Nowadays, environmental hazards caused by plastic wastes are a major concern in academia and industry. Utilization of biodegradable polymers derived from renewable sources for replacing common petroleum‐based plastics is a potential solution for reducing the problem. In this regard, starch has become one of the most promising alternatives to non‐biodegradable polymers for depleting plastic waste thanks to its low expense, abundance, renewability and biodegradability. However, the main drawbacks of starch are its poor processability, weak mechanical properties and severe hydrophilicity. In this work, thermoplastic starch (TPS) samples have been prepared using glycerol and sorbitol as co‐plasticizers in a laboratory co‐rotating twin screw extruder. Based on the mechanical test results, glycerol caused higher elongation to break but had lower tensile strength and elastic modulus compared to sorbitol plasticized starch. Fourier transform infrared spectroscopy and DSC results indicated that the hydrogen bond interaction between starch chains and plasticizers could be improved by replacing glycerol by sorbitol, which resulted in higher resistance against retrogradation proved by XRD results. TGA illustrated that the higher the sorbitol to glycerol ratio was, the more stable was the TPS. Using a proper amount of plasticizers (42 wt% total plasticizer, sorbitol to glycerol ratio 2:1) led to the preparation of a TPS sample with optimized properties including enhanced mechanical properties, high thermal stability, strong hydrogen bond formation and high resistance against retrogradation. © 2017 Society of Chemical Industry
Polylactic acid (PLA) and thermoplastic starch (TPS) are known as bio‐based and biodegradable thermoplastic polymers that can be used in different applications owing to their inherent physical and mechanical properties. In order to reduce the higher costs of PLA and tuning its physical and mechanical properties suitable for short life packaging applications, blending of PLA with the TPS, more economical biodegradable polymer, has been considered in academic and industrial researches. However, melt blending of PLA with TPS without compatibilization process caused some drawbacks such as coarsening morphology and declining mechanical properties and ductility because of thermodynamic immiscibility, which may restrict its usage in packaging applications. Subsequently, our approach in this research is compatibilization of PLA/TPS blends by utilization of primary well tuning of TPS formulation with a combination of sorbitol and glycerol plasticizers. In this work, the wide composition range of melt mixed PLA/TPS blends was prepared using a laboratory twin screw extruder. The effects of microstructure on the rheological and mechanical properties of PLA/TPS blends were studied using different methods such as scanning electron microscopy (SEM) images, contact angle, oscillatory shear rheological measurements, and tensile and impact strength mechanical tests. The rheological and mechanical properties were interpreted according to the morphological features and considering the possibility of plasticizer migration from TPS to PLA phase during melt blending. Reduction in complex viscosity and storage modulus of PLA matrix samples indicates the improved melt processability of blends. Finally, in comparison with mechanical results reported in literature, our simple approach yielded the blends with elastic modulus and ductility comparable with those of chemically compatibilized PLA/TPS blends.
Recently, the use of biodegradable polymers became the applicable solution to reduce the environmental concerns, which are created by plastic wastes as well as restrictions of petroleum-based synthetic polymers. By this point of view, polylactic acid (PLA) as a biodegradable and bio-based polymer is resolving both aforementioned issues. While, the high cost of PLA and its slow biodegradation rate make researchers to blend it with a faster one, for instance, thermoplastic starch (TPS). Adding TPS into PLA can influence on the morphological structure, thermal stability, and biodegradability. In this study, the well-tuned co-plasticized TPS via sorbitol/glycerol mixture was melt mixed with PLA for achieving the physically compatibilized PLA/TPS blend.Thermal properties and aerobic biodegradation behavior of samples were discussed in detail considering the morphology development in each blend composition. Thermogravimetric analysis of PLA/TPS blends showed the single degradation peaks, which indicated the fine interdependence between 2 phases. The continuity and content of TPS phase were strongly influenced on moisture absorption and biodegradability of PLA/TPS samples. Also, the presence of TPS accelerated the biodegradation rate of PLA/TPS samples.
Studying the flow-induced alignment of anisotropic liquid crystalline materials is of major importance in the 3D printing of advanced architectures. However, in situ characterization and quantitative measurements of local orientations during the 3D printing process are challenging. Here, we report a microfluidic strategy integrated with polarized optical microscopy (POM) to perform the in situ characterization of the alignment of cellulose nanocrystals (CNCs) under the shear-flow condition of the 3D printer's nozzle in the direct ink writing process. To quantify the alignment, we exploited birefringence measurements under white and monochromatic light. We show that the flow-induced birefringence patterns are significantly influenced by the initial structure of the aqueous CNC suspensions. Depending on the CNC concentration and sonication treatment, various structures can form in the CNC suspensions, such as isotropic, chiral nematic (cholesteric), and nematic (gel-like) structures. In the chiral nematic phase, in particular, the shear flow in the microfluidic capillary has a distinct effect on the alignment of the CNC particles. Our experimental results, complemented by hydrodynamic simulations, reveal that at high flow rates (Er ≈ 1000), individual CNC particles align with the flow exhibiting a weak chiral structure. In contrast, at lower flow rates (Er ≈ 241), they display the double-twisted cylinder structure. Understanding the flow effect on the alignment of the chiral liquid crystal can pave the way to designing 3D printed architectures with internal chirality for advanced mechanical and smart photonic applications.
Architected materials with nano/microscale orders can provide superior mechanical properties; however, reproducing such levels of ordering in complex structures has remained challenging. Inspired by Bouligand structures in nature, here, 3D printing of complex geometries with guided long‐order radially twisted chiral hierarchy, using cellulose nanocrystals (CNC)‐based inks is presented. Detailed rheological measurements, in situ flow analysis, polarized optical microscopy (POM), and director field analysis are employed to evaluate the chiral assembly over the printing process. It is demonstrated that shear flow forces inside the 3D printer's nozzle orient individual CNC particles forming a pseudo‐nematic phase that relaxes to uniformly aligned concentric chiral nematic structures after the flow cessation. Acrylamide, a photo‐curable monomer, is incorporated to arrest the concentric chiral arrangements within the printed filaments. The time series POM snapshots show that adding the photo‐curable monomer at the optimized concentrations does not interfere with chiral self‐assemblies and instead increases the chiral relaxation rate. Due to the liquid‐like nature of the as‐printed inks, optimized Carbopol microgels are used to support printed filaments before photo‐polymerization. By paving the path towards developing bio‐inspired materials with nanoscale hierarchies in larger‐scale printed constructs, this biomimetic approach expands 3D printing materials beyond what has been realized so far.
Finely controlled flow forces in extrusion-based additive manufacturing can be exploited to program the self-assembly of malleable nanostructures in soft materials by integrating bottom-up design into a top-down processing approach. Here, we leverage the processing parameters offered by direct ink-writing (DIW) to reconfigure the photonic chiral nematic liquid crystalline phase in hydroxypropyl cellulose (HPC) solutions prior to deposition on the writing substrate to direct structural evolution from a particular initial condition. Moreover, we incorporate polyethylene glycol (PEG) into iridescent HPC inks to form a physically cross-linked network capable of inducing kinetic arrest of the cholesteric/chiral pitch at length scales that selectively reflect light throughout the visible spectrum. Based on thorough rheological measurements, we have found that printing the chiral inks at a shear rate where HPC molecules adopt pseudonematic state results in uniform chiral recovery following flow cessation and enhanced optical properties in the solid state. Printing chiral inks at high shear rates, on the other hand, shifts the monochromatic appearance of the extruded filaments to a highly angle-dependent state, suggesting a preferred orientation of the chiral domains. The optical response of these filaments when exposed to mechanical deformation can be used in the development of optical sensors.
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