The block copolymers of poly(acrylic acid)-b-poly(vinyl alcohol) (PAA-b-PVA) were obtained from the hydrolysis of poly(methyl acrylate)-b-poly(vinyl acetate) (PMA-b-PVAc), which was synthesized by cobalt -mediated radical polymerization (CMRP) using the cobalt(II) porphyrin complex (Co-II(TMP)) as the mediator. The mechanical properties of the PAA-b-PVA free-standing films could be tuned by the pH of the aqueous solution used to cast the films. The block copolymer films showed a much higher tensile strain and fractural tensile strength than the films prepared from the blends of PAA and PVA homopolymers. FTIR and morphological characterizations suggested that the tensile properties of the films were governed by both the hydrogen bonding between PVA and PAA that led to interpolymer complexation and the phase-separated morphology. For a given type of material, the greater extent of interpolymer complexation attained at lower solution pH led to the film with better tensile properties. The difference in. the length scale of phase separation was responsible for the large difference in the tensileproperties between block copolymer and blend films, where the characteristic nanostructure formed in the block copolymer prescribed a considerably larger amount of interface which enhanced the tensile properties significantly.