Polymer-dispersed liquid crystals (PDLCs) show great application potential in the areas of displays and smart windows. However, their electro-optical (E-O) properties such as contrast ratio and threshold voltage still need further improvement. In this study, the effects of α-substituted acrylate monomers on the morphology and E-O properties of PDLC composite films were systematically studied. It was found that the large substituent tended to increase the void size of the polymer matrix, while the small fluorine substitution led to a microsphere-type polymer morphology, which deteriorated the E-O performance. Finally, a largely improved E-O performance of low threshold voltage (0.437 V/μm), low saturation voltage (1.012 V/μm), and high contrast ratio (27) was achieved in an 8 μm-thick film by the addition of a chlorine-substituted monomer. This study provides a new approach for optimizing PDLCs from a material perspective.