Hyperbranched Structures Research Articles

Non-aggregated hyperbranched phthalocyanines: single molecular nanostructures for efficient semi-opaque photovoltaics

A series of single molecular nanostructured hyperbranched phthalocyanines, HBMPc-COOH (M = H2, AlCl, Co, Cu, Zn), have been synthesized, characterized, and systematically studied as efficient semi-opaque sensitizers of TiO2 by using ultraviolet-visible absorption, steady-state and femtosecond time-resolved fluorescence, cyclic voltammetry, current–voltage and photoelectric measurements. The inherent steric effect within these hyperbranched structures effectively suppresses the aggregation of phthalocyanine rings on TiO2 surface, providing a facile approach for improving the photovoltaic performance of phthalocyanine-based dye sensitized solar cells (DSSCs). A power conversion efficiency of 1.15% along with a high incident photon to current conversion efficiency of 66.7% at 670 nm is achieved from HBZnPc-COOH sensitized solar cells. These results are consistent with findings from the femtosecond time-resolved fluorescence study, which reveals an ultrafast and efficient multi-phasic interfacial electron injection from both the Soret and Q bands to the conduction band of TiO2. The changing of the metal centers dramatically affects the optical, photophysical, electrochemical, and photovoltaic properties of the hyperbranched phthalocyanines. Zinc has been found to be the best metal center in the whole series for the design of hyperbranched phthalocyanine-based dyes for DSSC applications.

Facile and efficient synthesis of hyperbranched polyesters based on renewable castor oil

Hyperbranched polyesters with thioether linkages were facilely prepared from methyl 10-undecenoate, a castor oil-derived renewable chemical. The monomer was obtained in excellent yield through thiol–ene click chemistry in the presence of catalytic amounts of photoinitiator under UV irradiation. Subsequent bulk polycondensation via a transesterification process catalyzed by Ti(OBu)4, Sb2O3 or Zn(OAc)2 gave hyperbranched polyesters with high molecular weights and unusual crystalline properties. The degree of branching in the range 0.45 − 0.54 calculated from quantitative 13C NMR spectroscopy and low inherent viscosities of 0.16 − 0.25 dL g−1 strongly confirmed the hyperbranched structures of the resultant polymers. © 2012 Society of Chemical Industry

PEG-Stabilized Core–Shell Nanoparticles: Impact of Linear versus Dendritic Polymer Shell Architecture on Colloidal Properties and the Reversibility of Temperature-Induced Aggregation

Superparamagnetic iron oxide nanoparticles (SPIONs) have been widely used experimentally and also clinically tested in diverse areas of biology and medicine. Applications include magnetic resonance imaging, cell sorting, drug delivery, and hyperthermia. Physicochemical surface properties are particularly relevant in the context of achieving high colloidal nanoparticle (NP) stability and preventing agglomeration (particularly challenging in biological fluids), increasing blood circulation time, and possibly targeting specific cells or tissues through the presentation of bioligands. Traditionally, NP surfaces are sterically stabilized with hydrophilic polymeric matrices, such as dextran or linear poly(ethylene glycol) brushes. While dendrimers have found applications as drug carriers, dispersants with dendritic ("dendrons") or hyperbranched structures have been comparatively neglected despite their unique properties, such as a precisely defined molecular structure and the ability to present biofunctionalities at high density at the NP periphery. This work covers the synthesis of SPIONs and their stabilization based on poly(ethylene glycol) (PEG) and oligo(ethylene glycol) (OEG) chemistry and compares the physicochemical properties of NPs stabilized with linear and dendritic macromolecules of comparable molecular weight. The results highlight the impact of the polymeric interface architecture on solubility, colloidal stability, hydrodynamic radius, and thermoresponsive behavior. Dendron-stabilized NPs were found to provide excellent colloidal stability, despite a smaller hydrodynamic radius and lower degree of soft shell hydration compared to linear PEG analogues. Moreover, for the same grafting density and molecular weight of the stabilizers, OEG dendron-stabilized NPs show a reversible temperature-induced aggregation behavior, in contrast to the essentially irreversible aggregation and sedimentation observed for the linear PEG analogues. This new class of dendritically stabilized NPs is believed to have a potential for future biomedical and other applications, in which stability, resistance to (or reversible) aggregation, ultrasmall size (for crossing biological barriers or inclusion in responsive artificial membranes), and/or high corona density of (bio)active ligands are key.

