Breadth of glass transition temperature in styrene/ acrylic acid block, random, and gradient copolymers: Unusual sequence distribution effects

Block, random, and gradient copolymers of styrene (S) and acrylic acid (AA) are synthesized by conventional or controlled radical polymerization, and their glass transition temperature (T_g) behaviors are compared. The location and breadth of the T_gS are determined using derivatives of differential scanning calorimetry heating curves. Each S/AA random copolymer exhibits one narrow T_g, consistent with a single phase of limited compositional nanoheterogeneity. Block copolymers exhibit two narrow T _gs originating from nanophase separation into ordered domains with nearly pure S or nearly pure AA repeat units. Each gradient copolymer exhibits a T_g response with a ∼50-56 °C breadth that extends beyond the upper T_g of the block copolymers. For copolymers of similar composition, the maximum value in the gradient copolymer T_g response is consistent with that of a random copolymer, which has an enhanced T _g relative to poly( acrylic acid) due to more effective hydrogen bonding when AA units are separated along the chain backbone by S units. These results indicate that gradient copolymers with ordered nanostructures can be rationally designed, which exhibit broad glass transitions that extend to higher temperature than the T_gs observed with block copolymers.