The award is based on complementary research activities, including polymer synthesis, fabrication and characterization in Yang's lab in Materials Science and Engineering (MSE) at the University of Pennsylvania (Penn), theoretical modeling at the molecular and meta-structural (ordered array) levels in Li's lab at Penn/MSE, in collaboration with Xie's lab at General Motors Global Research & Development Center (GM). It builds upon their knowledge in shape memory polymer (SMP) chemistry and surface chemistry toward robust and industrial scale adhesion. The focus of this proposal is on the fundamentally unresolved question concerning the individual and combined roles of surface chemistry, topography, and compliance on adhesion at different length scales. Specifically, the PIs plan to: (1) fabricate SMP pillar arrays with precise control over size, aspect ratio, and spacing; (2) graft well-controlled polymer brushes to manipulate molecular interactions (e.g. H-bonding and ion-pi interactions) for both bonding and debonding; (3) systematically study adhesion and peeling off on complementary SMP pillars under deformation and recovery, and (4) compare experimental results with multiscale modeling throughout each relevant lengthscale and develop a complete mechanistic view of interlocking dry adhesion.
Adhesion between polymers plays an important role in a wide range of industrial applications. Liquid based adhesives offer strong adhesion, however their thermal curing is energy intensive and the adhesion is generally not reversible. Nature provides us with remarkable examples of reversible dry adhesion as manifested in burdock seeds and gecko foot hairs, where no liquid or lengthy curing is involved in the adhesive attachment. This proposal seeks to develop biomimetic superglue-like, yet reworkable, dry adhesives. It will not only provide important scientific insights, but also impact a wide range of technologies, including electronic packaging, automotive and airplane assemblies, and soft robotics. Students at all levels will be exposed to a diverse range of topics in chemistry, materials science and engineering, soft mechanics, nanofabrication and computational modeling through new training and outreach opportunities, including summer lectures by the industrial researcher from General Motors, integration of the research outcome in courses, engagement of high school and undergraduate students through summer research and senior design projects, and industrial internship by a PhD student at General Motors Global R&D Center. The research outcome will also create a significant opportunity to excite the general public, thereby engaging their interest in Science, Technology, Engineering, and Mathematics (STEM).
Adhesion between polymers plays an important role in a wide range of industrial applications, which often require the use of liquid adhesives that are not energy efficient nor reworkable. Gecko-like dry adhesives are reversible but typically not strong. The major goal of this proposal is to address the fundamentally unresolved question concerning the individual and combined roles of surface chemistry, topography and compliance on dry adhesion at different length scales, therefore, offering new insights of how to improve the dry adhesion strength. To achieve these goals, we fabricated shape memory polymer (SMP) pillars, which could change the mechanical property (from hard to soft state) and thus shape upon heating. The property and shape can be recovered when being reheated. When engaging two identical SMP pillar arrays in the soft state at a preload to buckle the pillars, the pillars interweaved or indented with each other, forming hooks and loops like Velcro. This resulted in a strong and anisotropic adhesion at room temperature (hard state). The average normal adhesion reached ~53.6N/cm2 and shear adhesion was ~71.9N/cm2 , much larger than the values from pillar-to-flat (gecko-like adhesion, ~ 10 N/cm2) and flat-flat surface. The bonded adhesive could be easily separated by reheating the pillars to the soft state, where the modulus played the major role to the adhesion reduction. We further demonstrated that the relative adhesion strength in the shear or normal direction could be tuned by varying the pillar spacing. We then created arrays of elastomeric micropillars decorated with silica particles (diameter of 100 nm to 1 µm) on the heads of pillars. We found that the high-density protrusions provided by silica particle assembly (1 µm diameter) on the heads of micropillars (5 µm diameter, aspect ratio =height/diameter=8, and spacing ratio=2) provided interlocking through the interweaving mechanism, leading to enhanced shear adhesion strength by an order of magnitude from 4.1 N/cm2 (between pristine micropillars) to 48.5 N/cm2. The above results have been presented in several publications, numerous conferences (ACS, MRS, Adhesion Society meeting, and Gordon Research Conference) and invited seminars. Students at all levels and postdocs involved in the project have been exposed to a diverse range of topics in polymer science, mechanical properties, materials science and engineering, and nano- and microfabrication techniques through new training and outreach opportunities, including REU activities, science demonstrations at K-12 level, development of a new nano-/microfabrication lab course by the PI, senior design projects, industrial internship, interaction with various research groups at Penn, MIT, Lehigh University, Seoul National University, and the industrial partner (GM and HRL Laboratories). Specifically, 3 PhD students, 1 postdoc fellow, 2 master students, and 1 high school student were partially involved in the proposed research. They learned about the nano- and microfabrication of structured polymers, characterization of shape memory effect, bulk modulus and adhesion strength. PhD student, Chi-Mon Chen, interacted frequently with industrial co-PI at GM R& D Center (later at HRL Laboratories) in sample preparation and characterization of SMPs. The postdoc involved in the project initiated a new international collaboration with Kahp-Yang Suh (late) at Seoul National University. The work by the PhD student, Chi-Mon Chen, landed him an industrial internship at Intel and later a job offer. Another PhD student, Jie Li, was also recruited by Intel. The postdoc became a faculty to lead bio-inspired research on adhesion and wetting. In the introductory polymer class and nano-/microfabrication class (a new lab course ) by the PI to juniors, seniors and graduate students at Penn, these new dry adhesives are case studied to relate applications to fundamental engineering questions. The PI also show cased the research at REU programs and annual science demos to excite the general public. The research could potentially impact a wide range of technologies, including electronic packaging, automotive and airplane assemblies, and soft robotics. In turn, it will improve the environment and energy efficiency.