New autophotosensitive semiaromatic hyperbranched polyimides with excellent thermal stabilities and low birefringences

A series of autophotosensitive semiaromatic hyperbranched polyimides were prepared by (A2 + B3)-type method and endcapped with 3-aminophenylacetylene. The photosensitive-hyperbranched polyimides (PHBPIs) showed excellent solubility, excellent optical properties, and low birefringences in the range of 0.0010–0.0027, caused by their special hyperbranched structures and alicyclic chain segments reduced both intramolecular and intermolecular charge transfer complex effect. The cured films also showed high glass transition temperatures between 204°C and 220°C, excellent thermal stabilities with 10% thermal decomposing temperatures in the range of 476–495°C, and good mechanical properties. The PHBPIs had photosensitivities ( Dg) in the range of 80–162 mJ/cm2 and showed a clear negative image with a line width of 8 µm when exposed to an ultraviolet light of 365 nm ( i-line).

Investigation of the Redox Property, Migration and Catalytic Performance of Ferrocene-Modified Hyperbranched Poly(amine) Ester

The electrochemical properties of a series of ferrocene modified hyperbranched poly(amine) esters (HPAE-Fc) were analyzed by cyclic voltammetry. HPAE-Fc was used as burn-rate catalytic components added into hydroxyl-terminated polybutadiene (HTPB)-based elastomers to simulate the original migration process in propellants to investigate the migration behavior of HPAE-Fc. The ultraviolet absorption properties of HPAE-Fc and their migration inhibition effects in HTPB elastomers under different aging conditions were analyzed. The diffusion coefficients (D) of the migration components were calculated according to Fick’s law. The catalytic performance of HPAE-Fc for thermal decomposition of hydroxyl-terminated polybutadiene was also investigated by non-isothermal measurement using the Ozawa method. HPAE-Fc maintains the special redox properties by electron transfer of ferrocene/ferrocenium in ferrocene groups; and, all the redox centers were equivalent with no interaction between them. The migration of HPAE-Fc in HTPB was minimized significantly by grafting ferrocenes onto the hyperbranched structures compared with that of simple ferrocene derivatives—ferrocenecarboxylic acid and tert-butylferrocene. HPAE-Fcs present efficient catalytic effects on the thermal degradation of HTPB; and, the catalytic reactions were characterized by decreased activation energy and increased rate constant.

Synthesis and Properties of Degradable Copolymers Composed of Poly(ε‐caprolactone) and 3,4‐Dihydroxycinnamic Acid

AbstractPhotoreactive and degradable hyperbranched (HB) copolymers with various 3,4‐dihydroxycinnamic acid (DHCA) compositions, poly(ε‐caprolactone)‐co‐poly(3,4‐dihydroxycinnamic acid) (PCL‐co‐PDHCA), were obtained by thermal melt‐polycondensation of PCL and DHCA. The HB structures and the branching degree (BD) of the PCL‐co‐PDHCA copolymers were confirmed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (1H NMR). The melting points (Tm) of the PCL‐co‐PDHCA copolymers changed depending on the PCL and DHCA composition by differential scanning calorimetry (DSC) measurements. Wide angle X‐ray diffraction (WXRD) analysis showed semi‐crystalline of the PCL and PCL‐co‐PDHCA polymers. The PCL‐co‐PDHCA copolymers showed good photoreactivities and fluorescent properties. Crosslinking of the cinnamoyl groups in the copolymers caused by UV irradiation affected the thermal stability and wettability slightly. Moreover, the hydrolysis experiments revealed that copolymers are facile degradable.

Ethynyl-Capped Hyperbranched Conjugated Polytriazole: Click Polymerization, Clickable Modification, and Aggregation-Enhanced Emission

Cu(I)-catalyzed azide–alkyne click polymerization, developed based on the click reaction, has become a powerful tool for the construction of functional polytriazoles with linear and hyperbranched structures. This method has, however, rarely been used for the preparation of functional hyperbranched conjugated polytriazoles (hb-CPTA). In this paper, soluble ethynyl-capped hb-CPTA with weight-averaged molecular weight of 39 500 was synthesized in high yield (84.4%) by the Cu(I)-catalyzed azide–alkyne click polymerization of tetraphenylethene containing diazide [1,2-bis(4-azidophenyl)-1,2-diphenylethene] and tetrayne [1,1,2,2-tetrakis(4-ethynylphenyl)ethane] in equal concentration. By taking advantage of the ethynyl groups on its periphery, the polymer could be efficiently postfunctionalized by azide–alkyne and thiol–yne click reactions. The polymers are thermally stable and loss 5% of their weights at temperatures higher than 340.0 °C. hb-CPTA also possesses high char yield (74.8%) at 800 °C. The polymers feature the unique characteristics of aggregation-enhanced emission. Furthermore, the PL intensities of the hb-CPTA and thiol–yne postfunctionalized polytriazoles increase linearly with water fraction in THF/water mixtures. Thanks to their rigid structures, the polymers could be fabricated into unimolecular nanoparticles with sizes of ca. 100 nm. Thus, this paper provides a powerful method to synthesize soluble ethynyl-capped hyperbranched polymers, which could be a useful platform for preparation of versatile functional polymers via postreactions.

