Block copolymers (BCPs) self-assemble into intricate nanostructures that enhance a multitude of advanced applications in semiconductor processing, membrane science, nanopatterned coatings, ...nanocomposites, and battery research. Kinetics and thermodynamics of self-assembly are crucial considerations in controlling the nanostructure of BCP thin films. The equilibrium structure is governed by a molecular architecture and the chemistry of its repeat units. An enormous library of materials has been synthesized and they naturally produce a rich equilibrium phase diagram. Non-equilibrium phases could potentially broaden the structural diversity of BCPs and relax the synthetic burden of creating new molecules. Furthermore, the reliance on synthesis could be complicated by the scalability and the materials compatibility. Non-equilibrium phases in BCPs, however, are less explored, likely due to the challenges in stabilizing the metastable structures. Over the past few decades, a variety of processing techniques were introduced that influence the phase transformation of BCPs to achieve a wide range of morphologies. Nonetheless, there is a knowledge gap on how different processive pathways can induce and control the non-equilibrium phases in BCP thin films. In this review, we focus on different solvent-induced and thermally induced processive pathways, and their potential to control the non-equilibrium phases with regards to their unique aspects and advantages. Furthermore, we elucidate the limitations of these pathways and discuss the potential avenues for future investigations.
Droplets capture an environment-dictated equilibrium state of a liquid material. Equilibrium, however, often necessitates nanoscale interface organization, especially with formation of a passivating ...layer. Herein, we demonstrate that this kinetics-driven organization may predispose a material to autonomous thermal-oxidative composition inversion (TOCI) and texture reconfiguration under felicitous choice of trigger. We exploit inherent structural complexity, differential reactivity, and metastability of the ultrathin (∼0.7–3 nm) passivating oxide layer on eutectic gallium–indium (EGaIn, 75.5% Ga, 24.5% In w/w) core–shell particles to illustrate this approach to surface engineering. Two tiers of texture can be produced after ca. 15 min of heating, with the first evolution showing crumpling, while the second is a particulate growth above the first uniform texture. The formation of tier 1 texture occurs primarily because of diffusion-driven oxide buildup, which, as expected, increases stiffness of the oxide layer. The surface of this tier is rich in Ga, akin to the ambient formed passivating oxide. Tier 2 occurs at higher temperature because of thermally triggered fracture of the now thick and stiff oxide shell. This process leads to inversion in composition of the surface oxide due to higher In content on the tier 2 features. At higher temperatures (≥800 °C), significant changes in composition lead to solidification of the remaining material. Volume change upon oxidation and solidification leads to a hollow structure with a textured surface and faceted core. Controlled thermal treatment of liquid EGaIn therefore leads to tunable surface roughness, composition inversion, increased stiffness in the oxide shell, or a porous solid structure. We infer that this tunability is due to the structure of the passivating oxide layer that is driven by differences in reactivity of Ga and In and requisite enrichment of the less reactive component at the metal–oxide interface.
Exploiting interfacial excess (Γ), Laplace pressure jump (ΔP), surface work, and coupling them to surface reactivity have led to the synthesis of undercooled metal particles. Metastability is ...maintained by a core–shell particle architecture. Fracture of the thin shell leads to solidification with concomitant sintering. Applying Scherer's constitutive model for load‐driven viscous sintering on the undercooled particles implies that they can form conductive traces. Combining metastability to eliminate heat and robustness of viscous sintering, an array of conductive metallic traces can be prepared, leading to plethora of devices on various flexible and/or heat sensitive substrates. Besides mechanical sintering, chemical sintering can be performed, which negates the need of either heat or load. In the latter, connectivity is hypothesized to occur via a Frenkel's theory of sintering type mechanism. This work reports heat‐free, ambient fabrication of metallic conductive interconnects and traces on all types of substrates.
Frustrating solidification of molten metal (undercooling) enables heat‐free fabrication of metallic interconnects. Mechanical or chemical sintering of undercooled metal particles circumvents the need for melting the metal during fabrication, leading to ambient fabrication of metallic components. The versatility of this approach allows for the formation of conductive traces on a variety of substrates irrespective of their stiffness, porosity, or wettability.
