Linear Viscoelastic Behavior Research Articles

Characterising the linear viscoelastic behaviour of an injection moulding grade polypropylene polymer

The linear viscoelastic behaviour of an injection moulding grade polypropylene is studied using theoretical and computational methods. Polypropylene has a variety of engineering applications as a component. However, it commonly exhibits viscoelastic deformations. This paper analyses the creep and recovery responses of the BJ368MO polypropylene copolymer using the Burgers and generalised Maxwell models. Within the linear viscoelastic regime, an experimental creep strain at 20 text{MPa} is used to determine the rheological constants of the models. These constants (springs and dashpots) are determined using a nonlinear least-squares curve fitting of the experimental creep. Then they are used to predict the creep and recovery responses of the polymer at three different stresses, 10 text{MPa}, 12.5 text{MPa} and 15 text{MPa}. The experiments are made using tensile specimens designed according to the ASTM D638-14standard. The theoretical evaluations are made using the creep and recovery equations derived from their constitutive. Whereas COMSOL Multiphysics software is used during the finite element (FE) analyses. The results of the theoretical and FE calculations are verified using creep and recovery experiments. Based on the validation analyses, both viscoelastic models showed lower deviations from the experimental results when a computational approach is used. In addition, the viscoelastic models are compared by evaluating the residuals of the creep and recovery strain predictions. The theoretical analyses showed better predictions at 12.5 text{MPa} and 15 text{MPa} stresses when the generalised Maxwell model is used. However, the improvements are attributed to the recovery predictions. When FE is used, the Burgers model showed lower mean absolute percentage errors (MAPEs) in all creep and recovery predictions. The model has a minimum of 6.37% error at the 10 text{MPa} stress and a maximum of 8.23% error at the 15 text{MPa}. By comparison, the generalised Maxwell model showed a minimum of 9.24% error at 12.5 text{MPa} and a maximum of 12.8% error at 15 text{MPa} stresses. The novelty of this paper is on predicting the creep and recovery behaviour of the polymer using the FE and theoretical approaches in the linear viscoelastic regime. The findings suggest that the FE analyses using the Burgers viscoelastic material model provide better predictions, with all calculated errors falling below 10%.

Rate dependent adhesion of nanoscale polymer contacts

The adhesive interactions between polymer nanofibers strongly influence the mechanical behavior of their networks in synthetic materials and biological systems. A treatment of the adhesive interactions at polymer contacts must take into account the viscoelastic behavior of the material in the contact region and the associated energy dissipation. This study focuses on the rate-dependent adhesion of polyacrylonitrile (PAN) nanofibers which interact with strong van der Waals forces. Experimental measurements of the detachment force between PAN fibers with diameters 400 and 4000 nm showed that the apparent work of adhesion increases with the rate of unloading by a factor of two within a three orders of magnitude increase in the unloading rate. In order to obtain further insights into this rate-dependent adhesion, the Maugis-Dugdale (M-D) elastic contact model for normal detachment was extended to include a linear viscoelastic behavior of the PAN nanofibers. The extended model predicted the normal pull-off force instabilities in good agreement with the experiments, capturing well the effect of the unloading rate. The calculated viscoelastic time constants were of the order of milliseconds, suggesting fast relaxations in the contact region, which explain the instantaneous full-strength reattachment of nanoscale polymer fibers during slip-stick experiments.

Rheological modeling of thermoplastic vulcanizates (TPVs) using the Kaye–Bernstein, Kearsley, Zapas (K–BKZ) constitutive law

Thermoplastic vulcanizates (TPVs) comprise a thermoplastic polypropylene (PP) matrix and a dynamically vulcanized ethylene propylene diene monomer (EPDM) rubber. Several model TPVs of known composition were studied rheologically in both shear and elongation to understand and model their response using the Kaye–Bernstein, Kearsley, Zapas (K–BKZ) rheological law. A new technique is proposed to capture the effects of yielding and wall slip using the K–BKZ model. Moreover, the linear viscoelastic behavior of the TPVs was investigated, and it was found that all the TPVs exhibit a universal relaxation modulus (power-law with added yield). All TPVs exhibited a yield stress due to the presence of curing at the PP/EPDM interface evident from simple shear and uniaxial extensional experiments. The damping function was also determined, and it was shown to follow the universal Zapas relation. Finally, due to their elastomeric nature, TPVs slip significantly. Overall, it is shown that the use of the K–BKZ with yield stress and slip can capture well their rheological response.

