Crystal structure and orientation behavior of transversely compressed poly(ethylene-co-1-octene) filaments.

INTRODUCTION

Polyethylene has become an important commercial thermoplastic, since its low density form (LDPE) was first produced in the 1930s by ICI (1). A high density linear form (HDPE) was synthesized in the 1950s by Ziegler et al. (2-4) and by Phillips Petroleum researchers (5-8). Ziegler catalyst-based copolymers known as linear low density polycthylene (LLDPE) were introduced in the 1970s (9-11). Recently, new types of polyethylene have also become commercially available through the application of improved catalysts, notably metallocene catalysts. These produced ethylene copolymers with narrow molecular weight distribution and homogeneous comonomer distribution (12-14). In 1991, Exxon Chemical first commercially made metallocene polyethylene (15). Exxpol[R] catalyst technology was used to produce Exact[R] plastomers. The products have a density between 0.87 and 0.915 g/[cm.sup.3] and a molecular weight (Mw) of 40,000-120,000 g/mol. These plastomers are ethylene-[alpha]-olefin copolymers, which include 1-butene, 1-hexence, and 1-octene comonomer. In 1995, Exxon Chemical extended the Exact plastomer by the Exceed[R] family, a LLDPE made in a fluidized bed gas phase reactor targeting film applications. The density is above 0.915 g/[cm.sup.3] and 1-hexene is the sole comonomer. In 1993, Dow Chemical was able to commercialize ethylene 1-octene copolymers from their solution process. The catalyst used was of the single-site, "constrained geometry" type (16-18). The whole process is called Insite[R] technology. The Dow metallocene polymers are known as Engage[R] (elastomers, density below 0.90 g/[cm.sup.3]) and Affinity[R] (plastomers, density between 0.90 and 0.915 g/[cm.sup.3]. These polymers have 1-octene content of up to 20 wt%. In 1995, BASF introduced the first family of metallocene polyethylene from a heterogeneous polymerization process. The Luflexen[R] family (ethylene-1-butene plastomers with densities from 0.903 to 0.917 g/[cm.sup.3]) is made in a slurry-loop process. At the same year, Mitsui Petrochemical introduced metallocene catalysts to a fluidized bed reactor. An ethylene-1-hexene copolymer is sold under the trade name Evolve[R]. In addition, Mitsui Petrochemical is involved in producing plastomers in a solution process. There is also an elastomer grade available under the trade name Tafmer[R].

Many studies have been performed on this new material. Special attention was given to the crystal structure of it. Usually, HDPE, LDPE, and LLDPE will show only orthorhombic crystal structure under quiescent conditions (19). However, for ethylene-[alpha]-olefin copolymer with random-distributed and higher-content comonomer, some researchers reported that other crystal structures also exist. Ruiz de Ballesteros et al. (20) took the WAXD pattern on oriented ethylene-propylene copolymer synthesized with metallocene catalyst. The comonomer were randomly distributed and the content of ethylene was 75 mol%. Three equatorial reflections were observed and indexed as (100), (110), and (200) of a pseudo-hexagonal cell with a = 4.94 [Angstrom]. The well-defined layer lines indicated the nearly trans-planar conformation of the chains. (c = 2.54 [Angstrom]). The broadness of all the nonequatorial peaks indicated the occurrence of some packing disorder. Androsch et al. (21) studied the crystal structure of homogeneous poly(ethylene-co-octene) with 13.3 mol% 1-octene, which was synthesized using Dow Chemical's Insite technology. In addition to the amorphous halo, three Bragg reflections at d = 4.55, 4.15, and 3.77 [Angstrom] were observed. The second and third reflections were identified as the (110) and (200) of the orthorhombic phase. The first was assigned to the (100) of the hexagonal form. Simanke et al. (22) analyzed a series of random ethylene copolymers (synthesized with a metallocene catalyst), which were received different thermal treatment. Comonomers studied include 1-decene, 1-hexene, 4-methylpentene, norbornene, and dicyclopentadiene in a range of concentrations that includes semicrystalline and amorphous copolymers at ambient temperature. Only orthorhombic structure was found in these copolymers. No extra peak superimposed on the halo as reported by Androsch et al. (21) was identified. However, an ethylene-1-decene with 15.2 mol% quenched and annealed at 23[degrees]C for 6 months showed a very small peak superimposed on the amorphous halo. These authors concluded that only after very long annealing times will random copolymers develop a poorly ordered crystalline structure consistent with the reported pseudo-hexagonal structure. Hu and Sirota (23) studied a series of homogeneous ethylene-1-butene and ethylene-1-octene copolymer (synthesized with ExxonMobil metallocene catalyst) with different comonomer contents (7.5, 9.3, 12.0, 15.6, 16.6, and 19.7 mol% for 1-butene copolymer, 12.9, 13.0, and 13.7 mol% for 1-octene copolymer) using WAXS and solid state NMR. They observed an extra reflection peak at 4.6 [Angstrom] besides orthorhombic peak for copolymers with comonomer contents larger than 9 mol% and assigned it to monoclinic phase. No other reflection peak from monoclinic phase was observed.

