Novel nanostructured microfibrillated cellulose–hydroxypropyl methylcellulose films with large one-dimensional swelling and tunable permeability

Microfibrillated cellulose (MFC) films containing the water soluble and pharmaceutically approved polymer hydroxypropyl methylcellulose (HPMC) exhibited an unexpected decrease in permeability and excellent one-dimensional swelling properties above a threshold in HPMC content. It is proposed that the observed material characteristics derive from the influence of HPMC on the aggregation behavior of MFC in such a way that above a critical HPMC content the films are created through self assembly into a layered structure, composed of low swelling layers with swellable inter layer regions. The suggested structures were supported by high resolution microscopy. The findings should hold potential for direct applications, but even more as a concept for future material design

High Performance Polysodium Acrylate Superabsorbents Utilizing Microfibrillated Cellulose to Augment Gel Properties

Microfibrillated cellulose was utilized at low concentrations as a filler material, added prior to free radical polymerization, in cross-linked superabsorbent polysodium acrylate hydrogels. The effect of microfibrillated cellulose concentration on equilibrium swelling, shear modulus after synthesis, and shear modulus at equilibrium swelling was studied at different degree of cross-linking. For the characterization of the microfibrillated cellulose optical microscopy, atomic force microscopy, and transmittance analysis were used. The shear modulus of the samples was determined using uniaxial compression analysis. The swelling of the gels was determined using classical gravimetrical measurements. It was found that microfibrillated cellulose was highly efficient in increasing the shear modulus of the gels. Furthermore, the microfibrillated cellulose was found to have the same effect on the swelling and shear modulus at equilibrium swelling as the same mass of the conventional covalent cross-linker N,N'-methylenebisacrylamide (MBA), while in fact improving the fracture resistance of the gels. In conclusion, microfibrillated cellulose shows great potential as an additive to enhance the performance of soft materials

Determination of the glass transition of powder samples using Dynamic Mechanic Analysis on compacts

Dynamic Mechanic Analysis, DMA, is a sensitive method to determine the glass transition temperature (Tg) of materials. Several different sample geometries such as three point bending, stretched films or compressed cylinders, are commonly used. The Tg of a powder is related to its “sticky point”, i.e. the temperature when the powder particles interact instead of flowing freely. The determination of powder Tg is not straightforward using DMA due to the limiting geometry. Here it is demonstrated that the Tg of powder samples can be effectively and correctly determined by DMA on compacts in compression mode, using a standard wedge shaped probe usually utilized in three point bend analysis. The analysis of compacts holds the benefits of analysing powder samples as received from manufacturers, being relatively robust with regard to deformation as the material becomes rubbery, and enabling easy sample preparation and handling

Effect of calcium neutralization on elastic and swelling properties of crosslinked poly(acrylic acid) - correlation to inhomogeneities and phase behaviour

Crosslinked hydrogels of poly(acrylic acid) neutralized with calcium hydroxide were synthesized using free radical co-polymerization. The effects of the amount added calcium on the elastic modulus and swelling properties of the gels and the correlation to phase behaviour and structural changes within the gels were studied using texture analysis, gravimetric analysis, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and atomic force microscopy (AFM). It was found that the elastic modulus decreased nonlinearly with increasing amount of added calcium and that the swelling's dependence on the amount of added calcium was complicated. The maximum swelling increased with increasing amount of added calcium until a critical calcium content was reached, upon which the gels phase separated, with a strong decrease in swelling as a result. The changes in properties are explained by the fact that calcium affects the structure of the polymer network during synthesis and by the phase behaviour of the gels. Changes in the gel structure with the addition of calcium were detected with AFM. Furthermore, AFM revealed different phases on the nanometre scale for the sample with a calcium content around which phase separation is macroscopically observed. Finally, it was shown that the sulphur from the initiator potassium persulfate formed crystal like regions with high sulphur and calcium content upon drying of the hydrogels

Effect of ethanol on the water permeability of controlled release films composed of ethyl cellulose and hydroxypropyl cellulose

The robustness of controlled release formulations when co-ingested with alcohol is a current concern expressed by regulatory authorities, especially with regard to dose dumping. One such controlled release formulation commonly used is film coating composed of ethyl cellulose (EC) and hydroxypropyl cellulose (HPC). The aim of this study was to investigate how the presence of ethanol in the dissolution medium affects the water permeability of such films. Film samples were prepared in various EC–HPC compositions, and the effect of different ethanol concentrations in the dissolution medium on the permeability was studied using a modified Ussing chamber and tritiated water. It was found that the effect of ethanol on the film permeability varied depending on the composition of the films. The results were interpreted in terms of swelling of the EC in the films, where the swelling increased with increasing ethanol concentration. Thus, for films with low HPC content (non-interconnected pores), the water permeability of the films increased with increasing ethanol concentration as the diffusion through the ethyl cellulose increased due to swelling. However, for films with higher HPC content (having interconnected pores through the films), the permeability decreased, likely due to the swelling of the ethyl cellulose blocking the pores. The interpretation of the results was supported by dynamic mechanic analysis and SEM analysis

Computational homogenization of liquid-phase sintering with seamless transition from macroscopic compressibility to incompressibility

Liquid phase sintering of particle agglomerates is modeled on the mesoscale as the viscous deformation of particle-particle contact, whereby the single driving force is the surface tension on the particle/pore interface. On the macroscale, a quasistatic equilibrium problem allows for the prediction of the shrinkage of the sintering body. The present paper presents a novel FE2 formulation of the two-scale sintering problem allowing for the transition to zero porosity, implying macroscale incompressibility. The seamless transition from compressibility to incompressibility on the macroscale is accomplished by introducing a mixed variational format. This has consequences also for the formulation of the mesoscale problem, that is complemented with an extra constraint equation regarding the prolongation of the volumetric part of the macroscopic rate-of-deformation. The numerical examples shows the sintering of a single representative volume element (RVE) which is sheared beyond the point where the porosity vanishes while subjected to zero macroscopic pressure. © 2013 The Authors

On the variationally consistent computational homogenization of elasticity in the incompressible limit

Background Computational homogenization is a well-established approach in material modeling with the purpose to account for strong micro-heterogeneity in an approximate fashion without excessive computational cost. However, the case of macroscopically incompressible response is still unresolved. Methods The computational framework for Variationally Consistent Homogenization (VCH) of (near) incompressible solids is discussed. A canonical formulation of the subscale problem, pertinent to a Representative Volume Element (RVE), is established, whereby complete macroscale incompressibility is obtained as the limit situation when all constituents are incompressible. Results Numerical results for single RVEs demonstrate the seamless character of the computational algorithm at the fully incompressible limit. Conclusions The suggested framework can seamlessly handle the transition from (macroscopically) compressible to incompressible response. The framework allows for the classical boundary conditions on the RVE as well as the generalized situation of weakly periodic boundary conditions