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Structures and tensile properties of a magnetically and mechanically oriented liquid crystalline copolyester, Xydar.

INTRODUCTION

Controlling the orientation of liquid crystalline co-polyesters during processing could improve the physical and mechanical properties of the final product. Injection molding causes orientations through shear, but the possibility of controlling the orientations is limited. In some cases, skin-core structures are formed, leading to inhomogeneous orientations. On the other hand, it is well known that liquid crystalline copolyesters are susceptible to magnetic fields (1-4) because of the diamagnetic anisotropy of their backbone structures and to the propensity to form mesophases. Since the magnetic field is flexible in application (and penetrates the material) compared to shear, it could be an additional means of controlling the orientation. Though a stronger magnetic field is required for rapid buildup of orientation for the actual application, the use of high-field magnets (5) supports this situation.

A number of studies have been reported on the magnetic orientation of liquid crystalline copolyesters (6-11), the tensile properties of mechanically oriented liquid crystalline copolyesters (12-17), and liquid crystalline copolyester-based blends (18-22), but there are few studies reporting the structures and the physical properties of magnetically oriented liquid crystalline copolyesters (23-27). It is reported (23) that the magnetic field can achieve a more enhanced mechanical strength than the mechanical stretching, especially in the case of thicker samples, probably because of the homogeneous penetration of a magnetic field. We report on the tensile properties of magnetically oriented films of Xydar, a thermotropic liquid crystalline copolyester composed of p-hydroxybenzoic acid, terephthalic acid, and p,p[prime]-biphenol, in comparison with those of mechanically oriented films of similar thickness in order to elucidate the intrinsic tensile properties attained by means of magnetic fields. The difference in tensile properties is interpreted in terms of the structures observed by means of the X-ray diffraction, the scanning electron microscopy, and the polarizing microscopy.

EXPERIMENTAL

Materials and Sample Preparation

Low viscosity Xydar (SRT 900, Amoco Polymers, Inc.) (19, 28-32) used in this study was supplied by Nippon Petrochemicals Co. Ltd., in the form of powder, biaxially stretched films ([approximately]60-70 [[micro]meter]), uniaxially stretched films ([approximately]80-90 [[micro]meter]), and fibers, without fillers. The nominal melting point was 346 [degrees] C. The melt showed a nematic texture pattern. The powder was dried in vacuum at 100 [degrees] C for 10 h, hot-pressed at 360 [degrees] C for 5 min, and cooled at room temperature. The film thus obtained was of 90 to 110 [[micro]meter] in thickness and was used for the tensile test. The film exhibited a slight orientation that was probably attributable to the shear applied during the hot press. The powder was also hot-pressed at 320 [degrees] C for 5 min to obtain films for additional heat treatment in magnet. The film was about 150 to 170 [[micro]meter] in thickness and showed no orientation.

Magnetic Orientation

An Oxford super conducting magnet of 6T and an electromagnet (at the Institute for Materials Research at Tohoku University) generating 1 and 3T were used. The apparatus and the method used to prepare magnetically oriented samples are reported in the reference (27). Samples used for the tensile test were prepared by heat treating hot-pressed films at 380 [degrees] C for 5 min or 10 min under a 6T magnetic field followed by slow cooling ([approximately]40 min) in the magnet. The experimental errors of temperature were about [+ or -] 5 [degrees] C, and the error of the orientation degrees of the magnetically oriented samples was [+ or -] 0.02.

Microscope Observation

Texture patterns were observed by using an Olympus BHSP polarizing microscope equipped with a Linkam TH-600RMS hot stage at magnifications of 200x, 400x, 600x in the temperature range of 20 to 450 [degrees] C. A JEOL JSM-5410 was used for scanning electron microscopy (SEM) at magnifications of 1,000x, 2,000x, 5,000x, 10,000x, and 20,000x.

Wide Angle X-ray Measurements

Wide angle X-ray diffraction (WAXD) measurements were carried out by using a MAC Science MXP system operating at 40kV and 150mA to generate Ni-filtered Cu[K.sub.[Alpha]] X-ray beam. Azimuthal scans were carried out at 20 = 19.58 [degrees] to determine the orientation degrees (OD) by using a relation, OD = (180 [degrees] - [H.sub.w])/180 [degrees], where [H.sub.w] is the half-width of the peak.

