Liquid Crystalline Polymer Fibrils Research Articles

Crystallization kinetics of compatibilized blends of a liquid crystalline polymer with polypropylene

Blends of a maleic anhydride-grafted polypropylene (m-PP) and a liquid crystalline polymer (LCP) based on a copolyester of hydroxynapthoic acid and hydroxybenzoic acid were fabricated. The morphology and isothermal and nonisothermal crystallization kinetics behavior of the m-PP copolymer and m-PP/LCP blends were investigated using polarizing optical microscopy, depolarized light intensity, and differential scanning calorimetry. A polarizing optical micrograph revealed that the m-PP is very effective to promote a finer dispersion of the LCP phase in the PP matrix. Consequently, the LCP domains or fibrils acted as potential sites for the spherulite nucleation. The isothermal kinetics measurements also indicated that the rate of crystallization is enhanced in the maleated PP/LCP blends which exhibit transcrystallinity. In general, the nonisothermal kinetics results were in good agreement with those obtained from the isothermal kinetics measurements. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 707–715, 1997

Thermal behavior, morphology, and mechanical properties of blend strands consisting of poly(ethylene telephthalate) and semiaromatic liquid crystalline polymer

Polymer blends of poly(ethylene terephthalate) (PET) and a liquid crystalline polymer (LCP) [random copolymers of the poly(ethylene telephthalate) and poly(hydroxybenzoic acid)] were prepared by using a twin-screw extruder. Strands were extruded from a capillary die. Extruded strands were stretched in an oven at 80°C. DSC and SEM were employed to investigate the structural properties of the strands. Mechanical properties of the strands were evaluated by a sonic propagation method. DSC investigation suggested that LCP phases may act as a nucleating agent of PET and the orientation-induced crystallization of PET was accelerated by the presence of LCP. An SEM micrograph shows that the LCP phases formed finely spherical domains with a diameter of 0.1-1.0 μm in the PET matrix and large parts of LCP spherical droplets were deformed to fibrils. In the case of unstretched strands, sonic moduli increased linearly with increasing LCP content, because PET was reinforced by LCP fibrils as in the case of glass fiber-reinforced PET. The degree of crystallization of PET also increased with increasing LCP contents. In the case of stretched strands, sonic moduli increased with an increasing stretching ratio due to the orientation-induced crystallization of PET. A larger increasing of the sonic modulus was shown in LCP-containing strands in the regions of a low stretching ratio (1-5), since the orientation-induced crystallization of PET was accelerated by the presence of LCP phases.

Poly(p‐phenylene sulphide)/liquid crystalline polymer blends. I. Miscibility and morphologic studies

The miscibility in the melt and solid state of blends made of poly(p-phenylene sulphide) (PPS) with a liquid crystalline polymer (LCP) from DuPont was studied by polarized light optical microscopy (PLOM) and dynamic thermal mechanical analysis. Both techniques showed that the PPS and the LCP are immiscible in both states, and that the critical concentration for the formation of fibrils C*, in this particular system, was located between 20 and 25 wt % LCP. The resultant blend morphology was studied by PLOM and scanning electron microscopy (SEM). It was observed that when LCP fibrils are formed in the PPS matrix, the PPS macromolecules will crystallize around the LCP fibrils by forming columnar layers called transcrystallites. These transcrystallites are the result of the LCP acting as a nucleating agent for the PPS, promoting heterogeneous nucleation. © 1996 John Wiley & Sons, Inc.

Poly(p-phenylene sulphide)/liquid crystalline polymer blends. I. Miscibility and morphologic studies

The miscibility in the melt and solid state of blends made of poly(p-phenylene sulphide) (PPS) with a liquid crystalline polymer (LCP) from DuPont was studied by polarized light optical microscopy (PLOM) and dynamic thermal mechanical analysis. Both techniques showed that the PPS and the LCP are immiscible in both states, and that the critical concentration for the formation of fibrils C*, in this particular system, was located between 20 and 25 wt % LCP. The resultant blend morphology was studied by PLOM and scanning electron microscopy (SEM). It was observed that when LCP fibrils are formed in the PPS matrix, the PPS macromolecules will crystallize around the LCP fibrils by forming columnar layers called transcrystallites. These transcrystallites are the result of the LCP acting as a nucleating agent for the PPS, promoting heterogeneous nucleation. © 1996 John Wiley & Sons, Inc.

