LCP Fibrils Research Articles

Highly Efficient Reinforcement of Poly-l-lactide Materials by Polymer Blending of a Thermotropic Liquid Crystalline Polymer

A novel polymer blend system consisting of poly(l-lactide) (PLLA) and a thermotropic liquid crystalline polymer (LCP: an aromatic polyester comprising poly(4-hydroxybenzoate) sequences) was investigated in the presence and absence of a polycabodiimide (PCD). Scanning electron micrographs of the injection-molded polymer blends revealed the formation of fibrous structure of LCP in the PLLA matrix, supporting the efficient toughening. In particular, the LCP fibrils became semimicrometer in diameter in the presence of PCD with which both PLLA and LCP had reacted during the melt blending to form their block and graft copolymers working as compatibilizer. The blend specimens containing LCP in 20-30 wt % were found to hold high dynamic storage-moduli (E') at high temperature. In addition, the E' value of the specimens containing 30 wt % of LCP reached 10.7 GPa at room temperature, being significantly higher than that of PLLA.

PP/LCP composites: effects of shear flow, extensional flow and nanofillers

PP/LCP composites moulded under shear flow and extensional flows were prepared in this work. It was observed that the predominating shear flow during injection moulding was unable to induce in-situ reinforcements in the Rodrun LC5000/PP blend. With the introduction of an additional drawing process, fibrous LCP was developed. The fibrillation of the LCP phase was found to increase with the draw speed and the formation of LCP fibrils was manifested as higher strength and stiffness. However, due to the poor adhesion between the pre-generated LCP fibrils and PP matrix, the strength of the extensional PP/LCP composites was lower than that of direct injection moulded LCP/PP blend. In another approach, silica fillers were introduced to the LCP/PP blend and the LCP fibrillation was found to improve. The nanosilica was believed to behave as a viscosity thickening agent and was able to promote the LCP droplet-fibril transition effectively. The nanofiller/LCP/thermoplastic system possessed dual reinforcements which imparted good properties to the composite.

Flow Properties and Morphology of PC/LCP Blends Affected by the Addition of Glass Fiber and Resulted Mutual Influences

Abstract In-situ hybrid composites containing matrix polycarbonate, glass fiber and LCP are prepared. The morphology of LCP and the rheological behavior of the composites are described. The relationship between flowability and morphology is discussed. It is interesting to find that in some hybrid systems, composites containing more glass fiber have better flowability than composites containing less glass fiber. Glass fiber aspect ratio is an important factor that determines the flowability. Materials with short glass fiber have lower viscosities than the materials with long glass fiber. LCP morphology is another important parameter that determines the flowability of the composites. If LCP forms fibrils, the composites have better flowability. If spheres dominate the morphology of LCP, the materials have larger viscosity. The addition of glass fiber brings about new factors that influence the morphology of LCP. The LCP morphology is not only affected by LCP content, but also by glass fiber content and glass fiber length. LCP fibrils are promoted in some in-situ hybrid composites. A model is proposed to describe how fibrillation of LCP is promoted due to the addition of glass fiber.

Processability of LCP‐nylon‐glass hybrid composites

AbstractA hybrid composite consisting of rubber toughened nylon 6,6, short glass fibers and thermotropic LCP was investigated by varying the content of LCP. It was found that a hybrid composite offered better processability over the glass fiber reinforced polymers alone. The total torque in melt mixing increased with short glass fiber addition but decreased with an increase in LCP content. The thermal stability of the glass fiber reinforced composite was improved by blending with LCP. However, a minimum of 15 wt% LCP was required to realize reinforcement effect from the hybrid composite. The fracture morphology was examined using SEM techniques. Some LCP fibrils could be observed on the tensile fracture surface.

Molecular orientation of individual LCP particles in injection-moulded PPS/LCP blends

Polarized light microscopy was used to investigate the presence of preferred molecular orientation in the LCP phase of PPS/LCP blends after injection moulding. Normal birefringence effects appeared to be complicated by artifacts due to sample preparation and by the complex nature of polarized light transmission through a multicomponent sample. It was found, however, that, during low-temperature cutting of optically transparent thin sections on a standard microtome, individual LCP particles could be separated from the PPS matrix, and their birefringence analyzed separately. Preferred orientation was detected only in LCP fibrils which dominated in skin regions, but not in droplet-shaped particles which had formed in core regions. Quantitative measurements indicated that the molecular orientation of the fibrils increased linearly with their length-to-diameter aspect ratios which ranged from 15 to 50. Even for the highest aspect ratios, however, the degree of orientation was always less than that which could easily be introduced into pure LCP thin-film samples by manual shearing.

