Synthesis of New Azo Based Block Copolymers and Their Properties

Abstract

Azo based block copolymers (BCPs) with azobenzene-containing liquid crystalline (LC) block show great potential as a functional material for the control of their microphase separated morphologies such as spheres, cylinders, and lamellaes. The domain sizes can be controlled by the photoisomerization of azobenzene groups in the BCPs and also by different thermal (or solvent) treatment of the azo LC block. Herein, we report synthesis of a series of novel di- and triblock copolymers, which are composed of crystalline hard block and amorphous soft one, and their characteristics. The hard and soft blocks in the BCPs are derived from the RAFT polymerization of either new crystalline monomers such as 2-[2-(4-cyano-azobenzene-4`-oxy)ethylene-oxy]ethyl methacrylate (CAEMA) and 4-dodecylphenyl-N-acrylamide (DOPAM), or conventional amorphous monomers such as methyl methacrylate (MMA), n-butyl methacrylate (BMA), and low molecular weight poly(ethylene glycol) (PEG), respectively. The combination of the hard and the soft blocks generates various hard-hard (PCAEMA-b-PDOPAM), hard-soft (PDOPAM-b-PEO), and hard-soft-hard (PCAEMA-b-PBMA-b-PDOPAM, PCAEMA-b-PEO-b-PDOPAM, or PCAEMA-b-PMMA-b-PDOPAM) BCPs. Composition ratio and molecular weight of each block in the BCPs are controlled by changing the molar ratio of second monomers (20 to 150 mol%) relative to macro-chain transfer (MCTA) that is prepared by the RAFT polymerization of first monomer. Kinetic studies on the BCP systems verifies the controlled living manner of RAFT polymerization. The chemical structure, phase transition temperature, and composition ratio of the prepared BCPs are determined by 1H NMR spectroscopy, DSC, and GPC, respectively. The resulting azo BCPs with different azo block contents of 23 to 81wt% have low polydispersity, Mw/Mn < 1.55, and they show LC behaviors (enantiotropism) on both heating and cooling cycles. Especially, the BCPs with high azo contents display strong and sharp endothermic peaks of LCs as compared to those with low contents. They exhibit fan-shaped or batonnet-like textures in the range of 116 oC and 47 oC on a cooling cycle, confirming a typical smectic phase. The LC textures are observed by using an optical polarizing microscope (OPM). The synthesized BCP thin films are annealed under the vapor of mixed solvent (THF/cyclohexane or THF/toluene) to develop different microphase separated morphologies such as sphere, lamellae, or hexagonally arranged sphere nanostructure. The nonostructures formed onto the thin films are dependent on the weight fraction ratio of different block segments (PCAEMA, PDOPAM, PEO, PBMA or PMMA). It is found that the azo microdomains in the PCAEMA-b-PDOPAM thin films are changed from lamellae nanostructure into hexagonally arranged sphere when the PDOPAM content increases over 50 wt%. On the other hand, the PCAEMA-b-PBMA-b-PDOPAM and the PCAEMA-b-PMMA-b-PDOPAM BCPs form morphological features of lamellae and hexagonally packed sphere nanostructures perpendicular on a silicon substrate. The morphology of the block copolymer thin films also depends on the nature of the mixture of solvents, their volume fractions, and solvent annealing time. On the basis of the above result, our newly designed azo based block copolymers with excellent properties may pave the way for new nanotechnology applications such as nanotemplating, nanolithography, and nanoporous membranes.