Viscoelastic Properties of Inert Solid Rocket Propellants Exposed to a Shock Wave

AbstractInert solid rocket propellant samples were subjected to dynamic inflation experiments, to characterize the viscoelastic response at high strain rates. An oxyacetylene‐driven shock tube created the shock wave, which was used to dynamically pressurize the surface of the samples during the inflation experiments. Two high‐speed cameras captured the deforming samples, which were speckled to measure the full‐field surface displacements using the digital image correlation (DIC) algorithm. An inverse finite element analysis (iFEA) was used to calibrate parameters of a generalized Maxwell model (i. e. Prony series), which was used to characterize the propellants’ viscoelastic response to shock wave exposure. The viscoelastic parameters calibrated using a Prony series with one Maxwell branch provided a better fit with the out‐of‐plane displacement data from DIC. At least 50 % of the energy dissipated, within the inert solid rocket propellant, occurred within 5 ms following shock wave exposure. The softening phenomenon, due to debonding of the particles embedded in the inert solid rocket propellant, occurred since there was a decrease in instantaneous elastic modulus with increased strain rate. The results of this study will add to the limited knowledge of the linear viscoelastic behavior of inert HTPB propellant at high strain rates and may improve the predictive capabilities of health‐monitoring sensors that assess the solid rocket propellant's structural integrity.

Micromechanical modelling of the anisotropic creep behaviour of granular medium as a fourth-order fabric tensor

The main objective of the present work is to develop a micromechanics approach to predict the macroscopic anisotropic creep behaviour of granular media. To this end, the linear viscoelastic behaviour of the inter-particle interaction at contact is adopted, and the contact distribution is characterized by a fourth-order fabric tensor in the local scale. Then, fourth-order tensor fabric-based micromechanical approaches based on Voigt and Reuss localization assumptions are applied to granular media in the Laplace–Carson space. With help of the inverse Laplace–Carson transformation of these obtained models, the macroscopic anisotropic creep behaviour of granular media submitted to a constant external loading is examined. Finally, the obtained results by specializing the Burgers model into the obtained models are compared with the numerical simulations in the particle flow code (PFC2D) to illustrate the validation and the accuracy of the analytical models for the macroscopic anisotropic creep behaviour of granular media.

The Impact of Titanium Dioxide Type Combined with Coffee Oil Obtained from Coffee Industry Waste on Sunscreen Product Performance

(1) Background: Titanium dioxide (TiO2) consists of three polymorphs, including anatase, rutile and brookite. This work aimed to elucidate the influence of rutile and anatase forms in the performance of sunscreens formulated with green coffee oil (GCO) from coffee beans discarded in the agri-food industry. (2) Methods: TiO2 particles were characterized in terms of size and wettability. The sunscreens formulated with GCO were characterized regarding the droplet size, rheology, texture profile analysis (TPA), in vitro Sun Protection Factor and Water Resistance Retention. Topical delivery and permeation studies were performed to confirm caffeine release and skin penetration. (3) Results: Particle size distributions of rutile and anatase TiO2 particles were similar, however, smaller droplets as well as decreased viscosity and increased thixotropy were obtained for anatase TiO2 and GCO formulation compared to rutile form formulations. Notwithstanding, all formulations exhibited linear viscoelastic behavior. Regarding the TPA, a wide range of mechanical properties improved mainly by GCO rather than TiO2 form has been demonstrated. The influence of TiO2 form on UV protection was better evidenced in absence of GCO. The sunscreen formulations containing GCO presented a favorable topical delivery as confirmed by caffeine release and permeation. (4) Conclusions: Both TiO2 forms combined with GCO provided suitable properties including an effective ultraviolet (UV)-light protection.

