The architectures and surface behavior of highly branched molecules

Surface modification of carbon nanotubes with dendrimers or hyperbranched polymers

The different methods for modification of carbon nanotubes with dendrimers or hyperbranched polymers are summarized, and recent development is highlighted by discussing some representative examples in detail.

Self-assembly of hyperbranched polymers and its biomedical applications

Hyperbranched polymers (HBPs) are highly branched macromolecules with a three-dimensional dendritic architecture. Due to their unique topological structure and interesting physical/chemical properties, HBPs have attracted wide attention from both academia and industry. In this paper, the recent developments in HBP self-assembly and their biomedical applications have been comprehensively reviewed. Many delicate supramolecular structures from zero-dimension (0D) to three-dimension (3D), such as micelles, fibers, tubes, vesicles, membranes, large compound vesicles and physical gels, have been prepared through the solution or interfacial self-assembly of amphiphilic HBPs. In addition, these supramolecular structures have shown promising applications in the biomedical areas including drug delivery, protein purification/detection/delivery, gene transfection, antibacterial/antifouling materials and cytomimetic chemistry. Such developments promote the interdiscipline researches among surpramolecular chemistry, biomedical chemistry, nano-technology and functional materials.

One-pot synthesis of brush-like polymers via integrated ring-opening metathesis polymerization and polymerization of amino acid N-carboxyanhydrides

We report here the integration of ring-opening metathesis polymerization (ROMP) and ring-opening polymerization of the amino acid N-carboxyanhydride (NCA) to allow facile synthesis of brush-like polymers containing polypeptide as the brush side chains. ROMP of N-trimethylsilyl norbornenes rendered the preparation of poly(norbornene)s bearing pendant N-TMS groups. With no need to purify the resulting polymers, such macromolecular initiators could subsequently initiate controlled NCA polymerizations. Brush-like poly(norbornene)s with grafted polypeptides or block copolypeptides were readily obtained with controlled molecular weights and narrow molecular weight distributions. Because numerous ROMP and NCA monomers are widely available, this novel polymerization technique will allow easy access to numerous brush-like hybrid macromolecules with unprecedented properties and broad applications.

Atom transfer radical polymerization in aqueous dispersed media

During the last decade, atom transfer radical polymerization (ATRP) received significant attention due to its exceptional capability of synthesizing polymers with pre-determined molecular weight, well-defined molecular architectures and various functionalities. It is economically and environmentally attractive to adopt ATRP to aqueous dispersed media, although the process is challenging. This review summarizes recent developments of conducting ATRP in aqueous dispersed media. The issues related to retaining “controlled/living” character as well as colloidal stability during the polymerization have to be considered. Better understanding the ATRP mechanism and development of new initiation techniques, such as activators generated by electron transfer (AGET) significantly facilitated ATRP in aqueous systems. This review covers the most important progress of ATRP in dispersed media from 1998 to 2009, including miniemulsion, microemulsion, emulsion, suspension and dispersed polymerization.

Structure and properties of functionalized bithiophenesilane monodendrons

This study reports a focal group modification of bithiophenesilane monodendrons and its effect on their molecular ordering in solution, bulk, and surface. We investigated hydrophobic MDn monodendrons and COOH-functionalized MDn-COOH monodendrons with generations, n=0, 1, 2, and 3. We observed that increasing the number of branches led to the progressive blue shift, indicating distorted packing of branched thiophene fragments of MDn. In contrast, MDn-COOH monodendrons showed a progressive red shift with the increasing generation number, indicating gradual domination of sigma-pi interactions. Moreover, the introduction of a focal carboxylic group resulted in the formation of a highly crystalline state for the linear MD0-COOH compound with separated alkyl tail-thiophene packing, which limits pi-pi interactions. Increasing branching in the COOH-containing monodendrons resulted in a hydrophobic-hybrophilic balance sufficient to form stable and uniform Langmuir monolayers at the air-water at a modest surface pressure (<10 mN/m), easily transferrable to a solid substrate. However, a further increase in the thickness of the surface layers from tens to hundreds of nanometers via Langmuir-Blodgett (LB) deposition or spin casting is limited by the formation of globular surface aggregates because of strong intermolecular interactions. A modest red shift observed for condensed Langmuir monolayers indicates densification of thiophene branches and limited intramonolayer crystallization, which preserves photoluminescence. In contrast, thicker surface films showed a significant red shift, confirming a dense molecular packing with strong pi-pi interactions, which results in photoluminescence quenching.

Bulk and surface assembly of branched amphiphilic polyhedral oligomer silsesquioxane compounds

This study probes the behavior of two series of organic-functionalized core-shell silsesquioxane (POSS-M)p-(x/y) derivatives with various hydrophobic-hydrophilic terminal group compositions in the bulk state and within mono- and multilayered films at the air-water interface and on solid surface. POSS-M refers to mixed silsesquioxane cores, in contrast to the geometrically specific POSS compounds. It is composed of polyhedra, incompletely condensed polyhedra, ladder-type structures, linear structures, and all the possible combinations thereof and attracts great interest because of its facile preparation, low polydispersity, high yield, and low cost. The two series of (POSS-M)p-(x/y) molecules are different in hydrophobic-hydrophilic balance of their terminal groups, with x and y respectively referring to the molar percent of -OCONH-C(18)H(37) tails and -OH for p = 1 and the percent of -OCONH-C(18)H(37) tails and -OCO-C(6)H(4)COOH terminal groups for p = 2. In the bulk state, the presence of aromatic rings in (POSS-M)2-(x/y) series resulted in a lower symmetry crystal structure than the (POSS-M)1-(x/y) series. Moreover, the (POSS-M)p-(x/y) molecules that contain a sufficient amount of -OCONH-C(18)H(37) tails exhibit double endothermic transition, which attributed to the melting of alkyl chains followed by the melting of the unit cells of (POSS-M) cores. The surface morphologies for the various hydrophobic-hydrophilic combinations at surface pressure p = 0.5 mN/m are similar to that observed for the classical amphiphilic star polymers. However, at higher surface pressure (p > or = 5 mN/m), the POSS-M compounds with lower content of hydrophilic groups form a uniform monolayer.

Responsive brush layers: from tailored gradients to reversibly assembled nanoparticles

We present a condensed overview of the recent developments of novel responsive thin polymer films from end-tethered chains (polymer brushes), which are different from conventional, uniform, and planar brush layers. For this discussion, we selected two types of recently introduced surface layers: binary brush layers with variable chemical composition forming a controllable gradient of composition and properties in a selected direction and brush layers either grafted directly to inorganic nanoparticles to form hybrid core-shell structures or combined with inorganic nanoparticles embedded into this layer. Unlike traditional brush layers, such a design brings a novel set of responsive surface properties allowing for capillary-driven microfluidic motion, combinatorial-like multiplexing response, reversible aggregation and dis-assembly of nanoparticles, fabrication of ultrahydrophobic coatings, and switchable mass transport across interfaces.