Porous films by the self-assembly of inorganic rod-b-coil block copolymers: mechanistic insights into the vesicle-to-pore morphological evolution

Abstract

The self-assembly in thin films of polyphosphazene block copolymers [N = P(O₂C₁₂H₈)]_n-b-[N = PMePh]_m (O₂C₁₂H₈ = 2,2′-dioxy-1,1′-biphenyl; 2a: n = 50, m = 35; 2b: n = 20, m = 70, and 2c: n = 245, m = 60), having different volume fractions of the rigid [N = P(O₂C₁₂H₈)]_n block, has been studied. BCP 2a spontaneously self-assembled into well-defined round-shaped macroporous films, observing also, as a minor morphology, spherical vesicles in regions where the film was not formed. A detailed study by SEM, TEM and AFM of the structure of the vesicles, the morphology of the pores (inverted mushroom-shaped), and the behaviour of the copolymers with shorter (2b) and longer (2c) [N = P(O₂C₁₂H₈)]_n rigid blocks provided sufficient experimental evidence to propose a vesicle-to-pore morphological evolution as the most likely mechanism to explain the pore formation during the self-assembly of 2a. Moreover, by changing the volume fraction of the rigid block and the speed of solvent evaporation, it was possible to vary the pore morphology (and their diameter) from isolated regular groups to 3D interconnected pore networks.