Effect of fillers on the structure and mechanical properties of LCP/PP/SiO 2 in-situ hybrid nanocomposites

Fibrillation of LCP in a LCP/PP blend was enhanced through the introduction of nanosized silica particles to the PP matrix. LCP/(PP/SiO 2) composites with various silica concentrations were injection moulded into 1 mm plaques and characterized using structural, morphological and mechanical methods. The results revealed the transformation of short LCP fibrils to high aspect ratio fibrous structures upon the addition of the nanofillers. The silica particles have promoted the shear-induced fibrillation of the LCP phase. The wide angle X-ray diffraction results indicated that high orientation was achieved with rising silica content. The injection moulded samples also showed increased mechanical anisotropy with rising filler content. Consequently, both the in-situ fibrillation of LCP and silica reinforcements imparted good tensile strength and modulus to the composites along the flow direction. Such an improvement was achieved from the basis of increasing the matrix viscosity and raising the Capillary number which is a dimensionless factor governing the fibrillation process. This novel approach opens a new insight to the feasibility of producing in-situ hybrid nanocomposites with synergetic reinforcements.

LCP droplet deformation in fiber spinning of self-reinforced composites

The development of morphology during fiber spinning of blends of polyesters (PET and PBT) with LCP (PET/HBA) was studied. The quantitative predictions of the blend morphologies under isothermal and non-isothermal conditions were obtained using a droplet deformation criteria based on the reduced capillary number and the affine deformation theory. The temperature, radius, velocity, and strain rate profiles of 60/40 polyester/LCP fibers along the spin line were calculated using Kase and Matsuo's theory for fiber spinning under the steady-state condition. Simulation of breakup of LCP phase during fiber spinning of the blend based on the dimensionless breakup time indicated the absence of their breakup. The flow curves of the blends at various temperatures required to carry out simulation were obtained by a capillary rheometer. An Arrhenius type equation was established for the viscosity variation with the temperature. An interfacial tension between the components was calculated from the interfacial thickness observed by SEM. In the calculation, various viscosity ratios between thermoplastics and LCP components were applied. Initial and final sizes of LCP phase were examined by SEM and image analyzer. The calculated diameters of LCP fibrils in the fibers were found to be in agreement with the measured values.

Self-Reinforced Composites of Various Polyesters with PET/HBA Based LCP

Four different polyesters, PET, PBT, PEN, and copolyester, were blended with LCP based on poly(ethylene terephthalate)/p-hydroxybenzoic acid, PET/HBA, by using a single screw extruder with a static mixer followed by a die. Unidirectional sheets and fibers of the blends of various LCP contents were prepared at various operating conditions including die geometry, die temperature, and extension ratio to find optimum processing conditions required for improvement of mechanical properties. Depending on matrix and processing conditions, the blends revealed different LCP domain defonnation, size, shape, and, consequently, mechanical properties. Among the four different polyesters, the PET matrix was the most efficient in obtaining unifonn LCP phase distribution and large aspect ratio in blends, which led to nanocomposites of PET/LCP blends. However, the blends based on the copolyester matrix showed the best mechanical properties with addition of LCP phase. The mechanical properties and morphologies of injection moldings made by reprocessing thermoplastic/LCP fibers at the melt temperature below the LCP melting point were investigated and compared with properties of the original fibers. Among various blends, the LCP fibrils preexisting in PEN/LCP fibers were best preserved in moldings. The addition of a compatibilizer to the injection molding of the PET/LCP blend revealed little increase in tensile strength and modulus.

Effect of composition and molecular structure on the LC phase of PHB-PEN-PET ternary blend

Poly(p-hydroxybenzoic acid) (PHB)-poly(ethylene terephthalate) (PET) 8/2 thermotropic liquid crystalline copolyester, poly(ethylene 2,6-naphthalate) (PEN), and PET were mechanically blended to pursue the liquid crystalline (LC) phase of ternary blends. The torque values of blends with increasing PHB content abruptly decreased above 40 wt % of PHB content because the melt viscosity of ternary blends dropped. Glass transition temperature and melting temperature of blends increased with increasing PHB content. The tensile strength and initial modulus of blends were low at 10 and 20 wt % PHB. However, the blends containing above 30 wt % PHB were improved with increasing PHB content due to the formation of fibrous structure. The blend of 20 wt % PHB formed irregularly dispersed spherical domains, and the blends of 30-40 wt % PHB showed LCP ellipsoidal domains and fibrils. In the polarized optical photographs, the blends of 40 wt % PHB showed pseudo LC phases. The degree of transesterification and randomness of blends were increased with blending time.

