Effect of inorganic material surface chemistry on structures and fracture behaviours of epoxy resin.

The mechanisms underlying the influence of the surface chemistry of inorganic materials on polymer structures and fracture behaviours near adhesive interfaces are not fully understood. This study demonstrates the first clear and direct evidence that molecular surface segregation and cross-linking of epoxy resin are driven by intermolecular forces at the inorganic surfaces alone, which can be linked directly to adhesive failure mechanisms. We prepare adhesive interfaces between epoxy resin and silicon substrates with varying surface chemistries (OH and H terminations) with a smoothness below 1 nm, which have different adhesive strengths by ~13 %. The epoxy resins within sub-nanometre distance from the surfaces with different chemistries exhibit distinct amine-to-epoxy ratios, cross-linked network structures, and adhesion energies. The OH- and H-terminated interfaces exhibit cohesive failure and interfacial delamination, respectively. The substrate surface chemistry impacts the cross-linked structures of the epoxy resins within several nanometres of the interfaces and the adsorption structures of molecules at the interfaces, which result in different fracture behaviours and adhesive strengths.

Epitaxial Growth of Surface Perforations on Parallel Cylinders in Terraced Films of Block Copolymer/Homopolymer Blends.

Due to incommensurability between initial thickness and interdomain distance, thermal annealing inevitably produces relief surface terraces (islands and holes) of various morphologies in thin films of block copolymers. We have demonstrated three kinds of surface terraces in blend films: polygrain terraces with diffuse edges, polygrain terraces with step edges, and pseudo-monograin terraces with island coarsening. The three morphologies were obtained by three different thermal histories, respectively. The thermal histories were imposed on blend films, which were prepared by mixing a homopolystyrene (hPS, 6.1 kg/mol) with a weakly segregated, symmetry polystyrene-block poly(methyl methacrylate) (PS-b-PMMA, 42 kg/mol) followed by spin coating. At a given weight-fraction ratio of PS-b-PMMA/hPS = 75/25, the interior of the blend films forms parallel cylinders. Nevertheless, the surface of the blend films is always dominated by a skin layer of perforations, which epitaxially grow on top of parallel cylinders. By oxygen plasma etching at various time intervals to probe interior nanodomains, the epitaxial relationship between surface perforations and parallel cylinders has been identified by a scanning electron microscope.

In-situ shearing process observation system for soft materials via transmission electron microscopy.

We developed an in-situ shear test system suitable for transmission electron microscopy (TEM) observations, which enabled us to examine the shear deformation behaviours inside soft materials at nanoscale resolutions. This study was conducted on a nanoparticle-filled rubber to investigate its nanoscale deformation behaviour under a large shear strain. First, the shear deformation process of a large area in the specimen was accurately examined and proven to exhibit an almost perfect simple shear. At the nanoscale, voids grew along the maximum principal strain during shear deformation. In addition, the nanoscale regions with rubber and silica aggregates exhibited deformation behaviours similar to the global shear deformation of the specimen. Although the silica aggregates exhibited displacement along the shearing directions, rotational motions were also observed owing to the torque generated by the local shear stress. This in-situ shear deformation system for TEM enabled us to understand the nanoscale origins of the mechanical properties of soft materials, particularly polymer composites. Graphical Abstract.

Direct Visualization of the Self-Alignment Process for Nanostructured Block Copolymer Thin Films by Transmission Electron Microscopy.

Herein, this work aims to directly visualize the morphological evolution of the controlled self-assembly of star-block polystyrene-block-polydimethylsiloxane (PS-b-PDMS) thin films via in situ transmission electron microscopy (TEM) observations. With an environmental chip, possessing a built-in metal wire-based microheater fabricated by the microelectromechanical system (MEMS) technique, in situ TEM observations can be conducted under low-dose conditions to investigate the development of film-spanning perpendicular cylinders in the block copolymer (BCP) thin films via a self-alignment process. Owing to the free-standing condition, a symmetric condition of the BCP thin films can be formed for thermal annealing under vacuum with neutral air surface, whereas an asymmetric condition can be formed by an air plasma treatment on one side of the thin film that creates an end-capped neutral layer. A systematic comparison of the time-resolved self-alignment process in the symmetric and asymmetric conditions can be carried out, giving comprehensive insights for the self-alignment process via the nucleation and growth mechanism.

