All issues / Volume 15 (2021) / Issue 8 (August)
This is an editorial article. It has no abstract.
Influence of accelerated weathering on the physical and structural properties of poly(lactic-acid)/poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PLA/PHBV) blends
A. Antunes, A. S. Luyt, A. Popelka, A. Mahmoud, O. Aljarod, M. K. Hassan, P. Kasak
Vol. 15., No.8., Pages 687-707, 2021
Vol. 15., No.8., Pages 687-707, 2021
The paper aims to study the poly(lactic acid) (PLA) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) blend properties after their degradation under 2000 h of accelerated weathering conditions following the ASTM D4329 standard. PLA/PHBV blends form a biphasic system shown by the presence of two distinct glass transition temperatures and melting temperatures in differential scanning calorimetry (DSC) analysis, attributed to the neat polymers regardless of the blend composition. Scanning electron microscopy (SEM) indicated that the addition of high PHBV content to the blend results in a very heterogeneous surface with cavities available for moisture and UV penetration resulting in easier bulk and in-depth degradation of the samples. Thus, the wettability properties (evaluated by contact angle measurements) of the blends changed significantly over the weathering time. On the other hand, the PLA content delays the degradation process of the blends because of the resultant crystallinity from weathering degradation which exhibits a physical barrier to protect the PHBV fraction. PHBV improves the toughness of the blend and acts as a nucleating agent for PLA, promoting its crystallinity during sample preparation. Fourier-transform infrared (FTIR) confirmed photodegradation of all the blends via a Norrish II mechanism.
Natural materials often consist of hierarchical architectures, which are extremely efficient in mechanical terms. Whereas the structure-function relationship is well-studied in natural hard materials, soft materials are not getting equal attention, despite their high prevalence in nature. These soft materials are usually constructed as fiber-reinforced composites consisting of diverse structural motifs that result in an overall unique mechanical behavior. In this study, as a proof-of-concept, a soft biomimetic composite was fabricated from a hierarchical electrospun polyamide fiber, reinforcing a hydrogel matrix and creating a simple synthetic analog for natural soft composites. This material system investigates the structure-function relationship between the structure and mechanical function by mimicking different structural motifs. The polyamide-hydrogel composite exhibited large deformations and nonlinear material behavior. Varying degrees of crimping enabled a controlled strain stiffening behavior and engineered transition from matrix-dominated to fiber-dominated behavior. We also observed that the individual nanofibers in our bundles created cross-bridges with the matrix and within the bundle, making the material system more resistant to failure. Our bio-inspired composite demonstrated mechanical behaviors similar to natural soft composites, which can aid in the future design and development of the next generation of soft architectural composites.
Poly(lactic acid) (PLA) has been found to be important in various applications, such as in the medical, pharmaceutical, and packaging industries. However, the long-term associated degradation process of PLA is a limiting factor for some applications. Therefore, in this research, the influence of corona and radio-frequency (RF) surface plasma treatment on the degradation of PLA in accelerated weathering tests was studied. The accelerated weathering test was applied using standard UV irradiation for up to 2000 h. The morphological/topographical, chemical, crystallization, mechanical, and thermal changes were analyzed after 500, 1000, and 2000 h of accelerated weathering time. The introduction of the polar functional groups caused by plasma treatment on the PLA surface improved its wettability, and therefore, hydrolytic degradation was promoted over the accelerated weathering time. It was revealed that the plasma treatment enhanced the hydrolytic and UV degradation of the PLA, as was confirmed by investigation of the physical, chemical, mechanical, and thermal properties. Moreover, the RF plasma was more pronounced than the corona plasma in the degradation of the PLA. Such an approach represents a pathway to promote and tailor PLA degradation.
Wood-plastic composites (WPCs) are a group of emerging and a sustainable class of high-performance materials, consisting of polymers reinforced with wood particulates, having a wide range of applications in the field of building, infrastructure, and transportation. However, the main drawback of the WPCs is their high flammability. Fire retardants usually enhance WPCs’ flame retardancy but at the expense of mechanical properties. This paper reviews the available literature on flame retardant WPCs in developing an optimum condition between the flammability and mechanical properties, i.e., the addition of wood-flour and flame retardant (FR) in order to find a balance between the flammability resistance and their mechanical properties. It concentrates on the recent advances in the mechanical properties and flammability studies of wood flour-polymer composite products. The applications and durability of these flame retardant WPCs with future remarks are also highlighted.
For more effective removal of the hidden troubles from early minor damages before their spreading out, hyperbranched polyurethane crosslinked by dioxyphenylalanine-Fe3+ (DOPA-Fe3+) and histidine-Zn2+ (His-Zn2+) coordination bonds is synthesized. By taking advantage of the cascading variation of the two types of metal-ligand complexations under the applied force, the growth of minor damages is firstly blocked, and then rehabilitation of the blocked damages takes place without manual intervention. Moreover, the experimental results indicate that the specific structure of hyperbranched macromolecules, which possess plenty of functional groups and great mobility, benefits to construct a stronger network with rapid response than that derived from the linear macromolecules. As a result, the stress-induced micro-voids in the crosslinked hyperbranched polyurethane are much smaller, and its robustness is allowed to be maintained to a higher extent, which is consistent with the concept of timely repairing upon destruction.
Adenine containing phthalonitrile (ADCN)/graphene/Fe3O4 composites with excellent EMI performance, outstanding thermal, thermo-oxidative stability, and heat resistance ae introduced. The interactions between ADCN and nanofillers combined with high-energy ball milling-molding sintering processing method ensure the uniform dispersion of nanofillers in the matrix. The EMI shielding effectiveness (EMI SE) of ADCN/G/Fe3O4 is up to 50 dB with only 0.5 mm thickness, which is one of the best EMI performance compared with previous reports on conductive polymer composites (CPCs). The high-performance ADCN polymer introduces outstanding thermal, thermo-oxidative stability and heat resistance into the composites. The T5% in the air of composites could still maintain more than 491 °C, and the EMI value of composites after flame retardancy test could reach 40 dB. The results prove that ADCN/G/Fe3O4 composite is a promising candidate that could be used in the aerospace industry and under extreme working conditions.
The influence of excimer argon fluoride (ArF) laser cutting of poly(L-lactide) (PLLA) on its physicochemical properties was studied. The samples in the form of micro-bars of the size comparable to the width of typical vascular stents struts were fabricated in order to focus during the analysis on heat-affected zone and surfaces exposed to laser radiation and plasma. The threshold fluence and ablation rate of PLLA were measured. The ablated polymer’s surface was analyzed using Scanning Electron Microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR-ATR). The produced microstructures with different laser repetition rates and beam size were analyzed using differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) techniques. It allowed tracking the thermal history changes of PLLA, estimating the fraction of modified polymer within micro-bars, and determining the extent of the polymer degradation by observing the molecular weight changes. In many cases, DSC revealed no significant changes in the material’s thermal history; however, GPC showed that each laser fabricated sample has a low molecular weight fraction resulting from multi-pulse exposition of the micro-bar walls to UV irradiation, laser-induced plasma, UV irradiation and heat. Nevertheless, most of the micro-bars preserve their original properties within the vast majority of their volume what gives the potential to use them in biomedical applications after cleaning the components from low molecular weight fragments localized at the cut walls.