Tianbai He

Single Crystals of Polythiophene with Different Molecular Conformations Obtained by Tetrahydrofuran Vapor Annealing and Controlling Solvent Evaporation

Single crystals of poly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT) have been prepared by tetrahydrofuran vapor annealing and controlling solvent evaporation, respectively. The morphology and structure of the single crystals are characterized using optical microscopy, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and wide-angle X-ray diffraction. It is observed that in P3HT single crystals, the molecules are packed with pi-pi stacking direction perpendicular to the length axis of the crystals and main chains parallel to the substrate, whereas in P3OT single crystals, the molecules are packed with pi-pi stacking direction parallel to the length axis of the crystal and main chains parallel to the substrate. In the field effect transistors, the current flow is parallel to the length axis of the single crystals, and the mobility is 1.57 x 10(-3) cm(2)/Vs for a P3HT single crystal and 0.62 cm(2)/Vs for a P3OT single crystal. The single crystals of P3HT and P3OT showed high anisotropic electrical properties. The influences of molecular conformation and alkyl chain length on the electrical properties of P3ATs are discussed.

Large-area 2D gold nanorod arrays assembled on block copolymer templates.

Various large-area 2D gold nanorod arrays are achieved on plasma-etched block copolymer templates. With the help of capillary forces, aqueous gold nanorods assembled on the templates show good position selectivity and high coverage of the grooves. Furthermore, these nanorod arrays can transform into gold nanowires by a post-etching process.

Direct measurement of plowing friction and wear of a polymer thin film using the atomic force microscope

Nanometer-scale plowing friction and wear of a polycarbonate thin film were directly measured using an atomic force microscope (AFM) with nanoscratching capabilities. During the nanoscratch tests, lateral forces caused discrepancies between the maximum forces for the initial loadings prior to the scratch and the unloading after the scratch. In the case of a nanoscratch test performed parallel to the cantilever probe axis, the plowing friction added another component to the moment acting at the cantilevered end compared to the case of nanoindentation, resulting in an increased deflection of the cantilever. Using free-body diagrams for the cases of nanoindentation and nanoscratch testing, the AFM force curves were analyzed to determine the plowing friction during nanoscratch testing. From the results of this analysis, the plowing friction was found to be proportional to the applied contact force, and the coefficient of plowing friction was measured to be 0.56 +/- 0.02. Also, by the combination of nanoscratch and nanoindentation testing, the energetic wear rate of the polycarbonate thin film was measured to be 0.94 +/- 0.05 mm(3)/(N m).

Study on the Single Crystals of Poly(3-octylthiophene) Induced by Solvent-Vapor Annealing.

Needle-like single crystals of poly(3-octylthiophene) (P3OT) have been prepared by tetrahydrofuran-vapor annealing. The morphology and structure of the crystals were characterized with optical microscopy, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and wide-angle X-ray diffraction. It is observed that the P3OT molecules are packed with the backbones parallel to the length axis of the crystal and the alkyl side chains perpendicular to the substrate. The field effect transistor based on the P3OT single crystal exhibited a charge carrier mobility of 1.54 x 10(-4) cm(2)/(Vs) and on/off current ratio of 37, and the molecular orientation of the crystal is ascribed to account for the device performance. The time-dependent morphological evolution demonstrated that the crystals underwent Ostwald ripening when annealed.

Improved thermal fractionation technique for chain structure analysis of ethylene/α-olefin copolymers

In this report, we describe an improved thermal fractionation technique used to characterize the polydispersity of crystalline ethylene sequence length (CESL) of ethylene/alpha -olefin copolymers. After stepwise isothermal crystallization, the crystalline ethylene sequences are sorted into groups by their lengths. The CESLs are estimated using melting points of known hydrocarbons. The content of each group is determined using the calibrated peak area. The statistical terms: the arithmetic mean (L) over bar (n), the weighted mean (L) over bar (w) and the broadness index I = (L) over bar (w)/(L) over bar (n) are used to describe the distribution of CESL. Results show that improved thermal fractionation technique can quantitatively characterize the polydispersity of CESL with a high degree of accuracy.

The effect of the preferential affinity of the solvent on the microstructure of solution-cast block copolymer thin films.

We have studied the microstructures of the solvent-cast films of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer with PMMA cylinder-forming composition. The casting solvents, 1,1,2-trichloroethane (TCE), toluene (Tol) and their binary mixtures, were controlled to evaporate at a certain rate R approximately 0.01 mL/h, and the effect of the preferential affinity of the solvents for a certain block P2VPng microstructures is investigated. 1,1,2-Trichloroethane and toluene, with similar volatility, are good solvents for both PS and PMMA blocks but having opposite preferential affinity for each block (TCE has preferential affinity for the minority PMMA block and Tol has preferential affinity for the majority PS block), and the preferential affinity of the solvents is modulated by mixing the two solvents. As the preferential affinity of the solvents for the majority PS block increases, a series of microstructures including perforated lamella with ringlike morphologies, PMMA cylinders, PMMA spheres, and PMMA cylinders covered with a layer of PS microdomain have been observed. The results are discussed in view of the effective volume fraction of each block induced by the solvent preferential affinity and the special mechanical strain field brought by the solvent evaporation.

