| The phase behavior expressed between the symmetric AB and ABC copolymer states was investigated for triblock copolymer systems differing only in the linear sequence of the same three constituent blocks: polystyrene (PS), polyisoprene (PI), and polyethylene oxide (PEO). Ten poly(styrene-b-isoprene-b-ethylene oxide) (PS-PI-PEO) and thirteen poly(isoprene-b-styrene-b-ethylene oxide) (PI-PS-PEO) triblock copolymers distributed along the same compositional isopleth (fA = fB) were characterized using a combination of small angle x-ray scattering, transmission electron microscopy, rheology, and static birefringence measurements. Differences in phase behavior exhibited by these two series of molecules are explained in terms of contrasting thermodynamic relationships imposed by the specific connectivity of each system. Only a single morphology, albeit a previously unreported non-cubic network phase, was found to span the two- and three-domain lamellar regions bounding the symmetric states in the PI-PS-PEO system. Both the simplicity of behavior and the formation of a new morphology are discussed in terms of the primary interaction parameter symmetry (χAB ∼ χ BC) and the lack of frustration (χAC > χAB and χBC in these molecules. In direct contrast, the frustration condition (χAC < χBC and/or χAB ) and primary interaction parameter asymmetry (χBC > χ AB) inherent to the PS-PI-PEO molecules was found to generate behavior of comparatively greater complexity. In this case, four distinct morphologies were identified between the two lamellar states, including core-shell analogs of the cylinder and gyroid phases, and two tentatively assigned lamellar-like structures both possessing regions of A/C interfacial contact. |
