Ternary block copolymer (BCP)-homopolymer (HP) blends offer a simple method for tuning nanostructure sizes to meet application-specific demands. Comprehensive dissipative particle dynamic (DPD) simulations were performed to study the impact of polymer interactions, molecular weight, and HP volume fraction (φ HP ) on symmetric ternary blend morphological stability and domain spacing. DPD reproduces key features of the experimental phase diagram, including lamellar domain swelling with increasing φ HP , the formation of an asymmetric bicontinuous microemulsion at a critical HP concentration φ * HP , and macrophase separation with further HP addition. Simulation results matched experimental values for φ * HP and lamellar swelling as a function of HP to BCP chain length ratio, α = N HP /N BCP . Structural analysis of blends with fixed φ HP but varying α confirmed that ternary blends follow the wet/dry brush model of domain swelling with the miscibility of HPs and BCPs depending on α. Longer HPs concentrate in the center of domains, boosting their swelling efficiencies compared to shorter chains. These results advance our understanding of BCP-HP blend phase behavior and demonstrate the value of DPD for studying polymeric blends.

The thin film phase behavior of ternary blends consisting of symmetric poly(styrene) (PS)‐b‐poly(dimethylsiloxane)(PDMS), PS, and PDMS was investigated using X‐ray reflectivity (XRR) and atomic force microscopy (AFM). This system is strongly segregated, and the homopolymers are approximately the same length as the corresponding blocks of the copolymer. The XRR and AFM data are used to quantify changes in domain spacing (L) and morphology evolution with increasing homopolymer content (Φ H). In 100 nm thick films, from Φ H = 0 to 0.20, the system maintains a perfect parallel lamellar structure and domains swell as predicted based on theory; however, from Φ H = 0.30 to 0.50, a morphology transition to a “dot pattern” morphology (tentatively identified as perforated lamellae) and mixed morphologies were observed before macrophase separation. In thicker films, dot patterns were observed for a broad range of Φ H before macrophase separation. The absence of the bicontinuous microemulsion phase reported for bulk blends and thin films of perpendicular lamellae and the presence of dot patterns/perforated lamellae are attributed to preferential migration of the PDMS homopolymer to the wetting layers located at the substrate and free air interfaces, which leads to an asymmetric composition within the film and morphology transition. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1443–1451