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

Abstract

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.

Anomalous dilute solution properties of segmented polydimethylsiloxane–polyurea copolymers in isopropyl alcohol

“…The investigators concluded that in these dilute aqueous solutions of triblocks that differ so significantly in intermolecular interactions, the concentration of the polymer played a major role in globular domain formation. In spite of working with concentrations that were lower than C* (the critical chain overlap concentration), interchain aggregation leading to hydrophilic assembly formation seems to explain the results best [29].…”

“…Other investigators have recently looked at dilute solution properties of poly(methacrylic acid) and PDMS block copolymers of various block molecular weights [29]. These ABA systems have hydrophobic/hydrophilic pairings as do our materials.…”

Optical Properties of Polyelectrolytes

“…In many cases, spherical aggregates are formed from self-assembly of the insoluble blocks into micelle-type structures consisting of a hydrophobic core surrounded by a shell or corona of solvated blocks. The sensitivity of the fluorescence spectrum of pyrene to its environment [63,64] has been exploited to determine [158,159,171,174,176,178,179] the critical micelle concentration (CMC) following aggregation of the block copolymer chains: at low concentrations, only molecularly dissolved chains (unimers) are present which are unable to solubilize pyrene. The use of luminescence spectroscopy has featured prominently in the study of block copolymer micelles: for a representative sample of this extensive literature see, for example, references [71, and references therein.…”

“…Dependent on the constituent components within the block, stimuli responsive micelles can be formed which are critically dependent on pH and salt content. The vibrational fine structure of the emission spectrum of pyrene has also been used to probe the behavior of micelles formed in a variety of diblock copolymer systems in response to changes in the pH [166,173,174] and salt content [158,174]. Valuable information concerning the nature and stability of block copolymer micelles has been derived from fluorescence spectroscopic techniques such as excimer formation [162,163], energy transfer [164,168,169,172,175], and TRAMs [161,165,180].…”

Synthesis and micellization of polydimethylsiloxane– carboxy-terminated poly(ethylene oxide) graft copolymer in aqueous and organic media and its application for the synthesis of core-shell magnetite particles

“…From the fluorescence spectra of pyrene it is possible to assess the local polarity in the probe neighborhood based on the intensity ratio of the first to the third vibronic bands (I 1 /I 3 , referred to a polarity index); higher or lower values of this index are associated with polar or nonpolar environments of the probe, respectively [26]. The excimer formation depends on the proximity and mobility of the probe and provides a reliable method for monitoring local chain conformations in a given system [27].…”