Molecular heterogeneity of metallocene short-chain branched polyethylenes and their fractions

A comparison on the molecular heterogeneity of two whole metallocene short-chain branched polyethylenes (SCBPEs) with different hexene comonomer (butyl branching) content and their Cross-Fractionated (CF) fractions has been made. As elucidated by differential scanning calorimetry (DSC) thermal segregation experiments and the subsequent isothermal crystallization kinetics investigations, the whole metallocene SCBPEs were found to possess both inter- and intramolecular heterogeneity. The crystallization kinetics of whole SCBPEs are substantially different for samples with or without a pre-multiple-step annealing treatment. For the CF fractions, the inter-molecular heterogeneity is not evident, but intra-molecular heterogeneity exists. Nevertheless, another self-induced SCBPE fraction, as a model sample, shows good homogeneity both inter- and intramolecularly. The crystal morphology observed via transmission electron microscopy (TEM) shows that multiple-step crystallized whole polymers exhibit large scale (molecularly) phase separation, whereas the CF fractions demonstrate microscopic (segmental) segregation only.

The polydispersity of ethylene sequence in metallocene ethylene/α-olefin copolymers III: crystallization and melting behavior of short ethylene sequence

Crystallization and melting behavior of short ethylene sequence of metallocene ethylene/alpha -olefin copolymer with high comonomer content have been studied by standard DSC and modulated-temperature differential scanning calorimetry (M-TDSC) technique. In addition to high temperature endotherm around 120 degreesC, a low temperature endotherm is observed at lower temperatures (40-80 degreesC), depending on time and temperature of isothermal crystallization. The peak position of the low temperature endotherm T-m(low) varies linearly with the logarithm of crystallization time and the slope, D, decreases with increasing crystallization temperature T-c. The T-m(low) also depends on the thermal history before the crystallization at T-c, and an extrapolation of T-m(low) (30.6 degreesC) to a few seconds has been obtained after two step isothermal crystallization before the crystallization at 30 degreesC. The T-m(low) is nearly equal to T-c, and it indicates that the initial crystallization at low temperature is nearly reversible. Direct evidence of conformational. entropy change of secondary crystallization has been obtained by using M-TDSC technique. Both the M-TDSC result and the activation energy analysis of temperature dependence suggest that crystal perfection process and conformational entropy decreasing in residual amorphous co-exist during secondary crystallization.

Coupling of Microphase Separation and Dewetting in Weakly Segregated Diblock Co-polymer Ultrathin Films

We have studied the coupling behavior of microphase separation and autophobic dewetting in weakly segregated poly(ε-caprolactone)-block-poly(L-lactide) (PCL-b-PLLA) diblock co-polymer ultrathin films on carbon-coated mica substrates. At temperatures higher than the melting point of the PLLA block, the co-polymer forms a lamellar structure in bulk with a long period of L ∼ 20 nm, as determined using small-angle X-ray scattering. The relaxation procedure of ultrathin films with an initial film thickness of h = 10 nm during annealing has been followed by atomic force microscopy (AFM). In the experimental temperature range (100-140 °C), the co-polymer dewets to an ultrathin film of itself at about 5 nm because of the strong attraction of both blocks with the substrate. Moreover, the dewetting velocity increases with decreasing annealing temperatures. This novel dewetting kinetics can be explained by a competition effect of the composition fluctuation driven by the microphase separation with the dominated dewetting process during the early stage of the annealing process. While dewetting dominates the relaxation procedure and leads to the rupture of the ultrathin films, the composition fluctuation induced by the microphase separation attempts to stabilize them because of the matching of h to the long period (h ∼ 1/2L). The temperature dependence of these two processes leads to this novel relaxation kinetics of co-polymer thin films.

Gold nanoparticle arrays assembled on the reconstructed surface of block copolymer thin films

Poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) block copolymer thin films were used as templates for the large-area site-selective assembly of as-synthesized gold nanoparticles. The negatively-charged nanoparticles were placed on the exposed positively-charged protonated P2VP phase mainly through electrostatic interaction. Various nanoparticle arrays with different density and position selectivity were obtained rapidly and straightforwardly by controlling the assembly time. Highly ordered linear gold nanoparticle chains could be fabricated through the optimization of the assembly process. Moreover, the surface evolution of the block copolymers templates such as surface reconstruction was found to be a critical factor that significantly affects the assembly position of the nanoparticles, since the solvent from the nanoparticle solution could alter the block copolymer surface in our case. Due to the tunable plasmonic property, it is believed that these ordered nanoparticle arrays have a wide variety of potential applications in optical, sensing and SERS fields.