Viscosity of entangled polystyrene thin film melts: Film thickness dependence

Roles of aqueous nonsolvents influencing the dynamic stability of poly-(n-butyl methacrylate) thin films at biologically relevant temperatures

Poly-(n-butyl methacrylate) (PnBMA) is an important polymer in biomedical applications. Here we study the stability of PnBMA thin films prepared on top of slippery silicon substrates and exposed to nonsolvent aqueous incubation media like water and phosphate-buffered saline (PBS) at temperatures relevant to biological applications (37 °C, 25 °C and 4 °C). Dewetting hole growth experiments allowed us to probe the instability in PnBMA films upon incubation followed by thermal annealing. From the early stage of dewetting hole growth dynamics, we inferred that the stability of the thin PnBMA films decreases as a function of the duration and temperature of incubation, even though the films were found not to readily dewet at room temperature after incubation. It is also observed that water incubation makes films more unstable than incubation in PBS. We explained our observations as a combined effect of (i) an increase in surface energy of the PnBMA film due to incubation, (ii) an increased destabilizing effect due to the dominant polar interactions between the incubation medium and the PnBMA film and (iii) the plasticization effect of PnBMA films by the incubation media. Plasticization resulted in a decrease in the modulus of PnBMA thin films as a function of incubation time. The viscosity of PnBMA films upon incubation was found to be coupled to the decreasing modulus. Thus we infer that incubation in common aqueous nonsolvents can detrimentally affect the stability of polymers limiting their specific usages through a complex interplay of multiple molecular level phenomena.

Dynamics and Structure Formation of Confined Polymer Thin Films Supported on Solid Substrates

The stability/instability behavior of polystyrene (PS) films with tunable thickness ranging from higher as-cast to lower residual made on Si substrates with and without native oxide layer was studied in this paper. For further extraction of residual PS thin film (hresi) and to investigate the polymer-substrate interaction, Guiselin's method was used by decomposing the polymer thin films in different solvents. The solvents for removing loosely adsorbed chains and extracting the strongly adsorbed irreversible chains were selected based on their relative desorption energy difference with polymer. The PS thin films rinsed in chloroform with higher polarity than that of toluene showed a higher decrease in the residual film thickness but exhibited earlier growth of holes and dewetting in the film. The un-annealed samples with a higher oxide film thickness showed a higher decrease in the PS residual film thickness. The effective viscosity of PS thin films spin-coated on H-Si substrates increased because of more resistance to flow dynamics due to the stronger polymer-substrate interaction as compared to that of Si-SiOx substrates. By decreasing the film thickness, the overall effective mobility of the film increased and led to the decrease in the effective viscosity, with matching results of the film morphology from atomic force microscopy (AFM). The polymer film maintained low viscosity until a certain period of time, whereupon further annealing occurred, and the formation of holes in the film grew, which ultimately dewetted the film. The residual film decrement, growth of holes in the film, and dewetting of the polymer-confined thin film showed dependence on the effective viscosity, the strength of solvent used, and various involved interactions on the surface of substrates.

Kinetics of sub-spinodal dewetting of thin films of thickness dependent viscosity

An alternative explanation of the time varying and very low growth exponents in dewetting of polymer films like polystyrene films is presented based on non-linear simulations. The kinetics of these films is explored within the framework of experimentally observed thickness dependent viscosity. These films exhibit sub-spinodal dewetting via formation of satellite holes in between primary dewetted holes under favorable conditions of excess intermolecular forces and film thicknesses. We find that conditions responsible for sub-spinodal dewetting concurrently lead to remarkable changes in the kinetics of dewetting of even primary holes. For example, the radius of the hole grows in time with a power-law growth exponent sequence of [Formula: see text], in contrast to the usual  ∼4/5. This is due to the cumulative effect of reduced rim mobility due to thickness dependent viscosity and hindrance created by satellite holes.

Spatial distribution of entanglements in thin free-standing films

We simulate entangled linear polymers in free-standing thin film geometries where the confining dimension is on the same scale as or smaller than the bulk chain dimensions. We compare both film-averaged and layer-resolved, spatially inhomogeneous measures of the polymer structure and entanglement network with theoretical models. We find that these properties are controlled by the ratio of both chain- and entanglement-strand length scales to the film thickness. While the film-averaged entanglement properties can be accurately predicted, we identify outstanding challenges in understanding the spatially resolved character of the heterogeneities in the entanglement network, particularly when the scale of both the entanglement strand and the chain end-to-end vector is comparable to or smaller than the film thickness.

