Polarized Optical Microscopy Research Articles

Fabrication of amorphous lamellar freestanding poly(N‐dodecyl acrylamide) films and their mechanical properties

AbstractAn amorphous lamellar freestanding film composed of poly(N‐dodecyl acrylamide) (pDDA) was prepared by annealing a micrometer‐thick film under humid conditions (humid‐annealing). pDDA was blade‐coated onto a glass substrate, and its lamellar formation was studied by XRD and polarized optical microscopy (POM). The XRD pattern indicates that pDDA formed a highly oriented lamellar structure by humid‐annealing at 80 °C for 6 h. Moreover, the POM image of the cross‐section of the film indicates that lamellar formation started at the polymer–substrate interface, and the amorphous lamella covered the entire film area without randomly oriented regions. In contrast, the open Nicol microscopic image exhibited large cracks in the highly oriented film owing to shrinkage of the film. A crack‐free film was prepared using water‐saturated chloroform as the solvent for blade coating. Water is thought to adsorb to an amide moiety to induce a ‘pre‐lamellar’ structure in the solution. The formation of pre‐lamellae and the decrease in the humid‐annealing temperature reduced the shrinkage of the film by lamellar formation to form a crack‐free film. The mechanical properties of the crack‐free, freestanding pDDA films were studied using a simple tensile test. The film became increasingly stronger with the formation of an amorphous lamellar structure. These results indicate that the mechanical properties of polymer films can be improved without crystallization. © 2024 The Author(s). Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Mortars in the Archaeological Site of Hierapolis of Phrygia (Denizli, Turkey) from Imperial to Byzantine Age

Hierapolis of Phrygia, an archaeological site in southwestern Turkey, has been a UNESCO World Heritage Site since 1988. During archaeological campaigns, 71 mortar samples from public buildings were collected, dating from the Julio-Claudian to the Middle Byzantine period. The samples were analyzed using a multi-analytical approach including polarized optical microscopy (POM), digital image analysis (DIA), X-ray powder diffraction (XRPD) and SEM–EDS to trace the raw materials and understand the evolution of mortar composition and technology over time. During the Roman period, travertine and marble were commonly used in binder production, while marble dominated in the Byzantine period. The aggregates come mainly from sands of the Lycian Nappe and Menderes Massif, with carbonate and silicate rock fragments. Variations in composition, average size and circularity suggest changes in raw material sources in both Roman and Byzantine periods. Cocciopesto mortar was used in water-related structures from the Flavian to the Severan period, but, in the Byzantine period, it also appeared in non-hydraulic contexts. Straw became a common organic additive in Byzantine renders, marking a shift from the exclusively inorganic aggregates of Roman renders. This study illustrates the evolving construction technologies and material sources used throughout the city’s history.

Crystallization, thermal, and compatibility performances of poly (butylene succinate) (PBS)/poly((R)‐3‐hydroxybutyrate‐co‐(R) 3‐hydroxyhexanoate) (PHBHHx) blends

AbstractBiodegradable materials provide protective properties comparable to conventional plastics while diminishing reliance on nonrenewable resources. Environmentally friendly polymer blends were prepared by melting poly(R)‐3‐hydroxybutyrate‐co‐(R)‐3‐hydroxyhexanoate) (PHBHHx) with poly (butylene succinate) (PBS). PBS was synthesized through esterification and polycondensation using 1,4‐butanediol and succinic acid, with tetrabutyltitanate as the catalyst. The PHBHHx is supplied by Bulepha®PHA Company of China, model number BP330. The effects of PHBHHx content on thermal behavior, compatibility, and thermal stability of PBS/PHBHHx blends were systematically investigated. Differential scanning calorimetry (DSC) results indicated that the addition of PHBHHx influenced the crystallization behavior, the crystallization temperature (Tc) of PBS decreased meanwhile, the crystallization half‐time of PBS is reduced to 1.5 min compared with neat PBS (5 min) after isothermal is crystallized at 95°C. Polarized Optical Microscopy (POM) images also revealed that crystallization rate accelerated in the blend with increasing PHBHHx content. Rheological and Scanning Electron Microscopy (SEM) analysis demonstrated that incorporating a significant content of PHBHHx reduced compatibility between PBS and PHBHHx. Mechanical property testing indicates that the addition of PHBHHx reduced the toughness of blends. Additionally, the addition of a small amount of PHBHHx imparts a higher crystallinity (56%) and heat resistance of the PBS/PHBHHx blends (78°C) was higher than neat PBS (74°C).Highlights Laboratory synthesis of high molecular weight PBS. Optimal ratios improved crystallinity and thermal stability of blends. A systematic study of the crystallization properties of blends.

