Atomistic-scale analysis of the deformation and failure of polypropylene composites reinforced by functionalized silica nanoparticles

Interfacial adhesion between polymer matrix and reinforcing silica nanoparticles plays an important role in strengthening polypropylene (PP) composite. To improve the adhesion strength, the surface of silica nanoparticles can be modified by grafted functional molecules. Using atomistic simulations, we examined the effect of functionalization of silica nanoparticles by hexamethyldisilazane (HMDS) and octyltriethoxysilane (OTES) molecules on the deformation and failure of silica-reinforced PP composite. We found that the ultimate tensile strength (UTS) of PP composite functionalized by OTES (28 MPa) is higher than that of HMDS (25 MPa), which is in turn higher than that passivated only by hydrogen (22 MPa). To understand the underlying mechanistic origin, we calculated the adhesive energy and interfacial strength of the interphase region, and found that both the adhesive energy and interfacial strength are the highest for the silica nanoparticles functionalized by OTES molecules, while both are the lowest by hydrogen. The ultimate failure of the polymer composite is initiated by the cavitation in the interphase region with the lowest mass density, and this cavitation failure mode is common for all the examined PP composites, but the cavitation position is dependent on the tail length of the functional molecules. The present work provides interesting insights into the deformation and cavitation failure mechanisms of the silica-reinforced PP composites, and the findings can be used as useful guidelines in selecting chemical agents for surface treatment of silica nanoparticles.

Frank-van der Merwe Growth in Bilayer Graphene

<p>Bilayer graphene has attracted interest for its unique properties, including interesting electrical behavior when one layer is slightly rotated relative to the other. However, the quality of large-area bilayer graphene is often limited by the layer-plus-island growth mode in which islands of thicker graphene present as unavoidable impurities. Here, we report the observation of the layer-by-layer, Frank-van der Merwe (FM) growth mode in bilayer graphene where multilayer impurities are suppressed. Instead of the conventional surface adhesive energy, it is found that interface adhesive energy is possible to be tuned with an oxidative pretreatment. The FM-grown bilayer graphene is of AB-stacking or with small-twisting-angle (θ = 0-5°), which is more mechanically robust compared to monolayer graphene, facilitating a free-standing wet transfer technology.</p>

Frank-van der Merwe Growth in Bilayer Graphene

<p>Bilayer graphene has attracted interest for its unique properties, including interesting electrical behavior when one layer is slightly rotated relative to the other. However, the quality of large-area bilayer graphene is often limited by the layer-plus-island growth mode in which islands of thicker graphene present as unavoidable impurities. Here, we report the observation of the layer-by-layer, Frank-van der Merwe (FM) growth mode in bilayer graphene where multilayer impurities are suppressed. Instead of the conventional surface adhesive energy, it is found that interface adhesive energy is possible to be tuned with an oxidative pretreatment. The FM-grown bilayer graphene is of AB-stacking or with small-twisting-angle (θ = 0-5°), which is more mechanically robust compared to monolayer graphene, facilitating a free-standing wet transfer technology.</p>

Competition between delamination and tearing in multiple peeling problems

Adhesive attachment systems consisting of multiple tapes or strands are commonly found in nature, for example in spider web anchorages or in mussel byssal threads, and their structure has been found to be ingeniously architected in order to optimize mechanical properties: in particular, to maximize dissipated energy before full detachment. These properties emerge from the complex interplay between mechanical and geometric parameters, including tape stiffness, adhesive energy, attached and detached lengths and peeling angles, which determine the occurrence of three main mechanisms: elastic deformation, interface delamination and tape fracture. In this paper, we introduce a formalism to evaluate the mechanical performance of multiple tape attachments in different parameter ranges, where an optimal (not maximal) adhesion energy emerges. We also introduce a numerical model to simulate the multiple peeling behaviour of complex structures, illustrating its predictions in the case of the staple-pin architecture. Finally, we present a proof-of-principle experiment to illustrate the predicted behaviour. We expect the presented formalism and the numerical model to provide important tools for the design of bioinspired adhesive systems with tuneable or optimized detachment properties.

