PPS Bicomponent Fibers Revealed

We present a small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) characterization of the semicrystalline structure of homo- and...
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Modified Crystallization in PET/PPS Bicomponent Fibers Revealed by Small-Angle and Wide-Angle X‑ray Scattering Edith Perret,†,* Felix A. Reifler,† Rudolf Hufenus,† Oliver Bunk,‡ and Manfred Heuberger† †

Laboratory for Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland ‡ Paul Scherrer Institut, 5232 Villigen, Switzerland S Supporting Information *

ABSTRACT: We present a small-angle X-ray scattering (SAXS) and wideangle X-ray scattering (WAXS) characterization of the semicrystalline structure of homo- and heterobicomponent core−sheath fibers melt-spun from poly(ethylene terephthalate) (PET) and poly(phenylene sulfide) (PPS). The two-dimensional SAXS/WAXS patterns of the various bicomponent fibers are found to reflect the mutual influence of the components on their thermal profiles along the spinline, leading to modified crystallization of the PET component and a larger strain rate in the PPS component. The predominant scattering features in the SAXS patterns are four-point reflections, an intense equatorial streak, and a central anisotropic diffuse scattering. The four-point reflections are attributed to tilted crystalline lamellar stacks of PET. The lamellar stack dimensions and the orientation of their lamellar surfaces are determined. We find that the heterobicomponent arrangement can promote the formation of equally spaced and uniformly sized crystallites in the PET phase and highly oriented crystallites in the PPS phase.

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2D SAXS patterns.8−10 Few publications deal with the structure of coated fibers or polymer blends.11,12 However, to our knowledge, there are no publications revealing anisotropic 2D SAXS patterns of core/sheath bicomponent fibers. In this article, anisotropic 2D SAXS patterns of PPS−PET, PET−PPS, PET−PET, and PPS−PPS bicomponent core− sheath fibers for various draw ratios are presented. In a SAXS pattern of bicomponent fibers, it is generally difficult to distinguish between the scattering features arising from the two different constituents. This particular problem is addressed in the discussion part of this article. The dimension of lamellar stacks, the orientation of the lamellar surfaces and the widths of long structures along the fiber axis as a function of the draw ratio are evaluated by a series of longitudinal and transversal slices. Additionally, some 2D WAXS patterns are presented, which shed light on the degree of orientation of PET and PPS crystals. In 2D WAXS patterns the two components can be distinguished from one another since diffraction peak positions depend on the component. During melt spinning, the polymers usually experience fast cooling followed by mainly isothermal conditions during drawing. The cooling rate can be influenced by modifying the temperature profile of the extruded filament along the spinline: Delayed quenching with a postheater installed under the spinneret is a well-known approach to avoid too fast cooling of the filament surface, resulting in lower freeze-point stress and

INTRODUCTION The specific modification of synthetic fiber properties is of high interest for a wide range of applications. An increasingly common way to improve fiber properties is to combine two polymers in a single fiber. For example, fibers made of poly(ethylene terephthalate) (PET) are sensitive to high temperatures or corrosive chemicals as opposed to fibers made of poly(phenylene sulfide) (PPS). A thin sheath of PPS is thus sufficient to protect PET fibers against corrosive environments.1 Bicomponent fibers are manufactured by extruding two polymers into one fiber. Different types can be produced; common examples include core−sheath, side-byside, or island-in-the-sea fibers.2,3 In this article we focus on the structure determination of coaxial core−sheath PPS−PET and PET−PPS fibers (herein after referred to as “heterobicomponent” fibers) and, for reference purposes, core−sheath PET− PET and PPS−PPS fibers (herein after referred to as “homobicomponent” fibers). The molecular structure of unoriented polymers is conveniently analyzed by either one-dimensional (1D) radial scans or circularly averaged scans obtained from isotropic twodimensional (2D) SAXS data. However, a different analysis of 2D SAXS data is required for oriented polymers, since in this case the scattering pattern is anisotropic. For example, the diffuse scattering of uniaxially drawn fibers is typically elliptically shaped. Although a vast amount of literature is available on the structure determination of PET films or fibers by 2D SAXS patterns,4−7 there are only a few publications on the structure determination of PPS films or fibers by circular integration of © 2012 American Chemical Society

Received: October 9, 2012 Revised: December 17, 2012 Published: December 31, 2012 440

