Photoresponsive liquid crystalline polymer single-chain nanoparticles

Abstract

A new type of multifunctional polymer single-chain nanoparticle (SCNP) is reported, which is prepared from a side-chain liquid crystalline polymer (LCP), namely, poly{6-[4-(4-methoxyphenylazo)phenoxy]hexylmethacrylate-co-4-methyl-[7-(methacryloyl)oxy-ethyl-oxy]coumarin} (PAzoMACMA). The polymer's side groups comprise photoisomerizable azobenzene in majority and photodimerizable coumarins in minority, with the former as mesogens and the latter for intra-chain photocrosslinking. Despite the sub-15 nm size, confinement and crosslinking, the liquid crystalline (LC) phases of bulk PAzoMACMA persist in SCLCPs. Such LC-SCNPs exhibit a number of interesting and peculiar properties. While their dispersion in THF is non-fluorescent, when dispersed in chloroform, the nanoparticles appear to agglomerate to a certain degree and display significant fluorescence that is different for SCNPs rich in the trans or cis isomer of azobenzene. The azobenzene LC-SCNPs also undergo photoinduced deformation, similar to azobenzene micro- or colloidal particles. However, the elongational deformation of the nanoparticles is dependent on the linearly polarized excitation wavelength. While under polarized 365 nm UV irradiation the SCNP stretching direction is perpendicular to the light polarization, under polarized 400–500 nm visible light irradiation, the stretching takes place along the light polarization direction. Finally, an all-polymer nanocomposite was prepared by dispersing the LC-SCNPs in poly(methyl methacrylate) (PMMA), and mechanically stretching-induced orientation of azobenzene mesogens developed along the chain direction. The interesting properties of LC-SCNPs unveiled in this study suggest new possibilities for applications.