Mechanics of bio-inorganic interface

Strength and toughness are two mechanical properties that are generally mutually exclusive but highly sought-after in the design of advanced composite materials. There has only been limited progress in achieving both high strength and toughness in composite materials. However, the fundamental underlying mechanics remain largely unexplored, especially at the nanoscale. Inspired by the lamellar structure of nacre, here a layered graphene and polyethylene nanocomposite with tunable interfacial cross-links is studied via coarse-grained molecular dynamics simulations in order to achieve both high strength and toughness. Our simulations indicate that, as the cross-link density increases from 0 to about 25%, strength and toughness of the nanocomposite experience a surprising 91% and 76% increase respectively. This strengthening mechanism can be well explained by the extent of increased non-boned contacts between polymer chains (van der Waals interaction) during the stretch and exceptional stretchability of each polymer chain (dihedral interaction) due to interfacial cross-links by comparing nanocomposites with and without cross-links. As the strength of cross-links increases, both mechanical strength and toughness of graphene-based polymer nanocomposite increase as expected. It may be attributed to the intra-chain bond and angle interaction among polymer chains which may be negligible for nanocomposites with weak cross-links but play a key role in enhancing both strength and toughness for nanocomposites with strong cross-links. Overall, our findings unveil the fundamental mechanism at the nanoscale for tough-and-strong polymer composites via interfacial cross-links as well as offer a novel way to design bioinspired nanocomposites with targeted properties via tunable interfacial cross-links.

polymer-graphene composite fracture
Top view of the graphene-polyethylene composites during tensile tests without cross-links when strain is (a) 0% (b) 50% (c) 150% (d) 200%; composites with cross-links (23.69 percent) when strain is (e) 0% (f) 50% (g) 150% (h) 200%; zoomed-in snapshots for the sample marked by (i) blue (j) red ellipse.

 

mechanical properties of polymer-graphene composites
Mechanical properties of polymer-graphene composites (Young’s modulus, ultimate strength, and toughness) versus the stiffness of the cross-links when the cross-link density is about 25 percent.

Interfacial load transfer capacity of fiber-polymer interfaces plays critical roles to the bulk performance of fiber-reinforced polymer nanocomposites, and is thus a primary factor in selecting reinforcing nanofiller materials for making light, strong and durable polymer nanocomposites. Boron nitride nanotubes (BNNTs) have been regarded as a promising nanofiller material due to their extraordinary structural and mechanical properties, many of which rival those of their pure carbon counterparts, carbon nanotubes (CNTs). However, the load transfer capacity of BNNT-polymer interfaces remains largely unexplored. Here, we report the first experimental measurement on the strength of binding interfaces between individual BNNTs and polymer matrices by means of in situ electron microscopy nanomechanical single-tube pull-out techniques. Our nanomechanical measurements show that the maximum interfacial shear strengths of respective interfaces formed by BNNTs with epoxy resins and poly(methyl methacrylate) (PMMA) reach 323 and 219 MPa (median values), respectively. Our studies reveal that the mechanical strengths of BNNT-polymer interfaces are substantially higher than those of the comparable interfaces formed with CNTs. Molecular dynamics (MD) simulations show that the high load transfer capacity of BNNT-polymer interfaces is ascribed to both the strong van der Waals interactions and Coulomb interactions on BNNT-polymer interfaces, the latter of which is due to the polarized nature of B-N bonds and is otherwise absent on CNT-polymer interfaces. The findings of extraordinary load transfer capacity of BNNT-polymer interfaces suggest that BNNTs may be superior to CNTs as reinforcing additives for nanocomposite applications, and help to boost the development of light-weight and high-strength BNNT-based polymer nanocomposites.

BN-polymer interface
Left: In situ nanomechanical single-tube pull-out measurement inside a high resolution SEM. (a) 2D schematic; (b) a BNNT-epoxy thin-film composite sample with protruding BNNTs (scale bars 500 nm); (c) HRTEM images of three protruding BNNTs (scale bars 10 nm); (d) Selected snapshots of one representative single-tube pull-out measurement: (upper) a selected protruding tube from a BNNT-epoxy composite sample was welded to an AFM tip by EBID of Pt; (lower) the tube after being pulled out from the polymer (scale bars 200 nm). Right: The calculated trajectories of the intermolecular interaction energy between each model polymer chain and the same BNNT during the polymer relaxation process. The green curve shows the binding energy contributed by the carbon atoms in the aromatic rings in the model epoxy chain. The dashed lines indicate the respective average steady-state binding energies.

Publications

N. Liu, X. Zeng, R. Pidaparti, and X. Wang, "Tough and Strong Bioinspired Nanocomposites via Interfacial Cross-linking", Nanoscale, under review.

X. Chen, L. Zhang, M. Zheng, C. Park, X. Wang and C. Ke, "Quantitative Nanomechanical Charaterization of the van der Waals Interfaces between Carbon Nanotubes and Epoxy", Carbon, 82: 214-228, 2015.

X. Chen, L. Zhang, C. Park, C. C. Fay, X. Wang and C. Ke, "Mechanical Strength of Boron Nitride Nanotube-Polymer Interfaces",Applied Physics Letters, 107: 253105, 2015..

M. J. Razavi and X. Wang, "Morphological Patterns of a Growing Biological Tube in a Constrained Environment with Contacting Boundary", RSC Advances, 5: 7440-7449, 2015.

Collaborators

Dr. Changhong Ke (Bingham University)

Dr. Xiaowei Zeng (University of Texas at San Antonio)