Triphenylene-Based Side Chain Liquid Crystalline Block Copolymers Containing a PEG block: Controlled Synthesis, Microphase Structures Evolution and Their Interplay with Discotic Mesogenic Orders

Abstract

Triphenylene (TP) derivatives are typical and probably the most widely studied discotic liquid crystalline (DLC) materials. Through polymer analogous reactions to attach TP mesogens to the well-synthesized poly(ethylene glycol)-b-poly(2-hydroxyethyl acrylate) (PEG–PHEA) by ATRP, a series of well-defined side chain DLC diblock copolymers PEG–poly(TPm) (m = 6 or 10) with DLC block weight fraction (fw,DLC) ranging from 37% to 90% have been successfully prepared with narrow molecular weight distribution (PDI ≤ 1.11). An intriguing microphase-separated superstructure evolution and the correlation between overall morphologies and discotic mesogenic orders as a function of fw,DLC and temperature have been demonstrated by combination of DSC, POM, and variable temperature SAXS/WAXS. Those copolymers with lower DLC contents (fw,DLC = 37% and 43%) and at lower temperatures formed lamellar structures of variant periods and underwent order–order transitions upon PEG region crystallization at 45 °C and different discotic mesophases of ND or Ncol transition at about 25 °C. For the copolymer with intermediate fw,DLC = 62%, a high temperature hexagonal packed cylinder (HPC) structure of amorphous PEG nanocylinders in the matrix of DLC was formed above 35 °C, while upon cooling below 35 °C it turned into a mixed lamellar structure with PEG region crystallization. The higher fw,DLC (67% ∼ 80%) copolymers exhibited HPC structures with the DLC matrix showing Ncol or ND mesophases. For copolymers with the highest fw,DLC around 90%, an overall ND phase was developed in sharp contrast to the ordered columnar phase formed by their corresponding DLC homopolymers, which was quite inspiring and might suggest another pathway of attaining this important nematic discotic phase through introducing a suitable copolymerized block. The better understanding of the interrelation of microstructures and discotic mesogenic orders constitutes the key basis for utilizing such type of organic semiconductor materials and could help to guide the design of complex DLC polymer materials with hierarchical structures for variant applications.