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Structure development in melt-spinning syndiotactic polystyrene and comparison to atactic polystyrene.

INTRODUCTION

Syndiotactic polystyrene (s-PS) was first announced in 1986 by Idemitsu Kosan (1), who commercialized it in succeeding years. Today it is produced by both Idemitsu and Dow Chemical.

s-PS has the same [T.sub.g] (about 100[degrees]C) as atactic polystyrene (a-PS) and a relatively high melting point around 273[degrees]C. The heat of fusion of 100% crystalline polymer was determined to be 53.2 J/g (2). The crystal structure of s-PS has been investigated by various researchers. Different crystalline forms have been described that possess chains with TTTT or TTGG chain conformations. Melt-crystallized s-PS generally exhibits the hexagonal [alpha]-form or orthorhombic [beta]-form (3). And, polymorphic behavior depends on cooling rate (4) or crystallization temperature (5). The literature (3) also describes zigzag [beta]' and [beta]" structures. Immirzi et al. studied Helical TTGG [gamma] and [delta] forms with solvent treatment and their transformation (6). Mesomorphic forms of s-PS have also been described (7, 8).

Studies of the processing of a-PS date to the 1930s. It was realized that processing induces mechanical property changes, which were associated with polymer chain orientation (9). This orientation can be measured using birefringence. In 1978, Oda et al. (10) showed that the birefringence developed in melt-spun filaments and sheared samples was linearly related to the principal stress difference applied at vitrification. The coefficient connecting them was the same as that which relates birefringence and stress in the flowing melt. This relationship was further supported by the study by Choi et al. (11) of the relationship between processing-induced birefringence and applied stress in tubular film extrusion. There have been few investigations of structure development in processing of s-PS. Hsiung and Cakmak have published studies of uniaxial stretching of compression molded s-PS films (12) and injection molding (13).

In the present paper, we describe a study of structure development in melt-spinning s-PS and a-PS. The filaments are characterized by differential scanning calorimetry, X-ray diffraction, and birefringence. The mechanical properties of the filaments were measured.

EXPERIMENTAL

The s-PS and a-PS were obtained from The Dow Chemical Company. The a-PS was Dow Styron 615 ([M.sub.w] = 172,000) and s-PS was Dow Questra QA101 ([M.sub.w] = 200,000). The pellets of the two polymers were examined with a DuPont 951 Thermogravimetric analyzer (TGA) at a heating rate of 20[degrees]C/min. [N.sub.2] and air atmospheres were used. The complex shear viscosity ([eta]*) of a-PS was initially measured at 235[degrees]C and 290[degrees]C and s-PS at 290[degrees]C in a Rheometrics Mechanical Spectrometer (ARES).

s-PS filaments were melt-spun from an Instron capillary rheometer using a 1.593-mm capillary die with a plunger speed of 0.127 mm/s at temperatures of 270[degrees]C, 280[degrees]C, and 290[degrees]C. This is equivalent to a die wall shear rate of 22.8 [s.sup.-1]. The filaments were quenched in both ice water and air. The spinline tensions in the filaments were measured with a Rothschild tensiometer. a-PS filaments were melt-spun similarly at a processing temperature of 235[degrees]C and 290[degrees]C.

The melt-spun filaments were characterized by differential scanning calorimetry (DSC) using a DuPont 910 instrument at a scanning rate of 10[degrees]C/min. The DSC scans were interpreted using a heat of crystallization of 53.2 J/g for s-PS (2).

We characterized the melt-spun filaments by wide angle X-ray diffraction using a General Electric X-ray generator with Cu-[k.sub.[alpha]] radiation, 2[theta] scans were obtained using a Rigaku X-ray generator. The birefringence of the filaments was measured with a Leitz polarized light microscope with a 4th-order Berek compensator.

The uniaxial stress-strain characteristics of the filaments were measured with an Instron tensile testing machine at room temperature. The gauge length was 4.2 mm and the crosshead speed was 5 mm/s.

RESULTS

Material Properties

The TGA scans for the s-PS and a-PS are shown in Fig. 1. The s-PS and a-PS lost 100% of their weight at 450[degrees]C in air and 463[degrees]C in [N.sub.2], respectively. At the same temperature of 290[degrees]C, the zero shear viscosity of s-PS showed much higher values (507 Pa*s) than the a-PS (107 Pa*s). At the melt-spinning temperature (235[degrees]C) of a-PS, the zero shear viscosity for a-PS was 1350 Pa*s. Both melts were non-Newtonian, with the shear viscosity decreasing with increasing shear rate (Fig. 2).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Filament Properties

The spinline stresses in the melt-spinning experiments for the a-PS and s-PS are shown in Fig. 3 as a function of a drawdown ratio. The spinline stress vs. drawdown ratio increased roughly and linearly for both polymers. The s-PS has a higher spinline tension than a-PS at the temperature of 290[degrees]C. At the lower meltspinning temperature (235[degrees]C) of a-PS, the spinline stress was higher than that of s-PS.

