Morphology and mechanical properties of liquid crystalline copolyester and polyester elastomer blends

Blends of a polyester elastomer (PEL) having a hard segment of polyester (PBT) and soft segment of polyether (PTMG) 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 of the LCP/PEL blends was characterized under different processing conditions. To determine what conditions were necessary for the development of a fibrillar morphology of LCP, we have studied the effect of processing method (extrusion and injection molding), injection molding temperature (below and above the melting point of LCP), and gate position in the mold (direct gate and side gate). SEM studies revealed that some extensional flow was required for the fibrillar formation of LCP and the fibrillar structure of LCP was controlled by the processing method. The morphology of the blends was found to be affected by their compositions and processing conditions. SEM studies revealed that finely dispersed spherical domains of LCP were formed in the PEL matrix and the inclusions were deformed in fibrils from the spherical droplets with increasing LCP content and injection temperature. The mechanical properties of the LCP/PEL blends were also found to be affected by their compositions and processing conditions. The mechanical properties of LCP/PEL blends were very similar to those of polymeric composite. An attempt was made to correlate the structure of the blends from the scanning electron microscope with the measured mechanical properties. All of the aspects of the morphology were possible to explain in terms of the mechanical properties of the blends. A DSC study revealed that the crystallization of PEL was accelerated by the addition of LCP in the matrix and a partial compatibility between LCP and PEL was predicted. The rheological behavior of the LCP/PEL blends was found to be very different from that of the parent polymers, and significant viscosity reductions were observed in the blend consisting of only 5 wt% of LCP.     

Manufacturing and characterisation of microfibrillar reinforced composites from liquid crystalline polymers and poly(phenylene ether)

Abstract

The purpose of the present study was to investigate the fibrillisation process of liquid crystalline polymers (LCPs) in an amorphous poly(phenylene ether) (PPE) matrix during melt blending and a subsequent drawing operation, as well as to analyse the relationship between morphology and mechanical properties of the fibrillar reinforced LCP/PPE blends. In order to understand the effect of the compatibility between the blend partners, an additional set of LCP/PEE blends, containing different amounts of a compatibiliser, was studied too. The processing steps included: (i) melt extrusion and continuous hot stretching for fibrillisation of the LCP component in the different LCP/PPE blends, and (ii) compression (CM) or injection moulding (IM) of the drawn blends at temperatures below the melting temperature (T_m) of the LCPs. Samples from each processing stage were characterised by means of scanning electron microscopy (SEM), wide and small angle X-ray scattering (WAXS and SAXS), and mechanical testing. SEM and WAXS showed that the as extruded blends were isotropic, but after hot stretching the LCP components became highly oriented, with a high aspect ratio and a diameter of the fibrils between 0·4 and 3 μm. The fibrillated structure of the LCPs in the blends could be preserved after the compression and injection moulding only at temperatures below T_m of the LCPs. Addition of a compatibiliser to the LCP/PPE blend did not remarkably improve the adhesion between the components, as a result of the large difference between the coefficients of thermal expansion of the blend partners, which leads to different shrinkage conditions of the LCP fibrils and the PPE matrix. The flexural modulus (E) of all IM blends increased stepwise with an increase in the weight (wt) fraction of the LCP. At the same time, the highest values for the flexural strength (σ) were obtained for the LCP/PPE blends containing 5 wt-% LCP.     

Rheological hybrid effect in dually filled polycarbonate melt containing liquid crystalline polymer

Polycarbonate (PC)/liquid crystalline polymer (LCP) blends dually filled with glass fiber and nano‐SiO₂ were prepared by melt blending, with the use of a commercial Vectra A130 as the source of LCP and glass fiber. In these dually filled PC/LCP melts, rheological hybrid effect occurred, confirmed by the melt viscosity of the quadruple polymer blends decreased with increasing nano‐silica loading, influenced by the minor LCP phase in the blend. The drastic viscosity reduction closely correlates with the deformation and fibrillation of LCP droplets in the system. The LCP fibrillation was controlled jointly by the thermodynamic and hydrodynamic driving forces. Finally, the dually filled PC/LCP melt had decreased viscosity lower than those of pure PC, silica‐filled PC, and PC/Vectra A130 blends, and furthermore had decreased glass fiber breakage, shown by larger average aspect ratio than that in PC/Vectra A130 blends. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Binary Blends of Polytetrafluoroethylene and Liquid Crystalline Polymer