Metastable Tetragonal Cu2Se Hyperbranched Structures: Large‐Scale Preparation and Tunable Electrical and Optical Response Regulated by Phase Conversion

Despite the promising applications of copper selenide nanoparticles, an in-depth elucidation of the inherent properties of tetragonal Cu(2)Se (β-Cu(2)Se) has not been performed because of the lack of a facile synthesis on the nanoscale and an energy-intensive strategy is usually employed. In this work, a facile wet-chemical strategy, employing HCOOH as reducing agent, has been developed to access single-crystalline metastable β-Cu(2)Se hyperbranched architectures for the first time. The process avoids hazardous chemistry and high temperatures, and thus opens up a facile approach to the large-scale low-cost preparation of metastable β-Cu(2)Se hyperbranched architectures. A possible growth mechanism to explain the formation of the β-Cu(2)Se dendritic morphology has been proposed based on time-dependent shape evolution. Further investigations revealed that the metastable β-Cu(2)Se can convert into the thermodynamically more stable cubic α-Cu(2-x)Se maintaining the dendritic morphology. An increase in electrical conductivity and a tunable optical response were observed under ambient conditions. This behavior can be explained by the oxidation of the surface of the β-Cu(2)Se hyperbranched structures, ultimately leading to solid-state phase conversion from β-Cu(2)Se into superionic conductor α-Cu(1.8)Se, which has potential applications in energy-related devices and sensors.

Emergence of localized in-gap states in conjugated polymers of branched topology

Using cryogenic scanning tunneling microscopy and spectroscopy, we investigated cross-linked phenyelene-based polymers with various branched morphology at a single-molecule resolution. We found that localized states that are in the band gap of un-branched polymers emerge at the branch junctions. These in-gap states can be shifted close to the Fermi level through three means: extending the length of branch arms, coupling adjacent branches in hyperbranched structures, or increasing the number of branch arms. Single-band tight-binding calculations can reproduce all experimental results and provide quantitative relationships between the energy level shifts and the branching geometry. Our discovery evidences the emergence and development of in-gap states in the band structures of conjugated systems with fractional dimensions.

Novel redox‐active polycarbazole‐functionalized polycatechol network films produced by controlled electropolymerization

AbstractA novel precursor, 1,2‐bis[6‐(9H‐carbazol‐9‐yl)hexyloxy] benzene (BCHB), was successfully synthesized. Its polycarbazole‐functionalized polycatechol network films, poly{1,2‐bis[6‐(9H‐carbazol‐9‐yl)hexyloxy] benzene} (PBCHB), with good redox activity were formed by the direct anodic oxidation of BCHB in CH2Cl2 and boron trifluoride diethyl etherate binary solvent solution. Ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, 1H‐NMR, and matrix‐assisted laser desorption ionization–time of flight mass spectrometry were used to characterize the polymers. The results indicate that the network polymers could be synthesized electrochemically with different polymerized units by controlled electropolymerization. The PBCHB films prepared at low potential were oligomers with short conjugation lengths and were soluble in common organic solvents, whereas the polymers with long conjugation lengths and hyperbranched network structures obtained at high potential were insoluble. The electrosynthesized polymers exhibited blue emission maxima around 450 nm and were much more redshifted than their monomer. The emissions were also brighter; this indicated the polymers are potential good blue‐light emitters. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Quantification of Branching in Model Three-Arm Star Polyethylene

The versatility of a novel scaling approach in quantifying the structure of model well-defined 3-arm star polyethylene molecules is presented. Many commercial polyethylenes have long side branches, and the nature and quantity of these branches varies widely among the various forms. For instance, low-density polyethylene (LDPE) is typically a highly branched structure with broad distributions in branch content, branch lengths and branch generation (in hyperbranched structures). This makes it difficult to accurately quantify the structure and the inherent structure–property relationships. To overcome this drawback, model well-defined hydrogenated polybutadiene (HPB) structures have been synthesized via anionic polymerization and hydrogenation to serve as model analogues to long-chain branched polyethylene. In this article, model 3-arm star polyethylene molecules are quantified using the scaling approach. Along with the long-chain branch content in polyethylene, the approach also provides unique measurements...