Impact of statistical poly(propylene co-ethylene) on the thermo-mechanical properties of high-density polyethylene

A series of high-density polyethylene and a statistical copolymer of poly(propylene-co-ethylene) blends in a wide range, namely (0, 10, 20, 30, 40, 50, 60,70, 80, 90, 100) abbreviated as HDPE/VM were systematically investigated by using a rheometer, differential scanning calorimetry (DSC) measurements, polarized optical microscopy (POM) and tensile tests. Rheometer results show that adding VM decreases dynamic viscosity, storage, and loss modulus. Han plot shows that HDPE and VM are compatible and miscible in the range from 20 to 60 VM % in the molten state. DSC results show little nucleation effect of VM on HDPE (HDPE’s melt crystallization temperature shifts 2 °C higher). Moreover, a linear composition dependence of ∆cp ∆Hc, ∆Hm shows that PE and VM are most probably compatible in the molten state in composition range from 20 to 60%. However, upon crystallization, the VM and PE domains occur distinctively. The results of the tensile tests demonstrated a decrease in elastic modulus, yield stress, and ultimate tensile strength as VM content increased. At low VM content (less than 20%), high elongation at break was detected for the blends, and very fine spherulites of HDPE were found across the sample by POM.

Liquid crystal and emulsifying properties of carboxymethyl glucoside as water-soluble surfactant

ABSTRACT Three derivatives of glucosides derived from n-dodecyl alcohol in a highly pure β-anomer were synthesised with the aim to yield the liquid crystalline phase at ambient temperature with improved water solubility. Effect of variation in headgroup polarity on thermal properties, liquid crystalline phases, and structures of these derivatives of n-dodecyl β-D-glucoside (β-GlcC12) has been examined through differential scanning calorimetry, optical polarising microscopy, and small- and wide-angle X-ray scattering, respectively. Derivative bearing two carboxymethyl and two hydroxyl groups at the sugar ring i.e. n-dodecyl 2,3-bis-O-[carboxymethyl]-β-D-glucopyranoside (2,3-diCOOH-β-GlcC12) exhibited lamellar phase at the room temperature in anhydrous condition by significantly reduced the melting and clearing temperatures compared to parent compound β-GlcC12. Under excess water condition, 2,3-diCOOH-β-GlcC12 was completely soluble to form a normal micellar solution. We further investigated its foaming performance and emulsifying ability in n-octane, n-dodecane, and n-hexadecane as the oil phase. Both aqueous solution of 2,3-diCOOH-β-GlcC12 emulsified with n-dodecane and n-hexadecane formed oil-in-water emulsion layer of more than 95% of the total system volume with n-dodecane/water system consistently maintains smaller particle sizes and a narrower polydispersity index. This supports the stronger emulsifying ability and better colloidal stability of the 2,3-diCOOH-β-GlcC12 in the n-dodecane/water system than in the n-hexadecane/water system.

Liquid Crystal Behavior of Uniform Short Rods Made from Computationally Designed Parallel Coiled Coil Building Blocks.

