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A new class of degradable dextrin-based hydrogels (dextrin-HEMA) was developed. The hydroxyethyl methacrylate ester (HEMA) hydroxyl groups were activated with N,N'-carbonyldiimidazole (CDI), followed by their coupling to dextrin, yielding a derivatized material that can be polymerized in aqueous solution to form hydrogels. A comparative study of the stability of the dextrin-HEMA hydrogels and dextrin-vinyl acrylate (dextrin-VA, produced in previous work) revealed that only the firsts are effectively hydrolyzed under physiological conditions. A severe mass loss of dextrin-HEMA gels occurs over time, culminating in the complete dissolution of the gels. Rheologic analysis confirmed that physical structuring is less pronounced when dextrin is modified with methacrylate side groups. The biocompatibility results revealed that the dextrin hydrogels have negligible cell toxicity, irrespective of the hydrogel type (HEMA and VA), allowing cell adhesion and proliferation. Gathering the biocompatibility and the ability to tailor the release profiles, we consider dextrin a promising biomaterial for biomedical applications, namely for controlled release.

This work aims to produce poly(ethylene terephthalate)/multilayer graphene oxide (mGO) nanocomposites via continuous melt mixing in twin‐screw extrusion, and to study the changes in crystallization and melt flow behavior. Three mGO contents (0.05, 0.1, and 0.3 wt%) were used. Differential scanning calorimetry analyses showed that at 0.1 wt%, mGO acted best as nucleating agent, increasing the crystallization kinetics as well as the melt crystallization temperature (Tmc) by more than 20%. It was also observed that mGO increases the crystals perfection. The nucleating behavior was confirmed by X‐ray diffraction and small angle X‐ray scattering analyses, which showed a decrease in the composites' crystalline lamella thickness (lc) and long period. X‐ray microtomography data confirms that this behavior is significantly affected by the mGO agglomerates distribution and specific surface area inside the polymer matrix. The rheological behavior was studied under two different conditions. It was noticed that under lower shear stresses the mGO particles hinder the polymer flow, increasing the composites viscosity and the pseudo‐solid character. However, under higher shear stresses, for example, when flowing through a die, the nanomaterial enters its "superlubricity state," acting as a lubricant to the flow. This is industrially interesting, because it may allow the use of less severe processing parameters to produce the nanocomposites. [ABSTRACT FROM AUTHOR]

Controlling the structure and the rheological properties of colloidal suspension is essential in numerous applications to control the phenomenon known as shear-thickening. Here, we report on the nontrivial interplay between hydrodynamic and frictional interactions using mesoscopic characterization of semidense, φ = 0.48, and dense, φ = 0.58, colloidal suspensions. Monitoring computationally both rheology and microstructure of these complex fluids under an external deformation, we show that in the semidense regime the interactions in colloidal suspensions are dominated by hydrodynamics while the fraction of frictional bonds remains negligible and consequently the size of frictional clusters remain small. For these systems, the normal stresses remain negative and large. For dense suspensions, frictional forces are necessary to capture discontinuous shear-thickening (DST); however, the microstructure and rheology are sensitive to the level of roughness of colloidal particles. Furthermore, we show that the frictional bonds in the dense and semidense regime follow the same statistics as random networks introduced by Erdős–Rényi where the presence of frictional bonds in dense suspensions promotes formation of a Giant percolated cluster. We show that for both semidense and dense regimes hydroclusters initially form, within which the frictional contacts nucleate. In the case of dense suspensions these nuclei grow and percolate and form a frictional network. We show that the presence of such a percolated cluster is also necessary for DST to occur. [ABSTRACT FROM AUTHOR]