Blends of polytetrafluoroethylene (PTFE) and liquid crystalline polymer (LCP) were prepared by varying the amount of LCP when using a melt mixing technique. The rheological, morphological, thermal, and dynamic mechanical properties of the blends were investigated. The viscosity of the blends was decreased with the increasing LCP content in the blends up to 20% of LCP. With further loadings of LCP a slight improvement in viscosity was observed. This decrease in the viscosity was attributed to be the interlayer slip of the polymers due to formation of the elongated fibrils of LCP, whereas at higher amounts of LCP the fibril nature of LCP was not apparent, which is revealed by the scanning electron microscopic study. The differential scanning calorimetry (DSC) results showed a shift in the melting temperature (T_m) of PTFE with the addition of LCP. This shift in the T_m of PTFE is due to increasing crystallinity. This is further supported by the X-ray diffraction (XRD) and dynamic mechanical analysis (DMA) studies.     

Blends of a bottle grade polyethylene terephthalate copolymer and a liquid crystalline polymer. Part I: Injection molding morphology and rheological properties

Blends of a bottle grade polyethylene terephthalate copolymer (PET) with a liquid crystalline polymer (LCP) were prepared by injection molding. The thermal transitions, the morphology and the rheological properties of the pure components and of the blends were measured by dynamic mechanical analysis (DMTA), scanning electron microscopy (SEM) and capillary and parallel plates rheometry, respectively. The blends displayed only one T_g; the B60 and B80 compositions showed the highest LCP β‐transition, which has been correlated to good barrier properties. In all the blends a “skin‐core” type morphology was observed; the core region had two phases while the skin region showed only one fibrillar phase. The viscosity measurements gave an indication that the interface was strong, probably due to transterifications reactions that occurred during the tests. On creep recovery, the increasing addition of the LCP to the PET increased the blends elastic recovery. On stress growth, the highest stress overshoot was displayed by the pure LCP; this polymer actually presented two overshoots that were also observed in some of the blends at high shear rates.     

Electrical conductivity of polymer blends containing liquid crystalline polymer and carbon black

This paper presents results of a study of melt‐processed immiscible polymer blends of high impact polystyrene (HIPS), liquid crystalline polymer (LCP) and carbon black (CB). Relationships between composition, electrical resistivity and morphology of the blends produced by Brabender mixing followed by compression molding, extrusion through a capillary rheometer, extrusion through a single‐screw extruder and injection molding were investigated. The LCP phase morphology in the blends was found sensitive to the processing conditions. A blend composition of at least 20 wt% LCP and 2 phr CB is necessary to preserve the conductivity of filaments produced over a wide range of shear rates. Enhancement of conductivity of blends containing CB and 30 wt% or more LCP was observed, under processing at 270°C and increasing levels of shear rate. An important role of the skin region in determining the resisitivy of injection molded samples was found. A good agreement between resistivity values of extruded or injection molded blends with resistivity values of filaments produced at similar conditions by a capillary rheometer was shown. Hence, the study of shear rate effect on resistivity of capillary rheometer filaments may serve as a predictor of resistivity behavior in real processing procedures. Polym. Eng. Sci. 44:528–540, 2004. © 2004 Society of Plastics Engineers.     

Elaboration of PEG/PA66 blends in a twin-screw batch-type mini-extruder: Process study and modelling

This paper deals with the development of the morphology in polyethylene glycol (PEG) and polyamide 66 (PA66) immiscible blends exhibiting an extremely low viscosity ratio (η_PEG/η_PA66=3-4×10^-5). These materials were obtained by melt mixing, under different operating conditions, using a twin-screw batch-type DSM mini-extruder.Scanning electron microscopy, followed by quantitative image analysis was used to determine PEG particles size distribution (PSD) as a function of blends composition and screw rotation speed. Experiments carried out with two mixing time (5 and 10 min) showed no significant difference of PSD. So, to avoid thermal degradation of the products, the mixing time was set up at 5 min for all experiments. The influence of PEG concentration and screw rotation speed on PSD appeared to be similar to that obtained in a previous study for the same blends elaborated in a Haake internal mixer. The results clearly showed that the average particle diameters decreased as screw rotation speed increased and as PEG concentration decreased. However, this decrease is less important using the twin-screw batch-type mini-extruder with which the particle sizes are smaller. The particles sizes were then correlated to blend composition, shear rate and viscosity ratio owing to an extension of Serpe's model. The unknown parameters of the corresponding model were estimated on the basis of experimental data. This enabled then to predict with a good precision the influence of the process operating conditions on the morphology of the dispersed phase.     