Parallel, homotetrameric coiled coils were computationally designed using 29 amino acid peptides. These parallel coiled coils, called "bundlemers", have C2 symmetry, with all N-termini displayed from one end of the nanoparticle and all C-termini from the opposite end. This anisotropic display of the peptide termini allowed for the functionalization of two sets of nanoparticles with either maleimide or thiol functionality at the N-terminal region of the constituent peptides. The thiol-Michael conjugation reaction between the N-terminal end of complementary bundlemer nanoparticles formed monodisperse, rigid bundlemer dimer, called "dibundlemer", rods. The constituent, individual bundlemer nanoparticles were characterized with small-angle X-ray scattering (SAXS), Förster resonance energy transfer (FRET), and circular dichroism (CD) spectroscopy to confirm the parallel assembly of the coiled coils, consistent with the computational design. The dibundlemer rods were characterized with SAXS to reveal the uniform dibundlemer nature of the rods. Optical birefringence is observed in concentrated samples of the rods, with polarized optical microscopy (POM) revealing a nematic liquid crystalline behavior.

The Influence of Carbon Fiber on Crystallization and Morphology of Poly(Ether Ketone Ketone) Composites Towards High Rate Thermoplastic Manufacturing Processes

Recent interest in thermoplastic composites for aerospace applications is driven by the need for high-volume manufacturing of materials that display excellent mechanical, thermal, and solvent resistive properties. Specifically, a large component of the composite properties of thermoplastic composites are highly dependent on the crystalline fraction of the semi-crystalline polymer matrix of the reinforced system. This work investigates the melting behavior of poly (ether ketone ketone) (PEKK) and the influence of temperature and fiber on crystallization kinetics and morphology of (PEKK) composites utilizing, differential scanning calorimetry, Velaris-Sefaris modelling, and polarized optical microscopy (POM). Annealing temperatures of 380°C and above were required to fully erase crystalline ordering in the PEKK matrix. Furthermore, carbon fibers accelerated crystallization kinetics in PEKK at a range of temperatures observed via DSC, and Velaris-Sefaris modelling implied a shift from bulk to surface induced nucleation by the addition carbon fiber to the PEKK matrix, reducing activation energy of crystallization events. This nucleation behavior was visually confirmed through POM with observation of transcrystalline domains nucleated from the surface of a single carbon fiber.

The Influence of Carbon Fiber on Crystallization and Morphology of Poly(Ether Ketone Ketone) Composites Towards High Rate Thermoplastic Manufacturing Processes

Recent interest in thermoplastic composites for aerospace applications is driven by the need for high-volume manufacturing of materials that display excellent mechanical, thermal, and solvent resistive properties. Specifically, a large component of the composite properties of thermoplastic composites are highly dependent on the crystalline fraction of the semi-crystalline polymer matrix of the reinforced system. This work investigates the melting behavior of poly (ether ketone ketone) (PEKK) and the influence of temperature and fiber on crystallization kinetics and morphology of (PEKK) composites utilizing, differential scanning calorimetry, Velaris-Sefaris modelling, and polarized optical microscopy (POM). Annealing temperatures of 380°C and above were required to fully erase crystalline ordering in the PEKK matrix. Furthermore, carbon fibers accelerated crystallization kinetics in PEKK at a range of temperatures observed via DSC, and Velaris-Sefaris modelling implied a shift from bulk to surface induced nucleation by the addition carbon fiber to the PEKK matrix, reducing activation energy of crystallization events. This nucleation behavior was visually confirmed through POM with observation of transcrystalline domains nucleated from the surface of a single carbon fiber.

Designing dicationic organic salts and ionic liquids exhibiting high fluorescence in the solid state