Model thermoplastic polyurethanes (TPUs) are prepared with the aim of investigating the effect of soft-to-hard segment ratio on the phase transition and the resulting structure that forms upon isothermal exposure to temperatures near their phase transition temperature. The dynamic rheological properties of TPUs before exposure to these isothermal conditions show a phase transition at high temperatures that is directly related to the content of hard segments. The extensional viscosity data indicate a strain-hardening behavior that becomes less pronounced with the increase of hard segments. After isothermal treatment, the DSC results show that the high-temperature endotherm peak narrows and shifts to higher temperatures, suggesting a transition in structure. Small angle X-ray scattering, wide angle X-ray scattering, and atomic force microcopy results indicate a phase-separated system in which the hard domain sizes and crystallinity change during this process. The rheological data collected after recrystallization show a significant increase in both moduli, transitioning from viscoelastic fluid-like to glassy behavior. Concurrently, the uniaxial elongation viscosity presents a significant increase in absolute values, but with a shift from strain-hardening to strain-softening behavior for all strain rates. A transition from traditional phase separated viscoelastic melt behavior to more brittle rupture is also observed, marking a significant fundamental difference in properties before and after recrystallization. [ABSTRACT FROM AUTHOR]

In this paper, covalently linked graphene oxide-poly(ethylene glycol) methyl ether methacrylate-reversible addition-fragmentation chain transfer (GO-PEGMEMA-RAFT) and physically mixed GO-PEGMEMA hydrogel nanocomposites are synthesized. Spectroscopic and imaging techniques such as UV-vis, Fourier transform infrared, Raman spectroscopy, and transmission electron microscopy show that the PEGMEMA is successfully grafted on GO sheets. The rheology of the nanocomposites is studied by small angle oscillatory shear, which shows a competition between reinforcement and lubrication behavior of GO. In the case where lubrication effect dominates reinforcement, the covalently linked GO--PEGMEMA-RAFT has higher G' compared to the physically mixed GO--PEGMEMA. Hence, in the covalently linked system, the grafted polymer chains appear to minimize the lubrication effect. [ABSTRACT FROM AUTHOR]

This work aims to examine whether or not interfacial failure can occur in a compatible polymer bilayer system under large shear and elongation deformations, as well as to probe the sensitivity of nonlinear transient rheology to the presence of interface/interphase at neighboring layers. For this, stress relaxation after a step strain and fast startup in simple shear and uniaxial extension experiments have been performed on healed and coextruded poly(methyl methacrylate)/poly(vinylidene fluoride) bilayers with the presence of a robust diffuse interphase as evaluated by energy dispersive X ray. For unhealed bilayers, interfacial failure occurred in shear flows at intermediate deformations, while for healed bilayers the interphase greatly delayed the onset of interfacial failure to larger deformation steps and to a higher deformation rate in the startup shear. Extensional rheology demonstrated that the presence of an interphase in the bilayers greatly enhanced the transient extensional viscosityηE+(t) as well as the tensile relaxation modulus E(t) of the structure, even though the entanglement density was relatively low. Moreover, models are presented to predict the nonlinear relaxation behavior of bilayers and to estimate the relaxation behavior of the interphase. Fitting of the tube model to the shear relaxation indicates a dilated tube diameter in the interphase, confirming its weak entanglement intensity and its readiness to flow-induced disentanglement under large external deformations. Finally, the physics of the interfacial failure was assessed based on some recent molecular dynamic theories. [ABSTRACT FROM AUTHOR]

The most promising method capable of providing accurate measurements of the first and second normal-stress differences in shear flows at shear rates typical of polymer processing is the so-called hole-pressure method, but its use has not been as widespread as would be expected, namely due to the experimental difficulties associated with performing such experiments accurately. In this work, we use a small-scale modular slit die to assess the practical utility of the method for in-line monitoring of polymer melt flow. We provide a quantitative analysis of intrinsic error sources and use state-of-the-art data acquisition tools to minimize errors associated with pressure transducers. Our results demonstrate that the method can be used to accurately measure the viscosity and first normal-stress difference in melts but probably not the second normal-stress difference because the intrinsic errors are too high, even when the influence of all the potential error sources is minimized or eliminated. [ABSTRACT FROM AUTHOR]

Abstract: The present work presents a predictive direct fiber simulation of fiber suspensions under uniaxial extensional flows by Stokesian dynamics, considering both near- and farfield hydrodynamic intrafiber interactions and lubrication approximation for interfibers. Initially, code validation is performed and simulation predictions compared with experimental data. Afterwards, the orientation tensor under flow is discussed as a function of volume fraction, aspect ratio and flexibility. Based on the results, a general constitutive equation with a proposed parameter is proposed and verified by comparing experimental data of high volume fraction and high aspect ratio systems. [Copyright &y& Elsevier]

