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Effect of compatibilizers on mechanical properties and morphology of in-situ composite film of thermotropic liquid crystalline polymer/polypropylene.

INTRODUCTION

The modification of polymers through the blending of a thermotropic liquid crystalline polymer (TLCP) with thermoplastics (TP) providing superior rheological and mechanical properties of the composite has drawn considerable attention (1-4). The processing of an incompatible TLCP/TP blend under an elongational flow condition is known to produce an oriented TLCP-fiber phase. Hence, the term "in-situ composite" was coined (5) for this type of polyblend, which means self-reinforcement due to the fibers formed during processing.

Blends of Hoechst Celanese Vectra A900 TLCP and SEBS (styrene-ethylene butylene-styrene) thermoplastic elastomer were intensively studied by De Boer et al. (6-8). They reported the formation of TLCP fibers in an almost pure shear flow condition, which contradicted previous works suggesting that the fibers only form in an elongational flow condition.

In-situ composites produced mostly by fiber spinning and injection molding have a higher modulus than sheet or film (9) because of a fibrillar structure that can be obtained more effectively by elongational force in the spinning process. In-situ composite film has only recently gained much interest for applications such as high-strength balloons (10). However, the main problem in the TLCP blend system in such an application has been due to the high degree of anisotropy of the mechanical properties, i.e., the properties along the machine direction (MD) are different from those along the transverse direction (TD). Chinsirikul et al. (11) attempted to reduce the anisotropy using a counter-rotating die in film extrusion. Another approach to improve the properties was investigated by Datta et al. (12) through the addition of a compatibilizer. In their work, a blend of polypropylene (PP) and TLCP (Vectra A) with a maleic anhydride-grafted polypropylene (MA-PP) added as a compatibilizer was produced as an extruded strand and injection-molded sheet. An increase of about 25% in modulus was found in injected tensile bars containing about 30% TLCP. A similar investigation by O'Donnell (13) using MA-PP as a compatibilizer in a PP/Rodrun LC3000 (70/30 weight ratio) system also reported about a 30% increase in the modulus of the blend. It was concluded that the added compatibilizer helped produce more finely dispersed TLCP fibrils and consequently improved the tensile strength and modulus.

The objective of the present work is to improve the mechanical properties of in-situ composite film based on TLCP/PP using block and grafted copolymers as compatibilizers at various concentrations. Cast films were characterized in morphological and mechanical aspects.

EXPERIMENTAL

Materials

The thermoplastic polymer matrix used in this study was an injection grade polypropylene (PP6331) with a melt flow rate (MFR) of 12g/10 min (230 [degrees] C, 2.16 kg load). A thermotropic liquid crystalline polymer was a copolyester comprising 60 mol% of p-hydroxybenzoic acid and 40 mol% of poly(ethylene terephthalate) (Rodrun LC3000) purchased from the Unitika Company. The crystal-nematic and nematic-isotropic transitions of LC3000 are 220 [degrees] C and 280 [degrees] C, respectively. A triblock thermoplastic elastomer of styrene ethylene butylene styrene (SEBS, styrene/rubber ratio 29/71, Kraton G1652) and maleic anhydride-grafted SEBS (Kraton FG1901X, containing 1.8 wt% maleic anhydride) was provided by Shell Chemical Co. Maleic anhydride-grafted polypropylene (MA-PP), containing about 0.1 wt% maleic anhydride, was provided by the Mitsubishi Co.. The materials were vacuum dried at 60 [degrees] C for 12 h before use.

Blending

Melt blending of PP and 10 wt% TLCP was performed using a co-rotating twin screw extruder (PRISM TSE-16TC) with a screw diameter of 16 mm, L/D = 25, intermeshing, at an extrusion rate of 150 rpm. The processing temperature profile was 180/220/220/225/225 [degrees] C (14), representing temperatures at the hopper zone, the three barrel zones, and the heating zone in the die head, respectively. The strand exiting the extruder was immediately quenched in a water bath and subsequently was pelletized.

