Electrical degradation of homo- and copolymers of styrenes and of styrene/2-vinylnaphthalene

A variety of different styrene monomers (styrene (S), p-fluorostyrene (p-FS), pentafluorostyrene (PFS), p-chlorostyrene (p-CIS), p-bromostyrene (p-BrS), p-methylstyrene (p-MS), p-tert-butylstyrene (p-tBS), p-methoxystyrene (p-MOS), p-ethoxystyrene (p-EOS), and p-propoxystyrene (p-POS)) have been polymerized by radical and cationic mechanisms. Copolymers of S/PFS, S/p-CIS, S/2-vinyl-naphthalene (2-VN), and S/p-phenylstyrene (p-PhS) were prepared by radical polymerization. All polymers were fully amorphous and transparent and exhibited only one glass transition temperature in the temperature range 77 to 141 degrees C. Thin films of the polymers were subjected to external electrical partial discharges (PD). Typically, 20 PD experiments were conducted for each polymer and the data for the time to breakdown were adapted to the two-parameter Weibull distribution function. The resistance towards PD of the various polymers was found to depend strongly on the monomeric structure. Complementary PD experiments on a series of narrow molecular mass polystyrenes confirmed that the minor variation in molecular mass between the different styrene polymers is of no importance for the PD resistance. The PD resistance of the homopolymers decreased in the following order: PS approximately equal to P(p-MS) > P(p-tBS) approximately equal to P(p-BrS) > P(p-POS) > P(p-CIS) approximately equal to P(p-MOS) > P(p-EOS) approximately equal to P(p-FS) > P(PFS). The time to breakdown for copolymers of S and PFS decreased monotonously with increasing PFS content. Styrene copolymers with low molar contents of 2-VN and p-PhS exhibited a higher resistance towards PD than PS. The results are discussed with reference to various properties of the studied polymers, such as segmental mobility, ionization potential, resonance stabilization, and reactivity of the radicals formed.