Sub-10 nm Self-Assembly of Mesogen-Containing Grafted Macromonomers and Their Bottlebrush Polymers

Stabilizing a Double Gyroid Network Phase with 2 nm Feature Size by Blending of Lamellar and Cylindrical Forming Block Oligomers

Molecular dynamics simulations are used to study binary blends of an AB-type diblock and an AB₂-type miktoarm triblock amphiphiles (also known as high-χ block oligomers) consisting of sugar-based (A) and hydrocarbon (B) blocks. In their pure form, the AB diblock and AB₂ triblock amphiphiles self-assemble into ordered lamellar (LAM) and cylindrical (CYL) structures, respectively. At intermediate compositions, however, the AB₂-rich blend (0.2 ≤ x _AB ≤ 0.4) forms a double gyroid (DG) network, whereas perforated lamellae (PL) are observed in the AB-rich blend (0.5 ≤ x _AB ≤ 0.8). All of the ordered mesophases present domain pitches under 3 nm, with 1 nm feature sizes for the polar domains. Structural analyses reveal that the nonuniform interfacial curvatures of DG and PL structures are supported by local composition variations of the LAM- and CYL-forming amphiphiles. Self-consistent mean field theory calculations for blends of related AB and AB₂ block polymers also show the DG network at intermediate compositions, when A is the minority block, but PL is not stable. This work provides molecular-level insights into how blending of shape-filling molecular architectures enables network phase formation with extremely small feature sizes over a wide composition range.

Star-Shaped ROMP Polymers Coated with Oligothiophenes That Exhibit Unique Emission

A series of oligo(thiophene)-modified "soluble" star-shaped ring-opening metathesis polymerization (ROMP) polymers were prepared by sequential living ROMP of norbornene and a cross-linking agent using a molybdenum-alkylidene catalyst, followed by Wittig-type coupling for termination with oligo(thiophene) carboxaldehydes. The resultant star-shaped ROMP polymers displayed unique emission properties affected by the core size and arm repeat units as well as the kind of oligothiophene coated. The effects of the thiophene groups on photophysical properties of star-shaped/linear polymers were studied via time-resolved fluorescence spectroscopy. Fluorescence lifetimes were determined in THF as 400, 640, 730, and 820 ps for Star 3TPh, Linear 3TPh, Star 4T, and Linear 4T, respectively. A significant enhancement of the nonradiative rate constants k _nr in the star-shaped polymers results in relatively lower fluorescence quantum yields and shorter fluorescence lifetimes compared to the corresponding linear polymers.

Sub-5 nm homeotropically aligned columnar structures of hybrids constructed by porphyrin and oligo(dimethylsiloxane)

A series of tetraphenylporphyrin-based thermotropic liquid crystals containing oligo(dimethylsiloxane) were synthesized. These disc-coil hybrids form ordered nanostructures with periodic sizes on the sub-5 nm scale, including oblique columnar, lamellar, and hexagonal columnar phases. Films with sub-5 nm line patterns and homeotropically aligned columnar structures can be obtained by substrate-induced self-assembly.

100th Anniversary of Macromolecular Science Viewpoint: Opportunities for Liquid Crystal Polymers in Nanopatterning and Beyond

Liquid-crystal polymers (LCPs) integrate at a molecular level the characteristics of two important material classes, i.e., liquid crystals (LCs) and polymers. As a result, they exhibit a wide variety of intriguing physical phenomena and have useful properties in various settings. In the nearly 50 years since the discovery of the first melt-processable LCPs, there has been a remarkable expansion in the field encompassing the development of new chain architectures, the incorporation of new classes of mesogens, and the exploration of new properties and applications. As engineering materials, LCPs are historically best known in the context of high strength fibers. In a more contemporary study, the pairing of LC mesophase assembly with block copolymer (BCP) self-assembly in LC BCPs has resulted in a fascinating interplay of ordering phenomena and rich phase behavior, while lightly cross-linked networks, LC elastomers, are extensively investigated as shape memory materials based on their thermomechanical actuation. As this Viewpoint describes, these and other examples are active areas of research in which new, compelling opportunities for LCPs are emerging. We highlight a few selected areas that we view as being potentially significant in the near future, with a particular emphasis on nanopatterning. Here, the ability to readily access small feature sizes, the fluidity of the LC mesophase, and LC-based handles for achieving orientation control present a compelling combination. Opportunities for LCPs are also presented under the broad rubric of "beyond nanopatterning", and we discuss relevant challenges and potential new directions in the field.

Ordered structures and sub-5 nm line patterns from rod-coil hybrids containing oligo(dimethylsiloxane)

Sub-5 nm ordered nanostructures including lamellar, double gyroid, and columnar phases are formed by a series of oligo(dimethylsiloxane) (ODMS)-based rod-coil liquid crystals with accurate molecular weights. Films with well-oriented line patterns can be obtained by substrate-induced directed self-assembly, which may be further used as lithographic templates.

Design, Synthesis, and Self-Assembly of Janus Bottlebrush Polymers

Janus bottlebrush polymers are a class of special molecular brushes, which have two immiscible side chains on the repeating unit of the backbone. The characteristic architectures of Janus bottlebrush polymers enable unique self-assembly properties and broad applications. Recently, remarkable advances of Janus bottlebrush polymers have been achieved for polymer chemistry and material science. This review summarizes the synthetic strategies of Janus bottlebrush polymers, and highlights the self-assembly applications. Finally, the challenges and opportunities are proposed for the further development.

