Changes in molecular dynamics during the bulk polymerisation of an epoxide/boroxine mixture as studied by dielectric relaxation spectroscopy, revealing direct evidence for a floor temperature for reaction

Real-time dielectric relaxation spectroscopy (DRS), has been used to monitor changes in the dynamics of chain dipoles and of ions during the bulk polymerisation of an epoxide/boroxine system at different reaction temperatures T-R in the range 60-120 degreesC. For T-R < 72<degrees>C the dielectric alpha -process moved to ultra-low frequencies indicating the formation of a chemically unstable glass. For T-R > 72 degreesC the process moved from high frequencies to settle at audio or higher frequencies, giving a loss peak independent of time that indicated the formation of a chemically stable elastomer. We analysed our results in terms of changes in molecular mobility in the reaction mixture with time and have determined the floor-temperature TF below which a glass is formed and above which an elastomer is formed. The DRS data were also presented as complex impedance Z(omega) which emphasises changes of ionic conductance during reaction. Below T-F the peak in the loss impedance Z(m)" moved to ultra-low frequencies during reaction in a time-scale far shorter than that for the dielectric loss peak epsilon (m)". Comparison of the two representations of data made well above T-F show that the Z " data become independent of time before the epsilon " data. DRS data presented as dielectric permittivity epsilon(omega) provides a good indicator of glass-formation below TF and of elastomer-formation above TF Our data are considered in relation to the TTT-diagram of Gillham and coworkers that describes the onset of gelation, glass-formation and elastomer-formation during cure and we extend this diagram to describe how T-g evolves with time for different values of T-R (C) 2001 Elsevier Science Ltd. All rights reserved.