Controlling the Periodically Ordered Nanostructures in Ceramics: A Macromolecule-Guided Strategy.

Microscopic structures have a significant influence on the properties of ceramics. The development of macromolecular self-assembly has allowed for control over microscopic structures of ceramics to prepare ceramics with diverse compositions and ordered nanostructures. Herein, recent progress in the preparation of ceramics with periodically ordered nanostructures guided by phase-separated macromolecules are reviewed, which can be summarized as a general strategy termed the "macromolecule-guided strategy." Moreover, two different subcategories, namely, the macromolecule-templated method and the macromolecule-precursor method, are illustrated. In the former method, amphiphilic macromolecules are used as templates to guide the assembly of inorganic species into ordered nanostructures, which are subsequently converted into ceramics; in the latter method, amphiphilic macromolecules containing non-volatile elements are used as the single-source precursors for ordered ceramics. It is believed that the unique diversity and tunable features of macromolecular self-assembly might offer unprecedented opportunities in the development of functional ceramics for various applications.

Engineering self-assembly of giant molecules in the condensed state based on molecular nanoparticles.

In biological systems, it is well-known that the activities and functions of biomacromolecules are dictated not only by their primary chemistries, but also by their secondary, tertiary, and quaternary hierarchical structures. Achieving control of similar levels in synthetic macromolecules is yet to be demonstrated. Most of the critical molecular parameters associated with molecular and hierarchical structures, such as size, composition, topology, sequence, and stereochemistry, are heterogenous, which impedes the exploration and understanding of structure formation and manipulation. Alternatively, in the past few years we have developed a unique giant molecule system based on molecular nanoparticles, in which the above-mentioned molecular parameters, as well as interactions, are precisely defined and controlled. These molecules could self-assemble into a myriad of unconventional and unique structures in the bulk, thin films, and solution. Giant molecules thus offer a robust platform to manipulate the hierarchical structures via precise and modular assemblies of building blocks in an amplified size level compared with small molecules. It has been found that they are not only scientifically intriguing, but also technologically relevant.

Pathway toward large two-dimensional hexagonally patterned colloidal nanosheets in solution.

We report the solution self-assembly of an ABC block terpolymer consisting of a polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer tail tethered to a fluorinated polyhedral oligomeric silsesquioxane (FPOSS) cage in 1,4-dioxane/water. With increasing water content, abundant unconventional morphologies, including circular cylinders, two-dimensional hexagonally patterned colloidal nanosheets, and laterally patterned vesicles, are sequentially observed. The formation of toroids is dominated by two competing free energies: the end-cap energy of cylinders and the bending energy to form the circular structures. Incorporating the superhydrophobic FPOSS cages enhances the end-cap energy and promotes toroid formation. Lateral aggregation and fusion of the cylinders results in primitive nanosheets that are stabilized by the thicker rims to partially release the rim-cap energy. Rearrangement of the parallel-aligned FPOSS cylindrical cores generates hexagonally patterned nanosheets. Further increasing the water content induces the formation of vesicles with nanopatterned walls.

Stimuli-responsive nanocomposite: potential injectable embolization agent.

Liver cancer remains a significant medical problem and one promising therapeutic approach is to embolize the tumor. One emerging embolization strategy is to use thermoresponsive materials that can be injected but gel at the tumor site. It is now reported on thermoresponsive nanocomposites generated by grafting poly(N-isopropylacrylamide) chains on bacterial cellulose nanowhiskers. Chemical and physical evidences are provided for grafting and demonstrated a sol-gel transition when the temperature is increased above 34.3 °C. Cytotoxicity test in human umbilical vein endothelial cells indicates the excellent biocompatibility of these nanocomposites for use as embolic materials. These results suggest that the nanocomposites offer appropriate properties for embolization of hepatocellular carcinoma.

Thermoresponsive bacterial cellulose whisker/poly(NIPAM-co-BMA) nanogel complexes: synthesis, characterization, and biological evaluation.

