Structural Changes of Gel Drawn, Ultra-High Molecular Weight Polyethylene Fibers With Kerosene as a Solvent.
High performance, ultra-high molecular weight polyethylene (UHMW-PE) fiber, which is a light and tough material with a high tensile strength, between 2 and 7 GPa, and a high Young's modulus, above 100 Gpa, has been commercially produced [1, 2]. This fiber is usually made of PE, having an average molecular weight of above 1 X [10.sup.6]. The appearance of this fiber upsets the traditional idea that fibers having high tenacity and a high modulus have to be prepared from fibrous polymers with a rigid chain structure. Unlike the wholly aromatic polyamide or polyarylate macromolecules with very high rigidities, linear PE is a typical macromolecule with a soft chain structure, only having atoms of C and H on its main chain and usually in a random coil in the state of melt or solution, which easily entangles within or among molecules. The fibers prepared by a regular forming technology have a folded chain structure in which the parts of molecules with order arrangement to form crystalline regions in the fiber are li nked by means of a few tie-molecules between the crystalline regions. Therefore, the real tensile strength and tensile modulus are far lower than the values estimated according to the C-C covalent bond energy. Gel spinning enables polymer molecules with very high molecular weight to disentangle sufficiently during the dissolving process and the disentangling state can be kept as-spun by quenching. When the as-spun state undergoes ultra hot drawing, the disentangled fold chain structure is easily transformed into an extended one. Since the extended chain structure has fewer molecular terminals and a higher orientation and crystallinity, the fibers possess better mechanical properties than those formed by a regular spinning technology.
In a previous paper , we reported that an UHMW-PE fiber with such excellent properties was prepared by means of a gel fiber drawing method, using general kerosene as the solvent and gasoline as the extraction solvent. The primary results of studies on the structure and properties of the fibers showed that the sonic velocity orientation factor and the degree of crystallinity of the fiber increased slowly when the draw ratio was over 30, and the tensile strength and tensile modulus kept increasing with a further increasing draw ratio. Moreover, the morphology of macromolecular chains was changed from the folded state to an extended chain structure with the increase of the draw ratio. in addition, the crystal form of the fiber also changed during drawing process. These changes were more evident when the draw ratio exceeded about 20.
The crystal form of PE belongs to orthorhombic phase in general, but other crystalline modifications can also appear in PE fibers under special conditions. For example, during the axial compression of gel drawn UHMW-PE film, accompanied by the formation of the kink bands at angles of 90[degrees] and 75-80[degrees] to the fiber axis, mainly due to a (100) [less than]001[greater than] and (110) [less than]001[greater than] slip, the monoclinic phase also formed . Similar phenomena were also observed in the deformation of isotropic polyethylene [5-7]. Other examples are that the orthorhombic phase of PE tends to transform into a hexagonal or pseudo-hexagonal phase under high temperatures or high pressures [8-10]. Such transformations were also observed in the cases of surface-grown fibers at a temperature of above 150[degrees]C under a constrained state [11, 12], in drawn gel fibers  and in drawn samples of UHMW-PE reactor powder extrusions above the static melting temperature . In addition, Smook et al. attributed the equatorial reflection near [2.sup.0] = 19.3[degrees] possibly to a small amount of triclinic phase in the UHMW- PE filaments produced by the gel spinning and hot drawing at a drawing temperature of 148[degrees]C . It is thus clear that PE is of not only high flexibility and strong crystallinity, but also has several crystalline modifications, which depend on the process of spinning formation or the processing conditions.
Our attention was focused on the morphology of a gel-drawn UHMW-PE fiber using a general kerosene and gasoline as the solvent and extraction solvent, respectively. It was found that with an increasing draw ratio, the transformation of a folded chain structure into an extended chain structure, an increase in the density of the fiber and the orthorhombic crystallite size moving towards a narrow distribution in the fiber occurred simultaneously. The orthorhombic unit cell dimensions of the fiber with a draw ratio of 40 were a = 0.732 nm, b = 0.491 nm, c = 0.254 nm. In addition, kink bands were observed because of the effect of winding tension and were seen as more evident with a smaller winding roller diameter. It was suggested that local crystalline slip plays an important role in the formation of kink bands at angles of 75-80 [degrees] to the fiber axis.
