Critical interaction strength for surfactant-induced mesomorphic structures in polymer-surfactant systems

The critical interaction strength to induce mesomorphic structures in flexible polymers by complexing with surfactants is determined by using surfactants with different hydrogen-bonding strengths;. Two essential requirements have to be satisfied: (i) the association has to be strong enough, otherwise the structure dissociates. (ii) The polar-nonpolar repulsion has to be sufficiently strong to induce microphase separation. If the latter requirement is not satisfied, but the association is sufficiently strong, a characteristic comb polymer-like SAXS peak will still be present in the homogeneous melt state. To address these issues experimentally, poly(4-vinylpyridine) (P4VP) was combined with different classes of hydrogen-bonding surfactants: alkyl aromatic alcohols, alkyl carboxylic acids, alkyl aliphatic alcohols, and alkyl aliphatic amines. FTIR, SAXS, WAXS, and optical microscopy demonstrate that alkyl aromatic alcohols, such as 1-dodecyl 3,4,5-trihydroxybenzoate yield mesomorphic structures at not too high a temperature. Alkyl carboxylic acids are somewhat less effective. In the melt state, the SAXS data still show a peak, but because no birefringence could be found, the peak is attributed to block copolymer-like concentration fluctuations in an otherwise homogeneous state. Aliphatic alkyl alcohols yield still weaker hydrogen bonding. In the melt the short alkyl aliphatic alcohol surfactants form a homogeneous mixture with P4VP, but a SAXS peak is no longer observed. The longer ones macrophase separate. Both the alkyl carboxylic acids and the short alkyl aliphatic alcohols partly macrophase separate at lower temperatures due to crystallization