Shenghong A. Dai

Synthesis and montmorillonite-intercalated behavior of dendritic surfactants

A series of novel dendrons serving as surfactants for surface modification of montmorillonite (MMT) have been synthesised via a convergent approach. The most distinguishing characteristic of the prepared dendrons is the long alkyl chain at the periphery. This would certainly bring about good solubility, high yield and easy purification. By simple acidification of the “head group” upon the dendron and then ion-exchange with Na+-MMT, the interlayer distance could be enlarged significantly between the layered silicates. The organic/inorganic ratio determined by thermogravimetric analysis (TGA) is in the range of 60%–91%. This also confirms the occurrence of dendron ion-exchange with Na+-MMT. Transmission electronic microscopy (TEM) reveals the intercalated and exfoliated morphology of the dendron-modified MMTs. Modification using the generation 2 dendritic surfactant (G2) with molecular weight (Mw) = 2887.5 has achieved the exfoliated morphology in this work.

Superhydrophobic waxy-dendron-grafted polymer films viananostructure manipulation

To imitate the superhydrophobicity of salient epicuticular wax on lotus leaves (hereafter “Lotus effect”), waxy dendrons were synthesized and subsequently grafted on amine-containing polystyrenes. To achieve a low surface energy and a specific surface morphology, the waxy dendron design is composed of two parts—the focal part possessing plenty of hydrogen bonding sites, and the peripheral part rich in van der Waals forces. The enhanced van der Waals force accompanied with increasing generation of dendrons helps induce self-assembly and phase separation in the preparation process of the polymer films. By different coating processes, three different films (thin film, honeycomb-like film, and three-dimensional rod-co-valley-like film) were obtained with contact angles of 95°, 130°, and 165°, respectively. The three-dimensional rod-co-valley film samples were able to imitate the superhydrophobic property (i.e. Lotus effect), as well as utilize the built-in strong hydrogen bonds to adhere water droplets on surfaces or substrates.

Side chain dendritic polyurethanes with shape-memory effect

In this study, thermally reversible polyurethanes (PUs) with shape memory effect were developed by using hydrogen bonding to enhance physical interactions. Two different types of PUs were synthesized: (1) a linear PU whose hard segment consists of methylene di-para-phenyl isocyanate (MDI) and di(ethylene glycol) (DEG); its soft segment is made of Tone 0260 polyol, a polyester polyol; (2) a side-chain dendritic PU which replaces DEG with different generations of dendritic chain extenders. By incorporation of the dendritic structure with peripheral long alkyl chains (strong van der Waals forces), the miscibility between hard and soft segments can be significantly improved. Consequently, the hydrogen bonding reinforced hard segments (malonamide linkages) of side chain dendritic PUs result in greatly enhanced mechanical properties and shape memory effect. During cyclic shape memory tests, one of the series can effortlessly achieve complete recovery in less than 10 second without any deficiency.

Organo-clay hybrids based on dendritic molecules: preparation and characterization

Three generations of dendrons (G1, G2 and G3) with phenyl end-groups were intercalated into montmorillonite (MMT) layered silicates in dimethylformamide/water cosolvent. These dendrons synthesized via the convergent route were different in size and shape with molecular weights ranging from 930 to 5975?g?mol?1. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) indicate that the respective intercalations of MMT with G1, G2 and G3 dendrons exhibited organized characteristics, and the interlayer spacings were 38, 77 and 115?? for G1/MMT, G2/MMT and G3/MMT, respectively. Furthermore, the modified dendron/MMT hybrids could be well dispersed into organic solvents, such as DMF, due to the presence of hydrophobic dendritic molecules. Our results indicate that organic/inorganic hybrids resulting from the association of dendrons and inorganic layered silicate can be obtained not only by ionic exchange reaction, but possibly by a direct organized route as well. Moreover, the preparation of dendron/MMT hybrids was investigated at different dendron/MMT molar ratios regarding the changes of interlayer distance. When the molar ratio is within the range of 0.25?1.0 cationic exchange capacity (CEC) equivalents, the d spacings increase from 19 to 38?? for G1/MMT, 15 to 77?? for G2/MMT and 14 to 115?? for G3/MMT, revealing a conformation change of the intercalating dendrons.

