Use of time-resolved X-ray scattering methods for investigation of structural formation in polymer liquid crystals (original) (raw)
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Macromolecules, 2000
The phase transitions in two liquid crystalline polyesters have been studied by diffraction techniques employing synchrotron radiation. The two polyesters, derived from p,p′-bibenzoic acid and 3-methyl-1,6-hexanediol, only differ on the optical activity of the chiral spacer: both the active and racemic polymers have been investigated. Time-resolved experiments at variable temperature were performed, analyzing the cooling from the isotropic melt and the subsequent melting. The analysis of the different diffraction profiles (covering WAXS, MAXS, and SAXS regions) shows important differences in the phase behavior of the two polyesters, more important than those anticipated from the analysis of the DSC curves. Nevertheless, a common feature is the obtention of S C mesophases in both polymers, characterized by a continuous increase of the tilting angle as the temperature is decreased (before the appearance of high-order mesophases).
Liquid crystalline behavior of polymers; fundamentals, synthesis and characterizations
2019
As it is well known that, materials that have liquid crystalline properties are early discovered long time ago. These type of materials can carry both type of different types of matter (conventional liquid and solid crystal). Upon inserting such properly to the backbone of any polymer structure. It can enhance its amazing properties to be considered as one of the most and effective smart materials in the last few decades. The current work presents a short summary about the historical overview and some important basic fundamentals of liquid crystalline materials and its different types of calcifications. In addition, a detailed study about liquid crystalline polymers (LCps) have been described classifications and synthetic method. Both types of these polymers were discussed including main chain or side chain polymers. The effects of mesogenic groups, structural unit, substituents and flexible spacers on the thermotropic properties were also displayed. Finally, a brief overview for th...
Thermal and Optical Characterization of Polymer-Dispersed Liquid Crystals
International Journal of Polymer Science, 2012
Liquid crystals are compounds that display order in the liquid state above the melting temperature and below the mesogenic isotropic temperature. Polymer-dispersed liquid crystals (PDLCs) are composite materials in which liquid crystalline material is dispersed within a polymer matrix to form micron-sized droplets. The aim was to prepare several cholesteryl esters or alkoxybenzoic acid PDLCs and characterise thermal and optical properties. Differential scanning calorimetry and polarized optical microscopy were employed. The matrix polymer was a one-component UV-curable epoxy-acrylate resin. PDLCs were formed through entropy controlled phase separation resulting from UV-initiated crosslinking. The liquid crystals, both as mesogenic moieties and as dispersed droplets, exhibited various textures according to their molecular order and orientation. These textures formed in constrained regions separated by phase boundaries that occurred at temperatures characteristic of each liquid crystal used. The PDLC phase transitions occurred at temperatures lower than those exhibited by the mesogenic components in the neat state.
European Polymer Journal, 2009
In order to understand nucleation; crystallization and other phase transitions in polymers, polymer based composites, or in liquid crystals simultaneous experiments with a combination of different methods are useful. Due to different sample geometry, contact faces with the sample holder, and thermal conditions it is usually difficult to compare the results of several individual experiments. As an important supplement to the classical techniques for studying crystallization like X-ray scattering, or differential scanning calorimetry, measurements which test molecular mobility like dielectric or mechanical spectroscopy are of interest during isothermal and non-isothermal crystallization. From such simultaneous experiments one can learn about the existence of pre-ordered structures before formation of crystals, as detected by DSC or X-ray scattering.
Macromolecular Research, 2004
The structural changes occurring in the isothermal crystallization processes of polyethylene (PE), polyoxymethylene (POM), and vinylidene fluoridetrifluoroethylene (VDFTrFE) copolymer have been reviewed on the basis of our recent experimental data collected by the time-resolved measurements of synchrotron-sourced wide-angle (WAXS) and small-angle X-ray scatterings (SAXS) and infrared spectra. The temperature jump from the melt to a crystallization temperature could be measured at a cooling rate of 600–1,000°C/min, during which we collected the WAXS, SAXS, and infrared spectral data successfully at time intervals of ca. 10 sec. In the case of PE, the infrared spectral data clarified the generation of chain segments of partially disordered trans conformations immediately after the jump. These segments then became transformed into more-regular all-trans-zigzag forms, followed by the formation of an orthorhombic crystal lattice. At this stage, the generation of a stacked lamellar structure having an 800-Å-long period was detected in the SAXS data. This structure was found to transfer successively to a more densely packed lamellar structure having a 400-Å-long period as a result of the secondary crystallization of the amorphous region in-between the original lamellae. As for POM, the formation process of a stacked lamellar structure was essentially the same as that mentioned above for PE, as evidenced from the analysis of SAXS and WAXS data. The observation of morphology-sensitive infrared bands revealed the evolution of fully extended helical chains after the generation of lamellae having folded chain structures. We speculate that these extended chains exist as taut tie chains passing continuously through the neighboring lamellae. In the isothermal crystallization of VDFTrFE copolymer from the melt, a paraelectric high-temperature phase was detected at first and then it transferred into the ferroelectric low-temperature phase at a later stage. By analyzing the reflection profile of the WAXS data, the structural ordering in the high-temperature phase and the ferroelectric phase transition to the low-temperature phase of the multidomain structure were traced successfully.
X-ray studies of the phases and phase transitions of liquid crystals
Acta Crystallographica Section A Foundations of Crystallography, 2005
A short review is given of recent X-ray diffraction studies of the phases and phase transitions of thermotropic liquid crystals. The areas covered are twistgrain-boundary phases, antiferroelectric phases studied with resonant X-ray diffraction and smectic phases within gel structures. In all areas, X-ray diffraction has played a key role. Nonetheless, open questions remain: the nature of the smectic C variant of the twist-grain-boundary phase, the origin of antiferroelectric phases, and whether novel Bragg glass states exist for smectic A gel samples.
Dynamic x-ray diffraction studies of liquid-crystalline polyesters
Journal of Polymer Science Part B: Polymer Physics, 1990
Mesophase transitions in liquid-crystalline (LC) polyesters were studied by dynamic x-ray diffraction using a synchrotron radiation source. Powder and fiber samples were examined by continuous heating from 50°C to 270°C in a hot stage. The polymer systems consisted of two types of thermotropic polyesters with mesogenic cores composed of combinations of substituted terephthalate, oxybenzoate and hydroquinone units combined with aliphatic spacers placed in the main chain. One of these samples was a chemically homogeneous LC polyester (HTH12) while the other LC polyester possessed chemical heterogeneity (BP6). BP6 could also be processed to form fibers which showed thermal transition behavior by x-ray diffraction and no detectable melting or clearing transition by thermal measurements. LC textures were observed using polarized light microscopy. Results of the dynamic x-ray diffraction studies of these two LC polymers are described.
Properties of polymer liquid crystals: choosing molecular structures and blending
Polymer, 1990
Monomer liquid crystals (MLCs), polymer liquid crystals (PLCs) and heterogeneous composites are compared. PLCs are classified into categories in function of their molecular structure. Molecular composites (MC) are characterized. Phase diagrams are analysed, and new ways of presenting information on coexisting phases including glassy ones are proposed. Rheoiogy and methods of processing of PLCs and MCs are outlined. Characterization of PLCs including their dielectric behaviour is discussed. Blending of PLCs and properties of the blends of MCs are related to present as well as potential applications of liquid-crystalline and rigid rod materials.