Synthesis and Characterization of Polycarbonate-containing All-organic High-χ Block Copolymers for Directed Self-assembly

To extend the scaling beyond the most widely used block copolymer (BCP), poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA), a new organic high-chi BCP based was developed. Polystyrene-b-polytrimethylene carbonate (PS-b-PTMC) BCP was synthesized using ring opening polymerization (ROP) of trimethylene carbonate from hydroxy-functional polystyrene (PS-OH) with diazabicyclo[5.4.0]undec-7-ene (DBU) as the base catalyst. The resulting BCP was characterized by H-1 nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) to confirm the complete chain extension of the polystyrene macroinitiator. For the DBU catalyzed BCP, the GPC trace revealed a trimodal distribution indicating the presence of coupling product and homo-PTMC impurity. Using purification techniques, homo-PTMC impurity was isolated to afford purified BCP. Thin-film morphologies of the non-purified and the purified BCPs on poly(methyl methacrylate) (PMMA) coated substrates followed by short thermal annealing were characterized using atomic force microscopy (AFM) analysis. The non-purified BCP showed island morphology with a step-height of 17 nm whereas the purified PS-b-PTMC showed a flat film with parallel cylinders of 16.4 nm pitch row-to-row distance. The ability for the PS-b-PTMC to self-assemble below 20-nm pitch resolution confirms the high interaction parameter, chi, between the PS and PTMC blocks and is a promising candidate as a high-chi BCP for directed self-assembly (DSA) application.