Preparation of UHMW-PE Fibers
The fibers used in this study were obtained by dissolving 5% by weight of UHMW-PE ([M.sub.w] = 3 x [10.sup.6], Beijing Auxiliaries Plant No. 2, China) in kerosene (boiling point 160-180[degrees] C into which 0.3% by weight of antioxidant was added. Gel spinning of the UHMWPE was carried out according to the previous method . The kerosene out of the as-spun by phase separation occurred after spinning was first removed. The residual solvent in the gel as spun was then extracted with gasoline (boiling point 120[degrees]C). The gel as-spun thus obtained was dried at the room temperature and then drawn. Because one-step drawing cannot easily reach a draw ratio high enough, a two-step drawing process was employed. Dried gel as-spun were first drawn to a ratio of 1-10 at [less than or equal to]100[degrees]C. The first-stage drawn fibers were then subjected to s second-stage drawing to various ratios above 10 at [less than or equal to]1480[degrees]C on a self-made mini-drawing apparatus. The draw ratio at each of the two steps is defined as the ratio of the velocity of the drawing roller to that of the feeding roller, and the total draw ratio as the product of the two. Following mentioned always means the total draw ratio except specified.
Wide-angle X-ray diffraction( WAXD) traces were made at transmission mode with a Rigaku 2080 model X-ray diffractometer at 40kV and 150 mA, and nickel-filtered Cu-[[Ku.sup.[alpha]] radiation was used as the X-ray source. WAXD patterns were obtained using a Toshiba X-ray generator (XC-40H) equipped with a flat-plate camera. Microscopic observations were carried out with an Olympus polarizing microscope (POM). Densities ([delta]) of the fibers were measured by means of a density gradient column with heptan and carbon tetrachloride.
RESULTS AND DISCUSSION
In order to decrease the degree of chain entanglement, a pseudo-dilute spinning solution was prepared and then spun and quenched to form gel as-spun. in the as-spun thus obtained, molecules stay in a disentangling state and have a proper long-range order along the direction of the fiber axis.
The WAXD patterns of UHMW-PE fibers with various draw ratios are shown in Fig 1. Figure 1a is the characteristic of gel as-spun. At first glance, it looks like an amorphous halo, but considering the results of Figs. 3 and 5, it can be seen that this apparent halo is in fact the superimposition of the true amorphous halo of the (110) and (200) crystalline reflections. This result indicates that when the UHMW-PE gel-spinning solution under a disentanglement state flows through the spun nozzle, the orientation by shear flow is a little more stable than that of general chemical fibers. In other words, the effect of the disorientation is relatively weaker as the spun streamlet gets away from the spun nozzle, so evidence of orientation is observed even in the as spun. When draw was initiated, the amorphous halo rapidly decreased in intensity and concentrated on the equator. The orthorhombic (110) and (200) reflection arcs appeared at [2.sup.0] = 21.9[degrees] and [2.sup.0] = 24.0[degrees] transformed to the sharpe st spots. In the WAXD patterns of the fibers with a higher draw ratio, (210), (020), and (310) reflections and those on the layer line, (011), (111), (201) and (211) could be observed. In addition, even for the fiber with a draw ratio of 5, the reflection at [2.sup.0] = 19.5[degrees] inside the strongest reflection (110) can be observed as well. Investigations of UHMW-PE reactor powder extrusion drawn to various draw ratios showed that the weak hexagonal (100) reflection at about [2.sup.[theta]] - 20[degrees] presented a significantly broad arc due to the very poorly oriented hexagonal crystals before drawing and disappeared gradually through drawing at [less than or equal to]150[degrees]C because the transition from the hexagonal to the orthorhombic phase occurred under such a drawing . But in this study, the reflection at [2.sup.[theta]] = [19.5[degrees]] had a tendency to increase in intensity instead during the drawing process, so this reflection is closer to that of monoclinic crystalline phase.
