Covalent grafting of polyacrylamide-based hydrogels to a polypropylene surface activated with functional polyperoxide

Polyacrylamide (PAAm)-based hydrogels covalently bonded to a polypropylene (PP) substrate have been synthesized using modification of the PP with a new functional polyperoxide (FPP). The FPP initiates radical polymerization from the PP’s surface by decomposing the peroxide groups. Two different approaches for forming the hydrogel network have been used. In the first, a grafted hydrogel network is formed by radical polymerization of the acrylamide on the peroxidized PP substrate (“grafting from”) in the presence of N,N′-methylene-bis-acrylamide (a cross-linking agent) and potassium persulfate (an additional initiator in bulk). In the second approach, a grafted hydrogel network is synthesized by: (i) grafting the PAAm macromolecules from the peroxidized PP substrate and (ii) following intermolecular condensation of the surface-grafted PAAm and adding to the reactive bulk PAAm and poly-N-(hydroxymethyl)acrylamide (used as a cross-linking macromolecule), resulting in the formation of a hydrogel network grafted to the PP substrate.The covalent attachment of hydrogel networks has been verified for both approaches by contact angle measurements, dye adsorption by the hydrogel residues after network removal and extraction, and mechanical failure tests. The intermolecular condensation method has been found to be more advantageous for the formation of covalently bonded hydrogels on the PP substrate, while grafted network formation by radical polymerization was detected only for a limited range of monomer and cross-linking agent concentrations.     

Shear-induced gelation of associative polyelectrolytes

Copolymer of N,N-dimethylacrylamide (DMA) and acrylic acid (AA) was functionalized with pendant alkyl groups. Their dynamic mechanical properties in aqueous solution were investigated using continuous shear and oscillatory shear. Shear flow showed an abrupt divergence of the viscosity at a critical shear stress (σ_c). Oscillatory shear showed, with increasing applied stress, slight shear thinning followed by strong shear thickening above σ_c. The effect of the polymer concentration and the oscillation frequency (ω) was investigated. The behaviour at all concentrations and frequencies was fully determined by the product of the oscillation frequency and the terminal relaxation time (τ) of the systems at rest. Master curves of the data determined at different concentrations and frequencies were obtained if the reduced shear modulus was plotted versus the reduced applied stress at constant ω.τ. The effect of shear increased with decreasing value of ω.τ. At low frequencies the storage shear modulus crossed the loss modulus with increasing shear. A model is proposed for this phenomenon of shear-induced gelation.     

Quantitative analysis of the cross-linked structure of microgels using fluorescent probes

Properties of polymeric microgels are influenced by the internal polymer cross-linked structure, but tools to quantitatively analyze this internal structure are limited. With the finding that polymer networks alter the diffusivity and subsequent excimer formation of pyrene, this study used the ratio between pyrene excimer and monomer emission to determine the number of cross-links (N) and average pore size (ξ) in poly(ethylene glycol) diacrylate (PEGDA) microgels. A calibration curve to relate pyrene emission to N and ξ in PEGDA hydrogels was prepared and used to calculate N and ξ in PEGDA microgels. The pyrene emission indicated that PEGDA microgels had a higher cross-linking density and a smaller average pore size when compared with bulk cross-linked hydrogels of the same PEGDA concentration. The analytical method demonstrated in this study may be useful for fine-tuning polymeric microgel properties for a broad array of applications.     

Influence of molecular weight of polymer matrix on the structure and rheological properties of graphene oxide/polydimethylsiloxane composites

The structure and rheological properties of graphene oxide (GO)/polydimethylsiloxane (PDMS) composites are examined as the molecular weight of PDMS and concentration of GO are varied. Clusters formed by GO sheets get smaller and disperse better with increasing molecular weight of PDMS, which results in the higher critical concentration to form network (C_cr). Moreover, at GO concentration just above C_cr, the plateau modulus of samples decreases with the molecular weight of PDMS. During shear experiments, negative normal stress differences (ΔN) are observed in composites with PDMS molecular weight lower than critical entanglement molecular weight (M_c). However, positive ΔN is found in samples with PDMS molecular weight above M_c. It can be concluded that the vorticity alignment of GO clusters induces the negative ΔN based on the optical shear experiments. The possible mechanism for the positive ΔN is also proposed.     

