Annealing Effects on DC Flashover Performance of PMMA in Vacuum

Commercial atactic poly (methyl methacrylate) (aPMMA) were tested to study the annealing effects on its DC flashover characteristics in vacuum. The PMMA samples were machined to 60 mm in diameter, 8 mm in height. After careful cleaning, the samples were prepared at Sub-Tg or near Tg in a vacuum oven for 2 hours, and cooled naturally. Samples were clumped between two steel electrodes in a vacuum chamber. All DC tests were done with the pressure below 3E-3 Pa, discharge current less than 20 mA. As expected, the annealing process did increase the surface flashover strength of PMMA. A notable phenomenon is that when the samples that annealed near Tg were polished again, their first flashover electrical strength was 80% higher than samples annealed at sub-Tg, and their conditioned flashover electrical strength was 90% higher. X-ray photo-electron spectroscopy (XPS) and dielectric spectroscopy analysis were performed to investigate the structure of PMMA. It is speculated that the stereo-complex formation in PMMA causes the great improvement in DC surface flashover performance in vacuum