Kinetically engendered subspinodal length scales in spontaneous dewetting of thin liquid films

Numerical simulations reveal emergence of subspinodal length scales in spontaneous dewetting of nonslipping unstable thin liquid films on homogeneous substrates if the liquid viscosity decreases with decrease in film thickness.

Role of surface in apparent viscosity of glasses

Two problems have intrigued experts for a long time: The one is within the context of the legend of flowing cathedral glass windows and the second is the inaccuracy appearing in very old thermometers of famous scientists. We relate this with the role of the surface on the apparent viscosity of glasses. The apparent viscosity could deviate from the bulk viscosity if the fraction w of the surface molecules, of small samples, is sufficiently large. The effect is more prominent at low temperatures, correspondingly at high viscosities. The interpretation is within the Avramov and Milchev viscosity model, combined with the predictions of the change of heat capacity for extremely small samples. We find that the apparent glass transition temperature could depend on the sample size, in agreement with experimental observations existing in the literature. In addition to glasses, the present results could be of importance for thin films and foams.

Onset of plasticity in thin polystyrene films

Polymer glasses have numerous advantageous mechanical properties in comparison to other materials. One of the most useful is the high degree of toughness that can be achieved due to significant yield occurring in the material. Remarkably, the onset of plasticity in polymeric materials is very poorly quantified, despite its importance as the ultimate limit of purely elastic behavior. Here, we report the results of a novel experiment which is extremely sensitive to the onset of yield and discuss its impact on measurement and elastic theory. In particular, we use an elastic instability to locally bend and impart a local tensile stress in a thin, glassy polystyrene film, and directly measure the resulting residual stress caused by the bending. We show that plastic failure is initiated at extremely low strains, of the order 10(-3) for polystyrene. Not only is this critical strain found to be small in comparison to bulk measurement, we show that it is influenced by thin film confinement--leading to an increase in the critical strain for plastic failure as film thickness approaches zero.

Viscosity measurements of thin polymer films from reflow of spatially modulated nanoimprinted patterns

We present a method to measure the viscosity of polymer thin films. The material is spin coated onto a silicon substrate and specially designed nanopatterns are imprinted on the film using thermal nanoimprint. A brief reflow is performed during which patterns flow under surface tension. Spectral densities of the topology before and after annealing are compared and the rheologic properties, such as viscosity, are extracted as fitting parameters of an evolution model. Contrary to previous similar approaches, emphasis was put on the spatial description rather than the temporal decay of the patterns. We used this method to measure the viscosity of polystyrene for two molecular weights at various temperatures and successfully recovered results of previous authors.

Application of colloid probe atomic force microscopy to the adhesion of thin films of viscous and viscoelastic silicone fluids

The adhesive characteristics of thin films (0.2-2 μm) of linear poly(dimethylsiloxane) (PDMS) liquids with a wide range of molecular weights have been measured using an atomic force microscope with a colloid probe (diameters 5 and 12 μm) for different separation velocities. The data were consistent with a residual film in the contact region having a thickness of ∼6 nm following an extended dwell time before separation of the probe. It was possible to estimate the maximum adhesive force as a function of the capillary number, Ca, by applying existing theoretical models based on capillary interactions and viscous flow except at large values of Ca in the case of viscoelastic fluids, for which it was necessary to develop a nonlinear viscoelastic model. The compliance of the atomic force microscope colloid beam was an important factor in governing the retraction velocity of the probe and therefore the value of the adhesive force, but the inertia of the beam and viscoelastic stress overshoot effects were not significant in the range of separation velocities investigated.

Nonequilibrium behavior of thin polymer films

The rheological behavior of 100-nm-thick polystyrene films cast from various solvents was examined using an electric field to weakly perturb the free surface of the polymer melt. The effective viscosity and residual stresses of the as-spun films are seen to strongly depend on the properties of the casting solvent and the solvent quality. Both effects are explained in terms of the coil dimension at the solvent-polymer composition at which the film vitrifies. The more compact chains in a near-Θ-solvent are less entangled and less deformed when quenched to the dry melt compared to the more swollen chains in an athermal solution. Despite chain conformations that are further from equilibrium for the Θ-solvent cast chains, these films have reduced stored stresses compared to the chains cast in films from athermal solvents. A more detailed analysis of the data suggests that the formation of a surface-near region with more strongly deformed chains during spin coating. Since thermal equilibration of spin-cast high-molecular-weight films is unpractical, solvent vapor annealing was used to equilibrate films on timescale of a few hours.

