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3D printing technologies have matured to produce complex structures, still they are often limited to static materials. Introducing alkoxyamine bonds into 3D printed structures offers unprecedented possibilities for post‐synthetic modification through nitroxide exchange reaction and nitroxide‐mediated polymerization. This study provides a comprehensive molecular and macroscopic characterization of 3D‐printed alkoxyamine‐containing dynamic covalent adaptable networks. The study provides new insights into their dynamic structural and mechanical alterations, making them promising candidates for advanced applications ranging from biomedical engineering to flexible electronics. [ABSTRACT FROM AUTHOR]

The present study highlights a range of surface and volume extrudate patterns that can be detected during the extrusion flow of long‐chain branched polymers. Thus, four linear low‐density polyethylenes (LDPEs) have been extruded using a single‐screw extruder coupled to an inline optical imaging system. The selected LDPEs are selected to outline the influence of molecular weight and long‐chain branching on the types of melt flow extrusion instabilities (MFEI). Through the inline imaging system, space–time diagrams are constructed and analyzed via Fourier‐transformation using a custom moving window procedure. Based on the number of characteristic frequencies, peak broadness, and whether they are surface or volume distortions, three main MFEI types, distinct from those typically observed in linear and short‐chain branched polymers, are identified. The higher molecular weight, low long‐chain branching LDPEs exhibited all three instability types, including a special type volume instability. Independently of the molecular weight, higher long‐chain branching appeared to have a stabilizing effect on the transition sequences by suppressing volume extrudate distortions or limiting surface patters to a form of weak intensity type. [ABSTRACT FROM AUTHOR]

The extrusion flow instabilities of commercial polybutadiene (PBD) are investigated as a function of the different extrusion die geometries, such as round capillary, slit, and complex cross‐section profile slit dies via capillary rheology. Qualitative models are used to fit the experimental data for the spatial characteristic wavelength (λ) of the appearing extrusion flow instabilities. A new qualitative model for the slit die geometry, rectangular cross‐section, is derived based on the theoretical concept of the "two layers" extrudate and the force balance at the die exit region. The proposed qualitative model for the slit die geometry is used to predict the spatial characteristic wavelength (λ) for extrudates obtained by complex cross‐section profile slit die geometries similar to industrial manufacturing. Correlation between the ratio of the extensional (Ys) and shear (σx) stress at the die exit area and the characteristic dimension, height H for slit dies and diameter D for round capillary dies, is presented. Moreover, a geometry‐dependent model is used to predict the spatial characteristic wavelength (λ) of the extrusion flow instabilities from a round capillary die to a slit die and vice versa. [ABSTRACT FROM AUTHOR]

The fatigue behavior of 28 amorphous and semi‐crystalline thermoplastic polymers and elastomers is tested under strain controlled sinusoidal tension‐tension (TT) and torsion (T) at room temperature and analyzed via strain‐life (total strain amplitude versus fatigue lifetime) and crack propagation rate versus total strain amplitude curves, analogous to Paris' law. Investigating fatigue is extremely time‐ and resources consuming, so universal relationships between the exponents of the strain‐life power‐law and material properties are of high importance. For a brittle failure mechanism (I), the strain‐life curves are found to have fixed exponents of BTT,I = −0.27 and BT,I = −0.22, respectively, while the crack propagation versus strain amplitude in TT has an exponent of mda/dN,I = 4. For ductile failure (II), fixed strain‐life curve exponents in TT of BTT,II = −0.35 and in torsion of BT,II = −0.48 with mda/dN,II = 2.8 are obtained. In torsion, most semi‐crystalline polymers show brittle and ductile failure depending on the applied strain amplitude, so the strain‐life curve exponent changes accordingly. The universal exponents for strain‐life and crack growth‐strain amplitude curves offer a significant simplification to rapidly estimate, predict, and simulate the fatigue behavior of polymers. [ABSTRACT FROM AUTHOR]

The extrusion flow instabilities of two commercial styrene‐butadiene rubbers are investigated as they vary in isomer content (1,4‐cis, 1,4‐trans, and 1,2 conformation) of the butadiene monomer and the molecular architecture (linear, branched). The investigated samples have similar multimodal molecular weight distribution. Two geometries of extrusion dies, slit and round capillary, are compared in terms of the type and the spatial characteristics of the flow instabilities. The latter are quantified using three methods: a highly pressure sensitive slit die, online and offline optical analysis. The highly pressure‐sensitive slit die has three piezoelectric pressure transducers (Δt ≈ 10−3 s and Δp ≈ 10−5 bar) placed along the die length. The characteristic frequency (fChar.) of the flow instabilities follows a power law behavior as a function of shear rate to a 0.5 power for both materials, fChar.∝γ˙app.0.5. A qualitative model is used to predict the spatial characteristic wavelength (λ) of the flow instabilities from round capillary to slit dies and vice versa. Slip velocities (Vs) are used to quantify the slippage at slit and round capillary dies as well. [ABSTRACT FROM AUTHOR]

In this study, the ability of cationic poly(acrylamide‐co‐(3‐acrylamidopropyl)trimethylammonium chloride) hydrogels to desalinate seawater is explored, where the salt separation is based on the partial rejection of mobile salt ions by the fixed charges along the polymer backbone. Water absorbency measurements reveal that artificial seawater‐containing divalent ions (Mg2+, Ca2+, and SO42−) drastically decrease the swelling capacity of previously employed anionic poly(acrylic acid‐co‐sodium acrylate) hydrogels, whereas no influence on the swelling behavior of the synthesized cationic hydrogels is found. The swelling behavior and mechanical properties are studied by varying the degree of crosslinking and degree of ionization systematically in the range of 1–5 and 25–75 mol%, respectively. Finally, artificial seawater (csea = 0.171 mol L−1) is desalinated in a custom‐built press setup with an estimated efficiency of Em³ = 17.6 kWh m−³ by applying an external pressure on the swollen hydrogels. [ABSTRACT FROM AUTHOR]