Dicationic ionic liquids (DILs) are emerging as a powerful, next-generation approach to designing applied ILs because of their superior physicochemical properties as well as their diverse complexity and tunability for task specific applications. DILs are scarce in the literature compared to monocationic ILs (MILs), and one of their main issues is their expected tendency to possess higher melting temperatures. A series of 1,4-bis[2-(4-pyridyl)ethenyl]benzene and 1,4-bis[2-(2-pyridyl)ethenyl]benzene quaternary salts (Q-BPEBs) with different counterions (bromide, tosylate, and triflimide) and carbon chain lengths (C6, C9, and C12) have been synthesized for their potential as DILs with thermal properties and strong photoluminescent properties in the solid state. All Q-BPEB salts demonstrated robust thermal stabilities as determined by thermogravimetric analysis (TGA). The differential scanning calorimetry (DSC) thermograms for Q-BPEB tosylates and triflimides displayed crystalline polymorphisms before melting transitions as verified by polarizing optical microscopy (POM). The Q-BPEB bromide and tosylate salts all showed high melting points of above >170 °C because of their dicationic rigid structures and strong ionic interactions of their anions. Once the Q-BPEB tosylates were exchanged with triflimide ions, para- isomers 1aTf2N-1cTf2N still possessed very high melting points (>225 °C), however, the ortho- isomers 2aTf2N, 2bTf2N, and 2cTf2N exhibited melting points lower than 100 °C, classifying them as DILs. Their photoluminescent properties were also studied in methanol with the emission values of λem = 476-482 nm for the para- isomers and those of λem = 448-453 nm for the ortho- isomers. In the solid state, the Q-BPEB salts exhibited strong fluorescence with quantum yields of up to 50%. The relatively simple synthesis of these fluorescent dicationic organic salts and ILs are pertinent towards the scarcity of these materials in the literature and provide a deeper insight on the design of fluorescent ILs containing more than one charge center.

Esh-Shaheinab: The archetype of the Sudanese Neolithic, its premises and sequels.

Esh-Shaheinab is a landmark in the African Neolithic. This site gave the name Shaheinab Neolithic to the Neolithic period in central Sudan, becoming its archetype. Excavated in the late 1940s by A.J. Arkell, it bears witness to the processes of domestic animal introduction from the Middle East into North and East Africa. Its excavation also uncovered the remains of an earlier Mesolithic or Early Khartoum (ca. ninth-sixth millennia BC) and a Late Neolithic occupation (ca. fourth millennium BC), providing essential insights into the Neolithic's premises and sequels. Although the influence of Esh-Shaheinab has been recognized for more than seventy years, our knowledge of its material culture has remained as it was then. In 2001, one of the present authors (EAAG) had permission to restudy the ceramic collection at the National Museum in Khartoum and subsequently export samples for laboratory analyses. Here, for the first time, we provide a multi-scale analysis of the Esh-Shaheinab ceramic material from the Early Khartoum to the Late Neolithic periods by integrating the chaîne opératoire approach into the local landscape. By combining the results of macroscopic and microscopic analyses, we performed petrographic investigations on the composition and manufacturing technology of the ceramic pastes using polarized optical microscopy (POM) and scanning electron microscopy (SEM-EDS). Organic residue analysis (ORA) was also carried out, to provide information on diet, vessel use, and subsistence practices. The results of our combined analyses showed that the inhabitants of Esh-Shaheinab developed an adaptation specific to the ecological niche they inhabited. They lived in the western valley of the Nile, which was narrower and offered different environmental conditions than the eastern bank. This resulted in partial continuity in manufacturing traditions and ceramic recipes, including more mixed wadi materials and a strong emphasis on wild meat consumption as the narrower alluvial plain restricted animal herding.

Carbon dots induced homeotropic alignment in a negative dielectric nematic liquid crystal material

Abstract Recently, doping guest materials such as quantum dots (QDs) into liquid crystals (LCs) has been of great interest since their addition substantially enhances the properties of LC and opens new avenues for scientific advancement. Here, we report the induction of homeotropic alignment in cells without alignment layers of the negative dielectric nematic liquid crystal, N-(4-Methoxybenzylidene)-4-butylaniline (MBBA) by doping with carbon dots (CDs ~2.8±0.72 nm). The CDs-MBBA composites (CDs concentration: 0.002, 0.01, 0.03, 0.1 and 0.3 wt%) were investigated using optical polarising microscopy, electro-optical and dielectric techniques. Polarizing optical micrographs and voltage dependent optical transmission revealed the induced homeotropic alignment for all the composites under investigation. Interestingly, the least concentrated sample, 0.002 wt% exhibited partial homeotropic alignment. However, due to light leakage, the optical transmission value below threshold voltage was relatively higher than the rest of the composites. MBBA is a negative dielectric material, hence the application of a voltage across the cell was able to switch the alignment from a dark to a bright state for all composites. However, above a certain voltage (> threshold voltage), the bright state produced some instabilities. The value of dielectric permittivity was observed to decrease with increasing concentration, confirming the effect of CDs in producing homeotropic alignment in MBBA. Measurements as a function of temperature were conducted to examine the thermal stability of the induced alignment. The alignment was found to be stable throughout the nematic phase of MBBA. The induction of such alignment without conventional alignment (i.e., rubbing of polyimides) technique can be helpful in addressing the evolving display demands by making liquid crystal displays (LCDs) and other display devices cost effective.

