The aim of this work is to investigate the influence of interfacial chemistry on the properties of fluoropolymer composites, independent of effects derived from changes in morphology, in particular particle dispersion state. A comparative study of solvent cast composites of poly (vinylidene fluoride - trifluoroethylene) with barium fitanate particles, at concentrations up to 60 vol%, was carried out using pristine hydrophilic particles, and particles hydrophobized with three organosilanes, bearing different functional groups (-CH3, -CH2NH2 and -CF3). For each filler concentration, composites with good particle dispersion and comparable amount of porosity with all types of particles were prepared and characterized. While pristine particles provided higher permittivity, all silanes decreased the dielectric losses due to Maxwell-Wagner-Sillars dispersion, although to different extents. Moreover, only the aminosilane provided better thermomechanical stability to the highly filled composites. These results provide useful insights into the advantages and disadvantages of the use of different approaches to improve the dispersion of hydrophilic particles in poly (vinylidene fluoride) copolymer based composites.

Published as: Oliveira F., Leterrier Y., Månson J.-A.E., Sereda O., Neels A., Dommann A., Damjanovic D., Process Influences on the Structure, Piezoelectric and Gas-Barrier Properties of PVDF-TrFE Copolymer, J. Polym. Sci. B: Polym. Phys., 52, 496–506 (2014). DOI 10.1002/polb.23443. © 2014. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

We review the process rates and energy intensities of various additive processing technologies and focus on recent progress in improving these metrics for laser powder bed fusion processing of metals, and filament and pellet extrusion processing of polymers and composites. Over the last decade, observed progress in raw build rates has been quite substantial, with laser metal processes improving by about 1 order of magnitude, and polymer extrusion processes by more than 2 orders of magnitude. We develop simple heat transfer models that explain these improvements, point to other possible strategies for improvement, and highlight rate limits. We observe a pattern in laser metal technologies that mimics the development of machine tools; an efficiency plateau, where faster rates require more power with no change in energy nor rate efficiency.

Nonisothermally fusion bonded butt joints were prepared by overmolding thermoplastic elastomers (TPEs) onto isotactic polypropylene (iPP) inserts in order to investigate the effect of processing conditions on the bond strength and interfacial microstructure. The mold temperature (Tm) was the most important factor for bond strength, as determined from interfacial mechanical tests. Extensive melting and recrystallization took place at the surface of the iPP insert at high Tm, promoted by migration of plasticizer from the TPE, whereas the original structure of the iPP remained intact at low Tm. Bond strengths of at least 50% of the cohesive strength of the TPE were nevertheless obtained at low Tm, suggesting intimate contact between the TPE melt and the iPP surface to be sufficient to provide useful adhesive bond strengths in these materials. The influence of pressure was less marked than the Tm, high pressures not being necessary to achieve intimate contact for the bonding times of about 5 s used here. However, the combination of a low bonding pressure with a high Tm typically led to poor quality bonds in thick specimens owing to uncompensated shrinkage during solidification, and voiding at the interface and in the melt zone of the iPP insert. POLYM. ENG. SCI., 58:E82–E92, 2018. © 2017 Society of Plastics Engineers

In this work, resins based on diglycidyl ether of bisphenol A (DGEBA) epoxy and a latent hardener, dicyandiamide (DICY), as well as carbon fiber (CF) composites based on them, were prepared with three commercial accelerators: a methylene bis (phenyl dimethyl urea), a cycloaliphatic substituted urea, and a modified polyamine. The curing kinetics of the three DGEBA/DICY/accelerator systems were investigated by chemorheology and differential scanning calorimetry (DSC), in isothermal and over temperature change conditions. Differences in the reaction onset temperature, and in the glass transition temperature (Tg) were highlighted. For curing of thick resin samples, a slow curing cycle at the lowest possible temperature was used, followed by high temperature (160 - 180 °C) post-curing. Indeed, fast curing at higher temperatures caused the formation of hot spots and led to local burning of the samples. The obtained thermomechanical properties, assessed by ultimate tensile testing and dynamic mechanical analysis (DMA) in single cantilever configuration, were all in the expected range for epoxy resins, with tensile moduli close to 3 GPa and Tg > 140 °C. The longterm stability of these resins at room temperature was verified by DSC. Composite samples were prepared by hand lay-up by manually impregnating four layers of 5-harness satin CF textile, and curing in vacuum bag. Impregnation quality and void content were assessed by optical microscopy. The flexural properties of the post-cured composites were assessed by three-point bending test at room temperature and showed no relevant differences, all composites having bending moduli of 45 - 50 GPa. Finally, composites cured with a faster high temperature curing cycle (20 min at 140 °C) were prepared with the DGEBA/DICY/ methylene bis (phenyl dimethyl urea) system, obtaining similar properties as with the slower curing cycle, showing that the prepreg system allowed more flexibility in terms of curing cycle than the bulk resin samples.

