Effect of molecular weight on network formation in linear ABC triblock copolymers

A series of linear poly(isoprene-b-styrene-b-ethylene oxide) (ISO) triblock copolymers were synthesized to investigate the effects of molecular weight and segregation strength on block copolymer network phases. Analysis of small-angle X-ray scattering (SAXS), dynamic mechanical spectroscopy (DMS), and transmission electron microscopy (TEM) data shows that several factors influence network formation. Increasing the polymer molecular weight 50-100% above the value necessary to place the order - disorder transition temperature within an experimentally accessible range leads to disruption of long-range order at compositions associated with the orthorhombic O ⁷⁰ phase in freeze-dried and annealed specimens. Comparable molecular weight three-domain lamellae (LAM ₃) behave differently, exhibiting well-ordered morphologies that closely resemble the correlated layers found in diblock copolymers. These findings are interpreted on the basis of the reduction in molecular diffusion that accompanies increasing molecular weight and segregation strength, bridging across the poly(styrene) middle block, and differences in domain topology. These kinetic restrictions can inhibit establishment of equilibrium phase behavior in ABC triblock copolymers but do not necessarily eliminate formation of multifunctional triply periodic materials.