AVS 53rd International Symposium
    Applied Surface Science Thursday Sessions
       Session AS-ThA

Paper AS-ThA2
Phenomenological Relationships Between Chemistry and Mechanical Properties Derived from ToF-SIMS and Nanoindentation Observations

Thursday, November 16, 2006, 2:20 pm, Room 2005

Session: Combined Methods or Multiple Methods
Presenter: G.L. Fisher, Physical Electronics
Authors: G.L. Fisher, Physical Electronics
C. Szakal, The Pennsylvania State University
N. Winograd, The Pennsylvania State University
J.G. Swadener, Los Alamos National Laboratory
Correspondent: Click to Email
Remarkable correlations between the chemistry and the mechanical properties of polytetrafluoroethylene (PTFE) have been revealed as a function of ionizing radiation fluence. Specifically, changes in the hardness are explained in terms of the macromolecular evolution during @alpha@ particle (5.5 MeV @super 4@He@super 2+@) irradiation. Chemical changes in the surface region of the polymer, i.e. macromolecular evolution, were evaluated using a ToF-SIMS instrument that was equipped with a 20 keV C@sub 60@@super +@ ion probe. The mechanical properties of hardness and elastic modulus were monitored in the near-surface (~2.5 µm) region of the polymer by nanoindentation. Nanoindentation shows that scissioning of the linear polymer chains by @alpha@ particles is offset by crosslinking at moderate doses and leads to a peak in the elastic modulus at ~10@super 8@ Rad. The elastic modulus falls off with continued irradiation due to an increase in molecular mobility that arises by a reduction in the average molecular weight of the matrix molecules (i.e. fragmentation). The magnitude of the hardness indicates that crosslinking continues to a dose of ~5x10@super 9@ Rad and thereafter declines with continued irradiation due to fragmentation. The ToF-SIMS data also reveal a peak in the level of crosslinking at ~5x10@super 9@ Rad that is followed by extensive fragmentation of the matrix molecules. In contrast to nanoindentation, the method by which ToF-SIMS data is used to evaluate the relative degree of crosslinking in a polymer matrix is immature. Therefore, the molecular fragments that were evaluated, the mechanism(s) by which they arise in the mass spectrum, and the treatment of the data will be discussed. These developments represent a significant step forward in the application of ToF-SIMS to address the properties of soft materials.