DSC scans were made for the melt-spun fibers in Fig. 4. In Fig. 4a and 4b, the s-PS and a-PS filaments exhibited a [T.sub.g] at about 98[degrees]C. The s-PS exhibited a cold crystallization peak at about 130[degrees]C and a subsequent melting peak at about 270[degrees]C. Figure 4b shows that all air-quenched a-PS filaments are totally amorphous, only exhibiting a [T.sub.g] at about 98[degrees]C.

Consider now the s-PS filament that exhibits a DSC crystallization peak and a DSC melting peak. One finds that filaments with low drawdown ratios exhibit only about 21% crystallinity upon melting. Filaments at high drawdown ratios exhibit about 54% of the crystallinity observed upon melting. We plot the computed crystallinity of s-PS melt-spun fibers as a function of drawdown ratio in Fig. 5.

[FIGURE 3 OMITTED]

[FIGURE 4A OMITTED]

[FIGURE 4B OMITTED]

X-ray Diffraction Studies of Filaments

The a-PS filaments appear amorphous and show only a single broad amorphous halo.

Equatorial and meridional X-ray diffractometer scans of the melt-spun filaments of s-PS at various drawdown ratios are shown in Figs. 6a and 6b. With increasing drawdown ratio/stress, the diffraction patterns evolved and diffraction peaks became clear.

[FIGURE 5 OMITTED]

X-ray diffraction film patterns for melt-spun s-PS fibers are shown in Fig. 7. At low drawdown ratios, s-PS filaments showed two broad halos suggesting mesomorphic structure. At high drawdown ratio, diffraction patterns evolve from glassy/mesomorphic film pattern images to sharp diffraction peaks. We have annealed the fibers at 200[degrees]C for 5 minutes to develop the crystallography better. The results are shown in Fig. 8.

We identify the sharpest diffractions in Fig. 8. These WAXD reflections correspond to the zigzag [alpha]-crystalline form (3, 14) of s-PS. These are (i) 7.597 [Angstrom] on the equatorial representing (110) crystallographic planes and (ii) 13.16 [Angstrom] on the equatorial representing (300) crystallographic planes, respectively.

Birefringence of the Filaments

Birefringence measurements of a-PS and s-PS melt-spun filaments are contained in Fig. 9. Both a-PS and s-PS filaments exhibited negative birefringence.

It has long been known that [DELTA]n for a-PS is correlates with spinline stress [sigma], for melt-spun filaments (10). In Fig. 9, we plot the absolute birefringence for both a-PS and s-PS vs. spinline stress. The value for a-PS is much lower than s-PS at the same spinline stress, i.e.,

|[DELTA]n|[.sub.s-PS] > |[DELTA]n|[.sub.a-PS] (1)

We found that the a-PS data for different melt-spinning temperatures correlates reasonably well with stress. As described in the previous section, the melt-spun s-PS fibers were annealed. This made the filaments opaque, and it was not possible to determine their birefringence.

Mechanical Properties

Uniaxial engineering stress (F/[A.sub.0])--strain ([DELTA]L/[L.sub.0]) curves for melt-spun a-PS and s-PS filaments are shown in Fig. 10a for a-PS and Fig. 10b for s-PS.

For the a-PS, it can be seen that the oriented filaments produced by high spinning stresses have increased tensile strength and elongation to break. Notably, the strain to break was less than 0.1 at low drawdown, but it increased more than 0.4 at high drawdown ratio.

[FIGURE 6A OMITTED]

[FIGURE 6B OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

For the s-PS filaments, it was found that the tensile strength and elongation to break are at first increased. However, there was a maximum in tensile strength and elongation to break, and at higher drawdown ratios, they were reduced.

DISCUSSION AND INTERPRETATION

Crystalline Characteristics of s-PS and Comparisons With PP

The tendencies of isotactic and syndiotactic polystyrene to crystallize are well known. Isotactic polystyrene generally forms TGTG 3/1 helices when it crystallizes into a trigonal unit cell (15). s-PS, as noted earlier, crystallizes both into all zigzag TTTT structures and TTGG helical structures. In this paper, where we have melt-spun filaments found under uniaxial stress, we have found glassy/mesomorphic products only at low drawdowns/stresses, and all trans-conformation [alpha]-crystalline fibers at higher drawdowns. The situation with polypropylene has similarities. Isotactic polypropylene crystallizes also into TGTG 3/1 helices (16). Syndiotactic polypropylene melts crystallize into both TTTT zigzag and TTGG helix forms (17, 18).

[FIGURE 10A OMITTED]

[FIGURE 10B OMITTED]

Our study of melt-spinning s-PS produced glassy/mesomorphic fibers at low drawdown ratios/stresses and TTTT [alpha]-structure at higher drawdown ratios. A recent study (18) from our laboratories on melt-spinning syndiotactic polypropylene found well-formed TTGG helical structures at low spinline stress and all TTTT zigzag conformations at high spinline stresses. Clearly there is a weaker tendency to form TTGG helices in s-PS as compared to syndiotactic polypropylene.

