Order-Disorder Transition: Diblock versus Triblock Copolymers

A nearly symmetric cis-1,4-polyisoprene-1,2-polybutadiene (1,4-PI-1,2-PB) diblock copolymer was anionically polymerized and partially coupled to form a mixture of diblock and 1,4-PI-1,2-PB-1,4-PI triblock copolymers. This mixture was subsequently hydrogenated and fractionated, yielding a homologous pair of poly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) diblock and PEP-PEE-PEP triblock copolymers, along with three intermediate mixtures containing both molecular architectures. The orderdisorder transition temperature, T_ODT, which was determined for each mixture by dynamic mechanical spectroscopy, was found to vary significantly with molecular architecture. For the triblock material T_ODT was 72 °C higher than that for the homologous diblock specimen, which compares favorably with the 61 °C difference anticipated by mean-field theory. Above T_ODT the dynamic mechanical properties are insensitive to variations in molecular architecture, while below the order-disorder transition subtle variations in the low-frequency viscoelastic response are apparent.