Damage evolution in fibrous composites caused by interfacial debonding

Interfacial debonding between fibers and matrix is one of the dominant damage types in fibrous composites. This paper investigates weakening effect due to the interfacial debonding. For simplification, the fibers are assumed to be rigid since the modulii and strength of fibers are much greater than those of matrix, and the distribution of the radii of fibers is assumed to obey the logarithmic normal distribution. The matrix is assumed to be a viscoelastic material. The boundary of the composite is subjected to transverse loading condition, the direction of which is perpendicular to that of fibers. The interfacial debonding between fibers and matrix is analyzed by the energy criterion, and the evolution formula of nucleated porosity due to the debonding is derived by the statistical approach. A newly defined volume average method is proposed to establish the macroscopic constitutive relation of the composites. The effect of the material parameters of matrix, as well as the size of fibers on the critical stress for the interfacial debonding and damage evolution are discussed in detail. The results obtained in this paper indicate that the macroscopic strain rate, the dispersion degree of the fiber's radii, the adhesive energy at the interface, and loading condition play key roles in the overall mechanical properties of the composites.

On the Issue of Predicting the Strength Characteristics of Dispersed-Hardened Polymeric Composite Materials at the Design Stage of Engineering Products

In the article a technique is provided that allows to optimize the process of choosing a rational composition of particulate filled polymer composite materials (PFPCM) based on the estimation of specific adhesive energy. Specific adhesive energy is calculated assuming the oriented surface layer formation and using a structure of representative volume. Strength tests for a variety of PFPCMs were conducted. Comparison of analytical results with the results of strength tests showed that that the higher the value of the specific adhesion energy that the system possesses, the higher its strength characteristics. Thus, this technique provides an assessment of strength characteristics of PFPCM based on data on its granulometric composition, filling ratio and type of binder.

Bioinspired Ultra-Low Adhesive Energy Interface for Continuous 3D Printing: Reducing Curing Induced Adhesion

Additive manufacturing based on liquid resin curing is one of the most promising methods to construct delicate structures. However, precision and speed are limited by the vertical adhesion of in situ cured resin at the curing interface. To overcome the unavoidable adhesion and to develop a general curing interface, we propose a slippery surface taking inspiration of the peristome surface of the pitcher plant. Such surface shows ultra-low adhesive energy at the curing interface due to the inhibition of the direct contact between the cured resin and the solid surface, which also increases the refilling speed of liquid resin. This ultra-low adhesive energy interface is effective for continuous 3D printing and provides insights into the physical mechanisms in reducing vertical solid-solid interfacial adhesion.

Adhesion between Asphalt and Recycled Concrete Aggregate and Its Impact on the Properties of Asphalt Mixture

To study and evaluate the adhesion between recycled concrete aggregate and asphalt, the contact angles (CAs) between droplet (water and ethanol) and recycled concrete aggregate (RCA), natural aggregates, and solid bitumen (matrix asphalt, SBS modified asphalt) were tested via the sessile drop method with an optical microscope. The surface free energy was then calculated. The CAs between hot asphalt and RCA and natural aggregates were tested via the hanging slice method. The adhesive energy between asphalt and RCA and natural aggregates were calculated based on the test results of the surface free energy and CAs. Then, the influence of RCA on the water stability and fatigue performance of the asphalt mixture was analyzed by testing the water stability and fatigue properties of hot mix asphalts containing RCA (HMA-RCA) with different aggregates and RCA dosages. The surface energy of the various aggregates and the CAs between aggregates and asphalts were sorted as follows: Granite > RCA > serpentinite > limestone. The surface energy and CA of RCA were very close to that of serpentinite. The adhesive energy between various aggregates and asphalt were sorted as follows: Limestone > serpentinite > RCA > granite. The adhesive energy between RCA and asphalt was also very close to that of serpentinite. The residual Marshall stability, tensile strength ratio, and fatigue performance of the HMA-RCAs were gradually reduced along with the increasing RCA dosage. This effect may be attributed to the fact that the adhesive energy between the RCA and the asphalt was less than that of water and that the asphalt was easily stripped from the RCA surface. Excessive RCA content in the aggregate can lead to excessive porosity of the HMA-RCA. The CAs and adhesive energy between RCA and asphalt showed significant effects on the water stability and fatigue performance of HMA-RCA.