Search

Nanomechanical cantilevers (NMC) were used for the characterization of the film formation process and the mechanical properties of colloidal monolayers made from polystyrene (PS). Closely packed hexagonal monolayers of colloids with diameters ranging from 400 to 800 nm were prepared at the air-water interface and then transferred in a controlled way on the surface of NMC. The film formation process upon annealing of the monolayer was investigated by measuring the resonance frequency of the NMC (≈12 kHz). Upon heating of non-cross-linked PS colloids, we could identify two transition temperatures. The first transition resulted from the merging of polymer colloids into a film. This transition temperature at 147 ± 3 °C as measured at ≈12 kHz remained constant for subsequent heating cycles. We attributed this transition temperature to the glass transition temperature T g of PS which was confirmed by dynamic mechanical thermal analysis (DMTA) and using the time temperature superposition principle. The second transition temperature (175 ± 3 °C) was associated with the end of the film formation process and was measured only for the first heating cycle. Furthermore, the transition of the colloidal monolayer into a homogeneous film preserved the mass loading on the NMC which allowed determination of the Young's modulus of PS (≈3 GPa) elegantly. © 2011 American Chemical Society.

We present a method to measure the absorption of water molecules from the liquid and the vapour phase into polymer nano-films and the diffusion inside these films. Film thickness can be down to 45 nm. To demonstrate the possibilities of this method we use polymer films that are deposited on the upper side of a silicon cantilever by plasma polymerization of norbornene. When a microdrop of water is deposited onto the initially straight cantilever, the drop causes the cantilever to bend while it evaporates. Evaporation of such small water drops usually takes less than a second. An upwards bending is due to capillary forces and a downwards bending is due to the diffusion of water into the polymer film – and the consequent volume expansion (swelling) of the film. The magnitude of the capillary forces and the extent of swelling continuously change during drop evaporation. When drop evaporation is over the cantilever returns to its initial straight position. We simulate the time dependent bending with a numerical model that qualitatively agrees with the experiment. From the time dependence of cantilever bending we are able to determine the diffusion coefficient of water in the thin polymer film.

a b s t r a c t We investigated the reinforcement of blends made from poly(ethyl methacrylate) (PEMA) and PEMA- grafted nanoparticles at a nanometer length scale. The reinforcement was probed by nanowear experiments based on scanning force microscopy (SFM). In addition to imaging, the SFM is applied to wear the surface of samples at the length scale of nanoparticles. Blends with different miscibility of nanoparticles were prepared by varying the molecular weights of polymer grafted from the nanoparti- cles (N) and polymer forming the matrix (P). We were able to associate a critical force for the onset of nanowear by analyzing the nanowear patterns resulting from different normal loads during the nanowear experiment. The definition of this critical force allows quantitative comparison of nanoparticle-polymer systems of different composition. Nanowear tests indicated that only mixtures where N/P > 1 reinforced the composite material compared to the pure homopolymer. Under these conditions the grafted polymers were swollen and the nanoparticles acted as additional anchor sites. As reference experiments we used a blend made from PEMA homopolymer and unmodified particles. Non-grafted nanoparticles clearly did not account for any reinforcement.

The interface roughness of adjacent films which were made by plasma polymerization of hexamethyldisiloxane were investigated. Multilayered structures were made by using different plasma conditions in alteration resulting in different mechanical properties within each layer. Scanning force microscopy on the face side of fractured pieces of the multilayer structures revealed a significant phase contrast between the layers. The direct visualization of the interface using the mechanical contrast between layers allowed the estimation of the interfacial roughness. We found that the interfaces between hexamethyldisiloxane films deposited at a radio frequency (RF) input power of 90 W in the presence of oxygen on top of films made by 48 W without oxygen resulted in an interface roughness of ≈10 nm. In the reverse case, a significantly lower interface roughness of ≈3 nm was determined. We attribute the increase of the interfacial roughness compared to the surface roughness being

A new technique for patterning polymer brushes on the micrometer scale has been developed in which an inkjet printer was used to deposit droplets of acid on a surface-immobilized initiator for atom transfer radical polymerization (ATRP). The acid cleaved an ester bond in the ATRP initiator in a saponification reaction. As a result, the ATRP initiator was rendered inactive. To control the degree of defunctionalization, a new initiator containing a weak ester bond was derived from a tertiary alcohol. Comparison to an established ATRP initiator, derived from a primary alcohol, showed that the novel initiator was defunctionalized with a higher efficiency. Control of the reaction time allowed to partially defunctionalize the initiator molecules, leading to control of grafting densities within the written patterns.

