Surface mechanical properties of thin polymer films investigated by AFM in pulsed force mode.

Atomic force microscopy in the pulsed force mode (PFM) is applied in this work to the study of thin dewetting patterns formed by drying an aqueous solution of poly(N-isopropylacrylamide) (PNIPAM) and sodium dodecyl sulfate (SDS) on mica. This technique allows the automated acquisition of typically 4 x 10(6) force-distance curves on the sample surface together with maps showing nanodomains differentiated by their stiffness and adhesion to the tip. Topography images of dry films revealed a morphology formed by droplets distributed on the substrate. Adhesion and stiffness images with good lateral resolution show droplets containing polymer and surfactant contrasting with the substrate and also nanosized heterogeneities inside these droplets. They also revealed very small dewetted structures which could not be observed in the topography map by noncontact AFM. Adhesion interactions between the AFM tip and the polymer or the dewetted mica substrate were measured in terms of adhesion force and detachment energy, and can be used as new information to understand dewetting patterns containing silica particles, PNIPAM, and SDS. Other surface mechanical parameters such as stiffness, maximum indentation, hardness, compliance, hysteresis, and Young's modulus were obtained by sampling many points and used to characterize the PNIPAM/SDS films formed in the dewetting process.

Liquid templating for nanoparticle organization into complex patterns.

Dewetting of thin films of charged polymer solutions produces complex patterns that can be applied to direct nanoparticle organization on solid substrates. The morphology produced by dewetting can be controlled by the solution properties, temperature, and substrate wetting. In this work, new results on this liquid-template self-assembly system are presented, with special emphasis on producing large arrays of organized nanoparticles. On a hydrophilic substrate with complete wetting, the patterns include polygonal networks and parallel-track arrays that extend over several hundreds of microns. These large structures are formed under well-controlled drying conditions and characterized by scanning electron microscopy, which is better suited for the examination of large as well as small areas than atomic force microscopy. On partial wetting substrates, new patterns are observed, including a complex set of parallel curved bands with variable particle number densities.

Formation of calcium crystallites in dry natural rubber particles.

In this paper, the effects of drying and aging of natural rubber particles of Hevea brasiliensis are studied. The evolutions of the particle morphology and of the elemental distribution are investigated using electron-energy-loss spectroscopy imaging in a low-energy transmission electron microscope (ESI-TEM). It is found that when the sample is aged, calcium salt crystallites are formed around the particles. Fusion of these crystallites with time to form larger crystals shows evidence of ion mobility in the dry rubber matrix. Electron diffraction patterns and elemental mapping analysis indicate the crystals to be calcium sulfate. These crystallites are closely associated with membrane materials of the rubber particle and are surprisingly compatible with the hydrocarbon matrix of the rubber particle. It is proposed that polar sites on the membrane materials provide nucleation sites for the crystallization of calcium and sulfate ions from the latex serum.

Controlled nanoparticle assembly by dewetting of charged polymer solutions.

In this paper, we present an alternative approach for controlled nanoparticle organization on a solid substrate by applying dewetting patterns of charged polymer solutions as a templating system. Thin films of charged polymer solutions dewet a solid substrate to form complex dewetting patterns that depend on the polymer charge density. These patterns, ranging from polygonal networks to elongated structures that are stabilized by viscous forces during dewetting, serve as potential templates for two-dimensional nanoparticle organization on a solid substrate. Thus, while nanoparticles dried in pure water undergo self-assembly to form close-packed arrays, addition of charged polymer in the dispersion leads to the formation of open structures that are directed by the dewetting patterns of the polymer solution. In this study, we focus on the application of elongated structures resulting from dewetting of high-charge-density polymer solutions to align nanoparticles of silica and gold into long chains that are several micrometers in length. The particle ordering process is a two-step mechanism: an initial confinement of the nanoparticles in the dewetting structures and self-assembly of the particles within these structures upon further drying by lateral capillary attractions.

Skim and cream natural rubber particles: colloidal properties, coalescence and film formation.

Cream and skim fractions of freshly tapped natural rubber latex have been studied using atomic force microscopy and scanning electric potential microscopy to elucidate the topology and charge properties in film formation. Elemental distribution maps of the particles have also been obtained using electron energy-loss imaging in a low-energy transmission electron microscope. The two rubber fractions are obtained by centrifugation. The cream fraction is stable while rapid coagulation occurs in the skim fraction. After removal of the coagulum, no further coagulation occurs and the remaining skim rubber particles are stable. The rubber particles from the cream rubber particles contain higher amount of adsorbed protein-phospholipid materials compared to those in the "self-cleaned" skim fraction. This difference in membrane property has a significant impact on the spreading of the cis-1,4-polyisoprene cores, their coalescence and film formation behavior. Coalescence of cream particles appears to be hindered by the membrane materials, forming a rough film that retains the topology of individual particles. Skim particles coalesce more readily, forming relatively smooth films.