UV-photopolymerisation of poly(methyl methacrylate)-based inorganic-organic hybrid coatings and bulk samples reinforced with methacrylate-modified zirconium oxocluster

In this work, we developed and optimised a synthetic route which enables to produce by spray-coating hard, transparent and stable inorganic-organic hybrid coatings for a wide variety of different substrates (e.g. stone, stainless steel, polymethylmethacrylate, polyethylene, wood, anodized aluminum). Chemically and thermally stable acrylate-based hybrid materials embedding the zirconium oxocluster Zr₄O₂(OMc)₁₂, OMc(CH₂C(CH₃)C(O)O) were prepared and UV-cured.The coatings of different compositions on different substrates were characterized by numerous analytical and spectroscopic methods such as FT-IR spectroscopy, Angle Resolved-XPS (AR-XPS), Secondary Ion Mass Spectrometry (SIMS), X-ray Absorption Spectroscopy (XAS), Environmental Scanning Electron Microscopy (ESEM) and Energy Dispersive X-ray analysis (EDX). Bulk samples were also prepared for additional characterizations. The bulk samples were analysed by FT-IR, whereas the cross-linking degree was qualitatively evaluated by swelling experiments. As far as the mechanical properties are concerned, the shear storage modulus (G′) and loss modulus (G″) were measured by Dynamical Mechanical Thermal Analysis (DMTA) technique. Moreover, the best conditions for the curing and cross-linking processes of the hybrid materials were studied up to 200 °C by using Differential Scanning Calorimetry (DSC). The thermal stability of the hybrid samples was evaluated by Thermogravimetric Analysis (TGA).     

Structure–rheology relationship in complex quaternized polysulfones/solvent/nonsolvent systems

Polymer chains consisting of water-soluble polysulfones with various amounts of ionic chlorine have been prepared via the quaternization reaction of chloromethylated polysulfones with N,N-dimethylethanolamine. Rheological investigations have reflected the importance of electrostatic interactions, hydrogen bonding, and association phenomena from the ternary systems consisting of a proton-acceptor polymer (polysulfones with different ionic chlorine content), a proton-donor solvent (N,N-dimethylformamide), and a proton-donor solvent (methanol). The specific interactions, the polyelectrolyte effect induced by enhanced dissociation of the ionizable groups and mixed solvents’ quality modify the rheological functions, i.e., dynamic viscosity, elastic shear modulus and viscous shear modulus, as well as the thermodynamic parameters obtained from the rheological properties, such as apparent energy of activation and flow activation entropy. These results were correlated with the surface properties of the polysulfonic films formed from solutions in solvent/nonsolvent mixtures.     

Measuring the Surface and Bulk Modulus of Polished Polymers with AFM and Nanoindentation

Abstract

A new method to determine the elastic modulus of a material using the atomic force microscope (AFM) was proposed by Tang et al. [Nanotechnology 19, 495713 (2008)] and is used in this study. This method models the cantilever and the sample as two springs in a series. The properties of both the spring and cantilever are determined on two reference samples with known mechanical properties and these properties are then used to find the elastic modulus of an unknown sample. The indentation depth achieved with AFM is in the nanometer range (30–130 nm in this study); and hence when this technique is performed on polymers, whose surface structure is different from their bulk structure, AFM gives a measure of the surface elastic modulus. In the present study, after employing AFM to measure the surface modulus of five polymers, traditional depth-sensing nanoindentation, with penetration depths of about 1 μm, was used to determine the elastic modulus in the bulk. The mean values for elastic modulus from the AFM were within 5–50% of the nanoindentation results, suggesting the existence of a surface modulus for polished polymers.     

Comparison of the Adhesive Shear Modulus in Bulk and Bonded States

A method is proposed for determining the in situ shear modulus of a structural adhesive from a sandwich beam loaded in 3-point bending in which the adhesive is contained as a thin layer. Expressions for calculating the elastic shear modulus of the adhesive layer from compliance data on the beam are derived, and experimental tests to validate the theory are conducted. To verify the test results, tensile tests are also conducted, and the shear modulus for bulk adhesive is determined using the constitutive equation for an isotropic material relating tensile modulus and Poisson's ratio to shear modulus.

