Phase diagrams of thin films and wires

Starting from a relation between phonons, defects and surfaces, a model is developped, dealing with the stability of crystalline phases. It is shown that:

• 1.1) the disordering temperature decreases with decreasing thickness;
• 2.2) phase diagrams of thin films might differ from bulk ones;
• 3.3) surface and interface contributions might modify the previous effects.

     

High pressure Raman studies on the structural conformation of oligophenyls

The goal of this combined experimental and computational study is to investigate the structural conformation of oligo(para-phenylenes) in the crystalline phase, in particular the planarity of this type of molecules. To this end we have performed Raman experiments on para-terphenyl and para-quaterphenyl in a pressure range from 0 to 70 kbar and at temperatures from 10 to 300 K. The positions and the relative intensities of the C–C interring stretch mode at 1280 cm⁻¹ and the C–H in-plane bend mode at 1220 cm⁻¹ have been tracked. We find that upon increasing temperature at ambient pressure the intensity ratio I₁₂₈₀/I₁₂₂₀ drops rapidly at temperatures that coincide with the crystallographic phase transition for the investigated materials. At ambient temperature also, this intensity ratio drops rapidly upon increasing pressure up to about 15 kbar. In the computational part, the Raman frequencies and activities of isolated 3P and 4P molecules were calculated within restricted Hartree–Fock formalism with the interring tilt angles varying from 0 to 90°. These calculations confirm that the I₁₂₈₀/I₁₂₂₀ intensity ratio can be related to the planarity of the molecules. Three-dimensional bandstructure calculations within density functional theory were applied to determine phonon frequencies and estimate Raman activities for the polymer poly(para-phenylene). These simulations show that the same conclusions hold for crystalline environment.     

Flexural behavior of ECC hollow beams incorporating different synthetic fibers

Twelve ECC beams with three different fiber types, along with four normal concrete beams, were tested in this study to evaluate the influence of cross-sectional hollowing on their flexural performance. The fiber types used were nylon monofilament (NM), low-cost untreated polyvinyl alcohol (PVA), and polypropylene (PP). Three different square hole sizes of 60, 80, and 100 mm with cross-sectional hollowing ratios of 0.16, 0.28, and 0.44, respectively, were adopted for each group of beams in addition to a solid beam. All beams were tested under four-point loading using a displacement-controlled testing machine. The test results showed that ECC beams can mostly withstand higher cracking and ultimate loads compared to their corresponding normal concrete versions. The results also showed that both the ductility and toughness of the ECC beams are higher than those of the normal concrete beams and that the ductility values of the hollow beams with a hole size of 60 mm are higher than those of the corresponding solid beams. Moreover, hollow ECC beams with hole sizes of 60 and 80 mm exhibited a higher ductility than a solid normal concrete beam.