Chain Orientation and Crystallinity

Applied spinline stress enhanced the chain orientation, and that helped material crystallization. The filaments melt-spun at low drawdown ratios were mesomorphic and did not give sharp WAXD patterns. In Fig. 5a, we can also see that crystallinity was increased by increasing drawdown ratio/spinline stress during melt-spinning. This caused a reduced area of the DSC cold crystallization peak at higher drawdowns. Finally, the crystallinity of the melt-spun s-PS filaments was increased from 21% to 54% by increasing spinline stresses (Fig. 5).

Crystalline Orientation in s-PS

Uniaxial crystalline orientation is generally represented by the Hermans-Stein orientation factors (19-21)

[f.sub.j] = [3[bar.[cos.sup.2][[phi].sub.j]] - 1]/2 (2)

where j represents the crystallographic axis.

For our hexagonal s-PS we may use the (110) crystallographic planes together with

[f.sub.a] = [f.sub.b] (3a)

[f.sub.c] = 1 - ([f.sub.a] + [f.sub.b]) (3b)

From the WAXS reflection we may determine the three orientation factors [f.sub.a], [f.sub.b], [f.sub.c]. It is possible to compute the values of the orientation function only above a stress level of 1.7 MPa, where we can observe WAXS patterns. The [f.sub.a] and [f.sub.a] were negative. The [f.sub.c] was positive and increased to a value of 0.7 MPa at a high spin-line stress (Fig. 11). As we worked at three different melt temperatures, Fig. 11 represents a true correlation in terms of stress.

Birefringence

Birefringence is well known to be a measurement of orientation (10, 18-22)

[[[DELTA]n]/[[DELTA].sup.0]] = [[3[bar.[cos.sup.2][phi]] - 1]/2] = f (4)

where [[DELTA].sup.0] is the intrinsic birefringence and f is the Hermans orientation factor.

The birefringence in molten a-PS varied with stress according to the Rheo-Optical law (in one dimension)

[DELTA]n = C[sigma] (5)

We found, as did Oda et al. (10), that Eq 6 was also valid for melt-spun a-PS fiber. This is from Eq 4 equivalent to

[FIGURE 11 OMITTED]

f = [C/[[DELTA].sup.0]][sigma] (6)

In partially crystalline polymeric fibers, the crystalline orientation has always been found higher than the average orientation (22, 23). If we write

[DELTA]n = (1 - X)[DELTA][n.sub.am] + X[DELTA][n.sub.c] (7)

where X is the function crystallinity, [DELTA][n.sub.am] and [DELTA][n.sub.c] are the amorphous and crystalline birefringences, and we neglect form birefringence (24). Equation 7 may be written:

[[[DELTA]n]/[[DELTA].sup.0]] = f = (1 - X)[f.sub.am] + X[f.sub.c] (8)

where [f.sub.am] and [f.sub.c] are the amorphous and crystalline orientation factor, respectively. The amorphous and crystalline intrinsic birefringences are the same. This would seem reasonable with the zigzag all trans s-PS conformations in the crystalline state.

From the data of Fig. 9, we can see that at a low spin-line stress, the birefringence of s-PS is about the same as that of glassy polystyrene. However, at a higher spin-line stress, the value of [DELTA]n for s-PS is much higher than that of a-PS. This clearly comes about, as may be seen from the DSC scans of Fig. 4b, the rising level of crystallinity by increasing drawdown ratio/spinline stress associated with this crystalline fraction are values of [f.sub.c] that are much higher than [f.sub.am].

Above 0.5 MPa of spinline stress, it was impossible to measure the birefringence of melt-spun s-PS fibers because of opaqueness of the annealed fibers. This was probably associated with enhanced crystallinity.

Mechanical Properties

For a-PS, it is well known that increasing uniaxial orientation improves mechanical properties (9). With increases in drawdown ratio, Young's modulus, tensile strength, and elongation to break were all increased. In case of s-PS, Young's modulus and tensile strength were similarly increased. Elongation to break was increased at a low drawdown ratio, and decreased at high spinline stress (Fig. 10).

CONCLUSIONS

Syndiotactic polystyrene (s-PS) may be melt-spun into filaments. The crystallinity of s-PS was increased by increasing spinline stress. The crystallographic form of melt-spun s-PS evolves from glassy/mesomorphic to TTTT [alpha]-form by increasing drawdown ratio/spinline stress. Orientation in melt-spun s-PS fiber was much higher than in a-PS fiber. The Young's modulus and tensile strength of s-PS filaments were increased by uniaxial chain orientation, like a-PS. Elongation to break was enhanced at a low drawdown ratio, and decreased at a high drawdown ratios.

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MYEONG-HO HONG and JAMES L. WHITE

Department of Polymer Engineering

The University of Akron

Akron, OH 44325-0301
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Author:Hong, Myeong-Ho; White, James L.
Publication:Polymer Engineering and Science
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Date:Nov 1, 2004
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