The mechanical properties of crystalline cyclopentyl polyhedral oligomeric silsesquioxane

The anisotropic elastic constants of crystalline octacyclopentyl polyhedral oligomeric silsesquioxane (CpPOSS) were determined using molecular dynamics. The force field used for these calculations was shown to model accurately the rhombohedral and triclinic crystal structures of octasilsesquioxane and CpPOSS, respectively, as well as the vibrational frequencies of octasilsesquioxane. The moduli for CpPOSS are anisotropic, with a Reuss-averaged bulk modulus of 7.5 GPa, an isotropic averaged Young's modulus of 11.78 GPa, and an isotropic averaged shear modulus of 4.75 GPa. These isotropic averages or, alternatively, the full anisotropic stiffness tensor of the crystal can be used with micromechanical composite models to calculate the effective elastic properties of polymer nanocomposites that contain crystalline aggregates of CpPOSS.     

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.     

Mechanical characterization of Ormosil films on plastics: Young's modulus determination by three points bending

A protective layer has been deposited to improve the scratching properties of polycarbonate organic glasses. The starting solution consists of a mixture of organosilicon compounds, silicon alkoxides or silica colloidal solutions. The deposition is carried out using the dip coating technique. The thickness is about 2–5 µm for the film and 1 mm for the substrate.Young's moduli of the films are obtained by a new three points bending apparatus. Young's modulus of coating depends both on silica content and on the nature of the silica used as a filler.     

Elastic properties of poly(hydroxybutyrate) molecules

Elasticity of various poly(hydroxybutyrate) (PHB) molecules of regular and irregular conformational structure was examined by the molecular mechanics (MM) calculations. Force - distance functions and the Young's moduli E were computed by stretching of PHB molecules. Unwinding of the 2(1) helical conformation H is characterized at small deformations by the Young's modulus E = 1.8 GPa. The H form is transformed on stretching into the highly extended twisted form E, similar to the beta-structure observed earlier by X-ray fiber diffraction. The computations revealed that in contrast to paraffins, the planar all-trans structure of undeformed PHB is bent. Hence, a PHB molecule attains the maximum contour length in highly straightened, but slightly twisted conformations. A dependence of the single-chain moduli of regular and disordered conformations on the chain extension ratio x was found. The computed data were used to analyze elastic response of tie (bridging) molecules in the interlamellar (IL) region of a semi-crystalline PHB. A modification of the chain length distribution function of tie molecules tau(N) due to secondary crystallization of PHB was conjectured. The resulting narrow distribution tau(N) comprises the taut tie molecules of higher chain moduli prone to overstressing. The molecular model outlined is in line with the macroscopically observed increase in the modulus and brittleness of PHB with storage time.     

Hydrogel microspheres: influence of chemical composition on surface morphology, local elastic properties, and bulk mechanical characteristics

Hydrogel microspheres, beads, and capsules of uniform size, differing in their chemical composition, have been prepared by electrostatic complex formation of sodium alginate with divalent cations and polycations. These have served as model spheres to study the influence of the chemical composition on both surface characteristics and bulk mechanical properties. Resistance to compression experiments yielding the compression work clearly identified differences as a function of the composition, with forces at maximal compression in the range of 34-455 mN. The suitability and informative value of atomic force microscopy have been confirmed for the case where surface characterization is performed in a liquid environment equivalent to physiological conditions. Surface imaging and mechanical response to indentation revealed different average surface roughness and Young's moduli for all hydrogel types ranging from 0.9 to 14.4 nm and from 0.4 to 440 kPa, respectively. The hydrogels exhibited pure elastic behavior. Despite a relatively high standard deviation, resulting from both surface and batch heterogeneity, nonoverlapping ranges of Young's moduli were reproducibly identified for the selected model spheres. The findings indicate the reliability of contact mode atomic force microscopy to quantify local surface properties, which may have an impact on the biocompatibility of alginate-based hydrogel materials of different composition and conditions of preparation. Moreover, it seems that local elastic properties and bulk mechanical characteristics are subject to analogous composition influences.     

Mechanical properties of hybrid inorganic-organic framework materials: establishing fundamental structure-property relationships

The mechanical properties of hybrid framework materials, including both nanoporous metal-organic frameworks (MOFs) and dense inorganic-organic frameworks, are discussed in this critical review. Although there are relatively few studies of this kind in the literature, major recent advances in this area are beginning to shed light on the fundamental structure-mechanical property relationships. Indeed research into the mechanical behavior of this important new class of solid-state materials is central to the design and optimal performance of a multitude of technological applications envisaged. In this review, we examine the elasticity of hybrid frameworks by considering their Young's modulus, Poisson's ratio, bulk modulus and shear modulus. This is followed by discussions of their hardness, plasticity, yield strength and fracture behavior. Our focus is on both experimental and computational approaches. Experimental work on single crystals and amorphized monoliths involved primarily the application of nanoindentation and atomic force microscopy to determine the elastic moduli and hardness properties. The compressibility and bulk moduli of single crystals and polycrystalline powders were studied by high-pressure X-ray crystallography in the diamond anvil cell, while in one instance spectroscopic ellipsometry has also been used to estimate the elastic moduli of MOF nanoparticles and deposited films. Theoretical studies, on the other hand, encompassed the application of first principles density-functional calculations and finite-temperature molecular dynamics simulations. Finally, by virtue of the diverse mechanical properties achievable in hybrid framework materials, we propose that a new domain be established in the materials selection map to define this emerging class of materials (137 references).     

