Time-intensity transformation and internal stress in UV-curable hyperbranched acrylates

The photocuring of three different highly functional acrylates—Di-pentaerythritol penta/hexaacrylate (DPHA) and two hyperbranched molecules (HBP), one with a stiff polyester and one with a more flexible polyether structure—was investigated by means of photorheology, photo differential scanning calorimetry, and beam bending. Special attention was paid to the influence of the composition of DPHA/HBP reactive blends and UV intensity on gelation and vitrification and the resulting dynamics of the internal stress. It was found that adding HBPs to DPHA did not influence gelation significantly, but shifted the onset of vitrification to higher conversions and thus caused lower internal stresses in the material. Increasing UV intensity increased both the conversion at vitrification, thus retarding the build-up of internal stresses, and the ultimate conversion, thus increasing the final stress level. The obtained conversion, gelation, and vitrification data were assembled into time-intensity transformation diagrams, thus providing a useful tool for optimizing photocurin

Evaluation of thin film adhesion to compliant substrate by the analysis of progressive buckling in fragmentation test

The interface toughness of a thin coating/compliant substrate system is estimated based on the evolution of coating buckle patterns in the fragmentation test. The linear density of coating buckles as a function of applied strain is determined experimentally for a SiOx coating deposited on a polyethylene terephthalate film. A three-dimensional non-linear finite element model is developed to simulate the process of buckle formation in a single narrow coating strip. The elastic energy released during buckling-driven delamination is obtained from the energy balance in the system before and after the buckling event. Both the interface adhesion and the total energy release rate, which includes the plastic dissipation in the substrate during debonding, are evaluated. The apparent interfacial toughness, equal to 15 J/m2 at the onset of buckling, is found to increase with strain. This is tentatively explained by the probabilistic features of the buckle accumulation process, reflected also in the random locations of buckles evolving towards a log-normal distribution of buckle spacings at high strains

The effect of processing conditions on the morphology, thermomechanical, dielectric, and piezoelectric properties of P(VDF-TrFE)/BaTiO3 composites

In this study (0-3) P(VDF-TrFE)/BaTiO3 composites containing up to 60vol% of ceramic phase were prepared by solvent casting or compression molding. Their thermomechanical, dielectric, and piezoelectric properties were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. The crystalline structure of the P(VDF-TrFE) matrix was found to be highly dependent on the processing route, while the structure of BaTiO3 was not affected by any of the processing steps. The mechanical properties of the solvent cast materials showed a maximum at 30vol% BaTiO3, while they increased monotonically with BaTiO3 content for compression molded materials. This difference was attributed to a higher amount of porosity and inhomogeneities in the solvent cast composites. Permittivity as high as 120 and piezoelectric coefficient d 33 up to 32pC/N were obtained for compression molded composites, and the observed decrease in d 33 with aging time was attributed to the effect of mechanical stress release in the polymer matri

Effect of annealing and silylation on the strength of melt-spun Ni-Mn-Ga fibres and their adhesion to epoxy

Single crystals of ferromagnetic Ni-Mn-Ga shape memory alloys show magnetic-field and stress induced twinning, leading to shape memory. Adaptive composites can thus be produced by embedding single crystalline particles or bamboo-structured Ni-Mn-Ga fibres into a polymer matrix. To guarantee a durable performance of these composites, adhesion between reinforcement phase and matrix should be quantified and optimised. The influence of annealing and surface treatment with an aminosilane of melt-spun Ni-Mn-Ga fibres on their strength and adhesion to an epoxy matrix was investigated using single fibre tension and fragmentation tests. Annealing of the melt-spun Ni-Mn-Ga fibres changed the surface from a "pimpled" to a smooth microstructure. This resulted in a reduced adhesion of the annealed fibres in comparison to the as-spun fibres embedded in an epoxy matrix. As-spun fibres exhibited an interfacial shear strength (IFSS) comparable to the shear strength of the epoxy matrix so that the silylation did not change the adhesion significantly. For the annealed fibres, the silane treatment improved the IFSS by 67%. Furthermore, the silylation increased the fracture strength of the Ni-Mn-Ga fibres due to surface flaw healing or forming of a protective surface coating. (C) 2014 Elsevier Ltd. All rights reserved

Conversion and shrinkage analysis of acrylated hyperbranched polymer nanocomposites

The photo-curing behavior of composites containing nanosized SiO2 in an acrylated hyperbranched polymer matrix was investigated by means of photo differential scanning calorimetry. The chemical conversion data were analyzed using an autocatalytic model, paying close attention to the influence of composition and UV intensity. It was shown that the reaction order and the autocatalytic exponent were independent of UV intensity and filler fraction, whereas the rate constant showed strong intensity dependence, but weak filler dependence. Maximum conversion was independent of UV intensity, but was reduced when a filler was present. The dispersion state influenced the gel-point of the composites, but had no influence on the overall cure kinetics. Cure shrinkage reduction of ~33% could be achieved by adding 20 vol% of filler. This was attributed to the reduced double bond conversion of the matrix due to the presence of the filler. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 1954–1963, 200

