Characterizing the interfaces of block copolymers with high $\chi $

In order for block copolymer (BCP) directed self-assembly (DSA) to be able to pattern features below 10 nm there must be materials which can spontaneously assembly at the required length scales. For the smallest features this will require phase separation where the total chain lengths are under 50 monomer units, demanding very large interaction parameters ($\chi )$ to have an order-disorder transition. One of the key parameters for DSA will be the interfacial width between the blocks, which is expected to be correlated to the interaction parameter and will help determine the line edge roughness (LER). We have used resonant soft X-ray reflectivity to investigate a series of high $\chi $ BCPs with different compositions and molecular weights to determine the interfacial width and degree of phase separation. We use these results to estimate the value of $\chi $ and determine relationships between $\chi $ and the interfacial mixing.