The interfacial tension between a thermotropic liquid crystalline copolyester and polyethersulfone: A comparison of methods

Three dynamic methods to determine the interfacial tension between the thermotropic liquid crystalline polymer (TLCP) Vectra A900 and polyethersulfone were evaluated: (1) thread breakup, (2) fiber retraction and (3) dynamic shear rheometry. The thread breakup and retraction methods, were suitable for measuring the interfacial tension, provided that the shear thinning flow behavior of the TLCP was taken into account. The viscosity of the TLCP during breakup or retraction was estimated from steady-shear measurements at the observed overall rate of deformation during growth of capillary instabilities or during retraction. The calculation of the interfacial tension from breakup rates of TLCP threads was improved by accounting for transient flow behavior during distortion growth using a single-element Maxwell model. Determination of the interfacial tension by oscillatory shear measurements on TLCP/PES dispersions using the emulsion model of Palierne, was not applicable for this system. Only for dispersions containing low TLCP volume fractions (e.g. 9 vol%) was there reasonable agreement between the emulsion model and measurements. At higher volume fractions agreement was poor, possibly because of different dynamic flow behavior of the TLCP in the pure form and in blends. The interfacial tension values obtained from thread breakup and fiber retraction ranged from 4 to 6 mN/m, which demonstrate that in-situ determination of the interfacial tension is possible for blends containing TLCPs, despite their complex flow behavior.     

Study on the miscibility and rheological properties of thermotropic liquid crystalline polymers in thermoplastic matrices

The interfacial properties of polymer blends of the engineering thermoplastics (TPs) polycarbonate (PC) and polyethersulfone (PES) with thermotropic liquid crystalline polymers (TLCPs) were studied using FTIR and DSC. The TLCP/TP blend systems were mainly immiscible. The viscosity properties of the TLCP/TP blends were analysed. The mechanism of the viscosity variation of the blends is discussed. Based on the assumptions given in this paper, a reasoned theoretical formula of the blend viscosity is derived to express the viscosity reduction of the TLCP/TP blends.     

Effect of processing history on the morphology and properties of polypropylene/thermotropic liquid crystalline polymer blends

Preparation, morphology, and mechanical properties were studied of blends of a thermotropic liquid crystalline polymer (TLCP) with two different grades of polypropylene, one with and one without overlap in processing temperatures, using two different blending methods. The highly viscous grade (PP-1) was of sufficient thermal stability to be blended with the TLCP (Vectra A950) in a single-screw extruder with an Egan mixing section on the screw. The low viscous grade (PP-2) could not be processed at the same temperature as the TLCP because of degradation. Its blends were, therefore, prepared by a special coextrusion technique, i.e. feeding the two components from two separate extruders to a Ross static mixer. In both methods drawing of the extrudate is necessary to obtain satisfactory mechanical properties. The PP-1/TLCP blends had to be extruded twice in order to obtain proper mixing. The morphology of these blends ranges from a pronounced skin-core morphology at low extrudate draw ratio (DR = 3) to a high-aspect ratio fiber/matrix morphology at high draw ratio (DR = 15). The morphology of the PP-2/TLCP blends was always a high-aspect ratio fiber/matrix morphology even at low draw ratios. The TLCP fibers were generated in this coextrusion process under conditions where the viscosity of the dispersed phase was higher than the viscosity of the matrix. Breakup experiments demonstrate that fibers of a thickness of approximately 1 μm disintegrate into droplets within a few seconds at temperatures above the melting point of the TLCP. This is probably the cause of the skin-core morphology obtained with single-screw extrusion. Tensile modulus and strength of all blends increase with extrudate draw ratio. The deformation of the TLCP phase in the drawn blends is less than affine, probably because of slip between the phases. The moduli of the PP-1/TLCP blends as a function of the draw ratio can be described well by a modified Halpin-Tsai equation taking into account both changes in aspect ratio and molecular orientation of the TLCP fibers. The level of reinforcement in the PP-2/TLCP blends is lower than expected, probably because of the low temperature of drawing. This demonstrates a limitation of the coextrusion process: blending at temperatures that are too low reduces mechanical properties.     

