Thermodynamics of Block Copolymers with and without Salt

Modulation of thermodynamic and kinetic inverted phase behavior of block copolymers by inorganic polyoxometalates

The Keggin polyoxometalates (POM) H₃PW₁₂O₄₀ (PW) electrostatically complexed with poly(styrene-block-2-vinyl pyridine) (PS-b-P2VP) in DMF, and ordered microphase separation occurred through solvent evaporation. The phase behaviors of PS-b-P2VP/PW in bulk were systematically investigated by using SAXS and TEM to discover the effect of POM content and molecular weight of the block copolymers. Computational simulation was also performed to reveal the same phase transition sequence as the experimental results. As the POM content increases, the PS-b-P2VP/PW complex with a low molecular weight changed from lamellar phase (LAM) to hexagonal cylindrical phase (HEX), and finally transited into spherical phase (SPH). Unexpectedly, PS-b-P2VP/PW complexes with a high molecular weight were inclined to form a kinetic-trapped intermediate phase (inverted HEX). The mechanism of formation of inverted phases was proposed based on simulation that asymmetric swelling in the concentrated DMF solution would result in the ultimate kinetic-trapped nanostructure in the bulk.

Bicontinuous Microemulsions in Partially Charged Ternary Polymer Blends

We describe the phase behavior of a partially charged ternary polymer blend model system, comprising a compositionally symmetric poly[(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) propyl sodium sulfonate methacrylate)]-b-polystyrene (POEGMA23-PS) diblock polymer and the constituent POEGMA23 and PS homopolymers, along the volumetrically symmetric isopleth, where 23 denotes the percentage of charged monomers in the POEGMA chain. Small-angle neutron and X-ray scattering and dynamic mechanical spectroscopy measurements reveal morphological transitions from a layered superlattice to swollen lamellae to a bicontinuous microemulsion (BμE), followed by macroscopic phase separation, with increasing homopolymer content. The BμE channel occurs between 85 and 90% homopolymer addition, positioned approximately at the isotropic Lifshitz composition predicted by mean-field theory for neutral systems. The resulting BμE morphology exhibits a periodicity of 26 nm, yielding a mesoscopically structured but macroscopically disordered bicontinuous structure. That this structure can be achieved in a charged polymer system is surprising, given the huge asymmetries typically induced by adding charge to either diblock copolymers or binary polymer blends.

Field-Theoretic Study of Salt-Induced Order and Disorder in a Polarizable Diblock Copolymer

We study a salt-doped polarizable symmetric diblock copolymer using a recently developed field theory that self-consistently embeds dielectric response, ion solvation energies, and van der Waals (vdW) attractions via the incorporation of segment polarizabilities and fixed dipoles. This field theory is amenable to direct simulation via the complex Langevin sampling technique and, thus, requires no approximations beyond the phenomenology of the underlying molecular model. We measure the shift in the order-disorder transition (ODT) of a diblock copolymer with salt-loading in field-theoretic simulations and observe rich behavior in which solvation, dilution and charge screening effects compete to determine whether the ordered or disordered phase is stabilized. At low salt concentrations, the salt behaves as a selective solvent, localizing into the high-dielectric domains and stabilizing the ordered phase. At high salt concentrations, however, the salt localization vanishes due to charge screening effects, and the salt behaves as a nonselective solvent that screens vdW attractions and stabilizes the disordered phase.

Superlattice by charged block copolymer self-assembly

Charged block copolymers are of great interest due to their unique self-assembly and physicochemical properties. Understanding of the phase behavior of charged block copolymers, however, is still at a primitive stage. Here we report the discovery of an intriguing superlattice morphology from compositionally symmetric charged block copolymers, poly[(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) propyl sodium sulfonate methacrylate)]-b-polystyrene (POEGMA-PS), achieved by systematic variation of the molecular structure in general, and the charge content in particular. POEGMA-PS self-assembles into a superlattice lamellar morphology, a previously unknown class of diblock nanostructures, but strikingly similar to oxygen-deficient perovskite derivatives, when the fraction of charged groups in the POEGMA block is about 5-25%. The charge fraction and the tethering of the ionic groups both play critical roles in driving the superlattice formation. This study highlights the accessibility of superlattice morphologies by introducing charges in a controlled manner.

