On the Difference in Mechanical Behavior of Glass Bead-Filled Polyamide 12 Specimens Produced by Laser Sintering and Injection Molding.

An increasing demand for additively manufactured polymer composites with optimized mechanical properties is manifesting in different industries such as aerospace, biomedical, and automotive. Laser sintering (LS) is an additive manufacturing method that has the potential to produce reinforced polymers, which can meet the stringent requirements of these industries. For the development of a commercially viable LS nylon-based composite material, previous research studies worldwide have focused on adding glass beads to the powder material with the goal to produce fully dense parts with properties more representative of injection molded (IM) thermoplastic composites. This led to the development of a commercially available glass bead-filled polyamide 12 (PA12) powder. Although this powder has been on the market for quite a while, an in-depth comparison of the mechanical behavior of laser sintered versus IM glass bead-filled PA12 is lacking. In this study, laser-sintered glass bead-filled PA12 samples were built in different orientations and compared to IM counterparts. After sample production, the mechanical performance of the produced LS and IM parts was tested and compared to evaluate the quasistatic and dynamic mechanical performance and failure mechanisms at different load levels. In addition, the glass bead-filled PA12 properties were also compared to those of standard (unfilled) LS PA12 to assess whether glass beads actually improve the mechanical performance and fatigue lifetime of the final LS samples, as suggested in literature. Results in this work present and explain the increased stiffness but decreased fatigue life of glass bead-filled polyamide parts made by LS and IM. This research can be regarded as a "benchmark" study, in which samples produced from commercially available, filled and unfilled, PA12 powder grades are compared for both LS and conventional production techniques.

Upcycling Humins via Esterification Reactions of Hydroxyl Groups: From Functional Powders to PLA Foams and Compatibilized Blends.

The valorization of humins side streams from bio-refineries holds significant economic and sustainability potential. One plausible strategy involves using them as building blocks to create new materials. However, in their natural state, humins pose conceptual challenges due to their high viscosity, processing difficulties, and temperature sensitivity. This article presents a synthetic strategy for modifying humins properties to make them thermally stable and processable. Employing a sequence of esterification reactions and varying the reagent steric length, we showcase the selective transformation of humins into thermally-stable fine powders and low-viscosity liquids. We further extend this approach by reacting humins with polyesters such as polylactic acids and polycaprolactone. In particular, we detail a one-pot single-step synthesis of micro-phase separated compatibilized blends of polylactic acid and humins capped with the polylactic acid arms. Processed via solution-casting, the obtained materials behave as high-strength thermoplastic elastomers having uniform foam morphologies and material characteristics superior to the pure polylactic acid. By varying the content of D-enantiomers, we demonstrate an additional possibility to manipulate the cellular structures of the foams. Finally, we provide a solution to the product circularity by reporting a dissolution recycling method.

On the gelation of humins: from transient to covalent networks.

Humins are a by-product of many acid-catalyzed biorefinery processes converting polysaccharides into platform chemicals. The valorization of humin residue to increase the profit of biorefinery operations and reduce waste is a field that is growing interest as the production of humins continues to increase. This includes their valorization in materials science. For successful processing of humin-based materials, this study aims to understand the thermal polymerization mechanisms of humins from a rheological perspective. Thermal crosslinking of raw humins leads to an increase in their molecular weight, which in turn leads to the formation of a gel. Humin's gels structure combines physical (thermally reversible) and chemical (thermally irreversible) crosslinks, and temperature plays an essential role in the crosslink density and the gel properties. High temperatures delay the formation of a gel due to the scission of physicochemical interactions, drastically decreasing their viscosity, whereas upon cooling a stronger gel is formed combining the recovered physicochemical bonds and the newly created chemical crosslinks. Thus, a transition from a supramolecular network to a covalently crosslinked network is observed, and properties such as the elasticity or reprocessability of humin gels are influenced by the stage of polymerization.

Magnetic Self-Healing Composites: Synthesis and Applications.

Magnetic composites and self-healing materials have been drawing much attention in their respective fields of application. Magnetic fillers enable changes in the material properties of objects, in the shapes and structures of objects, and ultimately in the motion and actuation of objects in response to the application of an external field. Self-healing materials possess the ability to repair incurred damage and consequently recover the functional properties during healing. The combination of these two unique features results in important advances in both fields. First, the self-healing ability enables the recovery of the magnetic properties of magnetic composites and structures to extend their service lifetimes in applications such as robotics and biomedicine. Second, magnetic (nano)particles offer many opportunities to improve the healing performance of the resulting self-healing magnetic composites. Magnetic fillers are used for the remote activation of thermal healing through inductive heating and for the closure of large damage by applying an alternating or constant external magnetic field, respectively. Furthermore, hard magnetic particles can be used to permanently magnetize self-healing composites to autonomously re-join severed parts. This paper reviews the synthesis, processing and manufacturing of magnetic self-healing composites for applications in health, robotic actuation, flexible electronics, and many more.

