Using Positron Annihilation as a Method to Characterise Epoxy Networks with Regards to Chemical Resistance

Free volume is a topic of great discussion in the field of polymers and whilst there are a number of techniques available to measure such a property, positron annihilation lifetime spectroscopy (PALS) is seen as the preferred choice. Free volume has been shown in several studies to highly influence properties concerning permeability and diffusion. The work presented in this research concerns chemically resistant protective coatings based on DGEBF epoxy resins and as such, free volume is an important property to study.
Positron beam techniques were utilised in order to measure free volume properties such as Doppler broadened annihilation energies using variable energy positron spectroscopy (VEPS). Whilst VEPS indicated free volume defects, mono energetic positron spectroscopy yields a direct measure of free volume. This latter technique was used to measure free volume as a function of implantation depth in epoxy networks.
In addition positron beam studies, a digital PALS system was successfully constructed for the study of free volume at the University of Sheffield. The system is based on fast plastic scintillators and a timing resolution of 470 ± 26 ps was achieved. This system was then ultimately used throughout this research.
Both PALS and positron beams studies showed that when similar chemistries are probed for free volume, only minimal changes are witness regardless of the preparation method. However, changes in chemistry see significant changes in free volume with PALS and MEPS showing similar free volume trends.
A wide range of diffusion studies were performed on epoxy resins by means of methanol ingression. Studies concerning the production of epoxies were carried out in addition to varying the chemical formulations. Industrial applicability was considered with pigmentation and solvation of epoxies employed.
Ultimately, this research compared the glass transition temperature (Tg) to free volume and the effects on the diffusive nature of methanol in epoxy networks were observed. Overall findings showed that whilst both free volume and Tg are sensitive to changes in diffusion on many occasions, it is free volume which can detect changes in free volume between chemistries and due to strong correlations, indicate properties of diffusion.