Influence of Solvent Quality on the Self-Organization of Archetypical Hairy Rods−Branched and Linear Side Chain Polyfluorenes: Rodlike Chains versus “Beta-Sheets” in Solution

Abstract

We report on the nanoscale structure and solvent-induced phase behavior of two, nearly similar π-conjugated hairy-rod polymers, branched side chain poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (PF2/6) and linear side chain poly[9,9-dioctylfluorene-2,7-diyl] (PFO or PF8), in good and bad (or poor) solventssdeuterated toluene and deuterated methylcyclohexane (MCH)sat 20°C. Small-angle neutron scattering (SANS) measurements exploiting contrast variation with side chain deuterated PFO polyfluorene have been employed and complemented by optical absorption measurements. In toluene both PF2/6 and PFO adopt an elongated (rodlike) conformation containing predominantly only a single polymer chain (diameter of the order of 1 nm), which indicates dissolution down to the molecular level. In contrast, in MCH, PF2/6 shows an elongated structure while PFO forms sheetlike structures (characteristic thickness of 2-3 nm), thus dissolving down to the "colloidal" level. The elongated structure of PFO consists of individual polymer chains adopting dominantly a conformational isomer C R. The thickness of sheetlike PFO particles corresponds to that of around two polymer layers and side chain contrast variation gives an evidence for an even distribution of the backbones within the sheets. These sheets are potentially an initial stage of PFO crystallization and also contain conformational isomer C of those chains observed in the so-called beta-phase (or beta-sheets) in the solid state. The observed phenomena were not found to depend on concentration over the concentration range 5-10 mg/mL.

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.…”

“…Only PF10 is soluble in MCH or MCH-d 14 at room temperature at the applied concentration. To achieve a complete solution at elevated temperatures and subsequent gelation [26,32] at low temperatures, the mixtures were prepared via a heating-cooling cycle. In a typical procedure the mixtures were first heated up to 80-85…”

“…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.…”

“…Stiff chains tend to pack in a unidirectional fashion within the polymer assemblies justifying the term segmental alignment [27]. In MCH, the sheetlike assemblies appear with decreasing temperature [26] or by room temperature aging driven by macrophase separation, which leads to coexistence of an isotropic phase and an assembly-rich phase with local heterogeneities [30]. The gel-like appearance and optical characteristics of the β phase follow from the polymer assemblies and a sharp transition back to the isotropic phase is observed upon heating.…”

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

“…However, on the basis of recent SANS investigations, it can be concluded that the PFO/MCH system contains sheetlike objects whose thickness is only 2-3 nm, corresponding to the thickness of 2-3 stiff polymers. 31 Such a structure, lacking any threedimensional order, cannot strictly be regarded as a true crystal but is closer to the anisotropic lamellar structures seen in certain ordered smectic phases. In recent unpublished SANS studies, the q range was extended up to 0.45 Å -1 , and no interference maximum or "Bragg peak" was observed.…”