Electrophysical properties of nanocomposites based on poly(p-xylylene) and copper nanoparticles

Poly(p-xylylene)-based nanocomposites containing various amounts of copper have been prepared by vacuum codeposition on a cooled substrate. On the basis of the relationship between the electrical conductivity of composites and the volume content of copper, the percolation threshold has been estimated as 10%. This value coincides with the corresponding parameter derived from the concentration dependence of the electrical resistance temperature coefficient. It has been shown that the conductivity of the nanocomposites increases with the moisture content and their moisture sensitivity decreases with an increase in the content of copper from a concentration of 4%. An analysis of the kinetics of the sensor response measured for various moisture contents has shown that the capacity structure of the samples rearranges during adsorption of water. According to the advanced model, the value of the sensor response is determined by a reduction in the height of the potential barrier between nanoparticles and a change in the fractal characteristics of nanocomposites that results from the adsorption of water molecules.     

Atomic force microscopy of human serum albumin (HSA) on poly(styrene/acrolein) microspheres

Atomic Force Microscopy (AFM) in the tapping mode was used for the observation of bare poly (styrene/acrolein) P(SA) microspheres and microspheres with attached HSA. Prior to the AFM observations the P(SA) microspheres were immobilized covalently on the surface of quartz slides modified with -aminopropyltriethoxysilane. Atomic Force Microscopy pictures were registered for the dry samples. The partial coalescence of the P(SA) microspheres connected to the quartz surface with amino groups has been observed. The AFM pictures of the single P(SA) microspheres revealed that the surface of these particles is smooth and that any irregularities, if present, do not exceed 1 nm. The surface of microspheres with attached HSA has very clearly different morphology with regular pattern of HSA macromolecules. Cracks on the surfaces of some microspheres with HSA revealed that protein macromolecules are attached to these particles in several layers. In the case of some other microspheres the defects in protein attachment allowed the observation of the border between the bare surface of the P(SA) microspheres and the surface covered with protein macromolecules. Comparison of the thickness of the HSA layers on the P(SA) microspheres with the dimensions of HSA macromolecules, determined earlier from the x-ray studies, suggests that the first layer, 3.0±0.2 nm thick, is formed of the HSA macromolecules arranged flatly on the surface whereas protein macromolecules in the subsequent layers, each 8.6±1 nm thick, are adsorbed protruding from the surface.     

Single molecule force spectroscopy on poly(vinyl alcohol) by atomic force microscopy

The elastic properties of poly(vinyl alcohol) (PVA) were investigated on the nanoscale using the new technique of single molecule force spectroscopy by atomic force microscopy (AFM). It was found that the elastic properties of PVA molecules scale linearly with their contour lengths. This finding corroborates that the deformation of individual PVA chains is measured. The force spectra of PVA show a kink at around 200 pN and cannot be fitted by an extended Langevin function. The deviation of the elastic behavior of PVA from a freely jointed chain model may indicate the presence of a suprastructure of PVA in NaCl solution.     

Surface ionization state and nanoscale chemical composition of UV-irradiated poly(dimethylsiloxane) probed by chemical force microscopy, force titration, and electrokinetic measurements

The surface chemistry and ionization state of cross-linked poly(dimethylsiloxane) (PDMS) exposed to UV/ozone were studied as a function of treatment time. Various complementary and independent experimental techniques were utilized, which yielded information on the macroscopic as well as the nanometric scale. The average chemical composition of the PDMS surface was quantitatively investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). It was found that the top 1-2 nm surface layer was dominated by silanol groups (-SiOH) for which the concentration increased with increasing treatment dose. The lateral distributions of the silanol groups were analyzed on the nanometer scale by means of atomic force microscopy (AFM) with chemically functionalized tip probes in aqueous buffer solutions at varying pHs. Spatially dependent pull-off force curves (also called "force volume" imaging) indicated the presence of strong chemical heterogeneity of the probed surface. This heterogeneity took the form of patches of silanol functionalities with high local concentration surrounded by a matrix of predominantly hydrophobic domains at low pH. The average pull-off forces for the entire surface scanned were significantly reduced for pH values larger than a characteristic pK(a) constant (in the range between 4.5 and 5.5). The extent of the decrease in the pull-off force and the particular value of pK(a) were found to be a function of treatment time and to differ from the commonly reported values for silanol functional groups on a homogeneous silica surface. These dependences were ascribed to the evoking of a protonation/deprotonation process of the surface silanol groups which was sensitive to the hydrophobic/hydrophilic balance of their close molecular environment. Intermolecular hydrogen bonding may also account for the shifts in the surface pK(a). Furthermore, depending on the nature of the electrolyte, a third effect related to double layer composition, as determined by specific ion adsorption, was quantitatively analyzed by streaming potential measurements in the presence of sodium chloride and phosphate electrolytes.     

