Dynamics Of Thin Polymer Films Research Articles

Measurement of the depth-dependent local dynamics in thin polymer films through rejuvenation of ultrastable glasses

The depth dependence of structural relaxation dynamics is a key part of understanding thin glassy films. Despite this importance and decades of research, a method to provide this information has proved elusive. We measure the isothermal rejuvenation of stable glass films of poly(styrene), and demonstrate that the propagation of the front responsible for the transformation to a supercooled-liquid state serves as a highly localized probe of the local dynamics of the supercooled liquid. We use this connection to probe the depth-dependent relaxation rate with nanometric precision for a series of polystyrene films over a range of temperatures near the bulk glass transition temperature. The analysis shows the spatial extent of enhanced surface mobility and reveals the existence of an unexpected large dynamical length scale in the system. The results are compared with the cooperative-string model for glassy dynamics. The data reveals that the film-thickness dependence of whole film properties arises mainly from the volume fraction of the near-surface region. While the dynamics farthest from the free surface shows the expected bulk-like temperature dependence, the dynamics in the near-surface region shows very little dependence on temperature. This technique can be used in a broad range of thin film materials to gain previously unattainable information about localized structural relaxation.

Molecular Probe Diffusion in Thin Polymer Films: Evidence for a Layer with Enhanced Mobility Far above the Glass Temperature.

We studied experimentally the influence of interfaces on the dynamics in thin polymer films at temperatures far above the glass temperature (Tg + 80 °C). Polyisoprene (PI) was employed as a model system. We examined glass substrate supported films with thicknesses (d) spanning the range from 10 μm to 10 nm that correspond to d/Rg from 400 to 1, where Rg is the polymer radius of gyration. We employed fluorescence correlation spectroscopy (FCS) to monitor the translational diffusion of small fluorescent tracer molecules, dispersed at nanomolar concentrations in the PI matrix. In thick films, a single diffusion process correlated to the bulk segmental dynamics of the matrix polymer was present. However, when the film thickness was smaller than the normal dimension of the FCS observation volume, a second, faster diffusion process appeared, reflecting enhanced segmental dynamics near the free surface. Our results provide direct experimental evidence for the existence of a layer with enhanced mobility near the free surface of supported PI films at temperatures as high as 80 °C above the bulk Tg.

A wrinkling-based method for investigating glassy polymer film relaxation as a function of film thickness and temperature.

We investigate the relaxation dynamics of thin polymer films at temperatures below the bulk glass transition Tg by first compressing polystyrene films supported on a polydimethylsiloxane substrate to create wrinkling patterns and then observing the slow relaxation of the wrinkled films back to their final equilibrium flat state by small angle light scattering. As with recent relaxation measurements on thin glassy films reported by Fakhraai and co-workers, we find the relaxation time of our wrinkled films to be strongly dependent on film thickness below an onset thickness on the order of 100 nm. By varying the temperature between room temperature and Tg (≈100 °C), we find that the relaxation time follows an Arrhenius-type temperature dependence to a good approximation at all film thicknesses investigated, where both the activation energy and the relaxation time pre-factor depend appreciably on film thickness. The wrinkling relaxation curves tend to cross at a common temperature somewhat below Tg, indicating an entropy-enthalpy compensation relation between the activation free energy parameters. This compensation effect has also been observed recently in simulated supported polymer films in the high temperature Arrhenius relaxation regime rather than the glassy state. In addition, we find that the film stress relaxation function, as well as the height of the wrinkle ridges, follows a stretched exponential time dependence and the short-time effective Young's modulus derived from our modeling decreases sigmoidally with increasing temperature-both characteristic features of glassy materials. The relatively facile nature of the wrinkling-based measurements in comparison to other film relaxation measurements makes our method attractive for practical materials development, as well as fundamental studies of glass formation.

The architecture of the adsorbed layer at the substrate interface determines the glass transition of supported ultrathin polystyrene films.