Long-Range Electrostatic Interaction between a Charged Wall and Two Similarly Charged Colloidal Spheres at Low Surface Potentials

The long-range electrostatic interaction between a pair of similarly charged colloidal spheres and a charged planar wall at low surface potentials is theoretically investigated. The linear Poisson-Boltzmann equation (PBE) and the point charge approximation of the charged sphere are used. The electrical potential distribution in the electrolyte solution is found from the PBE at the constant surface potentials using the image charge method. The electrostatic forces acting on the spheres are then calculated. The results show that the repulsive interaction between a pair of similarly charged colloidal spheres clearly decreases when a charged wall appears nearby, but it is impossible for an attractive force to emerge at the scaled surface potentials less than 1. There is, however, an attractive force between the charged wall and the similarly charged colloidal spheres, when the surface potential zetap on the wall is sufficiently higher than the surface potential zetas on the spheres to make zetap > zetasexp(kappah) (h is the distance from the wall to the sphere center). In this case, there are negative surface charges on the spheres at positive surface potential zetas. It is these negative charges that produce the above attraction. Copyright 1999 Academic Press.     

Tight-binding molecular dynamics study of the role of defects on carbon nanotube moduli and failure

We performed tight-binding molecular dynamics on single-walled carbon nanotubes with and without a variety of defects to study their effect on the nanotube modulus and failure through bond rupture. For a pristine (5,5) nanotube, Young's modulus was calculated to be approximately 1.1 TPa, and brittle rupture occurred at a strain of 17% under quasistatic loading. The predicted modulus is consistent with values from experimentally derived thermal vibration and pull test measurements. The defects studied consist of moving or removing one or two carbon atoms, and correspond to a 1.4% defect density. The occurrence of a Stone-Wales defect does not significantly affect Young's modulus, but failure occurs at 15% strain. The occurrence of a pair of separated vacancy defects lowers Young's modulus by approximately 160 GPa and the critical or rupture strain to 13%. These defects apparently act independently, since one of these defects alone was independently determined to lower Young's modulus by approximately 90 GPa, also with a critical strain of 13%. When the pair of vacancy defects adjacent, however, Young's modulus is lowered by only approximately 100 GPa, but with a lower critical strain of 11%. In all cases, there is noticeable strain softening, for instance, leading to an approximately 250 GPa drop in the apparent secant modulus at 10% strain. When a chiral (10,5) nanotube with a vacancy defect was subjected to tensile strain, failure occurred through a continuous spiral-tearing mechanism that maintained a high level of stress (2.5 GPa) even as the nanotube unraveled. Since the statistical likelihood of defects occurring near each other increases with nanotube length, these studies may have important implications for interpreting the experimental distribution of moduli and critical strains.     

A SURFACTANT-ASSISTED APPROACH FOR PREPARING COLLOIDAL AZO POLYMER SPHERES WITH NARROW SIZE DISTRIBUTION

A surfactant-assisted method for preparing colloidal spheres with narrow size distribution from a polydispersed azo polymer has been developed in this work. The colloidal spheres were formed through gradual hydrophobic aggregation of the polymeric chains in THF-H2O dispersion media, which was induced by a steady increase in the water content. Results showed that the addition of a small amount of surfactant （SDBS） could significantly narrow the size distribution of the colloidal spheres. The size distribution of the colloidal spheres was determined by the concentrations of azo polymer and the amount of surfactant in the systems. When the concentrations of polymer and surfactant amount were in a proper range, colloidal spheres with narrow size distribution could be obtained. The colloidal spheres formed by this method could be elongated along the polarization direction of the laser beams to be a new type of the colloid-based functional materials. upon Ar^＋ laser irradiation. The colloidal spheres are considered     

Particle and substrate charge effects on colloidal self-assembly in a sessile drop

By direct video monitoring of dynamic colloidal self-assembly during solvent evaporation in a sessile drop, we investigated the effect of surface charge on the ordering of colloidal spheres. The in situ observations revealed that the interaction between charged colloidal spheres and substrates affects the mobility of colloidal spheres during convective self-assembly, playing an important role in the colloidal crystal growth process. Both ordered and disordered growth was observed depending on different chemical conditions mediated by surface charge and surfactant additions to the sessile drop system. These different self-assembly behaviors were explained by the Coulombic and hydrophobic interactions between surface-charged colloidal spheres and substrates.     

