Local mechanical and dielectric behavior of the interacting polymer layer in silica nano-particles filled SBR by means of AFM-based methods

Connecting Dynamics and Thermodynamics in Polymer-Resin Cured Systems

This work connects the calorimetric responses of different rubber-resin blends with varying resin contents with their alpha relaxation dynamics. We used differential scanning calorimetry and broadband dielectric spectroscopy to characterize the calorimetric and dielectric responses of styrene-butadiene, polybutadiene, and polyisoprene with different resin contents. To model the results, we used the Gordon-Taylor equation combined with an extension of the Adam-Gibbs approach. Thus, we propose a simple and effective model that allows us to estimate the blend dynamics from the temperature dependence of the relaxation times of the pure components and the calorimetric measurement of the glass transition temperature of only one blend composition. By estimating an effective interaction parameter from calorimetry, we achieved accurate alpha relaxation dynamics predictions for different resin concentrations. Our highly predictive approach provides a realistic description of the expected dynamics. This study offers valuable insights into the dynamic properties of polymer compounds, paving the way for the fast and effective development of advanced and more sustainable materials.

Tracking the evolution of the morphology and stress distribution of SIS thermoplastic elastomers under tension using atomic force microscopy

Styrene-based ABA-type triblock copolymers and their blends are widely investigated thermoplastic elastomers (TPEs). The design of tough TPE materials with high strength and resilience requires further clarification of the relationship between microstructure and macroscopic properties of stretched samples. Here, we applied atomic force microscopy (AFM)-based quantitative nanomechanical mapping to study the deformation behavior of poly(styrene-b-isoprene-b-styrene) blends under tension. The results indicated that the glassy polystyrene (PS) domains deformed and inhomogeneous stress distributions developed in the initial stretching stage. At 200% strain, the glassy PS domains started to crack. The change in the peak value in the JKR Young's modulus diagram during stretching was consistent with the stress - strain curve. Analysis of the particles before and after stretching suggested that the glassy domains separated and reorganized during stretching.

Insights into the Structural and Dielectric Behavior of Composites Produced from EPDM Waste Processed through a Devulcanization Method and SBR

Ethylene propylene diene monomer (EPDM) is one of the most used polymers in the world. It is an elastomer, which means that the existing cross-linking between the main chains of the polymer created during the vulcanization process makes its recycling difficult. In this paper, a possible solution to this issue is studied. The devulcanization of EPDM is achieved by a thermomechanical process followed by microwave irradiation. These combined treatments suppress the cross-linking, yielding a material (EPDMd) that can be successfully blended to form composites. A common elastomer, styrene butadiene rubber (SBR), has been selected as the matrix. The new SBR/EPDMd composites can be useful as elastomeric dielectric materials and can contribute to the recycling of the discarded EPDM. To provide a better understanding of their microstructure and its relationship with their micro- and macroscopic behavior, samples containing 20 and 40% of EPDMd have been tested by thermogravimetric and dielectric analysis, focusing on variables such as the thermal properties of the blends, permittivity, electric modulus, conductivity, and activation energies. The results show interesting changes linked to the presence of EPDMd in the SBR matrix, such as the displacement of the β dielectric relaxation toward higher frequencies. The correct integration between the two phases is confirmed by the absence of any Maxwell-Wagner-Sillars type relaxation in their dielectric behavior. The presence of additives in the EPDMd samples has an effect on the conductivity, mainly due to the conductive aluminum silicate present in the EPDMd, which acts toward increasing some key dielectric features like conductivity and permittivity and decreasing the insulation of the final SBR/EPDMd materials. The inclusion of EPDMd also affects the α relaxations (low frequencies) and suppresses the β relaxations (high frequencies). The samples showed a non-Debye dielectric behavior. In short, a compact and well-integrated material with a dielectric behavior is created, which exhibits interesting differences from the reference SBR matrix. Finally, it is concluded that the compounds tested are suitable for applications as electrical insulators.

Polymer Nanocomposite Dielectrics: Understanding the Matrix/Particle Interface

Polymer nanocomposite dielectrics possess exceptional electric properties that are absent in the pristine dielectric polymers. The matrix/particle interface in polymer nanocomposite dielectrics is suggested to play decisive roles on the bulk material performance. Herein, we present a critical overview of recent research advances and important insights in understanding the matrix/particle interfacial characteristics in polymer nanocomposite dielectrics. The primary experimental strategies and state-of-the-art characterization techniques for resolving the local property-structure correlation of the matrix/particle interface are dissected in depth, with a focus on the characterization capabilities of each strategy or technique that other approaches cannot compete with. Limitations to each of the experimental strategy are evaluated as well. In the last section of this Review, we summarize and compare the three experimental strategies from multiple aspects and point out their advantages and disadvantages, critical issues, and possible experimental schemes to be established. Finally, the authors' personal viewpoints regarding the challenges of the existing experimental strategies are presented, and potential directions for the interface study are proposed for future research.

