Segmented liquid crystalline copolyesters and blending with poly(ethylene terephthalate)

Summary A series of segmented copolyesters with semi-regular structure was synthesized. In these copolymers, fully aromatic triad hard segments-HB-T-HB-, acting as mesogenic units, are linked each other by poly(ethylene terephthalate) (PET) segments with different average chain lengths as flexible spacers. The liquid crystallinity of the copolymers, i.e. the meso-phase forming ability, was studied against length of the spacer. In subsequent blending of these copolymers with PET matrix, results were compared with those from a commercial TLCP of PHB/PET random copolyester. Better mechanical properties were gained as expected since the compatibility of the segmented liquid crystalline copolyesters with the matrix is believed to be improved.     

Modification reactions on thermotropic liquid-crystalline poly(ethylene terephthalate-co-oxybenzoate)s with bi- and trivalent 2-oxazoline couplers

Carboxy-terminated liquid-crystalline poly(ethylene terephthalate-co-oxybenzoate)s with 60 and 75 mol-% oxybenzoate units were modified with phenyl-1,4-bis(2-oxazoline) and phenyl-1,3,5-tris(2-oxazoline) in plasticorder and extruder. The modification gave rise to significant increase in molecular weights and also slight crosslinking and branching. The course of the reaction was followed by means of torque-vs.-time curves measured during the plasticorder modification. In spite of strong increase in melt viscosities the processability of the modified copolyesters was not worsened. Samples were injection-moulded and tested. Modifications improved the mechanical properties, distinctly. Especially, samples which were slightly crosslinked with phenyl-1,3,5-tris(2-oxazoline) demonstrated outstanding tensile and impact strengths. Significant differences in fracture behaviour could be evidenced by recording impact bending tests.     

Synthesis and characterization of liquid crystalline poly(p-vinylbenzyl ether)s

Summary The synthesis and characterization of poly(p-vinylbenzyl ether)s containing 4-oxybiphenyl or 4-oxy-4-methoxybiphenyl mesogens or a 4-oxy-4-methoxybiphenyl mesogen which is connected to the benzyl ether group through one, two or three oligo(oxyethylene) units, are presented. The first two polymers exhibit a nematic mesophase while the last three a highly ordered smectic mesophase.     

Fast crystallization of liquid crystalline copolyesters based on poly(ethylene terephthalate)

Crystallization of a series of liquid crystalline copolyesters prepared from p‐hydroxybenzoic acid (HBA), hydroquinone (HQ), terephthalic acid (TA), and poly(ethylene terephthalate) (PET) was investigated by using differential scanning calorimetry (DSC). It was found that these copolyesters are more crystalline than copolyesters prepared from PET and HBA. Insertion of HQ–TA disrupts longer rigid‐rod sequences formed by HBA and thus enhances molecular motion and increases the crystallization rate. The effects of additives on the crystallization of the copolyesters were also studied. Sodium benzoate (SB) and sodium acetate (SA) increase the crystallization rate of the copolyesters at low temperature, but not at high temperature. It is most likely that liquid crystalline copolyesters do not need nucleating agents, and small aggregates of local‐oriented rodlike segments in nematic phase could act as primary nuclei. Chain scission of the copolyesters caused by the reaction with the nucleating agents was proved by the determination of intrinsic viscosity and by the IR spectra. Diphenylketone (DPK) was shown to effectively promote molecular motion of chains, leading to an increase in the crystallization rate at low temperature, but it decreased the crystallization rate at high temperature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 497–503, 2001     

Transition temperature control of liquid crystalline poly(heptene sulphone)s

Liquid crystalline poly(heptene sulphone)s containing cholesteryl undecylenic ester and cholestanyl undecylenic ester were synthesized. The polymers displayed cholesteric mesophases during thermal transition. The clearing temperatures were from 114 to 172 °C depending on the molar ratio of mesogenic esters in the polymers. Received: 27 March 1997/Revised: 24 June 1997/Accepted: 27 June 1997     

Triplet energy transport of liquid crystalline poly(dodecene sulphone)s

Summary The geometric arrangement of liquid crystalline poly(dodecene sulphone)s containing cholestanyl pentenoic ester chemically bonded in the side-chains was characterized by monitoring the triplet transport via both steady-state emissions and decays of delayed fluorescence and phosphorescence. The increase in the mesogenic cholestanyl pentenoic side-chains shows an increase in the heterogeneity exponent or degree of disorder, h, which is greater than zero, suggesting that the explored system exhibits the non-classical diffusion-limited or fractal-like transport. The value of h is highly sensitive to the ratio of mesogenic to olefinic side-chains. Received: 20 August 1998/Revised version: 25 January 1999/Accepted: 28 January 1999     

