Mesomorphic Transition Research Articles

Metal-induced self-assemble transition: Triple responses on photophysical, liquid crystalline and gel properties of Hg(II)-coordination tetraphenylene dimer

The metal-coordination self-assemble system played an important role on constructing novel supramolecular smart-response materials. Although the single and double-response materials based on metal-induced self-assembly had been studied extensively, the triple responses for this kind of supramolecular material was seldom reported. In this work, the first metal-induced self-assemble system with triple responses on photophysical, liquid crystalline and gel properties was presented based on Hg2+-coordination tetraphenylene dimer (Bis-TPE). Bis-TPE was prepared by bridging two alkyl-substituted tetraphenylene with amido chain. It possessed the fluorescence response from the strong emission for Bis-TPE to the faint emission for Bis-TPE-Hg2+ complex. The self-assemble mechanism of Bis-TPE-Hg2+ was clarified as that Hg2+ was located in the cavity of the amido-ether chain of Bis-TPE. The Hg2+-induced mesomorphic transition was also observed from smectic mesophase for Bis-TPE to hexagonal columnar mesophase for Bis-TPE-Hg2+ complex. The unique Hg2+-induced gel transition was further distinguished from low-order spherical morphology for Bis-TPE to high-order fibrous morphology for Bis-TPE-Hg2+ complex. The maximum fluorescence wavelength exhibited the giant shift from 494 nm for Bis-TPE in gel to 606 nm for Bis-TPE-Hg2+ in gel. This kind of triple responses on photophysical, liquid crystalline and gel properties for metal-coordination self-assemble system was described for the first time, affording a new idea on design of new multiple-response materials with unique properties.

Impact of core polarity on smectic B-induced hydrogen bond liquid crystals.

The novel series of hydrogen bond liquid crystals were synthesized from the 2-methylglutaric acid (MGA) and 4-alkyloxybenzoic acid (nOBA) compounds. The induced smectic B phase with different texture (spine texture, needle texture, mosaic texture, natural mosaic texture and marble texture) were identified by polarizing optical microscope. Due to breaking of in-plane rotational symmetry within molecular layers, smectic B phase is tempted by suppressing other usual mesophases. The mesomorphic transition temperature, enthalpy and entropy values were calculated by differential scanning calorimeter which strongly proves the existence of mesomorphism. H-bond interaction and functional groups were confirmed by the observed peak between 2910 and 2954cm-1 in the FTIR spectra. Thermal stability and extended mesophase width (for MGA + 12OBA = 31.1) of Sm B mesophase were reported and it clearly reveals the existence of mono-phase variance in the MGA + nOBA HBLC complex. Due to the steric effect, and the increased molecular core polarity, the highly stabilized Sm B phase with different textures were observed while varying alkyloxy carbon number n = 7 to 12. Further, the origination of Sm B phase and its detailed characteristics were reported.

Preparation and DFT Study for New Three-Ring Supramolecular H-Bonded Induced Liquid Crystal Complexes.

Supramolecular three-ring Schiff base novel liquid crystal complexes have been prepared and investigated. Schiff bases of para-substituted aniline derivatives and para-pyridine carbaldehyde have been prepared and then mixed in equimolar quantities with para-alkoxy benzoic acids. On one side, the alkoxy chain length varies from 8 to 16 carbon atoms. On the other side, terminal small compact groups substituting aniline with various polarities are used. Hydrogen-bonding interaction was elucidated by FTIR spectroscopy. The mesomorphic thermal and optical characteristics of the samples were obtained by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). All samples exhibit enantiotropic mesophases. Experimental results obtained for the induced mesophases were correlated with density functional theory (DFT) theoretical calculations. The results revealed that both the polar compact groups’ polarity and the alkoxy chain lengths contribute strongly to mesomorphic characteristics and thermal stabilities of the mesophases. Surprisingly, the observed values of enthalpy changes associated with the crystalline mesomorphic transitions lie in the range of 2.2–12.5 kJ/mol. However, the enthalpy changes corresponding to the mesomorphic–isotropic transitions vary from 0.9 to 13.9 kJ/mol, depending on the polarity of para-attached groups to the aniline moiety.

