Reactive processing with low-molar-mass modifiers is a well-known method to create long-chain branched (LCB) structures in a poly(ethylene terephthalate) (PET) melt. However, less is known about the ...elongational flow properties of LCB-PET. Therefore, the aim of this contribution is (a) to generate LCB molecules and (b) to evaluate the influence of the branching level on the transient elongational behavior. For this purpose, a commercial, linear PET and different contents (0.1–0.3 wt %) of the tetrafunctional modifier pyromellitic dianhydride (PMDA) were reactively processed. All samples were analyzed by size exclusion chromatography coupled with a light scattering device and characterized by shear and elongational rheometry. It was found that the molar mass distribution of the modified materials exhibit a high molar mass shoulder, leading to an increase of the weight-average molar mass and a broadening of the molar mass distribution. Moreover, the Mark–Houwink plot of the modified materials displays deviations from the power law toward lower intrinsic viscosities, which indicate the existence of LCB molecules. The shear viscosity shows a pronounced shear thinning behavior and a remarkable increase at low frequencies compared to the linear PET. Considering the transient elongational viscosity, a distinguished strain hardening is observed, which increases with increasing PMDA content and with increasing strain rate. From the results of the rheological and molecular characterization and by considering the chemical reaction mechanisms, it can be concluded that the PET modified with high PMDA contents has a treelike branch-on-branch architecture, which is well-known from low-density polyethylene melts.
A crucial property for the evaluation of bioinks, besides biocompatibility, is printability, which is determined by resolution and shape fidelity. Recently, fiber reinforcement was used to overcome ...rheological limitations and introduce biomimetic structuring. This study provides a systematic approach to evaluate the printability of fiber reinforced hydrogels. Alginate and Pluronic hydrogels were blended with cellulose nanofibers (CeNF) and polycaprolactone (PCL) microfibers. SEM imaging revealed fiber-induced structural changes. Oscillatory rheological experiments showed that the addition of fiber fragments significantly altered the complex viscosity. A customized setup was utilized to determine strut spreading behavior in a real extrusion printing process. Strikingly, the data displayed excellent correlation with viscoelastic model-based predictions. CeNF increased the shape fidelity of both hydrogels, while PCL microfibers increased the viscosity but resulted in a time dependent loss of structural integrity in Pluronic. The results emphasize the need to complement shear-rheological analysis of bioinks by print-related customized analytical tools.
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Magnetic composites made of LSR (Liquid Silicone Rubber) and a magnetic filler have been investigated with respect to magnetic field guidance for transcutaneous energy transport. Effects of particle ...size and size distribution on the effective initial magnetic permeability are analyzed. A simple model is presented to describe the permeability as a function of the degree of filling, particle size and size distribution. From this, a calculus for the relevant size average of the filler was developed at 20 vol% of magnetic filler. The calculus showed self-consistency and can be used to tailor optimized powders for better field guidance.
•Discussion of influencing factors on the breakdown strength of silicone elastomers.•Development of a semi-empirical model on the influence of air voids.•Development of a model depending only on the ...main influencing factor, the compression.•Increase in dielectric breakdown strength by a factor of 8.7.•Addition of barium titanate has the same effect as the increase through compression.
Dielectric elastomers are utilized as soft electroactive polymers for actuators, sensors or generator applications to transduce mechanical to electrical energy and vice versa. They are characterized by the ability to change shape while an electrical field is applied. Indeed, this effect is most significant for electrical fields near the electrical breakdown strength, therefore it is a limiting factor for application. This work investigates different factors influencing the dielectric breakdown strength of soft silicone elastomers such as the sample thickness, the force and the radius of the upper electrode, the filling degree and the size of air voids. It can be shown that the main influence is the compression of the sample, thus a novel master curve can be created and fitted by a semi empirical model function. Therefore a comparison of all different experimental set-ups is possible.
