Nitroxide-mediated controlled radical polymerization is used to synthesize gradient and block copolymers of styrene (S) and 4-acetoxystyrene (AS), and conventional free radical polymerization is used to synthesize S/AS random copolymers. The S/AS copolymers are hydrolyzed to yield S/4-hydroxystyrene (HS) copolymers. Gel permeation chromatography and 1 H NMR of aliquots taken during polymerization yield proof of the controlled nature of the gradient copolymer structures. The glass transition temperature (T g ) responses are compared using the derivative of differential scanning calorimetry heat curves, with the temperature range over which the derivative exceeds a base level being equated to the T g breadth. A single, narrow T g is obtained in each random copolymer, consistent with a single phase of limited compositional nanoheterogeneity. Two narrow T g s are evident in each block copolymer, consistent with well-developed nanophases containing nearly pure S or nearly pure AS or HS units with a very narrow interphase yielding no indication of an intermediate T g . In contrast, T g breadths of ∼65-80 °C are observed in many S/HS gradient copolymers, consistent with ordered nanostructures in which the unit cell composition varies sinusoidally. The possibility of capitalizing on the broad T g of gradient copolymers in damping applications is discussed.

Gradient copolymers with gradients in composition along the chain lengths are synthesized by controlled radical polymerization, and their damping behaviors are compared to those of random and block copolymers. The effect of comonomer incompatibility is studied by comparing behaviors of strongly segregating styrene/4‐hydroxystyrene (S/HS) and moderately segregating styrene/n‐butyl acrylate (S/nBA) copolymers. The effect of composition gradient steepness is studied by designing “constant” or “increasing” gradients via the comonomer addition rate during semibatch polymerization. Dynamic mechanical analysis (DMA) is used to compare the temperature dependences of the storage modulus (E′), loss modulus (E″), and tan δ of the materials. A glass transition breadth (ΔTg) is defined by a temperature range over which E′ decreases from 109 Pa to 108 Pa. The gradient copolymer ΔTgs are at least four times larger than the random copolymer ΔTgs. The S/nBA gradient copolymers show strong effects of gradient steepness on ΔTg, with ΔTg being much larger for the increasing gradient than for the constant gradient. DMA data are compared to predictions by Hashimoto et al. for tapered block copolymers in the limit where the taper extends across the entire chain. The shapes of their calculated temperature dependences of E′ and E″ correspond well to the gradient copolymers with large ΔTg values, providing strong support for symmetric gradient copolymers forming nanoscale, ordered domains with sinusoidal composition profiles. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 48–58, 2008

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 (ΔT gs). 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), ΔT 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), ΔT 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)), ΔT gs 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 ΔT gs.

No abstract