Phase Behavior and Photoresponse of Azobenzene-Containing Polystyrene-block-poly(n-butyl methacrylate) Block Copolymers

It is well-known that polystyrene-block-poly(n-butyl methacrylate) (PS-b-PnBMA) block copolymers (BCPs) exhibit an hourglass-shaped phase diagram, resulting from the simultaneous appearance of an upper order-to-disorder transition (UODT) and a lower disorder-to-order transition (LDOT). Here, the phase behavior of three azobenzene-containing PS-b-PnBMA BCPs was investigated by small-angle neutron scattering (SANS). As to their molecular weights of Mn = 24 000, 40 000, and 47 000, the three copolymers are designated as Azo24k, Azo40k, and Azo47k, respectively. Temperature-dependent SANS data show that Azo24k and Azo40k BCPs are closer to their corresponding LDOT and UODT, respectively, while Azo47k BCP locates in the middle of temperature interval between UODT and LDOT. This indicates that after incorporating azobenzene functionalities into PnBMA blocks, the azobenzene-containing PS-b-PnBMA BCPs still exhibit both LDOT and UODT, characteristic of the "compressibility" similar to their parent PS-b-PnBMA BCPs. After exposing the films of three azobenzene-containing PS-b-PnBMA BCPs to UV light with a wavelength of 365 nm, the photoisomerization of azobenzene produces light-induced motions and thereby gives rise to conformational increase and density variation, which can result in a significant increase in entropy. As a consequence, three disordered azobenzene-containing PS-b-PnBMA BCPs are driven toward LDOT.