Rheological behavior, crystallization properties, and foaming performance of chain-extended poly (lactic acid) by functionalized epoxy

The branched/micro-crosslinked structure formed by chain extension reaction between EGMA and PLA improved the rheological behavior and crystallization properties of PLA, which ameliorated the foaming performance of various PLA samples.

Tailored viscoelasticity of a polymer cellular structure through nanoscale entanglement of carbon nanotubes

A three-dimensional carbon nanotube (CNT) cellular structure presents a unique revelation of microstructure dependent mechanical and viscoelastic properties. Tailored CNT-CNT entanglement demonstrated a direct impact on both the strength and viscosity of the structure. Unlike traditional foams, an increase in the CNT-CNT entanglement progressively increases both the strength and the viscosity. The study reveals that an effective load is directly transferred within the structure through the short-range entanglements (nodes) resulting in an enhanced mechanical strength, whereas the long-range entanglements (bundles) regulate the energy absorption capacity. A three-dimensional structure of entangled CNT-CNT shows ∼15 and ∼26 times enhancement in the storage and loss moduli, respectively. The higher peak stress and energy loss are increased by ∼9.2 fold and ∼8.8 fold, respectively, compared to those of the cellular structures without entanglement. The study also revealed that the viscoelastic properties i.e. the Young's modulus, stress relaxation, strain rate sensitivity and fatigue properties can be modulated by tailoring the CNT-CNT entanglements within the cellular structure. A qualitative analysis is performed using finite element simulation to show the impact of CNT-CNT entanglements on the viscoelastic properties. The finding paves a way for designing a new class of meta-cellular materials which are viscous yet strong for shock absorbing or mechanical damping applications.

Scanning Techniques for Nanobioconjugates of Carbon Nanotubes

Nanobioconjugates using carbon nanotubes (CNTs) are attractive and promising hybrid materials. Various biological applications using the CNT nanobioconjugates, for example, drug delivery systems and nanobiosensors, have been proposed by many authors. Scanning techniques such as scanning electron microscopy (SEM) and scanning probe microscopy (SPM) have advantages to characterize the CNT nanobioconjugates under various conditions, for example, isolated conjugates, conjugates in thin films, and conjugates in living cells. In this review article, almost 300 papers are categorized based on types of CNT applications, and various scanning data are introduced to illuminate merits of scanning techniques.

Characterization of Stereocomplex Polylactide/Nanoclay Nanocomposites

Poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA)/nanoclay nanocomposites with nanoclay contents ranging from 1% to 8% w/w were prepared by melt blending using an internal mixer. Wide-angle X-ray diffraction (XRD) and differential scanning calorimetry (DSC) results confirmed that complete stereocomplex polylactide (PLA) crystallites without any homocrystallites were produced when equal amounts of PLLA and PLDA were mixed. The nanoclay in the stereocomplex polylactide nanocomposites acted as a nucleating agent that significantly enhanced stereocomplex crystallization, resulting in smaller and finer spherulites. Compared to neat PLLA, the melting temperature of the stereocomplex polylactide and its nanocomposites was about 55°C higher. The crystallization temperature of the stereocomplex nanocomposites was also 16°C and 55°C higher than that of the stereocomplex PLA and neat PLLA, respectively. These significant increases in transition temperatures improved the thermal stability of the stereocomplex nanocomposites compared to regular polylactide, which was confirmed by thermogravimetric analysis (TGA). The TGA results also showed that increasing nanoclay content increased the thermal stability of the stereocomplex nanocomposites. Finally, XRD and transmission electron microscopy showed an intercalation nanoclay basal spacing of 3.22 nm in the stereocomplex nanocomposites; a slight increase from the 1.86 nm basal spacing in the as-received nanoclay.

Investigation of Thermal and Thermomechanical Properties of Biodegradable PLA/PBSA Composites Processed via Supercritical Fluid-Assisted Foam Injection Molding

Bio-based polymer foams have been gaining immense attention in recent years due to their positive contribution towards reducing the global carbon footprint, lightweighting, and enhancing sustainability. Currently, polylactic acid (PLA) remains the most abundant commercially consumed biopolymer, but suffers from major drawbacks such as slow crystallization rate and poor melt processability. However, blending of PLA with a secondary polymer would enhance the crystallization rate and the thermal properties based on their compatibility. This study investigates the physical and compatibilized blends of PLA/poly (butylene succinate-co-adipate) (PBSA) processed via supercritical fluid-assisted (ScF) injection molding technology using nitrogen (N₂) as a facile physical blowing agent. Furthermore, this study aims at understanding the effect of blending and ScF foaming of PLA/PBSA on crystallinity, melting, and viscoelastic behavior. Results show that compatibilization, upon addition of triphenyl phosphite (TPP), led to an increase in molecular weight and a shift in melting temperature. Additionally, the glass transition temperature (T_g) obtained from the tanδ curve was observed to be in agreement with the T_g value predicted by the Gordon–Taylor equation, further confirming the compatibility of PLA and PBSA. The compatibilization of ScF-foamed PLA–PBSA was found to have an increased crystallinity and storage modulus compared to their physically foamed counterparts.

