The exact knowledge of hydrogel microstructure, mainly its pore topology, is a key issue in hydrogel engineering. For visualization of the swollen hydrogels, the cryogenic or high vacuum scanning electron microscopies (cryo-SEM or HVSEM) are frequently used while the possibility of artifact-biased images is frequently underestimated. The major cause of artifacts is the formation of ice crystals upon freezing of the hydrated gel. Some porous hydrogels can be visualized with SEM without the danger of artifacts because the growing crystals are accommodated within already existing primary pores of the gel. In some non-porous hydrogels the secondary pores will also not be formed due to rigid network structure of gels that counteracts the crystal nucleation and growth. We have tested the limits of true reproduction of the hydrogel morphology imposed by the swelling degree and mechanical strength of gels by investigating a series of methacrylate hydrogels made by crosslinking polymerization of glycerol monomethacrylate and 2-hydroxyethyl methacrylate including their interpenetrating networks. The hydrogel morphology was studied using cryo-SEM, HVSEM, environmental scanning electron microscopy (ESEM), laser scanning confocal microscopy (LSCM) and classical wide-field light microscopy (LM). The cryo-SEM and HVSEM yielded artifact-free micrographs for limited range of non-porous hydrogels and for macroporous gels. A true non-porous structure was observed free of artifacts only for hydrogels exhibiting relatively low swelling and high elastic modulus above 0.5 MPa, whereas for highly swollen and/or mechanically weak hydrogels the cryo-SEM/HVSEM experiments resulted in secondary porosity. In this contribution we present several cases of severe artifact formation in PHEMA and PGMA hydrogels during their visualization by cryo-SEM and HVSEM. We also put forward empirical correlation between hydrogel morphological and mechanical parameters and the occurrence and intensity of artifacts.

At present time the number of implantations of joint replacements as well as their revisions increases. Higher demands are required on the quality and longevity of implants. The aim of this work was to determine the degree of oxidative degradation and the amount of free/residual radicals in selected ultra-high molecular weight polyethylene (UHMWPE) components of the joint replacements and demonstrate that the measured values are closely connected with quality and lifetime of the polymer components.

The multiple roles of organic nanofillers in biodegradable nanocomposites (NC) with a blend-based matrix is not yet fully understood. This work highlights combination of reinforcing and structure-directing effects of chitin nanowhiskers (CNW) with different degrees of deacetylation (DA), i.e., content of primary or secondary amines on their surface, in the nanocomposite with the PCL/PLA 1:1 matrix. Of importance is the fact that aminolysis with CNW leading to chain scission of both polyesters, especially of PLA, is practically independent of DA. DA also does not influence thermal stability. At the same time, the more marked chain scission/CNW grafting for PLA in comparison to PCL, causing changes in rheological parameters of components and related structural alterations, has crucial effects on mechanical properties in systems with a bicontinuous structure. Favourable combinations of multiple effects of CNW leads to enhanced mechanical performance at low 1% content only, whereas negative effects of structural changes, particularly of changed continuity, may eliminate the reinforcing effects of CNW at higher contents. The explanation of both synergistic and antagonistic effects of structures formed is based on the correspondence of experimental results with respective basic model calculations.

The pendant drop method is a widely used technique for measuring boundary tension (the term involves both surface and interfacial tension). The method is based on the evaluation of the shape of a drop in mechanical equilibrium. The method was often declared suitable for viscous materials. Contrary to this, an opposite opinion can sometimes be encountered. The explanation is that the higher limit of the viscosity range where the method is applicable (units or tens of Pa·s) is higher than that for many traditional methods. However, the measurement of materials with a viscosity of hundreds of Pa·s or more still brings complications. Waiting for equilibrium can be inconvenient or impossible. The relaxation of a pendant drop has not been mathematically described. However, the relaxation of an ellipsoid was described; it forms the principle of the deformed drop retraction (DDR) measurement, which is more appropriate for highly viscous materials. This paper brings together the findings from various papers dealing with errors in boundary tension measurements by the pendant drop method caused by mechanical disequilibrium. It also explores how an understanding of the relaxation process of the ellipsoid drop can help to estimate boundary tension in the pendant drop method when reaching mechanical equilibrium is a problem. The shape evolution of the pendant drop has been approximated using equations derived from the ellipsoid drop relaxation model. Recently, it has been found that the equations based on the DDR model can describe a small subset of drop behaviour, and that the parameters found in the equations vary across individual drops of the same material. Consequently, the tested equations do not permit the evaluation of boundary tension from drop shape evolution using the principles of the DDR method. Nonetheless, they can be used to extrapolate the equilibrium shape parameters for the pendant drop method. To determine boundary tension using the DDR approach, a specific mathematical model describing the relaxation of the pendant drop has to be developed. In addition, a model for non-Newtonian fluids would be useful for polymers. Such a model would require capturing the complex behaviour of these materials to improve measurement accuracy. Graphical Abstract