Presented are transmission electron microscopic observations of micron-sized single crystals of poly(9,9-di-n-octyl-2,7-fluorene) (PFO) prepared from thin films in the melt state. A preliminary determination of unit cell dimensions and molecular packing (orthorhombic, a = 2.56 nm, b = 2.34 nm, c = 3.32 nm, 8 chains, with space group P212121 and density 1.041 g mL-1) was made via combined considerations of the selected-area electron diffraction (SAED) pattern obtained along the [00l] zone of the single crystals, the SAED “fiber” pattern obtained from shear-oriented films, and the “powder” pattern from X-ray diffraction of melt-crystallized thick films in the absence of preferred orientation. In this model, PFO backbones are generally separated by transversely extended alkyl side chains, consistent with the dominance of single-chromophore emissions and the general lack of interbackbone delocalization of PFO chains in this ordered state as indicated by earlier photophysical studies. In addition, microscopic evidence for the presence of a vast number of nanograins was presented, and its implications in the crystallization process of semirigid PFO chains were discussed.

Structural evolution and morphological development in films of poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV, drop-cast from toluene solutions) upon isothermal heat treatment at elevated temperatures were studied by means of a combination of differential scanning calorimetry, polarized light microscopy, X-ray diffraction, transmission electron microscopy, ultraviolet−visible spectroscopy, and photoluminescence spectroscopy. Results indicated that MEH-PPV is mesomorphic in nature (optically biaxial, showing nematic-like texture under cross-polarization), with glass transition temperature T g = ca. 80 °C and isotropization temperature T i = ca. 290 °C. Upon short-term (i.e., 5 min) heat treatment at elevated temperatures (T a) below T i, MEH-PPV chains stack into boardlike entities (ca. 1.6 nm in thickness and ca. 0.4 nm in interbackbone spacing) within beadlike domains ca. 10−20 nm in size, which in turn aggregate transversely into wormlike features ca. 200 nm in length. Shearing at an elevated temperature results in disintegration of the wormlike agglomerates, leaving the beadlike domains arrayed into wavy lines transverse to the shear direction. Accompanying the morphological changes, ultraviolet−visible light absorption and photoexcited emission spectra vary systematically with improvement or disruption of the mesomorphic order.

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PEG is used to reinforce chitosan‐based hydrogels through the formation of ester and amide linkages. The reinforced PEG/chitosan (RPC) hydrogels exhibit significant enhancements in tensile modulus and elongation compared with neat chitosan. Other properties are thoroughly investigated and indicate that the physicochemical and in vitro degradation properties of the RPC hydrogels depend on the amount and molecular weight of the PEG. The RPC hydrogels can control evaporative water loss at a suitable rate to maintain a moist environment. In terms of in vitro biological properties, 3T3 fibroblasts show good viability with the RPC hydrogels, which indicates that the RPC hydrogels may be used as wound dressing materials.

The quantum efficiencies of photoluminescence (PL) and electro‐luminescence (EL) of poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) were significantly increased by heat treatments under vacuum with further removing the undissolved portion. The UV–vis absorption was found to decrease with heating time, while PL intensity increased. The maximum PL quantum yield was 6.5 times that of the untreated MEH‐PPV, which was attributed to the reduction of chain aggregations and the interruption of conjugation length. The maximum EL quantum yield of their prepared ITO/PANI/MEH‐PPV/Ca/AL light emitting diodes (PLED) was 46 (at 3 V) times that of the untreated sample. A typical turn‐on voltage of 2.5 V for MEH‐PPV PLED was able to decrease to 1 V after heat treatments, which was believed to result from the decrease of cis linkages in the polymer chains as revealed by the 1H NMR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1705–1711, 2005