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summarized in Table 1; the polymer volume ratios of core to sheath (c/s) and the draw ratios (DR), diameters and mass densities of the fibers are listed:

lower preorientation, and therefore better drawability and higher final tensile strength.13−16 The primary structural characteristic responsible for mechanical properties is the degree of molecular orientation created in the fiber.17 Although the isothermal crystallization kinetics of pure PET and PPS are both well-studied,18−23 fast cooled and drawn bicomponent fibers could have a significantly different molecular morphology. The core−sheath geometry is particularly interesting because the thermal history of PPS and PET are both altered with respect to the monocomponent equivalents. The thermal conductivity of PPS is 2 orders of magnitude larger than the one in PET.24 Furthermore, PPS exhibits a higher melting temperature (Tm ∼ 280 °C)18 than PET (Tm ∼ 254 °C),25 and PPS is known to have a dual crystallization mechanism.26 Therefore, PPS releases its heat of fusion (HF ∼ 60 J/g) at a temperature just above PET solidification. In a bicomponent system, PET can thus be kept longer at its molten state. Along the same lines, it is also expected that a PET sheath can thermally isolate and thus delay the cooling of a PPS core above 280 °C followed by a release of its heat of fusion (HF ∼ 60 J/g) just below PPS solidification, which could lead to transient annealing effects. Hence, the thermal profile of each component along the spinline can be influenced by the presence of the other component. Furthermore, the component which solidifies first can experience higher elongational stress near to the solidification point in the spinline compared to single-component spinning under the same conditions.27 Because of such larger strain rate, it can develop higher orientation and undergo orientation-induced crystallization. Simultaneously, the elongational stress in the other component decreases due to stress relaxation, because no further elongation can occur. Hence, the structure formation of the latter component can be suppressed compared to the corresponding single component melt spinning.28 Furthermore, crystallization and structure development of both polymers depend on their repartition in the fiber cross-section. The crystallinity of each component in the bicomponent melt-spun filament can be very different to that of the respective homopolymer filaments spun under similar conditions.29−35 Evidence of such phenomena will be discussed in this article by comparing the structure of the heterobicomponent and the homobicomponent fibers.

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Table 1. Fiber Properties no.

core−sheath material

volume ratio

draw ratio

diameter (μm)

linear mass density (tex)

A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 F1 F2 F3

PPS−PET* PPS−PET* PPS−PET* PPS−PET* PPS−PET* PPS−PET* PET−PPS PET−PPS PET−PPS PET−PPS PET−PPS PET−PPS PPS−PPS PPS−PPS PPS−PPS PET−PET PET−PET PET−PET

1/2 1/2 1/2 2/1 2/1 2/1 1/2 1/2 1/2 2/1 2/1 2/1 1/1 1/1 1/1 1/1 1/1 1/1

2 3 4 2 3 4 2 3 4 2 3 3.5 2 3 3.5 2 3 3.5

95.6 78.0 63.5 76.7 67.2 60.8 93.8 83.0 82.1 91.9 74.3 74.9 93.4 81.5 76.3 68.3 65.0 60.0

