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 intermediates mixtures containing both molecular architectures. The order-disorder 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-degrees-C higher than that for the homologous diblock specimen, which compares favorably with the 61-degrees-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.