We can conclude from the above description that for randomly distributed ethylene copolymer with high comonomer content (usually larger than 9 mol%), there appears an addition peak besides those from orthorhombic crystal structure on WAXD pattern. Different opinions existed for the nature of this peak. It was well established that for HDPE, additional crystal structure will appear after certain treatments. Generally, after receiving drawing or compression at room temperature, the monoclinic crystal structure will appear (24-29), Although after being heated up under high pressure, the hexagonal crystal structure will appear near its melting temperature (30). The detailed discussion can be found in our review article (31). Many researches have been reported on the structural changes during melt spinning and subsequent drawing of polyethylene (28), (29), (32), (33). However, few articles were on LLDPE and no articles on metallocene polyethylene copolymers. There is also no article on structural changes in compression of melt-spun filaments of polyethylene and its copolymers. The purpose of this article is thus first to give a clear understanding on how crystal structure develop during melt spinning and subsequent compression. Also, with a detailed and careful study on the mechanism of crystal structure transformation during melt spinning and cold compression, we hope to solve the long existing discrepancy viewpoint on the crystal structure of metallocene polyethylene copolymers with high comonomer contents.

EXPERIMENTAL

The polymers used in this study are described in Table 1. These include a high density polyethylene (EO958) and four copolymers with up to 13.3 mol% 1-octene, which are prepared by Dow's Insite constrained geometry catalyst and process technology. The samples are identified with the initials EO (ethylene/1-octene), followed by a number corresponding to the density of the copolymer.

TABLE 1. Material used in this study.

Polymer            EO958      EO916       EO902       EO885   EO870
designation

Density            0.958      0.916       0.902       0.885   0.870
(g/[cm.sup.3]

Type of             None      Octene      Octene     Octene  Octene
comonomer

Content of           0         4.2         5.9        9.7     13.3
comonomer (mol%)

Melting point      133.0      122.9        99.9       80.0     61.2
([degrees]C)

DSC                 70.5       36.1        28.3       23.2     16.5
crystallinity
(%)

Melt index (g/10    0.95       1.0         1.0        1.0      5.00
min)

Catalyst          Ziegler   Insite[R]   Insite[R]    Insite  Insite
                                                       [R]     [R]

Grade             Alathon   Elite 5400  Affinity PL  Engage  Engage
                   M6210                 1880         8003    8200

Manufacturer      Equistar     Dow         Dow       DuPont  DuPont
                            Chemical    Chemical       Dow     Dow

Melt-spun fibers were prepared using a capillary rheometer (Instron) and a take-up device. Figure 1a shows the schematic drawing of the melt-spinning apparatus we used. Molten polymers were melt-spun as a monofilament through a capillary die (diameter: 1.6 mm, length-to-diameter ratio 19.3). The filaments were normally spun through ambient air of room temperature. An electronic tensiometer (Rothschild R-1192) was used to measure the filament spinline tension.

[FIGURE 1 OMITTED]

The melt-spun fibers were transversely compressed in a Wabash compression molding press at room temperature under 27.6 MPa pressure for 10 min (Fig. 1b).

