Hardness Enhancement and Crosslinking Mechanisms in Polystyrene Irradiated with High Energy Ion-Beams

Abstract

In the past, radiation effects studies on polymers were mostly aimed at understanding polymerization mechanisms in radiation curing and synthesis, and radiation induced degradation mechanisms. However, little effort has been expended to improve mechanical properties of polymers, because radiation sources such as e-beams, y -rays, and ultraviolet light, which have been used frequently, produce either small improvement due to crosslinking or more often cause degradation due to scission. In recent work at ORNL, surface hardness values several times larger than that of steel were produced by employing high energy ion-beams (HEIB) in the range of several hundred keV to MeV. Studies show that high linear energy transfer (LET) is important for crosslinking. At low LET conditions, induced active free radicals along the track are so sparsely dispersed that little interaction occurs among radicals. Input energy tends to localize at an intra-molecular chain segment, leading to chain scission. On the other hand, at high LET condition, massive ionization induces a high concentration of free radicals over many neighboring molecular chains facilitating crosslinking.Detailed crosslinking mechanisms are studied by analyzing hardness variations in response to irradiation parameters such as ion species, energy, and fluence. Effective crosslinking radii at hardness saturation are derived based on experimental data for 350 keV € I + and 1 MeV Ai-+ irradiation of polystyrene. ABSTRACTIn the past, radiation effects studies on polymers were mostly aimed at understanding polymerization mechanisms in radiation curing and synthesis, and radiation induced degradation mechanisms. However, little effort has been expended to improve mechanical properties of polymers, because radiation sources such as e-beams, y -rays, and ultraviolet light, which have been used frequently, produce either small improvements due to crosslinking or more often cause degradation due to scission. In recent work at ORNL, surface hardness values several times larger than that of steel were produced by employing high energy ion-beams (HEIB) in the range of several hundred keV to MeV. Studies show that high linear energy transfer (LET) is important for crosslinking. At low LET conditions, induced active fiee radicals along the track are so sparsely dispersed that little interaction occurs among radicals. Input energy tends to localize at an intra-molecular chain segment, leading to chain scission. On the other hand, at high LET condition, massive ionization induces a high concentration of free radicals over many neighboring molecular chains facilitating crosslinking. Detailed crosslinking mechanisms are studied by analyzing hardness variations in response to irradiation parameters such as ion species, energy, and fluence. Effective crosslinking radii at hardness saturation are derived based on experimental data for 3 50 keV H' and 1 MeV A r ' irradiation of polystyrene. INTRODUCTIONOrganic polymers are inherently soft due to the lack of chemical bonds between ...

Proton-Radiation Tolerance of Silicon and SU-8 as Structural Materials for High-Reliability MEMS

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Cold plasma-induced surface modification of heat-polymerized acrylic resin and prevention of early adherence of <i>Candida albicans </i>

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