Thermodynamic Interactions in Model Polydiene/Polyolefin Blends

Polydiene/polyolefin blends have been used in commercialized elastomer manufacturing for a while and are model system to study thermodynamics. But the previous study mostly focused on polydiene/polydiene or polyolefin/polyolefin blends. There is little quantitative information available for polydiene/polyolefin blends. Most of the polymer blends are phase separated. Upon phase separation, the properties and morphologies are inter-related. To fully understand properties of a given system, it’s essential to predict its phase behavior. The phase behavior is related to the free energy of mixing which can be computed through Flory-Huggins interaction parameter χ.χ is used to describe the interactions between polymers and is of crucial importance in determining the processing conditions for polymer mixtures and block copolymers. We have chosen the 1,2-polybutadiene (1,2-PBD) based system, which provides multiple attractive features: 1,2-PBD is an amorphous polymer and doesn’t crystalize. Therefore we have a wide temperature range over which can be characterized, and we employed anionic polymerization techniques, allowing facile control over 1,2 content and producing polymers of low polydispersity (<1.1). It is possible to achieve very high 1,2 content (>99%) and therefore we avoid the complication of a random copolymer system. The temperature dependence of χ between 1,2-PBD and saturated 1,2-PBD was investigated. The synthesized 1,2-PBD was saturated with deuterium to provide contrast for small-angle neutron scattering (SANS). SANS technique enables isotope labeling to provide contrast without changing structure drastically. Two series of blends were prepared with different molecular weight. Values of χ were extracted from fitting the Random Phase Approximation (RPA) to SANS data. Additionally, χ was extracted by Zimm analysis, using the low-angle scattering intensity. χ extracted by RPA and Zimm analysis were in good agreement. The temperature dependencies of χ characterized for two separate blends of different molecular weight were consistent with one another. The measured χ (1/T) behavior of our system were compared to literature values of other systems. At low temperatures, χ of the 1,2- PBD system approaches that observed in a blend of polyisoprene (PI) and saturated PI of high 1,4-content. χ of the 1,2-PBD system decreases more rapidly at high temperatures than that of the 1,4-PI system. At low temperatures, the χ parameters for both the 1,2-PBD and 1,4-PI systems are significantly higher than that measured for a blend of 1,4-PI and saturated 1,2-PBD. The strong temperature dependence of our system indicates its highly tunable properties. Our measured χ(1/T) behavior provided quantitative information for processing.