Synthesis, Thermal and Optical Characterizations of New Lateral Organic Systems

New laterally OCH3-substituted optical organic Schiff base/ester series, namely 4-(4-(hexyloxyphenyl)iminomethyl)-3-methoxyphenyl 4-alkoxybenzoates, were prepared and characterized with different thermal, mesomorphic, and photoactive techniques. The prepared group constitutes five homologues that differ from each other in the number of carbons in the terminal alkoxy chain (n), which varies between n = 6, 8, 10, 12, and 16 carbons. The laterally protruded methoxy group is attached to the central benzene ring that makes an angle of 120° with the molecular long axis. Molecular structures of all newly prepared homologues were fully elucidated via FT-IR, 1H and 13C NMR spectroscopy. Mesomorphic transitions were determined via differential scanning calorimetry (DSC) and the phases identified by polarized optical microscopy (POM). Independent of the length of the terminal alkoxy chain attached to phenyl ester ring, only a monomorphic nematic (N) phase was observed for all the synthesized compounds. A comparative study was made between the present lateral methoxy-substituted homologues and their corresponding laterally-neat analogues. The results revealed that, depending on the length of the alkoxy chain and the presence or absence of the lateral methoxy group, different mesophases with different thermal stability and temperature ranges were observed. Finally, UV-vis spectra showed that the present nematogenic series possess photoactive properties that are of importance for many applications.

Synthesis of well-defined PS-based Azo-liquid crystals with control of phase transitions and photo-behaviors for liquid crystal networks with photomechanical deformation

Benefiting from their good mechanical processability and high tunability, Si–H and vinyl-functionalized polystyrenes (PSs), i.e., poly(4-vinylphenyl)-dimethyl-silane (PVPDMS/PS-SiH) and poly(4-vinylphenyl)-1-butene (PVSt/PS-Vinyl), produced by living anionic polymerization (LAP) were treated with four vinyl- and Si–H-terminated azobenzene mesogens (Azo-Vinyl, Azo-SiH, AzoB-Vinyl, and AzoB-SiH, referred to as M1, M2, M3, and M4, respectively) via hydrosilylation for the fabrication of PS-based side-chain liquid crystal polymers (SCLCPs) containing different spacers, i.e., PS-SiC/SiOSi-Azo/AzoB. As a strategy for basic molecular control in PS-based SCLCPs, the cooperative influences of the spacers and the degree of polymerization (DP) on the mesomorphic transitions were explored. The phase transitions could be tuned slightly based on the DP but dramatically by adjusting the spacers. For example, the temperature ranges of mesomorphic formation (ΔT) were in the order PS-[SiOSi]-Azo (ΔT = 34–49 °C) < PS-[SiC]-Azo (ΔT = 49–60 °C) < PS-[SiOSi]-AzoB (ΔT = 154–164 °C) < PS-[SiC]-AzoB (ΔT = 162–193 °C). Taking advantage of the quantitative structure-property correlation of PS-based SCLCPs, PS-based liquid crystal networks (LCNs) were further designed for the photomechanical deformation. PS-based LCNs show good mechanical properties with a storage modulus of 120 MPa, controlled phase transitions, and glass transition temperatures (Tg) around room temperature. Crosslinked PS-based LCNs of rigid PS homopolymers with selective crosslinking can exhibit directional bending upon irradiation with UV light at 50 °C after stretching, and this was shown to be a photomechanical deformation, which expands the applicability of these materials.

Design of new Schiff base polymers containing thiadiazole rings and study of their liquid crystalline behavior

New liquid crystalline thiadiazole dibenzaldehyde monomers labeled as THDB1–THDB3 were successfully synthesized by alkylation of thiadiazole’s potassium salt with 4-(bromomethyl) benzaldehyde. A number of polymers consisting of thiadiazole and azomethine coded PTDAZ1–PTDAZ5 were synthesized via condensation reaction of the presynthesized monomers THDB1–THDB3 with aromatic diamines. The chemical structures of the prepared materials were confirmed using Fourier-transform infrared spectroscopy and proton nuclear magnetic resonance techniques. The liquid crystalline behavior of the studied monomers and polymers was examined by differential scanning calorimetry and hot stage polarized optical microscopy (POM) techniques. All these compounds were found to demonstrate mesomorphic transitions belonging to smectic and nematic liquid crystals. The studied monomers exhibited fan-like texture of smectic mesophase under POM cooling investigation. Fan-like texture of smectic and nematic mesophases was observed under POM for PTDAZ1 and PTDAZ2, while PTDAZ3 and PTDAZ4 revealed clay and schlieren textures of the smectic and nematic mesophases, respectively, and nematic texture has been found for the polymer PTDAZ5.

