The synthesis of a new series of cationic tail-end polyamphiphiles of the type, poly(ωpyridinium alkyl methacrylate)s with various 4-substituted pyridinium bromide groups and with spacers of 8, 12, and 16 methylene units, obtained by free radical polymerization in aqueous micellar solution, is described, and the thermal and structural characteristics in the solid-state have been studied. All of the polymers show a single glass transition in the range 39-105 °C, whose value depends strongly both on the spacer length, due to internal plasticization, and on the specific nature of the pyridinium moiety, where higher T g's can be related to greater rigidity of the pyridinium group. The polymers are all birefringent between crossed polarizers, in some cases up to their degradation temperatures. X-ray diffraction investigations indicate that they tend to self-organize into an amorphous lamellar morphology that appears to be better defined the longer the alkyl spacer and whose details depend also on the specific pyridinium moiety. Thus, it is shown that the effects of ionic interactions and of the amphiphilic character of the polymers on their thermal and structural properties are clearly modulated by the alkyl spacer length and by mesogen-related parameters such as rigidity and bulkiness.
A methylpyridinium tail-end polyamphiphile with a 12-methylene spacer was complexed
with n-alkyl sulfonate surfactants of 4, 8, and 16 carbon lengths. The thermal and structural properties
of these poly(12-methacryloyloxydodecyl-4-methylpyridinium sulfonate)s in the solid state were studied.
The complexes with the two shorter surfactants self-assemble into a disordered lamellar-like morphology,
with relatively short correlation lengths, similar to the parent Br-neutralized homopolymer. Two glass
transition-like events are observed in the DSC thermograms and are ascribed to nanophase-separated
nonionic and ionic subplanes, respectively. The complex with the longest surfactant displays thermotropic
liquid crystalline behavior, with transitions between two different partially crystalline phases (related
to surfactant tail-end crystallization), a smectic A-like mesophase, and the isotropic phase. Surprisingly,
the lamellar spacings for all of the complexes are very similar to that of the parent polymer. Two possible
structural models taking this into account are discussed.
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