Hierarchical cross-linking in physical alginate gels: a rheological and dynamic light scattering investigation

We investigate the dynamics of alginate gels, an important class of biopolymer-based viscoelastic materials, by combining mechanical tests and non-conventional, time-resolved light scattering methods. Two relaxation modes are observed upon applying a compressive or shear stress. Dynamic light scattering and diffusive wave spectroscopy measurements reveal that these modes are associated with discontinuous rearrangement events that restructure the gel network via anomalous, non-diffusive microscopic dynamics. We show that these dynamics are due to both thermal activation and internal stress stored during gelation and propose a scenario where a hierarchy of cross-links with different life times is responsible for the observed complex behavior. Measurements at various temperatures and sample ages are presented to support this scenario.Comment: To appear in Soft Matte

Concentrated suspensions of Brownian beads in water: dynamic heterogeneities trough a simple experimental technique

Concentrated suspensions of Brownian hard-spheres in water are an epitome for understanding the glassy dynamics of both soft materials and supercooled molecular liquids. From an experimental point of view, such systems are especially suited to perform particle tracking easily, and, therefore, are a benchmark for novel optical techniques, applicable when primary particles cannot be resolved. Differential Variance Analysis (DVA) is one such novel technique that simplifies significantly the characterization of structural relaxation processes of soft glassy materials, since it is directly applicable to digital image sequences of the sample. DVA succeeds in monitoring not only the average dynamics, but also its spatio-temporal fluctuations, known as dynamic heterogeneities. In this work, we study the dynamics of dense suspensions of Brownian beads in water, imaged through digital video-microscopy, by using both DVA and single-particle tracking. We focus on two commonly used signatures of dynamic heterogeneities: the dynamic susceptibility, $\chi_4$, and the non-Gaussian parameter, $\alpha_2$. By direct comparison of these two quantities, we are able to highlight similarities and differences. We do confirm that $\chi_4$ and $\alpha_2$ provide qualitatively similar information, but we find quantitative discrepancies in the scalings of characteristic time and length scale on approaching the glass transition.Comment: The original publication is available at http://www.scichina.com and http://www.springerlink.com http://engine.scichina.com/publisher/scp/journal/SCPMA/doi/10.1007/s11433-019-9401-x?slug=abstrac

Finite Element Analysis Investigate Pulmonary Autograft Root and Leaflet Stresses to Understand Late Durability of Ross Operation

Ross operation might be a valid option for congenital and acquired left ventricular outflow tract disease in selected cases. As the pulmonary autograft is a living substitute for the aortic root that bioinspired the Ross operation, we have created an experimental animal model in which the vital capacity of the pulmonary autograft (PA) has been studied during physiological growth. The present study aims to determine any increased stresses in PA root and leaflet compared to the similar components of the native aorta. An animal model and a mathematical analysis using finite element analysis have been used for the purpose of this manuscript. The results of this study advance our understanding of the relative benefits of pulmonary autograft for the management of severe aortic valve disease. However, it launches a warning about the importance of the choice of the length of the conduits as mechanical deformation, and, therefore, potential failure, increases with the length of the segment subjected to stress. Understanding PA root and leaflet stresses is the first step toward understanding PA durability and the regions prone to dilatation, ultimately to refine the best implant technique

Analysis of the aging effects on the viscoelasticity of alginate gels

The effect of aging on the mechanical behaviour of ionically cross-linked alginate gels is studied in detail. Relaxation experiments upon both unconfined compression and torsion are performed on samples at different aging times. The elastic moduli of the gel are found to increase with the aging time, whereas the internal (constitutive) mechanism of the relaxation of the solid component of the gel is found to be unaffected by aging. It is demonstrated that the Linear Visco-Elastic Stress/Diffusion Coupling model [D. Larobina, F. Greco, J. Chem. Phys., 2012, 136, 134904], recently developed by two of the present authors, is able to quantitatively reproduce the experimental data for differently aged samples, at early-to-intermediate relaxation times. Moreover, it is shown that the gel always undergoes a spontaneous expulsion of water (syneresis) and some spontaneous deformation for a sufficiently long observation time, even in the absence of any externally imposed strain. The latter phenomenology progressively slows down with increasing of the gel age. By proper time shifting of the late relaxation decays, i.e., by properly defining an "effective time", master curves can be obtained in all cases, with all data pertaining to differently aged samples collapsing on a single relaxation law for each deformation history. The dependence of the shift factors on the aging time is found to follow a power law behavior, with an exponent of 1.39