Morphology of ternary polymer blends containing a liquid crystalline polymer

Morphology was studied in ternary polymer blends containing a polycarbonate (PC) matrix, a thermotropic HBA/HNA copolyester (LCP), and a poly(ethyleneterephthalate) (PET) or a poly-ϵ-caprolactone (PCL) as the minor components. LCP is immiscible in the other three polymers, while PET and PC exhibit a partial mutual miscibility, and PC/PCL is a miscible blend in the studied composition range, as evidenced by T g measurements. The melt rheology of the binary and ternary blends, studied using a capillary rheometer over a wide range of shear rates, shows unexpected dependency on the blend composition, which may partially stem from some degree of possible intermolecular reactions during melt blending, but is mainly due to morphological effects. The capillary extrudate morphology of the miscible blends (90PC/10PCL and 90PC/10PET) is that of a single-phase system. The morphology of the 85PC/15LCP and 60PC/40PET blends consists of a dispersed fibrillar minor phase oriented along the flow direction. However, no LCP fibrils, but rather particles, were observed in the [90/10]/15 [PC/PET or PCL]/LCP filaments. An intriguing morphology was observed in the [60/40]/15 [PC/PET]/LCP extrudates. As in the PC/LCP filaments, LCP particles, either spherical, elongated, or fibrillar, depending on the filaments' preparation conditions, were also observed in the ternary blends. However, they were engulfed by a PET-rich PET/PC layer. The preferred localization, engulfing the LCP particles rather than separately dispersed, of the PET-rich minor phase is expected by consideration of thermodynamic (interfacial tensions and “spreading coefficients”) and kinetic (viscosity ratios) parameters.

The effect of the viscosity ratio of dispersed phase to matrix on the rheological, morphological, and mechanical properties of polymer blends containing a LCP

The effect of the viscosity ratio of the dispersed LCP phase to the polystyrene/poly(phenylene oxide) (PS/PPO) thermoplastic matrix on the rheological, morphological, and resultant mechanical properties of the LCP blends was investigated. The viscosity of PS/PPO is largely dependent on the blend composition, so that different levels of viscosity ratios of dispersed LCP phase to PS/PPO thermoplastic matrix are obtained by using PS/PPO premixtures of different blend ratios as a thermoplastic matrix. When the viscosity of the LCP dispersed phase is lower than that of the thermoplastic matrix, finely distributed fibril structure of LCP is obtained. Tensile modulus of injection molded specimens show a positive deviation from the additive rule when the viscosity ratio (η LCP /η matrix ) is smaller than unity. These improvements in tensile modulus are attributed to the formation of finely distributed LCP fibrils.

Morphology and Mechanical Properties of Pc/Lcp in Situ Composites With Compatibilizer

AbstractA bisphenol A epoxy resin is used to modify compatibility between PC and LCP (Vectra B950, a wholly aromatic liquid crystalline polymer) during injection molding. LCP can effectively reinforce PC, since it forms in situ micro-fibrils in the thermoplastic PC matrix during melt flow. However, the immiscibility of PC and LCP affects the mechanical performance of the LCP in situ composite. Preliminary results of this work indicate that epoxy might improve the interfacial bonding between PC and LCP fibrils, and also modify the tensile properties of the PC/LCP system.

The relation between spinnability of thermotropic liquid-crystalline polymers and their submicrometers reinforcing function in polymer blends

Six thermotropic liquid-crystalline polymers (TLCPs) were spun on a CSI 194 Mini-Max extruder in their optimized spinning-temperature ranges. Based on the diameter and the diameter distribution of the resultant fibres, these TLCPs were classified into two groups with good and poor spinnability, respectively. The viscosity ratio of TLCP to matrix resin, η d/ η m, in the optimized spinning-temperature range was used as a parameter for controlling TLCP fibrillation in the resin matrix. For a TLCP having good spinnability, its morphology in different resins was observed and correlated with the viscosity ratio in the range of 10 −2 to 10. With decreasing η d / η m from 16 to 0.05, the dispersed TLCP phase changes from spheres through ellipsoids and short rods to fibrils. Under the same condition, i.e. in the η d / η m range of 0.01 to 0.1, TLCPs with good spinnability generate fibrils in injection-moulded samples, while poor spinnability ellipsoids and rods only are generated. The results of image analysis show that two TLCPs with good spinnability generate fibrils of smaller diameters, narrower diameter distributions and larger intrinsic aspect ratios. It is shown that there is a correlation between the TLCP spinnability in air and TLCP fibrillation in the resin melt. This could be taken as a prerequisite for the achievement of submicrometre reinforcing with LCP fibrils.