11.2 4.7 5.7 10.1 6.8 5.1 13.0 8.6 6.6 11.4 7.6 6.6 11.5 7.7 6.7 11.7 7.9 6.7

Small Angle X-ray Scattering (SAXS). The SAXS experiments were carried out at the coherent small-angle X-ray scattering (cSAXS) beamline of the Swiss Light Source (SLS) at the Paul Scherrer Institute (PSI) in Switzerland. In order to match the horizontally elongated focus, all fibers (Table 1) were mounted horizontally with vertical spacing. Nevertheless, for convenience, the fiber axis is shown vertically in all drawings. A helium-filled 2 m long flight tube was positioned in between the PILATUS 2M37 detector and the fibers in order to avoid air scattering. The detector-to-fiber distance was F = 2.148 m. A photon wavelength of 1 Å (12.4 keV) was selected. The beam was focused onto the fiber (focus size: H × V of 540 × 100 μm2). A background image, taken through direct illumination of the detector, was subtracted from all SAXS patterns. The zero intensity at locations of detector module gaps or at the pixels behind the beamstop wire was complemented with intensities from the SAXS patterns rotated around the origin by 180 degrees. For each background corrected SAXS pattern, an area of interest was selected and converted to a binned image (3 × 3 pixels binning). For the reminder of the article, “binned pixels” shall be termed “pixels”. Wide Angle X-ray Scattering (WAXS). WAXS experiments were performed on fiber bundles (8−20 single filaments depending on filament fineness) with a linear mass density of approximately 60 tex (mg·m−1) mounted on a custom-made sample holder. WAXS patterns were recorded using a Xcalibur PX four-circle single crystal diffractometer38 (Mo Kα1 radiation, λ = 0.7 Å). SAXS Data Analysis. For the sake of simplicity, SAXS patterns of monocomponent fibers are considered first. SAXS patterns of oriented polymers mostly show three prominent features: (i) the discrete meridian or off-meridian lamellar reflections from periodic crystalline structures, (ii) an equatorial streak from elongated fibrillar structures, and (iii) the central diffuse scattering from nonperiodic inhomogeneities. Even though some publications treat the 2D SAXS patterns with elliptical coordinates,6,7,39−41 we decided to analyze 2D SAXS patterns in the common way by a series of longitudinal and transversal 1D slices42,43 to minimize the number of parameters, as indicated in Figure 1. Typical models6,29,39,44−46 of semicrystalline polymer structures explaining various four-point SAXS patterns are shown in Figure 2. A melt-spun polymer fiber consists of fibrils, where each fibril is a linear array of slightly misaligned lamellar stacks. A four-point pattern (Figure 2a, left) is obtained when the lamellar surface is tilted away

EXPERIMENTAL SECTION

Materials. The raw material used for the here discussed bicomponent fibers were PPS pellets (FORTRON 0320C0 Ticona, Germany) and two types of PET. One PET type (GL6105, Kuag Elana Oberbruch, Germany) was used for the PET−PPS and PET−PET (core/sheath) fibers and the other PET type (Clariant Hunique, France) for the PPS−PET* fibers. This second PET* type is denoted with asterisk throughout the article. Information about the physical properties of the pellets is given by Houis et al.1 Melt spinning. Melt spinning was carried out on Empa’s custommade pilot melt spinning plant built by Fourné Polymertechnik (Alfter-Impekoven, Germany). This plant, with features corresponding to an industrial plant, enables the production of mono- and bicomponent fibers with various fiber cross sections and material combinations with a throughput of 0.1−5 kg·h−1. For this study, the two polymers were melted using two single screw extruders, and coaxially combined in a spinneret for bicomponent monofilaments with core/sheath-geometry. The spin pack temperature was kept between 260 and 305 °C for all fibers. More detailed information about the melt spinning equipment and the manufacturing parameters for the bicomponent fibers used for this study is given by Hufenus et al.36 and Houis et al.,1 respectively. The sample nomenclature is 441

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Figure 1. Typical features in a SAXS pattern of oriented polymer fibers: (i) lamellar reflections, (ii) equatorial streak, and (iii) central diffuse scattering. Transversal and longitudinal slices are indicated. from the fiber axis, due to shear during fiber crystallization.6 Lamellar stacks arranged on a 2D-lattice (cf. “lattice points” in Figure 2, right), which revolve around the fiber-axis are shown in Figure 2a on the right-hand side. Randomly oriented lamellar stacks with constant long spacing L would give rise to lamellar reflections, which fall on a circle. However, it has been reported that four-point patterns often lie on an elliptic trace. Yet, the origin of the elliptic trace is not fully understood. Murthy et al. attributed the elliptic trace to an affine deformation of the macrolattice.29,39,46 Their simulations showed that a mixture of lamellar stacks,29,46 with increased long spacing L for increased tilt angles lead to an elliptic trace in the diffraction pattern. The long spacing L, the tilt angle χ/2, the stack diameter D and the coherence length H are derived from the reciprocal space properties as indicated in Figure 2b (left). The coherence length H of the lamellar stack is retrieved from the width of the lamellar reflections along a longitudinal slice through the center of mass of the reflection and it reflects the variations in lamellar size and long spacing. Note if there is a correlation between L and lamellar tilts as suggested by Murthy et al.29,46 the widths of the four-point reflections are also influenced by a rotational factor. Therefore, we emphasize that values for the coherence length H are only approximated values. A detailed explanation of the extraction of all structural parameters from fourpoint patterns as shown in Figure 2b (left) is given in the next section. Figure 2c (left) shows an X-shaped diffuse pattern. Such a pattern has been attributed to a transition state between a two-point and a fourpoint pattern.44 If a system of large parallel lamellae (yield two-point pattern), arranged perpendicular to the direction of loading is stretched, the lamellae tilt and different long spacings are introduced (Figure 2c, right) yielding an X-shaped diffraction pattern (Figure 2c, left).44,45 If the fiber is further stretched the lamellae break down and fibrils are formed leading to a four-point diffraction pattern. Matlab routines were written in order to determine the structural parameters (long spacing L, tilt angle χ/2, stack diameter D, coherence length H and fibril length LF) from the SAXS patterns (Figure 2b). The analysis is explained in the following subsections. In case of heterobicomponent fibers, the same scattering features as in Figure 2 are observed, but features arising from core and sheath overlap. The difficulty to distinguish between the overlapping scattering features is addressed in the discussion part. Lamellar Reflections. An approximation of the coaxial long spacing L is calculated from the average Bragg spacing Δy of the lamellar reflections, measured parallel to the meridian (Figure 2b). L is calculated from the expression

