Thermodynamic Interactions in Polydiene/Polyolefin Blends
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Thermodynamic interactions in blends of polydienes and polyolefins were investigated using small angle neutron scattering (SANS). Model polydienes with controlled molecular weight, dispersity, and precise chemical structure were synthesized using anionic polymerization, and subsequently saturated with hydrogen or deuterium to form model polyolefins. The polydienes involved in this study include a series polybutadienes (PBD) with varying amounts of 1,2 and 1,4 microstructure units and 1,4-polyisoprene (1,4-PI); the corresponding polyolefins included are poly(ethyl ethylene) (PEE), polyethylene (PE), PE-co-PEE random copolymers, and poly(ethylene-alt-propylene) (PEP). Additionally, a model atactic polypropylene (aPP) prepared using a metallocene catalyst was used in this study. SANS data were interpreted using Zimm analysis and the Random Phase Approximation model to extract the Flory-Huggins interaction parameter, and chain dimensions of the pure components. The thermodynamic interactions in blends of polydienes and polyolefins were demonstrated to be repulsive, quantitatively large, and strongly temperature dependent. The chain dimensions of the components were mostly expanded compared to literature values. Interactions in blends of polydienes and polyolefins were uncorrelated to the statistical segment length disparity between the components. Predictions of the interaction parameters from pure component PVT properties were found to qualitatively describe the relative trends of the measured interaction parameters, with some exceptions, but failed to capture the magnitude of the interaction parameters. The Lattice Cluster Theory (LCT) also did not describe the absolute values of interaction parameters, proposed to originate from non-random mixing effects in the enthalpic contribution to the interaction parameters, which are not included in the theory. Blends were also examined containing random copolymers of polydienes and polyolefins, in order to modulate the large polydiene/polyolefin interaction parameters. The random copolymer theory (RCT) quantitatively predicted interaction parameters for some of the polydiene/polyolefin blends, including 1,2-PBD-co-PEE/PEE blends, 1,4-PI-co-PEP/PEP blends, and 1,2-PBD-co-PEE/1,4-PI blends, which indicates the interactions were dominated by dispersive forces. Sequence effects were also unimportant for the model polydiene-polyolefin copolymers employed in this study. We observed that RCT failed in some systems which contain complex random copolymer repeat units such as multiple olefinic repeat units in PE-co-PEE/1,4-PI blends. The existing theories were unable to explain the departure from RCT, and modified theories need to be developed.