Dielectric analysis of the upper critical solution temperature behaviour of a poly(acrylamide-co-acrylonitrile) copolymer system in water
A copolymer consisting of acrylamide (AAm) and acrylonitrile (AN) in aqueous solution was investigated using broadband dielectric spectroscopy at frequencies between 10−1 Hz and 106 Hz in the temperature range from 2 °C to 60 °C. This system shows an UCST phase behavior. The phase transition and aggregation behavior is monitored by both the temperature and frequency dependence of the complex conductivity σ*(f, T), where the AN fraction and the concentration of the solution were varied. Additionally, the dielectric data are compared with the results obtained from dynamic light scattering measurements. The temperature dependence of the DC conductivity (σDC) of the copolymer solution is monitored and the phase transition temperature (PTT) of the poly(AAm-co-AN) copolymer is deduced from a change in the T-dependence of the DC conductivity. The change in σDC can be explained by decreased effective charge carrier mobility and a reduction of the effective charge number density at temperatures below the phase transition temperature of the poly(AAm-co-AN) solution. A pronounced interfacial polarization effect on the frequency dependence of the real part of the conductivity (σ′) is observed at temperatures below the phase transition temperature. The charge carriers are blocked at the formed aggregates giving rise to this interfacial polarization. The dependence of the interfacial polarization on the acrylonitrile fraction in the copolymer and the concentration of the solution is studied in detail and conclusions concerning the internal structures of the copolymer aggregates are drawn.