Observation of elastic modulus inhomogeneities in thermosetting epoxies using AFM – Discerning facts and artifacts

Spatial Distribution of the Network Structure in Epoxy Resin via the MAXS-CT Method

We have succeeded in visualizing the spatial heterogeneity of the reaction ratio in epoxy resins by combining medium-angle X-ray scattering (MAXS) and computed tomography (CT). The reaction ratio is proportional to the degree of cross-linking between epoxy and amine in epoxy resins. The reaction ratio and its spatial inhomogeneity affect the toughness of epoxy resins. However, there has been no non-destructive method to measure the spatial inhomogeneity of the reaction ratio, although we can measure only the spatially averaged reaction ratio by Fourier-transform infrared spectroscopy (FT-IR). We found that the scattering peak reflected the cross-linking structures in the q region of MAXS and that the peak intensity is proportional to the reaction ratio. By reconstructing CT images from this peak intensity, we visualized the spatial heterogeneity of the reaction ratio. The application of this method may not be limited to epoxy resins but may extend to studying the heterogeneity of cross-linked structures in other materials.

Network Formation and Physical Properties of Epoxy Resins for Future Practical Applications

Epoxy resins are used in various fields in a wide range of applications such as coatings, adhesives, modeling compounds, impregnation materials, high-performance composites, insulating materials, and encapsulating and packaging materials for electronic devices. To achieve the desired properties, it is necessary to obtain a better understanding of how the network formation and physical state change involved in the curing reaction affect the resultant network architecture and physical properties. However, this is not necessarily easy because of their infusibility at higher temperatures and insolubility in organic solvents. In this paper, we summarize the knowledge related to these issues which has been gathered using various experimental techniques in conjunction with molecular dynamics simulations. This should provide useful ideas for researchers who aim to design and construct various thermosetting polymer systems including currently popular materials such as vitrimers over epoxy resins.

Blend configuration in functional polymeric materials with a high lignin content

Lignins upgrade the lignocellulosic cell-wall domains in all vascular plants; they embody 20-30% of terrestrial organic carbon. For 50 years, mistaken assumptions about the configuration of lignin have hindered the development of useful polymeric materials with a lignin content above 40 wt%. Now, polymeric materials composed only of methylated softwood lignin derivatives can exhibit better tensile behavior than polystyrene. Marked improvements may be achieved with small quantities (5-10 wt%) of miscible blend components as simple as poly(ethylene glycol). These observations contradict commonly held views about crosslinking or hyper-branching in lignin chains. The hydrodynamic compactness of the macromolecular lignin species arises from powerful noncovalent interactions between the lignin substructures. Individual lignin components undergo association to form macromolecular complexes that are preserved in plastics with a very high lignin content. Material continuity results from interpenetration between the peripheral components in adjoining lignin complexes. Through interactions with the peripheral domains, miscible blend components modulate the strength and ductility of these utterly original lignin-based plastics.

MORPHOLOGY AND NANOMECHANICAL CHARACTERISTICS OF NR/SBR BLENDS

Tire tread materials are generally blends of two or more rubbers, fillers, and other materials, resulting in a rubber compound with complex multiphase morphology. The bulk properties of these blends are influenced by morphology and microphase characteristics, hence the desire for techniques that can both discriminate between phases in a blend and provide quantitative information about their physical properties. The effect of polymer ratio and microstructure of SBR on blend morphology and nanomechanical mapping of unfilled NR/SBR blends will be discussed. With nanoindentation techniques available through the use of atomic force microscopy, nanomechanical properties are determined and compared with macroscopic values obtained by dynamic mechanical analysis.

Insights into Epoxy Network Nanostructural Heterogeneity Using AFM-IR

The first direct observation of a chemically heterogeneous nanostructure within an epoxy resin is reported. Epoxy resins comprise the matrix component of many high performance composites, coatings and adhesives, yet the molecular network structure that underpins the performance of these industrially essential materials is not well understood. Internal nodular morphologies have repeatedly been reported for epoxy resins analyzed using SEM or AFM, yet the origin of these features remains a contentious subject, and epoxies are still commonly assumed to be chemically homogeneous. Uniquely, in this contribution we use the recently developed AFM-IR technique to eliminate previous differences in interpretation, and establish that nodule features correspond to heterogeneous network connectivity within an epoxy phenolic formulation.