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

INTRODUCTION

Growing industrial activities create a continual demand for improved materials that satisfy more stringent requirements, such as high tensile strength and modulus. These requirements, which often involve a combination of many difficult-to-attain properties, can often be satisfied by utilizing a composite material whose constituents act synergistically to solve the needs of the application. As technology advances, it becomes increasingly important to understand the properties, performance, costs, and the potential of the available composite material. These materials are unique engineering materials, which may be tailored by controlling constituents and microstructure to meet desired stiffness and strength requirements. There has been a considerable interest in liquid crystalline polymers (LCPs), especially after the development of high modulus and high strength fibers prepared from the lyotropic aromatic polyamides (aramids) by DuPont (1, 2). Thermotropic liquid crystalline polymers (TLCPs) are attractive because of their potential application as ultra-high strength fibers and molded articles. The blends of LCPs with other engineering plastics to enhance the mechanical properties are also of interest (3). The ease of processing combined with high performance characteristics makes the blend composed of thermotropic LCP and engineering plastics very attractive (3, 4). By blending thermotropic main-chain LCPs with engineering thermoplastics, the highly ordered fibrous structure and good properties of LCPs can be transferred into a more flexible matrix polymer. LCPs are blended with thermoplastics mainly to reinforce the matrix polymer or to improve its dimensional stability. A small amount of a thermotropic LCP can also make some thermoplastics easier to process because of its relatively low melt viscosity.

Several researchers have studied blends of thermotropic LCPs and thermoplastics during recent years. Commercial or experimental LCPs have been blended with many thermoplastics, e.g., poly(butylene terephthalate) (PBT) (3, 5), poly(ethylene terephthalate) (PET) (6-10), and polycarbonate (PC) (5, 1115). The blends have been processed by extrusion, melt spinning, injection molding, and compression molding.

In this paper we focus on twin-screw extruder blended and injection molded blends of a polyester-type thermotropic LCP with poly(ethylene 2.6-naphthalate). PBT, PET, and PC are matrix polymers that have been widely studied previously. But PEN is a matrix polymer that has not been studied before. A matrix polymer of poly(ethylene 2,6-naphthalate) (PEN) was manufactured on a pilot scale by Kolon Ltd. We have investigated the mechanical and morphological characteristics of these blends and the observed mechanical properties were correlated to the morphology observed.

EXPERIMENTAL

The matrix polymer of poly(ethylene 2.6-naphthalate) (PEN) was KP 785 (I.V = 0.51: inherent viscosity of a solution in phenol/o-dichlorobenzene at 25 [degrees] C) manufactured by Kolon Ltd, The reinforced polymer of LCP was Vectra A950 manufactured by Hoechst Celanese Corp.

Materials were carefully dried at 120 [degrees] C under vacuum for at least 8 h, then blended by using a twin-screw extruder (Automatic ZCM 32/36) at 285 [degrees] C and at fixed rotor speed at 240 rpm, immediately quenched in a water bath, and finally pelletized. The blends were carefully redried at 120 [degrees] C under vacuum for at least 8 h. The contents of LCP in the blends were varied from 1 to 100 wt%. Molding was performed by using an Engel Es-1 injection machine and an appropriate mold producing ASTM testing specimens. The blend was injected at 290 [degrees] C (above the melting temperature of the LCP) [TABULAR DATA FOR TABLE 1 OMITTED] into a mold, which was kept at a constant temperature of 70 [degrees] C. Test specimens for tensile and flexural measurements were prepared with dog-bone (ASTM D638) and flexural bars (ASTM D790). The mechanical properties of the ASTM specimens were determined with an Instron model 1123 tensile tester operated at a 5 mm/min cross-head speed and a gauge length of 70 mm for dumbbell specimens at room temperature. The average values of at least ten measurements were taken. To study the morphology of the blends, the tensile specimens were quenched in liquid nitrogen and fractured at cryogenic temperature. These fractured surfaces were coated with gold palladium for the microscopy and observed with a JEOL JSM-35CF scanning electron microscope. Wide-angle X-ray diffraction (WAXD) patterns of LCP, PEN, and their blends were obtained in the reflection mode using a Rigaku Geigerflex X-ray diffractometer having a Ni-filtered Cu-Ka radiation (37.5 KV, 17.5 mA). The density measurements were made by using a Toyoseiki autodensimeter. The samples for the density measurements were taken from ASTM specimens. Dynamic mechanical measurements were carried out at a frequency of 110 HZ using a TOYO model DDV-II Rheovibron viscoelastometer. The sample dimensions used in all these measurements were 0.368 x 0.320 x 3.00 cm. All samples were subjected a temperature sweep from 25 [degrees] C to 200 [degrees] C under a heating rate of 3 [degrees] C/min.

