Microstructural changes in polyethylene-polypropylene blends as revealed by microhardness

Microhardness is used to examine the microstructural changes of a series of polyethylene (PE) /polypropylene (PP) blends in a wide composition range. This study complements previous hardness results obtained on high-density/low-density polyethylene systems. The use of isotactic polypropylene, as a blend component allows investigation of a material in which the hardness of the amorphous phase, contrary to PE, differs from zero. The influence of treatments such as crystallization of the PP-phase in the presence of molten PE, within the blend, or annealing the PE phase, while leaving the PP component unmodified, are discussed with reference to the additivity hardness values of the single componentsH _PE andH _PP. It is shown that the coexistence of the PP and PE phases inhibits the crystallization capability of one phase and modifies the annealing behaviour of the other phase leading, as a result, to depressedH _PP andH _PE values. The observed deviations ofH _blend, throughout the composition range, from the additivity law of single components are quantitatively justified in the light of crystallinity changes of the PP phase and in terms of the population of modified lamellae of the PE component.     

Microhardness of carbon fiber reinforced epoxy and thermoplastic polyimide composites

We have examined the micro indentation hardness of a series of carbon fiber reinforced epoxy and thermoplastic polyimide (TPI) composites. In the epoxy systems, the influence of Nylon particles was studied. The effect of crystallization of the thermoplastic polyimide upon the microhardness values of the resin was also investigated. The microstructure of the TPI-composites was characterized by X-ray diffraction. The results show that the addition of carbon fibers to the neat resins greatly increases the microhardness and thus the yield stress of the composite. The value of the microhardness technique is highlighted in emphasizing the heterogeneity of the CFRC.     

Metastable Polymorphic Phases in Monolayer TaTe2

Polymorphic phases and collective phenomena—such as charge density waves (CDWs)—in transition metal dichalcogenides (TMDs) dictate the physical and electronic properties of the material. Most TMDs naturally occur in a single given phase, but the fine-tuning of growth conditions via methods such as molecular beam epitaxy (MBE) allows to unlock otherwise inaccessible polymorphic structures. Exploring and understanding the morphological and electronic properties of new phases of TMDs is an essential step to enable their exploitation in technological applications. Here, scanning tunneling microscopy (STM) is used to map MBE-grown monolayer (ML) TaTe₂. This work reports the first observation of the 1H polymorphic phase, coexisting with the 1T, and demonstrates that their relative coverage can be controlled by adjusting synthesis parameters. Several superperiodic structures, compatible with CDWs, are observed to coexist on the 1T phase. Finally, this work provides theoretical insight on the delicate balance between Te…Te and Ta–Ta interactions that dictates the stability of the different phases. The findings demonstrate that TaTe₂ is an ideal platform to investigate competing interactions, and indicate that accurate tuning of growth conditions is key to accessing metastable states in TMDs.