Development of Intermolecular Structure and Beta-phase of Random Poly[9,9-bis(2-ethylhexyl)fluorene]-co-(9,9-dioctylfluorene) in Methylcyclohexane

Abstract

We present small-angle neutron scattering (SANS) and optical absorption studies of random poly[9,9-bis(2-ethylhexyl)fluorene]-co-(9,9-dioctylfluorene) (PF2/6-F8) mixed with deuterated methylcyclohexane (MCH-d14) at room temperature. The polymers studied have identical main chains and the same number of side chain carbons, but they differ in terms of side chain branching such that the fraction of F8 repeat units with linear octyl side chains was 50, 90 or 95%. Poly[9,9-bis(2-ethylhexyl)fluorene] (PF2/6) and poly(9,9-dioctylfluorene) (PF8) homopolymers were studied for comparison. On the scale of 100 nm or less, the data imply that PF2/6 and PF2/6-F8 (50:50) appear as separated stiff chains (the diameter of the order of 1 nm). PF8 and PF2/6-F8 (10:90 and 5:95) aggregate into stiff sheetlike aggregates (the lateral size of tens of nanometers, the thickness of about 2 nm). This aggregation tendency is an inverse function of side chain branching. On scales over 100 nm, the locally dissolved polymers overlap and form macroscopically isotropic network. In contrast, the polymer sheets form cross-linked ribbonlike agglomerates. This tendency, too, varies with the degree of side chain branching. The extent of β-phase follows the existence of sheetlike structures increasing sharply with increasing F8 content.