Development of special techniques for bonding nanoscale polymer devices is currently a priority in the medical industry for various drug delivery applications. Methods of bonding polymer nanostructures without introducing organic solvents, adhesives or higher processing temperatures are being explored. One of the methods uses supercritical CO2 as an intermediate processing agent. In the presence of high pressure CO2, there is depression in glass transition temperature (Tg) and reduction in viscosity and inter facial tension, all of which facilitate bonding at near room-temperature environment. However, due to insufficient understanding of the dynamics taking place at the molecular level, optimized fabrication of stable nanoscale structures is still a huge challenge. Molecular scale modeling is a viable option to understand the atomic-scale physics. Bonding of polymer nanostructures requires a thorough understanding of the mechanism ranging from atomic scale interactions (~ 0.1-1 nm) to polymer dynamics at the free surface and near the substrate (~ 1-10 nm) to the mechanics at the device level (~ 10-100 nm). Modeling such phenomenon requires a multiscale approach.
A molecular dynamics (MD) based multi-scale computational design framework is developed in this work to understand the effect of these mechanisms on polymer thin film due to the size dependence, free surface, high pressure CO2 and the type of substrate used. Polystyrene (PS) is used to model the polymer. Physical properties such as density, free volume, segmental motion across the thickness and end group mobility are also studied to gain insights into the polymer dynamics. The model is used to study the Tg of finite polystyrene (PS) system in the presence of
high pressure CO2. It is shown that interactions between PS and CO2 is instrumental in governing the Tg of the PS-CO2 system. The effectiveness of the model is established by comparing with the experimental free-volume data from positronium annihilation lifetime spectroscopy (PALS). An important observation made is that polymer samples exposed to high pressure CO2 develop a distinct surface layer that is swollen (less dense) and contains polymer segments that are highly mobile. The swelling along with the enhanced chain motion at the surface facilitates strong interface bonding between polymer samples. Bonding process with different CO2 pressures was simulated in this work. The bond-strength was determined by measuring extent of chain diffusion and chain entanglement across samples and the local stress at the site of bonding when strained. All these observations are further quantified to serve as design information to develop an optimized bonding process for polymer-nanostructure fabrication.
To scale up the computational model to experimental length scales, a new rigid body based coarse-grained molecular dynamics model for polystyrene is also developed. In this new representation, a PS monomer is replaced by an effective four beads structure where the phenyl side group is replaced as one rigid body entity. An order of magnitude scale advantage in terms of system size and timescale is achieved. The results have been validated against regular explicit system representation. This model is used to solve realistic nano-systems (~40-100nm) for thin film properties.