Extrusion Film Casting

TLCP/PP blend pellets were extruded using a 16mm mini-extruder (Randcastle RCP-0625) equipped with a cast film line. The temperature profile was 190/220/230/240 [degrees] C for the hopper zone, two barrel zones, and slit-die, respectively. The screw speed was 70 rpm. The gap of the die lip was adjusted at 0.65 mm and the width fixed at 152 mm. Extruded film was drawn downward as a molten blend exiting the die outlet and then quenched on a water-cooled roll. The draw ratio (slit width-to-film thickness ratio) was controlled by adjusting the take-off speed. The highest draw ratio used in this experiment was about 33. The film thickness was varied from 20 to 70 [[micro]meter].

Mechanical Testing

Tensile testing was conducted using an Instron mechanical tester (Model 4301) with a grip length of 25 mm, a crosshead speed of 50 mm/min, and a full-scale load of 10N. Tensile properties of the dumbbell-shaped specimens (70 mm x 4 mm) were measured in the flow (machine) and transverse directions (ASTM D412). Data were taken and averaged from at least ten specimens for each blending system.

Impact testing was performed using a pneumatic driving impact tester Radmana ITR-2000 at a constant temperature and humidity (ASTM D3763). The test films were about 70 [[micro]meter] thick. The results are averaged values of at least ten measurements for each blending system.

Morphology

The distribution of TLCP fibrils in the PP matrix was directly observed under an optical microscope at a magnification of 100-400 times. In order to inspect the size and shape of the fibers more clearly, composite films were extracted in boiling xylene and the remaining fibers were dried before observation. The observation of the fractured surface of the composite films was performed using a scanning electron microscope (SEM, Hitachi 82500) operated at 15 kV. Fractured surfaces were prepared by fracturing the composite film in liquid nitrogen. Palladium film was coated on the specimens using a Hitachi E102 ion sputtering coater.

Order Parameter

The order parameter or orientation function (S) (1517) defined as the degree of alignment of liquid crystal molecules with a preferred direction, was determined from the infrared dichroic ratio, R = [A.sub.[parallel]]/[A.sub.[perpendicular to], where [A.sub.[parallel]] and [A.sub.[perpendicular to]] are absorbance values for plane polarized light with the electric vector parallel and perpendicular to the preferred direction, respectively. For a band whose transition moment is parallel to the major molecular axis, S = (R - 1)/(R + 2). The IR absorption spectra of composite films about 25 [[micro]meter] thick were recorded using a Perkin-Elmer FTIR (System 2000) with an aluminum wire-grid polarizer placed between the sample and the light source. The polarization directions of the polarizer were adjusted parallel and perpendicular to the machine direction of the film. Each spectrum was collected in a transmission mode at a resolution of 4 [cm.sup.-1] and 25 scans. An area under the peak at 1601.5 [cm.sup.-1], (C-C stretching vibration of para-substituted benzene ring of p-hydroxybenzoic acid) corresponding to the parallel transition moment was used to determine the order parameter of the TLCP.

RESULTS AND DISCUSSION

Mechanical Properties

Young's moduli of TLCP/PP/compatibilizer films with varied amounts of compatibilizer are shown in Fig. 1. Young's moduli of pure polypropylene film produced under the same processing conditions (not shown) were also determined and compared: 616 [+ or -] 66 MPa and 586 [+ or -] 44 MPa in machine direction (MD) and transverse direction (TD), respectively. The film evidently exhibited a slight anisotropy in its moduli. The addition of 10 wt% TLCP resulted in an increase in the modulus in MD by almost twice as much. The modulus in TD, however, increased slightly. It is evident that the composite film exhibited a high degree of anisotropy due to the preferred fiber orientation in the composite film. The effect of a compatibilizer on the film modulus, especially in MD, varies by different extents depending on the type and amount of compatibilizer. SEBS improved the film modulus in MD to the greatest extent with a peak value at 3 wt% SEBS. The improvement was about 46% over that without the compatibilizer (1,592 MPa vs. 1,091 MPa). The modulus decreases with an increasing SEBS content above 3 wt%. However, at 8 wt% SEBS the modulus is still slightly higher than without SEBS. Films with added MA-SEBS exhibited a similar effect as those with SEBS but to a lesser extent. A peak in modulus was found at 1.5 wt% MA-SEBS with a 21% improvement over that with no compatibilizer. On the other hand, MA-PP did not appear to have a significant effect on the film modulus.

