Anisotropic and Interconnected Nanoporous Materials from Randomly End-Linked Copolymer Networks

Microphase separation within randomly end-linked copolymer networks (RECNs) provides access to disordered bicontinuous morphologies over a wide composition range of the constituent network strands. Here, we rely on end-linking of telechelic hydroxyl-terminated polystyrene (PS) and poly­(d,l-lactide) (PLA) chains of equal molecular weight, with a tetrafunctional isocyanate cross-linker in a good solvent for both strands, followed by solvent removal to induce microphase separation, and finally etching of the PLA phase to yield nanoporous materials. Transmission electron microscopy (TEM) tomographic reconstructed 3D images along with gravimetric measurements and small-angle X-ray scattering (SAXS) indicate the formation of highly interconnected structures over a range of ∼40–70 vol % of PLA, while N₂ adsorption measurements indicate narrowly distributed pore sizes that can be tuned by varying the strand molecular weights. Stretching of the PS/PLA copolymer networks above the glass transition temperatures of both components prior to etching the PLA phase provides a straightforward means to introduce controlled anisotropy into the 3D interconnected porous materials

Stress-Induced Orientation of Cocontinuous Nanostructures within Randomly End-Linked Copolymer Networks

Randomly end-linked copolymer networks (RECNs) provide a robust route to self-assembled cocontinuous nanostructures. Here, we study the orientation of cocontinuous polystyrene/poly­(d,l-lactide) (PS/PLA) RECNs induced by uniaxial stretching above the glass transition temperatures of the components. Small-angle X-ray scattering (SAXS) reveals that the domains initially undergo nonaffine stretching at low strain (ε < 0.4), followed by domain rotation at larger strains, yielding a “soft elastic” response and providing a high degree of orientation. Transmission electron microscopy (TEM) tomography confirms that stretching leads to topological changes in the nanostructure, corresponding to reorganization of domain interfaces. The combination of orientation at the molecular and nanostructural levels provides substantial improvements in yield strength, toughness, and stiffness. In addition to possibilities for improving mechanical properties, cocontinuous nanostructures with controlled levels of orientation have potential in a variety of contexts including directional ion transport and energy absorption

Chiral Arrangements of Au Nanoparticles with Prescribed Handedness Templated by Helical Pores in Block Copolymer Films

Fabrication of films with plasmonic nanoparticles (NPs) arrays arranged in chiral configurations of prescribed handedness is highly attractive for the design of new functional materials; however, this remains a formidable challenge in nanotechnology. In this study, we demonstrated the controlled arrangements of gold (Au) NPs into helical structures templated by helical pores created in cross-linked block copolymer (BCP) films. d- and l-tartaric acid (TA) were used to direct the self-assembly of achiral poly­(1,4-butadiene)-<i>b</i>-poly­(ethylene oxide) BCPs into helical cylindrical morphologies with prescribed handedness, i.e., D or L. Helical pores were generated by BCP cross-linking followed by TA extraction. Helical Au NP arrays, subsequently arranged within the helical pores, exhibited the chiral optical response. The helical structures of NPs arrays and the resulting optical handedness were tunable simply by using either D- or L-porous templates. This simple strategy offers a straightforward pathway for the fabrication of chiral porous BCP films and helical NPs arrays with chiral optical properties

Rapid, Large-Area Synthesis of Hierarchical Nanoporous Silica Hybrid Films on Flexible Substrates

We report a simple strategy for the creation of large-area nano­porous hybrid films of silica, carbon, and gold on poly­ethylene tere­phthalate via photo­thermal processing. This method enables the selective heating of light-absorbing thin films on low-temperature substrates using sub-milli­second light pulses generated by a xenon flash lamp. The film contains gold nano­particles as the nano­heaters to convert light energy to heat, a sacrificial block copolymer surfactant to generate mesopores, and cross-linked poly­hedral oligo­meric silsesqui­oxane as the silica source to form the skeleton of the porous structure. Hier­archical porous structures are achieved in the films after photo­thermal treatment, with uniform meso­pores (44–48 nm) on the surface and inter­connected macro­pores (>50 nm) under­neath resulting from a foaming effect during release of gaseous decomposition products. The loading of gold nano­particles is up to 43 wt % in the product, with less than 2 wt % organic residue. This rapid and large-area process for the synthetis of porous structures is compatible with roll-to-roll manufacturing for the fabrication of flexible devices

