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keep reading-23

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An all-natural bioinspired structural material for plastic replacement, Guan, Yang, Han, Ling, Yu, Nature Communications, 2020

Novelty/impact/significance:

A strong (stiffness 20 GPa and strength 281 MPa), tough (11.5 MPa m1/2), lightweight (1.7 g/cm3), and thermal-resistant (thermal expansion coefficient 7x10-6K-1) composite material is created, using all-natural raw materials through a mass-producible method.

This composite material can potentially replace the massively used but environmental-threatening plastics, with its high performance (much superior to engineering plastics), low cost, and great sustainability.

 

Scientific question:

How to develop a structural material that can replace the heavily used petroleum-based plastics, considering the performance (mechanical, thermal, density), economy (raw material cost, mass-producibility), and sustainability?

 

Key of how:

By developing a directional deforming assembly method using cellulose nanofiber (CNF) and TiO2-coated mica microplatelet (TiO2-mica) together with APTES pretreatment and Ca2+ crosslinking, the fabricated composite material has remarkable mechanical properties originating from its highly-ordered nacre-like brick-and-mortar structure, exceptional thermal properties due to its constituents, low cost by the abundance of raw materials through the mass-producible processing, and favorable sustainability by its all-natural components.

Key processing factors for the integrated structure to show high strength/stiffness and toughness may be the APTES pretreatment and Ca2+ crosslinking, which substantially enhance the attachment of TiO2 to CNF and the interaction between CNFs.

 

Major points:

1. In addressing the pressing issue of extensively used environmental-challenging plastics, one approach is to develop sustainable, high-performance structural materials that have superior (or at least equivalent) mechanical properties and processing cost.

2. Mimicking the nacreous brick-and-mortar structure for exceptional mechanical properties shows to be a good option, while the complex fabrication process based on all-natural constituents needs to be simplified.

3. The work presented here provides such an approach: using sustainable constituents, CNF (from plants, 1D nanoscale building block, high strength, low thermal expansion coefficient, abundant functional groups) and TiO2-mica (commercially available, consisting of TiO2 nanograins on surface resembling the aragonite platelets in nacre) as the biopolymer matrix and inorganic reinforcement, respectively, to make a hydrogel of surface-pretreated (APTES) 2D TiO2-mica and 1D CNF (the hydrogel is crosslinked by CaCl2), then directional pressing this hydrogel to reduce the thickness while keep the in-plane dimension unchanged, during which a highly ordered brick-and-mortar structure is achieved: the TiO2-mica uniformly aligned with the CNF evenly distributed in between.

This one-step method is simple (80oC, 100 MPa for 1 hour) and robust, allowing for making large-sized nacre-mimetic materials faster and low cost.

PS: what are the key mechanisms of such a directional deforming assembly for achieving the highly orientated TiO2-mica and uniformly distributed CNF, e.g., which processing parameters account for which key structural features?

4. The composites with APTES treatment and Ca2+ crosslinking show highest flexural strength and modulus than the non-treated and non-crosslinked ones. This is due to that the surface modification facilitates the attachment of TiO2-mica to CNF and the Ca2+ crosslinking enhances the interaction between CNFs.

PS: This shows that the chemical treatment and crosslinking enhance the internal bonding between building blocks in the composites; while how the directional pressing leads to the aligned brick-and-mortar structure? 

5. The designed all-natural nacre-mimetic brick-and-mortar composites have both high crack initiation and propagation toughness values (~6.7 and ~11.5 MPa m1/2 for KIC and KJ-c, respectively), much superior to the natural nacres, due to (1) the robust, highly ordered brick-and-mortar structure with improved platelet-biopolymer interface bonding enhancing stress transfer and resisting crack initiation, and (2) the extrinsic toughening mechanisms such as crack deflection, delamination, crack branching, crack bridging, and frictional sliding during TiO2 pullout.  

6. By specific experimental studies with typical plastics (PMMA, ABS, PA, PC), the designed all-natural nacre-mimetic composites show significantly superior thermal properties for long-term use at high or variable temperature with reliable mechanical performance, e.g., thermal expansion coefficient as low as 7x10-6 K-1, storage modulus unchanged to 200 oC and no visible change at 250 oC, and higher thermal diffusion, due to the high crystallinity of CNF, good thermal stability of TiO2-mica, and the highly ordered brick-and-mortar structure with surface chemical modification.

7. Compared with a variety of most plastics, the strength and modulus are twice and five times higher, much harder and tougher, with significantly lower coefficient of thermal expansion and higher thermal conductivity, demonstrating the exceptional mechanical performance and its reliability at high or variable temperatures. Also, the process is effective and scalable for mass production, and by different raw materials different colors for the materials can be fabricated.

 

The clearly demonstrated success in exceptional mechanical properties at high and variable temperatures due to obtaining highly ordered brick-and-mortar structure with surface modification by the directional deforming assembly method is exciting and impressive.

Here is the link of the fulltext: https://www.nature.com/articles/s41467-020-19174-1

Several personal thoughts: (1) the thermal properties and sustainability of the constituents (CNF and TiO2-mica) generally determine thermal and sustainability of the designed all-natural nacre-mimetic composites? The interface modification, the crosslinking, and the brick-and-mortar architecture guarantee an integrated structure mostly accounting for the mechanical properties? (2) what is comparing the mechanical properties of the designed nacre-mimetic composite with those of the TiO2-mica and with those of the TiO2-mica composite with simple alignment? (3) how to define/think the environmental sustainability? Can this composite degrade into non-environmentally challenging/non-poisonous substances naturally?

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