To elucidate the mechanism underlying the effect of polymer/solid interfacial interactions on the dynamics of thin polymer films, the glass transition of thin end-functionalized polystyrene films supported on SiO2-Si, such as proton-terminated PS (PS-H), α,ω-dicarboxy-terminated PS (PS-COOH), and α,ω-dihydroxyl-terminated PS (PS-OH), was investigated. All the PS films exhibited a substantial depression in Tg with decreasing film thickness, while the extent of such depression was strongly dependent on the chemical structure of the end groups and molecular weights. It was found that T - T of the various PS films increased linearly with increasing hads/Rg, in which hads is the thickness of the interfacial adsorbed layer and Rg is the radius of gyration of PS. The hads/Rg is a direct reflection of the macromolecular chain conformation within the adsorbed layer which was affected by its end groups and molecular weights. These findings are in line with the work of Napolitano, and present direct experimental evidence.

A unifying framework to quantify the effects of substrate interactions, stiffness, and roughness on the dynamics of thin supported polymer films.

Changes in the dynamics of supported polymer films in comparison to bulk materials involve a complex convolution of effects, such as substrate interactions, roughness, and compliance, in addition to film thickness. We consider molecular dynamics simulations of substrate-supported, coarse-grained polymer films where these parameters are tuned separately to determine how each of these variables influence the molecular dynamics of thin polymer films. We find that all these variables significantly influence the film dynamics, leading to a seemingly intractable degree of complexity in describing these changes. However, by considering how these constraining variables influence string-like collective motion within the film, we show that all our observations can be understood in a unified and quantitative way. More specifically, the string model for glass-forming liquids implies that the changes in the structural relaxation of these films are governed by the changes in the average length of string-like cooperative motions and this model is confirmed under all conditions considered in our simulations. Ultimately, these changes are parameterized in terms of just the activation enthalpy and entropy for molecular organization, which have predictable dependences on substrate properties and film thickness, offering a promising approach for the rational design of film properties.

Arresting Tautomerization in a Single Molecule by the Surrounding Polymer: 2,7,12,17-Tetraphenyl Porphycene

2,7,12,17-Tetraphenyl porphycene (TPPo) is known to undergo ultrafast (<1 ps) double-hydrogen transfer in solution. Fluorescence studies of single TPPo molecules embedded in a polymer matrix and excited with an azimuthally polarized laser beam reveal, for a considerable fraction of the population, double-lobe spatial emission patterns attributed to the absence of tautomerism. In consecutive image scans, these patterns change their orientation to nearly orthogonal. In some cases, switching of the orientation direction occurs through an intermediate case with a doughnut-shaped pattern, indicating the presence of fast tautomerism. These findings demonstrate that local polymer environment can drastically change the thermodynamics or kinetics of tautomerization. They also point out the contribution of the motion of peripheral substituents in the hydrogen-transfer coordinate. TPPo is proposed as a good probe for studying relaxation dynamics in thin polymer films.

Capillary Wave Dynamics of Thin Polymer Films over Submerged Nanostructures

The surface dynamics of thin molten polystyrene films supported by nanoscale periodic silicon line-space gratings were investigated with x-ray photon correlation spectroscopy. Surface dynamics over these nanostructures exhibit high directional anisotropy above certain length scales, as compared to surface dynamics over flat substrates. A cutoff length scale in the dynamics perpendicular to the grooves is observed. This marks a transition from standard over-damped capillary wave behavior to suppressed dynamics due to substrate interactions.

