CHAIN DYNAMICS IN SOLID POLYMERS AND POLYMERIZING SYSTEMS AS REVEALED BY BROADBAND DIELECTRIC SPECTROSCOPY

Abstract

A number of techniques are used to study the chain‐dynamics of solid polymers, including those of dielectric relaxation [1–4], dynamic mechanical thermal analysis (DMTA) [1, 5], multinuclear NMR relaxations [6], quasi‐elastic dynamic light scattering [7] and neutron scattering [8] (QELS & QENS) and transient fluorescence depolarization (TFD) [9]. Each technique has its own particular probe of the dynamics in a material. e.g. dielectric relaxation gives information on the angular motions of molecular chain‐dipoles (for dipole relaxation) and the translational motions of ions (for f‐dependent electrical conduction); NMR relaxations relate to the angular motions of chemical bonds; QELS relates to fluctuations in local refractive index; QENS to the time‐dependent van Hove correlation function (suitably‐defined) for proton‐containing groups; TFD to the angular motions of fluorescent groups in a chain. Due to its relevance to practical applications of materials, DMTA is pre‐eminent among the many physical techniques applied to solid polymers, but interpretations of behaviour in terms of molecular properties remain difficult since the direct link between an applied macroscopic stress and the molecular response of polymer chains in a bulk material remains an unsolved problem.Of the above techniques, Broadband Dielectric Spectroscopy (BDS) offers several advantages.(a) Materials may be studied in the frequency range 10−6 to 1010 Hz, over wide ranges of temperature and applied pressure, using commercially‐available instrumentation.(b) Since the electrical capacitance of a film is inversely proportional its thickness, free‐standing and supported films may be studied down to nm‐thicknesses, giving e.g. information on the behaviour of the dynamic Tg as sample thickness approaches molecular dimensions.(c) Theoretical interpretations of dielectric relaxation and a.c. conduction are well‐established in terms of Fourier transforms of molecular time correlation functions (TCFs) for motions of chain dipoles and ionic species. Large scale “Molecular dynamics” simulations have been used to calculate molecular TCFs, and hence the dielectric properties, of model bulk amorphous polymer materials [10, 11].