Structural Diversity and Phase Behavior of Brush Block Copolymer Nanocomposites

Brush block copolymers (BBCPs) exhibit attractive features for use as templates for functional hybrid nanomaterials including rapid ordering dynamics and access to broad ranges of domain sizes; however, there are relatively few studies of the morphology of the BBCPs as a function of their structural variables and fewer still studies of BBCP composite systems. Here we report the structural diversity and phase behavior of one class of BBCP nanocomposites as a function of the volume fractions of their components and the side chain symmetry of the BBCPs. We conducted a systematic investigation of gold nanoparticle (NP) (∼2 nm) assembly in a series of poly­(<i>tert</i>-butyl acrylate)-<i>block</i>-poly­(ethylene oxide) (P<i>t</i>BA-<i>b</i>-PEO) BBCPs with a fixed side chain length of P<i>t</i>BA (<i>M</i>_n = 8.2 kg/mol) but with different PEO brush lengths (<i>M</i>_n = 5.0, 2.0, or 0.75 kg/mol) as well as volume fractions (<i>f</i>_PEO = 0.200–0.484). The gold NPs are selectively incorporated within the PEO domain via hydrogen bond interactions between the 4-mercaptophenol ligands of the gold NPs and the PEO side chains. A number of morphological transitions were observed and were dependent on the total volume fraction (<i>f</i>_NP/PEO) of NPs and PEO domain. Symmetric or asymmetric lamellar morphologies of NP arrays were readily created through simple variation of <i>f</i>_NP/PEO. Interestingly, a lamellar structure was obtained at a small <i>f</i>_NP/PEO of only 0.248 for nanocomposites based on BBCPs with comparable side chain lengths (MW_PEO/MW_PtBA = 0.63). In contrast, NP morphological transitions from wormlike through cylindrical to lamellar structures were observed with the increase of <i>f</i>_NP/PEO for nanocomposites based on BBCPs with a large difference in side chain length (MW_PEO/MW_PtBA = 0.09). Highly deformed cylinders were observed in the cylindrical morphology as clearly identified by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) tomography. This work represents a starting point for understanding BBCP composite phase behavior, and it provides new insight toward strategies for control over the microstructure of NP arrays assembled in BBCP templates, which is essential for functional materials design

Locally Favored Two-Dimensional Structures of Block Copolymer Melts on Nonneutral Surfaces

Self-assembly of block copolymers (BCPs) into arrays of well-defined nanoscopic structures has attracted extensive academic and industrial interests over the past several decades. In contrast to the bulk where phase behavior is controlled by the segmental interaction parameter, the total number of segments in BCPs and volume fraction, the morphologies and orientations of BCP thin films can also be strongly influenced by the substrate surface energy/chemistry effect (considered as a “substrate field”). Here, we report the formation of locally favored structures where all constituent blocks coexist side-by-side on nonneutral solid surfaces irrespective of their chain architectures, microdomain structures, and interfacial energetics. The experimental results using a suite of surface-sensitive techniques intriguingly demonstrate that individual preferred blocks and nonpreferred blocks lie flat on the substrate surface and form a two-dimensional percolating network structure as a whole. The large numbers of solid-segment contacts, which overcome a loss in the conformational entropy of the polymer chains, prevent the structure relaxing to its equilibrium state (i.e., forming microdomain structures) even in a (good) solvent atmosphere. Our results provide direct experimental evidence of the long-lived, nonequilibrium structures of BCPs and may point to a new perspective on the self-assembly of BCP melts in contact with impenetrable solids