Anisotropic ternary composites comprising poly(methy-methacrylate) (PMMA), carbon black (CB), and carbon nanotubes (CNTs) were extruded using a capillary rheometer and the electrical conductivities ...of the composites were measured and presented in a detailed contour plot covering a large range of filler fractions (up to 30 vol% CNTs, 20 vol% CB). A recent generic conductivity model for ternary composites was successfully validated using the conductivity measurements. When analyzing the conductivity measurements using four traditional definitions of 'synergy' between two conductive fillers, no clear synergetic effect was observed between CB and CNT. Also, when all the conductivity data for ternary CNT/CB composites from the existing literature was carefully gathered and analyzed, the number of confirmed occurrences of strong and convincing CNT/CB synergies was surprisingly low. Finally, a novel definition of synergy based on the physical aspect, in particular, its maximum, the 'synergasm', was defined in order to obtain a more precise instrument for revealing regions of potential synergy.
In this study, poly(methyl methacrylate) (PMMA)/graphene nanoplatelets (GNPs) conductive composite films with different morphologies were fabricated from the same constituent materials using four ...fabrication techniques, solution casting (SC), SC followed by hot pressing (SCP), melt mixing followed by SC (MSC), and melt mixing followed by hot pressing (MP). Morphologies of dispersed GNPs and electrical properties in both in‐plane and perpendicular direction were investigated and compared systematically. The corresponding percolation thresholds (Φc) of the composites varied from 0.42 ± 0.13 vol% to 3.26 ± 0.48 vol%. The conductivities varied up to two orders of magnitude and decreased in the sequence of SC > MSC > SCP > MP. These variations were explained in terms of GNPs size, GNPs orientation, distribution and dispersion state of fillers. The contribution of the above factors in each procedure were discerned individually, the results were discussed and compared with other experimental studies and simulations as well.
In this letter, both the viscoelastic and electrical properties of poly(methyl methacrylate)/carbon black (PMMA/CB) nanocomposites with different CB concentrations prior to and after annealing were ...investigated. The linear steady-state recoverable compliance of pure PMMA is independent of annealing as it reflects only polymer-polymer interactions. While the conductivity of PMMA/CB nanocomposites increases gradually with annealing, and the viscoelastic properties show different regions of response depending on the CB concentration. At CB contents below the percolation threshold the recoverable compliances increase after annealing, whereas the reverse trend is found at CB contents above the percolation threshold. This behavior is related to the interplay between network formation and particle-particle interactions.
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•The linear steady-state recoverable compliance of pure PMMA is constant.•Recoverable compliance of the composite with low content increases after annealing.•At high content, recoverable compliance decreases after annealing.
The main challenge in extrusion‐based bioprinting is to develop inks which must comprise a manifold of characteristics before, during, and after printing. To tackle the challenge of good shape ...fidelity and printability of low concentration inks, in this study hydrogel microparticles (HMPs) are proposed to produce internally pre‐crosslinked inks. Alginate (Alg) and oxidized alginate (OA)‐based HMPs are fabricated and used as Ca2+‐releasing reservoirs. OA HMPs are used to demonstrate the versatility of this approach and to show its suitability also for chemically modified alginate. Embedded in either fresh Alg or OA solution, HMPs are used to pre‐crosslink the inks. Rheological measurements revealed that HMP pre‐crosslinking increases the yield stress and viscosity while reducing the loss angle of bioinks. Moreover, printing experiments reveal that being able to tailor rheological properties is an effective tool to improve printability. Furthermore, pre‐crosslinking significantly alters the hydogel internal microstructure. In vitro studies show that NIH/3T3 cells proliferate in HMP pre‐crosslinked bioinks modified with gelatin. Altogether, a low‐cost and easy to use setup to prepare HMPs is presented and for the first time, the possibility of using such HMPs as pre‐crosslinking agent to tailor the printability of alginate‐based bioinks is demonstrated.
Hydrogel microparticles (HMPs) are used to produce internally pre‐crosslinked alginate‐based bioinks for 3D bioprinting. Alginate and oxidized alginate‐based HMPs are fabricated and used as Ca2+‐releasing reservoirs. Printing experiments reveal improved printability. The approach is based on low‐cost, easy to prepare HMPs demonstrating the possibility of using HMPs as pre‐crosslinking agent to tailor the printability of alginate‐based bioinks.
Poly(methyl methacrylate) (PMMA) composites containing carbon fibers (CFs) with the controlled aspect ratio (AR) at various concentrations were produced and hot pressed into rectangular sheets with a ...variety of thickness. The orientation of CFs on the surface and inside the specimen was obtained, and the conductivity σ in three different directions (X‐length, Y‐width, Z‐thickness) were investigated. A preferential orientation of CFs in XY plane was found inside the samples, resulting in higher σX and σY. The orientation of CFs in Z direction increases with the sample thickness increasing, leading to an increasing σZ. Thus the anisotropic conductivity between σX (or σY) and σZ reduces from three orders of magnitude to one order of magnitude. The percolation thresholds (Φc) in three directions of the samples were revealed by fitting with McLachlan GEM equation, which lead to Φc,X ≈ Φc,Y ≈ Φc,Z, independent of the thickness and the measuring direction. The values of exponent t in GEM equation were found to be tX > tY > tZ, while tZ increases with the sample thickness. For the first time, the relationship between the exponent t and the sample thickness was revealed and discussed.