However, the bulk shear modulus as traditionally determined from a tensile test was up to an order of magnitude greater than the in situ shear modulus obtained from the 3-point bend test. A finite element simulation and sensitivity study replicated the experimental results of the 3-point bend tests, and showed that using the shear modulus obtained from the tensile tests would result in significant errors in predicting material and joint behavior. In addition, torsion tests were conducted on bonded cylinders to measure directly the shear modulus. The shear modulus from the torsion test was in agreement with the in situ modulus obtained from the 3-point bend test. This combined experimental-computational approach validated the 3-point bend test as a means to determine the in situ adhesive shear modulus. Finally, micrographs of the interface of the 3-point bend specimen indicated that adhesion occurred by the extension of adhesive pillars to the surface of the adherends. This pillaring phenomenon may have resulted in a lack of bonding along significant portions of the interface, and may explain the compliance of the 3-point bend specimens and, subsequently, the lower shear modulus. The repeatability of the experiments and the substantiation of the results of the experiments by finite element analysis suggest that this pillaring phenomenon may be a mechanism of adhesion.     

Mapping distributions of mechanical properties and formation ability on the ternary B―C―N phase diagram

Based on first-principles calculations, we present the distributions of mechanical properties and formation ability of amorphous B_xC_yN_z solids on the ternary B-C-N phase diagram. Along the C-BN isoelectronic line, the formation energy shows symmetric distributions; the Young's modulus and ratio of bulk modulus and shear modulus (B/G) show zonal distributions. Amazingly, for some peculiar compositions (B: 13-17 at.%; C: 48-52 at.%; N: 33-35 at.%), B-C-N solids exhibit certain ductile characteristic that is comparable to metals. On the phase area (B: 15-30 at.%; C: 50-60 at.%; N: 20-30 at.%), B-C-N solids possess both excellent hardness and good formation ability. These theoretical results provide valuable guidance for intentionally synthesizing B_xC_yN_z materials with desirable mechanical properties.     

Systematics of elastic and thermodynamic properties of super-hardness cubic boron nitride under high pressure

The elastic constants of cubic boron nitride (c–BN) are calculated by the first-principles method at ambient condition. The dependence of the elastic constants, the bulk modulus B, and the shear modulus G on the applied pressure are presented. The calculated results are in good agreement with the comparable experimental and theoretical values. The deviation from the Cauchy relation and the elastic anisotropy are investigated. The normalized elastic constants c_ij ′ are introduced to investigate elasticity of c–BN in more detail. Moreover, the thermodynamic properties (Debye temperature, melting temperature) and sound velocity have been investigated under high pressure for the first time.     

Small angle neutron scattering studies on structural inhomogeneities in polymer gels: irradiation cross-linked gels vs chemically cross-linked gels

A comparison of network structure in a solvent was made for two types of poly(N-isopropylacrylamide) gels cross-linked by chemical reaction with N,N′-methylenebisacrylamide (BIS) (chemical gels) and by γ-ray irradiation (γ-ray gels). The cross-linking density dependence for these gels was examined by small angle neutron scattering (SANS). The SANS results indicated an increase of frozen inhomogeneities with an introduction of cross-links for both chemical and γ-ray gels. However, it was found that the effect of cross-linking is much stronger in the chemical gels than in the γ-ray gels. The differences in the structure were successfully interpreted by a statistical-mechanical theory of gels proposed by Panyukov-Rabin (Phys. Rep. 269 (1996) 1). The degree of polymerization between cross-links, N, was a decreasing function of cross-linking content for both types of gels, while that for the γ-ray gels was a weak function of irradiation dose. Quantitative analyses on BIS concentration and γ-ray dose dependence led to an experimental evidence of the existence of cross-linking saturation threshold.     

Investigating the elastic,mechanical, and thermal properties of polycrystalline Mo2C under high pressure and high temperature

Understanding pressure-induced acoustic velocity-elasticity behavior has extraordinary significance in the fields of materials science and earth science. Herein, we used an advanced high-pressure high-temperature (HPHT) method to synthesize pure-phase β-Mo₂C bulk ceramics. The sound velocity-elasticity behavior, thermodynamic properties, and mechanical behavior of the synthesized β-Mo₂C ceramics were systematically investigated by in-situ high-pressure ultrasonic interferometry and theoretical calculations. The compressive and shear wave velocities, isothermal bulk modulus, shear modulus, Vickers hardness, fracture toughness, Debye temperature, and melting temperature of polycrystalline β-Mo₂C exhibited a monotonic increase with increasing pressure. The experimental and calculation results showed that β-Mo₂C had strong ductile behavior and was a ductile ceramic. Additionally, the temperature-hardness relationship of the as-synthesized polycrystalline β-Mo₂C was investigated by in-situ high-temperature Vickers indentation measurements. The hardness of β-Mo₂C gradually decreased with increasing temperature, and this ceramic still maintained a hardness as high as 12.3 GPa at 500 °C. These results suggest that the intrinsic mechanical and thermodynamic properties of β-Mo₂C are dominated by its unique electronic structure and bonding mode.     