The effect of a compatibilizer on the film modulus in TD is less pronounced than that found in MD. It appears that Young's modulus in TD was not significantly affected by the type and content of the compatibilizer, with an exception of an improvement of about 28% for film with 1.5 wt% SEBS.

The yield stress of composite films will now be discussed. Figure 2 shows the yield stress of composite films containing various amounts of compatibilizer. For comparison, yield stress values of pure PP film were 20.2 [+ or -] 1.4 and 15.4 [+ or -] 1.5 in MD and TD, respectively (not shown in the Figure). The composite films were found to have a slightly higher yield stress than PP in both directions. A slight improvement of the yield stress in MD was found in the composite film with SEBS. In other cases, it was found that yield stress in both MD and TD decreases with an increasing amount of compatibilizer. In all cases, the yield stress in MD is higher than that in TD.

From the mechanical results presented above, it is rather surprising that such a soft elastomer such as SEBS improved the modulus of the composite films more than MA-SEBS, which in turn improved it more than MA-PP. This contradicts the expectation that the presence of a reactive MA group in the latter two compatibilizers could form a chemical or hydrogen bond with TLCP, which consequently, should improve the interfacial adhesion between the two phases. Such bonding between an MA group, and TLCP domains was probably formed during processing as previously reported by O'Donnell (13) and Seo (18). However, this kind of bonding may retard the extension of the TLCP fibers by elongational flow. Hence, using MA-SEBS may give rise to thicker TLCP fibers flower fiber aspect ratio) than those obtained in the case of SEBS. Since the modulus is measured at low strain, the difference in modulus would indicate a difference in the structure of the composite film, i,e., the size and aspect ratio of fiber formed in the film. The better properties of composite with the elastomeric compatibilizer may be due to the effect of a compatibilizer on the viscosity of the system. In order to support this assumption we measured the melt flow rate (MFR) (using 2.16 kg force at 230 [degrees] C) of these blend systems, It was found that MFR reduces from 19.5g/10 min for the composite without a compatibilizer to 12.5, 13.2, and 17.6g/10 min for the blends containing 3 wt% SEBS, 1.5 wt% MA-SEBS, and 3 wt% MA-PP, respectively. A further addition of the compatibilizers showed no further significant change in MFR [ILLUSTRATION FOR FIGURE 3 OMITTED]. The results reveal that SEBS and MA-SEBS increase the viscosity of the blend to a much greater extent than MA-PP, and hence, aid the formation of TLCP fibers. The effect of the viscosity ratio of LC3000/PP blends on their morphology was investigated by Heino et al.(14). They reported that the most fibrous structure was achieved when the viscosity ratio ranged from about 0.5 to 1. At the lower viscosity ratio the fiber structure was coarser, while at viscosity ratio above unity, the TLCP domains tended to be spherical. Similarly, in our work, the addition of SEBS to the TLCP/PP blend may affect the matrix viscosity to help form the fibrous structure of TLCP. The evidence from the morphology study will be discussed later.