Salinity gradients between seawater and river water is a renewable source of energy having a worldwide potential capacity of about 3.1 TW. This energy can be extracted by e.g., an osmotic engine, using hydrogels with high water uptake capacity. Consecutive exposing hydrogels to fresh and saline water makes swelling–shrinking cycles, which can be utilized to move a piston in an osmotic engine. The production of power with this method is significantly suppressed by gelblocking, where voids between particles are blocked so that the water flow is limited and the absorbency significantly retarded. To improve the power production, the gelblocking is minimized within this article by using spherical mono‐dispersed hydrogels made by microfluidic technique. In this study mono‐disperse poly(acrylic acid‐co‐sodium acrylate) hydrogels with varying diameters (100‐600 µm) and varying degrees of neutralization (DN = 10–75 mol%) are synthesized. In addition, hydrogels with different DN are utilized for additional surface crosslinking to fabricate core–shell particles. The maximum power of 0.67 W kg−1 is obtained for hydrogels with a diameter of 105 µm, degree of crosslinking (DC) = 1.7 mol%, DN = 75 mol%, and a core‐shell architecture, which is three times higher compared to hydrogels having undefined size without a core–shell framework. [ABSTRACT FROM AUTHOR]

Different carbon-based fillers such as carbon nanotubes (CNTs), graphite, and thermally reduced graphene oxide (TrGO) are melt mixed with an isotactic poly(propylene) (iPP) and the mechanical properties of the resulting composites in the solid and melt state are analyzed. The Young's modulus of composites is increased around 25% relative to the neat iPP at concentrations above 10 wt% of CNTs or graphite whereas composites with TrGO are increased around 40% at similar concentrations. These results are compared with theoretical models showing that the filler agglomeration and surface area are key parameters. The rheological results of the composites under oscillatory shear conditions at the melt state show that the viscous raw polymer melt experiences a solid-like transition at a threshold concentration that strongly depends on the filler used. This transition appears at 10 wt% for CNTs, 8 wt% for TrGO, and 40 wt% for graphite. The viscosity of iPP/TrGO composites is further increased by adding CNTs particles, although the Young's modulus does not increase. [ABSTRACT FROM AUTHOR]

Rubber materials filled with reinforcing fillers display nonlinear rheological behavior at small strain amplitudes below γ0 < 0.1. Nevertheless, rheological data are analyzed mostly in terms of linear parameters, such as shear moduli ( G′, G″), which loose their physical meaning in the nonlinear regime. In this work styrene butadiene rubber filled with carbon black (CB) under large amplitude oscillatory shear (LAOS) is analyzed in terms of the nonlinear parameter I3/1. Three different CB grades are used and the filler load is varied between 0 and 70 phr. It is found that I3/1(φ) is most sensitive to changes of the total accessible filler surface area at low strain amplitudes (γ0 = 0.32). The addition of up to 70 phr CB leads to an increase of I3/1(φ) by a factor of more than ten. The influence of the measurement temperature on I3/1 is pronounced for CB levels above the percolation threshold. [ABSTRACT FROM AUTHOR]

This publication is concerned with the development of a novel high sensitivity instability detection die for a laboratory size extruder. The high sensitivity system consists of piezoelectric transducers placed along the die length to monitor local pressure fluctuations, having temporal and pressure resolution of up to Δ t ≈ 10−3 s and Δ p ≈ 10−5 bar, at a nominal pressure of 500 bar. The system is tested on short chain branched polyethylenes, namely on the detection and characterization of the sharkskin instability. It is shown that the determined instabilities are related to characteristic peaks in the corresponding in situ pressure Fourier-transform spectra and that other peaks are caused by different, but defined, sources. The in situ results obtained are validated through optical image analysis of the instabilities. [ABSTRACT FROM AUTHOR]

A combined system for solidification and instability detection during the forced extrusion of polymers is constructed and first investigations are presented. The system comprises a home built Halbach-type NMR magnet, monitoring the NMR relaxation at different positions along the extrudate length, and a highly sensitive transducer in the die recording mechanical pressure fluctuations. The system is complemented by an optical extrudate visualization setup. First results are shown for low density and high density polyethylene (LDPE and HDPE), which have different branching structures. Three relevant processing parameters were varied during the investigations, i.e., extruder rotation speed, draw ratio and extrusion temperature. Overall, the characteristics of NMR relaxation show a clear dependence on the magnet position along the extrudate length, which can be related to the cooling, onset and propagation of crystallization, and solidification of the polymer melt. The coupling of a low-field NMR with a high sensitivity instability detection system shows the potential of creating a versatile tool for a unique online control method in polymer processing. In the future, this combination could provide insight into the influence of branching structures on polymer processing of polyethylene, for example. [ABSTRACT FROM AUTHOR]

Front Cover: In article number 1800586, Manfred Wilhelm and co‐workers present studies on the molecular dynamics, morphology, and flow behavior of crystallizing polymers. Material properties are correlated on broad length and time scales by using hyphenations of rheology and in‐situ nuclear magnetic resonance, small angle X‐ray scattering, and optical microscopy. [ABSTRACT FROM AUTHOR]