The plasma polymer of hexamethyl disiloxane (HMDSO) was investigated as an adhesive for the gluing of silicon-to-silicon for potential application in microelectronic device and sensor fabrication. The aim of this work was to determine whether nm-thick films of HMDSO deposited in excess oxygen could be used as an adhesion layer for the bonding of silicon or glass to itself or other materials. The adhesive should be stable at high temperature and it should not outgas. The chemical nature of the thin films was analysed using Fourier transform infra red spectroscopy (FTIR) and contact angle measurements. Thermal stability was investigated using gas chromatography combined with mass spectroscopy (GC/MS). Adhesive strength was determined using an Instron tensile testing apparatus. Results show that a combination of plasma surface modification and plasma deposition reactions allow for the fabrication of Si–Si sandwiches that show shear strength up to 6 MPa, which would be sufficient for bonding applications in many microelectronic devices. The deposited materials were temperature stable. Sources of errors and methods to overcome these are discussed.

We report measurements of structure, mechanical properties, glass transition temperature, and contact angle of a novel nanocomposite material consisting of swellable silsesquioxane nanoparticles with grafted poly(ethyl methacrylate) (PEMA) brushes and PEMA matrices with varying molecular weight. We measured the interparticle distance at the surface of the composites using scanning probe microscopy (SPM) and in the bulk of ∼0.5 μm thick films by grazing incidence small angle X-ray scattering (GISAXS). For a given molecular weight of the brush unstable dispersions at high molecular weight of the matrix indicate an intrinsic incompatibility between polymer-grafted-nanoparticles and homopolymer matrices. This incompatibility is affirmed by a high contact angle between the polymer-grafted-nanoparticles and the high molecular weight matrix as measured by SPM. For unstable dispersions, we measured a decreased glass transition temperature along with a decreased plateau modulus by dynamic mechanical thermal analysis (DMTA) which indicates the formation of a liquid-like layer at the brush-matrix interface. This proves the ability to decouple the structural and mechanical properties from the potential to be swollen with small molecules. It opens a new area of use of these soft nanocomposites as slow release materials with tailored mechanical properties.

The Kelvin equation relates the vapor pressure of a volatile liquid to the curvature of the liquid surface. It describes phenomena such as capillary condensation, capillary adhesion, nucleation, and the adsorption of vapors into porous media. Here we propose an extension of the Kelvin equation, which takes into account changes of the vapor pressure due to electric fields. The presence of electric fields reduces the saturation vapor pressure and leads to field-induced condensation. Field-induced condensation can explain the presence of water bridges in scanning probe nanolithographic methods such as anodic oxidation.

In this paper we report on the unprecedented deformation behavior of stratified ultrathin polymer films. The mechanical behavior of layered nanoscale films composed of 8-12 nm thin plasma polymerized hexamethyldisiloxane (ppHMDSO) films on a 70 nm thick film of polystyrene was unveiled by atomic force microscopy nanoindentation. In particular, we observed transitions from the deformation of a thin plate under point load to an elastic contact of a paraboloid of revolution, followed by an elastic-plastic contact for polystyrene and finally an elastic contact for silicon. The different deformation modes were identified on the basis of force-penetration data and atomic force microscopy images of residual indents. A clear threshold was observed for the onset of plastic deformation of the films at loads larger than 2 μN. The measured force curves are in agreement with an elastic and elastic-plastic contact mechanics model, taking the amount of deformation and the geometry of the layer that presumably contributed more to the overall deformation into account. This study shows that the complex deformation behavior of advanced soft matter systems with nanoscale dimensions can be successfully unraveled.

The irradiation of polymer surfaces with ion beams leads to pronounced chemical and physical modifications when the ions are scattered at the atoms in the polymer chain. In this way, different products of decomposition occur. Here we show that by changing the ion fluence and the mass of the ion the local mechanical properties as Young's modulus of a polystyrene surface layer can be tailored. By annealing prestretched irradiated PS near the glass transition, surface rippling occurs in the irradiated areas only, which can be described with an elastic model. The moduli obtained from rippling periodicities and elastic model assumptions are in the range between 8 and 800 MPa at the glass transition and characterize the irradiated PS as rubberlike. From these values the network density and the molar mass of entanglement are quantified. The obtained network density equals the density of hydrogen vacancies generated through the scattered ions, as confirmed by simulations of the atomic scattering and displacement processes. The obtained molar mass of entanglement reveals that the PS locally was densely cross-linked. Our results show that even for nondiscrete layered polymer systems relevant polymer parameters can be derived from the well-known surface rippling without the need for costly chemical analysis.

V2O5 nanowires act as biomimetic catalysts resembling vanadium haloperoxidases (V-HPO). The nanowires display iodinating activity as confirmed by a colorimetric assay using thymol blue (TB), UV/Vis spectrophotometry and mass spectrometry (FD-MS). In the presence of dopamine these nanowires catalyze the fast and efficient synthesis of melanin-like biopolymers under mild conditions (aqueous solution, neutral pH and room temperature). The resulting melanin-like biopolymer obtained by the V2O5 nanowire catalysts was characterized by scanning electron microscopy (SEM), X-ray diffraction, UV-Vis, FT-IR and electric conductivity resembling the natural biopolymer both in its chemical and morphological features. In addition, this synthetic biopolymer self-assembles into fibril-like structures by forming stacks due to π interactions between the aromatic rings.