Characterization of viscoelastic properties of asphalt mixture at low temperatures using DC(T) creep test

This study presents a framework to identify the low-temperature viscoelastic properties of asphalt concrete using the disk-shaped compacted tension (DC(T)) creep test. Due to the DC(T) geometry, the stress–strain state in the DC(T) geometry is not simple, and a geometry coefficient is needed to characterize the relationship between the mechanical and geometrical properties. The geometry coefficient of DC(T) was calculated by invoking the correspondence principle in viscoelasticity. The DC(T) creep test was conducted on different asphalt mixtures comprised of various components and modifiers at three temperature levels of 0, −12, and −24 °C. The creep compliance function for each of these mixtures was modeled using a generalized Voigt-Kelvin spring-dashpot phenomenological representation. The numerical implementation of the generalized Voigt-Kelvin model was developed in the finite element code Abaqus via a user material subroutine (UMAT). Numerical simulations of DC(T) creep tests using the identified viscoelastic properties are presented, which indicate the capability of the proposed approach to characterize the low-temperature linear viscoelastic behavior of the investigated asphalt mixtures. To further validate the viscoelastic properties obtained from DC(T) test through different stress–strain states, numerical simulation results from an Indirect Tensile Creep Test were compared to experimental results. The close agreement found between the results of indirect tension creep tests and numerical simulations indicates the capability of the proposed approach for identification of viscoelastic properties of asphalt mixtures at low temperatures, which opens the door to avoid the intricate experimental setup and poor repeatability of the indirect tensile creep test at low temperatures.

Thermo-Viscoelastic Response of Protein-Based Hydrogels.

Because of the bioactivity and biocompatibility of protein-based gels and the reversible nature of bonds between associating coiled coils, these materials demonstrate a wide spectrum of potential applications in targeted drug delivery, tissue engineering, and regenerative medicine. The kinetics of rearrangement (association and dissociation) of the physical bonds between chains has been traditionally studied in shear relaxation tests and small-amplitude oscillatory tests. A characteristic feature of recombinant protein gels is that chains in the polymer network are connected by temporary bonds between the coiled coil complexes and permanent cross-links between functional groups of amino acids. A simple model is developed for the linear viscoelastic behavior of protein-based gels. Its advantage is that, on the one hand, the model only involves five material parameters with transparent physical meaning and, on the other, it correctly reproduces experimental data in shear relaxation and oscillatory tests. The model is applied to study the effects of temperature, the concentration of proteins, and their structure on the viscoelastic response of hydrogels.

Strain Sweep Fatigue Testing of Sand Asphalt Mortar to Investigate the Effects of Sample Geometry, Binder Film Thickness, and Testing Temperature

This study evaluated the viscoelastic fatigue behavior of binder using strain sweep fatigue testing of cylindrical sand asphalt mortar (SAM) samples. The SAM samples can represent the realistic film thickness (such as 10–70 μm thick) of the binder in mixtures, while testing repeatability-efficiency can still be met as a result of the use of a standard sand as a load carrier between binder films. A proper testing protocol is still under development, and one of the unknowns in the field is a set of testing conditions that can provide repeatable and case-sensitive test results. Toward that end, SAM samples with different dosages of binder in three geometries were tested at varying temperatures in this study. A PG 64-34 binder was used, and a strain sweep test using a dynamic shear rheometer was conducted to compare the test results from the different cases. Four parameters resulting from the SAM testing were examined: two accounting for material linear viscoelastic behavior and two accounting for fatigue damage characteristics. The parameters were incorporated with statistical analyses to quantitatively evaluate data variability and sensitivity influenced by the binder film thickness, SAM specimen geometry, and testing temperature. The coefficient of variation was less than 20% for all the cases, which indicated the validity of the SAM method attempted in this study. Results also indicated that fatigue behavior was independent of the SAM geometries used in this study, while binder film thickness and testing temperature significantly affected test results.

