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.

Surface Characterization of Lipid Biomimetic Systems

Zeta potential and dipole potential measures are direct operational methodologies to determine the adsorption, insertion and penetration of ions, amphipathic and neutral compounds into the membranes of cells and model systems. From these results, the contribution of charged and dipole groups can be deduced. However, although each method may give apparent affinity or binding constants, care should be taken to interpret them in terms of physical meaning because they are not independent properties. On the base of a recent model in which the lipid bilayer is considered as composed by two interphase regions at each side of the hydrocarbon core, this review describes how dipole potential and zeta potential are correlated due to water reorganization. From this analysis, considering that in a cell the interphase region the membrane extends to the cell interior or overlaps with the interphase region of another supramolecular structure, the correlation of dipole and electrostatic forces can be taken as responsible of the propagation of perturbations between membrane and cytoplasm and vice versa. Thus, this picture gives the membrane a responsive character in addition to that of a selective permeability barrier when integrated to a complex system.

Effects of nano-sized silica particles on the off-axis creep performance of unidirectional fiber-reinforced polymer hybrid composites

A multi-step homogenization approach is presented to predict the off-axis creep response of hybrid polymer matrix composites (HPMCs) reinforced with unidirectional carbon fibers and silica nanoparticles. The first step deals with evaluating the viscoelastic properties of silica nanoparticle-polymer nanocomposite using the Mori-Tanaka micromechanical model. Two essential features affecting the behavior, including silica nanoparticle agglomeration and interphase region generated due to the interaction between the nanoparticle and polymer are taken into account. In the second step, the off-axis viscoelastic behavior of HPMCs is extracted from the homogenized nanocomposite and carbon fiber properties using a unit cell-based micromechanical model. Some comparative studies between the predictions and available experiment are directed to verify the homogenization process. All the model predictions are in good agreement with the experimental data. The results indicate that with increasing the fiber off-axis angle from 0° to 90°, the presence of silica nanoparticles leads to a reduction in the HPMC creep compliance. Also, the proposed multi-step homogenization approach is applied to investigate the effects of volume fraction, size and agglomeration degree of silica nanoparticles; thickness and material properties of the interphase region; and off-axis angle and volume fraction of the carbon fiber on the HPMC creep response.

Subsurface and At-Depth Interphase Characterization in Hygrothermal Aged Carbon Fiber Reinforced Polymer Matrix Composite

The interphase region between the carbon monofilament and epoxy matrix in Carbon Fiber Reinforced Polymer Composites (CFRPs) is immensely vital to transfer stress between carbon monofilament and bulk matrix material. To the best of the authors’ knowledge, no other research group has studied the interphase region at subsurface and at-depth levels on hygrothermal aged CFRPs. The composite samples were exposed to 60° C and 90% humidity for one and two years. Moisture absorptions were measured periodically to assess water gain in the material. The advanced Atomic Force Microscopy (AFM) based Peak Force Quantitative Nanomechanics Mapping Technique was used to study the physics of the interphase. PFQNM allows non-destructive and simultaneous capture of imaging and mechanical property data with nanometer resolution. The interphase thickness was increased with increased hygrothermal exposure time. The interphase surrounding carbon monofilaments exhibited nonuniform thickness, ranging from ∼85 to 95 nm in the subsurface level, and from ∼10 to 75 nm on the at-depth level. Aged samples showed a decrease in average surface roughness, likely due to swelling of the epoxy matrix caused by the moisture absorption. The water diffusion generally followed Fickian.

Nanoscale Interphase Characterization of Porous CNT Buckypaper Composites in Correlation to Interlaminar Mode I Fracture

In this conference paper, nanoscale material property data and ASTM mode I interlaminar fracture results for three-phase buckypaper samples are presented and analyzed. Vacuum filtration and surfactant-free methods were used to manufacture buckypaper membranes. Epoxy infused buckypaper membranes were placed in front of the crack tip in a stitch bonded carbon fiber polymer matrix composite. Peak Force Quantitative Nanomechanical Mapping (PFQNM), using probes with nominal tip radius in the range of 5–8 nm were used. PFQNM characterized the interphase region between a three-phase sample of carbon monofilament, epoxy resin, and multi-walled carbon nanotube (MWCNT) buckypaper. This experiment captured reproducible nanoscale morphological, viscoelastic, elastic and energy properties of porous MWCNT buckypaper samples. An enlarged interphase region surrounding the CNT buckypaper was found. The buckypaper and epoxy interphase thickness was found to be 50nm, higher than the 10–40nm reported for epoxy and carbon monofilaments. The observed MWCNT structure provides explanation of the increased surface roughness compared to the smooth carbon monofilaments. The increased surface roughness likely improves mechanical interlocking with the epoxy of adjacent lamina. The nanoscale interphase and subsurface characterization data provide explanation for a change in crack propagation toughness. Buckypaper exhibited inhomogeneous properties at micrometer length scales.