Post extrusion hot drawing of polycarbonate/liquid crystalline polymer blends

AbstractThe post extrusion hot drawing of polycarbonate/liquid crystalline polymer (PC/LCP) blends, over the entire composition range, was studied. The extruded filament morphology and elastic modulus were followed as a function of blends composition, initial phase morphology, and draw ratio (DR). Hot drawing was found to cause further orientation to the already existing partially oriented LCP phase at the die exit, as reflected by the increased blends modulus. The additional orientation depends on the initial filaments structure, the blend composition, and the DR. Moreover, the orientability of the LCP phase is much higher, similar to that of neat LCP, for blends in which the LCP forms the continuous phase. In low LCP content blends, a critical DR was identified, beyond which the LCP fibrils undergo fragmentation and voids at the fibrils/matrix interface are formed, resulting in a decrease in the drawn filament modulus.

In situ reinforcement of polypropylene with liquid-crystalline polymers: effect of maleic anhydride-grafted polypropylene

The effect of maleic anhydride-grafted polypropylene (MAP) on the mechanical properties of three polypropylene/liquid-crystalline polymer (PP/LCP) blends was investigated. Three LCPs were used in the study. One LCP was a poly(ester amide) and the other two LCPs were copolyesters. The tensile properties of the PP/liquid-crystal (LC) poly(ester amide) blend were affected by the addiction of MAP to a greater extent than were the tensile properties of the two PP/LC copolyester blends. The tensile strength of the former blend increased without reaching a plateau as the level of MAP was increased, whereas the tensile strength of the latter blends showed that a plateau or maximum in the strength existed. The tensile modulus of the PP/LC poly(ester amide) blend showed a 19% increase as the amount of MAP was increased above 5 wt%. Only small changes occurred in the modulus of the PP/LC copolyester blends. The effect of MAP on the mechanical properties of PP/LCP blends indicated that MAP interacted with the amide group preferentially over the ester group. An excess of MAP was also shown to change the morphology from larger elongated structures to smaller structures that can lead to reduced tensile moduli. Measurement of the contact angles of liquids on PP,PP/MAP and the three LCPs indicated that MAP lowered the interfacial tension for all three PP/LCP blends. The reduced interfacial tension and increased adhesion indicated that MAP compatibilized the PP/LCP blends. While in most polymer blends compatibilization leads to finely dispersed drops of the minor components, LCP fibrils were obtained over a large range of compatibilizer concentrations. Finally, isolation of the PP/MAP phase from the LCP phase indicated that MAP did not react with the LCP. Instead, an interaction such as hydrogen bonding was believed to be responsible for the compatibilizing effect of MAP on PP/LCP blends.

Morphology and mechanical properties of fibers from blends of a liquid crystalline polymer and poly(ethylene terephthalate)

AbstractThe domain morphology and mechanical properties of fibers spun from blends of a thermotropic liquid crystalline polymer, Vectra A‐900, and poly(ethylene terephthalate) (PET) have been studied across the entire composition range. The PET phase was removed by etching to reveal fibrillar LCP domains in the blends of all compositions. The 0.5μm fibril appeared to be the basic structural entity of the LCP domains. A primary effect of composition was the change from discontinuous fibrils when the composition was 35 and 60% by weight LCP to continuous fibrils when the composition was 85 and 96% LCP. This transition had major ramifications on the mechanical properties: the modulus increased abruptly between 60 and 85% LCP, and a change in the fracture mode from brittle fracture to a splitting mode was accompanied by an increase in fracture strength. Different models were required to describe the mechanical properties of the discontinuous and continuous fibril morphologies. Analytic models for short aligned fibers of Nielsen, and Kelly and Tyson were applicable when the LCP fibrils were discontinuous, while modulus and strength of blend fibers with continuous LCP fibrils were discribed by the rule of mixtures.