Networks with controlled chirality via self-assembly of chiral triblock terpolymers.

Nanonetwork-structured materials can be found in nature and synthetic materials. A double gyroid (DG) with a pair of chiral networks but opposite chirality can be formed from the self-assembly of diblock copolymers. For triblock terpolymers, an alternating gyroid (GA) with two chiral networks from distinct end blocks can be formed; however, the network chirality could be positive or negative arbitrarily, giving an achiral phase. Here, by taking advantage of chirality transfer at different length scales, GA with controlled chirality can be achieved through the self-assembly of a chiral triblock terpolymer. With the homochiral evolution from monomer to multichain domain morphology through self-assembly, the triblock terpolymer composed of a chiral end block with a single-handed helical polymer chain gives the chiral network from the chiral end block having a particular handed network. Our real-space analyses reveal the preferred chiral sense of the network in the GA, leading to a chiral phase.

Generalizing the effects of chirality on block copolymer assembly.

We explore the generality of the influence of segment chirality on the self-assembled structure of achiral-chiral diblock copolymers. Poly(cyclohexylglycolide) (PCG)-based chiral block copolymers (BCPs*), poly(benzyl methacrylate)-b-poly(d-cyclohexylglycolide) (PBnMA-PDCG) and PBnMA-b-poly(l-cyclohexyl glycolide) (PBnMA-PLCG), were synthesized for purposes of systematic comparison with polylactide (PLA)-based BCPs*, previously shown to exhibit chirality transfer from monomeric unit to the multichain domain morphology. Opposite-handed PCG helical chains in the enantiomeric BCPs* were identified by the vibrational circular dichroism (VCD) studies revealing transfer from chiral monomers to chiral intrachain conformation. We report further VCD evidence of chiral interchain interactions, consistent with some amounts of handed skew configurations of PCG segments in a melt state packing. Finally, we show by electron tomography [3D transmission electron microscope tomography (3D TEM)] that chirality at the monomeric and intrachain level ultimately manifests in the symmetry of microphase-separated, multichain morphologies: a helical phase (H*) of hexagonally, ordered, helically shaped tubular domains whose handedness agrees with the respective monomeric chirality. Critically, unlike previous PLA-based BCP*s, the lack of a competing crystalline state of the chiral PCGs allowed determination that H* is an equilibrium phase of chiral PBnMA-PCG. We compared different measures of chirality at the monomer scale for PLA and PCG, and argued, on the basis of comparison with mean-field theory results for chiral diblock copolymer melts, that the enhanced thermodynamic stability of the mesochiral H* morphology may be attributed to the relatively stronger chiral intersegment forces, ultimately tracing from the effects of a bulkier chiral side group on its main chain.

Perovskite Quantum Dots with Near Unity Solution and Neat-Film Photoluminescent Quantum Yield by Novel Spray Synthesis.

In this study, a novel perovskite quantum dot (QD) spray-synthesis method is developed by combining traditional perovskite QD synthesis with the technique of spray pyrolysis. By utilizing this new technique, the synthesis of cubic-shaped perovskite QDs with a homogeneous size of 14 nm is demonstrated, which shows an unprecedented stable absolute photoluminescence quantum yield ≈100% in the solution and even in the solid-state neat film. The highly emissive thin films are integrated with light emission devices (LEDs) and organic light emission displays (OLEDs). The color conversion type QD-LED (ccQD-LED) hybrid devices exhibit an extremely saturated green emission, excellent external quantum efficiency of 28.1%, power efficiency of 121 lm W-1 , and extraordinary forward-direction luminescence of 8 500 000 cd m-2 . The conceptual ccQD-OLED hybrid display also successfully demonstrates high-definition still images and moving pictures with a 119% National Television System Committee 1931 color gamut and 123% Digital Cinema Initiatives-P3 color gamut. These very-stable, ultra-bright perovskite QDs have the properties necessary for a variety of useful applications in optoelectronics.