Investigation on composition-dependent properties of Mg5xAl23−5xO27+5xN5−5x (0 ≤ x ≤ 1): Part II. Mechanical properties via first-principles calculations combined with bond valence models

Due to the application in transparent windows, armors and domes, it is essential to have a deep insight into the mechanical properties of spinel-type MgAlON solid solution. Mg_5xAl_23?5xO_27+5xN_5?5x were chosen for the investigation of composition-dependent mechanical properties via combining first-principles calculations with bond valence models. The calculated mechanical properties showed a consistence with experimental values, supporting the validity of the investigation. The simulated elastic constants were used to check the mechanical stability and analyze the shear modulus of Mg_5xAl_23?5xO_27+5xN_5?5x. The mechanical properties of Mg_5xAl_23?5xO_27+5xN_5?5x weakened with increasing x. The composition-dependent mechanical properties were explored from the perspective of chemical bonds. The results demonstrated that decline trend of the mechanical properties, including hardness, bulk modulus and shear modulus of Mg_5xAl_23?5xO_27+5xN_5?5x is mainly impacted by the bonds in tetrahedra, since the linear density of bond valence and bond force constant in tetrahedra decreased dramatically, while those in octahedra were almost unchanged.     

Kinetics of Adhesion Interaction of Polyolefins with Metals Under Conditions of Contact Thermooxidation

The formation of gradients of macromolecular transformations at contact oxidation in the adhesive layer during formation of adhesive joints of pure and peroxide-containing polyolefins with steel was studied. Indirect methods were used, such as measurement of fluidity in adhesive samples, measurement of shear creep and shear modulus of a polymer melt layer for different adhesive thickness, and some others. Effective viscosity, μ, was calculated from the experimental shear creep curves as a function of the thickness, b, of a cross-linking PE melt layer for several levels of contact time. The experimentally-measured values of viscosity were regarded as the complex average values of a certain viscosity gradient. It was found that in the case of catalytically-inactive substrate (Cellophane), there was no gradient. The dependence of the shear modulus on the layer thickness was determined. The measurements of fluidity of adhesive samples scraped at different distances from the interface for various contact times confirmed that the gradient of macromolecular transformations also originated when steel was in adhesion contact with polyethylene and ethylene-vinylacetate copolymer which did not contain peroxide. In the adhesive layers near the adhesive-steel interface, the catalyzed oxidative cross-linking prevailed. Within the adhesive layers located far enough from the interface, the oxidative destruction was dominant. For a catalytically-inert substrate (Teflon), the oxidative destruction prevailed over the whole thickness of the polymer layer.     

Structural and Mechanical Properties of Fats Quantified by Ultrasonics

Since the velocity of an ultrasonic wave through a material depends on its density, bulk modulus (K), and shear modulus (G), a new approach to determine the shear elastic modulus and the mass fractal dimension (D) in a fat crystal network was developed. An ultrasonic chirp wave containing a range of frequencies and amplitudes, was used to estimate the structural and mechanical properties of palm oil based fats, crystallized under shear at three different temperatures (20, 25, and 30 °C). Considering the fat crystal network as a two-phase system (i.e. liquid and solid fat) the velocity of sound in both phases was obtained separately, assuming that the speed of sound in the oil phase was inversely dependent on the temperature. A constant shear modulus for the solid fraction was obtained experimentally by rheology, which was independent of the sample’s nature. These parameters were used for the determination of sample compressibility and its corresponding shear modulus by ultrasonic velocimetry. In addition fractal dimensions (D) were determined by using the relationship of the shear elastic modulus (G) to the mass fraction of the solid fat (φ) in a weak-link regime. The obtained results are comparable and consistent with previously reported fractal dimension values. This method allows online determination of the shear modulus of fats and could be potentially applied for quality control purposes in manufacturing.     