Mechanical properties of bituminous mixtures under sinusoidal cyclic loadings: Experiment and modelling

Mechanical behaviour of bituminous mixtures is characterized by the great thermal sensitivity and the large viscous effects. This paper focuses on the linear viscoelastic (LVE) behaviour of bituminous mixtures that is considered for pavement design. The studied material is a GB3 mix (GB in French is “Grave Bitume”) which is often used for base course construction in France. Complex modulus tests are performed to determine the LVE properties of bituminous mix. Sinusoidal cyclic loadings in tension and compression for small strain amplitudes (up to 10-4 m/m) are applied on cylindrical samples at different temperatures (from -23.4°C to 39.1°C) and different frequencies (from 0.03 to 10Hz). The complex modulus E* and complex Poisson’s ratio ν* are obtained for these large ranges of temperature and frequency. From all these data, it is shown that within the linear viscoelastic domain and in the 3D case, the Time Temperature Superposition Principle (TTSP) is applicable and verified. A model with a continuum spectrum called 2S2P1D (2S2P1D means two Springs, two Parabolic elements, one Dashpot), developed at the Ecole Nationale des Travaux Publics de l’Etat (ENTPE), is used to simulate the 3D LVE behaviour of tested bituminous mixture.
 Keywords:
 linear viscoelasticity; bituminous mixture; modelling; complex modulus; complex Poisson’s ratio.

Dilute dispersion of compound particles: deformation dynamics and rheology

Compound particles are a class of composite systems in which solid particles encapsulated in a fluid droplet are suspended in another fluid. They are encountered in various natural and biological processes, for e.g. nucleated cells, hydrogels, microcapsules etc. Generation and transportation of such multiphase structures in microfluidic devices is associated with several challenges because of the poor understanding of their structural stability in a background flow and the rheological characteristics of their dispersions. Hence, in this work, we analyse the flow in and around a concentric compound particle and investigate the deformation dynamics of the confining drop and its stability against breakup in imposed linear flows. In the inertia-less limit (Reynolds number, $Re \ll 1$ ) and assuming that the surface tension force dominates the viscous forces (low capillary number, $Ca$ , limit), we obtain analytical expressions for the velocity and pressure fields up to ${O}(Ca)$ for a compound particle subjected to a linear flow using a domain perturbation technique. Simultaneously, we determine the deformed shape of the confining drop correct up to ${O}(Ca^2)$ , facilitating the following. (i) Since ${O}(Ca^2)$ calculations account for the rotation of the anisotropically deformed interface, the reorientation dynamics of the deformed compound particles is determined. (ii) Calculations involving the ${O}(Ca^2)$ shape of the confining interface are found to be important for compound particles as ${O}(Ca)$ calculations make qualitatively different predictions in generalised extensional flows. (iii) An ${O}(Ca)$ constitutive equation for the volume-averaged stress for a dilute dispersion of compound particles was developed to study both shear and extensional rheology in a unified framework. Our analysis shows that the presence of an encapsulated particle always enhances all the measured rheological quantities such as the effective shear viscosity, extensional viscosity and normal stress differences. (iv) Moreover, linear viscoelastic behaviour of a dilute dispersion of compound particles is characterised in terms of complex modulus by subjecting the dilute dispersion to a small-amplitude oscillatory shear (SAOS) flow. (v) Various expressions pertaining to a suspension of particles, drops, and particles coated with a fluid film are also derived as limiting cases of compound particles.

A Stochastic Model Based on Fiber Breakage and Matrix Creep for the Stress-Rupture Failure of Unidirectional Continuous Fiber Composites 2. Non-linear Matrix Creep Effects

