Molecular dynamics simulations of the effects of carbon dioxide on the interfacial bonding of polystyrene thin films

Fabrication of nanoscale polymer‐based devices, especially in biomedical applications, is a challenging process due to requirements of precise dimensions. Methods that involve elevated temperature or chemical adhesives are not practicable due to the fragility of the device components and associated deformation. To effectively fabricate devices for lab‐on‐a‐chip or drug delivery applications, a process is required that permits bonding at low temperatures. The use of carbon dioxide (CO₂) to assist the bonding process shows promise in reaching this goal. It is now well established that CO₂ can be used to depress the glass transition temperature (T_g) of a polymer, allowing bonding to occur at lower temperatures. Furthermore, it has been shown that CO₂ can preferentially soften a polymer surface, which should allow for effective bonding at temperatures even below the bulk T_g. However, the impact of this effect on bonding has not been quantified, and the optimal bonding temperature and CO₂ pressure conditions are unknown. In this study, a molecular dynamics model is used to examine the atomic scale behavior of polystyrene in an effort to develop understanding of the physical mechanisms of bonding and to quantify how the process is impacted by CO₂. The final result is the identification of a range of CO₂ pressure conditions which produce the strongest bonding between PS thin films at room temperature. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011