269783 Mechanics of Thin Polystyrene Films: Role of Additives and Chain Architecture
For many applications, additives are included in polymer formulations to provide the desired processing characteristics or functionality. The use of additives is particularly important for the microelectronics industry where polymeric photoresists are used to transform UV exposure to physical patterns. These photoresists are highly engineered formulations with photoactive small molecules that catalyze the chemical modification of the polymer to generate solubility differences to create the features. Additionally, the casting solvent generally does not fully evaporate. The nanoscale features formed by these photoresists have commonly been utilized as practical motivation for examination of the properties of polymer thin films. Feature fidelity is predominately associated with the modulus of the polymer, so the mechanical properties of polymers as a function of film thickness provide some indication if confinement will adversely impact the feature quality.
However, very few studies have examined how additives impact the physical properties of polymer thin films. The work of Torkelson and co-workers has illustrated that small molecules that interact favorably with the polymer through relatively strong interactions (H-bonding, pi-pi overlap) can eliminate the nanoconfinement effect in the glass transition temperature. We have previously shown that one of these additives can also eliminate any thickness dependence in the elastic moduli as well. Here, we will discuss how a series of small molecules with vastly different solubility parameters impact the thin film modulus of polystyrene. Further, the polystyrene chain architecture is varied through controlled branching to demonstrate how available conformations of the polymer in solution can potentially impact how nanoconfinement impacts the mechanical properties of polystyrene.