Kinetics and Thermodynamics of Poly(9,9-dioctylfluorene) β-Phase Formation in Dilute Solution

Abstract

Poly(9,9-dioctylfluorene) (PFO) adopts a particular type of conformation in dilute solutions of the poor solvent methylcyclohexane (MCH) below 273 K, which is revealed by the appearance of a red-shifted absorption peak at 437-438 nm. The formation of this ordered conformation depends on the temperature but is independent of polymer concentration over the range studied (3-25 µg/mL). On the basis of absorption, steadystate, and time-resolved fluorescence data, the new absorption peak at 437-438 nm is assigned to a highly ordered conformation of PFO chains, analogous to the so-called -phase first identified in PFO films. From the study of PFO solutions in MCH as a function of temperature, we conclude that these ordered segments ( -conformation) coexist with less ordered domains in the same chain. When the ordered domains are present, they act as efficient energy traps and the fluorescence from the disordered regions is quenched. The transition between the disordered and the ordered PFO conformations is adequately described by a mechanism that involves two steps: a first, essentially intramolecular, one from a relatively disordered (R) to an ordered conformation ( ), followed by aggregation of chains containing -conformation into anisotropic ordered domains. From the temperature dependence of the 437-438 nm peak intensity, the transition temperature T ) 261 K, enthalpy ∆H ) -18.0 kcal mol -1 , and entropy ∆S ) -68.4 cal K -1 mol -1 were obtained. The formation of the -conformation domains were also followed as a function of time at 260 K. The rate constants at 260 K were determined, showing an order of magnitude around 10 -3 s -1 (k Rf ) 5.9 × 10 -4 s -1 ; k fR ) 9 × 10 -4 s -1 ; k agg ) 2.3 × 10 -3 M -1 s -1 ; k diss ) 4.4 × 10 -4 s -1 ). This small magnitude explains the long times required for a "complete" conversion to the -conformation.

Polyfluorene based blends for white light emission

“…The LaPPS10 photoluminescence decays in degassed chloroform solution (10 À5 mol L À1 ) (k exc = 320 nm, k em = 440 nm) is bi-exponential with s 1 = 0.48 ns (54%) and 0.79 ns (46%) ( Table 1) and becomes mono-exponential in films with a lifetime of 0.31 ± 0.07 ns. This lifetime range is in accordance with those reported for polyfluorenes in diluted solution (from 0.08 to 5.0 ns) [36,[49][50][51][52]. The decrease in lifetime in the solid state can be attributed to the formation of aggregates as well as to several possibilities of quenching processes in the solid state, apart from resonant energy transfer from disordered to b phases.…”

Modulating the Emission Intensity of Poly-(9,9-bis(6‘-N,N,N-trimethylammonium)hexyl)-Fluorene Phenylene) Bromide Through Interaction with Sodium Alkylsulfonate Surfactants

“…The longest decay time is assigned to the pure polymer lifetime, since it is the dominant component at sufficiently high surfactant concentrations and its value is of a similar order of magnitude to that of other polyfluorene lifetimes: 360 ps for PBS-PFP in water 13a and 340 ps for poly(9,9-dioctylfluorene) in MCH at 298 K. 25 From the fluorescence quantum yield (0.53) and lifetime (508 ps) of HTMA-PFP in 4% v/v DMSO-water in the absence …”

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

“…At room temperature, the optical characteristics matching those of the PF8 β phase are found for PF7 and PF9 in MCH both at high (∼1 vol %) [32] and low (∼0.002 vol %) [34] concentrations, while the same is seen for PF10 in thin films [35], with its thermal dependency following the lower T * mem [33]. The presence of sheetlike assemblies [26] or membranes [33] in PFs mixed with MCH (or PF N -MCH) is well established and discussed in terms of their internal structure [32] and macrophase separation [30], thermal behavior [30,32], and the existence of β phase [25,34,35]. Attention should also be placed on the density fluctuations or aggregation of the sheetlike assemblies.…”

“…Elsewhere, Kitts and Vanden Bout [24] worked with PF8 (M n = 34.8 kg/mol) in 0.001 wt% (or ∼0.0009 vol %) toluene and Dias et al [25] with PF8 (M n = 188 kg/mol) in 3 μg/mL (or 0.0003 vol %) MCH and observed optical characteristics of β phase when cooling the solutions (down to ∼200 K, for example). Our simple consideration gives values c * ∼ 0.09 vol % and ∼ 0.05 vol %, implying that the condition c c * holds for both cases.…”

“…In our notation this factor is characterized by the number of side chain beads, N. [24] studied PF8 and demonstrated how various solvents have significant effects on the optical characteristics matching the twisted and planar polymer chains, with the latter being favored by poor solvents. 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.…”