Many semicrystalline polymers undergo crystal phase transitions prior to melting. The crystal to crystal transition observed for polyamides is known as the Brill transition. The existence of such a transition is however still not quite clear in the case of PA6. Through a detailed analysis of the XRD data, Murthy et al were the first authors to report a Brill transition for PA6: they showed that the RT monoclinic  phase transforms into a high-temperature monoclinic ’ form beyond 160°C. Furthermore, the authors claim that the H-bonded sheets structure is preserved during the transition [1]. The biaxial stretching of thermoplastic films is usually performed below the melting point in view of improving both mechanical and barrier properties. Nevertheless polyamides such as PA6 display a poor ability for biaxial orientation when compared to polyolefins [2-4]. The presence of the sheet-like H-bonded structure is usually evoked to explain this behavior. The present study deals with the influence of the  phase “perfection” on the Brill transition and its consequences on the biaxial stretching behavior of PA6. PA6 films in predominantly mesomorphic phase were obtained by a melt-cast process and quenched at RT. To prepare samples in  form with similar supramolecular organization (comparable lamella thickness, number of tie molecules, spherulite size…), two annealing procedures have been used. In the first one, films were annealed at 190°C under vacuum during 1 hour and are referenced as PA6-1. In the second one, cast films were annealed in superheated water at 150°C for 40 min in an autoclave (PA6-2). Both annealing procedures allow formation of samples in predominantly monoclinic -form. However, slight differences in the RT crystalline structure have been identified; in particular the gap between the H-bonded chains and the H-bonded sheets is more important for PA6-2. In-situ variable temperature XRD measurements seem to reveal a Brill transition beyond 165°C. However, while a HT monoclinic phase is clearly evidenced for PA6-2 in good agreement with literature, a HT pseudohexagonal form is achieved from PA6-1 samples. Furthermore, in the latter case, no more diffraction peak originating from H-bonded planes is observed, suggesting a random distribution of H-bonds around the chain axis. The simultaneous biaxial stretching behavior has been studied below and above the Brill transition temperature for both samples. Poor ability for biaxial orientation is observed for PA6-2 whatever the temperature. By contrast, a huge improvement of biaxial drawability is obtained for PA6-1 at temperature above the Brill transition. This result thus confirms that there is no more sheet-like organization in the HT pseudo-hexagonal phase of PA6-1. REFERENCES 1. N. S. Murthy, S. A. Curran, S. M. Aharoni, H. Minor, Macromolecules 24, 3215-3220 (1991) 2. S. Rhee, J. L. White, J. Polym. Sci 40, 2624-2640 (2002). 3. O. Persyn, V. Miri, J.M. Lefebvre, V. Ferreiro, T. Brink, A. Stroeks J. Polym. Sci: Part B Polym. Phys 44, 1690-1701 (2006). 4. A.K. Taraiya, I.M. Ward J. Appl. Polym. Sci. 59, 627-638 (1996).