Microphase-separated structures of block copolymers (BCPs) are of increasing interest for the application in next-generation nanolithography. The periodicity (domain-spacing, d) of the phase-separated structure, which dictate the feature size in the lithography process, can be decreased by reducing the degree of polymerization (DP). This approach, however, has a fundamental limit that the phase-separation does not occur and disordered state is achieved when the DP of each block is reduced below a critical value. Here, we reports a novel strategy that is capable of downsizing the domain-spacing independent of the molecular weight by restricting the BCP chain dimensions via intramolecular cross-linking reaction. In this study, polystyrene-b-poly(rac-lactide) (PS-b-PLA) was selected as a model case (Figure). A PS-b-PLA possessing pendant olefin groups on the PS block (SBS-b-LA; Mn,SBS = 7,980, Mn,LA = 8,210, Mw/Mn = 1.05) was synthesized via living anionic copolymerization of styrene (S; 70 mol%) and p-3-butenyl styrene (BS; 30 mol%) followed by ring-opening polymerization of rac-lactide (LA). The cross-linkable SBS-b-LA was then subjected to the olefin metathesis reaction using 2nd generation Grubbs' catalyst (G2) under highly diluted condition to yield the intramolecularly cross-linked SBS-b-LA (SBS(cl)-b-LA). Small angle X-ray scattering (SAXS) analyses were carried out on SBS-b-LA and SBS(cl)-b-LA in the bulk. Primary scattering peaks (q*) as well as the higher-ordered scattering peaks at the 2q* and 3q* positions were observed in the SAXS profiles for both BCPs, confirming the well-ordered lamellar formations (Figure). Importantly, the d value for the resulting morphology of SBS(cl)-b-LA (14.3 nm) was found to be smaller than that of SBS-b-LA (16.3 nm) despite their comparable molecular weight. This result demonstrates that the intramolecular cross-linking is a promising approach for decreasing the domain-spacing of microphase-separated structures.

Intramolecular cross-linking of poly(styrene-b-lactide)s for decreasing domain sizes in microphase-separated structures

255th National Meeting (2018)

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