Imine based chiral liquid crystals: Effect of varying the terminal substituent and orientation of ester linking unit

New liquid crystalline compounds composed of three-benzene-ring molecular core comprising both the imine and ester linkage units, (S)-3,7-dimethyloctyloxy chiral unit at one of terminals and n-dodecyl/dodecyloxy group as other side chain have been synthesized and characterized to study the effect of varying terminal chains, the orientation of polar ester linking group and the presence of molecular chirality on mesophase properties of imine based calamitic compounds. The liquid crystalline properties of the compounds were investigated by differential scanning calorimetry, optical polarizing microscopy and electro-optic methods. As a result of the strong effect of chirality of (S)-3,7-dimethyloctyloxy chain, all Schiff bases (imines) exhibit enantiotropic chiral smectic C (SmC*), chiral nematic (N*) as well as SmX phase just below the SmC* phase and blue phase (BP) appearing monotropically. The SmC* phase shows ferroelectric switching behaviour with PS values in the range 70–130 nC cm−2. The alteration of the n-alkyl tail with an alkoxy chain gives rise to a significant increase up to 30 °C in mesomorphic transitions. The change in the orientation of polar ester linking group has no effect on the type and sequence of observed LC phase. As compared with (S)-4-(3,7-dimethyloctyloxy)phenol based imine derivatives (7a, 7b), the chiral nematic (N*) mesophase temperature range of (S)-4-(3,7-dimethyloctyloxy)benzoate based compounds (8a, 8b) significantly broadened. Additionally, a decrease on the phase transition temperatures occurred with regardless of introducing n-dodecyl/dodecyloxy group as side chains.

Combining Fast Imaging and Variable Temperature Studies in Atomic Force Microscopy

AbstractFast imaging in Atomic Force Microscopy enhances the capability of studying phase transitions and surface properties of materials at variable temperatures. This is demonstrated by measurements of several polymers [poly(diethylsiloxane), low-density polyethylene and ethylene-octene copolymer] and bitumen at low (down to -20°C) and high (up to +150°C) temperatures. Monitoring of structural transitions was performed at small and large (up to 40 μm) areas with 1-5°C/min cooling/heating rates. Novel data about dynamics and structural transitions of mesomorphic transitions and crystallization were obtained.

Self-Assembling Polyhedral Silsesquioxanes - Structure and Properties

Polyhedral oligomeric silsesquioxanes (POSS) are perfect three-dimensional elemental building blocks that can be used for construction of diverse inorganic/organic hybrid materials. Well-defined inorganic frameworks, tailor-made physicochemical properties and symmetric distribution of organic groups make the hybrid organo-silica compounds ideal nano-fillers for polymer composites. Understanding the phenomena directing the molecular packing morphology of neat polyhedral silsesquioxanes is very important for an effective control of hierarchal structures and physicochemical properties of POSS-based hybrid materials, especially for the adjustment of polymer-nanoparticle interfacial interactions. Keywords: Polyhedral silsesquioxanes, self-assembly, crystal structure, solid-state interactions, plastic crystals, mesomorphic transitions.

Dependence of mesomorphism on flexibility of lateral and terminal groups of chalconyl esters

ABSTRACTA novel liquid crystalline homologous series of chalconyl vinyl esters with a lateral bromo substituent RO─C6H4─CH═CH─COO─C6H3─(Br)─CO-CH═CH─C6H4─C12H25(n) has been synthesized and studied with a view to understanding and establishing the relation between the molecular structure and liquid crystal (LC) behavior in terms of molecular flexibility of the lateral and terminal groups. The novel homologous series consists of thirteen (C1–C18) homologues, whose enantiotropic nematic and smectic mesomorphism commences from the C4, (C4–C18) and the C5, (C5–C18) homologue, respectively. The rest of the homologues (C1, C2, C3) are nonliquid crystals (NLC). Mesomorphism and transition temperatures were examined using an optical polarising microscope (POM) equipped with a heating stage. Textures of nematic phase are threaded or Schlieren and those of the Smectic-A or Smectic-C are focal conic. Analytical, thermal and spectral data supported molecular structures of the novel homologues. Thermal for smectic and nematic are 87.5°C and 101.1°C, respectively, whose, total mesophase length ranges from 17°C to 30°C at C14 and C7 homologue, respectively. The mesomorphic transition temperature ranges are between 70°C and 113°C.

