No abstract

We have examined the microstructure, solution processing, thermal transitions, and mechanical properties of regioregular poly(nonylbithiazole) (PNBT). PNBT can adopt three distinct colors in the solid stateyellow, red, and metallic greendepending on processing conditions such as polymer concentration, solvent evaporation rate, temperature, and pressure. Microstructural variations were observed between samples of different colors, including the intensification of optical absorption with the degree of crystalline order and/or crystal size. Yellow samples lacked well-developed three-dimensional order, whereas red and green samples were semicrystalline. The development of a lyotropic mesophase was observed at intermediate concentrations in a “diffusion couple” geometry. A crystal model with two planar, π-stacked chains per unit cell was shown to be consistent with the experimental diffraction data. The simulated orthorhombic unit cell parameters are a = 2.38 nm, b = 0.72 nm, and c = 0.79 nm.

No abstract

Certain 4,4'-alkyl substituted 2,2'-bithiazole and bithiazole-thiophene oligomers display an endothermic transition in their DSC trace below their respective melting points. Variable-temperature FTIR, MAS-1H NMR, UV-vis spectra, and XRD all indicate that the thermal transition is due to a crystal-crystal phase transition that we have labeled alpha --> beta. FTIR shows a stepwise increase in the concentration of gauche defects at the alpha --> beta transition temperature, but MAS NMR spectra show little increase in the side chain motion until the mp is reached. UV-vis spectra demonstrate that the conjugated main chains remain essentially planar through the alpha --> beta transition, and significant deviations from planarity occur only at higher temperatures, but well below the mp. The close similarity of this behavior to the phase transitions in long chain n-paraffins and the "side-chain melting" phenomenon in poly(3-alkylthiophenes), P3ATs, suggests that the latter may actually be more accurately described as a crystal-crystal phase transition of the crystalline fraction, driven by side chain disorder.

Figure 4. Electroluminescence spectra of an ITO||PPV: MPNBT (80:20 blend)||Al LED (top) and of an ITO||PPV|MPNBT||Al two-layer LED (bottom).