Sequential self-repetitive reaction toward wholly aromatic polyimides with highly stable optical nonlinearity

A sequential self-repetitive reaction (SSRR) based on carbodiimide (CDI) chemistry was utilized for preparing a high-yield wholly aromatic polyimide. The polyimide was synthesized with 4,4′-methylene-diphenylisocyanate (MDI) and a di(acid-ester) compound which was derived from the ring-opening reaction of 3,3′,4,4′-oxydiphthalic dianhydride (ODPA) at room temperature by the addition of equimolar methanol. Poly-CDI was first synthesized from MDI. The di(acid-ester) compound was then reacted with poly-CDI to form poly(N-acylurea). After curing process, N-acylurea moiety was converted to di(ester-amide) structure viaSSRR and further subjected to a ring-closure reaction to form the wholly aromatic polyimide with a Tg of 247 °C. This approach was further taken to prepare thermally stable nonlinear optical (NLO) materials. Similarly a diimide-diacid containing chromophore was reacted with poly-CDI to obtain an intermediate, poly(N-acylurea). The poly(N-acylurea) with the ester side groups would exhibit excellent organosolubility, which enabled the fabrication of high quality optical thin films. After in situ poling and curing processes, N-acylurea moiety was converted to di(ester-amide) structure viaSSRR and further subjected to a ring-closure reaction to form the wholly aromatic NLO polyimide with an electro-optical coefficient, r33 of 25 pm/V (830 nm). Excellent temporal stability at elevated temperatures (200 °C) and a waveguide optical loss of 2.5 dB cm−1 at 1310 nm were also obtained.

Orderly Arranged NLO Materials Based on Chromophore-Containing Dendrons on Exfoliated Layered Templates

Three chromophore-containing dendrons were intercalated into montmorillonite layered silicates via an ion-exchange process. Enlarged d spacings ranging from 50 to 126 A were achieved for these novel organoclays. After the organoclays were blended with a polyimide, the steric bulkiness of the dendrons and the interaction between dendron and polyimide resulted in an ordered morphology. The orderly arranged nanocomposites were characterized by a UV-visible spectrophotometer, a variable-temperature infrared spectrometer, and electro-optical modulation. The dendrons in layered silicates were capable of undergoing a critical conformational change into an ordered structure, indicated by the drastic changes of interlayer distances at certain packing densities. Electro-optical coefficients increased sharply from 0 to 6 pm/V while the conformational change occurred. Furthermore, the addition of a polyimide capable of interaction-induced orientation was found to exert an enhancing effect on the degree of the noncentrosymmetric alignment.

Enhanced shape memory performance of polyurethanes via the incorporation of organic or inorganic networks

A diol compound with a reactive azetidine-2,4-dione group was prepared and introduced as a side chain moiety of poly(e-caprolactone) (PCL) based polyurethane (PU). The PUs with reactive pendants were crosslinked by 1,6-diaminohexane, or modified by an alkoxysilane (3-aminopropyltriethoxysilane, APTS) followed by a sol–gel reaction to bring about crosslinked PUs for shape memory applications. Thermal and mechanical properties of the crosslinked PUs are strongly dependent on the chemical structure of the interchain-linker between the polymer chains. Higher tensile strengths would be achieved for the PU samples crosslinked with alkoxysilanes in comparison with the ones crosslinked with an aliphatic diamine, while higher values of elongation at break were observed for the latter. As compared to the PCL based linear and side-chain PUs, much better shape memory performance was observed as the PU sample was crosslinked with the aliphatic diamine, or by the sol–gel reaction of alkoxysilanes, particularly the latter. Higher shape retention (91%) and recovery (99%) ratios were observed for the CPU samples with inorganic networks. By introducing the chemically crosslinked structures, the deformed PU samples completely recovered their original shape in less than 10 seconds without any deficiency during shape recovery measurements.

