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 g s 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 g s 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 g s observed with block copolymers.