Solution‐crystallization and related phenomena in 9,9‐dialkyl‐fluorene polymers. II. Influence of side‐chain structure

Abstract

Solution‐crystallization is studied for two polyfluorene polymers possessing different side‐chain structures. Thermal analysis and temperature‐dependent optical spectroscopy are used to clarify the nature of the crystallization process, while X‐ray diffraction and scanning electron microscopy reveal important differences in the resulting microstructures. It is shown that the planar‐zigzag chain conformation termed the β‐phase, which is observed for certain linear‐side‐chain polyfluorenes, is necessary for the formation of so‐called polymer‐solvent compounds for these polymers. Introduction of alternating fluorene repeat units with branched side‐chains prevents formation of the β‐phase conformation and results in non‐solvated, i.e. melt‐crystallization‐type, polymer crystals. Unlike non‐solvated polymer crystals, for which the chain conformation is stabilized by its incorporation into a crystalline lattice, the β‐phase conformation is stabilized by complexation with solvent molecules and, therefore, its formation does not require specific inter‐chain interactions. The presented results clarify the fundamental differences between the β‐phase and other conformational/crystalline forms of polyfluorenes. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1492–1506

Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β -phase in Poly(9,9-dioctylfluorene)

“…The absorption spectrum for β-phase embedded PFO films displays the characteristic redshifted peak at about 432 nm, attributed to the S 0 → S 1 0-0 transition for the extended, planar-zigzag-type β-phase chain segments [65,118]. The β-phase absorption peak is especially well defined because (i) β-phase is understood to form with a certain minimum segment length [92], and (ii) the β-phase formation process-namely, co-crystallization with a small-molecular solvent [119]-inherently requires a distinct molecular geometry in terms of both the backbone and the side-chain arrangements [120,121]. Figure 18(b) shows PL spectra of the þ=− β TOF films normalized to their respective S 1 → S 0 0-0 vibronic peaks.…”

Controlling aggregate formation in conjugated polymers by spin‐coating below the critical temperature of the disorder–order transition

“…Here, we present a versatile and very facile approach that allows to manipulate the formation of such aggregates during solution deposition that can complement and be combined with existing methods. We exploit the fact that many π‐conjugated polymers undergo a disorder–order transition in solution upon cooling, and demonstrate that when spin‐coating a solution at a temperature below the critical temperature, aggregates already developed in solution dictate formation of important short‐range ordered features in the solid state. Beneficially, in many cases, the critical temperature for this transition is near room temperature and, thus, a solution can readily be brought to a temperature above or below as desired.…”

Spectroscopic properties of poly(9,9‐dioctylfluorene) thin films possessing varied fractions of β‐phase chain segments: enhanced photoluminescence efficiency via conformation structuring

“…14,15 In addition to this, the remarkably diverse polymorphism of PFO is also the focus of extensive research effort-both fundamental and application-driven-with its characteristic well-defined b-phase conformational isomer receiving particular attention. 8,9,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] When processed from solutions in good solvents (Hildebrand solubility parameter d 9.2 cal 1/2 cm 23/2 as for, e.g., chloroform or tetrahydrofuran) [16][17][18] that also allow for sufficiently rapid solvent evaporation during film deposition (e.g., toluene heated to 100 8C), 15 in-plane isotropic, "glassy" PFO films are obtained. 34 In such films the PFO chains adopt a range of disordered wormlike conformations, collectively also termed "glassy," featuring a broad distribution of intermonomer torsion angles.…”