Glass Transition Breadths and Composition Profiles of Weakly, Moderately, and Strongly Segregating Gradient Copolymers: Experimental Results and Calculations from Self-Consistent Mean-Field Theory

Gradient copolymers are prepared from comonomer systems with a range of segregation strengths and homopolymer glass transition temperature ( T g ) differences to explore the breadths that can be achieved by their single, continuous glass transition regions compared to random and block copolymers. A variety of chain architectures are synthesized using semibatch nitroxide-mediated controlled radical polymerization, including linear gradients, sigmoidal gradients, blocky gradients, and blocky random cases. The derivative of the differential scanning calorimetry heat curve is used to extract T g breadths (Δ T g s). For the first time, these T g breadths are compared against values derived from nanophase separation levels predicted by self-consistent mean-field theory and found to be in good accord. In moderately segregating systems (styrene (S)/ n -butyl acrylate and S/ tert -butyl acrylate), Δ T g may be tuned dramatically via gradient structure and molecular weight; e.g., a T g breadth exceeding 100 °C, or >65% of the homopolymer T g difference, is obtained with a sigmoidal gradient copolymer of S/ n -butyl acrylate. In the very weakly segregating system (S/ n -butyl methacrylate), Δ T g remains narrow (<40% of the homopolymer T g difference), regardless of gradient design. In strongly segregating systems (S/4-vinylpyridine and S/4-acetoxystyrene (AS)), Δ T g s are observed spanning 70−80% of the homopolymer T g difference. Small-angle X-ray scattering applied to S/AS materials demonstrates a range of temperature-sensitive scattering intensities consistent with the level of segregation observed through their Δ T g s.