Brønsted-Acid-Catalyzed Exchange in Polyester Dynamic Covalent Networks

The effect of catalyst strength on polyester–alcohol dynamic covalent exchange was systematically studied using Brønsted acids and a low-<i>T</i>_g poly­(4-methylcaprolactone) vitrimer formulation. Relaxation times, activation energies, and Arrhenius prefactors are correlated with p<i>K</i>_a. Strong protic acids induce facile network relaxation at 25 °C on the order of 10⁴–10⁵ s, significantly faster than Lewis acid alternatives that function only above 100 °C. Activation energies span 49–67 kJ/mol and increase as p<i>K</i>_a decreases. The opposite trend is observed with the Arrhenius prefactor. We anticipate that the quantitative understanding of Brønsted acid effects disclosed herein will be of utility in future studies that exploit acid-catalyzed dynamic covalent bond exchange

Thin Film Self-Assembly of Poly(trimethylsilylstyrene‑<i>b</i>‑d,l‑lactide) with Sub-10 nm Domains

Integrating block copolymer self-assembly with existing lithography processes to enhance their patterning capability is a promising approach for manufacturing a variety of semiconductor devices and next-generation magnetic storage media. Sub-10 nm block copolymer domains are specifically targeted in many of these applications, yet there are relatively few block copolymers that can achieve these dimensions. Here the synthesis and self-assembly characteristics of a new block copolymer poly­(trimethylsilylstyrene-<i>b</i>-d,l-lactide) (PTMSS-<i>b</i>-PLA) capable of forming domains as small as ∼5 nm are described. Several lamellar and cylinder forming diblocks were synthesized with bulk domain periodicities of 12–15 nm which are among the smallest domains yet reported for any neat block copolymer. Such small domains are possible because this new material has a large segment–segment interaction parameter which is an order of magnitude higher than poly­(styrene-<i>b</i>-methyl methacrylate) (PS-<i>b</i>-PMMA) and twice as large as poly­(styrene-<i>b</i>-dimethylsiloxane) (PS-<i>b</i>-PDMS), two commonly studied polymers for these applications. Furthermore, the PTMSS-<i>b</i>-PLA blocks have glass transitions well above room temperature with a large reactive ion etch rate contrast between them (∼28) which is at least 4 times greater than PS-<i>b</i>-PMMA due to incorporation of a trimethylsilyl group into the styrene monomer

Structure, Stability, and Reorganization of 0.5 <i>L</i>₀ Topography in Block Copolymer Thin Films

The structure, stability, and reorganization of lamella-forming block copolymer thin film surface topography (“islands” and “holes”) were studied under boundary conditions driving the formation of 0.5 <i>L</i>₀ thick structures at short thermal annealing times. Self-consistent field theory predicts that the presence of one perfectly neutral surface renders 0.5 <i>L</i>₀ topography thermodynamically stable relative to 1 <i>L</i>₀ thick features, in agreement with previous experimental observations. The calculated through-film structures match cross-sectional scanning electron micrographs, collectively demonstrating the pinning of edge dislocations at the neutral surface. Remarkably, near-neutral surface compositions exhibit 0.5 <i>L</i>₀ topography metastability upon extended thermal treatment, slowly transitioning to 1 <i>L</i>₀ islands or holes as evidenced by optical and atomic force microscopy. Surface restructuring is rationalized by invoking commensurability effects imposed by slightly preferential surfaces. The results described herein clarify the impact of interfacial interactions on block copolymer self-assembly and solidify an understanding of 0.5 <i>L</i>₀ topography, which is frequently used to determine neutral surface compositions of considerable importance to contemporary technological applications

Improved Elastic Recovery from ABC Triblock Terpolymers

The promise of ABC triblock terpolymers for improving the mechanical properties of thermoplastic elastomers is demonstrated by comparison with symmetric ABA/CBC analogs having similar molecular weights and volume fraction of B and A/C domains. The ABC architecture enhances elasticity (up to 98% recovery over 10 cycles) in part through essentially full chain bridging between discrete hard domains leading to the minimization of mechanically unproductive loops. In addition, the unique phase space of ABC triblocks also enables the fraction of hard-block domains to be higher (fhard ≈ 0.4) while maintaining elasticity, which is traditionally only possible with non-linear architectures or highly asymmetric ABA triblock copolymers. These advantages of ABC triblock terpolymers provide a tunable platform to create materials with practical applications while improving our fundamental understanding of chain conformation and structure–property relationships in block copolymers

Electrocatalysis of CO₂ Reduction in Brush Polymer Ion Gels

The electrochemical characterization of brush polymer ion gels containing embedded small-molecule redox-active species is reported. Gels comprising PS–PEO–PS triblock brush polymer, 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide (BMIm-TFSI), and some combination of ferrocene (Fc), cobaltocenium (CoCp₂⁺), and Re­(bpy)­(CO)₃Cl (<b>1</b>) exhibit diffusion-controlled redox processes with diffusion coefficients approximately one-fifth of those observed in neat BMIm-TFSI. Notably, <b>1</b> dissolves homogeneously in the interpenetrating matrix domain of the ion gel and displays electrocatalytic CO₂ reduction to CO in the gel. The catalytic wave exhibits a positive shift versus Fc^+/0 compared with analogous nonaqueous solvents with a reduction potential 450 mV positive of onset and 90% Faradaic efficiency for CO production. These materials provide a promising and alternative approach to immobilized electrocatalysis, creating numerous opportunities for application in solid-state devices