Symmetrical homologous series of thiadiazole derivatives containing azomethine linkages: synthesis, characterization and mesomorphic properties

Symmetrical homologous series of liquid crystalline thiadiazole Schiff’s-base derivatives derived from 1,4-bis(4’-formylphenoxy)butane and 5-(4’-alkoxyphenyl)-1,3,4-thiadiazole-2-amine have been synthesized. Spectral analysis including FT-IR and 1H–NMR of every individual compound was carried out and were confirmed using elemental analysis. The microscopic textures of prepared compounds and their transition temperatures were observed using a polarizing optical microscope. The enthalpy change data value was measured using differential scanning calorimetry. Smectic and nematic phases were observed on heating as well as cooling cycles. The thermal stability of the prepared compounds was recorded using a thermogravimetric analysis.

Structural characterisation of a cyanomesogen by X-ray diffraction and computational methods

The work reports the structural analysis of a cyanomesogen namely, p-cyanobenzylidene p-nonyloxyaniline belonging to the Schiff base class of compounds. It exhibits a partial bilayer/inter-digitated smectic Ad phase and a nematic phase. Polarising Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC) studies have been carried out to study phase sequence and transition temperatures. Micro-structural parameters, like molecular length, spacing, crystallite size, activation energy and micro-strain, have been estimated and pair correlation function (PCF) studies have been done for exhibited mesophases using the X-ray diffraction technique. The study investigated the surface, electronic and optical properties of a compound by computational studies like Density Functional Theory (DFT) and Hirshfeld surface analysis (HSA) methods using the DFT B3LYP/6-311++G(d,p) level basis set. The results reveal that the polarity associated with the polar groups and alkoxy chain lengths, contributes majorly to the mesomorphic characteristics and stability. The Highest Occupied Molecular Orbitals (HOMO) and Lowest Unoccupied Molecular Orbitals (LUMO) analyses of the DFT optimised molecule give electronic and geometrical parameters, atomic charge distribution, polarisability factors which are used to interpret the characteristics of the mesophases and their suitability for electro-optical and non-linear optical applications of the compound in terms of the predicted data.

Structural investigation of the liquid crystalline phases of three homologues from the nOS5 series (n = 9, 10, 11) by X-ray diffraction

ABSTRACT Polarizing optical microscopy and differential scanning calorimetry are used to determine the phase sequence of three liquid crystalline 4-pentylphenyl-4’-n-alkyloxythiobenzoates with n = 9, 10, 11. The X-ray diffraction method is applied for structural characterisation of the liquid crystalline phases. The smectic layer spacing, tilt angle, average distance between the long axes of molecules and correlation length of the short-range order are determined as a function of temperature. For the crystal-like smectic phases with the hexagonal or herring-bone packing, the unit cell parameters are obtained. The presence of the tilted hexagonal phase for n = 10, 11 and tilted herring-bone phase for n = 9, 10 is indicated, although the direction of the tilt cannot be determined.

On the Use of Reflection Polarized Optical Microscopy for Rapid Comparison of Crystallinity and Phase Segregation of P3HT:PCBM Thin Films.