Although fused deposition modeling (FDM) is of great interest for the cost-effective manufacture of polymer parts with complex customized geometries, it currently provides insufficient mechanical integrity for the production of high performance functional structures, and is restricted to too limited a range of materials. The present work is aimed at investigating the suitability of new combinations of hard and soft thermoplastics for FDM. To this end, nonisothermal fusion bonding of polypropylene (iPP) and a thermoplastic elastomer (TPE) with a continuous plasticized iPP matrix was investigated by overinjecting the TPE onto a solid iPP insert. The influence of temperature and pressure was evaluated by tensile testing of butt joint specimens, and optical and electron microscopy. Interface temperatures approaching the iPP melting point gave high bond strengths for hold pressures above 200 bars, resulting in cohesive failure in the TPE as well as adhesive failure at the interface. The pressure was more critical at lower interface temperatures, but bond strengths sufficient to cause cohesive failure in the TPE could still be obtained at interface temperatures well below the iPP melting point if the maximum pressure was high enough. Moreover, too low a hold pressure led to poorer bonding at high interface temperatures than at low interface temperatures, an effect attributed to uncompensated shrinkage during solidification. Similar trends were seen in fusion deposition of TPE onto iPP, where bond strengths comparable with those obtained by overinjection were observed at low bed temperatures, while high bed temperatures resulted in poorer bonds. These results are discussed in terms of the interfacial morphology and the dominant bonding mechanisms in each case.

Diffusion of CO2 in polylactide was modelled by assuming the diffusion coefficient to depend on CO2 concentration, c, according to D[ c] = D[0]exp[ Ac], where D[0] and A are empirical constants, with the aim of optimizing impregnation of nominally amorphous and semicrystalline polylactide/CO2-based precursors for physical foaming. Numerical simulations provided a consistent description of desorption at different temperatures, T, from polylactide impregnated with liquid CO2 at 10℃ and 5 MPa, and D[0, T] could be represented analytically using Arrhenius or Williams–Landel–Ferry-type expressions, allowing interpolation and extrapolation. Sorption was argued on this basis to involve a step-like diffusion front, such that the CO2 content of a plate of thickness l increased as ( D[0] t)1/2 l−1 F[ Aco], where co is the value of c at saturation and F is a function of Aco only. A major practical concern with polylactide/CO2 precursors is that the glass transition temperature, Tg, decreases strongly with c, so that amorphous polylactide saturated with CO2 at 10℃ and 5 MPa degasses spontaneously at room temperature and pressure. However, it was inferred from the models and confirmed experimentally that partial impregnation in liquid CO2 for relatively short times could provide a relatively rapid means of preparing precursors with a roughly uniform CO2 content of around 0.1 g/g that were stable with respect to rapid CO2 loss on heating to room temperature. The resulting precursors gave satisfactory foam morphologies and densities on foaming at 100℃. Moreover, it was also possible to adapt the impregnation conditions so as to obtain partially foamed structures from semicrystalline polylactide under these conditions, in spite of its tendency to undergo cold crystallization during impregnation in liquid CO2, which suppressed expansion of saturated specimens at 100℃.

In this study, we demonstrate by simulation and experiment the antireflective (AR) performance of hyperbranched polymer (HBP) nanocomposites textures replicated from a moth-eye pattern using ultraviolet (UV) nanoimprint lithography. A UV-curable acrylated HBP and a hybrid composite of the HBP with silica nanoparticles with an organometal precursor were used. Using effective medium theory (EMT) simulations, optimal patterns were found to be arrays of paraboloids with stable AR performance over a large range of geometries. A good agreement was found between the simulated and measured optical behavior of such AR arrays with a normal reflectance within the visible range of approximately 4% for the use of a glass substrate. The photocurrent of AR-coated a-Si:H solar cells was found to be 2% higher than the reference value of an uncoated cell whereas, for AR-coated tandem thin film Si cells, the increase was 3% for the top a-Si:H cell and 2% for the bottom microcrystalline Si cell.

