Two-dimensional crystallization of cyclopolymers

Abstract

Free-standing 2D polymer materials with graphene-like crystalline layers hold great promise for many state-of-art applications. However, their fabrication remains challenging. A ferrocene tethered cyclopolymer (pFcMMA) is synthesized via the free radical polymerization of a divinyl monomer in this work. PFcMMA has a high degree of cyclization, but is not a stereo-regular structure like most vinyl polymers synthesized in free radical polymerization. PFcMMA can crystallize on the surface to form 2D crystals, which is evidenced by the birefringence phenomena, DSC and XRD analysis, and microscopic observation. Powder XRD of crystalline pFcMMA shows sharp interlayer diffraction, indicative of a highly crystalline phase. The crystallization process is supposed to follow the nonclassic crystallization pathway, i.e. the orientation of cyclopolymer main chains followed by the conformation transition of the side groups to form 2D crystalline layers. Variable temperature XRD combined with DSC analysis and polarizing optical microscopic observation demonstrate that the 2D crystalline phase begins to transition to a 3D crystalline phase at about 45 °C, the latter has a melting point at about 60 °C. The solid-solid crystalline transition is reversible and without intermediate melting, and the 2D crystalline phase is more stable at room temperature than the 3D phase. It is rationalized that nucleation from solutions is predominantly an entropy-driven process, while interactions between side groups and those between far separated main chains also benefit the formation of 2D crystalline phase. The commonalities of the crystallization processes of cyclopolymers are discussed. • A ferrocene tethered cyclopolymer forms 2D crystals on water surface. • The 2D crystalline phase undergoes a reversible solid-solid transition. • The 2D form appears to be the stable form at room temperature. • Formation of a 3D crystal at higher temperatures is observed. • The 2D crystallization process is entropy driven benefiting from group interactions.