Synthesis and characterization of thermotropic liquid crystalline poly(azomethine ether)s

Two series of monomers, namely 4,4′-diformyl-α,ω-diphenoxydecane and 4,4′-diformyl-3,3-methoxy-α,ω-diphenoxydecane, were prepared from 1,10-dibromodecane with p-hydroxybenzaldehyde and 4-hydroxy-3-methoxybenzaldehyde (vanillin), respectively. The poly(azomethine ether)s were prepared by solution polycondensation using 4,4′-diformyl-α,ω-diphenoxydecane, 4,4′-diformyl-3,3-methoxy-α,ω-diphenoxydecane with various diamines. The monomers and polymers were characterized by intrinsic viscosity, FT-IR, ¹H, and ¹³C NMR spectroscopy. The thermogravimetric analysis reveals that the polymers are stable up to 320-500 °C and decomposed with good char yield. The thermotropic liquid crystalline properties of the polymers were examined by differential scanning calorimetry (DSC) and their textures observed under hot stage optical polarized microscopy (HOPM). All the polymers were exhibited thermotropic liquid crystalline properties except tetramethylene diamines-based polymers.     

Thermal properties of blends of poly(ethylene terephthalate) and a liquid crystalline copolyester

Summary A thermotropic liquid crystal copolyester (CHQ/BP/TA/IA; 40/10/40/10) (LCP), and melt blends of poly (ethylene terephthalate) (PET) with LCP have been studied for thermal transition and crystallization behaviour. The LCP has a mesophase transition (KM) in the temperature range of 295–315°C. The endothermic peak showing mesophase to Isotropic (MI) transition is observed around 420°C. These transitions are supported by hot stage polarizing microscopy. In blends of PET/LCP, the mesomorphic transition is observed at temperature around 314°C, along with the melting transition of PET around 274°C. The dynamic calorimetric measurements reveal that the two polymers are at least partially miscible.     

Morphological studies of blends containing liquid crystalline polymers with poly(ethylene terephthalate)

The investigation involved the structure–property behavior of extruded cast films prepared from blends of thermotropic liquid crystalline copolyesters with poly(ethylene terephthalate) (PET). Data were obtained which showed not only the temperature dependence of the moduli and stress–strain behavior but also the orientation effects that must be prevalent in order to explain the differences between the moduli measured parallel and perpendicular to the extrusion direction. Only at high liquid crystal polymer (LCP) composition is the modulus particularly increased. The modulus enhancement with lower LCP content and utilization of process variables are discussed with respect to the induced morphological textures and nature of the process equipment. Specifically, the process variable extruder gear pump speed did not enhance Young's modulus at the same LCP content as extensively as did the process variable of extruder screw speed.     

Reactive blending of poly(ethylene terephthalate) with a liquid crystalline copolyester and polyhydroxyether

Poly(ethylene terephthalate) modified with a dianhydride (PET–anhydride) was melt‐blended with a liquid crystalline copolyester (Vectra A) in the presence of a small amount of a liquid crystalline polyhydroxyether. The mechanical properties of a blend consisting of PET–anhydride/Vectra A/polyhydroxyether were drastically improved compared to blends without polyhydroxyether or without anhydride. Melt‐spun fibers of PET–anhydride/Vectra A/polyhydroxyether in a 80/20/0.75 weight ratio displayed a much higher tensile modulus (17 GPa) and tensile strength (214 MPa) than did a 80/20 PET–anhydride/Vectra A blend (4 GPa and 60 MPa, respectively). A similar increase in modulus and strength was found for a 90/10/0.75 relative to a 90/10 blend. The tensile moduli of the blends can well be described by the Tsai–Halpin equation. A better fibril formation was observed, which was attributed to an improved viscosity ratio. Reactions between the various functional groups during melt processing were indicated by viscosity measurements. The polyhydroxyether may act as a reactive compatibilizer which improves the interfacial adhesion, chemically and/or physically. WAXD recordings of both blends showed a crystalline and highly oriented Vectra phase. The PET phase was unoriented and amorphous in a PET/Vectra blend and semicrystalline and weakly oriented in a PET/Vectra/polyhydroxyether blend. Postdrawing of the various blend fibers to λ = 4 increased the modulus by about 40% and the tensile strength by more than 100%, mainly through orientation of the PET phase. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1107–1123, 1999     

Crystallization kinetics of poly(ethylene terephthalate) with thermotropic liquid crystalline polymer blends