NNanoindentation tests were carried out on the surface of polymer nanocomposites exhibiting either graded or homogeneous distributions of Fe3O4@silica core-shell nanoparticles in a photocurable polymeric matrix. The results reveal a complex interplay between graded morphology, indentation depth and calculated modulus and hardness values, which was elucidated through numerical simulations. First, it was experimentally shown how for small (1 µm) indentations, large increases in modulus (up to +40%) and hardness (up to +93%) were obtained for graded composites with respect to their homogeneous counterparts, whereas at a larger indentation depth (20 µm) the modulus and hardness of the graded and homogeneous composites did not substantially differ from each other and from those of the pure polymer. Then, through a Material Point Method approach, experimental nanoindentation tests were successfully simulated, confirming the importance of the indentation depth and of the associated plastic zone as key factors for a more accurate design of graded polymer nanocomposites whose mechanical properties are able to fulfill the requirements encountered during operational life.

The overall aim of this work is to develop precursors suitable for the in-line production of lightweight bio-based foam core sandwich structures with consolidated wood particle facings, using a non-VOC blowing agent. A preliminary study of poly(D,L-lactide) (PDLLA)/CO2 showed that solid expandable precursors could be prepared by impregnation with CO2 under both subcritical and supercritical conditions. However, because the particleboard process involves temperatures, T, well above the effective glass transition temperature, T-g, of the PDLLA/CO2, foam collapse tended to occur during consolidation of facings. The PDLLA was therefore extrusion-compounded with poly(methyl methacrylate) (PMMA) in order to increase its T-g.That the thermal characteristics of the PLA could be modified by blending with PMMA was confirmed by differential scanning calorimetric measurements of T-g and the evolution of the corresponding alpha-transition from dynamic mechanical analysis over a wide range of compositions. The uptake of liquid CO2 during impregnation at 10 degrees C and 5 MPa was found to decrease with PMMA content, but the subsequent desorption rates were comparable with those obtained with unmodified PDLLA. It was consequently possible to adapt an impregnation and conditioning procedure developed previously for PDLLA to give solid granular precursors from PDLLA-50 wt% PMMA blends that were stable during handling at room T, but contained sufficient CO2 for the generation of low density foams at T in the vicinity of 100 degrees C, as required by the particle-board process, in which water vapour is the primary vector for heat transport.The free expansion behaviour of the precursors was investigated systematically as a function of T, in order to establish process windows and investigate process-structure- property relationships. Model foam-core particleboard sandwich structures were then successfully prepared using an open hydraulic press and a single thermo-mechanical cycle, in which consolidation of the facings using a conventional binder and foam expansion took place simultaneously.

Solvent cast and compression molded composite films based on poly(vinylidene fluoride-trifluoroethylene), containing pristine and silylated BaTiO3 particles, were poled, and their electromechanical response (d33), their dielectric properties, and the crystalline structure of the embedded BaTiO3 particles, probed by X-ray diffraction, were recorded during one week. Their d33 was also measured after long-term aging (> 3 years). During this time the d33 decreased for all composites. The d33 of solvent cast composites was higher when surface modified particles were used, both immediately after poling and after aging, while for compression molded composites only slight differences in the initial and aged d33 of materials containing pristine versus modified particles were found. However the decay rate of d33 in the short time after poling was highly affected by the surface modification of the particles for both the solvent cast and the compression molded composites. The results suggest that other phenomena besides the polarization of the ceramic particles and of the polymer matrix contribute to the measured electromechanical response of the composites. Charge accumulation and differences in the charge distribution due to the presence of the silane layer on the surface of BaTiO3 may play an important role.

