All issues / Volume 15 (2021) / Issue 5 (May)
This is an editorial article. It has no abstract.
Ultrasound micromolding (USM) preparation of hybrid scaffolds based on polylactide (PLA) and hydroxyapatite (HAp) particles has been evaluated. PLA was stable under the applied ultrasound source since a minimum degradation was detected. Porous materials were achieved using polyethylene glycol (PEG) and NaCl salts to the initial PLA and the subsequent leaching of the micromolded specimens. To avoid cavitation and decomposition problems during micromolding, it was necessary to use HAp free of typical synthesis impurities like carbonate and nitrate compounds. Compact PLA/HAp pieces allowed a maximum HAp load of 60 wt%, while porous specimens could be obtained with a maximum load of 38 wt%. Physical characterization of new scaffolds was performed by X-ray diffraction, spectroscopic and calorimetric techniques, stress-strain tests and contact angle measurements. Results indicated that a degree of porosity of 35% and relatively good mechanical properties could be achieved (i.e., 580 MPa, 4%, and 15.6 MPa for the Young modulus, elongation at break, and tensile strength, respectively). Scaffolds showed the positive effect of HAp and porosity on cell proliferation; this latter was 40% higher than that detected for non-porous PLA specimens.
A new method for the quantitative description of the phase-specific localization of hybrid fillers in rubber blends was developed based on the wetting concept. Using the new concept, the kinetics of phase-specific localization of silica (Si)/carbon black (CB) hybrid fillers in styrene-butadiene rubber (SBR)/natural rubber (NR) blends was experimentally characterized during the single-step mixing and the multi-step mixing process. It could point out that the localization of the hybrid fillers at the end of the mixing process always reaches the stationary stage regardless of the mixing regime. The stationary state of filler localization is related to the migration of silica and CB during the mixing process, determined by the thermodynamic driving forces and well predicted by the Z model. The adsorption of the curing additives on the filler surface markedly alters the surface energy of silica and CB, which in turn influences the localization of hybrid fillers in rubber blends, particularly at low filler loading. That is why the morphological investigation made by electron microscopy for low filled rubber blends can hardly be applied to describe the filler localization of hybrid fillers in rubber blends at high filler loading.
In this work, a series of reactive in-situ polyurethane (PU) nanocomposites based on the triblock copolymer of PCL1000-PEG1000-PCL1000, chemically cross-linked by hydroxyl-functionalized MWCNTs (um-MWCNT) and PCL-grafted MWCNTs (mod-MWCNT), were synthesized. In order to optimize the shape memory performance, crystallization mechanisms of the soft domains were tuned. The nanoparticles, acting as phase controller of the block copolymer, affected the chain’s confinement and crystals’ morphology leading to a wide range of shape fixity (84–100%) and shape recovery (78–97%) ratios. Non-isothermal crystallization studies revealed that using mod- MWCNTs increased the melting temperature (Tm) as an indication of higher thermal stability of the formed crystallites. Moreover, isothermal DSC measurements, fitted to the Avrami equation, were used to measure the changes in the growth rate and morphological features of the formed crystallites. The results indicated an increase in Avrami exponent (n) from 1.43 to 3.11, and crystallization half-time (t0.5) decreased from 6.16 to 2.67 minutes for crystallization temperature (Tc) of –25 °C, attributed to the effect of PCL grafts on PUs’ microstructure. In addition, the results of cell viability, evaluated by HFF cells, proved a proper cytocompatibility. Culturing hMSCs also showed good adhesion and cell spreading, as a function of hydrophilicity. The optimum sample, containing 0.5% PCL-g-MWCNT, showed 97% shape recovery at body temperature (37°C).
This work reports the synthesis of a series of shape memory polyimides by polycondensation of 2,2′-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane (m-6FBAPP), 4,4′-(hexafluoroisopropylidene) (6FDA) and PA-terminated OAPS. The influence upon shape memory properties by the introduction of PA-terminated OAPS was investigated systematically in both experiments and simulations. PA-terminated OAPS acted as both a plasticizer and a rigid cage structure, preventing excessive movement of the polymer chain. Thus, both strain and shape recovery values of the resulting polyimide composites strongly depended on the content of PA-terminated OAPS. The shape memory polyimide composites showed high strain and recovery values equal to 247 and 94.3%, respectively. Additionally, these polyimide composites showed multiple shape memory effects and excellent shape memory cycle stability.
In this work, nickel citrate (NC) was combined with ammonium polyphosphate (APP) to improve the flame retardance of thermoplastic polyurethane (TPU). And, the influence of nickel citrate on the flame retardance of TPU/APP composites was investigated by cone calorimeter test (CCT), thermogravimetric analysis/infrared spectrometry (TG-IR), scanning electron microscope (SEM), gas chromatography/mass spectrometry (GC-Ms), and X-ray photoelectron spectrometer (XPS), respectively. It has been found the combination of NC and APP improved the flame retardancy and reduced the total heat release (THR) of TPU composites. The THR value of TPU/APP/NC0.5 was reduced by 20.7% at 750 s compared with that of TPU/APP. The flame retardancy of TPU composites and the graphitization degree of char residue from TPU composites were improved by the combination of both NC and APP. This work provided a new way to improve the flame retardancy of polymer composites.
The photomechanical effect has been studied so far mainly in elastomers, but recently more and more publications, in which this effect is studied for other materials too, so-called glassy or amorphous polymers (in particular polyimides), have appeared. This mini-review describes the photomechanical effect in polyimides containing light-sensitive derivatives of azobenzene and, in some cases, for comparison, in polyamides and polystyrenes. The work aims to present the influence of chemical structure factors on the bending angle of free-standing polymer films and their long-term stability. We focused on aspects of polymer structure such as the content of azo-moieties, the flexibility of the backbone, kind of azobenzene attachment (covalent/non-covalent), degree of crystallinity, influence the intermolecular H-bonds. The paper should help in designing new amorphous azopolymers with properties tailored to specific applications.
This study deals with investigating the morphological, mechanical, dynamic-mechanical, and shape memory properties of novel Polycaprolactone (PCL)/polypropylene carbonate (PPC)-based nanocomposite blends reinforced with GlycidylIsobutyl-functionalized polyhedral oligomeric silsesquioxane (G-POSS) nanoparticles. Scanning Electron Microscopy images of blends revealed droplet-matrix morphology with distinct domains where increased PPC content led to increased number and size for PCC droplets. Introducing nanoparticles by compatibilizing the blends changed the droplet-matrix morphology to a co-continuous one and consequently improved mechanical and shape memory properties. Distribution of nanoparticles up to 5 wt% remained uniform for samples with higher PPC contents. The shape memory analysis results revealed that the shape memory properties were highly dependent on the mixing ratio of polymers and nanoparticles content. Besides, introducing the nanoparticles caused a considerable reduction in the recovery time. Finally, the composition with a PCL/PPC ratio of 20/80 as well 5 wt% G-POSS content was suggested as the sample with optimum shape memory properties with Tg = 35 °C, fixity ratio = 98%, recovery ratio = 95%, and the remarkable recovery time of 17 seconds, enjoying the elastic modulus of 772 MPa, the tensile strength of 85.2 MPa, and elongation at break of 450% that shows the high potential of this blend for different applications, especially in the biomedical field.