Abstract
The precise control of crystal orientation provides a methodology for directional manipulation of the properties of crystalline polymers. The use of nanoscale templates to spatially confine the crystallization process is an effective approach to create crystal orientation over a large length scale. This thesis presents the systematic studies of the preferred orientation of polymer crystallites developed in the confined spaces templated by block copolymer microdomains and anodic alumina oxide (AAO) nanochannels. Two critical problems were examined here, namely, the mechanism of the development of the crystal orientation from the early stage of crystallization and the variable orientational order of the crystallites developed in the 2-D confined space templated by AAO nanochannels. Using time-resolved WAXS with synchrotron radiation, we explored the time evolution of the preferred crystal orientation within one-dimensionally confined space constructed by the lamellar microdomains of two crystalline block copolymers, PE-b-PDLLA and PLLA-b-PE, where the developments of the parallel and the perpendicular orientation of PE and PLLA crystallites, respectively, were monitored from the early stage of crystallization. Both types of crystallites were randomly oriented at the early stage of formation. As crystallization proceeded further, the ensemble-average orientation progressively improved toward the preferred orientation type and the rate of establishing the orientation exhibited the same dependence on crystallization temperature (Tc) as the crystallization kinetics. Further examination of the effectiveness of enhancing the average orientation with respect to the increase of crystallinity supported the postulate that the perpendicular orientation of PLLA crystallites arises from the tendency to attain long-range crystal growth, while the parallel crystal orientation of PE is driven by the excluded volume interaction between the crystallites as a result of the intrinsically high nucleating power of PE. The orientation of PEO crystallites developed within the AAO nanochannels has been investigated. PEO was infiltrated into AAO nanochannels by solution and melt infiltration method to yield tubular and rod confinement space, respectively, for the crystallization. The effects of crystallization temperature (Tc), PEO molecular weight (MPEO) and AAO channel diameter (DAAO) on the crystal orientation attained have been systematically examined. The PEO crystal orientation was revealed from the azimuthal scan of the (120) diffraction arcs in 2-D WAXS patterns. Two modes of crystal orientation were identified here, namely, perpendicular orientation with the (120) plane aligning along the channel axis and tilt orientation with the (120) planes tilted 45o away from the channel axis. For both tube and rod geometry, decreasing Tc tended to transform the orientation from perpendicular type to tilt configuration with 45o of tilt angle, whereas increasing MPEO and DAAO suppressed the population of crystallites with perpendicular orientation. The crystal orientation was postulated to be governed by the strength of confinement to the crystal growth. Higher nucleation density at the larger degree of supercooling created a crowded environment where the growth along the channel axis was highly restricted; in this case, the crystallites tilted 45o from the channel axis to maximize the growth distance of the two (120) planes. On the other hand, the smaller tube thickness and rod diameter and larger curvature prescribed by smaller DAAO tended to force the crystallites to adopt perpendicular orientation at the early stage of crystallization at which the nucleation density was low. The window of pure perpendicular orientation was apparently narrower for the nanorods, since rod morphology created weaker confinement, and therefore increased the population of the tilt-oriented crystallites.