Dielectric relaxations in amorphous polymers with complex chain architectures
By combining thermostimulated current and dynamic dielectric spectroscopies, the dielectric properties characteristic of the molecular mobility can be described over a very broad frequency range. The study of dielectric relaxations of phosphorus-containing dendrimers in the sub-glass transition region, and of poly(n-alkyl methacrylates) at and above the glass transition region allows us to gain insight into the molecular dynamics of complex architecture systems. From these sets of data we make an attempt to propose a structural interpretation of molecular mobility through the glass transition.
Dielectric study of local relaxations in dendritic macromolecules
The aim of this work is to characterize the dielectric properties of amorphous dendritic macromolecules as a function of architectural complexity. By combining thermostimulated currents and dynamic dielectric spectroscopy, the dielectric permittivity has been obtained as a function of temperature, in a very broad frequency range (10−4–106 Hz). Generations g = 0 to g = 2 were investigated. The molecular origin of the quasi-Debye mode observed for g = 0 at low temperature is common to all generations: localized reorientations of aldehyde end groups. The interactions with the phosphorus/sulfur (P = S) dipoles are probably responsible for the broadening pointed out in upper generations.
Rayleigh‐brillouin spectroscopy of syndiotactic poly(n‐butyl methacrylate) near the glass transition
We present Rayleigh-Brillouin light scattering data of highly syndiotactic poly(n-butyl methacrylate) [PBMA] whose glass transition temperature as measured by DSC is 55°C. The Brillouin peak shifts, Brillouin peak widths, and Landau-Placzek ratios from −15 to 130°C are reported. The Brillouin peak widths decrease continuously through the glass transition region. This indicates a continual decrease in the strength of processes whose relaxation times are about 10−10 s with decreasing temperature even as the system becomes glassy. The Landau-Placzek ratio above the glass transition is about 3, indicating the high optical purity of our sample. This low Landau-Placzek ratio arises from the sample's homogeneous stereochemistry. Some of the anomalous behavior observed around 40–50°C in previous PBMA studies is explained in terms of syndiotactic regions within a largely atactic sample.