Simulating Local T_g Reporting Layers in Glassy Thin Films

Abstract

One of the most notable deviations from bulk fluid properties is the onset of a thickness-dependent glass transition temperature (T g ) for nanometrically thin polymer films. Experimental and theoretical observations suggest that this behavior is a response to the interfaces, which perturb the local properties of a film and play an increasingly important role in influencing the global properties of a film as its thickness decreases. In this work, we probe the global and local properties of free-standing films using our limited mobility (LM) model, which is a simple kinetic lattice model that simulates free volume and mobility in a fluid. We provide insight about the role of mobility in affecting the thicknessdependent film-average T g of free-standing polymer films by characterizing the depth to which mobility propagates from a free surface, i.e., the "mobile layer depth". We also consider the effect of "stacking" free-standing polymer films, where confinement by interfaces composed of the same material yields T g suppression intermediate to that of substrate supported and free-standing films. In order to characterize the local properties of a film, we utilize "reporting layers" located near the free surface and film interior, from which we compute local glass transition temperatures and make connections with experimental results reported for real polymer films.

Local Glass Transition Temperature Tg(z) Within Polystyrene Is Strongly Impacted by the Modulus of the Neighboring PDMS Domain

“…A localized fluorescence method is used to measure the local T g ( z ) of a thin pyrene-labeled PS probe layer placed at a distance z from the PS/PDMS interface. These findings demonstrate that the breadth of the compositional interface between the two dissimilar polymers and the modulus of the neighboring domain are key factors controlling the T g ( z ) behavior, providing insight for related theoretical efforts in the field ,,,,− into the control parameters responsible for this phenomenon. Characterization of the local properties near the interface of PS/PDMS, in particular, are relevant for a range of applications from mechanical reinforcement of polymers , to the buckling-based metrology used to measure the modulus of ultrathin glassy films. − …”

Role of “Hard” and “Soft” Confinement on Polymer Dynamics at the Nanoscale

“…In the bulk, the difference between the measurement temperature T and glass transition temperature T g , ( T – T g ), is generally a good predictor of relative segmental relaxation rates, provided that the Vogel–Fulcher–Tamann (VFT), or equivalent Williams–Landel–Ferry (WLF), relationship holds and associated physical constants of the polymer are known. With regard to thin films, the situation can be more complex, due largely to effects associated with the close proximity of the external interfaces. , Typically, a decrease of the T g occurs with decreasing film thickness h for sufficiently thin ( h ∼ tens of nanometers) freely standing films. , Near a free surface, simulations − reveal the existence of a mobile surface layer with thicknesses on the order of nanometers and significantly faster dynamics than the bulk for linear chain polymers. For asymmetrically confined films (hard substrate/film/air), the T g increases with decreasing h when the interactions between the polymer segments and the substrate are strong, e.g., hydrogen bonding; , interactions of chain segments in contact with a “wall” reduce the relaxation rates of the chains because of the associated increase of activation barriers for torsional relaxations.…”

“…The T g depression is corroborated by simulations performed by Simmons and co-workers. , Other experiments − reveal an enhancement of chain dynamics with respect to the bulk in the presence of a fluid-like layer at the interface. Simulations by Tito et al and DeFelice et al predict that within the interfacial region of a polymer film in contact with a material composed of high-mobility molecules, the segmental mobility in the polymer film is enhanced. This is consistent with the behavior we observedenhanced mobility of a film in contact with a more mobile confining polymer.…”

Nanoscale depth-resolved polymer dynamics probed by the implantation of low energy muons

“…For PB a length scale for the mobile layer depth that increases with M w is very clearly seen and the details are being reported elsewhere [15]. Simulating the generic surface behaviour of glasses with a coarse grained model has had some recent success in describing the dependence of T g in polymer films of varying thickness [28], however describing the M w dependence that is unique to polymer glasses require another modelling approach [29] that is based on the model of de Gennes [5] for transmission of mobility from the surface into the bulk of the film via conformational fluctuations propagating along the polymer chains [15].…”