On rheological properties of charge-stabilized spherical colloids

Exact relations are derived which represent the high-frequency elastic moduli of colloidal suspensions of charged spherical particles in terms of integrals of the static structure factor. The only assumptions are the form of the interparticle potential as a screened Coulomb potential and a sufficiently high charge per macroion so that the radial distribution function at contact is zero.     

Amphiphilic azo polymer spheres, colloidal monolayers, and photoinduced chromophore orientation

In this work, azobenzene-containing colloidal spheres have been fabricated and used to construct photoresponsive monolayers. The colloidal spheres were prepared from an amphiphilic azobenzene-containing random copolymer through hydrophobic aggregation of the polymer chains, which was induced by adding the selective solvent (H2O) into a THF solution of the polymer. The size and size distribution of the spheres depended on the initial concentration of the azo polymer in THF and the H2O/THF ratio. Adjusting those factors and optimizing other preparation conditions, uniform colloidal spheres could be obtained. Monolayers composed of hexagonally close-packed colloidal spheres were prepared by the capillary-force-driven method. The colloidal monolayers showed obvious dichroism after laser irradiation due to the photoinduced azo-chromophore orientation occurred in the spheres. The orientation order parameter was related to the irradiation time and estimated to be 0.09 at the photostationary state. The colloidal spheres and their monolayers can potentially be used as building blocks or media for reversible optical data storage, photo-switching, sensors, and other photo-driven devices.     

Colloidal crystals of core-shell type spheres with poly(styrene) core and poly(ethylene oxide) shell

Elastic modulus and crystal growth kinetics have been studied for colloidal crystals of core–shell type colloidal spheres (diameter = 160–200 nm) in aqueous suspension. Crystallization properties of three kinds of spheres, which have poly(styrene) core and poly(ethylene oxide) shell with different oxyethylene chain length (n = 50, 80 and 150), were examined by reflection spectroscopy. The suspensions were deionized exhaustively for more than 1 year using mixed bed of ion-exchange resins. The rigidities of the crystals range from 0.11 to 120 Pa and from 0.56 to 76 Pa for the spheres of n = 50 and 80, respectively, and increase sharply as the sphere volume fraction increase. The g factor, parameter for crystal stability, range from 0.029 to 0.13 and from 0.040 to 0.11 for the spheres of n = 50 and 80, respectively. These g values indicate the formation of stable crystals, and the values were decreased as the sphere volume fraction increased. Two components of crystal growth rate coefficients, fast and slow, were observed in the order from 10⁻³ to 10¹ s⁻¹. This is due to the secondary process in the colloidal crystallization mechanism, corresponding to reorientation from metastable crystals formed in the primary process and/or Ostwald-ripening process. There are no distinct differences in the structural, kinetic and elastic properties among the colloidal crystals of the different core–shell size spheres, nor difference between those of core–shell spheres and silica or poly(styrene) spheres. The results are very reasonably interpreted by the fact that colloidal crystals are formed in a closed container owing to long-range repulsive forces and the Brownian movement of colloidal spheres surrounded by extended electrical double layers, and their formation is not influenced by the rigidity and internal structure of the spheres.     

Thermodiffusion of interacting colloids. II. A microscopic approach

A microscopic approach is presented to describe the contribution to the thermal diffusion coefficient of colloids due to intercolloidal particle interactions. An exact expression for the leading-order virial coefficient of the thermal diffusion coefficient of interacting colloidal spheres is derived in terms of the intercolloidal pair-interaction potential and hydrodynamic interaction functions. This general expression is explicitly evaluated for hard-core interactions and for spheres with a short-ranged attractive potential. The derivation is based on a Smoluchowski equation that is generalized to include temperature gradients. For short-ranged attractive potentials, a negative Soret coefficient is predicted under certain conditions, when the depth of the attraction increases with increasing temperature.     

Determination of the elastic moduli of polymer films by a new ultrasonic reflection method

The development and first applications of a new ultrasonic reflection method for determination of the viscoelastic properties of polymer films are reported. The complex shear modulus G* and the complex longitudinal modulus L*=K*+ 4/3 G* of the samples are derived from the measured complex reflection coefficients of an ultrasonic shear and longitudinal wave, respectively. From G and L the Young's modulus E, the compression modulus K and the Poisson ratio ν can be calculated for isotropic materials. A LiNbO₃-transducer (10° rotated Y-cut) is used for the simultaneous excitation of longitudinal and transversal ultrasonic waves, which allows to determine different elastic constants by one measurement. A measuring cell with normal incidence of the ultrasonic waves is used. The equipment has been applied to study the time dependence of the moduli during film formation from an aqueous polymer dispersion and the isothermal curing of an epoxy resin. Furthermore, the temperature dependence of the elastic constants of a carbon-black filled rubber and during non-isothermal crystallization of a semi-crystalline polymer has been studied.     

