Planar Orientation and Transparency of Nanoporous-Crystalline Polymer Films

The dependence of polymer film transparency on different kinds of planar orientations of nanoporous-crystalline (NC) phases of syndiotactic polystyrene (sPS) and poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) is described. The two polymers exhibit opposite behavior: film transparency is improved for sPS and PPO by orientations of the chain axes of the NC phases being parallel (c(parallel to)) and perpendicular (c(perpendicular to)) to the film plane, respectively. This behavior can be rationalized by the negative and positive birefringence of sPS and PPO chains, respectively. In fact, to maximize transparency, the refractive index of the NC phase along the perpendicular to the film plane has to be increased to come closer to that of the corresponding amorphous phase. This can be pursued by controlling the orientation of the NC phases and hence of the associated refractive index ellipsoids.