Determination of the melting point of solid polymers by rheological method

The rheological properties of polymer systems determine their deformation behavior and the relationship between stresses, strains and strain rates. We present the results of determining the melting points of solid polymer compounds by the rheological method. Tests were carried out using a rotary rheometer in an oscillating mode. Using the obtained thermomechanical dependences, the experimental data were compared with the values determined by standard analytical (capillary method) and thermal (differential scanning calorimetry) methods. The viscous and elastic behavior of the samples were analyzed using the dependence of the oscillating stress or strain on the angular velocity or frequency. It is shown that the temperature dependences of the storage and loss moduli, as well as the angle of mechanical losses, determine the physical and relaxation transitions in polymers upon their heating. The obtained results can be used in analysis of the effect of conditions of polymer processing on their properties, as well as in optimization of the technological processes of product manufacture and modeling the behavior of materials under operation conditions at the stage of development.

Investigation of the rheological behavior of ABS plastic grades for the production of filaments for 3D printing by layer-by-layer deposition

A comprehensive study of the rheological properties of ABS-plastic grades used for the manufacture of fi laments for 3D printing by the FDM method has been carried out. It is shown that under the printing temperature-speed conditions, the viscous properties of melts of different grades and activation energy of their viscous flow differ significantly. The temporal parameters of the thermal stability of melts at an elevated (250°C) temperature were determined by the rheological method. It is shown that, under printing conditions, the polymer does not undergo noticeable degradation, assessed by the change in its effective viscosity. Viscoelasticity, which determines the dimensional accuracy of products and the thickness of the deposited layer, was evaluated by the degree of swelling of the extrudate under different flow conditions. Criteria for the applicability of ABS-plastic grades for fi lament production are proposed.

Rheological effects of hypertonic saline and sodium bicarbonate solutions on cystic fibrosis sputum in vitro

Background Cystic fibrosis (CF) is a life-threatening multiorgan genetic disease, particularly affecting the lungs, where recurrent infections are the main cause of reduced life expectancy. In CF, mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) protein impair transepithelial electrolyte and water transport, resulting in airway dehydration, and a thickening of the mucus associated with abnormal viscoelastic properties. Our aim was to develop a rheological method to assess the effects of hypertonic saline (NaCl) and NaHCO3 on CF sputum viscoelasticity in vitro, and to identify the critical steps in sample preparation and in the rheological measurements. Methods Sputum samples were mixed with hypertonic salt solutions in vitro in a ratio of either 10:4 or 10:1. Distilled water was applied as a reference treatment. The rheological properties of sputum from CF patients, and the effects of these in vitro treatments, were studied with a rheometer at constant frequency and strain, followed by frequency sweep tests, where storage modulus (G′), loss modulus (G″) and loss factor were determined. Results We identified three distinct categories of sputum: (i) highly elastic (G′ > 100,000 Pa), (ii) elastic (100,000 Pa > G′ > 1000 Pa), and (iii) viscoelastic (G′ < 1000). At the higher additive ratio (10:4), all of the added solutions were found to significantly reduce the gel strength of the sputum, but the most pronounced changes were observed with NaHCO3 (p < 0.001). Samples with high elasticity exhibited the greatest changes while, for less elastic samples, a weakening of the gel structure was observed when they were treated with water or NaHCO3, but not with NaCl. For the viscoelastic samples, the additives did not cause significant changes in the parameters. When the lower additive ratio (10:1) was used, the mean values of the rheological parameters usually decreased, but the changes were not statistically significant. Conclusion Based on the rheological properties of the initial sputum samples, we can predict with some confidence the treatment efficacy of each of the alternative additives. The marked differences between the three categories suggest that it is advisable to evaluate each sample individually using a rheological approach such as that described here.

Effect of Surfactant Molecular Structure on Emulsion Stability Investigated by Interfacial Dilatational Rheology

Polyglycerol polyricinolate (PGPR) and polyglycerol-2 dioleate were selected as model surfactants to construct water-in-oil (W/O) emulsions, and the effect of interfacial rheological properties of surfactant film on the stability of emulsions were investigated based on the interfacial dilatational rheological method. The hydrophobicity chain of PGPR is polyricinic acid condensed from ricinic acid, and that of polyglycerol-2 dioleate is oleic acid. Their dynamic interfacial tensions in 15 cycles of interfacial compression-expansion were determined. The interfacial dilatational viscoelasticity was analyzed by amplitude scanning in the range of 1–28% amplitude and frequency sweep in the range of 5–45 mHz under 2% amplitude. It was found that PGPR could quickly reach adsorption equilibrium and form interfacial film with higher interfacial dilatational viscoelastic modulus to resist the deformation of interfacial film caused by emulsion coalescence, due to its branched chain structure and longer hydrophobic chain, and the emulsion thus presented good stability. However, polyglycerol-2 dioleate with a straight chain structure had lower interfacial tension, and it failed to resist the interfacial disturbance caused by coalescence because of its lower interfacial dilatational viscoelastic modulus, and thus the emulsion was unstable. This study reveals profound understanding of the influence of branched structure of PGPR hydrophobic chain on the interfacial film properties and the emulsion stability, providing experimental reference and theoretical guidance for future design or improvement of surfactant.