Tensile Test

Tensile measurements were carried out by using a newly developed tensile test machine, a TMC-500L of Tokyo Koki Co. Ltd. (33), equipped with stress relief chucks and a video microscope monitoring the fracture surface, at a strain rate of 2 mm/min. The temperatures used in the experiments were 23, 80, 150, 200, and 250 [degrees] C. Six to ten experiments were carried out on strips of 80 x 3 mm cut from films and the average was made. The tension direction coincided with the orientation direction of films except for the biaxially oriented film for which the tension direction was taken between two orientation directions. The reproducibility was within 10 to 15% for mechanically stretched films and 20 to 25% for magnetically oriented films.

Thermal Analysis

A thermal mechanical analysis was conducted on a Seiko TMA/SS analyzer at room temperature at a strain rate of 100 [[micro]meter]/min to estimate the elastic modulus of a monofilament.

RESULTS AND DISCUSSION

Time evolution of magnetic orientation at temperatures of 380 and 390 [degrees] C under 1, 3, and 6T is shown in Fig. 1. The rate of orientation seems to increase with the temperature and the strength of magnetic field though there are not enough data points for 1 and 3T. This tendency is reasonable because the rate of orientation is proportional to the inverse of the viscosity and the square of the field strength (8). Similar trends were observed for the magnetic orientation of other liquid crystalline polymers (10). Though the rate of orientation depends on the temperature and the magnetic field strength, the rapid increase in orientation degree (OD) is noted in the first 10 min. The maximum OD is higher for the higher temperature and for the higher magnetic field strengths. The maximum OD values are compared with the value of 0.82, which was obtained for the uniaxial mechanically oriented film. The temperature of the heat treatment in magnets was limited to the range used in this experiment because of a low rate of orientation at lower temperatures and because of the thermal degradation at elevated temperatures.

The tensile tests were carried out for five different samples: the magnetically oriented films with OD = 0.77 and 0.83, the uniaxially stretched film (OD = 0.82), the biaxially stretched film (OD = 0.64), and the hot-pressed film (OD = 0.65). The stress-strain curves are shown in Fig. 2. We see that the ultimate tensile strength of the magnetically oriented films is about a half of that of the uniaxially stretched and hot-pressed films. The curves for the magnetically oriented films are located between those for the uniaxially stretched film and the hot-pressed film though the magnetically oriented film of a lower OD (0.77) is closer to the hot-pressed film. The biaxially stretched films exhibit a significantly large value of elongation at break, while its ultimate tensile strength is comparable to that for the magnetically oriented films.

Figures 3a, b, c show the dependence of the tensile properties on the orientation degree measured at room temperature. Although the available data points are limited, we could say that the elastic modulus and the ultimate tensile strength of the magnetically oriented samples are about half of those of the mechanically stretched samples. Thermal degradation occurring during a prolonged heat treatment in the magnet, noted by discoloration of the sample, could reduce the tensile properties to some degree. The difference in the elastic moduli, however, is not regarded very large in comparison with the elastic modulus of the monofilament sample (OD = 0.95), which is 57 GPa. On the other hand, the magnetically oriented films and the uniaxially stretched film show similar values of the elongation at break.

Temperature dependence of the tensile properties of the uniaxially stretched film, the blaxially stretched film, the pressed film, and the magnetically oriented film (OD = 0.83) are shown in Figs. 4a, b, c. The elastic modulus and the ultimate tensile strength decreased as the temperature was increased in each sample. The elongation at break shows a decrease with increasing temperature, followed by a slight increase, which is probably attributable to a partial melting of the samples at temperatures above 200 [degrees] C.

Fracture surfaces of the uniaxially stretched film taken after the tensile test at different temperatures are shown in Fig. 5a. For most samples, the rupture runs perpendicular to the stretching direction and the fracture surfaces are straight. In the magnetically oriented films of the same orientation degree, on the other hand, the rupture runs inclined against the orientation direction and the fracture surfaces exhibit zigzag patterns in some samples [ILLUSTRATION FOR FIGURE 5B OMITTED]. The zigzag patterns and the rupture direction vary from sample to sample. These observations indicate that the structure of the uniaxially stretched film is uniformly oriented over the scale of several millimeters, but it is not the case for the magnetically oriented film.