Molecular structure effect of thermotropic liquid crystalline polymers on interfacial adhesion with polycarbonate

The effect of molecular structure of thermotropic liquid crystalline polymers (TLCP) on the interfacial adhesion with polycarbonate (PC) was investigated by varying the flexible spacer length in the main chain of the TLCP. The interfacial adhesion was estimated by two methods: (1) the debonding stress from the melt-contacted TLCP/PC plates was measured by a modified pin-pull test; (2) the mechanical response of the melt-mixed blend was analyzed by the composite theory. Both methods indicated that the interfacial adhesion of the TLCPs with flexible spacer was improved over that of a rigid TLCP (i.e., Vectra A950), and that it was further enhanced by an increase in the length of a flexible unit.     

Formation,stability, and properties of in-situ composites based on blends of a thermotropic liquid crystalline polymer and a thermoplastic elastomer

This paper describes the preparation and properties of in-situ composites based on polymers with no overlap in processing temperatures. The polymers used were Vectra A900, a thermotropic liquid crystalline copolyester (TLCP), and Arnitel em630, a thermoplastic elastomer. Blends were generated by feeding the two components from separate extruders into a Ross static mixer. Different morphologies were obtained by varying the number of mixing elements of the static mixer. Using 8 mixing elements led to a stratified morphology of Vectra layers in Arnitel, using 11 mixing elements resulted in the desired continuous fiber/matrix morphology whereas a pronounced skin-core morphology was obtained with 14 mixing elements. It is argued that in-situ composites can be generated by a distributive mixing process without the formation of an intermediate droplet/matrix morphology as occurs in common dispersive blending equipment. Tensile modulus and strength of all blends increased with extrudate draw ratio as a result of increased molecular orientation of the TLCP phase. The level of reinforcement, however, was lower than expected, probably due to the low temperature of drawing. Annealing and capillary instability experiments showed that above the melting point of the TLCP the fiber/matrix morphology rapidly breaks up into a droplet/matrix morphology. This process takes just a few seconds for fibers of thickness ∼ 1 μm. It is shown to be the probable cause of the skin-core morphology obtained in case of 14 mixing elements.     

Surface Tension in Polymer Blends of Isotactic Poly(propylene) and Atactic Polystyrene

The surface tension of atactic polystyrene (PS), isotactic poly(propylene) (PP) and PS/PP‐blends, and additionally the interfacial tension between PP/PS have been measured in the temperature range between 200 and 280°C using the pendant drop method. Within the temperature range studied, the surface tension decreased linearly with increasing temperature for all systems whereas the surface tension of neat PP is approximately 7 mN/m smaller than the value of PS. The interfacial tension between PS and PP is in the range of approximately 4 mN/m and this indicates a strong incompatibility. It results a heterogeneous PP/PS blend morphology. A significant increase of the surface tension of the blends as a function of composition is observed only when the PS content exceeds 60 wt.‐%. Furthermore, microscopic observations indicate that even if the bulk matrix material is PS, a thin layer of PP can be detected by atomic force microscopy on the droplet surface used for surface tension measurements.     

Study of the Processing and Mechanical Properties of In Situ Composite Materials of Thermoplastics with Thermotropic Liquid Crystalline Polymers

By blending thermoplastics (TPs)—polycarbonate (PC) and polyethersulfone (PES)—with thermotropic liquid crystalline polymers (TLCPs)—KU9221 and KU9231—and then extruding the blends to form fibers, the in situ reinforcing characteristics were studied. The injection experiment of blends was compared with the extrusion experiment. According to the experimental results, in situ reinforcing characteristics of these processes were analyzed theoretically. These researches have come to some important conclusions. TLCP domains can be transformed to form fibers that are oriented in the direction of flow during processing; these TLCP microfibers result in improved mechanical properties of the TP/TLCP blends. The extruding flow is more effective in orienting TLCP domains and results in better in situ reinforcement than that of injection molding, and the extruded fibers have better mechanical properties. The mechanical properties of the blend fibers are improved greatly with increasing tensile ratio of melt drawing and the content of TLCPs.     

