pH responsive fibers were fabricated from a set of architecturally designed copolymers of acrylonitrile and acrylic acid derivatives. The copolymers, which were prepared in a block type structure using method of regulated dozing, were found to impart a domain morphology to the spun fibers [1,2]. This morphology, having separate domains of acrylonotrile and acrylic acid, was found to significantly enhance both the diffusion of water through the fiber for faster response and physical bonding of chains for better mechanical properties compared to fibers prepared from random copolymers. In this study, the responsive domains of acrylic acid were modified by replacing acrylic acid (AA) with methacrylic acid (MA) in the above copolymer system. The effect of bulky side group ($-CH_{3}$) on pH response and mechanical properties of the pH responsive fibers were investigated. Thus, pH sensitive block copolymers, poly(acrylonitrile-co-acrylic acid:58:42) (AA40B) and poly(acrylonitrile-co-methacrylic acid:57:43) (MA30B), were synthesized using regulated dosing of more reactive monomer-acrylic acid/methacrylic acid monomers by free radical polymerization. The copolymers were converted to fibers by solution spinning and annealed at $120^{circ}C$ to get a stable morphology. The annealed fibers exhibited reversible response to changing pH and were stable to repeated cycling. Expectedly, the fibers from MA30B showed significantly higher volumetric swelling at 5010% compared to 3610% in AA40B. The results implied that the presence of bulky $-CH_{3}$ group in methacrylic acid, could open up the responsive domains of the fibers. However, the bulky group also affected the bonding in the acrylonitrile domains indicated by somewhat lower mechanical properties of the fibers.