Figure 6 is a microscopic photograph of an unoriented sample melt at 390 [degrees] C taken under crossed polars. A typical texture of nematic liquid crystals with threads are observed. Microscopic photographs of the mechanically and magnetically aligned structures of the same orientation degree (thickness of [approximately]15 [[micro]meter]) are displayed in Figs. 7a, b. The observation without cross polars clearly shows a band-like structure developed in the uniaxially stretched film, which is in contrast to that reported for other liquid crystalline polymer systems after the cessation of shear (34-38). The band width is of the order of 50 [[micro]meter] scale, beyond which no significant inhomogeneity of transmitting light is seen. The observation under cross polars with the polarizer parallel to the orientation direction gives a uniformly dark view, indicating a uniform orientation. However, the magnetically oriented film displays an inhomogeneous texture. The observation under cross polars indicates that the bright regions are attributable to the deviation of the director orientation off from the magnetic field. A wider survey under cross polars shows that the bright and dark regions alternate on the scale of sub-millimeter.

Figures 8a and b are SEM observations of an internal alignment of the uniaxially stretched film and the magnetically oriented film of the same orientation degree. The photographs were taken after the skin layer was removed. Both of them exhibit highly elongated fibrils of similar size, which are comparable to those observed in the Xydar fiber used in this study.

Figure 9 shows the X-ray diffraction of the powder sample. Less significant peaks around 2[Theta] = 22 [degrees] and 28 [degrees] indicate a low crystallinity. No significant difference in crystallinity was observed among the investigated samples irrespective of the processes that the samples experienced.

The main equatorial peak around 2[Theta] = 19.6 [degrees] shifts slightly depending on the samples. Figure 10 plots the mean interchain spacing against the orientation degree measured at room temperature. For the film samples, the values of the mean spacing are almost the same irrespective of the orientation degree or the orientation method. However, the mean spacing for the biaxially oriented film is significantly large and that for the fiber is significantly small. In Fig. 11, the elastic moduli of the investigated samples are plotted against the mean interchain spacing. A dense packing observed in the fiber sample could be related to its high elastic modulus. The slight difference in the mean spacing observed between the magnetically and mechanically oriented films, however, does not explain the difference in tensile properties.

CONCLUSION

A random thermotropic liquid crystalline copolyester, one from Xydar series, was aligned in the magnetic field of 1, 3, and 6T. The tensile properties of the thin aligned films were investigated in relation to those of the mechanically oriented films of similar thickness in order to assess the intrinsic tensile properties attained by means of magnetic fields. The elastic modulus of the magnetically oriented films was lower than that of the mechanically oriented films of the same orientation degree, but it was comparable to or even higher than the elastic moduli of the mechanically oriented films of lower orientation degrees. The ultimate tensile strengths of the magnetically oriented films were lower than those of the mechanically oriented films in this study. However, we could regard the tensile properties attained by magnetic fields as being in an acceptable level for actual applications: the tensile properties attained by mechanical methods are usually lower compared to the intrinsic tensile properties in the case of thicker sheets or molds because of insufficient alignment inside the specimen, while the tensile properties attained by means of magnetic fields are expected to be as high as the intrinsic ones even for thicker specimens owing to the penetration of magnetic fields. Structure analyses ranging from a macroscopic order to a microscopic order, carried out by means of optical microscope observations, SEM observations, and X-ray diffraction show that the inhomogeneity including a non-uniform orientation of sub-millimeter scale is a reason for the inferior tensile properties of the magnetically oriented samples. These structural inhomogeneities could be arising from disclinations inherent in liquid crystalline systems in addition to wall or loop structures appearing under the application of magnetic fields. Usage of higher magnetic fields could reduce these inhomogeneities.

ACKNOWLEDGMENT

The authors would like to thank Nippon Petrochemicals Co. Ltd., for supplying the copolyester samples and allowing to use the polarizing microscope and the hot-stage; the Institute for Materials Research at Tohoku University for the assistance in operating the electromagnet; and JEOL for the assistance in SEM operation. This work was partially supported by a Grant-in-Aid for the 1992 Special Research Project and by a Grant-in-aid for the 1993 Special Research Project both from Tokyo Metropolitan University.

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Author:Kossikhina, S.; Kimura, T.; Ito, E.; Kawahara, M.
Publication:Polymer Engineering and Science
Date:Feb 1, 1997
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