Dispersions of poly(N-isopropylacrylamide-co-butyl methacrylate) (PNB) nanogels are known to exhibit reversible thermosensitive sol-gel phase behavior and can consequently be used in a wide range of biomedical applications. However, some dissatisfactory mechanical properties of PNB nanogels can limit their applications. In this paper, bacterial cellulose (BC) whiskers were first prepared by sulfuric acid hydrolysis and then nanosized by high-pressure homogenization for subsequent use in the preparation of BC whisker/PNB nanogel complexes (designated as BC/PNB). The mechanical properties of PNB was successfully enhanced, resulting in good biosafety. The BC/PNB nanogel dispersions exhibited phase transitions from swollen gel to shrunken gel with increasing temperature. In addition, differential scanning calorimetry (DSC) data showed that the thermosensitivity of PNB nanogels was retained. Rheological tests also indicated that BC/PNB nanogel complexes had stronger gel strengths when compared with PNB nanogels. The concentrated dispersions showed shear thinning behavior and improved toughness, both of which can play a key role in the medical applications of nanogel complexes. Furthermore, the BC/PNB nanogel complexes were noncytotoxic according to cytotoxicity and hemolysis tests. Concentrated BC/PNB nanogel dispersion displayed gel a forming capacity in situ by catheter injection, which indicates potential for a wide range of medical applications.

Construction of CdS quantum dots via a regioselective dendritic functionalized cellulose template.

Using regioselective dendritic functionalized cellulose, CdS quantum dot nanoparticles were prepared and their photo-optical properties and morphology as well as the preliminary biocompatibility of the hybrid were investigated.

Simultaneously strong and tough ultrafine continuous nanofibers.

Strength of structural materials and fibers is usually increased at the expense of strain at failure and toughness. Recent experimental studies have demonstrated improvements in modulus and strength of electrospun polymer nanofibers with reduction of their diameter. Nanofiber toughness has not been analyzed; however, from the classical materials property trade-off, one can expect it to decrease. Here, on the basis of a comprehensive analysis of long (5-10 mm) individual polyacrylonitrile nanofibers, we show that nanofiber toughness also dramatically improves. Reduction of fiber diameter from 2.8 µm to ∼100 nm resulted in simultaneous increases in elastic modulus from 0.36 to 48 GPa, true strength from 15 to 1750 MPa, and toughness from 0.25 to 605 MPa with the largest increases recorded for the ultrafine nanofibers smaller than 250 nm. The observed size effects showed no sign of saturation. Structural investigations and comparisons with mechanical behavior of annealed nanofibers allowed us to attribute ultrahigh ductility (average failure strain stayed over 50%) and toughness to low nanofiber crystallinity resulting from rapid solidification of ultrafine electrospun jets. Demonstrated superior mechanical performance coupled with the unique macro-nano nature of continuous nanofibers makes them readily available for macroscopic materials and composites that can be used in safety-critical applications. The proposed mechanism of simultaneously high strength, modulus, and toughness challenges the prevailing 50 year old paradigm of high-performance polymer fiber development calling for high polymer crystallinity and may have broad implications in fiber science and technology.

Poly(beta-alanoid-block-beta-alanine)s: synthesis via cobalt-catalyzed carbonylative polymerization and self-assembly.

The titled diblock copolymers are synthesized via cobalt-catalyzed living carbonylative polymerization of N-alkylaziridines under moderate pressures followed by a deprotection step. The poly(beta-alanine) block is solubilized by the poly(beta-alanoid) block in chloroform and remains fully hydrogen-bonded in the form of a sheet-like assembly.

Nucleation control in polymer crystallization: structural and morphological probes in different length- and time-scales for selection processes.

Thermodynamically, polymer crystallization is a first-order transition that involves overcoming an energy barrier. Building a molecular kinetic model that links this macroscopic concept with experimental observations has been and still remains a difficult issue. It requires a physical picture that can show how a three-dimensionally random linear macromolecule is converted to a chain-folded crystalline state despite the loss of entropy in the process. There are a number of dynamic molecular pathways during polymer crystallization, and previous analytical models have used a 'mean-field' approach. In polymer crystallization, every macromolecule has to go through several selection processes on different length- and time-scales. In this article, we try to identify these selection processes and lay down some basic principles of polymer crystallization. Experimental observations on stem configurations, helical conformations, crystal structures, fold lengths, global macromolecular conformations and lamellar single-crystal morphologies have been used as probes to identify these selection processes.