A Bruker AXS HI-STAR general area detector diffraction system with graphite monochromatized Cu K[alpha] was used to obtain the diffraction patterns of the filaments. The generator was operated at 40 kV and 40 mA. Because it was only possible to collect a portion of the WAXD pattern with the area detector, the sample stage was rotated 45[degrees] to obtain a quarter portion patterns. However, the information is enough for us to analyze the crystal structure and calculate the orientation factors. An intensity-2[theta] curve was obtained through integrating the intensity-2[theta] curve was obtained through integrating the intensity at equatorial position. To eliminate the effect from sample size and exposure time, the intensity was normalized according to the total area between 2[theta] of 12.5 [degrees] to 25.5[degrees].

The Hermans-Stein orientation factors [f.sub.a], [f.sub.b], [f.sub.c] (12), (28), (29), (34) were calculated using Eq. 1

[f.sub.i] = [[3[cos.sup.2][[empty set].sub.j] - 1]/2] (1)

where [[phi].sub.j] is the angle between the a, b, and c crystallographic axes and the fiber axis. [[bar].cos.sup.2][[phi].sub.j] indicates an average over all the polymer chains around the solid angle. For the case of complete parallel alignment of crystallographic axis with the fiber, [[phi].sub.j] is zero and [f.sub.j] becomes unity. For the case of crystallographic axis aligned perpendicular to the fiber axis, [[phi].sub.j] is 90[degrees], and [f.sub.j] is-0.5. Here,

[bar.[cos.sup.2][[empty set].sub.j]] = [[[integral].sub.0.sup.[[pi]/2]]I([[empty set].sub.j])sin [[empty set].sub.j][cos.sup.2][[empty set].sub.j]d[[empty set].sub.j]/[[integral].sub.0.sup.[[pi]/2]]I([[empty set].sub.j])sin [[empty set].sub.j]d[[empty set].sub.j]] (2)

which was calculated from the intensity distribution around the Debye ring of a specific reflection plane.

RESULTS

Melt-Spun Fibers

The formation of the melt-spun fibers of this study has already been described elsewhere (32). The melt-spun fibers generally showed a Crystallinity level independent of draw-down conditions. The level of Crystallinity decreases with increasing 1-octene (Table 3). It was also found that the final properties of the melt-spun fibers correlate with the spinline stress.

Figure 2 shows the WAXD patterns of fibers of polyethyene and its copolymers, which were melt spun at low, medium, and high spinline stress. Figure 3 shows the normalized intensity-2[theta] curves of fibers spund at low spinline\ stress. For those spun at medium and high spinline stress, the curves are same, indicating the same crystal structures formed in these fibers. As we can see from Fig. 3, for those copolymers with highests octene content, the peak height decreases, which is due to the lower Crystallinity; the peak width increase, which is due to the smaller size of the crystals. Using the Bragg's law, we further calculate the d spacing of each peak, which will allow us to compare the data from literature and further determine the crystal structure (19). As we cans see from Fig. 3, the EO958, EO916, EO902, and EO885 only show WAXD peaks, that is, at 4.1 and 3.7 [angstrom] of the Bunn orthorhombic crystal structure, corresponding to its (110) and (200) peaks. However, the EO870 shows these peaks plus an additional broad peak at 4.5. The EO870 has the highest 1-octene content (13.3 mol%) among the five polymers are investigated. This confirms our findings with previous studies (20-23). The assignment of this additional peak will be discussed later.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The corresponding crystal orientation factors of the orthorhombic structure of polyethylene and its copolymers were calculated using Eqs. 1 and 2. The results are given in Table 2. EO958, EO916, and EO902 all exhibited strong b axis orientation perpendicular to the filament axis at low spinline stresses. There was at the same time low a axis and c axis orientation in the fiber axis direction. With increasing spinline stress, the c axis became increasingly oriented in the machine direction and the a axis perpendicular to the fiber axis. The crystal orientation behavior is correlated to the different crystal morphology formed under different spinline stress, as been explained in our former article (32), (34). Under low spinline stress, the lamellae will grow radially outward in the form of twisted ribbons (Keller/Machine I morphology), with their growth axis parallel to the b axis. While under high stress, the radially grown lamellae extend directly outward without twisting (Keller/Machin II morphology). The mEO885 and mEO870 showed monotonic increasing of c axis orientation and decreasing of a and b axis orientation, with increasing spinline stress. The difference in b axis orientation behavior between EO985, EO916, EO902 and EO885, EO870 is also due to the different crystal morphology existing in these materials (32).

