Deformation of Poly-l-lactid acid (PLLA) under Uniaxial Tension and Plane-Strain Compression

The ability of PLLA, either amorphous or semicrystalline, to plastic deformation to large strain was investigated in a wide temperature range (Td = 70–140 °C). Active deformation mechanisms have been identified and compared for two different deformation modes—uniaxial drawing and plane-strain compression. The initially amorphous PLLA was capable of significant deformation in both tension and plane-strain compression. In contrast, the samples of crystallized PLLA were found brittle in tensile, whereas they proved to be ductile and capable of high-strain deformation when deformed in plane-strain compression. The main deformation mechanism identified in amorphous PLLA was the orientation of chains due to plastic flow, followed by strain-induced crystallization occurring at the true strain above e = 0.5. The oriented chains in amorphous phase were then transformed into oriented mesophase and/or oriented crystals. An upper temperature limit for mesophase formation was found below Td = 90 °C. The amount of mesophase formed in this process did not exceed 5 wt.%. An additional mesophase fraction was generated at high strains from crystals damaged by severe deformation. After the formation of the crystalline phase, further deformation followed the mechanisms characteristic for the semicrystalline polymer. Interlamellar slip supported by crystallographic chain slip has been identified as the major deformation mechanism in semicrystalline PLLA. It was found that the contribution of crystallographic slip increased notably with the increase in the deformation temperature. The most probable active crystallographic slip systems were (010)[001], (100)[001] or (110)[001] slip systems operating along the chain direction. At high temperatures (Td = 115–140 °C), the α→β crystal transformation was additionally observed, leading to the formation of a small fraction of β crystals.

Engineering Mesophase Stability and Structure via Incorporation of Cyclic Terminal Groups

We report on the characterisation of a number of liquid-crystalline materials featuring cyclic terminal groups, which lead to significant enhancements in the temperature range of the mesomorphic state. Materials with only short terminal chains are able to support lamellar mesophase formation by appending a large terminal cyclic unit at the end of a short methylene spacer. X-ray scattering experiments reveal that the layer spacings of the lamellar smectic phase are significantly larger when a cyclic end-group is present than for equivalent linear unsubstituted materials, but there is no effect on orientational order. Fully atomistic molecular dynamics simulations faithfully reproduce experimental layer spacings and orientational order parameters, and indicate that the cyclic terminal units spontaneously segregate into diffuse sub-layers and thus cause the increased layer spacing. This shape segregation predicted by molecular dynamics simulations is observed in the crystalline solid state by X-ray diffraction.

Compositional Fibers Based on Coal Tar Mesophase Pitch Obtained by Electrospinning Method

This research examines the use of coal-processing wastes of Shubarkol deposit (Kazakhstan) in obtaining useful materials such as carbon fibers. For our experiments, mesophase pitch was obtained by coal tar heat treatment at 773 K. Spinnable solution was prepared by crushing mesophase pitch into the pieces with adding poly(methylmethacrylate) as a fiber-forming material and 1,2-dichloroethane as a solvent. Elemental analysis revealed that the chemical composition of mesophase pitch (С – 91.48 %; О – 8.52 %; S – 0.00 %) showed that heat treatment up to 773 K leads to the complete removal of sulfur-containing components which affect the mesophase formation. Raman data of the obtained pitch revealed the appearance of D (1366 cm-1) and G (1605 cm-1) peaks, which are responsible for carbon materials; another peak at 2900 cm-1 shows the presence of C–H bonds. Carbon fibers with the diameter of 0.8–1.75 μm were obtained by electrospinning method in laboratory settings.

