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Solid‐state Polyelectrolyte Complexes of Branched Poly(ethylenimine) and Sodium Lauryl Sulfate

Three lamellar phases (two crystalline and one liquid crystalline) were identified by X‐ray diffraction, DSC, and polarizing optical microscopy in an ionic complex obtained by precipitation on mixing sodium lauryl sulfate (SLS) and branched poly(ethylenimine) (PEI). One of the crystalline structures, which has an orthogonal, bilayer, orthorhombic (crystal E‐like) morphology, is attributed to a small amount of non‐complexed, phase‐separated SLS. The PEI‐SLS complex is characterized by a smectic C bilayer mesophase that is stable up to degradation temperatures, in contrast to the much lower stability of complexes with carboxylate surfactants. It also shows a low degree of crystallinity at lower temperatures, attributed to tail‐end crystallization of the complexed side‐chains, the crystalline phase having a tilted bilayer lamellar structure with hexagonal packing (F or I‐type mesophase). In general, ionic interactions in PEI‐surfactant complexes result in nanophase segregation into ionic and alkyl sublayers giving rise to lamellar liquid crystalline structures whose stability depends on the strength of the ionic interactions.