Electrostatic adsorption of asymmetric dimethyl arginine (adma) on poly (2-hydroxyethyl methacrylate-acrylic acid) nanoparticles

Separation and purification of asymmetric dimethylarginine (ADMA) molecule, which is an important biomolecule in terms of cardiovascular diseases, is of great importance. Among the methods, the adsorption technique is of considerable demand, and as an adsorbent, the nanoparticles are widely used. In this study, an ADMA isolation was performed via a novel method. Therefore, ADMA adsorption was achieved using a poly (2-hydroxyethyl methacrylate-acrylic acid) (poly(HEMA-AA)) nanoparticles. The change in the adsorption capacity was investigated in terms of changing interaction time, initial ADMA concentration, stirring rate, temperature and ionic strength. The functional group on the polymeric nanoparticles was characterized using Fourier Transform Infrared Spectroscopy (FT-IR); the surface morphology was determined via scanning electron microscopy (SEM), transient electron microscopy (TEM) and surface area (BET) analyzes. The Elisa Spectrophotometer was used for the quantitative analysis of the ADMA molecule. The adsorption capacity of the nanoparticles was determined as 23.76 mg/g. The adsorption process was characterized according to the isotherm calculation.     

Estimating the thickness of hydrated ultrathin poly(o-phenylenediamine) film by atomic force microscopy

A novel method to measure ultrathin poly(o-phenylenediamine) (PPD) film electropolymerized on gold electrode in liquid was developed. It is based on the force versus distance curve (force curve) of atomic force microscopy (AFM). When 1-0.25 μm/s was chosen as the rising rate of the scanner, and 50% of the confidence interval (CI) as the qualifying threshold value, the thickness of the hydrated polymer film could be calculated. This result was compared with one obtained from an AFM image. A step-like electrode fabricated by a photolithographic process was used. The height difference of the electrode before and after the PPD coating was imaged in liquid, and then the real thickness, 19.6±5.2 nm, was obtained. The sample was also measured by estimating the transition range of the force curve of hydrated PPD film, and the thickness of the hydrated PPD film was determined to be 19.3±8.2 nm. However, the results calculated by integrating the electropolymerized charge for the oxidation process of o-phenylenediamine (o-PD) was only one-third as large as it was when using the two previously described methods. This indicated that the structure of hydrated PPD film might have been swollen.     

Atomic force microscopy and enzymatic degradation of oyster shell protein and poly(aspartate)

Oyster shell protein (OSP), an aspartate-enriched regulator of crystallization, was readily observed in its natural condition by atomic force microscopy (AFM) of fragments of oyster shell. The fragments of shell consisted of layers of calcite mineral, termed folia, to which arrays of protein molecules are attached. Modification and removal of the OSP following treatment with several proteolytic enzymes such as subtilisin, carboxypeptidase B, and endoproteinase Glu-C were also observed by AFM. Similarly, poly (aspartate), a polypeptide analog of the OSP, was visualized by AFM on both calcite and mica. Images of poly(aspartate) before and after treatment with lipase demonstrated the potential utility of AFM in degradation studies. The mechanism of hydrolysis is not clear in that lipase normally is considered to be an esterase and not a peptidase.     

Electrostatic characteristics of nanostructures investigated using electric force microscopy

Nanosized materials possess many interesting physical and chemical properties that differ significantly from their macroscopic counterparts. Understanding the size- and shape-dependent properties of nanostructures are of great value to rational design of nanomaterials with desired functionality. Electric force microscopy (EFM) and its variations offer unique opportunities to deepen our insights into the electrical characteristics of nanostructures. In this paper, we review recent progress of this versatile technique and its applications in studying the electrical properties of nanosized materials. A variety of important issues in EFM experimentation and theoretical modeling are discussed, with an emphasis on the ongoing efforts to improve the precision in quantitative measurements of charge density and dielectric properties of nanostructures.     