In a current procedure for periodontal tissue regeneration, enamel matrix derivative (EMD), which is the active component, is mixed with a propylene glycol alginate (PGA) gel carrier and applied directly to the periodontal defect. Exposure of EMD to physiological conditions then causes it to precipitate. However, environmental changes during manufacture and storage may result in modifications to the conformation of the EMD proteins, and eventually premature phase separation of the gel and a loss in therapeutic effectiveness. The present work relates to efforts to improve the stability of EMD-based formulations such as Emdogain™ through the incorporation of arginine, a well-known protein stabilizer, but one that to our knowledge has not so far been considered for this purpose. Representative EMD-buffer solutions with and without arginine were analyzed by 3D-dynamic light scattering, UV-Vis spectroscopy, transmission electron microscopy and Fourier transform infrared spectroscopy at different acidic pH and temperatures, T, in order to simulate the effect of pH variations and thermal stress during manufacture and storage. The results provided evidence that arginine may indeed stabilize EMD against irreversible aggregation with respect to variations in pH and T under these conditions. Moreover, stopped-flow transmittance measurements indicated arginine addition not to suppress precipitation of EMD from either the buffers or the PGA gel carrier when the pH was raised to 7, a fundamental requirement for dental applications.

From the experimental analysis of the photocuring process in terms of reaction kinetics as well as modulus and shrinkage build-up, the residual stresses arising during the photopolymerization of functionally graded composite coatings based on an acrylate matrix and Fe3O4@SiO2 core@shell nanoparticles are evaluated through a Finite Element Modeling approach. Owing to the monotonous variation of volume fraction of the constituent phases that influences the local conversion of the polymeric matrix, these coatings are able to decrease the residual stresseS at the coating/substrate interface by as much as approximate to 25% compared to those encountered in composites with homogeneous compositions, and by as much as approximate to 40% compared to those arising in the pure polymer. The influence of substrate stiffness, nanoparticle stiffness and conversion degree of the polymer matrix was also analyzed, providing further information for the optimization of the stress reduction mechanism in graded nanocomposite coatings. (C) 2015 Elsevier B.V. All rights reserved.

Solid heat-expandable foam precursors were prepared by impregnating melt-blended poly( d,l -lactide) (PDLLA)-poly(methyl methacrylate) (PMMA) blends with liquid carbon dioxide (CO 2 ). The phase behavior of these blends was strongly dependent on the processing steps, but impregnation with liquid CO 2 led to phase separation regardless of the prior thermomechanical history, and crystallization in blends containing a low-D grade of PDLLA suppressed subsequent expansion. On the other hand, blends containing nominally amorphous high-D PDLLA were found to be unstable with respect to expansion under ambient conditions when saturated with CO 2 . It was therefore necessary to reduce the overall CO 2 content by allowing it to desorb partially at 10 °C immediately after impregnation. Under these conditions, the amorphous PDLLA-50 wt% PMMA precursors were stable at ambient temperature and pressure, and showed peak expansion ratios at significantly higher temperatures than pure PDLLA, thanks to the increase in effective glass transition temperature with increasing PMMA content. It was hence demonstrated that blending with PMMA may provide a convenient means of tailoring the process window for heat-expandable polylactide foams, as well as improved heat stability.

The morphology of polymer/nanofibrillated cellulose (NFC) composite sheets produced using different techniques and its influence on low strain stiffness were assessed by optical and transmission electron microscopy. Solvent processing led to relatively homogeneous NFC dispersions and significant reinforcement of the in-plane Young's modulus. The continuous cellular networks obtained by wet comingling of polylactide powder or latex with NFC also provided efficient and essentially scale independent reinforcement, in spite of the extensive agglomeration of the NFC. However, the irreversible nature of these networks is incompatible with low pressure thermoplastic processing routes such as physical foaming, and while they may be broken up by e.g. extrusion, this led to substantial loss in reinforcement, particularly at temperatures above the glass transition temperature of the matrix, consistent with the observation of isolated low aspect ratio NFC aggregates in the extruded specimens.