Polyblends containing a liquid crystalline polymer

Polyblends of a liquid crystalline thermotropic polymer (LCP) and an amorphous polyamide (PA) were prepared by melt blending. The blends' rheological behaviour was found to be very different from that of the individual components and very significant viscosity reductions were observed for blends consisting of only 5% LCP. The blends viscosity was always much lower than that of the parent polymers. The tensile mechanical behaviour of LCP/PA blends is very similar to that of polymeric composites. The blends' two phase morphology was found to be affected by their compositions. The LCP phase changed gradually with increasing LCP content from ellipsoidal particles to rod-like and fibrillar structure. A good interphase adhesion was observed.     

Rheological and physical properties of polyarylate/LCP blend systems

A polyarylate Unitika U-Polymer 100 (PAR) was melt blended with a thermotropic liquid crystalline polymer (LCP) Vectra A950, and the processingmorphology-properties relations were investigated. Inclusion of LCP slightly reduced T_g of PAR. The PAR/LCP blend with the LCP content higher than 50 wt% exhibited a noticeable yield stress, particularly in the vicinity of crystal-to-nematic transition temperature (T_cn). LCP lowered the blend viscosity above T_cn and seemed to play a role as processing aid. The tensile strength of the blends was increased with increasing spin draw ratio and level of LCP, and the spinning temperature influenced tensile strength. The relaxation behavior under dynamic shear and resultant blend morphology based on WAXD and SEM analyses are discussed as well.     

Supercritical carbon dioxide assisted blending of polystyrene and poly(methyl methyacrylate)

A method for blending polystyrene and poly(methyl methacrylate), (PMMA), with the addition of supercritical carbon dioxide has been investigated. The first series of blends was a PMMA and polystyrene with similar melt viscosities. The second series of blends was a PMMA and polystyrene with a viscosity ratio (ηPMMA/ηpolystyrene) of about 20. The results show that a reduction in the size of the minor or dispersed phase is achieved when supercritical carbon dioxide is added to the blend system. A high-pressure mixing vessel was used to prepare the blends under pressure with carbon dioxide for batch blending. The solubilities of CO₂ in PMMA and polystyrene, measured in the high-pressure mixing vessel at 200°C and 13.78 MPa (2000 psi) was 5.8 and 3.6 wt%, respectively. A single screw extruder was used to study the effects of carbon dioxide on the viscosity of polymer melts. The melt viscosity of PMMA was reduced by up to 70% with approximately 0.4 wt% CO₂. The melt viscosity of polystyrene was reduced by up to 56% with a CO₂ content of 0.3 wt%. A twin screw extruder was used to study the effects of injecting carbon dioxide in a continuous compounding operation.     

Shear rate dependence of viscosity and first normal stress difference of LCP/PET blends at solid and molten states of LCP

The liquid crystalline polymer (LCP) and polyethylene terephthalate (PET) were blended in an elastic melt extruder to make samples having 20, 40, 60, 80, and 100 wt % of LCP. Morphology of these samples was studied using scanning electron microscopy. The steady state shear viscosity (η), dynamic complex viscosity (η*) and first normal stress difference (N₁) were evaluated and compared at two temperatures: 265°C, at which LCP was in solid state, and 285°C, at which LCP was in molten state. The PET was in molten state at both the temperatures. The shear viscosity of the studied blends displayed its dependence on composition and shear rate. A maxima was observed in viscosity versus composition plot corresponding to 80/20 LCP/PET blend. The N₁ increased with LCP loading in PET and with the increased asymmetry of LCP droplets. The N₁ also varied with the shear stress in two stages; the first stage demonstrated elastic deformation, whereas second stage displayed dominant plastic deformation of LCP droplets. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2212–2218, 2007     

The morphology and rheology of polymer blends containing a liquid crystalline copolyester