A drop in the Young's modulus of the composite film at a high SEBS content will now be discussed. Since SEBS is a triblock copolymer with a styrene block at both ends and an ethylene/butylene block in the middle, it would be expected that the two ends containing aromatic rings would be compatible with the TLCP phase, while the rubbery EB block would be compatible with the PP matrix. Accordingly, SEBS should be present at the interface to promote interfacial adhesion and to help disperse the TLCP phase. At a high content, however, the amount of compatibilizer at the interface is likely to exceed the saturation limit (critical micelle concentration) and phase separation of the compatibilizer may take place (19, 20), resulting in an overall decrease in the properties of the blend. A further increase in the SEBS content lowers the Young's modulus of the composite due to the soft nature of the added rubber. However, even at 8 wt% SEBS, the composite film still shows a higher modulus than the film without a compatibilizer. This lowering effect at a high SEBS concentration was in agreement with the decrease in the shear modulus of PP on blending with SEBS (no reinforcing fibers), as reported by Gupta et al. (21).

The addition of MA-PP to the composite film shows only a slight increase in the modulus in MD. Although it is likely that the compatibilizer helps disperse the TLCP phase, it appears to be less effective than MA-SEBS. This may be due the lower concentration of the MA-group in MA-PP than in MA-SEBS and because MA-PP does not affect the viscosity of the blend [ILLUSTRATION FOR FIGURE 3 OMITTED].

Morphology

Figure 4 shows optical micrographs of composite films with: a) no compatibilizer, b) 3 wt% SEBS, c) 1.5 wt% MA-SEBS, and d) 3 wt% MA-PP. At these concentrations of compatibilizer, a maximum value of tensile modulus is observed in each system. It is evident that the number of fibers per unit area as well as the aspect ratio of the TLCP fiber increase with the addition of a compatibilizer. The incorporation of a compatibilizer results in a more finely dispersed TLCP phase, and hence, more fibers are formed under shear and elongational forces. The addition of a compatibilizer, therefore, has a similar effect to increasing the fiber loading, giving rise to the enhancement of the modulus. This is in good agreement with the results reported by O'Donnell (13), In addition to the difference in shape of the dispersed TLCP fibers, the birefringence characterizing the frozen nematic phase of TLCP domains also differs. Without a compatibilizer, the TLCP phase exhibits a birefringence less homogeneous than that of the systems containing a compatibilizer, suggesting a better molecular orientation of the TLCP domains in the latter case. The result is supported by the increase in the order parameter of the TLCP phase, to be discussed later. To study the size and shape of fibers more clearly, composite films were extracted using boiling xylene, a good solvent for PP but a nonsolvent for TLCP. Figure 5 shows photomicrographs of TLCP fibers extracted from films, taken at the same magnification. Fibers extracted from films containing compatibilizers [ILLUSTRATION FOR FIGURES 5B-D OMITTED] appear to be thinner and longer than those obtained from films with no compatibilizer [ILLUSTRATION FOR FIGURE 5A OMITTED]. Among the three compatibilizers, SEBS yields the thinnest, and hence, the highest fiber aspect ratio, resulting in the highest Young's modulus discussed above.

Figure 6 shows SEM micrographs of the fractured surface of composite blends, broken along the direction normal to the flow direction. The film with no compatibilizer [ILLUSTRATION FOR FIGURE 6A OMITTED] shows a number of TLCP-fiber pullouts with a smooth surface, suggesting a poor fiber-matrix interfacial adhesion. In the case of a composite with a SEBS compatibilizer [ILLUSTRATION FOR FIGURE 6B OMITTED], different features are observed: fiber breakage, fiber surface roughness, and fiber-tip bending. Similar features are also observed in the case of using an MA-SEBS compatibilizer [ILLUSTRATION FOR FIGURE 6C OMITTED]. However, in the presence of MA-PP [ILLUSTRATION FOR FIGURE 6D OMITTED], the fiber surface is rather smooth with spike fiber tips and less fiber pull-out than in Fig. 6a, suggesting a better adhesion in the compatibilized composite. The appearance of fiber surface roughness in composites with an elastomeric compatibilizer may arise from an uneven extension of the TLCP phase caused by nonuniform friction due to the elastomer partly adhering at the interface. Such surface roughness was recently reported by Seo (22) for the ternary blend systems, nylon/TLCP/MA-EPDM, and PBT/TLCP/MA-EPDM. Though with higher viscosity of TLCP than the matrix, fibrillation of TLCP could be obtained at a low shear rate. Seo proposed the mechanism for the existing surface roughness: That it is a result of the relaxation of the elastomer surrounding the elongated TLCP phase.