RESULTS AND DISCUSSION

Mechanical Properties

The blends with various compositions were injected molded to investigate the potential of the LCP/PEN blends as in-situ composites. Table 1 and Figs. 1 through 4 show the mechanical properties of the LCP/PEN blends. Figure 1 shows tensile strength as a function of LCP content for the blends along with the calculated lines according to the rule of mixtures. As shown in Fig. 1, tensile strength was found to increase with increasing LCP content. It was found that the LCP acts as a reinforcing agent for the PEN matrix. DSC studies have also shown that LCP and PEN are incompatible. The constant glass transition temperature (Tg) corresponding to PEN was shown in the DSC scans for the blends. Tensile strength was improved over the rule of mixtures at high LCP contents (above 30 wt%). It is believed that an in-situ composite was developed successfully. Typically, for the blends containing more than 60 wt% LCP, tensile strength exceeds that of pure LCP. A layered structure (skin-core) was observed in the LCP/PEN blends. This may be explained by the high orientation in the outer layer, due to a fountain flow phenomenon (16). High LCP content moldings showed a skin layer of substantial thickness, believed to be responsible for the increased load bearing capacity. Weng et al. (22) have proposed a hierarchical structure resulting from complex flow patterns of pure LCP. They observed maximum molecular orientation in the skin layer. The observed skin morphology is likely to agree with the model proposed for the pure LCP by Weng (22). Below 10 wt% LCP, the core layer dominated because the skin layer was limited, but the skin-core structure began to develop in these blends from 10 wt% LCP. Especially in the LCP/PEN 70/30 blend, the skin layer was fully developed and the core portion was reduced considerably. At this LCP content, the skin layer consisted of nearly continuous LCP fibrils oriented in the flow direction with a large aspect ratio in the PEN matrix. It was found that the tensile strength was increased and a synergistic effect was observed in the LCP/PEN 70/30 blends. In fact, the highest tensile strength obtained was 201 MPa in the LCP/PEN 70/30 blend and that was twice that of PEN homopolymer and exceeded that of pure LCP. This is due to the development of a highly oriented skin layer with long and continuous LCP domains well distributed in the PEN matrix. This skin morphology will be substantiated by morphological studies presented below.

To observe the effect of the degree of orientation of the LCP phase on the mechanical properties of the 70 wt% LCP blend and the parent pure LCP, the orientation angle and the degree of orientation of the LCP phase in the 70 wt% LCP blend and the pure LCP were measured by the penetration method using a Rigaku Geiger X-ray diffractometer having a Ni-filtered Cu-Ka radiation (50 kv, 180 mA). The orientation angle of the LCP phase in the 70 wt% LCP blend was lower than that of the pure LCP (44 [degrees] in the 70 wt% LCP blend and 53 [degrees] in the pure LCP), whereas the degree of orientation of the LCP phase in the 70 wt% LCP blend was higher than that of the pure LCP (76% in the 70 wt% LCP blend and 70% in the pure LCP). These results could be accounted for mainly by higher orientation due to the fountain flow phenomenon. These results are consistent with the synergy in mechanical properties.

Figure 2 shows the elongation at break as a function of LCP content for the LCP/PEN blends. The elongation at break decreased significantly with increasing LCP content. This means that the addition of LCP made the matrix stiffer and more brittle. Such behavior is a characteristic of all fiber reinforced composites. Figure 3 shows the flexural strength of the LCP/PEN blends. The flexural strength increased with increasing LCP content. This behavior is characteristic of polymeric composites. The flexural strength was improved over the rule of mixtures at entire LCP contents. It was found that a synergistic in-situ composite was developed successfully. In fact, the highest flexural strength (174 MPa) was obtained in a blend with 70 wt% LCP, exceeding that of pure LCP. It was found that LCP might act as a reinforcing component.

