The present work discusses four types of new experiments that can improve the current theoretical description of nonlinear rheology of entangled polymers. First, a slowly imposed strain is found to result in nonmonotonic evolution of the state of chain entanglement during quiescent relaxation, consistent with the idea of chain disentanglement after step shear. Second, the stress relaxation upon a sizable step strain is found to be identical to that for small step strain, consistent with a molecular scenario that a strained entangled melt has an entropic barrier to resist chain retraction. Third, the ability of a step-strained polymer to undergo elastic recovery is found to be the same up to strain amplitude of unity, and a sample sheared for a period much longer than the Rouse time is shown to still undergo nearly full elastic recovery. Fourth, an entangled melt, stretched at a rate significantly lower than the Rouse relaxation rate, undergoes full elastic recovery until the point of tensile force maximum. We have discussed an alternative conceptual framework to describe these nonlinear responses of entangled polymers despite the possibility that the tube model might be further remedied to characterize the new rheometric measurements presented in this work.

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In rheological characterization of polymeric materials, the time−temperature superposition (TTS) principle allows us to acquire a wider spectrum of information on polymer dynamics. Although there are reports of the failure of TTS when both chain dynamics and local segmental dynamics are accessible at each of several temperatures, we have assumed that TTS would apply to describe the temperature dependence of transient responses of entangled melts to fast startup deformation. In this work we show that at the same effective rate (e.g., the same Rouse−Weissenberg number Wi R equal to the product of the Hencky rate and the Rouse time) of uniaxial extension the nonlinear responses of several polymer melts are each different at different temperatures, pointing to an evident breakdown of the TTS. Specifically, we will show that for the same Wi R well-entangled polymer melts rupture at relatively low temperatures, yet still 20−30 deg above the glass transition temperature T g , but undergo necking-like failure at higher temperatures. Thus, at the same Wi R , stress−strain curves are significantly different at different temperatures. Moreover, at the lower temperature, these polymer melts can reach an extreme level of extensibility at a critical Wi R , which is completely unattainable at higher temperatures. These TTS violating phenomena present a serious challenge to our existing theoretical understanding of nonlinear rheology of entangled polymeric liquids.

We have demonstrated a novel synthesis of mesoscopically ordered stimulus-responsive silica-based hybrid materials through a hierarchical self-assembly process. Two cationic surfactants with a special functional chromophoric group at one end of a carbon chain and a trimethylammonium headgroup at the other end were synthesized and used as both structure-directing agents and functional nano building blocks. Using these two cationic ammonium surfactants with different chromophoric groups at their hydrophobic tails, the micrometer-sized particles with a two-dimensional hexagonal mesostructure and spherical particles with a wormlike mesostructure were produced, respectively. Both of the mesostructured composite particles exhibit strongly fluorescent and pH-responsive behaviors with a rapid and recyclable signal response. Significantly, the symmetry of the self-assembled architecture demonstrated here not only guarantees a high filling and total homogeneous distribution of organic functionalities but also affords precise molecular-scale control over the spatial distribution of the organic component in the mesoscopically ordered inorganic network, which endows the hierarchical materials with mechanical robustness, improved thermal stability, and faster responses to chemical stimuli.

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Molecules constructed from a combination of zero-dimensional ([60]fullerene (C60)) and two-dimensional (porphyrin (Por)) nanobuilding blocks represent an intriguing category of sphere-square "shape amphiphiles". These sphere-square shape amphiphiles possess interesting optoelectronic properties. To efficiently synthesize a large variety of C60-Por shape amphiphiles, a facile route based on Steglich esterification was developed. The synthetic strategy enables the preparation of hydroxy-functionalized Por precursors (9-12) with high purity in a one-pot procedure. All of the C60-Por shape amphiphiles (1-5) can be readily synthesized in good yields through subsequent Steglich esterification with a highly soluble carboxylic acid derivative of methanofullerene (13). Photophysical studies indicated weak electronic coupling between the C60 and Por moieties and suggest an edge-to-face alignment for the moieties. The fluorescence of electronically excited Por portions of each amphiphile was efficiently quenched, which was indicative of electron transfer from (1)Por to the C60 group(s). Increasing the number of C60 groups on the shape amphiphiles led to more pronounced quenching of the Por fluorescence, which indicated the potential for more effective generation of charge-separated species, C60(-.)Por(+.), from the photoexcited C60-Por shape amphiphiles.

Smart nanomaterials: The orientational organization of small organic semiconductors (anthracene, in this case) within periodic nanoscale silica channels (see figure) is achieved through a novel hierarchical self-assembly approach. This elicits interesting optical effects and improved mechanical properties that could be of potential importance for functional materials.A novel hierarchical organic-inorganic self-assembly approach is proposed in driving the orientational organization of small organic semiconductors (anthracene, in this case). A cationic surfactant with the special organic semiconductor anthracene at the hydrophobic tail was synthesized and used as both the structure-directing agent and as functional nanobuilding blocks. The self-assembly procedure was rapid and allowed for the uniform and molecular-level controllable organization of the organic semiconductors within periodic nanoscale silica channels. A range of techniques were used to demonstrate that the photophysical and photochemical nature of anthracene is significantly altered in the inorganic host, consistent with orientational packing of the organic semiconductors and excimer formation within the channels, from which energy migration and significant emission occur. The nanocomposite has also been demonstrated to show an interesting selective sensor function with respect to small solvent molecules. We suggest that this method could be used to drive the assembly of a wide range of organic semiconductor guests, offering the development of a variety of useful, smart nanomaterials that are able to self-assemble in a controllable and robust fashion.