Sheet extrusion of microcomposites based on thermotropic liquid crystalline polymers and polypropylene

This work is concerned with the extrusion of sheets from pellets of polypropylene (PP) containing pregenerated microfibrils of thermotropic liquid crystal polymers (TLCPs), referred to as microcomposites. The TLCPs used were HX6000 and Vectra A950. The microcomposites are produced by drawing strands of PP and TLCPs generated by means of a novel mixing technique and pelletizing the strands. The work was undertaken in an effort to improve on the properties for in situ composites in which the TLCP fibrils are generated in contractions in the die and the subsequent drawing step. In situ composites usually exhibit highly anisotropic mechanical properties and the properties do not reflect the full reinforcing potential of the TLCP fibers. Factors affecting the mechanical properties of the composite sheets considered include the effect of in situ composite strand properties and TLCP concentration. In addition, the properties of the extruded sheets are compared to those of microcomposites processed by means of injection molding. It is shown that the sheets produced using microcomposites have a good balance between the machine and transverse direction properties (ratios of these properties ranging from 0.8 to 1.2) and those properties compare well to those obtained by processing microcomposites in injection molding. The tensile modulus of the composite sheets increases with increasing in situ composite strand modulus. The moduli of the 20 wt% Vectra A950 and HX6000 composites are about equal to the modulus of 20 wt% glass reinforced PP (about 2.1 GPa), while the tensile strength of the TLCP reinforced composites is 28% lower than that of the glass reinforced PP. Furthermore, it is shown that the tensile modulus of the 10 wt% TLCP composites approach the predictions of composite theory, while at 20 and 30 wt% TLCP negative deviations from the predictions of composite theory are seen. Finally, it is concluded that the properties of the sheets produced through the extrusion of microcomposites may be further improved by improving the modulus of in situ composite strands and reducing the TLCP fiber diameter.     

Characterization of thermotropic liquid crystalline polyester/polycarbonate blends: Miscibility,rheology, and free volume behavior

Miscibility, rheology, and free volume properties of blends of thermotropic liquid crystalline polymers (TLCPs) (Vectra A950) and polycarbonate (PC) are studied in this work. Despite the unusual increase in T_g of the PC phase, the blends are found to be generally immiscible. Transesterification may occur during blending and be the cause of the increase of T_g of the PC phase and the partial miscibility of the blends at high TLCP concentrations. With regard to the melt rheology of these materials, according to a three‐zone model, dynamic moduli of Vectra A950 show plateau‐ and transition‐zone behavior, while PC exhibits terminal‐zone behavior. The blends show only terminal‐zone behavior at low Vectra A950 contents (≤50%) and terminal‐ and plateau‐zone behavior at higher Vectra A950 contents. The relaxation time of Vectra A950 is much longer than PC and the blends have relaxation times greater than additivity. Both the complex and steady shear viscosities of the blends increase with the addition of Vectra A950. This is attributed to interfacial association, which retards the reorientation and alignment of the Vectra A950 phase in the molten state. The Cox–Merz rule holds true for PC but not for Vectra A950 and the blends. Free volume properties on an angstrom scale evaluated by positron annihilation lifetime spectroscopy (PALS) indicate that Vectra A950 has smaller, fewer free volume cavities than PC and the variation of free volume behavior in the blends can be explained in terms of blend miscibility. The measured densities of the blends agree well with the free volume fractions of the blends determined from PALS. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2319–2330, 2000     