Induced Wide Nematic Phase by Seven-Ring Supramolecular H-Bonded Systems: Experimental and Computational Evaluation

New seven-ring systems of dipyridine derivative liquid crystalline 2:1 supramolecular H-bonded complexes were formed between 4-n-alkoxyphenylazo benzoic acids and 4-(2-(pyridin-4-yl)diazenyl)phenyl nicotinate. Mesomorphic behaviors of the prepared complexes were investigated using a combination of differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Fermi bands attributed to the presence of intermolecular H-bond interactions were confirmed by FT–IR spectroscopy. All prepared complexes possessed an enantiotropic nematic phase with a broad temperature nematogenic range. Phases were confirmed by miscibility with a standard nematic (N) compound. A comparison was constructed to investigate the influence of the incorporation of the azophenyl moiety on the mesomeric behavior of corresponding five-membered complexes. It was found that the present complexes observed induced a wide nematic phase with relatively higher temperature ranges than the five aromatic systems. Density functional theory (DFT) suggested the nonlinear geometry of the formed complex. The results of the DFT explained the nematic mesophase formation. Moreover, the π–π stacking of the aromatic moiety in the phenylazo acid plays an effective role in the mesomorphic thermal stability. The energy difference between the frontier molecular orbitals, HOMO (highest occupied) and LUMO (lowest occupied), and the molecular electrostatic potential (MEP) of the prepared complexes were estimated by DFT calculations. The results were used to illustrate the observed nematic phase for all H-bonded supramolecular complexes. Finally, photophysical studies were discussed which were carried out by UV spectroscopy connected to a hot stage.

One-Step Densification of Carbon/Carbon Composites Impregnated with Pyrolysis Fuel Oil-Derived Mesophase Binder Pitches

Carbon/carbon (C/C) composites are conventionally manufactured by liquid-phase impregnation (LPI), in which the binder pitches and phenolic resins are impregnated into the composites, and by chemical vapor infiltration (CVI). However, CVI has certain limitations in that expensive gases, such as methane and propane, are used and a long reaction time is required. Therefore, LPI is more widely used, as it employs economical pitches. In this study, the effects of one-step preparation on mechanical properties of C/C composites impregnated with mesophase binder pitches and phenolic resins have been investigated. The C/C composites containing four types of 20 wt.% mesophase binder pitches had differences in softening point (SP) and quinoline insoluble (QI) contents. After conducting trials on mesophase formation using different heat treatment temperatures and times, the best density and mechanical properties of the C/C composites were achieved using the mesophase binder pitches with 170 °C SP. However, when SP 200 °C was used, the density of the C/C composites was not further improved. This is because the binder pitches were not properly impregnated into the composites due to the high viscosity and QI of the binder pitches. Furthermore, the C/C composites fabricated with 20 wt.% pitch 2 exhibited the highest mechanical properties.

Structural and Mechanical Strength of Proton Radiation Processed Polyethylene Terephthalate

The change in structural and mechanical behavior of polyethylene terephthalate (PET) due to 2.4 MeV proton has been studied. Radiation processing of PET polymer is carried out using different low doses such as 0.2, 2.0, and 20 kGy. The Physics of microstrain and radiation-induced mesophase formation are analysed. X-ray investigation indicates that proton-induced structural modification takes place in the material. Apart from usual diffraction peaks, a low intensity broad peak is observed at small angle of about 2q =10º, when the fibre axis is mounted parallel to the X-ray direction. Such peak is absent in the diffraction spectrum when the fibre axis is mounted perpendicular to the beam direction. The appearance of the extra peak in a particular orientation confirms that, the phase is 2-dimensionally oriented (mesophase). The Young’s modulus (Y) of this irradiated PET sample is found to be more than that of the virgin sample with the highest value recorded for a dose of 2.0 kGy. The decrease in Y for higher dose (20 kGy) may be due to enhanced ion-induced microstrain in the sample, causing degradation in mechanical strength.

Self-assembly and mesophase formation in a non-ionic chromonic liquid crystal: insights from bottom-up and top-down coarse-grained simulation models

New coarse-grained models are introduced for a non-ionic chromonic molecule, TP6EO2M, in aqueous solution.