Ion mobility time-of-flight measurements: isolating the mobility of charge carriers during an epoxy–amine reaction

Abstract

Ion mobility time-of-flight (ITOF) measurements can isolate the mobility of the charge carriers during a chemical reaction. The ITOF technique complements conductivity measurements. Together they more accurately monitor changes in the chemical and physical properties such as viscosity of a material as it polymerizes. The use of conductivity due to migration of charges to monitor the advancement of reaction has been widely reported. It is commonly assumed that a correlation should exist between the decrease in conductivity, and the increasing viscosity and/or extent of reaction. This is based on the assumption that the number of charge carriers in the polymer-forming system is constant. Unfortunately, this assumption is not correct in an epoxy/amine system where the number of OH groups and NH groups changes as the reaction advances. The epoxy–amine reaction (diglycidyl ether of bisphenol-A (DGEBA)/amine 4,4′-methylene bis[3-chloro 2,6-diethylaniline] (MCDEA)) was investigated by ITOF and dielectric measurement. Together these two techniques were used to separately monitor the changes in the ion mobility and the number of charge carriers. Parameters, such as voltage, pulse length and sensor geometry, were determined and optimized to obtain the repeatable results. The results were compared to dynamic mechanical measurements and the reaction advancement. A most interesting finding is the significant changes in the number of charge carriers during the reaction at the three different temperatures measured. An explanation of the observed changes in ion mobility and the number of charge carriers is offered in terms of the inter-relationship of the changing chemical structure and the density of an epoxy during cure.