Structure, Photophysics and the Order-Disorder Transition to the Beta Phase in Poly(9,9-(di -n,n-octyl)fluorene)

X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the beta phase in poly(9,9-(di n,n-octyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperature-dependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called beta phase. Rapid thermal quenching is employed for PF8 studies of pure alpha phase samples while extended low-temperature annealing is used for improved beta phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the beta phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative beta phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band. The crystalline phase, as observed in annealed thin-film samples, has scattering peaks which are incompatible with a simple hexagonal packing of the PF8 chains.Comment: Submitted to PRB, 12 files; 1 tex, 1 bbl, 10 eps figure

Poly(trimethylene teraphthalate) crystal structure and morphology in different length scales

Poly(trimethlene teraphthalate) (PTT) is one of the terephthalic polyester family members which can crystallize between its glass transition and melting temperatures. The crystal structure has been determined using both electron diffraction (ED) on single crystals and wide angle X-ray diffraction on powder and oriented fibers. The unit cell is triclinic with dimensions of a = 0.46 (3) nm, b = 0.61 (2) nm, c = 1.86 (1) nm, alpha = 97.5 degrees, beta = 92.1 degrees, and gamma = 110 degrees. The bulk PTT samples usually crystallize to form spherulites in the crystallization temperature (T-c) region studied. Between T-c = 135 and 165 degreesC, the spherulites show a banded texture, and the band spacing increases with increasing T-c. The radial direction in the spherulites has been determined to be the crystal a-axis. Observations of successive ED patterns taken along the radial direction of a spherulite within one band reveal twist of lamellar crystals along the spherulite radial direction. The chain-folding direction, determined using a polyethylene decoration method, is along the a-axis direction in the lamellar crystals and parallel to the radial direction in the spherulites. Linear growth rates of the spherulites have also been measured and the maximum growth temperature is located at 165 degreesC. This temperature is also the upper-limit temperature for PTT banded texture spherulite formation. (C) 2001 Elsevier Science Ltd. All rights reserved

Chain orientation and defects in lamellar single crystals of syndiotactic polypropylene fractions

Sectorization is frequently observed in elongated rectangular single crystals of syndiotactic polypropylene (s-PP) grown from the melt in thin films. The crystals are bound laterally by {100} and {010} growth planes. The constituent sectors are readily observed because of a difference in thicknesses; specifically, the sectors bound by the {100} planes [the (100) growth sectors] are thicker than those bounded by the {010} planes [the (010) growth sectors]. Dark field (DF) transmission electron microscopy (TEM) was utilized to examine the chain orientation and lattice defects in the different growth sectors of s-PP single crystals. The (020) DF images exhibited pairs of bright streaks that are more or less perpendicular to the (100) planes and cross over the whole width of the (100) sectors. In the (200) DF images, the (100) sectors also exhibited similar but dimmer streaks than those in the (020) DF images. This suggests that the crystal c-axis orientation in the (100) crystal sectors undergoes a periodic change in inclination along both the longitudinal (parallel to the b-axis) and the transverse axis (parallel to the a-axis) directions of the single crystal. The ripples in the (100) sectors, previously observed in TEM, were also seen with atomic force microscopy as sinusoidal-like periodic height changes along the longitudinal axis direction at both room temperature and high temperatures. This periodic height change accounted for the pairs of bright streaks in the (020) DF images. The ripple formation was explained by lamellar thickening in the (010) sectors during crystal growth. This thickening process causes lateral contractions, which accumulate mainly along the longitudinal axis direction of the single crystal. On the other hand, the (020) DF images exhibited a relatively uniform brightness in the (010) sectors, while in the (200) DF images, several dark zones in the (010) sectors were more or less along the diagonal directions of the single crystal. This observation indicates that the crystal c-axis in these zones is slightly deviated from the (200) planarity due possibly to the lateral contraction in the (010) sectors. A regular Moiré pattern in the (010) sectors was observed in the (020) DF images, and no Moiré patterns were found in the (100) sectors. Again, this was presumably due to sinusoidal-like ripples, which substantially affect the crystal plane orientation with respect to the lamellar crystal normal. In the (200) DF images, only random Moiré fringes could be found and, in particular, when the (010) and the (100) sectors overlapped. However, regular Moiré fringes were observed continuously over both sectors in (220) DF images

Polymorphism in syndiotactic polypropylene : thermodynamic stable regions for form I and form II in pressure-temperature phase diagram

Polymorphism is a general phenomenon observed in polymers. Syndiotactic polypropylene also exhibits polymorphism, and four limited-ordered modifications have been proposed so far. These modifications are commonly known as form I, II, III, and IV. Form I is normally obtained on cooling an isotropic melt at atmospheric pressure. By in-situ X-ray studies performed at elevated pressure-temperature, we show that, for the first time, form II can be also obtained on cooling the isotropic melt at high pressures. When investigated by NMR, the form II thus obtained is found to be free from conformational defects. We observed that above 1.5 kbar the melting temperature of form II is higher than that of form I. This is in contradiction to the melting behavior of the two different forms below 1.5 kbar; that is, the melting temperature of form II (crystallized at pressures greater than 1.5 kbar) is found to be always lower than for form I below 1.5 kbar. On cooling from the isotropic melt, below 1.5 kbar, the sample crystallizes in the ordered form I. These findings suggest presence of a thermodynamically stable region for form II in the pressure-temperature phase diagram. The difference in crystallization kinetics of the two phases has been also followed at different pressures