Formation of nanopillar arrays in ultrathin viscous films: the critical role of thermocapillary stresses

Experiments by several groups during the past decade have shown that a molten polymer nanofilm subject to a large transverse thermal gradient undergoes spontaneous formation of periodic nanopillar arrays. The prevailing explanation is that coherent reflections of acoustic phonons within the film cause a periodic modulation of the radiation pressure which enhances pillar growth. By exploring a deformational instability of particular relevance to nanofilms, we demonstrate that thermocapillary forces play a crucial role in the formation process. Analytic and numerical predictions show good agreement with the pillar spacings obtained in experiment. Simulations of the interface equation further determine the rate of pillar growth of importance to technological applications.

Nonsolvent-induced dewetting of thin polymer films

The process of nonsolvent-induced dewetting of thin polystyrene (PS) films on hydrophilic surfaces at room temperature has been studied by using water as a nonsolvent. It is observed that the process of nonsolvent-induced dewetting is greatly different from other previous dewetting processes. The PS film is found in nonviscous state in our study. A mechanism of nonsolvent-induced dewetting is deduced in an order of penetration, replacement, and coalescent, and it is different from other previous dewetting mechanisms. The results of experiments are analyzed from thermodynamics and dynamics to support the hypothetical mechanism.

Dewetting dynamics in miscible polymer-polymer thin film mixtures

Thin polystyrene films supported by oxidized silicon (SiOx/Si) substrates may be unstable or metastable, depending on the film thickness, h, and can ultimately dewet the substrate when heated above their glass transition. In the metastable regime, holes nucleate throughout the film and subsequently grow due to capillary driving forces. Recent studies have shown that the addition of a second component, such as a copolymer or miscible polymer, can suppress the dewetting process and stabilize the film. We examined the hole growth dynamics and the hole morphology in thin film mixtures composed of polystyrene and tetramethyl bisphenol-A polycarbonate (TMPC) supported by SiOx/Si substrates. The hole growth velocity decreased with increasing TMPC content beyond that expected from changes in the bulk viscosity. The authors show that the suppression of the dewetting velocity is primarily due to reductions in the capillary driving force for dewetting and to increased friction at the substrate-polymer interface. The viscosity, as determined from the hole growth dynamics, decreases with decreasing film thickness, and is connected to a depression of the glass transition of the film.

Dynamics of dewetting at the nanoscale using molecular dynamics

Large-scale molecular dynamics simulations are used to model the dewetting of solid surfaces by partially wetting thin liquid films. Two levels of solid-liquid interaction are considered that give rise to large equilibrium contact angles. The initial length and thickness of the films are varied over a wide range at the nanoscale. Spontaneous dewetting is initiated by removing a band of molecules either from each end of the film or from its center. As observed experimentally and in previous simulations, the films recede at an initially constant speed, creating a growing rim of liquid with a constant receding dynamic contact angle. Consistent with the current understanding of wetting dynamics, film recession is faster on the more poorly wetted surface to an extent that cannot be explained solely by the increase in the surface tension driving force. In addition, the rates of recession of the thinnest films are found to increase with decreasing film thickness. These new results imply not only that the mobility of the liquid molecules adjacent to the solid increases with decreasing solid-liquid interactions, but also that the mobility adjacent to the free surface of the film is higher than in the bulk, so that the effective viscosity of the film decreases with thickness.

Nature of solidification of nanoconfined organic liquid layers

A simple model is established for solidification of a nanoconfined liquid under nonequilibrium conditions. In terms of this model, the nature of solidification is the conjunct finite size and interface effects, which is directly related to the cooling rate or the relaxation time of the undercooled liquid. The model predictions are consistent with available experimental results.