Interfacial and Substrate Effects on Local Elastic Properties of Polymers Using Coupled Experiments and Modeling of Nanoindentation

AbstractThe mechanical properties of polymers near interfaces are important in a number of different fields. For almost two decades, the local dynamics of thin polymer films have been studied in great detail. However, development of an understanding of local mechanical properties has been hindered by the necessary proximity of stiff substrates: mechanical measurements are confounded by interaction with the substrate, convoluting polymer, and substrate properties. In this paper, local elastic properties of thin polymer films near interfaces are directly probed for the first time via nanoindentation experiments on thin films coupled with finite element modeling. A comprehensive set of experimental and numerical modeling results are presented for poly(methyl methacrylate) (PMMA) revealing separately the effects of substrate and interphase polymer. Results indicate the attractive surface significantly affects the properties up to hundreds of nanometers. This new, direct approach to measure local mechanical properties provides new fundamental understanding of interfacial and small‐scale behaviors in polymers and soft matter for application advances in nanocomposites, microelectronics, and biopolymers.

Confinement and flow dynamics in thin polymer films for nanoimprint lithography

In nanoimprint lithography (NIL) viscous flow in polymeric thin films is the primary mechanism for the generation and the relaxation of the structures. Here we quantify the impact of confinement on the flow rate. Pattern relaxation experiments were carried out above the glass transition temperature as a function of film thickness. The results are adequately fitted by a simple expression for the flow rate valid at all confinements. This expression, based on Newtonian viscosity, should be of use in NIL process design and for the measurement of the rheological properties of confined polymers.

Challenges in Glassy Dynamics of Polymers

AbstractThis work is focused on the glassy dynamics of polymers under conditions of one‐dimensional confinement (in thin films). Several important topics are addressed: (a) preparative factors with large impact on the molecular dynamics of thin polymer films (oxidative degradation, plasticizing effects, residual stresses leading to metastable states); (b) it is emphasized the necessity to characterize the glass transition by measuring relaxation time distribution functions; (c) it is shown that different experimental techniques do not necessarily deliver converging results when applied to characterise the dynamics of confined polymers; (d) a novel approach is demonstrated which grants a direct experimental access to interfacial polymer dynamics; (e) measurements on nearly isolated polymer chains are presented.magnified image

Dielectric Behavior of Glass Transition and Dynamics in Thin Polymer Films

Glass transition temperature (Tg) and dynamics of thin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) are investigated using dielectric relaxation spectroscopy and related methods. In case of thin films of polystyrene, the glass transition temperature could successfully be obtained as a function of film thickness and the obtained results are consistent with those observed using other techniques such as ellipsometry. In accordance with the decrease in the glass transition temperature in thin films, the dynamics of the α-process becomes faster with decreasing film thickness and the distribution of the α-relaxation times becomes broader. In case of thin films of poly(methyl methacrylate), the thickness dependence of the α- and β-processes could be observed using the dielectric relaxation spectroscopy. Both the temperatures Tα and Tβ, at which the dielectric losses due to the α- and β-processes show a maximum at a given frequency, decrease with decreasing film thickness. Below a critical thickness, both Tα and Tβ begin to decrease rapidly with decreasing thickness. In order to investigate the positional dependence of the dynamics of the α-processes for polystyrene thin films, we utilize bilayer films, where a thin film of PS labeled with a dye is located on top of the film of unlabeled PS. For such systems, we could extract the contribution only from the upper labeled layer of PS. The glass transition temperature of the upper labeled layer between unlabeled PS layer and aluminum electrode is lower than that of the bulk labeled polystyrene. This result supports that the sample geometry commonly used for dielectric measurements of ultra thin films is the same to that used for other methods as far as the glass transition dynamics are concerned.

Stabilizing Thin Film Polymer Bilayers against Dewetting Using Multiwalled Carbon Nanotubes

We have investigated the effect of the length of multiwalled carbon nanotubes (MWNT) on the dewetting dynamics of thin polymer films. The results indicated that long nanotubes were much more effective than short nanotubes in stabilizing the films against dewetting. The diffusion of polymer chains in the filled matrices was measured using neutron reflectivity, and the result indicated no significant effect on the diffusion coefficient by either short or long nanotubes. We therefore proposed a model whereby the nanotubes did not interact with the individual polymer chains. On the other hand, the long nanotubes formed an effective entangled network, which prevented long-range motion of the polymer films upon dewetting. This model was supported by rheological experiments on bulk samples where the nanotubes had a strong effect on G‘ of the polymer and only a negligible effect on G‘ ‘.

