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"AFM" : High elasticity, high toughness, high modulus, transparent nanofiber reinforced hydrogel 2021-06-07

Time : 2021-06-07 Hits : 12

Nanofibers can significantly improve the mechanical properties of the materials and are considered as ideal stiffening materials for the preparation of mixed hydrogels. However, the layered deposition or uneven distribution of opaque nanofibers in the hydrogel matrix will lead to the low mechanical properties and poor transparency of the mixed hydrogels. The preparation of a strong, stretchy and uniform nanofiber-reinforced ionic conducting hydrogel without compromising transparency remains a significant challenge.

In Situ Synthesis of Mechanically Robust, "In Situ Synthesis of Mechanically Robust," by Ding Bin and Zhang Shichao from School of Textile, Donghua University, In Advanced Functional Materials, Transparent nanofiber-reinforced (SFRHS) Hydrogels for Highly Sensitive Multiple Sensing with mechanical and electronic stability were reported.

A transparent nanofiber-reinforced SFRHS hydrogel was prepared by combining silica nanofibers and vinylsilane into sodium alginate (SA)/ polyacrylamide (PAM) hydrogel. PAM chains were covalently connected to the silica nanofibers through silane, forming a strong interfacial chemical bond. In addition, the interface between the silica nanofibers and PAM chains provides non-covalent interactions (such as hydrogen bonds) that may break and recombine during stretching, dissipating energy and homogenizing the hydrogel network.


The high aspect ratio of silica nanofibers and the stable interface between the nanofibers and the hydrogel network contribute to the excellent mechanical properties of SFRHs. The research team first prepared a silane-free silicon nanofiber reinforced hydrogel with only physical bonding at the interface. The hydrogel was identified as P-SFRHS. When the aspect ratio of silicon nanofibers increases from 50 to 400, the mechanical strength of the nanofibers increases greatly (from 0.11 MPa to 0.24 MPa), which may be related to the increase of the interface strength.

In order to improve the interfacial load transfer efficiency, TMSPMA was introduced between the nanofibers and the hydrogel matrix, and the hydrogel was recorded as C-SFRHS. The mechanical strength of C-SFRHS is 0.3MPa, the strain is 1400%, and the Young's modulus is 0.11MPa. The high modulus of C-SFRHS is far better than that of the existing tensile tenacity hydrogels with similar high water content.

In order to understand the strengthening mechanism of nanofibers, the effects of L/ D ratio and covalent bond on the interfacial properties of SFRH were quantified by using total atomic molecular dynamics (MD) simulation. Two kinds of silicon nanofibers with different L/ D ratios, P-SFRH5 and P-SFRH10, and C-SFRH10 with additional introduction of TMSPMA, were used as three models. The results show that the interaction energy increases from 1135 to 2241 kJ·mol-1 with the increase of L/ D ratio, which indicates that the physical interactions, such as van der Waals forces, Coulomb forces, and hydrogen bonds between the silicon nanofibers and the hydrogel matrix, are strengthened. After the introduction of stable covalent bond, the interface of the composite hydrogel can be significantly increased to 4000 kJ·mol-1. In addition, the interface friction of the three models is simulated by drawing, and the interface friction of the three models is calculated based on the loss of kinetic energy. The maximum friction of C-SFRH10 is 3.34 NN (P-SFRH5 and P-SFRH10 are 1.06 and 1.87 NN, respectively). This large friction force makes it difficult to slip between the silicon nanofibers and the hydrogel matrix, which is consistent with the highest modulus and strength observed for C-SFRHs.


Mechanical properties and strengthening mechanism of SFRHS

Compared with the small hysteresis of P-SFRHs, C-SFRHs showed a larger hysteresis in the loading and unloading cycle experiments, while the permanent deformation after unloading was almost negligible, which highlighted its toughness and superelasticity. After 1000 cyclic tensile tests, the Young's modulus, maximum stress and energy loss coefficient of C-SFRHS did not decrease significantly. Compared with brittle conventional hydrogels, SFRHs show strong compressive resilience and high compressive stress (2.6 MPa at 90% strain). Moreover, they can be repeatedly compressed under such large strains without structural collapse. In addition, it was found that with the increase of the content of SiO2 nanofibers, the impedance gradually decreased, indicating that the ionic conductivity of SFRHs showed a trend of increasing. 

The improvement of ionic conductivity is attributed to two effects: first, the small volume of non-conductive silica nanofibers in the composite hydrogel ensures the high mobility of ions in water. Secondly, the silica nanofibers with high aspect ratio and acidic surface (-OH group) attract counterions and form a continuous highly conductive channel throughout the hydrogel.


Superelasticity and ionic conductivity of SFRHS

SFRHS has high sensitivity and circulatory stability over a wide sensing range and can be used to monitor human movement and pulse. SFRHs can effectively monitor complex and small external stimuli, such as handwriting, etc., which has important potential applications in handwriting prevention and other fields.


SFRHS sensing performance

Thesis Title: In Situ Synthesis of Mechanically Robust, Transparent Nanofiber-Reinforced Hydrogels for Highly Sensitive Multiple Sensing

DOI: 10.1002 / adfm. 202103117

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