A Model for the Crystal Orientation of Polymers Confined in 1D Nanocylinders

Anodic aluminum oxide (AAO) templates with parallel aligned nanochannels provide an ideal scenario for constructing the one-dimensional (1D) nanoconfinement environment. In recent years, while many studies have been conducted on the orientation of crystalline polymers in AAO, a universal model is still absent for explaining the diverse or even contradictory observations in different polymer systems and further understanding the complicated evolution of orientation upon changing the crystallization conditions. In this work, the texture of isotactic polypropylene (iPP) in AAO template was studied by X-ray pole figure analysis, with two major modes of uniaxial orientation, b* or a*parallel to(n) over right arrow (pore axis), observed for iPP. Furthermore, the relative ratio of these two orientation modes varied with the crystallization conditions, indicating that their temperature dependence differed from each other. Specifically, the orientation degree of both b* and a*parallel to(n) over right arrow gradually decreased with raised cooling rate, and the changes in the latter were more pronounced. Moreover, < 110 >*parallel to(n) over right arrow orientation emerged with the increase of cooling rate, and the relative population of this orientation was also enhanced. Samples would be amorphous if quenched directly into liquid nitrogen. As the previous model apparently failed to explain these observations, a simple "1D lattice" model was proposed herein to numerically simulate the crystallization of polymer within 1D channel. In particular, it enabled to explore the influences of nucleation rate and crystal growth rate at a wide range of scales. According to the model established, orientation behavior of polymer in 1D nanocylinders can be divided into three zones. High nucleation rate combined with low growth rate will result in nearly isotropic structure, which corresponds to the crystallization under very large supercooling. The intermediate zone holds moderate nucleation rate and growth rate, orientation structure in which follows the rule of "direction of the fastest growth aligns with the channel axis". When the nucleation rate is very low and the growth rate is high, any (hk0) will grow freely to fill the whole channel under static conditions, which is the scenario described earlier by the "kinetic selection" model. In summary, comparison of experimental and simulation results proved that the complete model developed in this study can better explain those diverse observations recorded in literature.