Determination of interfacial tension between PA/PP and LCP/PP by IFR method and effect of compatibilization on interfacial tension

The dynamic imbedded fiber retraction (IFR) method was used to measure the interfacial tension between two molten polymers. The aim was to evaluate the applicability of this method for polypropylene (PP) used as matrix, and two polyamide (PA6, PA66) and thermotropic main-chain liquid crystalline polymer fibers. The effect on the interfacial properties of modifying the PP matrix with compatibilizers was studied as well. The IFR method was found to be suitable for evaluating the interfacial properties of these polymer blends. The measured interfacial tensions correlated well with the morphology and mechanical properties of the blends and values calculated from the harmonic mean equation. Although the measured interfacial tensions were generally lower than the theoretical ones, the order of the values for the different polymer pairs was similar. © 1995 John Wiley & Sons, Inc.     

Reinforcement of mechanical properties of a poly(ethylene terephthalate) and polycarbonate blend by the addition of thermotropic liquid crystalline polymer

Mechanical properties of the ternary blends of poly(ethylene terephthalate) (PET), polycarbonate (PC), and thermotropic liquid crystalline (TCLP, Vectra A950) were investigated. The ternary blends were prepared by varying the amount TLCP but fixing the ration of PET and PC. The fiber fallen freely through the capillary die had the highest initial modulus (1.46 GPa)/tensile strength (73 MPa) when 10% of TLCP was added. Above this TLCP content, however initial modulus and tensile strength decreased. The scanning electron microscope (SEM) micrographs of the TLCP phase which was extracted by dissolving PET/PC matrix from the blend showed the fine fibrils formed at 5 and 10% of TLCP, while the aggregated TLCP phases at 20 and 30% of TLCP. It was suggested that the decrease of the mechanical properties of the resulting blend was caused by the aggregation of TLCP phase above 10% of TLCP. A high draw ratio gave a rise to the formation of highly oriented fibrils of TLCP phase in the PET/PC matrix and the improvement of mechanical properties of the ternary blend.     

Properties of a thermotropic liquid crystalline polymer blended with different thermoplastics

Thermal, rheological, morphological, and mechanical properties of a thermotropic liquid crystalline polymer, TLCP (copolyester Vectra A-950 from Hoechst), blended with a polycarbonate (PC), a polyethylene glycol terephthalate (PETG), and a blend of PC and PETG (20/80) are presented and discussed. Important supercooling effects are observed for the TLCP. For the blends the glass transition temperature of the matrix is shown to decrease slightly, suggesting partial miscibility of the components. A finer dispersion is observed for the TLCP/PC blends, at least for TLCP concentrations lower than 20%, for which the mechanical properties are quite good. For higher TLCP concentrations, as well as for the other two matrices, the mechanical properties follow more or less the mixing rule, and the morphology of the blends suggests poor adhesion. We were unable to obtain fibrillar structures by extruding the blends through a capillary rheometer; in the TLCP/PC blends, the TLCP domains were too small, and for the other blends the extrudates had not enough melt strength.     

Thermal and mechanical properties of injection molded liquid crystalline polymer/amorphous polymer blends

Injection molded samples of binary blends of Vectra (LCP) and the three amorphous polymers polyethersulfone (PES), polycarbonate (PC), and aromatic poly(ester carbonate) (APEC) have been subjected to morphological and rheological characterization, and coefficients of linear thermal expansion and Young's moduli have been determined. The Young's modulus of the PES/LCP blends exhibited a near lower-bound behavior that could be predicted by the one-adjustable-parameter equations of Halpin-Tsai (ζ = 0.18) and Takayanaga (b = 0.23), whereas the coefficients of linear thermal expansion followed the Takayanaga equation with a value of b = 0.50. The chain orientation of the LCP component was essentially constant in all PES/LCP blends with a Herman's orientation parameter of 0.39 ± 0.03. Transesterification reactions led to randomization of the constituents of the PC/LCP and APEC/LCP blends. The effect was more pronounced in the PC/LCP blends. The introduction of the LCP into the PC/LCP blends led to no reduction in melt viscosity and no self-reinforcement. APEC/LCP exhibited self-reinforcement in blends with a content greater than 27 vol% LCP, and especially the blend with 67 vol% LCP. The self-reinforcement was caused by the presence of an oriented LCP phase, confirmed by X-ray diffraction, and by improved interfacial bonding, presumably resulting from the transesterification reactions occurring at the phase boundaries.     

