Three phosphate ester diluents were added to 50:50 blends of polystyrene (PS) with poly-(2,6-dimethyl-1,4-phenylene oxide), also known trivially as poly(xylylene ether) (PXE). One of the three diluents was also added to BPA polycarbonate (BPA-PC). The diluents were trioctyl phosphate (TOP), triphenyl phosphate (TPP), and tetraxylyl hydroquinone diphosphate (TXHQDP). The dynamics of these single-phase systems vs temperature were monitored by dynamic mechanical relaxation and phosphorus-31 NMR. Dynamic mechanical characterization revealed the rise of a new low-temperature relaxation when TOP or TPP was added to the PXE-PS blend. The addition of TOP to BPA-PC partially suppressed the well-known polycarbonate low-temperature relaxation. The P-31 chemical shift anisotropy line shape was used to detect the rotational mobility of the phosphate esters. In all four blend systems studied, a substantial fraction of the diluent was found to undergo rapid overall Brownian rotation at temperatures well below the DSC glass transition temperature. This behavior was exhibited in the spectra as a narrow line on top of a broader line. The fraction of mobile diluent associated with the narrow component of the line was estimated from a lattice model in which the portion of diluent molecules in contact with other diluent molecules in microclusters was calculated. A random distribution of repeat units and diluent molecules among the lattice sites was assumed with no phase separation. The P-31 line shapes were simulated as a function of temperature by assuming the presence of two populations of diluent molecules with different rotational rates. Apparent activation energies for the rates increased from TOP to TPP to TXHQDP, reflecting increasing rigidity of the diluent. The rate of rotation of TOP in polycarbonate is about 3 times slower than in the PXE-PS blend. This difference in rate is attributed to a stronger interaction between the diluent and the more polar polycarbonate matrix. In all the polymer-diluent systems studied, the more mobile fraction of diluent molecules has achieved a degree of rotational mobility characteristic of a liquid at temperatures below the DSC glass transition temperature. In some sense such systems could be regarded as having two glass transitions: one for the mobile diluent and one for the less mobile diluent and polymer repeat units.