Transparent poly(bisphenol A carbonate)-based nanocomposites with high refractive index nanoparticles

Transparent organic-inorganic nanocomposites were successfully synthesized from sulfonic acid-modified poly(bisphenol A carbonate) (SPC) and TiO₂ or ZrO₂ nanoparticles. The dispersibility of nanoparticles was significantly improved by both the surface treatment of nanoparticles with phosphoric acid 2-ethylhexyl esters (PAEH) and the introduction of a sulfonic acid moiety into the PC chain. It was found that in some cases, crystallization of the matrix caused a reduction in transparency. Efficient dispersion of nanoparticles and the absence of crystallization resulted in highly transparent nanocomposites with up to 42 wt% TiO₂ and 50 wt% ZrO₂ nanoparticles. The refractive indices of the nanocomposites based on SPC increased with the increasing amount of nanoparticles. Theoretical equation based on Maxwell-Garnett effective medium theory provided reasonably close estimation of the refractive indices to the experimentally observed values. The prepared nanocomposites had lower thermal stability than the host matrix polymers.     

Compatibility studies of polystyrene and poly(vinyl acetate) blends using electrostatic force microscopy

The effect of thermal treatment on the phase separation process of the components of a polymer blend was investigated using electrostatic force microscopy (EFM). EFM technique is an advance on conventional atomic force microscopy, which enables us to measure locally the dielectric properties of the samples under investigation providing compositional information. In this work, we studied the phase separation process of the polymer blend thin films made of polystyrene and poly(vinyl acetate) (PS/PVAc) (75/25 weight fraction). The samples were subjected to different thermal treatments. It was found that at low annealing temperature, PVAc forms many small islands within PS matrix. As the annealing temperature increases, the number of PVAc islands decreases with an increase in the size of the islands. These islands take spherical‐like shape when annealed at a temperature well above the glass transition temperatures of both the component polymers. Despite these morphological/topographical changes, EFM images evidence that there is no interdiffusion which was further confirmed by quantitatively measuring the value of the dielectric permittivity across the interphase. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1332–1338, 2011     

Behavior of thin films of poly(oxyethylene)-poly(oxybutylene) copolymers studied by brewster angle microscopy and atomic force microscopy

Surface films of two copolymers of ethylene oxide (E) and butylene oxide (B), namely E23B8 and E87B18, have been examined by Brewster angle microscopy (BAM) and atomic force microscopy (AFM). Isotherms taken on unsupported films of these copolymers at the air-water interface showed a clear gas to liquid phase transition for E57B18 and a barely discernible phase transition for E23B8. The BAM studies showed a gradual brightening of the films as the surface pressure was increased, which was associated with a film thickening and/or a film densification. Several bright spots were also observed within the films, with the number of spots increasing gradually as the film surface pressure was increased. AFM studies of these films did not show any localized ordering, which fits in with the results from our previous X-ray study of these copolymers [Hodges, C. S.; Neville, F.; Konovalov, O.; Gidalevitz, D.; Hamley, I. W.; Langmuir 2006, 22 (21), 8821-8825], where no long-range ordering was observed. AFM imaging showed two sizes of particulates that were irregularly spaced across the film. The larger particulates were associated with silica contaminants from the copolymer synthesis, whereas the smaller particulates were assumed to be aggregated copolymer. An analysis of the semidilute region of the isotherm showed that while both copolymers had intermixed ethylene oxide and butylene oxide units, the lower molecular weight E23B8 copolymer manifested significantly more intermixing than E87B18.     