In the context of tissue engineering, biofabrication techniques are employed to process cells in hydrogel-based matrices, known as bioinks, into complex 3D structures. The aim is the production of ...functional tissue models or even entire organs. The regenerative production of biological tissues adheres to a multitude of criteria that ultimately determine the maturation of a functional tissue. These criteria are of biological nature, such as the biomimetic spatial positioning of different cell types within a physiologically and mechanically suitable matrix, which enables tissue maturation. Furthermore, the processing, a combination of technical procedures and biological materials, has proven highly challenging since cells are sensitive to stress, for example from shear and tensile forces, which may affect their vitality. On the other hand, high resolutions are pursued to create optimal conditions for subsequent tissue maturation. From an analytical perspective, it is prudent to first investigate the printing behavior of bioinks before undertaking complex biological tests. According to our findings, conventional shear rheological tests are insufficient to fully characterize the printing behavior of a bioink. For this reason, we have developed optical methods that, complementarily to the already developed tests, allow for quantification of printing quality and further viscoelastic modeling of bioinks.
Polymer material exhibits a trade-off between toughness and stiffness, therefore, it remains difficult to develop a strategy that simultaneously realizes improved mechanical strength and toughness. ...Inspired by the bamboo-like structure, an efficient and simple melt sequential injection molding is proposed to fabricate a controllable skin-core structure of iPP samples with self-reinforcement and toughness. With increasing the melt injection number, the shear layers containing shish-kebabs are progressively thickened, resulting in an effective improvement of mechanical properties. The tensile strength increased from 35.1 to 55.3 MPa for the melt injection number from one to four, meanwhile the toughness was enhanced from 23.1 to 53.6 kJ/m2. A simple theoretical model was derived and utilized to describe the experimental data. Theoretically calculated tensile strength of sample with entirely shish-kebabs can even reach 222.9 MPa via this technology, which is much higher than that of most engineering plastics and polymer composites. This work not only provides an efficient way to fabricate high-performance general plastics for engineering purposes but also a new opportunity to understand the fundamental issues about the mechanism of the self-reinforced polymers.
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•A melt sequential injection molding (MSIM) technology is proposed.•MSIM sample with a thickened shear layer is obtained.•The tensile strength, stiffness and toughness are simultaneously increased.•A simple model is derived and utilized for the description of the tensile strength.
Selective serotonin reuptake inhibitors (SSRIs) are widely prescribed for the treatment of depression and anxiety-related disorders. These drugs target the serotonin transporter (5-HTT) and increase ...serotonin signalling. Although chronic SSRI administration has few reported side effects, recent observations suggest that it could have long-term effects on neurodevelopment. First, 5-HTT is transiently expressed in many brain areas during development. Second, 5-HTT blockade during development causes wiring defects in these areas. These effects are seen most clearly in the sensory systems. Third, the behavioural effects of 5-HTT blockade during development are sometimes dramatically different from the effects of 5-HTT blockade during adulthood. Most of this evidence was collected from studies with 5-HTT knockout mice and rats. However, the phenotypes associated with low or high functioning 5-HTT alleles in humans can result from similar developmental alterations in 5-HT levels. Here, we review the existing evidence on the long-term effects of developmental SSRI exposure.
In this study, poly(methyl methacrylate) (PMMA)/carbon black (CB), PMMA/carbon fiber (CF), and PMMA/carbon nanotube (CNT) conductive composite films with different filler concentrations are prepared ...using the solution casting technique. Both perpendicular and in‐plane direction conductivity of all the binary composite films are investigated, percolation thresholds (ϕc) of both directions of PMMA/CB, PMMA/CF, and PMMA/CNT composite films are investigated and the experimental data are fitted using McLachlan’s equation. For all the three investigated films, the perpendicular ϕc,⊥ and in‐plane ϕc,∥ with different fillers show totally different behaviors. Pristine CB, CF, and CNT as well as PMMA/CB, PMMA/CF, and PMMA/CNT composite films are discussed. The gravity effect of the fillers is found to be most significant in the PMMA/CB system. A schematic diagram of PMMA composite films with CB, CF, and CNT as filler prepared from solution casting process is presented to explain the distribution gradient of the fillers in the perpendicular direction of the film after solution casting. A power law behavior is revealed for different filler types (CB, CF, CNT) correlating the exponent t for McLachlan’s equation and corresponding ϕc for in‐plane and perpendicular directions.