Extrusion blow molding of microcomposites based on thermotropic liquid crystalline polymers and polypropylene

This work is concerned with the extrusion blow molding of bottles from pellets of polypropylene (PP) containing pregenerated microfibrils of thermotropic liquid crystal polymers (TLCPs), referred to as microcomposites. The TLCPs used are HX6000 and Vectra A950. The microcomposites are produced by drawing strands of PP and TLCPs generated by means of a novel mixing technique and pelletizing the strands. The work was undertaken in an effort to improve on the properties observed for in situ composites in which the TLCP fibrils are generated in elongational flow fields that occur in polymer processing operations and to determine if TLCP reinforced bottles could be produced by extrusion blow molding of microcomposites. In situ composites usually exhibit highly anisotropic mechanical properties and the properties do not reflect the full reinforcing potential of the TLCP fibers. Factors considered include the effect of TLCP concentration and in situ composite strand properties on the mechanical properties and anisotropy of bottles made from microcomposites. Specifically, strands having three different draw ratios are used to produce bottles at 10 and 20 wt% TLCP. Increasing the in situ composite strand modulus is shown to cause an increase in both the machine and transverse direction moduli of the composite bottles. The mechanical properties of the bottles increase with increasing TLCP composition. Finally, the machine and transverse direction properties are not balanced in the composite bottles produced in this study (degrees of anisotropy ranging from 1.5 to 1.8). The mechanical anisotropy is probably the result of a low blow up ratio (2) in the bottles and the TLCP fibers being oriented primarily in the machine direction due to the shear flow in the die.     

In situ composites based on blends of a polyetherimide and thermotropic liquid crystalline polymers subjected to shearfree deformations

The effects of shearfree elongational flows on the morphology and mechanical properties of blends of a polyetherimide (PEI) with thermotropic liquid crystalline polymers (TLCP) have been investigated. Extruded sheets and injection molded plaques of PEI/Vectra A and PEI/HX1000 blends, with a TLCP concentration of ≤30 wt%, were subjected to uniaxial elongation, planar and biaxial deformations at 240°C, above the glass transition temperature of the PEI, and at 265°C, which is below the melting point of the TLCPs. Experimental results revealed that each particular mode of shearfree deformation had a distinct effect on the morphology and properties of the blends. For instance, TLCP droplets were deformed into elongated fibrils by application of uniaxial elongation, deformed into elongated ribbon-like structures after planar deformation, and deformed into a disc-like shape by application of equibiaxial flow. Regarding mechanical properties, it was observed that the tensile modulus and strength of molded plaques of PEI/HX1000 80/20 wt% increased to about twice their initial values (from 5.13 to 10.40 GPa and from 105 to 198 MPa, respectively) after a strain of 0.75 was applied in a direction parallel to the initial direction of the TLCP fibers. In addition, samples exhibiting equal values of flow and transverse direction tensile modulus of ∼5.0 GPa were obtained when molded plaques of PEI/HX1000 80/20 wt% were subjected to planar stretching in a direction transverse to the initial direction of the fibers. Thus, by subjecting injection molded plaques to planar stretching, it was possible to obtain a sample exhibiting balanced flow and transverse direction mechanical properties and, consequently, reduced anisotropy.     

