Cellulose nanofibrils (CNF) form hydrogels at low concentrations. These hydrogels are held together by transient interactions such as entanglement of fibrils, non-specific ionic interactions and hydrogen bonds; and are thus vulnerable for changes in the chemical environment or the influence of mechanical forces. By a covalent crosslinking of the fibrils, stable permanent gels can be formed. In this study we have produced CNF by using TEMPO mediated oxidation followed by fibrillation. During this procedure, carboxyl and aldehyde groups are introduced on the CNF surfaces. The aldehyde groups are suitable sites for crosslinking, as aldehydes readily form covalent bonds to primary amines through formation of Schiff bases. For this purpose the diamines ethylenediamine and hexamethylenediamine, differing with four carbon atoms in the chain, were used as crosslinker molecules. The results show that by varying the concentration and length of the crosslinker molecules, the elastic modulus of the gels could be controlled. The reversible gels were in this way transformed to irreversible gels by a simple water based reaction. Controlling gel strength is one important premise for the use of CNF in applications such as tissue engineering.
Micron-sized polymer particles with nanoscale metal coatings are essential in conductive adhesives for electronics assembly. The particles function in a compressed state in the adhesives. The link between mechanical properties and electrical conductivity is thus of the utmost importance in the formation of good electrical contact. A custom flat punch set-up based on nanoindentation has been developed to simultaneously deform and electrically probe individual particles. The set-up has a sufficiently low internal resistance to allow the measurement of sub-Ohm contact resistances. Additionally, the set-up can capture mechanical failure of the particles. Combining this data yields a fundamental understanding of contact behavior. We demonstrate that this method can clearly distinguish between particles of different sizes, with different thicknesses of metal coating, and different metallization schemes. The technique provides good repeatability and physical insight into the behavior of these particles that can guide adhesive design and the optimization of bonding processes.
Micron-sized metal-coated polymer spheres are frequently used as filler particles in conductive composites for electronic interconnects. However, the intrinsic electrical resistivity of the spherical thin films has not been attainable due to deficiency in methods that eliminate the effect of contact resistance. In this work, a four-point probing method using vacuum compatible piezo-actuated micro robots was developed to directly investigate the electric properties of individual silvercoated spheres under real-time observation in a scanning electron microscope. Poly(methyl methacrylate) spheres with a diameter of 30 lm and four different film thicknesses (270 nm, 150 nm, 100 nm, and 60 nm) were investigated. By multiplying the experimental results with geometrical correction factors obtained using finite element models, the resistivities of the thin films were estimated for the four thicknesses. These were higher than the resistivity of bulk silver.
Recently, there has been an increasing interest in silver thin film coated polymer spheres as conductive fillers in isotropic conductive adhesives (ICAs). Such ICAs yield resistivities similar to conventional silver flake based ICAs while requiring only a fraction of the silver content. In this work, effects of the nanostructure of silver thin films on inter-particle contact resistance were investigated. The electrical resistivity of ICAs with similar particle content was shown to decrease with increasing coating thickness. Scanning electron micrographs of ion milled cross-sections revealed that the silver coatings formed continuous metallurgical connections at the contacts between the filler particles after adhesive curing at 150°C. The electrical resistivity decreased for all samples after environmental treatment for 3 weeks at 85°C/85% relative humidity. It was concluded that after the metallurgical connections formed, the bulk resistance of these ICAs were no longer dominated by the contact resistance, but by the geometry and nanostructure of the silver coatings. A figure of merit (FoM) was defined based on the ratio between bulk silver resistivity and the ICA resistivity, and this showed that although the resistivity was lowest in the ICAs containing the most silver, the volume of silver was more effectively used in the ICAs with intermediate silver contents. This was attributed to a size effect due to smaller grains in the thickest coating.
The flash diffusivity method, also known as laser flash analysis (LFA), is commonly used to obtain the thermal diffusivity (a) and thermal conductivity (j) of materials, due to its relative simplicity, rapid measurements, small sample size requirement, and standardized commercially available instruments. In this work, an epoxy adhesive was filled with a large fraction of homogeneous micron-sized polymethylmethacrylate spheres coated with thin silver films, such that a percolating metallic network that dominated the electric and thermal transport formed through the polymer at <3 vol. % silver. Specific heat capacity (C p) was measured from the LFA measurements by a comparative method and from the total and reversible heat flow signals of modulated differential scanning calorimetry (MDSC). j was estimated as the product of a, C p , and density (q) and was found to vary significantly with the method to find C p. The electron contribution was found from the electrical conductivity by the Wiedemann-Franz law and was used to elucidate the thermal transport mechanisms in the composite. A theoretical background for the various methods is included.
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