When poly(d-lactide) (PDLA) is mixed with poly(l-lactide) (PLLA) of similar molecular weight and crystallized from the melt in ultrathin films (30 nm), hexagonal single crystals are obtained at the equimolar composition, and triangular single crystals at non-equimolar compositions. Here, stereo complexes are formed by blending PDLA with a PLLA incorporated as the central block of a triblock copolymer made with poly(2-dimethylaminoethyl methacrylate) (PDMAEMA-b-PLLA-b-PDMAEMA or BAB); the resulting single crystals are studied by in-situ atomic force microscopy. The imbalance between the PDLA and PLLA characteristics, due to the presence of the B blocks of the triblock copolymer, leads to triangular single crystals at equimolar compositions, the hexagonal single crystal morphology being shifted to off-equimolar compositions. The importance of the shift depends upon the A/B ratio and the molecular weight of each species. (C) 2017 Elsevier Ltd. All rights reserved.