Influence of accelerated aging on nanomechanical properties, creep behaviour and adhesive forces of PDMS C. A. Charitidis*, E. P. Koumoulos, V. P. Tsikourkitoudi, D. A. Dragatogiannis and G. Lolas In the present study, the nanoindentation creep behaviour of untreated and ultraviolet (UV) treated polydimethylsiloxane (PDMS) samples was investigated, accompanied with adhesion analysis and Fourier transform infrared spectroscopy (FTIR) characterisation. Different hold times, in the range of 5–2000 s (at 10 mN of applied load), were incorporated into each nanoindentation measurement. The increase in hold time results in an increase in change in depth of the indenter in both samples. The error in hardness/modulus due to creep can be neglected for hold times of y400 s or more for untreated PDMS and y200 s or more for UV treated PDMS. The FTIR obtained data revealed surface deterioration, while the bulk nanomechanical properties were almost identical. A decrease in adhesive energy in the case of UV treated PDMS was observed, indicating that adhesive forces play a significant role at the nanometre scale in the indentation tests (real contact area determination during nanoindentation measurements). Keywords: Nanoindentation, Polydimethylsiloxane, Creep, Adhesion, Viscoelasticity, Accelerated aging This paper is part of a special issue on Durability of composite systems Introduction Polydimethylsiloxane (PDMS) Polydimethylsiloxane, a silicon based elastomer, exhi- bits several intriguing properties, including biological and chemical inertness, extremely low T g , high tem- perature and oxidation resistance and vapour perme- ability. PDMS consists of repeating [–OSi(CH 3 ) 2 –] units with a hydrophobic, low surface energy, methyl terminated surface. Owing to its properties, PDMS is used in a wide variety of applications in bioengineer- ing, electronics and microelectromechanical systems, including micromachined mechanical and chemical sensors, 1 stamp material for soft lithography, 2,3 micro- fluidics devices 4–6 and cell growth. However, in many applications, it cannot be used as it is, due to its hydrophobic surface. In these cases, the surface of PDMS should be modified in order to increase adhesion and improve wetting of aqueous solvents. As is well known, surface treatment results in changes in the surface properties. Previous studies 7–9 reported PDMS changes in its surface chemistry and its ionisation state exposed to ultraviolet (UV)/ozone irradiation. Accelerated aging – UV irradiation In the case of a polymer’s irradiation with UV light, primary and secondary processes occur. First, transfor- mations as primary processes occur when the polymer chain absorbs radiation doses exceeding its ionisation energy, leading to the formation of charge pairs, free charges, singlet excitation and, in much smaller amount, triplet excitation and finally decomposition, following either a free radical path with yields of macromolecular radicals and protons, or the molecular path when molecular hydrogen is produced with the formation of double bonds in the macromolecule (high irradiation energies usually result in the formation of long chained radicals, which contribute to polymer decomposition). Secondary processes include rearrangements, where the previously formed hydrogen atoms or molecules either recombine with the macromolecules, forming molecular hydrogen–radical pairs or saturating double bonds, or the unsaturated double bonds can migrate along the macromolecular backbone. Ionic cross-linking and physical cross-linking (entanglements) occur possibly as a consequence of increased entropy, enhancing the cross-link density of the irradiated polymer. 10 A large number of studies are reported in the literature describing modifications of the PDMS sur- faces. In order to change the surface energy (and hence the nanomechanical properties hardness H and elastic modulus E) and introduce various polar groups, corona discharges, 11,12 UV irradiation in combination with ozone 13–16 and oxygen plasma 17–24 treatments (among National Technical University of Athens, Heroon Polytechneiou 9, Athens 15780, Greece *Corresponding author, email charitidis@chemeng.ntua.gr 94 ß Institute of Materials, Minerals and Mining 2012 Published by Maney on behalf of the Institute Received 7 December 2010; accepted 28 January 2011 DOI 10.1179/1743289811Y.0000000013 Plastics, Rubber and Composites 2012 VOL 41 NO 2