Nanoscale hydrophobic recovery: A chemical force microscopy study of UV/ozone-treated cross-linked poly(dimethylsiloxane)

Chemical force microscopy (CFM) in water was used to map the surface hydrophobicity of UV/ozone-treated poly(dimethylsiloxane) (PDMS; Sylgard 184) as a function of the storage/recovery time. In addition to CFM pull-off force mapping, we applied indentation mapping to probe the changes in the normalized modulus. These experiments were complemented by results on surface properties assessed on the micrometer scale by X-ray photoelectron spectroscopy and water contact-angle measurements. Exposure times of < or = 30 min resulted in laterally homogeneously oxidized surfaces, which are characterized by an increased modulus and a high segmental mobility of PDMS. As detected on a sub-50-nm level, the subsequent "hydrophobic recovery" was characterized by a gradual increase in the pull-off forces and a decrease in the normalized modulus, approaching the values of unexposed PDMS after 8-50 days. Lateral imaging on briefly exposed PDMS showed the appearance of liquid PDMS in the form of droplets with an increasing recovery time. Longer exposure times (60 min) led to the formation of a hydrophilic silica-like surface layer. Under these conditions, a gradual surface reconstruction within the silica-like layer occurred with time after exposure, where a hydrophilic SiOx-enriched phase formed < 100-nm-sized domains, surrounded by a more hydrophobic matrix with lower normalized modulus. These results provide new insights into the lateral homogeneity of oxidized PDMS with a resolution in the sub-50-nm range.     

Electrostatic force microscopy analysis of lipid miscibility in two-component monolayers

Electrostatic force microscopy (EFM) was used to assess lipid miscibility and phase behavior in two-component Langmuir-Blodgett (LB) monolayers composed of cationic dioctadecyldimethylammonium bromide (DOMA) and nonionic methyl stearate (SME) lipids. The surface potential measurements were calibrated by applying known bias voltages to the sample during several line scans, thus creating surface potential "scale bars" on the images from which it was determined that circular domains were 50 mV more positive than the surrounding phase. As the spatially averaged surface potential of DOMA was over 400 mV more positive than that of SME, this 50-mV surface potential difference is too low to correspond to lipid phase separation (immiscibility) in the two-component film. Rather, the surface potential contrast was attributed to an increased packing density and a more orthogonal orientation of lipids in the domains resulting in a greater contribution of dipoles to the measured (normal) surface potential. Monolayers prepared by sequentially spreading the two lipids resulted in irregular domains that were 50-450 mV more positive than the surrounding phase, representing varying degrees of lipid mixing, restricted by two-dimensional diffusion at the interface. Fluorescent images of monolayers stained with negatively charged dye supported the EFM miscibility prediction and assignment of surface potential. These results demonstrate a new approach using EFM to quantitatively measure surface potential in order to assess the lateral distribution of components in thin films as well as predict adsorption patterns to heterogeneous interfaces.     

Pull-off force measurements between rough surfaces by atomic force microscopy

Direct measurements of the pull-off (adhesion) forces between pharmaceutical particles (beclomethasone dipropionate, a peptide-type material, and lactose) with irregular geometry and rough polymeric surfaces (series of polypropylene coatings, polycarbonate, and acrylonitrile-butadiene-styrene) were carried out using the atomic force microscope. These measurements showed that roughness of the interacting surfaces is the significant factor affecting experimentally measured pull-off forces. A broad distribution of pull-off force values was noted in the measurements, caused by a varying adhesive contact area for a particle located on rough substrate. The possibility of multiple points of contact between irregularly shaped pharmaceutical particles and substrate surfaces is demonstrated with nanoindentations of the particle in a fluoro-polymer film. Force-distance curves showing the "sawtooth" pattern are additional evidence that particles make contact with substrates at more than one point. Reduced adhesion of 10- to 14-microm-diameter lactose and peptide material particles to the polypropylene coatings with a roughness of 194 nm was found in this study. Similar pull-off force versus roughness relationships are also reported for the model spherical particles, silanized glass particle with a size of 10 microm and polystyrene particle with a diameter of 9 microm, in contact with polypropylene coatings of varying roughness characteristics. It was found that the model recently proposed by Rabinovich et al. (J. Colloid Interface Sci. 232, 1-16 (2000)) closely predicts the pull-off forces for glass and lactose particles. On the other hand, the adhesion of the peptide material and polystyrene particle to polypropylene is underestimated by about an order of magnitude with the theoretical model, in which the interacting substrates are treated as rigid materials. The underestimate is attributed to the deformation of the peptide material and polystyrene particles.     

