Session A55: Confined Polymer Glasses I: Influence of Irreversibly Adsorbed Layers and Free Surfaces

Unraveling the Dynamics of Thin Films of a Miscible PVME/PS Blend

Dielectric spectroscopy (BDS) was employed to investigate the dynamics of thin films (7 – 200 nm) of a Poly (vinyl methyl ether) (PVME) / Polystyrene (PS) blend (50:50 wt%). For the BDS measurements Nano-Structured Capacitors (NSC) were employed, where films have a free surface. This method was applied for film thicknesses up to 36 nm. Thicker films were prepared between Crossed Electrodes Capacitors (CEC). The spectra of the films showed multiple processes. The first process was assigned to the a-relaxation of a bulk-like layer. For films measured by NSC, its rates were higher compared to that of the bulk blend. This behavior was related to a PVME-rich free-surface layer. A second process was observed for films measured by CEC (process X) and the 36 nm film measured by NSC (process X2). This process was assigned to fluctuations of PVME constraint by PS. Its activation energy was found to be thickness dependent, due to the evidenced thickness dependency of the compositional heterogeneity. Finally, a third process with an activated temperature-dependence was observed for all films measured by NSC (process X1). It resembled the molecular fluctuations in an adsorbed layer found for films of pure PVME.
  

Mechanisms of Irreversible Polymer Adsorption

Controlling polymer/substrate interfaces without modifying chemistry is nowadays possible by finely tuning the formation of adsorbed layers. The complex processes leading to irreversible attachment of chains onto solid substrates are governed by two mechanisms: molecular rearrangement and potential-driven adsorption. Here we describe an analytical method to differentiate these two mechanisms. By analyzing experiments and simulations, we investigate how changes in thermal energy and interaction potential affect equilibrium and non-equilibrium components of the adsorption kinetics. We find that adsorption process is thermally activated, with activation energy comparable to that of local non-cooperative processes. On the other hand, the final adsorbed amount depends on the interface interaction only (i.e. it is temperature independent in experiments). We identify a universal linear relation between the growth rates at short and long adsorption times, suggesting that the monomer pinning mechanism is independent of surface coverage, while the progressive limitation of free sites significantly limits the adsorption rate.   

Growth Kinetics and Molecular Mobility of Irreversibly Adsorbed Layers in Thin Polymer Films

In well-annealed thin polymer films, with non-repulsive polymer/substrate interactions, an irreversibly adsorbed layer is expected to form. These adsorbed layers have shown great potential for technological applications. However, their growth kinetics and molecular dynamics are still not fully understood. This is partially due to the hard accessibility of these layers in thin films. Here, the irreversibly adsorbed layers of homopolymer thin films are revealed by solvent-leaching experiments. First, the growth kinetics of these layers is investigated as a function of annealing times and original film thickness. The thickness, topography and quality of the adsorbed layer is controlled with Atomic Force Microscopy (AFM). Secondly, the molecular mobility of the adsorbed layer is investigated with Broadband Dielectric Spectroscopy (BDS). A recently developed nanostructured capacitor is employed to measure the adsorbed layers with a free surface layer. The results are quantitatively compared and discussed with respect to recently published work.   

Effect of Chain Adsorption on the Mobility of Polymer Chains

Irreversible adsorption of polymer chains on solid substrates have a huge impact on the wetting, glass transition, aging and polymer chain mobility in thin films. In recent years the segmental mobility of the adsorbed chains and neighbouring unadsorbed chains is under discussion. We have studied the effect of polymer chain adsorption on the mobility of polymer chains in thin films for linear and star polymers. The vertical mobility of polystyrene (PS) chains was determined by probing interdiffusion between protonated PS (hPS) and deuterated PS (dPS) layers using neutron reflectivity. By varying the amount of deuterated adsorbed chains in the bottom layer for linear dPS, 4-arm star dPS and 8-arm star dPS we have determined that the mobility of polymer chains did not decrease with increasing adsorption. The apparent diffusion coefficient for 8-arm star PS was the largest even though X-ray reflectivity measurements showed that 8-arm star PS chains form adsorbed layers with the largest normalized thickness among these three polymers.   

