Characteristics of lamellar mesophases in strongly segregated broad dispersity ABA triblock copolymers

We report the synthesis and characterization of a series of 13 strongly segregated poly(lactide-b-1,4-butadiene-b-lactide) (LBL) triblock copolymers, in which a broad dispersity center B segment (D = M_w/M_n ∼ 1.7-1.9) is embedded between two narrow dispersity L end blocks (D ≤ 1.20). Derived from chain transfer ring-opening metathesis polymerization (ROMP-CT) of 1,5,9-cyclododecatriene in the presence of 1,4-diacetoxy-2-butene, α,ω-dihydroxytelechelic poly(1,4-butadienes) serve as ring-opening transesterification polymerization (ROTEP) macroinitiators for the parallel synthesis of LBL triblock copolymers with M_n = 12.4-28.7 kg/mol and volume fractions f_B = 0.44-0.79. By determining the Flory-Huggins interaction parameter χ_LB = 0.192 at 155 °C from mean-field theory analyses of synchrotron X-ray scattering profiles for a narrow dispersity LB diblock copolymer, we estimate that the segregation strengths associated with the broad dispersity LBL copolymers range χ_LBN = 35.1-83.6. As compared to their narrow dispersity analogues reported herein, broad B segment dispersity shifts the composition-dependent lamellar phase window in LBL triblocks to higher values of f_B = 0.52-0.75. Contrary to previous reports of substantial dispersity-induced, lamellar domain spacing dilation in weakly segregated AB diblock and ABA triblock copolymers, strongly segregated LBL copolymers exhibit surprisingly similar domain sizes and scaling relations (d ∞ N^0.72±0.06) to their narrow dispersity analogues. This finding suggests that the magnitude of χ_AB determines the moment of the molar mass distribution that controls the observed lamellar domain spacing.