Without considering the reflection near [2.sup.[theta]] = 19.5[degrees], according to the (200) and (020) reflections on the equator, the dimensions of the unit cell a-axes and b-axes could be estimated. From the first layer line reflection on the patterns, the fiber period, that is the length of the unit-cell c-axis, could be estimated . The results are shown in Table 1. It is evident that the length of the a-axis and b-axis and the value of a/b tend to decrease with the increase of the draw ratio, but the c-axis changes little, and the density of the fiber keeps increasing linearly with a draw ratio up to about 30 (Fig. 2). These results are quite consistent with those of our previous work , which revealed that with an increasing draw ratio, the crystallinity of the fiber increases and transformations of the folded chain structure to an extended chain structure and orthorhombic to monoclinic crystallinity occur. It may thus be concluded that the increase of the density of the fiber with a draw ratio is attributed to the increase of the density of the crystalline phase in the fiber and the increase of the global degree of crystallinity.
The meridional X-ray diffractograms of fiber samples are shown in Fig. 3. It is evident that even the dried as-spun gel is of a certain crystallinity because of the strong crystallinity of PE. This kind of crystalline structure is a folded chain lamella, and the orientation is extremely weak in the direction of the fiber axis. The difference of the X-ray diffraction profile on the equator with that on the meridian is slight. When drawing takes place, the folded chain lamella transforms into an extended chain structure, for which the most evident characteristics are the gradually disappearing of the (hk0) reflection on the meridian and the apparent increase in intensity of the (002) reflection near [2.sup.[theta]] = 74.8[degrees]. In the diffractogram of the fiber with a draw ratio of 40, the (hk0) reflection could not be observed any more. The degree of orientation of the macromolecules in the fiber was estimated by using the ratio of the intensity of the meridional (002) reflection [I.sub.(002)] to t he sum of the intensities of the equatorial  reflection [I.sub.(110)] and the meridional (002) reflection [I.sub.(002)], i.e., [I.sub.(002)]/[[I.sub.(002)] + [I.sub.(110)]], and it was defined as the meridional orientation parameter ([F.sub.MOP]). The results thus obtained are shown in Fig. 4. It can be seen that the [F.sub.MOP] of low drawn fiber samples was relatively small but increased very rapidly with the draw ratio. It thus can be seen that the macromolecules in the gel as-spun are basically in a disentangled state just as they were in the spinning solution, but they fold or crimp along the fiber axis. Even drawn to a low draw ratio, their long-range order can be considerably improved. With a further increasing draw ratio ([greater than]20), the degree of orientation tends to increase slowly.
By measuring the azimuthal intensity distribution of the meridional (002) reflection, i.e., I([phi]), the macromolecular orientation function [f.sub.c] can be determined by the following formula (15):
[f.sub.c] = 3 [less than] [cos.cup.2] [phi] [greater than] - 1 / 2 (1)
where [less than] [cos.sup.2] [phi] [greater than] = [[[integral of].sup.[pi]/2].sub.0] I([phi])sin[phi][cos.sup.2][phi]d[phi] / [[[integral of].sup.[pi]/2].sub.0] I([phi]) sin [phi]d[phi] (2)
The results thus obtained are shown in Fig. 5, which presents the changes of the degree of crystalline orientation with the draw ratio. From Fig. 5 it can be seen that the degree of crystalline orientation essentially increases with the increase of the draw ratio.
From the half-width values and the peak positions the crystallite dimensions in the fibers could be estimated from the well-known Scherrer equation :
[D.sub.hkl] = [lambda].(180/[pi]) / [([H.sup.2] - 0.01).sup.1/2].cos[theta] (3)
where [lambda] = 0.15418 nm is the wave-length of the X-rays and 2 is the Bragg-angle in the corresponding (hkl) reflection. [D.sub.hkl] is the size of a crystallite perpendicular to its diffracting plane and H represents the half-width of the corresponding (khl) reflection in the diffractogram. To correct for the instrument broadening of the diffractometer, the experimentally determined factor 0.01  has been inserted in the Scherrer equation. The results thus calculated show that the dimensions of the crystallite in the fiber increase with the increase of the draw ratio. From this line broadening analysis of the (002) reflection in the fiber of drawn 40, the average crystallite length of the c-axis was estimated to be about 50 nm and from [D.sub.200] the lateral width, 20 nm. Investigations by Smook et al. on the UHMW-PE fiber by gel spinning and hot drawing to 70 with paraffin oil as the solvent show that the corresponding values are about 70 and 20 nm, respectively. It is evident that with the increase of the draw ratio, the folded chains absorb energy to transform into extended chains with a higher three-dimensional order by melting-rearrangement-recrystallizing. During this process the crystallinity and the dimensions of the crystallite in the fiber have also increased. It should be pointed out that because the assessment of crystallite dimensions with the Scherrer equation is based on the assumption that the line broadening is entirely determined by the dimensions of the crystallites and does not take into account the possibility of line broadening being caused by the presence of lattice distortions such as internal defects, the magnitude of the crystallite dimensions obtained in this way have to be regarded as only first approximations.