Frozen Local Conformation of Interfacial Polyisoprene Chains at Room Temperature

The performance of a polymer composite material, in which inorganic fillers are dispersed, is closely related to the aggregation states and dynamics of polymer chains at the filler interface. In this study, the local conformation of polyisoprene (PI) at a quartz substrate interface was studied as a model system for the rubber/filler composite material. Films of PI were prepared from a toluene solution onto quartz substrates by a spin-coating method. Sum-frequency generation vibrational spectroscopy revealed that the local conformation of interfacial PI chains at the quartz interface depended on the spinning rate. The tilt angle of methyl groups increased with the rotational speed probably due to the centrifugal force applied to chains during the solidification process. This result indicates that the interfacial orientation of PI chains can remain even at room temperature, which is approximately 90 K higher than the bulk glass transition temperature (T_g^b). The interfacial orientation disappeared at a temperature approximately 120 K higher than T_g^b.   

The Interfacial Zone Around Nanoparticles in Polymer Nanocomposites and Its Relation to the Average Scale of Collective Motion within the Polymer Matrix

There has been much interest in quantifying the spatial scale over which the interfacial dynamics of glassy materials is perturbed by the presence of solid substrates and nanoparticles and in factors that influence the dynamics within this “interfacial zone”. In general, there are many interfacial zones to consider in thin polymer films and nanocomposites, but we focus on the interfacial zone about nanoparticles (NPs) in a polymer matrix, a topic of intense recent experimental investigation because of the relevance of such layers for achieving significant property changes in polymer materials. We model our nanocomposites using a coarse-grained model of both the NPs and the polymer matrix that we have studied extensively in the past. We find that the width ξ of the interfacial zone about the NPs is typically on the order of a few nm and that this scale grows progressively upon cooling where the temperature dependence and order of magnitude of the interfacial zone are in qualitative accord with estimates by Sokolov and coworkers, and others. We also find that the NP interfacial zone scale ξ grows nearly linearly with the extent of string-like collective motion within the film, as found before for the interfacial zone near the polymer-air interface of thin supported polymer films.   

Glass transition and molecular mobility in polymers under nanoscale confinement

In the last couple of decades, a plethora of studies have shown that confined polymer glasses with free interface exhibit significantly modified vitrification kinetics, resulting in substantial T_g suppression. Beside this observation, a number of studies, where the molecular mobility is measured by techniques probing the linear response, show the presence of a dominant component with bulk-like dynamics. To explain these results, two main hypotheses have been recently formulated. On one side, it has been argued that in nanoscale confinement T_g and molecular dynamics are decoupled. Beside, the role of free surface to speed dynamics, which has been shown in several studies, has been invoked to explain T_g suppression in confinement.
This contribution aims to provide compelling experimental evidence on the decoupling between glass transition and molecular mobility in polymeric glasses under geometrical confinement. Special attention is devoted to calorimetric techniques where the two aspects can be unequivocally separated by applying temperature perturbations either in the linear and non-linear regime. A number of examples are provided where, despite the ubiquitous presence of a prominent component with bulk-like linear dynamics, the T_g is suppressed. This outcome is discussed in view of the conceptual difference between the two aspects of glass dynamics.
  

Diminishing Effect of Bound Polymer Layers with Decreasing Nanoparticle Size

The attractive interactions between polymers and nanoparticles (NPs) in polymer composites can leads to the formation of a "bound" layer around the nanoparticle (NP) with very slow dynamics. We explore the degree to which bound polymer affects nanocomposite relaxation as NP size becomes progressively smaller. To explain the effects of NP size, we study the structural and dynamical (relaxation time, glass transition temperature) of polymer composites. First, we examine the dependence of relaxation on the distance from the nanoparticle interface. We then look at how the overall relaxation, which averages over the relaxation gradient, changes with NP size. Whether looking at local or overall relaxation, we show how the effects of NP on interfacial relaxation and bound polymer diminish as NP become smaller. In addition, we study the structural effects of NP size on the bound layer.   

Influence of a Crystalline Substrate on the Diffusion Coefficient of an Adsorbed Polymer

We address polymer diffusion on a crystalline substrate to assess its relevance. As such, Monte Carlo simulations were utilized to study two novel models. In one of the models, Model $A$, the equilibrium distance between monomers and substrate particles remains constant as we vary the size of the monomers. In the other model, Model $B$, this distance scales with the monomer-monomer equilibrium distance. In Model $A$, the monomer-monomer and monomer-substrate interactions have different ranges, which makes it anisotropic, while in Model $B$ these ranges are the same, which makes it isotropic. Results for Model $A$ show novel significant features of the diffusion coefficient on the ratio, $\gamma$, monomer size to lattice spacing. When this ratio is a multiple integer of some lattice vectors the diffusion constant diminishes dramatically suggesting that the monomers settle. In Model $B$, monomers do settle for $\gamma= \sqrt 5$, but otherwise the behavior of the diffusion coefficient remains monotonic. Additionally, this model shows that larger $\gamma$ values give similar diffusion coefficients, which might be of use to experimentalists.   

