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 (Delta 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. fit moderately segregating systems (styrene (S)/n-butyl acrylate and S/tert-butyl acrylate), Delta T-g may be tuned dramatically via gradient structure and molecular weight; e.g., a T-g breadth exceeding 100 degrees 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), Delta 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)), Delta 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-senstitive scattering intensities consistent with the level of segregation observed through their Delta T(g)s.