Phase behavior and structure of ionomer polymer blends

Date of Completion

January 2000


Engineering, Chemical|Engineering, Materials Science|Plastics Technology




Miscibility of two high molecular weight polymers usually requires the presence of exothermic, intermolecular interactions, since the combinatorial entropy of mixing is small for polymer mixtures. Specific, attractive intermolecular interactions such as hydrogen bonding, transition-metal complexation, dipole-dipole, ion-dipole, and acid-base interactions are possible with blends involving ionomers and another polymer containing a complementary functional group. One such blend system is lightly sulfonated polystyrene (SPS) mixed with poly(N,N-dimethylethylene sebacamide) (mPA). These blends exhibit lower critical solution temperature behavior (LCST). The phase separation process of this blend is quite unusual in that conventional spinodal decomposition kinetics were not followed, and the phase separation stalled after a period of time due to structure pinning. ^ In order to understand and elucidate the structure pinning mechanism in blends of SPS and mPA, small-angle neutron scattering and x-ray scattering were used to investigate these blends. Using neutron scattering, a scattering maximum was observed for all blends below the phase boundary, except for blends involving the highest sulfonated ionomer. This scattering maximum is indicative of a microdomain structure or inhomogeneity. It was found that the position of the maximum scaled inversely with the average end-to-end distance between sulfonate groups on the polystyrene chain. X-ray scattering revealed that during phase separation of SPS/mPA blends, ionic aggregate structures form, confirming a possible mechanism responsible for the structure pinning phenomenon. ^ Neutron scattering data was analyzed by several models that predict the scattering function from polymer blends. Significant, thermodynamic parameters such as the interaction parameter χ was calculated from these models. Qualitative agreement was found with the scattering data and de Gennes' model for a crosslinked polymer blend, and a modified scattering function based on de Gennes' theory is presented. ^ Miscibility between polymers may also be achieved through the “copolymer effect”, where the intra-repulsive forces along a polymer chain promotes mixing. One such blend system involves polystyrene ionomers and Bisphenol-A polycarbonate (PC), an upper critical solution temperature (UCST) blend. In this investigation involving SPS/PC blends, wide-angle neutron scattering was used to study the short-range order present in PC, and how this short-range order is affected during blending. ^