Degradation of poly(ethylene-co-methacrylic acid)-calcium carbonate nanocomposites

Composites of poly(ethylene-co-methacrylic acid) with 5 mass fraction percent of precipitated calcium carbonate nanoparticles were prepared by melt extrusion on a miniature melt-blender and medium-scale production equipment. The composites consisted mostly of isolated particles. The ultimate mechanical properties of the nanocomposites were consequently largely superior to composites with micron-sized filler. The calcium carbonate particles were shown to offer a large surface area for calcium salt formation during the thermal degradation of the material. This imparted a stabilizing effect to the copolymer that was comparable to the neutralization of the methacrylic acid units with calcium ions. The rate of calcium salt formation was fast at temperatures above 350 °C. Stearic acid surface coatings did not interfere significantly with the calcium salt formation. The oxidative stability of the composites was further largely improved by the formation of a diffusion barrier.     

Young's moduli of surface-bound liposomes by atomic force microscopy force measurements

Mechanical properties of layers of intact liposomes attached by specific interactions on solid surfaces were studied by atomic force microscopy (AFM) force measurements. Force-distance measurements using colloidal probe tips were obtained over liposome layers and used to calculate Young's moduli by using the Hertz contact theory. A classical Hertz model and a modified Hertz one have been used to extract Young's moduli from AFM force curves. The modified model, proposed by Dimitriadis, is correcting for the finite sample thickness since Hertz's classical model is assuming that the sample is infinitely thick. Values for Young's moduli of 40 and 8 kPa have been obtained using the Hertz model for one and three layers of intact liposomes, respectively. Young's moduli of approximately 3 kPa have been obtained using the corrected Hertz model for both one and three layers of surface-bound liposomes. Compression work performed by the colloidal probe to compress these liposome layers has also been calculated.     

Exploiting poly(dimethylsiloxane)-modified tips to evaluate frictional behavior by friction force microscopy

With the aim of investigating the effect of the surface properties on the friction behavior of self-assembled monolayers, we have modified tipless atomic force microscopy (AFM) cantilevers with a poly(dimethylsiloxane) (PDMS) lens. The friction coefficient using the silicon tip is strongly influenced by the mechanical properties of the substrate monolayer because hard, sharp silicon tips penetrate the surface of organic monolayers. However, the friction coefficient obtained for the PDMS-modified AFM cantilever is mostly due to the surface properties of the monolayer functional end group, rather than the viscoelastic deformation of the monolayer. The use of the PDMS tip was demonstrated as a novel means to investigate the effect of surface properties on the frictional behavior of self-assembled monolayers with various functional groups with less mechanical deformation.     

The characterization of high generation poly(amidoamine) G9 dendrimers by atomic force microscopy (AFM)

Scanning force microscopy (AFM) has been employed to characterize the generation‐9 (G9) poly(amidoamine) (PAMAM) dendrimer packing on a mica surface under various conditions. Well ordered 2‐D arrays from hexagonally packed particles of PAMAM (G9) dendrimers (11.4nm in diameter) were deposited on the mica surface. This may be one of the smallest regular monolayer arrays ever observed. The mechanism considered to be responsible for this 2‐D array packing is the interaction of forces between the dendrimer and the mica surface and between dendrimer molecules as well. Other factors such as molecular interpenetrating and the rigidity of the branch structure obviously play an important role in the 2‐D array formation.     

Morphological studies of spin-coated films of poly(styrene-block-methyl methacrylate) copolymers by atomic force microscopy

The surface structure of very thin (15–20 nm) spin-coated films of a symmetrical poly(styrene-b-methyl-methacrylate) block copolymer on silicon and mica is analyzed by atomic force microscopy (AFM). The films show a surface corrugation of a very regular 100 nm lateral periodicity and 6–8 nm amplitude. Film thickness is measured by AFM at induced film defects and checked by ellipsometry. XPS shows that both blocks are at the film surface. Selective degradation of the methyl methacrylate block is used for contrast enhancement and allows to assign poly(styrene) to the elevated surface regions and poly(methyl methacrylate) to the substrate/film interface.Friction interactions of the AFM tip with the film surface may be used to induce high orientational ordering of the morphological pattern perpendicular to the fast scan direction.