Viscoelastic properties and phase behavior of 12-tert-butyl ester dendrimer/poly(methyl methacrylate) blends

This study used refractometry, ultraviolet-visible spectroscopy, Fourier tr ansform infrared spectroscopy, differential scanning calorimetry, and diele ctric analysis to assess the viscoelastic properties and phase behavior of blends containing 0-20% (w/w) 12-tert-butyl ester dendrimer in poly(methyl methacrylate) (PMMA). Dendritic blends were miscible up through 12%, exhibi ting an intermediate glass-transition temperature (T-g; alpha) between thos e of the two pure components. Interactions of PMMA C=O groups and dendrimer N-H groups contributed to miscibility. T-g decreased with increasing dendr imer content before phase separation. The dendrimer exhibited phase separat ion at 15%, as revealed by Rayleigh scattering in ultraviolet-visible spect ra and the emergence of a second T-g in dielectric studies. Before phase se paration, clear, secondary p relaxations for PMMA were observed at low freq uencies via dielectric analysis. Apparent activation energies were obtained through Arrhenius characterization. A merged ap process for PMMA occurred at higher frequencies and temperatures in the blends. Dielectric data for t he phase-separated dendrimer relaxation (cu,) in the 20% blend conformed to Williams-Landel-Ferry behavior, which allowed the calculation of the appar ent activation energy. The a, relaxation data, analyzed both before and aft er treatment with the electric modulus, compared well with neat dendrimer d ata, which confirmed that this relaxation was due to an isolated dendrimer phase. (C) 2001 John Wiley & Sons, Inc.