Deconvolution of Stress Interaction Effects from Atomic Force Spectroscopy Data across Polymer−Particle Interfaces

BOTTS: broadband optimized time-temperature superposition for vastly accelerated viscoelastic data acquisition

Modern materials design strategies take advantage of the increasing amount of materials property data available and increasingly complex algorithms to take advantage of those data. However, viscoelastic materials resist this trend towards increased data rates due to their inherent time-dependent properties. Therefore, viscoelasticity measurements present a roadblock for data collection in an important aspect of material design. For thermorheologically simple (TRS) materials, time-temperature superposition (TTS) made relaxation spectrum measurements faster relative to, for example, very long creep experiments. However, TTS itself currently faces a speed limit originating in the common logarithmic discrete frequency sweep (DFS) mode of operation. In DFS, the measurement time is proportional (by a factor much greater than one) to the lowest frequency of measurement. This state of affairs has not improved for TTS for half a century or more. We utilize recent work in experimental rheometry on windowed chirps to collect three decades of complex modulus data simultaneously, resulting in a ∼500% increase in data collection. In BOTTS, we superpose several isothermal chirp responses to produce a master curve in a fraction of time required by the traditional DFS-TTS technique. The chirp responses have good, albeit nontrivial, signal-to-noise properties. We use linear error propagation and a noise-weighted least squares approach to automatically incorporate all the data into a reliable shifting method. Using model thermoset polymers, we show that DFS-TTS and BOTTS results are comparable, and therefore BOTTS data represent a first step towards a faster method for master curve generation from unmodified rheological measurement instruments.

Insights into the Mechanical Properties of Ultrathin Perfluoropolyether-Silane Coatings

Ultrathin perfluoropolyether-silane (PFPE-silane) films offer excellent functionality as antifingerprint coatings for display touchscreens due to their oleophobic, hydrophobic, and good adhesion properties. During smartphone use, PFPE-silane coatings undergo many abrasion cycles which limit the coating lifetime, so a better understanding of how to optimize the film structure for improved mechanical durability is desired. However, the hydrophobic and ultrathin (1-10 nm) nature of PFPE-silane films renders them very difficult to experimentally characterize. In this study, the cohesive fracture energy and elastic modulus, which are directly correlated with hardness and better wear resistance of 3.5 nm-thick PFPE-silane films were, respectively, measured by double cantilever beam testing and atomic force microscopy indentation. Both the cohesive fracture energy and modulus are shown to be highly dependent on the underlying film structure. Both values increase with optimal substrate conditions and a higher number of silane groups in the PFPE-silane precursor. The higher cohesive fracture energy and modulus values are suggested to be the result of the changes in the film chemistry and structure, leading to higher cross-linking density. Therefore, future work on optimizing PFPE-silane film wear resistance should focus on pathways to improve the cross-linking density. Subcritical fracture testing in humid environments reveals that humidity negatively affects the fracture properties of PFPE-silane films.

Direct Visualization of Interfacial Regions between Fillers and Matrix in Rubber Composites Observed by Atomic Force Microscopy-Based Nanomechanics Assisted by Electron Tomography

In order to explain or predict the macroscopic mechanical properties of polymer composites with complex nanostructures, atomic force microscopy (AFM)-based nanomechanics is one of the most appropriate tools because the local mechanical properties can be obtained by it. However, automatic force curve analysis based on contact mechanics would mislead us to the wrong conclusion. The purpose of this study is to elucidate this point by applying AFM nanomechanics on a carbon black (CB)-reinforced isoprene rubber (IR). The CB aggregates underneath the rubber surface prevent us from quantitatively evaluating the ratio of CB and interfacial polymer region (IPR), which is an important parameter to determine the macroscopic mechanical properties. In order to overcome this problem, transmission electron microtomography was incorporated to investigate the 3D structure in the same field of view as AFM nanomechanics. As a result, it was found that there are buried structures that do not appear in the AFM topographic image. In addition, we were able to reveal the existence of a force curve with an inflection point, which is characteristic of such "false" IPRs. To put it another way, we evidenced the existence of true IPRs for the first time by combining these state-of-the-art techniques.

MXene Enabling the Long-Term Superior Thermo-Oxidative Resistance for Elastomers

The ability of long-term thermo-oxidative resistance is very important for elastomers in application. However, many conventional antioxidants are difficult to realize the long-term thermo-oxidative resistance. To overcome this limitation, a design strategy is introduced by combing elastomers with MXene and natural rubber (NR) is chosen as a model material. MXene is efficient in absorbing oxygen and the generated free radicals in the NR matrix and can inhibit the diffusion of oxygen toward the interior. Moreover, MXene, like graphene and carbon black, absorbs molecular chains, inhibiting the migration of MXene toward the surface of the sample. Such characteristics of MXene endow NR/MXene with the long-term outstanding thermo-oxidative resistance. For example, after three days of the thermo-oxidative process for NR/MXene, the tensile strength is 19 MPa and the retention of tensile strength is 63%, which far exceeds the effects of conventional antioxidants. This work not only provides a good guide for the universal design of elastomers with long-term thermo-oxidative resistance but also expands the application of MXene.

Nanomechanical mapping of soft materials with the atomic force microscope: methods, theory and applications

Fast, high-resolution, non-destructive and quantitative characterization methods are needed to develop materials with tailored properties at the nanoscale or to understand the relationship between mechanical properties and cell physiology. This review introduces the state-of-the-art force microscope-based methods to map at high-spatial resolution the elastic and viscoelastic properties of soft materials. The experimental methods are explained in terms of the theories that enable the transformation of observables into material properties. Several applications in materials science, molecular biology and mechanobiology illustrate the scope, impact and potential of nanomechanical mapping methods.