Investigation of nanofluid bubble characteristics under non-equilibrium conditions

We report experimental and theoretical investigations of the bubble characteristics during the oscillatory growth period for several nanofluids. The nanoparticles were found to affect liquid–gas and solid surface tensions, which modulated the bubble contact angle, radius of triple line, bubble volume and the dynamics of bubble growth. To increase the accuracy of the Young–Laplace equation predictions during the bubble growth in the oscillatory period, a new method multi-section bubble (MSB) approach was developed. In this method, the bubble was divided into n sections (i.e., n = 1:N) and the Young–Laplace equation was solved for each section individually. As N increases, within each section the effects of inertia force and viscosity become reduced comparing to that of the liquid-gas surface tension. Unlike the conventional Young–Laplace approach (i.e., N = 1), the new approach is able to predict the bubble characteristics reliably in the following cases: (a) the oscillatory period when bubble is fluctuating; (b) the departure period when bubble is stretched upward, right before departure; and (c) the high shear stress condition when gas velocity is relatively high.     

Development of Novel Amidosulfobetaine Surfactant–Polymer Systems for EOR Applications

Experimental studies were conducted to evaluate the thermal stability and rheological properties of novel surfactant–polymer (SP) systems for enhanced oil recovery applications. One in-house synthesized amphoteric amidosulfobetaine surfactant 3-(N-pentadecanamidopropyl-N,N-dimethylammonium)propanesulfonate and three different polymers were evaluated. Polymer A was a terpolymer of acrylamide, acrylamido tert-butyl sulfonate, and acrylic acid, whereas polymers B and C were terpolymers of acrylamide, N-vinylpyrrolidone, and acrylamido tert-butyl sulfonate with different anionicity. Long-term thermal stability of the surfactant was assessed using FTIR, ¹H NMR, and ¹³C NMR. The surfactant was compatible with seawater at 90 °C and no precipitation was observed. Structural analysis showed good thermal stability and no structural changes were observed after aging at 90 °C. The effects of surfactant concentration, shear rate, salinity, and polymer concentration on rheological properties of the SP systems were determined. Polymer A showed highest viscosity among the investigated polymers in deionized and seawater. The interactions between the surfactant and polymer A were assessed using rheological measurements. In the presence of salts, the viscosity of all three polymers reduced significantly as a result of charge screening. At low shear rates, the added surfactant slightly decreased the viscosity and storage modulus of polymer A. At high shear rates, the effect of the surfactant on the viscosity and storage modulus of polymer A was insignificant.     

Correlations of the first normal stress difference with shear stress and of the storage modulus with loss modulus for homopolymers

The effects of temperature, molecular weight and its distribution, side chain branching, and the structure of polymers on the elastic behavior of bulk homopolymers were investigated, by using logarithmic plots of first normal stress difference (N₁) against shear stress (σ₁₂) and logarithmic plots of storage modulus (G′) against loss modulus (G″). For the investigation, we have used data from the literature as well as our recent experimental results, covering a very wide range of temperature and shear stress or loss modulus. It has been found that such plots are very weakly sensitive to (or virtually independent of) temperature and to the molecular weight of high molecular weight polymers, but strongly dependent upon the molecular weight distribution and the degree of side chain branching. A theoretical interpretation of the observed correlations is presented, using molecular theories.     

Synthesis and characterization of polyhedral oligomeric silsesquioxane-based waterborne polyurethane nanocomposites

A series of aqueous polyurethane nanocomposites were prepared using various amounts (0.3-4.6 wt%) of a diol functionalized polyhedral oligomeric silsesquioxane (POSS) by the prepolymer mixing method. N,N-bis(2-hydroxy ethyl-2-amino ethane sulfonic acid sodium salt (BES sodium salt) was used as the anionic internal emulsifier and ionic center. The molecular structure of the samples was characterized by ATR-FTIR spectroscopy. We investigated the effect of the POSS contents on the properties of the specimens by particle size and viscosity measurements, X-ray diffractometry, mechanical behavior assessment, dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis and morphological studies. The results showed that with increasing the POSS contents, particle size, viscosity, tensile strength, modulus, T _g , and thermal stability of the synthesized samples were improved. Also, SEM and TEM images indicated that a homogeneous morphology was obtained in the 1.2 wt% POSS-based sample. AFM results showed that the surface roughness increased as the POSS amounts increased.