Homochiral Evolution in Self-Assembled Chiral Polymers and Block Copolymers.

The significance of chirality transfer is not only involved in biological systems, such as the origin of homochiral structures in life but also in man-made chemicals and materials. How the chiral bias transfers from molecular level (molecular chirality) to helical chain (conformational chirality) and then to helical superstructure or phase (hierarchical chirality) from self-assembly is vital for the chemical and biological processes in nature, such as communication, replication, and enzyme catalysis. In this Account, we summarize the methodologies for the examination of homochiral evolution at different length scales based on our recent studies with respect to the self-assembly of chiral polymers and chiral block copolymers (BCPs*). A helical (H*) phase to distinguish its P622 symmetry from that of normal hexagonally packed cylinder phase was discovered in the self-assembly of BCPs* due to the chirality effect on BCP self-assembly. Enantiomeric polylactide-containing BCPs*, polystyrene-b-poly(l-lactide) (PS-PLLA) and polystyrene-b-poly(d-lactide) (PS-PDLA), were synthesized for the examination of homochiral evolution. The optical activity (molecular chirality) of constituted chiral repeating unit in the chiral polylactide is detected by electronic circular dichroism (ECD) whereas the conformational chirality of helical polylactide chain can be explicitly determined by vibrational circular dichroism (VCD). The H* phases of the self-assembled polylactide-containing BCPs* can be directly visualized by 3D transmission electron microscopy (3D TEM) technique at which the handedness (hierarchical chirality) of the helical nanostructure is thus determined. The results from the ECD, VCD, and 3D TEM for the investigated chirality at different length scales suggest the homochiral evolution in the self-assembly of the BCPs*. For chiral polylactides, twisted lamellae in crystalline banded spherulite can be formed by dense packing scheme and effective interactions upon helical chains from self-assembly. The handedness of the twisted lamella can be determined by using rotation experiment of polarized light microscopy (PLM). Similar to the self-assembly of BCPs*, the examined results suggest the homochiral evolution in the crystallized chiral polylactides. The results presented in this Account demonstrate the notable progress in the spectral and morphological determination for the examination of molecular, conformational, and hierarchical chirality in self-assembled twisted superstructures of chiral polymers and helical phases of block copolymers and suggest the attainability of homochiral evolution in the self-assembly of chiral homopolymers and BCPs*. The suggested methodologies for the understanding of the mechanisms of the chirality transfer at different length scales provide the approaches to give Supporting Information for disclosing the mysteries of the homochiral evolution from molecular level.

Chirality Control and Its Memory at Microphase-Separated Interface of Self-Assembled Chiral Block Copolymers for Nanostructured Chiral Materials.

Here, we show the induced chirality of an achiral chromophoric dye as a joint of polylactide-containing chiral block copolymers (BCPs*) driven by self-assembly, giving the achiral dyes preferentially arranged in a one-handed helical array at the microphase-separated interface. This helical arrangement of the achiral dyes can be "memorized" after hydrolysis of the polylactides in the BCPs* and serves as a chiral template for further chirality induction of different achiral dyes, probably through attractive aromatic π-π interactions at the interface, producing nanostructured chiral materials with tunable circular dichroism signals at desired wavelengths.

Gas-phase self-assembly of uniform silica nanostructures decorated and doped with silver nanoparticles.

