Synthesis of Semicrystalline/Fluorinated Side-Chain Crystalline Block Copolymers and Their Bulk and Thin Film Nanoordering

Stiffness Variable Polymers Comprising Phase-Changing Side-Chains: Material Syntheses and Application Explorations

Stiffness variable materials have been applied in a variety of engineering fields that require adaptation, automatic modulation, and morphing because of their unique property to switch between a rigid, load-bearing state and a soft, compliant state. Stiffness variable polymers comprising phase-changing side-chains (s-SVPs) have densely grafted, highly crystallizable long alkyl side-chains in a crosslinked network. Such a bottlebrush network-like structure gives rise to rigidity modulation as a result of the reversible crystallization and melting of the side chains. The corresponding modulus changes can be more than 1000-fold within a narrow temperature span, from ≈10²  MPa to ≈10²  kPa or lower. Other important properties of the s-SVP, such as stretchability, optical transmittance, and adhesion, can also be altered. This work reviews the underlying molecular mechanisms in the s-SVP's, discusses the material's structure-property relationship, and summarizes important applications explored so far, including reversible shape transformation, bistable electromechanical transduction, optical modulation, and reversible adhesion.

Chemically tailored high-χ block copolymers for perpendicular lamellae via thermal annealing

A chemically tailored high-χ block copolymer (BCP), polystyrene-block-poly[2-hydroxy-3-(2,2,2-trifluoroethylsulfanyl)propyl methacrylate] (PS-b-PHFMA), was designed to incorporate tailored surface affinities and chemical incompatibilities for engineering perpendicular lamellae using thermal annealing. PS-b-PHFMA was synthesized via the sequential anionic polymerization of styrene and glycidyl methacrylate and the post-polymerization functionalization of the glycidyl moieties with 2,2,2-trifluoroethanethiol. The bulk studies revealed lamellae with a minimum domain spacing of 9.6 nm and a large effective Flory-Huggins interaction parameter (χeff) of 0.191 at 25 °C. Furthermore, atomic force microscopy and scanning electron microscopy showed perpendicular lamellae of the PS-b-PHFMA prepared on thermally-annealed thin films. The introduction of hydrophobic trifluoroethyl moieties onto the hydrophilic glycidyl moieties successfully balanced the surface affinity of the PHFMA block relative to PS, while simultaneously increasing the strength of segregation. Thus, χeff of the chemically tailored BCP increased, and a perpendicular orientation was facilitated on the thin films using thermal annealing.

Liquid crystalline moiety-assisted perpendicular orientation of cylindrical domains within P4VP-b-PMA(Az) films with high aspect ratio

Block copolymer (BCP) films with perpendicularly aligned cylindrical domains of high aspect ratio have important applications in diverse fields. However, an aspect ratio of the cylinders as high as 200 has rarely been reported so far. Here we demonstrate an efficient route to the formation of normally aligned P4VP cylinders with high aspect ratio surrounded by a matrix of azobenzene-containing block (PMA(Az)) via hierarchical self-assembly. A crisscross structure, consisting of parallelly aligned liquid crystalline (LC) layers and normally aligned self-assembly domains, is expected to assist the formation of well-defined nanostructures. The LC layers in the cylindrical films self-assemble to form smectic phase after solvent annealing, as confirmed by WAXD and UV-vis spectra. We found that the aspect ratio of the vertical P4VP cylinders is up to 200 and the film thickness reaches 6 μm. P4VP is a functional polymer, making this P4VP-b-PMA(Az) film more suitable for advanced filters, multi-nanochannels, nanolithography, and high-density storage media, etc.

Polydimethylsiloxane-assisted alignment transition from perpendicular to parallel of cylindrical microdomains in block copolymer films

The orientation transition from perpendicular to parallel alignment of PEO cylindrical microdomains within PEO-b-PMA(Az) films has been demonstrated via introducing tiny polydimethylsiloxane (PDMS) into the block copolymers.

Transparent superhydrophobic coatings from amphiphilic-fluorinated block copolymers synthesized by aqueous polymerization-induced self-assembly

Preparation of transparent and superhydrophobic coatings by co-deposition of an aqueous solution of an amphiphilic fluorinated block copolymer with silica was developed.

Directed self-assembly of silicon-containing block copolymer thin films

The directed self-assembly (DSA) of lamella-forming poly(styrene-block-trimethylsilylstyrene) (PS-PTMSS, L0=22 nm) was achieved using a combination of tailored top interfaces and lithographically defined patterned substrates. Chemo- and grapho-epitaxy, using hydrogen silsesquioxane (HSQ) based prepatterns, achieved density multiplications up to 6× and trench space subdivisions up to 7×, respectively. These results establish the compatibility of DSA techniques with a high etch contrast, Si-containing BCP that requires a top coat neutral layer to enable orientation.

Structural evolution of low-molecular-weight poly(ethylene oxide)-block-polystyrene diblock copolymer thin film

The structural evolution of low-molecular-weight poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer thin film with various initial film thicknesses on silicon substrate under thermal annealing was investigated by atomic force microscopy, optical microscopy, and contact angle measurement. At film thickness below half of the interlamellar spacing of the diblock copolymer (6.2 nm), the entire silicon is covered by a polymer brush with PEO blocks anchored on the Si substrate due to the substrate-induced effect. When the film is thicker than 6.2 nm, a dense polymer brush which is equal to half of an interlamellar layer was formed on the silicon, while the excess material dewet this layer to form droplets. The droplet surface was rich with PS block and the PEO block crystallized inside the bigger droplet to form spherulite.