Figure 6 shows the POM photographs of two kinds of fibers. One is of a draw ratio of 5 (Figs. 6a and b) and the other is of a draw ratio of 40 (Figs. 6c and d). Under the POM diagonal position it was clearly observed that a certain number of discontinuous fine grooves randomly distributed along the fiber axis in the low drawn sample. As the draw ratio increased, these fine grooves gradually disappeared. When the sample was placed in the orthogonal position (Fig. 6b), there was no difference with Fig. 6a in nature, which suggests that the degrees of crystallization and orientation of the fiber drawn to such an extent are not high enough to enable the fiber to exhibit the characteristics of polarized anisotropic materials. It is well known that the original morphology in the porous asspun fiber consists of large lamella interconnected by several fibrils. Upon hot drawing of this filament the original morphology will gradually convert into a shish-kebab structure at a relatively low draw ratio (R[less than]10) and eventually into a smooth fibrillar structure in the fully drawn fiber with a draw ratio of [greater than]30 [tilde]40. For highly drawn fibers, as shown in Fig. 6c, a sample with a draw ratio of 40, observed was a smooth appearance morphology and they have a considerably increased compactness in their structure. It was more interesting that kink bands were observed at angles of 75[tilde]80[degree] to the draw axis and became most intense in the highly drawn samples. Figure 6d is the result from the corresponding orthogonal position. A bright field was observed even in the orthogonal position, which revealed that the three-dimensional order of macromolecules in the kink bands was damaged to a high extent.
A banded structure that developed during shear-deformed crystallization of thermotropic and lyotropic liquid crystalline polymers has been studied by many [19-23]. The structure can be observed under a POM as an image with a periodic alternation of dark and bright bands perpendicular to the direction of shear. Such a banded structure develops due to the constructional strain of shear-deformed liquid crystalline polymers. An elongated liquid crystalline polymer system is unstable and it tends to recover the original textures that correspond to a minimum of free energy distortion, which may be the origin of the contraction of the elongated liquid crystals. As a typical lyotropic liquid crystalline polymer, poly-p-phenylene terephthalamide (PPTA) can be spun from a liquid crystalline solution. On basis of electron microscopic studies, Dodd et al. proposed a so-called pleated sheet structure model for PPTA fibers [24,25]. In the polarized optical micrograph of PPTA fiber, Simmens et al.  observed transverse bands, which were identified with the pleated structure, with a 250[tilde]300 nm spacing at an angle of 90[degrees] to the fiber axis and interpreted them as the optical diffraction images of the banded structure of PITA fiber. Whereas poly(vinyl alcohol)(PVA) is one of the flexible polymers and on gel-drawn PVA fibers a series of bands with [tilde]1 [micro]m spacing were found to form at angles of 75[tilde]90[degrees] to the fiber axis (27). From stress-strain curves during drawing-contraction cycles it was suggested that the formation of the banded structure can also be due to contraction, which is induced during stress relaxation after the cessation of elongational stress. Based on Orowan's equation , the formation of band angles of [tilde]75[degrees] to the fiber axis is mainly due to the (101)[less than]010[greater than]slip. However, the banded structure observed in this study. as shown in Fig. 6, has a much longer spacing than PPTA or PVA but is quite close to that of liquid crystalline, wholly aromatic copolyester fibers at a low compressional deformation .
