Temperature Experiments for Improved Accuracy in the Calculation of Polyamide-11crystallinity by X-ray Diffraction

The calculation of the crystalline index (CI) of polymeric materials by X-ray diffraction (XRD) may be rendered difficult by the number of overlapping crystalline phases and amorphous halos that contribute to the diffraction pattern. Rilsan Polyamide-11 (PA-11), produced by Arkema Inc., is a semi-crystalline polymer with an eleven-carbon repeat unit belonging to the general class of Nylon polymers. This material presents the peculiarity of having a smectic phase along with three crystalline phases. Samples from melt-extruded Rilsan PA-11 oil pipeline liners that had been subjected to various aging conditions were analyzed by XRD. From wide-angle X-ray scattering (WAXS) patterns their CI was calculated, which were compared to values obtained from differential scanning calorimetry (DSC) measurements. Severe overlaps among the crystalline, amorphous and smectic phases of Rilsan PA-11 were observed, which were further convoluted by the amorphous phases of additives in the polymer matrix. Initial attempts to calculate CI reliably from XRD data were therefore unsuccessful. Subsequently, hot stage XRD was used to perform peak decomposition on three temperature patterns (90oC, 150oC, and 180oC). A linear evolution for smectic and amorphous peak positions was seen in this temperature range. Extrapolation of peak positions to room temperature conditions helped to refine the experimental patterns and greatly increased the accuracy of the crystalline index calculation by XRD, which is seen by comparison with DSC data. INTRODUCTION Rilsan PA-11 is a semi-crystalline polymer with an eleven-carbon repeating unit belonging to the family of Nylon polymers. It is a high-performance polymer used in a variety of applications such as airbrake tubing for semi-tractor trailers, coatings, and liners for high temperature crude oil and gas pipelines. Several studies completed on PA-11 have elucidated many of the crystalline forms of this polymer and have found that its X-ray patterns resemble those of Nylon 6,6 [1]. Indeed, Slichter reported that the reflections of PA-11 could be indexed using the triclinic structure described by Bunn and Garner for Nylon 6,6. Slichter [1, 2] also discussed the possibility for the polymer chains to pack parallel or anti-parallel to each other in ordered arrays. Kawaguchi and co-workers [3] reported that anti-parallel chain packing could be described with a triclinic cell as observed by Little [4], or by a monoclinic cell as they observed it. They also stressed that it would be unlikely statistically to create only parallel chain arrangements from either melt or solution, and that a combination of parallel and anti-parallel chain organization is more probable in PA-11 lamellar structure. Newman and co-workers [5] reported that the triclinic α-form transforms reversibly to the pseudo-hexagonal δ-form at 95°C, which is known as the Brill transition temperature, following a change in thermal expansion coefficient in the [010] direction. A disordered pseudo-hexagonal Copyright ©JCPDS International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48. 170