Injection molding of PC/PBT/LCP ternary in situ composite

AbstractA liquid crystalline polymer (LCP), Vectra B950, reinforced polycarbonate (PC) 60 wt%/polybutylene terephthalate (PBT) 40 wt% blend was studied using the injection molding process. Morphology and mechanical properties of ternary in situ LCP composites were investigated and compared with binary polycarbonate/Vectra B950 LCP composites. Good in situ fibrillation of LCP was observed in the direct injection‐molded LCP composites. Preliminary results of this work indicate that addition of PBT improves skin‐core distribution of LCP microfibrils in the matrix and also enhances adhesion between the matrix and Vectra B950, which contains terephthalic acid. The PC/PBT/LCP ternary system also exhibits lower viscosity than the PC/PBT blend and pure LCP. In a ternary system with 30 wt% of Vectra B950, tensile modulus and strength increase approximately threefold and twofold, respectively. The rule of mixtures (ROM) for continuous reinforcement can accurately represent the strengthening effects for the ternary LCP in situ composites. Generally, LCP reduces the ductility and impact strength of the thermoplastic blends; however, the relative loss is less in the ternary system than in the binary system.

Thermal conductivity and thermal expansivity of in situ composites of a liquid crystalline polymer and polycarbonate

AbstractThe thermal conductivity and thermal expansivity of extruded blends of a liquid crystalline polymer (LCP) and polycarbonate (PC) with volume fraction (Vf) of LCP between 0.09 and 0.8 have been measured as functions of draw ratios λ ranging from 1.3 to 15. At Vf < 0.3, the LCP domains are dispersed in a PC matrix and the aspect ratio of the domains increases with increasing λ. At Vf > 0.55, phase inversion has occurred and the LCP becomes the continuous phase. The axial thermal conductivity K∥ increases while the axial expansivity α∥ decreases sharply with increasing λ, as a result of the higher aspect ratio of the LCP fibrils and the improved molecular orientation within the fibrils. Since the transverse thermal conductivity and expansivity are little affected by drawing, the blends exhibit strong anisotropy in the thermal conduction and expansion behavior at high λ. At Vf < 0.3, the behavior of K∥ is reasonably modeled by the Halpin‐Tsai equation for short fiber composites. At high draw ratio (λ = 15), all the blends behave like unidirectional continuous fiber composites, so K∥ and α∥ follow the rule of mixtures and the Schapery equation, respectively.

In situ compatibility of polystyrene and liquid crystalline polymer blends

The styrene-glycidyl methacrylate (SG) copolymer can be used to compatibilize the immiscible and incompatible blends between polystyrene (PS) and liquid crystalline polymer (LCP) copolyester. The epoxy functional groups present in the SG copolymer can react with the carboxylic and/or the hydroxyl terminal groups of the LCP at interface to form the SG-g-LCP copolymer during melt processing. This in situ-formed graft copolymer tends to reside along the interface to reduce the interfacial tension in the melt state and to increase the interphase adhesion in the solid state. However, the compatibilized PS/LCP blends reduce the number of LCP fibrils formed and have a tendency to form droplet LCP domains. The overall mechanical properties, stiffness and toughness of the blends improved after compatibilization, but the extent of the improvement is not very significant. It appears that the gain from the adhesion enhancement is more than offset by the loss due to the reduction in LCP fibril formation. The ethyl triphenylsphosponium bromide catalyst can further improve the compatibility of the blends by increasing the rate of the graft reaction.

Mechanical properties of injection moulded blends of polypropylene with thermotropic liquid crystalline polymer

Blends of a thermotropic liquid crystalline polymer (LCP) with polypropylene (PP) were injection moulded. The LCP exhibited a higher viscosity than that of PP. Static and dynamic mechanical measurements, Izod impact tests, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) were performed on these blends. The static tensile tests show that the tensile modulus and strength of PP are improved with the addition of LCP. The improvement in mechanical properties is associated with the formation of LCP fibrils as evidenced by SEM observations. Dynamic studies on these blends show an increase in the storage modulus but a decrease in loss factor with the addition of LCP. Furthermore, TGA measurements show that the thermal stability of PP is improved substantially with the addition of LCP.