TABLE 2. Crystal structure and crystal orientation factor of melt spun
filaments.

Sample  Spinline stress     Crystal
        [[sigma] (MPa)     structure      [f.sub.a]  [f.sub.b]

EO958        0.3          Orthorhombic       0.30      -0.36
             1.3                             0.07      -0.32
             3.0                            -0.37      -0.45

EO916        0.9          Orthorhombic       0.22      -0.33
             2.1                             0.11      -0.33
             3.9                            -0.14       0.38

EO902        0.9          Orthorhombic       0.05      -0.07
             2.4                            -0.001     -0.20
             3.6                            -0.16      -0.34

EO885        0.2          Orthorhombic      -0.13      -0.17
             2.5                            -0.37      -0.45
             5.1                            -0.40      -0.47

EO870        0.2          Orthorhombic      -0.10      -0.09
             0.8              +             -0.32      -0.24
             1.51        Psendo-hexagonal   -0.37      -0.28

Sample  [f.sub.c]  Crystallinity

EO958     0.06         66.3
          0.25         69.8
          0.82         72.2

EO916     0.09         35.0
          0.22         35.5
          0.52         36.3

EO902     0.02         28.1
          0.20         27.0
          0.50         28.8

EO885     0.30         21.1
          0.83         26.6
          0.87         23.5

EO870     0.19         15.9
          0.56         16.2
          0.65         17.5

Transversely Compressed Fibers

Cold Compression Behavior of Melt-Spun Fibers. The filaments of polyethylene and its copolymers, which were initially spun under different spinline stresses, were transversely compressed under pressure at 25[degrees] C for 10 min. The gauge pressure reading was 27.6 MPa. After being compressed, the filament became a flat sheet with very thin thickness. No fibrillation or whitening phenomenon was observed. The schematic dimensional change during compression is shown in Fig .4. The detailed dimensional changes are given in Table 3.

[FIGURE 4 OMITTED]

TABLE 3. Dimensional changes of melt spun filaments after transversely
compression.

                        Before compression     After compression

Material    Spinline    Diameter  Length    Width  Thickness  Length
          stress (MPa)    (mm)     (mm)     (mm)      (mm)     (mm)

EO958         0.3         0.36     150       0.99     0.10     150
              1.3         0.145    155       0.30     0.05     155
              3.0         0.085    178       0.115    0.04     178

EO916         0.9         0.26     163       0.68     0.07     160
              2.1         0.18     157       0.33     0.06     153
              3.9         0.12     178       0.20     0.05     173

EO902         0.9         0.37     164       0.8      0.12     160
              2.4         0.13     179       0.17     0.08     169
              3.6         0.08     140       0.11     0.04     137

EO885         0.2         0.21     151       0.53     0.07     136
              2.5         0.16     180       0.24     0.06     166
              5.1         0.15     170       0.24     0.06     154

EO870         0.2         0.39     205       0.1      0.11     196
              0.8         0.32     186       0.80     0.09     174

                                    Deformation strain

            Spinline      [[epsilon].  [[epsilon].  [[epsilon].
Material  stress (MPa)     sub.1]       sub.1]       sub.1]

EO958         0.3           0            1.75        -0.72
              1.3           0            1.05        -0.66
              3.0           0            0.35        -0.53

EO916         0.9          -0.02         1.62        -0.73
              2.1          -0.03         0.83        -0.67
              3.9          -0.03         0.67        -0.58

EO902         0.9          -0.02         1.16        -0.68
              2.4          -0.06         0.31        -0.38
              3.6          -0.02         0.38        -0.50

EO885         0.2          -0.10         1.52        -0.67
              2.5          -0.08         0.50        -0.63
              5.1          -0.09         0.60        -0.60

EO870         0.2          -0.04         1.56        -0.72
              0.8          -0.06         1.5         -0.72

It can be seen that, for EO958, the length of the compressed filaments did not change, while for the other materials, the length of filaments did shrink a small amount For the compressed filaments of all materials, the width was larger and the thickness was smaller than its initial diameter. We defined the length elongation strain ([[epsilon].sub.1]), the width elongation strain ([[epsilon].sub.2]), and thickness elongation strain ([[epsilon].sub.3]) as follows:

[[epsilon].sub.1] = [[[l.sub.1] - [l.sub.0]]/[l.sub.0]],[[epsilon].sub.2] = [[w - d]/d],[[epsilon].sub.3] = [[t - d]/d],(3)

Where [l.sub.0] and d are the length and the diameter of the melt-spun filament; [l.sub.1], w, and t are the length, the width, and the thickness of the compressed filament.