Enhanced photovoltaic performance of inverted polymer solar cells by incorporating graphene nanosheet/AgNPs nanohybrids

A linear-dendritic block copolymer (LDBC) functionalized exfoliated graphene nanosheets (XGS)/silver nanoparticles (AgNPs) was prepared for using as the interfacial layer between the zinc oxide (ZnO) based electron-selective layer and poly(3-hexylthiophene) (P3HT)/fullerene derivative (PC61BM) blend based photoactive layer in an inverted polymer solar cell (PSC). The LDBCs were prepared by the respective addition reaction of a hydrophilic poly(oxyalkylene)amine with hydrophobic dendrons of various generations based on 4-isocyanate-4′-(3,3-dimethyl-2,4-dioxo-azetidine)-diphenylmethane (IDD). The dendrons having polyurea/malonamide functionalities were synthesized by using IDD as a building block. Moreover, the dendrons comprised not only hydrogen bond-rich malonamide linkages, but long alkyl chains at the peripheries as well. XGS were respectively grafted with these amphiphilic LDBCs to afford XGS-dendritic derivatives (XGS-G0.5, XGS-G1.5, and XGS-G2.5). Subsequently, a nanohybrid, XGS-G2.5/AgNPs was obtained through the reduction of Ag+ on the surface of LDBC modified XGS. The dendrons would serve as the effective templates for hosting AgNPs. By the incorporation of XGS-G2.5/AgNPs nanohybrid, it rendered the PSC an increased power conversion efficient (PCE) to 4.04%, a 19.5% improvement over a PCE of 3.38% for the bare inverted PSC, due to the localized surface plasmon resonance of AgNPs, and the improvement of compatibility and charge transfer capacity between ZnO based cathode and photoactive layer.

Sulfonated reduced graphene oxide catalyzed cyclization of hydrazides and carbon dioxide to 1,3,4-oxadiazoles under sonication

Acid catalysts facilitate many chemical reactions. Sulfonated reduced grapheneoxide (rGOPhSO3H) has shown to be an encouraging solid acid catalyst because of its efficiency, cost-effectiveness and safety of use. In this study, we prepared the rGOPhSO3H nano acid catalyst, with the introduction of aromatic sulfonic acid radicals onto GO by fractional removal of oxygenated functions. It was thoroughly characterized by FT-IR, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, energy dispersive spectroscopy (EDS) and solid state 13C MAS NMR (SSNMR). Here we report the conversion of CO2 (1.0 atm pressure, at = 50 °C, the source of C1 carbon feed stock) with hydrazides and a catalytic amount rGOPhSO3H, which through a cyclization reaction results in a new strategy for the synthesis of 5-substituted-3H-[1,3,4]-oxadiazol-2-ones (SOxdOs) under ultrasonic irradiation. Hence this concept of cyclization opens up for new insights

Synthesis of 5-substituted-3H-[1,3,4]-oxadiazol-2-one derivatives: a carbon dioxide route (CDR)

A carbon dioxide route (CDR) for making biologically important 5-substituted-3H-[1,3,4]-oxadiazol-2-ones (SHOs) has been accomplished through synthesis and cyclization of a variety of hydrazides as the key intermediates. All of these hydrazides were prepared readily in 89–97% yields by reacting acid chlorides with a hydrazine monohydrate in the initial step. Then, SHOs were obtained in high yields from hydrazides by reacting them with carbon dioxide under basic conditions. More notable than the high yields, is that the present CDR process for the first time has succeeded in providing a straightforward cyclization reaction leading to SHO formation with simple reagents in ethanol solution.