Abstract
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∥) and perpendicular (c⊥) 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.
Highlights
Amorphous polymer films are generally highly transparent, due to their homogeneous refractive index
The aim of this paper is to study the dependence of film transparency on the kinds of planar orientation of NC phases of syndiotactic polystyrene (sPS) and PPO
The kind and degree of the planar orientation of crystalline phases of polymer films can be determined by sending the X-ray beam parallel to the film plane and by collecting 2D-WAXD patterns with the so-called EDGE geometry.28−34 2D-WAXD EDGE patterns of NC films of sPS
Summary
Amorphous polymer films are generally highly transparent, due to their homogeneous refractive index. Semicrystalline polymer films, instead, exhibit a reduced transparency due to the different refractive index of their amorphous and crystalline phases. The density and refractive index of their crystalline phases are higher than for the corresponding amorphous phases (ncr > nam). In a few specific cases, e.g., isotactic poly(4-methyl-pentene-1),[6] the density and refractive index of crystalline and amorphous phases are similar, and the film transparency is poorly affected by the degree of crystallinity. Syndiotactic polystyrene (sPS)[7−9] and poly(2,6-dimethyl-1,4-phenylene)oxide (PPO),[10,11] exhibit nanoporous-crystalline (NC) phases, with an ordered distribution of empty space, the density and refractive index of which are definitely lower than for the corresponding amorphous phases (nNC < nam). Suitable procedures to increase the transparency of semicrystalline films generally imply a reduction of the crystal size to values smaller than the visible light wavelength, usually by adding nucleating agents.[1−5] The transparency of semicrystalline polymer films can be improved by procedures (generally quenching) leading to mesomorphic polymer phases, like for isotactic polypropylene.[12,13] Another well-established method to obtain highly transparent films involves polymer crystallization under mechanical strain, being generally biaxial.[14−18] strain-induced crystallization in polymer films leads to formation of crystallites of nanometric size[19−23] as well as to a high degree of orientation (mainly of their crystalline phases), generating high optical anisotropy.[24−26]
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