Molecular Dynamics Investigations of Polystyrene-Based Binary Thin Film Systems: Interfacial Properties and Mechanical Behavior

Implementation and development of novel nanoscale biomedical devices and procedures depends critically upon the ability to manufacture affordable nanoscale polymer structures. This, in turn, depends upon understanding of the properties of the polymer materials at these scales, where material properties are often divergent from those observed at larger scales. In this thesis, a molecular dynamics (MD) based approach is developed to analyze these properties at an atomic resolution. Under the hypothesis that the emergence of new properties in nanoscale polymers is due to the increased importance of interface effects, the primary focus of this work is the differentiation of material behavior at various interfaces.

The first study presented here analyzes a polystyrene (PS) - carbon dioxide (CO₂) interface. The main result of this study is a quantification of the impact of CO₂ on the glass transition behavior of PS. It is shown that introduction of CO₂ depresses the glass transition temperature T_g significantly and that this depression increases with increasing CO₂ pressure. For the highest CO₂ pressure studied (7 MPa), observed T_g for the polymer is more than 50 K below the value for bulk PS. In the study of this binary system, a number of techniques are developed for the measurement and analysis of properties like free volume and mobility. These techniques are applied to quantify the dependence of various important properties of PS on spatial location, temperature, and CO₂ pressure.

The results of the first study suggest that the bonding of PS should be facilitated by the introduction of CO₂, possibly enabling bonding at near room temperature. The second study presented here examines this possibility, using a MD model of PS thin films to study the impact of CO₂ on the structure of a symmetric PS-PS interface during bonding at 300 K. The properties of the interface are used to analyze the results of simulations which show that the strongest interface is produced for a CO₂ pressure of ~2 MPa. It is determined that this strength is largely determined by the development of chain segments that bridge the interface. The number of bridges that develop is shown to be dependent on atomic mobility near the interface, which is a maximum for PS in the presence of CO₂ at ~2 MPa.

The final study presented in this work examines free surface and silica substrate interface effects on the glass transition behavior of PS thin films. The change in T_g for freestanding and silica-supported PS thin films is estimated using MD. It is shown that a freestanding PS thin film roughly 20 nm in thickness exhibits a depression in T_g of 33 K below the bulk value. It is also shown that a silica-supported PS thin film exhibits a depressed T_g, but that the competition between polymer-substrate and polymer free surface effects limits this depression to 19 K for a film of identical composition. It is concluded that the deviation in the thin film T_g values is driven by changes in atomic mobility due to interface and free surface effects.