Microscopic chemical characterization of epoxy resin with scanning transmission electron microscopy - electron energy-loss spectroscopy

The structural characterization of epoxy resins is essential to improve the understanding on their structure-property relationship for promising high-performance applications. Among all analytical techniques, scanning transmission electron microscopy-electron energy-loss spectroscopy (STEM-EELS) is a powerful tool for probing the chemical and structural information of various materials at a high spatial resolution. However, for sensitive materials, such as epoxy resins, the structural damage induced by electron-beam irradiation limits the spatial resolution in the STEM-EELS analysis. In this study, we demonstrated the extraction of the intrinsic features and structural characteristics of epoxy resins by STEM-EELS under electron doses below 1 e-/Å2 at room temperature. The reliability of the STEM-EELS analysis was confirmed by X-ray absorption spectroscopy and spectrum simulation as low- or non-damaged reference data. The investigation of the dependence of the epoxy resin on the electron dose and exposure time revealed the structural degradation associated with electron-beam irradiation, exploring the prospect of EELS for examining epoxy resin at low doses. Furthermore, the degradation mechanisms in the epoxy resin owing to electron-beam irradiation were revealed. These findings can promote the structural characterization of epoxy-resin-based composites and other soft materials.

Rod-Coil Block Copolymer: Fullerene Blend Water-Processable Nanoparticles: How Molecular Structure Addresses Morphology and Efficiency in NP-OPVs

The use of water-processable nanoparticles (WPNPs) is an emerging strategy for the processing of organic semiconducting materials into aqueous medium, dramatically reducing the use of chlorinated solvents and enabling the control of the nanomorphology in OPV active layers. We studied amphiphilic rod-coil block copolymers (BCPs) with a different chemical structure and length of the hydrophilic coil blocks. Using the BCPs blended with a fullerene acceptor material, we fabricated NP-OPV devices with a sustainable approach. The goal of this work is to clarify how the morphology of the nanodomains of the two active materials is addressed by the hydrophilic coil molecular structures, and in turn how the design of the materials affects the device performances. Exploiting a peculiar application of TEM, EFTEM microscopy on WPNPs, with the contribution of AFM and spectroscopic techniques, we correlate the coil structure with the device performances, demonstrating the pivotal influence of the chemical design over material properties. BCP5, bearing a coil block of five repeating units of 4-vinilpyridine (4VP), leads to working devices with efficiency comparable to the solution-processed ones for the multiple PCBM-rich cores morphology displayed by the blend WPNPs. Otherwise, BCP2 and BCP15, with 2 and 15 repeating units of 4VP, respectively, show a single large PCBM-rich core; the insertion of styrene units into the coil block of BCP100 is detrimental for the device efficiency, even if it produces an intermixed structure.

Chemical Fingerprinting of Polymers Using Electron Energy-Loss Spectroscopy

Electron energy-loss spectroscopy (EELS) is becoming an important tool in the characterization of polymeric materials. The sensitivity of EELS to changes in the chemical structure of polymeric materials dictates its applicability. In particular, it is important for compositional analysis to have reference spectra of pure components. Here, we report the spectra of the carbon K-edge of six polymers (polyethylene, polypropylene, polybutylene terephthalate, and polylactic acid) including copolymers (styrene acrylonitrile and acrylonitrile butadiene styrene), to be used as reference spectra for future EELS studies of polymers. We have successfully decomposed the carbon K-edge of each of the polymers and assigned the observed peaks to bonding transitions. The spectra have been acquired in standard experimental conditions, and electron beam damage has been taken into account during establishment of spectral-structural relationships. We found that the more commonly available low-energy resolution spectrometers are adequate to chemically fingerprint linear saturated hydrocarbons such as PE, PP, and PLA. We have thus moved a step closer toward creating an atlas of polymer EELS spectra, which can be subsequently used for chemical bond mapping of polymeric materials with nanoscale spatial resolution.