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The crystallization of the PLLA block within the microphase-separated lamellar microdomain in a poly(styrene)-b-poly(l-lactide) (PS-PLLA) diblock copolymer was carried out from hard confinement (i.e., the crystallization temperature of PLLA (T_c,PLLA) < the glass transition temperature of PS (T_g,PS)) to soft confinement (i.e., T_c,PLLA ≥ T_g,PS), where interesting morphological evolution was observed. At T_c,PLLA < T_g,PS, a typical confined morphology was obtained, while at T_c,PLLA ≥ T_g,PS, a unique undulated morphology was seen for the first time. The amplitude and periodicity of the undulation instability are dependent upon the orientation of microphase-separated lamellae. The PLLA crystals under confinement formed perpendicular morphology having crystalline chains normal to the lamellae as evidenced by 2D small-angle X-ray scattering and wide-angle X-ray scattering. On the basis of morphological …