Humins Blending in Thermoreversible Diels-Alder Networks for Stiffness Tuning and Enhanced Healing Performance for Soft Robotics.

Humins waste valorization is considered to be an essential pathway to improve the economic viability of many biorefinery processes and further promote their circularity by avoiding waste formation. In this research, the incorporation of humins in a Diels-Alder (DA) polymer network based on furan-maleimide thermoreversible crosslinks was studied. A considerable enhancement of the healing efficiency was observed by just healing for 1 h at 60 °C at the expense of a reduction of the material mechanical properties, while the unfilled material showed no healing under the same conditions. Nevertheless, the thermal healing step favored the irreversible humins polycondensation, thus strengthening the material while keeping the enhanced healing performance. Our hypothesis states a synergistic healing mechanism based on humins flowing throughout the damage, followed by thermal humins crosslinking during the healing trigger, together with DA thermoreversible bonds recombination. A multi-material soft robotic gripper was manufactured out of the proposed material, showing not only improved recovery of the functional performance upon healing but also stiffness-tunable features by means of humins thermal crosslinking. For the first time, both damage healing and zone reinforcement for further damage prevention are achieved in a single intrinsic self-healing system.

The Extent of Interlayer Bond Strength during Fused Filament Fabrication of Nylon Copolymers: An Interplay between Thermal History and Crystalline Morphology.

One of the main drawbacks of Fused Filament Fabrication is the often-inadequate mechanical performance of printed parts due to a lack of sufficient interlayer bonding between successively deposited layers. The phenomenon of interlayer bonding becomes especially complex for semi-crystalline polymers, as, besides the extremely non-isothermal temperature history experienced by the extruded layers, the ongoing crystallization process will greatly complicate its analysis. This work attempts to elucidate a possible relation between the degree of crystallinity attained during printing by mimicking the experienced thermal history with Fast Scanning Chip Calorimetry, the extent of interlayer bonding by performing trouser tear fracture tests on printed specimens, and the resulting crystalline morphology at the weld interface through visualization with polarized light microscopy. Different printing conditions are defined, which all vary in terms of processing parameters or feedstock molecular weight. The concept of an equivalent isothermal weld time is utilized to validate whether an amorphous healing theory is capable of explaining the observed trends in weld strength. Interlayer bond strength was found to be positively impacted by an increased liquefier temperature and reduced feedstock molecular weight as predicted by the weld time. An increase in liquefier temperature of 40 °C brings about a tear energy value that is three to four times higher. The print speed was found to have a negligible effect. An elevated build plate temperature will lead to an increased degree of crystallinity, generally resulting in about a 1.5 times larger crystalline fraction compared to when printing occurs at a lower build plate temperature, as well as larger spherulites attained during printing, as it allows crystallization to occur at higher temperatures. Due to slower crystal growth, a lower tie chain density in the amorphous interlamellar regions is believed to be created, which will negatively impact interlayer bond strength.

Boosting PLA melt strength by controlling the chirality of co-monomer incorporation.

Bio-based and degradable polymers such as poly(lactic acid) (PLA) have become prominent. In spite of encouraging features, PLA has a low melt strength and melt elasticity, resulting in processing and application limitations that diminish its substitution potential vis-a-vis classic plastics. Here, we demonstrate a large increase in zero shear viscosity, melt elasticity, elongational viscosity and melt strength by random co-polymerization of lactide with small amounts, viz. 0.4-10 mol%, of diethylglycolide of opposite chiral nature. These enantiomerically pure monomers can be synthesized using one-step zeolite catalysis. Screening of the ester linkages in the final PLA chains by the ethyl side groups is suggested to create an expanding effect on the polymer coils in molten state by weakening of chain-chain interactions. This effect is suspected to increase the radius of gyration, enabling more chain entanglements and consequently increasing the melt strength. A stronger melt could enable access to more cost-competitive and sustainable PLA-based biomaterials with a broader application window. Amongst others, blow molding of bottles, film blowing, fiber spinning and foaming could be facilitated by PLA materials exhibiting a higher melt strength.

Viscoelastic cluster densification in sheared colloidal gels.

