Studies on Limit of Symmetry and Feature Size of Block Copolymer Lamellar Structure

Abstract

Block copolymers have been extensively investigated due to their various self-assembled structures such as lamellae, gyroids, cylinders and spheres depending on the volume fraction (f) of each block, the degree of polymerization (N), and the Flory−Huggins segmental interaction parameter (χ). In addition, 10 – 100 nm scale of self-assembled structures can provide great applicability for various fields. Especially, lamellar structure, one of the basic morphology of block copolymers has been widely explored due to the potential application to line nanopatterns in lithography. However, when lamellar structure is used for fabricating line nanopatterns, there are some inherent limitations which should be solved for next-generation lithography. Firstly, the available volume fraction range for lamellar structure of conventional AB diblock is commonly located from 0.35 to 0.65 in the strong segregation regime. Therefore, highly asymmetric lamellar morphology, where the width of each lamellar nanodomains is greatly different, cannot be produced by diblock copolymers. Secondly, there is a boundary where order-to-disorder transition happens when the degree of polymerization (N) decreases which means reduction in domain size at a given χ value. Because reducing feature size is a challenging issue to manufacture high density and miniaturized devices, block copolymers with huge χ value are required. In this thesis, I dealt with the phase behavior of binary blend of two block copolymers having hydrogen bonding interaction arising from the hydroxyl group in 4-hydroxystyrene and nitrogen atom in vinylpyridine to investigate experimentally the asymmetric lamellae forming mechanism and to enhance the asymmetry of lamellar structure by increasing the hydrogen bonding power between two blocks. Furthermore, I achieved sub-3 nm feature size of lamellar structure with polydihydroxystyrene-block-polystyrene (PDHS-b-PS) which has very huge χ value resulted from the large polarity of the dihydroxyl group in PDHS generates. In chapter 2, I investigated the phase behavior of (A-B)/(Aʹ-C) binary blend consisting of asymmetric and higher molecular weight of polystyrene-block-poly(2-vinylpyridine) copolymer (as-PS-b-P2VP) and asymmetric and lower molecular weight of deuterated polystyrene-block-poly(4-hydroxystyrene) copolymer (as-dPS-b-PHS), which has a hydrogen bonding between PHS block (B) and P2VP (C) block. A binary blend of these two block copolymers showed highly asymmetric lamellar microdomains. By using neutron reflectivity and curve-fitted scattering length density profile, I found that dPS chains do not exist in the middle of P2VP and PS microdomains, and the junction of two blocks in as-dPS-b-PHS is located at the interface of PS and P2VP microdomains. This result shows that phase transformation of a binary blend of as-dPS-b-PHS and as-PS-b-P2VP from BCC spherical microdomains to asymmetric lamellar microdomains is caused by the interface curvature change, which contracts long P2VP chains but stretches short PHS chains to enhance hydrogen bonding between P2VP and PHS, consistent with the strong segregation theory. In chapter 3, I investigated the effect of the degree of hydrogen bonding on phase transformation from spherical microdomains to highly asymmetric lamellar microdomains in (A-B)/(A-C) binary blends. Two binary block copolymer blends are considered: (1) asymmetric polystyrene-block-poly(4-vinylpyridine) copolymer (as-PS-b-P4VP) and asymmetric polystyrene-block-poly(4-hydroxystyene) copolymer (as-PS-b-PHS) and (2) as-PS-b-P2VP and as-PS-b-PHS with various molecular weights and volume fractions of each block copolymer. Because the degree of hydrogen bonding between P4VP and PHS is much stronger than that of P2VP and PHS, PS-b-P4VP/PS-b-PHS blend showed enhanced asymmetric lamellar microdomains having lamellar width ratio of 6:1, which is a 50% increase of that (4:1) obtained by PS-b-P2VP/PS-b-PHS blend. Furthermore, due to higher Flory-Huggins segmental interaction parameter (χ ~ 0.3) of PS/P4VP compared with that (~ 0.1) of PS/P2VP, I obtained a lamellar width of P4VP/PHS down to ~ 5 nm. In chapter 4, I synthesized, via a reversible addition-fragmentation chain-transfer (RAFT) polymerization, PDHS-b-PS showing lamellar and cylindrical microdomains by adjusting the volume fraction of PS block (fPS). I found that Flory-Huggins interaction parameter (χ) between PDHS and PS is very large, ~ 0.7 at 170 ℃ which is one of the largest value reported in the literature so far. Due to huge χ, the lamellar domain spacing of PDHS-b-PS with 2.1 kg/mol and fPS = 0.5 was only 5.9 nm; thus sub-3 nm feature size was successfully obtained. Furthermore, PDHS-b-PS with 4.2 kg/mol and fPS = 0.79 showed hexagonally packed cylinders with ~ 4 nm diameter of cylinder.