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

Force Volume Atomic Force Microscopy-Infrared for Simultaneous Nanoscale Chemical and Mechanical Spectromicroscopy

Photothermal atomic force microscopy-infrared (AFM-IR) combines the nanoscale spatial resolution of AFM with the chemical identification capability of infrared spectroscopy and has thrived in various applications. Currently executed in three major AFM modes (contact, tapping, and peak force tapping) we introduce a fourth variant built upon force volume mode comprising a defined engage, hold, and retract segment in each pixel. IR laser pulsing at a probe resonance frequency during the constant-force hold segment duplicates the resonance-enhanced AFM-IR detection principle of contact mode. However, force volume AFM-IR removes the lateral forces that cause tip wear and sample damage while adding the spatial resolution of tapping AFM-IR. As demonstrated on different materials, this imaging and spectroscopy technique integrates monolayer sensitivity, sub-10 nm spatial chemical resolution, simultaneous nanomechanical property sensing, and precise force control. The ability to sweep the infrared laser repetition rate in each pixel provides additional, rich information in the form of contact resonance curves, while compensating for mechanically induced probe resonance shifts in an alternative to conventional phase-locked loop based frequency tracking. Such sweeps inherently consider the Q-factor (i.e., mechanical damping) in the AFM-IR response, a little investigated aspect. Furthermore, the probing depth can be varied by selecting different resonances recorded within a single broad frequency sweep. Switching to the surface sensitive AFM-IR detection scheme during the hold segment additionally limits the probing depth. These qualities should position force volume AFM-IR as a valuable addition to established AFM-IR modes.

Potential-Dependent ATR-SEIRAS and EQCM-D Analysis of Interphase Formation in Zinc Battery Electrolytes

With the heightening interest in bivalent battery technology, there arises a necessity for a thorough investigation into zinc-ion battery (ZIB) electrolytes, accommodating their chemical attributes and potential-dependent structural dynamics. While the phenomenon of in situ solid electrolyte interphase formation is extensively documented in lithium-ion batteries, its analogous occurrences in ZIBs remain limited. Herein is a comparative study of three zinc electrolytes of interest: ZnSO4, ZnOTF, and Zn(TFSI)2/LiTFSI hybrid water-in-salt electrolyte. Additionally, the impact of an acetonitrile additive is scrutinized, with a comparative assessment of the interfacial behavior in aqueous solutions. Utilizing ATR-SEIRAS, potential-dependent alterations in the composition of the electrolyte/electrode interface were monitored, while EQCM-D facilitated a comprehensive understanding of variations in the mass and structural properties of the adsorbed layer. Aqueous ZnSO4 demonstrated the accumulation of porous Zn4SO4(OH)6·xH2O at negative potentials, leading to a mass of 1.47 μg cm-2 after five cycles. Bisulfate formation was observed at positive potentials. SEIRAS measurements for ZnOTF demonstrated reorientation and surface adsorption of CF3SO3- to favor CF3 at the surface for positive potentials, and acetonitrile showed increased stability for the electrode at negative potentials. The additive was also reported to lead to the accumulation of a substantial passivation layer with viscoelastic properties. The zinc water-in-salt showed exceptional surface stability at negative potentials and a widened potential window. A thin rigid zinc SEI layer is reported with a mass of 0.7 μg cm-2. The compositional intricacies of these surface structures are discussed in relation to their solvent conditions. This investigation not only sheds light on the initial charge/discharge cycles in ZIBs but also underscores their pivotal role in instigating enduring transformations that can significantly influence their long-term cycling performance.

Mapping the Nanoscale Heterogeneous Responses in the Dynamic Acceleration of Deformed Polymer Glasses

Understanding the evolution of local structure and mobility of disordered glassy materials induced by external stress is critical in modeling their mechanical deformation in the nonlinear regime. Several techniques have shown acceleration of molecular mobility of various amorphous glasses under macroscopic tensile deformation, but it remains a major challenge to visualize such a relationship at the nanoscale. Here, we employ a new approach based on atomic force microscopy in nanorheology mode for quantifying the local dynamic responses of a polymer glass induced by nanoscale compression. By increasing the compression level from linear elastic to plastic deformation, we observe an increase in the mechanical loss tangent (tan δ), evidencing the enhancement of polymer mobility induced by large stress. Notably, tan δ images directly reveal the preferential effect of the large compression on the dynamic acceleration of nanoscale heterogeneities with initially slow mobility, which is clearly different from that induced by increasing temperature.

