AFM-based Dynamic Scanning Indentation (DSI) Method for Fast, High-resolution Spatial Mapping of Local Viscoelastic Properties in Soft Materials

Probing Viscoelastic Properties and Interfaces in High-Density Polyethylene Vitrimers at the Nanoscale Using Dynamic Mode Atomic Force Microscopy

Vitrimers are a new class of heterogeneous polymers that combine the best features of thermosets with those of thermoplastics. The introduction of cross-links strongly changes the viscoelastic behavior of vitrimer materials. However, the characterization and understanding of the nanostructures and interfaces in vitrimers resulting from dynamic cross-linking formation remain a major challenge. Here, using dynamic modes of atomic force microscopy (AFM), namely intermodulation AFM (ImAFM) and AFM-based dynamic mechanical analysis (AFM-nDMA), local viscoelastic properties and interfaces at the nanoscale length of high-density polyethylene (HDPE) vitrimer materials are reported. ImAFM imaging in combination with the k-means clustering algorithm clearly reveals two distinct phases in the vitrimer system with highly different viscoelastic properties. AFM-nDMA further provides quantitative nanoviscoelastic properties at the nanoscale to confirm that there is a cross-linking-rich aggregation area forming a nanosize network structure in the cross-linking-poor matrix phase. The cross-linking-rich region shows a similar elastic modulus but much higher adhesion force measured by AFM compared to the cross-linking-poor HDPE matrix. Furthermore, the frequency influence on the local viscoelastic properties of HDPE vitrimer at the nanoscale was initially screened. The observed HDPE vitrimer nanostructures and viscoelastic properties at the nanoscale also provide explanations on the observed bulk HDPE vitrimer crystallinity decrease and dimensional stability increase compared to HDPE. Therefore, probing the viscoelastic properties and interfaces of HDPE vitrimer provides important insights into understanding of the correlations between the vitrimer nanostructure and the bulk mechanical and rheological behaviors.

Nanoscale Rheology: Dynamic Mechanical Analysis over a Broad and Continuous Frequency Range Using Photothermal Actuation Atomic Force Microscopy

Polymeric materials are widely used in industries ranging from automotive to biomedical. Their mechanical properties play a crucial role in their application and function and arise from the nanoscale structures and interactions of their constitutive polymer molecules. Polymeric materials behave viscoelastically, i.e., their mechanical responses depend on the time scale of the measurements; quantifying these time-dependent rheological properties at the nanoscale is relevant to develop, for example, accurate models and simulations of those materials, which are needed for advanced industrial applications. In this paper, an atomic force microscopy (AFM) method based on the photothermal actuation of an AFM cantilever is developed to quantify the nanoscale loss tangent, storage modulus, and loss modulus of polymeric materials. The method is then validated on styrene-butadiene rubber (SBR), demonstrating the method's ability to quantify nanoscale viscoelasticity over a continuous frequency range up to 5 orders of magnitude (0.2-20,200 Hz). Furthermore, this method is combined with AFM viscoelastic mapping obtained with amplitude modulation-frequency modulation (AM-FM) AFM, enabling the extension of viscoelastic quantification over an even broader frequency range and demonstrating that the novel technique synergizes with preexisting AFM techniques for quantitative measurement of viscoelastic properties. The method presented here introduces a way to characterize the viscoelasticity of polymeric materials and soft and biological matter in general at the nanoscale for any application.

Nanorheology and Nanoindentation Revealed a Softening and an Increased Viscous Fluidity of Adherent Mammalian Cells upon Increasing the Frequency

The nanomechanical response of a cell depends on the frequency at which the cell is probed. The components of the cell that contribute to this property and their interplay are not well understood. Here, two force microscopy methods are integrated to characterize the frequency and/or the velocity-dependent properties of living cells. It is shown on HeLa and fibroblasts, that cells soften and fluidize upon increasing the frequency or the velocity of the deformation. This property was independent of the type and values (25 or 1000 nm) of the deformation. At low frequencies (2-10 Hz) or velocities (1-10 µm s-1 ), the response is dominated by the mechanical properties of the cell surface. At higher frequencies (>10 Hz) or velocities (>10 µm s-1 ), the response is dominated by the hydrodynamic drag of the cytosol. Softening and fluidization does not seem to involve any structural remodeling. It reflects a redistribution of the applied stress between the solid and liquid-like elements of the cell as the frequency or the velocity is changed. The data indicates that the quasistatic mechanical properties of a cell featuring a cytoskeleton pathology might be mimicked by the response of a non-pathological cell which is probed at a high frequency.