Many biological materials, consumer products and industrial formulations are colloidal suspensions where the suspending medium is itself a complex fluid, and such suspensions are effectively soft matter composites. At rest, the distortion of the microstructure in the suspending fluid by the particles leads to attractive interactions between them. During flow, the presence of a microstructure in the viscoelastic suspending medium changes the hydrodynamic forces due to the non-Newtonian and viscoelastic effects. However, little is known about the structural development, the rheology and the final properties of such materials. In the present study, a model flocculated suspension in both a Newtonian and a viscoelastic medium was studied by combined rheological and rheo-confocal methods. To this extent, micrometer-sized fluorescent PMMA particles were dispersed in polymeric matrices (PDMS). The effect of fluid viscoelasticity is studied by comparing the results for a linear and a branched polymer. Stress jump experiments on the suspensions were used to de-convolute the rate dependence of the viscous and elastic stress contributions in both systems. These results were compared to a qualitative and quantitative analysis of the microstructure during flow as studied by fast structured illumination confocal microscopy, using a counter-rotating rheometer. At comparable interaction strength, as quantified by equal Bingham numbers, the presence of medium viscoelasticity leads to an enhanced densification of the aggregates during steady-state flow, which is reflected in lower limiting high shear viscosities. Following a strong preshear, the structural and mechanical recovery is also altered between the Newtonian and viscoelastic matrix with an increase in the percolation threshold, but with the potential to build stronger materials exploiting the combination of processing history and medium rheology at higher volume fractions.

Stress Contributions in Colloidal Suspensions: The Smooth, the Rough, and the Hairy.

The collective properties of colloidal suspensions, including their rheology, reflect an interplay between colloidal and hydrodynamic forces. The surface characteristics of the particles play a crucial role, in particular, for applications in which interparticle distances become small, i.e., at high concentrations or in aggregates. In this Letter, we directly investigate this interplay via the linear viscoelastic response of the suspensions in the high-frequency regime, using particles with controlled surface topographies, ranging from smooth to hairy and rough particles. We focus directly on the stresses at the particle level and reveal a strong impact of the surface topography on the short-range interactions, both dissipative and elastic. As the particle topography becomes more complex, the local stresses depend on how the topography is generated. The findings in this Letter, in particular, show how changes in topography can both screen or enhance the dissipation, which can be used to engineer the properties of dense or aggregated suspensions.

Covalent organic frameworks for membrane separation.

Covalent organic frameworks (COFs), which are constructed from organic linkers, are a new class of crystalline porous materials comprising periodically extended and covalently bound network structures. The intrinsic structures and the tailorable organic linkers endow COFs with a low density, large surface area, tunable pore size and structure, and facilely-tailored functionality, attracting increasing interests in different fields including membrane separations. Exciting research activities ranging from fabrication strategies to separation applications of COF-based membranes have appeared. This review analyzes the synthesis and applications of diverse continuous/discontinuous COF membranes, such as COF-based mixed matrix membranes (MMMs), COF-based thin film nanocomposite (TFN) membranes, and free-standing COF films. Special attention was given to pore size, stability, hydrophilicity/hydrophobicity and surface charge of COFs in view of determining proper COFs for membrane fabrication, along with the approaches to fabricate COF-based membranes, such as blending, in situ growth, layer-by-layer stacking and interfacial polymerization (IP). Moreover, applications of COF-based membranes in gas separation, water treatment (deaslination and dye removal), organic solvent nanofiltration (OSN), pervaporation and fuel cell are disscussed. Finally, we illustrate the advantages and disadvantages of COF-based membranes through a comparison with MOF-based membranes, and the remaining challenges and future opportunities in this field.

Quantifying the dispersion quality of partially aggregated colloidal dispersions by high frequency rheology.

An important parameter for the performance of nanomaterials is the degree by which the nanoparticles are dispersed in a matrix. Optical microscopy or scattering methods are useful to characterise the state of dispersion, but are not generally applicable to all materials. Electron microscopy methods are laborious in preparation and typically offer only quantitative information on a very local scale. In the present work we investigate how high frequency rheological measurements can be used for partially dispersed suspensions at intermediate to higher particle loadings, even for high viscous matrices. Although the contribution of the particles is particularly visible in the low frequency linear viscoelastic behaviour, a more direct relationship between rheological properties and degree of dispersion can be derived from the loss modulus in the high frequency limit. To this end, a home-built piezo shear rheometer is constructed to extend the frequency range typically accessible by commercial rotational rheometers. Measurements on spherical silica particles, with a varying degree of dispersion in low molecular weight PDMS, are used to demonstrate how high frequency rheometry can be used to quantify dispersion quality. The linear viscoelastic properties are compared to analytical scaling theories to demonstrate that a hydrodynamically dominated regime is reached. The dependence of the relative high frequency loss modulus on volume fraction is then compared to predictions of a hydrodynamic viscosity model for the derivation of a dispersion quality index. It is used to follow the evolution of the dispersion quality as a function of mixing time and consumed power.

