Directional crystallization of 20 nm width polymer nanorods by the inducement of heterogeneous nuclei at their tips

Abstract

Crystallization of polymers under confinement has emerged as a new tool to fall into some of the classical problems of the crystallization of polymers. Furthermore, these investigations are essential to obtain nanostructures with a customized behavior, as the most important properties of such nanostructure (mechanical, optical, electrical, etc.) strongly depend on the crystalline features. Here we show a new crystallization strategy for achieving a completely oriented crystallization of ultrasmall polymer rods (20 nm in diameter) by inducing crystal nucleation centers exclusively at one of their tips. Such heterogeneous nucleation takes place at a temperature tens of degrees higher than the temperature at which the crystal nuclei would be spontaneously formed (via a homogeneous mechanism), so the formation of additional nuclei along the nanorods can be ruled out. Therefore, the confined polymer crystallizes unidirectionally along the nanorod axis just by the growth of already formed crystals. This strategy opens up the possibility of preparing polymer nanorods presenting considerably improved crystalline features. Thus, our nanorods show single-crystal orientation and high crystallinity. Interestingly, the new strategy also allows studying for the first time the kinetics of the unidirectional crystal growth under confinement. Thus, we show that the crystallization of the polymer nanorods shows the time dependency of a diffusion process, i.e., t0.5. Moreover, the linear crystallization rate value of the confined flat on lamellae is of 2.7 × 10-7 cm/s. Since the polymer nanorods crystallize exclusively through of the growth of crystals heterogeneously nucleated at one of their tips, and just a single flat on lamella can be accommodated in a 20 nm in diameter pores due to space reasons, what we present in this paper may be the study of the crystallization of a single lamella within each pore, which would mean that single-crystalline nanorods are obtained from the melt. © 2013 American Chemical Society.