92 Aristeidis Papagiannopoulos and Stergios Pispas
2.4. Fluorescence Spectroscopy
A double-grating excitation and a single-grating emission
spectrofluorometer (Fluorolog-3, model FL3-21, Jobin Yvon-Spex) was used
with air-equilibrated solutions. The excitation wavelength was λ=335 nm and
emission spectra were recorded in the region of 350-500 nm (1nm increment)
with an integration time of 0.5 s. Slit openings of 1mm were used for both the
excitation and the emitted beam. The average of three measurements was used
to determine the I 1 /I 3 ratio (I 1 , I 3 are the intensities of the first and the third
peaks of the pyrene fluorescence spectra at 372 and 383 nm respectively).
RESULTS AND DISCUSSION
In Figure 1 the scattered intensity from mixed solutions of fixed SCPI
concentration (5.128x10-^4 g/ml) and varying DTMAB content are presented.
The corresponding range of the charge ratios (surfactant/polyelectrolyte) is
0.2-1.2. At higher charge ratios phase separation and precipitation took place.
For DTMAB concentrations lower than 4x10-^4 g/ml there is no significant
change in the molecular weight of the polymer aggregates caused by the
presence of the surfactant. At higher surfactant-to-polyelectrolyte mass ratios
the scattered intensity depends strongly on surfactant concentration and
increases systematically. This shows that there is secondary aggregation of the
polyelectrolyte chains due to the presence of surfactant molecules. In the case
of binding DTMAB on to negativelly charged poly(isoprene-b-methacrylate)
amphiphilic polyelectrolyte micelles [11] two regimes were also observed.
However binding of the surfactant in that case did not seem to lead to
secondary aggregation of the micelles. The break between the two regimes of
the surfactant concentration effect upon the scattered intensity was therefore
attributed to formation of surfactant micelles on the micelle corona chains.
The dependence of hydrodynamic radius on the surfactant concentration
clearly illustrates the two regimes of complexation (Figure 2). The
hydrodynamic radius of the diffusing objects is in the order of 60nm which is
much higher than what is expected for individual SCPI chains of the molecular
weight in this study [12]. The reason for the aggregated state of SCPI chains is
the presence of purely hydrophobic units and the intrinsic hydrophobic nature
of the backbone. In the first complexation regime (푐퐷푇푀퐴퐵<4x 10 −^4 g/ml)
the hydrodynamic radius of the SCPI aggregates reduces.