Dependence of mesomorphism on molecular rigidity of chalconyl liquid crystals with two phenyl rings

ABSTRACTA novel homologues series: RO-C6H4⋅CH=CH⋅CO⋅C6H4⋅OC16H33(n) (meta) of chalcones with two phenyl rings is synthesized and studied with a view to understanding and establishing the relationship between molecular structure and mesogenic behavior. The series consists of 13 homologues (C1 to C18). Mesomorphism commences from the second member of the series. C2 to C6 homologues are enantiotropicnematic and the rest of the homologues (C7 to C8) are monotropicnematic plus monotropicsmectic. C1 homologue is nonmesogenic. Transition temperatures were determined by an optical polarizing microscope (POM) equipped with a heating stage. Textures of nematic and smectic phases are threaded, Schlieren or smectic-A, respectively, as judged directly from the heating top of microscopic observations. Thermal, analytical and spectral data confirms the molecular structures. Thermal stabilities for smectic and nematic are few (1 or 2°C) degrees and 72.6°C, respectively. The mesophase lengths are relatively shorter with low mesomorphic transition temperatures (44.0°C to 85.0°C).

Effect of CO2on Mesomorphic Structure of Cold-Drawn Poly(ethylene therephthalate) Fibers by Dynamic Mechanical Analysis

Dynamic mechanical analysis (DMA) provides interesting information about the dynamic viscoelastic properties and molecular scale motions. This article presents the results of an investigation into the effects of draw ratio and supercritical CO2 (scCO2) exposure on structural changes and mesomorphic transitions in cold-drawn poly(ethylene terephthalate) (PET) fibers by focus on DMA. Samples of partially oriented yarn PET fibers were uniaxially drawn below the glass transition temperature (Tg) using various draw ratios to obtain filaments with different structures for subsequent exposure to scCO2 in the presence and absence of tension at a temperature of 80°C and under a pressure of 220 bar. Microstructural investigation of the samples was carried out by combining the results obtained from DMA, differential scanning calorimetry, as well as density and birefringence measurements, all of which exhibited a good correlation. The rigid amorphous region in the PET samples was considered for interpreting structure–property relations. Results showed that increasing draw ratio leads to increased values of rigid amorphous regions and distribution of molecular orientation in the amorphous regions followed by an increase in the relaxation time distribution. Depending on sample conditions, exposure to scCO2 was found to have a strong effect on the transformation of the transient structure into crystalline or amorphous regions. PET fibers exposed to scCO2 under tension yielded higher values of crystallinity and orientation compared to those under no tension. In the case of PET fibers exposed to scCO2 under no tension, CO2 molecules released from the structure possibly caused a metastable structure to form in the amorphous phase. Therefore, rearrangement of molecular chains during the dynamic mechanical test in the temperature sweep mode is expected to increase the storage modulus of the sample before its glass transition temperature is reached. As a result, an increment is observed in the storage modulus of samples exposed to scCO2 without tension that can be related to the release of stress stored during the depressurization process. scCO2 exposure can be, therefore, claimed to be an environmentally friendly method for inducing structural changes and enhancing crystallinity in PET samples.

Rosin based epoxy coating: Synthesis, identification and characterization

Ketone type derivative of rosin was prepared by dehydrocarboxylation of isomerized abietic acid. Coupling of dipimaryl ketone with maleic anhydride was performed by acetic acid catalyzed Diels–Alder reaction. Afterwards, the dipimaryl ketone was epoxidized to get the corresponding tetra glycidyl ester. The chemical structure of the synthesized products was confirmed by UV, FTIR and 1H NMR spectroscopic analyses. Cured resins using a rosin-based crosslinker and p-phenylene diamine (a commercial crosslinker) were evaluated using dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA) and some preliminary universal coating tests. Results showed that the fully rosin-based epoxy system gave better performance than commercially bisphenol-A based one. This finding was attributed to a liquid crystal behavior of the rosin-based crosslinker. Mesomorphic transition temperature and liquid crystalline texture of the rosin-based crosslinker were investigated by polarized optical microscopy (POM) and differential scanning calorimetry (DSC).