Factors influencing microstructure formation in polyblends containing liquid crystalline polymers

AbstractFour isotropic polymers, poly(butylene terephthalate) (PBT), polycarbonate (PC), polyethersulfone (PES) and polysulfone (PSU), were blended by extrusion with a thermotropic liquid crystalline polymer (LCP) at different temperatures. The morphology of extrudates was observed by means of scanning electron microscopy and the intrinsic aspect ratio of LCP fibrils and particles separated from matrix resin was measured with an image analysis. Special attention was paid to the LCP fibrillation in these four matrices in a wide temperature range from 270 to 360°C and the internal relations among the effects of processing parameters, such as viscosity ratio, extrusion temperature, and LCP concentration. The results show that the viscosity ratio of the dispersed LCP phase to the continuous phase is a decisive factor determining the formation of LCP fibrils, but its effect closely relates with the LCP content. In the range of viscosity ratios investigated, 0.004 to 6.9, and lower LCP content of 10%, significant fibrillation took place only at viscosity ratios below 0.01. It is predicted that the upper limit of the viscosity ratio for LCP fibrillation will increase with increasing LCP content. A comparison of the morphologies of LCP/PES blends with different LCP concentrations reveals that the LCP phase becomes continuous at a concentration of less than 50%, and high LCP content does not always favor the formation of long and uniform LCP fibrils. The extrusion temperature has a marked effect on the size of the minor LCP domains. For fibril forming systems, the percentage of LCP fibrils with larger aspect ratios increases with increasing extrusion temperatures. All these results are explained by the combined role of deformation and coalescence of the LCP disperesed phase in the blend.

A preliminary investigation of flash formation during injection molding of polyphenylene sulfide and liquid crystalline polymer blends

AbstractFlashing in pure polyphenylene sulfide (PPS) and a blend containing PPS and liquid crystalline polymer (LCP) during injection molding was investigated by differential scanning calorimetry and scanning electron microscopy. The shape of the flash was observed by use of a projector. Flashing was detected in pure PPS and 90/10 PPS/LCP blend but was not found in other compositions, including pure LCP. The DSC thermograms of the flash revealed both exothermic and endothermic peaks at around 120° and 285°C. The first peak, known as crystallization temperature on heating, occurred as a result of early crystallization of PPS. The observed double peaks indicated that the degree of crystallinity was lower in the flash than in the molded part. The morphological studies revealed the presence of LCP fibrils in the skin region and droplets in the core region of 90/10 PPS blend. The absence of flash was attributed to the diameters of the fibrils and droplets, which were found to increase with increasing LCP component.

Through‐thickness distribution of LCP in PPS/LCP blends

AbstractThrough‐thickness distribution of liquid crystalline polymer (LCP) of blends containing polyphenylene sulfide (PPS) and LCP was investigated using differential scanning calorimetry and scanning electron microscopy. The effect of the LCP distribution on the mechanical test was checked through bending testing of the various compositions of the injection molded samples. These studies showed a nonuniform distribution of LCP in the PPS‐rich region where the LCP content in the skin layer was higher than in the core layer or boundary between the two layers. The LCP component was uniformly distributed in the LCP‐rich region. The increase of bending modulus with increasing LCP content was attributed to the reinforcing nature of the LCP fibrils in the skin layer. © 1994 John Wiley & Sons, Inc.

Microstructure formation in polyblends containing liquid crystalline polymers

Based on rheological data, two polyblends of poly(ether sulfone) (PES) and polycarbonate (PC) with a liquid crystalline polymer (LCP) as the dispersed phase were prepared by extrusion. The process of LCP microstructure formation was investigated by studying the morphology of the blend obtained from different sections of the extruder. The deformation of LCP domains is controlled by the viscosity ratio of the dispersed phase to the continuous phase (ηdηm). LCP domains in LCP/PES blend have been deformed into oriented fibrils, due to a viscosity ratio of 0.01. A quantitative comparison of LCP droplet sizes before and after the extruder die shows that the morphology of extrudates has been determined before the melt enters the die. After the die, the diameter and length to diameter ratio of LCP fibrils were increased by coalescence and further deformation by the action of extensional flow. The morphology of LCP/PES extrudates after remaining at quiescence within the die for different time intervals indicates that even after a 60 s residence, LCP domains retained their fibrillar structure in the blend and no significant recoiling and breakup of LCP fibril occurred.

The mechanism of skin‐core morphology formation in extrudates of polycarbonate/liquid crystalline polymer blends

AbstractThe mechanism of skin/core morphology development and LCP (liquid crystalline polymer) fibril formation in polycarbonate/LCP blends was studied. A certain minimum concentration of the LCP phase must be present for the formation of continuous LCP fibrils in the extrudates. A skin‐core morphology characterizes the PC/LCP extrudates. Short LCP fibrils are formed in the capillary converging entrance section, through the elongation of LCP domains and their coalescence. Continuous fibrils were formed in the skin of extrudates emerging from cylindrical capillaries, through the coalescence of the short fibrils, provided the shear stresses are high enough and the LCP viscosity is equal or lower than that of PC. Increasing capillary length enhances the LCP lateral migration and fibrils formation. The high interfacial tension stabilizes the LCP fibrils. In the core region the short fibrils recoil or breakup, resulting in spherical or elongated droplets. Long and continuous fibrils cannot be formed in a zero length capillary, even at high flow rates.

