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Woo, Dong Jin
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article
Glass transition breadths and composition profiles of weakly, moderately, and strongly segregating gradient copolymers
Abstract
<p>Gradient copolymers are prepared from comonomer systems with a range of segregation strengths and homopolymer glass transition temperature (T <sub>g</sub>) 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 <sub>g</sub> breadths (ΔT <sub>g</sub>s). For the first time, these T <sub>g</sub> 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), ΔT <sub>g</sub> may be tuned dramatically via gradient structure and molecular weight; e.g., a T <sub>g</sub> breadth exceeding 100 °C, or >65% of the homopolymer T <sub>g</sub> difference, is obtained with a sigmoidal gradient copolymer of S/n-butyl acrylate. In the very weakly segregating system (S/n-butyl methacrylate), ΔT <sub>g</sub> remains narrow (<40% of the homopolymer T <sub>g</sub> difference), regardless of gradient design. In strongly segregating systems (S/4-vinylpyridine and S/4-acetoxystyrene (AS)), ΔT <sub>g</sub>s are observed spanning 70-80% of the homopolymer T <sub>g</sub> difference. Smallangle 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 ΔT <sub>g</sub>s.</p>