Session Z42: Recent Developments on the Physics of Polymer Adsorbed Layers

Accessing low energies in glasses with large free interface

In nature, the kinetics of approach to equilibrium in non-equilibrium systems can be either accelerated or retarded depending on environmental conditions. This is the case of non-equilibrium glasses; where it was shown that, increasing the amount of free interfacial area, i.e., decreasing the length scale of nanostructuring, accelerates approach to equilibrium, the so-called "physical aging". This acceleration is evident in the efficiency of maintaining equilibrium of these glasses, resulting in significant glass transition temperature (Tg) suppression. Importantly, moderate length scales of nanostructuring, acceleration of physical aging takes place with essentially bulk-like equilibrium properties. In this contribution, I will first introduce how calorimetric techniques, including fast scanning calarimetry, can be exploited to the charcaterize the kinetics of evolution of a glass thermodynamic state. Subsequently, by characterizing the enthalpy evolution during physical aging, I will show that mild enhancement of the free interfacial area may result in massive decrease of the glass energy toward the equilibrium liquid line. Finally, the ability of nanostructured glasses to equilibrate is exploited in polystyrene films and poly-t-butyl styrene spheres. In both cases, aging way below Tg for times not exceeding some days allows attaining glasses whose entropy equals that of the corresponding crystal. This outcome is interpreted as a signature of the existence of the ideal glass. Glasses exhibiting the thermodynamic state of the ideal glass show vibrational density of states (VDOS), as measured by inelastic neutron scattering (INS), resembling that of crystals. Specifically, suppression of the boson peak, normally observed in standard glasses, is observed. These results are discussed in the framework of the link between the macroscopic thermodynamic state of glasses and their vibrational properties.   

Revealing the nature of long-range interfacial effect on polymer dynamics under confinement

Interfaces play an important role in modifying the dynamics of polymers confined to the nanoscale. Proximity to substrate interface substantially slowed polymer dynamics, and the dynamics slowdown can be propagated over tens or even hundreds of nanometers into film interior, exhibiting a long-range interfacial effect. The nature of this surprisingly long-range effect remains unsolved. The central question is how the suppressed interfacial dynamics can be transmitted over such a long distance. In our investigations, we considered the essential role of formation of an irreversible adsorption layer with sluggish molecular dynamics atop the substrate surface. We exploited our ability to tune local conformation and molecular packing of chains in the adsorbed layers, together with the thin-film dewetting experiment, to show that there could be an interphase in between the adsorbed layer and polymer matrix due to the differences in chain conformations in the distinct regions (i.e., conformational asymmetry); the chains' topological interactions (e.g., chain interpenetration, entanglement and proximity to each other) in interphase region are crucial to the propagation of suppressed interfacial dynamics. The topological interactions increase motional coupling of chains and facilitate the propagation of suppressed dynamics originating at the interface. The results highlight the ability to manipulate interfacial effects and confined polymer mobility by conformation and packing state of chain in adsorbed nanolayers, as well as the importance of topological interactions between adsorbed chains and free chains in matrix.   

Multifaceted role of bound chains in reinforcement of polymer nanocomposites - structural and dynamical analysis

The addition of nanoparticles (NPs) to polymer matrices is a powerful route to improve mechanical and other properties and design next-generation engineering materials. This presentation focuses on a nanoscale understanding of mechanical property enhancement in polymer nanocomposites (PNC). A key is ascribed to polymer chains adsorbed on the NP surface that acts as polymer bridges between neighboring NPs, leading to a network-like microstructure that reinforces the PNC.

The first topic is to probe the NP network’s structures and dynamics (10^-3 < t < 10³ s) using X-ray photon correlation spectroscopy. Silica NPs dispersed in attractive poly(2-vinyl pyridine) melts are used. We map out the structures and dynamics as a function of the probed length scale, NP loadings, temperature, and molecular weight. In addition, we employ the dynamical mode-coupling theory that incorporates the effect of bridging microstructure on the structures/dynamics.

The second topic is to reveal the structural and dynamical features of bound polymer (BP) chains that are not in bridges, but rather at the level of individual chains. A carbon black (CB) filled polybutadiene (PB) system is used. We prepare the BP chains composed of hydrogenated PB using a solvent-based approach, and the “BP coated CB fillers” are then embedded into a deuterated PB matrix, resulting in a strong contrast between a matrix and the BP for neutron scattering and spectroscopy experiments. In addition, molecular dynamics simulations are performed to complement the experimental results and to unravel details that are not accessible experimentally.

These experimental, theoretical, and computational results are then linked to the mechanical properties. The details will be discussed in the presentation.   

Exchange lifetimes of the bound polymer layer on silica nanoparticles

Decades of research have emphasized the importance of a bound layer i.e. a thin layer of immobilized polymer adsorbed on the surface of a nanoparticle, as an important factor in determining the material property improvements in polymer nanocomposites (PNCs). By utilizing the contrast difference between partially deuterated poly (2-vinylpyridine) (P2VP) adsorbed on the surface of SiO₂ nanoparticles (NPs) suspended in a hydrogenated P2VP, a reduction in apparent bound layer thickness was observed with time at elevated temperatures, with the process being highly temperature dependent. This suggests that the bound layer can desorb from the surface at elevated temperatures, with the temperature dependence arising from the different valencies of the adsorbed polymer chains. Going beyond the structural reorganization of the bound layer, recent work has also observed long term structural reorganization of the NPs in a traditional PNC as well. As with the bound layer, this process is also highly activated and is observable at length scales well beyond the size of the matrix polymer chain or the bound layer. This opens up new avenues of controlling the structure and thermomechanical properties of PNCs using elevated temperatures.   

How fundamental material properties influence interfacial effects on polymer dynamics.

It is an enduring problem to make direct connections between the characteristic material properties of a system and its behavior. We have illuminated one path that begins with a thermodynamic characterization and leads to new understanding of various material responses, including the dynamics of local segmental relaxation.  Our initial work focused on bulk samples which allowed for substantive model testing. More recently we have turned our attention to confined systems, where one or more interfaces are present in the form of a substrate (or cap), or a free surface.  This presentation will focus on recent collaborative work in which our theoretical model leads to predictions and explanations for dielectric relaxation measurements involving several polymer systems. Results to be discussed include those showing the effects of surface roughness, as well as a direct comparison of the influence from proximity to supported versus free surfaces.