Variable Crystal Orientation of Poly(ethylene oxide) Confined within the Tubular Space Templated by Anodic Aluminum Oxide Nanochannels

The use of nanoscale templates to confine the crystallization process is an effective approach for creating large-scale crystal orientation. Here we incorporated poly(ethylene oxide) (PEO) into anodic aluminum oxide (AAO) nanochannels by a solution infiltration process to generate PEO nanotubes confined in the channels. The preferred crystal orientation of the PEO crystallites developed in the tubular space was then studied as a function of crystallization temperature (T_c), PEO molecular weight (M_PEO), and AAO channel diameter (D_AAO = 23 and 89 nm). Two distinct types of crystal orientation, i.e., perpendicular and tilt orientation with the (120) plane aligning along and tilting 45° away from the channel axis, respectively, were identified. Crystallization in the nanotubes templated by the AAO with D_AAO of 23 nm led predominantly to perpendicularly oriented crystallites except for the high molecular weight PEO (M_PEO = 95 000 g/mol), where a significant fraction of the crystallites showed tilt orientation. In the nanochannels with D_AAO = 89 nm, however, perpendicular orientation only dominated at the higher T_c (≥20 °C), as most crystallites developed at the lower T_c adopted tilt orientation with the invariant tilt angle of the (120) plane of 45°. On basis of the observed effects of the three parameters, the preferred crystal orientation attained was proposed to be governed by the strength of confinement to the crystal growth prescribed by the confinement geometry and the nucleation density of crystallization.