Surface hardening of poly(methyl methacrylate) by electron irradiation

Poly(methyl methacrylate) (PMMA), known as a radiation-degrading polymer, showed mechanically hardening behavior under a high-fluence electron irradiation. When the electron fluence was less than 1 × 1015 cm−2, PMMA showed normal degrading behavior. However, at electron fluence higher than 1 × 1017 cm−2, the mechanical properties of the irradiated PMMA were tremendously improved. Bulk plate PMMA samples and an electron energy of 60 keV were employed in the irradiation experiments. The surface hardness of the irradiated PMMA increased from a pristine value of 0.29 GPa up to 2.8 GPa and the elastic modulus increased from 5.6 GPa to 22.9 GPa at an electron fluence of 7.5 × 1017 cm−2. Both Raman spectra and XPS spectra showed that the surface hardening was attributed to the transformation of polymeric materials into hydrogenated amorphous carbon under the electron irradiation. Confocal Raman spectra showed that the transformation occurred from the surface to the depth of electron range.