Directed Self-Assembly of Silicon-Containing Block Copolymer Thin Films

The directed self-assembly (DSA) of lamella-forming poly­(styrene-<i>block</i>-trimethylsilylstyrene) (PS–PTMSS, <i>L</i>₀ = 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

Photopatternable Interfaces for Block Copolymer Lithography

Directly photopatternable interfaces are introduced that facilitate two-dimensional spatial control of block copolymer (BCP) orientation in thin films. Copolymers containing an acid labile monomer were synthesized, formulated with a photoacid generator (PAG), and coated to create grafted surface treatments (GSTs). These as-cast GST films are either inherently neutral or preferential (but not both) to lamella-forming poly­(styrene-<i>block</i>-trimethylsilylstyrene) (PS-<i>b</i>-PTMSS). Subsequent contact printing and baking produced GSTs with submicron chemically patterned gratings. The catalytic reaction of the photoacid generated in the UV-exposed regions of the GSTs changed the interfacial interactions between the BCP and the GST in one of two ways: from neutral to preferential (“N2P”) <i>or</i> preferential to neutral (“P2N”). When PS-<i>b</i>-PTMSS was thermally annealed between a chemically patterned GST and a top coat, alternating regions of perpendicular and parallel BCP lamellae were formed

Oligosaccharide/Silicon-Containing Block Copolymers with 5 nm Features for Lithographic Applications

Block copolymers demonstrate potential for use in next-generation lithography due to their ability to self-assemble into well-ordered periodic arrays on the 3–100 nm length scale. The successful lithographic application of block copolymers relies on three critical conditions being met: high Flory–Huggins interaction parameters (χ), which enable formation of <10 nm features, etch selectivity between blocks for facile pattern transfer, and thin film self-assembly control. The present paper describes the synthesis and self-assembly of block copolymers composed of naturally derived oligosaccharides coupled to a silicon-containing polystyrene derivative synthesized by activators regenerated by electron transfer atom transfer radical polymerization. The block copolymers have a large χ and a low degree of polymerization (<i>N</i>) enabling formation of 5 nm feature diameters, incorporate silicon in one block for oxygen reactive ion etch contrast, and exhibit bulk and thin film self-assembly of hexagonally packed cylinders facilitated by a combination of spin coating and solvent annealing techniques. As observed by small angle X-ray scattering and atomic force microscopy, these materials exhibit some of the smallest block copolymer features in the bulk and in thin films reported to date

A Hybrid Chemo-/Grapho-Epitaxial Alignment Strategy for Defect Reduction in Sub-10 nm Directed Self-Assembly of Silicon-Containing Block Copolymers

The directed self-assembly (DSA) of a 20 nm full-pitch silicon-containing block copolymer (BCP), poly­(4-methoxystyrene-<i>b</i>-4-trimethylsilylstyrene), was performed using a process that produces shallow topography for hybrid chemo-/grapho-epitaxy. This hybrid process produced DSA with fewer defects than the analogous conventional chemo-epitaxial process, and the resulting DSA was also more tolerant of variations in process parameters. Cross-sectional scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) confirmed that BCP features spanned the entire film thickness on hybrid process wafers. Both processes were implemented on 300 mm wafers initially prepatterned by 193 nm immersion lithography, which is necessary for economic viability in high-volume manufacturing. Computational analysis of DSA extracted from top-down SEM images demonstrates the influence of process parameters on DSA, facilitating the optimization of guide stripe width, guide stripe pitch, and prepattern surface energy. This work demonstrates the ability of a hybrid process to improve the DSA quality over a conventional chemo-epitaxial process and the potential for high-volume manufacturing with high-χ, silicon-containing BCPs

Interfacial Design for Block Copolymer Thin Films

Top coat design, coating, and optimization methodologies are introduced that facilitate the synthesis, application, and identification of neutral top coats for block copolymer (BP) thin films. Polymeric top coat composition, controlled via synthesis, determines interfacial wetting characteristics. Trimethylammonium salts of top coats improve solubility and coating uniformity. A “confined” island and hole test conveniently establishes (non)­preferential wetting at the top coat/BP interface, which depends upon top coat composition. The utility of these three concepts was demonstrated with two high-χ, lamella-forming BPs, poly­(styrene-<i>block</i>-4-trimethylsilylstyrene) (PS-<i>b</i>-PTMSS) having two periodicities <i>L</i>₀ = 18 and 22 nm and poly­(styrene-<i>block</i>-methyltrimethylsilylmethacrylate) (PS-<i>b</i>-PTMSM) with <i>L</i>₀ = 15 nm. The combination of neutral top and bottom interfaces resulted in a perpendicular orientation of lamellae independent of BP film thickness (1–3 <i>L</i>₀) when thermally annealed for 60 s or less