Blending of modified polyphenylene ether with a liquid crystalline copolyester

Blending of poly (2,6-dimethylphenylene ether) (PPE), polystyrene (PS) and a thermotropic liquid crystalline polymer (LCP) was performed using a continuous co-rotating twin-screw extruder. The influence of LCP content on the blending process was studied by changing the barrel heater temperature and the screw speed. The torque of the screw shafts generated during the blending process was influenced by LCP content; its influence was not simple. The generated torque was found to depend not only on the melt viscosity of LCP but also on its distortion temperature. Further, the effects of matrix viscosity on the morphology and mechanical properties of the PPE/PS/LCP blends were studied. Well-developed fine LCP fibrils were formed during the melt-drawing process when the matrix viscosity was high. The formation of well-developed fine fibrils was found to improve the mechanical properties of the PPE/PS/LCP blend. Two mechanisms are proposed for the formation of well-developed fine LCP fibrils.

Blends containing liquid crystalline polymers: Preparation and properties of melt‐drawn fibers, unidirectional prepregs, and composite laminates

AbstractThis paper discusses the effect of melt drawing on the mechanical properties and morphology of liquid crystalline polymer (LCP) and thermoplastic polymer blends. Extruded fibers and films of LCP/polymer blends were melt drawn to develop uniaxial orientation of the dispersed LCP phase. The longitudinal modulus increased with increasing draw. The increase in modulus was due to higher aspect ratio of the LCP fibrils and improved molecular orientation of the LCP chains within the fibrils. Laminated composites were prepared using the extruded sheets as prepregs. The mechanical properties and the coefficient of thermal expansion (CTE) of the prepreg and the laminates agreed well with predictions from conventional composite lamination theories.

Processing of thermotropic liquid crystalline polymers and their blends—analysis of an in‐situ LCP composite system

AbstractThermotropic LCP/LCP fiber blends were prepared by a combination of meltblending and hot‐drawing, using a wholly aromatic copolyester KU‐9211 (also called K161 from Bayer A.G.) and an aliphatic containing LCP (liquid crystalline polymer) PET/PHB60 (from Kodak Tennessee Eastman). Morphological evidence, including scanning electron (SEM) and transmission electron microscopy (TEM), showed that the dispersed phase consisted primarily of highly oriented, 0.5 to 2 μm diameter rigid‐rods of aromatic fibers imbedded in a matrix of predominantly aliphatic LCP fibrils with diameters in the range of 20 to 50 nm. An interphase of approximately 50 nm strongly bonded the two phases together. The fiber blends were characterized using dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis (TGA), gas chromotography/mass spectroscopy (GC/MS), and rheological measurements. It appears that the processing conditions employed for melt blending had caused PET/PHB60 to undergo chain scission, thereby creating chemical interactions between the two LCP components during the melt blending process. Differential scanning calorimetry (DSC) thermograms as well as nuclear magnetic resonance (NMR) spectra of the extracted fraction from the mixture of 30 wt% K161/70 wt% PET(PHB60) confirmed the chemical interaction between the two thermotropic liquid crystalline polymers.

Correlative STM, FESEM, and TEM studies of fibrillar structures in liquid crystalline polymers

Hierarchical fibrillar structures have been reported to be present in liquid crystalline polymer (LCP) extrudates such as fibers and tapes. Using conventional scanning and transmission electron microscopy (SEM, TEM), the smallest fibrils were shown to be about 50 nm wide and 5 nm thick. With the emergence of higher resolution imaging techniques such as scanning tunneling microscopy (STM) and field emission SEM (FESEM), we have been able to further explore the ultrastructures in the LCP fibrils and extend the LCP structural model.The LCP materials investigated consist of copolyesters of 4-hydroxybenzoic acid (HBA) and 2- hydroxy-6-napthoic acid (HNA) in the form of melt-spun fibers and extruded tapes. These materials exhibit unidirectionally-oriented structures as revealed by x-ray diffraction and microscopy studies. For STM and FESEM, samples were prepared by a peel-back method which reveals the internal structures of fibrils. In addition, ultrasonication was used to disintegrate the LCP fibers and tapes to provide fine “fibrillar” samples for STM, FESEM, and TEM studies. The original tape and fiber surfaces were also examined by STM and FESEM. Thin Pt coatings (ca. 5 nm) were deposited on the LCP samples using ion beam sputtering to provide a conducting surface for STM and FESEM imaging. These coatings have been shown to introduce minimal topography to the original sample surface.