Elsevier

Polymer

Volume 42, Issue 4, February 2001, Pages 1601-1612
Polymer

Structural changes during deformation of Kevlar fibers via on-line synchrotron SAXS/WAXD techniques

https://doi.org/10.1016/S0032-3861(00)00460-2Get rights and content

Abstract

On-line studies of structure and morphology changes in Kevlar 49 fibers during stretching were carried out using synchrotron simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. A unique two-dimensional (2D) image analysis method was used to extract quantitative information of crystal, amorphous and mesomorphic fractions from 2D WAXD patterns. Results showed that about 20% of the fraction (mass) in the Kevlar 49 fiber was mesophase, 50% was crystalline and 30% was amorphous. There were transitions between crystal, amorphous and mesomorphic fractions during deformation. The crystal orientation was obtained in terms of the Herman's orientation function f2 from the azimuthal scan of the (200) crystal reflection. The crystal orientation was found to be quite high in the Kevlar 49 fiber and increased with the stretch ratio. The fibril length and misorientation were also obtained from 2D SAXS patterns by using the Ruland method. Results showed that the fibril length decreased with the stretch ratio until 2.0% and then increased. The misorientation decreased with increasing stretch ratio.

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)

  • M.G. Northolt

    Eur Polym J

    (1974)
  • A.M. Hindeleh et al.

    Polymer

    (1989)
  • H.K. Herglotz

    J Colloid Interface Sci

    (1980)
  • M.G. Northolt et al.

    Polymer

    (1985)
  • M.G. Northolt

    Polymer

    (1980)
  • S.R. Allen et al.

    Polymer

    (1989)
  • L. Penn et al.

    Polymer

    (1979)
  • S. Bruckner et al.

    Prog Polym Sci

    (1991)
  • S. Bruckner et al.

    Prog Polym Sci

    (1991)
  • C.L. Jackson et al.

    Polymer

    (1994)
  • D.T. Grubb et al.

    Polymer

    (1991)
  • H.H. Yang

    Kevlar aramid fiber

    (1993)
  • M.G. Northolt et al.

    J Polym Sci, Polym Lett Ed

    (1973)
  • K. Haraguchi et al.

    J Appl Polym Sci

    (1979)
  • A.M. Hindeleh et al.

    J Macromol Sci, Phys B

    (1984)
  • M. Panar et al.

    J Polym Sci, Polym Phys Ed

    (1983)
  • C.O. Pruneda et al.

    Polym Prepr ACS Div Polym Chem

    (1981)
  • R. Barton

    J Macromol Sci, Phys B

    (1985)
  • R. Hagege et al.

    J Microsc

    (1979)
  • M.G. Dobb et al.

    J Polym Sci, Polym Phys Ed

    (1977)
  • M.G. Dobb
  • L.S. Li et al.

    J Macromol Sci, Phys B

    (1983)
  • R.J. Young et al.

    J Mater Sci

    (1992)
  • Y. Li et al.

    J Polym Sci, Part B: Polym Phys

    (1991)
  • B. Chu et al.

    J Polym Sci, Part C: Polym Lett

    (1990)
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