Rapid, nondestructive characterization techniques for evaluating the degree of crystallinity and phase segregation of organic semiconductor blend thin films are highly desired for in-line, automated optoelectronic device fabrication facilities. Here, it is demonstrated that reflection polarized optical microscopy (POM), a simple technique capable of imaging local anisotropy of materials, is capable of determining the relative degree of crystallinity and phase segregationof thin films of polymer:fullerene blends. While previous works on POM of organic semiconductors have largely employed the transmission geometry, it is demonstrated that reflection POM provides 3× greater contrast. The optimal configuration is described to maximize contrast from POM images of polymer:fullerene films, which requires Köhler illumination and slightly uncrossed polarizers, with an uncrossing angle of ±3°. It is quantitatively demonstrated that contrast in POM images directly correlates with 1) the degree of polymer crystallinity and 2) the degree of phase segregation between polymer and fullerene domains. The origin of the bright and dark domains in POM is identified as arising from symmetry-broken liquid crystalline phases (i.e., dark conglomerates), and it is proven that they have no correlation with surface topography. The use of reflection POM as a rapid diagnostic tool for automated device fabrication facilities is discussed.

Interface Structure and Properties of Polypropylene/Wood Flour Composites Reinforced with Grafted Polypropylene Compatibilizer

AbstractIn this work, a new compatibilizer, ternary monomer grafted polypropylene (hereafter shorted as: G‐PP), was successfully synthesized by solid phase grafting of maleic anhydride, methyl methacrylate, and butyl‐acrylate onto polypropylene. For all developed polypropylene matrix composites, the prepared G‐PP evidently showed enhancing mechanical properties on PP/wood flour (WF) composites, which is obviously superior to those from maleic anhydride grafted polypropylene (W‐PP) and virgin polypropylene (C‐PP). In particular, the wettability of G‐PP was greatly improved, while the melting temperature and β crystal phase amounts of the WF composites with G‐PP are much higher when compared to that of W‐PP and C‐PP. This can be reasonably explained by the formation of continuous micropores and filaments in WF composites containing G‐PP, as verified by hot stage polarizing optical microscope (POM). Meanwhile, the G‐PP effectively reduced movement of molecular chains of WPC and formed a better interface layer with WF, which contributed to penetrate more deeply and form micro‐crosslinking in the WF layer. We believe that the results reported in this work will increase the added value of polypropylene based wood plastics composites (WPC) and allow WPC to participate in lightweight and environmentally friendly products.

Investigation on induced smectic phases in double hydrogen bond liquid crystal complexes derived from aromatic carboxylic acids: Experimental and quantum chemical approaches

In the present study, double Hydrogen bond liquid crystal (HBLC) complexes in different mole ratio (1:1 and 1:2) have been isolated from symmetrical aromatic dicarboxylic acid of non-mesogenic compound 1,3-Phenylenediacetic acid (1,3-PDA) and mesogenic compound 4-n-dodecyloxybenzoic acid (12 OBA). Fourier transform infrared Spectroscopy (FTIR) explores the presence of functional groups in the title complexes. Formation of H-bond between 1,3-PDA and 12 OBA is experimentally confirmed by observing the FTIR peak at 3639 cm−1 (1:1 ratio). Induced mesophases and its textures along with transition temperatures are analyzed using polarizing optical microscope (POM) and differential scanning calorimetry (DSC). The layered structures (smectic phases) and their molecular mechanism has been analyzed using density functional theory (DFT). Further, the establishment of H-bond in the title complex has been investigated with the aid of optimized geomentry, molecular electrostatic potential (MEP) and reduced density gradient (RDG) analysis. The band gap energy of title complex (1:1) is calculated by UV–Vis spectrometer and the same has been justified by HOMO-LUMO studies. Maximum absorption in the UV region and moderate bandgap energy (Eg = 4.22 eV) of the title complex clearly indicates its potential application in NLO, optoelectronics and laser tuning devices. In addition to that, LC parameters and its effect on mesophase are reported using experimental and computational methods.