Stress rupture (sometimes called creep-rupture) is a time-dependent failure mode occurring in unidirectional fiber composites under high tensile loads sustained over long times (e. g., many years), resulting in highly variable lifetimes and where failure has catastrophic consequences. Stress-rupture is of particular concern in such structures as composite overwrapped pressure vessels (COPVs), tension members in infrastructure applications (suspended roofs, post-tensioned bridge cables) and high angular velocity rotors (e.g., flywheels, centrifuges, and propellers). At the micromechanical level, stress rupture begins with the failure of some individual fibers at random flaws, followed by local load-transfer to neighboring intact fibers through shear stresses in the matrix. Over time, the matrix between the fibers creeps in shear, which causes lengthening of local fiber overload zones around previous fiber breaks, resulting in even more fiber breaks, and eventually, formation clusters of fiber breaks of various sizes, one of which eventually grows to a catastrophically unstable size. Most previous models are direct extension of classic stochastic breakdown models for a single fiber, and do not reflect the micromechanical detail, particularly in terms of the creep behavior of the matrix. These models may be adequate for interpreting experimental, composite stress rupture data under a constant load in service; however, they are of highly questionable accuracy under more complex loading profiles, especially ones that initially include a brief “proof test” at a “proof load” of up to 1.5 times the chosen service load. Such models typically predict an improved reliability for proof-test survivors that is higher than the reliability without such a proof test. In our previous work relevant to carbon fiber/epoxy composite structures we showed that damage occurs in the form of a large number of fiber breaks that would not otherwise occur, and in many important circumstances the net effect is reduced reliability over time, if the proof stress is too high. The current paper continues our previous work by revising the model for matrix creep to include non-linear creep whereby power-law creep behavior occurs not only in time but also in shear stress level and with differing exponents. This model, thus, admits two additional parameters, one determining the sensitivity of shear creep rate to shear stress level, and another that acts as a threshold shear stress level reminiscent of a yield stress in the plastic limit, which the model also admits. The new model predicts very similar behavior to that seen in the previous model under linear viscoelastic behavior of the matrix, except that it allows for a threshold shear stress. This threshold allows consideration of behavior under near plastic matrix yielding or even matrix shear failure, the consequence of which is a large increase in the length-scale of load transfer around fiber breaks, and thus, a significant reduction in composite strength and increase in variability. Derivations of length-scales resulting from non-linear matrix creep are provided as Appendices in the Supplementary Material.

Investigating the Transition between Polymer Melts and Solutions in Nonlinear Elongational Flow

The Doi–Edwards tube model, coupled with relaxation mechanisms, such as reptation, contour length fluctuation, and constraint release, allows us to quantitatively predict the linear viscoelastic properties of entangled polymers. However, for nonlinear elongational flows, large discrepancies between theoretical predictions based on the tube model and experimental results still persist today. This is in particular obvious for the experimentally observed strong qualitative differences in extensional flow of entangled polystyrene (PS) melts and solutions despite having the same number of entanglements and exhibiting the same linear viscoelastic behavior. The cause of this non-universality is often attributed either to a monomeric friction reduction or to an interchain pressure effect. In this work, we investigate the changes in extensional flow behavior going from polymer solutions to the melt state. For this purpose, we measure with a filament stretching rheometer the nonlinear extensional responses of differently long PS chains, both in the melt state and diluted in short-chain matrices of the same polymer at varying concentrations. These concentrations have been chosen sufficiently high, such that the chains stay entangled. This allows us to discuss the influence of concentration and molar mass on the steady-state elongational viscosity to highlight scaling relations. The purpose of the present work is to conduct well-defined experiments to further investigate how the steady extensional viscosity of polymer solutions and blends varies with the concentration and with the molecular weight of the chains.

Structural response and sensitivity analysis of granular and asphaltic overlayment track considering linear viscoelastic behavior of asphalt

Abstract This study investigated the structural response of granular and asphaltic overlayment of rail track considering the linear viscoelastic behavior of asphalt. The calculation of the tensile strains at the bottom of the asphalt layer, the compressive stresses at the top of the subgrade layer, and the service life of the granular and the asphaltic overlayment rail track were conducted using the KENTRACK software. Furthermore, the sensitivity analysis by changing different factors was studied in this paper. The results of this study indicate that the asphaltic overlayment rail track structure has a much longer predicted service life than the granular rail track. It was also shown that the sub-grade compressive stress is more sensitive to the change in subgrade modulus than the change in ballast-sub-ballast-asphalt layer thickness and the change in binder type, respectively. In addition, the asphalt tensile strain is more sensitive to the change in asphalt layer thickness than the change in subgrade modulus and the change in binder type, respectively. These findings also enhance our understanding that subgrade compressive stress and asphalt tensile strain in the asphaltic overlayment track are more sensitive to the change in asphalt layer thickness than the change in binder type.

