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 (NN*). 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 NIN^*. For N>N*, stable membranes are predicted for T_IN^*mem^* and metastable membranes with nematic coexistence for TIN^*. 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 NN*. T_mem^*(N) decreases from 340 K to 280 K for N>=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>=7 and Tmem^* the tendency for membrane formation becomes stronger with increasing temperature.