AFM nano-indentation
The modifications of the mechanical properties were followed
as a function of the irradiation time using AFM nano-indentation.
The forceedistance curves were monitored under a constant
deflection (or constant load). Nano-indentations were performed
using a diamond tip (64.05 kHz). The given characteristics of the
diamond tip corresponded to a spring constant of 234.5 N/m and a
curvature radius of 25 nm. To determine the relative stiffness of the
polymer, three tests of nano-indentation on different areas of the
polymer were performed. Each test corresponded to nine indentations;
thus, 3 9 force curves were used with a load that
varied from 1 to 5 mN. In the literature, several models have been
proposed for measurement of mechanical properties based on the
nano-indentation testing method. One of these models is the Oliver
and Pharr [19,20] procedure (O&P). Thus, the data extracted from
the forceedisplacement curve obtained via AFM nano-indentation
measurements was treated with this O&P model. The O&P procedure
is based on Sneddon’s approach [21], in which the force F is
given as a power law of the displacement, F(h) ¼ ahm, where a and
m are constants that depend on the mechanical properties of the
polymer and indenter geometry, respectively. The model assumes
that the sample deformation is elastic and plastic, that the deformation
is time-independent, that the indenter is a rigid punch and
that there is no sink-in or pile-up. The elastic modulus (E) can be
directly calculated from the reduced modulus obtained using the