Poly(alanine): Structure and Stability of the D and L-Enantiomers.

High-performance, biobased materials can potentially be manufactured from polymerized α-amino acids (α-polypeptides). This paper reports on the synthesis, structure, and properties of both polyalanine enantiomers (PLAla and PDAla). The molecular structure of the polypeptide chains, their molecular weight, and polydispersity were investigated by (1)H NMR, MALDI-TOF, and size-exclusion chromatography. The secondary structure and crystalline order were probed via Fourier transform infrared spectroscopy, circular dichroism, and (synchrotron) wide-angle X-ray diffraction. The phase behavior and thermal stability were assessed by differential scanning calorimetry and thermogravimetric analysis. The kinetically trapped PAla chain conformation in the solid state, after synthesis or solvent treatments, is the α-helical shape. Upon heating, crystals from the α-helices convert into more stable crystals from ß-sheets at a temperature higher than 210 °C. This temperature is close to where polymer degradation sets in. The ß-sheet crystals combine melting with thermal degradation at temperatures above 330 °C. In the presence of superheated water, the conversion from α-helices to ß-sheets happens at lower temperatures, allowing for a conversion without degradation.

Simultaneous Synchrotron WAXD and Fast Scanning (Chip) Calorimetry: On the (Isothermal) Crystallization of HDPE and PA11 at High Supercoolings and Cooling Rates up to 200 °C s(-1).

An experimental setup, making use of a Flash DSC 1 prototype, is presented in which materials can be studied simultaneously by fast scanning calorimetry (FSC) and synchrotron wide angle X-ray diffraction (WAXD). Accumulation of multiple, identical measurements results in high quality, millisecond WAXD patterns. Patterns at every degree during the crystallization and melting of high density polyethylene at FSC typical scanning rates from 20 up to 200 °C s(-1) are discussed in terms of the temperature and scanning rate dependent material crystallinities and crystal densities. Interestingly, the combined approach reveals FSC thermal lag issues, for which can be corrected. For polyamide 11, isothermal solidification at high supercooling yields a mesomorphic phase in less than a second, whereas at very low supercooling crystals are obtained. At intermediate supercooling, mixtures of mesomorphic and crystalline material are generated at a ratio proportional to the supercooling. This ratio is constant over the isothermal solidification time.

Effect of confinement on droplet breakup in sheared emulsions.

The breakup of Newtonian droplets in a Newtonian matrix during shear flow is investigated in a counterrotating parallel plate device. For bulk conditions, the critical capillary number for breakup is known to be only determined by the viscosity ratio. Here, we show that the critical capillary number is also affected by the degree of confinement: for low viscosity ratios, confinement suppresses breakup, whereas for high viscosity ratios, confinement promotes breakup. This way, above a critical value for the degree of confinement, even droplets with a viscosity ratio larger than 4, which are unbreakable by shear in a bulk situation, can be broken in a simple shear flow field.

Structure development in confined polymer blends: steady-state shear flow and relaxation.

In this work, the structure development in immiscible polymer blends in confined geometries is systematically investigated. Poly(dimethylsiloxane)/poly(isobutylene) blends with a droplet-matrix structure are subjected to simple shear flows. The confined environment is created by using a Linkam shearing cell in which the gap is systematically decreased to investigate the transition from "bulk" behavior toward "confined" behavior. Small-angle light scattering experiments in a confinement, which have not yet been reported in the literature, and also microscopy are used to observe the morphology development during steady-state shearing and relaxation. These experiments indicate that the size and relaxation of single droplets in a confined environment are still governed by the relations that describe the structure development in bulk situations. Yet, depending on the applied shear rates and blend concentrations, the droplets organize in superstructures such as pearl necklaces or extended superstrings in a single layer between the plates. These structures are stable under flow. To observe a single layer, a critical ratio of droplet size to gap spacing is required, but this ratio is clearly below the one already reported in the literature.

Direct evidence for breakup of liquid fibrils via Rayleigh instabilities in model polymer blends in step-up experiments.

Droplet breakup in immiscible polymer blends after a sudden substantial increase in shear rate is studied using small-angle light scattering experiments. During the breakup of the fibrils, secondary streaks are visible in the scattering pattern, which indicate the occurrence of Rayleigh instabilities. A high-resolution camera allows quantitative determination of the evolution of the wavelength of the disturbances during the disintegration process. It is shown that this process is time-controlled rather than strain-controlled as suggested by theory.