The amorphous phase geometrically confined at the nanometric length scale in semi-crystalline polymers exhibits different conformational dynamics with respect to the pure amorphous situation. In this work, we have studied the influence of the crystalline phase on the structural relaxation phenomena occurring in the amorphous phase by characterizing the cooperative rearranging region (CRR) in poly(L-lactic acid), PLLA, a well known biodegradable polyester. More particularly, the evolution of the characteristic cooperativity length xi(Tg), at the glass transition is investigated in function of the crystallinity degree using Temperature Modulated Differential Scanning Calorimetry, TMDSC. Thermal analyses have been performed on PLLA cold crystallized at 80 degrees C for different durations in order to obtain a crystallinity degree varying between 0 and 42%. We show that two types of mobile amorphous phases exist: the amorphous matrix and the amorphous fraction trapped in the spherulites. For this latter one, a clear confinement effect is shown despite the fact that the amorphous domain size is larger than xi(Tg). This behavior is attributed to the chemical connection between the mobile and the rigid fraction of the polymer. (c) 2008 Elsevier Ltd. All rights reserved.