From Flexible to Hard Polyurethane Aerogels: The Effect of Molecular Functionality vs. Molecular Rigidity

ABSTRACTHigh performance all-polymer aerogels are not only interesting for their low thermal conductivity, but also for their mechanical strength and their conversion to porous carbons. The prevalent design rule dictates that crosslinking at the monomer level decreases solubility of the developing polymer and induces phase separation of nanoparticles with high surface to volume ratios. Hence, hyperbranched structures based on trifunctional single aromatic core monomers should have enhanced interparticle connectivity and rigidity compared to those based on either difunctional or multiple aromatic core monomers. That design rule is applied here to hyperbranched polyurethane (PU) aerogels synthesized from tris(4-isocyanatophenyl)methane (TIPM) and 1,1,1-tris(4-hydroxyphenyl)ethane (HPE) in anhydrous acetone using dibutyltin dilaurate (DBTDL) as catalyst. The resulting materials vary from highly flexible to rigid as the monomer concentration increases. FTIR and 13C solid NMR confirm formation of urethane. SEM shows that the flexible variety is macroporous with a beaded worm-like structure, while the rigid variety is mesoporous and nanoparticulate. Lower-density flexible aerogels are highly porous (92%) with surface area of 132 m2 g-1, while the rigid ones have higher surface areas, up to 256 m2 g-1. Polyurethane aerogels from other triols and diols, e.g., phloroglucinol (POL) and resorcinol (RES), respectively, have also been studied and for similar monomer concentrations shrinkage and bulk density increases.

Azide-alkyne click polymerization: An update

The great achievements of click chemistry have encouraged polymer scientists to use this reaction in their field. This review assembles an update of the advances of using azide-alkyne click polymerization to prepare functional polytriazoles (PTAs) with linear and hyperbranched structures. The Cu(I)-mediated click polymerization furnishes 1,4-regioregular PTAs, whereas, the metal-free click polymerization of propiolates and azides produces PTAs with 1,4-regioisomer contents up to 90%. The PTAs display advanced functions, such as aggregation-induced emission, thermal stability, biocompatibility and optical nonlinearity.

Organic Macromolecular High Dielectric Constant Materials: Synthesis, Characterization, and Applications

Hyperbranched and dendritic architectures have been targeted for various applications such as sensing, drug delivery, optical limiting, and light harvesting. One interesting development in this area has focused on utilizing the existence of long-range delocalization in hyperbranched structures to achieve high dielectric constants. In this Feature Article, we will review the creation and development of this concept, and we highlight our recent research progress in this aspect. In particular, we discuss (1) synthetic methods for a particular group of hyperbranched polymers; (2) detailed optical and electronic characterization of this group of hyperbranched polymers, revealing the design criteria for achieving a good combination of high dielectric constant and minimum loss in such materials; and (3) the importance and potential applications of these materials.

Hyperbranched Polyacetals with Tunable Degradation Rates

We report the first synthesis of hyperbranched polyacetals via a melt transacetalization polymerization process. The process proceeds via the self-condensation of an AB2 type monomer carrying a hydroxyl group and a dimethylacetal unit; the continuous removal of low boiling methanol drives the equilibrium toward polymer formation. Because of the susceptibility of the acetal linkage to hydrolysis, the polymer degrades readily under mildly acidic conditions to yield the corresponding hydroxyl–aldehyde as the primary product. Furthermore, because of the unique topology of hyperbranched structures, the rate of polymer degradation was readily tuned by changing just the nature of the end-groups alone; instead of the dimethylacetal bearing monomer, longer chain dialkylacetals (dibutyl and dihexyl) monomers yielded hyperbranched polymers carrying longer alkyl groups at their molecular periphery. The highly branched topology and the relatively high volume fraction of the terminal alkyl groups resulted in a significant lowering of the ingress rates of the aqueous reagents to the loci of degradation, and consequently the degradation rates of the polymers were dramatically influenced by the hydrophobic nature of the terminal alkyl substituents. The simple synthesis and easy tunability of the degradation rates make these materials fairly attractive candidates for use as degradable scaffolds.

