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A typical example of viscoelastic matrices is represented by polymeric particles devoted to the controlled release of active agents (drugs) that swell when put in contact with an external solvent, typically a physiological medium. Although, in general, physiological fluids can be considered Newtonian, this is not always the case, let us consider, for example, the mucus present in different organs such lungs when affected by chronic obstructive disease (bacterial infections and cystic fibrosis, for instance). Swelling allows the release of the embedded drug due to the enlargement of the meshes of the polymeric network. The demand for more and more sophisticated drug delivery systems requires to theoretically study in high detail the mass transport phenomena involved in the swelling and release processes. The present study concerns with the mathematical modelling of both transport processes in the case of a viscoelastic matrix swollen by a viscoelastic fluid. While matrix viscoelasticity is accounted for by the generalized Maxwell model, a mean relaxation time accounts for fluid viscoelasticity. The model assumes that solvent uptake gives origin to an internal stress, due the polymeric network reaction to enlargement, able to affect solvent transport and thus, drug release. Despite the complexity connected to the theoretical description of stress and deformation in the swelling matrix, it is generally assumed that the stress state can be approximated by a scalar that can be viewed as an osmotically induced viscoelastic swelling pressure related to the trace of the stress tensor. At the same time, assuming incompressible materials and vanishing deformation gradient inside the matrix, deformation can be simply represented by a scalar quantity (the radial deformation in the case of spheres). Model simulations indicate that the higher the swelling solvent relaxation time, the lower the effect of matrix viscoelastic properties on mass transport phenomena

Laponite dispersions have been widely studied for their appealing industrial applications as well as for their interesting and peculiar properties. The anisotropy of the Laponite nanoparticles, combined with the competition between attractive and repulsive interactions originates the complexity of the phase diagram of such colloidal systems. Depending both on nanoclay and salt concentrations, different final arrested states (gel, repulsive or attractive glass) can be attained after quite different aging times. The present study concerns the aging evolution of salt-free aqueous dispersions with Laponite concentrations ranging from 0.33 to 2.0 % wt/wt. Rheological measurements were performed under linear and nonlinear conditions after different days from sample preparation, also in combination with Low-Field NMR relaxometry tests. The time evolution of the most significant rheological parameters (zero shear viscosity, yield stress and storage modulus) is similar for all the systems examined. The sigmoidal profiles of the individual data sets can be superposed onto a master curve through a double shifting procedure, using a proper coordinate frame. Accordingly, the effects of concentration on aging kinetics of Laponite dispersions are synthetically described by the asymptotical values of the rheological parameters and the characteristic induction time. Additionally Low-Field NMR tests were carried out to evaluate the dependence of the relaxation time (T2)) of the water hydrogens on laponite concentration and its evolution with aging time and to recognize possible correlations between NMR relaxation and rheological parameters of nanoclay dispersions.

Nanocellulose is a renewable, biocompatible and biodegradable material with excellent mechanical properties and hygroscopic character. Two main types of cellulose nanoparticles are nanocrystals (nanocrystalline cellulose, CNC) and nanofibrils (nanofibrillated cellulose, CNF). Aqueous suspensions of CNF show stable gel properties at low solids content, due to the hygroscopic character as well as the high aspect ratio and specific surface area of nanofibrils. CNF is commonly obtained by mechanical disintegration of wood in aqueous medium. To enhance the specific surface area of nanofibrils and to prevent their agglomeration different chemical pretreatment methods, such as carboxylation, have been developed. TEMPO (2,2,6,6- tetramethylpiperidine-1-oxyl) mediated oxidation pretreatment, which converts the primary alcohol groups of the cellulose Dglucose units into carboxylate groups, provides the necessary electrostatic repulsion between fibrils. Recently, TEMPO-CNF gels have been studied and designed to meet requirements for applications such as drug delivery, wound covering, tissue engineering, cell encapsulation, aerogels, rheology modifier, nanofiber-reinforced composites. The present study concerns the development of rheological properties of TEMPO-CNF hydrogels prepared from a commercially available TEMPO-CNF powder. Ageing of hydrogels with concentrations from 1 to 3 wt % of TEMPO-CNF was investigated by monitoring shear flow and linear viscoelastic properties after different days from sample preparation. For quantitative representation of the results time dependencies of zero shear viscosity, yield stress and storage modulus are given. By superposition of individual data sets master curves displaying normalized values of the most significant rheological parameters are constructed, showing the effect of concentration on ageing kinetics. Constant parameters' values are reached in about 90 days of storage. The zero shear viscosity and storage modulus values increase for 2 and 1 decade, respectively.