Facile synthesis of polyester-polyether block copolyols for the sustainable high-performance polyurethane reactive hot melt adhesives

The presence of polyols with both polyester and polyether structures in polyurethane enhances bonding strength and facilitates easy curing. In this study, polyester-polyether block polyols with varying molecular weights (2500–3500) were synthesized through ring-opening polymerization (ROP) using l-lactide (LA) and ε-Caprolactone (CL). The chemical structure of the polyols was characterized using 1H nuclear magnetic resonance (1H NMR), Fourier transform infrared (FTIR), and gel permeation chromatography (GPC). Differential scanning calorimetry (DSC) and polarized optical microscopy (POM) were employed to examine the crystalline morphology of the polyols. Subsequently, the synthesized polyols were combined with 4,4′-diphenylmethane diisocyanate (MDI) and other additives to prepare polyurethane reactive hot melt adhesives. It was observed that, in PCL-based polyurethane reactive hot melt adhesives, increasing the molecular weight of the polyols resulted in improved crystalline morphology, increased lap shear strength and tensile strength, and enhanced thermal stability. On the other hand, PLA-based polyurethane reactive hot melt adhesives exhibited low bonding strength and lacked practical application value due to the poor toughness of PLA. Interestingly, the bonding strength and tensile strength of PLA-based polyurethane reactive hot melt adhesives improved when physically blended with PCL-based polyurethane reactive hot melt adhesives. These polyols offer a novel approach for utilizing LA and CL in adhesive applications.

Azobenzene-Based Main-Chain Liquid Crystalline Polymers Bearing Periodically Installed Alkyl, PEG, or Fluoroalkyl Segments and Photoisomerization Studies

ABSTRACT In the present study, liquid crystalline polymers (LCPs) were prepared by melt-transesterification of azobenzene-bearing diols and diethyl malonate (DEM) derivatized monomers. The effect of the nature and length of the installed pendant segments on the mesomorphic properties of the prepared LCPs was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and polarized optical microscopy (POM) studies. The formation of layered morphology was inferred from X-ray scattering analysis arising from the colocation of rigid azobenzene moieties in one layer and the flexible pendant segments in alternate layers due to the zigzag folding of the polymer backbone. The linear variation of the interlamellar spacing with the number of carbon atoms in the pendant alkyl segments corroborates the proposed zigzag backbone folding in the polymers. The rate constant values obtained from the photoisomerization studies of the isomeric polymers and model compound, which are in the same ballpark, reveal that the photoisomerization process of such polymers is not affected by the position or size of the azobenzene unit.

Aryl/fluoroaryl-substituted triphenylene discotic liquid crystals: Synthesis, mesomorphism, and photophysical properties

Fluorine aromatic hydrocarbons are ubiquitous optoelectronic functional materials and their efficient synthetic methods are still the subject of rapid, ongoing development. Here, several aryl- and fluoroaryl-substituted triphenylenes (Ar-TP/FAr-TP) were synthesized via either Suzuki-Miyaura cross-coupling or aromatic nucleophilic substitution (SNAr) of perfluoroarenes, respectively, starting indifferently from 2-bromo-3,6,7,10,11-pentakis(hexyloxy)triphenylene (Br-TP). These compounds possess very high thermal stability with initial decomposition temperature over 300 °C, as measured by thermal gravimetric analysis (TGA). Differential scanning calorimetry (DSC), polarized optical microscopy (POM), and small/wide angle X-ray scattering (S/WAXS) revealed that they all self-organize into hexagonal columnar (Colhex) mesophase over broad temperature ranges, occurring at room temperature for some of them, with higher stability than the archetypical 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene parent compound. Under the combined influence of the conjugated π-system and core-side arm distortion, the clearing temperatures and mesophase ranges increase from phenyl- (P-TP), naphthyl- (N-TP), to biphenyl-triphenylene (BP-TP). The number of fluorine atoms on the side fluoroaryl group also impacts the columnar mesomorphism: the more fluorine atoms, the higher the clearing temperatures and the wider the Colhex phase ranges, concomitantly with increasing arene-fluorarene intermolecular interactions. In addition, these compounds show blue-violet photoluminescence in solution: the largest fluorescent absolute quantum yield is measured for BP-TP (45 %), while the smallest is for 7FN-TP (10 %). Perfluoroaryls tend to lower the LUMO, which results in fluorescence red-shift. The arene-perfluoroarene polar-π interactions improve the physical properties of these FAr-TP discotic liquid crystals.