As opposed to a rigid polymer, the extended chain structure of UHMW-PE, a typical flexible polymer, is transformed from a folded structure under certain conditions, which leads to various chain morphological structures and various mechanisms of deformation for the same flexible polymer. For example, under a compression load, extruded linear PE with a folded chain structure yields a series of kink bands at an angle of about 50[degrees] to the extrusion direction, whereas the gel-drawn UHMW-PE fiber with an extended chain structure has a band angle of 75[degrees] or 90[degrees]. For the formation of the banded structure, the former is attributed to the slips between the macromolecules in the amorphous region and the latter to the slips of certain crystal planes [4, 30]. Our present work shows that banded structure is not observed in the fiber samples with a low draw ratio or those unwound when drawn. It tends to be weakened with an increase of the winding roller diameter. It thus seems reasonable to assume tha t the formation of the banded structure in this case is due to the contraction strain at the inside of the fiber being greater than at the outside when the highly elongated fiber is bent. This induces a greater stress relaxation at the inside of the fiber than at the outside, as shown in Fig. 7. The more the fiber is bent, the greater the difference of the contraction strain between the two sides of the fiber, and therefore the more apparent the banded structure formed. A similar kink band structure can also be observed in geldrawn UHMW-PE fiber with decalin as the solvent, but the density of the bands in the fiber is much less.
According to Orowan's equation, in the crystalline phase of UHMW-PE fiber the (110)[less than]001[greater than] slip in the orthorhombic crystallite plays the most important role in the formation of the kink bands with angles of 75[tilde]80[degrees]. Considering the fact that the (110) planes orient nearly perpendicularly to the fiber surface, it seems reasonable to assume that the kink bands thus formed are oblique to the fiber surface. It therefore can be concluded that the (1l0)[less than]001[greater than] slip plays an important role in the formation of the kink bands in the highly drawn UHMW-PE fiber with an extended chain structure. It should be pointed out that a few of the defects in the micro-structure of the high-drawn fiber samples, i.e., distractions in the orientation of the macromolecules in the banded structure, are not big enough to affect the overall structure (Figs. 3 and 5) and mechanical properties of the fibers .
Changes in the morphology structure of gel-spun UHMW-PE fiber with kerosene as the solvent during drawing process were studied. From the above results and discussion, these conclusions were made:
1. As draw ratio increases, the folded macromolecular chains transform into extended ones. At the same time, in the orthorhombic crystallite block in the fibers, the unit cell a- and b-axes decrease, but the c-axis hardly changes and the dimensions of the crystallite tend to increase. For a fiber with a draw ratio of 40, the unit cell parameters are a = 0.732 nm, b = 0.491 nm and c = 0.244 nm.
2. The degree of macromolecular orientation in the meridian can be depicted as the ratio of the intensity of meridional (002) reflection to the sum of the intensities of the (002) and (110) reflections, that is, [F.sub.MOP] = [I.sub.(002)]/[I.sub.(002)] + [I.sub.(110)]]. The results thus obtained are identical with those obtained from the orientation function method.
3. In the fiber with a draw ratio of 40, kink bands at angles of 75[tilde]80[degrees] to the fiber axis yielded due to the local crystalline slip which was induced by the contraction strain when the fiber was bended during drawing process and tended to be more evident with the increasing draw ratio and the decreasing winding roller diameter.
4. In terms of this study, any effect of kink bands in the high drawn fiber on the orientation and clystallization of the fibers as a whole, as well as on its mechanical properties, was not observed.
The authors gratefully acknowledge Prof. T. Takahashi from the Fukui University, Japan, for helpful discussions and assistance in the POM experiments.
(1.) P. J. Lemstra and H. C. Booij, J. Polym. Sci., Polym. Phys. Edn., 19, 877 (1981).
(2.) C. F. Xiao, Y. F. Zhang, S. L .An, and G. X. Jia, J. AppL Polym. Sci., 59, 931 (1996).
(3.) T. Takahashi, K. Suzuki, and K. Sakurai, J. Macromot Sci., Phys. Edn. (B), 29, 101 (1990].
(5.) P. B. Bowden and R J. Young, J. Mater. Sci., 9, 2034 (1974).
(6.) L. Lin and A. S. Argon, J. Mater. Sci., 29. 294 (1994).
(7.) F. B. Michael, M. D. Athene, B. Wim, R. M. Geoffrey, E. D. Gareth, and R Anthony, Macromolecules, 28, 6383 (1995).