Solid state 13C-NMR, infrared, X-ray powder diffraction and differential thermal studies of the homologous series of some mono-valent metal (Li, Na, K, Ag) n-alkanoates: A comparative study

A comparative study of the molecular packing, lattice structures and phase behaviors of the homologous series of some mono-valent metal carboxylates (Li, Na, K and Ag) is carried out via solid state FT-infrared and 13C-NMR spectroscopes, X-rays powder diffraction, density measurements, differential scanning calorimetry, polarizing light microscopy and variable temperature infrared spectroscopy. It is proposed that, for lithium, sodium and potassium carboxylates, metal-carboxyl coordination is via asymmetric chelating bidentate bonding with extensive intermolecular interactions to form tetrahedral metal centers, irrespective of chain length. However, for silver n-alkanoates, carboxyl moieties are bound to silver ions via syn–syn type bridging bidentate coordination to form dimeric units held together by extensive head group inter-molecular interactions. Furthermore, the fully extended hydrocarbon chains which are crystallized in the all-trans conformation are tilted at ca. 30°, 27°, 15° and 31° with respect to a normal to the metal plane, for lithium, sodium, silver and potassium carboxylates, respectively. All compounds are packed as lamellar bilayer structures, however, lithium compounds are crystallized in a triclinic crystal system whilst silver, sodium and potassium n-alkanoates are all monoclinic with possible P1 bravais lattice. Odd–even alternation observed in various physical features is associated with different inter-planar spacing between closely packed layers in the bilayer which are not in the same plane; a phenomenon controlled by lattice packing symmetry requirements.All compounds, except silver carboxylates, show partially reversibly first order pre-melting transitions; the number of which increases with increasing chain length. These transitions are associated, for the most part, with lamellar collapse followed by increased gauche-trans isomerism in the methylene group assembly, irrespective of chain length. It is proposed that the absence of mesomorphic transitions in their phase sequences is due to a lack of sufficient balance between attractive and repulsive electrostatic and van der Waals forces during phase change. The evidence presented in this study shows that phase behaviors of mono-valent metal carboxylates are controlled, mainly, by head group bonding.

Novel Cinnamate Esters—Synthesis and Mesomorphic Properties in Relation to Molecular Structure

A novel homologous series 4-[4′-n-alkoxy cinnamoyloxy] benzyl cinnamates consisting of 11 homologs was synthesized and studied with a view to understanding and establishing the relation between molecular structure and mesomorphic behavior of a substance. Mesomorphic behavior of the series commences from the third homolog and then continues until the last hexadecyloxy homolog. The first and second members of a series are non-mesomorphic. Nematogenic mesophase formation is observed from the propoxy homolog to the hexadecyloxy homolog, but the smectogenic mesophase formation is observed from the hexyloxy homolog to the tetradecyloxy homolog. All mesomorphic transitions are enantiotropic in nature. Transition temperatures were determined by an optical polarizing microscopy equipped with a heating stage. The textures of the nematic mesophase are threaded or Schlieren in type and that of the smectic mesophases are focal conic fan shaped of the smectic A or smectic C type. A phase diagram of the series shows a normal behavior of the transition curves with a minor abnormality of the last three homologs for the nematic-isotropic transition curve. The average thermal stability for smectic and nematic mesophases is 176.0°C and 219.3°C, respectively. Analytical and spectral data agree with the molecular structures of homologs. Mesomorphic phase length varies between 22°C and 76°C. Smectic phase length varies from 6°C to 33°C and nematic phase length varies from 22°C to 54°C. Thus, the novel present series is predominantly nematogenic and partly smectogenic. Mesomorphic properties of the novel series are compared with structurally similar other known homologous series.

Effect of drawing temperature on the structure and free volume of semicrystalline polyester yarns

Supercritical carbon dioxide (scCO2), an environmentally friendly solvent, can change the fine structure of fibers depending on treatment temperature and pressure. This article presents the results of an investigation into the effects of drawing temperature, scCO2 exposure and tension on structural changes, and mesomorphic transitions in partially oriented yarn (POY) and oriented poly(ethylene terephthalate) (PET) yarns. Samples of POY and PET yarns were uniaxially drawn at different temperatures to obtain filaments with different structures for subsequent exposure to scCO2 in the presence and absence of tension at a temperature of 80oC and under a pressure of 220 bar. Structural investigation of the samples was performed by combining the results obtained from differential scanning calorimetry, Fourier transform infrared, mechanical deformation, and density and birefringence measurements, all of which exhibited a good correlation. The fractional free volume in the PET samples was considered for interpreting structure–property relations. Results showed that the development of a transient structure strongly depends on process temperature and that for an identical draw ratio, PET yarns drawn at 23oC are significantly more oriented than those drawn at 68oC. Two crystallization mechanisms, that is strain-induced and thermal-induced crystallization, are involved that are frequently used to explain the structural changes during yarn deformation at different temperatures. The degree of crystallinity and orientation factors for PET yarns does not proportionally increase with increasing temperature of the drawing process; indeed, the highest value is obtained at a temperature of 100oC, whereas the lowest is observed at a temperature of 68oC, which is near the glass transition temperature. Tension under scCO2 exposure also produces significant effects in term of causing structural changes. POLYM. ENG. SCI., 55:2030–2041, 2015. © 2014 Society of Plastics Engineers