Detection of the Order-to-Disorder Transition in Block Copolymer Electrolytes Using Quadrupolar 7Li NMR Splitting

The order-to-disorder transition temperature (T_ODT) in a series of mixtures of polystyrene-b-poly(ethylene oxide) (SEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt is identified by the disappearance of a quadrupolar ⁷Li NMR triplet peak splitting above a critical temperature, where a singlet is observed. The macroscopic alignment of ordered domains required to produce a quadrupolar splitting occurs due to exposure to the NMR magnetic field. Alignment is confirmed using small-angle X-ray scattering (SAXS). The T_ODT determined by NMR is consistent with that determined using SAXS.

Nanoscale Resolution of Electric-field Induced Motion in Ionic Diblock Copolymer Thin Films

Understanding the responses of ionic block copolymers to applied electric fields is crucial when targeting applications in areas such as energy storage, microelectronics, and transducers. This work shows that the identity of counterions in ionic diblock copolymers substantially affects their responses to electric fields, demonstrating the importance of ionic species for materials design. In situ neutron reflectometry measurements revealed that thin films containing imidazolium based cationic diblock copolymers, tetrafluoroborate counteranions led to film contraction under applied electric fields, while bromide counteranions drove expansion under similar field strengths. Coarse-grained molecular dynamics simulations were used to develop a fundamental understanding of these responses, uncovering a nonmonotonic trend in thickness change as a function of field strength. This behavior was attributed to elastic responses of microphase separated diblock copolymer chains resulting from variations in interfacial tension of polymer-polymer interfaces due to the redistribution of counteranions in the presence of electric fields.

Anomalous Self-Assembly and Ion Transport in Nanostructured Organic-Inorganic Solid Electrolytes

Nanostructured solid electrolytes containing ion-conducting domains and rigid nonconducting domains are obtained by block copolymer self-assembly. Here, we report on the synthesis and characteristics of mixtures of a hybrid diblock copolymer with an organic and inorganic block: poly(ethylene oxide)-b-poly(acryloisobutyl polyhedral oligomeric silsesquioxane) (PEO-POSS) and a lithium salt. In the neat state, PEO-POSS exhibits a classical order-to-disorder transition upon heating. Dilute electrolytes exhibit a dramatic reversal; a disorder-to-order transition upon heating is obtained, indicating that the addition of salt fundamentally changes interactions between the organic and inorganic chains. At higher salt concentrations, the electrolytes primarily form a lamellar phase. Coexisting lamellae and cylinders are found at intermediate salt concentrations and high temperatures. The conductivity and shear modulus of PEO-POSS are significantly higher than that of an all-organic block copolymer electrolyte with similar molecular weight and morphology, demonstrating that organic-inorganic block copolymers provide a promising route for developing the next generation of solid electrolytes for lithium batteries.

Ionic Liquids as Additives to Polystyrene- Block-Poly(Methyl Methacrylate) Enabling Directed Self-Assembly of Patterns with Sub-10 nm Features

Polystyrene- block-poly(methyl methacrylate) (PS- b-PMMA) is one of the prototypical block copolymers in directed self-assembly (DSA) research and development, with standardized protocols in place for processing on industrially relevant 300 mm wafers. Scaling of DSA patterns to pitches below 20 nm using PS- b-PMMA, however, is hindered by the relatively low Flory-Huggins interaction parameter, χ. Here, we investigate the approach of adding small amounts of ionic liquids (ILs) into PS- b-PMMA, which selectively segregates into the PMMA domain and effectively increases the χ parameter and thus the pattern resolution. The amount of IL additive is small enough to result in limited changes in PS- b-PMMA's surface and interfacial properties, thus maintaining industry-friendly processing by thermal annealing with a free surface. Three different ILs are studied comparatively regarding their compositional process window, capability of increasing χ, and thermal stability. By adding ∼3.1 vol % of the champion IL into a low-molecular-weight PS- b-PMMA ( M_n = 10.3k- b-9.5k), we demonstrated DSA on chemically patterned substrates of lamellar structures with feature sizes <8.5 nm. Compatibility of the PS- b-PMMMA/IL blends with the standardized processes that have been previously developed suggests that such blend materials could provide a drop-in solution for sub-10 nm lithography with the processing advantages of PS- b-PMMA.

Ion Correlation Effects in Salt-Doped Block Copolymers

We apply classical density functional theory to study how salt changes the microphase morphology of diblock copolymers. Polymers are freely jointed and one monomer type favorably interacts with ions, to account for the selective solvation that arises from different dielectric constants of the microphases. By including correlations from liquid state theory of an unbound reference fluid, the theory can treat chain behavior, microphase separation, ion correlations, and preferential solvation, at the same coarse-grained level. We show good agreement with molecular dynamics simulations.

