Dielectric Constant Enhancement of Epoxy Thermosets via Formation of Polyelectrolyte Nanophases

Hierarchical polyion complex vesicles from PAMAM dendrimers

Hierarchical dendrimer-based polyion complex (PIC) vesicles with multiple compartments have attracted considerable attention as functional delivery vehicles and nano-carriers. Formation of these vesicles relies on the electrostatic assembly of asymmetric polyelectrolytes, namely branched dendrimers with linear polyion-neutral diblock copolymers. However, successful incorporation of dendrimers in vesicle lamellae is challenging due to the compact structure of dendrimers, and therefore, vesicles reported so far are prepared mainly with low generation dendrimers which lack the cavity required for carrier functions. Here, we present a new assembly combination of amine-terminated dendrimer polyamidoamine (PAMAM) with polyion-neutral diblock copolymer poly (styrene sulphonate-b-ethylene oxide) (PSS-b-PEO). The strong charge interaction between the building blocks leads to stable and well-defined PIC vesicles that can tolerate not only different PSS block lengths but, more importantly, also different dendrimer generations from 2 to 7. As a consequence, high generation dendrimers with a cavity can be packed in the vesicle wall, and one obtains hierarchical PIC vesicles with multiple compartments, namely the dendrimer cavity for loading small hydrophobic cargo, and the vesicle lumen for encapsulating hydrophilic macromolecules. Our study demonstrates that combining proper building blocks enables to manipulate the charge interactions, which is essential for controlling the dendrimer packing and the formation of PIC vesicles. These findings should be helpful for understanding the assembly of asymmetric (linear / branched) polyelectrolyte complexes, as well as for designing new hierarchical PIC vesicles for controlled delivery of multiple active substances.

Investigation of Azobenzene Photoisomerization Effect on Morphologies and Properties of Nanostructured Thermosets Involving Epoxy and a Diblock Copolymer

This work highlights the effect of azobenzene photoisomerization on the morphologies and properties of the nanostructured thermosets involving epoxy and a diblock copolymer. First, a diblock copolymer composed of poly(ethylene oxide) (PEO) and poly(6-(4-(4-cyanophenylazo)phenoxy)hexyl methacrylate) (PCPHM) was synthesized, and this diblock copolymer was composed of an epoxy-philic block (i.e., PEO) and an azobenzene moiety-beating block (viz., PCPHM). This diblock copolymer was introduced into epoxy to obtain the nanostructured thermosets via reaction-induced microphase separation approach. To control the configuration of azobenzene moieties of the PCPHM block, the curing reactions were performed in the absence and/or presence of ultraviolet (UV) irradiation, respectively. It was found that, without UV irradiation, the PCPHM microdomains were generated with the trans isomers of azobenzene. Under UV irradiation, however, the PCPHM microdomains were formed with the cis configuration of azobenzene moieties. The ultraviolet-visible light (UV-vis) spectroscopy showed that the trans and cis configurations of azobenzene moieties were significantly fixed with the occurrence of curing reactions. The photoluminescent measurements showed that the nanostructured thermosets with trans-azobenzene moieties can emit fluorescence, which was in sharp contrast to those with cis-azobenzene moieties. The results of small-angle X-ray and atomic force microscopy showed that the nanostructured thermosets with trans and cis isomers of azobenzene moieties had quite different morphologies. It was found that the sizes of the PCPHM microdomains with cis configuration of azobenzene moieties were significantly larger than those with trans configuration. The difference in configuration of azobenzene moieties also resulted in the difference in glass-transition temperatures and dielectric properties of the materials. The results suggest a new approach to modulate the morphologies and physical properties of the nanostructured thermosets by means of photoisomerization of azobenzene moieties.

Fluorescence Enhancement Induced by Curing Reaction in Nanostructured Epoxy Thermosets Containing a Diblock Copolymer

In this work, a novel curing-induced fluorescence (FL) enhancement phenomenon in the nanostructuring process of epoxy thermosets was investigated. Toward this end, a diblock copolymer composed of poly(ethylene oxide) and poly(((4-vinylphenyl)ethene-1,1,2-triyl)tribenzene) (PTPEE) blocks was introduced into epoxy thermosets. Before curing reaction, the mixtures of epoxy precursors with the diblock copolymer only emitted feeble FL under ultra-visible (UV) irradiation. However, photoluminescence was significantly enhanced after the curing reaction was carried out. It was found that the novel FL enhancement phenomenon resulted from the aggregation-induced emission behavior of PTPEE blocks, which was triggered by curing reaction. In the nanostructured thermosets, the fluorophore blocks (viz. PTPEE) of this diblock copolymer were segregated into aggregates, that is, a reaction-induced microphase separation occurred. Owing to the generation of PTPEE microdomains, the epoxy nanocomposites significantly displayed the enhanced dielectric constants due to the promoted contribution from electron polarizations via π-π conjugation in the materials.

Synthesis of Hierarchical Silica/Titania Hollow Nanoparticles and Their Enhanced Electroresponsive Activity

Wrinkled silica nanoparticle (WSN)-based hollow SiO₂/TiO₂ nanoparticles (W-HNPs) with hierarchically arrayed internal surfaces were prepared via the combination of sol-gel, TiO₂ coating, and etching of core template techniques. The hierarchical internal surface of W-HNPs was attained using WSNs as a core template. Compared with SiO₂ sphere-templated hollow SiO₂/TiO₂ nanoparticles (S-HNPs) with flat inner surfaces, W-HNPs displayed distinctive surface areas, TiO₂ loading amounts, and dielectric properties arising from the hierarchical internal surface. The unique properties of W-HNPs were further investigated as an electrorheological (ER) material. W-HNP-based ER fluids exhibited ca. 1.9-fold enhancement in the ER efficiency compared to that of S-HNP-based ER fluids. Such enhancement was attributed to the unique inner surface of W-HNPs, which effectively enhanced the polarizability by increasing the number of charge accumulation sites, and to the presence of the high-dielectric TiO₂. This study demonstrated the advantages, in terms of practical ER applications, of hollow nanomaterials having uniquely arrayed internal spaces.

Organic-Inorganic Nanocomposites via Self-Assembly of an Amphiphilic Triblock Copolymer Bearing a Poly(butadiene-g-POSS) Subchain in Epoxy Thermosets: Morphologies, Surface Hydrophobicity, and Dielectric Properties

Organic-inorganic nanocomposites composed of polyhedral oligomeric silsesquioxane (POSS) and epoxy resin were prepared via self-assembly of an amphiphilic triblock copolymer bearing a poly(POSS) midblock in epoxy thermosets. First, this organic-inorganic amphiphilic triblock copolymer was synthesized via hydrosilylation of heptaphenylhydro POSS with an existing triblock copolymer containing a short polybutadiene midblock. It was found that this novel amphiphilic block copolymer can self-assemble into nanophases in epoxy thermosets. In the presence of preformed nanophases, the curing reaction was performed, and the organic-inorganic nanocomposites containing poly(POSS) microdomains were thus obtained. Compared with plain epoxy, the as-obtained thermosets exhibited enhanced surface hydrophobicity; the enhanced surface hydrophobicity is attributed to enrichment of the POSS component at the surface of the materials. Owing to the formation of poly(POSS) microdomains, the dielectric constants of the materials significantly reduced, whereas the dielectric loss remained almost unchanged.