Control over phase behavior and solution structure of hairy-rod polyfluorene by means of side-chain length and branching

Abstract

We present guidelines on how the solution structure of pi -conjugated hairy-rod polyfluorenes is controlled by the side-chain length and branching. First, the semiquantitative mean-field theory is formulated to predict the phase behavior of the system as a function of side-chain beads (N). The phase transition at N=N{ *} separates a lyotropic phase with solvent coexistence (N<N{ *}) and a metastable membrane phase (N>N{ *}). The membrane phase transforms into the isotropic phase of dissolved rodlike polymers at the temperature T_{mem}{ *}(N), which decreases both with N and with the degree of side-chain branching. This picture is complemented by polymer demixing with the transition temperature T_{IN}{ *}(N), which decreases with N . For N<N{ *}, the lyotropic phase turns isotropic with increasing T at T_{IN}{ *} . For N>N{ *}, stable membranes are predicted for T_{IN}{ *}<T<T_{mem}{ *} and metastable membranes with nematic coexistence for T<T_{IN}{ *}. Second, in experiment, samples of poly(9,9-dialkylfluorene) with N=6-10 were mixed in methylcyclohexane. For N=8 the side-chain branching was controlled by (9,9-dioctylfluorene)/(9,9-bis(2-ethylhexyl)fluorene) (F8/F2/6) random copolymers. The proportion of F8 to F2/6 repeat units was 100:0, 95:5, 90:10, 50:50, and 0:100. In accordance with the theory, lyotropic, membrane, and isotropic phases with the corresponding phase transitions were observed. For N<N{ *} approximately 6 only the lyotropic phase is present for attainable temperatures. The membrane and isotropic phases are present for N>N{ *}. T_{mem}{ *}(N) decreases from 340 K to 280 K for N > or = 8 . For copolymers, the membrane phase is found when the fraction of F8 units is at least 90%, T_{mem}{ *} decreasing with this fraction. The membrane phase contains three material types: loose sheets of two polymer layers, a better packed beta phase, and dissolved polymer. For N > or = 7 and T<T_{mem}{ *} the tendency for membrane formation becomes stronger with increasing temperature.

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

“…Attention should also be placed on the density fluctuations or aggregation of the sheetlike assemblies. This system may be called a lyotropic phase with solvent coexistence [33] and the fluctuations called "heterogeneities in the liquid crystalline domains" as suggested by Chen et al [30]. We should also be aware of the coexistence of these density fluctuations and assemblies themselves and the coexisting assembly types, i.e., membranes, β sheets, and free chains [33].…”

“…The assemblies are favored by decreasing N both in toluene [31] and MCH [32]. If the assemblies are assumed to be quasi-two-dimensional membranes, the isotropic phase transition temperature (T * mem ) for the membrane generally increases with decreasing N [33] and is unattainable for PF6. 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].…”

“…If the assemblies are assumed to be quasi-two-dimensional membranes, the isotropic phase transition temperature (T * mem ) for the membrane generally increases with decreasing N [33] and is unattainable for PF6. 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].…”

“…Our previous work identified sheetlike assemblies of linear side chain PFs that can either be "loose" or "ordered" [33]. The majority of these assemblies appeared as loose bilayers and were denoted as "membranes."…”

“…The solvent and solution phases coexist inside the cuplike region. We learn from experiment that the case N = 10 is in the single phase region at room temperature [33,35] and therefore has lowχ . We can thus mark the concentration f 10 = f exp .…”