Self-Assembly of Hierarchical High-χ Fluorinated Block Copolymers with an Orthogonal Smectic-within-Lamellae 3 nm Sublattice and Vertical Surface Orientation

Hierarchical structure-within-structure assemblies offer a route toward increasingly complex and multifunctional materials while pushing the limits of block copolymer self-assembly. We present a detailed study of the self-assembly of a series of fluorinated high-χ block copolymers (BCPs) prepared via postmodification of a single poly(styrene)-block-poly(glycidyl methacrylate) (S-b-G) parent polymer with the fluorinated alkylthiol pendent groups containing 1, 6, or 8 fluorinated carbons (termed trifluoro-ethanethiol (TFET), perfluoro-octylthiol (PFOT), and perfluoro-decylthiol (PFDT), respectively). Bulk X-ray scattering of thermally annealed samples demonstrates hierarchical molecular assembly with phase separation between the two blocks and within the fluorinated block. The degree of ordering within the fluorinated block is highly sensitive to synthetic variation; a lamellar sublattice was formed for S-b-GPFOT and S-b-GPFDT. Thermal analyses of S-b-GPFOT reveal that the fluorinated block exhibits liquid crystal-like ordering. The complex thin-film self-assembly behavior of an S-b-GPFOT polymer was investigated using real-space (atomic force microscopy and scanning electron microscopy) and reciprocal-space (resonant soft X-ray scattering (RSoXS), grazing incidence small- and wide-angle scattering) measurements. After thermal annealing in nitrogen or vacuum, films thicker than 1.5 times the primary lattice spacing exhibit a 90-degree grain boundary, exposing a thin layer of vertical lamellae at the free interface, while exhibiting horizontal lamellae on the preferential (polystyrene brush) substrate. RSoXS measurements reveal the near-perfect orthogonality between the primary and sublattice orientations, demonstrating hierarchical patterning at the nanoscale.

Tunable Thin Film Periodicities by Controlling the Orientation of Cylindrical Domains in Side Chain Liquid Crystalline Block Copolymers

A facile approach to block copolymer (BCP) domain orientation control in thin films has been demonstrated by employing a BCP with liquid crystalline semifluorinated side chains by tuning the composition of the copolymers of the bottom surface layer (BSL). 1H,1H,2H,2H-Perfluorodecanethiol was attached to a precursor polymer, polystyrene-block-poly(glycidyl methacrylate) (PS-b-PGMA), to obtain a novel BCP with a C8F17-containing liquid crystal (LC) side chain (PS-b-P8FMA). Anisotropic hexagonally packed cylinder domains in a bulk state were first characterized by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The observed morphology transition of BCPs with different fluorinated side chain lengths of –CF3, –C4F9, and –C6F13 suggested the decisive effects of LC side chain ordering on the anisotropic nanostructures. In the thin film study, poly(methyl methacrylate-random-2,2,2-trifluoroethyl methacrylate-random-methacrylic acid) (PMMA-ran-PTFEMA-ran-PMAA) solution was used as BSLs for tuning the desired periodicities. The surface free energy (SFE) of BSL was simply tailored by changing the composition of comonomers. In atomic force microscopy (AFM) characterization, long-range ordered perpendicularly oriented BCP domains in a hexagonally packed array or parallel oriented BCP domains as striation patterns were easily fabricated on non-preferential or preferential BSL, respectively. The study presents a novel approach to tunable thin film periodicities without changing or modifying BCPs, which is desired in next-generation BCP lithography.

Molecular engineering of stimuli-responsive, functional, side-chain liquid crystalline copolymers: synthesis, properties and applications

This review describes recent progress made in designing stimuli-responsive, functional, side-chain, end-on mesogen attached liquid crystalline polymers (LCPs).

Photoinduced Reorientation in Thin Films of a Nematic Liquid Crystalline Polymer Anchored to Interfaces and Enhancement Using Small Liquid Crystalline Molecules

The photoinduced reorientation of the side-chain mesogens in nematic liquid crystalline (LC) polymer thin films triggered by the axis-selective photo-Fries rearrangements of side-chain phenyl benzoate moieties is studied to understand the regulation of the anisotropic nanostructures supported by LC polymers. The influence of the substrate surface in anchoring the side-chain mesogens near the interfaces is examined by comparing the reorientation of 30- and 120-nm-thick films. Irradiation with linearly polarized ultraviolet (UV) light and subsequent annealing causes the side-chain mesogen reorientation to align perpendicular to the electric field of the incident UV light. The inplane order in the 30-nm-thick films is lower than that in the 120-nm ones. On the other hand, the annealing period required for mesogen alignment is independent of the film thickness. It is suggested that the substrate surfaces anchor the LC mesogens to fix their orientation, rather than slowing down the reorientational motion. In addition, it is demonstrated that small LC molecules miscible with the nematic LC polymer enhance photoinduced reorientation through cooperative molecular interaction with the side-chain mesogens, remarkably accelerating the orientation and improving the inplane order of the unidirectionally aligned mesogens.

Self-assembly of a silicon-containing side-chain liquid crystalline block copolymer in bulk and in thin films: kinetic pathway of a cylinder to sphere transition

The self-assembly of a high-χ silicon-containing side-chain liquid crystalline block copolymer (LC BCP) in bulk and in thin films is reported, and the structural transition process from the hexagonally packed cylinder (HEX) to the body-centered cubic structure (BCC) in thin films was examined by both reciprocal and real space experimental methods. The block copolymer, poly(dimethylsiloxane-b-11-(4'-cyanobiphenyl-4-yloxy)undecylmethacrylate) (PDMS-b-P(4CNB11C)MA) with a molecular weight of 19.5 kg mol-1 and a volume fraction of PDMS 27% self-assembled in bulk into a hierarchical nanostructure of sub-20 nm HEX cylinders of PDMS with the P(4CNB11C)MA block exhibiting a smectic LC phase with a 1.61 nm period. The structure remained HEX as the P(4CNB11C)MA block transformed to an isotropic phase at ∼120 °C. In the thin films, the PDMS cylindrical microdomains were oriented in layers parallel to the substrate surface. The LC block formed a smectic LC phase which transformed to an isotropic phase at ∼120 °C, and the microphase-separated nanostructure transformed from HEX to BCC spheres at ∼160 °C. The hierarchical structure as well as the dynamic structural transition of the thin films were characterized using in situ grazing-incidence small-angle X-ray scattering and grazing-incidence wide-angle X-ray scattering. The transient morphologies from the HEX to BCC structure in thin films were captured by scanning electron microscopy and atomic force microscopy, and the transition pathway was described.