We report a systematic study of the controlled gas-phase synthesis of silver-silica hybrid nanostructures (Ag-SiO2 NP) using the concept of evaporation-induced self-assembly. The approach includes the use of a direct gas-phase electrophoresis for size classification and in situ characterization of mobility size. Transmission electron microscopy and ultraviolet-visible light spectroscopy were employed complementarily to determine the morphology and surface plasmon resonance of Ag-SiO2 NP. Results show that two types of Ag-SiO2 NPs were successfully synthesized: (1) AgNPs decorated on a SiO2-NP (Ag-T-SiO2 NP), and (2) AgNPs doped in a cluster of SiO2-NPs (Ag-C-SiO2 NP). The physical size, morphology, and compositions of Ag-SiO2 NPs were tunable through the adjustments of precursor concentrations and the selected mobility sizes. The results also show that SPR performance, colloidal stability, and dispersibility of AgNPs enhanced significantly in an aqueous environment after the hybridization with SiO2-NP (especially for Ag-C-SiO2 NP). The results and corresponding methodology summarized here provide the proof of concept to fabricate high-purity AgNP-based hybrid nanostructures through gas-phase evaporation-induced self-assembly for future biomedical applications (e.g., hyperthermal therapy, targeted drug delivery, and antibacterial applications).

Surface PEGylation of Silver Nanoparticles: Kinetics of Simultaneous Surface Dissolution and Molecular Desorption.

A quantitative study of the stability of silver nanoparticles (AgNPs) conjugated with thiolated polyethylene glycol (SH-PEG) was conducted using gas-phase ion-mobility and mass analyses. The extents of aggregation and surface dissolution of AgNPs, as well as the amount of SH-PEG adsorption and desorption, were able to be characterized simultaneously for the kinetic study. The results show that the SH-PEG with a molecular mass of 6 kg/mol (SH-PEG6K) was able to adsorb to the surface of AgNP to form PEG6K-HS-AgNP conjugates, with the maximum surface adsorbate density of ∼0.10 nm(-2). The equilibrium binding constant for SH-PEG6K on AgNPs was calculated as ∼(4.4 ± 0.9) × 10(5) L/mol, suggesting a strong affinity due to thiol bonding to the AgNP surface. The formation of SH-PEG6K corona prevented PEG6K-HS-AgNP conjugates from aggregation under the acidic environment (pH 1.5), but dissolution of core AgNPs occurred following a first-order reaction. The rate constant of Ag dissolution from PEG6K-HS-AgNP was independent of the starting surface packing density of SH-PEG6K on AgNP (σ0), indicating that the interactions of H(+) with core AgNP were not interfered by the presence of SH-PEG6K corona. The surface packing density of SH-PEG6K decreased simultaneously following a first-order reaction, and the desorption rate constant of SH-PEG6K from the conjugates was proportional to σ0. Our work presents the first quantitative study to illustrate the complex mechanism that involves simultaneous aggregation and dissolution of core AgNPs in combination with adsorption and desorption of SH-PEG. This work also provides a prototype method of coupled experimental scheme to quantify the change of particle mass versus the corresponding surface density of functional molecular species on nanoparticles.

Controlled Handedness of Twisted Lamellae in Banded Spherulites of Isotactic Poly(2-vinylpyridine) as Induced by Chiral Dopants.

Herein, we suggest a unique approach to control the handedness of twisted lamellae in banded spherulites of a stereoregular polymer, isotactic poly(2-vinylpyridine) (iP2VP). When (R)- or (S)-hexahydromandelic acid (HMA), which can associate with iP2VP, was introduced as a chiral dopant, mirror-image CD spectra in the complex systems showed induced circular dichroism (ICD) of the iP2VP by chiral HMA. Banded spherulites resulting from lamellar twisting due to the imbalanced stresses at the opposite folding surfaces could be formed by crystallization of the iP2VP/HMA complexes, which had a crystalline structure similar to that of neat iP2VP. A preferential sense of the twisted crystalline lamellae was found in the iP2VP/HMA complex, thus suggesting homochiral evolution from conformational to hierarchical chirality.