Undercooling metals relies on frustration of liquid–solid transition mainly by an increase in activation energy. Passivating oxide layers are a way to isolate the core from heterogenous nucleants ...(physical barrier) while also raising the activation energy (thermodynamic/kinetic barrier) needed for solidification. The latter is due to composition gradients (speciation) that establishes a sharp chemical potential gradient across the thin (0.7–5 nm) oxide shell, slowing homogeneous nucleation. When this speciation is properly tuned, the oxide layer presents a previously unaccounted for interfacial tension in the overall energy landscape of the relaxing material. We demonstrate that 1) the integrity of the passivation oxide is critical in stabilizing undercooled particle, a key tenet in developing heat‐free solders, 2) inductive effects play a critical role in undercooling, and 3) the magnitude of the influence of the passivating oxide can be larger than size effects in undercooling.
The potential effect of the static surface tensor for metal alloy surfaces is found to be significantly larger than size‐related effects as demonstrated by oxide shell and stabilizing ligand composition. Considering these finds, the free‐energy equation related to solid–liquid phase transformation, under the classical nucleation theory, is extended to incorporate the stress‐component resulting from the passivating shell.
We report a method for the directed self-assembly (DSA) of block copolymers (BCPs) in which a first BCP film deploys homopolymer brushes, or “inks”, that sequentially graft onto the substrate’s ...surface via the interpenetration of polymer molecules during the thermal annealing of the polymer film on top of existing polymer brushes. By selecting polymer “inks” with the desired chemistry and appropriate relative molecular weights, it is possible to use brush interpenetration as a powerful technique to generate self-registered chemical contrast patterns at the same frequency as that of the domains of the BCP. The result is a process with a higher tolerance to dimensional and chemical imperfections in the guiding patterns, which we showcase by implementing DSA using homopolymer brushes for the guiding features as opposed to more robust cross-linkable mats. We find that the use of “inks” does not compromise the line width roughness, and the quality of the DSA as a lithographic mask is verified by implementing a robust “dry lift-off” pattern transfer.
Thermal percolation in polymer nanocompositesthe rapid increase in thermal transport due to the formation of networks among fillersis the subject of great interest in thermal management ranging ...from general utility in multifunctional nanocomposites to high-conductivity applications such as thermal interface materials. However, It remains a challenging subject encompassing both experimental and modeling hurdles. Successful reports of thermal percolation are exclusively found in high-aspect-ratio, conductive fillers such as graphene, albeit at filler loadings significantly higher than the electrical percolation threshold. This anomaly was attributed to the lower filler–matrix thermal conductivity contrast ratio k f/k m ∼104 compared to electrical conductivity ∼1012–1016. In a randomly dispersed composite, the effect of a low contrast ratio is further accentuated by uncertainties in the morphology of the percolating network and presence of other phases such as disconnected aggregates and colloidal dispersions. Thus, the general properties of percolating networks are convoluted as they lack a defined structure. In contrast, a prototypical system with controllable nanofiller placement enables the elucidation of structure–property relations such as filler size, loading, and assembly. Using self-assembled nanocomposites with a controlled 1,2,3-dimension nanoparticle (NP) arrangement, we demonstrate that thermal percolation can be achieved in spite of using spherical, nonconductive fillers (k f /k m ∼60) at a low volume fraction (9 vol %). We observe that the effects of volume fraction, interfacial thermal resistance, and filler conductivity on thermal conductivity depart from effective medium approximations. Most notably, contrast ratio plays a minor role in thermal percolation above k f /k m ∼60a common range for semiconducting nanoparticles/polymer ratios. Our findings bring new perspectives and insights to thermal percolation in nanocomposites, where the limits in contrast ratio, interfacial thermal conductance, and filler size are established.
