By means of transmission electron microscopy (TEM) and X-ray diffraction (XRD) we demonstrate the presence of a previously unspecified modification (R′) of the crystalline (R) phase in poly(9,9-di-n-octyl-2,7-fluorene) (PFO). The R′ form is metastable and lower in melting temperature than the R form, and its formation is kinetically favored at temperatures lower than 130°C. The modification involves mainly a slight increase in the b axis from 2.34 to 2.38 nm and lowered symmetry along this axis, whereas the a axis tends to lie preferentially normal to film surface. Using specimens composed selectively of R and R′ phases, respectively, we show that films of the two forms are characteristically different in routine 1-D XRD profiles and absorption spectra. These are attributable to the oriented nature of the R′ form as well as minor but significant differences in molecular packing and backbone coplanarity. However, photoexcited emission spectra of these films remain highly similar in the vibronic structure to the purely nematic film, with only minute shifts in emission maxima that are inconsistent with the significantly red-shifted absorption edge upon crystallization. This appears better explained by assuming that the liquid-crystalline matrix rather than crystalline domains dominates optical emission of PFO.

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Structural evolution and its effect on the optical absorption/emission behavior of poly(2,5,2‘,5‘-tetrahexyloxy-8,7‘-dicyano-di-p-phenylenevinylene) (DH-CNPPV) upon short-term isothermal heat treatment at elevated temperatures were studied by means of a combination of X-ray diffraction, transmission electron microscopy, UV−vis spectroscopy, and photoluminescence spectroscopy. Results indicated kinetic preference for mesomorphic order in bulk DH-CNPPV, with the isotropization temperature (T i) located in the vicinity of 210 °C. Upon heat treatment at elevated temperatures (T a) below T i, supramolecular self-assembly of DH-CNPPV chains results in lamellar structure ca. 2.0 nm in layer spacing. This lamellar phase exists in the form of nanodomains (ca. 10−20 nm in diameter) that agglomerate into wormlike features with DH-CNPPV backbones and the mesomorphic lamellae lying transverse to the worm's long axis. Details in the selected-area electron diffraction pattern of shear-oriented specimens prescribe a short intraboard spacing of 0.38 nm and longitudinal displacement by one-third of the repeating unit between neighboring backbones, such that adequate interaction between the cyano group and the phenylene ring is allowed for. Accompanying the thermally induced morphological changes, UV−vis light absorption behavior varies consistently with improvement or disruption of the lamellar order whereas the major emission in the photoexcited spectrum shifts from 590 to 640 nm upon emergence of mesomorphic order, which is directly excitable and hence attributable to either extended backbone conjugation or enhanced interchain delocalization in the mesomorphic state. This is in dramatic contrast to the closely related case of poly(2,5-di-n-octyl-oxy-1,4-phenylenevinylene) (DO-PPV) without cyano substitutions in the backbone (for which excimer emission is the preferred mode upon morphological aggregation), signifying the role of specific cyano−phenylene interaction in the ground state of aggregated chains in the mesomorphic phase.

By means of in situ small-angle X-ray scattering/wide-angle X-ray scattering/differential scanning calorimetry (SAXS/WAXS/DSC), structural evolution of poly(9,9-di-n-octyl-2,7-fluorene) (PFO) in a cold crystallization process was quantitatively examined. After removal of background scattering from the fractal-like structure, SAXS profiles obtained during programmed heating of an amorphous PFO specimen unveil a sequence of stages of structural evolution. These include (1) a frozen-in stage below the glass transition temperature (T g ≈ 62 °C), (2) the nucleation of prolate nanograins with radius of gyration R g ≈ 3.3 nm from 65 to 85 °C, (3) growth of the prolate ellipsoids up to R g ≈ 5.0 nm between 95 and 105 °C, and (4) coalescence (and thickening) of the nanocrsytals into oblates (of R g ≈ 10 nm) upon further heating to 145 °C. There were no further changes in morphological features in the subsequent isothermal annealing at 145 °C for up to 1 h, as the size of the coalesced nanograins quickly reached a threshold value where the orientation and attachment of neighboring nanograins via thermally activated Brownian rotation became seriously hindered. Developments in the DSC-determined crystallinity (X c,DSC) and the degree of heterogeneity (Q inv from SAXS) coincided with the nucleation and growth stages, respectively; in contrast, buildup of the WAXS-determined crystallinity (X c,WAXS) proceeded mainly in the subsequent coalescence stage where large nanocrystallites were developed.