(8.) D. C. Bassett and B. Turner, Nature (Phys. Sci.), 240, 146 (1972).
(9.) D. C. Bassett, S. Block, and C. J. Piermarini, J. AppL Phys. 45, 4146 (1974).
(10.) H. Tanaka and T. Takemura, J. Polym., 12, 355 (1980).
(11.) A. J. Pennings and A. Zwijnenburg, J. Polym. Sci., Polym. Phys. Ed., 17, 1011 (1979).
(12.) P. J. Lemstra, N. A. J. M. Vanaerle, and C. W. M. Bastiaansen, J. Polym., 19, 85 (1987).
(13.) N. A. J. M. Vanaerle, P. J. Lemstra, and A. W. M. Braam, Polym. Commun, 30, 7 (1989).
(14.) H. Uehara, T. Kanamoto, and S. Muurukami, Macromolecules. 29, 1540 (1996).
(15.) J. Smook and J. Pennings. Colloid Polym. Sci., 262, 712 (1984).
(16.) S. Kavesh and J. M. Schultz, J. Polym. Sci., (A-2)8, 243 (1970).
(17.) A. Zwijnenburg, P. F. Vanhutten, A. J. Pennings, and H. D. Chanzy, Colloid Polym. Sci., 256, 729 (1978).
(18.) H. M. Heuvel, R. Huisman, and K. C. B. J. Lind, Polym. Sci., Polym. Phys. Ed., 14, 921 (1976).
(19.) G. Kiss and R. S. Porter, Mol. Cryst Liq. Cryst, 60, 267 (1980).
(20.) J. Takahashi, K. Shibata. S. Nomura, and M. Kurokawa, Sen-i Gakkaishi 38, T-375 (1982).
(21.) A. M. Donald, C. Viney, and H. Windle, Polymer, 24, 155 (1983).
(22.) Y. Nishio, T. Yamane, and T. Takahashi, J. Polym. Sci., Polym. Phys. Edn., 23, 1053 (1985).
(23.) P. Navard and A. E. Zacharides, J. Polym. Sci., Polym. Phys. Edn., 25, 1089 (1987).
(24.) M. G. Dodd, D. J. Johnson, and B. P. Aville, J. Polym. Set., Polym. Phys. Edn., 15, 2201 (1977).
(25.) M. Panar, P. Avakiem, R. C. Blume, K. H. Gardner, T. D. Gierke, and H. H. Yang, J. Polym. Sci., Polym. Phys. Edit., 21, 1955 (1983).
(26.) S. C. Simmens and J. W. S. Hearle, J. Polym. Sci., Polym. Phys. Edn., 18, 871 (1980).
(27.) T. Takahashi, K. Suzuki, T. Aoki, and K. Sakursi, J. Macromol. Sci., Phys., (B)30. 101 (1991).
(28.) E. Orowan, Nature, 149, 643 (1942).
(29.) T. Takahashi, C. F. Xiao, and K. Sakurai, Sen-I Gakkaishi, 47, 397 (1991).
(30.) K. Shigematsu, K. Imada, and M. Takayanagi, Polym. Sci., Polym. Phys. Edn., 13, 71 (1975).
Drawing Ratio Dependencies of Lattice Parameters and Specific Volume of UHMW-PE Fibers. Draw Ratio a-axis (nm) b-axis (nm) c-axis (nm) a/b As-spun 0.749 0.499 0.254 1.501 5 0.748 0.498 0.255 1.502 10 0.747 0.497 0.253 1.503 15 0.742 0.497 0.253 1.493 20 0.740 0.495 0.254 1.495 25 0.739 0.495 0.253 1.493 30 0.737 0.494 0.253 1.492 35 0.734 0.492 0.254 1.492 40 0.732 0.491 0.254 1.491 Draw Ratio Specific Volume ([cm.sup.3]/g) As-spun 1.033 5 1.029 10 1.024 15 1.022 20 1.018 25 1.016 30 1.014 35 1.013 40 1.011
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|Author:||XIAO, C. F.; ZHANG, Y. F.; AN, S. L.; JIA, G. X.|
|Publication:||Polymer Engineering and Science|
|Article Type:||Statistical Data Included|
|Date:||Jan 1, 2000|
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