Electrosteric Control Of The Aggregation And Yielding Behavior Of Concentrated Portlandite Suspensions

Portlandite (calcium hydroxide: CH: Ca (OH)2) suspensions aggregate spontaneously and form percolated fractal aggregate networks when dispersed in water. Consequently, the viscosity and yield stress of portlandite suspensions diverge at low particle loadings, adversely affecting their processability. Even though polycarboxylate ether (PCE)-based comb polyelectrolytes are routinely used to alter the particle dispersion state, water demand, and rheology of similar suspensions (e.g., ordinary portland cement suspensions) that feature a high pH and high ionic strength, their use to control portlandite suspension rheology has not been elucidated. This study combines adsorption isotherms and rheological measurements to elucidate the role of PCE composition (i.e., charge density, side chain length, and grafting density) in controlling the extent of PCE adsorption, particle flocculation, suspension yield stress, and thermal response of portlandite suspensions. We show that longer sidechain PCEs are more effective in affecting suspension viscosity and yield stress, in spite of their lower adsorption saturation limit and fractional adsorption. The superior steric hindrance induced by the longer side chain PCEs results in better efficacy in mitigating particle aggregation even at low dosages. However, when dosed at optimal dosages (i.e., a dosage that induces a dynamically equilibrated dispersion state of particle aggregates), different PCE-dosed portlandite suspensions exhibit identical fractal structuring and rheological behavior regardless of the side chain length. Furthermore, it is shown that the unusual evolution of the rheological response of portlandite suspensions with temperature can be tailored by adjusting the PCE dosage. The ability of PCEs to modulate the rheology of aggregating charged particle suspensions can be generally extended to any colloidal suspension with a strong screening of repulsive electrostatic interactions

Ultrafast Stiffening of Concentrated Thermoresponsive Mineral Suspensions

Extrusion-based 3D printing with rapidly hardening polymeric materials is capable of building almost any conceivable structure. However, concrete, one of the most widely used materials for large-scale structural components, is generally based on inorganic binder materials like Portland cement. Unlike polymeric materials, a lack of precise control of the extent and rate of solidification of cement-based suspensions is a major issue that affects the ability to 3D-print geometrically complex structures. Here, we demonstrate a novel method for controllable-rapid solidification of concentrated mineral suspensions that contain a polymer binder system based on epoxy and thiol precursors as well as one or more mineral fillers like quartz and calcite. The thermally triggered epoxy-thiol condensation polymerization induces rapid stiffening of the hybrid suspensions (0.30 ≤ ϕ ≤ 0.60), at trigger temperatures ranging between 50 °C and 90 °C achieving average stiffening rates up to 400 Pa/s. The use of nucleophilic initiators such as 1-methylimidazole provides control over the activation temperature and curing rate, thereby helping to achieve an adjustable induction period and excellent thermal latency. By using multiple techniques, we provide guidelines to create designer compositions of mineral suspensions that utilize thermal triggers to achieve thermal latency and ultrafast stiffening – prerequisite attributes for 3D-manufacturing of topologically-optimized structural components

Investigations of Highly Concentrated Emulsions Incorporating Multi-walled Carbon Nanotubes

