Double-stranded helices of 101 and 91 it-PMMA (10 units per turn and 9 units per turn, respectively) were constructed based on reported bond lengths, bond angles and internal rotation angles.16,24 Crystal models were assembled by arranging eight identical double-stranded helices in a hexagonal packing mode within a periodically replicated orthorhombic unit cell (Fig. 1a). The fiber axes of the chains were oriented in the z-direction and to eliminate the terminal melting propensity of finite polymeric chains, each it-PMMA fragment was connected to its periodic image in the z-direction, mimicking an infinite PMMA crystal (Fig. 1a). Each cell displayed initial dimensions of 4.20 and 2.43 nm in the x- and y-directions respectively, and varied in length along the z-direction depending on the helical pitch of the chains; 2.10 and 1.84 nm for the 101 and 91 it-PMMA crystals respectively. We examined structural perturbations associated with the handedness (right or left) and orientation (down/down or up/down) of the helices by considering four packing motifs