The physical properties of polycarbonate blends containing the nonlinear optical dye lead tetracumylphenoxy phthalocyanine were characterized. Blends with up to 20 wt% dye were prepared and characterized in terms of density, refractive index, glass transition temperature, loss modulus, subambient relaxation behavior, and free volume hole size from positron annihilation lifetime spectroscopy. The dye strongly affected the physical properties of the blend. The initial 0.1 wt% dye produced a dramatic increase in the density. A similar trend in the refractive index was accounted for by the change in density using a relationship between density and refractive index derived from the Lorentz-Lorenz equation. Increasing the dye content to 8 wt% led to a large reduction in the glass transition temperature. An increase in E' and a decrease in the subambient gamma-relaxation intensity accompanied the large decrease in T-g. This behavior fit the conventional concept of antiplasticization, which has been described for other low molecular weight diluents in PC. Recognizing that the dye was present as a mixture of monomer, dimer and higher aggregates, it was shown that the monomer form was responsible for the antiplasticization. In the glass, dimer and higher aggregates were viewed as nanoparticle fillers. It was confirmed that the antiplasticization effect was due to a reduction in excess hole free volume of the polymer. (c) 2006 Elsevier Ltd. All rights reserved.