<p>The refining characteristics of highly concentrated water-in-oil (w/o) emulsions, wherein the dispersed phase constitutes greater than 90 wt% of the total emulsion, have been investigated. The dispersed phase of the emulsion comprises a supersaturated solution of inorganic salts, and the continuous phase consists of a mixture of an emulsifier in a blend of two oils. The development of microstructure at various stages of the emulsification process has been studied in detail and an empirical correlation between the characteristic droplet size and refining time has been proposed. As the refining time was increased, the Sauter mean diameter (d32) of the aqueous phase droplets decreased exponentially and the width of the droplet-size distribution reduced. The evolution of rheological characteristics of the emulsion during the refinement of the microstructure has also been investigated through different protocols of the dynamic and steady-state rheology. The increase in the refining time led to an increase in the elastic modulus, the yield stress and the viscosity of the emulsion. The network structure of the dispersed phase, the droplet-size distribution and the corresponding interdroplet interactions all govern the rheological characteristics of the final emulsion. The dependence of the elastic modulus and the yield stress on the characteristic droplet-size has also been discussed.<br>Multi-walled carbon nanotubes (MWCNTs) were incorporated into the oil phase of highly concentrated w/o emulsions with the aim of achieving ‘network-like’ structure of MWCNTs throughout the entire continuous phase of the emulsion, which can The refining characteristics of highly concentrated water-in-oil (w/o) emulsions, wherein the dispersed phase constitutes greater than 90 wt% of the total emulsion, have been investigated. The dispersed phase of the emulsion comprises a supersaturated solution of inorganic salts, and the continuous phase consists of a mixture of an emulsifier in a blend of two oils. The development of microstructure at various stages of the emulsification process has been studied in detail and an empirical correlation between the characteristic droplet size and refining time has been proposed. As the refining time was increased, the Sauter mean diameter (d32) of the aqueous phase droplets decreased exponentially and the width of the droplet-size distribution reduced. The evolution of rheological characteristics of the emulsion during the refinement of the microstructure has also been investigated through different protocols of the dynamic and steady-state rheology. The increase in the refining time led to an increase in the elastic modulus, the yield stress and the viscosity of the emulsion. The network structure of the dispersed phase, the droplet-size distribution and the corresponding interdroplet interactions all govern the rheological characteristics of the final emulsion. The dependence of the elastic modulus and the yield stress on the characteristic droplet-size has also been discussed.<br>Multi-walled carbon nanotubes (MWCNTs) were incorporated into the oil phase of highly concentrated w/o emulsions with the aim of achieving ‘network-like’ structure of MWCNTs throughout the entire continuous phase of the emulsion, which can ultimately modify the emulsion characteristics. By keeping the same aqueous-to-oil phase ratio, the amount of MWCNTs in the oil phase was systematically adjusted to investigate their effects on the refining characteristics, microstructure and rheology of the emulsion. The concentration of the MWCNTs in the emulsions for the investigation has been varied from 0.5 to 4 wt% of the oil phase of the emulsion; the corresponding concentration of the MWCNTs in the emulsion varied from 0.0325 to 0.26 wt% of the total emulsion. The refining characteristics of nanotube-incorporated emulsions have been investigated. The incorporation of MWCNTs led to a finer emulsion microstructure with reduced droplet size and narrowed droplet-size distribution. The decrease in droplet size with the addition of MWCNTs is mainly due to the increase in the viscosity of the oil phase which, in turn, results in an increased applied stress during emulsion refining. However, the state of dispersion of MWCNTs within the emulsion also plays a crucial role in determining the final microstructure of the nanotube-incorporated emulsions.<br>The state of dispersion of MWCNTs in the emulsions was investigated through cryo-FEG-SEM analysis, however, from the fractured surface morphology, it was hard to unequivocally conclude the selective dispersion of MWCNTs in the continuous phase of the emulsion. Rheological properties of nanotube-incorporated were characterised as a function of the emulsification time, as well as the MWCNTs concentration. The rheological behaviour of the nanotube-incorporated emulsions was identical to that of the neat emulsions, and primarily governed by the droplet drop size and droplet-size distribution. However, the strain behaviour, especially the yield strain and crossover stain are independent droplet size of the droplet size and the polydispersity of the emulsion. Emulsions that have smaller droplets exhibited higher storage modulus (G^'), yield stress (τ_Y) and apparent viscosity (η). For all the studied refining times, nanotube-incorporated emulsions have higher G^', τ_Y, and η values when compared to the neat emulsion, and these values further increased with the MWCNTs concentration. This is primarily due to the decrease in droplet size with the addition of MWCNTs. Furthermore, our findings suggest that the incorporated MWCNTs did not induce any significant changes in the rheological behaviour of emulsions with identical droplet sizes and it remained essentially unchanged with the MWCNTs concentration. However, the nanotube-incorporated emulsions possessed the solid-like behaviour up to a higher applied stress when compared to the neat emulsion of identical droplet size.<br>Two tetra-alkylated pyrenes have been designed and synthesized for the noncovalent surface modification of MWCNTs, namely, 1,3,6,8-tetra(oct-1-yn-1-yl)pyrene (TOPy) and 1,3,6,8-tetra(dodec-1-yn-1-yl)pyrene (TDPy). The modifier molecules were designed in such a way that they could facilitate better dispersion of individualised MWCNTs in the continuous phase of the emulsion. Moreover, the adsorbed modifiers facilitate the MWCNTs, which are incorporated in the emulsions, to be localised in the continuous phase of the emulsion through the interaction between oil and the alkyl chains of the modifiers. Scanning electron microscopic and transmission electron microscopic analyses suggested that the modifier molecules have been adsorbed on the MWCNT surface, which subsequently resulted in the ‘debundling’ of MWCNT ‘agglomerates.' The red-shift in the C‒H wagging vibrational bands in the FTIR spectroscopy and the G-band shift in Raman spectroscopic analysis for the modified MWCNTs, and the fluorescence quenching of the alkylated pyrene molecule in the presence of the MWCNTs, have confirmed the π–π interaction between the modifier molecules and MWCNTs.<br>The modified MWCNTs were then incorporated into highly concentrated water-in-oil emulsions, and the effect of the noncovalent surface modification on the emulsion morphology was investigated. The concentration of modified MWCNTs was varied between 0.25 ‒ 2 wt% of the oil phase of the emulsion while maintaining the identical droplet size. In the modified MWCNT-incorporated emulsion, there was a significant reduction in the average agglomerate size and the area ratio of the remaining MWCNT agglomerates in the emulsion matrix when compared to the corresponding emulsions that comprise unmodified MWCNTs.<br>The dispersion and localisation of modified and unmodified MWCNTs in the oil phase was assessed through the electrical conductivity measurements. For the MWCNT‒oil blend dispersions, there was a significant improvement in the electrical conductivity (an increase of the order of ~106 in the DC electrical conductivity with 1 wt% MWCNTs). Emulsions with 1 wt% and 2 wt% MWCNTs exhibited a low DC electrical conductivity as opposed to the purely insulating behaviour of the neat emulsion. This change could be an indication of the change in emulsion morphology due to the presence of incorporated MWCNTs. However, the enhancement in the electrical conductivity of the emulsions was very low when compared to the enhancement in oil blend with the addition of MWCNTs. The electrical conductivity measurements of the emulsions did not suggest the formation of a complete and effective percolation network up to an MWCNT content of 2 wt% of the oil phase.<br>In the present study, the first of its kind, an attempt has been made to investigate the effect of incorporation of MWCNTs into the highly concentrated w/o emulsions. A significant level of understanding has been gleaned about the effect of MWCNT incorporation on the morphology and rheology of the HCEs through different microscopic techniques, rheological analysis and various spectroscopic analyses.</p