Structure and dynamics of thin polymer films using synchrotron X-ray scattering

Recent measurements of the scattering function and of the dynamics of surface and interfacial fluctuations in thin supported molten films and bilayers using synchrotron X-ray diffuse scattering and photon correlation spectroscopy in reflection geometry are reported. The results for monolayer films thicker than four times of the radius of gyration of polystyrene show behavior of normal over-damped capillary waves expected for the surface fluctuations of a viscous liquid. However, thinner films show deviations indicating the need to account for viscoelasticity. The theory has been extended to the surface and interfacial modes in a bilayer film system. The results are discussed in terms of surface tension, viscosity and shear modulus. Also recent experiments to measure the isothermal compressibility of supported polystyrene films by studying `bulk' scattering from the interior of the films is discussed.

Dewetting of thin UHV-deposited organic films

The dewetting of thin organic molecular films UHV-deposited on He-cooled substrate is analyzed by optical and AFM microscopies. By controlled annealing to room temperature such films can be transformed into patterns of isolated droplets. The description of the dewetting process takes advantage of the close similarity to pattern formation and dynamics in thin polymer films (polystyrene). Typical dewetting stages are discerned. Experimental results on films of a fulgide derivative, deposited on quartz glass substrates at thicknesses of 10 and 21.5 nm, are presented as example.

Qualitative discrepancy between different measures of dynamics in thin polymer films⋆

We have used ellipsometry to measure the initial stages of interface healing in bilayer polystyrene films. We also used ellipsometry to measure the glass transition temperature T(g) of the same or identically prepared samples. The results indicate that as the film thickness is decreased, the time constant for the interface healing process increases, while at the same time the measured glass transition temperature in the same samples decreases as the film thickness is decreased. This qualitative difference in the behavior indicates that it is not always possible to make inferences about one probe of polymer dynamics from measurements of another. We propose a reason for this discrepancy based on a previously discussed origin for reduction in the T(g) value of thin films.

Dynamics of thin polymer films: Recent insights from incoherent neutron scattering

AbstractIncoherent neutron scattering is presented as a powerful tool for interpreting changes in molecular dynamics as a function of film thickness for a range of polymers. Motions on approximately nanosecond and faster timescales are quantified in terms of a mean‐square atomic displacement (〈u2〉) from the Debye–Waller factor. Thin‐film confinement generally leads to a reduction of 〈u2〉 in comparison with the bulk material, and this effect becomes especially pronounced when the film thickness approaches the unperturbed dimensions of the macromolecule. Generally, there is a suppression (never an enhancement) of 〈u2〉 at temperatures T above the bulk calorimetric glass‐transition temperature (Tg). Below Tg, the reduction in the magnitude of 〈u2〉 depends on the polymer and the length scales being probed. Polymers with extensive segmental or local mobility in the glass are particularly susceptible to reductions of 〈u2〉 with confinement, especially at the Q vectors probing these longer length scales, whereas materials lacking these sub‐Tg motions are relatively insensitive. Moreover, a reduced 〈u2〉 value correlates with reduced mobility at long time and spatial scales, as measured by diffusion in these thin polymer films. Finally, this reduced thin‐film mobility is not reliably predicted by thermodynamic assessments of an apparent Tg, as measured by discontinuities or kinks in the T dependence of the thermal expansion, specific volume, index of refraction, specific heat, and so forth. These measurements illustrate that 〈u2〉 is a powerful and predictive tool for understanding dynamic changes in thin polymer films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3218–3234, 2004

Dynamics in thin polymer films by dielectric spectroscopy.