Thiol‐ene clickable hyperscaffolds bearing peripheral allyl groups

AbstractRadical catalyzed thiol‐ene reaction has become a useful alternative to the Huisgen‐type click reaction as it helps to expand the variability in reaction conditions as well as the range of clickable entities. Thus, direct generation of hyperbranched polymers bearing peripheral allyl groups that could be clicked using a variety of functional thiols would be of immense value. A specifically designed AB2 type monomer, that carries two allyl benzyl ethers groups and one alcohol functionality, was shown to undergo self‐condensation under acid‐catalyzed melt‐transetherification to yield a hyperbranched polyether that carries numerous allyl end‐groups. Importantly, it was shown that the kinetics of polymerization is not dramatically affected by the change of the ether unit from previously studied methyl benzyl ether to an allyl benzyl ether. The peripheral allyl groups were readily clicked quantitatively, using a variety of thiols, to generate an hydrocarbon‐soluble octadecyl‐derivative, amphiphilic systems using 2‐mercaptoethanol and chiral amino acid (N‐benzoyl cystine) derivatized hyperbranched structures; thus demonstrating the versatility of this novel class of clickable hyperscaffolds. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

From sequence to 3D structure of hyperbranched molecules: application to surface modified PAMAM dendrimers

The molecular modeling of hyperbranched molecules is currently constrained by difficulties in model building, due partly to lack of parameterization of their building blocks. We have addressed this problem with specific relevance to a class of hyperbranched macromolecules known as dendrimers by describing a new concept and developing a method that translates monomeric linear sequences into a full atomistic model of a hyperbranched molecule. Such molecular-modeling-based advances will enable modeling studies of important biological interactions between naturally occurring macromolecules and synthetic macromolecules. Our results also suggest that it should be possible to apply this sequence-based methodology to generate hyperbranched structures of other dendrimeric structures and of linear polymers.

Controlling the evolution of polypropylene microstructure during melt free radical modification using zinc dithiocarbamate with different N-substituted groups

The effects of zinc dithiocarbamates on degradation and branching of polypropylene (PP) were studied during melt radical modification using a tri-functional monomer (trimethylol propane triacrylate (TMPTA)). High-temperature size-exclusion chromatography (HT-SEC) coupled with differential refractive index detector (DRI), light scattering detector (LSD) and viscometer detector (VD) and rotational rheometry were used to analyse the microstructure of modified PP samples. The chemical structure of the N-substituted group showed an important influence in controlling the evolution of PP microstructure during melt radical reaction. The chain cleavage of PP was controlled in the presence of zinc N, N-dimethyldithiocarbamate (ZDMC), and a substantial long chain branched (LCB) fraction was formed. Without co-agent, the molecular weight of PP decreased measurably, and only a minor LCB fraction was formed. Importantly, considerable amounts of highly branched (microgel and hyperbranched) structures were formed in this case.

Enhanced Mechanical and Dielectric Properties of PU Networks with Hyperbranched Structures

We report a new polyurethane (PU) dielectric containing hyperbranched structures and metal nanoparticles. Silver nanoparticles were incorporated into the PU networks by using hyperbranched polyester (HBP) as a reductive template as well as the cross-linker. Simultaneously enhanced mechanical and dielectric properties were observed in the hyperbranched PU dielectrics. The branched macromolecular fractal provides a “covalent bridge” between the PU matrix and Ag nanoparticles, which contributed to the simultaneously enhanced mechanical and dielectric properties.

Characterising layered structure of MgPS<SUB align=right>3 and new application as a hydrogen storage material

X-ray diffraction (XRD) pattern exhibits sharp 001 reflections of the MgPS 3 layered material with interlayer gap (d-spacing) of 6.4 A. Scanning electron microscope (SEM) shows a pattern of conglomerates of layers quite uniform in size, around 100–200 nm. TEM images show homogeneous distribution of nanoparticles of about 60–200 nm in size. These particles tend to be agglomerated in hyperbranched structures. Thermal stability was detected by DSC-TGA, which shows decomposition temperature up to 740°C relying on the rate of heating. Strong peak of n(PS 3 ) 2– stretching at 570 cm –1 of pure MgPS 3 was shown by infrared (IR). New application as a hydrogen storage material is investigated. The average gravimetric capacity is about 16 g H 2 /kg of MgPS 3 at 25 bars and cryogenic temperature.