The consequence of the lack of selective reflections on the modelization of TGB structures

Contrary to our previous calculations, selective reflection bands related t o the TGB block thickness could not be put in evidence by spectroscopic way . For TGB(C) phases this lack of reflections confirms the penetration of th e twist inside the blocks instead of an expulsion into the grain-boundaries . So, along the helical axis of the TGB(C) structure the molecular director orientation is nearly continuous. For TGB(A) phases our results are consis tent with recent X-rays measurements performed on compounds of the same ser ies and showing that the number of smectic blocks per pitch is higher than previously admitted. So the fluctuations undergone by the director at the b oundaries are very small. Anomalous optical activity properties have been observed in reflection mode between crossed polars. Those phenomena can be explained by a cholesteric like twist that confirms the above structures proposed for TGB(C) and TGB(A ) phases.