Both perpendicular and in‐plane conductivity of poly(methyl methacrylate) (PMMA)/carbon black (CB), PMMA/carbon fiber (CF), and PMMA/carbon nanotube (CNT) composite films are investigated. The electrical percolation thresholds (ϕc) of the films are fitted using McLachlan’s equation. A power law behavior is revealed for different filler types (CB, CF, CNT) correlating the exponent t for McLachlan’s equation and corresponding ϕc for perpendicular and in‐plane directions.
Mixing of inorganic particle into polymer material is widely used to tailor polymer with desired physical properties and processibility, in which the size and concentration of the former play a ...critical role in polymer processing. In this study, rigid spherical silica particle with varying size (average diameter of 7 nm, 40 nm, and 9 μm) were used to reinforce polylactide (PLA) by melt compounding. The dependence of rheological properties of PLA/silica composites on the silica size and concentration were examined by dynamical mechanical and creep-recovery experiments. Our results demonstrate that mixing of silica particles into PLA matrix could increase the thermal stability of PLA. Oscillatory shear tests in the linear viscoelastic regime revealed a strong concentration-dependent behavior for the storage, loss moduli, and complex viscosity of the PLA/silica composites by the addition of nanosilica, while these properties were slightly affected by the addition of microsilica at low frequency range. A linear relationship between the silica concentration and the logarithmic form of zero shear viscosity (logη0) of the composites is found when the concentration is below the rheological percolation threshold, whereas the growth rate is inversely influenced by the silica particle size. Creep-recovery experiments indicated that the elastic properties of PLA were more sensitive by the addition of silica than the viscous properties. Even for microsilica, a remarkable enhancement of the elastic properties was found at low silica concentration. A model based on the radius of gyration of polymer matrix Rg and the mean distance between particles D is proposed to describe the interactions between polymer matrix and particles. When D is larger than 2Rg, the particles-polymer interactions are suggested to be responsible for the rheological properties. On the other side, when D is smaller than Rg, a silica network structure will be formed and the rheological properties of the composites are dominated by the interactions of particle–particle and particle-polymer.
Posterior lamellar transplantation of the eye' s cornea (DSAEK, DMEK) currently is the gold standard for treating patients with corneal endothelial cell and back surface pathologies resulting in ...functional impairment. An artificial biomimetic graft carrying human corneal endothelium could minimize the dependency on human donor corneas giving access to this vision-restoring surgery to large numbers of patients, thus reducing current long waiting lists. In this study, four groups of electrospun nanofibrous scaffolds were compared: polycaprolactone (PCL), PCL/collagen, PCL/gelatin and PCL/chitosan. Each of the scaffolds were tissue-engineered with human corneal endothelial cells (HCEC-B4G12) and analyzed with regard to their potential application as artificial posterior lamellar grafts. Staining with ZO-1 and Na
/K
-ATPase antibodies revealed intact cell functionalities. It could be shown, that blending leads to decreasing contact angle, whereby a heterogeneous blend morphology could be revealed. Scaffold cytocompatibility could be confirmed for all groups via live/dead staining, whereby a significant higher cell viability could be observed for the collagen and gelatine blended matrices with 97 ± 3% and 98 ± 2% living cells respectively. TEM images show the superficial anchoring of the HCECs onto the scaffolds. This work emphasizes the benefit of blended PCL nanofibrous scaffolds for corneal endothelial keratoplasty.
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