from their linear analogues of otherwise identical chemical structure. The equilibrium contact angles of macroscopic droplets composed of star-shaped macromolecules on silicon oxide substrate, are as much as one order of magnitude smaller than their linear analogues, provided that f is equal to or greater than 8 and the degree of polymerization per arm length, Narm, is sufficiently small. The corresponding line tensions of the star polymers are as much as two orders of magnitude smaller. The dewetting characteristics of the linear and star polymers also differ. Linear PS chains dewet leaving an adsorbed layer of nanoscopic dimensions; this layer is also structurally unstable and breaks up into nano-droplets, leaving a precursor layer at the boundary of the macroscopic droplets. The thickness of this layer is consistent with expectations based on the shape of the effective interface potential. The corresponding nanoscopic layer of star-shaped polymers, of sufficiently large f and sufficiently small Narm, remains structurally stable. These effects are discussed in terms of the role of molecular architecture and entropic effects on the structure and dynamics of macromolecules at interfaces.   

Surface Segregation of Well-Defined Comb Polymers

Blending polymers with different chain architectures may prove useful in controlling interfacial properties by controlling interfacial segregation. A linear response theory by Wu \textit{et al.} predicts that a long-chain branched polymer blended with its linear analog will be preferentially segregated to the surface and interface of the blend film. The comb architecture is particularly promising for achieving substantial surface segregation. In particular, its high degree of branching provides a substantial driving force for surface segregation when chain ends prefer the surface. Comb polystyrenes with well-defined architectural details were prepared by living anionic polymerization via the ``grafting-through'' approach. Neutron reflectivity (NR) and secondary ion mass spectrometry (SIMS) analyses reveal that the comb polymers that are still miscible with linear analogs in the bulk segregate so strongly to the surface that the surface concentration is nearly 100 vol{\%}. The effect on the surface segregation of bulk concentration and chain end chemistry will be discussed.   

Size Dependant Nucleation of Confined 2-Decanol

We have studied freezing and melting of physically confined 2-decanol in nano porous silica using a Differential Scanning Calorimeter (DSC). Both melting and freezing temperatures are suppressed for physically confined 2-decanol. In the presence of bulk, freezing of the confined system is triggered by freezing of the bulk where nucleation is heterogeneous. There is, however, a cutoff size between 100 nm and 300 nm where phase transition is no longer initiated through heterogeneous nucleation. Below the cutoff size, nucleation is homogeneous where the confined system has to be supercooled further before any phase transition can occur. Melting of the confined system, on the other hand, is not influenced by the presence or absence of the bulk.