Role of surface structure/composition in altering solidification behavior is demonstrated through undercooling of core–shell metal particles. Autonomous surface speciation in the passivating oxide ...plays a critical role in the relaxation of a molten metal due to divergence in the concentration and composition of oxidizing species across the thickness of the oxide layer. In an unreactive environment, surface speciation is dictated by flux, cohesive energy density, and surface energy minimization. Under oxidizing conditions (e.g., ambient), however, reduction potential, curvature, and surface plasticity dictate the spatial order and concentration(s) across thin passivating oxide layers. It is therefore important to redefine solubility beyond the limitations of Hume–Rothery rules by substituting electronegativity for redox potential and cohesive energy density. Increasing number of components in an alloy does not necessarily lead to increased undercooling, but a maximum is observed around two to three components. These results lead to an empirical observation that ΔG LS (Gibbs’ free energy for liquid–solid transition) around the freezing point can be understood from enthalpy and surface tension balance, with the degree of undercooling being a proportionality term for the enthalpic component. This work extends our understanding of classical nucleation theory by illustrating the importance of asymmetry in surface work in phase change, largely due to changes in structure of nanoscale passivating oxides. This phenomena is critical in development of low-temperature solders.
Conventional fabrication of microfluidic channels/devices faces challenges such as single use channels, material incompartibility, and/or significant time consumption. We propose a flexible platform ...for fabricating microfluidic channels simply through indentation on a smart compositethe so-called ST3R (Stiffness Tuning through Thermodynamic Relaxation) composite. The application of the ST3R composite allows rapid fabrication of microfluidic channels by hand or with a prefabricated stamp and precise prototyping of complex designs using a 2D plotter. Indenter geometry, applied stress, filler loading, and number of repeated indentations affect channel dimensions and/or shape. These channels further exhibit the following: (i) substantial improvement against swelling by organic solvent, in part due to the high modulus of the solidified metal network, and (ii) channel reconfigurability by heating the solidified undercooled metals. ST3R composite slabs have the potential to serve as microfluidic “breadboards”, from which complex channels can be integrated in a flexible manner.
Recent developments in smart responsive composites have utilized various stimuli including heat, light, solvents, electricity, and magnetic fields to induce a change in material properties. Here, we ...report a thermodynamically driven mechanically responsive composite, exploiting irreversible phase-transformation (relaxation) of metastable undercooled liquid metal core shell particle fillers. Thermal and mechanical analysis reveals that as the composite is deformed, the particles transform from individual liquid droplets to a solid metal network, resulting in a 300% increase in Young's modulus. In contrast to previous phase change materials, this dramatic change in stiffness occurs autonomously under deformation, is insensitive to environmental conditions, and does not require external energy sources such as heat, light, or electricity. We demonstrate the utility of this approach by transforming a flat, flexible composite strip into a rigid, 3D structure that is capable of supporting 50× its own weight. The ability for shape change and reconfiguration are further highlighted, indicating potential for multiple pathways to trigger or tune composite stiffness.
Hierarchical assemblies from block copolymer (BCP)-based supramolecules have shown immense potential as programmable materials owing to their versatility for incorporating functional molecules and ...provide access to arrays of hierarchical structures. However, there remains a knowledge gap on the formation of the supramolecule in solution. Here, we applied NMR techniques to investigate the solution-phase behavior of the most studied supramolecular systems, polystyrene-block-poly(4-vinylpyridine)(3-pentadecylphenol) (PS-b-P4VP(PDP)r). The results show that the supramolecule likely adopts a coil-comb conformation, despite the small molecule’s (PDP) rapid exchange between the bonded and free states. The exchange rate (>104 s–1) exceeds the NMR time scale at the frequency of interest. The supramolecules form under dilute conditions (∼2 vol %) and are attributed to the enthalpic gain of the hydrogen bonding between the PDP and 4VP. As the solute concentration increases (>10 vol %), the supramolecule forms micelle-like aggregates with PDP accumulated within the comb-block’s pervaded volume based on analysis of the apparent molecular weight, viscosity, and chain dynamics. This work sheds light on the long-standing question regarding the evolution of the constituents in the BCP-based supramolecule in solution and provides valuable guidance toward their solution-based processing and morphological control.
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