Abstract: The present work presents a predictive direct fibre simulator for concentrate fibre suspension based on the Particle Simulation Method (PSM), considering near-field hydrodynamic interactions and collision forces to prevent fibres from overlapping. To validate the software, rheological properties such as the intrinsic viscosity [η], the shear viscosity at steady state η for various fibre concentrations and aspect ratios, and both the time transient viscosity η + and time transient first normal stress difference under shear are discussed by comparing theories and experimental data available in the literature. It is confirmed that the software developed in this work is able to predict and analyze rheological properties quantitatively as well as qualitatively. Finally, η + , and the second-order orientation tensor, a 2, are discussed for a variety of aspect ratio and concentrations up to large strains. [Copyright &y& Elsevier]

Abstract: Flexible fibre suspensions in a Newtonian fluid experimentally show shear-thinning and normal stress differences in steady state. This paper discusses properties of the shear viscosity, η, and the first and second normal stresses, N 1 and N 2, using the help of direct fibre simulation based on PSM (particle simulation method), considering near-field hydrodynamic interactions, collision without attractively interacting forces and friction forces between fibres. The software reproduces the tendencies obtained experimentally such as shear-thinning and decreasing N 1. In addition to this result, when the aspect ratio is low (<10) and the volume fraction varies between ϕ v =0.05 and 0.1, shear-thickening and increasing N 1 are predicted. These tendencies are analyzed using the orientation of the end-to-end vector and the radius of gyration. The prediction of N 2 as a function of is also shown in the paper. [Copyright &y& Elsevier]

The article presents a study on the aging behavior of polyvinyl chloride (PVC) plastisols made with different types of resin. The results showed that the emulsion-based plastisol is more prone to aging than the microsuspension-based plastisol. For the emulsion plastisol, both the viscosity and the apparent yield stress increase while the microsuspension plastisol keeps the same viscosity, apparent yield stress, and gelation behavior. The microsuspension was found to be very stable in time.

Polypropylene (PP) foams are important in various industrial applications due to their unique mechanical and electrical properties. However, fabrication of microcellular PP foams is still limited because of polymer's rheological properties. In this work, the influence of rheological properties and the layer confinement effect on the microcellular morphology of foam/film structure and the mechanical and dielectric properties were investigated. Two types of long chain branched polypropylenes (BPPs) were used and named as BPP1 and BPP2, respectively. Talc micro-particles were used as a nucleating agent for crystallization. In uniaxial extensional flow, both BPPs exhibited strong strain-hardening behavior with the Trouton ratios well above 3 at all extensional rates, regardless of the talc content; however, BPP2 exhibited more strain-hardening than BPP1. 16 and 32 layered foam/film structures were then fabricated with BPP2 and 32 layered foam/film was fabricated with BPP1. The lower strain-hardening of BPP1 caused cell collapse and showed very little evidence of film layers separating the foam layers, while the stronger strain-hardening of BPP2 resulted in stable cell layers near the surface of the sheet and a non-coalesced cell structure. Mechanical properties in compression showed large differences in compressive strain due to the difference in cell structures. Dielectric property measurement on biaxially oriented 32-layer BPP2 foam/film, the one that showed the best cell structure and mechanical properties, showed much higher apparent dielectric constant (6–7) and higher release and stored energy densities than those of neat biaxially oriented polypropylene films. [ABSTRACT FROM AUTHOR]

In the present work an experimental investigation was conducted to obtain a correlation for the determination of convective heat transfer coefficients of stirred yoghurt in a plate heat exchanger. A rheological study was carried out in order to characterise the stirred yoghurt flow behaviour, evaluating its dependency both on shear rate and temperature. A shift in the temperature dependency was evidenced at 25 °C. It is also shown that the material shows a complex flow behaviour, changing from a Bingham fluid to a power-law fluid at shear stresses in excess of approximately 6.7 Pa. As regards the heat transfer behaviour of the non-Newtonian stirred yoghurt a correlation for the convective heat transfer coefficient was obtained that reveals the large effects of the thermal entry length due to the high Prandtl numbers and to the short length of the plate heat exchanger. [Copyright &y& Elsevier]

