Self-assembled hierarchical structure evolution of azobenzene-containing linear-dendritic liquid crystalline block copolymers

In this paper, the phase behaviors and self-assembled structures of a series of liquid crystalline linear-dendritic block copolymers (LDBCs) in the bulk state have been systematically investigated in combination with DSC, POM, SAXS, WAXS, and TEM. The LDBCs consist of a linear poly(ethylene glycol) (PEG) block (degree of polymerization, DP = 49) and dendritic polyamidoamine (PAMAM) segments of generation G0 to G3, functionalized with photoactive azobenzene mesogenic units bearing an octyloxy tail and a flexible ten-methylene spacer (AZO). An unprecedented hierarchical structure evolution with increasing dendron generation has been demonstrated for this series of azobenzene-containing LDBCs in bulk. Well-defined lamellar structures are formed for liquid crystalline LDBCs of G0, G1, and G2 with only a slight change in layer spacing between 12.2 and 13.0 nm. A simple lamellar structure is generated by mPEG-G0-(AZO)(2), while tetragonal-within-lamellar and lamellar-within-lamellar hierarchical structures are created in mPEG-G1-(AZO)(4) and mPEG-G2-(AZO)(8), respectively, due to further suborganization between PAMAM and AZO segments within the dendritic blocks. For the highest generation copolymer investigated in this work, mPEG-G3-(AZO)(16), the structure changes dramatically from lamellar for lower generations into rectangular columnar with lattice parameters a = 8.5 nm and b = 6.5 nm or higher temperature disordered columnar mesophases. The introduction of the alkyloxy tail in the azobenzene mesogenic unit is of crucial importance for constructing such hierarchical structures. Moreover, those complex lamellar mesophases transform into micellar or network cubic structures upon cooling to ambient solidification temperature with PEG crystallization showing a complete inverse phase change tendency in virtue of the curvature effect as a result of the specific linear-dendritic architecture. Very interestingly, the films of azobenzene-containing liquid crystalline LDBCs exhibit generation/morphology dependent photophysical characteristics, which may afford a convenient way for designing and fine-tuning novel optical storage materials.