Photopolymerized hydrogels are commonly used for a broad range of biomedical applications. As long as the polymer volume is accessible, gels can easily be hardened using light illumination. However, in clinics, especially for minimally invasive surgery, it becomes highly challenging to control photopolymerization. The ratios between polymerizationvolume and radiating-surface-area are several orders of magnitude higher than for ex-vivo settings. Also tissue scattering occurs and influences the reaction. We developed a Monte Carlo model for photopolymerization, which takes into account the solid/liquid phase changes, moving solid/liquid-boundaries and refraction on these boundaries as well as tissue scattering in arbitrarily designable tissue cavities. The model provides a tool to tailor both the light probe and the scattering/absorption properties of the photopolymer for applications such as medical implants or tissue replacements. Based on the simulations, we have previously shown that by adding scattering additives to the liquid monomer, the photopolymerized volume was considerably increased. In this study, we have used bovine intervertebral disc cavities, as a model for spinal degeneration, to study photopolymerization in-vitro. The cavity is created by enzyme digestion. Using a custom designed probe, hydrogels were injected and photopolymerized. Magnetic resonance imaging (MRI) and visual inspection tools were employed to investigate the successful photopolymerization outcomes. The results provide insights for the development of novel endoscopic light-scattering polymerization probes paving the way for a new generation of implantable hydrogels.

Micron- and submicron-sized barium titanate (BaTiO 3 ) particles, untreated and surface modified with aminopropyl triethoxy silane, were incorporated in poly(vinylidene fluoride–trifluoroethylene) to fabricate composites with up to 60 vol% of ceramic phase. The morphology and structure of solvent cast and compression-molded films, and their thermal, viscoelastic, and dielectric properties were investigated. When surface-modified BaTiO 3 was used, it was possible to decrease both the viscoelastic and the dielectric losses of highly filled solvent cast films, while their storage modulus and relative permittivity either increased or remained equal, owing to reduced porosity and improved matrix-filler compatibility. The effect of BaTiO 3 surface modification on the morphology of compression-molded films was less marked, leading to unchanged viscoelastic properties, and lower permittivity and dielectric losses. For all composites the frequency dependency of the dielectric properties at low frequencies was suppressed with modified BaTiO 3 .

ABSTRACTNanocomposites based on 10 to 60 vol % cellulose nanofibers (NFC) in a photopolymerizable hyperbranched acrylate matrix were prepared. Unmodified NFC and NFC chemically modified with a silane coupling agent and with ceric ammonium nitrate for direct polymer grafting from the cellulose surface were used. A homogeneous dispersion of NFC in the matrix was obtained in each case, leading to a marked improvement in oxygen barrier (up to nine times) and thermomechanical properties (storage modulus increased up to seven times). The mechanisms involved in the permeability reduction were investigated, revealing non‐monotonic trends in the evolution of the solubility and diffusion coefficients with NFC content. Most significantly, the inherent moisture sensitivity of the oxygen permeability of the NFC was found to be drastically reduced when it was dispersed in the polymer matrix, particularly after chemical modification, underlining the promise of the present approach for the production of robust, high barrier organic films. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40604.

The evolution of the morphology and degree of crystallinity was investigated postmortem in initially amorphous specimens of a commercial poly( dl -lactide) with a relatively low d -lactide content, after different immersion times in liquid CO 2 at 10 °C and 5 MPa. Relatively high concentrations of CO 2 induced a crystalline phase that remained stable at room temperature after desorption of the CO 2 , but was distinct from those generally associated with melt crystallization of polylactides (PLA), as demonstrated by transmission electron microscopy and wide-angle X-ray diffraction, consistent with previous observations. Crystallinity developed at the surface of the specimens within relatively short times compared with those necessary for the overall CO 2 content to reach saturation, resulting in a well-defined semicrystalline layer, whose thickness increased with immersion time. This behaviour was argued to be consistent with the existence of a well-defined diffusion front, associated with a step-like CO 2 concentration gradient that reflected a strong increase in the diffusivity of the CO 2 with the local CO 2 content. Crystallization led to a reduction in both the rate of CO 2 uptake and the CO 2 concentration at saturation compared with that observed for a poly( dl -lactide) with a significantly higher d -lactide content and little tendency to crystallize in the presence of liquid CO 2 . Assuming the CO 2 to be concentrated in the amorphous regions of semicrystalline PLA, a simple model for non-linear Fickian diffusion based on data from previous desorption measurements was used to show that diffusion through the semicrystalline surface layer should dominate impregnation kinetics in initially amorphous specimens that undergo rapid crystallization above a certain critical CO 2 concentration, consistent with the observed rates of CO 2 uptake.