Nonlinear dynamic buckling analysis of imperfect viscoelastic composite laminated plates

The aim of this paper is to study the effects of linear viscoelastic behavior on dynamic buckling response of imperfect composite laminated plates subjected to in-plane dynamic loads by implementing semi-analytical finite strip method (FSM). The semi-analytical FSM converges with a comparatively small number of strips and correspondingly small number of degrees of freedom. Thus, it is usually implemented more easily and faster than many other computational methods. The governing equations are derived by using classical laminated plate theory (CLPT) and the behavior of plate is assumed to be geometrically nonlinear through Von-Karman assumptions. The Newmark's implicit time integration method in conjunction with the Newton-Raphson iteration are employed to solve the nonlinear governing equation. A Kelvin-Voigt viscoelastic model is considered, and the effects of viscosity coefficient, thickness of the layers of the composite plate and boundary conditions on the nonlinear dynamic buckling response are discussed. In order to justify the accuracy of the results, some of them are verified against those available in other sources. It is also shown that the nonlinear dynamic buckling response of an imperfect viscoelastic composite laminated plate is significantly different from the elastic one by considering different viscosity coefficients.

Analysis of delaminated multilayered functionally graded viscoelastic beam

The present paper is concerned with analysis of a delaminated multilayered functionally graded cantilever beam with linear viscoelastic behaviour of the material. The beam under consideration is made of adhesively bonded longitudinal vertical layers which have different widths and material properties. Besides, each layer is functionally graded along its width. The delamination crack is located arbitrary between layers. A model consisting of two linear springs and a linear dashpot is used for treating the viscoelastic behaviour. The modulii of elasticity of the springs and the coefficient of viscosity of the dashpot are distributed continuously along the width of each layer. A time-dependent solution to the strain energy release rate is derived with considering the viscoelastic behaviour. The solution is verified by applying the compliance method. The solution derived is used to study the viscoelastic behaviour induced variation of the strain energy release rate with the time. It is found that the strain energy release rate increases with the time. Effects of the delamination crack location along the beam width and the material inhomgeneity on the time-dependent strain energy release rate are also studied. The influence of the ratios of the modulii of elasticity and the coefficients of viscosity in the layers on the time-dependent strain energy release rate is evaluated too.

Large amplitude oscillatory shear (LAOS) for nonlinear rheological behavior of heterogeneous emulsion gels made from natural supramolecular gelators

The linear and nonlinear rheological behaviors of heterogeneous emulsions gels made from natural glycyrrhizic acid (GA) nanofibrils and sitosterol-oryzanol mixtures (sterols) were investigated using small amplitude oscillatory shear (SAOS) and large amplitude oscillatory shear (LAOS). The nonlinear rheological response was qualitatively analyzed using normalized Lissajous-Bowditch curves. The microstructure of the emulsion gels strongly depended on the concentration of sterols in the oil phase, and showed a percolated segregated network at 10–20 wt% sterols due to the partial coalescence of droplets, and a jamming transition without coalescence at higher sterols concentration of 30 wt%. The microstructure differences led to different linear and nonlinear viscoelastic behaviors of these emulsion gels. SAOS tests showed that the oil phase structuring by the sterols significantly enhance the viscoelasticity of GA nanofibril emulsion gels, and the percolating emulsion gels exhibited higher elasticity than the jammed emulsion gel, as evidenced by a lower damping factor and frequency power-law exponent. The data of crossover strain, phase angle, and the normalized Lissajous-Bowditch curves from LAOS tests further revealed that compared to the samples in a jammed state or without oil phase structuring, the emulsion gels with a percolating segregated network showed higher structural elasticity and thus were more resistant to large deformations, probably due to the slow relaxation of rigid, hydrodynamically interacting clusters of partially coalesced droplets. These findings could potentially aid in the design of novel emulsion gels, based on all-natural and sustainable building blocks, with specific textural and functional properties for foods, cosmetics, and pharmaceutical applications.

Mapping cellular nanoscale viscoelasticity and relaxation times relevant to growth of living Arabidopsis thaliana plants using multifrequency AFM