The morphology of blends of polycarbonate and nylon 6,6 with a copolyester of 60 mole percent p-hydroxybenzoic acid/40 mole percent poly(ethylene terephthalate) was characterized under different processing conditions. In particular, single-screw extrusion, steady simple shear flow, and flow through a capillary were studied to determine what conditions were necessary for the development of a fibrillar morphology of the liquid crystalline polymer (LCP). Results indicate that some extensional flow is required for the coalescence and extension of the particulate LCP phase. The viscosity of the blends was determined both in a cone-and-plate geometry of a Rheometrics Mechanical Spectrometer at low shear rates and in the Instron Capillary Rheometer at higher rates. In general, only a small (10 or 30 percent) weight fraction of LCP was required to reduce the viscosity of the thermoplastics to that of the polymeric liquid crystal. An attempt was made to correlate the structure of the blends seen under the scanning electron microscope with the observed rheology. Not all aspects of the morphology were possible to explain in terms of the viscous properties of the blends.     

Random orientation of in situ composites based on liquid‐crystalline polymers by compression moulding

In this study, randomly oriented in situ composites based on liquid‐crystalline polymers (LCPs) were prepared by thermal compression moulding. The LCP employed was a semi‐flexible liquid‐crystalline copolyesteramide with 30 mol% of p‐aminobenzoic acid (ABA) and 70 mol% of poly(ethylene terephthalate) (PET). The matrices were poly(butylene terephthalate) (PBT) and polyamide 66 (PA66). The rheological properties, compatibility and morphological structures of these in situ composites were investigated. The results showed that PA66‐LCP and PBT–LCP component pairs of the composites are miscible in the molten state, but partially compatible in the solid state. The ratios of viscosity, λ₁ = η_LCP/η_PA66 and λ₂ = η_LCP/η_PBT, are all greater than 1.0. However, the melt viscosity of the LCP/PBT and LCP/PA66 blend is much lower than that of PBT and PA66, and it decreases markedly with increasing LCP content. When the LCP/PA66 or LCP/PBT blends are compression moulded, the LCP/PA66 or LCP/PBT melts and flows easily due to their low viscosity, and the LCP phases in the melts deform easily along the flow directions, which are random. It leads to uniformly dispersed LCP micro‐fibres randomly orientation in the thermoplastic matrix due to the compatibility between the blending components. © 2003 Society of Chemical Industry     

Characterization of liquid crystalline polyester polycarbonate blends

Mechanical and rheological properties of blends of a thermotropic liquid crystalline polyester with a polycarbonate have been investigated. The blends are fibrillar in character and exhibit great hardness and toughness due to high degree of molecular orientation which develops during the melt blending and processing steps. Increases of the Young modulus by 100 percent are observed for blends containing only 10 percent of liquid crystalline polymer, LCP. Time-dependent behavior of the blends was investigated by performing solid state relaxation measurements and the relaxation modulus was also found to increase by the addition of LCP. The effect is relatively small in the glassy zone of viscoelastic response, but increases through the transition and viscous flow regions. The melt viscosity of the polycarbonate is slightly shear thinning whereas that of the unblended LCP increases rapidly with decreasing shear rate at low shear rate. This suggests the presence of yield stresses as confirmed by measurements on the Rheometics RSR in the stress sweep mode. The melt viscosity of the blends was found to be similar to that of the unblended polycarbonate, but more shear-thinning and less viscous. Preliminary results of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) are also presented.     

Effect of ethylene/propylene ratio on reinforcing characteristics of liquid crystalline polymer (LCP) in EPDM-LCP composites

Abstract

Two ethylene/propylene diene monomer (EPDM) polymers were blended with a liquid crystalline polymer (LCP) at concentrations of 10, 20, 30 and 40 wt-%. The effects of ethylene/propylene (EP) ratio on the in situ fibrillation, and hence the reinforcing characteristics, of the LCP in EPDM-LCP blends were studied. The fibre forming capacity of the LCP depended on the viscosity of the EPDM rubber. Under high temperature processing conditions (at 300°C), the high EP ratio EPDM, which had the higher viscosity, facilitated the fibrillation of the LCP. Further melt processing at 100°C, followed by curing at 150°C, decreased the reinforcing effects of the LCP owing to breakage of the fibrils under the high shear stresses developed in the high viscosity matrix. However, this degradation of fibre lengths depended on the LCP concentration. After curing, the more viscous EPDM formed blends with higher stiffnesses and strengths than those obtained from the low viscosity EPDM. Both the nucleation and growth of crystal domains in the EPDM matrix were promoted by small amounts of LCP. Again the effects were more pronounced in the EPDM with the higher EP ratio.     