Synthesis of new wide nematic diaryl-diacetylenes containing thiophene-based heteromonocyclic and heterobicyclic structures, and their birefringence properties

New monocyclic and bicyclic thiophene-based diaryl-diacetylene liquid crystal (LC) materials were synthesised and demonstrated to exhibit wide enantiotropic nematic phases. One thiophenyl-benzene derivative, in particular, displayed a stable nematic phase across a temperature range of over 100°C. The birefringence properties of the compounds were measured using a multiple-beam interference (MBI) method, with the highest value found to be for the same thiophenyl-benzene molecule (Δn = 0.57, 550 nm) at 10°C above the crystal-to-nematic transition temperature. Furthermore, the effects of the cyclic structure on the mesomorphic behaviours, transition temperatures, and birefringence properties were investigated in detail from both experimental and theoretical viewpoints.

Study of microstructure of oriented PET fibres exposed to supercritical carbon dioxide

Samples of partially oriented yarn (POY) PET fibers were uniaxially drawn at temperatures below, near, and above the glass transition temperature at a constant draw ratio before exposure to supercritical carbon dioxide (scCO2) in the presence of tension at a temperature of 80 °C and a pressure of 220 bar. The effects of drawing temperature and scCO2 exposure on structural changes and on mesomorphic transitions, in particular, were investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and birefringence and density measurements. A good correlation was obtained among the results obtained from various techniques. Results indicated that the development of a transient mesophase structure depended strongly on process temperature. By drawing PET fibers in the samples at temperatures below the glass transition (cold-drawing), a mesophase structure developed in which the highly extended chains played a key role in structural changes incurred. Meanwhile, exposure to scCO2 led to the plasticization of the samples accompanied by their reduced glass transition and cold crystallization temperatures. This process also gave rise to the appearance of a second melting peak at about 135 °C that is related to the melting of imperfect and thin crystals, thereby inducing structural changes in the treated fibers. In the case of samples subjected to cold drawing and to scCO2 exposure, the transformation of the mesophase structure into the crystalline phase was found to be strongly affected by scCO2 exposure, while this same effect was negligible in the case of hot drawn samples.

Fluorescence-enhanced organogelators with mesomorphic and piezofluorochromic properties based on tetraphenylethylene and gallic acid derivatives

Two novel aggregation-induced emission compounds derived from tetraphenylethylene and gallic acid were synthesized and characterized. Both of the emission compounds possessed mesomorphic properties and exhibited mesomorphic phases over a wide temperature range and the thermal-induced mesomorphic transition from metastable to stable phases accompanied by a change of the luminescent color. The mesomorphic transition may be ascribed to liquid crystalline phase transition. The emission compounds possess different gelation behavior in organic solvents. The emission intensities of the emission compounds can be reversibly changed with the gel–solution transition by alternate cooling and heating. Moreover, the color and emission of these organogelators can be repeatedly switched by various external stimuli including pressing, heating and solvent-fuming. These new compounds may offer potential as external stimuli-responsive materials.

Effects of tension on mesomorphic transitions and mechanical properties of oriented poly(ethylene terephthalate) fibers under supercritical CO2 exposure

Samples of partially oriented yarn PET fibers were uniaxially drawn below the glass transition temperature (cold-drawing) before exposure to supercritical carbon dioxide (scCO2) in the absence and presence of tension at a temperature of 80 °C and a pressure of 220 bar. The effects of draw ratio, scCO2 exposure, and tension on structural changes and on mechanical properties in particular were investigated using differential scanning calorimetry, tensile deformation, and birefringence and density measurements. A good correlation was obtained among the results obtained from various techniques. Results indicate that exposure to scCO2 not only induces structural changes but also develops crystallization in the samples. Tension under scCO2 exposure also produces significant effects in terms of causing structural changes and transforming the oriented chains in the mesophase into the crystalline or noncrystalline domains. PET fibers exposed to scCO2 under no tension yields lower values of crystallinity, orientation, tenacity, and Young’s modulus but higher values of breaking elongation compared to samples exposed under tension. This is an indication of higher plasticity of the chains in the amorphous domains in samples exposed under no tension. It is also found that mechanical measurements confirm the structural changes taking place in exposed PET samples.