A simplified mathematical model was given based on the actual experimental data from a laboratory-scale poly (ethylene terephthalate) (PET) melt spinning. PET fibres were produced using three virgin ...and two recycled PET materials at two different processing temperatures of 270 °C and 280 °C. Filament emerging from a spinneret was drawn by high-velocity air moving through an aspirator, where six take-up pressures were used. It was proved that the model with only two adjustable parameters could effectively predict the variation of PET fibre diameter as a function of take-up pressure, and one of the parameters could be estimated using the zero shear viscosity of materials.
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•A simple model is proposed, effectively predicting the PET fibre diameter versus take-up pressure in melt spinning.•Based on huge amounts of experimental data from five PET materials at two processing temperatures and six take-up pressures.•Only two adjustable parameters in this model.•One parameter can be estimated using the zero shear viscosity of materials.
The conductivity and morphology of melt-spun PMMA/carbon fiber (CF) composites have been investigated and analyzed. Using a two-step melt mixing procedure, CF aspect ratios of 6.3, 9.2 and 12.0 were ...achieved. Fiber composites with different CFs concentrations were melt spun using a capillary rheometer, to induce CF orientation, and the morphologies of both the surface and the cross-section were observed and discussed. The conductivity of composite fibers at room temperature was compared with the McLachlan GEM equation and Balberg excluded volume theory. The percolation threshold as well as the relationship between the percolation behavior and the aspect ratio were also determined. Taking into consideration the filler orientation, the slope K = 3.4 in excluded volume theory was estimated for the first time.
The miscible amorphous/semi-crystalline polymer blends with shape memory potential has received increasing interests in recent years. In this work, the shape memory mechanism of the miscible ...amorphous PMMA/semi-crystalline PLA blends was investigated using thermo-mechanical and rheological approaches. With the incorporation of PMMA into PLA, a broad glass transition and increased glass transition temperature Tg were observed by differential scanning calorimetry (DSC) measurements. The broadening of glass transition is attributed to the local nanoscale heterogeneities in the miscible blend system which is related to the self-concentration of the components. The degree of molecular entanglement of the blends was derived based on the oscillatory rheological measurements, showing that the dissimilar chains are more likely to entangle with each other than the similar ones, and the entanglement density ve is enhanced with increased PMMA content up to 50% where a 100% recovery of the initial shape is yielded. After that, ve is reduced with the addition of PMMA. The mechanism underlying the shape memory property is further validated by performing the shape memory test on the PLA/PMMA blend films at respective Tg. It is evident that for the semi-crystalline blends (PLA rich), PLA crystallites and molecular entanglement provide physical cross-links for shape recording, while a negative effect of crystallinity on the shape recovery ratio is obtained due to the strain-induced crystallization and chain slippage between the crystalline and amorphous chains. In contrast, for the amorphous blends (PMMA rich), the shape recovery ratio shows a strong positive linear dependence on ve, and the entanglement network is regard as the most important factor in the shape memory performance.
Average entanglement molar mass Me, Me,PLA-PMMA and entanglement density of ve PLA/PMMA blends versus composition. Display omitted
•The shape memory mechanism of miscible amorphous/semicrystalline polymer blends is studied.•Entanglement network could be formed in the PLA/PMMA blends.•The dissimilar chains are more likely to entangle with each other than the similar ones.•Entanglement network and crystallites are responsible for the shape memory performance.
Highly oriented electrospun conductive nanofibrous biocomposites (CNBs) of polylactic acid (PLA) and polyaniline (PANi) are fabricated using electrospinning. At the percolation threshold (φc), the ...growth of continuous paths between PANi particles leads to a steep increase in the electrical conductivity of fibers, and the McLachlan equation is fitted to identify φc. Annealing generates additional conductive channels, which lead to higher conductivity for dynamic percolation. For the first time, dynamic percolation is investigated for revealing time-temperature superposition in oriented conductive nanofibrous biocomposites. The crystallinity (χc) displays a linear dependence on annealing temperature within the confined fiber of CNBs. The increase in crystallinity due to annealing also increases the Young’s modulus E of CNBs. The present study outlines a reliable approach to determining the conductivity and elasticity of nanofibers that are highly desirable for a wide range of biological tissue applications.
Jane Jacobs's famous book The Death and Life of Great American Cities (1961) has challenged the discipline of urban planning and led to a paradigm shift. Controversial in the 1960s, most of her ideas ...became generally accepted within a decade or so after publication, not only in North America but worldwide, as the articles in this volume demonstrate. Based on cross-disciplinary and transnational approaches, this book offers new insights into her complex and often contrarian way of thinking as well as analyses of her impact on urban planning theory and the consequences for planning practice. Now, more than 50 years after the initial publication, in a period of rapid globalisation and deregulated approaches in planning, new challenges arise. The contributions in this book argue that it is not possible simply to follow Jane Jacobs's ideas to the letter, but instead it is necessary to contextualize them, to look for relevant lessons for cities and planners, and critically to re-evaluate why and how some of her ideas might be updated. Bringing together an international team of scholars and writers, this volume develops conclusions based on new research as to how her work can be re-interpreted under different circumstances and utilized in the current debate about the proclaimed 'millennium of the city', the 21st century.
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