Elastic and Dynamic Heterogeneity in Aging Alginate Gels

Anomalous aging in soft glassy materials has generated a great deal of interest because of some intriguing features of the underlying relaxation process, including the emergence of “ultra-long-range” dynamical correlations. An intriguing possibility is that such a huge correlation length is reflected in detectable ensemble fluctuations of the macroscopic material properties. We tackle this issue by performing replicated mechanical and dynamic light scattering (DLS) experiments on alginate gels, which recently emerged as a good model-system of anomalous aging. Here we show that some of the monitored quantities display wide variability, including large fluctuations in the stress relaxation and the occasional presence of two-step decay in the DLS decorrelation functions. By quantifying elastic fluctuation through the standard deviation of the elastic modulus and dynamic heterogeneities through the dynamic susceptibility, we find that both quantities do increase with the gel age over a comparable range. Our results suggest that large elastic fluctuations are closely related to ultra-long-range dynamical correlation, and therefore may be a general feature of anomalous aging in gels

Molecular simulation of carbon dioxide sorption in nanoporous crystalline phase of sydiotactic polystyrene

Nanoporous crystalline δe-form of syndiotactic polystyrene (sPS) is characterized by the rather peculiar behavior of being able to absorb considerable amounts of low molecular weight penetrants, in contrast to the general behavior reported for the crystalline phase of polymers that is impervious to penetrants. In particular, the δe nanoporous crystalline form of sPS displays a sorption capacity of penetrants that is several times higher than the one of the amorphous phase of sPS. In this paper, sorption thermodynamics of carbon dioxide in the δe nanoporous crystalline form of semicrystalline sPS is analyzed by means of Grand Canonical Monte Carlo (GCMC) molecular simulation methods, evaluating sorption isotherms as well as isosteric heats of sorption based on a semi-empirical molecular force-field. In fact, in the last years, this technique has been successfully used to investigate sorption properties of periodic crystals of a wide range of materials, including zeolites and polymers, supplying reliable estimates and representing a valid support to the experimental activity. While computational techniques allow direct determination of sorption properties of purely crystalline systems, experimental characterization of sorption in a semicrystalline polymer, as is the case of sPS under investigation, does not give a straight estimation of sorption capacity of the crystalline phase since sorption measurement incorporates other contributions related to the amorphous phase and interphases, as well as to possible defects of the crystalline phase. However, in the case of sPS at the investigated gas pressures, contribution of the amorphous and non-crystalline phases to carbon dioxide sorption can be neglected, and it has been possible to directly compare simulation predictions with experimental results, showing that GCMC computations supply excellent estimates for sorption isotherms and isosteric heats of sorption

Modeling of the reticulation kinetics of alginate/pluronic blends for biomedical applications

In this work, blends of alginate/pluronic (F127) for biomedical applications were investigated. In particular, the kinetics of alginate chain reticulation by bivalent cationswas studied by experimental and modeling approaches. Two kinds of sodium alginate were tested to obtain hard gel films. The thicknesses of the reticulated alginate films were measured as function of the exposure time and of the reticulating copper (Cu2+) solution concentration. The kinetics was described by a proper model able to reproduce the experimental data. The model parameters, evaluated based on the measurements of thicknesses as function of Cu2+ concentration and exposure time, were further validated by comparing the prediction of the modelwith another set of independent measurement; here, the depletion of Cu2+ ions in the conditioning solution above the reacting gel is measured as function of time. The tuned model could be used in the description of the future applications of the blend

Molecular Mechanism of H2O Diffusion into Polyimides: A Model Based on Dual Mobility with Instantaneous Local Nonlinear Equilibrium

The mass transport mechanism of water into polyimide films has been analyzed and modeled on the basis of the relevant findings of an in situ FTIR spectroscopic analysis, performed previously, which (i) identified a molecular mechanism of diffusion based on two water species, i.e., H2O molecules interacting with the carbonyl groups of the polyimide and self-associated water, and (ii) evidenced the establishment of an instantaneous nonlinear equilibrium between these species. To model water transport, it has been assumed that concurrent diffusion occurs of two species, i.e., single water molecules and water dimers, which display different mobilities. A nonlinear instantaneous equilibrium between the local concentrations of these two species has been imposed. This equilibrium relationship has been derived on the basis of a two-layer BET (Brunauer, Emmett, and Teller) theory for the water sorption isotherm. The proposed approach is able to give a good qualitative and quantitative interpretation of both sorption equilibrium and of sorption/desorption kinetics data collected for each of the two water species identified by FTIR spectroscopy