57th Annual Report on Research 2012 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 investigated the possibility of creating high aspect ratio epoxy based SMP pillar arrays at micro-scale. We then demonstrated strong dry adhesion between SMP pillar arrays in hexagonal array (1 μm in diameter, 4 μm in height, 2-3 μm in pitch). The SMP pillars were first heated to 80^oC, above the glass transition temperature (T_g) of SMP, where the Young’s modulus dropped from 2.2 GPa (glassy state) to 3.1 MPa (rubbery state). When brought into contact under a load above the buckling threshold of the pillars, the pillars became interweaved or indented with each other, forming micro-scale hooks and loops. The deformed structure was locked after cooling to room temperature, leading to strong adhesive force, up to ~54±25 N/cm² in the normal direction and ~72±23 N/cm² in the shear direction for pillars with an aspect ratio (=height/diameter) of 4. More indented pillars were observed in the shear direction compared to that in the normal direction, leading to more pillar-to-pillar contact and higher pull-off force. When reheated to 80^oC above the glass transition temperature of SMP, the pillars could be easily separated and the adhesion between two sets of SMP pillars was greatly reduced to ~4±2 N/cm² for normal load and ~8±1 N/cm² for shear load, respectively. This dramatic reduction in adhesion can be explained by the significantly decreased modulus of SMP above T_g, thus, much less elastic energy stored in the pillars for separation.

In summary, we demonstrated a novel dry adhesive based on buckling and interlocking, which could be easily separated on demand. Investigation of the mechanical behaviors of SMP micropillars will offer new insights to design dry adhesives beyond structural properties, but also tuning their intrinsic mechanical properties and chemistry.