Effect of Organic Modification on Multiwalled Carbon Nanotube Dispersions in Highly Concentrated Emulsions

Highly concentrated water-in-oil emulsions incorporating multiwalled carbon nanotubes (MWCNTs) are prepared. Homogeneous and selective dispersions of MWCNTs throughout the oil phase of the emulsions are investigated. The practical insolubility of carbon nanotubes (CNTs) in aqueous and organic media necessitates the disentanglement of CNT "agglomerates" through the utilization of functionalized CNTs. The design and synthesis of two tetra-alkylated pyrene derivatives, namely, 1,3,6,8-tetra(oct-1-yn-1-yl)pyrene (TOPy) and 1,3,6,8-tetra(dodec-1-yn-1-yl)pyrene (TDPy), for the noncovalent organic modification of MWCNTs are reported. The modifier molecules are designed in such a manner that they facilitate an improved dispersion of individualized MWCNTs in the continuous-oil phase of the highly concentrated emulsion (HCE). Transmission electron microscopic analyses suggest that the alkylated pyrene molecules are adsorbed on the MWCNT surface, and their adsorption eventually results in the debundling of MWCNT agglomerates. Fourier transform infrared, Raman, and fluorescence spectroscopic analyses confirm the pi-pi interaction between the alkylated pyrene molecules and MWCNTs. The noncovalent modification significantly improves the effective debundling and selective dispersion of MWCNTs in HCEs

Ultrafast Stiffening of Concentrated Thermoresponsive Mineral Suspensions

Extrusion-based 3D printing with rapidly hardening polymeric materials is capable of building almost any conceivable structure. However, concrete, one of the most widely used materials for large-scale structural components, is generally based on inorganic binder materials like Portland cement. Unlike polymeric materials, a lack of precise control of the extent and rate of solidification of cement-based suspensions is a major issue that affects the ability to 3D-print geometrically complex structures. Here, we demonstrate a novel method for controllable-rapid solidification of concentrated mineral suspensions that contain a polymer binder system based on epoxy and thiol precursors as well as one or more mineral fillers like quartz and calcite. The thermally triggered epoxy-thiol condensation polymerization induces rapid stiffening of the hybrid suspensions (0.30 ≤ φ ≤ 0.60), at trigger temperatures ranging between 50 °C and 90 °C achieving average stiffening rates up to 400 Pa/s. The use of nucleophilic initiators such as 1-methylimidazole provides control over the activation temperature and curing rate, thereby helping to achieve an adjustable induction period and excellent thermal latency. By using multiple techniques, we provide guidelines to create designer compositions of mineral suspensions that utilize thermal triggers to achieve thermal latency and ultrafast stiffening - prerequisite attributes for 3D-manufacturing of topologically-optimized structural components

Electrosteric Control of the Aggregation and Yielding Behavior of Concentrated Portlandite Suspensions

Portlandite (calcium hydroxide: CH: Ca(OH)2) suspensions aggregate spontaneously and form percolated fractal aggregate networks when dispersed in water. Consequently, the viscosity and yield stress of portlandite suspensions diverge at low particle loadings, adversely affecting their processability. Even though polycarboxylate ether (PCE)-based comb polyelectrolytes are routinely used to alter the particle dispersion state, water demand, and rheology of similar suspensions (e.g., ordinary portland cement suspensions) that feature a high pH and high ionic strength, their use to control portlandite suspension rheology has not been elucidated. This study combines adsorption isotherms and rheological measurements to elucidate the role of PCE composition (i.e., charge density, side chain length, and grafting density) in controlling the extent of PCE adsorption, particle flocculation, suspension yield stress, and thermal response of portlandite suspensions. We show that longer side-chain PCEs are more effective in affecting suspension viscosity and yield stress, in spite of their lower adsorption saturation limit and fractional adsorption. The superior steric hindrance induced by the longer side chain PCEs results in better efficacy in mitigating particle aggregation even at low dosages. However, when dosed at optimal dosages (i.e., a dosage that induces a dynamically equilibrated dispersion state of particle aggregates), different PCE-dosed portlandite suspensions exhibit identical fractal structuring and rheological behavior regardless of the side chain length. Furthermore, it is shown that the unusual evolution of the rheological response of portlandite suspensions with temperature can be tailored by adjusting the PCE dosage. The ability of PCEs to modulate the rheology of aggregating charged particle suspensions can be generally extended to any colloidal suspension with a strong screening of repulsive electrostatic interactions