Quantifying nanosheet graphene oxide using electrospray-differential mobility analysis.

We report a high-resolution, traceable method to quantify number concentrations and dimensional properties of nanosheet graphene oxide (N-GO) colloids using electrospray-differential mobility analysis (ES-DMA). Transmission electron microscopy (TEM) was employed orthogonally to provide complementary data and imagery of N-GOs. Results show that the equivalent mobility sizes, size distributions, and number concentrations of N-GOs were able to be successfully measured by ES-DMA. Colloidal stability and filtration efficiency of N-GOs were shown to be effectively characterized based on the change of size distributions and number concentrations. Through the use of an analytical model, the DMA data were able to be converted into lateral size distributions, showing the average lateral size of N-GOs was ∼32 nm with an estimated thickness ∼0.8 nm. This prototype study demonstrates the proof of concept of using ES-DMA to quantitatively characterize N-GOs and provides traceability for applications involving the formulation of N-GOs.

Protein-silver nanoparticle interactions to colloidal stability in acidic environments.

We report a kinetic study of Ag nanoparticles (AgNPs) under acidic environments (i.e., pH 2.3 to pH ≈7) and systematically investigate the impact of protein interactions [i.e., bovine serum albumin (BSA) as representative] to the colloidal stability of AgNPs. Electrospray-differential mobility analysis (ES-DMA) was used to characterize the particle size distributions and the number concentrations of AgNPs. Transmission electron microscopy was employed orthogonally to provide visualization of AgNPs. For unconjugated AgNPs, the extent of aggregation, or the average particle size, was shown to be increased significantly with an increase of acidity, where a partial coalescence was found between the primary particles of unconjugated AgNP clusters. Aggregation rate constant, kD, was also shown to be proportional to acidity, following a correlation of log(kD) = -1.627(pH)-9.3715. Using ES-DMA, we observe BSA had a strong binding affinity (equilibrium binding constant, ≈ 1.1 × 10(6) L/mol) to the surface of AgNPs, with an estimated maximum molecular surface density of ≈0.012 nm(-2). BSA-functionalized AgNPs exhibited highly-improved colloidal stability compared to the unconjugated AgNPs under acidic environments, where both the acid-induced interfacial dissolution and the particle aggregation became negligible. Results confirm a complex mechanism of colloidal stability of AgNPs: the aggregation process was shown to be dominant, and the formation of BSA corona on AgNPs suppressed both particle aggregation and interfacial dissolution of AgNP samples under acidic environments.

Lamellar-twisting-induced circular dichroism of chromophore moieties in banded spherulites with evolution of homochirality.

Banded spherulites are formed by crystallization of a chiral polymer that is end-capped with chromophore. Induced circular dichroism (ICD) of the chromophore can be found in the crystallized chiral polymers, giving exclusive optical response of the ICD. The ICD signals are presumed to be driven by the lamellar twisting in the crystalline spherulites, and the exclusive optical activity is attributed to the chirality transfer from molecular level to macroscopic level. To verify the suggested mechanism, the sense of the lamellar twisting in the crystalline spherulite is determined using PLM for the comparison with the ICD signals of the chromophore in the electron circular dichroism spectrum. The conformational chirality of the chiral polymer is determined by the vibrational circular dichroism spectrum. On the basis of the chiroptical results, evolution of homochirality from helical polymer chains (conformational chirality) to lamellar twisting in the banded spherulite (hierachical chirality) is suggested.

Shifting networks to achieve subgroup symmetry properties.

A simple method for the preparation of nanomaterials with new functionality by physical displacement of a network phase is suggested, giving a change in space group symmetry and hence properties. A double gyroid structure made by the self-assembly of block copolymers is used as a model system for the demonstration of shifting networks to achieve single gyroid-like scattering properties. Free-standing single gyroid-like network materials can be fabricated to give nanophotonic properties, similar to the photonic properties of a butterfly wing structure.

