Saša Andjelić

Monitoring of reactive processing by remote mid infra-red spectroscopy

Abstract A novel experimental facility for remote mid infra-red (m.i.r.) spectroscopy has been assembled in our laboratory. We can conduct in situ real-time monitoring of practically any combination of processes and operating conditions, while, at the same time, enjoying the benefits of an unmatched wealth of molecular-level information contained in the m.i.r. range of the electromagnetic spectrum. The principal parts of our set-up are a host of optical components, gold-coated waveguides and a Fourier transform infra-red ( FT i.r.) spectrophotometer. The signal generated in the remote mode was sharp, clear and reproducible, and the results were in every aspect as good as those obtained with conventional FT i.r. spectroscopy. The applicability of our remote m.i.r. set-up to in situ real-time monitoring of processing of reactive polymers was demonstrated by analysing the kinetics of cure of a multifunctional epoxy/amine formulation composed of diglycidylether of bisphenol-F (DGEBF) and 4,4′-methylene dianiline (MDA). A particularly interesting discovery was made concerning the necessary correction of the ‘standard’ epoxy peak at 915 cm −1 , which must be considered in order to preserve the reliability of kinetic results in the later stages of cure.

In situ real-time monitoring of reactive systems by remote fibre-optic near-infra-red spectroscopy

A remote fibre-optic near-infra-red (n.i.r.) spectroscopy set-up was recently assembled in our laboratory and successfully used for in situ real-time monitoring of various reactive systems. A disposable probe was designed and employed, enabling us to study processing of thermosetting polymers. A detailed description of our experimental set-up and several selected examples are presented in this communication.

Impedance spectroscopy of reactive polymers. 5. Impedance as a measure of chemical and physical changes in glass formers

A study was conducted aimed at establishing the nature of chemical and physical phenomena in polymeric and nonpolymeric glass formers that can be observed by impedance measurements. Various systems were investigated that undergo a temporal evolution of structure as a result of chemical reactions and physical processes such as crystallization, vitrification, or phase separation. Distinct and systematic changes in impedance during crystallization and vitrification confirmed that these events could be monitored by impedance spectroscopy. Of particular interest was the potential use of impedance measurements in detecting gelation in crosslinking polymers. It was shown that the experimentally observed knee in imaginary impedance during reaction shifts with frequency and, hence, cannot be used to measure gelation. But a new insight at the molecular level was obtained by employing a novel experimental approach based on simultaneous dielectric-infrared measurements. Evidence was generated to support the formation of a hydrogen-bonded complex in the vicinity of gel point in polymer networks, which affords a vehicle for the migration of intrinsic charges and provides a contribution to the overall conductivity. This finding should be explored further because it suggests the possibility of correlating dielectric response with gelation.

Anomalous trends in conductivity during epoxy—amine reactions

Measurements were carried out of conductivity due to migrating charges as a function of extent of reaction for a series of epoxy—amine formulations. All systems were characterised by the reaction mechanism common to this generic group of materials and by an increase in the steady shear viscosity during reaction. The measured conductivity, however. followed one of three distinct patterns as a function of extent of reaction. Intuitively unexpected, and hence particularly interesting, was the observed increase in conductivity during reaction in several formulations. This communication marks the first time that such response has been reported in the literature. An explanation of the observed trends was offered in terms of an interplay between the contributions of extrinsic and intrinsic migrating charges to the overall (measured) conductivity. Without such information, all correlations between the measured conductivity and the major processing parameters (viscosity, extent of reaction) remain strictly batch-specific and empirical.