Nanomechanical Characterization of Ugelstad Polymer Particles

The Ugelstad method is a well known and versatile technology for manufacturing of mono-sized polymer particles. The technology has proven highly successful within biotechnology. Recently Conpart has sparked a renewed interest in exploiting this technology also for use in the manufacturing of electronics and micro-systems, where there are numerous potential applications. Until now, the focus has been on metal plated particles for use in Anisotropic Conductive Adhesive, but also applications as spacers for Liquid Crystal Displays are in the pipe-line. The mechanical properties of the polymer particles are of crucial importance to both these applications. Due to the inherent complexity of the spherical geometry, characterization of the nanostructured polymer particles is different from traditional material characterization and possesses great challenges. Together, NTNU Nanomechanical Lab and Conpart have developed state of the art technology for measuring the mechanical properties of such micrometer sized particles, based on a commercial available Nanoindenter. A special tool has been invented, called a “flat punch”, by which individual micrometer sized particles are characterized. The contact stress-strain behavior and contact modulus are obtained from Nanoindentation measurements based on a specially developed contact mechanics method. The data can be used to determine compression modulus of the particles, as well as non-elastic behavior. Particle of same composition but different size shows near identical stress-strain behavior, indicating very homogenous particles. Varying chemical composition on the other hand gives significant variations in mechanical properties. The preparation of the test samples as well as the parameters set on the Nanoindenter has shown to have a significant effect on the results, including large variations in the results. A procedure for preparing the test samples have therefore been developed, to obtain repeatable results. This technique also makes it possible to locate the individual particles tested in the SEM afterwards, for the investigation of possible failure-mode for the particles. A number of different particles have been tested representing different chemical composition, different sizes, metallised particles as well as porous particles. Depending on particle chemistry and size, the 10mN maximum load is sufficient to observe failure of the particles, which is particle cracking during the load cycle. Ramp test having a maximum load of 1mN and relaxation test having a 1μm maximum displacement are performed to analyze particle viscoelastic-plasticity in finite strain. Furthermore the effects of loading rate and contact strain rate are investigated and contributed by the deformation mechanism of polymer sphere. A significant size effect is observed under deformation of 10 percent and 20 percent. Results demonstrate that Nanoindentation-based flat punch approach is an effective method of studying the mechanical behavior of polymer particles.