Adhesion and Contact Aging of Acrylic Pressure-Sensitive Adhesives to Swollen Elastomers.

Fluid-infused (or swollen) elastomers are known for their antiadhesive properties. The presence of excess fluid at their surface is the main contributor to limiting contact formation and minimizing adhesion. Despite their potential, the mechanisms for adhesion and contact aging to fluid-infused elastomers are poorly understood beyond contact with a few materials (ice, biofilms, glass). This study reports on adhesion to a model fluid-infused elastomer, poly(dimethylsiloxane) (PDMS), swollen with silicone oil. The effects of oil saturation, contact time, and the opposing surface are investigated. Specifically, adhesion to two different adherents with comparable surface energies but drastically different mechanical properties is investigated: a glass surface and a soft viscoelastic acrylic pressure-sensitive adhesive film (PSA, modulus ∼25 kPa). Adhesion between the PSA and swollen PDMS [with 23% (w/w) silicone oil] retains up to 60% of its value compared to contact with unswollen (dry) PDMS. In contrast, adhesion to glass nearly vanishes in contact with the same swollen elastomer. Adhesion to the PSA also displays stronger contact aging than adhesion to glass. Contact aging with the PSA is comparable for dry and unsaturated PDMS. Moreover, load relaxation when the PSA is in contact with the PDMS does not correlate with contact aging for contact with the dry or unsaturated elastomer, suggesting that contact aging is likely caused by chain interpenetration and polymer reorganization within the contact region. Closer to full saturation of the PDMS with oil, adhesion to the PSA decreases significantly and shows a delay in the onset of contact aging that is weakly correlated to the poroelastic relaxation of the elastomer. Additional confocal imaging suggests that the presence of a layer of fluid trapped at the interface between the two solids could explain the delayed (and limited) contact aging to the oil-saturated PDMS.

Interface Stabilization in Adhesion Caused by Elastohydrodynamic Deformation.

Interfacial instabilities are common phenomena observed during adhesion measurements involving viscoelastic polymers or fluids. Typical probe-tack adhesion measurements with soft adhesives are conducted with rigid probes. However, in many settings, such as for medical applications, adhesives make and break contact from soft surfaces such as skin. Here we study how detachment from soft probes alters the debonding mechanism of a model viscoelastic polymer film. We demonstrate that detachment from a soft probe suppresses Saffman-Taylor instabilities commonly encountered in adhesion. We suggest the mechanism for interface stabilization is elastohydrodynamic deformation of the probe and propose a scaling for the onset of stabilization.

Contribution of Surface Energy to pH-Dependent Underwater Adhesion of an Acrylic Pressure-Sensitive Adhesive.

Maintaining the underwater adhesive performance over a broad range of solution pH is challenging but necessary for many biomedical applications. Therefore, understanding how environmental conditions influence the mechanisms of bonding and debonding of pressure-sensitive adhesives (PSAs) can provide guidelines for materials design. We investigate how the presence of acrylic acid as a co-monomer impacts the adhesion of a model PSA in aqueous environments of varying pH. The adhesives under investigation are poly(2-ethylhexyl acrylate), or poly(2-EHA), and poly(2-EHA) co-polymerized with 5 wt % acrylic acid, or poly(2-EHA- co-AA). We characterize bonding and debonding (adhesion) of the adhesives using probe tack measurements with a spherical hydrophobic glass probe. We analyze the performance of the two PSAs in air and in low-ionic-strength buffered aqueous solutions of pH 3- 11. We find that the presence of the acrylic acid co-monomer increases the cohesiveness of the PSA and leads to stronger adhesion under all conditions investigated. We also observe that the presence of the acrylic acid co-monomer imparts the PSA with a strong dependence of adhesion on the solution pH. Dynamic contact angle and ζ potential measurements support the hypothesis that deprotonation of the acrylic acid groups at higher pH causes the decrease in adhesion at higher pH. Rheological measurements do not show changes in the dynamic mechanical properties of the PSAs after exposure to solutions of pH 3- 11. Our measurements allow us to isolate the effect of the solution pH on the surface and bulk properties of the PSA. In the absence of the acrylic acid co-monomer, the bulk dissipation and the surface properties of the PSA are independent of the solution's pH.

Importance of Substrate Functionality on the Adhesion and Debonding of a Pressure-Sensitive Adhesive under Water.

We investigate the effect of an aqueous environment on the adhesion of a model acrylic pressure-sensitive adhesive (PSA) composed of 2-ethylhexyl acrylate-co-acrylic acid. We use probe-tack adhesion measurements accompanied by in situ imaging of the contact region during bonding and debonding. Within the probe-tack tests, we use both hydrophilic (piranha and plasma treatment) and hydrophobic (C18-silanization) surface treatments to investigate the contribution of the probe's surface energy on the underwater adhesion. In examining contact formation in air and underwater, we find that the presence of water when contact is made leads to different modes of PSA relaxation and contact formation. For all probes investigated, the adhesive strength between the PSA and the probe decreases when measured underwater. Additionally, we observe that the presence of water during debonding has a more pronounced effect on the adhesive strength of the PSA when probed by a hydrophilic surface as opposed to a hydrophobic surface. Using fingering wavelength analysis, we estimate the surface energy of the PSA in situ and find that when submerged in water, the PSA has a significantly higher surface energy compared to in air. Therefore, combining the observation of different modes of contact formation, the increase in surface energy, and the importance of the surface energy of the probe, we suggest that the decrease in adhesive strength in water can be explained by the hydration of the PSA and by trapped water defects between the PSA and the probe.