Autohesion of semi-crystalline PEEK near and under the glass transition temperature

Autohesion of semi-crystalline polymers under the glass transition temperature (Tg) is fundamentally difficult due to severe restriction in molecular chain mobility. We demonstrate that enhanced autohesion of semi-crystalline polyetheretherketone (PEEK) at temperatures close or under Tg can be achieved through plasma immersion ion implantation (PIII) treatment. Autohesive bonding strength of PEEK increased linearly with PIII bias voltage up to tenfold compared to untreated controls at 160 °C (Tg +10 °C). At 130 °C (Tg −20 °C), PIII treatment increased bonding strength of PEEK by more than fivefold compared to the untreated control. PIII treated surface exhibited higher surface energy, polar component and oxygen content. A model based on the formation of covalent bonds on the plasma treated PEEK by radicals present at the interface was found to predict the bonding strength above and below Tg. The major mechanism responsible for improved autohesion of PEEK near and under Tg is believed to be free radicals based covalent bonding assisted by an increase of surface wettability and reduction in surface crystal structure as was evidenced by SEM images.