Romano Lapasin

Rheology of industrial polysaccharides : theory and applications

The Polysaccharides: Sources And Structures. Industrial Applications of Polysaccharides. Rheology. Rheology of Polysaccharide Systems. Rheometry. Appendix. Index

Starch–methylcellulose based edible films: rheological properties of film-forming dispersions

Abstract The rheological properties of edible film-forming dispersions containing corn starch, methylcellulose (MC) and glycerol were studied using oscillatory and steady shear flow tests. The combined effects of glycerol content and blending levels of MC with starch on the rheological properties of dispersion were evaluated. The flow curves showed shear-thinning behaviour. Mechanical spectra and Cox–Merz superposition of steady-shear viscosity and dynamic viscosity were consistent with polymer solutions containing topological entanglement interactions of chains. Dispersion stability results showed total recovery of the viscoelastic properties of dispersions subject to high strains, as expected for entangled polymers. MC was the main factor influencing apparent viscosity and viscoelastic properties.

Drug release from an ensemble of swellable crosslinked polymer particles

This paper presents a new model suitable to describe the drug release from drug delivery systems constituted by an ensemble of drug loaded crosslinked polymer particles. The model accounts for the main factors affecting the drug release such as the particle size distribution, the physical state and the concentration profile of the drug inside the polymeric particles, the viscoelastic properties of the polymer-penetrant system and the dissolution-diffusion properties of the loaded drug. In order to check the validity of the model, release experiments were performed by using crosslinked polyvinyl-pyrrolidone (PVP) particles and two different model drugs, MAP (medroxyprogesterone acetate) and TEM (Temazepam). MAP and TEM were chosen because of their completely different dissolution behaviours in water. In particular, TEM undergoes a phase transition to the crystalline state upon dissolution when it is loaded in the polymeric network in the amorphous state. The comparison with the experimental results confirms that the most important factors determining the drug release kinetics can be properly accounted for.

Synergistic gelation of xanthan gum with locust bean gum: a rheological investigation.

Many industrial products often include in their formulation more than one polysaccharide to achieve the desired properties during and after processing. Many such mixed systems behave as would be expected from the known properties of the individual polymers. In others, however, their properties are superior to those of either component alone, or may be qualitatively different. In many polysaccharide systems, the combination of a gelling polymer with a nongelling one gives rise to strong synergistic effects, as a consequence of interaction among different chain polymers and formation of mixed junction zones.Probably, the most exploited mixed gels, especially by the food industry, are those involving the microbial polysaccharide xanthan gum (XG) and the plant galactomannans, like locust bean gum (LBG). Concentrated aqueous systems of LBG and XG display quite different rheological properties: the former show the behaviour typical of hyperentangled macromolecular solutions, whereas the flow and viscoelastic properties of XG systems correspond to those of tenuous, weak-gel networks. Interestingly, when mixed together these macromolecules interact to form a firm, thermoreversible gel with synergistic effects.In the present paper we report the results of a thorough investigation of both polymer concentration and temperature effects on the rheological properties of mixed LBG-XG systems in 20 mM KCl under continuous and oscillatory flow conditions.Under continuous shear at 25°C, pure LBG shows the flow properties of a macromolecular solution, with a shear-thinning behaviour and a Newtonian region at low shear rates, whereas the rheological behaviour of XG and all LX mixed systems is that typical of weak-gels. Furthermore, in the mixed systems the viscosity values do not increase monotonically with increasing xanthan concentration, but the synergistic effect has a maximum in accordance with the XG:LBG ratio 1:1. As the temperature is increased from 25°C to 85°C, whilst the LBG system do not show any qualitative change but there is only a parallel, downward shift of viscosity values, in the case of xanthan there is a dramatic change in the corresponding curve profiles, due to the thermally induced helix-coil conformational transition.The differences in the rheological behaviour of the systems examined can be better shown through dynamic tests at 25°C. The strain sweeps performed at constant frequency of oscillation reveal that the mixed systems show higher sensitivity to strain amplitude, and lower strain values must be attained to ensure linear viscoelastic properties. The mechanical spectra clearly show the influence of composition on the viscoelastic properties of these biopolymer systems. All LX systems show the mechanical spectra typical of polysaccharide gels: G′ is always much greater than G″ and is nearly independent of the applied frequency over a wide frequency range. In addition, the marked gap between the elastic responses of the pure LBG and the LX 1:3 systems demonstrates the strong effect of the initial addition of xanthan to the pure LBG, especially in the low frequency range, whereas the highest synergistic effect is attained for the LX 1:1 system. A comprehensive description of the frequency dependence of both moduli can be suitably obtained through the four-parameter Friedrich model, which belongs to the class of fractional derivative approaches viscoelasticity.The same thermal effect is observed for the XG and all LX mixed systems considered, indicating a progressive change from the behaviour of a typical gel to that of a quasi-solution state, when temperature is increased from 25°C to 85°C. Among all mixed systems, the LX 1:1 has the highest values of the moduli at any temperature considered, and is characterized by the highest gel-sol transition temperature. In all LX systems, the temperature sweeps show that the gel-sol transition follows a two-step process, characterized by the presence of two inflection points in the relevant G* vs T curves. The first step could be reasonably ascribed to the melting process of the mixed xanthan-locust bean gum junction zones, in which the association of XG with LBG is occurring with the xanthan component in its fully ordered helical conformation. The second step, occurring at higher temperature, can be attributed to the conformational transition of the xanthan chains.