Generally, [[epsilon].sub.1] was negative and its magnitude was between 0 and -0.1; [[epsilon].sub.2] was positive and its magnitude was between 0.3 and 1.8; [[epsilon].sub.3] was negative and its magnitude was between 0.4 and 0.8 We also found that, for filaments spun under higher spinline stress, the magnitude of the [[epsilon].sub.2] and the [[epsilon].sub.3] was smaller (see Table 3).

Summarizing, during transversely compression, the filament was stretched along the width direction, while shrinking along the length and the thickness direction. The elongational strain of the filament, which was initially spun under low spinline stress, was larger than that initially spun under higher spinline stress. This is the first time that such phenomenon was reported.

WAXD Investigations. WAXD patterns of the five transversely compressed polyethylene and copolymer filaments, initially spun under different spinline stress, are shown in Fig. 5. Figure 6 shows the normalized intensity 2 [theta] curves of compressed fibers, initially spun at low spinline stress. Using Bragg's law, we determined the d spacing of each peak, which are given in Table 4. For EO958, three extra peaks beyond those of the orthorhombic crystal structure were observed. However, for EO916, only one extra peak was observed. However, for EO916, only one extra peak was observed. The intensity of this peak is lower. For EO902 and EO885, we did not observe any additional peak except some concentration at the meridian position at ca. 4.5 [Angstrom]. For EO870, the pattern is similar to that of its melt-spun filaments. It is difficult to say whether an additional peak appeared after transverse compression.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

TABLE 4. d-Spacing of transversely compressed filaments of polyethylene
and its copolymers.

Polymer                Seto et   EO958     EO916       EO902
designation            al. (27)

Octene content           0         0        15          20
(wt%)

Crystal         (hkl)                     d spacing/
structure                                 [Angstrom]

Orthorhombic    (110)            4.105    4.125       4.151
                (200)            3.711    3.743       3.759
                (020)            2.467    2.476

Other peaks            4.552     4.508    4.512
                       3.855     3.810
                       3.520     3.522

Pseudo-         (100)
hexagonal
Amorphous halo                   ca. 4.5  ca. 4.5     ca. 4.5

Polymer designation          EO885    EO870
Octene content (wt%)          30       38

Crystal structure     (hkl)

Orthorhombic          (110)  4.143    4.190
                      (200)  3.778    3.803
                      (020)

Other peaks

Pseudo-hexagonal      (100)           4.534
Amorphous halo               ca. 4.5  ca. 4.5

The crystal orientation behavior can also be observed in Fig. 5. To provide a better understanding, we incorporate sketches, which give the general crystal orientation behavior of the (110) and the (200) planes of orthorhombic crystal structure in Fig. 7. As we can see, after the filaments, initially spun under low spinline stress, were transversely compressed, the (110) and the (200) peaks of the orthorhombic crystal structure concentrate at the meridian position (Fig. 7a). For the compressed filaments, initially spun under high spinline stress, the crystalline orientation behavior seems more complex, For EO958, EO916, and EO902, besides the original equatorial orientation of the (110) and (200) peaks of orthorhombic structure, they are also located at the meridian position (Fig. 7b). However, for EO885 and EO870, the original equatorial orientation of the (110) and (200) peaks disappeared and relocated in the meridian position. (Fig. 5d and e).

[FIGURE 7 OMITTED]

DISCUSSION

Crystal Structure of Transversely Compressed Polyethylene Copolymer Filaments

For the compressed EO958 filaments, we observed three extra reflections, beyond those from orthorhombic crystal structure. These occur at Bragg d spacing of 4.508, 3.810, and 3.522 A, which are the exact same positions of the d spacings of the extra reflection of those we found earlier for cold-drawn EO958 filaments (34). They are also similar to those observed by Tanaka et al. (26), (27). We can assign these reflections to their monoclinc crystal structure. For EO916, only one peak from monoclinic crystal structure was observed, with d spacing at 4.512 [Angstrom] The intensity of it is lower than that of EO958. Since it received same deformation history as that of EO958, we should also assign this peak to the monoclinic phase. The same conclusion was also reached in the cold-drawn case (34).