Structure design, fabrication and property investigation of water-based polyesters with notable surface hydrophilicity

Environmentally friendly waterborne polyesters (WPEs) were obtained by copolymerization of diethylene glycol (DEG), isophthalic acid (IPA) and 5-sulfoisophthalic acid monosodium salt (5-SSIPA).

Incipient microphase separation in short chain perfluoropolyether-block-poly(ethylene oxide) copolymers

Incipient microphase separation is observed by wide angle X-ray scattering (WAXS) in short chain multiblock copolymers consisting of perfluoropolyether (PFPE) and poly(ethylene oxide) (PEO) segments. Two PFPE-PEO block copolymers were studied; one with dihydroxyl end groups and one with dimethyl carbonate end groups. Despite having a low degree of polymerization (N ∼ 10), these materials exhibited significant scattering intensity, due to disordered concentration fluctuations between their PFPE-rich and PEO-rich domains. The disordered scattering intensity was fit to a model based on a multicomponent random phase approximation to determine the value of the interaction parameter, χ, and the radius of gyration, R_g. Over the temperature range 30-90 °C, the values of χ were determined to be very large (∼2-2.5), indicating a high degree of immiscibility between the PFPE and PEO blocks. In PFPE-PEO, due to the large electron density contrast between the fluorinated and non-fluorinated block and the high value of χ, disordered scattering was detected at intermediate scattering angles, (q ∼ 2 nm^-1) for relatively small polymer chains. Our ability to detect concentration fluctuations was enabled by both a relatively large value of χ and significant scattering contrast.

Thermodynamics and phase behavior of acid-tethered block copolymers with ionic liquids

We investigate the phase behavior of acid-tethered block copolymers with and without ionic liquids. Two phosphonated block copolymers and their sulfonated analogs were synthesized by fine-tuning the degree of polymerization and the acid content. The block copolymers carrying acid groups with ionic liquids exhibited rich phase sequences, i.e., disorder-lamellae (LAM), gyroid-LAM, gyroid-hexagonal cylinder (HEX), and gyroid-A15 lattice, and the cation/anion ratio in the ionic liquid exerted profound effects on the segregation strength and topology of the self-assembled structures. Additionally, using ionic liquids with excessive cation content was found to enhance the effective Flory-Huggins interaction parameter, χ_eff, of the samples. However, as the anion content of the ionic liquids increased the segregation strength decreased. This is attributed to the packing frustration accompanied by the prevailing repulsive electrostatic interactions of the anions in the ionic liquid and the polymer matrix. As the hydrophobicity of the ionic liquids increased, well-defined ordered phases emerged in the phosphonated block copolymers with increased anion content, contrary to the disordered phases of the sulfonated samples. Thus, the balance between solvation energy of the anions and the electrostatic interactions is a key determinant of the thermodynamics of acid-tethered block copolymers containing ionic liquids.

Strategies for Inorganic Incorporation using Neat Block Copolymer Thin Films for Etch Mask Function and Nanotechnological Application

Block copolymers (BCPs) and their directed self-assembly (DSA) has emerged as a realizable complementary tool to aid optical patterning of device elements for future integrated circuit advancements. Methods to enhance BCP etch contrast for DSA application and further potential applications of inorganic nanomaterial features (e.g., semiconductor, dielectric, metal and metal oxide) are examined. Strategies to modify, infiltrate and controllably deposit inorganic materials by utilizing neat self-assembled BCP thin films open a rich design space to fabricate functional features in the nanoscale regime. An understanding and overview on innovative ways for the selective inclusion/infiltration or deposition of inorganic moieties in microphase separated BCP nanopatterns is provided. Early initial inclusion methods in the field and exciting contemporary reports to further augment etch contrast in BCPs for pattern transfer application are described. Specifically, the use of evaporation and sputtering methods, atomic layer deposition, sequential infiltration synthesis, metal-salt inclusion and aqueous metal reduction methodologies forming isolated nanofeatures are highlighted in di-BCP systems. Functionalities and newly reported uses for electronic and non-electronic technologies based on the inherent properties of incorporated inorganic nanostructures using di-BCP templates are highlighted. We outline the potential for extension of incorporation methods to triblock copolymer features for more diverse applications. Challenges and emerging areas of interest for inorganic infiltration of BCPs are also discussed.