In this study, the effect of organoclay as a compatibilizer in immiscible poly(lactic acid) (PLA) and natural rubber (NR) blends was studied. Structural development of PLA90/NR10 blend was examined depending on the clay content (0.5–10 wt%). The drop size of the dispersed NR phase initially decreased and then remained constant as the clay content increased. Below the critical concentration (3 wt%), the clay was selectively localized mostly at the interface between the matrix phase (PLA) and the dispersed phase (NR), whereas the clay particles were additionally observed in the matrix phase above 3 wt%. XRD analysis revealed that the gallery structure of the clay was highly dependent on the concentration and selective location of the clays. The development of the structure and the physical properties of the blend were also investigated according to the clay contents by measuring the linear viscoelasticity and surface tension. The degree of clay exfoliation was analyzed quantitatively in terms of the modulus ratio of PLA/NR/clay and PLA/clay. The elongational behavior and mechanical properties of the blends were strongly dependent on the location of the clays. The clay worked as an efficient compatibilizer below the critical content, while it worked as a reinforcing filler above the threshold, making the matrix more rigid. [ABSTRACT FROM AUTHOR]

In this work, the rheological and structural changes induced by uniaxial extensional flows are studied for two aromatic TPUs: an elastomeric (soft) material composed both of hard and soft segments and an amorphous glass (hard) one composed almost exclusively by hard segments. The uniaxial extensional viscosity was measured on a Sentmanat fixture (SER) at single temperature of 175 °C, showing that at high strain rates both materials initially strain-soften before eventually strain-hardening at higher Hencky strains. This effect is smoother and occurs at lower strain rates for the soft TPU. Optical microscopy, atomic force microscopy and X-ray scattering show this behavior to be related with structural changes induced by the flow. Strain-softening is attributed to the orientation of hard domains in the flow direction. In the soft TPU strain-hardening is caused by the stretching of entangled soft segments but in the hard one it appears to be associated with microcracks and with the development of a hitherto unreported hierarchical structure in the molten material. [ABSTRACT FROM AUTHOR]

Abstract: Blends of nylon 6 and a silsesquioxane additive, aminopropyl isobutyl POSS (APOSS) were prepared containing 0–5% POSS by weight. APOSS was previously demonstrated by us to be an ineffectual additive at concentrations of more than approximately 3% by weight, lacking solubility and favorable interactions with the polymer matrix. Significant reinforcement was only obtained under the extreme confinement and orientation conditions induced by melt-spinning, under which it was suggested the APOSS might undergo forced-assembly into elongated, rebar-like reinforcement structures. In the present work, we show that while the structure–property relationships of these systems, measured by WAXS, DSC and Raman spectroscopy revealed there are no major changes in the chemical structure induced by molecular interactions, rheological and SEM investigations on the APOSS-polymer systems suggest that at temperatures above the APOSS melting point, solubility and dispersion is slightly enhanced. We also show that two conditions seem to be necessary for improvement of properties: i) A sufficient concentration of APOSS needs to exist in the system to diffuse in significant quantities into the matrix upon re-melting; ii) The remaining non-diffused APOSS agglomerates need to be below a critical size around 4 μm in diameter, which does not happen for concentrations above approximately 3%. [Copyright &y& Elsevier]

Abstract: Capillary rheometry is the simplest and most popular system to measure the viscosity of fluids at high shear rates and, because of this, it is normally used to simulate industrial processes in the polymer processing industry. Thus, traditionally the rheometers have been equipped with high-pressure transducers, which means that they present some limitations in the characterization for low viscosity (lower than O(101) Pas) fluids. The aim of this work is to present an accessory to a laboratory capillary rheometer in order to allow for low viscosity fluids to be tested. This is a simple adaptation that works as a extension of the rheometer and contains a new low-pressure transducer with a pressure range of 0–120Psi. The concept is introduced and validated with Newtonian liquids and initial results are presented for non-Newtonian fluids, namely emulsion-based and micro suspension-based PVC plastisols for textile coating. [Copyright &y& Elsevier]