Gelation and Its Effect on the Photophysical Behavior of Poly(9,9-dioctylfluorene-2,7-diyl) in Toluene

Abstract

We probed the gelation and its effect on the photophysical property of a conjugated semiconducting polymer, poly(9,9-dioctylfluorene-2,7-diyl) (PF8), in toluene. The conformational structure characterized by the radius of gyration and the persistence length of PF8 in the dilute toluene solution depended strongly on temperature, where the conjugated chains exhibited a more extended conformation as the temperature was raised. The interchain interaction above the overlap concentration induced aggregation of PF8 to form network clusters in the solution. Gelation took place when the semidilute PF8 solutions were cooled and subsequently aged at −20 °C. The gel composed of the dynamically arrested PF8-enriched phase and the isotropic phase due to the occurrence of a macrophase separation at the subambient temperature. The PF8-enriched phase was mesomorphic, consisting of sheetlike aggregates or membranes. A fraction of the PF8 chains or segments within these sheetlike aggregates formed the β-phase which dominated the photoluminescence of the gel. The gel structure could be disrupted by heating to ca. 45 °C, above which the corresponding PL spectra displayed a blue shift because of reduced amount of the β-phase.

Network structure of polyfluorene sheets as a function of alkyl side chain length

“…Chen et al [29] used PF8 with M n = 128 kg/mol and measured R g ≈ 250 and 460Å in toluene at T = 25 • C and 50…”

“…The values of the parameters used for the repeat length and persistence length for linear side chain PFs were the repeat length of PF8 [39] (∼8.3Å) and the average of the persistence length found for PF8 in toluene in Refs. [19] (85.5 ± 10.5Å) and [29] (∼98Å). The total lengths of the polymers estimated from the degree of polymerization and the repeat length of PF8 fall within the limits 1000-2000Å.…”

“…PF8 in toluene or methylcyclohexane (MCH) [25,26] is an illustrative example of how polymer aggregation is enhanced in the poorer solvent (MCH) or by increasing polymer concentration, decreasing temperature, or aging. In toluene, PF8 is dissolved locally down to the single polymer level when concentration is sufficiently low ( 10 mg/mL) at room temperature but it assembles into a network with associated domains, if the concentration is increased ( 30 mg/mL) [27,28] or if the system is cooled down [29], as comprehensively shown by Chen and co-workers. Stiff chains tend to pack in a unidirectional fashion within the polymer assemblies justifying the term segmental alignment [27].…”

“…The prerequisite for both the agglomerates of polymer assemblies and presumably, gel formation, is polymer overlap as discussed thoroughly by Chen et al [29]. Following the arguments of Ying and Chu [36] and Graessley [37], the polymer overlap concentration is defined as the polymer mass per unit volume:…”

Estudo de sistemas foto- e eletroluminescentes contendo polifluoreno fluorado e rubreno para aplicação em diodos emissores de luz poliméricos.

“…De um modo geral, foi possível obsevar que para diversos tipos de polifluorenos, com variações nas ramificações na posição C-9 e com diversas formas copoliméricas são estruturas que apresentam temperaturas de fusão na faixa entre as temperaturas de 140°C e 170°C e podem apresentar grau de cristalinidade variados, até valores próximos de 50% [88][89][90][91] . Não foi localizado nenhum estudo feito da dopagem do poli[2,7(9,9-dioctilfluoreno)-alt-1,4-fluorfenileno] com o rubreno ou outros dopantes.…”