Coherent crystal branches: the impact of tetragonal symmetry on the 2D confined polymer nanostructure.

Ziying Liang,Shuailin Zhang,Ziwei Lai,Bo Ni,Yan Cao,Nan Zheng,Hui Niu

doi:10.1107/s2052252521000774

Abstract

The symmetry of polymer crystals greatly affects the optical, thermal con-ductivity and mechanical properties of the materials. Past studies have shown that the two-dimensional (2D) confined crystallization of polymer nanorods could produce anisotropic structures. However, few researchers have focused on understanding confined nanostructures from the perspective of crystal sym-metry. In this research, we demonstrate the molecular chain self-assembly of tetragonal crystals under cylindrical confinement. We specifically selected poly(4-methyl-1-pentene) (P4MP1) with a 41 or 72 helical conformation (usually crystallizing with a tetragonal lattice) as the model polymer. We found a coherent crystal branching of the tetragonal crystal in the P4MP1 nanorods. The unusual 45°- and 135°-{200} diffractions and the meridional 220 diffraction (from 45°-tilted crystals) have shown a uniform crystal branching between the a 1-axis crystals and the 45°-tilted crystals in the rod long axis, which originates from a structural defect associated with tetragonal symmetry. Surprisingly, this chain packing defect in the tetragonal cell can be controlled to develop along the rod long axis in 2D confinement.

Highlights

The Scanning electron microscopy (SEM) images of the 70 and 300 nm rods of P4MP1 are shown in Figs. 1(b) and 1(c), respectively, while the Transmission electron microscopy (TEM) BF morphologies of the P4MP1 rods with average diameters of 70 and 300 nm are shown in Figs. 1(d) and 1(e), respectively
A detailed structure analysis based on wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and SEM experiments indicates that tetragonal crystals are formed in rods with diameters of 30, 50, 70 and 300 nm
The 45 angle between the a1-axisoriented and branched crystals indicates that a tetragonal structural defect takes place along the rod long axis, which helps the new crystals to grow epitaxially from the a1-axis-oriented crystals in 2D confinement and results in a coherent crystal branching under cylindrical confinement

Summary

Introduction

Confinement through a nanoporous template is an effective way to produce anisotropic polymer nanostructures (Steinhart et al, 2006; Higuchi et al, 2012; Jinnai et al, 2012; Zhu et al, 2001b; Dai et al, 2018; Lai et al, 2020; Yu et al, 2019; Zeng et al, 2019; Hsiao et al, 2008; Ho et al, 2005). The above symmetry studies on monoclinic or triclinic crystals under cylindrical constraints mainly illustrate the hierarchical nanostructures based on 21 and 31 helical chains assembling in confined spaces. An interesting structural feature of tetragonal or similar cell symmetry is that, as found by Lotz and Lovinger, chain packing defects occur during the growth process (Ruan et al, 2006a; Lovinger et al, 1993). For these reasons, in this study, we selected poly(4-methyl-1-pentene) (P4MP1) (most occurring tetragonal crystals) as the model polymer to demonstrate the impact of tetragonal crystal symmetry on the 2D confined polymer nanostructure. Detailed experiments on the morphologies, thermal analysis and crystal structure of the P4MP1 nanorods are presented below

Morphologies

Multiple nucleation of P4MP1 nanorods under confinement

Conclusion

Sample and materials