Concentration- and pH-Dependent Conformational Changes and Aggregation of Block Copolymers of Poly(methacrylic acid) and Poly(dimethylsiloxane) in Aqueous Media, Based on Fluorescence Spectra of Pyrene and Potentiometry

• Published in: Macromolecules 2004-06, Vol.37 (12), p.4623-4634
• Main Authors: Pinteala, Mariana, Epure, Virginia, Harabagiu, Valeria, Simionescu, Bogdan C, Schlick, Shulamith
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Inorganic and organomineral polymers; Physicochemistry of polymers; Properties and characterization
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma0496697

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