The shapes of living organisms are formed and maintained by precise control in time and space of growth, which is achieved by dynamically fine-tuning the mechanical (viscous and elastic) properties of their hierarchically built structures from the nanometer up. Most organisms on Earth including plants grow by yield (under pressure) of cell walls (bio-polymeric matrices equivalent to extracellular matrix in animal tissues) whose underlying nanoscale viscoelastic properties remain unknown. Multifrequency atomic force microscopy (AFM) techniques exist that are able to map properties to a small subgroup of linear viscoelastic materials (those obeying the Kelvin-Voigt model), but are not applicable to growing materials, and hence are of limited interest to most biological situations. Here, we extend existing dynamic AFM methods to image linear viscoelastic behaviour in general, and relaxation times of cells of multicellular organisms in vivo with nanoscale resolution (~80 nm pixel size in this study), featuring a simple method to test the validity of the mechanical model used to interpret the data. We use this technique to image cells at the surface of living Arabidopsis thaliana hypocotyls to obtain topographical maps of storage E′ = 120–200 MPa and loss E″ = 46–111 MPa moduli as well as relaxation times τ = 2.2–2.7 µs of their cell walls. Our results demonstrate that (taken together with previous studies) cell walls, despite their complex molecular composition, display a striking continuity of simple, linear, viscoelastic behaviour across scales–following almost perfectly the standard linear solid model–with characteristic nanometer scale patterns of relaxation times, elasticity and viscosity, whose values correlate linearly with the speed of macroscopic growth. We show that the time-scales probed by dynamic AFM experiments (microseconds) are key to understand macroscopic scale dynamics (e.g. growth) as predicted by physics of polymer dynamics.

Evaluation of rheological behaviour of flowable dental composites reinforced with low aspect ratio micro-sized glass fibres

ObjectiveExperimental investigation is carried out to determine the flowability and stickiness of the developed composite material for dental restoration containing low aspect ratio (AR ≤ 100) surface treated micro-sized glass fibres. MethodsSpecimens are manufactured by mixing low AR (50/70/100) micro-sized glass fibres with two different weight fractions (5%/10%) into UDMA/TEGDMA based resin. Particulate filler composite (PFC) containing 55% glass fillers is used as the control group. Dynamic oscillatory strain sweep tests are conducted to analyse the linear viscoelastic behaviour. Solid-to fluidic transition behaviour of dental composites is also calculated in terms of flow and yield stresses. Furthermore, the oscillatory frequency sweep tests are conducted at three different strains (0.5%, 5% and 50%) resembling the positioning of unset paste onto restorations for different real-life clinical situations. Additionally, stickiness of dental composites with handling instrument (steel) and dentine covered with bonding agent is also evaluated. ResultsThe results suggested the all the FRC groups exhibited non-Newtonian, shear-thinning behaviour. It is further established that inclusion of 5% of 50/70AR fibres into dental composites does not affect the flowability. Simultaneously, stickiness with dentine covered with bonding agent is more for these two compositions as compared to that of handling instrument (steel). SignificanceThis study suggest that visco-elastic properties of dental composites are greatly affected by the type of filler (spherical shaped particulate fillers or rod-shaped fibres) as well as fibre weight fraction/fibre AR. This phenomenon can be attributed to the varying interactions between micro-sized fibres of different AR/weight fraction, particulate fillers and monomers.

Development and physicochemical characterization of a new grass pea (Lathyrus sativus L.) miso.

Western consumers interest in Eastern fermented foods has been growing, due to their nutritional and healthy properties. In this study, new sweet misos and salty misos were produced using grass pea (Lathyrus sativus L.) - traditional Portuguese legume from local producers - to promote its consumption and preservation. The evolution of the new misos was evaluated in comparison to traditional miso (made from soybean), through analysis of the chemical composition, colour, texture and linear viscoelastic behaviour. Throughout the fermentation process, the ascorbic acid and phenolic compounds content - with important nutritional value - increased in all misos, mainly in misos produced using grass pea, besides, grass pea sweet miso presented the fastest evolution and darkest colour. The texture parameters (firmness and adhesiveness) of misos decreased over time: grass pea sweet miso showed the highest firmness reduction (51.63 N to 6.52 N) and soybean sweet miso the highest adhesiveness reduction (27.76 N to 3.11 N). Viscoelastic moduli also decreased, reflecting a reduction in the degree of internal structuring for all misos. However, grass pea misos presented more structured internal systems with faster maturation kinetics than soybean misos, for which stabilization started earlier. Two innovative misos were developed from grass pea. After 4 months, the texture parameters and viscoelastic moduli for grass pea misos, were similar to the control misos made from soybean, showing that grass pea can be used as a raw material to produce a sustainable miso with potentially healthy properties. © 2020 Society of Chemical Industry.