Modified interfacial tensions measured in situ in ternary polymer blends demonstrating partial wetting

An in situ Neumann triangle-focused ion beam-atomic force microscopy (NT-FIB-AFM) method has been used to measure modified PS/HDPE interfacial tensions in ternary PS/PP/HDPE blends prepared by melt mixing and demonstrating partial wetting. The ternary blend was modified with SEB, SB and SEBS copolymers. Results related to the position of the PS droplet at the interface show that a symmetrical diblock copolymer is somewhat more efficient in decreasing the interfacial tension compared to an asymmetrical one of similar molecular weight, while the SEBS triblock copolymer appears to have no effect at all. Using the NT-FIB-AFM method, the lowest modified PS/HDPE interfacial tension is 3.0 ± 0.4 mN/m for the symmetric diblock, compared to 4.2 ± 0.6 mN/m (N = 34) for the unmodified interface. This corresponds to an apparent areal density in SEB copolymer equal to 0.16 ± 0.03 molecules/nm², which is near reported saturation values. By varying the concentration of the copolymer, an emulsification curve reporting the value of the PS/HDPE modified interfacial tension as a function of the apparent areal density of the copolymer at the PS/HDPE interface has been obtained. The interfacial tension values obtained by the NT-FIB-AFM approach are significantly higher than the 0.5 ± 0.2 mN/m (N = 3) result obtained by using the classical breaking thread method with the same materials. This discrepancy does not appear to be due to a poor migration of the copolymer to the PS/HDPE interface, but could instead be attributed to the interfacial elasticity of the compatibilized interface, a phenomena that has not been accounted for so far in experimental studies on the morphology of compatibilized multicomponent polymer blends.     

Morphological effects on glass transition behavior in selected immiscible blends of amorphous and semicrystalline polymers

The effects uncompatibilized immiscible polymer blend compositions on the T_g of the amorphous polymer were studied in the systems polystyrene/polypropylene (PS/PP), polystyrene/high density polyethylene (PS/PE) and polycarbonate/high density polyethylene (PC/PE). In the two similar systems of PS/PP and PS/PE, the T_g of PS increased with decreasing PS percentage in the blends. This variation in glass transition is attributed to the polymer domain interactions resulting from the different morphologies of various blend compositions. Experiments were conducted to study these effects by preparing blends with various polymers that varied the relationship between the T_g of the amorphous polymer and the crystallization behavior of the semicrystalline polymer. Results show that the variation in amorphous component T_g with composition depends strongly on the physical state of the semicrystalline domains. Whereas the T_g of PS in PS/PE blends changed with composition, the T_g of PC in the PC/PE blend did not change with composition.     

Polycarbonate blends with polystyrene and polypropylene

Blends of polycarbonate/polystyrene (PC/PS), polycarbonate/polypropylene (PC/PP) and ternary blends of the three components (PC/PS/PP) were studied. Extrudate swell of the molten blends increased with increasing concentrations of the minor components and leveled off at characteristic blend compositions. These compositions corresponded to the limits of compatibility as judged by the onset of brittleness in tensile tests. Both PS and PP appear to have some limited practical compatibility with PC. The change in extrudate swell behavior with concentration may be a rapid and convenient test for the effective concentration limits of partially miscible polymers.     

Rheological properties of polystyrene and poly(methyl methacrylate) blends

Rheological properties of the polystyrene (PS) and poly(methyl methacrylate) (PMMA) blends were studied by Advanced Rheometric Expansion System (ARES). Storage modulus and loss modulus of the PS and PMMA blends were measured, and the interfacial tension of the PS and PMMA blends were obtained with various emulsion models by using the storage modulus and loss modulus of the blends. The value of interfacial tension estimated from the Palierne emulsion model was found to be 2.0 mN/m. Also, the interfacial tension between PS and PMMA was calculated by a theoretical model. The values of interfacial tension of the PS and PMMA blends obtained by the experiment and theoretical model were found to be in good agreement.     

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