We should further discuss the reason why a monocline phase is formed after material received cold work. Tanaka et al. (26), (27) proposed a detailed orthorhombic-monoclinic transformation mechanism: After receiving stress, the molecular chains will move to different positions from that of the orthorhombic crystal structure. This forms a monoclinic phase. We can conclude that monoclinic phase comes from its parent phase--the orthorhombic phase. Now, from what we observed in EO958 and EO916, we can conclude that the amount of monoclinic phase in EO916 is lower than that in EO958. As we have found that the fraction crystallinity of EO958 is ca. 0.71, while that of EO916 is only ca. 0.36 (32), we may conclude: With decreased crystallinity level, less orthorhombic-monoclinic transformation will occur.

The WAXD pattern of cold-compressed EO902 and EO885 show a strong extra reflection at ca. 4.5 [Angstrom], even though their crystallinity is only 0.28 and 0.23. They do not show reflections at 3.810 and 3.522 [Angstrom]. The intensity of this reflection increased with a decrease of crystallinity. This seems contrary to what we concluded above for orthorhombic-monoclinic phase transformation. A possible explanation is that this extra reflection is from another phase, not the monoclinic phase. When we compare the WAXD pattern of melt-spun and cold-drawn filaments, it is possible to conclude that this extra reflection is from the pseudo-hexagonal mesophase. For EO870, we observe a WAXD pattern with reflections similar to that of melt-spun filament. The pattern is also quite similar to that of cold-compressed EO885. It seems reasonable to assign this extra reflection to the pseudo-hexagonal mesophase.

Crystal Orientation Behavior of Transversely Compressed Filaments

As we have described in the Results section, after filaments being transversely compressed, the (110) and (200) peaks of orthorhombic structure become located at the meridian position, which means that the (110) and (200) planes are oriented perpendicular to the fiber axis direction.

We tried to explain the crystalline orientation behavior during compression through analyzing the strain distribution along the fiber. As we can see from Fig. 2 and Table 3, after we applied a compression force p perpendicular to the fiber using two parallel plates, the filament was deformed along three different directions. It was elongated along the width direction and shrunk along the length and the thickness direction. We may thus regard it as essentially drawing filament along its width direction, which is the fiber radius direction. As we have found in our previous article (34), after polyethylene and its copolymer filaments are being cold drawn, the (110) and (200) peaks become located at the equatorial position. Based on this, we may estimate that, after filament is being cold compressed, the (110) and the (200) peaks are located at the meridian direction, that is, the (110) and (200) planes are perpendicular to the fiber axis direction.

CONCLUSIONS

In this article, we studied the structure development during transverse compression of polyethylene and its copolymer filaments, which were initially spun under different spinline stresses. On being compressed, the filament is stretched along the width direction, while it shrinks along the length and the thickness direction. The elongation strain of the filament, which was initially spun under low spinline stress, is larger than that initially spun under higher spinline stress, The monoclinic phase appears in the EO958 and EO916 filament after being compressed. The amount of monoclinic phase decreases with increase of octene and decrease of the crystallinity of the material. An extra reflection appears after the EO902 and EO885 filament have been compressed and may possibly be assigned to the pseudo-hexagonal phase. The amount of pseudo-hexagonal structure increases with increase of 1-octene and decrease of the crystallinity of the material. For EO870, we observe a similar WAXD pattern with that of melt-spun and cold-drawn filaments, the extra reflection is again assigned to the pseudo-hexagonal phase. After being transversely compressed, the (110) and (200) of orthorhombic crystal planes become oriented perpendicular to the fiber axis direction. The reason can be explained through analyzing the strain distribution along the fiber: the compression deformation of filament induces elongation along its width direction and shrinkage along its length and thickness direction.

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Haifeng Shan, James L. White

Department of Polymer Engineering, Institute of Polymer Engineering, University of Akron, Akron, Ohio 44325

Correspondence to: James L. White; e-mail: [e-mail] DOI 10.1002/pen.21109

Published online in Wiley InterScience (www.interscience.wiley.com).