Structural changes during deformation of Kevlar fibers via on-line synchrotron SAXS/WAXD techniques
Introduction
Kevlar aramid fiber, having a chemical structure of poly(p-phenylene terephthalamide) (PPD-T), is a high-performance fiber that possesses high tensile modulus, strength and thermal stability [1]. In the past two decades, its structure, morphology and the relationship between structure and properties have been studied extensively. For example, Northolt and van Aartsen [2], [3] assumed that it had a centered monoclinic (pseudo-orthorhombic) unit cell and proposed a crystal lattice model of PPD-T. Haraguchi, Kajiyama and Takayanagi observed a second crystal form of PPD-T when it coagulated from a nonaqueous liquid [4]. Hindeleh et al. [5], [6] and Panar et al. [7] studied the apparent crystallite size, orientation and also calculated the crystallinity from the equatorial X-ray diffraction using a conventional X-ray source; while Herglotz [8] observed the discrete scattering maximum in small-angle X-ray scattering of PPD-T. Panar et al. [7] proposed a crystalline structure model, where the stacks of crystalline layers are perpendicular to the fiber axis and separated by defect layers that are composed of chain bends and possibly half the chain ends. Pruneda et al. [9] proposed a somewhat similar defect model for the Kevlar (PPD-T) fiber. They assumed that the polymer chain ends congregate in a defect plane where breaks are more likely to occur. Panar et al. [7] proposed that the Kevlar fiber had a paracrystalline structure with the crystalline correlation length of 80–100 nm according to the results by Hindeleh et al. [5] and Barton [10]. The Kevlar fiber was observed to exhibit a pleat structure by optical microscopy with a spacing of 500–600 nm [7], [11], [12], [13], [14].
The structural changes of the aramid fibers during deformation have also been investigated in several studies by a variety of methods including mechanical testing [15], [16], [17], [18], WAXD [19], [20] and Raman spectroscopy [21], [22], [23]. Recently simultaneous WAXD/SAXS methods have become a unique tool to investigate the structure and morphology of polymer [24]. Synchrotron radiation provided us a more powerful means to carry out on-line research using simultaneous WAXD/SAXS for the study of fiber deformation [25], [26], [27]. In the present work, our goal is to obtain a more in-depth understanding of the structural changes during fiber deformation.
Recent experimental findings in polymer fibers [28], [29], [30] indicate that the fiber structure should include an intermediate phase between crystalline and amorphous fractions, which may be due to the lattice dislocations or one- and two-dimensional (1D and 2D) disordering of polymer chains in the drawn fibers. The intermediate phase (mesophase) represents a state of order between the zero long-range ordering (amorphous state) and the three-dimensional (3D) crystalline ordering. The mesophase has been reported in some polymer fibers, such as polypropylene [31], [32], [33], polyethylene [28] and PET [30]. Although the mesophase in the Kevlar fibers has not been discussed extensively, Dobb et al. [34] have stated that there was evidence for 2D ordering in the Kevlar fibers based on the layer-like streaking in electron diffraction patterns. English et al. [35], [36], [37], [38], [39] studied the structural dynamics of the Kevlar fiber using 2H NMR spectroscopy and pointed out that the dynamics was quite heterogeneous in structure. Thus, a less perfectly ordered structure could exist, probably from chains residing mostly at or close to the surface of the crystallites. The difficulty for the study of the mesophase is that no straightforward method that is able to extract the fraction of the mesophase quantitatively exists. In this study, we introduce an integrative X-ray fiber diffraction image analysis approach that based on the model is capable of extracting the fraction of the mesophase quantitatively. We will also illustrate other additional methods that can extract structural and morphological information from the 2D WAXD/SAXS patterns.
Section snippets
Experimental
The fibers studied were commercial Kevlar 49 fiber. Synchrotron measurements were carried out at the State University of New York (SUNY) X3A2 beam line in the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). The wavelength used was 1.54 Å. A 3-pinhole collimator system [40], [41] was used to reduce the beam size to 0.6 mm in diameter. The 2D WAXD patterns were recorded by a MAR CCD X-ray detector (MARUSA) for quantitative image analysis. Separate simultaneous SAXS
Background correction
In the data analysis of 2D WAXD and SAXS patterns, the elimination of background scattering from the air pathway and instrumentation is critical and non-trivial. In addition, the air scattering also changes due to the sample thickness change and incident beam fluctuation during deformation. In this study we have developed a new method to solve this problem using the PIN-diode beam-stop with the following procedures.
As shown in Fig. 1, we assume that the incident X-ray beam i0(t) is stable in
Results and discussion
The WAXD patterns from fiber deformed at different stretch ratios are shown in Fig. 2. Two strong reflections located on the equator can be indexed as (110) and (200), respectively. These two reflections show a small spread along the azimuthal direction indicating that the fiber has a high degree of crystal orientation along the fiber axis. The spread became narrower with increasing stretch ratio indicating that the orientation increases with increasing stretch ratio. The successive layer lines
Conclusions
A novel 2D image analysis method was introduced to extract quantitative information about the fractions of the crystal, amorphous and mesomorphic phases from WAXD fiber patterns. The average percentage of the mesophase in the Kevlar 49 aramid fiber using this method was about 20%, with the corresponding crystal phase being about 50% and the amorphous phase being about 30%. The mesophase may represent the highly oriented fraction of the chains with lattice registrations too poor to be considered
Acknowledgements
The authors are grateful for the financial support of this work by a grant from the US Army Research Office (DAAG559710022). A discussion with Dr R. Barton from DuPont was most useful for the preparation of this work.
References (50)
Eur Polym J
(1974)- et al.
Polymer
(1989) J Colloid Interface Sci
(1980)- et al.
Polymer
(1985) Polymer
(1980)- et al.
Polymer
(1989) - et al.
Polymer
(1979) - et al.
Prog Polym Sci
(1991) - et al.
Prog Polym Sci
(1991) - et al.
Polymer
(1994)
Polymer
Kevlar aramid fiber
J Polym Sci, Polym Lett Ed
J Appl Polym Sci
J Macromol Sci, Phys B
J Polym Sci, Polym Phys Ed
Polym Prepr ACS Div Polym Chem
J Macromol Sci, Phys B
J Microsc
J Polym Sci, Polym Phys Ed
J Macromol Sci, Phys B
J Mater Sci
J Polym Sci, Part B: Polym Phys
J Polym Sci, Part C: Polym Lett
Cited by (160)
Superinsulating nanocellulose aerogels: Effect of density and nanofiber alignment
2022, Carbohydrate PolymersWide-angle X-ray diffraction and small-angle X-ray scattering studies of elastomer blends and composites
2022, Elastomer Blends and Composites: Principles, Characterization, Advances, and ApplicationsIn-situ SAXS study on pore structure change of PAN-based carbon fiber during graphitization
2021, Microporous and Mesoporous Materials