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Biomimetic nanocoatings with exceptional mechanical, barrier, and flame

Time:2019-02-11 04:41Turbochargers information Click:

E with Biomimetic nanocoatings

Large-scale biomimetic organic/inorganic hybrid nanocoatings with a nacre-like microstructure were prepared via a facile coassembly process. Different from conventional polymer nanocomposites, these nanocoatings contain a high concentration of nanosheets, which can be well aligned along the substrate surface. Moreover, the nanosheets and polymer matrix can be chemically co–cross-linked. As a result, the nanocoatings exhibit exceptional mechanical properties (high stiffness and strength), barrier properties (to both oxygen and water vapor), and flame retardancy, but they are also highly transparent (maintaining more than 85% of their original transmittance to visible light). The nanocoatings can be applied to various substrates and regular or irregular surfaces (for example, films and foams). Because of their excellent performance and high versatility, these nanocoatings are expected to find widespread application.

INTRODUCTION

Through millions of years of evolution, many biological systems have developed to realize virtually perfect unification of their structures and thus optimized properties. They are usually made of organic and inorganic components arranged in a complicated but amazingly hierarchical structure, enabling them to have a unique combination of remarkable stiffness, strength, toughness, low density, and possibly extra functionality (, ). One of the most outstanding and representative examples is nacre.

Nacre is an organic/inorganic composite with outstanding strength, stiffness, and toughness (–). Nacre is composed of ca. 95 volume percent (volume %) of inorganic calcium carbonate (in the form of aragonite) and ca. 5 volume % of organic biopolymers (β-chitin and silk fibroin proteins), both having ordinary mechanical properties (, ). The striking contrast between the exceptional mechanical properties of nacre and their ordinary components has inspired materials scientists to synthesize organic/inorganic hybrids with a similar structure for practical applications. The key structural features of nacre are a high concentration of well-aligned nanosheets (fig. S1) and a strong interface. Nature has adopted an elaborate strategy to create nacre (–), involving a multistep biomineralization process (). Although this process has been mimicked in vitro (, ), it is very difficult to scale up this highly delicate biological process. In addition to mineralization (, ), a number of approaches, including ice-templated synthesis (, ), layer-by-layer (LbL) self-assembly (–), and electrophoretic deposition (), have been explored to form a nacre-like microstructure. Although each of the above approaches has its own advantages, it remains a huge challenge to achieve large-scale continuous mass production of large-sized samples.

It is well known and intuitively understandable that flow can help induce orientation (–). However, concentrated suspensions of fillers can pose difficulties in achieving filler alignment (). Here, we design to create a low-viscosity liquid flow containing both inorganic nanosheets and polymer binders, to help align nanosheets on a substrate surface along the flow direction. During the flow-induced orientation, the nanosheets and polymer chains are expected to coassemble to form a highly ordered layered structure with dozens of layers within a single step, whereas the ratio of the nanosheets and polymer can be easily adjusted, both of which make this process distinctively different from and easier than the LbL assembly ().

RESULTS

Coassembled nanostructured hybrid nanocoatings and structural characterization

Montmorillonite (MMT) can exfoliate into individual single-layer nanosheets with a thickness of ca. 1.0 nm () in aqueous system with the assistance of ultrasonication (). The transmission electron microscopy (TEM) image and lateral dimension distribution (260 ± 60 nm) of the exfoliated MMT nanosheets are shown in fig. S2. Meanwhile, polyvinyl alcohol (PVA) was dissolved in water to form a solution. Then, the two components were mixed at predetermined ratios. Extra water can be added to adjust the overall concentration. Because of the weak interactions, mainly hydrogen bonding and van der Waals interactions (), between the MMT nanosheets and the PVA chains, the PVA chains can attach to the MMT nanosheet surface () (, , ), which is critical for the following coassembly process. The dispersion was coated on polylactic acid (PLA) films (or other substrates) through a very simple dip-coating process, and the films were subsequently hung vertically so that the MMT nanosheets can be aligned with the assistance of the flow of a thin layer of liquid () induced by the gravity. A dispersion of 1.5 weight % (wt %) of solids (MMT + PVA) was chosen to ensure a low viscosity to create a quick flow and a thin liquid layer, both of which are highly desirable to achieve a high level of orientation of nanosheets (). The coated PLA films were dried at 60°C in an oven, during which PVA was dried and sandwiched between well-aligned MMT nanosheets. The coated samples were labeled as PLA (or other polymer)–PVA/MMT-X-C (or N), where X is the mass percentage of MMT in the mixture of MMT and PVA in the initial formulation; C refers to “cross-linked,” and N refers to non–cross-linked.

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