Morphology–Density Relations in Polyamide 12

O. Verkinderen1, P. Adriaensens2, P. Van Puyvelde3 and B. Goderis1

1Polymer Chemistry and Materials, KU Leuven, Heverlee, Belgium
2Applied and Analytical Chemistry, UHasselt, Diepenbeek, Belgium
3Soft Matter, Rheology and Technology, KU Leuven, Heverlee, Belgium

A two-phase model consisting of alternating amorphous and crystalline layers is often used to describe the morphology of semi-crystalline polymers. However, this simple model – at least in the case of polyamide 12 (PA12) – does not allow rationalizing the outcome of different techniques which are sensitive to particular features of the semicrystalline morphology. Therefore, several authors argued for the existence of a third phase [1], [2]. This third phase is the rigid amorphous fraction (RAF) and is in fact a phase with a higher density and lower mobility than the amorphous fraction but without the order of the crystalline. Although the existence of this RAF seems beyond dispute, there clearly is no consensus on its topology. Based on a combination of temperature dependent WAXD, SAXS, solid state NMR, and density measurements a new morphological model is proposed for PA12, which consists of alternating solid and mobile (liquid) amorphous layers. The solid layers are in turn composed of crystalline and rigid amorphous patches with the density of the latter being intermediate between that of mobile amorphous and crystalline matter. This morphology, which includes a clear picture of the RAF topology, leads to a similarity in the WAXD and SAXS based crystallinity as well as to matching SAXS based dense and NMR based rigid fractions. The model adequately describes the SAXS patterns and produces overall densities that are identical to experimentally observed ones.

[1] B. Goderis, P. G. Klein, S. P. Hill, and C. E. Koning, Prog. colloid Polym. Sci., 130, 40 (2005). (link)
[2] C. Hedesiu, D. E. Demco, R. Kleppinger, G. Vanden Poel, W. Gijsbers, B. Blümich, K. Remerie, and V. M. Litvinov, Macromolecules, 40, no. 11, 3977 (2007). (link)