The effect of main chain stiffness and side chain flexibility on the elastic modulus and glass transition temperature (Tg) of thin polymer films is investigated using non-traditional polymers formed from 5-(2-phenylethylnorbornene). Depending on the polymerization route chosen, the resulting polymer backbone is comprised of either bicyclic (norbornyl) units, which leads to a relatively rigid polymer with a high bulk Tg, or monocyclic (cyclopentyl) units, which leads to a more flexible structure with a lower bulk Tg. The modulus and Tg of the rigid bicyclic polymer is thickness independent down to <10 nm, whereas the modulus and Tg of the more flexible monocyclic polymer both decrease with decreasing thickness. By hydrogenation of the pendant phenyl ring to the cyclohexyl counterpart, we illustrate that the relative flexibility of the side chain does not significantly impact the observed thin film behavior of either polymer. To further support our hypothesis that main chain stiffness plays a central role in confinement-induced changes in physical properties, a series of arylate/phosphonate copolymers, where the addition of phosphonate groups decreases the overall stiffness of the polymer backbone, is also examined. Similar to the polynorbornenes, the poly(arylate-co-phosphonate) polymers exhibit more pronounced thickness-dependent properties as the stiffness of the backbone decreases. Our results represent a paradigm shift for the thin film glass community as thickness dependent properties for polymer films appear directly related to chain flexibility, which has not been explored extensively.
Pub Type: Journals
mechanical properties, modulus, thin films, confinement, wrinkling, glass transition, polymers