Microcellular injection molding of recycled poly(ethylene terephthalate) blends with chain extenders and nanoclay

Poly(ethylene terephthalate) (PET) resin is one of the most widely used thermoplastics, especially in packaging. Due to thermal and hydrolytic degradations, recycled PET (RPET) exhibits poor mechanical properties and lacks moldability. The effects of adding chain extender (CE) and nanoclay to RPET were investigated. Melt blending of RPET with CE was performed in a thermokinetic mixer (K-mixer). The blended materials were then prepared via solid and microcellular injection molding processes. The effects of CE loading levels and the simultaneous addition of nanoclay on the thermal and mechanical properties and cell morphology of the microcellular components were noted. The addition of 1.3% CE enhanced the tensile properties and viscosity of RPET. The higher amount of CE (at 3%) enhanced the viscosity, but the margin of improvement in mechanical properties diminished. While the solid RPET and CE blends were fairly ductile, the samples with nanoclay and all microcellular specimens showed brittle fractural behavior. Finally, nanoclay and the increase of CE content decreased the average cell size and enlarged the cell density of the microcellular samples.

Processing and characterization of solid and foamed injection-molded polylactide with talc

Polylactide (PLA) suffers from poor processability due to its low melt strength and slow crystallization kinetics and it is thus very challenging to achieve uniformly distributed fine-celled PLA foams with high void fractions in injection molding process. In this work, the low-pressure structural foam molding of linear PLA with a relatively high void fraction of ∼30% was conducted and by fine tuning the talc content and the foaming and processing parameters, a relatively uniform fine-celled structure with improved cell size and cell density was successfully produced. The effects of twin-screw compounding and the addition of talc on the foaming behavior, structural uniformity, crystallinity, and mechanical properties of the solid and foamed PLA samples were investigated. The results showed that the addition of 5 wt.% talc significantly improved the foaming properties such as cell density, cell size, structural uniformity, and consequently improved the mechanical properties of foams. The twin-screw compounding before injection molding did not significantly change the foaming behavior, but adversely affected the mechanical properties of the solid and foamed PLA samples due to mechanical and thermal degradation. The changes in the mechanical properties were discussed in terms of the crystallinity, talc toughening effect, and foam quality.

Study of Microcellular Injection Molding with Expandable Thermoplastic Microsphere

Injection molding with expandable thermoplastic microspheres (ETM) containing blowing chemicals is capable of fabricating lightweight, dimensionally stable plastic parts while using less material. This paper presents the study of microcellular injection molding of low density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS) parts with various ETM contents. It was found that the molded parts exhibit relatively better surface quality than conventional foamed parts. The microcellular morphology and cell density of the fractured cross-sectional surfaces were characterized using a scanning electron microscope (SEM). As reflected by the testing results, the cell microstructure – such as cell size, cell density, and a layered structure with a foamed core sandwiched by skin layers – play an important role in the weight reduction, surface quality, and mechanical properties. A smaller cell diameter and a thicker skin layer help to improve the surface quality and tensile properties of the injection molded parts with ETM. Finally, an appropriate ETM content has a positive effect on cell microstructure and weight reduction, whereas too high a concentration of microspheres adversely affects the tensile properties and surface quality.

Comparisons of microcellular polylactic acid parts injection molded with supercritical nitrogen and expandable thermoplastic microspheres: Surface roughness, tensile properties, and morphology

Microcellular injection molding is capable of fabricating light weight, dimensionally stable plastic parts while using less material and energy. Two kinds of blowing agents, namely, supercritical nitrogen and expandable thermoplastic microspheres, were employed to produce foamed polylactic acid parts. The surface characteristics were evaluated with a 2D surface roughness analyzer and a 3D white-light interferometer surface profiler. Through surface roughness comparisons, injection molded ASTM tensile test bars with expandable thermoplastic microspheres exhibited better surface quality than their supercritical nitrogen counterparts. The tensile properties of injection molded polylactic acid tensile bars with nitrogen and expandable thermoplastic microspheres at various weight concentrations were investigated. The results showed that the polylactic acid/nitrogen parts possessed a better Young’s modulus and tensile strength. The microstructure on the fractured cross-sectional surfaces was characterized using a scanning electron microscope. As reflected by the testing results, the cell microstructure—such as cell size and cell density, and multi-layered structure with a foamed core sandwiched by skin layers—played an important role in the surface quality and mechanical properties. In addition, while an appropriate expandable thermoplastic microsphere content had a positive effect on the cell microstructure and weight reduction, too high of a concentration of expandable thermoplastic microsphere adversely affected the tensile properties and surface roughness of the microcellular polylactic acid tensile test bars.