Antibacterial surfaces based on functionally graded photocatalytic Fe3O4@TiO2 core-shell nanoparticle/epoxy composites

Functionally graded epoxy composites with various concentration profiles of Fe3O4@TiO2 core-shell nanoparticles (NPs) were synthetized and characterized, with focus on their antibacterial properties. The NPs consisted of rutile, anatase, magnetite and hematite. Graded composites were produced starting with homogeneous 2 vol% to 12 vol% NPs suspensions using a magnetophorese process, leading to an enrichment of TiO2 at the surface of the composite up to 16 vol% from an initial 4 vol%. Homogeneous composites were also produced as references. Graded composites with an initial 4 vol% of NPs inactivated E. coli bacteria in less than 2 hours under simulated solar light (50 mW cm(-2)), significantly faster than their homogeneous analogues. During bacterial inactivation the pH decreased from 6.8 to 5.0. Repetitive E. coli inactivation tests on these 4 vol% graded composites were stable up to 8 cycles and 5 min contact between the bacteria and the sample surface was enough to guarantee an adequate bacterial adhesion

Wireless, battery-free, and real-time monitoring of water permeation across thin-film encapsulation

Long-term bioelectronic implants require stable, hermetic encapsulation. Water and ion ingress are challenging to quantify, especially in miniaturized microsystems and over time. We propose a wireless and battery-free flexible platform leveraging backscatter communication and magnesium (Mg)-based microsensors. Water permeation through the encapsulation induces corrosion of the Mg resistive sensor thereby shifting the oscillation frequency of the sensing circuit. Experimental in vitro and in-tissue characterization provides information on the operation of the platform and demonstrates the robustness and accuracy of this promising method, revealing its significance for in-situ real-time monitoring of implanted bioelectronics

A novel synthetic strategy for bioinspired functionally graded nanocomposites employing magnetic field gradients

In order to mimic the complex architecture of many bio-materials and synthesize composites characterized by continuously graded composition and mechanical properties, an innovative synthetic strategy making use of magnetic field gradients and based on the motion of superparamagnetic Fe3O4@SiO2 core-shell nanoparticles is adopted. It is demonstrated that by lowering the viscosity of the system through particle functionalization, and increasing the magnetic force acting on the nanoparticles upon optimization of a simple set-up composed of two permanent magnets in repulsion configuration, the magnephoretic process can be considerably accelerated. Thus, owing to the magnetic responsiveness of the Fe3O4 core and the remarkable mechanical properties of the SiO2 shell, approximately 150 mm thick polymeric films with continuous gradients in composition and characterized by considerable increments in elastic modulus (up to approximate to 70%) and hardness (up to approximate to 150%) when going from particle-depleted to particleenriched regions can be synthesized, even in times as short as 1 hour. The present methods are highly promising for a more efficient magnetic force-based synthesis of inhomogeneous soft materials whose composition is required to be locally tuned to meet the specific mechanical demands arising from non-uniform external loads

Influence of process pressure on local facesheet instability for ultralight sandwich structures

The skin wrinkling phenomenon was investigated in the case of ultra-light sandwich structures with a honeycomb core manufactured by one-shot vacuum bag processing. The interplay between process pressure and compressive strength of the skin was established. It was observed that the size of the adhesive menisci between honeycomb cell walls and skin, and the waviness of the skin increased with process pressure. As these two effects exerted opposing influences on the compressive strength of the skin, an optimal process pressure equal to 0.7 bar was identified experimentally and confirmed by an analytical model

Time-Intensity Superposition for Photoinitiated Polymerization of Fluorinated and Hyperbranched Acrylate Nanocomposites

The validity of the time-intensity superposition principle for the photoinitiated polymerization of nanocomposites based on a monofunctional fluorinated acrylate and on a multifunctional hyperbranched polyether acrylate was investigated in this work. Master curves were obtained for the conversion as a function of time, measured by photo differential scanning calorimetry, by shifting on the time axis the curves obtained at different intensities. A power-law dependence of the shift factor on the intensity was found for all materials, with exponents equal to 0.45 ± 0.03 for the fluorinated acrylates and to 0.71 ± 0.05 for the hyperbranched polyether acrylates. Consequently it is inferred that the radiant exposure reciprocity law, implying linear dependence of the shift factor on intensity, does not apply to the studied compositions. The kinetics of the photopolymerization of materials based on the fluorinated acrylate was analyzed with the autocatalytic model. The final conversion was independent on intensity and filler content. The rate constants showed for all materials a power-law dependency on intensity, with exponents similar to those of the corresponding shift factors