DYNAMICS IN CONCENTRATED-SOLUTIONS OF POLY(METHYL METHACRYLATE) BIS(2-ETHYLHEXYL PHTHALATE)

Photon correlation spectroscopy (PCS) and dielectric spectroscopy (DS) are employed to study the dynamics of density fluctuations at temperatures above and below the glass transition temperature, T(g), and the dynamics of concentration fluctuations above T(g) in concentrated solutions of poly(methyl methacrylate)/bis(2-ethylhexyl phthalate) (PMMA/DOP). The PCS measurements covered the temperature range between 252 and 373 K, and the DS measurements were made between 200 and 398 K and in the frequency range between 10(-2) and 10(5) Hz. Above T(g) the polarized PCS correlation functions contain contributions from both density and concentration fluctuations for a range of temperatures. The concentration fluctuations are characterized by a relatively narrow distribution of relaxation times compared to the density fluctuations and display the usual q2-dependence. The dynamics of both processes conform to the Vogel-Fulcher-Tammann equation, and the coupling model of relaxation can explain their different temperature dependences. The alpha-relaxation bifurcates into alpha- and beta-relaxations below T(g) which can be followed respectively by PCS and DS. The additive alters significantly the dynamics of the alpha-relaxation and also has a small effect on the dynamics of the beta-relaxation which become faster. Increasing plasticizer content enhances the dielectric beta-relaxation but weakens the alpha-relaxation. At 20 % plasticizer content a single, albeit broad, dielectric relaxation exists (beta-relaxation) which is due to the concerted motion of the polymer side group with the polar additive. Similarities with the internal plasticization of PMMA are discussed.