Figure 2. SAXS patterns of oriented polymer structures: (a) straight four-point pattern,6 (b) elliptical four-point pattern,29,39,46 (c) diffuse X-shaped four-point pattern.44,45 Left: SAXS patterns. Arrows denote which direct space property is derived from a reciprocal space property like, e.g., the long spacing L from the peak position Δy or the coherence length H from the peak width LH. Right: Suggested polymer structure (i.e., lamellar stacks), illustrating the various structural parameters termed in this article (long spacing L, tilt angle χ/2, stack diameter D, coherence length H). The red “lattice points” refer to the 2D-lattice arrangement of the lamellar stacks.

longitudinal slices through the lamellar reflections are fitted to a suitable function by the least-squares method (Figure 4a). A variety of functions, such as Gaussian, Cauchy, Voigt, pseudo-Voigt, and Pearson VII47 have been used in the past for that purpose. In this article, Pearson VII functions (commonly used function in fiber diffraction6,40,41,43) of shape factor m = 2 were used. The function becomes a Lorentzian function for a shape factor m = 1, whereas for m → ∞ it approaches a Gaussian function.48,49 Fitted Pearson VII functions to the intensity distribution of longitudinal slices through the SAXS patterns are given by the following equation:

L = 2π /qLM = λ /(2 sin θLM ) = λ /[2 sin(arctan(Δy/F )/2)] (1) where qLM = 4π[sin θLM]/λ is the scattering vector, θLM is half the scattering angle at the lamellar reflection, λ is the wavelength, Δy is the average distance from the equator to the lamellar reflection (Figure 2b) and F is the fiber-to-detector distance. In order to determine the distances from the equator to the lamellar reflections, data points from

I(y) = Ii /[1 + (2(1/ m) − 1)(2(y − yi )/FWHM)2 ]m 442

(2)

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where I(y) is the intensity in arbitrary units as a function of y coordinates in pixels, Ii is the maximum intensity at the peak center yi, m is the shape factor and f whm is the full width at half-maximum of the function. The peak positions yi are tracked as a function of x (Figure 1, Figure 3a, open circles). Smeared reflections which do not lie on an

The sizes of the lamellar stacks were determined from the Scherrer equation50

size = 0.9λ /[Δ(2θ ) cos θ ] ≈ 0.9λF / FWHM2 − b2

(3)

