Self-association behavior of hydrophobically modified poly(aspartic acid) in water studied by fluorescence and dynamic light scattering techniques

Poly(succinimide) (PSI), derived from L-aspartic acid by polycondensation, is a reactive precursor for synthesis of various derivatives of poly(aspart ic acid) (PASP), a commercially interesting polyelectrolyte because of its biodegradable properties. Hydrophobically modified PASP was synthesized by reacting PSI with dodecylamine, followed by hydrolysis. The self-associatio n behavior of PASP having pendant dodecyl groups (DDA-PASP) in water was ch aracterized by fluorescence and quasielastic light scattering (QELS) techni ques as a function of the mole percent content of dodecyl groups in the pol ymer (f(Dod)) For fluorescence studies, the polymers were labeled singly wi th pyrene (Py-DDA-PASP) or doubly with naphthalene and pyrene (Np-Py-DDA-PA SP). The vibrational fine structure of pyrene fluorescence spectra and the pyrene fluorescence lifetime for an aqueous solution of Py-DDA PASP at pH = 9 indicated that associations of polymer-bound dodecyl groups started to o ccur at a low f(Dod) (near 3 mol %), and a significant fraction of pyrene l abels were incorporated in hydrophobic microdomains formed. Intrapolymer no nradiative energy transfer from naphthalene to pyrene in Np-Py-DDA-PASP ind icated that a significant contraction of the conformational dimension occur red in the region 20 < f(Dod) < 50 mol % at pH = 9, this f(Dod) region shif ting to 10-30 mol % upon decrease in pH to 4 or upon addition of 0.05 M NaC l at pH = 9. QELS results at pH = 9 in the presence of 0.05 M NaCl indicate d that DDA-PASP with f(Dod) less than or equal to 31 mol % forms two kinds of aggregates with well-defined hydrodynamic radii (R-h), R-h ranging 3-4 n m for one type of the aggregates and 60-90 nm for the other, and they are c oexisting together. The fraction of the larger aggregates increases with po lymer concentration. In contrast, DDA-PASP with f(Dod) = 48 mol % forms onl y small aggregates with R-h On the order of 4 nm independent of the polymer concentration studied (0.25-1.0 g/dL). The mean aggregation number of dode cyl groups in this small aggregate was determined to be ca. 68 using a time -resolved fluorescence method. Since this aggregation number agrees well wi th the number of dodecyl groups per polymer chain (ca. 64) calculated from the degree of polymerization along with f(Dod), this small aggregate is sug gested to be a unimolecular micelle. Accordingly, all the small aggregates with R-h = 3-4 nm found for the polymers with f(Dod) less than or equal to 31 more are also likely to be unimolecular micelles. Consequently, DDA-PASP exhibits a strong preference for intrapolymer hydrophobe self-associations to form a unimolecular micelle if the hydrophobe content is as high as 48 mol %.