Block copolymers of poly(dimethylsiloxane) and poly(methacrylic acid) (PDMS-b-PMA) with
different molar ratios and lengths of the hydrophobic PDMS and hydrophilic PMA blocks were synthesized
by radical polymerization of methacrylic acid (MA) in the presence of siloxane macroazoinitiators.
Conformational changes and aggregation of the copolymers were examined by fluorescence spectroscopy
of pyrene (P) as a probe and by potentiometric measurements. Fluorescence spectra were measured as a
function of polymer concentration (range from 1 × 10-8 to 0.1 M COOH) and pH (range 3−11), in the
absence and presence of NaCl (0.05 M), and the results were compared with the behavior of PMA as a
reference. The most important parameters deduced from the spectra were the intensity ratio of the first
(∼373 nm) to the third (∼383 nm) vibronic bands (the “polarity ratio” I
1/I
3 in the emission spectra) and
the intensity ratio I
338/I
333 in the excitation spectra for λem = 373 and 383 nm. As the probe is insoluble
in the PDMS blocks, data reflect probe sites associated with the PMA blocks. Conformational changes
suggesting a transition from a compact to an expanded coil were detected, based on the increase of I
1/I
3
for pH > 5. The variation of I
1/I
3 with pH is strongly affected by the PDMS blocks. The increase of I
1/I
3
was less than that in PMA at the same pH; and the pH interval characterizing the expanded coil was
narrower in the block copolymers compared to PMA; moreover, in the limits of high and low polymer
concentrations, the local site for P was more hydrophobic in the copolymers than in PMA. The critical
aggregation concentration of the copolymers, CAC, was determined as a function of pH and the presence
of salt by two methods: based on the intensity of the first vibronic peak, I
1, in the emission spectra and
on the ratio I
338/I
333 in the excitation spectra. CAC values are significantly higher at pH = 8 compared to
pH = 3, typically by 1 order of magnitude. The CAC results are difficult to correlate with the structural
differences such as MA/DMS ratios in the three copolymers. This conclusion may be a result of the
hydrophobic interactions between the methyl groups of PMA and siloxane groups and of pyrene location
in a range of sites that differ in their distance to the hydrophobic core of the aggregates. This study
provided evidence for the role played by the PDMS blocks and polymer concentration on the process of
chain uncoiling at higher pH values, on the appearance of hydrophobic domains, and on the hydrophobicity
of these domains.
The synthesis and viscometric behavior of poly(dimethylsiloxane-b- N-vinylpyrrolidone) in dilute to extremely dilute water solutions was investigated. The viscometric molecular weights, intrinsic viscosities and critical concentrations which separate the dilute-semi-dilute and dilute-extremely dilute regimes were studied. A dependence between reduced viscosity ηsp /c and concentration for extremely dilute regime was found. The deviation from Huggins dependence is discussed.
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