Tbr1 haploinsufficiency impairs amygdalar axonal projections and results in cognitive abnormality.

The neuron-specific transcription factor T-box brain 1 (TBR1) regulates brain development. Disruptive mutations in the TBR1 gene have been repeatedly identified in patients with autism spectrum disorders (ASDs). Here, we show that Tbr1 haploinsufficiency results in defective axonal projections of amygdalar neurons and the impairment of social interaction, ultrasonic vocalization, associative memory and cognitive flexibility in mice. Loss of a copy of the Tbr1 gene altered the expression of Ntng1, Cntn2 and Cdh8 and reduced both inter- and intra-amygdalar connections. These developmental defects likely impair neuronal activation upon behavioral stimulation, which is indicated by fewer c-FOS-positive neurons and lack of GRIN2B induction in Tbr1(+/-) amygdalae. We also show that upregulation of amygdalar neuronal activity by local infusion of a partial NMDA receptor agonist, d-cycloserine, ameliorates the behavioral defects of Tbr1(+/-) mice. Our study suggests that TBR1 is important in the regulation of amygdalar axonal connections and cognition.

Giant surfactants provide a versatile platform for sub-10-nm nanostructure engineering.

The engineering of structures across different length scales is central to the design of novel materials with controlled macroscopic properties. Herein, we introduce a unique class of self-assembling materials, which are built upon shape- and volume-persistent molecular nanoparticles and other structural motifs, such as polymers, and can be viewed as a size-amplified version of the corresponding small-molecule counterparts. Among them, "giant surfactants" with precise molecular structures have been synthesized by "clicking" compact and polar molecular nanoparticles to flexible polymer tails of various composition and architecture at specific sites. Capturing the structural features of small-molecule surfactants but possessing much larger sizes, giant surfactants bridge the gap between small-molecule surfactants and block copolymers and demonstrate a duality of both materials in terms of their self-assembly behaviors. The controlled structural variations of these giant surfactants through precision synthesis further reveal that their self-assemblies are remarkably sensitive to primary chemical structures, leading to highly diverse, thermodynamically stable nanostructures with feature sizes around 10 nm or smaller in the bulk, thin-film, and solution states, as dictated by the collective physical interactions and geometric constraints. The results suggest that this class of materials provides a versatile platform for engineering nanostructures with sub-10-nm feature sizes. These findings are not only scientifically intriguing in understanding the chemical and physical principles of the self-assembly, but also technologically relevant, such as in nanopatterning technology and microelectronics.

Transfer of chirality from molecule to phase in self-assembled chiral block copolymers.

Here, we report the mechanisms of chiral transfer at various length scales in the self-assembly of enantiomeric chiral block copolymers (BCPs*). We show the evolution of homochirality from molecular chirality into phase chirality in the self-assembly of the BCPs*. The chirality of the molecule in the BCP* is identified from circular dichroism (CD) spectra, while the handedness of the helical conformation in the BCP* is determined from a split-type Cotton effect in vibrational circular dichroism spectra. Microphase separation of the BCP* is exploited to form a helical (H*) phase, and the handedness of helical nanostructure in the BCP* is directly visualized from transmission electron microscopy tomography. As examined by CD and fluorescence experiments, significant induced CD signals and a bathochromic shift of fluorescence emission for the achiral perylene moiety as a chemical junction of the BCPs* can be found while the concentration of the BCPs* in toluene solution is higher than the critical micelle concentration, suggesting a twisting and shifting mechanism initiating from the microphase-separated interface of the BCPs* leading to formation of the H* phase from self-assembly.

Helical phase from blending of chiral block copolymer and homopolymer.

The phase behavior of the binary blends of polystyrene-b-poly(L-lactide) chiral block copolymer (BCP*) and polystyrene homopolymer (HS) is found to be strongly dependent on the molecular weight (M(n)) of the HS. A helical phase is formed in the blends with low-M(n) HS due to an enhancement of helical steric hindrance.