Estimating the Segregation Strength of Microphase-Separated Diblock Copolymers from the Interfacial Width

The ever-growing catalog of monomers being incorporated into block polymers affords exceptional control over phase behavior and nanoscale structure. The segregation strength, $\chi N$, is the fundamental link between the molecular-level detail and the thermodynamics. However, predicting phase behavior mandates at least one experimental measurement of $\chi N$ for each pair of blocks. This typically requires access to the disordered state. We describe a method for estimating $\chi N$ from small-angle X-ray scattering measurements of the interfacial width between lamellar microdomains, $t_x$, in the microphase-separated melt. The segregation strength is determined by comparing $t_x$ to self-consistent field theory calculations of the intrinsic interfacial width, $t_i$, as a function of the mean-field $\chi N$. The method is validated using a series of independent experimental measurements of $t_x$ and $\chi N$, measured via the order-disorder transition temperature, $T_{\text{ODT}}$. The average absolute relative difference between $\chi N$ calculated from $t_x$ and the value calculated from $T_{\text{ODT}}$ is a modest 11%. Corrections for nonplanarity of the interfaces are investigated but do not improve the agreement between the experiments and theory. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.