Morphology and rheology of polysulfone/Vectra-A950 blends

Blends of polysulfone (PSu) with a liquid crystalline copolyester (Vectra-A950; VA) have been prepared by melt mixing. Their morphology has been studied by scanning electron microscopy (SEM). Either blend specimens as obtained from the melt mixing or fibers drawn from the melt were used for the SEM analysis. Further information on the morphology of the blends was gained by extraction of the PSu phase with methylene chloride. Preliminary rheological characterization of the blends was made by measuring the viscosity curves at 290 and 300°C, with a capillary viscometer having a die of 1 mm diameter and L/D = 40. Finally, an attempt at improving the phase compatibility was made by synthesizing a copolyester, having the same structure of commercial VA, in the presence of preformed PSu and using the product as a possible compatibilizer. It was demonstrated that the blends are composed of two immiscible phase showing poor adhesion. The LCP droplets could, nevertheless, be deformed into oriented fibrils under elongational flow conditions. The LCP particles were shown to coalesce into large domains, and to migrate toward the outer layer of, e.g., extruded rods, under the influence of appropriate flow conditions, thus showing that there is a strong mutual influence between morphology and rheology of these materials. © 1995 John Wiley & Sons, Inc.     

Fracture toughness and morphology of phenolphthalein poly(ether sulfone)–thermotropic liquid crystalline polymer blends

Blends of a new phenolphthalein poly(ether sulfone) (PES-C) and a thermotropic liquid crystalline polymer (LCP) were prepared by melt-blending in a twin-screw extruder. Rheological properties, fracture toughness, K_IC, and morphology of the blends were studied. It was found that the addition of LCP could reduce the melting viscosity and improve the fracture toughness of the PES-C matrix. The morphology of the LCP phase for the fractured section changed with increasing LCP content in the blend from dropletlike to fibrillar and layered structure. Strong interfacial adhesion could be observed at a lower content of LCP. The toughening mechanisms by blending LCP were also discussed. © 1994 John Wiley & Sons, Inc.     

Comparative study on phase and properties between rPET/PS and LCP/PS in situ microfibrillar-reinforced composites

Microfibrillar-reinforced composites based on two dispersed phases, liquid crystalline polymer (LCP) and recycled poly(ethylene terephthalate) (rPET), and polystyrene (PS) were prepared using extrusion process. The rheological behavior, morphology, and thermal stability of LCP/PS and rPET/PS blends containing various dispersed phase contents were investigated. All blends and LCP exhibited shear thinning behavior, whereas Newtonian fluid behavior was observed for rPET. The incorporation of both LCP and rPET into PS significantly improved the processability. The potential of rPET as a processing lubricant by bringing down the melt viscosity of the blend system was as good as LCP. The elongated LCP domains were clearly observed in as-extruded strand. Although the viscosity ratio of rPET/PS system was lower than that of LCP/PS system, most rPET domains appeared as small droplets. An addition of LCP and rPET into PS matrix improved the thermal resistance in air significantly. The obtained results suggested the high potential of rPET as a processing aid and thermally stable reinforcing-material similar to LCP. The mechanical properties of the LCP-containing blends were mostly higher than those of the corresponding rPET-containing blends when compared at the same blend composition.     

Liquid crystal polymer/polyethylene blends for thin film applications

A series of liquid crystal polymer/polyethylene (LCP/PE) blends have been studied to determine the potential of such a system to produce a high modulus film material which retains fabrication and low temperature characteristics of some current PE films. The subject of liquid crystalline polymer blends has been the focus of significant attention for the last decade due to the novel rheological and mechanical properties of this class of polymers. It has been demonstrated that if an LCP blend is processed under elongational flow conditions, the partially ordered LCP meso-phase intermediate allows the development of an oriented fibrillar morphology which is retained upon solidification. In this study, blown films of blends of 5 and 15% LCP in PE have been produced which show an enhancement in modulus over the neat PE matrix. These results are discussed in terms of processing conditions, LCP reinforcement aspect ratio, fibril diameter, and LCP/PE modulus ratio.