1,720,981 research outputs found
Elastic and dynamic heterogeneity in aging alginate gels
Full text linkAnomalous 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
Investigation on the thermal gelation of Chitosan/β-Glycerophosphate solutions
Full text linkThis work deals with the effect of temperature on the thermal-gelation process of water solutions containing
chitosan β-glycerolphosphate disodium salt hydrate. In particular, the attention is focused on the role played by
temperature on the gel final properties, a very important aspect in the frame of drug delivery systems. The study
was performed by combining rheology and low field nuclear magnetic resonance, two approaches that revealed
to be highly synergic as they can detect different aspects of the developing polymeric network. This study
indicates that 30 °C represent a sort of threshold for both the gelation kinetics and the gel final properties.
Indeed, above this temperature, gelation kinetics was rapid and yielded to a strong gel. On the contrary, a slow
kinetics and a final weak gel occurred below 30 °C. Finally, rheology and low field NMR allowed, independently,
evaluating the time evolution of the network mesh size upon gelation
Gas sorption and transport in syndiotactic polystyrene with nanoporous crystalline phase
No full textWater diffusion in glassy polymers and their silica hybrids: an analysis of state of water molecules and of the effect of tensile stress
No full textIn-situ, time-resolved FTIR spectroscopy along with gravimetric anal. were used to study water sorption and transport in several glassy polymeric matrixes, characterized by different levels of interaction with water, and on polymer-silica hybrids. The polymers include tetraglycidyl-4,4'-diaminodiphenylmethane epoxy, cured with either 4,4'-diaminodiphenylsulfone or hexahydrophthalic anhydride; polyimide of pyromellitic dianhydride and oxydianiline (PMDA-ODA); and the hybrids of silica (from TEOS and g-glycidyloxypropyltrimethoxysilane) and the polyimide. The time-resolved FTIR technique was coupled to a dynamical-mech. analyzer to gather information on the water sorption kinetics and thermodn. in a polymer sample submitted to stretching deformation and load. Results were modeled by coupling the mass balance and momentum balance, using a theor. approach developed for elastic matrixes and low sorbed amts. by Larche and Cahn
A novel spectroscopic approach to investigate transport processes in polymers: the case of water-epoxy system
No full textAnomalous Aging and Stress Relaxation in Macromolecular Physical Gels: The Case of Strontium Alginate
No full textWe investigate macromolecular physical gels (strontium alginates) in a wide range of aging times and length scales by combining linear stress relaxation and dynamic light scattering (DLS) experiments. Stress relaxation shows an early logarithmic decay followed by a stretched exponential behavior, leading to define two characteristic times, which increase as distinct power laws of gel age. The DLS clearly displays anomalous microscopic dynamics, with compressed exponential decay of autocorrelation functions and ballistic wavelength dependence of the decay time. Thus, our results demonstrate that stretched stress relaxation can coexist with compressed intensity autocorrelation functions. In addition, comparison between rheology and DLS allows for the identification of two characteristic lengths that we interpret as the typical size of collapsing pores in the gel and of avalanche-like rearrangements. We discuss this scenario in terms of some recently proposed ideas on anomalous aging, such as ultralong-ranged dynamical correlations in closely athermal systems
A novel spectroscopic approach to investigate transport processes in polymers: the case of water-epoxy system
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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