Construction of Combinatorial Library to Study Adhesion in a Multilens Measurement Platform
Polymer brushes (end-grafted polymer chains) on a surface are known to be effective in the control and enhancement of adhesion between surfaces. Up to now, it remains unclear how the enhancement of adhesion correlates with the properties of polymer brushes, since it would require fabrication of a large number of polymer brushes with slight variations in properties such as chain length, grafting density, and chemical composition. Combinatorial and high-throughput techniques provide a rapid and efficient approach to study the influence of polymer brush properties on the work of adhesion between surfaces. At the NIST Combinatorial Methods Center, the multilens combinatorial adhesion test (MCAT), based on the Johnson, Kendall, and Robert (JKR) contact mechanics, has been developed to quantify the work of adhesion between an elastomer lens array of numerous probes and a transparent substrate consisting of a property gradient of interest. Contrary to the conventional JKR technique, a single MCAT measurement is capable of generating a large number of adhesion measurements in parallel.
To achieve the best accuracy of the multilens combinatorial adhesion test, a discrete polymer brush library was designed and fabricated on the surface of a glass substrate. The properties of the polymer brushes, such as thickness, grafting density and chain length, were changed stepwise. Polymer brushes were grafted from the surfaces of the glass substrates by means of atom transfer radical polymerization (ATRP). The brush gradient was constructed in a controlled manner based on understanding and manipulating the kinetics of ATRP. Several classes of polymer brush libraries were synthesized, including PnBMA, poly(glycidyl methacrylate). The properties of the polymer brushes were determined by means of visible monochromatic ellipsometry, contact angle measurement, and infrared spectroscopic ellipsometry. It is observed that the measured work of adhesion between the PDMS lens and the gradient substrate depends on the chemical composition, grafting density and chain length of polymer brush.