Autophobic Behavior of Polymers at the Melt−Elastomer Interface

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.

Complexity in Wetting Dynamics

The spreading dynamics of a droplet of pure liquid deposited on a rigid, nonsoluble substrate has been extensively investigated. In a purely hydrodynamic description, the dynamics of the contact line is determined by a balance between the energy associated with the capillary driving force and the energy dissipated by the viscous shear in the liquid. This balance is expressed by the Cox-Voinov law, which relates the spreading velocity to the contact angle. More recently, complex situations have been examined in which dissipation and/or the driving force may be strongly modified, leading to sometimes spectacular changes in wetting dynamics. We review recent examples of effects at the origin of deviations from the hydrodynamic model, which may involve physical or chemical modifications of the substrate or of the wetting liquid, occurring at scales ranging from the molecular to the mesoscopic.

Adhesion of fluid infused silicone elastomer to glass

Elastomers swollen with non-polar fluids show potential as anti-adhesive materials. We study the effect of oil fraction and contact time on the adhesion between swollen spherical probes of PDMS (polydimethylsiloxane) and flat glass surfaces. The PDMS probes are swollen with pre-determined amount of 10 cSt silicone oil to span the range where the PDMS is fluid free (via solvent extraction) up to the limit where it is oil saturated. Probe tack measurements show that adhesion decreases rapidly with an increase in oil fraction. The decrease in adhesion is attributed to excess oil present at the PDMS-air interface. Contact angle measurements and optical microscopy images support this observation. Adhesion also increases with contact time for a given oil fraction. The increase in adhesion with contact time can be interpreted through different competing mechanisms that depend on the oil fraction where the dominant mechanism changes from extracted to fully swollen PDMS. For partially swollen PDMS, we observe that adhesion initially increases because of viscoelastic relaxation and at long times increases because of contact aging. In contrast, adhesion between fully swollen PDMS and glass barely increases over time and is mainly due to capillary forces. While the relaxation of PDMS in contact is well-described by a visco-poroelastic model, we do not see evidence that poroelastic relaxation of the PDMS contributes to an increase of adhesion with glass whether it is partially or fully swollen.

Quantized contact angles in the dewetting of a structured liquid

We investigate the dewetting of a disordered melt of diblock copolymer from an ordered residual wetting layer. In contrast to simple liquids where the wetting layer has a fixed thickness and the droplets exhibit a single unique contact angle with the substrate, we find that structured liquids of diblock copolymer exhibit a discrete series of wetting layer thicknesses each producing a different contact angle. These quantized contact angles arise because the substrate and air surfaces each induce a gradient of lamellar order in the wetting layer. The interaction between the two surface profiles creates an effective interface potential that oscillates with film thickness, thus, producing a sequence of local minimums. The wetting layer thicknesses and corresponding contact angles are a direct measure of the positions and depths of these minimums. Self-consistent field theory is shown to provide qualitative agreement with the experiment.

Influence of the polymeric interphase design on the interfacial properties of (fiber-reinforced) composites

In fiber-reinforced composites, the interphase nanostructure (i.e., the extended region between two phases in contact) has a pronounced influence on their interfacial adhesion. This work aims at establishing a link between the interphase design of PS-based polymeric fiber coatings and their influence on the micromechanical performance of epoxy-based composite materials. Thiol-ene photochemistry was utilized to introduce a polymeric gradient on silica-like surfaces following a two-step approach without additional photoinitiator. Two complementary grafting-techniques were adapted to modify glass fibers: "Grafting-onto" deposition of PB-b-PS diblock copolymers for thin-film coatings (thickness<20 nm) at low grafting density (<0.1 chains/nm2)--and "grafting-from" polymerization for brush-like PS homopolymer coatings of higher thickness (up to 225 nm) and higher density. Polymer-coated glass fibers were characterized for polymer content using thermogravimetric analysis (TGA) and their nanostructural morphologies by scanning electron microscopy (SEM). Model substrates of flat glass and silicon were studied by atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). The change in interfacial shear strength (IFSS) due to fiber modification was determined by a single fiber pull-out experiment. Thick coatings (>40 nm) resulted in a 50% decrease in IFSS. Higher shear strength occurred for thinner coatings of homopolymer and for lower grafting densities of copolymer. Increased IFSS (10%) was found upon dilution of the surface chain density by mixing copolymers. We show that the interfacial shear strength can be increased by tailoring of the interphase design, even for systems with inherently poor adhesion. Perspectives of polymeric fiber coatings for tailored matrix-fiber compatibility and interfacial adhesion are discussed.

Static and dynamic properties of the interface between a polymer brush and a melt of identical chains

Molecular-dynamics simulations of a short-chain polymer melt between two brush-covered surfaces under shear have been performed. The end-grafted polymers which constitute the brush have the same chemical properties as the free chains in the melt and provide a soft deformable substrate. Polymer chains are described by a coarse-grained bead-spring model, which includes excluded volume and backbone connectivity of the chains. The grafting density of the brush layer offers a way of controlling the behavior of the surface without altering the molecular interactions. We perform equilibrium and nonequilibrium molecular-dynamics simulations at constant temperature and volume using the dissipative particle dynamics thermostat. The equilibrium density profiles and the behavior under shear are studied as well as the interdigitation of the melt into the brush, the orientation on different length scales (bond vectors, radius of gyration, and end-to-end vector) of free and grafted chains, and velocity profiles. The obtained boundary conditions and slip length show a rich behavior as a function of grafting density and shear velocity.

Negative excess interfacial entropy between free and end-grafted chemically identical polymers

We measured the temperature dependence of the interfacial tension between free melt and end-grafted chemically identical polymers. The excess interfacial entropy (Delta S(MB)) was found to be extremely small and negative. This is in sharp contrast to the case of liquid surfaces and interfaces between different liquids, where Delta S(MB) is usually much larger and positive, i.e., molecules at such surfaces and interfaces have a higher degree of freedom than in the bulk. A quantitative comparison with theoretical predictions revealed large differences which we attribute to the finite compressibility of the polymer melt, not yet taken into account by theory.