After a brief review on the recent developments of the dielectric studies in thin polymer films, our recent results on dynamics in thin films of poly(methyl methacrylate) (PMMA) and polyisoprene (PIP) are shown. For PMMA, the tacticity effect on dynamics in thin films has been investigated and the disappearance of this effect was found below a critical thickness. For PIP, the motion of entire polymer chains, namely, the normal mode, has been investigated. The dielectric-loss spectrum of the normal mode is much more sensitive to the decrease in film thickness than that due to the [Formula: see text]-process. The broadening of dielectric-loss spectra of the normal mode is observed for film thicknesses below about 150 nm, while the position of the loss peak does not change in the thickness range down to about 50 nm. Anomalous increase in dielectric loss between the [Formula: see text]-process and the normal mode was observed, which is consistent with the recent report on the existence of an additional relaxation process.

Glassy dynamics in thin polymer films: recent MD results

The influence of a film geometry on the glass transition is investigated via molecular dynamics (MD) simulations of a (non-entangled) polymer melt. The confinement is realized by two identical potential barriers of the form U wall= z −9, where z denotes the distance of a particle from the wall. Despite the geometric confinement, basic qualitative features of the system dynamics can be well described in the framework of the mode-coupling theory (MCT). Examples are the two-step relaxation of the incoherent intermediate scattering function, the time–temperature superposition property of the late time α-process and the space-time factorization of the scattering function on the intermediate time scale of the MCT β-process. A comparison of the dynamics in the film and in the bulk shows an acceleration of the relaxation processes due to the presence of the walls. This leads to a reduction of the critical temperature, T c, of MCT with decreasing film thickness. A comparison of the quantities like the static structure factor and the mean-square displacements for the bulk and for the film suggests that T− T c( D) is a relevant temperature scale for the dynamics at intermediate times. Furthermore, we also analyze the sharp rise of the relaxation times at low temperatures by the Vogel–Fulcher–Tammann (VFT) equation and thus estimate the VFT-temperature T 0( D). We observe that, similar to T c( D), also T 0( D) decreases for smaller D. As T 0⩽ T g⩽ T c, these results suggest that also the glass transition temperature should decrease for stronger confinement.

Relaxation dynamics in thin supported polymer films

The glass transition and relaxation dynamics of thin polymer films supported on a substrate have been investigated for three different polymers, atactic polystyrene, poly(vinyl acetate), and atactic poly(methyl methacrylate) by using dielectric relaxation and thermal expansion spectroscopy. The temperature, T α, at which the imaginary component of the susceptibility has a peak due to the α-process at a fixed frequency, is measured as a function of thickness and the molecular weight of the polymers. The observed thickness and molecular weight dependence of T α is discussed in comparison with that of T g observed in the freely standing films of polystyrene.

Structure and dynamics of thin polymer films: a case study with the bond-fluctuation model

This paper reports Monte Carlo simulation results of a polymer melt of short, non-entangled chains which are embedded between two impenetrable walls. The melt is simulated by the bond-fluctuation lattice model under athermal conditions, i.e. only excluded volume interactions between the monomers and between the monomers and the walls are taken into account. In the simulations, the wall separation is varied from about one to about 15 times the bulk radius of gyration Rg. The confinement influences both static and dynamic properties of the films: Chains close to the walls preferentially orient parallel to it. This parallel orientation decays with increasing distances from the wall and vanishes for distances larger than about 2Rg. Strong confinement effects are therefore observed for film thicknesses D≲4Rg. The preferential alignment of the chains with respect to the walls suppresses reorientations in perpendicular direction, whereas parallel reorientations take place in an environment of high monomer density. Therefore, they have a relaxation time larger than that of the bulk. On the other hand, the influence of confinement on the translation motion of the chains parallel to the walls is very weak. It almost coincides with the bulk behavior even if D≈1.5Rg. Despite these differences between translational and reorientational dynamics, their behavior can be well reproduced by a variant of Rouse theory which only assumes orthogonality of the Rouse modes and determines the necessary input from the simulation.