Figure 4 shows the flexural modulus as a function of LCP content of the blends along with the calculated lines, according to the rule of mixtures. As seen in Fig. 4, flexural modulus increased with increasing LCP content. The data of the flexural modulus obtained at all LCP contents are seen to lie above the line corresponding to the rule of mixtures. In particular, the highest flexural modulus (6.5 GPa) was obtained in a blend with 70 wt% LCP and was twice that of PEN homopolymer and exceeded that of pure LCP. This result is consistent with results of Joseph et al. (8), who studied injection molded blends of PHB/PET copolyester and PET. They found that the bending modulus of PET was increased with increasing LCP content, and the bending modulus of an LCP/PET 50/50 blend was tripled over that of pure PET. Our results agree with his, although a different type of LCP and matrix were used. Skin-core morphology was found to develop in injection molding samples that result in a remarkable improvement in the mechanical properties. This phenomenon was characteristic of the pure LCP or its blends. Apparently, the orientation at the skin layer was caused by the elongational flow in the advancing front, while orientation in the core layer was related to shear flow (16).

Figure 5 shows the flexural modulus and storage modulus as a function of LCP content for the injection molded specimens of the blends. As shown in Fig. 5, the flexural and storage moduli of injection molded LCP/PEN blends were found to increase with increasing LCP content. At low LCP contents, the results followed those predicted by the rule of mixtures. Above 10 wt% LCP, the values obtained exceeded the predicted ones. The flexural and storage moduli of the LCP/PEN 70/30 blend were more than twice those of the pure PEN matrix. This increase of stiffness can be related to the good orientation and fibril formation of the LCP phases in the PEN matrix, which can be seen in the SEM micrographs in Figs. 8 through 13. Figure 5 shows a good correlation between the curves of flexural and storage moduli. It was found that the mechanical properties were increased with increasing LCP content, and synergistic effects were observed in the LCP/PEN blends. Figure 6 shows wide angle X-ray diffraction patterns of LCP, PEN, and the LCP/PEN blends. The diffractograms of PEN were indeed typical, while those of LCP consisted of the broad peaks. The diffractograms of the LCP/PEN blends were increased in their intensity and sharpness with increasing LCP content. It was found that the LCP might act as a reinforcing agent in the blends (20). This is due to dispersion of LCP in a separate phase of the parent polymer structure (17). Figure 7 shows the specimen density of the LCP/PEN blends. The density of the LCP/PEN blends increased with increasing LCP content. It was found that the addition of LCP increased the density with a synergistic effect seen at 70 wt% LCP. In fact, the density of the LCP/PEN 70/30 blend exceeded that of the pure LCP, owing to higher crystallinity. To observe the effect of the heat of fusion on crystallinity of the LCP blend and the parent pure LCP, the heat of fusion of the LCP phase in the 70 wt% LCP blend and the pure LCP were measured by a Perkin Elmer DSC-7 thermal analyzer. The heat of fusion ([Delta][h.sub.f]) of the 70 wt% LCP blend was significantly higher than that of the pure LCP (10 J/g in the 70 wt% LCP blend and 0.4 J/g in the pure LCP). These results are consistent with the synergy in density and crystallinity of LCP/PEN blends at 70 wt% LCP.

Morphology

Morphological studies of the LCP/PEN blends were undertaken to correlate the mechanical properties with the morphology of these blends and to study the formation of LCP fibrils in the PEN matrix. It is widely accepted that the morphology depends mainly on the theological and interfacial properties, the blending conditions, and the volume fraction of the components (3, 21). In this experiment, the morphology of the blends was investigated by scanning electron microscopy (SEM).