Perpendicularly aligned nanodomains on versatile substrates via rapid thermal annealing assisted by liquid crystalline ordering in block copolymer films

The highly ordered perpendicularly aligned cylindrical and lamellar microdomains within block copolymer (BCP) films have important applications in diverse fields. However, the fast normal orientation of self-assembled nanostructures on arbitrary substrates without tedious pre- and postprocessing has been a challenging issue in manufacturing miniaturized devices. Here, we outline the potential for extending the hierarchical self-assembly within azobenzene-containing PS-b-PMA(Az) films to inherently assist in the formation of normally aligned domains using a rapid thermal annealing process (140 °C for 5 min). Liquid crystalline (LC) mesogens in PS-b-PMA(Az) films self-assemble to form a parallelly aligned sematic phase after thermal annealing, as confirmed by grazing-incidence small-angle X-ray scattering (GISAXS), wide-angle X-ray diffraction (WAXD) and ultraviolet-visible (UV-vis) spectra. This sub-phase contributes to broadening of the PS-cylinder-phase window (0.083 ≤ f _PS < 0.49) and ∼12 nm PS cylinder structures. Perpendicular cylinders or lamellae are observed on various substrates, such as silicon wafers, flexible polyethylene terephthalate (PET) sheets and conductive aluminum foils. Additionally, the good reactive ion etching (RIE) rate difference between the two blocks makes these BCPs more attractive for advancing the field of BCP lithographic applications for fabricating flexible microelectronic devices.

Precise control of grafting density in periodically grafted amphiphilic copolymers: an alternate strategy to fine-tune the lamellar spacing in the sub-10 nm regime

By exactly locating pendant PEG550 segments at varying intervals along a hydrocarbon-rich polyester backbone, the lamellar dimension has been precisely tuned.

Tailored silyl ether monomers enable backbone-degradable polynorbornene-based linear, bottlebrush and star copolymers through ROMP

Ring-opening metathesis polymerization of norbornene-based (macro)monomers is a powerful approach for the synthesis of macromolecules with diverse compositions and complex architectures. Nevertheless, a fundamental limitation of polymers prepared by this strategy is their lack of facile degradability, limiting their utility in a range of applications. Here we describe a class of readily available bifunctional silyl ether-based cyclic olefins that copolymerize efficiently with norbornene-based (macro)monomers to provide copolymers with backbone degradability under mildly acidic aqueous conditions and degradation rates that can be tuned over several orders of magnitude, depending on the silyl ether substituents. These monomers can be used to manipulate the in vivo biodistribution and clearance rate of polyethylene glycol-based bottlebrush polymers, as well as to synthesize linear, bottlebrush and brush-arm star copolymers with degradable segments. We expect that this work will enable preparation of degradable polymers by ROMP for biomedical applications, responsive self-assembly and improved sustainability.

Stereocomplexes of Discrete, Isotactic Lactic Acid Oligomers Conjugated with Oligodimethylsiloxanes

Discrete length block co-oligomers (BCOs) comprised of a crystalline and an amorphous block are a new class of materials that gives highly ordered lamellar morphologies at small length scales. Here, we show the preparation of discrete, isotactic oligo l- and d-lactic acid (olLA and odLA) homoblocks followed by ligation to oligodimethylsiloxane (oDMS), affording a library of crystalline-amorphous BCOs that vary in molecular weight and composition. Mixing the two enantiomeric BCOs or homoblocks results in the formation of the corresponding stereocomplex. The properties and phase behavior of the isotactic (block co)oligomers and the stereocomplexes thereof are studied using differential scanning calorimetry and small-angle X-ray scattering. A systematic study of the isotactic homoblock lengths and crystal structure confirmed the formation of a 103 helix with a monomeric rise of 0.3 nm, whereas the stereocomplex adopts a 31 helix. The same type of crystal structure was found for the isotactic and stereocomplex of BCOs giving rise to the formation of lamellar morphologies at room temperature as a result of crystallization of the oLA blocks. Distorted lamellar structures were found in BCOs that preorganize into nonlamellar morphologies prior to crystallization. The stereocomplex BCOs shows more crystal defects and a loss of long-range ordering in the microstructure due to the larger driving force for crystallization. Hence, the balance between chain length, block volume, and the crystallization strength are of major importance for the formation of the final structure with the least defects.

Grubbs' and Schrock's Catalysts, Ring Opening Metathesis Polymerization and Molecular Brushes-Synthesis, Characterization, Properties and Applications

In this review, molecular brushes and other macromolecular architectures bearing a bottlebrush segment where the main chain is synthesized by ring opening metathesis polymerization (ROMP) mediated by Mo or Ru metal complexes are considered. A brief review of metathesis and ROMP is presented in order to understand the problems and the solutions provided through the years. The synthetic strategies towards bottlebrush copolymers are demonstrated and each one discussed separately. The initiators/catalysts for the synthesis of the backbone with ROMP are discussed. Syntheses of molecular brushes are presented. The most interesting properties of the bottlebrushes are detailed. Finally, the applications studied by different groups are presented.