Structure and photoresponsive behaviors of multiwalled carbon nanotubes grafted by polyurethanes containing azobenzene side chains

Two kinds of photoresponsive azobenzene polyurethane-functionalized multiwalled carbon nanotubes (AzoPU-MWNTs) were successfully synthesized by in situ polycondensation of azobenzene monomer containing bishydroxyl (AzoM) with two types of diisocyanates, an aliphatic diisocyanate (HDI) as soft monomer and an aromatic diisocyanate (TDI) as rigid monomer, in the presence of MWNTs with terminated multihydroxyl groups (MWNT-OH), and their photochemical behaviors were investigated. Fourier transform infrared (FTIR), Raman, and ¹H NMR spectra and transmission electron microscopy (TEM) revealed that azobenzene polyurethanes were covalently grafted onto the surfaces of MWNTs, forming core-shell structures with MWNT as hard core and polymer layer as soft shell, and the average thickness of the grafted polymers was about 7-10 nm. Thereby, evident improvements in the solubility of insoluble MWNTs and thermal stability of the azo polyurethanes were simultaneously obtained. The AzoPU-MWNTs showed reversible photoisomerism behavior. When they were compared to their parent polymers (AzoPUs), the photoisomerization rate constants of AzoPU-MWNTs decreased due to the heat sinks and steric effects of MWNTs. However, the responsive rate constant of AzoPU-MWNT could be effectively controlled by adjusting the main-chain flexibility of the grafted polyurethanes on MWNTs. This would play a key role in developing novel high-performance optic and photonic nanodevices.