Mechanics and Adhesion at Complex Interfaces

Summary:

Our goal is to develop measurement methods that elucidate the mechanical integrity and robustness of the complex materials interfaces inherent to emerging technologies, such as next-generation electronics, advanced adhesives, and functional coatings. Our combinatorial approaches provide experimental designs and quantitative measurement methods for understanding adhesion between disparate materials classes, and the effect of interface structure on materials and device performance.

Description:

There are increasing demands placed on polymer materials interfaces, the performance of which depends on a host of factors, including interfacial energy, interfacial roughness, chemical interactions, polymer entanglements, and defects. Given this immense parameter space, combinatorial and high-throughput techniques offer a promising measurement solution for industrial researchers. To meet this interdisciplinary challenge, we are developing three measurement platforms for rapidly testing polymeric materials interfaces and films. For adhesion, the multilens contact adhesion test assesses weak interactions such as bio-related adhesion, while the combinatorial edge lift-off test provides parallel measurements of adhesion in coatings and structural adhesives. In addition, our film buckling-based measurement provides a rapid measure of modulus that can be applied to libraries of thin coatings and ultrasoft materials surfaces. To complement these high-throughput measurement techniques, we are designing a number of combinatorial libraries that express key factors that govern interfacial adhesion, including surface chemistry, adhesive thickness, and adhesive formulation composition. These libraries provide a foundation for systematically gauging and optimizing the effect of these parameters on performance.

JKR Test Inverted Microscope

Additional Technical Details:

Creating surface libraries that systematically express the chemical and morphological diversity that governs adhesion at complex interfaces is a central part of the high-throughput measurement solutions provided through this project. Polymer brushes present a versatile, stable route for modulating the properties of a surface through tailored brush chemistry and architecture.

In 2007, we developed a facile surface treatment of poly(dimethylsiloxane) (PDMS) that enables grafting of polymer brushes from the surface without altering the underlying mechanical properties of the substrate. Our modification scheme enables immobilization of different chemical groups on the surface of PDMS, while maintaining surface fidelity. We combined ellipsometry and ATR-IR to provide the first quantitative measures of brush thickness directly on the PDMS substrate, and showed how surface wrinkling can be used to measure the mechanical properties of the polymer brush layer.

Surface modification scheme for PDMS

We are using this new surface treatment to functionalize PDMS lenses used in our contact adhesion test, based on the theory of Johnson, Kendall, and Roberts (JKR), which we are developing into a platform for measuring biological and bio-inspired adhesion in polymeric materials. This new route for lens functionalization allows us to systematically assess the effect of surface chemical moieties on adhesion, while keeping the mechanical properties of the PDMS constant. In addition, we successfully designed and implemented an in-situ measurement cell that enables JKR tests to be performed under aqueous media. Both of these achievements advance our goal of rapidly screening surface libraries with gradients in composition, roughness, etc., to map the key parameters that govern the strength of bio-surfaces and interfaces.

We also made significant headway in measuring the mechanics of soft material interfaces. Elastic modulus is one of the most important design parameters in hydrogel materials for biomedical application, since it is related to a number of other properties, including the propensity for cell proliferation and growth, fouling,and comfort. Accordingly as part of a NCMC Focus Project with Vistakon, we are developing a high-throughput metrology based on surface wrinkling of a sensor film to measure the modulus of soft hydrogel devices, such as contact lenses. In 2007, we demonstrated that this method can discriminate fine differences in hydrogel formulations, including crosslink density and diluent concentration, and we are now extending this method to assess the effect of environmental factors such as humidity.

Illustration of JKR adhesion measurements

We reported this work via invited lectures at national meetings of the Materials Research Society and Adhesion Society, as well as at the American Society for Mechanical Engineers Applied Mechanics and Materials Conference. We also presented this work at two NIST Combinatorial Methods Center industry member workshops, one of which was devoted specifically to measurement needs and library design for complex interfaces.

Wrinkling of a sensor film on a hydrogel

Major Accomplishments:

Adhesives represent a global market worth more than $22 billion per year. New markets continue to emerge in areas such as RFID labels, biomedical adhesives and devices, and functional coatings. Nanotechnology requires that interfaces be engineered in order to achieve the desired enhancement in properties.
Our high-throughput approaches for measuring the performance of interfaces allow industry to develop and optimize new adhesive formulations.
A NIST collaboration with Vistakon, a division of Johnson & Johnson Vision Care Inc., focuses on developing high-throughput measurement methods of contact lens modulus, which is a key performance metric for comfort and wearability.
NIST Combinatorial Methods Center (NCMC) members include Air Products and Chemicals, Lord Corporation, Vistakon, BASF, and ICI/ National Starch.