The isothermal and dynamic crystallization behaviors of polyethylene terephthalate (PET) blended with three types of liquid crystal polymers, i.e., PHB60–PET40, HBA73–HNA27, [(PHB60–PET40)–(HBA73–HNA27) 50 : 50], have been studied using differential scanning calorimetry (DSC). The kinetics were calculated using the slope of the crystallization versus time plot, the time for 50% reduced crystallinity, the time to attain maximum rate of crystallization, and the Avrami equation. All the liquid crystalline polymer reinforcements with 10 wt % added accelerated the rate of crystallization of PET; however, the order of the acceleration effect among the liquid crystalline polymers could not be defined from the isothermal crystallization kinetics. The order of the effect for liquid crystalline polymer on the crystallization of PET is as follows: (PHB60–PET40)–(HBA73–HNA27) (50 : 50); HBA73–HNA27; PHB60–PET40: This order forms the dynamic scan of the DSC measurements. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1383–1392, 1998     

Impact modification of poly(ethylene terephthalate)

The tensile and impact resistance of impact‐modified poly(ethylene terephthalate) (PET) is investigated. The impact modifiers are polyolefin‐based elastomers or elastomer blends containing glycidyl methacrylate moieties to improve the adhesion with the polyester. The tensile properties are measured on injection molded specimens at room temperature while the Izod impact strength is measured from ?40 to 20°C. The blend morphology is observed by scanning electron microscopy and the dispersed phase average diameter is determined by image analysis. The relation between the impact resistance and the phase morphology is discussed, and the critical ligament size for PET is determined. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2919–2932, 2003     

Rheology and thermal properties of liquid crystalline copolyester and poly(ethylene 2,6-naphthalate) blends

Blends of a poly(ethylene 2,6-naphthalate) (PEN) and a liquid crystalline copolyester (LCP), poly(benzoate-naphthoate) were prepared in a twin-screw extruder. Specimens for thermal properties were investigated by means of an instron capillary rheometer (ICR) and scanning electron microscopy (SEM). The blend viscosity showed a minimum at 10 wt% of LCP and increased with increasing LCP content above 10 wt% of LCP. Above 50% of LCP and at higher shear rate, phase inversion occured and the blend morphology was fibrous and similar to pure LCP. The ultimate fibrillar structure of LCP phase appeared to be closely related to the extrusion temperature. By employing a suitable deformation history, the LCP phase may be elongated and oriented such that a microfibrillar morphology can be retained in the solid state. Thermal properties of the LCP/PEN blends were studied using DSC and a Rheovibron viscoelastomer. These blends were shown to be incompatible in the entire range of the LCP content. For the blends, the T_g and Tm were unchanged. The half time of crystallization for the LCP/PEN blends decreased with increasing LCP content. Therefore, the LCP acted as a nucleating agent for the crystallization of PEN. The dimensional and thermal stability of the blends were increased with increasing LCP content. In studies of dynamic mechanical properties, the storage modulus (E′) was improved with increasing LCP content and synergistic effects were observed at 70 wt% of LCP content. The storage modulus for the LCP/PEN 70/30 blend is twice that of PEN matrix and exceeded pure LCP.     

Microstructure of amorphous and crystalline poly(ethylene terephthalate)

The microstructures of amorphous and crystalline poly(ethylene terephthalate) (PET) homopolymers have been determined in terms of their trans and gauche conformational isomer contents by using a combination of infrared and density characterization techniques. The effects of isothermal crystallization (from the glassy state between 105–150°C), as well as the effects of different monomer units in the polymerization process, have been investigated. Results indicate that samples, polymerized from different monomer and catalyst systems, show different microstructures in terms of trans and gauche isomers.These variations result in significant differences in PET optical properties. Further investigations find that these dissimilar behaviors accompany conformational isomer variations in the amorphous phase, suggesting different transformation mechanisms of trans and gauche isomers at early stages of crystallization. These unlike microstructural transformation processes give rise to further changes, which are evident in terms of the intensity of Vv light scattering, haze values, thermal properties, and FTIR spectral results. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1965–1976, 1998     

Structure and properties of blend fibers from poly(ethylene terephthalate) and liquid crystalline polymer