Unraveling bilayer interfacial features and their effects in polar polymer nanocomposites

Polymer nanocomposites with nanoparticles dispersed in polymer matrices have attracted extensive attention due to their significantly improved overall performance, in which the nanoparticle-polymer interface plays a key role. Understanding the structures and properties of the interfacial region, however, remains a major challenge for polymer nanocomposites. Here, we directly observe the presence of two interfacial polymer layers around a nanoparticle in polar polymers, i.e., an inner bound polar layer (~10 nm thick) with aligned dipoles and an outer polar layer (over 100 nm thick) with randomly orientated dipoles. Our results reveal that the impacts of the local nanoparticle surface potential and interparticle distance on molecular dipoles induce interfacial polymer layers with different polar molecular conformations from the bulk polymer. The bilayer interfacial features lead to an exceptional enhancement in polarity-related properties of polymer nanocomposites at ultralow nanoparticle loadings. By maximizing the contribution of inner bound polar layer via a nanolamination design, we achieve an ultrahigh dielectric energy storage density of 86 J/cm3, far superior to state-of-the-art polymers and nanocomposites.

Insights into Mechanical Dynamics of Nanoscale Interfaces in Epoxy Composites Using Nanorheology Atomic Force Microscopy

Interfacial polymer layers with nanoscale size play critical roles in dissipating the strain energy around cracks and defects in structural nanocomposites, thereby enhancing the material's fracture toughness. However, understanding how the intrinsic mechanical dynamics of the interfacial layer determine the toughening and reinforcement mechanisms in various polymer nanocomposites remains a major challenge. Here, by means of a recently developed nanorheology atomic force microscopy method, also known as nanoscale dynamic mechanical analysis (nDMA), we report direct mapping of dynamic mechanical responses at the interface of a model epoxy nanocomposite under the transition from a glassy to a rubbery state. We demonstrate a significant deviation in the dynamic moduli of the interface from matrix behavior. Interestingly, the sign of the deviation is observed to be reversed when the polymer changes from a glassy to a rubbery state, which provides an excellent explanation for the difference in the modulus reinforcement between glassy and rubbery epoxy nanocomposites. More importantly, nDMA loss tangent images unambiguously show an enhanced viscoelastic response at the interface compared to the bulk matrix in the glassy state. This observation can therefore provide important insights into the nanoscale toughening mechanism that occurs in epoxy nanocomposites due to viscoelastic energy dissipation at the interface.

Comparison of nanostructure and nanomechanical properties of cast and air sides of polyimide films from different manufacturers

Polyimide films are widely applied in harsh environments because of their outstanding performance. High-quality polyimide films are often manufactured through a two-step process. The complicated procedure results in different properties on the two sides, i.e., the air side and cast side of the films, and the quality of products from different manufacturers varies notably. In the present work, polyimide films with two thicknesses (1 and 2 mm) from four manufacturers were investigated. Atomic force microscope and FT-IR spectrometer were employed to monitor morphology, roughness, nanomechanical properties, and corresponding relative imidization degree on the two sides of each film. Statistical tools were applied to analyze the data. T-test suggests that the two sides of the same film were significantly different in roughness, DMT modulus, and relative imidization degree (p < 0.05). The roughness on the air side was consistently smaller than that of the cast side. ANOVA was used to compare differences among the manufacturers. Manufacturer B provided the smoothest films with the highest DMT moduli and imidization degrees. A positive correlation was found between the DMT modulus and imidization degree (r = 0.7330). Nanostructure and nanomechanical properties could affect the quality of the film. Striped morphology and adhesion were found on the cast side of the 2-mm film from manufacturer D, which compromised the film tension in the direction perpendicular to the strips. Investigations of morphology and mechanical properties of polyimide film at the nanoscale would help us better characterize the film, assure its quality, and select suitable film and side for proper applications.

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.