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A polyimide (PI) aerogel with excellent combined thermal and dielectric properties was successfully prepared by the polycondensation of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 5-amino-2-(4-aminophenyl)benzoxazole (APBO) and octa(amino-phenyl)silsesquioxane (OAPS) crosslinker, followed by a supercritical carbon dioxide (scCO2) drying treatment. The developed PI aerogel exhibited an ultra-low dielectric constant (k) of 1.15 at a frequency of 2.75 GHz, a volume resistivity of 5.45·1014 Ω·cm, and a dielectric strength of 132 kV/cm. The flexible PI aerogel exhibited an openpore microstructure consisting of three-dimensional network with tangled nanofibers morphology with a porosity of 85.6% (volume ratio), an average pore diameter of 19.2 nm, and a Brunauer-Emmet-Teller (BET) surface area of 428.6 m2/g. In addition, the PI aerogel showed excellent thermal stability with a glass transition temperature (Tg) of 358.3 °C, a 5% weight loss temperature over 500 °C, and a residual weight ratio of 66.7% at 750 °C in nitrogen.
Vinyltrimethoxysilane (VTMOS) was grafted on to the backbone of poly(lactide) (PLA) through a free radical grafting reaction using reactive extrusion (REX) processing. The methoxy groups of the silane provide the modified PLA sites for crosslinking through a moisture induced pathway. VTMOS grafting efficiencies of up to 90% were obtained. The newly created methoxy functionality of the modified PLA readily undergoes hydrolysis and condensation forming siloxane crosslinks in the material. Crosslinking with VTMOS exhibited improved modulus, strength, and impact toughness while showing a decrease in ductility. Incorporating silanol-terminated poly(dimethylsiloxane) (OH-PDMS) resulted in the formation of longer siloxane crosslinks. These samples showed an increase in modulus and impact toughness due to the crosslinking, while the longer siloxane linkages resulted in improved ductility and tensile toughness. This is unusual for polymers toughened through crosslinking reactions. Scanning Electron Microscopy (SEM) of the fractured surfaces showed the presence of these elongated siloxane crosslinks. This enhanced ability for the modified PLA to deform and absorb energy results in the increase in both impact and tensile toughness.
Cyclodextrin-based cationic star polymers were synthesized using β-cyclodextrin (β-CD) core, and 2-(dimethylamino) ethyl methacrylate (DMAEMA) as hydrophilic arms. Star-shaped polymers were prepared via a simplified electrochemically mediated ATRP (seATRP) under potentiostatic and galvanostatic conditions. The polymerization results showed molecular weight (MW) evolution close to theoretical values, and maintained narrow molecular weight distribution (MWD) of obtained stars. The rate of the polymerizations was controlled by applying more positive potential values thereby suppressing star-star coupling reactions. Successful chain extension of the ω-functional arms with a hydrophobic n-butyl acrylate (BA) formed star block copolymers and confirmed the living nature of the β-CD-PDMAEMA star polymers prepared by seATRP. Novelty of this work is that the β-CD-PDMAEMA-b-PBA cationic star block copolymers were synthesized for the first time via seATRP procedure, utilizing only 40 ppm of catalyst complex. The results from 1H NMR spectral studies support the formation of cationic star (co)polymers.
The focus of this study is to explore the potential use of Polyamide 6 nanocomposite reinforced with nanocrystalline (nc) Fe20Ni80 alloy (Fe20Ni80/PA6 PNC) in electromagnetic applications and provide understanding of how the alloy particle geometry is controlling the nanocomposite’s physical properties. Thermomechanical rigidity, room-temperature soft magnetic performance and thermal soft magnetic stability of Fe20Ni80/PA6 PNCs based on spherical-sea urchin alloy particles (UMB2-SU) and necklace-like alloy chains (UMB2-NC) have been investigated. Both PNCs have considerably superior bulk properties compared to neat PA6 and UMB2-SU exhibits the most remarkable overall performance. Morphological observations disclose two relevant phenomena: i) improved dispersion and distribution of the SU alloy particles than the NC ones within PA6 matrix, leading to stronger filler-matrix interfacial interactions within the UMB2-SU as compared to the UMB2-NC and ii) presence of constraint polymer regions in between alloy segments within the UMB2-SU that provide secondary reinforcing and soft magnetic mechanisms. Such phenomena along with the lower alloy crystallite size and PA6 γ-crystal type content within the UMB2-SU than in the UMB2-NC, are considered the main responsible factors for the distinctive performance of UMB2-SU. Overall, compared to various ferromagnetic nanocrystalline metallic materials, the research proposes the SU nc Fe20Ni80 alloy as a valuable nanofiller in polymers for electromagnetic applications.
Polylactic acid (PLA) was melt-blended with Pinus radiata unmodified and modified (hydroxypropyled) bark polyflavonoids in order to use such polyphenolic building-blocks as functional additives for envisaged applications. Rheological, morphological, molecular, thermal, and flexural properties were studied. Polyflavonoids improved blend processability in terms of short-time mixing. Furthermore, hydroxypropylated polyflavonoids improve miscibility in binary and ternary blends. Blend-composition affects crystallization-, melting-, and glass transition-temperature of PLA, as well as thermal resistance, and flexural properties of the blends. Polyflavonoids induced PLA-crystallization, and polymer-chain decomposition. Modified and unmodified bark polyflavonoids from radiata pine can be used successfully in PLA-based green composites beyond the food-packaging applications. The high compatibility between PLA and hydroxypropyled polyflavonoids highlights the potential of such phenolic derivatives for PLA-based material design.
Radiation cross-linking of polyamide 66 with electron beams alters the material’s characteristics. This leads to a varied relationship amongst the process, structure, and properties for infrared welding cross-linked polyamide 66. A threedimensional network of covalent bonds results in an impeded melt flow and altered welding characteristics. Compared to non-cross linked polyamide, a changed energy input in the weld during infrared heating and a reduced meltdown can be observed. Such thermal developments and a reduced meltdown affect the resulting weld strengths. Welding factors of almost 60% of base material strengths can be achieved. A clear influence of the heating time on the weld strength can be observed. The scope of this article is to investigate the influence of radiation cross-linking on the material characteristics and, by extension, the resulting processing and welding characteristics. Mechanical and optical investigations serve to highlight the influence of radiation cross-linking on the infrared welding process of polyamide 66.
2D radial injection vacuum-assisted resin transfer molding experiments were performed using anisotropic plainwoven fabrics to determine the void distribution and the relationship between the void fraction and the resin flow velocity at arbitrary resin impregnation angles. The obtained void fraction values vary with the impregnation angle and velocity, while void formation is very difficult at the minimum-void angle oriented in neither the warp nor the weft direction. Moreover, the impregnation in the fabric microscopic structure is characterized by two patterns separated at the minimum-void angle. Based on the experimental results, a mathematical model for predicting the void fraction value at arbitrary impregnation angles and velocities and for calculating the minimum-void angle was developed. A comparison of the model predictions with the experimental results revealed a good agreement between them.