which makes use of small-angle approximations, e.g., cos θ ≈ 1. The obtained “size” either stands for the coherence length H of the lamellar stack or its diameter D. The full width at half-maximum (fwhm) relates to the corresponding width LH or to the height LD of the lamellar reflections (Figure 2b) and is directly retrieved from fitted longitudinal (Figure 4a) and transversal slices (Figure 4b) through the center of mass (COM) of the lamellar reflections. The widths of the peaks obtained from the curve fitting need to be corrected for the instrumental broadening b. The instrumental broadening was estimated from the beamline characteristics to be 0.22 mm in the vertical direction and 0.004 mm in the horizontal direction. The average and standard deviation of the sizes are finally calculated from the four sizes obtained from the four-point reflections. An example of a longitudinal and transversal slice fitted to Pearson VII functions of shape factor m = 2 is shown in Figure 4. Equatorial Streak. The intense central equatorial streak (Figure 1) can arise from various scattering objects of high aspect ratio that are aligned parallel to the fiber axes; such as amorphous low density regions between fibrils or lamellae, aggregates of fibrils or individual fibrils or a lower density region between fibrils. Although it is known that surface refraction effects can contribute to the streak,51 we assume in the following part that surfaces are not contributing to the equatorial streak in order to estimate average lengths of scattering objects. The width of the equatorial streak at the origin directly relates to the characteristic length of such scattering objects in the direction of the fiber axis. In order to retrieve the width of the intense streak, longitudinal slices through the streak were taken and the resulting peaks were fitted, again, to Pearson VII functions (m = 2). The average lengths of the scattering objects (either fibril length LF or other contributions) are calculated by applying the Scherrer equation to the width of the equatorial streak at the origin, which is obtained by extrapolating the fitted fwhm (e.g., Figure 3b) with a polynomial of second degree. The presented results of these object lengths are limited in resolution due to the pixel nature of the experimental detector. Central Diffuse Scattering. The typical central diffuse scattering is elliptical- (Figure 5a), propeller-, or diamond-shaped (not shown).

Figure 3. (a) Tracked positions of lamellar reflections (○) and equatorial streak (red ▼) for fiber C3. (b) Full width at half-maximum of fitted Pearson VII functions for equatorial streak (red ▼) and fitted polynomial function of second degree (dashed black curve), where (x) indicates the extrapolated fwhm at the origin. elliptic trace are not tracked. The effective average distance Δy from the equator to the lamellar reflections is calculated from all tracked absolute values of positions yi (Figure 3a, open circles), leading to the wanted characteristic long spacing L using eq 1. The standard deviation is also calculated from the Matlab-tracked positions. The average tilt angle χ/2 is calculated as the average of the arctan(xi/yi).

Figure 4. (a) Longitudinal slice through left lamellar reflections of fiber C3 (black curve), Pearson VII fits (red dashed curves). (b) Transversal slice through upper lamellar reflections of fiber C3 at center of mass (black curve), Pearson VII fits (blue dotted curves), and combined fits (red dashed curve). 443

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large reciprocal region is included to emphasize the lamellar reflections. Four-point lamellar reflections are observed for PPS−PET*, PET−PPS, and PET−PET bicomponent fibers (A2, A3, B2, B3, C3, D2, D3, F2) for draw ratios equal or larger than 3. Typically, the observed lamellar reflections lie on an elliptical trace and show a smearing toward the direct beam. The equatorial streak is observable in all SAXS patterns, which indicates that long structures (e.g., fibrils) are present in both components PET and PPS. The central diffuse scattering is mostly elliptical or diamondshaped. Some exceptions are as follows: (i) PPS−PPS fibers, which show for small draw ratios (DR ≤ 3) two-point diffuse reflections along the equator (E1, E2); (ii) PET−PET fibers, which show for high draw ratios (DR = 3.5) strong two-point diffuse reflections (F3); (iii) PET−PPS (c/s = 1/2) fibers (C1, C2), which show for low draw ratios (DR ≤ 3) similar diffuse scattering patterns as those of PPS−PPS fibers. Special scattering features such as tilted streaks are observed for the PET−PPS (c/s = 2/1) fiber (D1) and a meridional streak is observed for the PET−PET fiber (F3). Structural parameters deduced from lamellar reflections and equatorial streaks are summarized in the following subsections. Lamellar Reflections. The structural properties (long spacing L, stack diameter D, coherence length H, tilt angle χ /2) determined from lamellar reflections with elliptical trace, where available, are summarized in Table 2. Note, the transversal slices of the fibers A2, A3 and F2 unfavorably overlap with central diffuse scattering. Therefore, the stack diameters cannot be retrieved for these patterns using the method described above. The fiber (B2) PPS−PET* (c/s = 2/1) shows lamellar reflections that smear out toward the direct beam and overlap with central diffuse scattering close to the origin. Therefore, the method described above to extract stack diameter and coherence length was not applied. The pattern

Figure 5. Observed diffuse scattering (left) with corresponding proposed structure models (right). Previous literature states that the diffuse scattering is made of two components,52 with the two independent components corresponding to scattering from aligned and randomly distributed objects. It is impossible to distinguish scattering from dense regions dispersed in a less dense medium or vice versa. Accordingly, the observed central diffuse scattering could either arise from low-density regions or from crystals of higher density. Low density regions are either expected between lamellae53 (sizes