Skin-core morphology was found to develop in the LCP/PEN blends. This was substantiated by the SEM micrographs. Figures 8 and 9 show the core region of these blends, while Figs. 10 and 11 show the skin region. Figures 8 and 9 show the scanning electron micrographs of the fractured surfaces at the core of the injection molded bar of the blends. Figures 8 and 9, show that the morphology of the blends is different with increasing LCP content. At lower LCP contents, the LCP ellipsoidal particles are finely dispersed in the PEN matrix, while a distinctive fibril formation can be seen above 30 wt% of LCP content (16-19). But the adhesion of these fibrils with the PEN matrix was poor, as shown in Fig. 8b and Fig. 9a. This was substantiated by the fact that black rings could often be seen in these micrographs. The micrograph with 70 wt% of LCP content shows long and uniform LCP fibrils with a large aspect ratio. The uniformity and orientation of these LCP fibrils increased with increasing LCP content. Figure 9a shows the fractured surface of the 50 wt% LCP blend. It showed a forest of LCP fibrils distributed in the PEN matrix and showed no thin and long LCP fibrils. Some of these LCP fibrils have clustered together to form fibril bundles. The fractured surface of the 70 wt% LCP blend showed a totally different morphology. Here the fibrils are thin and long. They are not short and stubby as seen in Fig. 9b. The micrograph of the 70 wt% LCP blend shows numerous long and uniform LCP fibrils that have a large aspect ratio. It may be because of this that synergisti c effects in mechanical properties [ILLUSTRATION FOR FIGURES 1-4 OMITTED] are observed for these composites. Such a morphology may explain why the mechanical properties were improved tremendously above 30 wt% LCP content for these blends.

Figures 10 and 11 show the scanning electron micrographs of the fractured surfaces at the skin region of the blends. As seen in Figs. 10 and 11, the skin region shows a totally different morphology. Here the fibrils seem to be well oriented. The fibrils are thin and long. The LCP/PEN 70/30 blend shows numerous highly oriented LCP fibrils with a large aspect ratio.

Figures 12 and 13 are scanning electron micrographs of the fractured surfaces of the LCP/PEN blends. The samples were fractured along the flow direction in liquid nitrogen. Figure 12 shows the core region of these blends, while Fig. 13 shows the skin region. From Figs. 12 and 13 it was found that a layered-like structure, a so-called hierarchical structure, results from complex flows during the mold filling process (22). Similar morphology was observed in all the blends. It was found that the extent of skin-core development is highly dependent on the LCP contents. The region of the skin-core transition was clearly seen in this micrographs. The fibril orientation in skin layers is high, and the random orientation in core layer is shown in Figs. 12 and 13. The skin region shows long and continuous fibrils well oriented to the flow direction, while the core region shows short and randomly oriented fibrils. This means that the orientation at the skin layer was caused by elongational flow in the advancing front, while the orientation in the core was related to shear flow. It was found that the spherical particles of LCP were deformed into fibrillar structures in the skin layer. As shown in Fig. 13, LCP fibrils well oriented in the flow direction were observed, In the case of 70 wt% LCP, the fibril formation became more apparent, and the fibrils were perfect with a large aspect ratio and a large variation in diameter owing to the coalescence of the dispersed phases. This is the reason that synergistic effects in mechanical properties [ILLUSTRATION FOR FIGURES 1-4 OMITTED] were observed in these blends. These morphological results are in fair agreement with the results of studies of the mechanical properties.

CONCLUSIONS

Mechanical and morphological characteristics of the LCP/PEN blends have been studied. Blends were prepared in a twin-screw extruder. Specimens for mechanical testing were prepared by injection molding. Even though these blends were incompatible and the interfacial adhesion between the two polymers was poor, they were found to be industrially important. Inclusion of LCP results in a tremendous improvement in mechanical properties. In studies of mechanical properties of the blends, tensile strength and flexural modulus improved with increasing LCP content and synergistic effects were observed at 70 wt% LCP content. In fact, the tensile strength and flexural modulus of the LCP/PEN 70/30 blend were found to be 201 MPa and 6.5 GPa, respectively, which is twice those of the PEN matrix and greater than those of pure LCP. The studies of the morphology of the LCP/PEN blends revealed that the LCP domains were changed into rod-like and fibrillar structures from ellipsoidal particles with increasing LCP content, and the interfacial adhesion between the two polymers was poor. The morphology of the blends was found to be affected by their compositions, and a distinct skin-core morphology was found to develop in the injection molded samples of these blends. This is believed to have caused a synergistic effect on the mechanical properties of the LCP/PEN blends and resulted in the formation of self-reinforced composites. In these studies, mechanical properties and morphology of the LCP/PEN blends, were well correlated.

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Author:Jang, Sang Hee; Kim, Bong Shik
Publication:Polymer Engineering and Science
Date:Mar 1, 1995
Words:3467
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