Unusual Surface Aging of Poly(dimethylsiloxane) Elastomers

We have observed an unusual increase in adhesion hysteresis and frictional forces for poly(dimethylsiloxane) (PDMS) lenses sliding on smooth glassy surfaces after a period of aging in a laboratory environment. X-ray photoelectron spectroscopy, contact angle, and in situ surface-sensitive sum frequency generation spectroscopy (SFG) measurements show no differences between an aged and unaged lens, indicating that these changes in tribological properties cannot be due to surface contaminant or degradation. Instead, we observed that the SFG intensity of the PDMS Si−CH 3 symmetric band is 3 orders of magnitude higher after sliding the aged lenses. Such a large increase in the SFG signal can only arise from very well-ordered PDMS molecules induced by sliding and has important consequences in understanding adhesion hysteresis and friction.