58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Understanding and manipulation of interfacial interactions between two polymeric materials when in contact is important for many industrial applications.  For example, thermal curing of liquid adhesives between two bodies is a common practice in large-scale manufacturing and assemblies. However, such a curing process is energy intensive and wasteful. Nature has provides us with remarkable examples of reversible dry adhesion as manifested in burdock seeds and gecko foot hairs, where no heat or liquid is involved in adhesion. The principle of burdock seed adhesion lies in the collective interactions of many small hooks and loops, which can be detached simply by peeling.  The ability of gecko to cling on almost any surface is attributed to the split contact adhesion resulting from millions of hierarchical fibrillar structures on its toe pads via the weak non-covalent van der Waals force and/or capillary forces.

The state of the art of dry adhesive technology currently is such that while many gecko-inspired dry adhesives have been proposed and made at the microscale, the manipulation of local chemistry at molecular level, which affects intrinsic adhesion properties on such structures is often neglected or limited.  As a result, the demonstrated adhesion is rather weak for practical applications. Our aim is to develop a completely new dry adhesive that combines chemical and topological designs together with shape memory polymers (SMPs) with tunable materials bulk properties upon heating/cooling, mimicking the hooks and loops in Velcro®.

In this funding period, we have fabricated SMP pillars and Au nanorods with SMPs, which can change their modulus upon heating or exposure to light. Under a load, the polymer pillars are deformed (or buckled) or tilted. When two complementary pillars are heated and put against each other under the load, they will interlock with each other upon cooling. We studied the deformation behaviors of various polymer pillars, investigated the interlocking behaviors via SEM, performed finite-element simulation, and compared the experimental adhesion results to various mechanics models to understand the adhesion mechanism. Specifically, high-aspect ratio SMP pillar arrays (in hexagonal lattice with diameter of 1 μm and aspect ratio (height/diameter) of 4, and pillar-to-pillar spacing varied as 1 μm and 2 μm) have been investigated as a new type of dry adhesives based on buckling and interlocking mechanism. When two identical SMP pillar arrays were engaged at 80^oC, above the glass transition temperature at a preload larger than the critical buckling threshold, the pillars were deformed and became interweaved and/or indented with each other. After cooling down to room temperature, strong pull-off forces were observed in the normal (~ 53.6±25.1 N/cm²) and shear (~ 71.9±23.2 N/cm²) directions, both of which were much larger than those from pillar-to-flat surface (12.3±8.8 N/cm² and 15.0±2.3 N/cm², respectively) and flat-to-flat surface contact (7.1±5.0 N/cm² and 15.8±2.0 N/cm², respectively). Using finite element analysis (FEA), we visualized how the pillars were engaged, deformed and interacted with each other. Moiré patterns were observed in experiments; the type of interlocking mode between two pillar arrays (interdigitation, interweaving and/or indentation), and thus adhesion strength between the pillar arrays were found dependent on the rotation angle between the superimposed pillar array and the pillar geometry. From FEA simulation and comparison of measured and calculated adhesion values using different contact mechanics models, we showed that interweaved pillars was the main source that contributed to the pillar-to-pillar adhesion and the indented pillars set the lower limit, whereas the probability of interdigitation was very low. Further, we found that interweaved pillars were primarily responsible for the decreased adhesion strength and increased anisotropy when the pillar spacing became larger. We showed that the bonded pillars could be easily separated after reheating to 80^oC due to significant drop of modulus of SMPs.

In summary, the study of interlocking adhesion mechanisms based on buckling and tunable modulus is the first example in literature in study of dry adhesion. It offers important insights of how to control the interfacial contact between pillars, which are critical to the design of strong dry adhesives in practice. The quantitative study of interlocking mechanism and the magnitude suggest that interweaving is the main contributor to dry adhesion strength. It would open up a new path for creating hierarchical structures to introduce strong and tunable adhesives and for applications beyond adhesives, such as sensors.