Polyelectrolyte multilayers with a tunable Young's modulus: influence of film stiffness on cell adhesion

Mechanical properties of model and natural gels have recently been demonstrated to play an important role in various cellular processes such as adhesion, proliferation, and differentiation, besides events triggered by chemical ligands. Understanding the biomaterial/cell interface is particularly important in many tissue engineering applications and in implant surgery. One of the final goals would be to control cellular processes precisely at the biomaterial surface and to guide tissue regeneration. In this work, we investigate the substrate mechanical effect on cell adhesion for thin polyelectrolyte multilayer (PEM) films, which can be easily deposited on any type of material. The films were cross linked by means of a water-soluble carbodiimide (EDC), and the film elastic modulus was determined using the AFM nanoindentation technique with a colloidal probe. The Young's modulus could be varied over 2 orders of magnitude (from 3 to 400 kPa) for wet poly(L-lysine)/hyaluronan (PLL/HA) films by changing the EDC concentration. The chemical changes upon cross linking were characterized by means of Fourier transform infrared spectroscopy (FTIR). We demonstrated that the adhesion and spreading of human chondrosarcoma cells directly depend on the Young's modulus. These data indicate that, besides the chemical properties of the polyelectrolytes, the substrate mechanics of PEM films is an important parameter influencing cell adhesion and that PEM offer a new way to prepare thin films of tunable mechanical properties with large potential biomedical applications including drug release.     

Effect of Young's modulus on the detachment force of 7 microm particles

The detachment force of ground, 7-microm-diameter polyester particles overcoated with clusters of silica having a cluster radius of approximately 100 nm from ceramer substrates with varying Young's moduli has been measured. It was found that the detachment force varied inversely with the Young's modulus of the ceramer. The results are attributed to the role of the silica, acting as asperities on the particles, and the degree of embedment of the particles into the substrate.     

Elasticity of polyelectrolyte multilayer microcapsules

We present a novel approach to probe elastic properties of polyelectrolyte multilayer microcapsules. The method is based on measurements of the capsule load-deformation curves with the atomic force microscope. The experiment suggests that at low applied load deformations of the capsule shell are elastic. Using elastic theory of membranes we relate force, deformation, elastic moduli, and characteristic sizes of the capsule. Fitting to the prediction of the model yields the lower limit for Young's modulus of the polyelectrolyte multilayers of the order of 1-100 MPa, depending on the template and solvent used for its dissolution. These values correspond to Young's modulus of an elastomer.     

Characterization of short glass fiber filled polystyrene by fiber orientation and mechanical properties

A quasi-steady state, non-isothermal, compressible, inelastic, and creeping flow of polymer melt into a thin cavity is analyzed to predict fiber orientation states. Modified Cross model and Tait's state equation are adopted to consider shear-thinning behavior and compressibility of the polymer melt. Second order tensors are introduced to describe 3-dimensional fiber orientation. Flow-induced fiber orientation can be predicted by solving the equations of change for the orientation tensor with a suitable closure approximation. The orthotropic closure is applied except for the case of low interaction coefficient. Fiber orientation develops mainly due to shear flow in the skin layer and due to stretching effect in the core layer. It turns out that the compressibility, which induces additional velocity gradients during packing, reduces development of the fiber orientation. Results are dependent upon the magnitude of the interaction coefficient. The larger the interaction coefficient, the smaller the orientation development and the effect of compressibility. To predict orientation dependent mechanical properties, the orientation averaging for an arbitrary orientation is carried out from the properties of a transversely isotropic unit cell. The compressibility reduces the axial modulus and increases the transverse modulus. Opposite trends are observed for thermal expansion coefficients. It is also observed that the consideration of compressibility reduces the overall anisotropy of the molded product. Effects of compressibility on mechanical properties of the parts are reduced as the interaction coefficient becomes larger.     

New phase for one-component hard spheres

A completely new phase for one-component hard spheres is reported in an unexpected region of the phase diagram. The new phase is observed at compressibility factors intermediate between the solid and the metastable branches. It can be obtained from either Monte Carlo simulations alone or a combination of Monte Carlo and molecular dynamics calculations. An analysis of the intermediate scattering function data shows that the new phase is in a stable equilibrium. Radial distribution function data, configurational snapshots, bond order parameters, and translational order parameters obtained from molecular simulations indicate that the new phase is significantly different from the isotropic liquid, metastable, or crystalline phases traditionally observed in hard sphere systems. This result significantly changes our previous understanding of the behavior of hard spheres.