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.

Effect of Condensed Water at an Alumina/Epoxy Resin Interface on Curing Reaction

The adhesion of epoxy adhesives to aluminum materials is an important issue in assembling parts for lightweight mobility. Aluminum surfaces typically possess an oxide layer, which readily adsorbs water. In this study, the aggregation states of water and its effect on the curing reaction were examined by placing a water layer between an amorphous alumina surface and a mixture of epoxy and amine components. This study used molecular dynamics simulations and density functional theory calculations. Before the reaction, water molecules strongly adsorbed onto the alumina surface, aggregating excess water. Some water diffused into the epoxy/amine mixture, accelerating the diffusion of unreacted substances. This led to faster reaction kinetics, particularly in proximity to the alumina surface. The adsorption of water molecules onto the alumina surface and the aggregation of excess water were similarly observed even after the curing process. Subsequently, the interaction between the alumina surface and various functional groups of the epoxy/amine mixture was evaluated before and after the reaction. Epoxy monomers had little interaction with the alumina surface before the reaction, whereas hydroxy groups formed by the ring-opening reaction of epoxy groups exhibited notable interaction. Conversely, sulfonyl and amino groups in amine compounds formed hydrogen bonds with OH groups on the alumina surface before the reaction. However, after the reaction, amino groups weakened their interaction with the alumina OH groups as they transformed from primary to tertiary during the curing reaction. Both epoxy and amine monomers/fragments similarly interacted with water molecules, both before and after the reaction. The insights gained from this study are expected to contribute to a better understanding of the impact of moisture absorption on the application of epoxy resins.

Electronic Interaction of Epoxy Resin with Copper at the Adhered Interface

A better understanding of the aggregation states of adhesive molecules in the interfacial region with an adherend is crucial for controlling the adhesion strength and is of great inherent academic interest. The adhesion mechanism has been described through four theories: adsorption, mechanical, diffusion, and electronic. While interfacial characterization techniques have been developed to validate the aforementioned theories, that related to the electronic theory has not yet been thoroughly studied. We here directly detected the electronic interaction between a commonly used thermosetting adhesive, cured epoxy of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-diaminodiphenylmethane (DDM), and copper (Cu). This study used a combination of density functional theory (DFT) calculations and femtosecond transient absorption spectroscopic (TAS) measurements as this epoxy adhesive-Cu pairing is extensively used in electronic device packaging. The DFT calculations predicted that π electrons in a DDM molecule adsorbed onto the Cu surface flowed out onto the Cu surface, resulting in a positive charge on the DDM. TAS measurements for the Cu/epoxy multilayer film, a model sample containing many metal/adhesive interfaces, revealed that the electronic states of excited DDM moieties at the Cu interface were different from those in the bulk region. These results were in good accordance with the prediction by DFT calculations. Thus, it can be concluded that TAS is applicable to characterize the electronic interaction of adhesives with metal adherends in a nondestructive manner.

Atomistic Modeling of the Effect of Temperature on Interfacial Properties of 3D-Printed Continuous Carbon Fiber-Reinforced Polyamide 6 Composite: From Processing to Loading

The combination of continuous fiber-reinforced thermoplastic composites (CFRTPCs) and the continuous fiber 3D printing (CF3DP) technique enables the rapid production of complex structural composites. In these 3D-printed composites, stress transfer primarily relies on the fiber-resin interface, making it a critical performance factor. The interfacial properties are significantly influenced by the temperatures applied during the loading and forming processes. While the effect of the loading temperature has been extensively researched, that of the forming temperature remains largely unexplored, especially from an atomistic perspective. Our research aims to employ molecular dynamics simulations to elucidate the effect of temperature on the interfacial properties of continuous carbon fiber-reinforced polyamide 6 (C/PA6) composites fabricated using the CF3DP technique, considering both loading and forming aspects. Through molecular dynamics simulations, we uncovered a positive correlation between the interfacial strength and forming temperature. Moreover, an increased forming temperature induced a notable shift in the failure mode of C/PA6 under uniaxial tensile loading. Furthermore, it was observed that increasing loading temperatures led to the deterioration of the mechanical properties of PA6, resulting in a gradual transition of the primary failure mode from adhesive failure to cohesive failure. This shift in the failure mode is closely associated with the glass transition of PA6.