Order Parameter

For thin film specimens, it is rather simple to determine the molecular orientation from the anisotropy of the absorption spectra (dichroism), especially in the infrared region. Since the selected peak used to measure the dichroic ratio belongs to the benzene ring in the TLCP molecule, the calculated order parameter, therefore, represents only the order in the TLCP phase. The order parameter of TLCP in the composite films containing compatibilizers is shown in Fig. 7. The films with about 1.5 wt% SEBS and with 1.5 wt% MA-SEBS have an increase of order parameter in the TLCP phase from about 0.5 to 0.6 and are roughly unchanged beyond this value. In the case of MA-PP, the less pronounced increase in order parameter is observed. This is in good agreement with the increase in modulus of the composite films at a low percentage of compatibilizer. Better molecular orientation in the TLCP phase translates to better mechanical properties of TLCP (16, 23), and hence, to a better composite.

Impact Strength

Results obtained from the impact testing of TLCP/PP composites without and with compatibilizers are illustrated in Fig. 8. Break energy in J/m is plotted against the concentration of compatibilizer in percentage by weight. The composite without a compatibilizer has a very low impact strength (318 J/m) compared with that of pure PP (1,840 J/m). The results suggest poor interfacial adhesion between TLCP and the matrix, which is always a problem with in-situ composites. This results show an improvement of impact strength when an elastomeric compatibilizer was added. At a low level of compatibilizer concentration, e.g., up to about 5 wt% SEBS and MA-SEBS, the impact property improves slightly with an increasing amount of compatibilizer. With an 8 wt% compatibilizer, the impact strength increases steeply to about 1,300 J/m, approximately a four-fold increase. A similar result was observed by Gupta et al. (21, 24) and Setz et al. (25) for a PP/SEBS system with no fiber reinforcement. It is concluded that the addition of elastomeric compatibilizers not only improves the tensile modulus but also the impact strength of the composites. This large improvement of impact property at a high concentration of the elastomer may be due to its action as an impact modifier, the mechanism of which was explained in the established rubber-toughening theory (26). Evidently, MA-PP does not show any effect on the impact strength of the composite, since it is a thermoplastic in nature and is unable to absorb impact energy like the rubbery SEBS and MA-SEBS can.

CONCLUSIONS

All three compatibilizers used in this study, namely, SEBS, MA-SEBS, and MA-PP, were found to improve the dispersion and interfacial adhesion of the TLCP phase (Rodrun LC3000) and PP matrix, giving rise to the enhancement of the tensile modulus. In this study, thermoplastic elastomers, SEBS and MASEBS, were found to be more effective as compatibilizers than MA-PP. The addition of an elastomeric compatibilizer enhanced the viscosity of the system as evident in the decrease in its melt flow rate, which in turn affected the elongational flow of the dispersed phase to form thinner (and hence, higher aspect ratio) TLCP fibers. Surprisingly, SEBS was found to be a much more effective compatibilizer than MA-SEBS, despite the presence of an MA reactive group that could have improved the interfacial adhesion in the latter. An additional advantage of using an elastomeric compatibilizer is that it could also improve the impact strength of the in-situ composite due to its action as an impact modifier.

ACKNOWLEDGMENT

Support of this work by The Thailand Research Fund is gratefully acknowledged. The authors also would like to thank the Shell Chemical Co. for providing SEBS and MA-SEBS, and the Mitsubishi Company for providing MA-PP.

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Author:Bualek-Limcharoen, Sauvarop; Samran, Jareerat; Amornsakchai, Taweechai; Meesiri, Wiriya
Publication:Polymer Engineering and Science
Date:Feb 1, 1999
Words:3852
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