A library of Y-shaped poly(dimethylsiloxane) (PDMS)-b-poly(D,L-lactide) (PLA) diblock copolymers and their corresponding linear counterparts were synthesized, and their morphologies and feature sizes in bulk and thin films were compared using small-angle X-ray scattering (SAXS), scanning force microscopy (SFM), and grazing incidence small-angle X-ray scattering (GI-SAXS). For macromolecular isomers with approximately the same molecular weights and volume fractions (PLA f(L): 0.20 and 0.35), different thin film morphologies were obtained for the Y-shaped PDMS-b-PLA derivatives when compared to the corresponding linear derivatives. These data also gave us the option to determine some of the key parameters of these block copolymers. A relatively high chi value of 0.24 was found for these PDMS-b-PLA systems and was shown to be influenced by architecture.