To estimate the curing time of epoxy oligomer during nonisothermal polymerization process

The results of an experimental study and numerical modeling of the curing process of an epoxy binder under conditions of non-isothermal polymerization are presented. Object of research: twocomponent composition of "cold curing": epoxy resin L and hardener EPH 161, certified as a binder for aerospace composite materials. The measurements were performed on a Physica MCR 501 rotary rheometer in the oscillation mode under conditions of a programmable temperature change. The Cauchy problem for an ordinary differential equation describing the behavior of an oligomer during the curing process was used as the main model. The values of the material parameters necessary for performing the calculation operations were obtained by the rheological method, analyzing the dependence of the binder viscosity on the polymerization time and the rate of temperature change. Estimates of the model parameters obtained as a result of processing full-scale experiments aregiven. The effective values of the kinetic parameters are determined, at which the calculated yield loss time is in good agreement with the experimental one.

Development of a Polyacrylamide-Based Mud Formulation for Loss Circulation Treatments

Loss circulation materials in the last two decades have witnessed a lot of developments and implementations. New technologies and materials are introduced to treat various types of loss zones. However, the success rate is still very low due to many uncertainties in the selection of types and particle size of the bridging materials. In addition, there are many operational restrictions such as the risk of plugging and pumping difficulties when large size of particle is needed, especially in deep-water drilling. In this study, polyacrylamide (PAM) crosslinked with polyethylenimine (PEI) is introduced as polymer-based mud for loss circulation treatment. The PAM/PEI systems have wide applications in water shutoff for high water production zones and are known for their strong gel and exceptional rheological properties. This study provides a rheological method for screening of PAM/PEI-based drilling formulation with optimized molecular weight and concentrations. Comparative analysis of rheology of non-crosslinked and crosslinked polyacrylamide with other drilling fluids additives as well as proper mixing procedures are provided. The results achieved in this study are used as a strong tool to design a polymer-based mud with competitive rheological properties which achieved an 80% reduction in fluid loss when compared with other conventional loss circulation materials.

Probing Flow-Induced Biomolecular Interactions With Micro-Extensional Rheology: Tau Protein Aggregation

Biomolecules in solutions subjected to extensional strain can form aggregates, which may be important for our understanding of pathologies involving insoluble protein structures where mechanical forces are thought to be causative (e.g., tau fibers in chronic traumatic encephalopathy (CTE)). To examine the behavior of biomolecules in solution under mechanical strains requires applying rheological methods, often to very small sample volumes. There were two primary objectives in this investigation: (1) To probe flow-induced aggregation of proteins in microliter-sized samples and (2) To test the hypothesis that tau protein aggregates under extensional flow. Tau protein (isoform:3R 0 N; 36.7 kDa) was divided into 10 μl droplets and subjected to extensional strain in a modified tensiometer. Sixteen independent tests were performed where one test on a single droplet comprised three extensional events. To assess the rheological performance of the fluid/tau mixture, the diameter of the filament that formed during extension was tracked as function of time and analyzed for signs of aggregation (i.e., increased relaxation time). The results were compared to two molecules of similar and greater size (Polyethylene Oxide: PEO35, 35 kDa and PEO100, 100 kDa). Analysis showed that the tau protein solution and PEO35 are likely to have formed aggregates, albeit at relatively high extensional strain rates (∼10 kHz). The investigation demonstrates an extensional rheological method capable of determining the properties of protein solutions in μl volumes and that tau protein can aggregate when exposed to a single extensional strain with potentially significant biological implications.

Polyelectrolyte Polysaccharide–Gelatin Complexes: Rheology and Structure

General features of rheological properties and structural peculiarities of polyelectrolyte polysaccharide–gelatin complexes were discussed in this paper. Experimental results were obtained for typical complexes, such as κ-carrageenan–gelatin, chitosan–gelatin and sodium alginate–gelatin complexes. A rheological method allows us to examine the physical state of a complex in aqueous phase and the kinetics of the sol–gel transition and temperature dependences of properties as a result of structural changes. The storage modulus below the gelation temperature is constant, which is a reflection of the solid-like state of a material. The gels of these complexes are usually viscoplastic media. The quantitative values of the rheological parameters depend on the ratio of the components in the complexes. The formation of the structure as a result of strong interactions of the components in the complexes was confirmed by UV and FTIR data and SEM analysis. Interaction with polysaccharides causes a change in the secondary structure of gelatin, i.e., the content of triple helices in an α-chain increases. The joint analysis of the structural and rheological characteristics suggests that the formation of additional junctions in the complex gel network results in increases in elasticity and hardening compared with those of the native gelatin.