Stamp design effect on 100 nm feature size for 8 inch NanoImprint lithography

Sub-100 nm resolution on a 200 mm silicon stamp has been hot embossed into commercial Sumitomo NEB 22 resist. A single pattern, exposed with electron beam lithography, has been considered to define the stamp and thus make it possible to point out the impact of stamp design on the printing. These results may be considered as a first attempt to define rules to solve the proximity printing effects (PPEs). Moreover, a large range of initial resist thickness, from 56 to 506 nm, has been spin coated to assess the effect of polymer flow properties for the stamp cavity filling and the printed defects. A detailed analysis of the printed resist in dense hole patterns showed that the application volume conservation is enough to calculate the residual layer thickness as the height of the printed resist feature. Good accordance has been obtained between the theoretical approach and experimental results. Moreover, the impact of the pattern symmetry breakdown on mould deformation is clearly shown in this paper in the printed areas as well as in the unprinted areas.

Viscoelastic dewetting of a polymer film on a liquid substrate

The Dewetting of thin polymer films (60-300 nm) on a non-wettable liquid substrate has been studied in the vicinity of their glass transition temperature. In our experiment, we observe a global contraction of the film while its thickness remains uniform. We show that, in this case, the strain corresponds to simple extension, and we verify that it is linear with the stress applied by the surface tension. This allows direct measurement of the stress/strain response as a function of time, and thus permits the measurement of an effective compliance of the thin films. It is, however, difficult to obtain a complete viscoelastic characterization, as the short time response is highly dependant on the physical age of the sample. Experimental results underline the effects of residual stress and friction when dewetting is analyzed on rigid substrates.

Hole growth in freely standing polystyrene films probed using a differential pressure experiment

We have probed the whole chain mobility of polymer molecules confined to freely standing films by measuring the flow of gas through holes growing in the films at elevated temperatures using a differential pressure experiment. Freely standing polystyrene films were measured for the temperature range 92 degrees C<T<105 degrees C for films with two different molecular weights Mw=717 x 10(3) and 2240 x 10(3) , with thicknesses 51 nm<h<97 nm . This range of film thicknesses is of particular interest because large reductions in the glass transition temperature Tg have been measured previously for freely standing PS films in this thickness range. We find that hole formation and growth, and therefore substantial chain mobility, does not occur until temperatures close to the bulk value of the glass transition temperature T(bulk)g. The characteristic growth times tau for the thinnest films, which have reduced values of Tg, are not substantially less than those for thicker films, and we find that these small differences in tau can be understood in terms of the bulk phenomenon of shear thinning. We also show that the viscosity at the edge of the hole inferred from the characteristic growth times obtained in this and previous studies exhibit shear thinning with reduced shear strain rates beta that span twelve orders of magnitude.

Stratification kinetics of polyelectrolyte solutions confined in thin films

We have studied free horizontal liquid films made with semidilute polyelectrolyte solutions. A stratification phenomenon is observed during film thinning, with a step size close to the mesh size of the polymer network: dark domains nucleate and expand, the outer polymer layer dewetting a thinner film. The kinetics of dark spot expansion is not simply related to bulk viscosity and becomes faster when the film thickness decreases, suggesting an increase of the chain mobility of the confined polymer chains. These findings are similar to recent ones for other confined liquids and are the first reported so far for freely standing films.

Moving fronts in entangled polymeric films

Thin liquid films can become structurally unstable and dewet, forming holes which subsequently grow on the substrate. Considerable research has been conducted on the structural evolution and growth of holes, which invariably are shown to be circular. We show that morphologies characterized by circular holes comprise one of three possible morphological regimes. In polystyrene films, supported by silicon oxide substrates, two other regimes are observed with decreasing film thickness. In the second regime, the moving boundary of the growing hole may become unstable and form fingers. The spacing between the fingers is characterized by a well-defined wavelength lambda proportional to h(7/6) M(-1/2) , where h is the film thickness and M is the molecular weight. A dense branchlike morphology characterizes the peripheral regions of the holes in the third regime and is found only in the thinnest films.

Dewetting of thin-film polymers

In this paper we present a theoretical model for the dewetting of ultrathin polymer films. Assuming that the shear-thinning properties of these films can be described by a Cross-type constitutive equation, we analyze the front morphology of the dewetting film, and characterize the time evolution of the dry region radius, and of the rim height. Different regimes of growth are expected, depending on the initial film thickness, and on the power-law index involved in the constitutive equation. In the thin-films regime, the dry radius and the rim height obey power-law time dependences. We then compare our predictions with the experimental results obtained by Debrégeas et al. [Phys. Rev. Lett. 75, 3886 (1995)] and by Reiter [Phys. Rev. Lett. 87, 186101 (2001)].