Insight into the Experimental Error in the Mapping of Electrical Properties with Electrostatic Force Microscopy

Electrostatic force microscopy (EFM) is an emergent, powerful technique for nanoscale detection of electrical properties such as permittivity and charge distribution. However, the surface irregularity of samples has been unfortunately overlooked in most EFM studies. Herein, we use a polymer nanocomposite dielectric (PND) as the showcase and demonstrate that the morphological discontinuity at the matrix/particle interface can lead to major discrepancies or even incorrect results in the EFM study. First, the influence of the morphology, permittivity, and charge density of the interface is quantitively analyzed with a numerical method, proving that linking EFM results directly to sample properties is impracticable in the research based on classical interface configuration. Then, two methods are proposed to address the issue. The first method is numerical inversion, which takes heterogeneous materials and irregular surfaces into consideration. In this method, the influence of several experimental uncertainties, such as the radius of the nanoparticle and the permittivity of the matrix, is estimated. It is shown that the uncertainties related to geometry have a great impact on inversion and should be determined preferentially. In the second method, two standard configurations of the interface are recommended and compared for the interface study to bypass the morphological issue. This work provides quantitative results regarding the long-overlooked error in the EFM detection of the microregion with heterogeneous composition and surface irregularities and offers methods to tackle this issue.

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.

NANOMECHANICS OF THE RUBBER–FILLER INTERFACE

Atomic force microscope (AFM)–based nanomechanics is one of the most promising tools for accessing the rubber–filler interface while providing not only structural information but also mechanical–property evaluation. An AFM-based static modulus map is used to close in on the understanding of the filler reinforcement effect. As an example, a famous Guth–Gold equation is verified by comparing tensile testing and AFM. Two different novel methods are also introduced to visualize viscoelastic quantities such as storage and loss moduli, loss tangent, relaxation modulus, and viscosity. The difference in segmental dynamics between a rubber matrix and an interfacial region will be reviewed.

Characterization of Dielectric Nanocomposites with Electrostatic Force Microscopy

Nanocomposites physical properties unexplainable by general mixture laws are usually supposed to be related to interphases, highly present at the nanoscale. The intrinsic dielectric constant of the interphase and its volume need to be considered in the prediction of the effective permittivity of nanodielectrics, for example. The electrostatic force microscope (EFM) constitutes a promising technique to probe interphases locally. This work reports theoretical finite-elements simulations and experimental measurements to interpret EFM signals in front of nanocomposites with the aim of detecting and characterizing interphases. According to simulations, we designed and synthesized appropriate samples to verify experimentally the ability of EFM to characterize a nanoshell covering nanoparticles, for different shell thicknesses. This type of samples constitutes a simplified electrostatic model of a nanodielectric. Experiments were conducted using either DC or AC-EFM polarization, with force gradient detection method. A comparison between our numerical model and experimental results was performed in order to validate our predictions for general EFM-interphase interactions.

Nanoscale Mapping of Dielectric Properties of Nanomaterials from Kilohertz to Megahertz Using Ultrasmall Cantilevers

Electrostatic force microscopy (EFM) is often used for nanoscale dielectric spectroscopy, the measurement of local dielectric properties of materials as a function of frequency. However, the frequency range of atomic force microscopy (AFM)-based dielectric spectroscopy has been limited to a few kilohertz by the resonance frequency and noise of soft microcantilevers used for this purpose. Here, we boost the frequency range of local dielectric spectroscopy by 3 orders of magnitude from a few kilohertz to a few megahertz by developing a technique that exploits the high resonance frequency and low thermal noise of ultrasmall cantilevers (USCs). We map the frequency response of the real and imaginary components of the capacitance gradient (∂C(ω)/∂z) by using second-harmonic EFM and a theoretical model, which relates cantilever dynamics to the complex dielectric constant. We demonstrate the method by mapping the nanoscale dielectric spectrum of polymer-based materials for organic electronic devices. Beyond offering a powerful extension to AFM-based dielectric spectroscopy, the approach also allows the identification of electrostatic excitation frequencies which affords high dielectric contrast on nanomaterials.

DETERMINATION OF FILLER STRUCTURE IN SILICA-FILLED SBR COMPOUNDS BY MEANS OF SAXS AND AFM

The structure of the silica particles network in two different solution styrene–butadiene rubbers (S-SBRs) was studied by means of small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). S-SBR compounds with different silica contents were analyzed in comparison with their oil extended counterparts. A study into the application of SAXS experiments was defined to quantify the structures of silica primary particles and clusters in filled rubber compounds up to very high levels of filler content. We propose a modified structure model that is physically more sound than the widely used Beaucage model and that leads to more robust quantification of the silica structures. In addition, an independent characterization of the filler structure was performed by means of AFM. The cluster and particle sizes deduced from both techniques are in close agreement, supporting the proposed approach. The synergetic application of SAXS and AFM allows a consistent and robust characterization of primary particles and clusters in terms of size and structure. These results were compared and discussed in the framework of previously published works.

Effect of Interface Interaction on the Segmental Dynamics of Poly(vinyl acetate) Investigated by Local Dielectric Spectroscopy

The segmental dynamics of poly(vinyl acetate) (PVAc) thin films were measured in the presence of an aluminum interface and in contact with an incompatible polymer, poly(4-vinylpyridine). The local dielectric relaxation was found to be faster in thin films than in the bulk; however, no differences were observed for the various interfaces, including a PVAc/air interface. These results show that capping of thin films, even with a rigid material, does not necessarily affect the dynamics, the speeding up herein for capped PVAc was equivalent to that for the air interface. The insensitivity of the dynamics to the nature of the interface affords a means to engineer thin films while maintaining desired mechanical properties. Our findings for PVAc also may explain the discordant results that have been reported in general for the effect of air versus rigid interfaces on the local segmental relaxation of thin films.