Reproducibility of Adsorbed Layer Growth Under Different Sample Preparation Conditions and Its Possible Influence on the Physical Aging of Thin Polystyrene Films

We have recently observed an unexpected molecular weight dependence to the physical aging rate of 31 nm thick polystyrene (PS) films supported on silicon not present in bulk films. Thin films made from ultra high MWs ≥ 6,500 kg/mol exhibit a 40% faster aging response at an aging temperature of 40 ^oC than equivalent films made from (merely) high MWs ≤ 3,500 kg/mol. Because the presence of adsorbed layers to substrates has been considered a potential effect on the glassy dynamics in thin polymer films by several recent studies, we began examining the possible influence of chain adsorption on our unexpected physical aging results. We have investigated various experimental factors that influence the value of the adsorbed layer thickness measured h_ads and find that the growth of these adsorbed layers is highly dependent on the sample’s thermal history, both above and below the glass transition temperature Tg. Here, we discuss the reproducibility of h_ads under different sample preparation conditions and address the precision with which such thin h_ads values can be reliably determined using ellipsometry. Our results show that the presence of small or large adsorbed layers do not alter the observed molecular weight dependence to the physical aging behavior in thin PS films.   

Direct Observation of Polymer Surface Mobility via Nanoparticle Vibrations

Understanding the mechanical properties of polymers nanoparticles (NPs) is essential to optimizing their performance and processing. The glass transition temperature (T_g) plays a crucial role in processing these and other polymeric materials. Polymer NPs show T_g deviations similar to that of polymer thin films, which has been attributed to the mobility of a surface mobile layer with reduced T_g. However, due to the lack of an experimental means, the existence of a surface mobile layer has yet to be directly verified. We studied the vibrational dynamics of polystyrene NP clusters by Brillouin light scattering. Since eigenmodes of the NP cluster are controlled by the elastic moduli and the interactions between NPs, the temperature dependent variation in the spectra allows us to not only measure T_g of NPs, but also to verify the presence of a surface mobile layer. Below T_g, the contact adhesion is enhanced due to an active surface mobile layer which introduces unique thermal variation of eigenmodes. The calculated phonon dispersion supports that this abnormal thermal transition is induced by sudden increase in adhesion area among neighboring NPs at a temperature T_s that is lower than the bulk T_g.
  

Scale-Free Interfacial Dynamic Decoupling Drives Free-Surface Effects on the Glass Transition

A quarter-century of effort has been focused on the study of glass formation in thin films, in large part motivated by the hope that the behavior of these systems will elucidate underlying length scales and mechanisms of glass formation. Here we show that these alterations in dynamics at free surfaces are driven by a scale-free local decoupling phenomenon. We find that the apparent activation barrier is truncated in a nearly temperature-invariant manner near the interface, with a power law recovery of bulk-like activation behavior in the far field. We show that this result gives rise to the temperature dependence of ‘apparent’ length scales of interfacial dynamics observed in simulation and experiment, despite the absence of any true underlying length scale in the physics of the system. This scenario is difficult to reconcile with models centering around an underlying temperature-variant thermodynamic or dynamical length scale of glass formation, potentially narrowing the range of viable theoretical explanations for this phenomenon and pointing towards an important role for elastic barriers in supercooled liquid dynamics.   

Mobility Gradients, Growing Apparent Length Scales, and Layer-Dependent Decoupling in Free Standing Films

We employ the Elastically Collective Nonlinear Langevin Equation Theory of bulk and confined supercooled liquids to study macroscopic films with a single vapor interface. An improved description of the long range elastic displacement field fluctuation required to facilitate large amplitude activated hopping events has been developed. Long range and large spatial gradients of mobility and local Tg near the interface are predicted. The effective interfacial length scale based on a return to the bulk timescale criterion grows in a nearly Arrhenius manner with cooling, and is also connected with the temperature-dependent collective elastic barrier in the bulk liquid. The scale-free nature of the elastic barrier leads to layer-dependent, effective power law decoupling of the film relaxation time relative to its bulk analog. The decoupling exponent varies as an inverse power law with distance from the surface. These novel results have a purely dynamical origin since no changes of structure or thermodynamics in confinement enter the theory. The predictions are consistent with simulations of Simmons and coworkers, and have strong implications for thinking about the question of a dynamical length scale in the bulk liquid and how it is related to its interfacial analog.