Adhesion studies using contact mechanics

Abstract

We report adhesion studies using the JKR contact mechanics method and poly(dimethylsiloxane) (PDMS) cross‐linked networks. When such networks are extracted, interdigitation of tethered chains into extraction zones at the network‐network interface results in a significant enhancement of adhesion. When these networks come into contact with a silicon oxide surface, an increase in adhesion hysteresis is observed in the order of increasing molecular weight between crosslinks, with a scaling factor of 1/2. Subjecting the system to maximum load for 30 min before unloading results in a scaling factor of 1/3, probably due to molecular weight dependent network relaxation rates. PDMS cross‐link networks form H‐bonds with surface Si–OH groups that result in significant and time‐dependent adhesion increase. Systematic studies of interfacial H‐bonding and its effect on adhesion have been carried out using self‐assembled monolayer (SAM) surfaces. An increase in adhesion strength was observed in the order of increasing acidity of surface OH protons [Au/S(CH2)11OH < Au/S(C6H4)2OH < Au/S(CH2)15COOH < Au/S(CH2)11PO(OH)2]. When surface OH concentration was controlled, the strength of adhesion was found to be proportional to the number of surface OH groups, with nonlinear relationship, probably due to the collective nature of H‐bonding between the PDMS chain and surface OH groups. Isotope effect studies confirmed the formation of interfacial H‐bonding.

Force Titration of Langmuir−Blodgett Bilayers of Glycine Amphiphiles: JKR-Type Measurements Using the Surface-Force Apparatus

“…JKR measurements made using functionalized poly(dimethylsiloxane) (PDMS) lenses have been a powerful tool for fundamental adhesion studies, − but JKR measurements using the SFA give more information, including force−distance profile and the shape of the surfaces in contact. Using the SFA, JKR-type measurements have been made between bare mica samples, , Langmuir−Blodgett monolayers, thin polymer films, and gold−thiol self-assembled monolayers, yielding values for surface energy that agree well with those obtained from wetting studies in many cases.…”

Screening of hydrogen bonding interactions by a single layer graphene

“…The large adhesion hysteresis observed for PDMS lenses in contact with oxide surfaces (such as SiO 2 and sapphire) has been reported previously in the literature. 43,44,56,57 Choi et al also showed that the W a during approach is not different for the adhesion of a PDMS lens to surfaces functionalized with selfassembled monolayers having varying concentrations of polar OH headgroups. 58 However, in their experiments, the adhesion hysteresis increased with an increase in concentration of surface OH groups.…”

“…However, for SiO 2 /Si substrate, the W a calculated from pull-off is 360±5 mJ/m 2 (∼6 times the value obtained from loading) indicating a large adhesion hysteresis, consistent with literature. 43,44 Same is true for bare and graphene-coated sapphire, except for a slightly larger adhesion hysteresis for graphene-coated sapphire than graphene-coated SiO 2 /Si. The high adhesion hysteresis observed for bare substrates has been attributed to acid-base interactions between the acidic surface Si-OH (or Al-OH) groups of silicon wafer (or sapphire) and weakly basic siloxane (Si-O-Si) groups of PDMS.…”

“…The high adhesion hysteresis observed for bare substrates has been attributed to acid-base interactions between the acidic surface Si-OH (or Al-OH) groups of silicon wafer (or sapphire) and weakly basic siloxane (Si-O-Si) groups of PDMS. 43,44 The low adhesion hysteresis observed for graphenecoated substrates would thus indicate screening of polar acid-base interactions by a single layer graphene.…”

“…work done in separating the two surfaces from adhesive contact is larger than the work done in bringing the two surfaces in contact). 43,44 A comparison of the differences between bare and graphene-coated substrate in terms of adhesion behavior and sapphire OH peak shifts would provide a clear understanding of graphene's transparency. Our adhesion results indicate a significant reduction in adhesion hysteresis for graphene-coated substrates, indicating the screening of acid-base interactions by a single layer of graphene.…”