Dynamic orientation of azopyridine units within the shell of vesicles PNIPAM‐b‐PAzPyn copolymers

Vesicles with hydrophobic shells have been self-assembled through three kinds of amphiphilic block copolymers containing pendent azopyridine groups with different spacers, namely PNIPAM-b-PAzPyn (n=0, 2, 6), respectively. By polarization laser-trapping Raman spectroscopy, the photoinduced orientation behaviors of azopyridine groups within the vesicle shells have been investigated and it is found that spacer lengths affect the orientation of the azopyridine groups and the morphologic structure of the vesicle shells. The exact experimental results show that the orientation is dynamic for the pendent azopyridine groups with connecting spacers of 2 or 6 methylene units rather than those without spacers, so the vesicles of PNIPAM-b-PAzPy6 can be changed to show a typical soft character compared with its solid films when irradiated with a relatively weak polarized UV light of 190 mu W/cm(2). However, the vesicles of PNIPAM-b-PAzPy0 without spacers do not change even though the azopyridine units can be oriented. By quantitative Raman spectral analysis, it is found that the isomerization degree of azopyridine units is 70% for PNIPAM-b-PAzPy6 yet it is 10% for PNIPAM-b-PAzPy0, which shows a close relationship between aggregation and isomerization of azopyridine units under a weak UV light. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 415-421