The structure and properties of the as-spun fibers of poly(ethylene terephthalate) (PET) blends with a thermotropic liquid crystalline polymer (LCP), Vectra A900, were studied in detail. The DSC results indicate that the LCP component may act as a nucleating agent promoting the crystallization of the PET matrix from the glassy state but which inhibits its crystallization from the melt due to the existence of an LCP supercooled mesophase. The effect of the drawdown ratio on the orientation of the as-spun blend fibers is highly composition-dependent, which is mainly associated with the formation of LCP fibrils during melt spinning. The modulus of the as-spun blend fibers has a significant increase as the content of LCP reaches 10%, while the tensile strength has a slightly decreasing tendency. The mechanical properties of the as-spun blend fibers could be well improved by heat treatment because of a striking increase in the crystallinity of the PET matrix. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 217–224, 1997     

Morphology and mechanical properties of liquid crystalline copolyester and poly(ethylene 2,6-naphthalate) blends

Blends of poly(ethylene 2,6-naphthalate) (PEN) and a liquid crystalline copolyester (LCP), poly(benzoate-naphthoate), were prepared in a twin-screw extruder. Specimens for mechanical testing were prepared by injection molding. The morphology and mechanical properties were investigated by scanning electron microscopy (SEM) and an Instron tensile tester. SEM studies revealed that finely dispersed spherical domains of the liquid crystalline polymer (LCP) were formed in the PEN matrix, and the inclusions were deformed into fibrils from the spherical droplets with increasing LCP content. The morphology of the blends was found to be affected by their composition and a distinct skin-core morphology was found to develop in the injection molded samples of these blends. Mechanical properties were improved with increasing LCP content, and synergistic effects have been observed at 70 wt% LCP content whereas the elongation at break was found to be reduced drastically above 10 wt% of LCP content. This is a characteristic typical of chopped-fiber-filled composites. The improvement in mechanical properties is likely due to the reinforcement of the PEN matrix by the fibrous LCP phase as observed by scanning electron microscopy. The tensile and modulus mechanical behavior of the LCP/PEN blends was very similar to those of the polymeric composite, and the tensile strength and flexural modulus of the LCP/PEN 70/30 blend were two times the value of PEN homopolymer and exceeded those of pure LCP, suggesting LCP acts as a reinforcing agent in the blends.     

In situ composite fibers: Blends of liquid crystalline polymer and poly (ethylene terephthalate)

To augment the concept of in situ composites as alternatives to fiber-reinforced composites, polyblends of a thermotropic liquid crystalline polymer (LCP) and poly(ethylene terephthalate) (PET) were prepared. Fiber-spinning of the blends was performed on a piston-driven plastorneter. Blends of LCP and a low-intrinsic-viscosity PET resin showed poor mechanical performance, which was attributed to their processing behavior. Blends of LCP and a high intrinsicviscosity PET manifested an almost additive behavior with regard to tensile modulus and strength. Elongation of the blends, however, displayed a radical decline, which is reminiscent of fiber-reinforced composites. Heat treatment of the blend fibers modestly increased the tensile properties of the LCP-rich compositions. Blend fibers from PET-rich compositions exhibit a moderate decline in tensile properties owing to thermal relaxation of PET. The data demonstrate that in situ composites or blends of thermotropic LCPs and isotropic polymers present challenging alternatives to fiber-reinforced composite systems because of their ease of processing.     

Miscibility and crystallization in crystalline/crystalline blends of poly(butylene succinate)/poly(ethylene oxide)

Miscibility and crystallization behavior have been investigated in blends of poly(butylene succinate) (PBSU) and poly(ethylene oxide) (PEO), both semicrystalline polymers, by differential scanning calorimetry and optical microscopy. Experimental results indicate that PBSU is miscible with PEO as shown by the existence of single composition dependent glass transition temperature over the entire composition range. In addition, the polymer-polymer interaction parameter, obtained from the melting depression of the high-T_m component PBSU using the Flory-Huggins equation, is composition dependent, and its value is always negative. This indicates that PBSU/PEO blends are thermodynamically miscible in the melt. The morphological study of the isothermal crystallization at 95 °C (where only PBSU crystallized) showed the similar crystallization behavior as in amorphous/crystalline blends. Much more attention has been paid to the crystallization and morphology of the low-T_m component PEO, which was studied through both one-step and two-step crystallization. It was found that the crystallization of PEO was affected clearly by the presence of the crystals of PBSU formed through different crystallization processes. The two components crystallized sequentially not simultaneously when the blends were quenched from the melt directly to 50 °C (one-step crystallization), and the PEO spherulites crystallized within the matrix of the crystals of the preexisted PBSU phase. Crystallization at 95 °C followed by quenching to 50 °C (two-step crystallization) also showed the similar crystallization behavior as in one-step crystallization. However, the radial growth rate of the PEO spherulites was reduced significantly in two-step crystallization than in one-step crystallization.