Rheology of polysaccharide systems

The transport properties and, specifically, the rheological behavior of real and complex materials such as polysaccharide systems can be significantly affected by several factors, mainly related to molecular and supermolecular features. Most of these factors are common to all polymeric systems, as they have universal character; others are peculiar to carbohydrate polymers. If we bear in mind the concepts discussed in §1.4, we can easily imagine for polysaccharides a variety of structural conditions much wider than that generally observed for synthetic polymers. On both molecular and supermolecular scales, polysaccharides possess special characteristics that reflect specific behavior so that different classes of materials can be identified.

Flow properties of hydroxypropyl guar gum and its long-chain hydrophobic derivatives

Abstract This paper deals with the rheological properties under continuous shear flow conditions of aqueous systems of hydroxypropyl guar gum (HPG) and three derivatives, of the same or lower molecular weight (MW) than HPG, characterized by different contents of long-chain hydrophobic pendants. The shear-dependent behavior of HPG and of the derivative with low molecular weight and low degree of hydrophobization (LMLH) resembles that of isotropic polymer solutions; accordingly, it can be fairly well described by the Cross equation, the effect of temperature discussed in terms of activation energy for viscous flow and the dependence of the Cross parameters η0 and λ on polymer concentration expressed by simple power-laws. The other two derivatives considered, i.e. the one of low MW and high long-chain hydrophobic substitution and the other of a MW as high as that of HPG and a low hydrophobic pendant content (LMHH and HMLH, respectively) show an unusual behavior, characterized by the existence of a shear rate interval in which the shear viscosity undergoes a catastrophic breakdown, and which cannot be described by any simple rheological model. Nevertheless, as far as the temperature effect is concerned, all experimental data can be combined into one master curve by a simple shifting procedure. All systems exhibit marked time-dependent properties of the thixotropic type, which are described with a stretched exponential model and discussed in terms of the variation of the model parameters with temperature and polymer concentration. The peculiar features of the long-chain hydrophobic derivatives can be ascribed to a balance between inter- and intramolecular interactions, which is mainly governed by the local stress field.

A study of the rheological behavior of scleroglucan weak gel systems

Abstract This study deals with the rheological behavior of aqueous systems of scleroglucan, the neutral polysaccharide secreted exocellularly by certain fungi of the genus Sclerotium . We investigated several aqueous systems under different temperatures and polymer concentrations by means of continuous and oscillatory flow procedures. Continuous shear flow tests revealed that a transition exists between a sol-like and a weak gel behavior for those scleroglucan systems in a polymer concentration range between 0.2 and 0.3% w/w. All systems exhibit marked non-Newtonian properties, which change from shear-thinning in the solution domain to plastic in the gel domain; however, the shear-dependent behavior of all systems can be satisfactorily described by a modified Cross equation. As far as the time-dependent properties are concerned, all scleroglucan aqueous systems exhibit a thixotropic response; stress transient experiments show that a delay time of approximately 4–5 min is necessary to remove shear history and to reconfigure the unperturbed state. Both classical dynamic measurements and parallel superposed continuous and oscillatory shear tests confirm the sol-gel transition for scleroglucan aqueous systems, and highlight the weak, transient nature f this microbial polysaccharide gel state.

Experimental determination of the theophylline diffusion coefficient in swollen sodium-alginate membranes

In this paper attention is focused on the determination of the drug diffusion coefficient in a swollen polymeric membrane referring to a recent mathematical model (linear model). The main advantage deriving from its use is that, despite its analytical nature and its ability to account for the most important aspects characterising a permeation experiment, it can also be applied in the case of thick membranes. To check the model reliability, a comparison is made with a more complex numerical model and with a largely employed model in terms of data fitting quality. To this purpose, particular care is devoted to the experimental and theoretical tools employed to calculate the auxiliary parameters required by the three models, and with the aim of getting a drug diffusion coefficient value as accurate as possible. Theophylline was chosen as model drug owing to its wide employment in the pharmaceutical field. Membranes were prepared with sodium alginates hydrogels at three different polymer concentrations. The present analysis demonstrates the reliability of the linear model and reveals that the theophylline diffusion coefficient is not significantly affected by the polymer concentration. Indeed, such a parameter is reflected in different membrane thicknesses rather than in different mesh sizes of the polymeric network.

Flow behavior of fresh cement pastes. A comparison of different rheological instruments and techniques

Abstract An investigation on the rheological behavior of fresh cement pastes is reported. In particular, the results obtained by the application of various experimental procedures and measurement devices with different geometric characteristics are compared and discussed. The coaxial cylinder viscometers Rotovisko-Haake RV3 and RV11 were employed. The cement pastes were prepared from a Portland cement 324. The water/cement ratio was within the range 0.30 + 0.40.

Thixotropic behaviour of cement pastes

Abstract In the present work the thixotropic behaviour of fresh Portland cement pastes is examined on the basis of the stress transients determined at constant shear rate. Some significant quantities are considered: the difference between maximum and equilibrium shear stresses, the breakdown area and the density of the energy absorbed by thē material for the breakdown of the thixotropic structure. A distance parameter is proposed in order to evaluate the influence of the specific surface and water/cement ratio on these quantities.